Efficient hole transport in asymmetric coupled InGaN multiple quantum wells

InGaN based light emitting devices (LEDs) with asymmetric coupled quantum wells (AS-QWs) and conventional symmetric coupled quantum wells (CS-QWs) active structures were grown by metal-organic chemical vapor deposition technique. The LEDs with AS-QWs active region show improved light emission intensity and reduced forward voltage compared with LEDs with CS-QWs active region. Based on the electroluminescence measurements and the devices structure analysis, it can be concluded that these improvements are mainly attributed to the efficient hole tunneling through barriers and consequently the uniform distribution of carriers in the AS-QWs. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3254232]

InP Based PD/EAM Integrated Photonic Switch

A new compact three-port InP based PD/EAM (photo-detector/electro-absorption modulator) integrated photonic switch is reported. The device demonstrates bi-directional wavelength conversion over 20 nm at 2.5 Gbit/s with a low input optical power of about 20 mW.

Electromagnetic-field-induced cohesive force enhancement in a metallic nanowire

We theoretically study the conducting electronic contribution to the cohesive force in a metallic nanowire irradiated under a transversely polarized external electromagnetic field at low temperatures and in the ballistic regime. In the framework of the free-electron model, we have obtained a time-dependent two-level electronic wavefunction by means of a unitary transformation. Using a thermodynamic statistical approach with this wavefunction, we have calculated the cohesive force in the nanowire. We show that the cohesive force can be divided into two components, one of which is independent of the electromagnetic field (static component), which is consistent with the existing results in the literature. The magnitude of the other component is proportional to the electromagnetic field strength. This extra component of the cohesive force is originally from the coherent coupling between the two lateral energy levels of the wire and the electromagnetic field.

THz-field-induced electronic transmission step-structure for a quantum wire

We study theoretically the low-temperature electronic transport property of a straight quantum wire under the irradiation of a finite-range transversely polarized external terahertz (THz) electromagnetic (EM) field. Using the free-electron model and the scattering matrix approach, we show an unusual behaviour of the electronic transmission of this system. A sharp step-structure appears in the electronic transmission probability as the EM field strength increases to a threshold value when a coherent EM field is applied. We demonstrate that this effect physically comes from the inelastic scattering of electrons with lateral photons through intersubband transitions.

Electronic transport through self-assembled quantum disks

In this paper we study a single electron tunneling through a vertically stacked self-assembled quantum disks structure using a transfer matrix technique in the framework of effective mass approximation. In the disks, the electron is confined both laterally and vertically; we separate the motion in the vertical and lateral directions within the adiabatic approximation and treat the energy levels of the latter as an effective confining potential. The influence of a constant applied electric field is taken into account using an exact Airy-function formalism and the current density is calculated at zero temperature. By increasing the widths of the barriers, we find the peaks of the current density shift toward lower voltage region; meanwhile, they can become even sharper. (C) 2004 Elsevier Ltd. All rights reserved.

Effects of lattice vibration on the effective mass of quasi-two-dimensional strong-coupling polaron

The effects of lattice vibration on the system in which the electron is weakly coupled with bulk longitudinal optical phonons and strongly coupled with interface optical phonons in an infinite quantum well were studied by using Tokuda' linear-combination operator and a modified LLP variational method. The expressions for the effective mass of the polaron in a quantum well QW as functions of the well's width and temperature were derived. In particular, the law of the change of the vibration frequency of the polaron changing with well' s width and temperature are obtained. Numerical results of the effective mass and the vibration frequency of the polaron for KI/AgCl/Kl QW show that the vibration frequency and the effective mass of the polaron decrease with increasing well's width and temperature, but the contribution of the interaction between the electron and the different branches of phonons to the effective mass and the vibration frequency and the change of their variation with the well's width and temperature are greatly different.

Iris recognition algorithm based on point covering of high-dimensional space and neural network

In this paper, we constructed a Iris recognition algorithm based on point covering of high-dimensional space and Multi-weighted neuron of point covering of high-dimensional space, and proposed a new method for iris recognition based on point covering theory of high-dimensional space. In this method, irises are trained as "cognition" one class by one class, and it doesn't influence the original recognition knowledge for samples of the new added class. The results of experiments show the rejection rate is 98.9%, the correct cognition rate and the error rate are 95.71% and 3.5% respectively. The experimental results demonstrate that the rejection rate of test samples excluded in the training samples class is very high. It proves the proposed method for iris recognition is effective.

Growth and properties of GaN on Si (111) substrates with AlGaN/AlN buffer layer by NH3-GSMBE

We describe the growth of GaN on Si (111) substrates with a AlGaN/AlN buffer layer by NH3-GSMBE. The influence of the AlN and AlGaN buffer layer thickness on the crack density of GaN has been investigated. It is found that the optimum thickness is 120 nm and 250 nm for AlN and AlGaN layers, respectively. The full width at half maximum of the GaN (0002) peak in the triple-crystal x-ray rocking curve measurement is about 15 arcmin.

Application of multi-weighted neuron for iris recognition

In this paper, from the cognition science point of view, we constructed a neuron of multi-weighted neural network, and proposed a new method for iris recognition based on multi-weighted neuron. In this method, irises are trained as "cognition" one class by one class, and it doesn't influence the original recognition knowledge for samples of the new added class. The results of experiments show the correct rejection rate is 98.9%, the correct cognition rate and the error recognition rate are 95.71% and 3.5% respectively. The experimental results demonstrate that the correct rejection rate of the test samples excluded in the classes of training samples is very high. It proves the proposed method for iris recognition is effective.

The effect of the AlxGa1-xN/AIN buffer layer on the properties of GaN/Si(111) film grown by NH3-MBE

We describe the growth of GaN on Si(111) substrates with AlxGa1-xN/AlN buffer layer by ammonia gas source molecular beam epitaxy (NH3-GSMBE). The influence of the AlN and AlxGa1-xN buffer layer thickness and the Al composition on the crack density of GaN has been investigated. It is found that the optimum thickness is 120 and 250 nm for AlN and AlxGa1-xN layers, respectively. The optimum Al composition is between 0.3 < x < 0.6. (c) 2005 Elsevier B.V. All rights reserved.

Growth and characterization of InN on sapphire substrate by RF-MBE

Indium nitride (InN) films were grown on sapphire substrates by radio-frequency plasma-excited molecular beam epitaxy (RF-MBE). Atomic force microscopy (AFM), reflection high-energy electron diffraction (RHEED), double-crystal X-ray diffraction (DCXRD) and photoluminescence (PL) spectroscopy were used to characterize the InN films. The results show that the InN films have good crystallinity, with full-width at half-maximum (FWHM) of InN (0 0 0 2) DCXRD peak being 14 arcmin. At room temperature, a strong PL peak at 0.79eV was observed. At 1.9eV or so, no peak was observed. In addition, it is found that the InN films grown with low-temperature (LT) InN buffer layer are of better quality than those without LT-InN buffer layer. (c) 2004 Elsevier B.V. All rights reserved.

Q-switched and mode-locked diode-pumped Nd : YVO4 laser with an intracavity composite semiconductor saturable absorber

We have demonstrated a passively Q-switched and mode-locked Nd:YVO4 laser with an intracavity composite semiconductor saturable absorber (ICSSA). Stable Q-switched and mode-locked pulses with Q-switched envelope pulse duration of 180 ns and pulse repetition rate of 72KHz have been obtained. The maximum average output power was 1.45W at 8W incident pump power. The repetition rate of the mode-locked pulses inside the Q-switched envelope was 154 MHz. Experimental results revealed that this ICSSA was suitable for Q-switched and mode-locked solid-state lasers. (c) 2007 Elsevier Ltd. All rights reserved.

Characteristics of high Al content AlxGa1-xN grown by metalorganic chemical vapor deposition

The epitaxial growth of AlxGa1-xN film with high Al content by metalorganic chemical vapor deposition (MOCVD) has been accomplished. The resulting Al content was determined to be 54% by high resolution X-ray diffraction (HRXRD) and Vegard's law. The full width at half maximum (FWHM) of the AlGaN (0002) HRXRD rocking curve was about 597 arcsec. Atomic force microscopy (AFM) image showed a relatively rough surface with grain-like islands, mainly coming from the low surface mobility of adsorbed Alspecies. From transmittance measurement, the cut-off wavelength was around 280 nm and Fabry-Perot fringes were clearly visible in the transmission region. Cathodoluminescence (CL) measurement indicated that there existed a uniformity in the growth direction and a non-uniformity in the lateral direction. (C) 2007 Elsevier Ltd. All rights reserved.

Effects of growth temperature of AlGaN buffer layer on the properties of A1(0.58)Ga(0.42)N epilayer by NH3-MBE

Al0.58Ga0.42N epilayers are grown by ammonia gas source molecular beam epitaxy (NH3-MBE) on (0001) sapphire substrate using AlGaN buffer layer. The effects of the buffer layer growth temperature on the properties of Al0.58Ga0.42N epilayer are especially investigated. In-situ high-energy electron diffraction (RHEED), double-crystal X-ray diffraction (DCXRD), atomic force microscopy (AFM), photoconductivity measurement and cathodoluminescence (CL) are used to characterize the samples. It is found that high growth temperature of AlGaN buffer layer would improve the crystalline quality, surface smoothness, optical quality and uniformity of the Al0.58Ga0.42N epilayer. The likely reason for such improvements is also suggested. (C) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Gain measurement and anomalous decrease of peak gain at long wavelength for InAs/GaAs quantum-dot lasers

Mode gain spectrum is measured by the Fourier series expansion method for InAs/GaAs quantum-dot (QD) lasers with seven stacks of QDs at different injection currents. Gain spectra with distinctive peaks are observed at the short and long wavelengths of about 1210 nm and 1300 nm. For a QD laser with the cavity length of 1060 mu m, the peak gain of the long wavelength first increases slowly or even decreases with the injection current as the peak gain of the short wavelength increases quickly, and finally increases quickly before approaching the saturated values as the injection current further increases.