Investigation of the accuracy of the spectral photocurrent method for the determination of minority carrier diffusion length

Abstract

The non-scanning, junction-based photocurrent method is one of many techniques that have been devised to determine the minority carrier diffusion length in semiconductor materials. The accuracy of this method is the subject of this paper and the investigation focuses on the theoretical equation that relates the diffusion length to the measured photocurrent produced by a Schottky barrier diode. Specifically, the investigation concentrates on a commonly used assumption in the derivation of this equation, which is that carriers arriving at the edge of the depletion region are swept away from the depletion edge toward the surface barrier at a velocity that is much greater than the diffusion velocity. The subsequent analysis and the results presented in this paper are based on experiments using p-type GaN. The results obtained from numerical simulations show that both velocities have comparable magnitude and the assumption in question is technically incorrect, especially when the diffusion length is very short, as is common for p-type GaN. However, further analysis also shows that in practice this does not adversely affect the accuracy of the diffusion values calculated. In fact, when the diffusion length is very short, the common equation results in calculated diffusion length values that are more accurate compared to those derived from the more detailed equation. This is an indirect consequence of the choice of the effective depletion width and hence, the relative proportion of the drift and diffusion currents.