American Vacuum Society

Two cylindrically symmetric and complementary sputtering geometries, the post and hollow cathodes, were used to deposit thick (∼25-μ) coatings of various metals (Mo, Cr, Ti, Fe, Cu, and Al-alloy) onto glass and metallic substrates at deposition rates of 1000–2000 Å/min under various conditions of substrate temperature, argon pressure, and plasma bombardment. Coating surface topographies and fracture cross sections were examined by scanning electron microscopy. Polished cross sections were examined metallographically. Crystallographic orientations were determined by x-ray diffraction. Microstructures were generally consistent with the three-zone model proposed by Movchan and Demchishin [Fiz. Metal. Metalloved. 28, 653 (1969)]. Three differences were noted: (1) at low argon pressures a broad zone 1–zone 2 transition zone consisting of densely packed fibrous grains was identified; (2) zone 2 columnar grains tended to be faceted at elevated temperatures, although facets were often replaced by smooth flat surfaces at higher temperatures; (3) zone 3 equiaxed grains were generally not observed at the deposition conditions investigated. Hollow cathode deposition accentuated those features of coating growth that relate to intergrain shading.

For n- and p-doped III–V compounds, Fermi-level pinning and accompanying phenomena of the (110) cleavage surface have been studied carefully using photoemission at hν≲300 eV (so that core as well as valence band levels could be studied). Both the clean surfaces and the changes produced, as metals or oxygen are added to those surfaces in submonolayer quantities, have been examined. It is found that, in general, the Fermi level stabilizes after a small fraction of a monolayer of either metal or oxygen atoms have been placed on the surface. Most strikingly, Fermi-level pinning produced on a given semiconductor by metals and oxygen are similar. However, there is a strong difference in these pinning positions depending on the semiconductor: The pinning position is near (1) the conduction band maximum (CBM) for InP, (2) midgap for GaAs, and (3) the valence band maximum (VBM) for GaSb. The similarity in the pinning position on a given semiconductor produced by both metals and oxygen suggests that the states responsible for the pinning resulted from interaction between the adatoms and the semiconductor. Support for formation of defect levels in the semiconductor at or near the surface is found in the appearance of semiconductor atoms in the metal and in disorder in the valence band with a few percent of oxygen. Based on the available information on Fermi energy pinning, a model is developed for each semiconductor with two different electronic levels which are produced by removal of anions or cations from their normal positions in the surface region of the semiconductors. The pinning levels have the following locations, with respect to the VBM: GaAs, 0.75 and 0.5 eV; InP, 0.9 and 1.2 eV (all levels + 0.1 eV). The first energy given is assocaited with a missing anion and the second with a missing cation. For GaSb, only an acceptor due to a missing Sb has been located at 0.1 eV. Our work is found to correlate well with that on practical Schottky barriers. A detailed comparison is made with interface state positions and densities found by others on practical MIS structures, and it is suggested that the large density of these states on III–V’s as compared to Si is due to extrinsic interface states created by stoichiometric deficits of the III–V semiconductor. For GaAs, the dominant state is found at 0.7 eV and is associated with an As deficit. For InP, the major interface level is about 0.1 eV below the CBM. These positions are in good agreement with the existing data obtained from a wide variety of samples.

The unified defect model has been successful in explaining a wide variety of phenomena as oxygen or a metal is added to the III–V surface. These phenomena cover a range from a small fraction of a monolayer of adatoms to practical III–V structures with very thick overlayers. The tenets of the unified defect model are outlined, and the experimental results leading to its formulation are briefly reviewed. InP levels 0.4 and 0.1 eV and GaAs levels 0.7 and 0.9 eV below the conduction-band minimum (CBM) are associated with either missing column III or V elements. In InP, it has been found possible by a number of workers to ’’switch’’ between the two defect levels by variations in surface processing, temperature, and/or selection of the deposited atom. The need to apply the proper concepts for surface and interface chemistry and metallurgy is recognized, and the danger of using solely bulk concepts is emphasized. The reason for this is examined for certain cases on an atomic level. The need for new fundamental attacks on interface interaction is shown. The importance of semiconductor–oxide chemical stability is also recognized and, drawing on a large body of work from several laboratories, it is suggested that there will be more difficulties with ’’native’’ oxides on GaAs than on InP. It is concluded that ’’scientific engineering’’ of interfaces to give optimum performance should be a goal and test of the fundamental work described here. Specific possibilities are discussed for Schottky barriers on III–V’s.

In this review paper, the physico–chemical properties of various dielectric films used on semiconductor devices are compared according to their method of formation or deposition. The insulators which are discussed are silicon dioxide, phosphosilicate glass, silicon nitride, alumina, borosilicate glass, and other silicate glasses. The techniques of formation or deposition are thermal oxidation, CVD and pyrolytic deposition, plasma CVD, dc and rf reactive sputtering, rf sputtering, electron beam evaporation, and fused glass from sedimented powders.

Thick [1–10 mil (25.4–254.0 μm)] OFHC copper coatings were deposited on copper, tantalum, and stainless-steel substrates maintained at temperatures (T) in the 50 °–950 °C range, at rates of from 200 to 18 000 Å/min, using primarily hollow and also post-type cathode sputtering apparatuses at argon pressures of 1 and 30 mTorr. Coating structures were examined by preparing metallographic cross sections. Surface topographies and fracture cross sections were examined by scanning electron microscopy. Crystallographic orientations were determined by x-ray diffraction. No significant deposition rate influence was found on the low-temperature structure zones reported previously [J. A. Thornton, J. Vac. Sci. Technol. 11, 666 (1974)] or on the columnar nature of coatings formed at elevated T. Truly equiaxed grain structures were generally not observed with hollow cathodes. Annealing twins were found within the grains for T≳350 °C. Evidence of extensive recrystallization and grain growth was seen for T∠900 °C. Coatings deposited at low rates and T≳400 °C on Cu substrates exhibited epitaxial effects; those on dissimilar substrates showed evidence of agglomeration.

Chemisorption of oxygen on tin oxide is studied. Correlations between electric conductivity and electron paramagnetic resonance (EPR) measurements are reported.

The chemical structures of thin SiO2 films, thin native oxides of GaAs (20–30 Å), and the respective oxide–semiconductor interfaces, have been investigated using high-resolution x-ray photoelectron spectroscopy. Depth profiles of these structures have been obtained using both argon ion bombardment and wet chemical etching techniques. The chemical destruction induced by the ion profiling method is shown by direct comparison of these methods for identical samples. Fourier transform data-reduction methods based on linear prediction with maximum entropy constraints are used to analyze the discrete structure in oxides and substrates. This discrete structure is interpreted by means of a structure-induced charge-transfer model (SICT).

High-resolution electron energy loss spectroscopy has been applied to study the adsorption of acetylene and ethylene on Pt (111) and the surface reactions of these adsorbates as a function of temperature and in the presence of chemisorbed hydrogen. Making use of the surface selection rule and the observed frequencies a rather detailed picture is developed about the chemical nature of the adsorption states and reaction intermediates. Below room temperature, acetylene rehybridizes so as to form one strong di/σ-like bond to the surface and an additional bond to the surface with the remaining π orbital. Ethylene is essentially just di/σ bonded. Around room temperature, ethylene converts to a new chemical species which we identify to be ethylidene (CH3–CH=). Ethylidene is also formed from acetylene when the surface is first exposed to hydrogen, then to acetylene at low temperatures, and subsequently annealed to 350 K. The same result was obtained by exposing adsorbed acetylene to atomic hydrogen at 350 K. We further find that the hydrogenation of adsorbed acetylene to ethylidene proceeds via the intermediate –CH2–C≡ (2-ethyl–1-ylidine).

The alloyed AuGe-based contact is widely used to make ohmic connections to GaAs. It has been presumed that the regrown alloyed region is heavily doped so that carrier transport is by tunneling. The electrical and metallurgical properties of this heterogeneous system have been extensively studied and have been shown to be spatially nonuniform. Details of fabrication technique, analysis, and theoretical interpretation of its behavior will be discussed. It is suggested that the observed (doping)−1 dependence of the contact resistance is due to spreading resistance domination of current paths through submicron regions of the contact area where heavy doping occurs.