The narrow gap semiconductor alloys SnxGe1 - x and SnxSi1 - x offer the possibility for engineering tunable direct energy gap Group IV semiconductor materials. For pseudomorphic SnxGe1 - x alloys grown on Ge (001) by molecular beam epitaxy, an indirect-to-direct bandgap transition with increasing Sn composition is observed, and the effects of misfit on the bandgap analyzed in terms of a deformation potential model. Key results are that pseudomorphic strain has only a very slight effect on the energy gap of SnxGe1 - x alloys grown on Ge (001) but for SnxGe1 - x alloys grown on Ge (111) no indirect-to-direct gap transition is expected. In the SnxSi1 - x system, ultrathin pseudomorphic epitaxially-stabilized α-SnxSi1 - x alloys are grown on Si (001) substrates by conventional molecular beam epitaxy. Coherently strained α-Sn quantum dots are formed within a defect-free Si (001) crystal by phase separation of the thin SnxSi1 - x layers embedded in Si (001). Phase separation of the thin alloy film, and subsequent evolution occurs via growth and coarsening of regularly-shaped α-Sn quantum dots that appear as 4-6 nm diameter tetrakaidecahedra with facets oriented along elastically soft 〈100〉 directions. Attenuated total reflectance infrared absorption measurements indicate an absorption feature due to the α-Sn quantum dot array with onset at ∼0.3 eV and absorption strength of 8 × 103 cm-1, which are consistent with direct interband transitions. © 2001 Elsevier Science B.V. All rights reserved.

Transmission electron microscopy studies in both the scanning and parallel illumination mode on samples of two generic types of self-assembled semiconductor quantum dots are reported. III-V and II-VI quantum dots as grown in the Stranski-Krastanow mode are typically alloyed and compressively strained to a few %, possess a more or less random distribution of the cations and/or anions over their respective sublattices, and have a spatially non-uniform chemical composition distribution. Sn quantum dots in Si as grown by temperature and growth rate modulated molecular beam epitaxy by means of two mechanisms possess the diamond structure and are compressively strained to the order of magnitude 10%. These lattice mismatch strains are believed to trigger atomic rearrangements inside quantum dots of both generic types when they are stored at room temperature over time periods of a few years. The atomic rearrangements seem to result in long-range atomic order, phase separation, or phase transformations. While the results suggest that some semiconductor quantum dots may be structurally unstable and that devices based on them may fail over time, triggering and controlling structural transformations in self-assembled semiconductor quantum dots may also offer an opportunity of creating atomic arrangements that nature does not otherwise provide.