Independence of photoconductivity of p-type a-Si:H films on doping level and defect concentration

The temperature dependence of the photoconductivity of boron doped a-Si:H films was studied in detail before and after intense light soaking. At temperatures from 200 to 330 K the lack of dependence of the photoconductivity of p-type a-Si:H films on doping level and light induced defects was observed. These results are explained by a model based on the concept that non-equilibrium occupation of dangling bonds (DB) in p-type a-Si:H differs essentially from equilibrium one. This is mainly caused from asymmetry of the density of states distribution in conductance- and valence-band tails. According to the proposed model, the non-equilibrium concentration of D0-centers in p-type a-Si:H is determined not by the doping level and total DB concentration, but by the parameters of the effective trap level in the valence band tail. Numerical calculations taking into account the direct capture of free carriers by DB and tunneling recombination between trapped electrons in the conduction band tail and trapped holes in the valence band tail confirm our findings.