The dependence of impurity-free interdiffusion upon the properties of a dielectric cap layer was studied in both unstrained GaAs/AlGaAs and pseudomorphic GaInAs/GaAs multi-quantum well structures that had been grown by means of molecular beam epitaxy. Electron-beam evaporated SiO2 films, chemical vapor deposited SiOxNy films, and spun-on SiO2 films were used as cap layers during rapid thermal annealing at temperatures of between 850 and 950C. The photoluminescence at 10K was used to monitor interdiffusion-induced band-gap shifts, and to calculate the corresponding Al-Ga and In-Ga interdiffusion coefficients. The latter were found to increase with cap layer thickness (electron-beam SiO2) up to a limit which was governed by saturation of the out-diffused Ga concentration in the SiO2 caps. A maximum concentration of between 4 x 1019 and 7 x 1019/cm3 in the SiO2 caps was found by using secondary ion mass spectroscopic profiling. Larger band-edge shifts were also obtained when the O content of SiOxNy cap layers was increased, but the differences were insufficient to suggest a laterally selective interdiffusion process that was based upon variations in cap layer composition alone. Much larger differences were obtained by using various deposition techniques for the cap layers. This indicated that the porosity of the cap layer was a much more important factor than was the film composition in obtaining a laterally selective interdiffusion process. In the case of Ga0.8In0.2As/GaAs interdiffusion, the activation energies and pre-factors were estimated to range from 3.04 to 4.74eV and from 5 x 10-3 to 2 x 105cm2/s, respectively; depending upon the cap layer deposition technique and the depth of the multi-quantum well below the sample surface.

S.Bürkner, M.Maier, E.C.Larkins, W.Rothemund, E.P.O’Reilly, J.D.Ralston: Journal of Electronic Materials, 1995, 24[7], 805-12