PROCESS PARAMETER DEPENDENCE OF IMPURITY-FREE INTERDIFFUSION IN GAAS/ALXGA1-XAS AND INYGA1-YAS/GAAS MULTIPLE-QUANTUM WELLS

The dependence of the impurity-free interdiffusion process on the properties of the dielectric cap layer has been studied, for both unstrained GaAs/AlxGa1-xAs and pseudomorphic InvGa1-yAs/GaAs MQW structures grown by molecular beam epitaxy. The influence of the cap layer thickness, composition, and deposition technique on the degree of interdiffusion were all systematically investigated. Electron-beam evaporated SiO2 films of varying thickness, chemical-vapor-deposited SiOxNy films of varying composition, and spin-on SiO2 films were used as cap layers during rapid thermal annealing (850-950 degrees C). Photoluminescence at 10K has been employed to determine the interdiffusion-induced bandgap shifts and to calculate the corresponding Al-Ga and In-Ga interdiffusion coefficients. The latter were found to increase with the cap layer thickness (e-beam SiO2) up to a limit determined by saturation of the out-diffused Ga concentration in the SiO2 caps. A maximum concentration of [Ga] = 4-7 x 10(19) cm(-3) in the SiO2 caps was determined using secondary ion mass spectroscopy profiling. Larger band-edge shifts are also obtained when the oxygen content of SiOxNy cap layers is increased, although the differences are not sufficiently large for a laterally selective interdiffusion process based on variations in cap layer composition alone. Much larger differences are obtained by using different deposition techniques for the cap layers, indicating that the porosity of the cap layer is a much more important parameter than the film composition for the realization of a laterally selective interdiffusion process. For the calculated In0.2Ga0.8As/GaAs interdiffusion coefficients, activation energies E(A) and prefactors D-0 were estimated to ranging from 3.04 to 4.74 eV and 5 x 10(-3) to 2 x 10(5) cm(2)/s, respectively, dependent on the cap layer deposition technique and the depth of the MQW from the sample surface.