New results on diffusion lengths measurements in wide bandgap semiconductors, obtained from steady state photocarrier gratings (SSPG)

In this work we develop our previous measurements of diffusion lengths in gallium nitride using the steady-state photocarrier grating (SSPG) method. We show how the use of a monomode HeCd laser as well as a photoelectrical compensation scheme improves significantly the measurement, and demonstrate the ruggedness of the SSPG technique as well as the reliability of the obtained diffusion length, which is typically in the range of several hundreds of nanometers. The small-signal photocurrent decay is exponential, with decay times in the millisecond range, a clear indication of trap-limited dynamics. We use the photocarrier grating field quenching to estimate the majority carrier drift mobility. Finally, a thermalization-limited diffusion mechanism is proposed in order to overcome a serious conflict between the experimental data and the recombination-limited SSPG diffusion theory.