Impacts of ultra-high-pressure annealing on undoped and ion-implanted GaN studied by photoluminescence measurements

Selective-area doping of GaN using ion implantation (I/I) followed by ultra-high-pressure annealing (UHPA) under 1-GPa-N2 has emerged as a promising technique for GaN power devices. In this article, the impacts of UHPA on the midgap recombination centers (MGRCs) in undoped, Si-implanted, Mg-implanted, and Mg/N-implanted GaN epitaxial layers are described based on the results of photoluminescence (PL) and time-resolved PL measurements. For undoped GaN, PL lifetimes at 300 K for the near-band-edge emission decreased with increasing the annealing temperature (Ta), indicating higher Ta increased the concentration of MGRCs. In contrast, for I/I-GaN, PL lifetimes or PL intensities at 300 K for the near-band-edge emission increased with increasing Ta, indicating higher Ta decreased the concentration of MGRCs originating from the I/I-induced vacancy-type defects comprising Ga and N vacancies [e.g., (VGaVN)3]. Nevertheless, MGRC concentrations in I/I-GaN after UHPA remained several orders of magnitude higher than in undoped GaN. Additional N-I/I onto Mg-implanted GaN followed by UHPA at Ta = 1480 °C enhanced the activation of a Mg acceptor substituted on a Ga site (MgGa) and decreased the concentration of MGRCs. For Mg- and Mg/N-implanted GaN, the progressive activation of MgGa acceptors with increasing Ta was confirmed. However, the PL lifetimes for Mg- and Mg/N-implanted GaN with Mg concentrations higher than 1 × 1018 cm−3 were limited to shorter than 1 ps. To fabricate reliable p-contacts and electron inversion layers using Mg- or Mg/N-I/I combined with UHPA, it is crucial to eliminate or etch away residual defective layers near the surface, which limits the minority carrier lifetime to below 1 ps.