Theoretical analysis and experimental results on the modulated photocarrier grating technique

We present a complete theoretical analysis of the modulated photocarrier grating method (MPG). Our calculations,show that the response of the sample will be not only a photocurrent modulated with the frequency f of the illumination, but also a response in dc and with frequency 2f. In order to put to test our calculations, we implement experimentally the MPG technique and we apply it to a hydrogenated amorphous silicon sample. We verify the existence of a response at frequency 2f, and we measure the relative intensity of this signal which compares well with the result of our theoretical calculation. We also show that under certain conditions the density of states (DOS) at the quasi-Fermi level for the majority carriers is related to the ratio of the MPG and modulated photoconductivity (MPC) signals. We present simple formula that allows estimatuing the DOS as a function of energy by changing the temperature and the generation rate. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim