Parameter space mapping of InAs nanowire crystal structure

Crystal structure and defects have been shown to have a strong impact on III-Vnanowire properties. Recently, it was demonstrated that the issue of random stacking and polytypism in semiconductornanowires can often be controlled using accessible growth parameters (such as temperature, diameter, and V/III ratio). In addition, it has been shown that crystal phase can be tuned selectively between cubic zinc blende and hexagonal wurtzite within individual nanowires of III-V materials such as InAs. In order for such results to be generally applied to different growth setups, it is necessary to fully explore and understand the trends governing crystal phase dependencies on all accessible growth parameters, including how they relate to each other. In this study, the authors have systematically investigated the influence of temperature, diameter, V/III ratio, and total mass flow on the crystal structure of InAsnanowiresgrown by metal-organic vapor phase epitaxy over a broad parameter range. The authors observed that each of these accessible parameters can affect the resulting crystal structure, and that the trends for each parameter are affected by the magnitude of the others. The authors also noted that most of the parameter dependencies are nonlinear and, in fact, exhibit threshold values at which structure changes discontinuously. By optimizing each of the growth parameters, it is shown that pure ZB or pure WZ phase can be achieved for several different sets of growth conditions. The roles of nucleation kinetics, thermodynamics, and precursor chemistry are also discussed to compare the results to current nanowiregrowth models. The results in this work should facilitate comparison of data and transfer of knowledge between different growth systems and techniques, which, in turn, should lead to greater understanding of polytypism in nanowires and greater control and freedom in nanowire crystal phase engineering.This work was supported by the Nanometer Structure Consortium at Lund University nmC@LU, the Swedish Foundation for Strategic Research SSF, the Swedish Research Council VR, and the Knut and Alice Wallenberg Foundation

Silver as Seed-Particle Material for GaAs Nanowires-Dictating Crystal Phase and Growth Direction by Substrate Orientation

Here we investigate the feasibility of silver as seed-particle material to synthesize GaAs nanowires and show that both crystal phase and growth direction can be controlled by choice of substrate orientation. A (111)B substrate orientation can be used to form vertically aligned wurtzite GaAs nanowires and a (100) substrate orientation to form vertically aligned zinc blende GaAs nanowires. A 45-50% yield of vertical nanowire growth is achieved on the (100) substrate orientation without employing any type of surface modification or nucleation strategy to promote a vertical growth direction. In addition, photoluminescence measurements reveal that the photon emission from the silver seeded wurtzite GaAs nanowires is characterized by a single and narrow emission peak at 1.52 eV

Nanowire-Aperture Probe: Local Enhanced Fluorescence Detection for the Investigation of Live Cells at the Nanoscale

Fluorescence microscopy has tackled many of the burning questions in cellular biology. Probing low-affinity cellular interactions remains one of the major challenges in the field to better understand cellular signaling. We introduce a novel approach the nanowire-aperture probe (NAP) to resolve biological signatures with a nanoscale resolution and a boost in light detection. The NAP takes advantage of the photonic properties of semiconductor nanowires and provides a highly localized excitation volume close to the nanowire surface. The probing region extends less than 20 nm into the solution, which can be exploited as a local light probe in fluorescence microscopy. This confined detection volume is especially advantageous in the study of cellular signaling at the cell membrane, as it wraps tightly around the nanowire. The nanowire acts as a local nanoaperture, both focusing the incoming excitation light and guiding photons emitted by the fluorophore. We demonstrate a 20-fold boost in signal-to-background sensitivity for single fluorophores and membrane-localized proteins in live cells. This work opens a completely new avenue for next-generation studies of live cells

The Crystal Structure of III-V Semiconductor Nanowires: Growth and Characterization

This thesis concerns growth and characterization of gold-particle seeded nanowires of III-V semiconductor materials (III-V NWs). The nanowires were grown using metal-organic vapour phase epitaxy (MOVPE) and characterized by various microscopy techniques. In particular cross-sectional scanning tunnelling microscopy (XSTM) and cathodoluminescence microscopy (CL) were employed to investigate heterostructured nanowires. In addition, the formation of the crystal structure within the nanowires as an effect of growth conditions was investigated by transmission electron microscopy. This thesis first presents an extensive description of the crystal structure and phases which have been observed in III-V NWs. It then gives an overview of the main characterization methods that were used in the included papers. Another part that is included concerns epitaxial growth in general and MOVPE in particular. The last part of the thesis is an outlook in which some important questions and possible future directions are presented and discussed. The basis for this thesis is 9 appended papers: Paper I-II present results from XSTM investigations; papers III-IV and VIII-IX concerns investigations of how to control the crystal phases and the density of defects within III-V NWs; paper V present a study of the effect of gold-particle fabrication method and deposition method on MOVPE growth of GaAs NWs; papers VI-VII present measurements of the ambipolar diffusion length in III-V NWs using cathodoluminescence

Crystal Phases in III-V Nanowires: From Random Toward Engineered Polytypism

III-V nanowires (NWs) are promising for a wide range of applications, ranging from optics to electronics, energy, and biological sensing. The structural quality of NWs is of paramount importance for the performance of such future NW-based devices. Random structural defects and polytypism occur naturally in semiconductor NWs, but progress both on the theoretical understanding and experimental control have been achieved recently. Here, we review progress towards the realization of perfect wurtzite and zinc-blende phases in III-VNWs, eventually leading to true phase engineering in single NWs

Crystal phases in III-V nanowires: From random toward engineered polytypism

III-V nanowires (NWs) are promising for a wide range of applications, ranging from optics to electronics, energy, and biological sensing. The structural quality of NWs is of paramount importance for the performance of such future NW-based devices. Rando

Diffusion length measurements in axial and radial heterostructured nanowires using cathodoluminescence

We have measured the ambipolar diffusion lengths in nanowires with GaAs and AlGaAs core material using cathodoluminescence imaging. This was done by combining III-V semiconductor materials with different bandgaps in single nanowires. We show that it is possible to record intensity profiles of the emission from segments of lower bandgap material positioned along the nanowire length and in this way gain an insight on important carrier transport properties of the nanowire core material. We present diffusion data for GaAs and AlGaAs nanowire core material in different radially and axially heterostructured nanowires and show that the diffusion of carriers is greatly increased by capping the nanowires with a higher-bandgap material. In addition, we show how a decoupling of the radial and axial growth during particle-seeded growth is necessary in order to reach long diffusion lengths along the core of axially heterostructured nanowires. In addition, for ternary compounds (InGaAs and AlGaAs), we observe compositional differences for radial and axial nanowire growth. (C) 2010 Elsevier B.V. All rights reserved

GaAs-based Nanowires Studied by Low-Temperature Cathodoluminescence

We present cathodoluminescence data of nanowires (NWs) grown using size-selected gold particles as seeds. The NWs have a GaAs core with a diameter of 50 nm and a length of several mu m. The NWs in this study were generally covered with a shell of AlGaAs. With increasing growth temperature, the emission intensity increases significantly. From a variety of growth conditions, we conclude that the exposed sides of the NWs during growth play an important role in the emission intensity. The diffusion of carriers was studied by inserting a segment of GaInAs in GaAs NWs. By capping the NWs with an AlGaAs shell, we observe a tenfold increase in the diffusion length along the core

Determination of diffusion lengths in nanowires using cathodoluminescence

We used cathodoluminescence imaging to determine diffusion lengths in III-V semiconductor nanowires, grown by metal-organic chemical vapor deposition seeded by gold nanoparticles. Intensity profiles were recorded either from the interface between the substrate and homogeneous nanowires, or from segments in nanowires containing axial heterostructures to determine the diffusion length. We determined diffusion lengths of 0.10 to 0.90 mu m, the shortest for uncapped wires. The reduction is attributed largely to surface recombination. (C) 2010 American Institute of Physics. [doi:10.1063/1.3473829