Buffer layers in heterostructures

Abstract

This work deals with the problem of choosing buffer layer architectures in the development of standard heterostructure models with controlled levels of mechanical stresses and low defect densities in the bulk and at layer boundaries. It has been found that these parameters depend on the quality of the source wafer surface, the wafer preparation procedure for epitaxy and the composition of the buffer layers. We note that the quality of the wafer surface can be most objectively estimated from the bonding strength of the directly bonded plates. We furthermore show that bonding strengths of below 107Pa (this is the most often observed experimental value) indicate that the wafer surface has noticeable roughness and contains various contaminants and chemical compounds, clusters and dust particles, as well as structural defects of different dimensionality. Moreover the wafer surface is restructured so the broken bonds are restored. The actual wafer surface structure and the compatibility of the constituent materials have a major effect on the quality of the epitaxial film. The analysis provided in this work proves the favorable effect of the preliminary deposition of a low-temperature (LT) underlying layer onto the wafer for small lattice mismatch and the deposition of additional transition layers with changing component ratios or in the form of superlattices for large lattice mismatch.