LIGHT-INTENSITY DEPENDENCE OF THE PHOTOCONDUCTIVITY OF HYDROGENATED AMORPHOUS-SILICON

The physical meaning of the exponent gamma characterizing the light intensity dependence of the photoconductivity sigma-p in hydrogenated amorphous silicon is investigated on the basis of a dual-beam modulated photoconductivity (DBMP) analysis. The frequency-dependent spectrum can be resolved 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 gamma-i not-equal gamma. On this basis a model is established which allows firstly sigma-p to be related to the components separated by DBMP, secondly each component to be characterized by its relative weight a(i) such that SIGMA-a(i)gamma-i = 1, thirdly the dependence of gamma on the position of the quasi-Fermi level in the gap to be related to the variations in SIGMA-a(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 value 0.5 eV below the mobility edge and a standard deviation of about 0.1 eV.