Structural properties of epitaxial layers of CdTe, ZnCdTe and HgCdTe

Structural properties of epitaxial layers of CdTe, ZnCdTe and HgCdTe

Thin Solid Films, 131 (1985) 267-278 PREPARATION 267 AND CHARACTERIZATION STRUCTURAL PROPERTIES ZnCdTe AND HgCdTe* OF EPITAXIAL LAYERS OF CdTe, ...

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Thin Solid Films, 131 (1985) 267-278 PREPARATION

267

AND CHARACTERIZATION

STRUCTURAL PROPERTIES ZnCdTe AND HgCdTe*

OF EPITAXIAL

LAYERS OF CdTe,

J. H. DINAN U.S. Army Night Vision and Electra-Optics Laboratory, (U.S.A.)

DELNV-IRTD,

Fort Belvoir, VA 22060

S. B. QADRIT Naval Research Laboratory, Washington, DC 20375 (U.S.A.) (Received

August

20,1984;

revised January

24,198s;

accepted

May 8, 1985)

Structural properties of CdTe, Zn,Cd, _,Te and Hg, _,Cd,Te epitaxial layers and of interfaces between them were measured using a variety of X-ray and surface analytical techniques. CdTe and Zn,Cd, _,Te layers were grown by the technique of molecular beam epitaxy. Hg, _,Cd,Te layers were grown by several liquid and vapor phase techniques. CdTe and Zn,Cd, _,Te layers were single phase and did not contain the low angle grains commonly observed in bulk wafers. Compositional transition regions at the interfaces between layers grown by molecular beam epitaxy and their substrates were abrupt.

1.

INTRODUCTION

Hg, _,Cd,Te has become the preferred semiconductor material for applications which require a direct band gap energy near 0.1 eV. It has long been available in the form of single-crystal wafers. Recently, several applications have emerged which require epitaxial layers; some of the most interesting of these are best satisfied by a multilayer epitaxial technology. As a step toward the development of such a technology we have explored the growth of multilayers which are compatible with HgCdTe. CdTe, because of its ease of growth and favorable electronic properties, has been used extensively as a substrate’ and occasionally as an overlayer2. However, the lattice constant misfit3 at room temperature between CdTe and ternary compositions near Hg,,,Cde.,Te is 3 x 10m3. It has been demonstrated4 for a related material system that a misfit of 4 x 10e3 has an adverse effect on heterojunction device performance. Basson and Booyens have suggested criteria for choosing a lattice-matched material for HgCdTe. The optical and electronic properties of Zn,Cd, _,Te make it an attractive candidate. The choice of a growth method is an important one. A prime consideration is *Paper presented at the Sixth International Conference on Thin Films, Stockholm, 13-17, 1984. 7 Permanent address: Sachs/Freeman Associates, Bowie, MD, U.S.A. 0040-6090/85/$3.30

0 Elsevier Sequoia/Printed

Sweden,

August

in The Netherlands

268

J.

H. DINAN,

S. B. QADRI

the need to maintain the lowest possible substrate temperature to minimize diffusion effects. In the case of III-V compounds, the technique of molecular beam epitaxy (MBE) has been shown to allow the use of temperatures significantly lower than those of conventional techniques. There are grounds for optimism that this situation will also hold for II-VI compounds6. We used MBE for the study reported here. In this paper we report MBE growth and characterization of epitaxial layers of CdTe and Zn,Cd, _,Te and characterization of Hg,-,Cd,Te grown by other techniques. All three materials are available commercially in the form of wafers cut from boules grown by a Bridgman technique. We compared the quality of epitaxial layers with that of wafers and show that, in some instances, epitaxial layers are superior. We also measured properties of the interfaces between layers and substrates. We show that the MBE technique is capable of producing chemical transition regions more abrupt than those reported for other growth techniques. 2.

EXPERIMENTAL

METHOD

Layers were grown in a Varian 360 MBE system equipped with a substrate preparation chamber. A single furnace containing polycrystalline CdTe was used to grow CdTe layers’. The temperature of the furnace was chosen to provide a growth rate of 0.14 nm s- ‘. In keeping with the vapor pressure considerations put forth by Smitha, one additional furnace containing elemental zinc was used for Zn,Cd, _,Te growths. The composition of the ternary compound was varied by adjusting the temperature of the zinc furnace. Layers were typically grown to a thickness of 1000 nm. High quality epitaxy was possible only on substrates whose surfaces were properly prepared. Peaks characteristic of moderate concentrations of carbon and oxygen were evident in Auger spectra of surfaces as installed in the preparation chamber. Epitaxial growth did not occur on these surfaces. Heating, even to temperatures at which evaporation of substrate species was significant, did not reduce these concentrations. Argon ion sputtering was used to reduce the concentrations of these impurities to below the threshold sensitivity of the Auger apparatus. The sputtering beam was incident at an angle of 24” to the plane of the sample surface. Auger signals were multiplexed so that variations in concentrations of up to six species could be continuously monitored. A 10 kV electron gun was available in the growth chamber to allow display of reflection high energy electron diffraction (RHEED) patterns for surfaces before and during growth. Lattice constants of substrates and epitaxial layers were determined using a copper X-ray source in conjunction with a conventional X-ray diffractometer. The primary index of crystal quality was taken to be the full width at half-maximum (FWHM) of the Bragg reflection of Cu Kor, radiation measured with a doublecrystal X-ray diffractometer9,‘0. The Darwin-Prinz theory of reflection from a perfect crystal can be used to predict values of the FWHM for comparison with experimental values. Values calculated for major reflections of crystals of interest here are listed in Table I.

STRUCTURAL

PROPERTIESOF CdTe,ZnCdTe,HgCdTe

269

TABLE1 VALUES

OF THE FULL

WIDTH

AT HALF-MAXIMUM

CALCULATED

USING

THE DARWIN-PRINZ

Materid

Reflection

FWHM

InSb

220 333 400 220 333 400 333

32 23 9 32 23 9 24

CdTe

Hg, _,Cd,Te

FORMULA=

(seconds of arc)

a Here it was assumed that a sample was irradiated with the Ku, component of a {400} reflection from an InSb “first crystal” which was itself illuminated with X-rays from a copper target. Values of 1.5405 h and 5.811 x lo-“ 8, were used for the wavelength and spectral width respectively of the Cu Ku, component.

The photoluminescence response of substrates and layers at a temperature of 4.2 K was also measured. Luminescence was stimulated with 3 x lo4 W m-’ of 476.5 nm radiation from an argon ion laser. Chemical depth profiles were obtained using an Auger spectrometer in conjunction with an argon ion sputtering gun. Instrumental resolution was estimated to be of the order of 10 nm. 3. RESULTS 3.1. CdTejInSb Several researchers have pointed out the advantages of InSb{lOO} as a substrate for CdTe and have demonstrated high quality epitaxial growth7,‘0~12. We have reproduced these results and have extended them to other crystallographic orientations. { lOO}-oriented InSb wafers were chemically polished in a 25:4:1 solution of lactic, nitric and hydrofluoric acids. The sputtering procedure described above was applied to InSb. Auger spectra at various stages of preparation are shown in Fig. 1. A RHEED pattern, rocking curve and photoluminescence spectrum of a CdTe layer grown at 212 “C to a thickness of 1000 nm are shown in Fig. 2. The value of 24” for the FWHM of the CdTe peak in the rocking curve is within a factor of 3 of the DarwinPrinz value. The photoluminescence spectrum is dominated by near-band-edge emission. RHEED patterns and rocking curves of CdTe layers 1OOOnm thick grown epitaxially on (11 l)A- and { 1 lO}-oriented InSb wafers are shown in Fig. 3. These data indicate that single-crystal epitaxy is possible on three faces of technological importance. For comparison, rocking curves for four wafers of CdTe obtained commercially are shown in Fig. 4. Multiple peaks are probably due to the presence of low angle grains. All had FWHM values significantly greater than those for MBE layers. Identical effects have been reported by Farrow13.

In

In

(4

(‘3

0

Fig. 2. (a) RHEED

pattern,(b)

oo~&

/

OXYGEN

stages

W

(4

curve and (c) photoluminescence

InSb wafer at various

Cd-

Qm

X-ray rocking

Fig. 1. Auger spectra for a (lOO)-oriented and (d) for a CdTe layer 1 pm thick.

w

(4

V

I4

c

Sb

-

24

w

sputtering

and (c) after sputtering)

PHOTON ENERGY (sV)

grown by MBE.

(b) during structure

((a) as installed,

-

for a CdTe{ lOO/‘InSb

preparation spectrum

of surface

-

-ANGLE

19-

Y

8 u ,”

F

F

0

STRUCTURAL

PROPERTIES

OF

CdTcZnCdTe,HgCdTe

271

212

J. H. DINAN,

S. B. QADRI

3.2. CdTelHgCdTe Samples of HgCdTe were obtained commercially and from sources internal to the Night Vision and Electra-Optics Laboratory. Compositions were nominally Hg,,,Cd,,,Te. Samples were in the form of (11 l)A-oriented wafers cut from boules or of epitaxial layers grown on (11 l)A-oriented CdTe substrates by liquid phase epitaxy (LPE) or by close-spaced vapor phase epitaxy (CSVPE). All of these were used as “substrates” for heteroepitaxial growth of CdTe by MBE. X-ray rocking curves for several HgCdTe samples are shown in Fig. 5. The FWHM values for the best LPE layers are within a factor of 2 of the Darwin-Prinz values and are less than those for HgCdTe wafers. Rocking curves for CSVPE layers were quite broad even though RHEED patterns were indicative of excellent quality single crystals and were among the best observed in this study. The rocking curve width could be explained if layers were compositionally graded either perpendicular to or parallel to the surface normal. Separate studiesI have shown that the composition and therefore the lattice constant of ternary layers grown by the CSVPE technique are graded in a directional normal to the surface.

(4

(4

Fig. 5. X-ray rockingcurves for four(l1 l)A-oriented (c) CSVPE HgCdTe(l1 l)A; (d) wafer of HgCdTe.

Hg,.,Cd,,,Te

samples: (a),(b) LPE HgCdTe(l1

l)A;

Auger spectra at various states of surface preparation of an LPE-grown sample are shown in Fig. 6. The stoichiometry, as inferred from Auger spectra, of surface regions ofsputtered Hg,,,Cd,,,Te is identical to within experimental error with that expected for this composition. RHEED patterns were indicative of smooth highly ordered surfaces. Those from CSVPE layers had both narrower streaks and more highly developed Kikuchi lines than those from LPE layers. There was no improvement in these patterns when samples were annealed at temperatures as high as 180 “C.

STRUCTURAL

PROPERTIES

OF

CdTe, ZnCdTe,

HgCdTe

273

(b)

(a)

Cd-

Te Cd gg

tir;-(c)

-ANGLE

(d)

--)

Fig. 6. Auger spectra for a (11 l)A-oriented LPE-grown Hg,,,Cd,,,Te layer at vartous stages of surface preparation: (a) as installed;(b) during sputtering; (c)after sputtering;(d) after epitaxial growth of CdTe. Fig. 7. (400) reflections taken from an X-ray diffraction “substrate” consisting of an LPE-grown HgCdTe layer.

spectrum

of an MBE-grown

CdTe layer on a

A {333) reflection from an X-ray diffraction spectrum for a CdTe layer grown to a thickness of 1000 nm at a temperature of 200 “C on a (11 l)A-oriented LPE layer is shown in Fig. 7. The fact that Kcr, and Ka, reflections from both HgCdTe and CdTe are resolved is indicative of high structural quality. Values of 0.6490 nm and 0.6463 nm for the lattice constants of CdTe and HgCdTe respectively were obtained from these data. Additional characteristics of a CdTe(MBE)/HgCdTe(LPE) structure are shown in Fig. 8. The RHEED pattern for the CdTe is indicative of smooth singlecrystal epitaxy. The FWHM of the CdTe rocking curve is significantly broader than that shown in Fig. 2. An explanation for this will be suggested in Section 3.3. The width of the compositional transition region indicated by the Auger depth profile is of the order of 10 nm, the resolution limit of our apparatus. For comparison, values between 300 and 3000 nm have been reported for CdTe/HgCdTe structures grown by LPE’5.‘6. 3.3. ZnCdTejInSb Growth of Zn,Cd,

_,Te layers on InSb substrates

by vacuum

evaporation

of

274

J. H. DINAN,

S. B. QADRI

(b)

(a)

CdTs

d

-ANGLE

-

\

(4

Fig. 8. (a) RHEED pattern,(b) Auger depth profile and (c)X-ray rocking layer on a “substrate” consisting of an LPE-grown HgCdTe layer.

curve for an MBE-grown

CdTe

the ternary compound has been reported”~‘a but few structural characteristics are given. To our knowledge, growth of ZnCdTe by MBE has not been reported. We used { lOO}-oriented InSb wafers to demonstrate MBE of ZnCdTe. The ratio of the fluxes of zinc and cadmium was varied by adjusting the temperature of the zinc furnace. Auger spectra of the resulting layers were used to confirm the incorporation of zinc. Spectra for several ternary compositions are shown in Fig. 9. In all cases, increases in the zinc peak height were accompanied by decreases in the cadmium peak height. This was taken as evidence of substitutional incorporation of zinc into the lattice and it allowed estimates to be made of the composition of the ternary compound by linear extrapolation of the binary end points. Characteristics of a ternary layer grown at 212 “C with x = 0.1 are shown in Fig. 10. It should be noted that the epitaxial layer peak is considerably broader than that of the substrate and that of the CdTe layer shown in Fig. 2. Changes in composition were reflected in changes in lattice constant. (400) reflections from X-ray diffraction spectra for ternary compounds of several compositions are shown in Fig. 11. As the x value increases, the lattice constant decreases. Compositions whose lattice constants varied from that of CdTe to close

STRUCTURAL

PROPERTIES OF CdTe,ZnCdTe,HgCdTe

*-----------;

275

q___________;

Cd To

(4

Fig.9. Auger Zn,.,Cd,.,Te;

04

spectra

for ZnCdTe

MBE

layers

of various

compositions:

(a) Zn,,,Cd,,,Te;

(c) Zn,.,Cd,.,Te.

0 .

alcure

bAahe+ --IIyyL 4momxomeoooo cd Fig. 10. Characteristics

of a { lOO}-oriented

ZnCdTe/InSb

structure.

(b)

J. H. DINAN,

276

n

A

S. B. QADRI

rwi .: ::

I\

;: ii

:i

/

I1 \\

.:: .%=....

. ,,::i: ,q 1:\- i:: 5 i:::.: ::: ;::

\

; ::

\_A



-PNGl.EFig. 11. {400) reflections on InSb.

taken from X-ray diffraction

spectra for ZnCdTe

of three compositions

grown

to the value for ZnTe were grown. No miscibility gap is evident for ZnCdTe grown by MBE. It should be noted that the shapes of the diffraction peaks change as the x value increases. For low values, the Ka, and Kcr, reflections for substrate and epitaxial layer are resolved. For high values, the width of the peak increases and the reflections are no longer resolved. RHEED patterns also change as the composition changes. For low x values, the patterns all resembled that shown in Fig. 10. As the x value increased, streaks became rings and then a uniform glow. Rocking curves for low and high x values are shown in Fig. 12. The FWHM values for x values below about 0.04 are comparable with that for CdTe/InSb. For higher x values, the peaks become considerably broader.

-ANGLEFig. 12. X-ray rocking curves for ZnCdTe of two compositions grown on InSb{lOO}: ---, curve characteristic of layers whose composition results in low misfit to the substrate;---, curve characteristic of moderate values of misfit.

STRUCTURAL

PROPERTIES

CdTe, ZnCdTe,

OF

HgCdTe

277

We suggest that RHEED, X-ray diffraction and rocking curve data can be explained on the basis of a combination of strain and dislocations induced in the epitaxial layer by misfit between the layer and the substrateig*“. For comparison, a rocking curve for a ZnCdTe wafer is shown in Fig. 13. Multiple peaks are probably caused by low angle grains within the area of 1 mm x 6 mm covered by the X-ray beam. 32 SEC-

-

-ANGlEFig. 13. X-ray rocking

curve for a (11 l)A-oriented

ZnCdTe

wafer.

4. CONCLUSIONS We have shown that both CdTe and ZnCdTe layers of high structural quality can be grown by MBE on substrates of several orientations. Structural quality is a strong function of the quality of the substrate and of the misfit. In most cases, the quality of CdTe, Zn,Cd, _,Te and Hg, _,Cd,Te epitaxial layers is superior to that of wafers. This quality is present in layers as thin as 1 urn. A hybrid growth technology for heteroepitaxial layers lattice matched to HgCdTe has been demonstrated. The electronic characteristics of these junctions are being investigated. ACKNOWLEDGMENTS

We thank J. Bratton for making the photoluminescence Wilson for providing CSVPE HgCdTe.

measurements

and H.

REFERENCES 1 2 3 4

T. C. Harman, J. Electron. Mater., 8(1979) 191. P. Migliorato, R. F. C. Farrow, A. B. Dean and G. M. Williams, InfrraredPhys., 22 (1981) 331. J. H. van der Merwe, in M. H. Francombe and H. Sato (ed.), Single Crystal Films, Pergamon, Oxford, 1964, p. 139. S. Rotter, D. Kasemset and C. G. Fonstad, IEEE Electron Devices Lett., 3 (1982) 66.

278

5 6 7 8 9 10 11 I2 13 14 15 16 17 18 19 20

J. H. DINAN, S. B. QADRI

J. H. Basson and H. Booyens, Phys. Status Solidi A, 80 (1983) 663. J. P. Faurie, Proc. 1984 U.S. Workshop on the Physics and Chemistry of Mercury Cadmium TeNuride, San Diego, CA, May 15-I 7,1984, in J. Vat. Sri. Technol. A, 3 (1) (1985) 55. R. F. C. Farrow, G. R. Jones, G. M. Williams and I. M. Young, Appl. Phys. Lett., 39 (1981) 956. D. L. Smith, in B. R. Pamplin (ed.), Molecular Beam Epitaxy, Pergamon, Oxford, 1980, p. 33. J. C. Bobb, H. Holloway, K. H. Maxwell and E. Zimmerman, J. Appl. Phys., 37(1966) 3909. K. Kankamura, M. Hikita, H. Asano and A. Terada, Jpn. J. Appl. Phys., 21(1982) 665. T. H. Myers, Y. Lo, J. L. Schetzina and S. R. Jost, J. Appl. Phys., 53 (1982) 9233. K. Sugiyama, Thin Solid Films, 1 I5 (1984) 97. R. F. C. Farrow, Proc. 1984 U.S. Workshop on the Physics and Chemistry of Mercury Cadmium Telluride, San Diego, CA, May IS-1 7. 1984, in J. Vat. Sci. Technol. A, 3 (1) (1985) 60. J. H. Pollard, personal communication, 1984. M. Chu and C. C. Wang, J. Appl. Phys., 51(1980) 2255. J. L. Schmit and J. E. Bowers, Appl. Phys. Lett., 35 (1979) 457. S. I. Radautsan, 0. V. Kulivkova and I. V. Varlamov, Izv. Akad. Nauk S.S.S.R., Neorg. Mater., 19 (1983) 366. 0. V. Kulikova, 0. H. Haksimova and M. M. Markus, Izv. Akad. Nauk Mold S.S.R., 3 (1982) 61. I. Samaras, L. Papadimitriou, J. Stoemenos and N. A. Economou, Thin SolidFilms, 115(1984) 141. J. H. Dinan and S. B. Qadri, 2nd Int. Conf. on II-VI Compounds, Aussois, France, March 1985.