THE TRANSPORT MECHANISM IN MICROCRYSTALLINE SILICON

We investigate the transport mechanism of mu c-Si:H with a typical gra in 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 conduct ivity (and also the Hall mobility) vary by more than two orders of mag nitude with crystallinity. In contrast, there is little variation of t hese quantities with temperature from room temperature to 10 K. We dis cuss these experimental data by comparison with theoretical network ca lculations of the de conductivity and the Hall mobility for a random a ssembly of highly conducting crystalline and essentially non-conductin g amorphous grains of concentrations x and (1 - x), respectively. We s how, that for x larger than the critical percolation concentration, p( c), the sample conductivity can be less than the crystalline conductiv ity 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 carr iers to a good approximation. This agreement provides a theoretical ba sis for the use of Hall data in these inhomogeneous systems. The remai ning 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.