(Invited) Moderate Temperature Mg Diffusion Doping of GaN

Abstract

Ex-situ p-type doping of GaN has been a long-standing challenge and prevented adoption of numerous devices. While regrowth of p-GaN on etched surfaces and ion implantation have made progress, technical challenges remain. Diffusion of Mg into GaN has received considerably less attention, primarily due to the extremely low diffusivity of Mg in GaN as well as the instability of GaN at the temperatures required (typically >1000oC), as well as its propensity to form compensating defects. Here, we report on a new technique for diffusion doping of GaN with Mg at relevant temperatures (under 1000oC). This approach uses a bi-layer structure, with a solid Mg source in contact with the GaN combined with a metal capping layer. Using this approach, we have measured significant diffusion of Mg into GaN by Secondary Ion Mass Spectrometry at temperatures as low as 800oC and time on the order of minutes (Figure 1) with concentrations exceeding 1019 cm-3 and Mg extending >100 nm into the GaN film. Room temperature Hall effect measurements confirm p-type conductivity under a limited set of diffusion conditions. In other cases, compensation of the diffused Mg by formation of defects during diffusion resulted in highly resistive layers. Photo-Hall measurements using sub-band gap light of different wavelengths was used to excite both holes and electrons, depending on the wavelength. Samples with higher measured Mg concentration show correspondingly increased hole density under illumination with 2 eV light. These results strongly imply the diffusion of Mg through Ga sites, but with corresponding formation of compensating defects as the material becomes more p-type. Density functional theory (DFT) calculations on the diffusivity of Mg through various pathways as well as the formation energies of various compensating defects suggest a path towards achieving higher Mg concentrations with high free hole concentrations.This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344, LLNL-ABS-810935. This support does not constitute an express or implied endorsement on the part of the Government. The information, data, or work presented herein was funded in part by the Advanced Research Projects Agency-Energy (ARPA-E), U.S. Department of Energy, PNDIODES program monitored by Dr. Isik Kizilyalli.Figure 1: Mg profiles after diffusion at 800oC for 5 minutes Figure 1