Mg doping of GaN by molecular beam epitaxy

We present a systematic study on the influence of growth conditions on the incorporation and activation of Mg in GaN layers grown by plasma-assisted molecular beam epitaxy. We show that high quality p-type GaN layers can be obtained on GaN-on-silicon templates. The Mg incorporation and the electrical properties have been investigated as a function of growth temperature, Ga?:?N flux ratio and Mg?:?Ga flux ratio. It was found that the incorporation of Mg and the electrical properties are highly sensitive to the Ga?:?N flux ratio. The highest hole mobility and lowest resistivity were achieved for slightly Ga-rich conditions. In addition to an optimal Ga?:?N ratio, an optimum Mg?:?Ga flux ratio was also observed at around 1%. We observed a clear Mg flux window for p-type doping of GaN?: 0.31% < Mg?:?Ga < 5.0%. A lowest resistivity of 0.98???cm was obtained for optimized growth conditions. The p-type GaN layer then showed a hole concentration of 4.3 ? 1017?cm?3 and a mobility of 15?cm2?V?1?s?1. Temperature-dependent Hall effect measurements indicate an acceptor depth in these samples of 100?meV for a hole concentration of 5.5 ? 1017?cm?3. The corresponding Mg concentration is 5 ? 1019?cm?3, indicating approximately 1% activation at room temperature. In addition to continuous growth of Mg-doped GaN layers we also investigated different modulated growth procedures. We show that a modulated growth procedure has only limited influence on Mg doping at a growth temperature of 800??C or higher. This result is thus in contrast to previously reported GaN?:?Mg doping at much lower growth temperatures of 500??C.