Critical thicknesses (t c) of Ge-rich strained Si1-xGex layers grown on various Ge substrates are precisely determined experimentally, and t c is revealed to strongly depend on the substrate conditions. We find that t c of Si1-xGex on Ge-on-Si(111) is much lower than that on the Ge(111) substrate for x > 0.75 while, for x < 0.75, t c becomes equivalent between both substrates, origins of which can be discussed in terms of dislocation nucleation and surface ridge formation. This study provides critical design parameters for strained SiGe(111) based devices, such as high-mobility channels and spintronic devices on a Si platform.

This study presents the material design of Si1−xGex epitaxial films/Si for thin film thermoelectric generators (TFTEGs) by investigating their thermoelectric properties. The thermoelectric films composed of group-IV elements are advantageous due to their compatibility with the Si process. We fabricated Si1−xGex epitaxial films with various controlled x values and strains using various growth methods. Ge epitaxial films without strains exhibited the highest thermoelectric power factor (∼47 μW cm−1 K−2) among various strain-controlled Si1−xGex (x ≠ 1) epitaxial films, which is higher at room temperature than SiGe alloy-based bulks ever reported. On the other hand, strained Si1−xGex epitaxial films showed an ultralow thermal conductivity of ∼2 W m−1 K−1, which is close to the value for amorphous Si. In addition to strained SiGe films with the ultralow thermal conductivity, unstrained Ge films with a high thermoelectric power factor can also be used for future TFTEGs by applying a nanostructuring technique. A preliminary TFTEG of Ge epitaxial films was realized, which generated a maximum power of ∼0.10 μW cm−2 under a temperature difference of 20 K. This demonstrates that epitaxial films composed of group-IV semiconductors are promising materials for TFTEG applications.

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We report on the highest two-terminal magnetoresistance (MR) ratio at room temperature in semiconductor-based lateral spin-valve devices. From first-principles calculations, we predict energetically stable ferromagnet–semiconductor heterointerfaces consisting of Co2MnSi (CMS) and Ge(111) upon insertion of Fe atomic layers. Using low-temperature molecular beam epitaxy, we demonstrate L21-ordered CMS epilayers at 80 °C on Ge(111), where the CMS layer can be utilized as a spin injector and detector. Two-terminal MR ratios as high as 0.1% are achieved in n-Ge-based lateral spin-valve devices with CMS/Fe/Ge Schottky tunnel contacts annealed at 200 °C. This study will open a path for semiconductor-based spintronic devices with a large MR ratio at room temperature.

We demonstrate that the critical thickness for Ge-rich strained SiGe layers can be drastically increased by a factor of more then two by means of growth on mesa-patterned Ge-on-Si. The Si0.2Ge0.8 layer grown on sub-millimeter mesa Ge-on-Si is fully strained and free from ridge roughness, while the same Si0.2Ge0.8 layers grown on unpatterned Ge-on-Si and a Ge substrate are partially strain-relaxed with the surface covered by high-density ridge roughness. This demonstrates that the proposed patterning method can provide thick and stable strained SiGe films as promising templates for realization of strained SiGe-based optoelectronic and spintronic devices.