Radial boundary layer diffusion in a cylindrical sandwich composite with application to oxidation of GaAs/AlAs/GaAs

The boundary layer diffusion in a cylindrical sandwich composite has been investigated. The system consists of a thin layer A (in the nanometer range) sandwiched between two semi-infinite outer layers B, with the diffusivity of the diffusant in A (DA) being much greater than that in B (DB). The interdiffusion between layer A and layer B is weak. In the analysis, two diffusion processes, one of the constant surface concentration source and the other the instantaneous surface source, were treated, and their concentration distributions solved analytically. Numerical results were presented to illustrate the effects of the diffusivity ratio (DA/DB) and of the specimen size on concentration distributions. The diffusion theory has been used in the study of wet oxidation in cylindrical composites of GaAs/AlAs/GaAs. The theory and experiment are in good agreement.