Ambipolar transport in amorphous semiconductors in the lifetime and relaxation-time regimes investigated by the steady-state photocarrier grating technique

The ambipolar-transport equations including space-charge effects are solved for the case of a sinusoidal generation of photocarriers in amorphous semiconductors. In the ‘‘lifetime regime’’ where the dielectric relaxation time is much shorter than the lifetime no space-charge effects exist, i.e., electrons and holes move together even if their mobilities are different and an electric field is applied. In the ‘‘relaxation regime,’’ where the opposite relation between lifetime and relaxation time prevails, separation of electrons and holes occurs for different mobilities of the carriers. In any case, an electric field will separate the carriers in this regime. We apply the theory to examine experimental results for the diffusion length of photocarriers in a sample of hydrogenated amorphous silicon obtained by the steady-state photocarrier grating technique. We find that space-charge effects are not serious at low electric fields so that the true ambipolar diffusion length is obtained, but that separation of charges occurs at high electric fields.