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