The Hall mobility and its relationship with persistent photoconductivity of undoped GaN

Temperature-variable Hall effect measurements have been used to investigate the electrical properties of undoped GaN, which have the electron densities on the order of mid-10(16) cm(-3) and a Hall mobility varying from < 50 cm(2)/sV to > 500 cm(2)/sV. We found that very strong ionized impurity scattering limits the Hall mobility of GaN. Illumination even at 77 K has very little effect on the electron density but can lead to a noticeable persistent increase of the Hall mobility. The induced persistent photoconductivity (PPC) effect is therefore related to the Hall mobility through intrinsic electrically active defects. The properties of those defects were further investigated by monitoring a transient change of resistivity after removal of illumination at different temperatures. It reveals that the recapturing process of excited electrons into illumination-neutralized defects is the mechanism responsible for the PPC effect of undoped GaN.