Study of evaporated amorphous silicon films by frequency dependent conductivity and phase shift analysis of modulated photocurrent methods

Abstract

A.c. and d.c. conductivity measurements have been performed on amorphous silicon films produced by electron gun evaporation. The a.c. measurements have been performed at frequencies between 100 Hz and 2 MHz, and at temperatures between 150K and 400K. The d. c. behavior of the samples obey the T- 1/4 law between 77K and 250K. The a. c. conductivity of the films are well represented by the form Aw s , where A and s are determined to be temperature dependent parameters. The data are discussed in terms of classical models based on, pair approximation and a unified theory, the extended pair approximation, EPA. Although the a. c. behavior can be approximately explained by the Correlated Barrier Hopping model below 200K, the temperature and the frequency dependence are stronger than any classical model predicts. The a.c. data show a perfect agreement with the quasi-universal law predicted by the EPA calculations. However, quantitative calculations with the EPA results give unreasonable values for both the decay parameter a, and the rate parameter Ro. One of the major problems of the method of Phase shift analysis of modulated photocurrent, PSAMP, for studying the density of states in the energy gap of amorphous semiconductors has been the determination of the energy scale corresponding to this DOS profile. A new way of dealing with this problem is presented. A computer analysis is used to confirm the validity of this method and to demonstrate how it can be used. A simulation that is used to determine the sensitivity of the PSAMP method to the differences in the fine scale structures in the DOS distributions is presented. Four DOS distributions are considered and the expected data are obtained. The results show that the PSAMP method is very sensitive to such fine features in the DOS distributions. A comparison is also made with the sensitivity of other techniques commonly used in the determination of the DOS profiles.