Bi incorporation in GaN and AlxGa1-xN alloys

Bismuth has been recently suggested as a possible surfactant in the growth of GaN samples in place of As and Sb. In this work we use a density-functional first-principles technique to investigate the solubility and electrical activity of Bi impurities in GaN and AlGaN, two key parameters for surfactants that are so far unknown for bismuth. We find that the Bi solubility in GaN and AlGaN is very low. In n-type and moderate p-type conditions Bi behaves like a donor or an isoelectronic impurity substituting Ga at the cation site, while in extreme p-type conditions a multiple-donor Bi-N defect is formed that hinders efficient p-type doping. In AlGaN, Al and Bi substitutional defects show a tendency to pair, giving origin to an Al-Bi complex, whose electronic properties are similar to those of an isolated Bi-Ga impurity. The results of our investigations suggest that the use of Bi as surfactant is incompatible with an efficient p-type doping. Finally we examine the features of the defects extrinsic levels, identifying the current data on Bi-doped GaN luminescence as a conduction-band-to-donor recombination process for the Bi-Ga defect.