Photovoltaics based on nanowire arrays could reduce cost and materials consumption compared with planar devices but have exhibited low efficiency of light absorption and carrier collection. We fabricated a variety of millimeter-sized arrays of p-type/intrinsic/n-type (p-i-n) doped InP nanowires and found that the nanowire diameter and the length of the top n-segment were critical for cell performance. Efficiencies up to 13.8% (comparable to the record planar InP cell) were achieved by using resonant light trapping in 180-nanometer-diameter nanowires that only covered 12% of the surface. The share of sunlight converted into photocurrent (71%) was six times the limit in a simple ray optics description. Furthermore, the highest open-circuit voltage of 0.906 volt exceeds that of its planar counterpart, despite about 30 times higher surface-to-volume ratio of the nanowire cell.

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.

We report reproducible fabrication of InP-InAsP nanowire light emitting diodes in which electron-hole recombination is restricted to a quantum-dot-sized InAsP section. The nanowire geometry naturally self-aligns the quantum dot with the n-InP and p-InP ends of the wire, making these devices promising candidates for electrically-driven quantum optics experiments. We have investigated the operation of these nano-LEDs with a consistent series of experiments at room temperature and at 10 K, demonstrating the potential of this system for single photon applications.Nanowire light emitting diodes (NW LEDs) offer exciting new possibilities for opto-electronic devices. Growth of direct-bandgap NWs on Si 1, 2 will allow optically active elements to be integrated with already highly mature Si technology. For solid-state lighting applications, broad-area LEDs made from NW arrays have higher light-extraction efficiency than traditional planar LEDs 3 , and in the field of quantum optics, NWs offer the possibility to control electron transport at the single-electron level 4 and light emission at the single-photon level 5 .1Since the first demonstration of GaAs NW LEDs in 1992 6 , different geometries and materials have been used to produce NW LEDs operating over a wide range of wavelengths 3, 7-10 . Single-NW LEDs with doping modulation in the axial direction, which is the most interesting geometry for many applications, have been fabricated using GaN-GaInN multi-junctions 3 and a proof-of-principle device has been shown using InP 7 . In this letter we describe the fabrication and characterization of reproducible axial InP NW LED devices, and show that an active InAsP quantum dot region can be incorporated into these devices. The axial geometry allows for controllable injection of electrons and holes into the precisely defined active region, with the additional advantage of high light-extraction efficiency since the optically active region is not embedded in a high refractive index material. Unlike GaInN, InAsP emission can be tuned to infra-red telecommunications wavelengths where there is strong interest in electrically driven single-photon sources 11 .Nanowire p-n junctions were reproducibly grown in the vapor-liquid-solid (VLS) growth mode 12 by use of low-pressure metal-organic vapour-phase epitaxy (MOVPE). 20 nm colloidal Au particles were dispersed on (111)B InP substrates, after which the samples were transferred to a MOVPE system (Aixtron 200), and placed on a RF-heated gas foil rotated graphite disc on a graphite susceptor. The samples were heated to a growth temperature of 420 °C under phosphine (PH 3 ) containing ambient at molar fraction χ PH3 = 8.3×10 -3 , using hydrogen as carrier gas (6 l/min H 2 at 50 mbar). After a 30 s temperature stabilization step, the NW growth was initiated by introducing trimethyl-indium (TMI) into the reactor cell at a molar fraction of χ TMI = 2.2×10 -5 . During the first 20 minutes, hydrogen sulfide (χ H2S = 1.7×10 -6 ) was used for n-type doping, after which the p-type NW part was g...

A review and expansion of the fundamental processes of the vapor–liquid–solid (VLS) growth mechanism for nanowires is presented. Although the focus is on nanowires, most of the concepts may be applicable to whiskers, nanotubes, and other unidirectional growth. Important concepts in the VLS mechanism such as preferred deposition, supersaturation, and nucleation are examined. Nanowire growth is feasible using a wide range of apparatuses, material systems, and growth conditions. For nanowire growth the unidirectional growth rate must be much higher than growth rates of other surfaces and interfaces. It is concluded that a general, system independent mechanism should describe why nanowires grow faster than the surrounding surfaces. This mechanism is based on preferential nucleation at the interface between a mediating material called the collector and a crystalline solid. The growth conditions used mean the probability of nucleation is low on most of the surfaces and interfaces. Nucleation at the collector‐crystal interface is however different and of special significance is the edge of the collector‐crystal interface where all three phases meet. Differences in nucleation due to different crystallographic interfaces can occur even in two phase systems. We briefly describe how these differences in nucleation may account for nanowire growth without a collector. Identifying the mechanism of nanowire growth by naming the three phases involved began with the naming of the VLS mechanism. Unfortunately this trend does not emphasize the important concepts of the mechanism and is only relevant to one three phase system. We therefore suggest the generally applicable term preferential interface nucleation as a replacement for these different names focusing on a unifying mechanism in nanowire growth.

We demonstrate the use of nanoimprint lithography to define arrays of vertical InP nanowires. Each nanowire is individually seeded from a catalyzing gold particle and then grown via vapor−liquid−solid growth in a metal−organic vapor phase epitaxy system. The diameter and position of each nanowire can be controlled to create engineered arrays, demonstrated with a hexagonal photonic crystal pattern. This combination of nanoimprint and self-assembly of nanostructures is attractive for photonics and electronics, as well as in life sciences.

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We fabricate and demonstrate optically active quantum dots embedded in single nanowires. Observation of photon antibunching proves the zero dimensionality of these heterostructures that can be epitaxially grown on various substrates, including silicon. We show that the nanowire dots are intense single photon sources, typically an order of magnitude brighter than self-assembled quantum dots. Due to control over their composition, size, and position, nanowire dots are ideal building blocks for fully controlled quantum dot molecules.

Interest in nanowires continues to grow because they hold the promise of monolithic integration of high-performance semiconductors with new functionality into existing silicon technology. Most nanowires are grown using vapour-liquid-solid growth, and despite many years of study this growth mechanism remains under lively debate. In particular, the role of the metal particle is unclear. For instance, contradictory results have been reported on the effect of particle size on nanowire growth rate. Additionally, nanowire growth from a patterned array of catalysts has shown that small wire-to-wire spacing leads to materials competition and a reduction in growth rates. Here, we report on a counterintuitive synergetic effect resulting in an increase of the growth rate for decreasing wire-to-wire distance. We show that the growth rate is proportional to the catalyst area fraction. The effect has its origin in the catalytic decomposition of precursors and is applicable to a variety of nanowire materials and growth techniques.