Electrical conductivity in sputter-deposited chromium oxide coatings

Thin Solid Films, 127 (1985) 241-256

241

ELECTRONICS AND OPTICS

E L E C T R I C A L C O N D U C T I V I T Y IN S P U T T E R - D E P O S I T E D CHROMIUM OXIDE COATINGS R. C. KU AND W. L. WINTERBOTTOM

Research Staff, Ford Motor Company, Dearborn, M148121 (U.S.A.) (Received April 24, 1984; accepted December 21, 1984)

The electrical conductivity and thermal stability of sputtered chromium oxide films were investigated in order to assess their potential as protective coatings for high temperature battery electrode materials. Sheet conductivities as high as 10-15f~ -x cm -1 were achieved in air at a temperature of 350°C. Film property changes resulting in reduced electrical conductivity (several orders of magnitude) occurred when the films were heated in vacuum. Electrical conductivity and electron beam methods were used to study the nature of the film instability. It is proposed that the instability is related to both structural and compositional (oxygen) changes in the sputter-deposited films.

1. INTRODUCTION A number of oxides including Cr203, TiO 2 and ZrO z have been shown in static corrosion tests to be resistant to attack by molten sodium polysulfides and to be candidate materials for the sodium-sulfur battery 1. Unfortunately, the high electrical resistance of these oxides in bulk form makes them unsuitable as current collectors in the sodium-sulfur battery system and of limited utility. Crosbie et al. 2 have evaluated the electrical conductivity of doped C r 2 0 3 to determine the feasibility of such a material as a chemically stable electrode material. Dynamic corrosion tests of the bulk oxides in sodium polysulfide showed negligible surface attack and unstable (increasing) electrical resistance. This report summarizes a study of the electrical conductivity and thermal stability of sputtered chromium oxide coatings to assess their suitability for evaluation as an electrode coating material for the battery. The thermal stability studies of the sputtered films were carried out in controlled environments of vacuum and oxygen at various partial pressures. 2.

PROPERTIES OF BULK CHROMIUM OXIDE

2.1. L a t t i c e structure a n d stability

Chromium sesquioxide (Cr203) is the only solid chromium oxide which is 0040-6090/85/$3.30

© ElsevierSequoia/Printedin The Netherlands

242

R. C. KU, W. L. WINTERBOTTOM

stable at temperatures above 500 °C 3 7. The oxide has a corundum structure. At temperatures below 500°C several other oxygen-rich phases can exist in the composition range C r z O 3 - C r O 3. Diffusion of chromium and oxygen in C r 2 0 3 is very slow; the self-diffusion coefficient of chromium measured at 1500 K is less than s cm 2 s ~ with an activation energy greater than 255 kJ m o l - 1 (ref. 6).

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2.2. Electrical conductivity

The electrical conductivity measurements of Meadowcroft and Hicks 8 are shown in Fig. 1 for hot-pressed samples of high purity C r 2 0 3. Two temperature regions are apparent for conductivities above and below approximately 1400 K. At the higher temperatures, the conductivity is relatively insensitive to the partial pressure of oxygen and is characterized by an activation energy of approximately 193 kJ m o l - 1. Below 1400 K, the influence of oxygen is apparent and the activation energy is much lower (24kJ mol-1). Similar results were reported by Hagel and Seybolt 7 in their study of cation diffusion in C r 2 0 3. Although a quantitative description of the p-type electrical conductivity has not been given, these researchers interpret the conductivity characteristics as indicative of the behavior expected for complex defects. Crosbie et al. 2 report electrical conductivities for sintered doped C r 2 0 3 that are higher than those shown in Fig. 1 for the bulk oxide conductivity. Conductivities of 2.5 x 10- 1 ~-) 1 cm t and 1.7 × 10 t ~ - t c m - 1 were obtained at 350°C with sintered C r 2 0 3 doped with 1 mol.~o MgO and 1 mol.% N i O respectively. Air annealing at 1500°C was required to develop the high conductivity. Activation energies of 33 kJ m o l - 1 and 38 kJ m o l - 1 were reported for the MgO-doped and NiO-doped chromium oxides respectively in the temperature range 100-350 °C and were interpreted in terms of a fixed concentration of electron holes with a temperature-dependent mobility.

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ELECTRICAL CONDUCTIVITY IN SPUTTER-DEPOSITED C r 2 0 3

3.

COATINGS

243

PROPERTIES OF SPUTTER-DEPOSITED CHROMIUM OXIDE FILMS~ FILM STRUCTURE

AND COMPOSITION

In an Auger electron spectroscopy (AES) study of the structure and composition of sputter-deposited chromium oxide films, Bhushan 9 found that the asdeposited films were uniform in composition throughout their thickness and that variations in sputtering parameters produced no variations in composition. The AES analyses indicated that the compositions of the amorphous films were approximately (Cr-O). However, the possibility that different sputtering rates for the chromium and oxygen species might alter the composition of the surface layer being analyzed made the compositional analyses uncertain. X-ray analyses indicated that the coatings were amorphous and were crystallized to a Cr20 3 structure after a 700 °C heat treatment for 20 h in air. No electrical properties of sputter-deposited chromium oxide films have been found in any published reports. 4.

EXPERIMENTAL PROCEDURE

Thin chromium oxide films and multilayer films of chromium and nickel oxide films were sputter deposited onto quartz or stainless steel (E-brite) substrates by means of a conventional planar diode type of r.f. sputtering module. High purity sintered C r 2 0 3 (99.8~o) and NiO (99~o) obtained from Cerac Inc. were used as target cathodes in producing films ranging in thickness from 20 to 6000/~. The E-brite substrates (26~o Cr, 1.0~o Mo and remainder iron) were cut from sheet material and mechanically polished to a finish of 3 p.m. Both types of substrates were cleaned ultrasonically in acetone, then in alcohol and dried immediately with hot air as a final operation. Sputtering was carried out in a bell-jar vacuum system in argon at a pressure of 4 Pa. The deposition rates were approximately 400/~ h - 1 and the film thickness was monitored with a quartz oscillator device. The temperature of the substrates did not exceed 230°C during deposition. The multilayer films were deposited by sequentially sputtering, cooling in argon, exposing to air during the cathode change, re-establishing the argon atmosphere, then sputtering etc. The films were stored in air prior to testing. Film conductivities were measured with conventional two- and four-point techniques 1°'1~. The specimen configuration for conductivity measurements is shown in Fig. 2. Sputtered platinum or silver paint stripes were used as contacts. Measurements were carried out over a range of temperatures and oxygen partial pressures. Oscilloscope display of the current-voltage characteristics showed them to be linear over the range of temperatures studied, namely 25-540 °C. Measurements of the temperature dependence of the conductivity were made while the oxide films were thermally cycled. Heating and cooling rates of about 10 °C min - ~ were used. In addition to the direct measurements of the conductivity, the sputtered films were analyzed by standard AES methods and by monitoring the sample-to-ground current Is induced by a beam of low-to-medium energy electrons. The characteristics of the functional relationship of the sample current Is and the primary electron beam

R.C. KU, W. L. WlNTERBOTTOM

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5. RESULTSAND DISCUSSION 5.1. Film composition and structure X-ray analyses of the sputter-deposited chromium oxide films showed them to be amorphous as deposited. The as-deposited films were dark brown or gray with a green tint visible occasionally. Subsequent heat treatment in air and vacuum tended to increase the intensity of the green color in the films and probably indicates the presence of an increasing amount of microcrystalline C r 2 0 3. X-ray analysis of the films after heating to about 500 °C indicated that the amorphous character of the asdeposited films was retained. AES analyses of sintered crystalline C r 2 0 3, the as-deposited films and the heattreated films (Fig. 3) were made using peak-to-peak measurements of the Cr(529 eV), Cr(571 eV) and O(510eV) peaks and the Auger peak intensity and the sensitivity factors for the peaks which were determined from the elemental spectra in the Handbook of Auger Electron Spectroscopy 12. The ratio of the atomic concentration of oxygen to that of chromium in the as-deposited film was found to be about 1.1 which lies between unity and the ratio 1.5 expected for a C r 2 0 3 composition. Similar results for sputter-deposited chromium oxide films have been reported earlier by Bhushan 9. In the study reported here heat treatment to about 500 °C in air or vacuum did not alter this ratio significantly. 5.2. Electrical properties The present conductivity results determined in air over a range of temperatures

ELECTRICALCONDUCTIVITYIN SPUTTER-DEPOSITED C r 2 0 3 COATINGS

245

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246

R. C. KU, W. L. WINTERBOTTOM

Similarities are also apparent in other features of the measurements. The temperature dependence obtained during cooling is linear on the reciprocal temperature scale with the conductivities lying above those values measured during the initial heating of the films. In subsequent thermal cycling the film conductivities retraced the values obtained on the cooling curve as illustrated by the second heating and cooling cycle of the (Cr O) sample (sample 3 in Fig. 4). The activation energy derived from the slope of the cooling curve for the (Cr-O) film is about 24 kJ m o l i which is in rough agreement with the results reported by Kofstad 3 and Kingery et al. 14 for bulk Cr203 and Crosbie et al. 2 for doped CrzO3. The room temperature conductivities of each of the thermally cycled films were almost an order of magnitude higher than the initial conductivity of the as-deposited films. The multilayer films studied here were designed to evaluate the onset of diffusive transport of NiO in sputtered (Cr-O) films at elevated temperatures. Various configurations of layered ( C r - O ) / N i O films were generated and Auger methods and sample current measurements were used to follow the process. 10 2

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ELECTRICAL CONDUCTIVITY IN SPUTTER-DEPOSITED C r 2 0 3 COATINGS

247

Unfortunately, the stresses set up in the films at elevated temperatures by the thermal expansion differences between the film and the substrate caused delamination and crazing of the films when heated to the temperature range 600-800 °C. These temperatures were too low to facilitate diffusive transport as verified by Auger measurements of the persistence of thin N i O overlayers on (Cr-O) films. Within the accuracy of the conductivity and sample current measurements rep0rted here, there was no difference in the layered films and the (Cr-O) films. Figure 5 shows the room temperature conductivity dependence on the asdeposited film thickness. Measurements for both the single-layer (Cr-O) and the

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248

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multilayer films are reported to illustrate the high conductivities characteristic of sputtered films and the similarity of the single-layer and multilayer films. In vacuum the sputtered films displayed a conductivity dependence that is distinct from that in air. As Fig. 6 illustrates, there is a break in the conductivity dependence in the temperature range 120-220°C similar to that noted in air. At temperatures above 200 °C the conductivity values recorded are less than the values measured in air, On cooling, the conductivities lie below those found during the initial heating in marked contrast with the behavior noted in air. The room temperature conductivity values found after thermally cycling in vacuum are typically several orders of magnitude below the values noted for the as-deposited 102,

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ELECTRICAL CONDUCTIVITY IN SPUTTER-DEPOSITED C r 2 0 3

COATINGS

249

film. Reheating the film in air increased the conductivity slightly during the heating portion of the cycle and gave the characteristic behavior for cooling in air; the results in Fig. 6 should be compared with those in Fig. 4. The behavior noted for a film repeatedly cycled in vacuum is shown in Fig. 7 for a layered film initially cycled in air. A type of hysteresis behavior is observed under hard vacuum and oxygen at low partial pressures (4 × 10 - 3 Pa) in which the conductivity values found during heating lie above the values measured on cooling. In the temperature region above 400 °C, the conductivities merge into a singlevalued functional dependence. The hysteresis type of behavior is indicative of a timeand temperature-dependent process which alters the defect concentration within the film. No measurable dependence of the conductivity was found with changes in the partial pressure of oxygen at low partial pressures (Po2 < 10-3 Pa) as shown in Fig. 7. The influence of higher oxygen pressures is shown in Fig. 8. The measurements made in air, in a forepumped vacuum (Po2 = 2.7 × 10- 3 Pa) and in ultrahigh vacuum (Po2 = 1.3 × 10 - 6 Pa) provide a rough estimate of the oxygen dependence, since 1600 I

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250

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others 7 have shown that nitrogen has a negligible influence on the conductivity of sintered C r 2 0 3. The results indicate a strong dependence of the conductivity on the concentration of oxygen present in the ambient. Figure 9, curve 1, shows the relationship between the oxygen partial pressure and conductivity at 227 "C (500 K) derived from the data in Fig. 8. The slope of the oxygen dependence is roughly 6/16 or twice the value of 3/16 which can be derived from eqn. (2) in the paper of Meadowcroft and Hicks 8 ; their data in Fig. 1 ofref. 8 for sintered Cr203 at the same temperature, shown as curve 2 in Fig. 9, give a slope of about 3/16 with much lower conductivities. The different slope found with the sputtered films is not surprising, since the as-deposited sputtered films have been shown by X-ray methods to be amorphous or microcrystalline in structure and quite distinct from the corundum structure of the sintered material.

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5.3. Electron beam analysis: sample current characteristics In order to provide additional information on the nature of the change taking place in the as-deposited film as it is heated, the sputter-deposited films were studied using an electron beam technique. The sputtered films were deposited onto stainless steel substrates in the configuration shown in Fig. 2(b) and mounted in the Auger electron analysis position of an ultrahigh vacuum system for study 15. In this position, a beam of electrons was applied to the film surface by means o f a defocused Auger spectrometer electron source. The general features of the measurement are illustrated in Fig. 10 to highlight the various electronic currents and the sample-toground current that was recorded. A primary beam current ip of about 10-6 A was used to probe the surface and the sample currents produced were in the range from

ELECTRICAL CONDUCTIVITY IN SPUTTER-DEPOSITED C r 2 0 3 COATINGS

251

- 3 × 10 6 to + 10 × 10-6A. The electron emission current is from the surface that resulted from the interaction of the primary beam included both the elastically and inelastically scattered primary and true secondary electrons. This emitted electronic current was not measured directly in this study.

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The behavior of the sample current as the primary electron beam voltage is increased is shown in Fig. 11 for three different surfaces : as-polished stainless steel (air-formed surface oxide), a sputter-deposited (Cr-O) film and sintered C r 2 0 3. As shown, the features were distinctive for each surface studied. As shown in the inset to Fig. 11, the relationship between the primary beam current and the beam voltage was approximately linear in the range of voltages used in these studies, namely 1 0 - 1 0 0 0 V, and was not responsible for the sample current characteristics illustrated in the figure.

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252

R. C. KU, W. L. WINTERBOTTOM

Qualitatively, the sample current shows a characteristic rise to a maximum with increasing primary beam voltage and decreases at still higher voltages in a manner similar to the dependence of the secondary electron yield on the primary beam voltage 15,16. The explanation accepted for the secondary electron yield behavior is that the yield increases initially with increasing beam voltage as the penetration distance of the primary electrons increases and then diminishes as the depth of penetration exceeds the mean escape depth for secondary electrons in the solid. In the electron beam study being described, the sample current will be determined by the scattering process and the secondary electron yield for the incoming primary beam of electrons. However, since we are dealing with oxide films whose thicknesses are for the most part many times larger than the average penetration depths of the primary electrons (several hundred nanometers), the influence of surface charging and primary beam reflection can be expected to complicate the simple relationship between ip, I s and true secondaries. Some features of the relationship between film conductivity and sample current can be understood in terms of the surface charging induced by the primary beam when the yield is not unity. With limited electronic charge carrier mobility, surface charging will occur and a fraction of the primary beam electrons will be reflected. Presumably, the small sample current shown in Fig. 11 for the bulk A1203/Cr203 surface relative to the other surfaces is related to poor conductivity and surface charging effects. Where the electronic conduction is sufficient to prevent charging as in the sputtered oxide films and the stainless steel substrate, the differences in sample current are more closely related to variations in secondary electron emission. Figure 12 illustrates the insensitivity of the sample current to the film thickness

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253

ELECTRICAL CONDUCTIVITY IN SPUTTER-DEPOSITED C r 2 0 3 COATINGS

when the film conductivity is in the range of 10- 2 f~- i cm - 1. The scatter apparent in these results for three multilayer films ranging in thickness from 60 to 5700/~ is typical and is not believed to be indicative of a thickness dependence. The sample current for the stainless steel substrate is shown for comparison.

5.4. Thermal stability of the films A number of changes in sample current characteristics were noted when the sputtered films were heated in a vacuum environment. Figure 13 illustrates the differences for an as-deposited (Cr-O) film before and after being heated to temperatures as high as 850 °C. The sample current measurements were made with the films at room temperature. The defocused primary beam had no measurable effect on the sample temperature. The reduced sample current after heating is probably related to a lower secondary electron yield from the film and/or conductivity caused by structure and/or composition changes induced in the film.

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The thermal stability of the films was studied by monitoring the sample current while the film temperature was being monotonically increased (Fig. 14). For these measurements the beam voltage was held constant at a value (600 eV) corresponding to that giving the maximum sample current (see Fig. 12). The results for a multilayer film show the reduction in sample current occurring as the film temperature is raised. A minimum current was reached at about 150 °C before the sample current returned to its initial level at still higher temperatures. During cooling, the sample current

254

R . C. K U , W .

L. W I N T E R B O T T O M

decreased monotonically as illustrated in Fig. 14 indicating that the process responsible for the effect was irreversible. Several other effects were observed during the initial heating of as-deposited films. A color change was observed in the films during their initial heating (from gray or dark brown to light green) which implies that structural and compositional changes are taking place in the film at temperatures in the neighborhood of 150 °C. The temperature range in which the observation was made is indicated in Fig. 14. Appreciable outgassing of the films in the temperature range 150-200 °C was also observed. The species identified mass spectroscopically were H 2, H 2 0 , CO and

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5.5. Interpretation of results Most of the effects observed in studying the thermal stability of sputterdeposited chromium oxide films occur in a rather narrow range of temperatures and are consistent with a qualitative model of the film properties involving structural and compositional changes. The observed dependence of the film conductivity on the partial pressure of oxygen, AES surface analyses of the film composition, electron beam studies of the temperature dependence of the sample current, the observed changes in film color and the vacuum outgassing characteristics provide some insight into the nature of the films and the processes taking place. W e have found that r.f. sputter-deposited chromium oxide films have conductivities about six orders of magnitude higher than that of the bulk oxide in the as-deposited (near room temperature) state. The room temperature sheet conductivity of the amorphous films decreases with increasing film thickness and exceeds 10- 2 f~- 1 cm - ~ in films less than 1000 pm thick. Heating the films above 100 °C in air or vacuum results in a lowered rate of

ELECTRICAL CONDUCTIVITY IN SPUTTER-DEPOSITED C r 2 0 3

COATINGS

255

increase in conductivity with increasing temperature in the range I00-200 °C. Slight color changes in the films noted in this temperature range indicate that structural changes are occurring (possibly the formation of microcrystalline Cr/O3). The film also begins outgassing in this range and the oxygen-bearing vapor species indicate there may be some loss of oxygen from the film at this time. The electron beam studies indicate that there are probably changes in the secondary electron yield of the film in the temperature range between 100 and 200 °C which would be consistent with the interpretation of structural changes in the film. Furthermore, the electron beam results show that the structural changes taking place are irreversible in the range of temperature studied. Above 200°C the temperature dependence of the conductivity increases through the range of temperatures studied in this work (to 550 °C). The highest conductivities are achieved by annealing the films in air within the temperature range 200-550 °C. On cooling, the enhanced conductivity is retained throughout the range of temperatures from 550 to 25 °C (the room temperature conductivity is enhanced by about one order of magnitude). A decrease in conductivity occurs when the films are annealed in oxygen at partial pressures below 10-2 Pa, which indicates that there is a net oxygen loss in this pressure region in the temperature range above 200 °C. While there is a net oxygen pick-up during the annealing at higher partial pressures of oxygen, AES determination of the concentrations of chromium and oxygen in the film before and after thermal treatment indicates that this oxygen pickup is small. Vacuum annealing of films previously heated in air above 200 °C brings about a reduction in conductivity and indicates that the oxygen uptake is at least partially reversible. Presumably, the overall conductivity changes noted are due to small changes in film structure and composition. 6.

SUMMARIZING REMARKS

Sputter-deposited chromium oxide films are more conductive than the crystalline sesquioxide C r 2 0 3. Sheet conductivities as high as 10-15 f~ l c m - 1 at 350 °C have been achieved in air with films approaching 0.3 ~tm in thickness. The asdeposited films are unstable at temperatures above 100 °C where their conductivity becomes dependent on the partial pressure of oxygen. Electron emission variations, color changes and outgassing indicate that structural and compositional changes occur in the films at these temperatures. The study demonstrates that sputter-deposited chromium oxide films are conductive enough to be evaluated as a protective coating material for application in the temperature range from room temperature to 550 °C. The thermal instability of the as-deposited film at temperatures above 100 °C, however, will require that film stabilization procedures be developed and environmental stability be demonstrated prior to application. ACKNOWLEDGMENTS

The authors gratefully acknowledge the assistance of Messrs. C. M. Kukla and J. S. Badgley in developing the resistance measurement procedures and informative

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discussions with R. McCune, G, Crosbie and D. Hoffman. Review of the manuscript by R, Jaklevic and E. Logothetis is appreciated. REFERENCES 1 K.R. Kinsman and W. L. Winterbottom, The use of coatings in high temperature battery systems, Thin Solid Films, 83 (1981) 417-428. 2 G.M. Crosbie, G. J. Tennenhouse, R. P. Tischer and H. S. Wroblowa, Electrically conducting doped chromium oxides, J. Am. Ceram. Soc., 67 (1984! 498-503. 3 P. Kofstad, Nonstoichiometry, Diffusion and Electrical Conductivity in Binary Metal Oxides, WileyInterscience, New York, 1972, pp. 203-211. 4 O. Fukunaga and S. Saito, Phase equilibrium in the system CrO2--Cr203, J. Am. Ceram. Soc., 51 (1968) 362. 5 K. P. Lillerud and P. Kofstad, On high temperature oxidation of chromium, 1, oxidation of annealed, thermally etched chromium at 800-1100 °C, J. Electrochem. Soc., 127 (1980) 2397. 6 P. Kofstad and K. P. Lillerud, On high temperature oxidation ofchromium, II, properties of Cr20 a and the oxidation mechanism of chromium, J. Electrochem Soc., 127 (1980) 2410. 7 W. Hagel and A. U. Seybolt, Cation diffusion in CrzO3, J. Electrochem. Soc., 108 (1961) 1146. 8 D. B. Meadowcroft and F. G. Hicks, Electrical conduction processes and defect structure of chromic oxide, Proc. Br. Ceram. Soc., 23 (1973) 33. 9 B. Bhushan, Characterization of r.f. sputter-deposited chromium oxide films, Thin Solid Films, 73 (1980) 255-265. 10 L.B. Valdes, Resistivity measurements on germanium for transistors, Proc. Inst. Radio Eng., 42 (February 1954) 420. I 1 D.R. Zrudsky, H. D. Bush and J. R. Fassen, 4-point sheet resistivity techniques, Bey. Sci. Instrum., 37 (7) (1966) 885. 12 U E. Davis, N. C. MacDonald, P. W. Palmberg, G. E. Riach and R. E. Weber, Handbook of Auger Electron Spectroscopy, Physical Electronics Industries, Eden Prairie, MN, 1976, 13 F.J. Morin, Electrical properties of NiO, Phys. Rev., 93 (6) (1954) 1199-1204. 14 W.D. Kingery, H. K. Bowen and D. R. Uhlmann, Introduction to Ceramics, Wiley, New York, 2nd edn., 1976, p. 867. 15 R.E. Weber, Auger electron spectroscopy for thin film analysis, Res.Dev., 23 (10) (1972) 22. 16 A.J. Dekker, Solid State Phys., 6 (1957).