Crystal structure transfer in core/shell nanowires.

Structure engineering is an emerging tool to control opto-electronic properties of semiconductors. Recently, control of crystal structure and the formation of a twinning superlattice have been shown for III-V nanowires. This level of control has not been obtained for Si nanowires, the most relevant material for the semiconductor industry. Here, we present an approach, in which a designed twinning superlattice with the zinc blende crystal structure or the wurtzite crystal structure is transferred from a gallium phosphide core wire to an epitaxially grown silicon shell. These materials have a difference in lattice constants of only 0.4%, which allows for structure transfer without introducing extra defects. The twinning superlattices, periodicity, and shell thickness can be tuned with great precision. Arrays of free-standing Si nanotubes are obtained by a selective wet-chemical etch of the core wire.

The role of surface energies and chemical potential during nanowire growth.

We present an approach to quantitatively determine the magnitudes and the variation of the chemical potential in the droplet (Δµ), the solid-liquid (γ(SL)) and the liquid-vapor (γ(LV)) interface energies upon variation of the group III partial pressure during vapor-liquid-solid-growth of nanowires. For this study, we use GaP twinning superlattice nanowires. We show that γ(LV) is the quantity that is most sensitive to the Ga partial pressure (p(Ga)), its dependence on p(Ga) being three to four times as strong as that of γ(SL) or Δµ, and that as a consequence the surface energies are as important in determining the twin density as the chemical potential. This unexpected result implies that surfactants could be used during nanowire growth to engineer the nanowire defect structure and crystal structure.

Formation of wurtzite InP nanowires explained by liquid-ordering.

We report an in situ surface X-ray diffraction study of liquid AuIn metal alloys in contact with zinc-blende InP (111)(B) substrates at elevated temperatures. We observe strong layering of the liquid metal alloy in the first three atomic layers in contact with the substrate. The first atomic layer of the alloy has a higher indium concentration than in bulk. In addition, in this first layer we find evidence for in-plane ordering at hollow sites, which could sterically hinder nucleation of zinc-blende InP. This can explain the typical formation of the wurtzite crystal structure in InP nanowires grown from AuIn metal particles.

Paired twins and [112] morphology in GaP nanowires.

Formation of random as well as periodic planar defects can occur during vapor-liquid-solid growth of nanowires with the zinc-blende crystal structure. Here we investigate the formation of pairs of twin planes in GaP nanowires. In such pairs, the first twin plane is formed at a random position, rapidly followed by the formation of a second twin plane of which the position is directly related to that of the first one. We show that the triangular [112] morphology of the nanowire is a key element in the formation of these twin pairs. We have extended our previous kinetic nucleation model and show that this describes the development of the nanowire morphology and its relation with the formation of single and paired twin planes.

Single electron charging in optically active nanowire quantum dots.

We report optical experiments of a charge tunable, single nanowire quantum dot subject to an electric field tuned by two independent voltages. First, we control tunneling events through an applied electric field along the nanowire growth direction. Second, we modify the chemical potential in the nanowire with a back-gate. We combine these two field-effects to isolate a single electron and independently tune the tunnel coupling of the quantum dot with the contacts. Such charge control is a first requirement for opto-electrical single electron spin experiments on a nanowire quantum dot.

Generic nano-imprint process for fabrication of nanowire arrays.

A generic process has been developed to grow nearly defect-free arrays of (heterostructured) InP and GaP nanowires. Soft nano-imprint lithography has been used to pattern gold particle arrays on full 2 inch substrates. After lift-off organic residues remain on the surface, which induce the growth of additional undesired nanowires. We show that cleaning of the samples before growth with piranha solution in combination with a thermal anneal at 550 degrees C for InP and 700 degrees C for GaP results in uniform nanowire arrays with 1% variation in nanowire length, and without undesired extra nanowires. Our chemical cleaning procedure is applicable to other lithographic techniques such as e-beam lithography, and therefore represents a generic process.

Selective excitation and detection of spin states in a single nanowire quantum dot.

We report exciton spin memory in a single InAs(0.25)P(0.75) quantum dot embedded in an InP nanowire. By synthesizing clean quantum dots with linewidths as narrow as about 30 microeV, we are able to resolve individual spin states at magnetic fields on the order of 1 T. We can prepare a given spin state by tuning excitation polarization or excitation energy. These experiments demonstrate the potential of this system to form a quantum interface between photons and electrons.

Zinc incorporation via the vapor-liquid-solid mechanism into InP nanowires.

We report the incorporation of zinc atoms into vapor-liquid-solid grown indium phosphide nanowires via a gold catalyst particle. We demonstrate this by synthesizing axial pn-junctions, chemically etching them, and fabricating electrical contacts in a vertical configuration. Electrical measurements show clear diode behavior. Control of dopant incorporation is crucial for future applications and will eventually lead to full freedom of design.

Large photonic strength of highly tunable resonant nanowire materials.

We demonstrate that highly tunable nanowire arrays with optimized diameters, volume fractions, and alignment form one of the strongest optical scattering materials to date. Using a new broad-band technique, we explore the scattering strength of the nanowires by varying systematically their diameter and alignment on the substrate. We identify strong Mie-type internal resonances of the nanowires which can be tuned over the entire visible spectrum. The tunability of nanowire materials opens up exciting new prospects for fundamental and applied research ranging from random lasers to solar cells, exploiting the extreme scattering strength, internal resonances, and preferential alignment of the nanowires. Although we have focused our investigation on gallium phosphide nanowires, the results can be universally applied to other types of group III-V, II-VI, or IV nanowires.

Twinning superlattices in indium phosphide nanowires.

Semiconducting nanowires offer the possibility of nearly unlimited complex bottom-up design, which allows for new device concepts. However, essential parameters that determine the electronic quality of the wires, and which have not been controlled yet for the III-V compound semiconductors, are the wire crystal structure and the stacking fault density. In addition, a significant feature would be to have a constant spacing between rotational twins in the wires such that a twinning superlattice is formed, as this is predicted to induce a direct bandgap in normally indirect bandgap semiconductors, such as silicon and gallium phosphide. Optically active versions of these technologically relevant semiconductors could have a significant impact on the electronics and optics industry. Here we show first that we can control the crystal structure of indium phosphide (InP) nanowires by using impurity dopants. We have found that zinc decreases the activation barrier for two-dimensional nucleation growth of zinc-blende InP and therefore promotes crystallization of the InP nanowires in the zinc-blende, instead of the commonly found wurtzite, crystal structure. More importantly, we then demonstrate that we can, once we have enforced the zinc-blende crystal structure, induce twinning superlattices with long-range order in InP nanowires. We can tune the spacing of the superlattices by changing the wire diameter and the zinc concentration, and we present a model based on the distortion of the catalyst droplet in response to the evolution of the cross-sectional shape of the nanowires to quantitatively explain the formation of the periodic twinning.

Design of light scattering in nanowire materials for photovoltaic applications.

We experimentally investigate the optical properties of layers of InP, Si, and GaP nanowires, relevant for applications in solar cells. The nanowires are strongly photonic, resulting in a significant coupling mismatch with incident light due to multiple scattering. We identify a design principle for the effective suppression of reflective losses, based on the ratio of the nondiffusive absorption and diffusive scattering lengths. Using this principle, we demonstrate successful suppression of the hemispherical diffuse reflectance of InP nanowires to below that of the corresponding transparent effective medium. The design of light scattering in nanowire materials is of large importance for optimization of the external efficiency of nanowire-based photovoltaic devices.

Three-dimensional morphology of GaP-GaAs nanowires revealed by transmission electron microscopy tomography.

We have investigated the morphology of heterostructured GaP-GaAs nanowires grown by metal-organic vapor-phase epitaxy as a function of growth temperature and V/III precursor ratio. The study of heterostructured nanowires with transmission electron microscopy tomography allowed the three-dimensional morphology to be resolved, and discrimination between the effect of axial (core) and radial (shell) growth on the morphology. A temperature- and precursor-dependent structure diagram for the GaP nanowire core morphology and the evolution of the different types of side facets during GaAs and GaP shell growth were constituted.