Lattice Location Studies of the Amphoteric Nature of Implanted Mg in GaN

Despite the renewed interest in ion implantation doping of GaN, efficient electrical activation remains a challenge. The lattice location of $^{27}$Mg is investigated in GaN of different doping types as a function of implantation temperature and fluence at CERN's ISOLDE facility. The amphoteric nature of Mg is elucidated, i.e., the concurrent occupation of substitutional Ga and interstitial sites: following room temperature ultra-low fluence $(≈2 × 10^{10} \textrm{cm}^{-2})$ implantation, the interstitial fraction of Mg is highest (20–24%) in GaN pre-doped with stable Mg during growth, and lowest (2–6%) in n-GaN:Si, while undoped GaN shows an intermediate interstitial fraction of 10–12%. Both for p- and n-GaN prolonged implantations cause interstitial $^{27}$Mg to approach the levels found for undoped GaN. Implanting above 400 °C progressively converts interstitial Mg to substitutional Ga sites due to the onset of Mg interstitial migration (estimated activation energy 1.5–2.3 eV) and combination with Ga vacancies. In all sample types, implantations above a fluence of 10$^{14}$ cm$^{-2}$ result in >95% substitutional Mg. Ion implantation is hence a very efficient method to introduce Mg into substitutional Ga sites, i.e., challenges toward high electrical activation of implanted Mg are not related to lack of substitutional incorporation.