Anisotropic Conductivity of n-type a-plane GaN films grown on r-plane Sapphire Substrates

We investigated the electrical properties of Ti/Al/Ni/Au Ohmic contacts and the anisotropic conductivity of ntype a-plane (11-20) GaN grown on r-plane (1-102) sapphire substrates. The Ohmic contacts and sheet resistances were measured on several wafers using transfer length method (TLM) as a function of azimuthal angles. The best specific contact resistance of 4.34×10?? Ohmㆍ㎠ was achieved after annealing at 900℃. It was found that there was significant electrical anisotropy in a-plane GaN films on r-plane sapphire substrates, and the sheet resistance varied with azimuthal angles. The sheet resistance values in the direction parallel to the m-axis [1-100] are 25%~75% lower than those parallel to the c-axis [0001] directions. Thus, basal stacking faults (BSFs) are offered as a feasible source of the anisotropic mobility in faulted heteroepitaxial a-plane GaN films because of the band-edge discontinuities at the interface between BSFs and wurtzite GaN matrix.