Dislocation scattering in a two-dimensional electron gas of an AlxGa1-xN/GaN heterostructure

The theoretical electron mobility limited by dislocation scattering of a two-dimensional electron gas confined near the interface of an AlxGa1-xN/GaN heterostructure is calculated. The accurate wave functions and electron distributions of the three lowest subbands for a typical structure are obtained by solving the Schrodinger and Poisson equations self-consistently. Based on the model of treating dislocation as a charged line, a simple scattering potential, a square-well potential, is utilized. The estimated mobility suggests that such a choice can simplify the calculation without introducing significant deviation from experimental data. It is also found that the dislocation scattering dominates both the low- and moderate-temperature mobilities and accounts for the nearly flattening-out behavior with increasing temperature. To clarify the role of dislocation scattering all standard scattering mechanisms are included in the calculation.

Raman scattering of non-planar graphite: arched edges, polyhedral crystals, whiskers and cones

Planar graphite has been extensively studied by Raman scattering for years. A comparative Raman study of several different and less common non-planar graphitic materials is given here. New kinds of graphite whiskers and tubular graphite cones (synthetic and natural) have been introduced. Raman spectroscopy has been applied to the characterization of natural graphite crystal edge planes, an individual graphite whisker graphite polyhedral crystals and tubular graphite cones. Almost all of the observed Raman modes were assigned according to the selection rules and the double-resonance Raman mechanism. The polarization properties related to the structural features, the line shape of the first-order dispersive mode and its combination modes, the frequency variation of some modes in different carbon materials and other unique Raman spectral features are discussed here in detail.

Raman scattering and photoluminescence studies of Er-implanted and Er+ O coimplanted GaN

Raman measurements and photoluminescence (PL) were performed on the metal-organic chemical-vapor deposition epitaxially grown GaN before and after the implantation with Er and Er+O. Several Raman defect modes have emerged from the implantation-damaged samples. The structures around 300 and 595 cm(-1) modes are attributed to the disorder-activated Raman scattering, whereas the 670 cm(-1) peak is assigned to nitrogen-vacancy-related defect scattering. One additional peak at 360 cm(-1) arises after Er+O coimplantation. This Raman peak is attributed to the O-implantation-induced defect complex. The appearance of the 360 cm(-1) mode results in the decrease of the Er3+ -related infrared PL intensity for the GaN:Er+O samples. (C) 2004 American Institute of Physics.

Raman scattering of folded acoustic phonons in self-assembled Si/Ge dot superlattices

Self-assembled Si/Ge dot multilayers with small, uncorrelated dots fabricated by molecular beam epitaxy in the Stranski-Krastanov growth mode are studied by Raman scattering of folded longitudinal acoustic (FLA) modes. The FLA Raman spectra are analyzed and modeled with respect to mode frequencies and the spectral envelope of mode intensities. The deduced average superlattice properties are consistent with results from atomic force microscopy. The simple Rytov model used for Si/Ge layer structures reproduces very well the frequencies of the FLA modes up to 150 cm(-1). The nonlinearity of phonon dispersion curves in bulk Si for large momenta, however, becomes important for modeling the higher frequencies of observed FLA modes up to 22nd order. The effective dot layer width and an activation energy for thermal intermixing of 2.1+/-0.2 eV are determined from the spectral envelopes of FLA mode intensities of as-grown and annealed Si/Ge dot multilayers. (C) 2004 American Institute of Physics.

One-step growth of ZnO from film to vertically well-aligned nanorods and the morphology-dependent Raman scattering

We observed a transition from film to vertically well-aligned nanorods for ZnO grown on sapphire (0001) substrates by metalorganic chemical vapor deposition. A growth mechanism was proposed to explain such a transition. Vertically well-aligned homogeneous nanorods with average diameters of similar to 30, 45, 60, and 70 nm were grown with the c-axis orientation. Raman scattering showed that the E-2 (high) mode shifted to high frequency with the decrease of nanorod diameters, which revealed the dependence of nanorod diameters on the stress state. This dependence suggests a stress-driven diameter-controlled mechanism for ZnO nanorod arrays grown on sapphire (0001) substrates. (c) 2005 American Institute of Physics.

Raman scattering from GaP nanowires

A complete Raman study of GaP nanowires is presented. By comparison with the Raman spectra of GaP bulk material, microcrystals and nanoparticles, we give evidence that the Raman spectrum is affected by the one-dimensional shape of the nanowires. The Raman spectrum is sensitive to the polarization of the laser light. A specific shape of the overtones located between 600 and 800 cm(-1) is actually a signature of the nanowires. Some phonon confinement and thermal behavior is also observed for nanowires.

The microstructure of hydrogenated microcrystalline silicon thin films studied by small-angle x-ray scattering

The microstructures of hydrogenated microcrystalline silicon (tic-Si: H) thin films, prepared by plasma-enhanced chemical vapor deposition (PECVD), hot wire CVD(HWCVD) and plasma assisted HWCVD (PE-HWCVD), have been analyzed by the small angle x-ray scattering(SAXS) measurement. The SAXS data show that the microstructures of the μ c-Si: H films display different characteristics for different deposition techniques. For films deposited by PECVD, the volume fraction of micro-voids and mean size are smaller than those in HWCVD sample. Aided by suitable ion-bombardment, PE-HWCVD samples show a more compact structure than the HWCVD sample. The microstructure parameters of the μ c-Si: H thin films deposited by two-steps HWCVD and PE-HWCVD with Ar ions are evidently improved. The result of 45° tilting SAXS measurement indicates that the distribution of micro-voids in the film is anisotropic. The Fouriertransform infrared spectra confirm the SAXS data.

Raman scattering and photoluminescence studies of Zn1-xMnxO nanowires via vapor phase growth

In this paper, we investigated the Raman scattering and photoluminescence of Zn1-xMnxO nanowires synthesized by the vapor phase growth. The changes of E-2(High) and A(1(LO)) phonon frequency in Raman spectra indicate that the tensile stress increases while the free carrier concentration decreases with the increase of manganese. The Raman spectra exited by the different lasers exhibit the quantum confinement effect of Zn1-xMnxO nanowires. The photoluminescence spectra reveal that the near band emission is affected by the content of manganese obviously. The values Of I-UV/G decrease distinctly with the manganese increase also demonstrate that more stress introduced with the more substitution of Mn for Zn.

Subtraction of scattering parameters for small-signal modulation characteristics of laser diode

An extended subtraction method of scattering parameters for characterizing laser diode is proposed in this paper. The intrinsic response is extracted from the measured transmission coefficients of laser diode, and the parasitics of packaging net-work laser chip are determined from the measured reflection coefficient of laser diode simultaneously. It is shown that the theories agree well with the experimental results.

Electron mobility related to scattering caused by the strain variation of AlGaN barrier layer in strained AlGaN/GaN heterostructures

Using the measured capacitance- voltage curves of Ni Schottky contacts with different areas on strained AlGaN/ GaN heterostructures and the current- voltage characteristics for the AlGaN/ GaN heterostructure field- effect transistors at low drain- source voltage, we found that the two- dimensional electron gas (2DEG) electron mobility increased as the Ni Schottky contact area increased. When the gate bias increased from negative to positive, the 2DEG electron mobility for the samples increased monotonically except for the sample with the largest Ni Schottky contact area. A new scattering mechanism is proposed, which is based on the polarization Coulomb field scattering related to the strain variation of the AlGaN barrier layer. (C) 2007 American Institute of Physics.

A novel implantable multichannel silicon-based microelectrode

Silicon-based microelectrodes have been confirmed to be helpful in neural prostheses. The fabricated 7-channel silicon-based microelectrode was feasible to be implanted into the brain cortex. The manufacturing process by microelectromechanical system (MEMS) technology was detailed with four photolithographic masks. The microscopic photographs and SEM images indicated that the probe shank was 3mm long, 100 mu m wide and 20 mu m thick with the recording sites spaced 120 mu m apart for good signal isolation. To facilitate the insertion and minimize the trauma, the microelectrode is narrowed down gradually near the tip with the tip taper angle of 6 degrees. Curve of the single recording site impedance versus frequency was shown by test in vitro and the impedance declined from 150.5 k Omega to 6.0 k Omega with frequency changing from 10 k to 10MHz.

First and second order Raman scattering spectroscopy of nonpolar a-plane GaN

Nonpolar a-plane [(1120)] GaN samples have been grown on r-plane [(1102)] sapphire substrates by low-pressure metal-organic chemical-vapor deposition. The room-temperature first and second order Raman scattering spectra of nonpolar a-plane GaN have been measured in surface and edge backscattering geometries. All of the phonon modes that the selection rules allow have been observed in the first order Raman spectra. The frequencies and linewidths of the active modes have been analyzed. The second order phonon modes are composed of acoustic overtones, acoustic-optical and optical-optical combination bands, and optical overtones. The corresponding assignments of second order phonon modes have been made. (c) 2007 American Institute of Physics.

Raman scattering from an individual tubular graphite cone

　

Polarized Raman scattering studies of nonpolar a-plane GaN films grown on r-plane sapphire substrates by MOCVD

Nonpolar (1120) a-plane GaN thin films were grown on r-plane (1102) sapphire substrates by low-pressure metal organic chemical vapor deposition (MOCVD). The stress characteristics of the a-plane GaN films were investigated by means of polarized Raman scattering spectra in backscattering configurations. The experimental results show that there are strong anisotropic in-plane stresses within the epitaxial a-plane GaN films by calculating the corresponding stress tensors. The temperature dependence of Raman scattering spectra was studied in the range from 100 K to 550 K. The measurements reveal that the Raman phonon frequencies decrease with increasing temperature. The temperature at which nonpolar a-plane GaN films are strain free is discussed. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Raman scattering detection of stacking faults in free-standing cubic-SiC epilayer

We report on stacking fault (SF) detection in free-standing cubic-SiC epilayer by the Raman measurements. The epilayer with enhanced SFs is heteroepitaxially grown by low pressure chemical vapour deposition on a Si(100) substrate and is released in KOH solution by micromechanical manufacture, on which the Raman measurements are performed in a back scattering geometry. The TO line of the Raman spectra is considerably broadened and distorted. We discuss the influence of SFs on the intensity profiles of TO mode by comparing our experimental data with the simulated results based on the Raman bond polarizability (BP) model in the framework of linear-chain concept. Good agreement with respect to the linewidth and disorder-induced peak shift is found by assuming the mean distance of the SFs to be 11 angstrom in the BP model.