3,806 research outputs found
Electron Beam-induced Light Emission and Transport in GaN Nanowires
We report observations of electron beam-induced light from GaN nanowires grown by chemical vapor deposition. GaN nanowires were modified in-situ with deposited opaque platinum coatings to estimate the extent to which light is channeled to the ends of nanowires. Some evidence of light channeling was found, but wire microstructure and defects play an important role in light scattering and transport, limiting the extent to which light is confined. Optical interconnects are powerful components presently applied for high bandwidth communications among high-performance processors. Future circuits based on nanometer-scale components could similarly benefit from optical information transfer among processing blocks. Strong light channeling (and even lasing) has been observed in GaN nanowires, suggesting that these structures could be useful building blocks in a future networked electro-optical processor. However, the extent to which defects and microstructure control optical performance in nanowire waveguides has not been measured. In this study, we use electron microscopy and in-situ modification of individual nanowires to begin to correlate wire structure with light transport efficiency through GaN nanowires tens of microns long
Optical control of internal electric fields in band-gap graded InGaN nanowires
InGaN nanowires are suitable building blocks for many future optoelectronic
devices. We show that a linear grading of the indium content along the nanowire
axis from GaN to InN introduces an internal electric field evoking a
photocurrent. Consistent with quantitative band structure simulations we
observe a sign change in the measured photocurrent as a function of photon
flux. This negative differential photocurrent opens the path to a new type of
nanowire-based photodetector. We demonstrate that the photocurrent response of
the nanowires is as fast as 1.5 ps
One step growth of GaN/SiO2 core/shell nanowire in vapor-liquid-solid route by chemical vapor deposition technique
GaN/SiO2 core/shell nanowires are grown by cobalt phthalocyanine catalyst
assisted vapor-liquid-solid route, in which Si wafer coated with a mixture of
gallium and indium is used as the source for Ga and Si and ammonia is used as
the precursor for nitrogen and hydrogen. Gallium in the presence of indium and
hydrogen, which results from the dissociation of ammonia, forms Si-Ga-In alloy
at the growth temperature around 910 degree Celsius. This alloy acts as the
source of Si, Ga and In. A detailed study using a variety of characterization
tools reveals that these wires, which are several tens of micron long, has a
diameter distribution of the core ranging from 20 to 50 nm, while the thickness
of the amorphous SiO2 shell layer is about 10 nm. These wires grow along
direction. It has also been observed that the average width of these wires
decreases, while their density increases as the gallium proportion in the Ga-In
mixture is increased.Comment: 14 pages, 4 figure
Enhanced spin-orbit coupling in core/shell nanowires
The spin-orbit coupling (SOC) in semiconductors is strongly influenced by
structural asymmetries, as prominently observed in bulk crystal structures that
lack inversion symmetry. Here, we study an additional effect on the SOC: the
asymmetry induced by the large interface area between a nanowire core and its
surrounding shell. Our experiments on purely wurtzite GaAs/AlGaAs core/shell
nanowires demonstrate optical spin injection into a single free-standing
nanowire and determine the effective electron g-factor of the hexagonal GaAs
wurtzite phase. The spin relaxation is highly anisotropic in time-resolved
micro-photoluminescence measurements on single nanowires, showing a significant
increase of spin relaxation in external magnetic fields. This behavior is
counterintuitive compared to bulk wurtzite crystals. We present a model for the
observed electron spin dynamics highlighting the dominant role of the
interface-induced SOC in these core/shell nanowires. This enhanced SOC may
represent an interesting tuning parameter for the implementation of
spin-orbitronic concepts in semiconductor-based structures
Polariton Bose-Einstein condensate at room temperature in a Al(Ga)N nanowire-dielectric microcavity with a spatial potential trap
A spatial potential trap is formed in a 6.0 {\mu}m Al(Ga)N nanowire by
varying the Al composition along its length during epitaxial growth. The
polariton emission characteristics of a dielectric microcavity with the single
nanowire embedded in-plane has been studied at room temperature. Excitation is
provided at the Al(Ga)N end of the nanowire and polariton emission is observed
from the lowest bandgap GaN region of the nanowire. Comparison of the results
with those measured in an identical microcavity with an uniform GaN nanowire
and having an identical exciton-photon detuning suggests evaporative cooling of
the polaritons as they are transported across the trap in the Al(Ga)N nanowire.
Measurement of the spectral characteristics of the polariton emission, their
momentum distribution, first-order spatial coherence and time-resolved
measurements of polariton cooling provide strong evidence of the formation of
an equilibrium Bose-Einstein condensate, a unique state of matter in solid
state systems, in the GaN region of the nanowire, at room temperature. An
equilibrium condensate is not formed in the GaN nanowire dielectric microcavity
without the spatial potential trap.Comment: 28 pages, 6 figures, Submitted to the Proceedings of the National
Academy of Sciences of the United States of Americ
Radiation Effects on Wide Band Gap Semiconductor Transport Properties
In this research, the transport properties of ZnO were studied through the use of electron and neutron beam irradiation. Acceptor states are known to form deep in the bandgap of doped ZnO material. By subjecting doped ZnO materials to electron and neutron beams we are able to probe, identify and modify transport characteristics relating to these deep accepter states. The impact of irradiation and temperature on minority carrier diffusion length and lifetime were monitored through the use of the Electron Beam Induced Current (EBIC) method and Cathodoluminescence (CL) spectroscopy. The minority carrier diffusion length, L, was shown to increase as it was subjected to increasing temperature as well as continuous electron irradiation. The near-band-edge (NBE) intensity in CL measurements was found to decay as a function of temperature and electron irradiation due to an increase in carrier lifetime. Electron injection through application of a forward bias also resulted in a similar increase of minority carrier diffusion length. Thermal and electron irradiation dependences were used to determine activation energies for the irradiation induced effects. This helps to further our understanding of the electron injection mechanism as well as to identify possible defects responsible for the observed effects. Thermal activation energies likely represent carrier delocalization energy and are related to the increase of diffusion length due to the reduction in recombination efficiency. The effect of electron irradiation on the minority carrier diffusion length and lifetime can be attributed to the trapping of non-equilibrium electrons on neutral acceptor levels. The effect of neutron irradiation on CL intensity can be attributed to an increase in shallow donor concentration. Thermal activation energies resulting from an increase in L or decay of CL intensity monitored through EBIC and CL measurements for p-type Sb doped ZnO were found to be the range of Ea = 112 to 145 meV. P-type Sb doped ZnO nanowires under the influence of temperature and electron injection either through continuous beam impacting or through forward bias, displayed an increase in L and corresponding decay of CL intensity when observed by EBIC or CL measurements. These measurements led to activation energies for the effect ranging from Ea = 217 to 233 meV. These values indicate the possible involvement of a SbZn-2VZn acceptor complex. For N-type unintentionally doped ZnO, CL measurements under the influence of temperature and electron irradiation by continuous beam impacting led to a decrease in CL intensity which resulted in an electron irradiation activation energy of approximately Ea = 259 meV. This value came close to the defect energy level of the zinc interstitial. CL measurements of neutron irradiated ZnO nanostructures revealed that intensity is redistributed in favor of the NBE transition indicating an increase of shallow donor concentration. With annealing contributing to the improvement of crystallinity, a decrease can be seen in the CL intensity due to the increase in majority carrier lifetime. Low energy emission seen from CL spectra can be due to oxygen vacancies and as an indicator of radiation defects
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