Properties of two-phase cadmium stannate thin films

Thin Solid Films, 185 (1990) 97-I 10 PREPARATION AND CHARACTERIZATION

97

P R O P E R T I E S O F TWO-PHASE C A D M I U M S T A N N A T E T H I N FILMS J. M. BLACKMORE AND A. F. CATTELL* Royal Signals and Radar Establishment. St. Andrews Road, Great Malvern, Worcestershire ( U.K. ) (Received November 15, 1988; revised July 24, 1989; accepted July 24, 1989)

Thin films of cadmium stannate were prepared from a C d 2 S n O 4 target by r.f. sputtering. Optical and electrical properties were studied as functions of film thickness and related to changes in composition and structure; in particular; the presence of both amorphous and crystalline phases within the same film was demonstrated for the first time. The surface composition of the films, in contrast to the bulk composition, was shown to be significantly affected by heat treatment, with clear implications for the use of cadmium stannate as a transparent conductor in electronic devices. It is concluded that failure to control adequately parameters such as deposition temperature can lead to non-uniformity of structure and properties within a single run and variability from one run to another.

1. INTRODUCTION

Materials whose properties include both high electrical conductivity and high optical transmission are widely used in thin film form in a variety of flat displays and opto-electronic devices. Early studies focused on sheet resistance and average transparency in the visible range but it is now accepted that the transparent conductor may play a more active role and a more detailed understanding is essential. One such example is found in direct-coupled thin film electroluminescence (DCTFEL), where the transparent conductor commonly comprises the cathode. Simple D C T F E L devices, i.e. those in which the ZnS active layer is merely sandwiched between two electrodes, are prone to progressive or catastrophic failure by destructive breakdown events. Film preparation conditions such as the atmosphere in which the ZnS is deposited, particularly in the region of the cathode, have a marked effect on the stability of the resulting device, as does the subsequent post-deposition anneal 1. Early attempts to improve performance by empirical changes in growth parameters were severely hampered by the large variability in device behaviour under nominally identical production conditions. In this paper we describe the characteristics of one particular cathode material, cadmium stannate, * Present address: Thorn EMI Central Research Laboratories,Dawley Road, Hayes, Middlesex,U.K. 0040-6090

98

J . M . BLACKMORE, A. F. CATTELL

that was used in the D C T F E L studies described above, and show how its inherent complexity can give rise to a variable substrate for the deposition of ZnS layers. It has been proposed 2 that instability due to local breakdown in D C T F E L devices is a fundamental problem associated with an inherent tendency to negative resistance and that it is not solely a consequence of localized film imperfections. The interface between the transparent conducting cathode and the ZnS is expected to be of great significance to the properties and behaviour of the final device. The surface composition and topography of the cadmium stannate investigated in this work are therefore of special interest. Thin films of cadmium stannate prepared by r.f. sputtering have been reported by many researchers (e.g. refs. 3-8). The sputtering target can be a mixture of CdO and SnO 2 powders, a Cd2SnO4 pressed powder or a C d - S n alloy and, depending on parameters such as target composition, sputtering gas and substrate temperature, amorphous or polycrystalline films can be obtained. Post-deposition heat treatment is almost always required to achieve low resistivity and high optical transmission; values of 5 × 10-4f~ cm and 85% (at about 550 nm) respectively are commonly quoted. The films produced for this work were first described by Lloyd 9, who investigated the preparation conditions necessary to give material of stoichiometric Cd:Sn ratio with the required electrical and optical properties. The present authors have made a more detailed study of the structure and composition of these films with particular emphasis on uniformity within a single run and reproducibility from one run to another. 2.

EXPERIMENTAL DETAILS

2.1. Film preparation Details of the sputtering chamber and diode system used in this work were reported by Lloyd 9. The 5 in diameter Cd2SnO4 pressed powder target was supplied by Nordiko Inc. and the substrate material was Corning 7059 glass clipped to a water-cooled holder. Sputtering was carried out in 100% oxygen at a pressure of 2.5 x 10- 2 Torr and a flow rate of 5 standard cm 3 min- 1, using an r.f. power of 500 W. The system was pre-sputtered for 30 min and then the shutter was removed for the required deposition time. For D C T F E L devices this was normally 15 min, corresponding to a nominal thickness of 0.3 Ixm (originally chosen as an acceptable compromise between sheet resistance and optical transparency), but for many of the

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C d STANNATE THIN FILMS

99

experiments reported here it was systematically varied to assess the effect of changing thickness on film properties. The cadmium stannate was usually deposited in stripes through a stainless steel mask onto six substrates simultaneously, each substrate measuring 3 c m x 2 c m (Fig. 1). On a few occasions a different mask was used, enabling stripes to be deposited onto two 2in square substrates simultaneously. After deposition the cadmium stannate films were usually removed to a furnace, heated to 450 °C in a stream of forming gas (FG) (90~oN2-10~H2) and immediately cooled. Sometimes post-deposition annealing was performed in the sputtering apparatus used for ZnS deposition, in either vacuum or 9 0 ~ A r - 1 0 ~ H 2 at a pressure of 5 x 10 -2 Torr; on these occasions the temperature was raised to 400°C and maintained for 30 min. 2.2. Film assessment

(i) Thickness measurements were made on etched edges using a mechanical stylus technique (Tencor Alpha Step, model 10-00020). Each sample was measured in two places (see Fig. 1). (ii) Resistance measurements were made by the four-terminal method using Keithley digital multimeters. Again, each sample was measured in two places (the cadmium stannate stripes were narrower than shown in Fig. 1). (iii) Optical transmission was measured in a double-beam UV-visible spectrophotometer (Perkin-Elmer model 554). Each sample was scanned from 900 nm down to about 250 nm with uncoated glass (Corning 7059) in the reference beam, 1009/o transmission having been set with glass in both beams. (iv) Structure was assessed using scanning electron microscopy (SEM), crosssectional transmission electron microscopy (XTEM) and X-ray diffraction (XRD). Surface topography was examined in a Cambridge 250/3 microscope. A selection of films was thinned to electron transparency in cross-sectional configuration by sequential mechanical polishing and low voltage Ar ÷ ion milling. Thinned samples were examined in a Jeol 1200 microscope operated at 120 kV. Most of the XRD work was performed in a Philips powder diffractometer using Cu Kat radiation. It was not possible to rotate the sample so particular care was needed in comparing results with published data for randomly oriented powders. Sophisticated data handling via computer software was not available and simple peak height measurements sufficed in most cases. A few films were examined using a cylindrical texture camera 1° and CrKct radiation. Exact Bragg angles are impossible to determine using this technique but more information about the direction and extent of preferred orientation can be obtained. (iv) Composition was determined by XRD, electron probe microanalysis (EPMA) and Auger electron spectroscopy (AES). The compounds present in the samples were identified by X RD as described above. Cd:Sn ratios were obtained in a Philips PSEM 500 instrument fitted with a Link Analytical X-ray microanalysis system and calibrated using cadmium and tin as standards. The Auger spectrometer used for this work was a Varian 10 kV cylindrical mirror analyser system fitted with an air lock, so that samples could be under ultrahigh vacuum within 40min of preparation. It was necessary to raster the electron beam electronically over a

100

J. M. B L A C K M O R E , A. F. C A T T E L L

100 I~m square to avoid electron-beam-induced effects on the sample surface. Depth profiling was accomplished using crossed 3 kV Ar ÷ ion beams. 3. RESULTS 3.1. Thickness 3.1.1. Uniformity Measurements were made on a set of six samples produced in a single run, of nominal thickness 0.3 pm; overall, the mean thickness was 0.325 pm and the standard deviation 0.04 pm, corresponding to a relative standard deviation of 12~o. However, the figures showed that while samples in the central positions varied in thickness by no more than 10~o of the mean, there was an approximately 20~o decrease in thickness towards the outer halves of samples in positions 1, 3, 4 and 6 (see Fig. 1). 3.1.2. Deposition rate Using average thickness measurements made on samples from the central substrate positions of several runs of different deposition times, the deposition rate was calculated to be about 190 ~ min-1. This is considerably lower than that originally achieved by Lloyd using the same r.f. power and is probably due to the use of a different target. 3.2. Resistance 3.2.1. Uniformity Two batches of six as-deposited cadmium stannate stripes, nominally 0.3 p.m thick, were examined, each substrate being measured in two places. The mean resistance of each batch and the corresponding resistivity are given in Table I. The lowest resistance readings were obtained in the central positions of each batch, which would be expected if the films were thicker in these positions. However, the measured variation in thickness is insufficient to explain the large variation in resistance. Two batches of six FG-annealed stripes, again of nominal thickness 0.3 pro, were similarly measured and from the readings shown in Table I it is immediately apparent that the F G anneal results in a reduction in resistance of 2 orders of magnitude. In addition, the variation within a batch is reduced by l order of TABLE I RESISTANCE: UNIFORMITY AND EFFECT OF ANNEALING (NOMINAL FILM THICKNESS, 0.3 t,tln)

Sample

State

Mean resistance (f~)

Standard deviation (~)

Relative standard deviation

Resistivity (f~ cm)

(%) Batch Batch Batch Batch

1 2 3 4

As d e p o s i t e d As d e p o s i t e d FG annealed FG annealed

1230 1680 14.4 13.0

___390 + 750 _ 0.6 +0.7

32 44 4 5

4.3 5.9 5.0 4.6

× × × ×

10 2 10- 2 10 4 10-4

Cd

101

STANNATE THIN FILMS

magnitude. This provides conclusive evidence that the thickness variation is not the primary factor responsible for the resistance variation in as-deposited samples. 3.2.2. Resistance variation with film thickness Using samples from the same substrate position each time, the resistance of asdeposited and FG-annealed material was monitored as a function of film thickness (Fig. 2). The scatter is greater for the as-deposited data and shows that the resistance is very variable, especially in thinner films. FG annealing results not only in a hundredfold decrease in resistance but also in less scatter. Furthermore, since the graph for the annealed samples is a straight line through the origin, resistivity is uniform throughout. However, for the as-deposited samples, although the graph has a straight-line portion which can be extrapolated to pass through the origin, the situation is very confused below about 0.3 lain; the resistivity here appears to be lower.

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Fig. 2. Resistanceofcadmium stannateas a functionof filrnthickness. 3.3. Optical characteristics 3.3.1. Transmission Percentage transmission (~oT) data were obtained for a number of films of different thicknesses in the as-deposited state and heat treated by the methods described earlier. For a film of nominal thickness 0.3 pm, such as would be used to fabricate a DCTFEL device, all the heat treatments employed shifted the absorption edge towards the UV (FG causing the biggest shift) and increased the average

102

J . M . BLACKMORE, A. F. CATTELL

transmission at 550 nm from about 75% to at least 85%. In thicker films the greater effect of the F G anneal is more clearly seen. Figure 3 shows that an as-deposited film approximately 1.2 jam thick has a transmission of only about 55%; heat treatment in Ar-H2 or vacuum increases this to about 70% but after F G annealing the transmission rises to 82%. In this case the absorption edge (defined as the 50% absorption point) was shifted from about 520 nm to 470 nm by the F G anneal. In addition, the F G treated sample exhibited a decrease in T towards the IR, an observation shared by Nozik 3 and Haacke et al. 4 who attributed the phenomenon to free-carrier absorption. The large difference in behaviour of FG-annealed samples was reflected in the visual appearance of the films, as-deposited material being deep golden yellow and Ar-H 2- and vacuum-annealed samples only slightly paler, whereas F G annealed cadmium stannate was a much paler greeny yellow.

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3.3.2. R e f r a c t i v e i n d e x

Thicker cadmium stannate films gave rise to a large number of interference fringes and enabled the refractive index n to be calculated from the formula n--

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where t is the film thickness and 21 and 22 are the wavelengths of two adjacent interference maxima (or minima). The average value of the refractive index for the wavelength range 500-800 nm was 2.65. 3.3.3. A b s o r p t i o n

For a homogeneous material the fraction of light transmitted is proportional to e - " where ~ is the absorption coefficient and t is the film thickness. Plots of the

Cd

S T A N N A T E T H I N FILMS

103

logarithm of the reciprocal of the fraction transmitted against thickness were drawn for as-deposited samples and those heat treated in the three ways described above; the results at 460 nm are shown in Fig. 4. The graph for as-deposited films is a straight line and has the steepest slope, indicating the highest absorption coefficient (2.6 x 104 cm-1). The graph for FG-annealed films, however, appears to have a change in slope: for thicker films the absorption coefficient is 5.0 x 103 c m - x but below about 0.4 p.m it may be rather less. For Ar-H2 and vacuum-annealed samples the situation is less clear. There may be a change in slope at about 0.2 lam, indicating that the region with lower absorption coefficient is thinner in these samples. Certainly, for thicker films, the similarity in slope between as-deposited Ar-H2- and vacuum-annealed samples and the difference between these three and the FGannealed material is consistent with the visual appearance reported above.

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104

J.M.

B L A C K M O R E , A. F. C A T T E L L

3.4. Structure 3.4.1. Surface topography One of the original reasons for choosing cadmium stannate as the cathode material for D C T F E L devices was that it appeared to be much smoother and more featureless under the microscope than commercially available alternatives. SEM shows that for films nominally 0.3 ~tm and 1.5 ~tm thick the grain sizes are about 400/~ and 1000/~ respectively. No change in grain size on annealing was observed. 3.4.2. Cross-section A selection of as-deposited and FG-annealed samples was examined by XTEM. As-deposited films were composed of a first-to-grow amorphous layer 0.1-0.25 lain thick, which changed abruptly to a polycrystalline structure at a characteristically shaped junction, with initial lamellar crystallites arranged in a chevron pattern. Columnar growth began at 0.4-0.6 I~m, The increase in grain size implicit in the figures reported above apparently occurred during the onset of columnar growth, since the columns of a 1.5 lam thick film were reasonably parallel sided and approximately 1000 & wide, There was also some evidence to suggest that FG annealing might result in a slightly thinner amorphous region and improved columnar structure. These features are illustrated in Fig. 5.

Fig. 5. XTEM micrographs:(a) as-deposited(nominal thickness, 1.5 lam);(b) annealed in FG (nominal thickness, 1.0 lain). Two samples from a batch of six, nominally 0.5 ~tm thick and FG annealed, were compared: one was from a central position in the substrate arrangement and the other from an edge position, The outer sample was about 25~o thinner than the central sample, confirming the mechanical thickness measurements described earlier. In addition, approximately 40~o of the outer sample was amorphous compared with only about 15~o of the central sample. This means that the crystalline layer was approximately twice as thick in the central region (Table II).

3.5. Composition 3.5.1. X-ray diffraction Using the powder diffractomer, as-deposited films less than about 0.2 lain thick exhibited no peaks at all, indicating an amorphous structure. In films of device

Cd

STANNATE

THIN

105

FILMS

TABLE II VARIATION IN STRUCTURE WITH SUBSTRATE POSITION (NOMINAL FILM THICKNESS, 0 . 5

Position

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Thickness of crystalline layer

(/am)

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Total thickness (lam)

0.50 0.38

thickness (0.3 lam) two peaks were detected, at approximately 32 ° and 38.5 °. As film thickness increased further, the peak height at 32 ° increased linearly and the other in a more complex way (Fig. 6). Also, several more peaks began to appear, and Fig. 7 represents a typical fingerprint from a film about 1.2 ~m thick. All the peaks can be attributed to the cubic spinel structure first suggested by Siegal t 1, although their relative intensities indicate non-random orientation. The dominant peak, however, also shifted to higher Bragg angles and developed a shoulder as film thickness increased, and although it could not be resolved into its component parts the implication is that in thinner films the peak is attributable mostly, if not wholly, to CdO, whereas in thicker films there is an increasing contribution from Cd2SnO 4 (Fig. 8). This result is similar to that of Haacke et al. 4 Estimates of the Cd2SnO 4 contributions to the 38.5 ° peak were made in order to calculate the relative

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106

J . M . BLACKMORE, A, F. CATTELL

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intensities of the 32 ° peak, and the results indicated a decrease in the latter with increasing thickness. Texture camera images of thicker films confirmed the cubic spinel structure and the noding of the lines was consistent with a preferred orientation in the (100) direction. In addition, an extra line was observed which indicated the presence of CdO. N o n e of the annealing procedures employed in this work resulted in significant changes in the XRD pattern, although each one caused minor but characteristic differences in relative peak heights, dependent on thickness. These findings are also in agreement with Haacke et al. 4, who reported no change at annealing temperatures below about 700 °C. The powder diffractometer was used to investigate variations in the heights of the two main peaks as a function of substrate position. The major peak in samples from central positions was up to 10 times larger than that in samples from outer positions. Variations in thickness (either total thickness or the thickness of the crystalline layer) were insufficient to account for this. The 32 ° peak was only slightly greater in the central samples than in the outer samples, so that the intensity of this

Cd

STANNATE THIN FILMS

107

peak, relative to the intensity of the 38.5 ° peak, increased nearly 10 times from the centre to the edges of the deposition area. This implies that the material in the centre of the area either is more highly oriented than that at the edges and/or has a higher CdO content. Some preliminary work was done on the effect of different substrate arrangements. The main peaks from two sets of two samples, nominally 1.0 lxm thick, were compared with samples of corresponding thickness from the usual set of six samples. Reasoning similar to that above suggests that the material deposited in the set-oftwo arrangement is less crystalline and either is less oriented or contains less CdO than that in the set-of-six arrangement. 3.5.2. Electron probe microanalysis Cd:Sn ratios were determined for several films of device thickness (0.3 lam), both before and after annealing in FG. Values varied from 1.8:1 to 1.9:1, well within the range reported by Lloyd 9 in his initial study of this system. There was no significant change after heat treatment and again these results are in agreement with those of Lloyd 9 and Haacke et al. 4 Measurements on 1.0 [am and 1.6 I~m films gave very similar figures, showing that the overall stoichiometry is preserved with increasing film thickness. 3.5.3. Auger electron spectroscopy Several depth profiles were obtained for as-deposited films of nominal thickness 0.3 I~m. In order to minimize surface oxidation the samples used were freshly prepared and the transfers from deposition kit to spectrometer were made as quickly and cleanly as possible. Figure 9 shows typical profiles through the top 1000 A. or so. There is a small amount of surface oxidation and carbon contamination but below this is a near-surface region of cadmium depletion. After heat treatment in F G the amount of surface carbon increased but surface oxidation remained unchanged; cadmium depletion increased in both amount and depth while the tin and oxygen profiles were not altered. Typically, the top 100 A of annealed material was depleted of cadmium by 50~o compared with its bulk value. These results are consistent with the findings of Kirk ~2 who studied the surfaces of the as-deposited films grown for this work and then heated them in situ in vacuum and in hydrogen at low pressure; while the Sn:O ratio remained unchanged the Cd:O and Cd:Sn ratios dropped sharply above about 350 °C. Davies x3 carried out a similar investigation on the surfaces of these samples using X-ray photoelectron spectroscopy and found that cadmium loss began somewhere between 370 and 470 °C with possibly an accompanying loss of oxygen. These result suggest that the C d - - O bonding on the surface undergoes disssociation at temperatures above about 350 °C and, since free cadmium has a melting point of about 320°C and sublimes readily, it is pumped away, leaving a heat-treated surface that is largely composed of SnO2. 4.

DISCUSSION AND CONCLUSIONS

Thin films of Cd2SnO4 prepared by the r.f. sputtering system described here are clearly complex in both structure and composition. XTEM and XRD have demonstrated that initially growth is amorphous but, presumably due to an

108

J. M. BLACKMORE, A. F. CATTELL

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DEPTH( JL)

Fig. 9. Auger depth profiles of cadmium stannate films 0.3 pm thick: (a) as deposited; (b) annealed in FG.

increase in substrate temperature in the high power r.f. environment, despite the water cooling, a threshold is reached after which polycrystalline growth takes place. The variation in proportion of amorphous material within a batch of samples is probably also an effect of temperature gradient caused by variation in thermal coupling between the substrates and the holder or the mask and the substrates. Differences in crystallinity between the set-of-six and set-of-two configurations could also be explained by more efficient cooling in the set-of-two case. However, there is another possibility that has not yet been fully explored. The shape of the plasma is not symmetrical because the target (and the substrate holder) is off-centre in the chamber to such an extent that it is in close proximity to the chamber wall; this may affect 9niformity of deposition. Furthermore, the substrate holder is not deliberately earthed; recent experiments in which the substrate holder was deliberately earthed showed that the plasma becomes more clearly and centrally

C d STANNATE THIN FILMS

109

defined, perhaps resulting in hotter substrates by virtue of more electron bombardment. The influence of substrate bias is clearly a subject for further work, since it is well known that r.f. sputtering results in the subjection of the substrate to considerable ion bombardment, which is expected to have significant effects on the structure and composition of the growing film (e.g. ref. 14). In as-deposited material the amorphous and crystalline layers appear to have different resistivities, and it is the variation in thickness of the amorphous layer, rather than the variation in total thickness, that accounts for the large variation in the resistance results. It is interesting to note, however, that FG annealing results in homogeneous resistivity without eliminating the amorphous layer. However, the optical absorption of as-deposited films appears to be homogeneous despite the amorphous layer, while F G annealing results in different absorption coefficients for the amorphous and polycrystalline regions. The structure of these films, and their electrical and optical properties, change quite markedly between 0.2 and 0.4 ~m; the 0.3 ~tm layers used for devices are therefore subject to some variation, with consequent implications for final device properties. Although XRD has indicated that the composition changes more gradually than structure with film thickness it is clear that the Cd2SnOa:CdO ratio and/or the extent of preferred orientation of the Cd2SnO 4 still varies considerably within a single batch of samples and from one batch to another. Again, variation in effective growth temperature is probably responsible. Energy-dispersive X-ray studies and AES have shown that, although the overall or bulk Cd:Sn ratio remains unchanged on heating (within the limits of sensitivity of these techniques), this is far from the case at the film surface. Changes in the surface chemistry of the cadmium stannate are brought about not only by the normal anneal necessary to induce the required electrical and optical properties but also by the initial (pre-sputter) stages in the deposition of ZnS for DCTFEL. A study of the nucleation and growth of ZnS on the cadmium stannate might therefore be a profitable next step in achieving an understanding of the mechanisms involved in D C T F E L and work on this subject is already under way. Several factors are being considered in order to simplify this stage of the investigation as far as possible. It is important to test the devices for stability using cadmium stannate films of thicknesses well away from 0.2-0.4 om, where structure is at its most variable. Better control over deposition parameters such as substrate temperature is necessary to ensure better uniformity and reproducibility of structure, composition and properties. Finally, the complexity of cadmium stannate is due partly to its being a ternary compound; an understanding of the ZnS-cathode interface may be more easily achieved using a simpler binary material such as SnO2 or ZnO, provided, of course, that stable devices can be made. ACKNOWLEDGMENTS

The authors wish to express their gratitude to Dr. A. Cullis and N. Chew carrying out the XTEM work, to Dr. D. Rodway for the AES and to D. Dosser the SEM and EPMA. They would also like to thank I. Young and Dr. G. Brown their help in interpreting the XRD results. Thanks are also due to Dr. J. Kirton

for for for for

1 10

J. M. BLACKMORE, A. F. CATTELL

his valuable support, to P. Lloyd and Dr. K. Lewis for stimulating discussion and finally to J. Davis and P. Snell for preparing the films, Copyright © Controller HMSO London 1988. REFERENCES I 2 3 4 5 6 7 8 9 10 11 12 13 14

J. Blackmore, A. F. Cattell, K. F. Dexter, J. Kirton and P. Lloyd, J. Appl. Phys., 61 (1987) 714. A.F. Cattell, J.C. InksonandJ. Kirton, J. Appl. Phys.,61(1987)722. A.J. Nozik, Phys. Rev. B, 6 (1972) 453. G. Haacke, W. Mealmaker and L. A, Siegal, Thin Solid Films, 55 (1978) 67. N. Miyata, K. Miyake, K. Koga and T. Fukishima, J. Electrochem. Soc., 127 (1980) 918. F.T.J. Smith and S. L. Lyu, J. Electrochem. Soc., 128(1981) 1083. T. Stapinski, E. Leja and T. Pisarkiewicz, J. Phys., D., 17 (1984) 407. R.P. Howson and M. I. Ridge, Thin Solid Films, 77 (1981) I 19. P. Lloyd, Thin Solid Films, 41 (1977) 113. C. A. Wallace and R. C. C. Ward, J. Appl. Crvstallogr., 8 (1975) 545. L.A. Siegal, J. Appl. Crystallogr., 11 (1978) 284. D.L. Kirk, personal communication, 1984. M. Davies, Proc. lnt. Electroluminescence Workshop, Oregon, 1986. J.A. Thornton, in J. Vossen and W. Kern (eds.), Thin Films Processes, Academic Press, New York, 1978.