Titus Pankey

Epitaxial GaAs Films Deposited by Vacuum Evaporation

Epitaxial films of GaAs have been vacuum‐deposited onto GaAs and Ge single‐crystal substrates by a modified three‐temperature‐zone technique. The film properties have been investigated by reflection electron diffraction (RED) and also optically. At a deposition temperature of 375°C the films possess [100] fiber textures; below 375°C, the film properties follow those deposited on amorphous substrates. At 400°C the films become epitaxial with twinning on all (111) planes. Between 425° and 450°C, the films are highly ordered, twin free, on the b faces of GaAs and on Ge; in the same temperature range, thick films (> 10 000 A) and films deposited on a faces of GaAs exhibit extra reflections in RED and departures from the expected optical behavior. Films deposited above 450°C, under conditions for which excess Ga may exist, grow in an hcp modification; the reflectivity of these films agrees with bulk zinc blende GaAs.

Structural and Optical Characteristics of Thin GaAs Films

Structural, electrical, and optical measurements have been carried out on over 85 GaAs films deposited under high vacuum in a thickness range between a few hundred to 70 000 A, and for substrate temperatures between 175° and 450°C. The films were textured when deposited above 220°C, exhibiting [110] textures below 280°C and [111] textures above 320°C. The films were always p type and the lowest resistivity measured was 20 Ω·cm. Films deposited below 280°C exhibit a fundamental absorption edge shift to lower energies; this shift is correlated with x‐ray line shift and annealing‐out measurements of conductivity. The absorption edge shift is attributed to a high concentration of vacancies trapped in the film during growth at low temperatures. An acceptor level 0.18 eV above the valence band is associated with these vacancies. The films exhibiting the anomalous shift can yield bulk optical properties by prolonged heating at 380°C. Films deposited above 320°C always exhibit properties characteristic of high‐re...

Structural and Optical Evaluation of Vacuum‐Deposited GaP Films

Optical absorption‐edge shifts of up to 1.5 eV have been observed for GaP films deposited in textured polycrystalline states and in the amorphous state. The amorphous‐crystalline transition occurs for a substrate temperature Ts of about 240°C. The films deposited at 240°C≤Ts≤270°C exhibiting the large optical absorption‐edge shift are nontransparent, metallic‐appearing, and exhibit a strong [110] texture. The amorphous films deposited at just below 240°C exhibit the same optical properties. With increasing Ts, the films exhibit decreasing optical absorption‐edge shifts, becoming transparent‐yellow for Ts≥425°C. However, a bulk optical behavior is achieved only by annealing the films at temperatures up to 600°C. The principal textures found by x‐ray diffraction are [110] for 240°≤Ts≤270°C, and [111] for Ts>350°C. Reflection‐electron‐diffraction (RED) observations agree substantially with those obtained by x ray, but also indicate some mixed textures. A weak (1010) reflection is observed by both x rays and...

ION‐IMPLANTATION INDUCED OPTICAL ABSORPTION EDGE SHIFTS IN GaP

The effects of implantation of argon on the optical properties of GaP films and bulk have been investigated for doses of 1013 to 1015/cm2 and energies between 1.5 and 3 MeV. Implantation of ordered films and bulk GaP results in a marked change in the optical absorption edge from about 2.3 eV to less than 1.0 eV, similar to behavior observed for amorphous films and for those subject to neutron irradiation. A maximum value of 3.22 has been observed for the index of refraction of implanted films as compared with 3.06 for bulk or highly ordered films. Bulk optical behavior can be recovered for implanted GaP with annealing at temperatures below 600°C.

Optical Properties of Cold‐ and Hot‐Deposited Gold Films

Gold films have been vacuum deposited onto quartz substrates in a temperature range from 5° to 750°C. Films which are deposited below about 50°C exhibit decreases in reflectivity of up to 25% over bulk values. Associated with this decrease are large increases in resistivity and near amorphous x‐ray diffraction patterns yielding particle sizes of less than 250 A. Films deposited above 350°C exhibit a superficial matte appearance and reflectivities which decrease to lower values with increasing deposition temperature. Associated with the onset of the matte appearance and the decrease of the reflectivity (for hot deposition) is lamellar particle growth in the plane of the films which increases, both in extent and depth, with increasing deposition temperature. Bulk values of reflectivity and electrical resistivity are observed with deposition in the temperature range between 240° and 350°C. The decrease in reflectivity for cold deposited films is explained on the basis of the Maxwell Garnett—Gans theories.

POLYMORPHISM IN VACUUM‐DEPOSITED GaP FILMS

GaP films have been vacuum‐deposited onto amorphous substrates by a three‐temperature zone technique. When the films are deposited at elevated temperatures (>500°C) and under a condition of Ga excess, whisker, rod and platelet growth occur in the film. Transmission electron diffraction examination has shown a polymorphic form of GaP to exist in these deposits.

Temperature dependence of electron drift velocity in silicon

Time‐of‐flight electron drift velocity measurements have been made for Si between 300 and 415 °K in the range of electric fields near the velocity saturation knee. The results show a more pronounced decrease from saturation values at the higher temperatures.

Attenuated‐total‐reflection measurements of Si‐SiO2 interfaces

Preliminary experiments involving attenuated‐total‐reflection (ATR) spectroscopy at the Si‐SiO2 interface of thermally grown oxides showed discrepancies in structure from standard SiO2 transmission data. Absorption coefficients calculated from SiC and SiO2 transmission data were included in a computer program which theoretically generated the ATR results showing effects of the presence of a SiC layer at the Si‐SiO2 interface. The computed results were similar to the experimental results, thus corroborating earlier Auger‐electron‐spectroscopy measurements which indicated that residual graphitic carbon on the Si surface changed to a carbide form after oxidation and remained at the Si‐SiO2 interface.

Anomalous Optical Effects in Germanium Films

The surface of a polycrystalline germanium film, vacuum-deposited onto a heated substrate in a narrow range of temperatures, takes on a regular structure exhibiting optical interference effects whose nature is quite sensitive to the angle at which the film is deposited. For all films deposited between 550° and 620°C, Fraunhofer diffraction effects are observed which are independent of crystalline texture. In the case of perpendicular deposition, the diffraction is nearly isotropic about the normal to the film, while in the case of deposition at an acute angle, the diffraction effects are nonisotropic and the observed optical behavior is analogous to that observed with blazed gratings; electron-microscope observations confirm these particular surfaces to be “blazed” in character. The “blue–grey” character of germanium films and the “blue haze” observed on other surfaces is explained on the basis of these results.