Metal-semiconductor-metal ultraviolet photodetector based on GaN

A metal-semiconductor-metal (MSM) ultraviolet photodetector has been fabricated using unintentionally doped n-GaN films grown on sapphire substrates. Its dark current, photocurrent under the illumination with lambda = 360 nm light, responsivity, and the dependence of responsivity on bias voltage were measured at room temperature. The dark current of the photodetector is 1.03 nA under 5 V bias, and is 15.3 nA under 10 V bias. A maximum responsivity of 0.166 A/W has been achieved under the illumination with lambda = 366 nm light and 15 V bias. It exhibits a typical sharp band-edge cutoff at the wavelength of 366 nm, and a high responsivity at the wavelength from 320 nm to 366 nm. Its responsivity under the illumination with lambda = 360 nm light increases when the bias voltage increases.

The effect of substrate orientation on the morphology of InAs nanostructures on (001) and (11n)A/B(n=1-5) InP substrates

Strained InAs nanostructures have been grown by solid-source molecular beam epitaxy in In0.52Al0.48As matrix on different InP substrate surfaces ((0 0 1) and (1 1 n)A/B (n = 1 - 5)). The morphology of the nanostructures was characterized using atomic force microscopy (AFM). The AFM results reveal interesting differences in the size, shape, and alignment of the nanostructures between different oriented surfaces. It was found that some faceted nanostructures tend to form on A-type surfaces, the shape and the alignment of these nanostructures show clear dependence on the substrate orientation. Samples grown on (0 0 1) and B-type surfaces showed preferentially dense round dots. Dots formed on (1 1 3)B, (1 1 3)B and (1 1 5)B surfaces have a higher dot density and size homogeneity, which shows a potential for the production of high-quality and customized self-assembled quantum dots for photonics applications. (C) 2000 Elsevier Science B.V. All rights reserved.

Pulsed excimer laser annealing of Mg-doped cubic GaN

A KrF (248 nm) excimer laser with a 38 ns pulse width was used to study pulsed laser annealing (PLA) on Mg-doped cubic GaN alms. The laser-induced changes were monitored by photoluminescence (PL) measurement. It indicated that deep levels in as-grown cubic GaN : Mg films were neutralized by H and PLA treatment could break Mg-H-N complex. The evolution of emissions around 426 and 468 nm with different PLA conditions reflected the different activation of the involved deep levels. Rapid thermal annealing (RTA) in N-2 atmosphere reverts the luminescence of laser annealed samples to that of the pre-annealing state. The reason is that most H atoms still remained in the epilayers after PLA due to the short duration of the pulses and reoccupied the original locations during RTA. (C) 2000 Elsevier Science B.V. All rights reserved. PACS: 61.72.Vv; 61.72.Cc; 18.55. -m.

Scanning electron microscope studies of cubic AlxGa1-xN films grown on GaAs(100) by metal organic vapor phase epitaxy (MOVPE)

Cubic AlGaN films were grown on GaAs(100) substrates by MOVPE. Scanning electron microscope and photoluminescence were used to analyze the surface morphology and the crystalline quality of the epitaxial layers. We found that both NH, and TEGa fluxes have a strong effect on the surface morphology of AlGaN films. A model for the lateral growth mechanism is presented to qualitatively explain this effect. The content of hexagonal AlGaN in the cubic AlGaN films was also related to the NH3 flux. (C) 1999 Elsevier Science B.V. All rights reserved.

Transmission electron microscopy study of triple-ribbon contrast features in a ZnTe epitaxial layer

A transmission electron microscopy study of triple-ribbon contrast features in a ZnTe layer grown epitaxially on a vicinal GaAs (001) substrate is reported. The ribbons go through the layer as threading dislocations near the [<(11)over bar 2>](111) or [112](<(11)over bar 1>) directions. Each of these (with a 40 nm width) has two narrow parts enclosed by three partial dislocations (with a 20 nm spacing). By contrast analysis and contrast simulation, the ribbons have been shown to be composed of two partially overlapping stacking faults. Their origin is attributed to a forced reaction between two crossing perfect misfit dislocations.

Fabrication of GaN epitaxial films on Al2O3/Si (001) substrates

Single crystal GaN films of hexagonal modification have been fabricated on Al2O3/Si (001) substrates via a low pressure metalorganic chemical deposition (LP-MOCVD) method. The full width at half-maximum of (0002) X-ray diffraction peak for the GaN film 1.1 mu m thick was 72 arcmin. and the mosaic structure of the film was the main cause of broadening to the X-ray diffraction peak. Al room temperature, the photoluminescence (PL) spectrum of GaN exhibited near band edge emission peaking at 365 nm.

分布事务监控器集群框架研究

介绍了在分布事务监控器OnceTX中采用的服务器/集群管理器/应用域管理器的三层集群结构,并给出了两级负载平衡策略和自适应的域间拓扑结构管理算法。它们已在OnceTX中实现,并达到了预期的目的。

基于软件仓库的个体软件过程能力度量技术研究

自60年代出现软件危机以来，世界各国政府、计算机软件研究机构和组织在软件工程化方法、技术和工具的研究、开发和实践方面投入了大量的人力、物力和资金。人们认识到，要高效率、高质量和低成本地开发软件，必须以改善软件生产过程为中心，实施过程指导的软件生产与质量管理。个体软件过程(PSP)是由卡内基·梅隆大学软件工程研究所的Humphrey领导开发的。它是一种可用于控制、管理和改进个人工作方式的自我持续改进过程。随着软件工业界对软件过程改进需求的日益增长，PSP的研究成为了软件组织为达成完全(从宏观到微观)量化过程管理研究中的一个热点课题。研究表明高水平的个体软件过程能力是软件项目成功的关键，如何进行有效的个体软件过程能力度量是PSP中的一个核心问题。 软件过程能力度量的准确度依赖于历史数据的积累，只有积累了大量客观充分的历史数据，软件过程度量所得到的结果才会更准确，对未来的过程改进才有指导意义。然而，工业生产中常见的协同软件开发，使得PSP能力指标的收集十分困难，例如，当一个软件系统由多人编码实现时，PSP能力的度量就面临着如何识别其中每个开发者所贡献的代码量，所引入的缺陷率以及所带来的程序复杂性等问题。同时，PSP能力度量问题本身具有多指标输入输出、规模收益可变以及需要考虑决策者偏好的特点，因此亟需一种面向PSP能力度量的量化分析方法，用于解决具有这类特点的量化度量问题。 由于软件仓库 (版本控制系统及缺陷跟踪系统等) 已经被广泛应用于大多数的软件项目开发之中，同时其中蕴含了丰富且极具价值的历史开发数据，这些数据和整个项目开发周期中开发人员的行为紧密相关，是个体软件活动的最直接反映，为PSP能力度量研究提供了大量客观的数据支持。因而本文提出了一种基于软件仓库的个体软件过程能力度量的新方法。该方法可分为两个步骤：基于软件仓库的PSP能力指标挖掘，以及支持PSP度量的量化分析模型。 首先本文通过充分研究当前常用的软件仓库数据挖掘技术，重点分析针对版本控制系统和缺陷跟踪系统的数据挖掘方法，提出了一种在协同工作环境中，基于软件仓库的PSP能力指标挖掘方法，并定义了四组指标进行详尽的分析，从理论和实践的角度，保证了PSP指标数据集的准确、客观和合理性 其次本文提出了一种基于数据包络分析(DEA)和层次分析法(AHP)的混合模型—PSPADA—用于PSP能力的度量分析，更进一步，还从理论上证明了PSPADA模型的正确性和可行性，并建立了与之相关的三个核心算法(综合决策者偏好，建立参考集和估计规模收益)。该模型能够同时解决多目标决策、可变规模收益以及主观决策者偏好的问题。应用该模型进行PSP指标数据的度量分析，其反馈的量化结果更为客观、更易理解，能有效地指导个体开发者实施个人软件过程改进。 然后，本文还实现了该度量方法的原型工具PSPstat。PSPstat实现了PSP指标收集和PSP能力度量分析的功能。它支持从软件仓库中自动挖掘多种PSP能力指标数据，使用PSPADA进行评价计算，并提供丰富的图形界面，展示指标数据和度量结果。PSPstat易于扩展，在设计上考虑了对多种版本控制系统、多种缺陷跟踪系统、多种程序语言、多种度量指标以及多种量化方法的支持，为进一步的研究和工作准备了必要的基础。 最后，在实例研究中，通过两个实验对本文提出的PSP能力度量模型及方法进行了验证。实验一的研究对象是一个标准的PSP数据集，侧重于从理论角度对PSP能力度量模型中的PSPADA方法进行有效性验证，证明PSPADA方法在结合决策者偏好的前提下，能有效度量个体软件过程的能力。实验二则以一个开源软件项目jEdit 为实验对象，获得了一个包含近百名个体开发者的大型工业数据集，因此在实验中，着重展示了该方法从工业软件仓库中挖掘个体软件过程能力指标的优势。 从本文的研究中可以看出，该基于软件仓库进行PSP能力指标挖掘的方法，可以保证度量指标的客观公正性，且将指标收集过程自动化，节省了大量的人力物力。同时其中的PSPADA度量模型能够在考虑决策者偏好的同时，有效的进行多指标、规模收益可变的量化评估，给出合理的度量结果，并指导未来的改进方向。因此该PSP能力度量方法对度量个体软件过程的能力，帮助软件企业建立IPRP薪资策略将有着显著的推动和促进作用。

基于形象化标签推荐技术的聋儿康复资源系统

针对目前教育、教学资源系统中存在的问题,在以用户为中心的场景设计方法指导下,提出了简单、易学、易用和高效4个层次递进的系统设计开发目标,并重点分析研究了Web2.0的个性化服务:标签和推荐服务。结合这两种服务的优势,在提出和实现形象化标签推荐技术后,阐述了其设计与实现。结合聋儿康复资源系统的设计与开发,详细地介绍了整个系统的功能框架、实现流程以及形象化标签在系统中的运用。最后对全文进行了总结,并对下一步研究工作做出了展望。

提高固定膜光催化氧化净水器性能关键因素的研究

简单介绍了影响光催化氧化有机物反应速度的五个因素，通过分析找出影响光催化氧化有机物反应速度的主要因素，必须同时考虑反应内E_((r))和S_((r))相互综合因素对反应器自由基产生率的影响;通过实验方法找出对应不同功率光源的光催化载体最优半径r_(opt)具体数值，是提高固定膜光催化氧化净水器性能的关键，如果把固定光催化膜载体布置在半径r_(opt)的圆周上，就能充分利用光源的光能，在反应器有限的空间内使得适当强度的紫外光照射到有效光催化剂的面积适当，这时光催化反应器光能利用率最高。

Design and operation of integrated pilot-scale dimethyl ether synthesis system via pyrolysis/gasification of corncob

The integrated pilot-scale dimethyl ether (DME) synthesis system from corncob was demonstrated for modernizing utilization of biomass residues. The raw bio-syngas was obtained by the pyrolyzer/gasifier at the yield rate of 40-45 Nm(3)/h. The content of tar in the raw bio-syngas was decreased to less than 20 mg/Nm(3) by high temperature gasification of the pyrolysates under O-2-rich air. More than 70% CO2 in the raw bio-syngas was removed by pressure-swing adsorption unit (PSA). The bio-syngas (H-2/CO approximate to 1) was catalytically converted to DME in the fixed-bed tubular reactor directly over Cu/Zn/Al/HZSM-5 catalysts. CO conversion and space-time yield of DME were in the range of 82.0-73.6% and 124.3-203.8 kg/m(cat)(3)/h, respectively, with a similar DME selectivity when gas hourly space velocity (GHSV, volumetric flow rate of syngas at STP divided by the volume of catalyst) increased from 650 h(-1) to 1500 h(-1) at 260 degrees C and 4.3 MPa. And the selectivity to methanol and C-2(+) products was less than 0.65% under typical synthesis condition. The thermal energy conversion efficiency was ca. 32.0% and about 16.4% carbon in dried corncob was essentially converted to DME with the production cost of ca. (sic) 3737/ton DME. Cu (111) was assumed to be the active phase for DME synthesis, confirmed by X-ray diffraction (XRD) characterization.

Scale study of direct synthesis of dimethyl ether from biomass synthesis gas

We investigated the synthesis of dimethyl ether (DME) from biomass synthesis gas using a kind of hybrid catalyst consisting of methanol and HZSM-5 zeolite in a fixed-bed reactor in a 100 ton/year pilot plant. The biomass synthesis gas was produced by oxygen-rich gasification of corn core in a two-stage fixed bed. The results showed that CO conversions reached 82.00% and 73.55%, the selectivities for DME were 73.95% and 69.73%, and the space-time yields were 124.28 kg m- 3 h- 1 and 203.80 kg m- 3 h- 1 when gas hourly space velocities were 650 h- 1 and 1200 h- 1, respectively. Deoxidation and tar removal from biomass synthesis gas was critical to the stable operation of the DME synthesis system. Using single-pass synthesis, the H2/CO ratio improved from 0.98-1.17 to 2.12-2.22. The yield of DME would be increased greatly if the exhaust was reused after removal of the CO2.