Amorphous bismuth-germanium thin films. I. Structural and electrical properties

Abstract

Thin films of bismuth-germanium have been fabricated by the method of codeposit vapor quenching onto liquid nitrogen or water-cooled substrates. Structural, electrical, optical, and photoelectrical characterizations of these films have been accomplished. In this paper, Paper I of a two-paper series, the structural and electrical characterization results are reported. X-ray diffraction measurements indicate that a stable amorphous phase exists for the BixGe1−x thin-film system for 0 ≤ x ≤ 0.12. Low-field dc resistivity as a function of temperature from room temperature to liquid-nitrogen temperature and relative thermopower measurements show that in the amorphous range the BixGe1−x thin films are n-type semiconductors with a continuously decreasing thermal activation energy as x increases. The resistivity of amorphous bismuth-germanium films decreases with an increase in bismuth concentration. Current-voltage data taken on several films at 300 and 162°K indicate Ohmic behavior with no evidence of Poole-Frenkel, Schottky-barrier, or switching effects. A structural model for the amorphous bismuth-germanium system is proposed with covalent hybrid bonding of monoatomic bismuth to a Polk-Turnbull amorphous germanium random network. An energy-band density-of-states model is proposed based on the Weaire-Thorpe interpretation of the Polk-Turnbull network and considerations from the theory of heavily doped semiconductors. The proposed structural and energy-band models are utilized to explain the observed electrical and optical properties.