The transport mechanism in micro-crystalline silicon

We investigate the transport mechanism of mu c-Si:H with a typical grain size in the 200 nm range. We study in particular a series of doped mu c-Si:H samples with a wide range of crystallinities, x. The conductivity (and also the Hall mobility) vary by more than two orders of magnitude with crystallinity. In contrast, there is little variation of these quantities with temperature from room temperature to 10 K. We discuss these experimental data by comparison with theoretical network calculations of the de conductivity and the Hall mobility for a random assembly of highly conducting crystalline and essentially non-conducting amorphous grains of concentrations x and (1 - x), respectively. We show, that for x larger than the critical percolation concentration, p(c), the sample conductivity can be less than the crystalline conductivity by more than two orders of magnitude. We further show that for x > p(c), the Hall experiment, evaluated with the standard formulae valid for a homogeneous sample, yields the mean mobility of the mobile carriers to a good approximation. This agreement provides a theoretical basis for the use of Hall data in these inhomogeneous systems. The remaining temperature dependence of the conductivities for highly doped mu c-Si:H is ascribed to the affects of barriers between the crystalline grains. (C) 1998 Elsevier Science B.V. All rights reserved.