Double surface effect causes a peak in band-edge photocurrent spectra: A quantitative model

Band-edge photocurrent spectra are typically observed in either of two shapes: a peak or a step. In this study, we show that the photocurrent band-edge response of a GaN layer forms a peak, while the same response in GaN nanowires takes the form of a step, and both are red-shifted to the actual band-edge energy. This apparent inconsistency is not limited to GaN. The physics of this phenomenon has been unclear. To understand the physics behind these observations, we propose a model that explains the apparent discrepancy as resulting from a structure-dependent surface effect. To test the model, we experiment with a GaAs layer, showing that we can deliberately switch between a step and a peak. We use GaAs because it is available at a semi-insulating doping level. We demonstrate that using this quantitative model one may obtain the exact band-edge transition energy, regardless of the red-shift variance, as well as the density of the surface state charges that cause the red shift. The model thus adds quantitative features to photocurrent spectroscopy.