Persistent photoconductivity study in a high mobility AlGaN/GaN heterostructure

We have used the Shubnikov-de Haas and the Hill effects to investigate the effect of subband gap illumination on the transport properties of a very hi gh mobility, mu = 54 000 cm(2)/V s at T = 1.2 K, Al0.09GaN0.91/GaN heterost ructure. We have found that this illumination resulted in a photocurrent th at was persistent at low temperatures. This photocurrent, which led to an i ncrease in the carrier density, was used to study the dependence of the mob ility and the quantum scattering time on the carrier density. Unlike in pre vious studies of persistent photoconductivity in GaN and AlGaN/GaN by other researchers, we have found that the mobility did not always increase with the carrier density as a result of illumination. For small duration of illu mination, the mobility increased with the carrier density. However addition al illumination resulted in a decrease of the mobility despite the increase in the carrier density. We believe that screening is responsible for the i nitial increase in the mobility, while ionization of defects in GaN, interf ace charges, and alloy disorder may be responsible for the eventual decreas e in the mobility. (C) 2000 American Institute of Physics. [S0021-8979(00)0 1524-3].