The manipulation of optical energy in structures smaller than the wavelength of light is key to the development of integrated photonic devices for computing, communications and sensing. Wide band gap semiconductor nanostructures with near-cylindrical geometry and large dielectric constants exhibit two-dimensional ultraviolet and visible photonic confinement (i.e. waveguiding). Combined with optical gain, the waveguiding behavior facilitates highly directional lasing at room temperature in controlled-growth nanowires with favorable resonant feedback. We have further explored the properties and functions of individual ultralong crystalline oxide nanoribbons that act as subwavelength optical waveguides, nonlinear frequency converter and assess their applicability as nanoscale photonic elements and scanning probes. Semiconductor nanowires offer a versatile photonic platform due to the ability to specify material size, shape, and composition. The integration of multiple unique materials with distinct optical properties promises to enable advances for several applications ranging from solid state lighting, biochemical sensing to imaging and spectroscopy.

http://ieeexplore.ieee.org/iel5/4302825/4302826/04302854.pdf

Nanowire Photonics

Silicon nanowires have been identified as important components for future electronic and sensor nanodevices. So far gold has dominated as the catalyst for growing Si nanowires via the vapour-liquid-solid (VLS) mechanism. Unfortunately, gold traps electrons and holes in Si and poses a serious contamination problem for Si complementary metal oxide semiconductor (CMOS) processing. Although there are some reports on the use of non-gold catalysts for Si nanowire growth, either the growth requires high temperatures and/or the catalysts are not compatible with CMOS requirements. From a technological standpoint, a much more attractive catalyst material would be aluminium, as it is a standard metal in Si process lines. Here we report for the first time the epitaxial growth of Al-catalysed Si nanowires and suggest that growth proceeds via a vapour-solid-solid (VSS) rather than a VLS mechanism. It is also found that the tapering of the nanowires can be strongly reduced by lowering the growth temperature.

/pdf/epitaxial-growth-of-silicon-nanowires-using-an-aluminium-g48dsa5gl7.pdf

Epitaxial growth of silicon nanowires using an aluminium catalyst

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. (Less)

Preferential Interface Nucleation: An Expansion of the VLS Growth Mechanism for Nanowires

We investigate the growth of vertically standing [100] zincblende InP nanowire (NW) arrays on InP (100) substrates in the vapor-liquid-solid growth mode using low-pressure metal-organic vapor-phase epitaxy. Precise positioning of these NWs is demonstrated by electron beam lithography. The vertical NW yield can be controlled by different parameters. A maximum yield of 56% is obtained and the tapering caused by lateral growth can be prevented by in situ HCl etching. Scanning electron microscopy, high-resolution transmission electron microscopy, and micro-photoluminescence have been used to investigate the NW properties.

/pdf/position-controlled-100-inp-nanowire-arrays-2jjqbhg99f.pdf

Position-controlled [100] InP nanowire arrays

We demonstrate vertically aligned epitaxial GaAs nanowires of excellent crystallographic quality and optimal shape, grown by Au nanoparticle-catalyzed metalorganic chemical vapor deposition. This is achieved by a two-temperature growth procedure, consisting of a brief initial high-temperature growth step followed by prolonged growth at a lower temperature. The initial high-temperature step is essential for obtaining straight, vertically aligned epitaxial nanowires on the (111)B GaAs substrate. The lower temperature employed for subsequent growth imparts superior nanowire morphology and crystallographic quality by minimizing radial growth and eliminating twinning defects. Photoluminescence measurements confirm the excellent optical quality of these two-temperature grown nanowires. Two mechanisms are proposed to explain the success of this two-temperature growth process, one involving Au nanoparticle-GaAs interface conditions and the other involving melting-solidification temperature hysteresis of the Au-Ga nanoparticle alloy.

Twin-free uniform epitaxial GaAs nanowires grown by a two-temperature process

III-V nanowires have been fabricated by metal-organic vapor-phase epitaxy without using Au or other metal particles as a catalyst. Instead, prior to growth, a thin SiOx layer is deposited on the substrates. Wires form on various III-V substrates as well as on Si. They are nontapered in thickness and exhibit a hexagonal cross-section. From high-resolution X-ray diffraction, the epitaxial relation between wires and substrates is demonstrated and their crystal structure is determined.

Au-free epitaxial growth of InAs nanowires.

We directly image the interior of GaAs/AlGaAs axial and radial nanowire heterostructures with atomic-scale resolution using scanning tunneling microscopy. We show that formation of monolayer sharp and smooth axial interfaces are possible even by vapor-phase epitaxy. However, we also find that instability of the ternary alloys formed in the Au seed fundamentally limits axial heterostructure control, inducing large segment asymmetries. We study radial core-shell nanowires, imaging even ultrathin submonolayer shells. We demonstrate how large twinning-induced morphological defects at the wire surfaces can be removed, ensuring the formation of wires with atomically flat sides.

GaAs/AlGaAs nanowire heterostructures studied by scanning tunneling microscopy.

We report on high resolution photoelectron core level spectroscopy (HRCLS) and scanning tunnelling microscopy (STM) measurements of the decomposition of lysine adsorbed on InP(001) substrates. We find that components from the lysine can be present on the InP surface even after annealing to 600 degrees C and desorption of the native surface oxide. We further observe that while a crystalline surface phase can be observed on the epi-ready surface after annealing, the lysine treated surface still appears rough. We conclude that lysine residues inhibit the formation of a flat crystalline structure on the InP(001) surface. These results are discussed in terms of the lysine promotion of [001] nanowire growth.

The influence of lysine on InP(001) surface ordering and nanowire growth

Au nanoparticles and Au films for growth of nanowires on the GaAs(111)B surface have been studied by a combination of experimental and theoretical techniques. If Au is present in either form, annealing to temperatures relevant for nanowire growth results in the formation of a thin Au wetting layer. Based on density functional theory calculations and experimental data, a structural model is proposed with an Au atom on every third threefold hollow hcp site of the Ga lattice. The authors observe that the stability of Au nanoparticles is governed by the presence of the wetting layer and outdiffusion of Au from the nanoparticles.

/pdf/au-wetting-and-nanoparticle-stability-on-gaas-111-b-2sfvhbabb3.pdf

Jessica Eriksson

Papers

Au-free epitaxial growth of InAs nanowires.

GaAs/AlGaAs nanowire heterostructures studied by scanning tunneling microscopy.

The influence of lysine on InP(001) surface ordering and nanowire growth

Au wetting and nanoparticle stability on GaAs(111)B