Lattice location of Mg in GaN: a fresh look at doping limitations

Radioactive 27Mg (t1/2=9.5 min) was implanted into GaN of different doping types at CERN’s ISOLDE facility and its lattice site determined via beta− emission channeling. Following implantations between room temperature and 800°C, the majority of 27Mg occupies the substitutional Ga sites, however, below 350°C significant fractions were also found on interstitial positions ~0.6 Å from ideal octahedral sites. The interstitial fraction of Mg was correlated with the GaN doping character, being highest (up to 31%) in samples doped p-type with 2E19 cm−3 stable Mg during epilayer growth, and lowest in Si-doped n-GaN, thus giving direct evidence for the amphoteric character of Mg. Implanting above 350°C converts interstitial 27Mg to substitutional Ga sites, which allows estimating the activation energy for migration of interstitial Mg as between 1.3 and 2.0 eV.