The organometallic chemical vapour deposition of ZnS and ZnSe at atmospheric pressure

148

Journal of Crystal Growth 59 (1982) 148-154 North-Holland Publishing Company

T H E O R G A N O M E T A L L I C C H E M I C A L VAPOUR D E P O S I T I O N OF ZnS AND ZnSe AT ATMOSPHERIC PRESSURE P.J. W R I G H T and B. C O C K A Y N E Royal Signals and Radar Establishment, St. Andrews Road, Great Malvern, Worcs. WR14 3PS, UK

It is shown that thin film single crystal layers of ZnS, ZnSe and Z n S x S e 1 - x can be grown on to a variety of substrates by direct reaction at atmospheric pressure, of dimethyl zinc, hydrogen sulphide and/or hydrogen selenide, using hydrogen as the carrier gas. Growth has been observedbetween 350 and 750"C at growth rates within the range 0.5 to 10/xm h-1. The layers exhibit a uniformly high standard of surface morphologyand perfection.

1. Introduction The organometallic chemical vapour deposition growth technique, offers a potential way for producing epitaxial layers of many I I I - V and I I - V I compounds of interest in optoelectronics. Thus far the major interests have centred around I I I - V semiconductors [1] and cadmium mercury telluride [2], although ZnSe and ZnSxSe I _x have been grown by organometallic chemical vapour deposition in a vertical reactor [3] and at low pressures [4]. The present paper reports the growth of thin films of ZnS and ZnSe using an alternative approach to this technique in which a horizontal reactor operating at atmospheric pressure is employed. It is shown that such a system obviates some of the difficulties associated with a low pressure process and allows either single crystal or polycrystalline layers to be grown respectively on single crystal or amorphous substrates in a controlled manner. The relative merits of several different substrates are compared and the chemical and structural characteristics of the layers are also discussed.

2. The growth apparatus The apparatus used for the growth of ZnS, ZnSe and alloys thereof is shown schematically in fig. 1. It is a development of the equipment designed by Bass [5] for the growth of I I I - V corn-

pounds at atmospheric pressure by organometallic CVD. The reactor is mounted horizontally and consists of a water-cooled silica envelope containing a SiC coated graphite substrate pedestal which is heated by high frequency induction. The reactor can be evacuated to normal rotary pump pressures so that the graphite can be baked at high temperature ( ~ 1100°C) in vacuum prior to use. The reactant gases are controlled by mass flow controllers and conveyed to the reactor through a system manufactured entirely from stainless steel. The basic reactants, namely dimethyl zinc, hydrogen sulphide and hydrogen selenide were supplied respectively by Alfa products and BOC Special Gases Division. Dimethylzinc (DMZ) is a volatile liquid (Bpt. 46°C) and has to be cooled to between - 5 and - 1 0 ° C in order to obtain an appropriate vapour concentration. In this instance, the necessary cooling was provided by a circulatory system supplied by Grant Instruments, Cambridge, employing a methylated spirit/water mixture as coolant. The H2S and H2Se were supplied in the form of 5% mixtures in high purity hydrogen. The carrier gas used to transport the reactants was hydrogen, purified by passage through a standard Johnson-Matthey palladium diffuser. A critical feature of the gas mixing system is the inlet nozzle for the alkyl plus hydrogen Stream. This consists of a short silica tube positioned to prevent premature mixing with the H2S and H2Se in the narrow inlet tube to the reactor. Careful position-

0022-0248/82/0000-0000/$02.75 © 1982 North-Holland

P.J. Wright, B. Cockayne / OM-CVD of ZnS and ZnSe at atmospheric pressure WATER COOLING

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INDUCTION

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ing of this tube inhibits undesirable homogeneous gas phase reactions upstream from the susceptor. If this inlet tube projects too far into the reactor, much of the deposition occurs ~downstream from the susceptor and results in low growth rates on the substrate.

3. Substrate preparation All the substrates were cut as slices from single crystals grown by the Czochralski technique, followed by standard chemical and mechanical polishing to produce a mirror finish. The present reactor has no facility for in-situ cleaning of the substrates prior to growth and this leads to problems with silicon. However, GaAs, GaP and Ge were all successfully cleaned prior to insertion into the reactor. For GaAs, the cleaning method used by Stutius [6] was successful but a simpler 3 stage procedure was adopted: (1) The substrate was boiled in a detergent solution (Decon 75 supplied by B D H Ltd.) and then thoroughly washed in deionised water. (2) The substrate was etched for 2 min at 40°C in

a 5 : 1 : 1 solution of H 2 S O 4 ; H202 : H 2 0 and again thoroughly washed in deionised water. (3) The substrate was then blown dry from boiling propan-2-ol. A similar procedure was used for Ge except that in stage 2, the sulphuric acid-peroxide etch was replaced by a 3 : 5 : 3 solution of H F : H N O 3 : CH3COOH. For GaP the stage 2 procedure was replaced by a 30 s etch in aqua regia.

4. Growth of ZnSe The basic reaction used to produce ZnSe is (CH3)2Zn + n 2 S e -* ZnSe + 2 CH4,

(1)

but because dimethyl zinc and H2Se can react at room temperature to give an unwanted homogeneous gas phase reaction, low concentrations of the reactants, typically 5 × 10-5 mole fraction dimethyl zinc and 2 × 10 -4 mole fraction H2Se, are required together with a high flow rate of 4.5 1 m i n - l for the hydrogen carrier gas. These conditions coupled with the correct positioning of the inlet nozzle described earlier, allow the reaction to

150

P.J. Wright, B. Cockayne / OM-CVD of ZnS and ZnSe at atmospheric pressure

occur predominately at the susceptor and growth to take place on suitable substrates. GaAs (lattice constant 5.6535 ,~)and Ge (lattice constant 5.6577 ,~) single crystals provide a close lattice match to ZnSe (lattice constant 5.6686 ,~) and both of these have been used as substrates for the successful single crystal growth of ZnSe layers. Single crystal growth was found possible for H 2 S e : D M Z gas phase ratios between the limits 1:1 and 20:1 with most layers grown with the ratio 4: 1. The growth rate limiting reactant is the D M Z , since excess of this component with respect to H2Se induces polycrystallinity and a poor surface morphology. At the 4:1 ratio and a con-

stant temperature of 350°C, it was established that the growth rate for ZnSe could be varied from 0.5 to 10 /~m h - t by correspondingly adjusting the concentrations of reactants in the gas phase from 1.25 × 10 -5 mole fraction D M Z and 5 × 10 -5 mole fraction H2Se to 2.5 × 10 -4 mole fraction D M Z and 10 -3 mole fraction H2Se. Single crystal growth occurred at substrate temperatures within the range 350-750°C, although above 550°C growth rate decreased with increasing temperature in the manner attributed by Blaconnier et al. [3] to the re-evaporation of deposited surface species. Most of the layers were grown at 350°C, this being the lower limit for stable temperature control with the

Fig. 2. The surface morphology of epitaxial ZnSe layers grown on various substrates: (a) GaAs (100); (b) GaAs (100) orientated 3° towards a [111]; (c) GaAs (111)B; (d) Ge (100). Marker represents 70 ~m.

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all these substrate orientations except for the GaAs (111)A which produced a polycrystalline structure and a poor surface morphology characterised by large triangular shaped hillocks. The morphology of growth on the remaining orientations is shown in fig. 2 from which it is apparent that good surfaces are produced on the (100) and ( l l l ) B substrates. The major feature is an "orange peel" type surface with some hillocks. Layer thicknesses have ranged between 2000/~ and 20 #m. X-ray texture pattern studies of layers on GaAs (100), Ge (100) and GaAs ( l l l ) B showed an absence of major structural defects such as twinning despite the layer/substrate lattice mismatch of 0.25% for Z n S e / G a A s and 0.15% for Z n S e / G e . X-ray diffractometer scans of the (100) layers, shown in fig. 3, using Cu radiation show well resolved narrow peaks for both substrate and epitaxial layer, indicative of good epitaxial growth.

20

Fig. 3. X-ray diffraction profiles for the 400 reflection from ZnSe layers grown on: (a) GaAs (100); (b) Ge (100) substrates.

6 kW radio frequency generator employed. Growth was attempted on GaAs substrates using (100) surfaces, (100) surfaces orientated 3 ° towards a [111] direction, ( l l l ) B and ( l l l ) A surfaces. For the Ge substrates only (100) surfaces were used. Single crystal growth was possible using

5. Growth of ZnS

The basic reaction used to produce ZnS by organometallic CVD is analogous to eq. (1), namely (CH3)2Zn + H2S --, ZnS + 2

CH4,

(2)

and the growth of ZnS layers proved possible under almost identical conditions to those described earlier for ZnSe layers. The same flow rate

Fig. 4. The surface morphology of epitaxial ZnS layers on GaP substrates (a) (100) orientation; (b) (lll)B orientation. Marker represents 35 #m.

152

P.J. Wright, B. Cockayne / OM-CVD of ZnS and ZnSe at atmospheric pressure

for the H 2 carrier gas, 4.5 1 min-i, was employed and an H2S:DMZ ratio greater than unity was also required. Similar growth rates, between 0.5 and 10 /~m h -1, were produced by varying the concentration of the reactants in the carrier gas and the same temperature range of 350 to 750°C was also demonstrated to be usable with most of the layers again being grown at 350°C. The major differences were less evidence of homogeneous gas phase reaction, reflecting the greater thermal stability of H2S compared to H2Se and the need for different substrate materials to promote lattice matching. GaP (lattice constant 5.4505 A) and Si (lattice constant 5.4307 ,~) single crystal substrates provide a reasonable lattice match to ZnS (lattice constant 5.4093 .~) with respective lattice mismatches of 0.8% and 0.45%. However, the absence of an in-situ etching facility in the present apparatus prevented the total removal of the oxide from the silicon surface and led to the deposition of polycrystalline layers of ZnS but with a preferred orientation; on (100) Si slices the ZnS was cubic and grew with the [100] direction normal to the substrate surface. At a thickness greater than about 5/xm, such layers tended to crack and peel off from the substrate possibly as a result of thermal expansion mismatch. In contrast, growth of ZnS on both GaP (100) and (Ill)B surfaces generally yielded cubic single crystal layers with the good surface morphology shown in fig. 4.

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The lattice mismatch in growing either ZnS or ZnSe epitaxially onto the respective substrates described in earlier sections is small but finite. The growth of ZnS~Se~_ x offers the possibility of growing layers which are lattice matched to these substrates and therefore potentially free from residual strain. Fig. 5, in which Vegard's law is assumed, shows the alloy compositions which correspond to the respective lattice spacings of Ge, GaAs, Si and GaP. A major difficulty in the alloy growth is that the relationship between the HES:H2Se ratio in the gas phase and the S:Se ratio in the solid phase is

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Fig. 6. Composition of ZnSxSel_ x layers as a function of the gas phase ratio [H2S]/[H2S]+[H2Se]: ( A ) this work; ( O ) comparison points from low pressure work [6].

153

P.J. Wright, B. Cockayne / OM-CVD of ZnS and ZnSe at atmospheric pressure

non-linear. Consequently, the mass flows of H2S and H2Se have to be controlled very accurately, particularly where there is a large disparity in the gas phase concentrations required. The relationship between the gaseous and solid phases has been determined experimentally by growing alloy layers at various known gas phase compositions and determining the layer composition by EDAX. Layers at each composition were grown simultaneously on to both GaAs and GaP substrates and the mean composition used to plot the relationship established in fig. 6. Control of the gas phase ratio to achieve lattice matching with Ge and GaAs (x = 0.035 and 0.052 respectively) proved relatively simple because the gas phase concentrations of the two hydrides is relatively high. Precise lattice matching to GaP (x = 0.83) would prove more difficult because of the very low mass flow of HESe that would be required. The suitability of organometallic CVD for growing ZnSxSe 1-x alloys is demonstrated by fig. 7 which shows the surfaces of layers grown with x = 0 . 1 4 on to both GaP and GaAs (100) substrates. The morphology is very similar to that seen in the binary layers. The growth on GaP, fig. 7b, might have been expected to be inferior to that on GaAs, since the layer composition is much closer to ZnSe than ZnS. X-ray texture studies

a b a b

Fig. 8. A cross section obtained by angle lapping through a multilayer structure of ZnS and ZnSe grown on GaAs: (a) ZnS; (b) ZnSe. Marker represents 5/xm.

confirmed that both layers are single crystal. The uniformity of composition through the layer was confirmed by EDAX studies across a cleaved section. The versatility of the organometallic CVD techniques is demonstrated further by the growth of multilayer structures of ZnS and ZnSe. Fig. 8 shows

Fig. 7. The surface morphologyof ZnSxSe1_x (x =0.14) epitaxial layers grown on: (a) GaAs (100); (b) GaP (100). Marker represents 35 #m.

154

P.J. Wright, B. Cockayne / OM-CVD of ZnS and ZnSe at atmospheric pressure

a cross section through such a structure, obtained b y lapping at a shallow angle. The structure consists of alternate layers of ZnS and ZnSe grown on GaAs.

display technology. This aspect is discussed m o r e fully in a separate but related study [7].

Acknowledgements 7. Discussion and conclusions The present work clearly shows that single crystal thin films of ZnS, ZnSe and alloy mixtures of these two c o m p o u n d s can be grown by organometallic C V D at atmospheric pressures. G r o w t h is possible over the wide ranges of temperature and growth rate. Furthermore the films prod u c e d on single crystal substrates have g o o d surface morphology. The growth technique has a n u m b e r of i m p o r t a n t advantages. F o r instance, working at atmospheric pressure removes the need for a p u m p i n g system capable of handling the toxic and p y r o p h o r i c waste products. The need for only one temperature controlled hot zone is m u c h simpler than the multi-zone furnaces required in conventional chemical vapour transport processes. A notable feature is the low growth temperature used (350°C), which renders the technique compatible with the glass-based substrates used in

The authors also wish to thank O.D. Dosser for her assistance with the scanning electron microscopy. Copyright © Controller, H M S O , L o n d o n , 1982.

References [1] S.J. Bass and P.E. Oliver, in Proc. 6th Intern. Symp. on GaAs and Related Compounds, Edinburgh, 1976, Inst. Phys. Conf. Ser. 33b (Inst. Phys., London and Bristol, 1977) p.l. [2] J.B. Mullin, S.J.C. Irvine and D.J. Ashen, J. Crystal Growth 55 (1981) 92. [3] P. Blanconnier, M. Cerclet, P. Henoc and A.M. Jean-Louis, Thin Solid Films 55 (1978) 375. [4] W. Stutius, Appl. Phys. Letters 33 (1978) 656. [5] S.J. Bass, J. Crystal Growth 31 (1975) 172. [6] W. Stutius, J. Electron. Mater. 10 (1981) 95. [7] P.J. Wright, B. Cockayne, A.F. Cattell, P.J. Dean, A.D. Pitt and G.T. Blackmore, J. Crystal Growth 59 (1982) 155.