Controlled-growth and tuning of electronic properties in GaAs nanowires on Si substrates

III-As semiconductors in the form of free-standing nanowires have exhibited new potentials for a wide variety of future applications in nanotechnology, ranging from energy-efficient electronic switches to entangled-photon-pair sources for quantum information technology, including the possibility for monolithic integration in the mainstream Si technology. Using molecular beam epitaxy, we developed an in situ procedure (substrate annealing – Ga deposition – substrate annealing) in order to modify the surface of Si substrates and, thus, to achieve highly synchronized nucleation of self-catalyzed GaAs nanowire ensembles with well controlled dimensions and number density. Specifically, the radius can be as low as 10 nm, the distribution of lengths can be sub-Poissonian (due to the so-called nucleation anti-bunching), and the number density can be varied from 10 ⁶ to 10 ⁹ cm-2.
Furthermore, the GaAs nanowires can be hydrostatically strained when they are overgrown all-around with lattice-mismatched shells. The high surface-to-volume ratios allow for growing highly mismatched combinations without dislocations, beyond to what is possible in thin-film heterostructures. Here, we show that the mismatch strain inside the GaAs core can be engineered via the composition and the thickness of an (In, Ga)As or (In, Al)As shell. As a result, the electronic properties of GaAs can be widely tuned; the band gap and the electron effective mass can be reduced down to 60% of the strain-free values, rendering GaAs nanowires suitable for photonic devices across the near-infrared (800 – 1400 nm) range or for high-speed transistors.