Stochastic resonance in quantum-well semiconductor lasers

The quantum well (QW) semiconductor lasers have become main optical sources for optical fibre communication systems because of their higher modulation speed, broader modulation bandwidth and better temperature characteristics. In order to improve the quality of direct-modulation by means of the stochastic resonance (SR) mechanism in QW semiconductor lasers, we investigate the behaviour of the SR in direct-modulated QW semiconductor laser systems. Considering the cross-correlated carrier noise and photon noise, we calculate the power spectrum of the photon density and the signal-to-noise ratio (SNR) of the direct-modulated laser system by using the linear approximation method. The results indicate that the SR always appears in the dependence of the SNR on the bias current density, and is strongly affected by the cross-correlation coefficient of the carrier and photon noises, the frequency of modulation signal, and the photon lifetime in the laser cavity.

Whispering-gallery-likel modes in square resonators

The mode frequencies and field distributions of whispering-gallery (WG)-like modes of square resonators are obtained analytically, which agree very well with the numerical results calculated by the FDTD technique and Pade approximation method. In the analysis, a perfect electric wall for the transverse magnetic mode or perfect magnetic wall for the transverse electric mode is assumed at the diagonals of the square resonators, which not only provides the transverse mode confinement, but also requires the longitudinal mode number to be an even integer. The WG-like modes of square resonators are nondegenerate modes with high-quality factors, which make them suitable for fabricating single-mode low-threshold semiconductor microcavity lasers.

Electronic investigation of self-organized InAs quantum dots

Deep Level Transient Spectroscopy (DLTS) has been applied to investigate the electronic properties of self-organized InAs quantum dots. The energies of electronic ground states of 2.5ML and 1.7ML InAs quantum dots (QDs) with respect to the conduction band of bulk GaAs are about 0.21 eV and 0.09 eV, respectively. We have found that QDs capture electrons by lattice relaxation through a multi-phonon emission process. The samples are QDs embedded in superlattices with or without a 500 Angstrom GaAs spacing layer between every ten periods of a couple of GaAs and InAs layers. The result shows that the density of dislocations in the samples with spacer layers is much lower than in the samples without the spacer layers.

AlGaN/AlN/GaN/InGaN/GaN DH-HEMTs with improved mobility grown by MOCVD

AlGaN/AlN/GaN/InGaN/GaN double heterojunction high electron mobility transistors (DH-HEMTs) structures with improved buffer isolation have been investigated. The structures were grown by MOCVD on sapphire substrate. AFM result of this structure shows a good surface morphology with the root-mean-square roughness (RMS) of 0.196 nm for a scan area of 5 mu mx5 mu m. A mobility as high as 1950 cm(2)/Vs with the sheet carrier density of 9.89x10(12) cm(-2) was obtained, which was about 50% higher than other results of similar structures which have been reported. Average sheet resistance of 327 Omega/sq was achieved. The HEMTs device using the materials was fabricated, and a maximum drain current density of 718.5 mA/mm, an extrinsic transconductance of 248 mS/mm, a current gain cutoff frequency of 16 GHz and a maximum frequency of oscillation 35 GHz were achieved.

Heavily doped polycrystalline 3C-SiC growth on SiO2/Si(100) substrates for resonator applications

3C-SiC is a promising material for the development of microelectromechanical systems (MEMS) applications in harsh environments. This paper presents the LPCVD growth of heavily nitrogen doped polycrystalline 3C-SiC films on Si wafers with 2.0 mu m-thick silicon dioxide (SiO2) films for resonator applications. The growth has been performed via chemical vapor deposition using SiH4 and C2H4 precursor gases with carrier gas of H-2 in a newly developed vertical CVD chamber. NH3 was used as n-type dopant. 3C-SiC films were characterized by scanning electron microscopy (SEM), x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS), and room temperature Hall Effect measurements. It was shown that there is no voids at the interface between 3C-SiC and SiO2. Undoped 3C-SiC films show n-type conduction with resisitivity, Hall mobility, and carrier concentration at room temperature of about 0.56 Omega center dot cm, 54 cm(2)/Vs, and 2.0x 10(17) cm(-3), respectively. The heavily nitrogen doped polycrystalline 3C-SiC with the resisitivity of less than 10(-3) Omega center dot cm was obtained by in-situ doping. Polycrystalline SiC resonators have been fabricated preliminarily on these heavily doped SiC films with thickness of about 2 mu m. Resonant frequency of 49.1 KHz was obtained under atmospheric pressure.

Processing network models of energy/environment systems

Processing networks are a variant of the standard linear programming network model which are especially useful for optimizing industrial energy/environment systems. Modelling advantages include an intuitive diagrammatic representation and the ability to incorporate all forms of energy and pollutants in a single integrated linear network model. Added advantages include increased speed of solution and algorithms supporting formulation. The paper explores their use in modelling the energy and pollution control systems in large industrial plants. The pollution control options in an ethylene production plant are analyzed as an example. PROFLOW, a computer tool for the formulation, analysis, and solution of processing network models, is introduced.

语义Web服务匹配与发现技术研究

在开放的互联网环境中，面向服务的计算（Service-Oriented Computing, SOC）和面向服务的体系架构（Service-Oriented Architecture, SOA）使得信息资源和软件系统的主要形态发生了巨大变化，并引领着互联网计算技术和软件系统新的发展方向。服务是SOC和SOA的核心思想，服务的平台独立、高可重用和松散耦合等特性使之成为复杂分布式应用可行的解决方案。Web服务的出现和发展为实施与部署SOC和SOA提供了最佳支撑技术。目前web服务发现机制在服务描述的全面性和服务匹配方法的有效性方面存在不足之处，难以保证服务发现的性能。特别是在服务数量剧增的情况下，用户亟需一种自动快速而准确的服务发现机制。语义web技术不仅为web的表示内容提供了一种语义标识方法，推动了web内容的自动化和智能化搜索，也为web服务的描述提供了增加语义层次内容的有效方法，两者的结合形成了一个新的研究领域——语义web服务。该领域通过借鉴语义web的相关技术来增加web服务的语义描述，进而为web服务的发现、组合、调用和监控等环节提供语义层次的支持，增强web服务的自动化和智能化水平。 本文的研究目的就是利用语义web的思想，基于SOC和SOA的理念以及现有的web服务发现机制对web服务信息进行语义描述、组织，探索语义web服务匹配与发现的关键问题，提出有效的解决方案。本文的研究工作和贡献主要在以下四个方面： 1. 针对语义近似度的度量和计算问题基于OWL（Web Ontology Language）提出了一个语义近似度计算SSM模型，该模型中所有概念及概念之间的关系、所有的概念属性及属性之间的关系都被抽象定义并建模，根据概念、属性及其语义关系模型，构建了基于概念关系的拓扑近似度计算函数和属性近似度计算函数，从而进一步构建语义近似度计算函数。该语义近似度计算函数以代表OWL本体概念上下级关系和等价关系的树状结构为基础，利用概念间的拓扑关系和属性关系进行语义近似度的度量和计算。语义近似度函数的返回值为一个介于0与1之间的数值，代表函数的两个输入概念之间的语义近似度值。 2. 提出了一个基于概念语义近似度的以OWL-S为顶层本体的web服务语义匹配PSM模型。该模型将服务的匹配项分为服务能力、服务质量、服务名称和服务参数四个部分。基于该模型，提出了相应的PSM服务匹配算法。PSM算法能够对服务内部的服务流程及流程的组合进行与服务请求的语义匹配，并且避免了服务流程中互斥接口参数参与匹配。而且PSM匹配算法实现了服务接口的单射匹配，进一步提高服务发现的性能。在服务质量匹配计算上，提出了服务质量匹配模型PSM-QoS，为服务的QoS匹配建立了计算基础。PSM算法使用语义近似度计算函数度量和计算服务属性概念的语义近似度值，从而能够返回细粒度的服务匹配度值。 3. 在语义服务匹配PSM模型的基础上提出一种语义web服务发现架构系统PSM-SD模型。该模型将OWL-S语义元素引入UDDI服务描述，从而在UDDI中保存语义信息。通过PSM算法提高web服务匹配性能，并通过数据映射机制保证模型对于当前的服务发现标准UDDI基础架构的兼容以及UDDI标准操作接口的透明性。模型使用本体概念的索引机制提高服务发现的效率。在建立和维护索引的过程或服务的匹配过程中，模型使用近似概念搜寻算法进一步提高本体概念搜寻和服务匹配的效率。 4. 设计并开发了基于语义web服务发现架构模型的原型系统。该系统将语义近似度计算模型、语义web服务匹配模型和语义web服务发现架构模型进行软件实现。本文在该原型系统平台上对语义web服务发现的方法进行了实验，验证了方法的有效性。