cdte and related compounds; physics, defects, hetero- and nano-structures, crystal growth, surfaces...

CHAPTER II

CDTE and Related CompoDOI: 10.1016/B978-0-08-

Applications

Contents
IIa. Photorefractive CdTe 148
1. Introduction 148

2. The Photorefractive Properties 149

2.1. The photorefractive effect 149

2.2. Carriers photogeneration quantum yield and

wavelength sensitivity range 151

2.3. Trapping level in the photorefractive crystal 152

3. Experimental Results 153

3.1. Photorefractive properties of CdTe:V 153

3.2. Effect of Zn content on photorefractive

properties 169

4. Applications 178

4.1. DPCM experiments 178

4.2. DPCM between single-mode fibres 182

5. Conclusion 183

References 185

IIb. Cadmium Telluride-Based Solar Cells 187

1. Introduction 187

2. State of the Art 189

3. Device Properties 189

und096

3.1. Back contact effects 189

3.2. Inter-diffusion at the CdTe/CdS junction 193

4. Fabrication of Cells 196

4.1. Deposition of the absorber CdTe layer 196

4.2. Deposition of the CdS window layer 197

4.3. Post-growth annealing in chlorine 197

4.4. Back contacts 199

4.5. Transparent conducting oxide front contacts 201

4.6. Alternative structures 202

5. Manufacture of CdS/CdTe Modules 206

6. Degradation Mechanisms 207

7. Use of CdS/CdTe Modules in Large-Scale Power

Generation 208

s # 2010 Published by Elsevier Ltd.513-0.00002-9

145

146 Applications

8. Concluding Remarks 210

References 211

IIc. Applications of CdTe, CdZnTe, and CdMnTe Radiation

Detectors 214

1. Introduction 214

2. National Security and Nonproliferation Inspections 215

3. Medical Imaging 217

3.1. Gamma (g)-camera 217

3.2. Digital mammography 222

3.3. X-ray computed tomography (CT) 223

4. Space and Astrophysics 228

5. Nature and Development of CMT Detectors 233

6. Summary and Future Work 236

Acknowledgments 237

References 237

IId. Electro-optic Modulator Applications 239

1. Introduction 239

2. Practical Configurations 240

3. Issues and Limitations 241

3.1. Mechanical 241

3.2. Optical 242

3.3. Electrical 243

4. Successful and Contemplated Deployments 245

4.1. Laser Q-Switching 245

4.2. Laser Cavity Dumping 246

4.3. Optical Pulse Shaping 246

4.4. Free-Space Optical Communications 246

4.5. Optical Frequency and Phase Modulators 247

4.6. Laser Intracavity Modulation and Mode Locking 247

References 247

IIe. Optical Detectors Based on CdTe Pure Crystals for

High-Efficiency Optical Computers 248

1. Introduction 248

1.1. Optical computers based on semiconductor

structures 248

1.2. Optical registering media in contemporary

optical processors on mis structures 249

1.3. Fast optical registering media on semiconductor

M(TI)S-nanostructures 250

2. Processors for Digital Optical Computers Based

on n-p(TI)M Nanostructures of CdTe 250

Applications 147

3. Processors for Analog Optical Computers

of Incoherent Light on n-p(TI)M Nanostructures

on CdTe 252

4. Optoelectronic Image Correlator of Incoherent

Light Based on Analog Optical Processors 254

5. Conclusion 255

References 255

CHAPTER IIA

4 A Route Crech Argant, F

148

Photorefractive CdTe

J.-Y. Moisan

1. INTRODUCTION

For optical telecommunication networks, optical switching systems havebeen studied, and some systems using integrated optics have been pro-posed, but a spatial holographic interconnect is also an attractive solutionfor switching of high bite rate channels. Holographic gratings can be usedto steer the optical beams, emerging from an input matrix of single-modeoptical fibres to an output matrix of single-mode optical fibres. Twocharacteristics have to be fulfilled in such a system: it must be active atthe telecommunication signal wavelength, that is, 1.3 and 1.5 mm, andmust be managed as large a number of channels as possible.

Photothermoplastic devices have been proposed [1] and, in such anoptical configuration, two recording beams are used in the visible range(their wavelength depending on the sensitivity of the recording material)and their reading beams, at 1.3 or 1.5 mm, are deflected by the recordedgratings. In this case, the photothermoplastic device is not sensitive to thesignal wavelength.

With photorefractive materials, it is possible to imagine an optical sys-temwhere the signal beam is active itself. Thus, in two-wavemixing (TWM)experiments, which are commonly used to estimate the properties of photo-refractive crystals, one can consider that the studiedmaterial is active to thewavelength used. In anoptical switching systemused in anoptical network,it is essential that the photorefractive crystals are sensitive to the communi-cation wavelengths; this is the first requirement. The second requirement,concerning aberrations in optical configurations, is that the single-modefibres could be used as an input and output signal source, with little loss.

-22730 Tregastel, France

Photorefractive CdTe 149

First, the photorefractive effect will be presented and the propertiesdiscussed. Next results, obtained with CdTe materials, will be given anddiscussed. And finally, an optical configuration will be presented and thefirst results of a beam-steering system presented.

2. THE PHOTOREFRACTIVE PROPERTIES

2.1. The photorefractive effect

The photorefractive effect, which can be described as the combination oftwo properties, namely, photoconductivity and electro-optic effect, isexplained in Fig. 1. Two interfering beams are focused on the crystal; socarriers are photogenerated in the illuminated zones of the material(Fig. 1A). These carriers diffuse into the non-illuminated zones where theyare trapped (Fig. 1B), and a non-homogeneous density of carriers

Δn(z) ~ –Esc(z)

s�(z) ~ Esc(z) ~ ∫Psc(z)dz

rsc(z)

I(z)

–

– –

– – ––

–

++++

y

A

B

C

D z

z

z

z

z

fg

Λ

Figure 1 (A) Illuminating energy, following the interference pattern on the crystal, and

generation of carriers. (B) Inside density of carriers? (C) ESC, space charge field built up by

the carrier density. (D) Index modulation created in the crystal and phase shifted from

the interference pattern.

150 J.-Y. Moisan

reproduces the interference pattern. Because of the modulated density oftrapped carriers, a space charge field occurs (Fig. 1C),which is phase shiftedby p/2 from the interference pattern. When the electro-optic coefficient ofthe crystal is no longer negligible, a modulation of the refractive index canbe observed (Fig. 1D). So, a phase grating shifted from the recording inter-ference pattern can be read from the recording beams themselves or from areading beam at a wavelength not active for the material.

The basic equation that describes the index modulation Dn in thecrystal is

Dn0 ¼ 1

2n3r41ESC ð1Þ

where n0 is the refractive index, r41 the electro-optic coefficient dependingon the crystal orientation and ESC the space charge field.

The most commonly used optical experiment for studying the photore-fractive behaviour of materials is TWM, and the configuration for thisis shown in Fig. 2. Two beams interfere on the crystal: one – the pumpbeam–with a higher energy than the other – the signal beam. Because of thegrating recorded inside the crystal, the pump beam ‘sees’ this index gratingand part of its energy is deflected in the signal beam direction; if thisdeflected energy is higher than the losses due to the signal beam absorptionand the deflected energy of the signal beam, an amplification gain can bemeasuredwith the detector. Note that, in this case, the two beams are activeand are used as recording and reading beams. FromEq. (1), it is clear that n0and r41 are the intrinsic properties of the crystal and depend only on thechosen material and for a given wavelength, and that ESC depends on theexperimental parameters (and also on the crystal, briefly listed on Table 1).

Detector

PRC

Signalbeam BS

Mirror

Pumpbeam

Mirror

Figure 2 Two-beam coupling configuration: PRC, photorefractive crystal; BS, beam splitter.

Table 1 Figures of merit of photorefractive materials where the r values are r41for the crystals, except for organic materials

Material n0 r (pm V�1) n03r (pm V�1) l (mm)

LiNbO3 2.26 31 360 0.633

BaTiO3 2.36 1640 21500 0.546

Bi12SiO20 2.54 5 82 0.633

Bi12GeO20 2.55 3.5 58 0.633

GaAs:EL2 3.48 1.43 60 1.06GaAs:Cr 3.5 1.2 51 1.06

InP:Fe 3.29 1.34 48 1.06

CdTe:V 2.82 5.5 123 1.06

CdTe:V 2.82 5.5 120 1.52

Polymer 1.56 2.5 9.5 .514

Organic crystal 1.7 24 118 .676


2.2. Carriers photogeneration quantum yield and wavelengthsensitivity range

The absorption of a CdTe:V crystal is reported in Fig. 3. It is clear that thesemiconductive crystals can be used only at wavelength corresponding tolower energies than the band gap, where the crystals are sufficientlytransparent. In Fig. 3, the 1.3 and 1.5 mm wavelengths are indicated byvertical lines, and the residual absorption, which is said to come fromdoping agent V, is observed. A similar spectrum has been published [2]for ZnTe:V, and also the photoconductivity studied in this way; the

120000.0

0.25

0.5

0.75

1.0

10000

WAVENUMBERS CM–1

8000 6000

Figure 3 Absorption spectrum of CdTe:V. The ordinate is absorbance. The thickness

was 5 mm. Vertical lines indicate 1.3 and 1.5 mm wavelengths.

152 J.-Y. Moisan

formation time of the holographic grating was measured by following thetotal intensity of the two beams. It clearly appears that the formation timeobeys the same law as the photocurrent in the material. This means thatthe photorefractive effect is linearly related to the photogeneration quan-tum yield. Therefore one can expect from the spectrum in Fig. 3 that CdTe:V will be sensitive to the 1.3 and 1.5 mm wavelengths, which has alreadybeen demonstrated [3]. So, two aspects have to be emphasised: the gener-ation quantum yield governs the response time of the material, but thenumber of trapped carriers will govern the space charge field.

2.3. Trapping level in the photorefractive crystal

Because it comes from the trapped carrier density, a large space chargefield will be observed in two conditions: when the carriers are trapped indeep traps, and when the number of traps is large. For II–VI crystals, it iswell known [4] that doping with V, Ti or Ni can induce deep levels,especially in CdTe and ZnTe.

The dopant solubilities are larger in II–VI material than in III–V [5].This means that the photocurrent will be larger and the response timeshorter but, above all, that the number of trapped carriers will be largerand, as a consequence, the space charge field larger.

Consider now, in Fig. 1, the diffusion of carriers; when some of themare trapped in the dark zones, the space charge field begins to increase.However, a conduction current in the opposite direction appears, andequilibrium will limit the space charge field when the conduction currentbecomes equal to the diffusion current. In these conditions, the electricfield inside the crystal is

ESC ¼ 2plkBT

eð2Þ

where l is the wavelength used, kB is the Boltzmann constant and Tthe temperature. Different solutions have been proposed to exceed thislimit.

1. A continuous electric field is applied to the crystal [6], but the phaseshift between the interference pattern and the electric field modula-tion must be preserved. So another limit will now appear.

ESC ¼ eLNA

2p e e0ð3Þ

where L is the grating period, NA the number of effective traps and ethe dielectric constant of the material. Following the grating period, theeffective limit can come from the resistivity of the material, limiting theexternal applied electric field, especially in semiconductor crystals.


To avoid too large a dark current, highly resistive materials are needed(108–109 Ocm).

2. Two others solutions have been proposed to exceed the limit inEq. (3): (a) an applied d.c. field and a moving grating [7]; (b) anapplied a.c. electric field, of sinusoidal or square wave nature [8].However, the same requirement of high resistivity in needed forboth these conditions.

3. Note that the resonance mechanisms have also been proposed toincrease the gain in TWM experiment: a Franz-Keldysh effect nearthe band gap [9], and an intensity-dependent resonant behaviourdepending on the temperature of the material [10].

3. EXPERIMENTAL RESULTS

3.1. Photorefractive properties of CdTe:V

Due to its high electro-optic factor of merit, CdTe:V is very attractive[11, 12]. At 1.32 mm, TWM gains larger than 10 cm�1 have been obtainedby Ziari [11], with a large amplitude (23 kV/cm) high-frequency (23 kHz)square-shaped electric field at 75 mW/cm2 pump intensity. Similarresults have been observed with a CdTe:V sample [13, 14] calledDAV31. In order to present and discuss photorefractive properties ofCdTe crystals, three crystals (called DAV30, DAV31 and S2105) will bestudied. It allows observing the different measurement parameters effecton TWM gain presented previously.

3.1.1. Experimental detailsThe crystals were grown by the Bridgman technique, at CNRS Bellevuefor DAV30 and DAV31 and at CNRS Strasbourg for S2105. In the melt4% and 1% Zn was added respectively, which is expected to improvethe mechanical properties of the crystals. The main characteristics aregiven in Table 2. The crystals were cut in 5 � 5 � 5 mm3 cubes withedges aligned along the h110i, h111i and h112i crystallographic direc-tions. In TWM experiments, the polarisation of the two beams lieswithin the plane of incidence; these beams form a grating along theh111i direction. They are expanded to provide uniform illumination onthe sample.

Three wavelengths were used: 1.05, 1.32 and 1.54 mm.When necessarythe crystal temperature was stabilised, using a Peltier effect device,from 275 to 305 K. The electron–hole competition factor x was measured,without external field, using the experimental procedure described inRef. [18].

Table 2 Main characteristics of crystals DAV31 and S2105

DAV30 DAV31 S2105

[Zn] (%) 4 4 1

Starting [V] (cm�3) 1019 2 � 1019 9.4 � 1018

[V] (cm�3) 0.5 � 1017 1.7 � 1017 4.5 � 1017

(SIMS)a (SIMS)a (A.A.)b

r (O cm) 1.25 � 1010 1010 5 � 109

a (cm�1) (1.048 mm) 1.90 4.88 4.82(1.32 mm) 1.23 2.24 2.94

(1.535 mm) 0.86 1.85 2.62

x (1.048 mm) �0.12 (e�) �0.08 (e�) �0.69 (e)

(1.32 mm) 0.68 (hþ) �0.35 (e�)(1.535 mm) 0.84 (hþ) 0.87 (hþ) 0.05 (hþ)

Neff (1015 cm�3) (1.048 mm) 0.26 3

(1.32 mm) 1.37

(1.535 mm) 0.84 1.74

aSIMS: secondary ion mass spectrometry.bA.A.: atomic absorption.

154 J.-Y. Moisan

3.1.2. TWM gains without an external electric fieldIn the electro-optic configuration given above, the TWM gain G is calcu-lated, assuming low-intensity (cw) illumination, one deep-level modeland one small grating period L [15] as

G ¼ 2pl0 cosy

2

√3xn03r41

EdEq

EdþEqð4Þ

or, in more succinct, form

G ¼ AxEdEq

EdþEqð5Þ

where l0 is the wavelength, y the half-angle of the two beams in the TWMsetup, n0 the refractive index, Ed the diffusion field and Eq, the maximumspace-charge field.

Ed ¼ kBTk

eK; Eq ¼ e

eKNeffNeff; Neff ¼ nT0pT0

nT0þpT0ð6Þ

Here K (2p/L) is the grating vector, and Neff is the number of effectivetraps. The factor x [10] is

x ¼ sppT0 � snnT0ðsp

0pT0 þ sn0nT0Þð1þ Id=IoÞ ð7Þ


where

Id ¼ enthnT0 � ep

thpT0

sn0nT0 þ sp0pT0ð8Þ

and I0 is the mean illumination, sp0 and sn

0 are the optical emission rates,ep

th and enth are the thermal emission rates of holes and electrons, respec-

tively, nT0 and pT0 are the number of occupied and empty deep traps, ande is the dielectric constant.

Figure 4 shows the TWM gain as a function of the incident inten-sity for the DAV30 crystal at the 1.32 and 1.535 mm wavelengths.

0.01 0.1 1 10

Io, Intensity (mW/cm2)

0.0010

0.1

0.2

0

0.1

0.2

0.001

DAV30

DAV30

Γ (c

m–1

)Γ

(cm

–1)

Wavelength : 1.535 mm

Period : 2.00 mmβ : 100


Period : 1.72 mmβ : 100

0.01 0.1 1 10

Figure 4 TWM gain as a function of the incident intensity: the crystal is DAV30 sample

and the experimental conditions are indicated in the figure. The points denote the

experimental results, and the curves represent the fits according to G ¼ G0/(1 þ Id/Io),

with G0 ¼ 0.240 cm�1, Id ¼ 75 mW/cm2 at the 1.32 mm wavelength and G0 ¼ 0.260 cm�1,

Id ¼ 155 mW/cm2 at the 1.535 mm wavelength, respectively.

156 J.-Y. Moisan

The experimental results are fitted to the equation G ¼ G0/(1 þ Id/Io). Bestfits are reported in the figure caption. The grating periods are 1.72 and2.00 mm, respectively. The Id values are in good agreement with previ-ously published results [3]. A lowest gain (0.06 cm�1) is obtained at1.048 mm.

The sign of the dominant photorefractive carriers was found by sepa-rate measurements of the signs of the electro-optic coefficient and TWMgain [17]. Results are also given in Table 2. A change in the sign of carriers(from electrons to holes) is observed between 1.048 and 1.32 mm wave-lengths. The same phenomenon was reported with another CdTe:Vcrystal [16].

Figure 5 represents the plot of (K/G) against K2. The electron–holecompetition factor x is related [18] to the intercept ordinate axis Or by

x ¼ e=Or kB Tk A ð9ÞWe evaluated x for the three wavelengths (see Table 2). The following twopoints are to be noted:

(1) At 1.32 and 1.535 mm, positive x values with relatively large abso-lute values are found, as previously published [3]; this absolutevalue seems to be larger for a higher V content.

(2) On the other hand, the negative but small x values at 1.048 mmprovide a smaller gain. (Note that the increase of x versus wave-length can be explained by the variation of the electron and hole

20

DAV30

1007

9

11

13

15

17

30

K2

K/Γ


Ip : 10 mW/cm2

β : 100

Figure 5 Experimental plot of K/G versus K2. K is the grating wave vector, and G is the

TWM gain. The deduced value x is 0.72 � 0.01, and the Neff value is 9.0 � 1014 cm�2.


photoionisation cross sections sn0 and sp

0 of the V-related level.This result is presented by Bremond et al. [21], and provides a fullexplanation.)

This behaviour is different from that of the sample of CdTe:V used inref [16], for which the electron–hole competition was stronger at the1.32 mm than at the 1.06 mm wavelength.

For the S2105 crystal, a change of the sign of the dominating photo-refractive carriers is experimented between 1.32 and 1.54 mm (see Table 2).If one assumes that, in this crystal, grown by the same technique asDAV30 and DAV31, holes are still the thermally generated carriers, wemust now expect to observe an intensity resonance at 1.32 mm wave-length. The theory [10] predicts that this effect can be observed whenE0 � Ed and when the pump intensity is comparable with Id. This isverified in Fig. 6 (the optimum pump intensity being 1 mW/cm2 at roomtemperature). The same resonant effect is still expected at 1.05 mm and isstill displayed in Fig. 6. Note that in this case, the optimum intensity ismuch lower (more than one decade than at 1.32 mm).

00.01

290 K - 1.048 mm

300 K - ...

310 K - ...

300 K - ...

310 K - ...

290 K - 1.32 mm

0.1 1 10

Intensity (mW/cm2)

TWM Gain (cm–1)

S2105

Ip/Is : 100

100

0.4

0.8

1.2

1.6

2

2.4

2.8

3.2

3.6

4

Figure 6 TWM gain versus intensity with S2105 crystal at 1.048 and 1.32 mm. The

electron–hole resonant effect is observed at 1.048 mm (for less than 0.1 mW/cm2) and

1.32 mm (for about 1–2 mW/cm2).

158 J.-Y. Moisan

Using the same plot as shown in Fig. 5 and taking into account theslope P of the best fit, we can evaluate Neff as

Neff ¼ ePA x e

ð10Þ

The Neff values are reported in Table 2. As is generally reported for CdTe:V crystals [3, 16], Neff is much smaller, approximately 1015 cm�3, thanthe effective V content in the material, 5 � 1015 cm�3, measured bysecondary-ion mass spectroscopy. The variation of Neff with respect tothe wavelength could indicate that the one-level model is not perfectlyvalid. On the other hand, the experimental determination of Neff is lessprecise than the one of x. Indeed, with theses samples it is not possible touse grating periods short enough to show a clear maximum of the curveG(L) curves (which is related to Neff), and on the same reason the slope ofthe K/G against K2 is not well defined, whereas the uncertainty regardingthe value of the ordinate at the origin is not so large. The use of samplesoriented for contra-directional TWM could help to solve this difficulty.

3.1.3. TWM gain with an external electric filed3.1.3.1. Continuous electric field The electric field is applied along theh111i direction, through gold electrodes, on the crystal between crossedpolarisers. The high resistivity of the samples allows us to apply electricfiled as large as 15 kV/cm. The electric field distribution between theelectrodes was visualised by an electro-optic technique in the presenceof uniform illumination at 1.32 or 1.54 mm. This distribution appears to behomogeneous for the voltage/interelectrode distance ratio to as much as10 kV/cm and for light intensities to as much as 10 mW/cm2. Figure 7shows the dependence of the TWM gain on the applied electric field atdifferent grating periods. The gain value always remains smaller than1 cm�1 at the 1.32 mm wavelength, which is smaller than the absorptioncoefficient. But a large transient effect is observed, which indicates thatmuch higher gains can probably be obtained by the usual techniques tomaintain a p/2 phase shift between the interference pattern and the indexgrating, such as the moving-grating method or the periodic-field method.

We also studied gain versus intensity in the presence of a d.c. field todetect a resonant intensity–temperature effect similar to that observed inS2105 crystal or in InP:Fe [10]. Figure 8 shows that, at 1.32 mm and with agrating period of 12 mm (Ed ¼ 0.135 kV/cm), the gain versus intensitycharacteristics in the presence of a 2 kV/cm field have the same non-resonant form as the zero-field gain. One therefore can conclude that,because at 1.32 mm the dominant photorefractive carriers are holes, thethermal carriers in these samples are also holes.

These factors suggest that a temperature-dependant resonance effectmay be seen at 1.048 mmbecause the dominant photorefractive carriers, as

0.001

β : 100

0

0.05

0.1

0.15

0.2

0.25

0.01 0.1

lo, Intensity (mW/cm2)

1 10

Γ (c

m–1

)

DAV30


Period : 12 mm

0 kV/cm

2 kV/cm

Fit

Figure 8 Gain as a function of the incident intensity with the DAV30 crystal.

The curve represents the best fit with G0 ¼ 0.180 cm�1, Id ¼ 20 mW/cm2, as used

in Fig. 4.

00

0.2

0.4

Γ (c

m–1

) 0.6

DAV31

0.8

1

2 4 6 8 10 12 14 16

Eo (kV/cm)


lo : 5 mW/cm2

β : 100

Period : 10 mmPeriod : 4 mmPeriod : 2 mm

Figure 7 TWM gain in the DAV31 crystal versus a continuous electric field for

different grating periods.


Io, Intensity (mW/cm2)

0.1

DAV31

β : 100

1

Eo : 5 kV/cmPeriod : 10 mm

10


1000

1

2

Γ (c

m–1

)

3

T : 300 K

Figure 9 Gain as a function of the incident intensity with the DAV31 crystal.

160 J.-Y. Moisan

indicated in Table 2, are electrons. This is illustrated in Fig. 9, inwhich a clear resonant behaviour appears with an optimum intensity ofapproximately 1.5–2 mW/cm2 at 300 K.

3.1.3.2. Periodic square-shaped electric field A square-shaped electricfield is applied with a variable frequency F in the low-frequency regime,starting from a quasi-d.c. field regime (F ¼ 1 Hz) to as much as a fewhundred hertz. When F ¼ 1 Hz, the amplified signal level at the end ofeach half of the electric field period is the same as a continuous electricfield. In an intermediate regime (to as much as approximately 40 Hz in theconditions given in Fig. 10) the signal intensity exhibits an oscillatorybehaviour. In this regime G is defined from the signal amplificationreached at the end of each half-period of the electric field. For higherfrequencies the transient effect becomes negligible. In Fig. 10, the gain(measured as discussed above) is plotted versus the frequency for differ-ent square-shaped electric field amplitudes at a wavelength of 1.32 mm.Gains as high as 10 cm�1 are achieved with E0 ¼ 14 kV/cm. An optimumin frequency (slightly decreasing when field amplitude decreases) isclearly seen near 30–100 Hz. At 40 Hz the amplified signal residual oscil-lation is approximately 5%; whereas this oscillation becomes negligible at100 Hz. The same behaviour occurs at 1.535 mm, as illustrated in Fig. 11.In this case the gains are lower, for example, 7 cm�1 instead of 10 cm�1 at14 kV/cm.

A practical difficulty arises when one considers the experimentalinfluence of the frequency of a square-shaped field. Indeed, it is wellknown [11, 19, 20] that, in the presence of a relatively small departure

Frequency (Hz)

0

2

4

6

8Γ

(cm

–1)

10

1 10 100 1000

1

2

3

4

5

6

8

10


Period : 10.2 mm

Ip : 10 mW/cm2

12DAV31

14 kV/cm

β : 1000

Figure 10 Gain versus frequency of the square-shaped alternative electric field

(in kilovolts per centimetre), as indicated, with the DAV31 crystal.


from the ideal square-shaped electric field, the gain may be significantlyreduced. For a given high-voltage amplifier, the field shape and its fre-quency are correlated. Thus, one may suspect that the frequency depen-dence shown in Figs. 10 and 11 could be due to this effect rather than to amore fundamental cause. Therefore the results achieved with two differ-ent high-voltage amplifiers (with slew rates of 25 and 105 V/ms, respec-tively) are compared. The result is shown in Fig. 12 for a 6 kV/cm electricfield: there is no significant effect of the shape of the electric field forfrequencies lower than 300 Hz. Unless otherwise specified, the resultsreported here were obtained with the 25V/ms amplifier. It appears thatthe field shape can be considered to be square to as much as approxi-mately 150 Hz for a 12 kV/cm electric field and to roughly 1 kHz for a2 kV/cm electric field. These factors clearly mean that the experimentaloptimum in frequency (generally situated between 10 and 100 Hz) is notdue to the slew rate of the electric field. This frequency effect must beexplained by the intrinsic properties of the CdTe:V samples.

Frequency (Hz)

10

1

2

3

4

5TWM Gain (cm–1)

10 100 1000

6 kV/cm - 1.32 mm

5 mW/cm2

Slew-rate : 105 V/ms

Slew-rate : 25 V/ms

Figure 12 TWM gain versus frequency, with the DAV30 Crystal, with two

high-voltage amplifiers with different slew rates (105 and 25 V/ms).

Frequency (Hz)

1 10 100 1000

1 kV/cm

2 kV/cm

3 kV/cm

4 kV/cm

5 kV/cm

6 kV/cm

8 kV/cm

10 kV/cm

12 kV/cm

14 kV/cm

Γ (c

m–1

)

0

2

4

6

DAV31

8Wavelength : 1.535 mm

Ip : 10 mW/cm2

Period : 10 mmβ : 1000

Figure 11 Gain versus frequency of the square-shaped alternative electric field, with

the DAV31 crystal. The conditions are the same as in Fig. 10, except for the wavelength of

the incident light.

Frequency (Hz)

1

DAV31

10 100 1000

β : 1000Eo : 10 kV/cmPeriod : 15 mm


0.1

0.2

0.5

1

2

20

10

5 mW/cm2

Γ (c

m–1

)

0

2

4

6

8

10

Figure 13 The light intensity is indicated for each curve. The decrease of the gain,

for 10 and 20 mW/cm2, is probably due to a non-homogeneous pump beam intensity.


Figure 13 presents the G versus frequency curves at 1.535 mm fordifferent light intensities with an external field of 10 kV/cm. In the1–20 mW/cm2 range the optimum frequency F0 was found to vary asF0 ¼ 13.5 I0

0.6, where I0 is given in mW/cm2 and F0 is given in hertz. Theoptimum frequency is also highly sensitive to the grating period, asdemonstrated in Fig. 14. Experiments taking into account different valuesof the pump-to-signal intensity ratio b were conducted. It appears (seeFig. 15) that the optimum frequency decreases when b increases. The gainat optimum frequency increases regularly with b as plotted in the inset ofFig. 15. This behaviour resembles that reported by Ziari et al. [11] underrather different conditions.

By making an appropriate choice of the experimental parameters, itis possible to optimise the gain. The highest value (11 cm�1) wasachieved with the DAV31 sample under the following conditions (seeFig. 16): E0 ¼ 15 kV/cm, F ¼ 40 Hz, L ¼ 15 mm, l0 ¼ 1.32 mm, I ¼ 5 MW/cm2, b ¼ 1000. Approximately the same value was reported [11], alsowith a square-shaped field but under quite different conditions (E0 ¼23 kV/cm, F ¼ 230 Hz, L ¼ 7.5 mm, l0 ¼ 1.32 mm, I ¼ 75 MW/cm2, andb ¼ 10,000).

Frequency (Hz)

Wavelength : 1.32 mmIp : 10 mW/cm2

Eo : 10 kV/cmPeriod : 10 mm

Γ (c

m–1

)

Γ (c

m–1

)

DAV31

0

5

10

15

1 10

10

100

500

1000

5000

1010

5

10

100β

10,00010,000

100 1000

Figure 15 Gain versus frequency of the 10 kV/cm square-shaped alternative electric

field, with the DAV31 crystal for different beam ratios b. A plot of the gain for each

optimum frequency versus b is shown in the inset.

Frequency (Hz)

1 10 100 1000

Γ (c

m–1

)

0

2

4

6

8

10DAV3115.9mm

22.9mm8.9mm

5.4mm

Wavelength : 1.32 mmIo : 1 mW/cm2

Eo : 10 kV/cmβ : 1000

Figure 14 Gain versus frequency of the 10 kV/cm square-shaped alternative electric

field, with the DAV31 crystal. The grating period is indicated for each curve.

164 J.-Y. Moisan

DAV311.32 mm

Γ (c

m–1

)

00

5

10

15

4 8 12 16 20

Eo (kV/cm)

Ip : 5 mW/cm2

Fo : 40 HzPeriod : 15 mm β : 1000

Figure 16 Gain versus the square-shaped electric field amplitude E0 obtained

with the DAV31 crystal at the 1.32 mm wavelength.


3.1.3.3. Discussion on the frequency dependence of the gain Inadequacy ofthe standard one-level model

In a photorefractive crystal, assuming thermal and optical transitionsbetween one deep centre and both valence and conduction bands, theevolution of the space charge field E1 in the presence of an external d.c. orsquare-shaped field E0 follows the equation

tgdE1

dtþ E1 ¼ mESC ð11Þ

where ESC is the saturated space charge field and tg is the grating forma-tion constant. This equation (11) is valid when T and tg are much largerthan the other time constants involved in the space charge field formation.In most cases this condition is fulfilled when the carrier lifetime tR ismuch smaller than tg. The most common method for enhancing the TWMgain in a photorefractive crystal – based on the application of a periodicsquare-shaped field with a period T – satisfies the conditions

tR � T � tg ð12ÞIn this case, Stepanov and Petrov [22] showed that the space charge fieldreaches a constant value that does not depend on the period of theexternal field frequency. The results presented in Section 3.1.3.2 wereobtained with T values that are near tg. Therefore, they do not contradictthe prediction of Stepanov and Petrov. Moreover, Bylsma et al. [23]noticed, by applying a square-shaped electric field to an InP:Fe crystal,

166 J.-Y. Moisan

that the gain may be slightly improved by operation at a frequencynear the inverse grating lifetime; they suggest that some resonance occursnear 1/tg, but they did not give any mathematical evidence of thisassumption.

However, Mathey et al. [24] observed a very pronounced external fieldfrequency dependence of the gain in some Bi12GeO20 crystals when T isshorter than tR and near the carrier drift time. They showed that one caninterpret this result by considering second order terms in the space chargefield differential equation, which cannot be neglected under these condi-tions. However, the operating conditions for the results presented inSection 3.1.3.2 are completely different from these conditions.

In the frequency range used, experiments were carried out to know ifthe electric field built up inside the crystal can be considered to beinstantaneous, in contrast to some reported complex dynamic effectsobserved with InP:Fe crystals larger than 10 kV/cm [25]. Once we remem-ber that the effect of the high-voltage amplifier slew rate cannot explainthe results given above, all the above remarks indicate that the spacecharge field dynamics in these CdTe:V samples must be described by asecond order differential equation, even at low external field frequencies.This description should involve a physical mechanism different from thatgiven in [24].

The issues raised in the preceding section lead us to consider a two-level model to explain, at least qualitatively, the periodic field behaviourof these samples. Different variants of two-level models have been pre-sented to give a more accurate description of the characteristics of differ-ent photorefractive materials. For instance, the sub-linear photorefractiveresponse time of BaTiO3 [26] and the temperate dependence of the photo-refractive effect in InP:Fe [28] have been clarified with such models.A shallow trap may also have an effect through trap limited mobility [28].

A two-level modelIn this section, are shown the results obtained for the TWM gain in the

presence of a low-frequency square-shaped external field by consideringa two-level model (see Fig. 17) of the photorefractive effect with a princi-pal deep level situated near the mid-gap and optically (and thermally)coupled to both the conduction and the valence bands. Because the maingoal is to explain the frequency response at 1.32 and 1.54 mm, where themain (optical and thermal) carriers are holes, a secondary level is intro-duced, situated somewhere between the valence band and the principallevel that is expected to have an effect on the hole transport. Furthermore,the optical intensity is assumed to be moderate enough that one canconsider the dominant emission process for this level can be the thermalemission of holes. This thermal emission is supposed to be muchmore important than the emission of the deep level; this second level issupposed to be filled by electrons in the dark.

Valence band

Cp

– – – – – – – – – – –

– – – – – – – – – –

NT0

p′T0N′T0

n′T0

σ0n en

th

σ0p ep

th

nT0 pT0

e′p C′p

Cn

Conduction bandμn

μp

– – – – –

Figure 17 Energy diagram for the two-level model with the different parameters

used for the calculations.


Figure 18 shows, for a given set of material parameters, the calculatedcurve of a TWM gain as a function of the square-shaped frequency fordifferent values of the second level concentration N0

T. The parametersrelative to the principal deep level are sn

0 ¼ 2� 10–17 cm2, sp0 ¼ 2� 10�16

cm2, cn ¼ 10�8 cm3/s, cp ¼ 5 � 10�8 cm3/s (where cn and cp are the capturecoefficients of electrons and holes, respectively), mn ¼ 1000 cm2/Vs, mp ¼80 cm2/Vs (Where mn and mp are the mobilities of electrons and holes,

FREQUENCY (Hz)

10 100

two wave mixing gain (cm–1)

103

0

NT′

104 105

3×10141×1014

3×1013

1×1013

3×1012

1

5

10

15

20

0

Figure 18 TWM gain as a function of the square-shaped field frequency for

different values of the second level concentrations N0T (in cm–3): l0 ¼ 1.32 mm,

L ¼ 10 mm, I0 ¼ 1 mW/cm2, E0 ¼ 10 kV/cm.

168 J.-Y. Moisan

respectively), r41 ¼ 5.5 pm/V, er ¼ 10.4 (where er is the relative dielectricconstant), nT0¼ 1.6� 1016 cm�3, and pT0¼ 8� 1015 cm�3. For this level thethermal emissions of both types of carriers are neglected. The character-istics of the level are described by two parameters: its hole thermalemission coefficient e0p (¼ 2000 s�1) and its hole capture coefficient c’p(¼ 1.5 � 10�4 cm3/s). The parameters of the principal level (supposedlyrelated to the V doping) are not precisely known. The values of thephotoionisation cross sections are issued from measurements performedby deep-level optical spectroscopy [29].

All these characteristics start from the d.c. field value and then presentsome oscillations that correspond to the oscillations of the continuous casegrating formation. Then G increases to a maximum that may be more orless wide and finally decreases until reaching a limit value. With negligi-ble second level concentration, the limit value is equal to the maximum,and there is no decrease of gain with frequency (as long as F is muchsmaller that the inverse carrier lifetime). This result corresponds to thebehaviour described by Pauliat et al. [30]. When the second level con-centration increases, the limit decreases considerably, whereas the maxi-mum value also decreases, but not to the same extent. The curves inFig. 18 (particularly the curve corresponding to N0

T ¼ 1013 cm�3) lookvery similar to the experimental curves obtained with the faster high-voltage amplifier (Fig. 12). Note that, with the set of material parametersused in Fig. 18, the d.c. field gain value remains practically unchanged,whereas the periodic field results are strongly dependent on the secondlevel concentration. It has been verified that, when the concentrationbecomes comparable with that of the principal level, the d.c. field gainbecomes significantly reduced, in agreement with the predictions ofRana et al. [27].

The freedom to choose parameters is considerable; however, a quanti-tative agreement by use of the present parameters (see Fig. 18) is obtainedfor some important experimental characteristics, as given in Table 3: foroptical absorption, the electron hole competition factor and photoconduc-tivity, the agreement is quite good. As shown, the difference between the

Table 3 Theoretical DAV31 characteristics deduced from the two-level model

parameters, as used in Fig. 17, compared with experimental values as given in Table 2

DAV31 (theoretical values)

1.32 mm to 1 mW/cm2

(experimental values)

a 1.9 cm�1 2.24 cm�1

x þ0.67 þ0.68

Ip (10 kV/cm) 45 mA/cm2 55 mA/cm2

Neff 5.3 � 1015 cm�3 1.37 � 1015 cm�3


theoretical end experimental values of Neff presented in Section 3.1.2cannot be explained by the introduction of a second level with moderateconcentration (see Fig. 18). This difference can be explain if one takes intoaccount two facts: the lack of accuracy of the experimental determinationof Neff as discussed previously and the uncertainty of the photoionisationcross section values which results from preliminary measurements.

3.2. Effect of Zn content on photorefractive properties

In the previous section, results on photorefractive behaviour of the dif-ferent CdTe:V crystals has been presented, especially the effects of thedifferent experimental parameters on the TWM gain. In this section, phy-sico-chemical properties of the different crystals will be analysed andtaken into account to explain the photorefractive behaviour of CdZnTe:Vcrystals: two classes of material have been seen and are presented now.

3.2.1. Sample preparationSix crystals have been studied: CTV2, DAV31, DAV35, DAV36, DAV37(grown in CNRS-Bellevue) and S2105 (grown in CNRS-Strasbourg). Thestarting content of V in the melt is in the 1019 cm�3 range. Three of them(DAV31, DAV35 and DAV37) contain about 4% Zn, S2015 about 1% andthe other contain no Zn (see Table 4). Apart from DAV35 and S2105, thecadmium losses (due to the high Cd vapour pressure inside the ampouleduring the growth of the crystal) are compensated for: so, CTV2, DAV31,DAV36 and DAV37 are assumed to be ‘stoichiometric’ crystals. On theother hand, DAV35 and S2105 are supposed to have some cadmiumvacancies ([VCd] in Table 4).

3.2.2. MeasurementsThe results are presented by separating the crystals in two classes, follow-ing their behaviour in the presence of an external continuous electric field.All the measurements are carried out at 1.32 mm wavelength. With nofield applied, an electron hole competition factor is obtained for eachcrystal. Its sign reflects the dominating photocarriers and are precised inFig. 19. Other values at lower (1.048 mm) and upper (1.535 mm) wave-lengths had been presented in previous sections [31, 32].

3.2.2.1. Class I crystals Three crystals are in this class: DAV31, DAV35and DAV37. The left part of the Fig. 20 shows the TWM gain (G) versusthe intensity (I0) with a continuous 10 kV/cm electric field and a 10 mmgrating period, for theses crystals. The gain curves present the expectedS-shape due to the saturation of the space charge field inside the crystalin the limit of equilibrium between photoionisation of the carriers andre-capture process. As previously discussed, the optically and thermally

Table 4 Main physico-chemical specifications of the V-doped CdTe and CdZnTe crystals studied

[V]SIMS (cm�3) % Zn [VCd] [Acc.] x (l ¼ 1.32mm) [V2þ]EPR (cm

�3) [V3þ]EPR (cm�3)

DAV31 3 � 1017 4 Stoichiometric High þ0.68 3 � 1017 Not seen

DAV35 4 � 1017 4 Non-stoichiometric High þ0.23 3.6 � 1017 Not seen

DAV37 1 � 1017 4 Stoichiometric High þ0.22 1 � 1017 Not seen

S2105 2 � 1017 1 Non-stoichiometric Low �0.37 1.7 � 1017 Not seen

DAV36 7 � 1017 0 Stoichiometric High �0.57 Not seen 5 � 1015

CTV2 3 � 1017 0 Stoichiometric Low �0.60 Not seen 7 � 1014

170J.-Y.M

oisan

1.2

h+

e−

(μm)

1–1

–0.6

–0.2

0.2

0.6

1

Ele

ctro

n/ho

le C

ompe

titio

n Fa

ctor

1.4 1.6 1.8

Wavelength

DAV31

S2105� DAV36

CTV2

#DAV35DAV37

�

Figure 19 Electron–hole competition factor x against wavelength for the six studied

crystals. The results are mainly obtained for the 1.32 mm wavelength. The main optically

emitted carriers are holes in the upper part of the graph, and electrons in the lower part.


main emitted carriers have the same sign (holes), so no electron holeresonant effect is expected. This also means that the electron/hole com-petition factor x, which is equal, in a one deep-level model and in the limitof short grating period, to

x ¼ sp0pT0 � sn0nT0sp0pT0 þ sn0nT0

ð13Þ

is positive. Above equation can now be written as

sp0pT0 > sn0nT0 ð14ÞThe x values are reported in Table 4.

3.2.2.2. Class II crystals The right part of Fig. 20 reports the same experi-ments for class II crystals (CTV2, DAV36 and S2105). An electron/holeresonant effect is clearly seen at about 1 mW/cm2 intensity for this setof three crystals, as discussed previously. If one considers that semi-insulating crystals are residual p-type materials [33] one can concludethat in this class II, the main optically emitted carriers are electrons.It means that the x parameter is negative, thus

sp 0pT0 < sn 0nT0 ð15ÞThe experimental results are given in Table 4. For the given 1.32 mmwavelength, the major difference between the two classes of crystal istherefore a change of the dominant photoionised carrier. In a model ofonly one deep centre induced by V doping (i.e. with constant photo-ionisation cross section sn

0 and sp0 for all crystals) the reverse of the sign

would be explained by the reverse in the pT0/nT0 (or V3þ/V2þ) ratio.However the EPR analysis will show that this hypothesis is not consistent.

Intensity (mW.cm–2)

0.001–0.2

0.2

0.2

0.2

0.4

0.4

0

0

0

0.01 0.1 1 10 100 0.0010

0.1CTV24kV/cm1,32mmIp/Is: 100Period: 10mm

DAV31300K10kV/cm1,32mmIp/Is: 100Period: 10mm

DAV37-6300K10kV/cm1,32mmIp/Is: 100Period: 10mm

DAV35300K10kV/cm1,32mmIp/Is: 100Period: 10mm

6kV/cm1,32mmIp/Is: 100Period: 10mm

S21055kV/cm1,32mmIp/Is: 100Period: 10mm

0.2

0

1

2

0DAV36-8bDAV36-5b

1

2

TWM Gain (cm–1)TWM Gain (cm–1)

A

B

C0.01 0.1 1 10 100

Intensity (mW.cm–2)

310°300°290°

E

F

D

Figure 20 Two wave-mixing gain versus pump beam intensity under a continuous

electric field. (A), (B) and (C) are respectively DAV35, DAV37–6 and DAV31. Note the

S-shape in all cases. (D), (E) and (F) are respectively for S2105, DAV36 and CTV2 crystals.

An electron–hole resonant effect is clearly seen in this case. For convenience G has

been plotted as positive but the x factor is clearly measured as negative. Crystals with

different numbers have been cut from different place in the ingot (numbered from the

top to the tail of the ingot).

172 J.-Y. Moisan

3.2.3. EPR resultsEPR results have been carried out on both classes of crystals. V3þ hasbeen already observed in CdTe:V crystals [33] by EPR analysis and thecontent [V3þ] can be evaluated in this way. In CdTe:V crystals (DAV36and CTV2) V3þ is clearly identified with the experimental setup. Resultsare reported in Table 4. But V3þ has never been seen in CdZnTe alloy


crystals. This could be due to the low degree of compensation since thesecrystals at room temperature p-type conductive with hole concentrationof �1015 cm�3 only. However, from Eq. (15) and with the photoionisationcross-section values given by Bremond et al. [33], these crystals areexpected to have a V3þ content which would lie well above the limit ofdetection of the used EPR apparatus. This limit is estimated to be around3 � 1014 cm�3 for CdTe:V crystals and about one decade higher if oneconsiders a reasonable line-width increase due to the Zn alloying.

On the other hand, for Zn alloyed crystals, a signal attributed to V2þ isdetected [33]. In all the crystals where such a low C2v symmetric relatedsignal has been detected, the maximum of the spectrum follows thesegregation of Vmeasured by SIMS. This rules out the possible attributionto, for example, a V-related complex. So a quantitatively analysis hasbeen tentatively performed. First one sample (DAV37), where a weakresonance signal of an effective mass shallow donor is observed, is pro-ceeded. The hypothesis was done that in these crystal the [V2þ] is equalto [V] as determined by SIMS experiments. EPR results estimate a near-stoichiometric V charge state ([V]SIMS � [V2þ]EPR) for the four crystalswith a Zn content (see Table 4). Even though the non-observation of theV2þ spectrum in CdTe:V crystals has received possible interpretations[35] it is still a mater of discussion. The fact that S2105 crystal belongs tothe class II crystals, makes this sample an intermediate crystal which willbe discussed below.

3.2.4. DiscussionFollowing their photorefractive properties, two classes of crystals ofCdTe:V have been defined and the main difference is obviously expressedby Eqs. (14) and (15). The main optically emitted carriers, at 1.32 mmwavelength, are holes (hþ) in the class I crystals and electrons (e�) in theclass II ones. In both classes, holes are the thermally emitted carriers. Itcomes out that the resonant e�/hþ effect is only observed in the class IIcrystals (i.e. when x is negative).

In a first approach, if one considers that with or without a continu-ous electric field, the photorefractive properties are explained – in allcrystals – by one deep trap level due to the V doping effect, one canconclude the following (see Fig. 19):

1. The x value increases as the wavelength increases; so the ratiosp

0/sn

0 increases with the wavelength. This is confirmed by previ-ous deep-level spectroscopy (DLOS) experiments [35].

2. The ratio V3þ/V2þ is larger for class I crystals than for class II ones.This is not confirmed by EPR results (see Table 4).

The problem is therefore to find out what material parameters can explainthis behaviour.

174 J.-Y. Moisan

3.2.4.1. Effects of physico-chemical parameters In Table 4 some physico-chemical parameters are also summarised. The total V content evaluatedby SIMS ([V]SIMS) has been measured in each crystal. [Acc.] is the contentof acceptor like impurities (Li, Na, K, etc.) which could – as for Cd losses –influence the charge state of V (i.e. V3þ or V2þ) dopant, for which theconcentrations have been evaluated by EPR experiments. The results ofSIMS measurements on the six studied crystals are given in Fig. 21.Absolute contents are only given for [V] concentrations which lie between1017 and 7 � 1017 cm�3. For all the other elements only the relativeconcentration is indicated.

Na K ClAlLi

Rel

ativ

e C

once

ntra

tions

(A

rb. U

nit.)

DAV31

DAV35

DAV37

DAV36

CTV2

S2105

V

1.×1013

1.×1014

1.×1015

1.×1016

1.×1017

1.×1018

Zn

DAV31

DAV35

DAV37

DAV36

CTV2

S2105

Figure 21 SIMS results: in the upper part, absolute concentrations of V are given and

Zn contents are relative values as in the lower part for different impurities. Except for

Li, no significant differences are clearly seen for the two crystal classes.


The two DAV36 and CTV2 crystals have a Zn content as a residualimpurity, but in S2105, [Zn] is about four times lower than in the class Icrystals. No clear differences appear in the Al and the Cl content – whichare donor-type impurities – between the two classes of crystals; the samefeature for K (an acceptor type impurity). The Li content appears tovary in a large range: one decade higher crystals than in the others.From Table 4, it appears that the main difference between the class Iand class II crystals remains the Zn content. So, one can conclude that[Zn] is the only parameter which can explain the difference between thetwo classes. If one now suppose that we always involve the same deeptrap level (with the same photoionisation cross sections), it follows thatincreasing [Zn] should lead to a drastic increase in pT0 or [V

3þ], which isnot observed in EPR analysis.

DLOS has been already obtained on such crystals [34]. Then majorresult is the characterisation of a main deep level in the band gap ofV-doped CdTe or CdZnTe crystals, which is proposed to be the maintrap responsible for the photorefractive behaviour of these materials. Thislevel is located at an activation energy of 0.95 eV below the bottom ofthe conduction band. Its concentration seems to be proportional to theV content introduced in CdZnTe crystals, as determined by SIMS experi-ments. Moreover, DLTS experiments on an un-doped CdTe crystal [29]have revealed a deep trap level signature at 1.08 eV but in two decadelower concentrations range (�1013 cm�3). Actually, DLTS gives a positionenergy for the trap which is the sum of the thermal ionisation energyand possible thermal activation of capture cross section; explaining theapparent discrepancy for the 0.95 eV level attributed to V and the valueusually given in the literature (Ecb – 0.7 eV). Its photoionisation crosssection sn

0(hn) and sp0(hn) have been measured in absolute scale (better

than 50% of uncertainties) by electrical and optical DLTS, respectively.A band gap change is expected with the [Zn] increase: about 7 meV [36]have been experimented for 1% of Zn. One can assume that this increaseof the band gap has only a weak effect on the photoionisation crosssections. So the largest reasonable changes of this rates for an increase of28 meV (4% of Zn), does not provide full explanation for the differencesobserved for the crystals with and without Zn.

3.2.4.2. Effects of a second deep trap level DLOS has shown [37] thatanother deep trap level appears in CdTe:V crystals with a low (or no)Zn content. This level is situated at an activation energy of 0.78 eV belowthe bottom of the conduction band and can appear in a higher concentra-tion than for the 0.95 eV level. Similar results have been confirmed forDAV36 crystal by PICTS experiments; but DLTS characterisations on thiscrystal are not available. So no quantitative evaluation of the deep levelconcentrations are known in this class II crystal. Note that this second

176 J.-Y. Moisan

deep level was also present in DAV31 crystal [34] but in lower concentra-tion than for the 0.95 eV level.

Based on these results, a model (see above) has been developed withtwo traps in the band gap of the material: an acceptor type second deeplevel is supposed. This means that the Fermi level pinned near the main0.95 eV deep trap. DLOS experiments [37] indicate than one can neglectsp2

0 with regard to sn0 at 1.32 mm wavelength. Even though materialparameters have been deduced by DLTS and DLOS experiments onconverted n-type annealed samples, these results are expected to be alsovalid for as-grown high resistivity samples.

One can see in Fig. 22 some numerical computation results for the Gversus I0 curves for such a two deep traps model. Material parameters areprecised on the legend of the figure. They are mainly extracted fromDLTSresults of DAV31 [34] and S2105 [37] crystals. As the total concentration ofthe second deep trap level (NT2) increases from a negligible or low value(like in DAV31 or type I crystal) to a dominant value (like in S2015 ortype II crystals) a change is observed in the sign of the x (and also G) withan emergence of the resonant effect. The maximum of this resonant effectis situated around an intensity of 1 mW/cm2 which pleasantly agreeswith experimental results (see Fig. 20).

INTENSITY (mW.cm–2)

10–3–2.5

–2.

–1.5.GA

IN (

cm–1

)

–1.

–.5

0

.5

10–2 10–1 1. 10 102 103

1×10165×10167×1015

5×1015

0

NT2

Figure 22 TWM gain as a function of total intensity for different values of the second

deep level concentrations (NT2). Experimental parameters: l ¼ 1.32 mm, E0 ¼ 5 kV/cm.

Material parameters for 0.95 eV (A) and 0.78 eV (B) deep levels: er ¼ 10.4, r41 ¼ 5.5 pm/V,

nTA0¼ 3� 1015 cm�3, pTA0¼ 8� 1015 cm�3, mn¼ 1000 cm2/Vs, mp¼ 1000 cm2/Vs, snA0¼

2.4 � 10�17 cm–2, spA0 ¼ 7.4 � 10�17 cm�2, CnA ¼ 2 � 10�6 cm3/s, CpA ¼ 3 � 10�8 cm3/s,

ethnA ¼ 0.038 s�1, ethpA ¼ 0.56 s�1, spB0 ¼ 3 � 10�16 cm�2, snB

0 ¼ 10�18 cm�2, CnB ¼2 � 10�6 cm3/s, CpB ¼ 10�8 cm3/s, ethnB ¼ 0.01 s�1, ethpB ¼ 0.1 s�1.

INTENSITY (mW.cm–2)

NT2:a:1×1014

b:2×1015

c:4×1015

d:5×1015

e:7×1015

10–3 10–2 10–1 1. 10 102 103–.4

–2.

GA

IN (

cm–1

)

.8

a

b

c

d

e

.6

.4

.2

0

Figure 23 Same numerical computations as in Fig. 22 but for E0 ¼ 10 kV/cm and

material parameters closer for DAV31 crystal: nTA0 ¼ 7 � 1015 cm�3, pTA0 ¼ 6.7 � 1015

cm�3, CnA ¼ 6 � 10�5 cm3/s, CpA ¼ 1.5 � 10�8 cm3/s, ethnA ¼ 0.23 s�1, ethpA ¼ 0.56 s�1,

snB0¼ 3� 10�16 cm�2, spB

0¼ 10�18 cm�2, CnB ¼ 2� 10�6 cm3/s, CpB ¼ 10�8 cm3/s, ethnB¼ 0.01 s�1, ethpB ¼ 0.1 s�1.


In Fig. 23, some material parameters deduced from DLTS results ofDAV31 crystal [34] are used. One can see that forNT2 lower than for S2105crystal but not negligible, the G changes sign for low I0 values dependingon NT2. This effect has been experimentally detected (Fig. 20C) and hadbeen also theoretically predicted [38] when more than one trap level isinserted in the photorefractivity theory. This phenomenon is due to anintensity dependence of the x factor, x ¼ x(I0) and would explain thedifficulties to deduce precise material parameters with analytical expres-sions based one model with only one deep trap. Further analytical devel-opments based on two deep traps both in thermal and optical interactionwith both the valence end conduction band have to be carried out.

Apart from these results, some thermal spectroscopy was also carriedout of the resonance electron–hole effect for both crystals from class I(DAV37) and class II (DAV36 and S2105). The measurements have bothbeen realised at l ¼ 1.06 and 1.32 mm wavelengths in order to get aresonance effect also for class I crystals (see Fig. 19). If one holds thehypothesis of crystals with a dark conductivity due to holes (i.e. domi-nated by thermal emission of holes), Arrhenius plots of the optimumintensity (at the centre of the resonance) versus 1/Tk should allow oneto extract energies of activation of the deep level which dominate thephotorefractive effect in each crystal. As it is expected, one can see inFig. 24 that crystals of class I and class II are not ruled by the samedeep trap. Due to its energy of activation of only 0.67 eV, S2105 is con-firmed to be intermediate origin which could be due to a content of

1000/T (K–1)

2,6

Eo = 5kV/cmIp/Ip = 100Grating period = 10mm

–15

–14

–13

–12

–11

–10

–9

–8

–7

Eact = 0.81eV

Eact = 0.34eV

Eact = 0.54eVEact = 0.67eV

2,7 2,8 2,9 3 3,1 3,2 3,3 3,4 3,5 3,6

DAV36.5 (1.32mm)

DAV37.6 (1.06mm)

S2015 (1.06mm)

Figure 24 Arrhenius plots of the optimal intensity (at the centre of the resonance) for

crystals DAV37, DAV36 and S2105. From the slope of each strait line one can deduce an

energy of activation of deep traps which thermally dominates the photorefractive effect

in each crystal. Note that class I and class II crystals seem not to be dominated by the

same deep levels.

178 J.-Y. Moisan

only 1%. Note that energies of activation come from thermal emission ofholes; so they are referred to the maximum of the valence band. With anestimated band gap, at 0 K, of Eg ¼ 1.6 eV, – the effect of the Zn in therange 0 to 4% is neglected – it has been illustrated by a photorefractivemethod the general tendency of crystals governed by different deep levelswhich was deduced from DLTS and PICTS measurements.

4. APPLICATIONS

One of the greatest interest for CdTe:V is its high sensitivity at telecom-munication wavelengths (1.3 and 1.5 mm). This has been largely demon-strated in the previous sections. With these materials, double-phaseconjugated mirrors [39] (DPCMs) could become interesting for freespace bi-directional optical links, owing the self-aligning properties ofphase conjugation [40–42].

4.1. DPCM experiments

The same crystals (8 mm long) as studied above were used (DAV31L Aand B). High TWM gain is obtained under an alternating square-waveelectric field. Antireflection coatings have been deposited onto both


optical faces. Their reflectivity is smaller than 0.3% from 1.2 to 1.6 mm. Theabsorption at 1.54 mm of the two samples is, respectively, 1.35 and1.22 cm�1. TWM characterisation and DPCM experiments were per-formed with a 20 mW diode-pumped solid state laser. A TWM gain G of6 cm�1, reported above in another un-coated sample from the same ingot,was confirmed with these antireflection coated crystals under identicalexperimental conditions.

The analytical expression [43] for the DPCM threshold in the presenceof absorption a predicts for equal pump intensities a threshold value ofG � 6 cm�1 for the two 8-mm-long crystals, which is the value that isexperimentally obtained for 10 kV/cm. One therefore expects to observethe threshold electric field for DPCM near 10 kV/cm.

The experimental setup is presented in Fig. 25. In some cases cylindri-cal lenses are used to focus the two beams in the sample. The conversionefficiency � (defined by the ratio of the diffracted beam intensity to thetotal transmitted one) measured as a function of the applied field fre-quency for the two crystals is shown in Fig. 26. With the high slew rate(>105 V/ms) high-voltage amplifier used in all the experiments reportedhere, the field shape can be assumed to be perfectly square for a frequencyup to �500 Hz. Note that for the measurements reported in Figs. 26–28,

Cd Te Cystal Optical detector

Beamsplitter

collimatedinput beam

screen

camera

cylindrical lenses

Imaging lens for far fieldpattern observation

H.V.

Figure 25 Experimental setup used in all the experiments reported here. Cylindrical

lenses allow for the elimination of conical diffraction and were removed to yield the

experimental data of Figs. 26 and 27 pump depletion of collimated beams. HV, high

voltage.

Field frequency (Hz)

14

10

8

6

4

2

1 10 100 10000

12C

onve

rsio

n ef

ficie

ncy

(%) DAV31L - A

DAV31L - B

Figure 26 Experimental DPCM conversion efficiency versus applied field frequency

for the two 8-mm-long crystals. The grating period is L ¼ 5.5 mm. The intensity of each

pump is 10 mW/cm2 and E0 ¼ 10 kV/cm.

14

12

10

8

6

4

2

03000 4000 5000 6000

Applied field (V/cm)

7000 8000 9000 10000

Con

vers

ion

effic

ienc

y (%

)

Ip = 20 mW/cm2

Ip = 15 mW/cm2

Ip = 10 mW/cm2

Ip = 5 mW/cm2

Figure 27 Experimental DPCM conversion efficiency as a function of the applied field

for DAV31LA. The field frequency value (70 Hz) is optimised for pump intensity of

20 mW/cm2. Ip, pump intensity.

180 J.-Y. Moisan

24

18

12

6

02 4 6 8

6

7

8

9

Eo = 10kV/cm

Grating period (mm)

Con

vers

ion

effic

ienc

y (%

)

10 12 14 16 18 20 22

Figure 28 Spatial bandwidth characterisation of DPCM for different applied

fields E0. The field frequency is 70 Hz, and the pump intensity is 20 mW/cm2.


collimated beams was used so that the conical diffraction was not elimi-nated and � was measured from pump depletion. Figure 26 illustrates theinfluence of the frequency dependence under a square shape externalfield. Because the TWM gain at high field frequency is markedlydecreased, the DPCM efficiency can be severely reduced (DAV31LA) oreven quenched (DAV31LB) because in the latter case the product G � Lbecomes lower than the threshold value for the DPCM. Even though sucha difference could be explained by inhomogeneities of the second level, nodefinitive explanation is given for this behaviour for the two samplesoperating from different parts of the same ingot. In any case, one cannotexplain these by taking the absorption into account, because the highestefficiency is obtained with the most absorbing sample.

Pump depletion of the DPCM versus the applied field with differentpump intensities is illustrated in Fig. 27. All the experiments were per-formed at a field frequency of 70 Hz. This frequency enables to avoidfluctuations of the diffracted beam intensity with the highest illumination(20 mW/cm2 for each pump). At 70 Hz, the best result is obtained withthe highest illumination. The results obtained at low illumination could inprinciple be improved by reduction of the frequency [12, 14] with noincrease in the intensity fluctuations, as the response time of the photo-refractive effect is inversely proportional to the incident illumination.

182 J.-Y. Moisan

The threshold determined by the appearance of conical diffraction occursat E0 ¼ 6 kV/cm for all intensities. Note that for electric field lower thanthis threshold, the experimental values in Fig. 27 are due to a fanningeffect. It is believed that the unpredicted low threshold value arises froman underestimation of the maximum gain in TWM experiments, espe-cially for long crystals, because of the so-called large-signal effect. Indeed,with a TWM gain of 6 cm�1 a beam ratio of 500 at the input face of thecrystal is reduced to �4 at the output, leading to strong nonlinearities ofthe space charge field. The TWM gain versus intensity dependence, foroperation at an optimum external field frequency [14], may also lead tosome overestimation of the threshold field for the DPCM. Indeed, in aDPCM configuration, the crystal is illuminated from both sides, so thatvariations of illumination are much smaller than in the TWM experi-ments. Because of the good photosensitivity at 1.54 mm a DPCM isobtained by using only a 5 mW/cm2 of power for each of the pumps inCdTe:V, as shown in Fig. 26.

Figure 28 shows the experimental conversion efficiency versus gratingperiod for different applied fields. The maximum efficiency is obtainedfor a grating period of 8 mm. A larger optimum grating period (�15 mm)has been observed with a sample from the same ingot in TWM experi-ments [14]. This phenomenon, already known for InP:Fe [42, 44], could berelated to nonlinearities of the space charge field at high fringe modula-tion (as is the case in DPCM experiments) and occurs mainly when thesmall signal gain is the highest (i.e. near 15 mm). A maximum conversionefficiency of 22% for an applied field of 10 kV/cm is reported. This leadsto a maximum diffraction efficiency (calculated by inclusion of the opticalabsorption) of �7.4% for crystal DAV31LA.

4.2. DPCM between single-mode fibres

To realise true phase conjugation (i.e. without conical diffraction) theinsertion of aberrators or cylindrical lenses [45] was proposed. In thelatter geometry, two cylindrical lenses of 20 cm focal length focusthe beams in the incident plane, making diffraction impossible outsidethat plane (Fig. 25). A beam splitter is inserted into the path of one of thecollimated pump beam to be viewed without astigmatism by a camera.In Fig. 29, the far-field pattern is represented with and without cylindricallenses. The resolution of the imaging optics is too low to ensure that thedevice is diffraction limited. Insertion of single-mode fibres for moreprecise evaluation is being considered. In a configuration with cylindricallenses the input beams overlap inside the crystal in a range across theelectrodes but in a thin region in a perpendicular direction (h112i axishere). One possible application of such linear arrays of a DPCM is theadaptative coupling of the beams within output single-mode fibres into a

2°15′

4°30′

2°15′

4°30′

ZOOMX4

ZOOMX3

Figure 29 Experimental far-field pattern image (with an 300-mm lens) on a Vidicon

camera analysed with a PC-monitored beam analyser. The upper figure represents the

cross section of the conjugated beam without cylindrical lenses. Conical diffraction is

clearly seen; the lower figure shows its successful elimination with two cylindrical lenses.

Note the different scales for the two parts of the figure.


vector–matrix crossbar switch. Indeed, this architecture, in which thedifferent beam directions (corresponding to one output) are situated ina single plane, is well matched to one-dimensional phase conjugation.Single-mode fibres could be used in such an architecture, leading toreduce insertion losses and thus to high switch capacities and bit rates,even though cross-talk problem have to be characterised first.

5. CONCLUSION

This study was performed to demonstrate that optical beam steeringcould be possible using photorefactivity properties of crystals. The firstpart of this chapter has explained the photorefractive effect in a semicon-ductive crystal. Because in a optical beam steering system, optical beamsare issued from optical fibres at telecommunication wavelengths (i.e. innear-infrared range of wavelengths) the used crystals must have high

184 J.-Y. Moisan

photorefractive figure of merit. Two semiconductive materials have suchproperties: InP:Fe and CdTe:V as largely presented in the literature for thefirst one.

In the last part of this chapter, demonstration of beam steering isreported: an optical system is presented and the conditions for such anoptical function seems to be fulfilled. The environmental conditions forthe used CdTe:V crystal have been previously studied and applied: theywere obtained through the measurements of the optical gain in a TWMoptical experiment. It appeared that the following materials conditionshave to be fulfilled:

� The V content that creates a trap level in the band gap, as it is wellknown.

� The Zn content: Zn is known for its effect on crystal synthesis andis generally added for this reason. But it appears that Zn contenthas a drastic effect on the photorefractive properties following thewavelengths range.

To manage as precisely as possible the photorefractive properties of theCdTe:V material, it is clear that it is necessary to manage the semiconduc-tive behaviour of this material as precisely as possible. This is the object ofthe second part of this chapter.

So it has been demonstrated that CdTe:V crystal is a good candidatefor optical systems in the near infrared wavelengths range, but the con-ditions are to fulfil some chemical parameters (V and Zn contents) andthen the semiconductive behaviour of the material.

ACKNOWLEDGEMENTS

The author would like to thank the following colleagues from his labora-tory (CNET, nowOrange Labs): P. Gravey, N.Wolffer, G. Martel, V. Vieuxfor the optical set ups and G. Picoli, B. Lambert and M. Gauneau for thestudy of the properties of the materials.

The author also gratefully acknowledges the contributions of collea-gues from several different laboratories who participated in this study:R. Triboulet, Y. Marfaing and co-workers from the CNRS Lab of Meudon,P. Siffert, M. Hadj-Ali, J.M. Koebel, and co-workers from the CNRS Lab ofStrasbourg, G. Bremond and co-workers from INSA Lyon and M. Pugnetfrom INSA Toulouse. This collective work, coordinated via a formal workgroup, has resulted in the material presented here, thanks to a fruitfulcollaboration via friendly and open discussion.

It is thus a collective work that is presented here, but is conductedunder a common project that explains the diversity of the technicaldomains treated and the interest found therein by each of the participants.


REFERENCES

[1] P. Gravey, J.Y. Moisan, Proc. Soc. Photo-Opt. Instrum. Eng. 1507 (1991) 239–246.[2] M. Ziari, W.H. Steier, P.M. Ranon, S. Trivedi, M.B. Klein, Appl. Phys. Lett. 60 (9) (1992)

1052.[3] A. Partovi, J. Millerd, A.M. Garmire, M. Ziari, W.H. Steier, S. Trivedi, M.B. Klein, Appl.

Phys. Lett. 57 (9) (1990) 846.[4] J.M. Langer, Phys. Rev. B 38 (11) (1988) 7723.[5] W. Giriat, J.K. Furdyna, Semiconductors and Semimetals, vol.25, Academic Press, 1988.[6] N.V. Kukhtarev, Ferroelectrics 22 (1979) 949.[7] G.C. Valley, J. Opt. Soc. Am. B 1 (1984) 868.[8] S.I. Stepanov, M.P. Petrov, Opt. Commun. 53 (1985) 292.[9] A. Partovi, E. Garmire, J. Appl. Phys. 69 (10) (1991) 6885.[10] G. Picoli, P. Gravey, C. Ozkul, V. Vieux, J. Appl. Phys. 66 (8) (1989) 3798.[11] M. Ziari, W.H. Steier, P.M. Ranon, M.B. Klein, S. Trivedi, J. Opt. Soc. Am. B 9 (1992)

1461.[12] Y. Belaud, P. Delaye, J.C. Launay, G. Roosen, Opt. Commun. 105 (1994) 204.[13] J.Y.Moisan, P. Gravey, N.Wollfer, O.Moine, R. Triboulet, A. Aoudia, in: Topical Meeting

on Photorefractive Materials, Effects and Devices, PRM’93, Kiev, 1993, pp. 283–286.[14] J.Y. Moisan, N. Wolffer, O. Moine, P. Gravey, G. Martel, A. Aoudia, E. Repzka,

Y. Marfaing, R. Triboulet, J. Opt. Soc. Am. B 11 (1994) 1655.[15] J. Strait, J.D. Reed, N.V. Kukhtarev, Opt. Lett. 15 (1990) 209.[16] J.C. Launay, V. Mazoyer, J.P. Zielinger, Z. Guellil, P. Delaye, G. Roosen, Appl. Phys. A

55 (1992) 33.[17] A.M. Glass, M.B. Klein, G.C. Valley, Electon. Lett. 21 (1985) 220.[18] M.B. Klein, G.C. Valley, J. Appl. Phys. 57 (1985) 4901.[19] K. Walsh, A.K. Powell, C. Stace, Y.J. Hall, J. Opt. Soc. Am. B 7 (1990) 288.[20] N. Wolffer, P. Gravey, Ann. Phys. (NY) 16 (1991) 143.[21] G. Bremond, et al., E-MRS Spring Meeting 1994, Strasbourg, Opt. Mater. 4 (1995) 246.[22] S.I. Stepanov, M.P. Petrov, Opt. Commun. 53 (1985) 292.[23] R.B. Bylsma, A.M. Glass, D.H. Olson, Electon. Lett. 24 (1988) 360.[24] P. Mathey, G. Pauliat, J.C. Launay, G. Roosen, Opt. Commun. 82 (1991) 101.[25] K. Turki, G. Picoli, J.E. Viallet, J. Appl. Phys. 73 (1993) 8340.[26] P. Tayebati, D. Mahgerefteh, J. Opt. Soc. Am. B 8 (1990) 1053.[27] R.S. Rana, D.D. Nolte, R. Steldt, E.M. Monberg, J. Opt. Soc. Am. B 9 (1992) 1614.[28] P. Nouchi, J.P. Partanen, R.W. Hellwarth, in: Photoconductive Materials, Effects and

Devices, vol. 14 of 1991 OSA Technical Digest Series, Optical Society of America,Washington, DC, 1991p. 236.

[29] G. Bremond, Personal communication, (1993).[30] G. Pauliat, A. Villing, J.C. Launay, G. Roosen, J. Opt. Soc. Am. B 7 (1990) 1481.[31] J.Y. Moisan, P. Gravey, G. Martel, N. Wollfer, A. Aoudia, Y. Marfaing, R. Triboulet,

M.C. Busch, M. Hadj-Ali, J.M. Koebel, P. Siffert, Opt. Mater. 4 (1995) 219.[32] J.Y. Moisan, N. Wollfer, O. Moine, P. Gravey, G. Martel, A. Aoudia, E. Repka,

Y. Marfaing, R. Triboulet, J. Opt. Soc. Am. B 7 (1994) 1665.[33] B. Lambert, M. Gauneau, G. Grandpierre, M. Shoisswohl, H.J. Von Bardeleben,

J.C. Launay, V. Mazoyer, A. Aoudia, E. Rzepka, Y. Marfaing, R. Triboulet, Opt. Mater.4 (1995) 267.

[34] G. Bremond, A. Zerrai, G. Marrakchi, A. Aoudia, Y. Marfaing, R. Triboulet, M.C. Busch,J.M. Koebel, M. Hadj-Ali, P. Siffert, J.Y. Moisan, Opt. Mater. 4 (1995) 246.

[35] R.N. Schwartz, M. Ziari, S. Trivedi, Phys. Rev. B 49 (1994) 5274.[36] S. Adachi, T. Kimura, J. Appl. Phys. 32 (1993) 3496.

186 J.-Y. Moisan

[37] A. Zerrai, G. Bremond, G. Marrakchi, J.Y. Moisan, G. Martel, M. Gauneau, B. Lambert,P. Gravey, N. Wollfer, A. Aoudia, Y. Marfaing, R. Triboulet, J.M. Koebal, M. Hadj-Ali,P. Siffert, in: E-MRS Spring Meeting, Strasbourg, France, Elsevier Amsterdam, 1995.

[38] P. Tayebati, J. Opt. Soc. Am. B 9 (1992) 415.[39] S. Weiss, S. Sternklar, B. Fischer, Opt. Lett. 12 (1987) 114.[40] R.B. Bylsma, A.M. Glass, D.H. Olson, M. Cronin-Golomb, Appl. Phys. Lett. 57 (1990)

1968.[41] P.L. Chua, D.T.J.H. Liu, L.J. Cheng, Appl. Phys. Lett. 57 (1990) 858.[42] V. Vieux, P. Gravey, N. Wolffer, G. Picoli, Appl. Phys. Lett. 58 (1991) 2880.[43] N.Wolffer, P. Gravey, J.Y. Moisan, C. Laulan, J.C. Launay, Opt. Commun. 73 (1989) 351.[44] J.E. Millerd, E.M. Garmire, M.B. Klein, J. Opt. Soc. Am. B 9 (1992) 1499.[45] M.P. Petrov, S.L. Sochava, S.I. Stepanov, Opt. Lett. 14 (1989) 284.

CHAPTER IIB

Department of Physics, Un

Cadmium Telluride-BasedSolar Cells

A.W. Brinkman

1. INTRODUCTION

With a near-optimal room temperature band gap of 1.52 eV and a highabsorption coefficient (>105 cm�1) [1, 2] above the band edge, CdTe is inprinciple an ideal candidate for use as a thin film solar cell material. Thereis a correspondingly large literature on various aspects of CdTe-basedcells and the present review makes no pretence to be comprehensive.The aim instead, is to introduce some of the background, review thestate of the art, to briefly examine obstacles to the realisation of potentialperformance and to consider the degradation mechanisms that determineoperational lifetime.

The first reported CdTe-based solar cells were shallow p-n homojunc-tion devices that had conversion efficiencies of 4–6% [3–5]. Eventually,efficiencies of up to 13% were achieved [6], but it was already clear that acombination of high absorption in the shallow surface layer, excessivesurface recombination and problems in making a shallow p-n junctionmeant that the homojunction was unlikely to be successful and the focusof attention shifted to heterostructures, principally the n-CdS/p-CdTeheterojunction [7]. Other heterostructure devices were briefly investi-gated, for example, the ITO-CdTe [8], but the CdS/CdTe cells ultimatelyhave proved to be superior.

The CdS/CdTe cells are fabricated in the superstrate configuration(Fig. 1): a multilayered structure where layers of CdS and CdTe aredeposited in sequence on transparent conducting oxide (TCO) – typicallya doped tin oxide film (e.g. SnO2:F, InSnOx (ITO), etc.) – coated glass and a

iversity of Durham, Durham DH1 3LE, United Kingdom

187

Glass substrate

CdS Window Layer

Transparentcontact (ITO)

Back contact

CdTe Absorber layer

Sunlight

Figure 1 Superstrate configuration.

188 A.W. Brinkman

suitable back contact is applied to the CdTe layer. The wide band-gap CdSacts both as a widow layer admitting visible and infrared (l > 520 nm)light to the strongly absorbing CdTe and as the n-type limb of then–p junction [7].

Studies based on the detailed balance Schockley-Queisser model [9],suggest a maximum efficiency for an ideal CdS/CdTe cell of about 29%(AM1.5G) [10]; however, these calculations do not take into account thepolycrystalline nature of the CdS and CdTe layers with all the losses dueto grain boundaries, voids etc. (Fig. 2), and in practice, only just over halfof the theoretical best performance has been achieved in the laboratory.Ideally, the n-CdS layer is kept to the minimum (100 nm) commensuratewith proper junction formation and made as conducting as possible – nþpone-sided junctions are more tolerant of conduction band discontinuities.[10]. The CdTe side of the junction is typically a few micrometre thickand frequently both the CdS and the CdTe layers are subjected to somepost-deposition treatment, usually involving CdCl2 and/or heating in air.For a comprehensive review of the material issues see Durose et al. [11].

Glass

TCO

CdS

CdTe

Contact

Grain boundaries Voids

Figure 2 ‘Real’ polycrystalline cell.

Cadmium Telluride-Based Solar Cells 189

2. STATE OF THE ART

In principle, an efficiency of about 18% [12], (short circuit current density,JSC ¼ 27 mA/cm2, open circuit voltage, VOC ¼ 0.88 V and fill factor,FF ¼ 76%) should be possible for a polycrystalline CdS/CdTe cell.As listed in the latest solar cell efficiency tables [13], the best conversionefficiency to date for a laboratory-scale device is that recorded byWu et al.[14] in 2001, which had an efficiency of 16.5% (JSC ¼ 25.9 mA/cm2, VOC ¼0.845 V, FF¼ 75.5%, aperture area ¼ 1.03 cm2). The preceding decade hadseen steady if slow improvements, with a 16% cell reported by Aramotoet al. [15] in 1997, and in 1993 by Britt and Ferekides [16] of a 15.5%efficient cell. The incremental nature of these improvements suggestthat, either the performance of the Wu cell is close to what is in factattainable, or alternatively, there are some materials issues that are notfully understood; for example, the role of post-deposition treatment inCl/O2 heat treatments necessary for the fabrication of efficient cells, andwhich are discussed later.

According to the same solar cell efficiency tables the best moduleefficiency is 10.7% for a module produced by BP Solarex [17] with anaperture area of 4874 cm2. The test module delivered 3.205 A at an outputvoltage of 26.21 V with a fill factor of 62.3%. Although lifetimeand performance in the field have yet to be established (a field life of25 years is generally accepted as being a necessary requirement), mod-ules are being manufactured with production exceeding 6 MW in 2004and expected to rise above 20 MW by 2006 [18].

3. DEVICE PROPERTIES

3.1. Back contact effects

Producing good injecting contacts to p-CdTe is difficult. The combinationof its large electron affinity (w ¼ 4.28 eV) and moderately wide band gap(Eg ¼ 1.52 eV) means that in the Schottky limit a simple metal contactwould require a material with a work function greater than about 5.8 eV(Eg þ w) [19] to obtain the valence band flat-band condition; no such metalexists. Observed barrier heights (e.g. by internal photoemission, CV etc.)are almost always larger than this due to pinning by interface states thatcreate a dipole, in effect changing the electron affinity [20, 21], andmakingthe problem even more severe. As a result alternative contact ‘recipes’have to be adopted, such as doping the back surface to produce a narrowbarrier through which carriers can tunnel or introducing buffer layers toaccommodate the contact potential. Although a large variety of backcontact regimes have been developed – the technologies are discussed

190 A.W. Brinkman

in detail later – there is invariably some sort of residual barrier, whichdepending on the magnitude will adversely affect performance.

By confining carriers at the contact–CdTe interface, barriers increasesurface recombination losses. For a given surface recombination velocitySbc, the recombination current density JR qnSbc, that is, JR is proportionalto the electron density, n, at the rear surface. For a small barrier,<0.2 eV, n will also be small and so will JR even for sizeable valuesof Sbc. Consequently, the impact on efficiency is insignificant, but forlarger potential barriers the reverse is true.

3.1.2. RolloverWhen the current is constrained by a barrier the J–V characteristics oftendisplay ‘rollover’, where the current saturates at bias voltages just aboveVOC and frequently ‘crossover’ as well, where the dark and illuminatedJ–V curves intersect in the first quadrant (Fig. 3) [22]. Rollover, formallydefined [23] as the case when the second derivative of the J–V curvesevaluated at VOC is negative, is generally attributed to the back contactbarrier acting as a reversed biased diode in series with the output [24](Fig. 3, inset). The majority hole current is then limited by the reversesaturation current (JSbcd) of the back contact diode (BCD) (Fig. 3, curve b).

–0.015

0

0.015

0.030

0 0.4 0.8 1.2

V

JBack contact

Solar cell

(a)

(b)

(c)

(d)

Crossover Rollover

Voltage (V)

Cur

rent

den

sity

(J)

Figure 3 Current–voltage characteristics illustrating rollover and crossover:

(a) dark J–V with no BCD; (b) dark J–V with BCD; (c) photovoltaic response with BCD

but no contribution from minority electron current; (d) as for (c) but with electron

current contribution. Inset shows equivalent circuit of solar cell including BCD.


This will occur under both illumination and in the dark. Often rollover isobserved to be temperature dependent, becoming increasingly apparentas the temperature is reduced, because JSbcd is proportional to the thermalelectron velocity [24].

Niemegeers and Burgelman [22] have shown that the dark current willstart to saturate when the bias voltage reaches:

VSDark� AkBT

qln

JSbcdJSpn

� �ð1Þ

where JSpn is the reverse saturation current density of the main CdS/CdTejunction (with ideality factor, A), and KB, T and q are the Boltzmannconstant, temperature and electron charge, respectively. Under illumi-nation current saturation sets in at a higher bias voltage (determined byJL/JSbcd:JL is the photo-generated current):

VSLight � VSDarkþ AkBT

qln 1þ JL

JSbcd

� �ð2Þ

but the total current should still saturate at JSbcd (Fig. 3, curve c). Thecorresponding perturbation in the output characteristics can lead to asignificant deterioration in the fill factor if the barrier height is largeand/or the saturation current is very small.

As a series component, the BCD should not, in principle, affect VOC

nor should it significantly limit JSC. Under short circuit conditions (and inthe absence of other losses), hole current flow through the back contactshould induce an equal potential, DV, of opposite sign to that generatedacross the primary junction, thus the BCD is forward biased and no longerlimiting:

DV ¼ kBT

qln 1þ JL

JSbcd

� �ð3Þ

The fill factor, FF0, for an ideal cell (i.e. one in which series and shuntresistive losses can be neglected) is a function of the open circuit voltage.For voltages below the maximum power point, the total device current iseffectively constant and therefore so is the bias voltage across the BCD,DV. Consequently, the current–voltage characteristic for the non-idealdiode can be approximated by a translation of the ideal J–V curve by anamount DV. The non-ideal fill factor would then be approximated by theideal FF0

0 at (VOC – DV); but the actual open circuit voltage, of course, inthe non-ideal diode is VOC and not (VOC – DV). Thus the non-ideal FF isgiven by FF0

0, reduced by a factor dependent on the degree to which theBCD is limiting performance:

FF � FF00 1� kBT

qVOCln 1þ JL

JSbcd

� �� ð4Þ

192 A.W. Brinkman

Clearly, as long as JL/JSbcd 1 (i.e. JL not limited by JSbcd) any reduction inthe FF will be small and unimportant compared to other factors, but thatas the ratio increases, FF loss starts to become significant.

3.1.2. Open circuit voltageIn principle, CdS/CdTe cells should have high values of VOC, (a conse-quence of the relatively large band gap energy), and one reason for thehigh theoretical efficiency [10]. Predicted VOC values are seldom observedin any of the solar cell systems, but the VOC deficit (Eg–qVOC), as it istermed, is large in CdS/CdTe devices compared to other cells [20]. Thedeficit can arise from a variety of causes, although a potential barrier atthe back contact is in general involved. The interaction of carrier lifetimeand a back contact barrier is a case in point [23]. Where the back contact isnot dominant, but carrier lifetimes are short, space charge region recom-bination is high, and VOC is reduced. Longer lifetimes imply less spacecharge region recombination, and consequently an increased value ofVOC. If instead the back contact barrier is significant, then VOC, FF andhence efficiency are affected irrespective of the carrier lifetime.

Problems may also arise in thin cells if the space charge regionsassociated with the BCD and the main p-n junction overlap [25]. Thetwo independent diodes model [24] breaks down and the cell starts tobehave as a reach-through diode [26], where at some threshold voltage,VRT, the CdTe becomes fully depleted. The current is no longer controlledby either the main junction or the BCD and if this occurs in the fourthquadrant, the output voltage at open circuit will be VRT, that is, VOC is ineffect reduced.

Although for thick cells, with small barriers, reach-through is unim-portant, the need to cut costs leads inevitably to a reduction in CdTethickness with a concomitant increase in the probability of reach througheffects. More importantly, back contacts are not laterally uniform and themagnitude of the associated potential barrier will vary with position.Local points of weakness can arise creating reach-through ‘microdiodes’,even though the CdTe layer as a whole has not become depleted. Exceptat short circuit, the microdiodes are forward biased and constitute shuntcurrent paths, reducing VOC in the process. Clearly, this is more signifi-cant for thin CdTe layers where the incidence of microdiodes is likely tobe greater. Reach-through effects are contingent on the magnitude of theback contact potential barrier, demonstrating once more the need toreduce this as much as possible.

3.1.3. CrossoverCrossover is less well understood and there is still some debate [27] as towhether it is associated with the front part of the cell or with the backcontact. Niemegeers and Burgelman [22] calculated the electron surface


recombination current (due to photo-excitation) by solving the transportequation in the neutral zone (i.e. the region bounded by the edges of thep-n junction and BCD space charge regions). They showed that if thediffusion length (Ln) was sufficiently long in relation to the width ofthe field free region (d), then the electron current could make a significantcontribution to the total current. The total current would still saturate,but at a higher value than in the dark (Fig. 3, curve d).

Beier et al. [28] extended the model, to allow for the voltage andwavelength dependence of the contact saturation current. In derivingthe hole current, Jp, the contact saturation current had been assumed tobe a constant. In real devices it is often found to be dependent on thevoltage and the illumination. Electron transport across the neutral regionis given (in the dark) by the term exp (–d/Ln), that is, the probability thatan electron will travel from the edge of the junction space charge region tothe edge of the BCD depletion width. Under forward bias, (reverse biasfor the BCD), the BCD space charge region is increased, hence reducingthe width of the neutral region, and resulting in pronounced voltagedependence.

The intensity and spectral content of any illumination can also affectthe electron current due to barrier lowering by absorption at interfaceacceptor states.

Crossover has also been attributed to the effects of illumination ondeep levels in the CdTe. Kontges et al. [27] modelled the case where therewere a high density (1016 cm�3) of deep acceptor states with dissimilarcapture cross sections for holes and electrons of 10�18 and 10�13 cm2,respectively. The low acceptor concentration (Na < 5 � 1014 cm�3) inCdTe solar cells and the low hole mobility ( 50 cm2/(V s)), favour adiffusion model for current transport over a back contact barrier less than 0.5 eV. If under illumination acceptor traps are ionised, then the effec-tive doping at the back contact region is increased. In the diffusion modelfor transport over a barrier, current is proportional to Na

½, thus photo-ionisation of the acceptor traps in the vicinity of the back contact willincrease JSbcd, with crossover in the J–V characteristics as a result.

3.2. Inter-diffusion at the CdTe/CdS junction

Inter-diffusion across the CdS/CdTe junction is inevitable, creating inter-mediate ternary compounds of the form CdSxTe1–x [29–31]. Ohata et al.[32] were the first to publish the phase diagram for the CdS–CdTe pseu-dobinary system from high temperatures down to 700 �C. Nunoue et al.[33] extended the range to 650 �C. The two binaries are not miscible in allproportions except at high temperature and Ohata and co-workers wereonly able to obtain mixed crystals over almost the complete compositionalrange by annealing mixtures of CdS and CdTe at temperatures of 1000 �C

194 A.W. Brinkman

and quenching. Using XRD, the crystal structure of CdSxTe1–x was foundto be zincblende when x < 0.2 and wurtzite at higher values of x; it wasconcluded that at 1000 �C the miscibility gap was narrow. The liquidusand solidus were measured by differential thermal analysis and found toconverge to the same minimum temperature of 1071 �C at x ¼ 0.2. In thesolid phase they observed that the miscibility gap increased rapidly withdecreasing temperature, which they attributed to lattice strain due tothe large difference in the atomic radii of the group VI constituents. Atthe lower temperature of 700 �C, the CdSxTe1–x is zincblende for x < 0.2and wurtzite for x > 0.9 with a mixture of phases in between.

Jensen et al. [29] estimated the solubility of S in CdTe to be 5.8% at atemperature of 415 �C (a typical temperature for Cl doping in electro-plated cells, though significantly below temperatures encountered inclose space sublimation). As a result, the interface between the CdTeand the CdS tends to form two (or more) intermediate ternary com-pounds, a Te-rich layer (CdTe1–xSx) and a S-rich layer (CdTeyS1–y).Nakayama et al. [34] reported a structure comprising two Te-rich zinc-blende layers (y ¼ 0.97 and y ¼ 0.96) adjacent to the CdTe side of thejunction and a S-rich wurtzite layer (x ¼ 0.99) on the CdS side in screenprinted cells. Similar results were obtained by Rogers et al. [35]. Theyused synchrotron radiation to measure the lattice parameter variation inelectroplated CdTe/CdS cells as a function of CdTe layer thickness andanneal time at 450 �C (with andwithout chlorine), and found considerableintermixing. For example for a chlorinated structure where the thick-nesses of the CdS and CdTe layers were 80 nm and 400 nm respectively,the CdS was completely ‘consumed’ after only two minutes anneal toform CdS0.93Te0.07 which did not appear to change with further annealing.Over the same two minute period, cross-diffusion of S into the CdTe layercreated a non-stoichiometric layer of composition, CdTe0.98S0.02, adjacentto the original junction. Eventually after sufficient annealing time(15 min), the average composition of the 400 nm CdTe layer changedto CdTe0.95S0.05 in agreement with the solubility studies of Jensen et al.[29], the S being derived from the non-stoichiometric layer.

Lane and co-workers have also measured the variation in the CdTelattice parameter as a function of depth [36] and concluded that theCdTe layer must have been under an in-plane stress of 140 MPa nearthe interface, sufficient to introduce a significant level of structuraldefects. On annealing the lattice parameter was found to follow a decreas-ing trend, which was attributed to the in-diffusion of S, to give a Te-richmixed crystal layer with y ¼ 0.96, in good agreement with Nakayamaet al. [34].

The interfacial compounds play an important role in accommodatingthe 10% mismatch between CdS and CdTe [37] in particular reducingthe concentration of recombination centres. Carrier lifetime should be


increased and, as discussed above [23], rollover with its detrimental effecton FF should be less severe.

Inter-diffusion will also affect the band line-ups [38]. As alreadynoted, conversion of CdS and CdTe to the ternary compounds ‘consumes’parts of both. However, since the thickness of the CdS layer is only100 nm, while that of the CdTe is typically several mm, the effect onthe CdS is disproportionately greater. The residual CdS layer is sand-wiched between the nþ TCO and the smaller band gap ternary, creating a‘hump’ in the energy bands. Discontinuities in the band lineups on theCdTeyS1�y side produce barriers to minority hole injection across thejunction. Holes generated by absorption of light in the CdS, will driftinto the hump, which constitutes a potential minimum, from where theymay be emitted over the barrier or trapped in the deep acceptor states ofwhich there will be a high density. The trapped holes will neutralise someof the negative charge, thereby reducing the barrier height and increasingemission over the barrier. When externally biased, the potentials through-out the cell will self-adjust to allow the same current to flow through allseries connected parts of the device. Superposition no longer holds andcrossover will occur in the characteristics.

The spectral response (variation of quantum efficiency (QE) with wave-length of illumination, QE(l)) of CdS/CdTe solar cells displays a window-type characteristic, bounded at the short and long wavelength ends by theband gaps of the CdS window layer and the CdTe absorber layer.

The optical band gap energy of CdSxTe1–.x is given by the empiricalexpression [39]:

EgðxÞ ¼ 2:4xþ 1:51ð1� xÞ � 1:8xð1� xÞ ð5ÞThis differs from the expression given in Ref. [40], which underestimatesthe energy gap of CdS. Equation (5) is strongly bowed, with a band gapenergy less than that of CdTe for 0 > x 0.51 and a minimum valueat x ¼ 0.3 of 1.4 eV, corresponding to a wavelength of 884 nm. Inter-diffusion may be expected to modify QE(l). Compared with the absorp-tion spectrum of a pure CdTe film (band edge at l 820 nm), a smallamount of S diffusion will cause a red shift in the long-wavelength endof the QE(l) response. For CdTe1–xSx with x 0.05, that is, close tothe solubility limit [29], the band gap would be reduced to 1.47 eV(l 842 nm); this has been widely reported [29, 34, 41].

There is also a corresponding shift to longer wavelengths at the blueend of the spectral response, as a consequence of the diffusion of CdTeinto CdS. This is generally held to be undesirable [11], since it reducestransmission into the absorbing CdTe. Absorption in the CdS side of thejunction is not efficient, and it is better if the incident light is absorbed inthe CdTe; this dictates that the CdS layer be as thin as possible, and higher

196 A.W. Brinkman

values of JSC are indeed obtained [37, 42] for thinner layers of CdS.However, excessive shunting will result if the CdS is too thin due topinhole formation.

4. FABRICATION OF CELLS

One of the singular advantages of the CdS/CdTe solar cell, is that it maybe produced by a wide variety of low cost and scaleable techniques [43,44], including physical vapour deposition and chemical bath methodswell suited to industrial scale manufacture of modules with m2 dimen-sions. Other deposition processes that could be used for module produc-tion, for example, atomic layer deposition, spray pyrolysis and screenprinting etc., have not so far proved useful, either because of cost or thepoor performance of the resulting modules.

4.1. Deposition of the absorber CdTe layer

The CdTe layers in the most efficient cells [14–16] have all been depositedby close-spaced sublimation (CSS). Originally developed by Nicoll [45]over 40 years ago for the heteroepitaxial growth of GaAs on Ge, CSS is aphysical vapour deposition process, where, as the name implies, the gapbetween the sublimating source and substrate is very small; just a fewmillimetre. At this proximity, the substrate will necessarily be at a rela-tively high temperature given that dissociative sublimation of CdTe takesplace at temperatures above 500 �C in vacuum. High-performance cellsare typically deposited at substrate and source temperatures in the range400–700 �C in a vacuum of 102 to 5 � 103 Pa, conditions under whichsublimation is diffusion limited [16]. It is a comparatively rapid processwith growth rates in the order of 1–2 mm/min. The as-deposited grain sizein CSS films is usually 3–5 mm somewhat larger than is the case with othergrowth techniques. Layers grown at higher temperatures are dense with apronounced {111} orientation [37].

Cadmium telluride may also be deposited by electroplating [2, 46–48].Although once the preferred technique, it is less common now as CSS hasconsistently proved to give superior devices. The CdTe is cathodicallydeposited from aqueous solutions containing Cd and Te ions in concen-trations of 10�4 mol/l. Plating is carried out at temperatures of 80–90 �Cand the pH is typically adjusted to 2. Electrodeposited CdTe layers aren-type and resistive, and must be type converted to form a proper p-njunction. This is typically done by an empirical procedure where the cellsare heated in air at 400 �C for a few minutes. It is frequently carried outsimultaneously with the Cl treatment discussed below, resulting in someconfusion in the literature between the respective roles of the Cl and air


heating processes. Grain sizes in as-grown electroplated CdTe layers arerelatively small, <1.5 mm, with a columnar microstructure.

4.2. Deposition of the CdS window layer

Chemical bath deposition (CBD) and CSS are generally used for the CdSlayer, although in the Aramoto cell [15], the CdS was deposited by metalorganic chemical vapour deposition (MOCVD), probably not well suitedto large area production in spite of the authors’ claims.

CBD of CdS [49] entails the heterogeneous reaction of thiourea(CS(NH2)2) and a Cd salt in a heated (60–90 �C) basic aqueous solution[50, 51]. Ammonia is commonly used to control the pH of the solution,although its volatility is a problem and less volatile alternatives such asethylenediamene are sometimes used instead [51, 52]. Growth rates tendto be low, 1–20 nm/min, and grain sizes very small, 15–100 nm.

The as-deposited CdS layer is randomly oriented and often reportedas being in the meta-stable cubic sphalerite rather than the thermodynam-ically stable hexagonal wurtzite phase, or sometimes a mixture of phasesand polytypes [53].

4.3. Post-growth annealing in chlorine

Efficient cells invariably require some form of post-deposition heat treat-ment or activation, generally by heating the layer in air after exposure toCdCl2 [11, 54, 55]. There are essentially three techniques for introducingthe CdCl2: (i) to deposit a layer by briefly immersing the cell in a solutionof CdCl2 in methanol before heating it for20 min in air at400 �C; (ii) todeposit the CdCl2 layer (60–100 nm thick) directly by vacuum evapora-tion followed by the same heating step as (i); (iii) to heat the CdTe inCdCl2 vapour at the annealing temperature. Note the latter does notapparently involve air/oxygen, although residual O2 may be presentfollowing the deposition process.

The CdCl2 process is not fully understood, but comparative studies oftreated and untreated cells have confirmed that its use is critical [42, 56,57]. In these studies the Cl treatment was found to increase the efficiencyfrom 1.5–10% in nominally identical cells. Heating in the absence of Clresulted in a small increase of about a factor of two to 3%; it is notewor-thy that the most efficient cells [14–16] were all subjected to a CdCl2procedure. The treatment does increase grain size in those cases wherethe as-deposited grain size is small (i.e. in electro-plated cells), is oftenaccompanied by some degree of recrystallisation, and promotes inter-diffusion. It appears to be a necessary step even where the CdTe hasbeen deposited by CSS, that is, at high temperature, when inter-diffusionis inevitable [37] and grain sizes are already large.

198 A.W. Brinkman

The microstructure of as-deposited CSS-grown CdTe is normallycolumnar with a pronounced {111} texture. Recrystallisation disruptsthis, producing a more random structure [42] that results in higher cur-rents. Durose et al. [11] have suggested that this somewhat counterintui-tive observation could arise because the incidence of twinning parallel tothe interface is greater in a columnar microstructure and lamellar twins ofthis type may be more prejudicial to the current flow. The CdCl2 treat-ment has also been reported to promote inter-diffusion, and some havesuggested that it is a necessary requirement [42].

The CdCl2 process changes the principal dark current transport mech-anism from interface tunnelling/recombination to space charge recombi-nation [57, 58] implying a reduction in the density of interface statesor possibly grain boundary states. Heating in the absence of Cl had noeffect. In addition to changing the principal dark current transportmechanisms, the Cl treatment modifies the density and nature of thedeep defect levels. Deep level transient spectroscopy (DLTS) of CdCl2processed cells [57] indicated that the use of Cl introduced a deep level atEV þ 0.64 eV, tentatively assigned either to a doubly ionised Cd vacancy(VCd)

2� or more probably a singly ionised Cd–Cl complex (VCdCl)�. In a

systematic DLTS study intended to elucidate the effects of post-deposi-tion processing on the distribution of deep levels, Lourenco et al. [59]found the distribution to be dominated by a hole trap at 0.46–0.49 eVabove the valence band, which was present in all samples, irrespective ofthe post-deposition processing, although the concentration was muchgreater in the CdCl2-treated cells. Post-deposition processing was foundto introduce a number of other metastable levels, with properties thatdepended to some extent on previous sample history. A comparativeDLTS and admittance study by Versluys et al. [60] of the effects on deeplevels of carrying out the CdCl2 activation in two differentways; comparingCdCl2 activation in vacuum and in air, also found a level with an activationof 0.45 eV,which they assumed to be a barrier at the back contact. Addition-ally, they observed a range of shallow and deep levels in both air andvacuumannealed cells, includingwhat appeared to be the chlorineA-centre(VCd–ClTe) at EV þ 0.113 eV in air-annealed cells and a similar, though lesspronounced, level in the vacuum-annealed devices.

Optical beam induced current (OBIC) studies have also shown that theCdCl2 treatment improves uniformity [61]. In untreated cells, the majorityof the cell area was inactive at the wavelength of the He-Ne laser used forthe investigation, whereas CdCl2 treated devices displayed almost novisible structure, suggesting a high degree of uniformity. Replacing thelaser by a well focused (<100 mm2) monochromated light source indicatedthat a good window response was only observed for CdCl2 treated cells.When the light was directed onto poor regions of the cell, the responsepeaked at the CdTe band gap and decreased with decreasing wavelength,behaviour indicative of a buried junction.


4.4. Back contacts

As discussed above, the fabrication of an injecting, low resistance anddurable back contact to CdTe has proved difficult. The ensuing problemsof rollover and crossover in the J–V characteristics have already beendiscussed at length. It is important that the barrier height andwidth shouldnot cause the localisation of carriers, since this would result in excessiveback surface recombination. In practice, this places an upper limit on thebarrier height of 0.4 eV, unless the barrier is particularly thin.

Contact regimes fall into two generic groups (see review by Fahren-bruch [20]): CdTe-metal or more realistically CdTe-dipole-metal types;and a group where some appropriate intermediate or buffer layer isincorporated between the CdTe and the metal to form a heterogeneouscontact. Whatever the contact regime the CdTe surface is invariableetched, typically using a Br/methanol solution or the so-called N–P etch(an aqueous solution of HNO3 and H3PO4) both of which preferentiallyremove Cd leaving a conducting Te-rich surface. The etching step iscrucial; not only is the Te layer formed over the surfaces of grains, it isalso produced along the grain boundaries where it creates a conductivebridge between grains, increasing lateral conductivity [62]. However, theNP etch is aggressive and excessive etching will widen the gaps betweengrains. A brief etch of a few seconds in a stronger NP solution appears tobe preferable to a longer etch in a weaker solution. In the latter case, grainboundary broadening appears to be more significant allowing diffusionrelated shunt leakage paths to develop with a consequential loss ofperformance [62].

The impact of Fermi level pinning at the CdTe–metal interface onthe barrier height is clearly demonstrated in the study undertaken byDharmadasa et al. [63]. They measured the electrical properties of a largenumber of metal contacts (to single crystal (110) n-CdTe) and with theexceptions of Mn, Cr and V, found a constant barrier height of 0.72 eVindependently of the metal. A barrier of this magnitude is too great forefficient cell performance, and apart from cases where the objective ofthe study was to model contact barrier effects explicitly [22, 25], simplemetal-CdTe contacts, for example, evaporated Au [47] are seldom used.More commonly, the surface is heavily pre-doped with an acceptor suchas Cu to lower the resistivity before depositing the contact metal, forexample, Au, graphite, Mo, Ni [54], although the predisposition to selfcompensation in CdTe makes it difficult to achieve the necessary levels ofdoping (1018 cm�3).

The second generic group offer greater flexibility, but at the cost of anadditional junction and potentially another barrier. However, the judi-cious choice of materials can lead to an optimum situation where thebuffer layer is highly or even degenerately doped minimising any barrierto the outer metal. XPS and related techniques have demonstrated that on

200 A.W. Brinkman

exposure to air, the CdTe surface will be oxidised to TeO2 [64, 65], but theoxide layer is very thin (it is readily removed by briefly sputtering inUHV) and would not seriously impede quantum mechanical tunnellingof carriers. The elemental Te layer is several nm thick with a valence bandoffset, DVB, between Te and CdTe reported by Niles et al. [64] to be 0.26and 0.5 eV by Kraft et al. [65]. As observed by Kraft, this determines thelower limit for any back contact involving Te.

The most commonly used contacts are based on the use of Cu.A Te-rich surface is prepared as above, followed by the application of aCu-loaded graphite paste [37]. A brief heat treatment diffuses the Cu intothe CdTe surface where it reacts with the Te to form non-stoichiometricCu2–xTe (x 2); a degenerate semiconductor (due to the excessive densityof Cu vacancies, 1021 cm�3 [66]) with a band gap of 1.04 eV. A layer ofCu2–xTe may also be deposited by co-evaporation of Cu and Te [67] orelemental Cu may simply be deposited by evaporation [68]. Surfacescience studies [67] of Cu-based contacts have shown that the bandalignment between CdTe and Cu differs from that between CdTe andCu2–xTe, with corresponding barrier heights of 1.0 eV and 0.8 eV,respectively. Notwithstanding such large barriers, efficient contacts canbe readily made using Cu2–xTe. This paradox may reflect the fact that UPSand XPS measurements made in UHV are not representative of contactsmade in ordinary ambient. Alternatively, a very high density of surfacedoping is in fact achieved in these contacts, with the result that the barrieris sufficiently narrow to allow tunnelling. Wu et al. [69] have shown thatcontrol of the Cu2–xTe phase is critical. They noted that although theresistivity of the chalcopyrite phase (x ¼ 0) can be as low as 10�4 O cm,this does not always make the best contact. The most efficient of their testcells used a mixed phase consisting of CuTe þ Cu1.4Te which gave anefficiency of 12.9%. The degenerate character of the Cu2–xTe means that agreat variety of metals may be used as the outer contact, since barriers atthis interface will be particularly narrow.

Although widely used, Cu2–xTe contacts are unfortunately short lived.Copper diffuses into the semiconductor, depleting the contact and thusincreasing its resistance, while simultaneously decreasing the effective-ness of the main junction [20]. Copper will in a relatively short period oftime diffuse through the entire CdTe, creating defect states near the p-njunction [69] with a consequential reduction in the photo-generated car-rier life-time. Similar results have been found with HgTe and ZnTe:Cu[70] based contacts.

Alone among the possible contenders, ZnTe has a favourable valenceband alignment with CdTe, with a negligible DVB of only 0.05 eV [71] andwhen doped with N, is potentially a good Cu-free contact with highdoping levels, 1020 cm�3 [72]. Baron [73] has suggested that the dopingefficiency of N is higher in ZnTe than in CdTe, because in the tetrahedral


cell, NTe is more stable when surrounded by Zn atoms than by Cd atoms.They quote X-ray diffraction data which suggest the Zn–NTe bond lengthis 2.2 A close to metal–nitrogen bond lengths of 2.1 A. Nitrogen, therefore,should not diffuse so rapidly as Cu into CdTe. However, ZnTe and CdTeare miscible across their compositional range and inter-diffusion atthe CdTe/ZnTe interface to form a graded junction is to be expected.The extent of any such inter-diffusion and its potential effects (positive orotherwise) has not been systematically studied. Amin and co-workers [72]investigated the effects of a Cd1–xZnxTe layer at the back contact of a thinfilm of CdTe, but this was for x ¼ 0.5, larger than would be expected forany cross-diffusion, moreover the Cd1–xZnxTe layers were Cu doped.

Nitrogen-doped ZnTe layers can be grown by vacuum evaporation ofZnTe using a RF plasma source for the N-doping [70–72]. The thickness ofZnTe:N is kept below 1 mm. Although the work function of ZnTe is5.27 eV [70] and thus a high work function material such as Au or C mustbe used as the outer contact, this is not such an important issue as it iswith CdTe, due to the high doping levels. The metal/ZnTe:N barrier istherefore very narrow and carriers can tunnel through the potentialbarrier relatively easily.

Evidently, back contacts must be stable over periods of many yearsif CdS/CdTe modules are to be viable sources of solar energy. Antimony-based contacts have so far proved to be among the most stable.A 100 nm thick film of Sb is deposited onto freshly etched CdTe (NPetch) either by vacuum evaporation [62] or as a sputtered film of Sb2Te3[74] grown at 150 �C. Metallisation is provided by Au or Mo preferablydeposited without breaking vacuum. Using Sb2Te3/Mo contact efficien-cies of 12.5% [74] were achieved, although other researchers have foundonly modest performance [20]. Antimony telluride is a small band gapsemiconductor (0.3 eV), but since the electron affinity is not known thevalence band line up cannot be estimated. Current–voltage characteristicstypically show roll over [62] suggesting that there is a barrier in the orderof 0.4 eV at minimum. Batzner et al. [62] have suggested the better stabil-ity of Sb-based contacts (as compared with Cu-based contacts), may berelated to the relative sizes of the atomic radii – 1.28 and 1.45 A for Cu andSb, respectively, the smaller atom being able to diffuse more rapidly.

4.5. Transparent conducting oxide front contacts

In comparison with, say the problems encountered with the back contact,the TCO/CdS interface is not widely regarded as being important asa limiting factor in cell performance [11, 75]. There are, nevertheless,some issues to be considered.

Conventionally, SnOx often doped with F, has been used as the TCO inCdS/CdTe cells. It is a wide band gap (between 3.6 and 4.3 eV), direct

202 A.W. Brinkman

gap semiconductor which is strongly n-type or even degenerate, due tooxygen vacancies. Photoemission studies [75] imply that the conductionband minimum in the SnOx coincides with that in the CdS, making it alow-resistance interface. In theory SnOx should be an ideal TCO, but thecomparatively high sheet resistance 10 O/□ and modest optical trans-mission of 80%, represent some loss of efficiency. With a resistivity r 2 � 10�4 O cm (some 3–4 times lower than SnOx) and a greater trans-mission �80% over the relevant spectral range, ITO ((In2O3)0.9(SnO2)0.1)gives better performance. A variety of doped In2O3 materials have beensuggested including In2O3:Ge [76, 77] and In2O3:F [76], both of which arecomparable to ITO – r 2 � 10�4 O cm and transmission 85%.

Although the performance of ITO and related materials is quite accept-able, they are expensive, due to the high price of In. This has inspired asearch for alternative In-free TCOs, of which the most promising areCd2SnO4 (CTO) and Zn2SnO4 (ZTO) [78]. Cadmium stannate films haveresistivities (r 1.5 � 10�4 O cm) some 2–6 times lower than SnOx, a peaktransmission > 90% at l ¼ 500 nm and are an order of magnitude moresmooth. Zinc stannate (ZTO) is much more resistive (r10�2 O cm) [78],but offers several advantages when used as an integrated buffer layerbetween a low resistive TCO (i.e. CTO) and CdS. During the process offabricating cells, cross-diffusion will take place at both the CdTe/CdS andZTO/CdS interfaces, consuming CdS from both sides in the process. Zinccadmium sulphide has a larger band gap than CdS, moving the shortwavelength cut-off deeper into the blue, increasing JSC as more of theavailable spectrum is transmitted through the window layer into theabsorbing CdTe. In addition, this allows the initial deposition of thickerCdS films, reducing the incidence of pinholes and short circuits. Zincstannate also improves layer adhesion, a particular problem with theCdCl2 process where extended treatment can lead to blistering and delam-ination [78]. Diffusion between CdS and ZTO appears to relieve the inter-facial stress, allowing greater latitude in optimising the CdCl2 process.

4.6. Alternative structures

4.6.1. Lightweight flexible substratesFor space applications, CdS/CdTe cells fabricated onto glass substratesoffer no weight advantage compared to high-performance single-crystalSi or GaAs cells, and although these are expensive and susceptible toradiation damage, traditionally they have been used to provide the elec-trical power. In low earth orbits, cell degradation due to radiation damageis not a significant problem, but to provide global coverage for, saycommunications, would require a large number of satellites. Far fewerwould be needed in high Earth orbits, but here cell degradation is aproblem and the design of the solar panels becomes complicated [79].


However, the production of high specific power (kW/kg) thin film mod-ules on thin metal film foils, if it can be achieved, would bring consider-able benefits. Since CdS/CdTe cells are only 10 mm thick the supportfoils need only be 50 mm thick, and support structures can be kept to theminimum. In addition, unlike single crystal devices, the CdS/CdTe cellis already quite highly defected (due to lattice and thermal expansioncoefficient mismatch etc.), and in consequence it is expected that it shouldbe less susceptible to radiation damage. Finally, foil mounted cellsmaybe folded (in principle) into any shape.

There are in essence two principal device configurations for a foilmounted cell: with the CdTe adjacent to the foil (referred to as ‘backwall’) or with the CdS next to the foil (front wall). Front wall cells requirea light transmitting contact to the CdTe, and at that time no satisfactorytransparent contacts to p-CdTe were known. Although, a grid contactstructure could have been used, the high sheet resistance of CdTe meantthe grid would need to be dense and inefficient. Of course, back walldevices also suffer from the problems of making reliable contacts top-CdTe, as discussed earlier.

Mathew et al. [80] have reviewed the deposition of CdTe by a varietyof techniques including CSS onto several metal films, including stain-less steel (SS), Mo, Ni and Cu. Molybdenum was considered to be apotentially good substrate due to the close match in thermal expansioncoefficient with CdTe – important for the high temperatures involved inCSS. However, energy band analysis indicated that the Mo/p–CdTecontact would be rectifying and the CdTe at the interface would need tobe heavily doped to promote efficient quantum tunnelling through thebarrier (see Section 4.4). Virtually all the studies reviewed related tometal/n-CdTe Schottky devices and were aimed at determining chargetransport processes, trapping levels and barrier heights etc. With theexception of the study by McClure et al. [79], little work had beenreported, at that time, on the fabrication of heterojunction devices onflexible substrates.

McClure et al. focused on the back wall configuration, because of theproblems with the front wall design. Initially, they used low carbon steelsubstrates, as iron has a relatively high work function. The films adheredwell with a low contact resistance, but proved to be n-type, probably dueto Fe3þ substituting for Cd2þ. They also studied Ni and Mo. Of thecommon metals Ni has one of the highest work functions, but theyfound there were adhesion problems following annealing. Ultimately Moproved the most successful. To circumvent the contact problems, 50 nmlayers of Cu and Te were deposited in sequence to produce an inter-layerbetween the CdTe and the Mo. This procedure gave contact resistances ofless than 1 O cm2 due, it was assumed, to the formation of a thin CuxTelayer at the interface. A thin 0.5 mm top layer of CdS was deposited by

204 A.W. Brinkman

evaporation, but subsequent annealing steps resulted in the inter-diffu-sion of S and Te to form a p-type layer of CdSxTe1–x. Since this hadconsumed most of the n-CdS the device did not function correctly andanother layer of CdS had to be deposited after the anneal. The structurewas completed with reactively sputtered ITO giving a sheet resistanceof 10 O/□. Although only a few cells were fabricated and tested, theyreported values of VOC and JSC of 580 mV and 12 mA/cm2, respectively.

Using ‘lift-off’ processes Romeo and co-workers [81] have recentlydeveloped efficient cells on flexible polymer substrates. They have beenable to produce cells in both the superstrate and substrate configurations,by supporting the thin films on conventional soda-lime glass through thedeposition and annealing stages. Although most transparent polymersare not stable at the high temperatures (450–550 �C), some polyimidesretain sufficient transparency for use in solar cells. Initially, a thin bufferlayer of NaCl was evaporated onto the glass. A10 mm film of polyimide,which had been developed in-house, was spin-coated onto this and aftercuring at 430 �Cwas coated by a layer of ITO by RFmagnetron sputtering.CdS and CdTe layers were then deposited and activated by CdCl2 bytheir standard process. When complete, the cell was rinsed in water todissolve the NaCl and separate the device on its flexible polymer sub-strate from the glass carrier. These cells yielded high efficiencies of 11%,(VOC ¼ 842 mV, JSC ¼ 18.5 mA/cm2, FF ¼ 70.9%). The relatively low JSCwas due to absorption in the polyimide.

Clearly a practical manufacturing process will require a low cost,commercially available polymer, and after evaluating a number of com-mercially available polyimide films, an UpilexTM 10 mm thick film wasfound to be sufficiently transparent for use in CdS/CdTe solar cells.Cells produced on the Upilex films were marginally more efficient,11.4%, (VOC ¼ 765 mV, JSC ¼ 20.9 mA/cm2, FF ¼ 70.9%) due probablyto a slightly increased transparency. They also studied the inverse ‘sub-strate’ configuration, where the TCO was deposited onto the NaCl. Afterremoving the cell from the glass support by rinsing in water as before, thecells proved to be less efficient. Two TCOs, ZnO:Al and FTO, wereinvestigated and exhibited efficiencies of 6% and 7.3%, respectively. Inboth cases, the FFs were very low as were the open circuit voltages.However, the short circuit currents were greater than for the superstratecase, since loss due to absorption in the polyimide (now at the back ofthe device) was reduced. The high efficiency polyimide-based flexiblecells demonstrated a specific power potential of 2 kW/kg. If the cell-level laboratory-based process can be transferred to the factory, use ofcommercially available UpilexTM is an attractive option for an in-linemanufacturing process, while replacing the polyimide with a suitablemetal foil, for example Mo, makes this a promising contender for civilspace applications.


4.6.2. Bifacial CdTe solar cellIt has been suggested that with the use of multi-junction configurations,the next generation of CdTe-based cells could realistically have effi-ciencies of 25% [82]. Such devices can be either mechanically stacked(three or four terminals) or two terminal monolithic structures. In addi-tion to the usual transparent conducting top contact, both arrangementsrequire a conducting, transparent back contact to allow the transmis-sion of light to the underlying cell; i.e., a bifacial configuration. Theprincipal challenge is to produce a good transparent contact to the CdTe.

Simply sputtering ITO onto the CdTe (after a Br/methanol etch)produced a transparent contact (>85%) with a low sheet resistance of10 O/□ [83]. However, as-deposited cells proved to be inefficient, � 2.5%, though annealing in air at 350 �C and after light soaking the effi-ciency could be more than doubled. The deposition of a thin Cu layer justbefore sputtering the ITO improved the efficiency, but accelerated life-time tests produced results that depended on the thickness of the Cu.Cells with 3 nm Cu inter-layer degraded over a four year equivalenttime, but were comparatively stable thereafter, while the performanceof cells with <0.5 nm Cu improved slightly with accelerated ageing.Cells with thicker, 2.5 mm, absorber (CdTe) layers performed betterwhen illuminated from the front (� ¼ 10.3%) but less well under backillumination (� ¼ 2.1%). When the absorber layer was reduced to 1 mm theefficiency under front illumination was reduced to � ¼ 8.6%, but underback illumination was increased to 3.2% largely as a result of an increasein JSC. Marsilac et al. [84] investigated even thinner cells, CdS ¼ 0.13 mmand CdTe ¼ 0.68 mm. These ultra-thin cells followed the same generaltrend, with efficiencies of 5.7% and 5% when illuminated from the glassand back contact sides, respectively. The cells were fabricated on SnOx:F-coated soda lime glass with a reactively sputtered ZnTe:N and ITOback contacts to produce a glass/SnOx:F/CdS/CdTe/ZnTe:N/ITOstructure.

The external QE of front and back contact spectra were quite different.The profile of the former was similar to that of thicker cells, but with areduction in QE from 600–800 nm due to the reduced absorption in thethin CdTe. Light incident from the back contact side must first passthrough the ITO and the ZnTe:N which has a band gap of 2.2 eV, andalthough it was very thin (70 nm), the QE was less than expected atl 560 nm, implying there was little or no carrier collection at these wave-lengths. It was suggested that at these wavelengths, photons are absorbednear the back contact where the electric field is weak and carrier separa-tion and collection is as a result poor. At wavelengths in the range 400–800 nm, photons are increasingly absorbed near the principal CdTe/CdSjunction, leading to a progressive increase in QE with increasing wave-length up to the band edge.

206 A.W. Brinkman

5. MANUFACTURE OF CdS/CdTe MODULES

Commercial high-volume module production requires the conversionof laboratory-based batch processes into continuous or at least semi-continuous assembly lines. Transfer of laboratory expertise and experi-ence to the factory is not merely a change of scale, but a paradigmshift. The magnitude of the necessary investment dictates radically differ-ent perspectives. Questions of yield and price transcend the search forultimate performance. Birkmire and Eser [43], in their review of poly-crystalline cells, outlined the challenge facing manufacturers of thinfilm polycrystalline modules. They noted that a factory manufactur-ing 10 MWp (i.e. the maximum power the module can deliver under100 mW/cm2, AM1.5 insolation) CdTe modules (1.2 � 0.6 m2) per annum,assuming 100% yield and 83% process uptime would need to produce onemodule every 2½ min, working 3 shifts/day, 250 days/year. Moreover,this must be achieved at a cost that the market will accept.

In spite of the challenges, a number of companies are beginning tomanufacture CdS/CdTe modules on the MW per annum scale. By far thelargest, First Solar [85], was created in 1999 with the purchase of SolarCells, Inc. (SCI) and started production of commercial modules in 2002with a 25 MW plant at its Perrysburg, Ohio facility. By the end of 2007production capacity had been expanded to 307 MWp with increased pro-duction at its Ohio base and four more lines in Germany. Capacity isplanned to reach 1 GWp by the end of 2009 with the construction ofadditional plants in Malaysia. While the module design is essentially astandard CdS/CdTe superstrate configuration, First Solar is particularlysecretive about theirmanufacture process and very little is known about it.

However, it is believed to be a refined version of that used by itspredecessor SCI [86]. The SCI manufacturing process is described in anannual subcontract report to NREL in 1993 [87]. In the SCI process SiO2/SnOx-coated glass is passed in sequence through a series of four heatedvacuum chambers on ceramic rollers. After the glass superstrate has beenloaded into the first chamber, the entry valve is closed and the chamber isevacuated. While it is being pumped down, the superstrate is radiativelyheated to 600 �C before it is transferred to the second chamber for deposi-tion of the CdS layer. This is carried out in a single pass through elementalvapours from a crucible containing a powder source at 700 �C locatedabove the superstrate. Control is exercised by admitting nitrogen into thechamber at a pressure of 1 torr. The superstrate is then passed intothe CdTe zone after which it is subjected to what appears to be a standardCl-based heat treatment at 400 �C before being quenched using N2.Cadmium sulphide and cadmium telluride deposition typically takeonly 10 and 40 s, respectively. Nickel is used as the CdTe contact backedby a thicker layer of Al. Interconnects are formed by laser scribing and the


module is completed by laminating a second sheet of glass on to the backusing a layer or of ethyl vinyl acetate. The SCI process differs from theusual CSS methods, in at least two significant respects: the superstrate ispre-heated before it is admitted into the deposition zones and secondlythe crucibles containing the source powders are located above the super-strate and face away from it. At 600 �C the glass is close to its softeningtemperature and this arrangement will allow it to be supported by moreclosely spaced rollers.

In an enterprising innovation First Solar developed the first pre-funded module collection and recycling programme in the PV industry.Typically modules can deliver 75Wp at a conversion efficiency of 10.6%,with a manufacturing cost of $1.14 per Wp, though to compete againstconventional fossil fuels, manufacturing costs will have to be reduced to$0.7 per Wp [86].

The principal European manufacturer is ANTEC Solar GmbH, whohave established a line intended to manufacture [88] 100,000 m2 of mod-ules per annum (standard size 60 � 120 cm2), corresponding to a powergenerating capacity of 10 MWp. The plant, which has been in operationsince 2002, is based on the CSS deposition of both CdS and CdTe in anautomatic production line, giving a high throughput of about 1 m/min.The projected net saving in CO2 is 16,000 kg/m2 over a 30 year life span.

6. DEGRADATION MECHANISMS

Clearly for CdS/CdTe modules to be practical, they must be stable in thefield and under load for many years. Laboratory-based studies of individ-ual solar cells tend to focus on improving cell performance and acquiringan understanding of the device physics. Apart from being time consum-ing, field trials involve other extrinsic factors such as the integrity ofpackaging, temperature cycling, humidity etc. not normally carried outin laboratory-based studies. It is important therefore, to distinguishbetween external influences and inherent ageing effects [89]. Due toobvious time constraints, degradation studies are usually carried out inaccelerated conditions where the unit under test is maintained in a cli-mate chamber at high levels of temperature and humidity (e.g. 85 �C and85%, respectively) under 1 sun (or greater) illumination and in opencircuit. More sophisticated trials utilise a range of stressing protocolsthat aim to mimic in some degree actual use, for example, cyclingbetween maximum power, open circuit and short circuit conditions [90].

The loss of performance is generally manifested as a reduction in VOC

and FF (Eq. (4)), indicative of an increase in series resistance and/or thedevelopment of a blocking contact [68]. Usually, cells and modulesstressed under open circuit or in reverse bias show greater power loss

208 A.W. Brinkman

than those tested under short circuit or at maximum power [90]. Thisobservation implies that degradation processes originate in material andbehavioural changes within the active components of modules. The prin-cipal cause of instability is probably the back contact, and there is consid-erable evidence for the diffusion of back contact materials under heat andnear junction field stressing [62]. In a comparative study, Batzner et al.[74] demonstrated that even for rather short periods of heat stress, 200 �Cfor 30 min in air, considerable loss of performance was observed for allcontact regimes except those based on Sb or Sb2Te3 buffer layers and Mometallisation. Secondary ion mass spectroscopy analysis of cells that hadbeen subjected to more rigorous accelerated life time tests, where the cellswere light soaked at 65 �C under 1 sun in open circuit conditions in air,revealed that Cu and Au had diffused through the CdTe and accumulatedwithin the CdS and at the CdS/TCO interface. There was also some Sb,but this was an order of magnitude less and only observed from Mo/Sbbi-layer contacts. No Sb was detected from Mo/Sb2Te3 contacts, nor wasany Mo.

A blocking contact can also develop due to oxidation of the backcontact resulting in an insulating layer at the back surface of the CdTe[91] again leading to roll over in the J–V characteristics and a reduction inthe FF. The oxidation process will be accelerated in conditions of highhumidity, emphasising the need for proper encapsulation of modules.

Outdoor field trials [92] of sixteen standard modules (1.2 � 0.6 m2)with Sb-based contacts showed an average of 12–13% relative degrada-tion in power output after 18 months outdoor exposure. This was follow-ing an initial increase of 4% over the first 5 months, which correlatedwith a rise in ISC over the same period before saturating. The loss ofoutput thereafter was a consequence of the progressive reduction inVOC and FF. Small area samples were cut from modules that had sufferedmore than 10% degradation in an attempt to distinguish the role of inter-connects in the loss of output. Detailed measurement and analysis indi-cated that the interconnects did not contribute significantly to thedegradation and it was concluded that the reduction in maximum powerwas partly due to changes in the cells and partly to the front contact TCO.This is consistent with the findings of Batzner referred to earlier.

7. USE OF CdS/CdTe MODULES IN LARGE-SCALEPOWER GENERATION

With the exception of large hydroelectric systems, renewable sources ofenergy do not readily integrate into large grid-based distribution net-works. The electrical grid was developed in response to the need forreliable power; if one station went out of commission the supply to users


was not interrupted. If wide-scale implementation of photovoltaic(PV) power is to be practical, then ways must be found to scale up fromthe few tens of Watts typical of individual modules by orders of magni-tude to tens or hundreds of MW at a single location that is comparable toa medium scale thermal or nuclear facility. Another problem with PVpower generation is the variability of output power. There is obviously adiurnal variation, but that can be managed. Much more difficult is theshort-term variation due to, for example, the passage of clouds, and thisdictates that ideally any large-scale PV station should be sited in a desert.More than that, sandy deserts are not really suitable, due to the damagingeffects of sandstorms; only rocky or gravel deserts are suitable.

There are in principle, three important parameters. The energypayback time (EPT), the net CO2 emission rate (gm C/kWh) and thegenerating cost (price per kWh). The EPT is simply the time taken torecover the total energy investment in building and running the stationover the life cycle using its own net energy production. The net CO2

emission rate is the ratio of the total CO2 emission released by the stationover its life span to the total energy generation over the lifetime. It is auseful index in determining the effectiveness of the plant’s contribution toglobal warming. It is obvious that the generation cost must not be undulyout of line with other generating systems. An important factor whichmust be included in any cost analysis is the ‘balance of system’ (BOS)costs; that is, the additional expenditure in construction of the facility,connection to the grid, maintenance, dismantling etc.

Very large-scale PV stations (100 MW) have yet to be built, but therehave been a limited number of smaller, but still substantial stationsdelivering up to a few tens of megawatts, such as the Springfield, Arizonaplant [93]. This has an installed PV capacity of 4.6 MWp in a mix ofmulticrystalline (mc) Si (3.5 MWp) and thin film (1.1 MWp) modules.The output is used to power the water pumps at the Springfield CoalFired station (10 MWac-al), and when the water pumps are not operatingthe capacity is fed into the grid for general use. In the case of the Spring-field station, the EPT was 0.21 years for the BOS, which equates to about0.37 years for average US conditions. The life cycle green house gas emis-sions (BOS) are estimated to be 29–31 kg CO2/m

2. Larger facilities, forexample First Solar’s 40 MWBrandis solar farm in Germany, are beginningto be built, usually with government subsidised programs, as in Germany[86]. Currently First Solar is only selling panels for use in solar farms andcommercial rooftop installations. It is not selling to the public at large as allits output is being used to meet the larger scale commercial demands.

There have been a few simulation studies of larger stations, for exam-ple, Ito 100 MW [94]. This was a comparative study of all the main solarcell systems based on current performance, and included all equipment,transport, operational, maintenance costs as well as transmission losses

210 A.W. Brinkman

over a distance of 100 km. For CdTe modules the EPT and CO2 emissionrates were estimated to be 1.9 years and 12.8 g-C/kWh, respectively.Corresponding values for Si modules were 1.5 years and 9.4 g-C/kWh.An important conclusion was that module efficiency was importantfor energy production and CO2 reduction, because the build costs –foundations, cabling, troughs etc. – were reduced. This was more impor-tant in ‘cold’ deserts than in ‘hot’ deserts, for example, the Sahara, wherethin film systems were a viable proposition due to their lower price($/Watt).

8. CONCLUDING REMARKS

The increasingly large volumes of Si required for Si wafer solar cells placeSi-based PV in direct competition with conventional Si-microelectronicsfor semiconductor grade material, currently in short supply. There is as aresult, an opportunity for thin film technologies in general and CdS/CdTecells in particular to gain an increased market share [95].

That is not to say that there are not supply issues with thin film PV.The Cu-chalcogenide cells utilise indium a rare element, subject to com-petitive pressures from flat screen displays and LEDs. Tellurium is alsoclassified as a scattered or precious element, and also has other competinguses in metal alloying and thermoelectric applications. Rare elements ofthis kind are only economically viable as a by product of some other largevolume mineral extraction process, for example, Te is a by product ofCu-refining. Cadmium (which is plentiful) is a waste product from themining and refining of Zn. An additional concern for the widespreaddeployment of CdS/CdTe solar cells is the toxicity of Cd. The modulecollection and recycling programme instituted by First Solar is an attemptto find a solution to the problem and pre-empt further legislation restrict-ing the use of Cd, as is the case in the EU, where the sale of consumerproducts containing more than 0.1% by weight Cd was banned in 2006.PV modules are exempt for the present, but the exemption will be subjectto review every four years. However, there is a strong argument that thesequestration of Cd from the waste of Zn mining into PV modules whereit can be both used and controlled makes environmental sense. Thetoxicity of Cd and scarcity of Te both serve as drivers for more efficientmanufacturing processes and the development of extraction technologiesfrom CdTe/CdS scrap and spent modules [96].

In conclusion, thin film CdS/CdTe solar cells are emerging as a poten-tially viable source of PV power generation. There are outstanding ques-tions to be resolved, not least: the scarcity of Te, public acceptability ofCd containing products and their long term durability. The increasingproduction and use of CdS/CdTe modules in the field may provideanswers to these questions.


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CHAPTER IIC

Brookhaven National Labo

214

Applications of CdTe, CdZnTe,and CdMnTe RadiationDetectors

Ge Yang and R.B. James

1. INTRODUCTION

For more than four decades, CdTe has been known as a good candidatematerial for radiation detection. Its high atomic number ensures effectiveradiation-atomic interactions, while delivering good sensitivity andenergy resolution. Furthermore, CdTe detectors offer high conversionefficiency of photons to electronic charge carriers as compared to scin-tillators; consequently, for high-quality CdTe detectors, the energy reso-lution is expected to be considerably better than scintillators. Moreimportantly, CdTe radiation detectors operate at room temperature,thereby obviating the need for a complicated cooling system and, accord-ingly, affording greatly enlarged application fields. However, CdTesometimes displays polarization effect, wherein the counting rates orpeak positions change with time [1]. A relatively high leakage currentalso has proven to be a limiting factor during the development of large-volume CdTe radiation detectors. In addition, CdTe detectors are rela-tively expensive, because it is difficult to grow large-volume CdTecrystals of acceptable quality and yield.

On the basis of the utility of CdTe, CdZnTe (CZT) was exploredcarefully, because the introduction of Zn increases the band gap andreduces the leakage current (and noise) of radiation detectors. It also isless expensive and easier to produce large-volume single crystals withCZT than with CdTe. Since the first practical CZT gamma-ray detectorreported in 1992 [2], there have been many advances in the performance

ratory, Upton, NY 11973, USA

Applications of CdTe, CdZnTe, and CdMnTe Radiation Detectors 215

of the devices. The high-resistivity CZT crystals required for radiationdetection initially were grown using the high-pressure Bridgman method(HPB). In recent years, other methods were employed, such as thetravelling heater method (THM) and the modified Bridgman method(MB). Today, large-volume CZT single crystals up to hundreds ofcubic centimeters can be produced due to improvements in the growthtechniques [3]. At the same time, the performance-limiting factors ofCZT detectors have been better clarified with modern characterizationmethods, such as synchrotron radiation X-ray mapping, time-of-flightsecondary ion mass spectrometry (Tof-SIMS) technique, Pockels effectmeasurements, deep-level transient spectroscopy, X-ray diffraction meth-ods, and others [4–6], thereby reciprocally helping CZT researchersenhance the availability of these large-volume crystals. Such progress inbettering the quality of the CZT material for CZT radiation detectorsand electron-transport-only device designs, as well as improving therelated electronics greatly have enlarged their application fields andaccelerated their commercial marketing.

CdMnTe (CMT) is a relatively novel material for radiation detection.Burger et al. reported the first investigation of its potential for suchapplications in 1999 [7]. They proposed that CMT has two main advan-tages over CZT, namely, better homogeneity and the lower amount of Mnneeded to reach the desired band gap, so making it a good competitor toCZT. Most problems with CMT radiation detectors center on the poorquality of the crystals. Until now, it was difficult to obtain CMT singlecrystals with high resistivity and acceptable carrier transport properties.The limited availability of CMT crystals to researchers also inhibits itsdevelopment as a radiation detector. As a result, they remain at the stageof laboratory development and have not been incorporated practicallyin commercial detection systems. However, some recent progress sug-gests the possibility of a breakthrough in this field, leading to largeimprovement in the performance of CMT radiation detectors [8, 9].

In this chapter, we focus primarily on the applications of CdTeand CZT radiation detectors. The nature of and future trends in CMTradiation detectors will also be discussed.

2. NATIONAL SECURITY AND NONPROLIFERATIONINSPECTIONS

In undertaking national security and nonproliferation inspections, radia-tion detectors mainly serve to locate and identify special radioactivenuclear materials. These applications especially demand portable, light-weight radiation detectors with high resolution. CdTe and CZT radiationdetectors have proven well-suited for these purposes, because they are

216 Ge Yang and R.B. James

compact, robust, and with low maintenance and low-power consump-tion, while simultaneously providing good energy resolution and highdetection efficiency per unit detector volume.

The International Atomic Energy Agency (IAEA) has used CdTe andCZT detectors for over three decades [10, 11]. Recently, their usage sig-nificantly increased, stimulated by the improvement of material proper-ties, their improved availability, and the development of improveddetector-specific, low-noise integrated circuits for electronic readouts.The IAEA employs hemispheric CdTe/CZT detectors to verify irradiatednuclear material, that is, spent-fuel assemblies; generally, they must mea-sure high count rates and high gamma energies. In such cases, a shieldedand collimated detector normally must be placed very close to an irra-diated item to obtain its specific signature because accessible space oftenis limited. Therein, the small size and high efficiency of CdTe/CZTdetectors are essential prerequisites. A good example is the miniatureCZT detection probe with its integrated preamplifier that the IAEAemploys for light-water-reactor assemblies and collimators; the probes’volumes range between about 6 and 60 mm3 and their resolution between1% for 137Cs (7 keV for 662 keV) and about 3% (18 keV) [12]. In addition,the long-term stability of hemispheric CdTe/CZT detectors generally isvery good; they have survived for several weeks after burn-in tests. Forsome applications, the detectors operated over months without noticeablechanges in their spectral parameters.

Large-volume CZT detectors also are needed in nonproliferationinspections, because they potentially reduce the measurement time sig-nificantly. This is especially important in verifying un-irradiated nuclearmaterial where count rates are low and typical gamma lines of uranium orplutonium gamma spectra must be resolved. However, the large-volumeCdTe/CZT detectors required for these inspections presently are limitedby the lack of CZT single crystals whose active volume is more than about500 mm3. The dearth of larger volume CdTe/CZT single crystals must beresolved before their widespread application in nonproliferationinspections.

Recently, scientists at Brookhaven National Laboratory (BNL) devel-oped a hand-held gamma-ray spectrometer for nonproliferation inspec-tions based on virtual Frisch-grid CZT detectors. The whole systemachieves an effective detection volume of 19.2 cm3, that is, 10 times largerthan commercial co-planar grid (CPG) CZT detectors. Consequently,detection efficiency is improved significantly. The system employsan 8 � 8 virtual Frisch-grid CZT detector array (Fig. 1); each detector is5 � 5 � 12 mm3. By using front-end application-specific integrated cir-cuits (ASICs) developed at BNL, this spectrometer has a small profile andhigh energy-resolution. Further, its relatively simple configuration greatlylowers the cost. This achievement has allowed us to build an inexpensive,

Figure 1 Virtual Frisch-grid CZT radiation detectors developed at BNL.


large-volume detector array with high energy resolution and high detec-tion efficiency, so affording wide application potentials in national secu-rity and nonproliferation inspections. Moreover, the detector modules arescalable to address a larger range of efficiency requirements.

3. MEDICAL IMAGING

The unique advantages of CdTe and CZT detectors over other materialsin medical-imaging applications rest on their long-term stability, directdigitization, and spectrometer-mode imaging, among other benefits.Especially, because these detectors are made from tiny individual pixels,they are relatively small, thereby ensuring high spatial resolution andgood energy resolution. Therefore, the arrays of small CdTe/CZT detec-tors possess good image quality with low noise, as invariably neededin medical imaging to precisely localize possible lesions. Furthermore,the room temperature operation of CdTe/CZT detectors effectivelysimplifies the structure of medical-imaging equipment and reducesmaintenance costs.

3.1. Gamma (g)-camera

The research and development on CdTe-based gamma cameras havebeen largely determined by the progress of both low-noise dedicatedintegrated chips, as well as the pixelization of the detectors, like the“small pixel effect.” The first attempt to use a CdTe array for nuclearcardiac imaging goes back to 1991: a small probe developed by Scheiberet al. [13], including 12 independent 10 � 10 mm2 detectors. The first fullimaging systemwas presented in 1996 by Eisen et al. [14], who developed


a small field-of-view camera equipped with 40 � 32 detectors (each 4 �4 mm2). Since then, great efforts have been undertaken by several groupsworldwide to develop clinical systems using CdTe or CZT detectors.For example, CZT has been used in integrated pixel detector structuresby Barber et al. [15], Polichar et al. [16], and Doty et al. [17]. Most of theseearlier references are reported in a paper by Scheiber and Giakos [18].

The NUCAM mobile camera developed by Eisen et al. [14, 19] incor-porating 1280 detectors is shown in Fig. 2. The contrast resolution due toscatter rejection was proven superior in the CdTe NUCAM camera, whencompared to a scintillator-based Anger camera. Nearly during the sameperiod, in the frame of a European program, a consortium “BIOMED II”developed a heart-devoted camera with 2304 pixels using dedicated I.C.low-noise chips [20].

Subsequently, extensive researches were devoted to attain the bestcompromise between spectrometric performance and detection efficiencyto improve the feasibility of a g-camera based on CdTe/CZT. Theyincluded preventing the early recombination of the holes, optimizingthe electrodes’ geometry and the size and shape of the detector elements,as well as assuring signal acquisition by the associated specific integratedcircuits.

Figure 2 A photograph of the first version of a moveable gamma camera, NUCAM,

based on large arrays of CdTe detectors. (Source: Ref. [19], reprinted with permission

from Elsevier.)


In 2003, Siemens reported the imaging performance of a prototypeCZT g-camera system comprising 15 CZT modules, as depicted in Fig. 3[21]. This camera had a 12 � 20 cm2 active area, comprising 3 columnseach with 5 rows of modules. Each module had a square array of 16 � 16pixels. The pixels’ pitch was 2.46 mm. Figure 4 compares the higherquality images of a brain phantom acquired with the CZT camera withthose poorer ones taken on a standard commercial NaI(Tl) gammacamera.

10 k

CZT

Nal

50 k 100 k 500 k

Figure 4 At the top are CdZnTe images with count density increasing from left to right.

Beneath them are the corresponding NaI(Tl) images. The phantom was positioned 10 cm

from the collimator’s surface. (Source: Ref. [21], reprinted with permission from Elsevier.)

Figure 3 Photograph of the CdZnTe prototype camera showing the 3 � 5 arrangement

of the modules. The low-energy, ultra-high resolution (LEUHR) collimator is behind it.

(Source: Ref. [21], reprinted with permission from Elsevier.)


Collaborations between Verger et al. [22] and Mestais et al. [23] gen-erated the design of a type of new CZT g-camera based onmeasuring bothpulse height and fast rise time [22, 23]. This method brings the advantagesof high scatter rejection while supporting high detection efficiency.In 2004, Verger et al. described their latest developed CZT g-camera anddiscussed the performance of a small CZT imager of 256 discrete detectorsin an array of four platforms [24]. They compared and evaluated theplanar imaging performance of this small CZT detector imager with thatof a standard NaI(Tl) g-camera using two different phantoms. Figure 5shows the detector. Figure 6 demonstrates that the CZT camera’s image issuperior to the NaI(Tl) one for all cold rod diameters. Figure 7 also clearlyreveals that the better detector energy resolution of CZT noticeablyimproves the image contrast in a high-scatter environment for the samesystem spatial resolution.

In addition, Konstantinos et al. developed and tested the photon-counting CdTe g-camera shown in Fig. 8 [25]. It is built of eight individualdetector hybrids, each consisting of a pixel CdTe detector of 22� 11 mm2,solder bump bonded to a photon-counting, custom-designed ASIC. Theeffective pixel size (image pixel pitch) is 0.5 mm. The current full activeimaging area of the CdTe g-camera covers 44 � 44 mm2. The cameraoperates both in the real-time imaging mode with a maximum speed of100 frames/s, and in the accumulation mode with user-adjustable count-ing time; its dynamic range is 1:14,000,000. It exhibits excellent sensitivity.

Figure 5 Small 256 CZT detector imager. (Source: Ref. [24], reprinted with permission

from, reprinted with permission from IEEE, # 2004 IEEE.)

A

B

C

D

NaI (TI) camera

With

out p

lexi

glas

With

ple

xigl

as

CZT camera

Figure 6 Comparison of images from NaI(Tl) and CZT cameras with and without 2.5 cm

of plexiglas obtained from a “Jaszczak” phantom filled with a 99mTc solution and

composed of cold rods of 16, 12.7, 11.1, 9.5, 7.9, and 6.4 mm diameters. The incident activity

is the same for both cameras. The cameras are located at 10 cm from the LEHR colli-

mator. (Source: Ref. [24], reprinted with permission from IEEE, # 2004 IEEE.)

Cou

nts

(u.a

.)

0

1000

2000

3000

4000

5000

6000

Nal (Tl)

Nal (Tl) image CZT image

Cold rod

3cm

CZT

Distance (u.a.)

0 20 40 60 80 100 120

Figure 7 Image comparison and associated profiles of NaI(Tl) and CZT cameras with

large cold rods of 3 cm and a high-scatter environment. (Source: Ref. [24], reprinted with

permission from IEEE, # 2004 IEEE.)

Figure 8 Photograph of the developed gamma/X-ray camera. (A) Detector board with

eight detector hybrids. (B) Camera housing with the 0.5-mm pitch collimator (150-mmseptal thickness, 350-mm openings, and 1:15 aspect ratio). (Source: Ref. [30], reprinted

with permission from Elsevier.)


3.2. Digital mammography

Digital mammography offers the potential for improving image qualityand, subsequently, the possibility of better detecting breast cancer, par-ticularly in women with dense breasts, where current screen-filmmammography often is inadequate.

Tumer et al. developed hybrid pixel detector arrays with 50 � 50 mm2

pixel sizes for use in digital mammography with different detectionmaterials [26]; CZT and CdTe pixel detectors gave the best results. Theimages from CZT and Si pixel detectors are compared in Fig. 9, whichshows a finger phantom with an embedded human bone. Detailed bonestructures are visible in both. Although the silicon detector’s thicknesswas 1 mm, much larger than that of CZT, the quality of the image fromthe latter is much higher, reflecting the lack of angle blurring and thehigher detective quantum efficiency (DQE) achieved with the CZT pixeldetector, even though only holes are collected.

Further, the results from a standard mammography test and calibra-tion phantom, shown in Fig. 10, also demonstrate the improved contrast

Figure 9 Images of a human finger phantom from a 0.15-mm thick CdZnTe detector

(left) and a 1-mm thick Si detector (right). (Source: Ref. [26], reprinted with permission

from Elsevier.)

Figure 10 Comparison of partial phantom images from a breast model taken with a

CdZnTe detector (left) with that of a commercial first-generation digital mammography

unit (right). The phantom is the standard mammographic model RMI 156 with only the

wax insert. (Source: Ref. [26], reprinted with permission from Elsevier.)


of CZT pixel detectors as compared to first-generation digital mammog-raphy systems.

Additionally, Fig. 11 compares images of a small mosquito fish(Gambusia affinis), 20 mm long, taken with a 0.15 mm CZT detector(top) and a 0.15 mm CdTe detector (bottom). Both images clearly revealits bone structure. The CdTe image seems of slightly poorer qualitythan the CZT one, which might be due to the polarization effect in theformer material that causes a higher background noise.

3.3. X-ray computed tomography (CT)

In 1989, Zelenina et al. demonstrated the advantages of using CdTedetectors for medical X-ray CT according to their calculations for fourmain types of statistical noises in CdTe metal-semiconductor-metal

Figure 11 Two images of a 20-mm-long mosquito fish using a 0.15-mm thick CdZnTe

detector (top), and a 0.15-mm thick CdTe detector (bottom). (Source: Ref. [26], reprinted

with permission from Elsevier.)


(MSM) structures [27]. In 1992, a prototype tomography machine, able toscan 0.5-m-size high-density objects, was realized by Glasser et al. with 25CdTe detectors (25� 15� 0.8 mm3) [28]. It produced good-quality 1024�1024 tomographic images.

More recently, Sueki et al. developed amonochromatic X-ray CT usinga photon-counting 256-channel CdTe array detector that offers severaladvantages [29]. First, there is no beam-hardening effect. Second, the CTvalue has a linear attenuation coefficient. Furthermore, a subtractionimage can be obtained using dual monochromatic energy X-rays. Theenergy resolution of this CdTe X-ray CT system is quite adequate for itspurposes. Figure 12 is a schematic diagram of this system; Fig. 13 is a CTimage of the head phantom obtained with it.

256ch CdTe array detector

Goniometer

Phantom

filter

Fan Beam Collimator

CollimatorTarget

Fluorescent X-ray

Synchrotron RadiationWhite X-ray Beam

GoniometerController

PersonalComputer

Figure 12 Schematic diagram of the monochromatic X-ray CT system. (Source:

Ref. [29], reprinted with permission from Elsevier.)

Figure 13 X-ray CT image of the head phantom obtained with the X-ray CT

system. (Source: Ref. [29], reprinted with permission from Elsevier.)


In 2004, Konstantinos et al. also manufactured a real-time X-ray imag-ing sensor with high-resistivity p-type CdTe suitable for small-field CTand similar applications [30]. To reduce the dark current and to preventafterglow, a serious problem in real-time imaging, they included a recti-fying indium anode contact. Figure 14 depicts its structure along with aphotograph of this imaging sensor. The pixel size (pitch) is 100 mm, andthe number of pixels is 506 � 508. The CdTe crystal is 0.75 mm thick. Thesensitivity and resolution of this CdTe X-ray imaging sensor is excellent.Figure 15 shows the X-ray images of a printed circuit board with mountedcomponents acquired at a 70-kV tube voltage and 40-mA tube current.The board’s multilayered structure is clearly apparent. Because of thehigh sensitivity of the sensor voids in the balls of a ball grid, the array isvisible even in single-frame (20 ms) magnified images.

The National Institute of Radiological Sciences of Japan also devel-oped a dual-energy X-ray CT with CdTe array [31] consisting of 64elements, 0.8 mm wide by 5 mm high by 5 mm deep, which are alignedside by side at intervals of 0.1 mm. This CdTe array detector registers

0.75 mm

10 B

2.5 cm

Sensor PCB

A

B

Wire bonds

Interface PCB

CMOS pixel circuits

CdTe pixel detector

Bump bonds

Figure 14 (A) Schematic of the structure of the CdTe real-time X-ray imaging sensor

tested in Ref. [23]; (B) Photograph of a CdTe real-time X-ray imaging sensor constructed

of 8 detector tiles. (Source: Ref. [30], reprinted with permission from Elsevier.)


the energy of incident photons as well as the photon number and incidentposition. Therefore, it is not necessary to tune a monochromator twiceto produce two monochromatic X-rays. This method is extendable toan advanced approach using polychromatic X-rays produced by aconventional X-ray tube.

Recent progress has proven that CZT has potential for use in making acombined imager for X-ray CT and single photon-emission computedtomography (SPECT). By contrast with using SPECT alone, the simulta-neous measurement by X-ray CT and SPECT yields structural and func-tional correlations, and improves the quantification and localization of theradionuclide. However, fabricating such a combined imager is challeng-ing, as the performance demands of these measurements are very differ-ent. The X-ray CT detector must have a linear response across a widedynamic range at high count rates, while SPECT requires high detectionefficiency with good energy resolution at low count rates. CZT detectorsare the most promising candidates for this application, because theysatisfy both requirements while possessing better g-ray energy resolution

0.9 mm Void (diameter = 90 μm)

5 cm

Figure 15 Images of a printed circuit board (top) and a ball grid array (bottom). The

images on the left are single-frame images (20 ms). The integration time of the images on

the right side is 10 s (500 added frames). (Source: Ref. [30], reprinted with permission

from Elsevier.)


and higher count-rate capabilities than scintillation detectors. Further-more, the acceptable cost and the avoidance of cryogenic cooling help topromote the use of CZT in a combined CT/SPECT system. In 2003,William et al. produced a simultaneous CT/SPECT imager using a singleCZT detector [32], so allowing the capture of structural and functionalinformation at the same time. Figure 16 shows the CT and SPECT imagesthus collected from a hot lesion phantom containing 28.6 mCi of 99mTcsodium pertechnetate. The photo-peak efficiency and energy resolutionfor 140 keV gamma rays of 70% and 10%, respectively, are constant for afluence rate up to 103 cps. The smallest lesions visible in SPECT and in CTare, respectively, 9 mm and 4.5 mm in diameter. Count rates less than 103

cps are sufficient for radionuclide studies, and the energy resolution ofthe SPECT images is comparable to current clinical systems.

Figure 16 CT (left) and SPECT (right) images of a cylindrical phantom with hole

diameters ranging from 4.5 to 38 mm. The left image is a map of attenuation coefficients

derived from detecting X-rays (CT). The radionuclide image on the right was recon-

structed using the CT-generated attenuation map to compensate the 99mTc-emission

data for photon attenuation. (Source: Ref. [32], reprinted with permission from Elsevier.)


4. SPACE AND ASTROPHYSICS

Both CdTe and CZT detector arrays have crucial applications for spaceexploration and astrophysics investigations. Operating typically in the10-500 keV range, CdTe/CZT detectors possess obvious advantages com-pared with Ge detectors and scintillators. Unlike Ge detectors, the abilityof CdTe/CZT detectors to operate at room temperature obviates a com-plex cooling system, a feature that is especially important for low-powerradiation detection systems used in space. Furthermore, the energyresolution of CdTe/CZT detectors is also superior to that of scintillators.In this section, we discuss the latest progress of CdTe/CZT radiationdetectors in space exploration and astrophysics.

The International Gamma-Ray Astrophysics Laboratory’s (INTEGRAL)ISGRI imager of the European Space Agency (ESA) is the first spaceinstrument using good spectral resolution CdTe; it was launched in 2002by the Russian PROTON launcher. The ISGRI comprises 16,384 CdTedetectors of 4 � 4 � 2 mm3 [33, 34], representing a sensitive area of2621 cm2. Figure 17 is a view of the detection plane of the ISGRI cameraformed with eight independent modules [33].

The ISGRI mainly is devoted to detecting and precisely measuringcelestial gamma-ray photons between 15 keV and 1 MeV. It supportsresearch on violent processes occurring near black holes, neutron stars,or in supernovae. Figure 18 illustrates the quality of the spectacularimages obtained with a 128� 128 pixels array during a test at CEA-Saclay[34]. The practical performance of ISGRI is illustrated in Fig. 19 with a

Figure 17 View of the 8 ISGRI MDUs (white) at the bottom of the passive shield well

(black) after integration in the IBIS detection system, 2004. (Source: Ref. [33], reprinted

with permission from EDP Sciences.)

IBIS CEA/CNES

INTEGRALISGRI

Figure 18 Gammagraphy of a discobole statuette recorded by ISGRI detector array

during a test at CEA-Saclay. (Source: Ref. [34], reprinted with permission from Elsevier.)


picture of the Cygnus region in the energy range 15-40 keV, where at leastthree sources are clearly visible. This is one of the finest images obtainedso far in the soft gamma-ray domain. The results from the INTEGRAL/ISGRI imager show that CdTe stability is better than expected and itsinternal background is comparable to that of scintillators. Meanwhile, the

Figure 19 ISGRI image of the Cygnus region in the 15–40 keV energy range. (Source:

Ref. [33], reprinted with permission from EDP Sciences.)


spectroscopic degradation of this imager in space is slow, with a lifetimeof about 40 years in an eccentric orbit. As the first CdTe space gamma-rayimager in the world, the ISGRI has verified the possibility of employing ahuge quantity of CdTe crystals in space applications.

Presently, the Burst Alert Telescope (BAT) onboard the Swift gamma-ray burst (GRBs) explorer is the largest CZT gamma-ray imager in theworld. It was launched on November 20, 2004 as one of NASA’s medium-class explorer programs. The BAT instrument is equipped with CZTsemiconductor detectors beneath a D-shaped coded mask. The 32,768semiconductor detectors (4 � 4 mm2 area, 2 mm thick), which form a5243 detector plane, are built in 16 blocks. The smallest unit encompasses128 individual CZT wafers. A single XA1 ASIC reads out the signals fromeach unit. In orbit, the CZT detectors operate at a nominal temperature of20 � 1 �C. The nominal bias voltage is �200 V, where the designedmaximum is –300 V. Figures 20 and 21, respectively, show the structurediagram of BAT and a photograph of the CZT detector module [35, 36].

BAT is designed to detect sources of GRBs and bright transient X-raysto determine their positions with an accuracy of 1-4 arc-min. The BAT’senergy range is from 15 to 150 keV, with an energy resolution of 6 keV(FWHM) at 122 keV. Figure 22 shows reconstructed images demonstrat-ing the instrument’s burst imaging capabilities [36].

Still several other CdTe/CZT X-ray telescopes have been loadedon balloons for astrophysics investigations. The High Energy FocusingTelescope (HEFT) and the International Focusing Optics Collaboration form-Crab Sensitivity (InFOCmS) are two good examples, both of whichincorporate pixel-based CZT detectors for imaging.

2.4 meters CodedApertureMask

PassiveGraded-ZShielding

DetectorPlane

CZT Detector Module

Thermal Radiator

1 meterbetweenMask andDetector Plane

0.6 m

Figure 20 The Burst Alert Telescope (BAT) onboard the Swift spacecraft. The BAT has

a 3 m2 D-shaped coded aperture mask with 5 mm pixels. The CZT array is 5243 cm2

with 4 � 4 mm2 detectors. (Source: Ref. [35], reprinted with permission from Elsevier).

Figure 21 The BATdetector module (DM). Two sub-arrays of 8� 16 pieces of CZT tile lie

on the top surface of the DM. (Source: Ref. [36], reprintedwith permission from Springer.)


−4 −3

−3

−2

−1

0

1

2

−2 −1 0 1 2

Angle Y (degree)

Ang

le Y

(de

gree

)

Figure 22 Image of the letters “BAT”, demonstrating the burst imaging capabilities of

the instrument, taken by irradiating with gamma rays from a 57Co radioactive source on

to the BAT’s detector plane through a coded mask in a preflight ground calibration

test. (Source: Ref. [36], reprinted with permission from Springer.)


HEFT was developed in a program led by Caltech in collaborationwith Columbia University and the Danish Space Research Institute.Figure 23 illustrates the HEFT CZT focal-plane detector system [37].An individual sensor consists of a 1.2 � 2.4 cm2, 2-mm-thick CZT crystal,with the anode contact segmented into pixels with 500 mm pitch.

Figure 23 Photo showing a HEFT focal plane detector system.Two CdZnTe/ASIC hybrid

pixel sensors are mounted side-by-side. (Source: Ref. [37], reprinted with permission

from Springer.)


Gold stud/epoxy interconnects couple the sensor pixel’s contacts to acustom low-noise readout chip. HEFT has been flown once in Spring2005, demonstrating the angular resolution of 1.5 arcmin (HPD) andspectral resolution of 1 keV (FWHM) at 60 keV.

The Goddard Space Flight Center, NASA, leads the InFOCmS project,whose focal plane consists of a CZT detector with a 12� 12 array of 2 mmpixels. The telescope was flown twice, in 2001 and 2004, and achieved anangular resolution of 2.2 arc-min (HPD) and spectral resolution of 4at 32 keV. The good performance of the CZT detectors facilitated thesuccessful detection of the astrophysical source Cyg X-1, even with anon-target observation time of only about a minute.

Additionally, Japanese researchers are proposing to develop hard X-ray telescopes (HXI) for the Non-thermal Energy eXploration Telescope(NeXT) mission with CdTe detectors [38]. The current goal for the CdTedetector in the HXI is a pixel detector with both a fine-position resolutionof 200-250 mm and a high-energy resolution of better than 1 keV (FWHM)in the energy range from 5 to 80 keV. In addition, designers will use CdTepixel detectors and a stack of 24 Si DSSDs to assemble a semiconductorCompton Telescope. Figure 24 illustrates the prototype of the CdTe pixeldetectors and the spectra acquired by them. Efforts are underway toimprove the performance of CdTe pixel detectors as thick as 5 mm.

Furthermore, the Danish National Space Institute (DNSC) is develop-ing a CdTe/CZT detectors program to fabricate the miniature X- andgamma-ray sensor (MXGS) for the ESA-supported Atmosphere-SpaceInteractions Monitor (ASIM) mission, which is expected to be launchedto the International Space Station (ISS) in 2011.

5. NATURE AND DEVELOPMENT OF CMT DETECTORS

CMT is a diluted magnetic compound semiconductor, previously used asFaraday rotators, optical isolators, solar cells, lasers, magnetic field sen-sors, and infrared detectors. In 1999, Burger et al. first investigated thepotential application of this material as radiation detector. CMT displayssome advantages over CZT, making it a good candidate to competewith the latter in radiation-detector applications. First, the segregationcoefficient of Mn in CdTe is nearly equal to unity in all directions, whilethat of Zn in CdTe has a coefficient of 1.35 [39]. This difference is reflectedin the nearly uniform concentration of Mn in CdTe, compared to the highvariation of Zn concentration in CdTe. The superior compositional homo-geneity of CMT potentially enhances the yield of crystals suitable fordetectors; ultimately, this might lower the costs of producing large-areaarrays. Secondly, CMT has greater tunability of the band gap due to thelarge compositional influence of Mn. The addition of Mn increases

200

00 20

241Am. Cdte

40 60Energy [keV]

Ent

ry [C

ount

s/0.

5 ke

V]

400

600

Figure 24 (Left) Photo of large-area CdTe pixel detectors developed for the prototype

Si/CdTe Compton Camera. (Right) Energy spectrum from CdTe (1 pixel) with an 241Am

source. The energy resolution is 1.6 keV (FWHM) for 60 keV at 0 �C. (Source: Ref. [38],reprinted with permission from Elsevier.)


the room-temperature band gap at a rate of 13 meV/[%Mn], that is, morethan twice as large as the increase after adding Zn to CdTe [40]. Therefore,the band gap in the range 1.7-2.2 eV, which proved ideal for assuringoptimal signal/noise ratio in X-ray and gamma-ray detectors [41], can beattained by adding relatively less Mn. This merit diminishes many alloy-ing related problems.

However, several material properties must be improved before CMTcan be practically employed for X-ray and gamma-ray detections. First,compared to CZT, the bond ionicity of CMT is higher, entailing a greatertendency for crystallization into a hexagonal structure, but not in theexpected zinc-blende structure [42, 43]. Also, higher ionicity can generate


twins easily in crystals. Second, the resistivity of CMT crystals must beimproved. Normally, CMT crystals grown by the Bridgman methods arep-type materials due to their high concentration of cadmium vacancies(VCd) the dominant acceptors, and the resistivity of as-grown crystals canbe as low as 10–103 O cm, thus not satisfying the resistivity requirement ofX-ray and gamma-ray radiation detectors.

Recently, BNL and the Institute of Physics, PolandAcademy of Sciences(PAS) cooperatively addressed some of these issues using their com-prehensive material characterization techniques and improved crystal-growing capabilities. Significant progress was made, and better resultswere reported [8]. Detector-grade CMT crystals were grown, and thefirst CMT detector was fabricated (Cd0.94Mn0.06Te doped with Vanadium5 � 1016 cm�3), as shown in Fig. 25. This figure also shows the energyspectra obtained with a sealed Am-241 source. Accordingly, the mt-product of electrons is 2.1 � 10�4 cm2/V. Furthermore, Fig. 26 is theX-ray map from a CMT detector measured at BNL’s National SynchrotronRadiation Source (NSLS). It demonstrates that Synchrotron X-ray mapping

0

200

400

600

1000

800

1200

0 200

A

B400 600 800 1000

Channel number

Co

un

ts

Bias 350VBias 300VBias 250V

Figure 25 (A) Photo of CMT detector produced by BNL and PAS; (B) its 241Am spectra

(electron collection from the anode). (Source: Ref. [8], reprinted with permission from

SPIE.)

0

20

40

60

80

2040

6080

0

20

A B

4060

80

Figure 26 X-ray mapping of a CMT detector (1 � 1 mm2). (A) 2-D view; (B) 3-D view

showing pulse-height of the photopeak in the third dimension. (Source: Ref. [8],

reprinted with permission from SPIE.)


can be used to correlate how microscale defects, for example, twin bound-aries, Te inclusions, anddislocations, affect theCMTdetector’s performance.

However, compared with the performance of CZT detector, there stillis much room to improve CMT detectors before they can to be applied aspractical radiation detectors.

6. SUMMARY AND FUTURE WORK

As promising materials for radiation detection, CdTe/CZT have highstopping power for energetic photons, good sensitivity and energy reso-lution, and excellent room-temperature operation capacity. This chapterdiscussed the primary applications of CdTe and CZT radiation detectors,including national security and nonproliferation inspections, medicalimaging, and space exploration and astrophysics investigation. The lastdecade saw great progress in all these fields. However, material issues, forexample, improving the availability of large-volume CZT crystals, remainthe main limiting factor for further developing these radiation detectors.The combination of modern characterization methods and modifiedgrowth techniques is providing a deep understanding of CdTe/CZTmaterial properties, which might resolve these problems and acceleratethe related applications in the near future. CMT radiation detectors arein the elementary stages of development compared with CdTe/CZTdetectors, and as yet have no practical application in commercial


radiation detection due to their poorer material properties. Nevertheless,recent progress possibly predicts a breakthrough of this promisingradiation material.

ACKNOWLEDGMENTS

We would like to thank the authors of the references and the Institute ofElectrical and Electronics Engineers, Inc. (IEEE), the International Societyfor Optical Engineering (SPIE), Elsevier, Springer, JohnWiley & Sons, Inc.(Wiley), Materials Research Society (MRS), American Institute of Physics(AIP), and Edition Diffusion Presse Sciences (EDP), among others forpermission to reprint the figures in their works.

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CHAPTER IID

Carl J. Johnson is with II-VGary L. Herrit is with II-VIEric R. Mueller is with Coh

Electro-optic ModulatorApplications

Carl J. Johnson, Gray L. Herrit and Eric R. Mueller

1. INTRODUCTION

Since the discovery of infrared (IR) lasers in the mid-1960s, the potentialof CdTe as an electro-optic (EO) modulator material has been well-recognized. Experiments conducted at 1.0 mm in 1967 and 1970, at3.39 mm in 1969, at 10.6 mm in 1969 and 1971, and at 23 and 28 mm 1968demonstrated CdTe’s modulation capability over a wide range of IRwavelengths and generated the measurements of its EO coefficient asshown in Table 1.

With the emergence of instrument- and industrial-grade CO2

lasers around 1970, commercial interest in 10.6 mm modulators increasedand motivated considerable efforts to overcome problems impedingthe deployment of CdTe at that time. Rapid progress by material anddevice manufacturers to some degree mitigated a variety of ingot-size, ingot-cracking, optical-absorption and scatter, electrical-resistivity,crystal-fabrication, antireflection (AR)-coating, electroding, and high-costlimitations. By 1973, a reasonable capability existed to provide 10 � 10 �50 mm3, >108 ohm-cm, >99.5% transmissive, affordable CdTe EO-mod-ulator devices and components. Early applications that required substan-tial numbers of modulator crystals and components included the CO2

Laser Fusion Program at the Los Alamos Scientific Laboratory in theUnited States, space-communication development programs at bothHughes Aircraft Company in the United States and the European SpaceAgency, and the US Department of Defense CMAG Program.

I Incorporated, Saxonburg, PA 16056 USAIncorporated, Saxonburg, PA 16056 USAerent, Incorporated, Bloomfield, CT 06002 USA

239

Table 1 EO coefficients (r41) of CdTe versus wavelength

Wavelength (mm)

EO coefficient r41(m/V �10�12) Radiation source Reference

1.0 2.2 Monochromator [1]

5.3 [2]

3.39 6.8 HeNe laser [3]

10.6 6.8 CO2 laser [3]

6.2 CO2 laser [4]23 5.5 Water vapor laser [5]

28 5.0 Water vapor laser [5]

240 Carl J. Johnson, Gray L. Herrit and Eric R. Mueller

The EO effect in CdTe depends on the fact that an electric field appliedin certain crystallographic directions causes lattice distortions which, inturn, induce changes in the indices of refraction or birefringence encoun-tered by polarized light traveling through the crystal. Thus, it is possibleto electrically modulate the phase, polarization, or intensity of laser radi-ation using properly oriented CdTe crystals. Several derivations of theformulas that characterize EO modulation in zinc-blende structure crys-tals, such as CdTe, have been presented by others [6–8]. In summary, thiscrystal type gives rise to a “transverse” EO effect, where the electric fieldis applied normal to the direction of light propagation and the inducedchanges in refractive index vary as

Dn / n30r41V

d; ð1Þ

where n0 is the unperturbed refractive index, r41 is the EO coefficient, V isthe applied voltage, and d is the electrode separation.

2. PRACTICAL CONFIGURATIONS

Amplitude modulation (AM) is the most commonly used type of modu-lation. The configuration generating the maximum AM effect is shown inFig. 1(A), where light propagates in the [�110] direction, the electric field isapplied in the [110] direction, and the linear polarization of the incidentlight is also oriented along the [110] direction.

In this case, the amount of relative phase retardation in radians, Grel,experienced by the light at the exit of the CdTe modulator is

Grel ¼ 2pll

n30r41V

d; ð2Þ

where l is the crystal length and l is the free-space wavelength of the laserradiation. For extra-cavity AM of a laser beam, with a polarization ana-lyzer that is crossed with the input polarization following the crystal, theintensity, I, of the light passing through the analyzer is

[110][110]

Polarizer

A B

Analyzer

d

l l

E = V/d

[001]

[111][110]

Polarizer

Analyzer

d

E = V/d

[112]

Figure 1 Configurations for generating (A) maximum AM and (B) FM using CdTe crystals.

Electro-optic Modulator Applications 241

I

I0¼ sin2 Grel

2

� �; ð3Þ

where I0 is the intensity of the incident light. With a ¼-wave plate placedin series with the modulator crystal, the amplitude modulated outputintensity is

I

I0¼ sin2 Grel

2þ p

4

� �; ð4Þ

and good small-signal linearity results.For frequency modulation (FM), the configuration shown in Fig. 1(B)

is most commonly utilized. Light propagates in the [�110] direction withboth the light polarization and applied electric field oriented in the [111]direction. In this case, the amount of phase shift, G, produced is

G ¼ffiffiffi3

ppl

ln30r41

V

d: ð5Þ

3. ISSUES AND LIMITATIONS

3.1. Mechanical

CdTe EO modulator crystals can be manufactured in a variety of sizes.The grain sizes obtainable, at least in the case of Bridgman growth, limitmodulator cross-sections to about 10 � 10 mm2; however, the mostcommon cross-sections utilized in CO2 laser applications fall between2 � 2 and 6 � 6 mm2. Grain-size limitations also restrict modulatorlengths to about 50 mm and because these are transverse EO modulators,it is desirable for the crystals to be as long as possible.

As shown in the previous section, the modulation per volt obtainablewith a CdTe modulator is proportional to the length of the crystal and


inversely proportional to the electrode spacing. In theory, it is desirable tomake the cross-section as small as possible; however in practice, theminimum cross-section is limited by the physical strength of the material.CdTe is relatively soft and readily cleaves parallel to the (110) family ofcrystal planes, thus the manufacturing of small cross-section crystalspresents a serious challenge. Historically, 0.8 � 0.8 � 30 mm3 (aspectratio ¼ 37.5) AM-cut crystals have been fabricated; however, a morepractical size for the deployment of both AM- and FM-cut modulators isin the range of 2 � 2 � 50 mm3 (aspect ratio ¼ 25.0).

3.2. Optical

EO-grade CdTe has excellent optical properties in the mid- and far-IRregions. The highest quality material displays minimal near-IR opticalscatter and can be used at wavelengths as short as 1 mm. Some ingotshaving excellent transmission properties at 10 mm cannot be used below5 mm because of Te-precipitate induced, near-IR scatter in the bulkmaterial.

The generation of stress can occur during the cutting, grinding, andpolishing of CdTe crystals. Herrit and Reedy [9] showed that certain fabri-cation techniques can induce or reduce stress birefringence in CdTe mod-ulator crystals. In a later paper [10], they showed that residual stressbirefringence from the fabrication process, along with other crystal defects,can affect the operation of a modulator. Their measurements were accom-plished by focusing a CO2 laser beam through a CdTe modulator, scanningthe beam across the aperture, and measuring the phase shift at each posi-tion. Figure 2(A) shows a phase-shift plot indicating a notable amount ofresidual birefringence in a CdTe modulator having coarse-ground sides.Figure 2(B) shows the corresponding plot indicating lesser amounts ofbirefringence for a modulator having highly polished sides.

The 10.6 mm bulk-absorption coefficient of EO-grade CdTe is typically0.0005 cm�1. In high-power CO2 lasers, bulk absorption results in heating

0

A BSide

Top

12

34

21

34

–25

Grd. Electrode

Pha

se S

hift

(deg

.)25

50

0

Side

Top

12

34

21

34

–25

Grd. Electrode

Pha

se S

hift

(deg

.)25

50

Figure 2 Stress birefringence induced by (A) course-ground and (B) highly polished sides.


of the optical material, so low-bulk absorption is highly desirable. Unfor-tunately, the low-bulk absorption of CdTe is offset by its low thermalconductivity. In other words, although the material absorbs very littlelaser power, the small amount of heat generated is difficult to conductaway, which leads to some practical power limitations for CdTe modula-tors. Numerous laboratory experiments and end-user applications haveindicated that the practical, continuous-wave (CW), power limit for anair-cooled CdTe modulator is about 20 watts (W). If the modulator pack-age is water cooled, then this power handling limit can be increased toabout 50 W. Generally, the material does not damage at these powerlevels, but thermal lensing of the laser energy can be quite severe.

Through good thermal management and engineering, some excep-tions to the above limits have emerged over the years. Most notable isthe work done by DeMaria et al. in producing a modulator package that iscapable of withstanding significantly higher laser intensity levels [11].Their work was documented in US Patent 5,680,412. In this work, theysandwiched the CdTe crystal between two ZnSe windows, as shown inFig. 3. The windows were AR coated on one side and uncoated (UC) onthe opposite side, and the CdTe modulator was uncoated on both ends(UC/UC). The UC face of a ZnSe window was optically contacted to therespective UC faces of the modulator. Since, in many high-power laserapplications, surface damage is the ultimate failure mechanism, the fabri-cation technique of DeMaria et al. greatly extended the range of use ofthese modulators. Figure 3 shows the concept of this assembly method,wherein 10 is the UC/UC modulator and 12 and 14 are the UC/ARwindows.

3.3. Electrical

Electrically, CdTe modulators possess high resistivity (>108 ohm-cm) andlow capacitance (5-10 pf). The material has a very high intrinsic break-down voltage, but more practically, the modulator packages usuallybreakdown first due to surface conduction along the faces or sides of

LASEROUTPUT

LASERINPUT

12�

102� 104

14�

8�

10

Figure 3 Use of UC/AR end-plates to reduce damage to CdTe modulator crystals.


the crystals. This often occurs at field strengths of about 1 kV/mm ofelectrode spacing at atmospheric pressure. At lower ambient pressures,air breaks down at lower voltages, so care must be taken to ensurethat this does not happen. Most CdTe modulators are made with anelectrode spacing and a crystal length such that the ½-wave voltage isabout 1 kV/mm of electrode spacing. For example, a 5 � 5 � 50 mm3

crystal will have a ½-wave voltage of about 5 kV. Since the cross-section ofthe crystal is 5� 5 mm2, the electrodes are spaced 5 mm apart. This meansthat the air breakdown voltage and the ½-wave voltage for the crystal areroughly the same, thus it is advisable to avoid using this modulator in anapplication where ½-wave modulation is required. These rules do notapply when the voltage pulse is very fast (<1 ms) or if the modulatorpackage is sealed and a dry, inert gas is used to pressurize the package.

Certain electrically active impurities can give rise to deep-level trapsin the bulk material of a CdTemodulator. When a voltage is applied, thesedeep-level traps can distort the electric-field distribution, resulting infield-strength nonuniformities across the clear aperture and nonuniformmodulation across the laser beam. Figure 4 displays phase-shift plots for aCdTe modulator. Figure 4(A) indicates the pattern of residual birefrin-gence in the crystal at rest, that is, with no voltage applied. Figure 4(B)shows the phase-shift distribution with the ¼-wave voltage applied to themodulator. Ideally, this plot should be a flat plane at 90� of phase shift.Charge trapping typically results in a higher phase shift near the positiveelectrode and a lower phase shift near the ground electrode.

Trapping is primarily a problem that occurs when the user wants toapply a DC voltage or long-duration voltage pulse to the modulator. Sinceit takes on the order of 1/10 of a second for the charge traps to fill, anyvoltage pulse that is much shorter than this will not cause such nonuni-form behavior. Applications where a DC bias is required will suffer fromthis phenomenon.

0

ASide

Top

12

34

21

34

–25

Grd. Electrode

Pha

se S

hift

(deg

.)25

50

0

BSide

Top

12

34

21

34

–25

Grd. Electrode

Pha

se S

hift

(deg

.)25

50

Figure 4 Nonuniformity in phase retardation caused by charge trapping in CdTe

material.


4. SUCCESSFUL AND CONTEMPLATED DEPLOYMENTS

CdTe modulator technology can be combined with high-reliability RF-excited CO2 laser technology to enable a wide range of applications. Overthe years, many techniques utilizing CdTe modulators have been realizedin the laboratory, including laser Q-switching, laser cavity dumping,optical pulse shaping, frequency/phase modulation, laser mode locking,and optical free-space communications. To date, only the first two of thesehave found their way into commercial products; however, a number ofresearch groups continue to investigate wider deployment of some of theother applications.

4.1. Laser Q-switching

This application typically involves the Q-switching of an RF-excited,waveguide, CO2 laser by using the CdTe as a voltage-tunable birefringentelement as shown in Fig. 5. Initially the modulator (EO QS) voltage is setto 0 V. In this state, the cavity Q is very low because the light leaving thegain region and making a round-trip through the EO QS and reflective-phase retarder (RPR)-mirror pair is rotated in polarization by 90� andreflected out of the cavity by the thin film polarizer (TFP). The EO QSvoltage is then rapidly switched to the ¼-wave voltage. In this state, thecavity has a typical lasing Q value, since the energized EO QS cancels thepolarization rotation effect of the RPR-mirror pair. In this configuration,the lasing action will build-up and an output pulse will exit the cavity viathe partially reflecting (partial R) output coupler.

With this configuration, the termination of the light pulse is adjustablevia the timing for returning the EO QS voltage to 0 V. At that point intime, any optical energy remaining in the cavity will be “dumped out” byreflecting from the surface of the TFP.

The resulting optical pulses consist of an initial temporal pulse in theorder of 150 ns FWHM, followed by a lower energy tail whose length canbe adjusted up to a fewmicroseconds.When combined with folded-cavitygain sections, practical commercially packaged lasers with averagepowers in the range of 50 W and pulse repetition rates of 100 kHzhave been realized. The optical output from this type of laser has foundapplication in material micromachining.

RPR-Mirror

Gain Region(often folded) TFP EO QSPartial R

Output

Figure 5 Optical arrangement for a Q-switched CO2 laser.


4.2. Laser cavity dumping

In this application, the optical configuration is quite similar to that shownin Fig. 5, with the exception that the partial R is replaced by a 100%reflective mirror and the output is reflected from the surface of the TFP.

The operation is also similar to the Q-switched operation with theexception that once the circulating power in the laser builds up, the EOQS voltage is very rapidly returned to 0 V, thus dumping the opticalenergy out of the cavity. This results in a quasi-Gaussian temporal opticalpulse with a FWHM in the order of 15 ns. Practically sized, cavity-dump-ing lasers can have average output powers in the order of 30 W.

The Coherent EOM-10, pictured in Fig. 6, is a scientific laser of thistype having a pulse width of 12 ns and an average power of >10 W. Todate, the majority of applications for this laser have been in scientificresearch, but there are potential commercial applications in development.

4.3. Optical pulse shaping

The use of CdTe in optical pulse shaping again takes advantage of itsvoltage-tunable birefringence. The CdTe crystal is used in conjunctionwith polarization-selective passive optical elements and fast-drive elec-tronics, to either tailor the shape of an existing optical pulse (e.g., makingits rise- or fall-time shorter), or obtain pulses from a CW laser beam.

Material processing is the primary area of interest for this modulatorembodiment. To date, however, the cost of such a CdTe-based pulse shaperhas prevented its widespread adoption in commercial applications.

4.4. Free-space optical communications

With the availability of high-reliability, single-frequency compact CO2

lasers and the fairly good transmission of 10 mm light through theatmosphere, optical free-space communications might be feasible.

Figure 6 Photograph of a Coherent EOM-10.


At this point in time, there are a few research organizations pursuing thispossibility.

Most of the communication approaches under development utilizeCdTe as an amplitude modulator, but there are communication strategiesunder investigation, which utilize FM as well.

4.5. Optical frequency and phase modulators

As described in earlier sections, depending on the cut of the crystal, onecan realize a voltage-dependent optical phase shift using CdTe. If a time-varying voltage waveform is placed on such a crystal, then frequency orphase modulation can be realized. At this point, the only applicationsfor this embodiment have been in research.

4.6. Laser intracavity modulation and mode locking

Intracavity AM and FM of CO2 lasers, outside of the mode-lockingregime, have been utilized in various remote-sensing research programs,typically sponsored by government agencies. These programs are explor-atory in nature and there are no commercially significant applications atthe present time.

The use of CdTe for the mode locking of a CO2 laser has been realizedin both AM and FM configurations. While applications in this areahave thus far been entirely in research, the mode-locked or more exoticmode-locked, Q-switched, cavity-dumped format may find its way intomaterial processing applications in future.

REFERENCES

[1] O.M. Stafsudd, F.A. Hack, K. Radisavljevic, Appl. Opt. 6 (1967) 1276.[2] V.S. Bagaev, T.Ya. Belousova, Yu.N. Berozashvili, D.Sh. Lordkipanidze, Sov. Phys.

Semicond. 3 (1970) 1418.[3] J.E. Kiefer, A. Yariv, Appl. Phys. Lett. 15 (1969) 26.[4] I.V. Nikolaev, M.M. Koblova, Sov. J. Quantum Electron. 1 (1971) 158.[5] C.J. Johnson, Proc. Inst. Electr. Eng. 56 (1968) 1719.[6] C.S. Namba, J. Opt. Soc. Am. 51 (1961) 76.[7] J.E. Pankove, Optical Processes in Semiconductors, Prentice-Hall, Englewood Cliffs, NJ,

1971.[8] A. Yariv, Introduction to Optical Electronics, second ed., Holt Rinehart and Winston,

New York, 1976.[9] G.L. Herrit, H.E. Reedy, J. Appl. Phys. 65 (1) (1989) 393–395.[10] G.L. Herrit, H.E. Reedy, Electro-Opt. Mater. Switches, Coatings, Sensor Optics, Detec-

tors, SPIE 1307 (1990) 1509–1515.[11] A.J. DeMaria, J.T. Kennedy, R.A. Hart, Apparatus for Improving the Optical Intensity

Induced Damage Limit of Optical Quality Crystals, US Patent No. 5,680,412 (1997).

CHAPTER IIE

A.F. Ioffe Physico-Technica

248

Optical Detectors Based onCdTe Pure Crystals for High-Efficiency Optical Computers

P.G. Kasherininov and A.A. Tomasov

1. INTRODUCTION

At present, CdTe crystals with high electrical, optical, and electro-opticalcharacteristics are available. They are widely utilized as optical, roentgen,and nuclear radiation detectors. Pure (noncompensated) crystalswith high electrical resistance (r ¼ 107–108 O cm), low impurity densities(Nt < 1013 cm�3), and high carrier mobilities and lifetimes are of specialinterest in the field of detectors. Detectors based on such a materials atpresent are the main spectrometric detectors of nuclear radiation func-tioning without cooling. Such detectors are MSM—structures producedwith cold evaporation of metal electrodes (M) on the crystal surface (S).They function at high applied voltages and posses large workingvolumes, low dark currents, and low noises. They are not polarizedduring the radiation detecting. At the last time, it was discovered thatMSM structures of this type based on pure p-CdTe crystals with highelectrical resistance are promising as a new type of fast optical registeringmedia for high-efficiency optical computers.

1.1. Optical computers based on semiconductor structures

During the last 20 years, the efficiency of modern electronic computersis increasing due to the redoubling of transistor density in super largeintegrated microchips (SLIC), which takes place in each year and a half.

l Institute, St Petersburg 194021, Russia

Optical Detectors Based on CdTe Pure Crystals 249

Technology of integrated schemes has certain physical limits. Theselimits result in the stagnation of microelectronic systems’ efficiency. Con-temporary electronic computers exhaust their development. There is aneed of computers functioning on a new principle with higher rate. Oneof the substitutions of modern electronic computers is to create so-calledoptical computers. The information in these computers is transportedwith a light flux. Optical calculations are divided in digital optical calcu-lations and analog ones.

Digital optical calculations use light signals to perform operations ofdigital logic. They are targeted at the class of applications, which areperformed currently with electronic computers. The advantage of opticalcomputers is their ability to transport the information at the speed of lightand to substitute wires with wireless optical connections.

Analog calculations include analog operations above images. Such aprocessor is able to perform action with two-dimensional pictures at onestep; meanwhile, the computer instruction might also be a picture. Thedevelopment of analog two-dimensional optical processor is now of max-imum interest and allows one to look forward at producing of superefficient computers with an operation rate more than 1012 operations/sand processor operating frequency of 106 cycles/s. Such processors arestill not realized.

1.2. Optical registering media in contemporary opticalprocessors on MIS structures

Modern optical processors is based upon the registering media, which aremetal (M)-insulator (I)-semiconductor(S) structures (MIS structures) witha thick (1 mm) insulating layer opaque to the charge carriers [1–3]. Theexternal bias voltage is applied to such structures, and the recorded imageis projected on the surface of it with a light flux of a fixed exposure time.The image is recorded in structure as a two-dimensional electric chargedistribution with the density following distribution of brightness of theimage on a surface of structure. In such MIS structures, the recordedcharge cannot leak out of crystal through the insulating dielectric layerafter the recording light is off. Low operation rate of these structures andthe devices based on them (n ¼ 102–103 cycle/s) is predefined by thenecessity to carry out the erasing of charge. It takes up most of the timeof information recording cycle (t ¼ 10�2 10�3 s). Meanwhile, informationreading and recording of images takes only microseconds. The maincontemporary optical processors are based on such a structure: space-time optical modulators (PROM), liquid crystal light modulators(MIS-LC), etc. [1–3].

250 P.G. Kasherininov and A.A. Tomasov

1.3. Fast optical registering media on semiconductorM(TI)S-nanostructures

At present, devices based on MIS structures with a thin nanosizedielectric layer (TI) (M(TI)S nanostructures) are proposed as fast opticalregistering media [4–14]. The thickness of a dielectric is 2–5 nm. It wasestablished that photocurrent occurs in such M(TI)S nanostructuresunder illumination with “proper” light. The current is proportional tothe illumination intensity. Meanwhile, in crystal at the boundary ofdielectric layer (TI) the free photo carriers are created. Their density isproportional to the illumination intensity. This charge exists under therecording light only. At switching on (off) the recording light thecharge establishes (dissipates) at the scale of microseconds. It is notnecessary to erase the existing charge in order to record new image insuch structures. It is established that real contacts of metal-semicon-ductor in MSM structures which are produced by covering of realcrystal surface with metal appeared to have the following structure:metal (M)-thin insulator (TI) semiconductor (S)-thin insulator (TI)-metal(M). It occurs due to presence of a thin dielectric (2–5 nm) at the crystalsurface. They are shortly called M(TI)S(TI)M nanostructures [10–14].Such M(TI)S(TI)M nanostructures based on pure high-resistive CdTecrystals are proposed to be used in fast optical registering media torealize high efficient optical computers. It is convenient to use a modi-fication of such M(TI)S(TI)M nanostructure on pure CdTe crystal,which presents n-p transition with inverse bias at one side of thecrystal and thin nanosize dielectric (TI) layer at the opposite side (n-p(TI)M-nanostructures) [10–14].

2. PROCESSORS FOR DIGITAL OPTICAL COMPUTERS BASEDON N-P(TI)M NANOSTRUCTURES OF CdTe

Processors of optical digital computers are intended for realizationof operations of digital logic and represent the photon keys executed asn-p(TI)M nanostructure, illuminated with two light streams: controlling(I2) and information (I0) [13].

Figure 1 depicts a optical shutter (A) and commutator (1 � 2) (C)realized upon these structures. The functioning of photon switches isbased on reversible change of electric field strength in such nanostruc-tures under illumination with light stream. Photon switches functioningis based on transversal electro-optical effect. It is n-p(TI)M nanostructurewith high resistive electro-optical CdTe crystal with a thin nanosizeinsulating layer (TI) with a thickness 2–5 nm. The nanostructure is placedbetween to crossed polarizers. It works in a following way: narrow

0

1 2

3

+=0.

82m

km

–M2

M1

TI

+

–M2

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p-CdTe

p-CdTe

λ=1.3mkm

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λ=0.

82m

km

I0

I0

I2

I01

I2 Il

Ill

n-CdTe

n-CdTe

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bitr

ary

units 0.3

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t, mks

150

1

1

3A

B

C

2

21

3 4

234

200

Figure 1 Photon switches on semiconductor n-p(TI)M nanostructures utilizing electro-

optical crystals (CdTe). (A) Principle diagram of an optical gate and illumination geom-

etry: 1 and 3, polarizers; 2, n-p(TI)M nanostructures. (B) Oscillogram of transmission of

commutated (passing through) stable light beam (I0, l ¼ 1.3 mm) under the illumination

of the structure with rectangle pulses of controlling light (I2, l ¼ 0.82 mm) with

intensities I2 (mW/cm2): 1-0.5; 2-5; 3-7; 4-9 (U0 ¼ 400 V). (C) Principle diagram of

light-controlled optical commutator (1 � 2): 1, polarizer; 2, n-p(TI)M nanostructure;

3, polarization-sensitive prism (Glan prism).


information light beam (I0) is let through the area of volume charge of n-ptransition in the crystal. It does not undergo absorption in crystal.Controlling beam of “proper” light illuminates the structure surface indirection parallel to the electric field. Controlling light induces formationof electric charge at the boundary of insulating layer (TI). This charge


changes the electric field strength distribution in crystal on the wayof light beam. At the entry of the structure the intensity of informationlight beam rapidly changes due to transversal electro-optical effect.Deep impurity levels in crystals determine the rate of such structures.The following parameters are achieved in n-p(TI)M structures with pureCdTe crystals (r ¼ 108 O cm, Nt < 1013 cm�3) [13]:

Spectral range of the switched light, l (mm)
1–2
Spectral range of the controlling light, l (mm)
0.82–0.83
Cycle time, t (s)
10�5–10�6
Intensity of controlling light, I (W/cm2)
10�1
Working voltage, U0 (V)
400
Modulation depth (%)
>90
3. PROCESSORS FOR ANALOG OPTICAL COMPUTERS OFINCOHERENT LIGHT ON n-p(TI)M NANOSTRUCTURES ONCdTe

Analog optical processor is targeted at the treatment and comparison ofthe images. It might be designed on a basis of n-p(TI)M nanostructureswith CdTe [11, 12, 14]. Figure 2(A) represents the basic circuit of theprocessor and geometry of illumination. “Proper” light pulse containingof recording image is projected at the surface of such a processor from theside of inverse biased photosensitive n-p transition. On an oppositesurface of the processor the pulse of the reading “proper” light homo-geneously distributed on a surface of a crystal or carrying the image ofcompared object is projected.

Under recording light stream (from the side n-p transition) the two-dimensional electric charges are created in crystal. The density of thecharges is proportional to the brightness of image.

Reading the recorded information in such processor is made by regis-tering a magnitude of a photocurrent on an output of the processor fromaction of a reading light stream (with simultaneous illumination of pro-cessor with recording light). A photocurrent thus contains the informa-tion on magnitude and a configuration of an electric charge written downin a crystal (information of recorded image).

Reading of the recorded image in such processors is performed by aone-step process, and the instruction (reading light beam) itself can be animage. Light and photoresponse pulses are shown in Fig. 2B and C [11,12]. Frequencies of recording and reading light are equal, pulses are

00.1J,

mA

/cm

2I,

mW

/cm

2J,

mA

/cm

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mW

/cm

2

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R+ –

n p

M

Tl

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40

00

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10

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t, μS

t, μS

t, μS

τ=10μs

τ=40μs

τ0=20μsI0=10mW/cm2

τ1=50μsI1=4mW/cm2

Figure 2 Optical processor based on n-p(TI)M nanostructure with CdTe targeted at

registering and processing of image signals (A), structure and illumination geometry

(B, C). Time dependence of recording and reading light pulses and pulses of

photoresponse at the outlet of processor (l ¼ 0.82 mm). The delay time between

the pulses: (B) 40 ms (C) 10 ms.


shifted respectively toward each other with a delay time t¼ 40 ms (Fig. 2B)and 10 ms (Fig. 2C). At the overlap (t ¼ 10 ms, Fig. 2C) the stationaryelectric field strength distribution establishes in the structure.

It corresponds to the recording image. Photoresponse of reading lightpulse reaches maximum at overlap of recording and reading image. Afterthe recording light pulse, photoresponse of reading pulse diminishes to


its minimum value. When recording light pulse contains recording imageand reading light intensity is homogeneous over the sample surface,photocurrent pulse at the outlet of processor is proportional to the record-ing image square (Fig 2C). When reading light pulse contains comparedimage the processor functions as image correlator.

4. OPTOELECTRONIC IMAGE CORRELATOR OF INCOHERENTLIGHT BASED ON ANALOG OPTICAL PROCESSORS

Such analog computers can be applied to the development of an optoe-lectronic correlator aimed at image recognition [12]. For the benefit of itthe surface of a processor containing n-p transition is illuminated withreference image of a necessary object. The opposite surface is illuminatedwith the image that needs to be compared. Further, it is performed withtheir mutual shift along both axes and their adjustment in scale andangular orientation. At the moment the signal at the outlet of a processoris defined by the overlap integral of these images, that is, proportionalto the correlation function (CF) of reference and recognized images.At coincidence of reference image of the object and current image insize and angular orientation, the CF value reaches its maximum. Thedecision about the presence of a necessary object in the field of the systemvision is made after the CF reaches a fixed threshold value. The time ofcalculation of correlation integral in this correlator is to be defined by itsoperation rate and is expected to be 1 ms, while the calculation processitself is finished at one step independently on dimension of images. Thus,the rate of the correlator is to be defined by the input time of the images atthe surface of processor and not by time of calculating the correlationintegral. As a result the processor is to work at the rate of image input,that is, real-time rate. The mutual shift of reference and recognizedimages, their scaling, and rotating are to be performed by means ofknown electronic methods, used in television and optical image correla-tors. The suggested image correlator is purposed to the usage in intellec-tual systems of technical vision, functioning of which is related to thenecessity of image recognition. It can be used to solve the followingproblems: automatic assembly at the assembly line, face recognition,fingerprints, credit card identification via the photographs and finger-print of the owner, prevention of the undesirable access into thespecial areas, navigation and astro-navigation of the flying objects inaccord with ground signs and stars, automatic connecting of spacecrafts,and so on.


5. CONCLUSION

Thus, pure crystals of CdTe with low impurity density (Nt < 1013 cm�3)are very prospective as a material for fast optical registering media.Registering media with a cycle time t ¼ 10�6 s, spatial distribution 5–10couples lines/mm, and sensitivity 10�2W/cm2 are realized on the basis ofCdTe. The optical, digital, and analog computers and image correlatorscan be realized on this basis.

REFERENCES

[1] A.A. Vasilev, D. Kasasent, I.N. Kampaneez, A.V. Parfenov, Spatial Light Modulators,Radio and Communication, Moscow, Russia, 1987.

[2] M.P. Petrov, I.S. Stepanov, A.V. Homenko, Fotosensitive Electrooptical Medias inHolography and Optical Information Processing, Nauka, Leningrad, Russia, 1983.

[3] A.V. Boroshnev, N.F. Kovtonyuk, Russ. Appl. Phys. 6 (2000) 5–10.[4] J. Sewchun, A. Waxman, G. Warfield, Solid State Electron. 10 (12) (1967) 1165–1186.[5] W.E. Dahlke, S.M. Sze, Solid State Electron. 10 (8) (1967) 865–873.[6] M.A. Green, J. Shewchun, Solid State Electron. 17 (4) (1974) 349–365.[7] A.A. Gutkin, V.E. Sedov, Semiconductors 9 (9) (1975) 1155–1158.[8] A.Ya. Vul’, S.V. Kozyrev, V.I. Fedorov, Semiconductors 15 (1) (1981) 83–86.[9] A.Ya. Vul’, A.V. Sachenko, Semiconductors 17 (8) (1983) 865–875.[10] P.G. Kasherininov, A.V. Kichaev, A.A. Tomasov, Semiconductors 29 (11) (1995)

1092–1099.[11] P.G. Kasherininov, A.V. Kichaev, A.N. Lodygin, V.K. Sokolov, Proc. SPIE 5381 (2004)

292–301.[12] P.G. Kasherininov, A.N. Lodygin, V.K. Sokolov, Proc. SPIE 5066 (2003) 273–280.[13] P.G. Kasherininov, A.V. Kichaev, A.A. Tomasov, V.K. Sokolov, Proc. SPIE 6594 (2007)

65941G.[14] P.G. Kasherininov, A.V. Kichaev, A.N. Lodygin, A.A. Tomasov, V.K. Sokolov, Proc.

SPIE 6251 (2006) 625112–625124.

cdte and related compounds; physics, defects, hetero- and nano-structures, crystal growth, surfaces...

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