Effect of the Temperature of Formation on the Crystallinity and Electrical Properties of Germanium Films on Fluorite

Abstract

Crystalline germanium films were grown epitaxially by vacuum evaporation of germanium onto heated fluorite substrates. In the work reported here, the effect of substrate temperature during film evaporation on the crystallinity and electrical properties of the films has been explored. Thirty Ω‐cm p‐type germanium was evaporated in a conventional bell jar at pressures between 10⁻⁵ and 10⁻⁴ mm Hg. The germanium was evaporated from a resistance‐heated tantalum boat onto a fluorite plate. Substrate temperatures varied from 470° to 915°C, while the rate and duration of evaporation was held constant at about 1500 Å/minute and two minutes, respectively. The crystalline nature of the films was studied by electron diffraction. The transition from a nearly amorphous film at 470°C to a nearly single‐crystal film at 915°C was observed. Hall coefficients and electrical conductivities were measured for the films, which were p type as was the starting material. These results are presented along with the mobilities and carrier concentrations calculated from them. The mobilities increase rapidly with film evaporation temperature. Carrier lifetimes were found to be less than 1 μsec in the most crystalline film, whereas the original germanium had a carrier lifetime of about 12 μsec.