Layer intermixing in heavily carbon-doped AlGaAs/GaAs superlattices

Interdiffusion of Al and Ga in heavily C-doped Al_0.3Ga _0.7As/GaAs superlattice (SL) structures has been investigated quantitatively for a variety of ambient and surface encapsulation conditions. High-resolution photoluminescence (PL) at T=1.7 K was employed to evaluate the extent of layer intermixing after 24-h anneals at 825 °C. From the shifts to higher energies of the PL peaks due to n=1 electron-to-heavy hole transitions in the quantum wells of the annealed SLs relative to the position of this peak in the as-grown crystal, approximate Al-Ga interdiffusion coefficients (D _Al-Ga) have been determined for different annealing conditions. For all encapsulants studied the interdiffusion in C-doped crystals is accelerated with increasing As₄ pressure in the annealing ampoule. This result disagrees with previously observed trends for Group II-doped p-type structures, which have led to the charged point-defect model (Fermi-level effect) of Al-Ga interdiffusion. The Si₃N₄ cap has provided the most effective surface sealing against ambient-stimulated layer interdiffusion, and yielded D_Al-Ga≊1.5-3.9×10^-19 cm²/s. The most extensive layer intermixing has occurred for uncapped SL annealed under As-rich ambient (D_Al-Ga≊3.3×10^-18 cm ²/s). These values are up to ∼40 times greater than those previously reported for nominally undoped Al_xGa_1-xAs/GaAs SLs, implying that the C_As doping slightly enhances host-atom self-diffusion on the Group III sublattice, but significantly less than predicted by the Fermi-level effect. The discrepancies between the experimental observations and the model, are discussed.