FREQUENCY-DEPENDENT CONDUCTIVITY IN EVAPORATED AMORPHOUS-SILICON FILMS

A.c. and d.c. conductivity measurements have been performed on amorphous silicon films produced by electron gun evaporation. The a.c. measurements were performed at frequencies between 100 Hz and 2 MHz, and at temperatures between 150 K and 350 K. The d.c. measurements were performed at temperatures between 90 K and 400 K. The d.c. behaviour of the samples obeys the T-1/4 law between 90 K and 150 K. The a.c. conductivity of the films is well represented by the form A-omega-s where A and s are found 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. behaviour can be approximately explained by the correlated barrier hopping model below 200 K, the temperature and the frequency dependences 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 alpha and the rate parameter R0.