Classified abstracts 4186-4193

35 4186. Initial etching in an rf butane plasma. (GB) During deposition of carbon from a glow-discharge of C4H ~ o we have observed that the hydrocarbon fragments impinging on the substrate initially do not deposit, but rather etch the surface of the substrate. Gradually the etching process changes over to deposition after a few minutes. The addition of as much as 15 ~ air into the butane plasma had no influence on the rate of etched gold. However, the choice of substrate material strongly affects the amount of etching taking place. A simple model is outlined in order to describe the observed effect in terms of incoming flux of hydrocarbon fragments and sputter yields of the substrate and the formed disordered carbon film. (Sweden) L P Andersson and S Berg, Vacuum, 28 (10/11), 1978, 449-451.

35 4187. On the deconversion of semiconducting CdF2 crystals due to vacuum heat treatment. (GB) Crystals of semiconducting CdF2:Mn:Y have been heated under dynamic vacuum of 1 ,'< I0 -* Pa at temperatures ranging between 120 and 700°C for 1-6 h. The results of van der Pauw measurements indicate that the deconversion does not make any difference as far as the initial resistivity of the crystal is concerned if heating temperature is kept below 250°C. However, above 250°C resistivity increases in an irreversible manner due to heat treatment, the total rise depending on the duration and temperature of such treatment. Deconversion is favoured by a set of the atomic concentrations of Cd, Mn and F which have been computed on the basis of microprobe analysis data. For the computing program it was necessary to know (tz/p)(Mn, F K¢), i.e. the atomic absorption coefficient of FK= in Mn which has not previously been reported. The experimental value is 12 775. Concentration of Y was not measured since it was smaller than the detection level of the analyser (about I0 ppm). (Poland) E Kaminska et al, JPhys D: ApplPhys, 11 (18), L211-L214.

35 4188. Lowering of the degassing rate of stainless steels of the AISI 304 and 316 series. (France) A review of different techniques able to give low degassing rates on 304-316 stainless steel is proposed. We compare different results obtained with the following processes: finish degree of the surface, cleaning sequences, heat treatments (in situ bake-out, ultra-high vacuum furnace, oxydation in air, oxydation in pure oxygen), ionic bombardment. The best result is actually obtained by degassing in a furnace at 800-1000°C, typically: 10 times lower than by oxydation (in air or in pure oxygen), and 20-100 times than in situ bake-out. Ionic bombardment with argon is an excellent treatment to pull out segregations accumulated on the surface during treatment with the other processes. R Nuvolone, Vide, 33 (193), 1978, 171-182 (hi French).

35 4189. Comparison of the performances of some surface treatments. (France) The very long service lives required for aeronautical materials not only imply a judicious choice of the materials themselves but also of the protective treatments they receive. According to the utilization conditions under which they are used on aircraft helicopters, missiles or engines, fatigue, friction, corrosion or stress corrosion, embrittle- ment, problems will appear either alone or combined in a more or less acute manner. The good choice starts with the best possible knowledge of operational conditions during service life and will be supplemented by a series laboratory experiments to test the various possible solutions. Some examples concerning: fatigue, friction corrosion, hydrogen embrittlement, bolts and nuts protective treat- ments will show how all these problems are dealt with and what are the difficulties met during the tests. G Hilaire et al, Vide, 33 (193), 1978, 146-156 (in French).

36. VACUUM INSTRUMENTATION FOR SURFACE STUDIES 36

4190. Electron microprobe, principles and application to spectrochemical analyses of surfaces. (France) This introduction to microprobe analysis intends to underline only the most important features of both technique and theory. A schematic description of the instrument is given and the origin of X-ray spectra is overviewed. The 'ZAF' classical approach is first presented according to the historical development based on experimental

evidence. Then the Monte-Carlo calculation is applied to the simu- lation of electron trajectories. At last some data give the field of application of the method. J Heaoc, Vide, 33 (192) 1978, 105-113 (in French).

36 4191. Silver contact on GaAs (001) and InP (001). We report here the first results of our study of metal/Ill-V semi- conductor contacts Ag-GaAs (001) and Ag-InP (001). Clean GaAs (001) and InP (001) surfaces, under various conditions of stoichio- metry, are obtained by ion etching followed by anneling and arsenic or phosphorous adsorption and carefully characterized by AES, LEED and ELS. Using these techniques some correlations between surface stoichiometry, surface structures and electronic surface states have been found. On these surfaces the metallization has been carried out by an MBE-like technique. The results obtained show that the Schottky barrier height changes according to the conditions of surface preparation. To explain these results, different hypotheses are discussed. (France). J Massies et al, J Vac Sci Technol, 15 (4), 1978, 1353-1357.

36 4192. Schottky barriers on ordered and disordered surfaces of GaAs (110). (USA) The properties of gold Schottky barriers on the GaAs (110) surface were measured to determine the influence of atomic order, cleanliness and surface processing. Clean and ordered surfaces were prepared by ion bombardment and thermal annealing. Their chemical condition was monitored by AES and their atomic order by LEED. Some surfaces were subsequently disordered by further ion bombardment or by exposure to oxygen; others were not subjected to any further processing. Gold dots were evaporated on all surfaces without breaking the uhv. No significant differences among barrier height values measured by internal photoemission were observed, suggesting Fermi-level pinning for all junctions. Major and consistent differences in capacitance-voltage and in current-voltage measurements were found between clean and ordered surfaces, and disordered surfaces. The experimental results indicate that transport across the barriers is very sensitive to inhomogeneities in the layers adjacent to the metal- lurgical junction introduced during the various steps employed in cleaning and ordering the surface. Chemical polishing, sputter etching and annealing all affect the stoichiometry at the surface. Annealing by itself appears to produce an intrinsic layer and while it results in ordering of the surface, it does not eradicate defects and other spatial inhomogeneities, nor the drastic changes in the elec- tronic properties which are introduced in the surface by sputtering. Surfaces may have the same structure and ordering, yet differ widely in their electronic properties. Seeming inconsistencies among results of different measurement methods are ascribed to the fact that each method probes a different region and is sensitive to different conditions. A Amith and P Mark, J Vac Sci Technol, 15 (4), 1978, 1344-1352.

36 4193. Metal-semiconductor surface and interface states on (110) GaAs. (USA) Photoemission studies have been carried out for aluminium and gallium metal surfaces and metal-GaAs interfaces. Core chemical shifts and valence band changes have been simultaneously investigated with the wide photon energy range available on the 4 ° beam line at the Stanford Synchrotron Radiation Laboratory. These experiments show that although studies of the individual surfaces provide useful reference information, the separate character of the free surfaces is dominated upon interface formation by chemical changes and bond formation. The studies have shown that, for a simple metal such as aluminium, surface states and charge localization are important on specific crystalline faces. These states can affect the interface that forms between a metal and a semiconductor. For the Ga-GaAs interface investigated as the interface is formed, no Schottky barrier is found for Ga on p-type GaAs. Charge transfer from the surface As atoms to the Ga overlayers occurs, and some As migrates into the overlayer resulting in vacancies at the interface. Valence band changes with Ga coverage show that at least two occupied interface states arise. These states, at --4.2 and --5.8 eV, fall within the gap in the surface Brillouin zone for projected bulk states. With alu- minium an exchange reaction takes place, leaving an interfacial layer of AlAs and a GaAI alloy at the interface. R Z Bachrach, J Vac Sci Technol, 15 (4), 1978, 1340-1343.

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