Progress on and challenges of p-type formation for GaN power devices

The fabrication processes of p-type regions for vertical GaN power devices are investigated. A p-type body layer in a trench gate metal-oxide-semiconductor field-effect transistor requires precise control of the effective acceptor concentration, which is equal to the difference between the Mg acceptor concentration (N-a) and the compensating donor concentration (N-d). The carbon atoms incorporated during growth via metalorganic vapor phase epitaxy substitute nitrogen sites (C-N) and function as donor sources in a p-type GaN layer. Since interstitial H atoms ( H i ) also compensate holes, their removal from an Mg-doped layer is crucial. Extended anneals to release H atoms cause the formation of extra hole traps. The p(+) capping layer allows effective and rapid removal of H atoms from a p-type body layer owing to the electric field across the p(+)/p(-) junction. On the other hand, selective area p-type doping via Mg ion implantation is needed to control the electrical field distribution at the device edge. Ultrahigh-pressure annealing (UHPA) under a nitrogen pressure of 1GPa enables post-implantation annealing up to 1753K without thermal decomposition. Cathodoluminescence spectra and Hall-effect measurements suggest that the acceptor activation ratio improves dramatically by annealing above 1673K as compared to annealing at up to 1573K. High-temperature UHPA also induces Mg atom diffusion. We demonstrate that vacancy diffusion and the introduction of H atoms from the UHPA ambient play a key role in the redistribution of Mg atoms.