The influence of V/III ratio in the initial growth stage on the properties of GaN epilayer deposited on low temperature AlN buffer layer

The V/III ratio in the initial growth stage of metalorganic chemical vapor deposition has an important influence on the quality of a GaN epilayer grown on a low-temperature AIN buffer layer and c-plane sapphire substrate. A weaker yellow luminescence, a narrower half-width of the X-ray diffraction peak, and a higher electron mobility result when a lower V/III ratio is taken. The intensity of in situ optical reflectivity measurements indicates that the film surface is rougher at the beginning of GaN growth, and a longer time is needed for the islands to coalesce and for a quasi-two dimensional mode growth to start. A comparison of front- and back-illuminated photoluminescence spectra confirms that many threading dislocations are bent during the initial stage, leading to a better structural quality of the GaN layer. (C) 2007 Elsevier B.V. All rights reserved.

An adaptive controller for a class of nonlinear system using direction basis function

For a class of nonlinear dynamical systems, the adaptive controllers are investigated using direction basis function (DBF) in this paper. Based on the criterion of Lyapunov' stability, DBF is designed which guarantees that the output of the controlled system asymptotically tracks the reference signals. Finally, the simulation shows the good tracking effectiveness of the adaptive controller.

Effects of residual C and O impurities on photoluminescence in undoped GaN epilayers

Photoluminescence (PL) was investigated in undoped GaN from 4.8 K to room temperature. The 4.8 K spectra exhibited recombinations of free exciton, donor-acceptor pair (DAP), blue and yellow bands (Ybs). The blue band (BB) was also identified to be a DAP recombination. The YB was assigned to a recombination from deep levels. The energy-dispersive X-ray spectroscopy show that C and O are the main residual impurities in undoped GaN and that C concentration is lower in the epilayers with the stronger BB. The electronic structures of native defects, C and O impurities, and their complexes were calculated using ab initio local-density-functional (LDF) methods with linear muffin-tin-orbital and 72-atomic supercell. The theoretical analyses suggest that the electron transitions from O-N states to C-N and to V-Ga states are responsible for DAP and the BB, respectively, and the electron transitions between the inner levels of the C-N-O-N complex may be responsible for the YB in our samples. (C) 2002 Elsevier Science B.V. All rights reserved.

概念设计中基于笔式手势的交互计算研究

基于笔式手势的自然交互是支持概念设计创新的有效方式.提出了一种笔式手势的层次概念模型,结合基于约束的自由勾画和上下文感知技术描述了手势设计方式,进一步讨论了手势内部的约束建立和求解算法;基于手势应用的范式,给出了概念设计中特征手势建模的应用实例,通过与传统建模和交互方式的对比,验证了特征手势建模的方便性.给出一个自然、高效的方法,以基于手势的方式来完成概念设计构思过程,采用笔式交互快速自然地记录下设计思路,所实现的系统方便了用户的创新设计,改善了人机交互模式.

the tilt cursor: enhancing stimulus-response compatibility by providing 3d orientation cue of pen

In order to improve stimulus-response compatibility of touchpad in pen-based user interface, we present the tilt cursor, i.e. a cursor dynamically reshapes itself to providing the 3D orientation cue of pen. We also present two experiments that evaluate the tilt cursor’s performance in circular menu selection and specific marking menu selection tasks. Results show that in a specific marking menu selection task, the tilt cursor significantly outperforms the shape-fixed arrow cursor and the live cursor [4]. In addition, results show that by using the tilt cursor, the response latencies for adjusting drawing directions are smaller than that by using the other two kinds of cursors.

面向海量层次信息可视化的嵌套圆鱼眼视图

针对海量层次信息可视化问题,提出嵌套圆鱼眼视图及实现方法.首先分析了相关工作;然后构造了针对同层兄弟节点的变形函数,给出了针对焦点节点同层兄弟节点的外切圆变形排列算法、变形后节点的子节点的嵌套圆递归排列算法及将二维鱼眼视图映射为三维鱼眼视图的方法;最后将该方法应用于计算机文件系统可视化.实验结果表明:该方法在相对小的显示空间内,既能显示海量信息的整体视图,又能突出局部焦点细节信息,具有较高的任务完成效率与用户满意度.

黄土旱塬长期施肥对小麦产量及养分平衡的影响

基于长期定位试验,研究不同肥料施用对小麦产量及养分平衡的影响。结果表明,合理施肥可以明显提高小麦的产量,增加小麦对养分的吸收量,同时达到培肥地力的效果。氮、磷配施增产效果显著,增幅为111.7%;在单施氮肥、单施磷肥的基础上施有机肥,分别比单施氮肥和单施磷增产32.7%、155.7%。氮、磷及有机肥配施时氮、磷、钾总吸收量均最多,分别增加303.5%、128.6%、238.8%。肥料单施,土壤中的部分养分会出现亏缺,单施氮肥,土壤中磷素亏损100%;单施磷肥,氮素亏损100%。肥料配施,氮素、磷素都出现不同程度的盈余。农业生产中该区要重视施用有机肥及肥料的合理配施。

~7Be示踪坡耕地次降雨细沟与细沟间侵蚀

对于坡面细沟与细沟间侵蚀过程的了解是建立侵蚀预报模型的基础,但传统方法难以对其进行深入研究。利用7Be示踪技术并结合人工模拟降雨,考虑坡脚沉积作用,研究了25°坡耕地径流小区次降雨过程中细沟与细沟间侵蚀动态。结果表明:根据流出径流小区泥沙7Be含量变化计算坡面明显细沟出现时间,由于坡脚沉积作用使得A、B两试验小区这一时间比实际细沟出现分别延迟了45min和11min;根据坡面-侵蚀泥沙中7Be总量守恒和泥沙质量平衡原理,坡面细沟间侵蚀及细沟侵蚀在坡面总侵蚀、坡脚沉积区泥沙及流出径流小区泥沙中的比例被定量区分开;总体上,细沟间侵蚀量在径流泥沙中的比例逐渐减少,而细沟侵蚀量逐渐增加。两试验小区中7Be示踪计算坡面细沟侵蚀量和坡脚沉积量与实测值相比相对误差均较小,因此7Be示踪技术可以对土壤侵蚀进行较为准确地定量研究。