Does Bi form clusters in GaAs1 − xBi x alloys?

GaAs1 − xBix alloys attract significant interest due to their potentiality for several applications, including solar cells. Recent experiments link the crucial optical properties of these alloys to Bi clustering at certain Bi compositions. Using ab initio calculations, we show that there is no thermodynamical driving force for the formation of small GaBi clusters incorporating As substitutional sites. However, the Ga vacancies should gather Bi atoms leading to small Bi clusters, and the Ga vacancies can act as nucleation centers for phase separation. The formation energy of the GaAs1 − xBix with respect to GaAs and GaBi shows a maximum at intermediate Bi concentrations. Thermodynamics and kinetics of the GaAs1 − xBix film growth is discussed. High Bi solubility is obtained, if the Bi atoms on the energetically favorable atom positions in the subsurface layer are relatively frozen. The Ga vacancy concentration may be increased by the incorporation of Bi. The Bi atoms can also prevent the out diffusion of Ga vacancies. [ABSTRACT FROM AUTHOR]