Microstructure and strain analysis of GaN epitaxial films using in-plane grazing incidence x-ray diffraction

This paper investigates the major structural parameters, such as crystal quality and strain state of (001)-oriented GaN thin films grown on sapphire substrates by metalorganic chemical vapour deposition, using an in-plane grazing incidence x-ray diffraction technique. The results are analysed and compared with a complementary out-of-plane x-ray diffraction technique. The twist of the GaN mosaic structure is determined through the direct grazing incidence measurement of (100) reflection which agrees well with the result obtained by extrapolation method. The method for directly determining the in-plane lattice parameters of the GaN layers is also presented. Combined with the biaxial strain model, it derives the lattice parameters corresponding to fully relaxed GaN films. The GaN epilayers show an increasing residual compressive stress with increasing layer thickness when the two dimensional growth stage is established, reaching to a maximum level of -0.89 GPa.

Epitaxial Semiconductor Quantum Wires

The investigation on the direct epitaxial quantum wires (QWR) using MBE or MOCVD has been persuited for more than two decades, more lengthy in history as compared with its quantum dot counterpart. Up to now, QWRs with various structural configurations have been produced with different growth methods. This is a reviewing article consisting mainly of two parts. The first part discusses QWRs of various configurations, together with laser devices based on them, in terms of the two growth mechanisms, self-ordering and self-assembling. The second part gives a brief review of the electrical and optical properties of QWRs.

Structural and electrical properties of (110) ZnO epitaxial thin films on (001) SrTiO3 substrates

(110) oriented ZnO thin films were epitaxially prepared on (001) SrTiO3 single crystal substrates by a pulsed laser deposition method. The evolution of structure, surface morphology, and electrical conductivity of ZnO films was investigated on changing the growth temperature. Two domain configurations with 90 degrees rotation to each other in the film plane were found to exist to reduce the lattice mismatch between the films and substrates. In the measured temperature range between 80 K and 300 K, the electrical conductivity can be perfectly fitted by a formula of a (T) = sigma(0) + aT(b/2). implying that the electron-phonon scattering might have a significant contribution to the conductivity. (C) 2008 Elsevier Ltd. All rights reserved.

Structural, electrical, and optical properties of InAsxSb1-x epitaxial films grown by liquid-phase epitaxy

The InAsxSb1-x films were grown on (100) GaSb substrates by liquid-phase epitaxy, and their structural, electrical, and optical properties were investigated. The high-resolution x-ray diffraction results reveal that the single crystalline InAsxSb1-x films with a midrange composition are epitaxially grown on the GaSb substrates. Temperature dependence of the Hall mobility was theoretically modeled by considering several predominant scattering mechanisms. The results indicate that ionized impurity and dislocation scatterings dominate at low temperatures, while polar optical phonon scattering is important at room temperature (RT). Furthermore, the InAsxSb1-x films with the higher As composition exhibit the better crystalline quality and the higher mobility. The InAs0.35Sb0.65 film exhibits a Hall mobility of 4.62x10(4) cm(2) V-1 s(-1). The cutoff wavelength of photoresponse is extended to about 12 mu m with a maximum responsivity of 0.21 V/W at RT, showing great potential for RT long-wavelength infrared detection. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.2989116]

Zn2SiO4/ZnO Core/Shell Coaxial Heterostructure Nanobelts Formed by an Epitaxial Growth

Si-doped ZnO can be synthesized on the surface of the early grown Zn2SiO4 nanostructures and form core/ shell coaxial heterostructure nanobelts with an epitaxial orientation relationship. A parallel interface with a periodicity array of edge dislocations and an inclined interface without dislocations can be formed. The visible green emission is predominant in PL spectra due to carrier localization by high density of deep traps from complexes of impurities and defects. Due to band tail localization induced by composition and defect fluctuation, and high density of free-carriers donated by doping, especially the further dissociation of excitons into free-carriers at high excitation intensity, the near-band-edge emission is dominated by the transition of free-electrons to free-holes, and furthermore, exhibits a significant excitation power-dependent red-shift characteristic. Due to the structure relaxation and the thermalization effects, carrier delocalization takes place in deep traps with increasing excitation density. As a result, the green emission passes through a maximum at 0.25I(0) excitation intensity, and the ratio of the violet to green emission increases monotonously as the excitation laser power density increases. The violet and green emission of ZnO nanostructures can be well tuned by a moderate doping and a variation in the excitation density.

Raman scattering characterization of Mn composition and strain in Ga1-xMnxSb/GaSb epitaxial layers

Raman scattering (RS) experiments have been performed for simultaneous determination of Mn composition and strain in Ga1-xMnxSb thin films grown on GaSb substrate by liquid phase epitaxy technique. The Raman spectra obtained from various Ga1-xMnxSb samples show only GaSb-like phonon modes whose frequency positions are found to have Mn compositional dependence. With the combination of epilayer strain model, RS and energy dispersive x-ray (EDX) experiments, the compositional dependence of GaSb-like LO phonon frequency is proposed both in strained and unstrained conditions. The proposed relationships are used to evaluate Mn composition and strain from the Ga1-xMnxSb samples. The results obtained from the RS data are found to be in good agreement with those determined independently by the EDX analysis. Furthermore, the frequency positions of MnSb-like phonon modes are suggested by reduced-mass model. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Resonant tunneling through double-bended graphene nanoribbons

We investigate theoretically resonant tunneling through double-bended graphene nanoribbon (GNR) structures, i.e., armchair-edged GNRs (AGNRs) in between two semi-infinite zigzag GNR leads. Our numerical results demonstrate that the resonant tunneling can be tuned dramatically by the Fermi energy and the length and/or widths of the AGNR for both the metallic and semiconductorlike AGNRs. The structure can also be use to control the valley polarization of the tunneling currents and could be useful for potential application in valleytronics devices. (C) 2008 American Institute of Physics.

Tuning of energy levels and optical properties of graphene quantum dots

We investigate theoretically the magnetic levels and optical properties of zigzag- and armchair-edged hexagonal graphene quantum dots (GQDs) utilizing the tight-binding method. A bound edge state at zero energy appears for the zigzag GQDs in the absence of a magnetic field. The magnetic levels of GQDs exhibit a Hofstadter-butterfly spectrum and approach the Landau levels of two-dimensional graphene as the magnetic field increases. The optical properties are tuned by the size, the type of the edge, and the external magnetic field.

Raman and infrared properties and layer dependence of the phonon dispersions in multilayered graphene

The symmetry group analysis is applied to classify the phonon modes of N-stacked graphene layers (NSGLs) with AB and AA stacking, particularly their infrared and Raman properties. The dispersions of various phonon modes are calculated in a multilayer vibrational model, which is generalized from the lattice vibrational potentials of graphene to including the interlayer interactions in NSGLs. The experimentally reported redshift phenomena in the layer-number dependence of the intralayer optical C-C stretching mode frequencies are interpreted. An interesting low-frequency interlayer optical mode is revealed to be Raman or infrared active in even or odd NSGLs, respectively. Its frequency shift is sensitive to the layer number and saturated at about 10 layers.

Mechanical and tribological properties of epitaxial cubic boron nitride thin films grown on diamond

电子邮箱nataliya.deyneka@uni-ulm.de

Theory of huge tunneling magnetoresistance in graphene

We investigate theoretically the spin-independent tunneling magnetoresistance effect in a graphene monolayer modulated by two parallel ferromagnets deposited on a dielectric layer. For the parallel magnetization configuration, Klein tunneling can be observed in the transmission spectrum but at specific oblique incident angles. For the antiparallel magnetization configuration, the transmission can be blocked by the magneticelectric barrier provided by the ferromagnets. Such a transmission discrepancy results in a tremendous magnetoresistance ratio and can be tuned by the inclusion of an electric barrier.

Fabrication and transport properties of ZnO/Nb-1 wt %-doped SrTiO3 epitaxial heterojunctions

(110) ZnO/(001) Nb-1 wt %-doped SrTiO3 n-n type heteroepitaxial junctions were fabricated using the pulse laser deposition method. A diodelike current behavior was observed. Different from conventional p-n junctions or Schottky diodes, the diffusion voltage was found to increase with temperature. At all temperatures, the forward current was perfectly fitted on the thermionic emission model. The band bending at the interface can qualitatively explain our results, and the extracted high ideality factor at low temperatures, as well as large saturation currents, is ascribed to the deep-level-assisted tunneling current through the junction. (C) 2008 American Institute of Physics.

Resonant tunneling through S- and U-shaped graphene nanoribbons

We theoretically investigate resonant tunneling through S- and U-shaped nanostructured graphene nanoribbons. A rich structure of resonant tunneling peaks is found emanating from different quasi-bound states in the middle region. The tunneling current can be turned on and off by varying the Fermi energy. Tunability of resonant tunneling is realized by changing the width of the left and/or right leads and without the use of any external gates.

EPITAXIAL LATERAL OVERGROWTH OF GaN ON SILICON-ON-INSULATOR

From a single process, GaN layers were laterally overgrown on maskless stripe-patterned (111) silicon-on-insulator (SOI) substrates by metalorganic chemical vapor deposition. The influence of stress on the behavior of dislocations at the coalescence during growth was observed using transmission electron microscopy (TEM). Improvement of the crystallin equality of the GaN layer was demonstrated by TEM and micro-Raman spectroscopy. Furthermore, the benefits of SOI substrates for GaN growth are also discussed.

Epitaxial Growth of AlInGaN Quaternary Alloys by RF-MBE

AlInGaN quaternary alloys were successfully grown on sapphire substrate by radio-frequency plasma-excited molecular beam epitaxy (RF-MBE). AlInGaN quaternary alloys with different compositions were acquired by changing the Al cell's temperature. The streaky RHEED patterns were observed during AlInGaN quaternary alloys growth. Scanning Electron Microscope (SEM), Rutherford back-scattering spectrometry (RBS), X-Ray diffraction (XRD) and Cathodoluminescence (CL) were used to characterize the structural and optical properties of the AlInGaN alloys. The experimental results show that the AlInGaN quaternary alloys grow on the GaN buffer in the layer-by-layer growth mode. When the Al cell's temperature is 920 degrees C, the Al/In ratio in the AlInGaN quaternary alloys is about 4.7, and the AlInGaN can acquire better crystal and optical quality. The X-ray and CL full-width at half-maximum (FWHM) of the AlInGaN are 5arcmin and 25nm, respectively.