Undoped 69GaAs/71GaAs isotope superlattice structures were molecular beam epitaxially deposited onto n-type GaAs substrates which had been Si-doped to about 3 x 1018/cm3. They were then used to study Ga self-diffusion in GaAs (table 6). At temperatures ranging from 850 to 960C, secondary ion mass spectrometric data indicated an activation enthalpy of 4eV for Ga self-diffusion. This value was larger than those previously found for Ga self-diffusion and Al-Ga interdiffusion under thermal equilibrium and intrinsic conditions; which were characterized by an activation enthalpy of 6eV. Secondary ion mass spectroscopic, capacitance-voltage, and transmission electron microscopic data showed that the as-grown superlattice layers were intrinsic. They became p-type, with hole concentrations of about 2 x 1017/cm3, after annealing. This occurred because the layers contained C. Dislocations were also present, at a density of 106 to 107/cm2. However, the factor which was responsible for the larger Ga self-diffusivity values which were observed here appeared to be Si out-diffusion from the substrate. Such out-diffusion decreased the electron concentration in the substrate, and caused the release of Ga vacancies into the superlattice layers, where they became supersaturated. This Ga vacancy supersaturation led to enhanced Ga self-diffusion in the superlattice layers.

Disordering in 69GaAs/71GaAs Isotope Superlattice Structures. T.Y.Tan, H.M.You, S.Yu, U.M.Gösele, W.Jäger, D.W.Boeringer, F.Zypman, R.Tsu, S.T.Lee: Journal of Applied Physics, 1992, 72[11], 5206-12

Table 7

Diffusivity of Ga in GaAs