Structure Shift of GaN Among Nanowall Network, Nanocolumn, and Compact Film Grown on Si (111) by MBE

Morphology-Controlled Reststrahlen Band and Infrared Plasmon Polariton in GaN Nanostructures

Due to ultrabright and stable blue light emission, GaN has emerged as one of the most famous semiconductors of the modern era, useful for light-emitting diodes, power electronics, and optoelectronic applications. Extending GaN's optical resonance from visible to mid- and-far-infrared spectral ranges will enable novel applications in many emerging technologies. Here we show hexagonal honeycomb-shaped GaN nanowall networks and vertically standing nanorods exhibiting morphology-dependent Reststrahlen band and plasmon polaritons that could be harnessed for infrared nanophotonics. Surface-induced dipoles at the edges and asperities in molecular beam epitaxy-deposited nanostructures lead to phonon absorption inside the Reststrahlen band, altering its shape from rectangular to right-trapezoidal. Excitation of such surface polariton modes provides a novel pathway to achieve far-infrared optical resonance in GaN. Additionally, surface defects in nanostructures lead to high carrier concentrations, resulting in tunable mid-infrared plasmon polaritons with high-quality factors. Demonstration of morphology-controlled Reststrahlen band and plasmon polaritons make GaN nanostructures attractive for infrared nanophotonics.

Effects of the surface preparation and buffer layer on the morphology, electronic and optical properties of the GaN nanowires on Si

The role of Si (111) substrate surface preparation and buffer layer composition in the growth, electronic and optical properties of the GaN nanowires (NWs) synthesized via plasma-assisted molecular beam epitaxy is studied. A comparison study of GaN NWs growth on the bare Si (111) substrate, silicon nitride interlayer, predeposited AlN and GaO _x buffer layers, monolayer thick Ga wetting layer and GaN seeding layer prepared by the droplet epitaxy is performed. It is demonstrated that the homogeneity and the morphology of the NW arrays drastically depend on the chosen buffer layer and surface preparation technique. An effect of the buffer and seeding layers on the nucleation and desorption is also discussed. The lowest NWs surface density of 14 μm^-2 is obtained on AlN buffer layer and the highest density exceeding the latter value by more than an order of magnitude corresponds to the growth on the 0.3 ML thick Ga wetting layer. It is shown, that the highest NWs mean elongation rate is obtained with AlN buffer layer, while the lowest elongation rate corresponds to the bare Si (111) surface and it is twice as lower as the first case. It is found, that use of AlN buffer layer corresponds to the most homogeneous NWs array with the smallest length dispersion while the least homogeneous array corresponds to the bare Si substrate. Vertically aligned GaN NWs array on the wide bandgap GaO _x semiconductor buffer layer grown by plasma-enhanced chemical vapor deposition demonstrates its potential for electronic applications. Photoluminescence (PL) study of the synthesized samples is characterized by an intense optical response related to the excitons bound to neutral donors. The highest PL intensity is obtained in the sample with AlN buffer layer.

Influence of Structure on Electronic Charge Transport in 3D Ge Nanowire Networks in an Alumina Matrix

We demonstrate formation of material consisting of three-dimensional Germanium nanowire network embedded in an insulating alumina matrix. A wide range of such nanowire networks is produced using a simple magnetron sputtering deposition process. We are able to vary the network parameters including its geometry as well as the length and width of the nanowires. The charge transport in these materials is shown to be related to the nanowire surface per unit volume of the material, α. For low values of α, transport is characterized by space charge limited conduction and a drift of carriers in the extended states with intermittent trapping-detrapping in the localized states. For large values of α, charge transport occurs through hopping between localized electronic states, similar to observations in disorder-dominated arrays of quantum dots. A crossover between these two mechanisms is observed for the intermediate values of α. Our results are understood in terms of an almost linear scaling of the characteristic trap energy with changes in the nanowire network parameters.

Droplet epitaxy mediated growth of GaN nanostructures on Si (111) via plasma-assisted molecular beam epitaxy

We demonstrate that the use of a GaN seeding layer prepared prior to the growth of epitaxial GaN on Si (111) can lead to the formation of oriented arrays of Y-shaped nanoislands and nanowires and affects the surface density of the nanostructures.