Epitaxial Deposition of Germanium by Both Sputtering and Evaporation

Abstract

The structural characteristics of germanium films deposited onto germanium (111) and CaF2 (111) substrates have been characterized as a function of sputtering and evaporation parameters. Diagrams defining epitaxial temperatures and amorphous-polycrystalline transition temperatures have been obtained both for sputtered and evaporated films. It is demonstrated that these transitions are a function of the growth rate and background pressure only. Neither the particular deposition conditions which control the growth rate (voltage and current in sputtering, source temperature, and source-substrate distance in evaporation) nor the deposition techniques influence the transitions as long as the background pressure and growth rates are identical. A qualitative model based on the relation between growth rate and surface mobility is presented which appears to adequately describe the growth-rate-epitaxial-temperature relations, including background pressure effects. The experimental data yield an activation energy for the onset of epitaxy at very low background pressures (evaporation) of 18 000 cal/mole, which is in good agreement with the activation energy for surface diffusion of germanium on germanium. Other activation energies ranging from 10 800 cal/mole at 65 μ argon pressure to 17 400 cal/mole at 8 μ argon pressure, have been obtained for sputtered films. The pressure dependence of the activation energy has been accounted for by the model presented. It is also demonstrated that the growth rates in all states of the film (amorphous, polycrystalline, and monocrystalline) are decreasing functions of substrate temperature. At any given temperature, the observed growth rates are dependent only on the incidence rate, and are independent of the technique (sputtering or evaporation) and of the conditions used to achieve the incidence rate. The dependence of growth rate on substrate temperature is consistent with established nucleation theory and yields activation energies for single-crystal growth of approximately 10 000 cal/mole, for polycrystalline growth of approximately 3000 cal/mole, and for amorphous growth of approximately 600 cal/mole. These activation energies are independent of the deposition technique (evaporation or sputtering), the deposition conditions, and the incidence rate.