Electronic band structure of insulating hydrogenated carbon-germanium films

Abstract

Amorphous carbon-germanium films (a-GeXCY:H) produced by plasma-assisted chemical vapor deposition from tetramethylgermanium in a radio-frequency (13.56 MHz) glow discharge, were investigated. It had been found previously that these films, according to the discharge power P, could be obtained as an amorphous semiconductor or an amorphous insulator. The latter type of the films (deposited at P=5 W) is a subject of this article. Investigations on an optical absorption, bulk photogeneration, and internal photoemission were performed. On this basis the main parameters of the electronic band structure of the films, such as transport gap EG, optical gaps, Eopt and E04, electron affinity χ, and ionization potential J, were determined. These parameters change drastically as a result of an aging process. For the fresh films, EG=5.7 eV, Eopt=3.1 eV, and E04=3.6 eV, while for the aged films, EG=7.1 eV, Eopt=3.8 eV, E04=4.2 eV, χ=1.2 eV, and J=8.3 eV. It is suggested that the aging process consists in the oxidation of the films, which causes a reduction of the density of localized states in the bulk of the film as well as a drastic decrease in the surface states (from about 1018 m−2 V−1 for the fresh film to 1.6×1016 m−2 V−1 for the aged film).