Shadow Mask Technology

This study investigated molecular beam epitaxy MBE regrowth through shadow masks developed from AlGaAsGaAs layers on GaAs 001 substrates. Adjusting the directions of the molecular beams relative to the masks results in in situ lateral structuring. This enabled the authors to modify doping and composition within one layer, to shift it laterally, and even to split it or to reduce its width relative to the masks aperture. The resolution of this selective area epitaxy SAE technique is restricted by half shadows and surface diffusion. With diaphragms and thin masks it is possible to decrease half shadows to a few nanometers. However, diffusion mainly is a material parameter. The authors investigated III-V and II-VI semiconductors. The latter case was extremely dependent on crystal orientation. Along 1-10 diffusion lengths are very short. This is in agreement with ab initio calculations in their group. Thus, lateral structuring with a resolution of a few nanometers is achieved. The authors discuss the degrees of freedom, which arise from different chamber geometries and mask concepts. With simple striped masks complex structures such as stacked three-color detectors or selective contacts to embedded 2DEGs can be realized. Three novel mask concepts are presented. Deeply underetched masks serve as elastic substrates for misfit layers over the critical thickness without plastic relaxation. As an analogy to cleaved edge overgrowth, the authors use masks developed from AlGaAsGaAs superlattices. Finally interconnected structures with lateral periodicity of a few micrometers have been realized with grid masks. 8 figures, 16 refs.