Advances in III-V Semiconductor Nanowires and Nanodevices

III-V semiconductor nanowires are expected to play a significant role in future nanoscale electronic and optoelectronic devices. In this chapter, we attempt to review the general synthetic strategies for III-V compound nanowires. We first summarize various III-V nanowire growth techniques such as chemical vapor deposition, laser ablation, metal-organic chemical vapor deposition, molecular beam epitaxy, chemical beam epitaxy, hydride vapour phase epitaxy, wafer annealing, and low-temperature solution methods. Subsequently, we discuss mechanisms involved to generate III-V nanowires from different synthetic schemes and conditions, including vapor-liquid-solid, vapor-solid-solid, solution-liquid-solid, vapor-solid (self-catalytic, oxide-assisted, and axial screw dislocation), ligand-aided solution-solid, and reactive Si-assisted growth.

This book chapter provides an overview of the recent developments of III-nitride nanowire heterostructures consisting of GaN, AlN, InN, and their alloys. The growth techniques and mechanisms for IIInitride nanowires are first briefly reviewed, followed by detailed discussions on the structural, optical and electrical transport properties of various III-nitride nanowire heterostructures. Special attention is paid to the recent achievement of high quality InN, InGaN core-shell, as well as dot-in-a-wire nanoscale heterostructures. The emerging device applications of III-nitride nanowires, including nanoscale transistors, LEDs, lasers, and solar cells are presented, and the challenges and future prospects of III-nitride nanowires are also discussed.

In this chapter we review the latest developments in the field of InP and GaP nanowires. A major part of the chapter is devoted to the vapor-liquid-solid (VLS) growth (mechanism) of nanowires, but also ‘catalystfree’ growth mechanisms will be discussed. The chapter starts with methods to obtain control of nanowire diameter and position. Such control is essential to understand the details of the growth mechanism. It will be shown that the liquid particle can truly catalyze the growth of nanowires. The other main theme considers the structural and optical nanowire properties. Nanowires can have a different (wurtzite) crystal structure than the bulk (zinc blende) materials. Parameters, which can affect the formation of specific crystal structures, will be discussed. Closely related to the crystal structure is the formation of stacking faults, such as twin planes. Single twins, paired twin and twin superlattices are characterized and their occurrence is explained by a kinetic growth model. Photoluminescence has been used to assess the optical properties of single nanowires. It is shown that wurtzite InP has higher bandgap energy than zinc blende InP. Control of the crystal structure allows for a new type of band-engineering. Finally, the photonic properties of wire ensembles are discussed.

We describe and analyze the growth of III-V semiconductor nanowires by molecular beam epitaxy activated with gold particles. We focus on (Al)GaAs(P) and InP(As) compounds. Optimal conditions of substrate surface preparation and adequate growth parameters are reported. The catalyst particles are shown to be rich in group-III atoms and liquid when nanowires form. The favorable growth temperatures thus depend on which group-III element is used. We explain why the nanowires often adopt the unusual wurtzite structure, pointing out that the crystalline phase is determined at the nucleation stage and that high liquid supersaturation is necessary. The kinetics of nanowire elongation is investigated with an original method based on modulations of the incident fluxes. The group-III flux intercepted by the nanowire sidewall facets is the main contribution to axial growth. Conditions leading to a short diffusion length of the group-III adatoms on these facets, produce lateral growth that we use to form abrupt core-shell heterostructures. We also present a method to bury vertical freestanding nanowires by a planar epitaxial growth. This process induces the layer-by-layer transformation of the nanowire phase from wurtzite to zinc-blende. Finally, the statistics of nucleation at the liquid-solid interface is revealed by using the flux modulation method. We show that the diluted concentration of group-V atoms in the nano-sized catalyst drop is at the origin of the self-regulation of the nucleation events.

We review the recent progress in the growth, structural, optical and electrical properties of GaAs/GaAsSb, GaAs/GaSb, and InAs/InSb nanowires (NWs) grown by the vapor-liquid-solid (VLS) mechanism. The structural characterization of GaAsSb NWs reveals that they adopt zinc blende (ZB) crystal phase, whereas the same growth conditions (except the Sb flux) produce GaAs NWs with wurtzite (WZ) crystal phase. With increasing mole fraction of Sb, the ZB GaAsSb NW forms a lower density of twinning planes. Low-temperature photo-luminescence (PL) characterization of axially heterostructured GaAs/GaAsSb NWs shows that the linear polarization of the PL emission from the ZB GaAsSb inserts is opposite (parallel to NW axis) to the WZ GaAs segments (perpendicular to NW axis) due to a difference in the optical selection rules between ZB and WZ crystals. GaSb NWs have been grown on top of GaAs NWs by the Au-assisted VLS method. The two most striking differences between the GaSb and GaAs NWs are: 1) The diameter of the GaSb NWs is significantly larger than that of the GaAs NWs. 2) Whereas the GaAs NWs have WZ crystal phase with stacking faults, the GaSb NWs exhibit a defect-free ZB crystal phase. Undoped GaSb NWs have so far been determined to be p-type. Low-temperature PL measurements on GaSb NWs reveal a PL peak near 0.8 eV, with the energy position dependent on the V/III ratio. InSb NWs grown on top of InAs NWs show similar behavior in terms of diameter and crystal phase change as the GaAs/GaSb NWs.

In this chapter I will give an overview of the existing knowledge on the growth and the main properties of III-V ternary nanowires. I will describe bare ternary as well as heterostructure nanowires, both axial and radial. A particular emphasis is given to the competing behaviors occurring during the growth of this class of nanowires, that is different ad-atom incorporation due to different properties of impinging species of the same group: diffusion lengths, solution of the different element in the catalyst nanoparticle and the thermal stability of chemical bonds. These competing processes have effects on alloy composition, composition uniformity and nanowire shape.

Semiconductor nanowires may be used as the channel of a field-effect transistor device. In contrast to field-effect transistors made of epitaxial layers, the nanowire approach provides large material diversity and enables a fully surrounding gate contact. In this way, no carriers can escape from the channel and a higher transconductance is routinely observed. In contrast to carbon nanotubes the charge polarity can be selected and a metallic phase that may inhibit the channel depletion is avoided. In this chapter device concepts will be presented based on semiconductor band structure and heterostructures. Recent advances in device technology and selfassembly will be discussed. The corresponding DC performance of the different nanowire approaches is reviewed. Special emphasis is given on the measurement and the performance of nanowire field-effect transistors at high frequencies.

We describe the growth and optical properties of III-V semiconductor nanowires and their application to nanoscale photonic devices such as Fabry-Perot cavity, waveguides, optically-pumped lasers, and lightemitting diodes. The nanowires were grown by selective-area metalorganic vapor phase epitaxy (SA-MOVPE) on the (111) oriented substrates. Nanowires containing heterostructures in their radial direction, that is, core-shell heterostructures, have also been realized by controlling the growth mode during SA-MOVPE. The nanowires were characterized by micro-photoluminescence measurements and those detached from the grown substrate showed resonant peaks associated with Fabry-Perot cavity modes. It was simultaneously shown that core-shell hetereostructured nanowires exhibited stronger photoluminescence than bare nanowires due to reduced surface non-radiative recombination. Furthermore, core-shell nanowires exhibited lasing oscillation originating from the cavity formed by both end facets at pulsed-laser excitation. Meanwhile, electroluminescence from core-shell nanowires was also demonstrated.

This chapter reviews the implementation of semiconductor nanowires as the active optical elements of nanowire-based photovoltaics. Some essential principles for understanding the semiconductor p-n junction photoresponse are briefly covered, followed by a more detailed presentation of the arguments for turning to a nanowire geometry rather than the traditional planar case. These include the decoupling of the light absorption and carrier collection directions for a radial junction geometry, the relaxation of lattice-mismatching constraints in choice of material combinations for axial heterojunctions, and absorption enhancement via light scattering in nanowire arrays. The emphasis here is on semiconductor nanowires grown epitaxially from a substrate, rather than solution-based methods. Because of the application-oriented nature of the material discussed, though, this scientific literature review is complemented by some remarks on the relevant patent literature for this field, also including alternative synthesis methods, as a gauge of the readiness for moving from scientific research to technology development. Finally, an outlook on additional modifications or integrations into the semiconductor nanowire solar cell framework is presented, such as the incorporation of plasmonic cell elements.