BANDGAP FLUCTUATIONS IN COMPENSATED A-SIH (P, B) FILMS

N-type compensated films of a-Si: H (P, B) and a-Si: H (Li, B) are pre pared by the glow discharge decomposition of SiH4. Doping is achieved from the gas phase by addition of PH3 and B2H6 or after film depositio n by in-diffusion of Li. The statistical shift of the Fermi level infe rred from the temperature dependence of the dark conductivity is consi stent with a pronounced minimum in the density of states distribution in the upper half of the mobility gap. This minimum apparently shifts towards midgap when the boron content of the films is increased. The o ptical absorption indicates a decrease of the bandgap and an increase of the Urbach-tail parameter E(t) with increasing boron content. Both the statistical shift and the changes in the optical absorption are ex plained in terms of an apparent broadening of the bandtails due to bor on-induced fluctuations of the bandgap.