Effect of As Passivation on Vapor-Phase Epitaxial Growth of Ge on (211)Si as a Buffer Layer for CdTe Epitaxy

We report an investigation of epitaxial germanium grown by chemical vapor deposition (CVD) on arsenic-terminated (211)Si, which is the preferred substrate in the USA for fabrication of night-vision devices based on mercury cadmium telluride (MCT) grown by molecular-beam epitaxy (MBE). The films were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), cross-sectional transmission electron microscopy (XTEM), and x-ray diffraction (XRD). Arsenic passivation was found to be effective in preventing cross-contamination of unwanted residual species present inside the reactor chamber and also in prolonging the evolution of layer-by-layer growth of Ge for significantly more monolayers than on nonpassivated Si. The two-dimensional (2D) to three-dimensional (3D) transition resulted in Ge islands, the density and morphology of which showed a clear distinction between passivated and nonpassivated (211)Si. Finally, thick Ge layers (∼250 nm) were grown at 525°C and 675°C with and without As passivation, where the layers grown with As passivation resulted in higher crystal quality and smooth surface morphology.     

Transition metal doped cadmium manganese telluride: A new material for tunable mid-infrared lasing

Relatively new materials for mid-infrared tunable lasing using chromium-doped Cd_1-xMn_xTe and cobalt-doped Cd_1-xMn_xTe have been developed. Previously, ZnS and ZnSe were used as host materials for chromium to produce mid-infrared (MIR) lasing. Compared to these materials, large diameter CdMnTe is easier to grow (using the Bridgman technique) and can be made more homogeneous. Moreover, the ternary nature of Cd_1-xMn_xTe offers the unique opportunity to optimize the optical properties of the material through variation of chemical composition and lattice parameter. Using Cd_0.55Mn_0.45Te:Cr, we have demonstrated room temperature lasing from 2.1 to 3.0 m, and we have demonstrated quasi-continuous wave (cw) lasing. To our knowledge, the observed tuning range (∼840 nm) of Cr²⁺:Cd_0.55Mn_0.45Te is the largest ever reported from a transition metal ion laser. Furthermore, this is the first time that a room temperature quasi-cw laser operating at 3 m has been demonstrated using this type of material. Also, preliminary work on Cd_0.55Mn_0.45Te:Co indicates its potential for tunable mid-infrared lasing around 3600 nm at cryogenic temperatures. Results from inductively coupled plasma mass spectrometry (ICP-MS), which determine the concentration of dopant that has been incorporated in to the host lattice, will be reported, as will the materials characterization and lasing results. The processing issues for optimizing the laser performance in these material systems will also be discussed.     

Nanoscale FinFETs with gate-source/drain underlap

Using two-dimensional numerical device simulations, we show that optimally designed nanoscale FinFETs with undoped bodies require gate-source/drain (G-S/D) underlap that can be effectively achieved via large, doable straggle in the S-D fin-extension doping profile without causing S-D punch-through. The effective underlap significantly relaxes the fin-thickness requirement for control of short-channel effects (SCEs) via a bias-dependent effective channel length (L/sub eff/), which is long in weak inversion and approaches the gate length in strong inversion. Dependence of L/sub eff/ on the S/D doping profile defines a design tradeoff regarding SCEs and S/D series resistance that can be optimized, depending on the fin width, via engineering of the doping profile in the S/D fin-extensions. The noted optimization is exemplified via a well-tempered FinFET design with an 18-nm gate length, showing further that designs with effective underlap yield minimal parasitic capacitance and reduce leakage components such as gate-induced drain leakage current.     

Dependability modeling of real-time systems using stochastic reward nets

Dependability modeling plays a major role in the design, validation and maintenance of real-time computing systems. Typical models provide measures such as mean time to failure, reliability and safety as functions of the component failure rates and fault/error coverage probabilities. In this paper we propose a modeling technique that allows the coverage to be dependent upon the local (i.e. embedded at task level) and global (i.e. available at system level) fault/error detection and recovery mechanisms. This approach also ensures important savings in terms of the simulation time required for deriving the coverage probabilities. Stochastic. reward nets are employed as a unique dependability modeling framework. For illustrating the usefulness of this technique we analyze dependability of a railroad control computer.     

A workload-based analysis of software aging, and rejuvenation

We present a hierarchical model for the analysis of proactive fault management in the presence of system resource leaks. At the low level of the model hierarchy is a degradation model in which we use a nonhomogeneous Markov chain to establish an explicit connection between resource leaks, and the failure rate. With the degradation model, we prove that the failure rate is asymptotically constant in the absence of resource leaks, and it is increasing as leaks occur & accumulate, which confirms the resource leaks as an aging source. The proactive fault management (PFM) is modeled at the higher level as a semi-Markov process. The PFM model takes as input the degradation analysis from the low-level model, and allows us to determine optimal rejuvenation schedules with respect to various system measures.     

Cross-Layer Optimization of Multichannel Multiantenna WMNs

Use of multiple channels can significantly improve the throughput of wireless mesh networks (WMNs). Additionally, recent advances in radio technology have made it possible to realize software-defined radio (SDR), which is capable of switching from one channel to another dynamically. On the other hand, equipping wireless nodes with multiple antennas creates great potential for throughput improvement via interference suppression, spatial multiplexing, and spatial division multiple access techniques. In this paper, we investigate the joint optimization of routing and scheduling in multichannel WMNs, where nodes are equipped with a single SDR and multiple antenna elements. We analyze achievable throughput of these networks under four different multiantenna modes: single user single stream, single user multi stream, multi user single stream, and multi user multi stream, each mode integrates different combinations of multiantenna techniques. We mathematically model scheduling and interference constraints and formulate joint routing and scheduling optimization problem with the objective of maximizing the throughput by minimizing network schedule time such that traffic demands for a set of sessions are satisfied. A column generation-based decomposition approach is proposed to solve the problem. Simulation results are presented to evaluate the impact of number of antennas, number of channels, and number of sessions on the schedule time for the four proposed modes.     

Cross-Layer Design Using Superposition Coding Scheme for Multiuser OFDM Systems

In this paper, a cross-layer design is proposed for downlink of orthogonal frequency division multiplexing systems which uses superposition coding (SC) scheme. SC theorem allows two users to share the same subchannel. Firstly, the subchannel will be allocated to degraded user (who is far away from the base station) and then this subchannel will be allowed to be shared by potential user (who is near to the base station). We also employed a packet dependent scheduling at the medium access control layer which decides the transmission order of packets according to the delay, size and quality of service priority level of packets. The weight of each user can be calculated by summing up the weights of all packets in the queues of that user. We have considered that each user is having multiple heterogeneous traffic queues. Simulation results show that the algorithm proposed in this paper is better than the previously reported algorithm in terms of total throughput and packet delay with the same computational complexity.     

Non-parametric Blind Spectrum Sensing Based on Censored Observations for Cognitive radio

In cognitive radio, spectrum sensing is a challenging task. In this paper, a spectrum sensing method based on censored observations is proposed. We call it as Censored Anderson Darling (CAD) sensing. We present the performance of the CAD sensing method with receiver operating characteristics (ROC) in fading channels using simulations. It is observed that the proposed method outperforms the conventional energy detection (ED) at lower signal to noise ratio. It also provides better detection performance compared to Ordered Statistics (OS) based sensing method. We also use the CAD sensing method assuming noise uncertainty. It means noise variance at secondary user(SU) is unknown. We call it as Blind CAD sensing (B-CAD). We also show that the B-CAD outperforms the energy detection.     

Performance analysis of reservation media-access protocol with access and serving queues under bursty traffic in GPRS/EGPRS

Performance modeling of the contention-based reservation protocol in the general packet radio service (GPRS)/enhanced GPRS (EGPRS) under bursty traffic is of practical use in system design. Instead of using discrete event simulation, we construct an analytical model based on stochastic Petri net formalism to investigate GPRS uplink performance efficiently. The model is built to capture the major features of a realistic system, taking into account factors such as the traffic profile, the system's capture capability, the contention persistence, the multislot capability, and the use of the access queue and the round-robin virtual serving queue. As shown in numerical results, these factors have different impacts on the performance measures, such as link-control-layer frame-blocking probability, frame delay, system throughput, and packet-channel utilization. To avoid modeling and computational complexity in a GPRS/EGPRS system model that supports several mobile terminals, we make first-order approximations and use an iterative fixed-point scheme. Comparisons with discrete event simulations are conducted and show promising agreement with the proposed analytical-numerical model. We then use the model to investigate GPRS packet-data performance under different parameter settings and present numerical results, providing insights into network design and optimization.     

Annealing studies of undoped Hg1−xMnx Te bulk crystals at high temperatures

The effect of high temperature annealing treatments in varying mercury atmospheres on Hg_1−xMn_xTe crystals with long wavelength infrared/very long wavelength infrared cut off wavelengths has been studied. The undoped Hg_1−xMn_xTe crystals were grown using the traveling heater method with a tellurium solvent zone, and composition was verified by infrared transmission measurements. The crystals were subjected to annealing temperatures of 500 and 550°C under mercury pressures varying from Hg-rich conditions to Te-rich conditions. The samples were either air cooled or water cooled to room temperature. Hall effect measurements were carried out at 77K at magnetic fields varying from 500 Gauss to 10 kGauss. The hole concentration in the annealed crystals was found to be roughly inversely proportional to the partial pressure of Hg indicating that the material is essentially intrinsic at the anneal temperature. A defect model and a relationship between the mass action constants for the native acceptor defects of HgMnTe are presented.