ZnSe and ZnSe/Ge epi-layers grown on (100) Si by molecular beam epitaxy

Abstract

ZnSe layers in the thickness range 2 to 3 μm were grown on (100) Si substrates with intermediate Ge epi-layer buffers. Ge epi-layers (∼ 0.5 μm thick) were grown by Knudsen evaporation of Ge from a PBN crucible at Si substrate temperatures around 330 °C, the Ge layer growth-rate being typically 0.33 μm/h. (2x2) reconstructed Ge surfaces were observed during Ge deposition by RHEED, the RHEED patterns being indicative of extremely smooth Ge surfaces. ZnSe layers were grown on the Ge epilayers with the substrate temperature maintained at 330 °C with growth-rates of around 0.5 μm/h. In addition, ZnSe/Ge superlattices were grown as buffers between the ZnSe and Si materials. The superlattices consisted of alternate layers of ZnSe and Ge (both layer thicknesses being around 300 Å) produced by alternate evaporation of the respective elements from Knudsen ovens. The substrate temperature was maintained constant at ∼ 330 °C for both the superlattice growth and the final ZnSe layer growth. ZnSe layer quality was assessed using photoluminescence, SEM and cross-sectional TEM analyses, with layer quality achieved using superlattice buffers and single Ge epi-layer buffers being compared to that obtained by growing ZnSe directly on (100) Si. A dramatic improvement in ZnSe layer quality was observed by incorporating a two-period ZnSe/Ge superlattice buffer over that achieved either by direct growth or using a Ge epi-layer buffer. Excitonic linewidths around 2 meV were recorded at 4.2 K from ZnSe layers grown with a two-period superlattice buffer compared with linewidths in the range 6 to 8 meV obtained from layers grown directly on Si or with 0.5 μm thick Ge epi-layer buffers. In addition, the ZnSe layer morphology was far superior for the intermediate superlattice case. Cross-sectional TEM analysis showed the layers grown with a two-period superlattice buffer to contain a significantly reduced concentration of structural defects.