Thermoelectric power and electronic transport in thermal-LPCVD amorphous silicon-boron films

Abstract

The temperature dependence of the thermoelectric power S and the electrical conductivity σ of thermal low-pressure chemical vapour deposited (LPCVD) a-Si:B films have been measured. The films contain a solid-phase boron concentration between 2 and 40at.%. To explain the temperature dependence of S and σ quantitatively, we propose a three-path-conduction model, involving conduction via the valence band, the band tail and a boron-related impurity band. By introducing suitable functions for the density of states and the microscopic mobility, both σ and S can be calculated numerically using the Kubo formalism. An excellent fit between calculated and experimental data has been obtained. It is found that the impurity band plays a dominant role in the transport properties of these heavily doped films. The impurity band, assumed to have a Gaussian shape, peaks at about 0·2 eV above E_v.