Electronic properties of the precrystallization regime of germanium: A photoemission study.

Abstract

Ultraviolet photoemission spectroscopy with synchrotron radiation has been used to measure the electronic properties of germanium films grown under different conditions. The substrates were germanium (111) single crystals and oxidized tungsten. Measurements were performed on amorphous germanium and on hydrogenated amorphous germanium films annealed at different temperatures and on films grown on substrates kept at temperatures higher than room temperature. The evolution of the electronic properties from amorphous to crystal-like behavior indicates the existence of a precrystallization regime where germanium clusters---mainly (111) oriented---are embedded in an amorphous matrix. Cluster size depends on the substrate temperature. We also observed a large variation of the photoemission gap corresponding to a different degree of ordering of the grown films. A comparison of photoemission and optical gaps shows that the core-exciton binding energy increases with an increasing degree of disorder.