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Studies on sublimed GaAs films, anodic A12O3 films and A12O3/GaAs interfaces Yan, George


The structural and electrical properties of sublimed GaAs films, the dielectric properties of anodic A1₂O₃ films and the electrical properties of A1₂O₃/GaAs interfaces are of interest from the viewpoint of using GaAs and A1₂O₃ films in thin-film integrated circuits. A new method, the close-spaced sublimation (CSS) method, was developed and used to deposit GaAs films on sapphire. The effects of growth conditions on the structural properties of the films were investigated using optical and electron microscopy, an electron microprobe and X-ray diffraction techniques. Crystallites increased in size with increasing substrate temperature, from about 0.7μ to 20μ for substrate temperatures from 480 to 670°C. The degree of preferred orientation of crystallites in the films increased with increasing substrate temperature. The films exhibited <111> textures when substrate temperatures were above about 600°C. Single-crystal diffraction patterns were obtained from films deposited on substrates held at 630 to 640°C. Electron microprobe analysis indicated that the ratio of Ga to As in the films was stoichiometric to less than 2 wt %. The as-grown heteroepitaxial films were p-type with room-temperature hole Hall mobility up to 42 cm²/V-sec. The room-temperature resistivity ranged from 0.6Ω -cm to 1.6 x 10⁵Ω-cm. The resistivity of higher resistivity films was more temperature dependent than that of lower resistivity films. The electrical properties of the films are discussed in terms of the effects of space charge regions in the grains, potential barrier at the grain boundaries, deviation from stoichiometry, and compensation of impurities. Conductivity-type conversion of the as-grown films to n-type was done by postdeposition dopant diffusion. A room-temperature electron Hall mobility of 77 cm²/V-sec was obtained. While transistor action and rectification characteristics were observed in thin-film insulated-gate field-effect transistors and Au-Schottky barrier diodes made with GaAs films, better films are required before devices with characteristics competitive with bulk devices can be fabricated. An n-type homoepitaxial film was deposited by using the CSS method. The film's electron Hall mobility varied with temperature as T³ˡ², which is the form predicted by the Brooks-Herring formula for ionized impurity scattering. The room-temperature electron Hall mobility was 219 cm²/V-sec. A.c. bridge and step response methods were used to study the dielectric properties of anodic A1₂O₃ films. Metal/ A1₂O₃/Al capacitors were made using evaporated Al films on glass which-had been anodized in ammonium penta-borate dissolved in ethylene glycol. The dielectric constant and loss tangent of anodic A1₂O₃ decreased with increasing frequency over the range from 0.5 to 100 kHz. Step response currents followed a t⁻ⁿ law. For linear dielectric response, this corresponded to є”(ω) varying as ωⁿ⁻¹. Electrical properties of A1₂O₃/GaAs interfaces were studied using the metal-insulator-semiconductor (MIS) capacitance technique. Theoretical curves relating the capacitance of metal/ A1₂O₃/GaAs capacitor to the d.c. voltage applied across the capacitor were calculated and plotted. These capacitance-voltage (C-V) curves then served as a basis for the interpretation of experimental C-V curves. "Fast" surface state densities greater than 10¹²/cm²-eV were obtained.

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