Light intensity dependence of the photoconductivity of hydrogenated amorphous silicon

The physical meaning of the exponent γ characterizing the light intensity dependence of the photoconductiyity σ p in hydrogenated amorphous silicon is investigated on the basis of a dual-beam modulated photoconductiyity (DBMP) analysis. The frequency-dependent spectrum can be resolyed into three components which at high temperatures are due to thermal re-emission of electrons from D− centres, thermal re-emission of electrons from tail states and electron-hole recombination. It is found that each component follows its own generation rate dependence with an exponent γ i ≠ γ. On this basis a model is established which allows firstly σ p to be related to the components separated by DBMP, secondly each component to be characterized by its relative weight a i such that Σa i γ i,= 1, thirdly the dependence of γ on the position of the quasi-Fermi level in the gap to be related to the variations in Σa i and fourthly the weights a i and the subgap density of states to be quantitatively correlated. Experimental data are analysed along these lines and simulated by assuming that the D− centres have a Gaussian distribution with mean yalue 0.5 eV below the mobility edge and a standard deyiation of about 0.1 eV.