Antisite arsenic incorporation in the low temperature MBE of gallium arsenide: Physics and modeling

A stochastic model for simulating the surface growth processes in the low temperature molecular beam epitaxy of gallium arsenide is developed to investigate the incorporation of antisite As and its dependence on the growth conditions including the dynamics of the physisorbed As on the surface. Three different kinetic models with a combination of surface kinetic processes such as incorporation of antisite As, evaporation of antisite As and incorporation of regular As. The kinetic model with all three surface processes was accepted as the best model due to its physical soundness and reasonableness of its model parameters. The arsenic flux, temperature, and growth rate dependences of antisite arsenic (As_Ga) obtained from our simulation are in excellent agreement with the experimental results. The activation energy of 1.16 eV and a frequency factor of 4×10¹²/s for the evaporation of antisite arsenic obtained from our model are in good agreement with experimental and theoretical estimates. At a constant substrate temperature and growth rate, the antisite arsenic concentration increases with arsenic flux for low fluxes and saturates beyond a critical flux. The critical arsenic flux increases with temperature and the saturation value of the As_Ga concentration decreases with temperature. As the arsenic flux increases, the coverage of the physisorbed layer increases and at a critical flux dictated by the fixed temperature and growth rate, the coverage saturates at its maximum value of unity (a complete monolayer) and hence the concentration of As_Ga saturates. Lower As_Ga concentration results at higher temperature due to more evaporation of As_Ga. Additionally, an analytical model is developed to predict the As_Ga concentration for various growth conditions.     

Degree distribution optimization for LT codes and its application in link maintenance of cognitive radio

The degree distribution has a great influence on the performance of Luby transform (LT) codes.Based on the link maintain model of cognitive radio system,the novel degree distribution of LT codes was obtained by using the THOA (two-layer hierarchical optimization algorithm) to combine the IPD (improved poisson distribution) which achieved high decoding success rate with low overhead and the robust soliton distribution (RSD) which achieved high decoding success rate with high overhead.Simulation results show that the proposed method with application to link maintenance in cognitive radio system can improve the reliability and efficiency of secondary communication.     

虚轴上具有重极点的拉普拉斯变换与傅里叶变换相互计算的方法

提出了一种虚轴上具有重极点的拉普拉斯变换(LT)与傅里叶变换(CTFT)相互计算的方法。对虚轴上具有重极点的LT,通过部分分式展开,将LT在虚轴上的极点分离出来,使其成为虚轴上含极点和不含极点两部分之和,针对虚轴上含极点的LT部分,分区左极点和区右极点两种情况,导出了利用LT计算CTFT的方法;将信号的CTFT分解成解析部分与不解析部分之和,针对CTFT的不解析部分,分因果信号和反因果信号两种情况,导出了利用CTFT计算LT的方法。     

数字喷泉码及其应用的研究进展与展望

 基于TCP协议的大部分网络通信都将接收到的数据包当作一个有序序列进行处理.TCP协议的这种有序序列模式限制了其在大量数据传输或将数据分布到大量用户的应用.基于数字喷泉码的数据传输模式不需要有序的数据序列,从而简化了网络中数据的传输方式,使得数字喷泉码成为一类适用于可靠通信的有效编码技术,而具有广阔的应用前景.本文综述了几类典型数字喷泉码的原理及其优缺点;探讨了这几类数字喷泉码的具体应用;指出了数字喷泉码研究中需要解决的一些关键问题.最后,对数字喷泉码的发展前景及研究方向进行展望.     

便携式GPS接收机设计

提出一种基于EM411GPS接收模块和PIC18F2520单片机的手持式GPS接收机设计方案。该方案具有电路简单、成本低、灵敏度高等优点,并可将接收的数据以FAT文件格式保存到SD卡中,通过SD卡将GPS数据导入电子地图,便于野外作业和户外运动使用。     

用LT886的ADSL Modems作远程、高速率的线路驱动器

介绍一款用LT1886的ADSL Modems作远程，高速率线路驱动器。     

基于TMS320F2808的矩阵变换器的研究与实现

本文介绍了矩阵变换器的特点，重点阐述了基于32位DSP TMS320F2808的矩阵变换器的设计与实现，该系统主要由主控制器、输入电压状态检测、译码及隔离驱动、电流检测、上位机通讯等电路组成，最后给出了实验结果。结果表明该系统能输出频率、电压均可在线调节的正弦波。     

Ordering in InGaAs/InAlAs layers

The structure of InCaAs/InAlAs layers lattice matched to an InP substrate, grown on either (100) or on (110) with a 4° tilt toward [111] at 500 and 300°C has been investigated by transmission electron microscopy. High perfection resulted for the layers grown on [001] oriented substrates whereas growth on the near [110] substrates resulted in compositional nonuniformities, macrosteps formation, and ordering of the group III elements. This difference in structural perfection between the two sets of samples was also reflected in differences in electrical properties.     

Diffusion of zinc into gaas layers grown by molecular beam epitaxy at low substrate temperatures

We report the diffusion of zinc into low temperature (LT) GaAs grown by MBE at 200° C, the problems associated with using a silicon nitride film directly deposited on the LT GaAs as a Zn diffusion mask, and several schemes to avoid the problems. The Zn diffusion coefficient is measured (sealed-ampoule technique) to be about one order of magnitude higher in the LT GaAs than in normal GaAs, attributed to a large quantity of defects including arsenic antisites (As_Ga) in the LT GaAs. The effectiveness of silicon nitride as a Zn diffusion mask depends if the mask is deposited directly on the LT GaAs. The failure of the nitride directly deposited on the LT GaAs to stop the Zn is attributed to arsenic atoms outdiffusing from the As-rich LT GaAs (about 1 at. % excess As) into the nitride. Several structures are introduced including a 100-Å thick GaAs layer on the LT GaAs that are effective in preserving the diffusion mask properties of the silicon nitride.     

新型升压式DC/DC变换器人LT1930

介绍了一种新型升压式DC/DC变换器：T1930及其应用电路。