171 research outputs found
Polarized Raman and photoluminescence studies of a sub-micron sized hexagonal AlGaN crystallite for structural and optical properties
The polarized Raman spectroscopy is capable of giving confirmation regarding
the crystalline phase as well as the crystallographic orientation of the
sample. In this context, apart from crystallographic x-ray and electron
diffraction tools, polarized Raman spectroscopy and corresponding spectral
imaging can be a promising crystallographic tool for determining both
crystalline phase and orientation. Sub-micron sized hexagonal AlGaN
crystallites are grown by a simple atmospheric pressure chemical vapor
deposition technique using the self catalytic vapor-solid process under N-rich
condition. The crystallites are used for the polarized Raman spectra in
different crystalline orientations along with spectral imaging studies. The
results obtained from the polarized Raman spectral studies shows single
crystalline nature of sub-micron sized hexagonal AlGaN crystallites. Optical
properties of the crystallites for different crystalline orientations are also
studied using polarized photoluminescence measurements. The influence of
internal crystal field to the photoluminescence spectra is proposed to explain
the distinctive observation of splitting of emission intensity reported, for
the first time, in case of c-plane oriented single crystalline AlGaN
crystallite as compared to that of m-plane oriented crystallite.Comment: 24 pages, 4 figure, jourana
Far field photoluminescence imaging of single AlGaN nanowire in the sub-diffraction length scale using optical confinement of polarized light
Till now the nanoscale focussing and imaging in the sub-diffraction limit is
achieved mainly with the help of plasmonic field enhancement assisted with
noble metal nanoparticles. Using far field imaging technique, we have recorded
polarized spectroscopic photoluminescence (PL) imaging of a single AlGaN
nanowire (NW) of diameter ~ 100 nm using confinement of polarized light. The
nanowires on the substrate have a nematic ordering. It is found that the PL
from a single NW is influenced by the proximity to other NWs with the PL
intensity scaling as 1/(lxd), where l and d are the NW length and the
separation from the neighbouring NW, respectively. We show that this proximity
induced PL intensity enhancement can be understood, if we assume the existence
of reasonably long lived photons in the intervening space between the NWs. A
nonzero non-equilibrium population of such photons causes stimulated emission
leading to the enhanced PL emission with the intensity scaling as 1/(lxd). The
effect is analogous to the Purcell enhancement of polarized optical emissions
induced by confined photons in micro-cavities. The enhancement of PL emission
facilitated the far field spectroscopic imaging of a single semiconducting
nanowire in the sub-diffraction regime.Comment: 22 pages, 4 figures, Communicated to journa
Optically confined polarized resonance Raman studies in identifying crystalline orientation of sub-diffraction limited AlGaN nanostructure
An optical characterization tool of Raman spectroscopy with extremely weak
scattering cross section tool is not popular to analyze scattered signal from a
single nanostructure in the sub-diffraction regime. In this regard, plasmonic
assisted characterization tools are only relevant in spectroscopic studies of
nanoscale object in the sub-diffraction limit. We have reported polarized
resonance Raman spectroscopic (RRS) studies with strong electron-phonon
coupling to understand the crystalline orientation of a single AlGaN nanowire
of diameter about 100 nm. AlGaN nanowire is grown by chemical vapor deposition
technique using the catalyst assisted vapor-liquid-solid process. The results
are compared with the high resolution transmission electron microscopic
analysis. As a matter of fact, optical confinement effect due to the dielectric
contrast of nanowire with respect to that of surrounding media assisted with
electron-phonon coupling of RRS are useful for the spectroscopic analysis in
the sub-diffraction limit of 325 nm (lamda(2N.A.)^(-1)) using an excitation
wavelength (lamda) of 325 nm and near ultraviolet 40X far field objective with
a numerical aperture (N.A.) value of 0.50.Comment: 13 pages, 4 figures, journal. arXiv admin note: text overlap with
arXiv:1509.0019
Light-matter interaction of single semiconducting AlGaN nanowire and noble metal Au nanoparticle in the sub-diffraction limit
The near field scanning optical microscopy (NSOM) is not only a tool for
imaging of sub-diffraction limited objects but also a prominent characteristic
tool for understanding the intrinsic properties of the nanostructures. In order
to understand the light-matter interactions in the near field regime using NSOM
technique with an excitation of 532 nm (2.33 eV), we selected an isolated
single semiconducting AlGaN nanowire (NW) of diameter ~120 nm grown via vapor
liquid solid (VLS) mechanism along with metallic Au nanoparticle (NP) catalyst.
The role of electronic transitions from different native defect related energy
states of AlGaN are discussed in understanding the NSOM images for the
semiconducting NW. The effect of strong surface plasmon resonance absorption of
excitation laser in the NSOM images for Au NP, involved in the VLS growth
mechanism of NWs, is also observed. KeywoComment: 20 pages, 7 figures, Communicated to journa
Optical imaging of metallic and semiconductor nanostructures at sub wavelength regime
The near field scanning optical microscopy (NSOM) is not only a tool for
imaging of objects in the sub wavelength limit but also a prominent
characteristic tool for understanding the intrinsic properties of the
nanostructures. The effect of strong localized surface plasmon resonance
absorption of excitation laser in the NSOM images for Au nanoparticles is
observed. The role of electronic transitions from different native defect
related energy states of AlGaN are also discussed in understanding the NSOM
images for the semiconductor nanowire.Comment: arXiv admin note: substantial text overlap with arXiv:1605.0334
Interface Phonon Modes in the [AlN/GaN]20 and [Al0.35Ga0.65N/Al0.55Ga0.45N]20 2D Multi Quantum Well Structures
Interface phonon (IF) modes of c-plane oriented [AlN/GaN]20 and
Al0.35Ga0.65N/Al0.55Ga0.45N]20 multi quantum well (MQW) structures grown via
plasma assisted molecular beam epitaxy are reported. The effect of variation in
dielectric constant of barrier layers to the IF optical phonon modes of well
layers periodically arranged in the MQWs investigated.Comment: 17 page
Optical properties of AlGaN nanowires synthesized via ion beam techniques
AlGaN plays a vital role in hetero-structure high electron mobility
transistor by employing a two-dimensional electron gas and as electron blocking
layer in the multi quantum well light emitting diodes. Nevertheless, the
incorporation of Al in GaN for the formation of AlGaN alloy is limited by the
diffusion barrier formed by instant nitridation of Al adatoms by reactive
atomic N. Incorporation of Al above the miscibility limit, however can be
achieved by ion beam technique. The well known ion beam mixing (IBM) technique
was carried out with the help of Ar+ irradiation for different fluences. A
novel approach was also adopted for the synthesis of AlGaN by the process of
post irradiation diffusion (PID) as a comparative study with the IBM technique.
The optical investigations of AlGaN nanowires, synthesized via two different
methods of ion beam processing are reported. The effect of irradiation fluence
and post irradiation annealing temperature on the random alloy formation were
studied by the vibrational and photoluminescence (PL) spectroscopic studies.
Vibrational studies show one-mode phonon behavior corresponding to longitudinal
optical (LO) mode of A1 symmetry (A1(LO)) for the wurtzite phase of AlGaN
nanowires in the random alloy model. Maximum Al atomic percentage ~6.3-6.7% was
calculated with the help of band bowing formalism from the Raman spectral
analysis for samples synthesized in IBM and PID processes. PL studies show the
extent of defects present in these samples
Piezoelectric Domains in the AlGaN Hexagonal Microrods: Effect of Crystal Orientations
Presently, the piezoelectric materials are finding tremendous applications in
the micro-mechanical actuators, sensors and self-powered devices. In this
context, the studies pertaining to piezoelectric properties of materials in the
different size ranges are very important for the scientific community. The
III-nitrides are exceptionally important, not only for optoelectronic but also
for their piezoelectric applications. In the present study, we synthesized
AlGaN via self catalytic vapor-solid mechanism by atmospheric pressure chemical
vapor deposition technique on AlN base layer over intrinsic Si(100) substrate.
The growth process is substantiated using X-ray diffraction and X-ray
photoelectron spectroscopy. The Raman and photoluminescence study reveal the
formation of AlGaN microrods in the wurtzite phase and ensures the high optical
quality of the crystalline material. The single crystalline, direct wide band
gap and hexagonally shaped AlGaN microrods are studied for understanding the
behavior of the crystallites under the application of constant external
electric field using the piezoresponse force microscopy. The present study is
mainly focused on understanding the behavior of induced polarization for the
determination of piezoelectric coefficient of AlGaN microrod along the c-axis
and imaging of piezoelectric domains in the sample originating because of the
angular inclination of AlGaN microrods with respect to its AlN base layers.Comment: 25 pages, 10 figures, Journa
Effect of Scattering Efficiency in the Tip Enhanced Raman Spectroscopic Imaging of Nanostructures in the Sub Diffraction Limit
The experimental limitations in the signal enhancement and spatial resolution
in spectroscopic imaging have been always a challenging task in the application
of near-field spectroscopy for nanostructured materials in the sub-diffraction
limit. In addition, the scattering efficiency also plays an important role in
improving signal enhancement and contrast of the spectroscopic imaging of
nanostructures by scattering of light. We report the effect of scattering
efficiency in the Raman intensity enhancement, and contrast generation in
near-field tip-enhanced Raman spectroscopic (TERS) imaging of one dimensional
inorganic crystalline nanostructures of Si and AlN having a large variation in
polarizability change. The Raman enhancement of pure covalently bonded Si
nanowire (NW) is found to be two orders of higher in magnitude for the TERS
imaging, as compared to that of AlN nanotip (NT) having a higher degree of
ionic bonding, suggesting the importance of scattering efficiency of the
materials in TERS imaging. The strong contrast generation due to higher signal
enhancement in TERS imaging of Si NW also helped in achieving the better
resolved spectroscopic images than that of the AlN NT. The study provides an
insight into the role of scattering efficiency in the resolution of near-field
spectroscopic images.Comment: 7 figure
Near Field Scanning Optical Imaging of Gold Nanoparticles in the Sub-Wavelength Limit
The near-field scanning optical microscopic (NSOM) imaging of Au
nanoparticles with size in the sub-wavelength limit (<wavelength/2N.A.) is
reported. The NSOM imaging technique can resolve the objects which is beyond
the scope of optical microscope using visible light (wavelength=500 nm) with
objectives having a numerical aperture (N.A.) close to unity. The role of
evanescent waves which is an exponentially decaying field with higher momenta
i.e., lower wavelengths compared to that of normal light, in the metal
dielectric interface is realized for imaging of noble metal nanostructures with
sub-wavelength dimension in the near field. However, the confined light with
components of evanescent waves, emanating from the NSOM probe, interacts with
the oscillating dipoles present in the sub-diffraction limited nanostructures
and produce propagating waves, which can be recorded by the far field detector.
The light-matter interactions of Au nanoparticles of diameters in the range of
10-150 nm probed by the NSOM technique with a visible excitation of 532 nm are
reported. The strong surface plasmon resonance (SPR) related absorption of Au
nanoparticles is envisaged for explaining the contrast variations in the
recorded NSOM images.Comment: Presented at APOGEE 2017, BITS Pilani-Pilani Campu
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