ANALYSIS OF THE MOVING-PHOTOCARRIER-GRATING TECHNIQUE FOR THE DETERMINATION OF MOBILITY AND LIFETIME OF PHOTOCARRIERS IN SEMICONDUCTORS

We describe a technique for the determination of the carrier lifetime (τ) and the carrier mobilities (μn,μp) in semiconductors, namely the moving-photocarrier-grating method. This technique utilizes a moving intensity grating that is generated by the superposition of frequency shifted laser beams for the illumination of the sample. This results in a spatial and temporal modulation of the generation rate of photoelectrons and holes and, as a consequence, in a modulation of the photocarrier densities. The amplitudes and phases of the resulting carrier densities, space charges, and electric fields are calculated by solving the continuity and Poisson equations in the small signal approach. Their dependence on grating velocity and grating period are analyzed in the lifetime and the relaxation-time regimes. It is shown that a dc short circuit current jsc results from the action of space charge induced fields on the photogenerated electrons and holes that can be measured in an external circuit. The validity of this analysis is demonstrated by comparing the expression for jsc with data obtained for two a-Si:H samples that realize the lifetime and the relaxation-time regimes, respectively. Good fits are obtained over a wide range of grating periods and grating velocities for material parameters μn, μp, and τ that compare favorably with values obtained from other measurements. © 1995 The American Physical Society.