Generation of light-induced surface current in c-oriented InN epitaxial layers

It has been shown theoretically that an electromotive force (EMF) can be developed between two points on the surface of InN epitaxial films when illuminated with above-band gap light. This is possible if certain surface-attached groups/adatoms, which result in the formation of donor-like-surface states that lead to the formation of a two-dimensional electron gas (2DEG) channel just below the film surface, have a gradient of density between the two contacts. The magnitude and the orientation of the EMF are governed solely by the spatial variation of the surface potential between the two points. Experimentally, we have investigated several c-axis-oriented InN epitaxial films grown on c-sapphire substrates by various techniques. The study shows downward bending of the band at the surface suggesting the formation of a 2DEG channel there. In these layers, surface photo-EMF has indeed been found to develop between contact points. At the same time, the measurement of surface potential across these points shows the existence of a gradient.