Gap density of states in amorphous silicon-germanium alloy: Influence on photothermal deflection spectroscopy and steady-state conductivity measurements

Abstract

A series of amorphous hydrogenated silicon-germanium films has been produced by the glow-discharge dissociation of a GeH4 +SiH4 +H2 gas mixture. The subgap optical absorption has been measured with the photothermal deflection spectroscopy (PDS) technique, and the results are discussed and referred to a model density of states. The density of defects has been found meaningfully correlated to the bulk material, despite a number of possible complications intrinsic of the PDS technique and/or of the material itself, which can potentially prevent the correct determination of the density of defects from the subgap optical absorption. Namely, the defect density ranges from 1016 cm−3 in a-Si:H to 5×1017 cm−3 in a Ge:H, whereas the Urbach tail width is only slightly increased from 50 to 70 meV. The amount of disorder and the defect density of the material are compared to the predictions of an available thermodynamical model: the increasing number of defects brought about by germanium results in being an intrinsic feature of the material. Furthermore, the impact of the variable concentration of silicon and germanium dangling-bond states on the position of the Fermi level has been studied by means of the same model density of states: a noticeable correlation was found between the activation energy of the steady-state dark conductivity (EA ) and the volume density of silicon and germanium dangling bonds (Nd ). In other words, the three-step trend of EA vs x can be considered to be a fingerprint of the separate dangling-bond densities in germanium and silicon. These can be deconvolved, provided that the dangling-bond states in the forbidden energy gap are known as a function of the alloy composition.