Thermoelectric power, hall effect and density-of-states measurements on glow-discharge microcrystalline silicon

Abstract

The paper deals with transport measurements and their interpretation in microcrystalline silicon specimens deposited directly in a silane glow discharge, strongly diluted with hydrogen. Also included are the results of field-effect experiments. The thermoelectric power S and the Hall effect have been measured on a series of undoped and n-type specimens between 470 and 170 K. Using the carrier densities derived from the Hall effect, it is shown that a consistent and quantitatively correct interpretation of the temperature and doping dependence of S can be given on the basis of crystalline transport theory. From the analysis it is found that the effective density of states at ϵ_s is proportional to T^v , where v varies systematically with doping level. With an average value for the heat of transport of A _c = 2·5, the Fermi level position is deduced from the thermoelectric power data as a function of temperature and doping level. ϵ_c - ϵ_f vanishes at a donor density of about 5 × 10¹⁹ cm^−1. The results of transport and field-effect measurements are then used to obtain information on the density-of-states distribution g(ϵ) near the mobility edge ϵ_c. It is shown that a model distribution of the form g(ϵ) = C(ϵ - ϵ_A)² provides a consistent fit to all experimental results. It is concluded that ϵ_c - ϵ_A ≃ 0 for undoped microcrystalline specimens, but increases to 0·07 eV in the most highly doped samples.