Theoretical description of H behavior in GaN p-n junctions

The diffusion and reactions of hydrogen in GaN are described by applying differential equations for the concentration profiles of H species, charged dopants, and carriers with simultaneous solution of Poisson's equation. This approach dispenses with the simplifying assumptions of local equilibrium among states and local charge neutrality that were employed previously by us to treat high-temperature H behavior in uniform layers. The result is a more general modeling capability which encompasses nonequilibrium conditions and space-charge effects such as are encountered in devices. Density-functional theory, previously used by us to treat equilibrium H energies, is employed herein to examine activation barriers and wave-function overlaps affecting the rates of relevant H and carrier reactions, thereby guiding the selection of mechanisms to be included and influencing the evaluation of some rate parameters. The model is applied to H-containing p-n junctions, with detailed consideration of the reversible, metastable electrical activation of H-passivated Mg acceptors that has been observed experimentally under forward bias. The calculations point to interstitial H-2 as the state of the H resulting from such activation, and this conclusion is supported by good agreement between the predicted and observed onset temperatures for repassivation under open-circuit annealing. In modeling the more complex activation process, experimentally observed qualitative features are reproduced by choosing relative carrier-capture cross sections in accord with ab initio theoretical considerations. In other model calculations, H is shown to be expelled from the carrier-depleted zone of p-n junctions, causing H redistribution under reverse bias. (C) 2001 American Institute of Physics.