Mechanical Deformation Behavior of Nonpolar GaN Thick Films by Berkovich Nanoindentation

In this study, the deformation mechanisms of nonpolar GaN thick films grown on m-sapphire by hydride vapor phase epitaxy (HVPE) are investigated using nanoindentation with a Berkovich indenter, cathodoluminescence (CL), and Raman microscopy. Results show that nonpolar GaN is more susceptible to plastic deformation and has lower hardness than c-plane GaN. After indentation, lateral cracks emerge on the nonpolar GaN surface and preferentially propagate parallel to the < 11 (2) over bar0 > orientation due to anisotropic defect-related stresses. Moreover, the quenching of CL luminescence can be observed to extend exclusively out from the center of the indentations along the < 11 (2) over bar0 > orientation, a trend which is consistent with the evolution of cracks. The recrystallization process happens in the indented regions for the load of 500 mN. Raman area mapping indicates that the distribution of strain field coincides well with the profile of defect-expanded dark regions, while the enhanced compressive stress mainly concentrates in the facets of the indentation.

Effect of counterpart on the tribological behavior and tribo-induced phase transformation of Si

The tribological behaviors and phase transformation of single crystal silicon against Si3N4, Ruby and steel were investigated in this study. It was found that the strong chemical action between silicon and Fe was the key factor to the tribological behavior of silicon as slid against steel. SEM and Raman spectroscopy indicated that phase transformation of single crystal silicon occurred during the running-in period at low sliding velocity as slid against Si3N4 and Ruby. and gave birth to single or a mixture phase of Si-III, Si-XII and amorphous silicon. The high hardness of counterpart and the absence of chemical action between silicon and counterpart facilitated the phase transformation of single crystal silicon. (C) 2008 Elsevier Ltd. All rights reserved.

Structural phase transformations of ZnS nanocrystalline under high pressure

In-situ energy dispersive x-ray diffraction on ZnS nanocrystalline was carried out under high pressure by using a diamond anvil cell. Phase transition of wurtzite of 10 nm ZnS to rocksalt occurred at 16.0 GPa, which was higher than that of the bulk materials. The structures of ZnS nanocrystalline at different pressures were built by using materials studio and the bulk modulus, and the pressure derivative of ZnS nanocrystalline were derived by fitting the equation of Birch-Murnaghan. The resulting modulus was higher than that of the corresponding bulk material, which indicates that the nanomaterial has higher hardness than its bulk materials.

Effect of implantation of nitrogen into SIMOX buried oxide on its fixed positive charge density

In order to obtain greater radiation hardness for SIMOX (separation by implanted oxygen) materials, nitrogen was implanted into SIMOX BOX (buried oxide). However, it has been found by the C-V technique employed in this work that there is an obvious increase of the fixed positive charge density in the nitrogen-implanted BOX with a 150 out thickness and 4 x 10(15) cm(-2) nitrogen implantation dose, compared with that unimplanted with nitrogen. On the other hand, for the BOX layers with a 375 nm thickness and implanted with 2 x 10(15) and 3 x 10(15) cm(-2) nitrogen doses respectively, the increase of the fixed positive charge density induced by implanted nitrogen has not been observed. The post-implantation annealing conditions are identical for all the nitrogen-implanted samples. The increase in fixed positive charge density in the nitrogen-implanted 150 nm BOX is ascribed to the accumulation of implanted nitrogen near the BOX/Si interface due to the post-implantation annealing process according to SIMS results. In addition, it has also been found that the fixed positive charge density in initial BOX is very small. This means SIMOX BOX has a much lower oxide charge density than thermal SiO2 which contains a lot of oxide charges in most cases.

Study on mechanical properties of GaN epitaxy films grown on sapphire by MOCVD

GaN epitaxy films were grown on (0001) oriented sapphire substrate by metal-organic vapor deposition(MOCVD). AFM and SEM were used to analyze the surface morphology of GaN films. Hardness and critical load of GaN films were measured by an nano-indentation tester, friction coefficient by reciprocating UMT-2MT tribometer. It is found that the surface of GaN film is smooth and the epitaxial growth mechanism is in two-dimension mode, GaN epitaxy films also belong to ultra-hardness materials, whose hardness is 22.1 MPa and elastic modulus is 299.5 GPa. Adhesion strength of epitaxial GaN to sapphire is high, and critical load reaches 1.6 N. Friction coefficient against GCr15 ball is steadily close to 0.13, while GaN films turns to be broken rapidly by using Si3N4 ceramic ball as counterpart.

Nano-layer structure of silicon-on-insulator materials

Silicon-on-insulator (SOI) has been recognized as a promising semiconductor starting material for ICs where high speed and low power consumption are desirable, in addition to its unique applications in radiation-hardened circuits. In the present paper, three novel SOI nano-layer structures have been demonstrated. ULTRA-THIN SOI has been fabricated by separation by implantation of oxygen (SIMOX) technique at low oxygen ion energy of 45 keV and implantation dosage of 1.81017/cm2. The formed SOI layer is uniform with thickness of only 60 nm. This layer is of crystalline quality. and the interface between this layer and the buried oxide layer is very sharp, PATTERNED SOI nanostructure is illustrated by source and drain on insulator (DSOI) MOSFETs. The DSOI structure has been formed by selective oxygen ion implantation in SIMOX process. With the patterned SOI technology, the floating-body effect and self-heating effect, which occur in the conventional SOI devices, are significantly suppressed. In order to improve the total-dose irradiation hardness of SOI devices, SILICON ON INSULATING MULTILAYERS (SOIM) nano-structure is proposed. The buried insulating multilayers, which are composed of SiOx and SiNy layers, have been realized by implantation of nitride and oxygen ions into silicon in turn at different ion energies, followed by two steps of high temperature annealing process, respectively, Electric property investigation shows that the hardness to the total-dose irradiation of SOIM is remarkably superior to those of the conventional SIMOX SOI and the Bond-and-Etch-Back SOI.

Temperature dependence of photoluminescence of flat and undulated SiGe/Si multiple quantum wells

Photoluminescence (PL) of strained SiGe/Si multiple quantum wells (MQW) with flat and undulated SiGe well layers was studied at different temperature. With elevated temperature from 10K, the no-phonon (NP) peak of the SiGe layers in the flat sample has firstly a blue shift due to the dominant transition converting from bound excitons (BE) to free excitons (FE), and then has a red shift when the temperature is higher than 30K because of the narrowing of the band gap. In the undulated sample, however, monotonous blue shift was observed as the temperature was elevated from 10 K to 287 K. The thermally activated electrons, confined in Si due to type-II band alignment, leak into the SiGe crest regions, and the leakage is enhanced with the elevated temperature. It results in a blue shift of the SiGe luminescence spectra.

An investigation on the pressure-induced phase transition of nanocrystalline ZnS

An in situ energy dispersive x-ray diffraction study on nanocrystalline ZnS was carried out under high pressure up to 30.8 GPa by using a diamond anvil cell. The phase transition from the wurtzite to the zinc-blende structure occurred at 11.5 GPa, and another obvious transition to a new phase with rock-salt structure also appeared at 16.0 GPa-which was higher than the value for the bulk material. The bulk modulus and the pressure derivative of nanocrystalline ZnS were derived by fitting the Birch-Murnaghan equation. The resulting modulus was higher than that of the corresponding bulk material, indicating that the nanomaterial has higher hardness than the bulk material.

Photoluminescence properties of SiGe/Si single wells with fluctuating structural parameters

Photoluminescence properties of SiGe/Si single wells with fluctuating structural parameters are studied. Four SiGe/Si single wells have been grown on Si(001) at 750 degrees C by disilane and solid Ge molecular beam epitaxy with varied disilane cracking-temperatures. Intense NP and TO-phonon replicas are detected up to 70 K in the photoluminescence spectra and the activation energy of the thermal quenching of the photoluminescence is 28 +/- 4 meV. The high growth temperature and purposeful introduction of fluctuation of structural parameters may be responsible for the improvement of the thermal quenching property.

组合测试:原理与方法

组合测试能够在保证错误检出率的前提下采用较少的测试用例测试系统.但是,组合测试用例集的构造问题的复杂度是NP完全的.组合测试方法的有效性和复杂性吸引了组合数学领域和软件工程领域的学者们对其进行深入的研究.总结了近年来在组合测试方面的研究进展,主要内容包括:组合测试准则的研究、组合测试生成问题与其他NP完全问题的联系、组合测试用例的数学构造方法、采用计算机搜索的组合测试生成方法以及基于组合测试的错误定位技术.

关于可满足性问题的一些研究及其在模型检测中的应用

命题逻辑证明复杂性的研究自从70年代初期由Cook等人提出开始，就一直迅速发展。 Cook等人提出这一领域的初衷是通过证明NP<>CoNP来证明P<>NP。 这一领域近年来的研究内容包括如下几个方向：第一个方向就是“指数下界”问题:也就是证明对于某一个特定的命题逻辑证明系统，存在一个自然的不可满足的公式实例，该系统对这个实例最短的证明规模都是关于实例长度指数级别的； 第二个方向就是比较不同命题逻辑证明系统之间的能力强弱。另一方面，模型检验技术自从80年代由Clarke等人提出用以自动化的检验软硬件系统性质正确性以来，同样的发展迅速。贯穿模型检验技术发展的一个主线就是试图解决系统状态数目过多无法完全在计算机中存储表示的问题，这个问题我们一般称为“状态爆炸”。为了缓解状态爆炸问题，1999年Biere等人提出把命题逻辑可满足性求解工具应用在模型检测领域的限界模型检测方法，相比传统的模型检验算法，对于部分性质，可以极大的减少需要表示的系统状态空间数目，缓解状态爆炸问题。 本文主要包含两部分的内容：第一部分的内容是关于OBDD证明系统的一个理论结果和实际求解的一个解法器的描述。概括的讲，我们解决了Atserias等人在CP2004会议上提出的关于OBDD证明系统的一个问题：我们给出了针对鸽笼问题的多项式规模的OBDD证明系统的一个直接构造，并严格证明了该构造的多项式规模；并且在这个理论结果的基础之上，我们设计了基于OBDD证明系统的实际的求解器，这个求解器可以在多项式时间内解决鸽笼和其他与鸽笼问题结构上相似的实例，这是其他的已知的基于OBDD的求解器无法做到的。这部分内容可以看做是命题逻辑证明复杂性里面第二个方向下的内容，在这里，我们把鸽笼问题 作为一个标尺问题，可以把OBDD证明系统的能力同其他对鸽笼问题的证明为指数的命题逻辑证明系统区分开来。 第二部分的内容是关于限界模型检测的。限界模型检测，是利用命题逻辑可满足性问题求解器来解决一般模型检测问题的方法。对于原本用来描述系统时序性质的时序逻辑的算子，该方法用命题逻辑公式表达出来（在系统迁移关系步数受限的情况下），这个过程就是编码。编码之后，再使用命题逻辑可满足性求解工具来求解。在这部分工作中，我们关注的逻辑是CTL逻辑的fragments，即ECTL或者ACTL，我们详细研究了并改进了ECTL和ACTL到命题逻辑的编码，并且把这个编码过程实现到了开源的模型检测工具平台NuSMV 2上，通过实验来对比限界模型检测方法的效率。实验结果表明，对于某些性质来说，我们的方法相比于传统的基于OBDD的模型检测算法具有优势。

组合对象的搜索算法研究

现实世界中的大量问题涉及如何安排有限个离散的基本对象，使之满足特定约束，这类问题可通称为组合问题。本论文主要研究复杂的组合对象的存在性问题。我们特别关注较难求解的组合问题，从算法复杂度上来说，至少是NP难的。由于这些组合对象相应的约束一般都不太容易满足，因此局部搜索等非完备算法方法常常失效。我们致力于研究高效的完备搜索算法，能够在庞大的搜索空间中精准地找到解，当解不存在时，也能够给出确定性的回答。 作者在博士期间的工作可分为两部分：组合数学问题的自动求解和线性约束上的可满足性模理论的体积求解。 我们系统化地研究了组合数学问题的计算机求解，这部分研究包括三大类问题：幂等拟群的大集问题、正交拉丁方和正交数组。这些问题的共同特点是，要寻找的不是一个复杂的组合对象，而是一系列复杂的组合对象，这些对象相互之间还要满足一定的约束。这样一来，搜索空间比寻找单独的一个组合对象要庞大很多倍，问题的难度也大大增加了。对于每一类问题，我们都通过深入研究其结构特征，提出了有效的问题表示方法和搜索方法，发展了一系列新颖的消除同构技术来降低搜索空间，在搜索中我们应用了自动推理最新的技术和工具，如高效的一阶逻辑模型产生器和命题逻辑求解器等，取得了较好的效果。具体来说，我们的工作包括以下三点： (1) n阶幂等拟群的大集问题需要寻找n-2个不相交的幂等拟群，这比寻找单个的满足特定性质的拟群要困难的多。我们设计了约束分割的策略，解决了幂等拟群大集上的一系列开放性问题，特别是得出了LSPQ(11)不存在的结论。我们的方法不但可以回答特定的大集是否存在，还能够确定相互不相交的拟群的最大个数。这些数学结果对朱烈教授等数学家很有意义，并被收入``HandBook of Satisfiability"一书中。 (2) 正交拉丁方的存在性是组合数学中著名的问题。我们利用一阶逻辑有限模型产生器对该问题进行了尝试，提出了一种新的建模方法，称为横截建模，其效果比直接建模方法提高了若干个数量级。同时，我们也提出了一种新的打破对称策略。 (3) 正交数组是一种重要的组合数学结构，可用于软件组件的集成测试。我们设计了一个搜索正交数组的完备性算法，并提出了一些新颖的打破对称技术。与当前的其它工作不同，我们的算法是普适性的，对输入OA参数无任何限制，并且当搜索完成后总能得到确定性的答案。实验表明，我们的工具BOAS在搜索强度为3及其以上的正交数组时非常高效，而用传统的数学方法构造这些数组则很困难，局部搜索算法也对这些数组常常失效。而且，BOAS能够轻易地解决一批对于其它完备算法来说非常难的实例。 我们研究的第二个方面是线性理论上的SMT实例的体积计算问题。关于各类逻辑和约束语言的可满足性问题有大量的研究工作，比如SAT和可满足性模理论（SMT），另一方面，判定问题的计数版本在自动推理中也非常重要。我们研究了SMT的计数版本，即对给定的SMT实例（更精确地说，线性算术约束的布尔组合）如何计算解的个数，或者是说如何计算解空间的体积。这一问题同时扩展了命题逻辑的模型计数问题和凸多面体的体积计算问题。SMT实例的体积计算在许多领域都有潜在的应用价值，比如程序分析与验证和近似推理，但目前还没有相关的研究工作。我们首次研究了这一问题，提出了一个高效的体积计算方法——逐簇法，我们的算法集成了SMT求解中的最新技术。我们的原型工具在随机实例上产生了较好的效果，并可应用在实际的程序实例中，以寻找热门路径。

逆向工程中圆锥曲面重建技术和裁剪算法的研究

逆向工程技术是随着计算机技术的发展以及数据测量技术的进步而迅速发展起来的一门新兴学科与技术，在计算机视觉、计算生物、几何造型和数字化制造等领域具有广泛的应用。曲面重建和裁剪技术是逆向工程中的重要组成部分，直接决定着重建模型的质量和重建效率，具有重要的研究意义。曲面重建研究已经取得了很大成绩，但是仍然存在着一些有待解决的难点问题，其中包括重建曲面的复杂性较高；数据点集的重建质量较差；自动化程度不高，需要大量的人机交互等。本文对这方面的问题展开了研究，提出了基于一个简单曲面的全自动曲面重建算法，即圆锥形曲面重建方法，同时为曲面重建质量的保证提供了理论证明。在模型重建过程中，需要大量的裁剪计算，使得裁剪效率直接影响到模型重建的速度。本文也对裁剪计算进行了研究，提出了一种高效的任意多边形的线裁剪计算方法，能自适应地降低计算复杂度，进而降低模型重建速度。 本论文的创新工作主要是在以下四个方面： 1. 提出了圆锥形快速重建点云模型的方法。对具有n个点的圆锥形点云模型，在误差阈值为ε的情况下，该方法的时间复杂度为O(n/ε3)，较以往的最快时间复杂度为O(nlogn/ε4+n/ε5)的算法有了明显的提高，并在理论上证明了重建后的圆锥的体积小于原始模型体积的(1+ε)倍。 2. 在圆锥形快速重建点云模型方法的基础上进行了改进，改进后的算法将求取全局最优解的过程修改为求取局部最优解，减少了大量的计算，将时间复杂度降至O(n/ε)。实验结果表明该算法能收敛到全局最优解。 3. 提出了采用多个圆锥进行模型重建的近似方法。该方法是一个全自动的方法，它可以有效地解决具有任意轴向的线性点云模型的圆锥重建问题（该问题是一个NP-难问题）。该方法将点云模型自适应地划分成一些子集，并对每个子集用一个圆锥来重建，使得圆锥包含对应子集内所有点。 4. 提出了基于凸片段的一般多边形线裁剪算法。该算法将多边形窗口的边顺序地分割成一些片段，使得每个片段都能局部地形成一个凸多边形，称为凸片段，并建立一个二叉树来管理这些凸片段。在裁剪计算时，先根据二叉树快速找到与被裁剪线段相交的凸片段，再利用高效的凸多边形线裁剪算法对这些凸片段进行裁剪操作。该方法能有效地降低裁剪计算的时间复杂度，使其在O(logN)~O(N)之间自适应地变化，且大部分情况下是小于O(N)的。

参数化系统安全性的启发式符号验证

参数化系统(paramterized system)是指包含特定有限状态进程多个实例的并发系统,其中的参数是指系统内进程实例的数目,即系统的规模.反向可达性分析(backward reachability analysis)已被广泛用于验证参数化系统是否满足以向上封闭(upward-closed)集合表示的安全性(safety property).与有限状态系统验证相类似,参数化系统的验证同样也面临着状态爆炸(state explosion)问题,并且模型检测算法的有效性依赖于如何采用有效的数据结构表示状态集合.针对表示无穷状态的向上封闭集合,研究人员提出了多种基于约束(constraint-based)的符号表示方法.但这些方法依然面临着符号状态爆炸(symbolic state explosion)问题或者其包含判定问题,即判断一个约束条件集合符号化表示的实际状态集合是否为另一约束条件集合所对应的状态集合的子集,是Co-NP完全问题.因此,虽然有限状态验证技术能够验证一些具有一定规模的问题,但现有针对参数化系统的验证方法所能解决的问题的规模较为有限,需要近一步提高模型检测算法的效率.针对参数化系统提出了加快反向可达性分析的多个启发式规则,实验结果表明,这些启发式规则可以使算法的效率提高几个数量级,从而有助于解决现有参数化系统验证方法所存在的问题.

基于角色的管理模型隐式授权分析

基于角色的管理模型被用于管理大型RBAC(role-based access control)系统的授权关系,UARBAC具有可扩展、细粒度等优点.UARBAC的管理操作包含隐式授权.隐式授权分析说明UARBAC管理操作的两类缺陷,包括两个定义缺陷,即无法创建客体和虚悬引用,以及一个实施缺陷,即不支持最小授权.通过修改管理操作更正定义缺陷,提出实施缺陷的改进方案.定义实施最小授权的最小角色匹配问题,证明该问题是NP难,并给出基于贪心算法的可行方案,帮助管理员选择合适的管理操作将最小角色集合授予用户.