SUBBAND GAP ABSORPTION IN HYDROGENATED AMORPHOUS-SILICON THIN-FILMS USING CONSTANT PHOTOCURRENT METHOD

The constant photocurrent method has been used to measure sub-band gap absorption in amorphous silicon (a-Si:H). Various parameters, such as , absorption coefficient, slope of Urbach tail and density of defect s tates have been obtained. The distribution of density of states has be en calculated assuming distribution of states consisting of a gaussian peak superimposed on an exponential whose slope is the same as that o f the Urbach tail. The density of neutral dangling bonds has been calc ulated considering the area between experimentally measured absorption coefficient and extrapolated part of Urbach tail. We show that these measurements are insensitive to small structures in the gap state dist ribution because a good fit of experimental data can be obtained by as suming distribution of density of states with or without a peak.