Semiconductor growth in the nanowire geometry allows a greater range of materials combinations than is possible with planar growth. This follows a general trajectory of epitaxy (MBE, MOCVD) since its invention: growth with an ever increasing range of materials and materials combinations. Some of the constraints of planar semiconductor growth are that epitaxial layers generally have to be lattice-matched and have the same crystal structures as the substrate they are grown on; if they aren't, things like misfit dislocations and crystal relaxation become serious problems. Because nanowires can expand or contract laterally in two dimensions, they can accommodate much higher strain without relaxing. This allows new materials combinations, and growth on low cost, mismatched, heterovalent substrates such as silicon. Nanowires can be grown with multiple (different material) layers like an onion, or layered in a segmented fashion. An additional advantage of nanowire growth is that growth rates can be very fast compared to planar growth, and much less material is used to create a "layer," i.e. a dense forest of nanowires. Nanowires have anomalously high absorption compared to their fractional fill of the "layer" due to the tendency of light to form modes concentrated in the nanowires; conversely, it is easier to extract light from nanowires due to the lower average index of refraction of the layer. However, a challenge with nanowires is that they have high surface-to-volume ratio, and high density of crystal defects (stacking faults, twinning, polytypic errors, etc), generally yielding low quality material. It is not unusual in InAs nanowires for carrier lifetimes to be 100x-1000x shorter than in planar materials.
Type-II superlattices consist of semiconductor layers with staggered bandgaps to tune the properties of the composite layer. They can also be thought of as electronically coupled quantum wells. By tuning layer thicknesses and compositions, the bandgaps of the superlattice can be tuned over a wide range, and the bandstructure engineered to suppress Auger scattering, a serious type of nonradiative mechanism in narrow gap semiconductors.
Below is a presentation of some of our research results of InAs nanowires and superlattices grown on silicon substrates.
