FREQUENCY-DEPENDENT DIELECTRIC BEHAVIOR OF AMORPHOUS SILICON THIN-FILMS

The admittance Y=G+jC of amorphous silicon films, produced by electron-beam evaporation, has been measured at frequencies between dc and 107 Hz, and at temperatures between 150 and 400 K. At low frequencies the conductance G was nearly independent of frequency, an exponential function of temperature, and influenced by contacts. At medium frequencies G increased almost proportionally to s, with s0.9. In this region G was found to be contact independent and superlinearly dependent on temperature. In the high-frequency region a saturation behavior of G was observed. The capacitance C decreased monotonically from low to high frequencies. These measured properties have been attributed to a hopping between sites, randomly distributed in space and energy, and over the barriers separating the sites. The barrier height has been correlated to the intersite separation by assuming Coulomb-like potential wells. This model accounts both for the s behavior and for the saturation of G. However, to obtain good agreement between theory and experiment it was necessary to envisage a minimum relaxation time o not much smaller than 10-6 sec. In contrast, o has in previous work been interpreted to be of the order of an atomic vibrational period ( 10-12 sec). © 1979 The American Physical Society.