LO Phonon Research Articles

Longitudinal polar optical phonons in InN/GaN single and double het‐ erostructures

AbstractLongitudinal optical phonon energy in InN epi‐layers has been determined independently from the Raman spectroscopy and temperature dependent Hall mobility measurements. Raman spectroscopy technique can be used to obtain directly the LO energy where LO phonon scattering dominates transport at high temperature. Moreover, the Hall mobility is determined by the scattering of electrons with LO phonons so the data for the temperature dependence of Hall mobility have been used to calculate the effective energy of longitudinal optical phonons.The samples investigated were (i) single heterojunction InN with thicknesses of 1.08, 2.07 and 4.7 μm grown onto a 40 nm GaN buffer and (ii) GaN/InN/AlN double heterojunction samples with InN thicknesses of 0.4, 0.6 and 0.8 μm. Hall Effect measurements were carried out as a function of temperature in the range between T = 1.7 and 275 K at fixed magnetic and electric fields. The Raman spectra were obtained at room temperature. In the experiments, the 532 nm line of a nitrogen laser was used as the excitation source and the light was incident onto the samples along of the growth direction (c‐axis). The results, obtained from the two independent techniques suggest the following: (1) LO phonon energies obtained from momentum relaxation experiments are generally slightly higher than those obtained from the Raman spectra. (2) LO phonon energy for the single heterojunctions does not depend on the InN thickness. (3) In double heterostructures, with smaller InN thicknesses and hence with increased strain, LO phonon energy increases by 3% (experimental accuracy is < 1%) when the InN layer thickness increases from 400 to 800 nm (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

High pressure Raman scattering of silicon nanowires

We study the high pressure response, up to 8 GPa, of silicon nanowires (SiNWs) with ∼ 15 nm diameter, by Raman spectroscopy. The first order Raman peak shows a superlineartrend, more pronounced compared to bulk Si. Combining transmission electronmicroscopy and Raman measurements we estimate the SiNWs’ bulk modulusand the Grüneisen parameters. We detect an increase of Raman linewidth at ∼ 4 GPa, and assign it to pressure induced activation of a decay processinto LO and TA phonons. This pressure is smaller compared to the ∼ 7 GPa reported for bulk Si. We do not observe evidence of phase transitions, such asdiscontinuities or change in the pressure slopes, in the investigated pressure range.

Self heating in AlInN/AlN/GaN high power devices: Origin and impact on contact breakdown and IV characteristics

It is observed experimentally that high electron mobility transistor devices with short channel length processed from nitride AlInN/AlN/GaN heterostructures containing 2D electron gases (2DEGs) with densities beyond 2 × 1013 cm−2 exhibit temperatures up to 1000 K if they are driven at high drain-source voltages. Corresponding current-voltage (IV) characteristics show a peaklike behavior with a maximum saturation current (IDS,max) up to 2 A/mm. The goal of this article is to describe device heating in the framework of LO phonon statistics and its dependence on channel length, carrier density, and applied voltage. The strong channel heating, on the other hand, affects the transport mobility and must be taken into account to correctly interpret IV characteristics and resolve the discrepancy with metal oxide semiconductor field effect transistor models. Furthermore, the breakdown of ohmic contacts can directly be related to the channel temperature, i.e., the channel reaches the melting point of the contact metallization. In addition, the model correctly predicts the behavior of IV curves versus 2DEG density and transistor initial ambient temperature. For 2DEGs confined in triangular potential wells formed at the heterointerface, a maximum IDS is found for 2DEG densities between 2 × 1013 and 3 × 1013 cm−2.

Double strain state in a single GaN/AlN nanowire: Probing the core-shell effect by ultraviolet resonant Raman scattering

We report the demonstration of an ultra-sensitive Raman probing of single GaN/AlN nanowires (NWs). The high sensitivity of the Raman scattering by longitudinal optical phonon is achieved by using ultraviolet resonant excitation near the energy band-gap of GaN. Structural variations within one single nanowire are evidenced very accurately by strong LO phonons shifts in the UV Raman spectra recorded on different regions of the NW. They are interpreted as a fine probing of the double strain state experienced by GaN, due to the formation of an AlN shell in the bottom part of the NW. The core-shell structure has been confirmed in a statistical way by measuring the average strain in the NWs ensemble thanks to the Raman scattering excited in the visible range. Data have been comprehensively accounted for by considering an axial strain in GaN NW part covered by AlN shell, in the elastic regime, while the top GaN is relaxed.

Correlation between Photooxidation and the Appearance of Raman Scattering Bands in Lead Chalcogenide Quantum Dots

In this report, we carefully study the effects of photooxidation on the Raman spectra of lead chalcogenide (PbX) quantum dots (QDs). Photoexcitation of PbS, PbSe, and PbTe QD films at 488 nm with power densities as low as 30 W/cm2 gives rise to several peaks related to both lead(II) oxide and the group VI chalcogenates (PbXO4). The amplitudes of these peaks are shown to increase with continuous laser illumination in air, but are completely absent for samples illuminated under rigorously air-free conditions. These results suggest that the ∼135 cm−1 Raman peak often assigned to an intrinsic PbX LO phonon is more likely an artifact arising from photooxidation. The myriad of potential photooxidation products formed quickly in laser-illuminated, air-exposed PbX QDs suggest that caution should be used in the assignment and interpretation of phonon spectra and phonon-mediated exciton relaxation pathways of these materials, unless the processing and experiments are conducted under air-free conditions.

Influence of the substrate type on CVD grown homoepitaxial diamond layer quality by cross sectional TEM and CL analysis

To assess diamond-based semiconducting devices, a reduction of point defect levels and an accurate control of doping are required as well as the control of layer thickness. Among the analyses required to improve such parameters, cross sectional studies should take importance in the near future. The present contribution shows how FIB (focused ion beam) preparations followed by electron microscopy related techniques as TEM or CL allowed to perform analysis versus depth in the layer, doping and point defect levels. Three samples grown along the same week in the same machine with identical growth conditions but on different substrates (CVD-IIIa (110) oriented, CVD-optical grade (100) oriented and a HPHT-Ib (100) oriented) are studied. Even though A-band is observed by CL, no dislocation is observed by CTEM. Point defect type and level are shown to substantially change with respect to the substrate type as well as the boron doping levels that vary within an order of magnitude. H3 present in the epilayer grown on HPHT type of substrate is replaced by T1 and NE3 point defects for epilayers grown on the CVD type one. An increase of excitonic transitions through LO phonons is also shown to take place near the surface while only TO ones are detected deeper in the epilayer. Such results highlight the importance of choosing the correct substrate.

Generation of coherent LO phonons in optically driven biased quantum wells

AbstractWe present a theoretical analysis and numerical simulations of the resonant generation of coherent LO phonons in an electrically biased quantum well. The system is driven by a laser pulse short enough to simultaneously excite two transitions, which leads to a quantum beat that can resonantly create coherent phonons. The generation of incoherent phonons is also calculated on a quantum kinetic level and found to be dominant in terms of the energy transferred to the lattice.The simulations successfully reproduce the main features found in recent experiments [Mizoguchi et al., Appl. Phys. Lett. 94, 171105 (2009)]. Depending on the excitation conditions different generation mechanisms can be discerned. We discuss their characteristics and their relative efficiency in the special case of the quantum well model under consideration. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

One-dimensional ZnO exciton polaritons with negligible thermal broadening at room temperature

Single ZnO microwires are investigated by angle-resolved photoluminescence spectroscopy. We show that confined optical modes similar to whispering gallery modes can strongly interact with excitons to form one-dimensional exciton polaritons at room temperature, with normal mode splitting exceeding 200 meV. With such a splitting, which is much larger than LO phonon energy, a strong quenching of the polariton-phonon interaction is achieved, even at room temperature and for large excitonic fractions. Thus, a record figure of merit of 50 for the ratio of the Rabi splitting to the polariton full width at half maximum is achieved as a consequence of negligible thermal contribution to dephasing.

Structural, optical and electrical properties of undoped and Si-doped Al xGa 1− xN thin films on Si (1 1 1) substrate grown by PA-MBE

We have studied the structural properties of undoped and Si-doped Al x Ga 1− x N/GaN/AlN on Si (1 1 1) substrate prepared by plasma-assisted molecular beam epitaxy (PA-MBE) using high-resolution X-ray diffraction (HR-XRD) and atomic force microscopy (AFM). In comparison with undoped AlGaN, the roughness and dislocation density on the surface of the AlGaN layer decrease with Si doping. Full width half maximum (FWHM) of the undoped and Si-doped samples were equal to 0.69° and 0.52°, respectively. This indicates that the Si doping improves the crystalline quality of the Al x Ga 1− x N layer compared with the undoped one. Raman scattering measurement reveals that the optical phonon modes of A 1(LO) and E 2(H) of the AlGaN show a one-mode and two-modes behavior, respectively. The Fourier-transform infrared reflectance (FTIR) investigation confirms the one-mode (two-mode) behavior of the LO (TO) phonon in our samples. This is in good agreement with Raman measurement. Finally, the barrier height ( Φ B ) of undoped and Si-doped Al x Ga 1− x N samples was found to be 0.86 and 0.74 eV, respectively.

Variable Temperature Spectroscopy of As-Grown and Passivated CdS Nanowire Optical Waveguide Cavities

Semiconductor nanowire waveguide cavities hold promise for nanophotonic applications such as lasers, waveguides, switches, and sensors due to the tight optical confinement in these structures. However, to realize their full potential, high quality nanowires, whose emission at low temperatures is dominated by free exciton emission, need to be synthesized. In addition, a proper understanding of their complex optical properties, including light-matter coupling in these subwavelength structures, is required. We have synthesized very high-quality wurztite CdS nanowires capped with a 5 nm SiO(2) conformal coating with diameters spanning 100-300 nm using physical vapor and atomic layer deposition techniques and characterized their spatially resolved photoluminescence over the 77-298 K temperature range. In addition to the Fabry-Pérot resonator modulated emission from the ends of the wires, the low temperature emission from the center of the wire shows clear free excitonic peaks and LO phonon replicas, persisting up to room-temperature in the passivated wires. From laser scanning measurements we determined the absorption in the vicinity of the excitonic resonances. In addition to demonstrating the high optical quality of the nanowire crystals, these results provide the fundamental parameters for strong light-matter coupling studies, potentially leading to low threshold polariton lasers, sensitive sensors and optical switches at the nanoscale.

Angular Dependence of Optically Detected Magnetophonon Resonances in Miniband Transport in Semiconductor Superlattice under Tilted Magnetic Fields

The angular dependence of the optically detected magnetophonon resonance (ODMPR) for the frequency-dependent magnetoconductivity in a Ge-based (nonpolar) semiconductor superlattice under a tilted magnetic field is investigated. The frequency-dependent magnetoconductivity, including the miniband width, the periodic potential, the photon frequency, and the tilt angle of a magnetic field tilted relative to the superlattice axis, is calculated using the nonpolar LO phonon modes as a function of the magnetic field strengths. In particular, the angular dependence of the ODMPR lineshape, such as the splitting and the shift of the ODMPR peaks for the photon frequency, is discussed. Furthermore, the angular dependences for the appearance of the plateau region between two neighboring ODMPR peaks and the change in the ODMPR amplitude as a function of the tilt angle of the tilted magnetic field for various miniband widths and photon frequencies are discussed in detail.

Surface characterization of thin silicon-rich oxide films

The silicon-rich oxide (SiO x ) films were deposited using the LPCVD (Low Pressure Chemical Vapour Deposition) method at the temperature of 570 °C and with silane and oxygen as the reactant gasses. The films were deposited on silicon (1 1 1) substrates. The flows of oxygen and silan in the horizontal tube reactor were varied in order to deposit films with different values of oxygen content x. The roughness of the film surfaces and of the substrate-film interfaces were determined by X-ray specular reflection. A homogeneous surface with the root-mean square (r.m.s.) surface roughness less than 3 nm has been found. Scanning electron microscopy shows surface lateral structures smaller than 50 nm. Infrared absorption shows the broad peak of the TO 3 phonon mode at 1000 cm −1 which blue shifts with the increase of oxygen content x. The observed absence of the LO 3 phonon mode at 1260 cm −1 is an another indication of the low surface roughness. The Raman spectra show broad bands of the TA, LA, LO, and TO-like phonon bands typical of amorphous materials.

Synthesis and characterization of ZnO/NiO p–n heterojunctions: ZnO nanorods grown on NiO thin film by thermal evaporation

The present article describes the synthesis and characterization of ZnO nanostructures grown on a NiO thin film. The structure and morphology of the as-grown film of ZnO nanorods were characterized by scanning electron microscopy, and X-ray diffraction revealing a rather well ordered array of ZnO nanorods aligned normally to the substrate. Photoluminescence spectroscopy was used to study the optical properties providing evidence for several types of defects in the as grown nanostructures. A comparison between the off-resonance Raman spectra of the nanostructures and bulk ZnO showed a number of atypical Raman bands in the former whose origin is discussed in terms of resonantly enhanced LO phonons via the extrinsic Fröhlich interaction. p-NiO/n-ZnO heterojunction diodes are currently emerging as attractive materials with potential use in electronic and optoelectronic devices.

The Rashba Effect on the Bound Polaron in a Parabolic Quantum Dot

The bound polaron ground state energy is calculated by the variational method of Pekar considering the influence of the Rashba SO interaction on the condition of electric–LO phonon strong coupling in a parabolic quantum dot (QD). The relations on the bound polaron ground state energy with the parallel confinement length, the electron–LO phonon coupling constant, the perpendicular confinement length and the Coulomb binding parameter are derived for a parabolic quantum dot.

Polaron effects on the optical refractive index changes in asymmetrical quantum wells

The optical refractive index changes considering polaron effects in asymmetrical quantum wells are theoretically studied. It is shown that a pronounced optical refractive index changes dependence of the quantum well parameters can be obtained. Moreover, the theoretical values of the optical refractive index changes obviously increase when considering the electron–LO–phonon interaction.

Correlation effects for a quasi‐one‐dimensional polaron gas

AbstractIn this work, we investigate the plasmon–LO phonon interaction effects on the intrasubband structure factor, electron–electron effective potential, and plasmon energy associated with the lowest subband in a GaAs‐AlGaAs rectangular quantum‐well wire (QWW) as a function of the electronic density. Our calculations are performed using the self‐consistent field approximation, which includes the local‐field correction (LFC) within the Singwi, Tosi, Land, and Sjolander (STLS) theory, at zero temperature and assuming a three‐subband model, where only the first subband is occupied by electrons. We report for the first time dips in the structure factor spectra as a function of the quasi‐one‐dimensional (Q1D) plasmon–LO phonon wavevector that are directly related with the resonant split of the collective excitation energy into two branches due to the polaronic effects.

Study of field emission and dielectric properties of AlN films prepared by DC sputtering technique at different substrate temperatures

Nanocrystalline AlN thin films were prepared via DC sputtering technique at different substrate temperature. The crystal orientation and particle size of aluminum nitride thin films were investigated by XRD analysis. Study indicated that the sample contained pure phase hexagonal AlN nanoparticles with a single peak corresponding to the (100) planes. The peak at 665 cm−1 in the FTIR spectrum of film was assigned to the LO phonon of hexagonal AlN. The particle size of the film, prepared at substrate temperature 200°C was about 9.5 nm, as investigated by atomic force microscope. Field emission study indicated that it can be used as a good field emitter. Turn-on field (Eto) of 15.02 V/μm was observed for the AlN films synthesized at substrate temperature 200°C. Dielectric constant of the AlN film was found nearly independent of frequencies in the measured frequency range 1 KHz to 1 MHz, i.e. in the audio frequency range. The values of dielectric constant (e) were 10.07, 9.46 and 8.65 for the film prepared at 70°C, 150°C and 200°C, respectively, at frequency 1 KHz.

Electron–phonon interaction and size effect study in catalyst based zinc oxide thin films

Nanocrystalline zinc oxide (ZnO) thin films have attracted great attention due to their possible applications in optoelectronic devices. The dependence of size and catalyst (indium and fluorine) in ZnO thin films synthesized by spray pyrolysis technique was studied. To understand the chemical bonding structure and electronic states of the catalyst based ZnO thin films, XPS analysis was carried out. A comprehensive micro-Raman scattering study of the phonons in catalyst based ZnO thin films grown in ambient air was made. Low temperature dependent Raman spectroscopy studies were carried out for the detailed structures, vibrations, and in situ size variation of nanostructured thin films. The size and catalyst dependence of the coupling strength between electron and LO phonon was experimentally estimated. The coupling strength determined by the ratio of second to first-order Raman scattering cross sections was found to diminish with decreasing crystallite size, and the Fröhlich interaction plays the vital role in electron–phonon-coupling in ZnO. Influence of catalyst on the luminescence properties was investigated.

Investigation of Structured Green-Band Emission and Electron−Phonon Interactions in Vertically Aligned ZnO Nanowires

We have investigated the green band (GB) emission from vertically aligned ZnO nanowires (NWs) prepared by vapor−liquid−solid growth technique. At low temperatures, the GB emission consists of two sets of zero phonon lines and their phonon replicas independent of the laser excitation power. Temperature-dependent photoluminescence (PL) measurement is performed at temperatures from 10 to 300 K, and the thermal evolution of peak widths and the origin of PL quenching at high temperatures are discussed in detail. The multimode Brownian oscillator model is adopted to reproduce the structured GB and to determine the Huang−Rhys factor. It is found that the two electronic transitions are strongly coupled to different LO phonon modes, and the size-dependent electron−phonon coupling strength is examined.

Resonance effects in Raman scattering of quantum dots formed by the Langmuir-Blodgett method

The enhancement of Raman scattering by optical phonon modes in quantum dots was achieved in resonant and surface-enhanced Raman scattering experiments by approaching the laser energy to the energy of either the interband transitions or the localized surface plasmons in silver nanoclusters deposited onto the nanostructures. Resonant Raman scattering by TO, LO, and SO phonons as well as their overtones was observed for PbS, ZnS, and ZnO quantum dots while enhancement for LO and SO modes in CdS quantum dots with a factor of about 700 was measured in surface enhanced Raman scattering experiments. Multiple phonon Raman scattering observed up to 5th and 7th order for CdS and ZnO, respectively, confirms the high crystalline quality of the grown QDs.