Experimental and theoretical study of acceptor activation and transport properties in p-type AlxGa1-xN/GaN superlattices

Experimental and theoretical results of Mg-doped superlattices consisting o f uniformly doped AlxGa1-xN, and GaN layers are presented. Acceptor activat ion energies of 70 and 58 meV are obtained for superlattice structures with an Al mole fraction of x=0.10 and 0.20 in the barrier layers, respectively . These energies are significantly lower than the activation energy measure d for Mg-doped bulk GaN. At room temperature, the doped superlattices have free-hole concentrations of 2x10(18) cm(-3) and 4x10(18) cm(-3) for x=0.10 and 0.20, respectively. The increase in hole concentration with Al content of the superlattice is consistent with theory. The room temperature conduct ivity measured for the superlattice structures is 0.27 S/cm and 0.64 S/cm f or an Al mole fraction of x=0.10 and 0.20, respectively. X-ray rocking curv e data indicate excellent structural properties of the superlattices. We di scuss the origin of the enhanced doping, including the role of the superlat tice and piezoelectric effects. The transport properties of the superlattic e normal and parallel to the superlattice planes are analyzed. In particula r, the transition from a nonuniform to a uniform current distribution (curr ent crowding) occurring in the vicinity of contacts is presented. This anal ysis provides a transition length of a few microns required to obtain a uni form current distribution within the superlattice structure. (C) 2000 Ameri can Institute of Physics. [S0021-8979(00)03816-0].