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Materials Science in Semiconductor Processing

Materials Science in Semiconductor Processing 10 (2007) 252– 257

1369-80

doi:10.1

� Cor

E-m

journal homepage: www.elsevier.com/locate/mssp

Effect of 80 MeV Au8+ ions irradiation on CuInTe2 single crystalsgrown by CVT technique

P. Prabukanthan a, K. Asokan b, D.K. Avasthi b, R. Dhanasekaran a,�

a Crystal Growth Centre, Anna University, Chennai 600 025, Indiab Inter University Accelerator Centre, New Delhi 110 067, India

a r t i c l e i n f o

Available online 23 May 2008

PACS:

81.10.Bk

68.73.Ps

61.80.Lj

61.82.Fk

78.55.Ap

Keywords:

CuInTe2

Gold ion irradiation

XRD

AFM

Raman

01/$ - see front matter & 2008 Elsevier Ltd. A

016/j.mssp.2008.03.001

responding author. Tel.: +9144 22203572; fax

ail address: rdhanasekaran@annauniv.edu (R.

a b s t r a c t

Single crystals of CuInTe2 (CIT) have been grown by the chemical vapor transport (CVT)

technique using iodine as the transporting agent. CIT crystals were irradiated with 80 MeV

Au8+ ions at room temperature at different fluences. The surface roughness was measured

using an atomic force microscope (AFM). It was found to increase from 9.319 nm in the as-

grown sample to 61.169 nm in the sample irradiated with a fluence of 1�1013 ions/cm2.

The intensities of the X-ray diffraction peaks corresponding to the (112) and (004/200)

planes of the irradiated sample decrease with respect to the fluences. The full-width at

half-maximum (FWHM) of X-ray rocking curves was measured as a function of different

ion fluences. The FWHW increases with increase of ions fluences. This is attributed to the

irradiation-induced partial amorphization of the top surface of the CIT crystals. The fall in

absorption coefficients with photon energy is sharper for as-grown samples than

irradiated samples. The band gap value gradually decreases from 1.04 to 0.977 eV upon

Au8+ ions’ irradiation with a fluence of 1�1013 ions/cm2. Photoluminescence (PL)

measurements show a red shift compared to the as-grown CIT single crystals. The Raman

modes of A1 (high) and E and/or B2 (LO) are observed at 123 and 173 cm�1 in as-grown CIT

single crystals, respectively. As the ion fluence is increased, the Raman frequency

increases and the curves broaden. The above observed features are related to the large

electronic energy transfer of the Au beam to the CIT crystals.

& 2008 Elsevier Ltd. All rights reserved.

1. Introduction

The ternary compound of CuInTe2 (CIT), which belongsto the I–III–VI2 (I ¼ Cu, Ag, III ¼ Al, Ga, In and VI ¼ S, Se,Te) semiconductor materials, is the isoelectronic analogueof II–VI binary compound semiconductors [1–3]. Duringthe last decade, irradiation experiments of variousmaterials at room temperature with swift heavy ions(SHIs) have been performed. On irradiation, damage iscaused in the near surface region of the wafer leading toamorphization and stress in the microelectronic structureresulting in delamination, cracking, anomalous diffusionof dopants and void formation [4]. SHIs are a suitable

ll rights reserved.

: +9144 22352774.

Dhanasekaran).

means for inducing surface modifications. The surfacemorphology of the resulting nonequilibrium surfacesdepends sensitively on ion flux, ion fluence, energy andsample temperature. SHI passes through the solid surfacewith a velocity comparable to the Bohr velocity ofelectrons and loses its energy while traversing throughthe matter. The total energy loss can be expressed byelectronic energy loss (Se) due to the inelastic collisionswith electrons and nuclear energy losses (Sn) due to theelastic collisions with atoms of the solid with theprojectile ion. For example, a uranium ion of 2.7 GeVloses energy of 30 keV/nm in the time interval of2�10�17 s, almost exclusively by interaction with elec-trons of the target [5]. Depending upon the sensitivityof the solid, the degree of disorder can range frompoint defects to a continuous amorphization zone alongthe ion trajectory.

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Recently the solar cells based on CuInSe2 andCu(Ga,In)Se2 demonstrated high conversion efficiency closeto 19% [6], which is comparable to that for polycrystallineSi solar cells. In view of their potential low cost and lightweight, these solar cells have been expected to be usefulin space area in addition to the terrestrial domain. In thespace application of solar cells, it is necessary to clarify theradiation damage in cell materials and solar cells. It issuggested that the CIT solar cells with an energy gap of1.04 eV, lying close to CuInSe2, could also give a similarefficiency [7]. For this reason, CIT material is of consider-able interest for device applications. There are severalreports on CIT crystals studied for electrical properties,optical properties and lattice vibrations [8–16]. CuInSe2

single crystals, which have been implanted with oxygen,showed an increased photo-response with an associatedconductivity change from p-type to n-type and were alsoshown to have a very high resistance to radiation damage[17–18]. Irradiation with Au8+ ions is of interest since itsheavy mass induces high-energy density collision cas-cades that readily lead to amorphization, lattice disorderand phase change. As a result of high-energy irradiation, asurface amorphization takes place in the CIT singlecrystals. However, there is no detailed report on thehigh-energy Au8+ ions irradiation in CIT. In the presentwork, the effects of 80 MeV irradiation of Au8+ ions in CITsingle crystals at fluences of 1�1012, 5�1012 and1�1013 ions/cm2 were investigated by an atomic forcemicroscope (AFM), energy dispersive X-ray (EDAX) analy-sis, glancing angle X-ray diffraction (GAXRD), X-ray rock-ing curves, optical absorption spectra, photoluminescence(PL) and Raman spectra.

01

10

100

1000

NIE

L (

MeV

cm2 /g

)

Penetration depth (μm)

80 MeV Au8+ ions on CuInTe2

12108642

Fig. 1. NIEL as a function of penetration depth for 80 MeV Au8+ ions on

CuInTe2.

2. Experimental

CuInTe2 (CIT) single crystals have been grown by thechemical vapor transport (CVT) method using iodine asthe transporting agent at the growth and source zonetemperatures of 873 and 923 K, respectively. The obtainedCIT single crystals were black in color and the dimensionwas 15�5�3 mm3. The stoichiometric composition ofthe as-grown CIT single crystal was determined byscanning at several points using EDAX. No significantvariation was found in the composition of Cu, In and Te ofthe crystal, thus confirming homogeneity. The crystalsobtained were confirmed to have a chalcopyrite structureby the X-ray diffraction studies.

The as-grown crystals were cut with dimensions of5�5�1 mm3 and washed with methanol solution for10 min and then rinsed in de-ionized water before theirradiation studies. The CIT single crystals were irradiatedat room temperature with Au8+ ions of 80 MeV at a currentdensity of 3 pnA/cm2 of various fluences of 1�1012,5�1012 and 1�1013 ions/cm2 using a 16 MV Tandempelletron accelerator at Inter University AcceleratorCentre, New Delhi, India. The irradiation was carried outat a material science beam line with background pressureof 3�10�7 Torr. The projected range was calculated to be7.53 mm for the 80 MeV Au8+ ions using SRIM code(Stopping and Range of Ions in Matter) [5].

The crystal surface morphologies of as-grown and Au8+

ion-irradiated CIT samples were studied using an AFM[Nanoscope III—a multimode scanning probe microscope(SPM)]. The composition of the irradiated CIT singlecrystal with different ion fluences was determined bythe EDAX, INCA 200 system. Structural elucidation of theas-grown and irradiated CIT samples was carried outby GAXRD studies using Bruker AXS Diffract Plus/D8Advanced Spectrometer with CuKa (l ¼ 1.5405 A) radia-tion with a scan speed of 11 and increment of 0.021. Theglancing angle was fixed at 21. As a quality criterion of thecrystalline perfection we use the full-width at half-maximum (FWHM) of the (400) reflex from X-ray rockingcurves obtained with a four-crystal monochromator. Theoptical absorption spectra of the as-grown and irradiatedCIT single crystals samples were recorded using ShimadzuUV–Visible–FIR spectrometer in the range 200–2000 nm.The luminescence properties of the as-grown and irra-diated CIT samples were investigated in the range400–1500 nm using a He–Cd laser as an excitation source(325 nm) at room temperature. The Raman spectra of as-grown and irradiated CIT samples were recorded at roomtemperature. The excitation source was an argon ion laserbeam of 300 mW (l ¼ 488 nm) power with verticalpolarization focused to a spot size of 50mm onto thesample. The scattered light was collected in the back-scattering geometry using a camera lens [Nikkon, focallength 5 cm, f/1.2]. The collected light was dispersed in adouble-grating monochromator, SPEX model 14018 anddetected using thermoelectrically cooled photo-multipliertube model ITT-FW 130. The resolution obtained was atleast 5 cm�1.

3. Results and discussion

Fig. 1 shows the nonionizing energy loss (NIEL) as afunction of the penetration depth for 80 MeV Au8+ ion-irradiated CIT single crystals. The method of derivingNIEL values from the SRIM 2003 code is describedin the literature [19–20]. The calculated NIEL value is

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903.17 MeV cm2/g for CIT materials. The displacementdamage dose (Dd) is obtained by multiplying the NIELvalue and the ion fluence. The values of displacementdamage dose (Dd) are 9.0317�1014, 45.1585�1014 and9.0317�1015 MeV/g with respect to the ion fluences of1�1012, 5�1012 and 1�1013 ions/cm2, respectively.

The AFM surface pictures of as-grown CIT singlecrystals and irradiated with various fluences are shownin Fig. 2(a–d). The effect of 80 MeV Au8+ ions on thesurface is visible from the change in the surfacemorphology at different fluences. It is observed thatseveral pits and islands are formed due to irradiation.The root mean square values of surface roughnessevaluated from the AFM data are 9.319 nm for the as-grown sample and 13.087, 16.858 and 61.169 nm forsamples irradiated at a fluence of 1�1012, 5�1012 and1�1013 ions/cm2, respectively. The CIT sample surfaceshows dramatic surface modifications distributed over thesurface (Fig. 2(b–d)).

It is interesting to observe the formation of pits andislands in Fig. 2(b–d), which may be formed due to theamorphization and recrystallization process [21–22].During energy loss of the incident ions, a very high hotregime is formed in the wake of incident ions, which mayresult in the melting of the host material. Due to meltingof the host materials at high temperatures followed bycooling, amorphization takes place to give the observed

Figs. 2. Atomic force microscope images of CuInTe2 single crystals: (a) as-grow

features of pits and islands. The observed features showthat the damage depends upon the ion fluences. Theelectronic energy loss (Se) and nuclear energy loss (Sn) for80 MeV Au8+ ions in the CIT target are 2.293�104 and8.366�102 keV/mm, respectively [5]. The difference be-tween the two energy loss processes [Se and Sn] by twoorders of magnitude for gold ions indicates that theobserved features are caused mainly due to the electronicenergy loss.

GAXRD spectra of as-grown and irradiated (withfluences of 1�1012, 5�1012 and 1�1013 ions/cm2) crys-tals are shown in Fig. 3. The peaks observed at 24.91 and28.81 when the glancing angle was fixed at 2y correspondto the (112) and (004/200) peaks of the bulk CIT crystals.There is a drastic decrease of intensity of (112) and (004/200) XRD peaks with increasing fluences, which indicatesthe occurrence of partial amorphization of the top surfaceof a CIT single crystal. This result suggests that pointdefects such as broken bonds produced at higher fluencesin the irradiated samples cause the decrease in peakintensity. Similar results were obtained in the case ofhigh-energy 200 MeV Ag8+-irradiated Si [23] and high-energy heavy ion (Xe15+ and Au13+ ions)-irradiated TiO2

thin films [24].Fig. 4(a–c) shows the X-ray rocking curves for the as-

grown and Au8+ ion-irradiated CIT single crystals. TheFWHM increases from 6 arcsec for the as-grown crystal to

n; (b) 1�1012 ions/cm2; (c) 5�1012 ions/cm2; and (d) 1�1013 ions/cm2.

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20

Inte

nsity

(a.u

.)

Two theta (deg)

(112)

(004/200)

(a)

(b)

(c)

(d)

40383634323028262422

Fig. 3. GAXRD spectra of as-grown CuInTe2 single crystals and 80 MeV

energy Au8+ ions irradiated with different fluences: (a) as-grown;

(b) 1�1012 ions/cm2; (c) 5�1012 ions/cm2; and (d) 1�1013 ions/cm2.

-100 1000

200

400

600

800

1000

1200

6"

Diff

ract

ed X

-ray

inte

nsity

[c/s

]

Glancing angle [arc s]

-1000

200

400

600

800

1000

22"D

iffra

cted

X-r

ay in

tens

ity [c

/s]

Glancing angle [arc s]

-100 1000

200

400

600

800

25"

Diff

ract

ed X

-ray

inte

nsity

[c/s

]

Glancing angle [arc s]

500-50

100500-50

500-50

Fig. 4. X-ray rocking curves of (a) as-grown CuInTe2 single crystal and

80 MeV Au8+ ion-irradiated CuInTe2 single crystals at different fluences;

(b) 1�1012 ions/cm2; and (c) 5�1012 ions/cm2.

P. Prabukanthan et al. / Materials Science in Semiconductor Processing 10 (2007) 252–257 255

25 arcsec for the gold ion-irradiated sample fluence of5�1012 ions/cm2. It shows that the crystalline nature ofthe top surface decreases with increase of ion fluence.

The composition analysis of as-grown and gold ion-irradiated CIT single crystals were carried out using EDAX.The atomic percentages of the elements present are givenin Table 1. No significant changes in the atomic % of Cu, Inand Te due to the irradiation were observed.

The optical absorption spectra of the as-grown andirradiated samples were recorded between 200 and2000 nm at room temperature. Absorption coefficients (a)were estimated from the absorption spectra of irradiatedCIT single crystals with different fluences. Fig. 5 shows thevalue of (ahn)2 versus photon energy (hn). CIT single crystalsirradiated with Au8+ ions at various fluences show adecrease in the sharpness of the absorption coefficientwith photon energy edge. The induced lattice damagecreates defect energy levels below the conduction band andhence the band gap decreases. The lattice damage isproportional to the fluencies of irradiation and henceabsorption edge decreases continuously with the increaseof fluences. Band gap energy for the as-grown CIT is1.04 eV. The band gap value gradually decreases to 1.03,1.01 and 0.98 eV for the fluences 1�1012, 5�1012 and1�1013 ions/cm2 upon gold irradiation.

Fig. 6 shows the PL spectra of as-grown and Au8+ ionsof 80 MeV-irradiated CIT single crystals recorded at roomtemperature. The as-grown CIT single crystal exhibits anear band edge luminescence peak at 0.942 eV. There is adecrease in emission intensity and band gap(0.942–0.926 eV) for the irradiated CIT single crystals.The radiation-induced defects are responsible for theobserved change in PL spectra. Ion-induced lattice damagemay create defect energy levels below the conductionband and the transition takes place through the defectenergy level, which is below the conduction band of theirradiated CIT single crystals. The PL intensity decreaseswhen the defect density increases due to nonradiativeenergy transfer to the defect levels.

The Raman spectra analysis of the as-grown andirradiated CIT single crystals was carried out between 0and 300 cm�1. The spectra are shown in Fig. 7. Fig. 7(a)shows the Raman spectrum for the as-grown singlecrystal of CIT and the most intense line appears at123 cm�1. This is evidently due to the A1 mode becausethis is the intense peak generally observed in the Ramanspectra of I–III–VI2 chalcopyrite compounds [25–26]. Thenext high-intensity E and/or B2 (LO) mode appears at173 cm�1. In Fig. 7(b–d) the Raman frequencies corre-sponding to A1 and E and/or B2 (LO) modes increase and

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Table 1Composition analysis of as-grown CuInTe2 single crystals and 80 MeV energy Au8+ ions irradiated with different fluences by EDAX analysis

Sl. no. CIT single crystals EDAX data of as-grown and irradiated CIT crystals in atomic % value

Copper Indium Tellurium

1 As-grown 24.52 25.15 50.33

2 1�1012 ions/cm2 fluences 25.08 24.88 50.04

3 5�1012 ions/cm2 fluences 25.55 24.87 49.58

4 1�1013 ions/cm2 fluences 25.68 24.84 49.48

0.850.0

3.0x103

6.0x103

9.0x103

(d)

(c)

(b)

Photon energy (eV)

(a)

(αhν

)2 (eV

/cm

)2

1.101.051.000.950.90

Fig. 5. Optical absorption spectrum, (ahn)2, versus photon energy (hn) of

as-grown CuInTe2 single crystals and 80 MeV energy Au8+ ions irradiated

with different fluences: (a) as-grown; (b) 1�1012 ions/cm2;

(c) 5�1012 ions/cm2; and (d) 1�1013 ions/cm2.

(d)

(c)

(b)

PL in

tens

ity (

a.u.

)

Photon energy (eV)

0.9 0.95 1.05 1.11.00.85

(a)

Fig. 6. Room-temperature PL spectra of (a) as-grown CuInTe2 single

crystal and 80 MeV Au8+ ion-irradiated CuInTe2 single crystals at

different fluences; (b) 1�1012 ions/cm2; (c) 5�1012 ions/cm2; and

(d) 1�1013 ions/cm2.

Fig. 7. Raman spectra of (a) as-grown CuInTe2 single crystal and 80 MeV

Au8+ ion-irradiated CuInTe2 single crystals at different fluences;

(b) 1�1012 ions/cm2; (c) 5�1012 ions/cm2; and (d) 1�1013 ions/cm2.

P. Prabukanthan et al. / Materials Science in Semiconductor Processing 10 (2007) 252–257256

the curves broaden with increase in the fluences. Due tostress effect induced by SHI irradiation, pits and islandsare formed in the top of the CIT surface. The displacementin the position of the Raman A1 and E and/or B2 (LO)

modes can be attributed to the strain and lattice damage.The broadening of the Raman modes is closely related tothe presence of structural defects to form amorphizationon the irradiated surface or stress gradients in thescattering volume and is supported by our AFM resultswhere the swelling of the irradiated surface occurs.

4. Summary

The NIEL value of 903.17 MeV cm2/g for CIT is calculatedfrom SRIM 2003 code. Preliminary studies have beencarried out using AFM to assess the damage produced ona CIT single crystal surface by irradiation with Au8+ ions of80 MeV energy at different fluences. It has been shown thatAu8+ ion irradiation affects the roughness of the top surfaceof CIT and leads to rather dramatic changes in the surfacewith pits and islands structure. The root mean square valueof surface roughness increases with increasing ion fluences.GAXRD results reveal that CIT single crystals surface isheavily damaged and surface amorphization is observedupon Au8+ ion irradiation with increasing ion fluences. Thestudy of the X-ray rocking curves shows that the crystallinenature of the irradiated region decreases with increase of

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P. Prabukanthan et al. / Materials Science in Semiconductor Processing 10 (2007) 252–257 257

Au8+ ion fluences. Optical absorption spectra reveal that theband gap value of the CIT samples decreases from 1.04 to0.98 eV, with fluence. The PL studies show that there is asignificant red shift after Au8+ ion irradiation. Ramanmodes A1 and E and/or B2 (LO) have been observed in theas-grown and irradiated CIT single crystals in which the A1

and E and/or B2 (LO) modes are shifted to higher frequencyand the curves are broadened, indicating the formation ofstrain and lattice damage. Irradiation with high-energygold ions reduces the crystalline in the top surface of CITsingle crystals.

Acknowledgments

One of the authors, P.P., is grateful to the Council ofScientific and Industrial Research (CSIR), India, for theaward of Senior Research Fellowship (SRF). Technicalassistance rendered by pelletron group of Inter UniversityAccelerator Centre (IUAC), New Delhi, in carrying out thiswork is highly acknowledged. They also thank P.K. Kulriaand I. Sulania (IUAC) for GAXRD and AFM measurements.

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