Transient enhanced intermixing of arsenic-rich nonstoichiometric AlAs/GaAs quantum wells

The superlattice intermixing of arsenic-rich nonstoichiometric AlAs/GaAs quantum wells grown at low-substrate temperatures around 300 °C is enhanced by several orders of magnitude relative to diffusion in stoichiometric structures grown at ordinary substrate temperatures. The transient enhanced intermixing is attributed to a supersaturated concentration of group-III vacancies grown into the crystal by low-temperature growth conditions. The enhanced diffusion decays during moderate-temperature annealing between 600 °C and 900 °C for annealing times between 30 and 1000 s. First-order and second-order decay kinetics were both found to agree equally well with diffusion data obtained from isochronal and isothermal annealing. However, both of these kinetics require a thermally activated annihilation enthalpy to explain temperature-insensitive behavior observed in the time-dependent diffusion coefficient. The activation enthalpy $H_a$ for the decay is between 1.4 and 1.6 eV, which is compared with the migration enthalpy $H_m=1.8\mathrm{}\mathrm{eV}$ of the gallium vacancy in GaAs. For the strongest annealing, the diffusion length approaches the self-diffusion values observed in isotopic superlattices of stoichiometric GaAs.