P-Conductivity in Co-Implanted and Gyrotron Microwave-Annealed GaN: Optical and Electrical Studies

Abstract

A key step to the development of future (Al)GaN-based power devices is the ability to form selective-area p-type regions, and a promising method of accomplishing this is through ion implantation of the dominant acceptor dopant, Mg. Ion implantation in GaN induces lattice damage and creates point defects, including vacancies and interstitials. To remove this damage, heal defects, and activate the implanted dopants requires temperatures >1000 °C, beyond the thermodynamic equilibrium stability of GaN [1]. Removal of point defects, especially donor-like nitrogen vacancies (VN) in the material can be accomplished through high temperature annealing and may be aided by co-implantation with N [2]. It is speculated that compensation of implant-induced VN may be enhanced during the annealing process by diffusion of implanted Ni to the corresponding site in the N sublattice, however evidence for such compensation through suppression of VN-related photoluminescence has not been reported in p-type material. Methods of high-temperature nonequilibrium annealing such as symmetric multicycle rapid thermal annealing (SMRTA), microwave annealing, and laser annealing have been reported to achieve p-type material, however activation of Mg-implanted material remains an area of active study.In this study, we report our latest results in obtaining p-type conductivity in implanted GaN by repeated, short-duration thermal cycles using a high-power (kW) gyrotron microwave source and systematic study of Mg implantation and Mg+N co-implantation into GaN and the resulting defect microstructure [3]. We examine the effect of co-implantation to depths of 250 nm and 500 nm, and Mg:N concentration ratios of 1 and 2 with [Mg]=1019 cm-3. Implanted MOCVD-grown u-GaN is capped with AlN and annealed for <60 sec in 1.5–3 sec heating cycles at 1350 °C using a gyrotron under relatively low N2 overpressure of 3 MPa.The optical and electrical properties of the resulting films are characterized by temperature-dependent photoluminescence (PL) and I-V measurements of vertical p-i-n diodes. Highly resistive Mg+N co-implanted and annealed material shows dominant luminescence of the VN-related GL2 band at 2.37 eV and relatively lower-intensity acceptor-related UVL and UVL* bands (at 3.05–3.27 eV). However, implanted/annealed samples showing p-n diode behavior (Vth=3–3.1 V at room temperature) and hole concentrations in the range of 1015–1017 cm-3 show dominant/high UVL/UVL* bands and YL band centered at 2.30 eV. The YL is attributed to unintentionally introduced CN-ON complexes and is commonly observed in GaN grown by MOCVD but is typically not observed in implanted/annealed GaN. Additionally, samples implanted only with Mg and activated under the same annealing conditions show persistent dominance of the GL2 band in this range.This work is funded by the Advanced Research Projects Agency-Energy (ARPA-E), U.S. Department of Energy under the PNDIODES programs directed by Dr. Isik Kizilyalli.[1] G. Alfieri, et al. J. Appl. Phys., 123, 205303 (2018)[2] R. Tanaka et al. Japan. J. Appl. Phys., 59 , SGGD02 (2020)[3] V. Meyers et al. manuscript submitted for publication (May 2020) Figure 1