Semiconductor materials have fueled many technological breakthroughs over the last half century (wireless communication, computing, solid-state lighting, solar powering, etc.), leaving a monumental impact on our society. Switching, amplification and light-electricity (electricity-light) conversion are the most important functions they perform. The family of III-V compound semiconductors, in particular, is known for its high electron mobilities and direct band gaps. Most importantly, they are versatile materials due to the possibility to tailor their (opto)electronic properties by selecting their composition appropriately. When grown heteroepitaxially, though, this possibility is constrained by the lattice mismatch with the substrate.
The situation is significantly different in epitaxial nanostructures, where more possibilities for strain engineering are offered at the nanoscale and new physical phenomena with potential device applications can be explored. On the other hand, the growth of such nanostructures is very demanding and necessitates good understanding and precise control of the growth mechanisms. Two examples of extremely strained nanostructures will be presented in this talk. The first example concerns InN/GaN multiple quantum wells with a thickness of one monolayer each. The role of the "self-regulation mechanism" and the high lattice mismatch (11%) in the successful growth will be highlighted, and the signatures of two-dimensional electron conductivity and its topological nature will be presented. The second example concerns vertical GaAs/InxGa1-xAs core/shell nanowires on Si substrates. This system consists of three lattice-mismatched materials, but the unique geometry of nanowires allows for growing them defect-free. The self-catalyzed vapor-liquid-solid growth mode, the peculiar accommodation of misfit strain, and the application of this type of nanowires in light emitting diodes and modulation-doped heterostructures will be discussed. The monolithic integration of GaAs/InxGa1-xAs core/shell nanowires in Si-CMOS platforms would have an enormous technological impact as high electron mobilities would boost the performance of transistors and composition-tunable direct bandgaps would add efficient optoelectronic functionalities (more-than-Moore).