MODEL OF EL2 FORMATION IN GAAS

It is demonstrated that existing thermodynamic data on the native deep donor, EL2, in melt-grown and epitaxially grown GaAs are consistent with that defect having the atomic structure As(Ga)V(Ga). In melt-grown GaAs at high temperatures (approximately 1200-degrees-C) arsenic antisite defects appear as the complex As(Ga)V(As)V(Ga). As the temperature drops toward 1000-degrees-C and the equilibrium concentration of divacancies decreases this complex dissociates, the divacancies outdiffusing and the antisites capturing gallium vacancies to form EL2. In GaAs grown by organometallic vapor-phase epitaxy it is suggested that the arsenic interstitial is the dominant native defect produced in equilibrium with the vapor and that it dictates the deviation from stoichiometry of the epilayer. Below the growth interface these interstitials rapidly react with indiffusing divacancies to form primarily arsenic antisites. Other divacancies then react with the antisites to briefly form the complexes As(Ga)V(As)V(Ga) which, in the nonuniform temperature regime of the epilayer, dissociate into EL2 and arsenic vacancies. The model predicts [EL2] is-proportional-to (As/Ga)1/2 in agreement with selected data and predicts that the EL2 concentration will increase under a nonuniform thermal anneal. It also accounts for the formation of EL2 in GaAs grown by molecular-beam epitaxy when subsequently annealed at approximately 800-degrees-C in a nonuniform temperature environment.