Optical characterization of AlN/GaN heterostructures

AlN/GaN/sapphire heterostructures with AlN gate film thickness of 3–35 nm are characterized using photoreflectivity (PR) and photoluminescence (PL) spectroscopy. Under a critical AlN film thickness, the luminescence from the GaN channel layer near the interface proves to be excitonic. No luminescence related to the recombination of the two-dimensional electron gas (2DEG) is observed, in spite of high 2DEG parameters indicated by Hall-effect measurements. The increase of the AlN gate film thickness beyond a critical value leads to a sharp decrease in exciton resonance in PR and PL spectra as well as to the emergence of a PL band in the 3.40–3.45 eV spectral range. These findings are explained taking into account the formation of defects in the GaN channel layer as a result of strain-induced AlN film cracking. A model of electronic transitions responsible for the emission band involved is proposed. © 2003 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71050/2/JAPIAU-94-8-4813-1.pd

Luminescence of GaN nanocolumns obtained by photon-assisted anodic etching

GaN nanocolumns with transverse dimensions of about 50 nm were obtained by illumination-assisted anodic etching of epilayers grown by metalorganic chemical vapor deposition on sapphire substrates. The photoluminescence spectroscopy characterization shows that the as-grown bulk GaN layers suffer from compressive biaxial strain of 0.5 GPa. The majority of nanocolumns are fully relaxed from strain, and the room-temperature luminescence is free excitonic. The high quality of the columnar nanostructures evidenced by the enhanced intensity of the exciton luminescence and by the decrease of the yellow luminescence is explained by the peculiarities of the anodic etching processing. © 2003 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69916/2/APPLAB-83-8-1551-1.pd

Core–Shell Structures Prepared by Atomic Layer Deposition on GaAs Nanowires

GaAs nanowire arrays have been prepared by anodization of GaAs substrates. The nanowires produced on (111)B GaAs substrates were found to be oriented predominantly perpendicular to the substrate surface. The prepared nanowire arrays have been coated with thin ZnO or TiO2 layers by means of thermal atomic layer deposition (ALD), thus coaxial core–shell hybrid structures are being fabricated. The hybrid structures have been characterized by scanning electron microscopy (SEM) for the morphology investigations, by Energy Dispersive X-ray (EDX) and X-ray diffraction (XRD) analysis for the composition and crystal structure assessment, and by photoluminescence (PL) spectroscopy for obtaining an insight on emission polarization related to different recombination channels in the prepared core–shell structures

Resonance Raman Scattering in TlGaSe 2

The resonance Raman scattering for geometries Y(YX)Z and Y(ZX)Z at temperature 10 K and infrared reflection spectra in E∥a and E∥b polarizations at 300 K were investigated. The number of Aa (Ba) and Au (Bu) symmetry vibrational modes observed experimentally and calculated theoretically agree better in this case than when TlGa2Se4 crystals belong to D2h symmetry group. The emission of resonance Raman scattering and excitonic levels luminescence spectra overlap. The lines in resonance Raman spectra were identified as a combination of optical phonons in Brillouin zone center

Aero-TiO2 Prepared on the Basis of Networks of ZnO Tetrapods

In this paper, new aeromaterials are proposed on the basis of titania thin films deposited using atomic layer deposition (ALD) on a sacrificial network of ZnO microtetrapods. The technology consists of two technological steps applied after ALD, namely, thermal treatment at different temperatures and etching of the sacrificial template. Two procedures are applied for etching, one of which is wet etching in a citric acid aqua solution, while the other one is etching in a hydride vapor phase epitaxy (HVPE) system with HCl and hydrogen chemicals. The morphology, composition, and crystal structure of the produced aeromaterials are investigated depending on the temperature of annealing and the sequence of the technological steps. The performed photoluminescence analysis suggests that the developed aeromaterials are potential candidates for photocatalytic applications

LETTER TO THE EDITOR: Sharp variations in the temperature dependence of optical reflectivity from AlN/GaN heterostructures

Sharp variations in optical reflectivity were observed when cooling and heating AlN/GaN heterostructures on sapphire substrates between room temperature and 10 K. The reflectivity was found to decrease at a definite temperature Tk in the downward temperature run, and to recover at Tr > Tk in the subsequent upward temperature run. The temperature behaviour of reflectivity exhibits memory on the cooling–heating cycles previously subjected to samples.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/48936/2/s302l1.pd

Photoluminescence and resonant Raman scattering from ZnO-opal structures

The photoluminescence (PL) of ZnO-opal structures excited by a 351.1 nm laser line was studied. The fabrication of these structures was done by infiltration of ZnO from an aqueous solution of zinc nitrate into opal matrices. It was observed that the emission spectrum of thick ZnO layers grown on the surface of bulk opals exhibited narrow PL bands associated with the recombination of bound and free-excitons. The results showed that the strong exciton-LO phonon and exciton-Frohlich mode coupling in ZnO nanostructures was deduced from the analysis of multiphonon excitonic resonant Raman scattering.This work was supported by INTAS under Grant No. 01-0796. ISSP authors are grateful for the financial support received from the Russian Foundation for Basic Research (Project No. 01-02-97024) and from the Russian Federation Government under Contract No. 40.012.1.1.11.54.Peer Reviewe

Rapid Thermal Annealing Induced Change Of The Mechanism Of Multiphonon Resonant Raman Scattering From Zno Nanorods

Multiphonon Resonant Raman scattering (RRS) excited by 351.1 and 363.8 nm lines of an Ar+ laser was studied at temperatures from 10 to 300 K in as-grown and rapid thermal annealed (RTA) aluminum doped ZnO nanorods synthesized by an aqueous chemical deposition method using zinc sulfate, aluminum sulfate, and ammonia hydroxide as precursors. RTA of ZnO nanorods at temperatures 650-750{ring operator}C was found to result in changing the mechanism of RRS from incoming to outgoing. This change is suggested to be related to the RTA induced improvement of the optical properties of the nanorods. © 2007 Elsevier Ltd. All rights reserved

Raman scattering in right angle configuration on Cu2ZnSiSe4 single crystals

Polarized Raman scattering and resonance Raman scattering spectra of Cu2ZnSiSe4 crystals measured at temperature 300 and 10 K were investigated. Nine vibrational modes of A2 symmetry, seven modes of B2 symmetry and nine modes of B1 symmetry were determined in Raman spectra taken at right angle con amp; 64257;guration from the 2 1 0 crystal plane. A resonance Raman scattering with participation of 2LO, 3LO and more phonons was observed at photon energies higher than the ground state of exciton transition at low temperatur