AL-GA INTERDIFFUSION THROUGH GROUP III-VACANCY 2ND NEAREST-NEIGHBOR HOPPING

We report on a systematic study of impurity-free Al-Ga interdiffusion in AlGaAs/GaAs superlattices in sealed ampoules. Three ambients were explored: along the Ga-rich solidus, with no excess Ga or As in the evacuated ampoule, and with excess As less than that required to reach the As-rich solidus limit. In each of the ambients the Arrhenius dependence of the Al-Ga interdiffusion coefficient is represented by a single activation energy throughout the temperature range investigated (700-1050-degrees-C). These results were obtained using four structures with superlattice periods ranging from 90 to 520 angstrom. Excellent agreement was obtained for the Al-Ga interdiffusion coefficients measured using superlattices on Si-doped and undoped GaAs substrates. With proper normalization to a constant As overpressure, P(As4) = 1 atm, the Ga- and As-rich activation energies are 3.26 +/- 0. 12 and 4.91 +/- 0.23 eV, respectively. These activation energies are in the range predicted for Al-Ga interdiffusion mediated by group III-vacancy second nearest-neighbor hopping. The increase in energy when going from Ga- to As-rich conditions is attributed to a shift in the Fermi-level position toward the valence band with an increase in the ionized group III-vacancy concentration.