An approach to functional synthesis of solutions in mechanical conceptual design. Part III: Spatial configuration

These three papers describe an approach to the synthesis of solutions to a class of mechanical design problems; these involve transmission and transformation of mechanical forces and motion, and can be described by a set of inputs and outputs. The approach involves (1) identifying a set of primary functional elements and rules of combining them, and (2) developing appropriate representations and reasoning procedures for synthesising solution concepts using these elements and their combination rules; these synthesis procedures can produce an exhaustive set of solution concepts, in terms of their topological as well as spatial configurations, to a given design problem. This paper (Part III) describes a constraint propagation procedure which, using a knowledge base of spatial information about a set of primary functional elements, can produce possible spatial configurations of solution concepts generated in Part II.

Phase perfection in zinc Blende and Wurtzite III-V nanowires using basic growth parameters.

Controlling the crystallographic phase purity of III-V nanowires is notoriously difficult, yet this is essential for future nanowire devices. Reported methods for controlling nanowire phase require dopant addition, or a restricted choice of nanowire diameter, and only rarely yield a pure phase. Here we demonstrate that phase-perfect nanowires, of arbitrary diameter, can be achieved simply by tailoring basic growth parameters: temperature and V/III ratio. Phase purity is achieved without sacrificing important specifications of diameter and dopant levels. Pure zinc blende nanowires, free of twin defects, were achieved using a low growth temperature coupled with a high V/III ratio. Conversely, a high growth temperature coupled with a low V/III ratio produced pure wurtzite nanowires free of stacking faults. We present a comprehensive nucleation model to explain the formation of these markedly different crystal phases under these growth conditions. Critical to achieving phase purity are changes in surface energy of the nanowire side facets, which in turn are controlled by the basic growth parameters of temperature and V/III ratio. This ability to tune crystal structure between twin-free zinc blende and stacking-fault-free wurtzite not only will enhance the performance of nanowire devices but also opens new possibilities for engineering nanowire devices, without restrictions on nanowire diameters or doping.

Growth temperature and V/III ratio effects on the morphology and crystal structure of InP nanowires

The effects of growth temperature and V/III ratio on the morphology and crystallographic phases of InP nanowires that are grown by metal organic chemical vapour deposition have been studied. We show that higher growth temperatures or higher V/III ratios promote the formation of wurtzite nanowires while zinc-blende nanowires are favourableat lower growth temperatures and lower V/III ratios. A schematic map of distribution of zinc-blende and wurtzite structures has been developed in the range of growth temperatures (400-510 °C) and V/III ratios (44 to 700) investigated in this study. © 2010 IOP Publishing Ltd.

Epitaxy of III-V semiconductor nanowires towards optoelectronic devices

GaAs and InP based nanowires were grown epitaxially on GaAs or InP (111)B substrates by MOCVD via VLS mechanism. In this paper, I will give an overview of nanowire research activities in our group. © 2009 IEEE.

Prospects of III-nitride optoelectronics grown on Si.

The use of III-nitride-based light-emitting diodes (LEDs) is now widespread in applications such as indicator lamps, display panels, backlighting for liquid-crystal display TVs and computer screens, traffic lights, etc. To meet the huge market demand and lower the manufacturing cost, the LED industry is moving fast from 2 inch to 4 inch and recently to 6 inch wafer sizes. Although Al2O3 (sapphire) and SiC remain the dominant substrate materials for the epitaxy of nitride LEDs, the use of large Si substrates attracts great interest because Si wafers are readily available in large diameters at low cost. In addition, such wafers are compatible with existing processing lines for 6 inch and larger wafers commonly used in the electronics industry. During the last decade, much exciting progress has been achieved in improving the performance of GaN-on-Si devices. In this contribution, the status and prospects of III-nitride optoelectronics grown on Si substrates are reviewed. The issues involved in the growth of GaN-based LED structures on Si and possible solutions are outlined, together with a brief introduction to some novel in situ and ex situ monitoring/characterization tools, which are especially useful for the growth of GaN-on-Si structures.

Preliminary study on the photoproduction of hydroxyl radicals in aqueous solution with Aldrich humic acid, algae and Fe(III) under high-pressure mercury lamp irradiation

Under a high-pressure mercury lamp (HPML) and using an exposure time of 4 h, the photoproduction of hydroxyl radicals ((OH)-O-.) could be induced in an aqueous solution containing humic acid (HA). Hydroxyl radicals were determined by high-performance liquid chromatography using benzene as a probe. The results showed that (OH)-O-. photoproduction increased from 1.80 to 2.74 muM by increasing the HA concentration from 10 to 40 mg L-1 at an exposure time of 4 h (pH 6.5). Hydroxyl radical photoproduction in aqueous solutions of HA containing algae was greater than that in the aqueous solutions of HA without algae. The photoproduction of (OH)-O-. in the HA solution with Fe(111) was greater than that of the solution without Fe(III) at pH ranging from 4.0 to 8.0. The photoproduction of (OH)-O-. in HA solution with algae with or without Fe(111) under a 250 W HPML was greater than that under a 125 W HPML. The photoproduction of (OH)-O-. in irradiated samples was influenced by the pH. The results showed that HPML exposure for 4 h in the 4-8 pH range led to the highest (OH)-O-. photoproduction at pH 4.0.

Influence of V/III ratio on the structural and photoluminescence properties of In0.52AlAs/In0.53GaAs metamorphic high electron mobility transistor grown by molecular beam epitaxy

A series of metamorphic high electron mobility transistors (MMHEMTs) with different V/III flux ratios are grown on GaAs (001) substrates by molecular beam epitaxy (XIBE). The samples are analysed by using atomic force microscopy (AFM), Hall measurement, and low temperature photoluminescence (PL). The optimum V/III ratio in a range from 15 to 60 for the growth of MMHEMTs is found to be around 40. At this ratio, the root mean square (RMS) roughness of the material is only 2.02 nm; a room-temperature mobility and a sheet electron density are obtained to be 10610.0cm(2)/(V.s) and 3.26 x 10(12)cm(-2) respectively. These results are equivalent to those obtained for the same structure grown on InP substrate. There are two peaks in the PL spectrum of the structure, corresponding to two sub-energy levels of the In0.53Ga0.47 As quantum well. It is found that the photoluminescence intensities of the two peaks vary with the V/III ratio, for which the reasons are discussed.

Growth of (10(1)over-bar(3)over-bar) semipolar GaN on m-plane sapphire by hydride vapor phase epitaxy

The crystalline, surface, and optical properties of the (10 (1) over bar(3) over bar) semipolar GaN directly grown on m-plane sapphire substrates by hydride vapor phase epitaxy (HVPE) were investigated. It was found that the increase of V/III ratio led to high quality (10 (1) over bar(3) over bar) oriented GaN epilayers with a morphology that may have been produced by step-flow growth and with minor evidence of anisotropic crystalline structure. After etching in the mixed acids, the inclined pyramids dominated the GaN surface with a density of 2 X 10(5) cm(-2), revealing the N-polarity characteristic. In the low-temperature PL spectra, weak BSF-related emission at 3.44eV could be observed as a shoulder of donor-bound exciton lines for the epilayer at high V/III ratio, which was indicative of obvious reduction of BSFs density. In comparison with other defect related emissions, a different quenching behavior was found for the 3.29 eV emission, characterized by the temperature-dependent PL measurement. (C) 2009 Elsevier B.V. All rights reserved.

Effects of V/III ratio on InGaAs and InP grown at low temperature by LP-MOCVD

Effects of V/III ratio on heavily Si doped InGaAs and InP were studied using low pressure metalorganic chemical vapor deposition (LP-MOCVD) at a growth temperature of 550degreesC. In InGaAs, as the V/III ratio decreases from 256 to 64, the carrier concentration increases from 3.0 x 10(18) to 5.8 x 10(18) cm(-3), and the lattice mismatch of InGaAs to InP was observed to vary from -5.70 x10(-4) to 1.49 x 10(-3). In InP, when the V/III ratio decreases from 230 to 92, the same trend as that in Si doped InGaAs was observed that the carrier concentration increases from 9.2 x 10(18) to 1.3 x 10(19) cm(-3). The change of AsH3 was found to have stronger effect on Si incorporation in InGaAs at lower growth temperature than at higher growth temperature. (C) 2003 Elsevier B.V. All rights reserved.

Studies of high DC current induced degradation in III-V nitride based heterojunctions

We report experiments on high de current stressing in commercial III-V nitride based heterojunction light-emitting diodes. Stressing currents ranging from 100 mA to 200 mA were used. Degradations in the device properties were investigated through detailed studies of the current-voltage (I-V) characteristics, electroluminescence, deep-level transient Fourier spectroscopy and flicker noise. Our experimental data demonstrated significant distortions in the I-V characteristics subsequent to electrical stressing. The room temperature electro-luminescence of the devices exhibited a 25% decrement in the peak emission intensity. Concentration of the deep-levels was examined by deep-level transient Fourier spectroscopy, which indicated an increase in the density of deep-traps from 2.7 x 10(13) cm(-3) to 4.2 x 10(13) cm(-3) at E-1 = E-C - 1.1 eV. The result is consistent with our study of 1/f noise, which exhibited up to three orders of magnitude increase in the voltage noise power spectra. These traps are typically located at energy levels beyond the range that can be characterized by conventional techniques including DLTS. The two experiments, therefore, provide a more complete picture of trap generation due to high dc current stressing.

Kinetic study of MOCVD III-V quaternary antimonides

The kinetics of MOCVD GaInAsSb and AlGaAsSb was studied by the growth rate as a function of growth temperature and partial pressure of III and V MO species. The diffusion theory was used to explain the mass transport processes in MOCVD III-V quaternary antimonides. On the basis of the discussion about their growth kinetics and epilayer properties, the good quality multi-epilayers of these two quaternary antimonides and their photodetectors and arrays with wavelength of 1.8 similar to 2.3 mu m and detectivities of D* > 10(9) cm Hz(1/2) W-1 were obtained.

Characterization of hot-electron effects on flicker noise in III-V nitride based heterojunctions

We report experiments on hot-electron stressing in commercial III-V nitride based heterojunction fight-emitting diodes. Stressing currents ranging from 100 mA to 200 mA were used. Degradations in the device properties were investigated through detailed studies of the I-V characteristics, electroluminescence, Deep-Level Transient Fourier Spectroscopy and flicker noise. Our experimental data demonstrated significant distortions in the I-V characteristics. The room temperature electroluminescence of the devices exhibited 25% decrement in the peak emission intensity. Concentration of the deep-levels was examined by measuring the Deep-Level Transient Fourier Spectroscopy, which indicated an increase in the density of deep-traps from 2.7 x 10(13) cm(-3) to 4.21 x 10(13) cm(-3) at E-1 = E-C - 1.1eV. The result is consistent with our study of 1/f noise, which exhibited up to three orders of magnitude increase in the voltage noise power spectra. Our experiments show large increase in both the interface traps and deep-levels resulted from hot-carrier stressing.

Cr(III)盐在Pt电极和PbO_2电极上阳极氧化为络酸盐的机理的研究

关于Cr(III)阳极氧化为Cr(VI)的过程，在工业应用方面已有许多工作，但对机理尚不清楚。本文对Pt电极和PbO_2电极上的这一过程进行了动力学的研究，首次得到了Cr(VI)在这两种电极上阳极形成的真实动力学数据，并提出了与实验基本相符的反应机理。用“分解极化曲线法”和稳态极化曲线的测量，得到了[H_2SO_4]和[SO_4~=]恒定的不同浓度Cr_2(SO_4)_3溶液中Cr(VI)在光滑Pt电极上阳极形成的真实动力学数据，可用如下方程式来表达：在较低电位下φ = a + 0.25 log i_2 - 0.23 log [Cr(III)]在较高电位下φ = a' + 0.48 log i_2 - 0.44 log [Cr(III)]同时得到了氧的阳极发生的动力学数据，Tafel线性区的斜率接近于2.303 RT/ΔF (Δ ≈ 0.5)。动力学方程式的推导与实验的比较表明，在光滑Pt电极上Cr(VI)是由Cr(II)通过“活性氧”的氧化形成的，提出了如下反应机理：H_2O → (OH)_(ad) + H~+ + e~- 2(OH)_(ad) → (O)_(ad) + H_2O 2(O)_(ad) →O_2 (OH)_(ad) + [Cr(H_2O)_6]~(3+) → (CrO_2)_(ad) + 3H~+ + 5H_2O (CrO_2)_(ad) + H_2O → (CrO_3~-)_(ad) + 2H~+ + e~- (CrO_3~-)_(ad) + H_2O → HCrO_4~- + H~+ + e~- 2HCrO_4~- <-> Cr_2O_7~(2-) + H_2O 在[H_2SO_4]和[SO_4~(2-)]恒定的不同浓度Cr_2(SO_4)_3溶液中，测得了Cr(VI)在Δ-PbO_2电极上阳极形成的动力学数据：在较低电位下φ = a + 0.28 log i_2 - 0.30 log [Cr(III)]在较高电位下φ = a' + 0.55 log i_2 - 0.51 log [Cr(III)]氧的极化曲线的Tafel线性区斜率也为2.303RT/ΔF (Δ approx= 0.5)。PbO_2电极和Pt电极上分解极化曲线的比较表明，Cr(VI)在前一电极上阳极形成的过电位远低于在后一电极上，这可能是两电极上电流效率显著差别的原因。测得了PbO_2电极上不同过电位下电极反应的有效活化能，其数值均在10 Kcal mol~(-1)以上，且随着极化的增大而减小，据此在动力学处理中可以忽略扩散的作用，交流阻抗的研究进一步证实了这一点。溶液pH的增大或[H_2SO_4]的减小会降低Cr(VI)阳极形成的过电位，使反应加速。在[H_2SO_4]和[SO_4~(2-)]恒定的不同浓度Cr_2(SO_4)_3溶液中，测得了在不同电位极化下PbO_2电极的阻抗频谱。在较高电位下阻抗谱呈现明显的两个半圆，表明电极过程包括了中间吸附物的形成。求得的吸附电容Cad比双层电容Cd大1-2个数量级；Cd比通常光滑电极表面的Cd大得多，这可能与SO_4~(2-)，HCrO_4~-, Cr_2O_7~-以及[_((SO_4))~((H_2O)_2) Cr_((SO_4))~((OH)_2) Cr_((SO_4))~((OH_2)_2)]~(2-)等阳离子的特性吸附有关。从动力学的推导与实验的比较得出，在PbO_2电极上Cr(VI)也按“活性氧”机构形成，可能的机理如下：H_2O → (OH)_(ad) + H~+ + e~- (OH)_(ad) + H_2O → (O)_(ad) + H_3O~+ + e~- 2(O)_(ad) → O_2 [Cr(H_2O)_6]~(3+) + (O)_(ad) → (CrO_3~-)_(ad) + 4H~+ + 4H_2O (CrO_3~-)_(ad) + H_2O → HCrO_4~- + H~+ +e~- 2HCrO_4~- <-> Cr_2O_7~(2-) + H_2O.

结构因子相角的代数解法及K_8H_5[La(SiWMo_(10)O_(39))(SiW_3Mo_8O_(39))]·nH_2O的晶体结构

一 结构因子相角的代数解法晶体结构研究中的相角问题，一直是晶体学工作者努力探求的基本问题。几十年来，关于相角问题的研究已经取得了巨大的进展，直接法的成熟和普遍应用就是这个进展的显著标志。在直接法相角问题研究中，历来存在两种基本方法，即概率法和代数法。当前直接法的迅猛发展，则主要以概率法为基础，其应用的基本公式即是来自∑关系中的∑_2关系和由此发展出的TANGENT公式，并结合一系列的基本理论和辅助公式，从而使直接法的发展一直呈方兴未艾之势。与此形成鲜明对照的是，相角问题的代数法研究则有日趋下降之势，尤其近十年来则显著落后了。作者从近年来的∑关系研究中，看到要解决相角问题，代数法仍然有其潜在的应作价值，并认为在概率法可以得到应用的许多方面，在代数法那里都应找到对应的类似内容，而代数法在研究和应用中所表现出的不同于概率法的那些方面也正是在相角问题研究中最令人感兴趣的问题，这就是我们进行代数法研究的目的和出发点。（一）1.5和2.5阶代数式的提出及对±2~(II)型相角的估算。（二）一般类型相角的估算（三）结构相角通用代数式的推导。代数法不仅可用于结构半不变量O，π型相角的计算，而且可用于非结构半不变量的种种类型相角的估算，其中包括型和一般类型相角，而且其应用效果完全可以和概率法相比拟，在机时利用上前者有一定优势。通用代数式的获得将为代数法的普遍应用和计算机计算创造了方便条件。二稀土杂多酸根配合物K_8H_5[La(SiWMo_(10)O_(39))(SiW_3Mo_8O_(39))]·nH_2O的晶体结构采用低温技术，在-90 ℃的干燥氮气保护下，收集晶体衍射数据，以直接法解出结构。a = 17.485(5), b = 27.096(6), c = 21.642(5)A, β = 107.79(2)°, z = 4。最后的R值为0.097。中心离子La(III)同两个杂多酸根提供的8 个氧配合。而其中的W、Mo同氧的配位情况基本相同，者是与氧六配位，形成变形配位八面体。

MOCVD growth of high quality crack-free GaN on Si(III) substrates

High quality crack free GaN epilayers were grown on Si(111) substrates. Low temperature AlN interlayer grown under low V/III ratio was used to effectively eliminate the formation of micro-cracks. It is found that tensile stress in the GaN epilayer decreases as the N/Al ratio decreases used for AlN interlayer growth. The high optical and structural qualities of the GaN/Si samples were characterized by RBS, PL and XRD measurements. The RT-PL FWHM of the band edge emission is only 39.5meV The XRD FWHM of the GaN/Si sample is 8.2arcmin, which is among the best values ever reported.