Properties of a‐SiGe:H thin films: correlation between photosensitivity and density of states

Abstract

AbstractThin films of hydrogenated amorphous silicon germanium (a‐Si1‐xGex:H) alloy are prepared using DC magnetron sputtering method, at a substrate temperature of 150 °C. Silicon and germanium are co‐pulverized. All our films are deposited in the same preparation conditions except for those having a germanium content from 27% to 40%. Structural, optical and electrical properties are systematically studied from IR absorption, optical transmission and electrical measurements. Hydrogen content is determined by integrating the wagging mode infrared absorption. The increase of the Ge content from 27% to 40% is responsible for a decrease in optical bandgap of 1.59 to 1.43 eV and a reduction of the total content of hydrogen bonds in both Si‐H and Ge‐H configurations. A good correlation is observed between the reduction of the optical gap and the increase of one order of magnitude of the dark conductivity.The white light sensitivity shows a significant degradation due to germanium incorporation in the silicon matrix. Therefore, this degradation increases with germanium content. The density of states (DOS) located at the Fermi level g(EF) is estimated from the analysis of capacitance‐frequency measurements on Au/a‐Si1‐xGex:H/c‐Si(N+) Schottky structure, in zero bias condition. In our layers, the turn on condition which permits to measure the complete response of the deep states, for 1 kHz frequency, occurs at a relatively high temperature (over 75 °C). The measured density of states increases from 3x1016 cm‐3 eV‐1 to 3x1018 cm‐3 eV‐1 when the germanium content increases. This can highlight the narrow correlation between sensitivity degradation and density of states augmentation. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)