Materials Chemistry and Physics 80 (2003) 581–585

Growth and strain investigation of Cd0.96Zn0.04Te/GaAsby hot-wall epitaxy

B.J. Kim, J.F. Wang∗, G.M. Lalev, Y.-G. Park, D. Shindo, M. IsshikiInstitute of Multidisciplinary Research for Advanced Materials, Tohoku University, 1-1, Katahira 2-chome, Aobaku, Sendai 980-8577, Japan

Received 10 June 2002; received in revised form 18 July 2002; accepted 12 August 2002

Abstract

A higher quality and a nearly stoichometric composition of Cd1−yZnyTe (y = 0.04) epilayers have been successfully grown on a GaAssubstrate by hot-wall epitaxy (HWE). The growth conditions regarding preheating treatment and Cd reservoir temperature were optimized.The relationship between quality and thickness was examined by four-crystal X-ray rocking curves and the best value of 120 arcsec for fullwidth at half maximum (FWHM) was obtained. The dislocations on the interface, generated from the difference in lattice constants, weredirectly observed by high-resolution electron microscopy (HREM). The variation of strain with epilayer thickness shows that the densityof extended defects in the epilayer decreases rapidly increasing the thickness up to 5�m. When the epilayer thickness reaches 20�m, thestrain almost becomes zero. This result suggests that the high-quality epilayer, same as the bulk crystal, can be obtained by increasing thethickness. Photoluminescence (PL) spectra at 4.2 K show that bound-exciton (BE) emission is dominative. The strain relaxation by misfitdislocations were also explored by HREM.© 2002 Elsevier Science B.V. All rights reserved.

Keywords: CdZnTe; Hot-wall epitaxy; Heteroepitaxy; Interface; Defects

1. Introduction

CdZnTe (CZT) is an important material for the develop-ment of far-infrared and radiation detectors[1,2]. CZT isalso used as a substrate for growing HgCdTe (MCT) andHgCdZnTe (MCZT) epilayers. Particularly, Cd1−yZnyTe(y = 0.04) is very attractive since its band gap correspondsto a wave length of about 10�m [3] and the lattice constantis near to that of Hg1−xCdxTe (x = 0.2). For this reason,a CZT epilayer on GaAs has been focused as an interest-ing one. However, such a heteroepitaxy is a problematic,because a high density of defects is introduced as a resultfrom the large lattice mismatch and the difference in thethermal expansion coefficients.

In our previous study[4], the CZT epilayer on a GaAssubstrate was grown, using the same hot-wall epitaxy (HWE)apparatus as in the present study, but the examination ofthe preheating temperature of the substrate before growingthe epilayers was ignored. Since the quality of the epitaxiallayers strongly depends on this parameter[5], it is importantto investigate the effect of preheating condition on the grownepilayer.

∗ Corresponding author. Tel.:+81-22-217-5139; fax:+81-22-217-5139.E-mail address: wang@tagen.tohoku.ac.jp (J.F. Wang).

In this paper, in order to grow high-quality Cd0.96Zn0.04Teepilayers on GaAs substrates by HWE, the effect of pre-heating temperature is firstly examined. Then other growthparameters as Cd reservoir and source temperature, compo-sition (y) and crystalline quality are examined again. As aresult, the CZT epilayers, grown under the new optimumgrowth conditions, show the better quality than in our pre-vious experiments.

2. Experimental procedure

The CZT epilayers were grown by HWE with a Cdreservoir. To provide Cd, Zn and Te2 vapors for thegrowing CZT epilayer, a high-purity Cd1−yZnyTe sourcewith y = 0.2 was synthesized by the vertical Bridgmanmethod, using starting materials of 6N–Zn and 6N–Cd,purified by vacuum distillation and overlap zone melting[6], and 6N–Te. The semi-insulated (1 0 0) GaAs substratewas cleaned before chemical etching by an ultrasonicallycleaner with trichloroethylene, acetone, and ethanol in se-quence. The cleaned GaAs substrate was chemically etchedwith (3H2SO4:H2O2:H2O) at 60◦C for 90 s and, finally,the substrate was rinsed with deionzed water. After dryingwith N2 gas, the substrate was put in the holder of the

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582 B.J. Kim et al. / Materials Chemistry and Physics 80 (2003) 581–585

HWE apparatus. Prior to the growth, the GaAs substratewas preheated for removing the oxide layer and possibleexisting trace impurities on the substrate surface. The pre-heating temperature was examined in the range 823–913 K.During the epilayer growth, the vacuum was maintained at2.7 × 10−4 Pa.

The grown epilayers were characterized by several tech-niques. A surface profiler (ULVAC, DEKTAK3ST) wasused to measure the epilayer thickness. Electron probe mi-croanalysis (EPMA) with a wavelength-dispersive X-rayspectroscopic mode was employed to determine the Zncomposition (y). The four-crystal rocking curves and pho-toluminescence (PL) spectra were used to estimate thecrystalline quality. The PL spectra were measured at 4.2 K,using the SGH-532 nm line of an Nd:YAG (12 mW) as anexciton light source.

3. Results and discussion

Since the preheating temperature has a great influenceupon the quality of the epilayer[5], the optimum preheatingcondition should be examined. The experiments were startedwith the original growth conditions reported in[4].

Fig. 1 shows the relationship between the full width athalf maximum (FWHM) values of the CZT epilayer grownat different source and preheating temperatures. It is veryclear that the smallest FWHM value is at around 873 K forall of the examined three kinds of source temperatures. Thisimplied that the preheating temperature of 873 K providesthe best surface condition for growing CZT epilayers. Thisresult is different from the reported one in[4]. For this rea-son, the temperature of 873 K will be adopted hereafter.

Since the determined preheating temperature is differentfrom that in our previous experiments[4], the other growth

Fig. 1. Dependence of the FWHM from four-crystal rocking curves onthe preheating temperature.

Fig. 2. Source temperature dependence of the growth rate and composition.

conditions needed to be revised. As an important parameter,the source temperature influences not only the quality ofthe epilayer, but also its composition. This is because thegrowth rate plays a very important role for the quality ofthe epilayer, as well as the fact that Cd, Zn and Te possessdifferent vapor pressures at the same temperature. Anotherparameter that affects the composition (y) is the temperatureof the Cd reservoir.Fig. 2 shows the dependence of thegrowth rate and the composition upon source temperatureunder the three different reservoir temperatures. The growthrate increases exponentially with the source temperature. It issimilar to the temperature dependence reported in[4]. On theother hand, the composition increases with increasing of thesource temperature in the examined temperature range forthe sample, grown at the reservoir temperature of 458 K. Inthe case ofTres = 468 and 478 K, the composition increaseswith increasing of source temperature up toTsource= 763 K.After this temperature, the composition tends to be constant.This result is quite a bit different from our previous results[4].

Fig. 3shows the FWHM values dependence on the sourcetemperature for CZT epilayers, grown at three differentreservoir temperatures. In this case, the samples’ thicknessis roughly the same. From these results, similar to our pre-vious study[4], it could be determined that the optimumsource temperature is 763 K. These samples show the exactconcentration (y = 0.04), which is desired, in the presentwork.

Since the substrate temperature is the same as in our pre-vious study, i.e. 643 K, we can summarize the optimum con-ditions for growing Cd1−yZnyTe (y = 0.04) epilayer asfollowing: temperature of preheating, substrate, source andreservoir being 873, 643, 763 and 468 K, respectively.

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Fig. 3. Dependence of the FWHM on the source temperature.

The Cd1−yZnyTe (y = 0.04) epilayers with 0.3–35�min thickness were grown on (1 0 0) GaAs substrates underthe above-determined optimum growth conditions to inves-tigate the effect of the epilayer thickness on the crystallinequality. Fig. 4 shows the dependence of the FWHM valueson the CZT epilayer thickness, in comparison with our pre-vious results[4]. In both cases, the FWHM values abruptlydecrease with the increase of thickness up to about 5�m.These results indicate that the biaxial compressive stress inthe epilayers, grown on GaAs substrates, is significantly re-laxed with increasing the thickness. When the thickness ofepilayer becomes larger than 5�m, the FWHM values grad-ually decrease. On the other hand, the FWHM values of ourprevious results are higher than in the present ones for therange from 5 to 12�m of the epilayer thickness. The resultcan be interpreted by adopting a suitable preheating temper-

Fig. 4. Dependence of the FWHM on the epilayer thickness. The insertis a typical rocking curve of a CZT epilayer.

Fig. 5. Dependence of the FWHM on the epilayer thickness by diffractioncurves measured inω andω-2θ scan.

ature. On the whole, the crystallinity is better in the presentresults than in the past. The smallest value of 120 arcsec isfound at 21�m epilayer thickness.

In order to investigate further the crystallinity of the CZTepilayer, rocking curves were measured by four-crystalX-ray diffraction in theω scan andω-2θ scan mode.Fig. 5shows the dependence of the FWHM values on the epilayerthickness. The white and black dots indicate the FWHMvalues of theω andω-2θ scan, respectively. In the case ofthe ω scan mode, the FWHM values decrease very steeplyas the epilayer thickness increases up to 5�m. For epilayersthicker than 5�m, it becomes an almost constant value. Inthe case of theω-2θ scan mode, for epilayers thinner than5�m, the FWHM values decrease very slowly, but above5�m, they maintain again an almost constant value. Thisresult indicates that the lattice mosicity (lattice scattering)in thinner epilayers is more remarkable than in thickerepilayers.

The difference in the thermal expansion parameters be-tween the epilayer and substrate can generate a lattice strain.Therefore, it is necessary to estimate this strain. It can be de-termined by measuring the change in the lattice parameters.In order to evaluate the lattice parameters of the CdZnTe epi-layer and GaAs substrate, theω-2θ scan of CdZnTe/GaAswas analyzed. The lattice parameter of the (4 0 0) CdZnTeepilayer is determined to be larger than that of bulk CdZnTe(a0 = 0.6468 nm). Because the thermal expansion coeffi-cient (5.4×10−6 K−1) of Cd0.96Zn0.04Te is smaller than thatof GaAs (6.8 × 10−6 K−1), CZT epitaxial films are suffer-ing from two-dimensional compressive stress. The strain ina perpendicular direction can be obtained by the followingequation:

ε = −(

C11

2C12

) (a⊥ − a0

a0

)(1)

where Cij are the elastic stiffness constants (C11 =55.6 GPa,C12 = 37.1 GPa[7]), a⊥ the lattice parameter

584 B.J. Kim et al. / Materials Chemistry and Physics 80 (2003) 581–585

Fig. 6. Dependence of strain on CZT epilayer thickness.

perpendicular to the hetero-interface obtained fromfour-crystal X-ray diffraction, anda0 is the lattice constantof bulk Cd0.96Zn0.04Te. Fig. 6 shows the relationship be-tween the strain and the thickness of the Cd0.96Zn0.04Teepilayer. With increasing thickness, the compression straindecreases abruptly below 5�m, and then slowly decreasesabove 5�m. Therefore, for films thicker than 20�m, thecompression strain becomes almost zero.

A high-resolution electron microscopy (HREM) imageof the interface between Cd0.96Zn0.04Te epilayer and GaAssubstrate was obtained by the incident electron beams par-allel to the [1 1 0] direction (Fig. 7). It illustrates a part ofthe digitized lattice image. This HREM image is a typicalrepresentation of all interfaces of the epitaxial layers. Theinterface appears well-defined without the presence of ox-ides or foreign layers over the examined area. The arrowsin Fig. 7 indicate the misfit dislocations due to the latticemismatch. An array of misfit dislocations with 60◦ angletoward the interface is responsible for the relaxation of theelastic strain. These dislocations disappeared with increas-ing epilayer thickness. Since the dislocations was generatedonly on the interface between CZT and GaAs, the critical

Fig. 7. A high-resolution electron microscopy image of a Cd0.96Zn0.04Te epilayer on a GaAs substrate.

Fig. 8. PL spectra (4.2 K) of CZT epilayers with different thicknesses.

thickness of the ZnTe epilayer can be thought to be nearzero. This shows that the epilayer is very poor in quality atthe beginning of its growth. On the other hand, these dis-locations have an average spacing of about 3.53 nm. If thelattice parameter of Cd1−yZnyTe can be described using thefollowing relationship[8]:

a = (0.6484− 0.0381y) (nm) (2)

the lattice parameter of Cd0.96Zn0.04Te is calculated to be0.6469 nm. This is consistent with our experimental result

B.J. Kim et al. / Materials Chemistry and Physics 80 (2003) 581–585 585

(0.6468 nm). Therefore, the mismatch is 14.4% betweenCd0.96Zn0.04Te and GaAs (0.5653 nm). But our experimen-tal result, estimated from the HREM image, shows that itis about 17.6%. This difference is thought to be due to thesmall measuring range.

Fig. 8shows the PL spectra of ZnTe epilayers for variousthicknesses in the exciton emission region. Compared to ourprevious results, the bound-exciton (BE) emission is strongerand sharp, and the deep emission, related to the defects, isweak. This result confirms that the quality of the epilayersis improved in the present experiment.

4. Conclusions

A detailed study on growing Cd0.96Zn0.04Te epilayers onGaAs substrate was carried out, and the optimum growthconditions for preparing high-quality Cd0.96Zn0.04Te epilay-ers were determined. The growth of high-quality and nearlystoichometric Cd0.96Zn0.04Te epilayers has been achievedusing a CZT source withy = 0.2 and a Cd reservoir. Itwas found that the epilayer quality strongly depends on thepreheating and reservoir temperatures. Moreover, the effectof the CZT epilayer thickness on the crystalline quality hasalso been investigated. Summarized results show that theFWHM value abruptly decreases with increasing thicknessfrom 0.3 to 5�m, and the smallest FWHM value of 120arcsec is obtained at 20�m epilayer thickness. The relax-ation of the misfit is estimated from the distance between

equivalent (1 0 0) planes at the Cd0.96Zn0.04Te/GaAs inter-face. The average spacing of dislocations is determined tobe 3.53 nm. This value shows that many dislocations weregenerated on the interface and contributed for relaxing thecompressive strain.

Acknowledgements

This work was performed under the inter-universitycooperative research program of the Institute for MaterialsResearch, Tohoku University.

References

[1] P.W. Kruse, in: R.K. Willardson, A.C. Beer (Eds.), Semiconductorsand Semimetals, vol. 18, Academic Press, New York, 1981, p. 1.

[2] K. Zannio, in: R.K. Willardson, A.C. Beer (Eds.), Semiconductorsand Semimetals, vol. 13, Academic Press, New York, 1978, p. 164.

[3] D.D. Edwall, E.R. Gertner, E. Tennant, J. Appl. Phys. 55 (1984)1453.

[4] B.H. Koo, J. Wang, Y. Ishikawa, M. Isshiki, Jpn. J. Appl. Phys. 37(1998) 5674.

[5] S.N. Nam, J.K. Rhee, K.S. Lee, Y.D. Choi, G.N. Jeon, C.H. Lee, J.Crystal Growth 180 (1997) 47.

[6] Y. Ishikawa, B. Yang, K. Mimura, T. Tomizono, M. Isshiki, J. MiningMater. Process. Inst. Jpn. 110 (1994) 1175.

[7] N.H. Karam, R. Sudharsanan, A. Mastrovito, M.M. Sanfacon, F.T.J.Smith, M. Leonard, N.A. El-Masry, J. Electron. Mater. 24 (1995) 483.

[8] A. Ebina, J. Crystal Growth 59 (1982) 51.