Session W17: Optics of Semiconductor Nanowires

Scalable Synthesis of Vertically Aligned, Catalyst-Free Gallium Arsenide Nanowire Arrays -- Towards Optimized Optical Absorption and Reflection.

Vertically aligned, catalyst-free nanowires hold great potential for photovoltaic applications, where scalable synthesis and optimized optical absorption are critical. Here, we report using nanosphere lithography, scalable synthesis of vertical gallium arsenide nanowires grown by selected area MOCVD. A comparative study was done between regular nanowires arrays using electron beam lithography and slightly more defective nanowire arrays using nanosphere lithography.~ Reflection of light by the nanowire array has been used as a measure to study the effects of defects in the patterned structures using NSL both experimentally and by simulation. Both studies show similar reflection behavior between nanowire prepared by EBL and NSL. GaAs nanowires as short as 130 nm show reflection of $<$10{\%} over the visible range of solar spectrum. Optimized nanowire configuration to maximize the absorption has also been discussed.   

Theory of photoluminescence polarization reversal in GaAs nanowires

The polarization of photoluminescence (PL) in wurtzite (WZ) GaAs nanowires (NW) of diameter ~100 nm has been observed to reverse as a function of temperature from perpendicular to parallel to the NW axis. We use the weak confinement limit for excitons and the envelope function approximation to study this phenomenon. The WZ GaAs crystal field and spin-orbit splittings were determined using GW calculations and agree well with resonant Raman spectra on WZ NWs. In contrast to zincblende (ZB) NWs, the crystal field splitting in WZ NWs leads to a perpendicularly polarized exciton ground state. The first excited state, however, has a parallel component and can be mixed in at slightly elevated temperature, leading to a polarization reversal. We find that a reversal can only take place for much smaller crystal field splittings than the one obtained in pure WZ. Strain induced reduction of the crystal field splitting would require an unrealistically large strain. On the other hand, multiple twinning, can lead to a substantially lower crystal field splitting as obtained from our GW calculations for lower hexagonality polytypes, such as 4H GaAs, and can thus explain the observed polarization reversals.   

Phonon Spectrum and Thermal Properties of free standing $<$100$>$ and $<$111$>$ InGaAs alloy nanowires

The phonon spectra in zinc blende InAs, GaAs and their ternary alloy nanowires (NWs) are computed using an enhanced valence force field (EVFF) model. The physical and thermal properties of these nanowires such as sound velocity, elastic constant, specific heat (Cv), phonon density of states, phonon modes, and the ballistic thermal conductance are explored. The calculated transverse and longitudinal sound velocities along $<$100$>$ direction in these NWs are $\sim $25{\%} and 20{\%} smaller compared to the bulk velocities, respectively. These velocities along $<$111$>$ direction are about twice smaller than bulk values. The Cv for NWs are about twice as large as the bulk values due to higher surface to volume ratio (SVR) and strong phonon confinement in the nanostructures. The temperature dependent Cv for InAs and GaAs nanowires show a cross-over at 180K and 155K along $<$100$>$ and $<$111$>$ directions respectively. It happens due to higher phonon density in InAs nanowires at lower temperatures. With the phonon spectra and Landauer's model the ballistic thermal conductance is reported for these III-V NWs. The results in this work demonstrate the potential to engineer the thermal behavior of III-V NWs.   

How and why a magnetized quantum wire can act as an optical amplifier

The fundamental issues associated with the magnetoplasmon excitations are investigated in a quantum wire characterized by a confining harmonic potential and subjected to a perpendicular magnetic field. Essentially, we embark on the device aspects of the intersubband collective (magnetoroton) excitations which observe a negative group velocity between the maxon and the roton. The computation of the gain coefficient suggests an interesting and important application: the electronic device based on such magnetoroton modes can act as an optical amplifier.\footnote{M.S. Kushwaha, J. Appl. Phys. {\bf 109}, 106102 (2011).}   

Electronic band structure calculations of bismuth-antimony nanowires

Alloys of bismuth and antimony received initial interest due to their unmatched low-temperature thermoelectric performance, and have drawn more recent attention as the first 3D topological insulators. One-dimensional bismuth-antimony (BiSb) nanowires display interesting quantum confinement effects, and are expected to exhibit even better thermoelectric properties than bulk BiSb. Due to the small, anisotropic carrier effective masses, the electronic properties of BiSb nanowires show great sensitivity to nanowire diameter, crystalline orientation, and alloy composition. We develop a theoretical model for calculating the band structure of BiSb nanowires. For a given crystalline orientation, BiSb nanowires can be in the semimetallic, direct semiconducting, or indirect semiconducting phase, depending on nanowire diameter and alloy composition. These ``phase diagrams'' turn out to be remarkably similar among the different orientations, which is surprising in light of the anisotropy of the bulk BiSb Fermi surface. We predict a novel direct semiconducting phase for nanowires with diameter less than $\sim$15 nm, over a narrow composition range. We also find that, in contrast to the bulk and thin film BiSb cases, a gapless state with Dirac dispersion cannot be realized in BiSb nanowires.   

Gas phase interactions with bare and gold nanoparticle decorated gallium nitride nanowires by ultraviolet photoelectron spectroscopy

Ultraviolet photoelectron spectroscopy (UPS) has been used to characterize the interaction of CO and H$_{2}$O with the surface of bare and gold nanoparticle (Au NP) decorated gallium nitride nanowires at 298 K, 77 K and 20 K. The average diameter of the Au NPs is 4.5 $\pm $ 0.5 nm and the average nanowire diameter is 105 $\pm $ 75 nm. CO and H$_{2}$O do not bond to the surface of the bare GaN nanowires at 298K, 77K, or 20K. Temperature dependent UPS analysis reveals that CO and H$_{2}$O weakly physisorbed to the Au NP decorated GaN nanowires with heats of adsorption of 4.37 $\pm $ 0.03 meV and 1.25 $\pm $ 0.04 meV , respectively. The adsorption at 298K of 50 Langmuir of CO followed by 50 Langmuir of H$_{2}$O showed that CO adsorption promotes H$_{2}$O adsorption, while 50 Langmuir of H$_{2}$O followed by 50 Langmuir of CO showed that H$_{2}$O inhibits CO adsorption. The findings of this study that the adsorption of H$_{2}$O inhibits CO adsorption onto the Au NP-GaN nanowires explains previous studies of the gas sensing properties of mats of Au NP- GaN nanowires.   

Genomic design of strong direct-gap optical transition in Si/Ge core/multishell nanowires

Converting the electronically superior but optically impractical indirect-gap Si and Ge semiconductors into a strongly light-absorbing system has been a long-standing challenge, given that the phonon-assisted optical transition of the indirect gap has weak intensity, requiring thick absorbers. One of main strategies has been the use of two-dimensional (2D) layer-by-layer growth of Si/Ge superlattices (SLs). However, the maximum thickness of SLs that can be grown coherently on a substrate is limited by the lattice-mismatch-induced strain. This limitation can be greatly relaxed by changing from 2D SLs to one-dimensional quantum nanowire (NW), where much higher strain can be accommodated. With developed Vapor-Liquid-Solid based technique, experimental growth of Si/Ge core-multishell NWs has recently demonstrated a significant level of synthetic control. However, the number of possible core/multishell sequences and thicknesses might easily reach an astronomic value. We will present here a genomic search for targeted core/multishell NW geometries that give both a direct gap and a significantly enhanced dipole-allowed optical transition in the Si/Ge system, by using a combination of genetic algorithm with atomistic pseudopotential electronic-structure calculations.   

Axially-Resolved Luminescence of Individual ZnSe Nanowires

Axially resolved distributions of luminescence intensity and lifetimes along individual ZnSe nanowires were studied using two-photon excited luminescence imaging and time-correlated single photon counting techniques. The nanowires were grown on GaAs substrates via the self-catalyzed VLS mode. An intense tip, to which a gallium particle is attached, is found for the deep level (DL) emissions via luminescence imaging, while the intensity for the near band edge (NBE) emissions is more uniform. The luminescence decays at all locations of the nanowires are dominated by a fast process at early times, followed by a slow one. In addition, the shape of distribution of the lifetimes along the length of nanowire resembles a flattened letter ``U'' for the NBE emissions, but it resembles a long tailed letter ``L'' for the DL emissions. Possible explanations of these results will be discussed.   

Studies of electronic excitations of rectangular ZnO nanorods by electron energy-loss spectroscopy

Electronic excitations of single ZnO rectangular nanorod have been investigated by electron energy-loss spectroscopy in conjunction with scanning transmission electron microscopy (STEM-EELS). We focus primarily on the surface excitations greatly enhanced at the grazing incidence parallel to the surfaces of ZnO nanorods. An uncommon kind of surface excitation known as surface exciton polaritons occurring near interband transitions is found to dominate in the spectral range between the band gap at 3.4 eV and the surface plasmon peak at 15.8 eV. In addition, the dielectric function of ZnO up to 25 eV has also been derived from the bulk excitation spectra using the Kramers-Kronig analysis on a single nanorod. Theoretical EELS simulations are also compared with the experimental results and good agreements are obtained.   

Precise investigation of optical/electronic fine structures of nanostructures via cathodoluminescence spectroscopy

High special/energy resolution cathodoluminescence (CL) spectroscopy enables us to make precise investigation on the optical/electronic fine structures in nanostructures. The linear distribution of strain gradient from tensile to compression in bent ZnO nano/microwires provides ideal conditions to address the modification of the electronic structures by strain in semiconductor materials. Radial line scan of the CL spectroscopy along bent ZnO wires at liquid helium temperature shows very fines excitonic emission structures, which demonstrates systematic red shift towards the increase of tensile strain, and blue shift as well as excitonic peak splitting towards the increase of compressive strain. First-principle simulations reveal an electronic band structure evolution under continuously tuned strain.   

Raman scattering study of mixed metal oxide Ir$_{x}$Ru$_{1-x}$O$_{2}$ nanowires grown by chemical vapor deposition

We present Raman scattering results of mixed metal oxide Ir$_{x}$Ru$_{1-x}$O$_{2}$ nanowires that have been studied for their stability and for activity as electrocatalysts. For our study, 1-dimensional metallic mixed oxide single crystalline Ir$_{x}$Ru$_{1-x}$O$_{2}$ nanowires were synthesized, for the first time, via a simple physical vapor transport process by controlling relative ratios of two precursors, RuO$_{2}$ and IrO$_{2}$, respectively. We measured Raman spectra of Ir$_{x}$Ru$_{1-x}$O$_{2}$ nanowires using excitation laser sources with wavelengths of 488 nm and 632.8 nm. We observed that an E$_{g}$ phonon mode of an Ir$_{x}$Ru$_{1-x}$O$_{2}$ nanowire is being blue-shifted linearly with respect to the Ir contents. We could use our observation of frequency shift of the E$_{g}$ phonon to determine stoichiometry information of nanowires which we also measured and confirmed by using EDS. From the shape of the phonon modes we measured, we could get information regarding crystalline quality that was also measured by HRTEM. We show that Raman scattering spectroscopy can provide a simple, prompt, and effective mean to measure the stoichiometry and crystalline quality of mixed metal oxide nanowires.   

Strain effects in polytypical wurtzite/zinc-blend nanowhiskers

The recent interest on III-V nanowhiskers has led to the growth of high quality samples of systems with two different crystalline structures [1]. The crystals grown in [111]-direction for the zinc-blend (ZB) phase and [0001]-direction for the wurtzite (WZ) phase are very similar and can both be described as stacked hexagonal layers. The effect of two different structural phases coexisting in the same nanostrucure is known as polytypism and creates confinement profiles similar to a heterostructure. One can notice band offsets at the interface and the formation of electronic minibands that can be explored to design systems for device applications. Although some of the III-V compounds do not present WZ structure in bulk form, recent calculations [2] presented a theoretical prediction of their band structure. However, as they considered that ZB and WZ to have the same lattice parameters no strain effects should appear on a first approach. Our theoretical approach introduces strain effects in our previous model [3] by using group theory arguments. It allows the analysis of the biaxial strain effects for both structures in a single matrix. [1] P. Caroff et al. Nature Nanotech. 4, 50, 2009. [2] A. De and C. E. Pryor, Phys. Rev. B 81, 155210, 2010 [3] http://arxiv.org/abs/1012.022   

Quantum Zigzag Phase Transition in Quantum Wires

We use Quantum Monte Carlo (QMC) techniques to study the quantum phase transition of interacting electrons in quantum wires to a quasi-one dimensional zigzag phase. The phase diagram of particles with Coulomb interaction that undergo a linear to zigzag transition is relevant to electrons in quantum wires [Meyer et al, PRL 2007] and ions in linear traps [Simshoni et al., PRL 2011]. Interacting electrons confined to a wire by a transverse harmonic potential form a Wigner crystal at low densities; as density increases, symmetry about the axis of the wire is broken and the electrons undergo a transition to a quasi-one-dimensional zigzag phase. Using QMC, we characterize this phase transition by measuring the power spectrum and addition energies.   

Raman Spectroscopy and Strain Mapping in Individual Ge- Si$_{x}$Ge$_{1-x}$ Core-Shell Nanowires

Core-shell Ge-Si$_{x}$Ge$_{1-x}$ nanowires (NWs) are expected to contain large strain fields due to the lattice-mismatch at the core/shell interface. Here we report measurement of the core strain in such NW heterostructures by Raman Spectroscopy. We measure the diameter dependence of Raman spectra in individual Ge NWs, as well as Ge-Si$_{x}$Ge$_{1-x}$ core-shell NW heterostructures. We find that the bare Ge NWs show no diameter-dependence of the Ge-Ge peak at 300.5 $cm ^{-1}$. On the other hand, the Ge-Ge peak of the Ge-Si$_{x}$Ge$_{1-x}$ core-shell NW shows a blue shift by comparison to the bare Ge NWs. This blue shift increases with reducing the NW diameter as a result of larger compressive strain in the Ge core. While the elastic strain is expected to split the triply degenerate Ge-Ge mode into separate singlet and doublet peaks, only the singlet mode was observed in experiment, a finding explained by the NW absorption and emission anisotropy. Using lattice dynamical theory and the Raman spectroscopy results we determine the strain in Ge-Si$_{x}$Ge$_{1-x}$ core-shell NWs as a function of the NW diameter. We compare the experimental results with the strain values calculated using a continuum elasticity model.