Journal of the Korean Physical Society, Vol. 64, No. 8, April 2014, pp. 1105∼1109

Characteristics of a Planar-type Cd0.9Zn0.1Te Radiation Detector Grown byUsing the Low-pressure Bridgman Method

Manhee Jeong, Han Soo Kim, Young Soo Kim and Jang Ho Ha∗

Advanced Radiation Detection Instrument & Sensor Lboratory,Korea Atomic Energy Research Institute, jeongeup 580-185, Korea

(Received 8 January 2014, in final form 17 March 2014)

An indium-doped (7 ppm) Cd0.9Zn0.1Te single crystal for use in room-temperature radiationdetectors has been grown using a low-pressure Bridgman (LPB) furnace at the Korea AtomicResearch Institute. The single crystal has a (111) orientation and a high resistivity of ∼1 × 1012

Ω·cm. In addition, the mobility-lifetime products of the electrons and hole are 4.2 × 10−4 cm2/Vand 5 × 10−5 cm2/V, respectively. These values are simply derived by using a Hecht and a neuralequation and 5 MeV alpha particles emitted from an 241Am alpha source. To characterize theCd0.9Zn0.1Te grown by using the LPB method, we fabricated planar detectors with volume of 10× 10 × 2.5 mm3 from a 2-inch-diameter Cd0.9Zn0.1Te ingot.

PACS numbers: 29.30.Kv, 29.40.-n, 29.40.Kv, 65.40.-bKeywords: Cd0.9Zn0.1Te, Mobility-lifetime products, Resistivity, Gamma-ray spectrumDOI: 10.3938/jkps.64.1105

I. INTRODUCTION

Room-temperature semiconductor radiation detectorsrequire the semiconductor band gap energy to be appro-priately large to reduce the thermally-generated leakagecurrent. In particular, compound semiconductors of gen-eral interest for room-temperature operation have bandgap energies between 1.35 and 2.55eV, although not allcompound semiconductors are readily available due toeither growth difficulties or economics [1]. Presently, thecompound semiconductors receiving the most attentionfor radiation detector applications are cadmium zinc tel-luride (CdZnTe), cadmium telluride (CdTe), mercury io-dide (HgI2) and thallium bromide (TlBr) [1–3]. Amongthese candidates, CdZnTe has been the subject of re-search for the last two decades owing to its high atomicnumber (Cd: 48, Zn: 30, Te: 52, Zeff : 49.1), high den-sity (ρ = 5.78 g/cm3) and wide band gap (Eg = 1.572eV) [3]. Significant areas of commercial activity usingCdZnTe have been utilized in the development of imag-ing detectors for nuclear medicine imaging and hard X-ray astronomy with high energy and spatial resolution[4].

Three common techniques are used for the growthof single-crystal CdZnTe: the low-pressure Bridgman(LPB) method, the high-pressure Bridgman (HPB)method and the travelling heater method (THM) [5–8].Among these growth methods, we used the LPB method

∗E-mail: jhha@kaeri.re.kr; Fax: +82-63-570-3748

with indium doping in the initial charge to increase theresistivity of the CdZnTe materials up to ∼1012 Ω·cm. Inthis paper we report a growth of high-resistivity CdZnTeradiation detectors by using the LPB method and thecharacterization results for the fabricated samples, i.e.,the orientation, resistivity, charge transport and detectorperformances with alpha and gamma sources.

II. EXPERIMENTS AND DISCUSSION

Cd0.9Zn0.1Te crystals were grown using the LPBmethod with the starting material being CdZnTe pow-der, with a nominal purity of 5 N purchased from Alfar-Aesar. The Bridgman furnace used for the crystal growthhad 6 heaters, and the heat gradient was set to between10 and 15 ◦C/cm. At the inner wall of the ampoule,carbon (graphite) was coated to easily remove the ingotfrom the ampoule. Figure 1 shows a 5.08-cm-diameterCd0.9Zn0.1Te ingot grown by using the LPB method.The Cd0.9Zn0.1Te ingot was diced into wafers with thick-nesses of 5−10 mm by using a diamond wire saw. Thedetector samples were cut into different sizes, shaped,lapped and polished with several steps. Finally, theyhave been etched with 2% Br-methanol the surfaces ofthe samples had a mirror finish, as shown in Fig. 2, ex-hibiting an obvious improvement due to chemical polish-ing.

Figure 3 shows the prepared Cd0.9Zn0.1Te samples toevaluate the characteristics for room-temperature radi-ation detectors. A mixture with AuCl3 and DI water

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Fig. 1. (Color online) CdZnTe ingots, 1 and 2 inches indiameter grown in a LPB furnace.

Fig. 2. (Color online) Microscopic photographs of the sam-ple surface (a) before and (b) after Br-methanol chemicaletching.

Fig. 3. (Color online) Polished and etched CdZnTe detec-tors fabricated from a 2-inch-diameter ingot.

was used to make a planar-type Au contact electrode onboth the top and the bottom of the Cd0.9Zn0.1Te surface.All of the detectors side was polished again and coveredwith Teflon tape because only a polished surface wouldprovide a low surface leakage current, which would leadsless peak tailing and better energy resolution.

The crystallinity of the Cd0.9Zn0.1Te detector was de-termined using by X-ray diffraction (XRD) with an X-raydiffractometer (Rigaku Co.). Figure 4 shows the XRDspectrum, and only one diffraction peak, 23.758◦, whichcorresponds to the (111) diffraction peak, appears on thespectrum which means that the fabricated Cd0.9Zn0.1Tesample is a single crystal and has a (111) orientation.

For the bulk resistivity of the Cd0.9Zn0.1Te detector,

Fig. 4. (Color online) X-ray diffraction pattern of the fab-ricated CdZnTe sample for the crystallinity evaluation.

Fig. 5. (Color online) Graph of the leakage current of theLPB-grown Cd0.9Zn0.1Te detector contacted with Au.

current-to-voltage (I-V) measurements were carried outwith a semiconductor analyzer (Keithley 4200). Figure5 shows the graph of the leakage current vs. the appliedbias voltage for the prepared Cd0.9Zn0.1Te detector. Theleakage current of the Cd0.9Zn0.1Te detector depends onseveral factors, such as the contact deposition technique,surface passivation, and side surface treatments. To min-imize the effects from the surface leakage currents, wecovered the four outer surfaces of the Cd0.9Zn0.1Te de-tector with Teflon tape after the outer sides had beenpolished again. The maximum leakage current was ∼5nA at −200 V. From the I-V graph shown in Fig. 5, theresistivity of the Cd0.9Zn0.1Te detector, which has a vol-ume of 10 × 10 × 2.5 mm3, is calculated as ∼1 × 1012

Ω·cm.In addition, the alpha spectral performance was mea-

sured for the LPB-grown planar Cd0.9Zn0.1Te detectorby using a high bias supply (Ortec 659), a preampli-fier (eV-5093, eV-products), a shaping amplifier (Ortec

Characteristics of a Planar-type Cd0.9Zn0.1Te· · · – Manhee Jeong et al. -1107-

Table 1. Comparison of the characteristics of the mobility-lifetime products from different vendors.

Materials Vendor Size (mm3) μeτe (cm2/V) μhτh (cm2/V)

KAERI 10 × 10 × 2.5 4.188 × 10−4 5.043 × 10−5

CdZnTe eV-products [11] 5 × 5 × 2 6.35 × 10−4 1.39 × 10−4

Redlen [12] 15 × 15 × 5 8.6 × 10−3 N/Aa

CdTeKAERI 10 × 10 × 2 3.137 × 10−4 4.868 × 10−5

Acroradb 10 × 10 × 1 6.037 × 10−4 2.218 × 10−4

aNot available.bMeasured by KAERI.

Fig. 6. (Color online) 241Am alpha-particle pulse-heightspectra obtained with the LPB-grown planar Cd0.9Zn0.1Tedetector for varying applied bias from −40 to −300 V.

572), and a multichannel analyzer (MCA). The measure-ment was made at room temperature (23 ◦C). The alphaspectral response should represent a measure of the elec-tron transports uniformity. The full width at half maxi-mum (FWHM) should be near or less than 1% for goodgamma-ray spectroscopy, which means a high energy res-olution.

The mobility and the carrier lifetime products (μeτe

and μhτh) are the most important parameters to eval-uate the charge-transport properties of semiconductordetectors. Many methods have been suggested to deter-mine the electron and the hole mobility-lifetime productsby using the alpha spectra obtained at various appliedbias voltages and the Hecht equation, an induced laserbeam to generate electron-hole pairs near the detectorssurface, and gamma spectra with the neural equationfitting method. In this paper, we used a conventionalmethod to obtain the alpha spectra at various appliedbias voltages from −40 to −300 V to determine the elec-tron and hole mobility lifetime products, as shown inFig. 6.

The charge collection efficiency (CCE) and the energyresolution strongly depend on the μτ value of the charge

carriers. The CCE is given by the Hecht equation [9]:

CCE(x0) =μeτeE

D

[1 − exp

(−D − x0

μeτeE

)]

+μhτhE

D

[1 − exp

(− x0

μhτhE

)], (1)

where D is the detectors thickness, x0 is the distancefrom the cathode to the point of the interaction, and Eis the applied electric field given by E = V/D. When thealpha spectra obtained at various applied bias voltagesare used to determine the μτ values, the single-particleHecht equation, is

CCE =μτV

D2

[1 − exp

(− D2

μτV

)], (2)

can be used because the penetration depth of alpha parti-cle is relatively much smaller than the detector thickness.

The incident radiation can be characterized by its re-laxation free path (λ), defined as the average distancetraveled in the matter before an interaction takes place,which is equal to one over the sum of the probabilitiesper unit path lengths. The linear attenuation coefficient(1/μ) is the sum of the probabilities percent path lengthof the photoelectric effect, Compton scattering and pairproduction. The value of the linear attenuation coeffi-cient for CdZnTe can be obtained from National Insti-tute of Standards and Technology (NIST) XCOM [10].

The charge collection efficiency, where most chargecarriers are produced at the relaxation free path of theincident gamma-ray, can be acquired by using Eq. 2, andthis position determines the energy range of the full-energy peak in the energy spectrum. From the Hechtrelation the electron mobility-lifetime product (μeτe) canbe obtained. Furthermore, by using the neural equationderived from the two charge collection efficiencies at aspecific position, which has the maximum charge collec-tion efficiency, we can also calculate the hole mobility-lifetime product (μhτh) as

μhτh = μeτe/(D/λ − 1). (3)

The electron and the hole mobility life-time prod-ucts measured for the planar-type 10 × 10 × 2.5

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Fig. 7. (Color online) Graphs of the charge collec-tion efficiency versus applied bias for the (a) LPB-grownCd0.9Zn0.1Te and (b) CdTe detectors for the 5.5 MeV alphaparticle peak of an 241Am source.

mm3 Cd0.9Zn0.1Te detector were determined using themethodology explained above. Figure 7 shows graphs ofthe charge collection efficiency versus applied bias for the5.5 MeV alpha-particle peak of an 241Am source for thedetectors with Cd0.9Zn0.1Te and CdTe grown using theLPB method at KAERI. The measured values were 4.188× 10−4 cm2/V and 3.137 × 10−4 cm2/V for the electronand 5.043 × 10−5 cm2/V and 4.868 × 10−5 cm2/V for thehole, respectively. A comparison of the characteristics ofthe mobility-lifetime products from different vendors isgiven in Table 1 [11,12].

To compare the energy spectrum performance, we useda 133Ba gamma-ray source with a Cd0.9Zn0.1Te detectorand a CdTe detector, which were fabricated at KAERI.Figure 8 shows the energy spectra obtained by using aplanar-type Cd0.9Zn0.1Te detector with a −300 V ap-plied bias and by using a 10 × 10 × 2 mm3 planar-typeCdTe detector with a −250 V applied bias and a 0.5 μspeaking time. The Cd0.9Zn0.1Te detectors, which hasbetter charge transport characteristics than the CdTe

Fig. 8. (Color online) 133Ba gamma-ray spectrum of (a) a10 × 10 × 2.5 mm3 planar-type Cd0.9Zn0.1Te detector at a−300 V applied bias and a 0.5 μs peak time and (b) a 10 ×10 × 2 mm3 planar-type CdTe detector at a −250 V appliedbias and a 0.5 μs peak time.

detector, shows a better energy resolution and can accu-rately identify the low-energy region.

III. CONCLUSION

Cd0.9Zn0.1Te with an active volume of 10 × 10 × 2.5mm3 that had been manufactured by using the LPBmethod at KAERI for use in a room-temperature ra-diation detector has been studied in terms of its crys-tallinity, resistivity, charge transport characteristics, andenergy spectrum. It shows a (111) orientation and a highresistivity of about 1012 Ω·cm, indicating single crys-tallinity, and can sustain a high bias voltage for bet-ter charge collection efficiency. The fabricated planar-type Cd0.9Zn0.1Te detectors have good charge-transportproperties and can be used for low energy gamma-raydetectors and for X-ray imaging sensors for medical andindustrial purposes.

Characteristics of a Planar-type Cd0.9Zn0.1Te· · · – Manhee Jeong et al. -1109-

ACKNOWLEDGMENTS

The authors would like to gratefully acknowledge sup-port by the Nuclear R&D program of the Ministry ofScience, ICT & Future Planning (MSIP) of South Ko-rea (NRF-2010-0026096, NRF-2013M2A2A4023359, andNRF-2011-0030465).

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