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

Localized tip enhanced Raman spectroscopic study of impurity incorporated single GaN nanowire in the sub-diffraction limit

The localized effect of impurities in single GaN nanowires in the sub-diffraction limit is reported using the study of lattice vibrational modes in the evanescent field of Au nanoparticle assisted tip enhanced Raman spectroscopy (TERS). GaN nanowires with the O impurity and the Mg dopants were grown by the chemical vapor deposition technique in the catalyst assisted vapor-liquid-solid process. Symmetry allowed Raman modes of wurtzite GaN are observed for undoped and doped nanowires. Unusually very strong intensity of the non-zone center zone boundary mode is observed for the TERS studies of both the undoped and the Mg doped GaN single nanowires. Surface optical mode of A1 symmetry is also observed for both the undoped and the Mg doped GaN samples. A strong coupling of longitudinal optical (LO) phonons with free electrons, however is reported only in the O rich single nanowires with the asymmetric A1(LO) mode. Study of the local vibration mode shows the presence of Mg as dopant in the single GaN nanowires.Comment: 12 pages, 3 figures, 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

Role of polarized tip-enhanced Raman spectroscopy in the enhancement of interface optical phonon modes in AlGaN multi-quantum wells

Group III nitride based two-dimensional multi-quantum well (MQW) nanostructures find remarkable applications in the visible to ultraviolet light sources. The interface optical (IFO) phonon modes in a c-axis oriented superlattice of [Al0.35Ga0.65N (~1.75 nm)/Al0.55Ga0.45N (~2nm)]20 MQWs are observed using tip-enhanced Raman spectroscopic (TERS) studies. The near-field studies using TERS probe with an Au spherical nanoparticle of ~ 200 nm diameter were carried out at ambient conditions showing approximately two to three orders of enhancement in the Raman intensities. The interface phonon mode belonging to E1 symmetry [IFO(E1)] vibrating normal to the c-axis of MQWs appeared to be more prominent in the case of TERS measurement compared to that for the other interface phonon mode of A1 symmetry. The confined electric field of the polarized electro-magnetic excitation using TERS probe, parallel to the plane of the interface of MQW, is made responsible for the plasmonic enhancement of IFO(E1) phonon mode. The confinement was verified using finite-difference time-domain simulation

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

Raman Measurements and Stress Analysis in Gallium Ion Implanted Gallium Nitride Epitaxial Layers on Sapphire

In this article, we estimate hydrostatic stress developed in gallium ion implanted gallium nitride epitaxial layers using Raman measurements. We have calculated deformation potential constants for $E_2$(high) mode in these epi-layers. The presence of a polar phonon-plasmon coupling in these systems has also been demonstrated. In as-implanted samples, with an increase in implantation fluence, we have observed disorder-activated Raman scattering.Comment: 26pages, 3 figure

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

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

Role of Vanadyl Oxygen in Understanding Metallic Behavior of V2O5(001) Nanorods

Vanadium pentoxide (V2O5), the most stable member of vanadium oxide family, exhibits interesting semiconductor to metal transition in the temperature range of 530-560 K. The metallic behavior originates because of the reduction of V2O5 through oxygen vacancies. In the present report, V2O5 nanorods in the orthorhombic phase with crystal orientation of (001) are grown using vapor transport process. Among three nonequivalent oxygen atoms in a VO5 pyramidal formula unit in V2O5 structure, the role of terminal vanadyl oxygen (OI) in the formation of metallic phase above the transition temperature is established from the temperature-dependent Raman spectroscopic studies. The origin of the metallic behavior of V2O5 is also understood due to the breakdown of pdpi bond between OI and nearest V atom instigated by the formation of vanadyl OI vacancy, confirmed from the downward shift of the bottom most split-off conduction bands in the material with increasing temperature