A F Moses

Observation of free exciton photoluminescence emission from single wurtzite GaAs nanowires

Microphotoluminescence measurements are used to investigate the optical properties of single wurtzite GaAs nanowires grown by molecular beam epitaxy. The wurtzite GaAs nanowires exhibit a photoluminescence emission peak at 1.544 eV, 29 meV higher than the zinc blende GaAs free exciton energy. Temperature dependent photoluminescence measurements (4.4–70 K) show indications of defect and impurity related emissions at lower energies (1.53–1.54 eV) and the presence of nonradiative defects. High resolution transmission electron microscopy images show a low density of short zinc blende segments sandwiched in between a dominating wurtzite structure and weak photoluminescence emission related to such zinc blende segments is also observed.

Growth and Characterization of Wurtzite GaAs Nanowires with Defect-Free Zinc Blende GaAsSb Inserts

We have demonstrated the growth of a unique wurtzite (WZ) GaAs nanowire (NW) with a zinc blende (ZB) GaAsSb insert by Au-assisted molecular beam epitaxy. An abrupt interface from the WZ GaAs phase to the ZB GaAsSb phase was observed, whereas an intermediate segment of a 4H polytype GaAs phase was found directly above the ZB GaAsSb insert. A possible mechanism for the different phase transitions is discussed. Furthermore, low temperature microphotoluminescence (micro-PL) measurements showed evidence of quantum confinement of holes in the GaAsSb insert.

Engineering Parallel and Perpendicular Polarized Photoluminescence from a Single Semiconductor Nanowire by Crystal Phase Control

We report on a crystal phase-dependent photoluminescence (PL) polarization effect in individual wurtzite GaAs nanowires with a zinc blende GaAsSb insert grown by Au-assisted molecular beam epitaxy. The PL emission from the zinc blende GaAsSb insert is strongly polarized along the nanowire axis while the emission from the wurtzite GaAs nanowire is perpendicularly polarized. The results indicate that the crystal phases, through optical selection rules, are playing an important role in the alignment of the PL polarization in nanowires besides the linear polarization induced by the dielectric mismatch. The strong excitation power dependence and long recombination lifetimes ( approximately 4 ns) from the wurtzite GaAs and zinc blende GaAsSb-related PL emission strongly indicate the existence of type II band alignments in the nanowire due to the presence of nanometer thin zinc blende segments and stacking faults in the wurtzite GaAs barrier.

The effects of Sb concentration variation on the optical properties of GaAsSb/GaAs heterostructured nanowires

This thesis presents a quantitative high-angle annular dark eld scanning transmission electron microscopy (HAADF STEM) study on heterostructured GaAs-based nanowires (NWs). Quantitative HAADF STEM was employed to investigate Sb concentration variations in axial GaAsSb inserts within GaAs NWs and in GaAsSb NWs, as well as Al concentration variations in the AlGaAs shell in GaAs/AlGaAs core-shell NWs. The NWs were grown with the vapor-liquid-solid (VLS) growth mechanism using Ga- and Au-assisted molecular beam epitaxy (MBE).Compositional characterisation by quantitative HAADF STEM was realized by comparing experimental image intensities normalized to the incident beam intensity with simulated intensities. The HAADF STEM image simulations were performed using the frozen-phonon multislice approach. For correct compositional analysis of GaAs1-ySby, static atomic displacements (SAD) had to be included in the simulations, however this was not the case with AlxGa1-xAs.GaAsSb inserts within GaAs NWs and GaAsSb NWs were studied with non-corrected STEM at a relatively low magnication using NWs in plane. In this case the known thickness prole of the hexagonal NWs could be exploited in the quantitative HAADF STEM analysis. In the GaAsSb inserts, concentration gradients axially along as well as radially across the insert were identied. The Sb concentration in the insert decreased axially towards the upper and lower interfaces with GaAs, and radially towards the outer surfaces. The axial concentration gradients were attributed to the reservoir eect related to the VLS growth process. The eects of the axial concentration gradients on the NWs optical properties were investigated.The radial concentration gradients in the GaAsSb inserts result from a combined eect of radial GaAs overgrowth and out-diusion of Sb during the post-insert axial GaAs growth, causing an increased surface depletion of Sb with increasing post-insert GaAs growth time. In GaAsSb NWs, increased surface depletion of Sb towards the bottom of the NW was observed, and this was attributed to out-diusion of Sb during the NW growth.GaAs/AlGaAs core-shell NWs were studied with aberration corrected STEM, using cross-sectional specimens prepared with ultramicrotomy. A method for mapping Al concentration in AlxGa1-x As at unit cell spatial resolution using the atomic resolution HAADF STEM images was developed. The method is independent of the eective source size and higher order lens aberrations. With the method, Al concentration variations in the AlGaAs shell could be quantied at unit cell spatial resolution.

Correlated micro-photoluminescence and electron microscopy studies of the same individual heterostructured semiconductor nanowires.

To correlate optical properties to structural characteristics, we developed a robust strategy for characterizing the same individual heterostructured semiconductor nanowires (NWs) by alternating low temperature micro-photoluminescence (μ-PL), low voltage scanning (transmission) electron microscopy and conventional transmission electron microscopy. The NWs used in this work were wurtzite GaAs core with zinc blende GaAsSb axial insert and AlGaAs radial shell grown by molecular beam epitaxy. The series of experiments demonstrated that high energy (200 kV) electrons are detrimental for the optical properties, whereas medium energy (5-30 kV) electrons do not affect the PL response. Thus, such medium energy electrons can be used to select NWs for correlated optical-structural studies prior to μ-PL or in NW device processing. The correlation between the three main μ-PL bands and crystal phases of different compositions, present in this heterostructure, is demonstrated for selected NWs. The positions where a NW fractures during specimen preparation can considerably affect the PL spectra of the NW. The effects of crystal-phase variations and lattice defects on the optical properties are discussed. The established strategy can be applied to other nanosized electro-optical materials, and other characterization tools can be incorporated into this routine.

Photocurrent Spectroscopy of Single Wurtzite GaAs Nanowires

Photocurrent of single wurtzite GaAs nanowires grown by Au‐assisted molecular beam epitaxy is measured at room and low temperature (10 K). At room temperature a high photo‐response with more than two orders of magnitude increase of current is observed. The wavelength dependence of the photocurrent shows a sharp change near the zinc blende GaAs band gap. The absence of the free exciton peak in the low temperature photocurrent spectrum, and problems related to determining the exact position of the energy bandgap of wurtzite GaAs from the observed data are discussed.

Micro-photoluminescence study of single GaAsSb/GaAs radial and axial heterostructured core-shell nanowires

We report on the low temperature micro-photoluminescence (μ-PL) of single GaAs/AlGaAs core-shell nanowires (NWs) with axial GaAsSb core-inserts grown by Au-assisted molecular beam epitaxy. The GaAs NW core has hexagonal wurtzite (WZ) crystal structure, whereas the GaAsSb NW core-insert has cubic zinc blende (ZB) crystal structure. The upper GaAsSb/GaAs interface contains a few nm thin cubic ZB GaAs segment just before the crystal structure switches back to WZ. By adding a growth interruption (GI) directly after the GaAsSb insert, the ZB GaAs segment in the WZ GaAs barrier can be avoided. This GI-induced crystal structure change of the upper GaAs barrier next to the GaAsSb/GaAs interface makes a large difference on the PL properties from the GaAsSb NW core-inserts. This is believed to be due to that the GaAsSb/GaAs heterojunction band alignment is changed from type II to type I when the GaAs crystal structure is changed from ZB to WZ.

Photoluminescence Polarization Anisotropy in a Single Heterostructured III‐V Nanowire with Mixed Crystal Phases

Low temperature (10 K) micro‐photoluminescence (μ‐PL) of single GaAs/AlGaAs core‐shell nanowires with single GaAsSb inserts were measured. The PL emission from the zinc blende GaAsSb insert is strongly polarized along the nanowire axis while the PL emission from the wurtzite GaAs nanowire is perpendiculary polarized to the nanowire axis. The result indicates that the crystal phase, through the optical selection rules, has significant effect on the polarization of the PL from NWs besides the dielectric mismatch. The analysis of the PL results based on the electronic structure of these nanowires supports the correlation between the crystal phase and the PL emission.

Correlated micro-photoluminescence and electron microscopy study of a heterostructured semiconductor nanowire

Correlation between the optical and the structural properties of an individual heterostructured nanowire (NW) is crucial for optimising the NW synthesis and device design. In this work, low temperature (10 K) micro-photoluminescence (μ-PL), low and high voltage scanning transmission electron microscopy and conventional transmission electron microscopy are applied to the same individual NWs. The studied NWs, grown by Au-assisted molecular beam epitaxy, have a wurtzite GaAs core with a zincblende GaAsSb axial insert, enclosed with an AlGaAs radial shell and a GaAs capping layer. By subsequent analysis of one and the same NW by μ-PL and several electron microscopy techniques in different microscopes, we can relate the spectral features of a single NW to the structural features, such as different crystal phases, lattice defects and composition.

Growth of heterostructured III-V nanowires by molecular beam epitaxy for photonic applications

We report the results on the growth, structural and optical characterization of single wurtzite (WZ) GaAs nanowires as well as WZ GaAs/AlGaAs core-shell nanowires with ZB GaAsSb inserts. For the GaAs/GaAsSb heterojunction both the crystal material and crystal phase change plays a critical role in the exact band alignment. We show that ZB GaAsSb inserts with both WZ and ZB GaAs barriers can be grown and hence both type I and type II band alignment can be achieved which has large effects on the optical properties of the nanowires. We thus demonstrate that it is possible to engineer the band-structure at a semiconductor heterojunction by modulating the crystal material as well as the crystal phase.