Anisotropy of magnetic properties in ferrimagnetic ytterbium iron garnet under high magnetic fields

The magnetic anisotropy in ytterbium iron garnet (YbIG) is theoretically investigated under high magnetic fields (up to 160 kOe). According to the crystal field effect in ytterbium gallium garnet (YbGaG), a detailed discussion of crystal-field interaction in YbIG is presented where a suitable set of crystal-field parameters is obtained. Meanwhile, the influences of nine crystal-field parameters on the crystal-field energy splitting are analyzed. On the other hand, considering the ytterbium-iron (Yb-Fe) superexchange interaction of YbIG, the spontaneous magnetization is calculated at different temperatures for the [111] direction. In particular, we demonstrate that the Wesis constant lambda is the function of 1/T in YbIG. In addition, the field dependences of the magnetization for the [110] and [111] directions are theoretically described where a noticeable anisotropy can be found. Our theory further confirms the great contribution of anisotropic Yb-Fe superexchange interaction to the anisotropy of the magnetization in YbIG. Moreover, our theoretical results are compared with the available experiments.

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.

Low-frequency noise properties of GaN Schottky barriers deposited on intermediate temperature buffer layers

Metal-semiconductor-metal (MSM) structures were fabricated by RF-plasma-assisted MBE using different buffer layer structures. One type of buffer structure consists of an AlN high-temperature buffer layer (HTBL) and a GaN intermediate temperature buffer layer (ITBL), another buffer structure consists of just a single A IN HTBL. Systematic measurements in the flicker noise and deep level transient Fourier spectroscopy (DLTFS) measurements were used to characterize the defect properties in the films. Both the noise and DLTFS measurements indicate improved properties for devices fabricated with the use of ITBL and is attributed to the relaxation of residue strain in the epitaxial layer during growth process. (C) 2003 Elsevier Ltd. All rights reserved.

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.

Improved performance in organic light emitting diodes with a mixed electron donor-acceptor film involved in hole injection

Organic light emitting diodes using a mixed layer of electron acceptor 3, 4, 9, 10 perylenetetracarboxylic dianhydride and electron donor copper phthalocyanine (PTCDA:CuPc) on indium tin oxide (ITO) anodes were fabricated. The device properties were found to be strongly dependent on the thickness of the PTCDA:CuPc film: both the power efficiency and the driving voltage of the device were optimized with a thickness of PTCDA:CuPc ranging from 10 to 20 nm. As compared to the conventional ITO/CuPc hole injection structure, the ITO/PTCDA:CuPc hole injection structure could remarkably enhance both the luminance and the power efficiencies of devices. A mechanism of static-induced, very efficient hole-electron pairs generation in mixed PTCDA:CuPc films was proposed to explain the experimental phenomena. The structural and optical properties of PTCDA:CuPc film were examined as well. (c) 2007 American Institute of Physics.

Molecular beam epitaxy InAs dot arrays on InGaAs/GaAs

Periodical alignment of the InAs dots along the < 100 > and < 110 > directions was observed on an elastically relaxed InGaAs buffer layer grown at 500 and 450 degrees C, respectively, on the vicinal GaAs(001) substrate. Due to alignment along these directions, the InAs dots were arranged into a quasi-two-dimensional hexagonal lattice. Such a periodical arrangement of InAs dots may be explained in terms of modulation in strain as well as composition along [110] as observed by using cross-sectional transmission electron microscopy.

Synthesis of high quality n-type CdS nanobelts and their applications in nanodevices

High quality n-type CdS nanobelts (NBs) were synthesized via an in situ indium doping chemical vapor deposition method and fabricated into field effect transistors (FETs). The electron concentrations and mobilities of these CdS NBs are around (1.0x10(16)-3.0x10(17))/cm(3) and 100-350 cm(2)/V s, respectively. An on-off ratio greater than 10(8) and a subthreshold swing as small as 65 mV/decade are obtained at room temperature, which give the best performance of CdS nanowire/nanobelt FETs reported so far. n-type CdS NB/p(+)-Si heterojunction light emitting diodes were fabricated. Their electroluminescence spectra are dominated by an intense sharp band-edge emission and free from deep-level defect emissions. (c) 2006 American Institute of Physics.

The study of high temperature annealing of a-SiC : H films

A series of amorphous silicon carbide films were prepared by plasma enhanced chemical vapor deposition technique on (100) silicon wafers by using methane, silane, and hydrogen as reactive resources. A very thin (around 15 A) gold film was evaporated on the half area of the aSiC:H films to investigate the metal induced crystallization effect. Then the a-SiC:H films were annealed at 1100 degrees C for 1 hour in the nitrogen atmosphere. Fourier transform infrared spectroscopy (FTIR), X-Ray diffraction (XRD), and scanning electron microscopy (SEM) were employed to analyze the microstructure, composition and surface morphology of the films. The influences of the high temperature annealing on the microstructure of a-SiC:H film and the metal induced metallization were investigated.

Space grown semi-insulating gallium arsenide single crystal and its application

Low noise field effect transistors and analogue switch integrated circuits (ICs) have been fabricated in semi-insulating gallium arsenide (SI-GaAs) wafers grown in space by direct ion-implantation. The electrical behaviors of the devices and the ICs have surpassed those fabricated in the terrestrially grown SI-GaAs wafers. The highest gain and the lowest noise of the transistors made from space-grown SI-GaAs wafers are 22.8 dB and 0.78 dB, respectively. The threshold back-gating voltage of the ICs made from space-grown SI-GaAs wafers is better than 8.5 V The con-elation between the characterizations of materials and devices is studied systematically. (C) 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved.

Effects of piezoelectricity and spontaneous polarization on electronic and optical properties of wurtzite III-V nitride quantum wells

A theoretical model accounting for the macropolarization effects in wurtzite III-V nitrides quantum wells (QWs) is presented. Energy dispersions and exciton binding energies are calculated within the framework of effective-mass theory and variational approach, respectively. Exciton-associated transitions (EATs) are studied in detail. An energy redshift as high as 450 meV is obtained in Al0.25GaN0.75/GaN QWs. Also, the abrupt reduction of optical momentum matrix elements is derived as a consequence of quantum-confined Stark effects. EAT energies are compared with recent photoluminescence (PL) experiments and numerical coherence is achieved. We propose that it is the EAT energy, instead of the conduction-valence-interband transition energy that is comparable with the PL energy. To restore the reduced transition rate, we apply an external electric field. Theoretical calculations show that with the presence of the external electric field the optical matrix elements for EAT increase 20 times. (C) 2001 American Institute of Physics.

Fermi-edge singularity observed in a modulation-doped AlGaN/GaN heterostructure

In this letter, we report on the observation of Fermi-edge singularity in a modulation-doped AlGaN/GaN heterostructure grown on a c-face sapphire substrate by NH3 source molecular beam epitaxy. The two-dimensional electron gas (2DEG) characteristic of the structure is manifested by variable temperature Hall effect measurements down to 7 K. Low-temperature photoluminescence (PL) spectra show a broad emission band originating from the recombination of the 2DEG and localized holes. The enhancement in PL intensity in the high-energy side approaching Fermi level was observed at temperatures below 20 K. At higher temperatures, the enhancement disappears because of the thermal broadening of the Fermi edge. (C) 1998 American Institute of Physics. [S0003-6951(98)02543-1].

High responsivity ultraviolet photodetector based on crack-free GaN on Si (111)

A Schottky-based metal-semiconductor-metal photodetector is fabricated on 1 mu m-thick, crack-free GaN on Si (I 11) substrate using an optimized AlxGal-xN/AlN complex buffer layer. It exhibits a high responsivity of 4600A/W at 366nm which may be due to both a crack-free sample and high internal gain. The relationship between responsivity and bias voltage is also investigated. The experiment results indicate that the responsivity increases with the bias voltage and shows a tendency to saturate. (c) 2007 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim.

The study of high temperature annealing of a-SiC : H films

A series of amorphous silicon carbide films were prepared by plasma enhanced chemical vapor deposition technique on (100) silicon wafers by using methane, silane, and hydrogen as reactive resources. A very thin (around 15 A) gold film was evaporated on the half area of the aSiC:H films to investigate the metal induced crystallization effect. Then the a-SiC:H films were annealed at 1100 degrees C for 1 hour in the nitrogen atmosphere. Fourier transform infrared spectroscopy (FTIR), X-Ray diffraction (XRD), and scanning electron microscopy (SEM) were employed to analyze the microstructure, composition and surface morphology of the films. The influences of the high temperature annealing on the microstructure of a-SiC:H film and the metal induced metallization were investigated.

Valence band offset of MgO/TiO2 (rutile) heterojunction measured by X-ray photoelectron spectroscopy

The valence band offset (VBO) of MgO/TiO2 (rutile) heterojunction has been directly measured by Xray photoelectron spectroscopy. The VBO of the heterojunction is determined to be 1.6 +/- 0.3 eV and the conduction band offset (CBO) is deduced to be 3.2 +/- 0.3 eV, indicating that the heterojunction exhibits a type-I band alignment. These large values are sufficient for MgO to act as tunneling barriers in TiO2 based devices. The accurate determination of the valence and conduction band offsets is important for use of MgO as a buffer layer in TiO2 based field-effect transistors and dye-sensitized solar cells.

CERIUM SILICIDE FORMATION IN THIN CE SI MULTILAYER FILMS

Alternating layers of Si(200 angstrom thick) and Ce(200 angstrom thick) up to 26 layers altogether were deposited by electron evaporation under ultrahigh vacuum conditions on Si(100) substrate held at 150-degrees-C. Isothermal, rapid thermal annealing has been used to react these Ce-Si multilayer films. A variety of analytical techniques has been used to study these multilayer films after annealing, and among these are Auger electron spectroscopy, Rutherford backscattering, X-ray diffraction, and high resolution transmission electron microscopy. Intermixing of these thin Ce-Si multilayer films has occurred at temperatures as low as 150-degrees-C for 2 h, when annealed. Increasing the annealing temperature from 150 to 400-degrees-C for 1 h, CeSi2 forms gradually and the completion of reaction occurs at approximately 300-400-degrees-C. During the formation of CeSi2 from 150-400-degrees-C, there is some evidence for small grains in the selected area diffraction patterns, indicating that CeSi2 crystallites were present in some regions. However, we have no conclusive evidence for the formation of epitaxial CeSi2 layers, only polycrystals were formed when reacted in the solid phase even after rapid thermal anneal at 900-degrees-C for 10 s. The formation mechanism has also been discussed in combining the results of the La-Si system.