On the origin of the two-dimensional electron gas at the AlGaN/GaN heterostructure interface

Bare surface barrier heights (BSBHs) of AlGaN/GaN heterostructures, with varying AlGaN layer thickness and similar to35% Al alloy composition, have been measured using UV laser induced transients. The BSBH has been observed to vary with AlGaN thickness, before it saturates beyond a critical thickness, and the variation found to be very similar to that of the two-dimensional electron gas (2DEG) density at the interface. Such a trend can be explained by considering the presence of surface donor states distributed in the band gap. The density of the surface donor states has been calculated from the variation of the surface barrier with the 2DEG density, and found to be almost constant at similar to1.6x10(13) cm(-2) eV(-1) in the energy range of similar to1.0-1.8 eV from the conduction band. After SiNx passivation of the surface, the BSBH reduces, and sheet charge density increases, indicating the presence of positive charges in the passivation layer. (C) 2005 American Institute of Physics.