Elastic strain engineering in silicon and silicon-germanium nanomembranes

Strain in crystalline materials alters the atomic symmetry, thereby changing materials properties. Controlling the strain allows tunability of these new properties. Elastic strain engineering in crystalline nanomembranes (NMs) provides ways to induce and relax strain in thin sheets of single-crystalline materials without exposing the material to the formation of extended defects. I use strain engineering in NMs in two ways: (1) elastic strain sharing between multiple layers using the crystalline symmetry of the layers to induce unique strain distributions, and (2) complete elastic relaxation of single-crystalline alloy NMs. In both cases, NM strain engineering methods enable the introduction of unique strain profiles or strain relaxation in ways not compatible with conventional bulk processing, where strain destroys the long-range crystallinity.;Elastically strain-shared NMs are fabricated by releasing multi-layer thin film heterostructures from the original host substrate. If one layer of the original heterostructure contains strain, the strain will share between the layers of the freestanding NM. The extent of strain sharing will depend on the relative thicknesses, the ratio of the elastic moduli between the materials, and elastic symmetry of the layers. I calculate strain distributions in flat NMs between layers with 2-fold and 4-fold elastic symmetry. I verify my calculations with experimental proof of two examples: (1) strain sharing between biaxially isotropic layers, Si/SiGe/Si(001), and (2) strain sharing between biaxially anisotropic layers, Si/SiGe/Si(110).;Strain engineering in NMs is also used to relax strain elastically in thin materials that are difficult to fabricate with conventional bulk crystal growth techniques. Thin films of SiGe grow uniformly and elastically strained on Si substrates. I release the SiGe layer from the Si growth template with NM fabrication processes and allow the SiGe allow to relax elastically to the appropriate bulk lattice constant. I confirm the high structural quality and strain uniformity of these new materials, and demonstrate their use as substrates for technologically relevant epitaxial films by growing strained Si layers and thick, lattice-matched SiGe alloy layers on them. I compare the structural quality of epitaxial films grown on SiGe NMs to those grown on plastically relaxed SiGe substrates.