Interpretation of the temperature-dependent transport properties of GaN/sapphire films grown by MBE and MOCVD

Electron transport in Si-doped and unintentionally-doped GaN films grown on sapphire by MOCVD and MBE has been analysed assuming that a parallel condu cting channel, via an impurity band, is present. No dependence on growth me thod or dopant type was observed, but other trends were apparent: a) the ac tivation energy for the impurity band feh with increased doping; b) the tem perature of the minimum in the Hall carrier density versus temperature curv es increased with doping. but did not depend strongly on the absolute value of mobility; c) the ratio of the mobility in thr GaN conduction band to th at in the impurity band also showed systematic behaviour, possibly arising from structure-related scattering processes. An STM study of the surface ch aracteristics of some of these samples suggests that potential variations a ssociated with particular structural features may be important in influenci ng the electrical properties.