In order to obtain AlxOy layer for quantum device application, the lateral sidewall oxidation of AlAs layers grown by molecular beam epitaxiy (MBE) on GaAs (n11)A-oriented substrates (n=1,2,3,4) within heterostructures was investigated. The time dependence of the oxidation rate was found to follow a linear law for the initial process, and later a parabolic law. The direction dependence of the oxidation rate for each type of substrate was measured between 390℃ and 510℃. It was found that the oxidation rate is highly anisotropic and that the anisotropy is related to the symmetry of the crystal structure. The temperature dependence of the oxidation rate was also measured between 390℃ and 600℃. Oxidation was observable for temperatures above 390℃ and the surface of the samples degraded when the temperature exceeded 540 ℃. The degradation seemed to be related to the build up of pressure inside the epilayers due to gases evoling from oxidation reaction. Optical reflection spectroscopic measurements suggested that oxidized layers are more compact than those reported previously. It was also found that AlxOy-AlGaAs-GaAs distributed bragg reflectors (DBRs) have reflectance spectra close to theoretically expected ones.