Hall-effect measurements were used to investigate the electrical properties of undoped material. Very strong ionised impurity scattering limited the Hall mobility. Even illumination at 77K had little effect upon the electron density, but could lead to a persistent increase in the Hall mobility. The latter was related to the persistent photoconductivity effect through charged dislocation plus point-defect complexes. The properties of the defects were also investigated by monitoring transient changes in resistivity after removal of the illumination at various temperatures. The recapture of excited electrons by illumination-neutralized defects was responsible for the persistent photoconductivity effect in undoped material. The potential barrier to the recapture of excited electrons into illumination-neutralized defect complexes was about 0.0133eV. A reduced density of dislocations, obtained by optimising the growth conditions of GaN buffer layers, effectively increased the Hall mobility and weakened the persistent photoconductivity effect.

The Hall Mobility and its Relationship to the Persistent Photoconductivity of Undoped GaN G.Li, S.J.Chua, W.Wang: Solid State Communications, 1999, 111[11], 659-63