Systematic characterization of Cl2 reactive ion etchingfor improved ohmics in AlGaN/GaN HEMTs

Pre-metal-deposition reactive ion etching (RIE) was performed on an Al_0.3Ga_0.7N/AlN/GaN heterostructure in order to improve the metal-to-semiconductor contact resistance. An optimum AlGaN thickness for minimizing contact resistance was determined. An initial decrease in contact resistance with etching time was explained in terms of removal of an oxide surface layer and/or by an increase in tunnelling current with the decrease of the AlGaN thickness. The presence of a dissimilar surface layer was confirmed by an initial nonuniform etch depth rate. An increase in contact resistance for deeper etches was experienced. The increase was related to depletion of the two-dimensional (2-D) electron gas (2-DEG) under the ohmics. Etch depths were measured by atomic force microscopy (AFM). The contact resistance decreased from about 0.45 Ωmm for unetched ohmics to a minimum of 0.27 Ωmm for 70 Å etched ohmics. The initial thickness of the AlGaN layer was 250 Å. The decrease in contact resistance, without excessive complications on device processing, supports RIE etching as a viable solution to improve ohmic contact resistance in AlGaN/GaN HEMTs     

A series active power filter based on a sinusoidalcurrent-controlled voltage-source inverter

A series active power filter working as a sinusoidal current source, in-phase with the mains voltage, has been developed and tested. The amplitude of the fundamental current in the series filter is controlled through the error signal generated between the load voltage and a pre-established reference. The control allows an effective correction of power factor, harmonic distortion and load voltage regulation. Compared with previous methods of control developed for series active filters, this method is simpler to implement because it is only required to generate a sinusoidal current, in-phase with the mains voltage, the amplitude of which is controlled through the error in the load voltage. The proposed system has been studied analytically and tested using computer simulations and experiments. In the experiments, it has been verified that the filter keeps the line current almost sinusoidal and in-phase with the line voltage supply. It also responds very quickly under sudden changes in load conditions, reaching its steady-state in about two cycles of the fundamental     

Film cracking and debonding in a coated fiber

A fracture mechanics based methodology for the determination of interface fracture toughness from crack spacing in a thin coated fiber is presented. The coating (film) may be regarded as the matrix material in typical experiments employing this configuration. Matrix crack spacing is considered to be the result of a competitive process between matrix segmentation and interface debonding which are assumed to be governed by critical energy release rate criteria. Matrix cracks are assumed to form by the process of channeling in the circumferential direction and steady state conditions are assumed at the matrix crack front in the channeling direction. Energy release rates are determined using domain integral procedures in conjunction with the finite element method. The minimum crack spacing is obtained as a function of applied stress for different values of interface fracture toughness. A methodology to relate the saturated crack spacing to interface fracture toughness is developed. Interfaces are classified into three categories: weak, intermediate and strong. It is shown that in experiments of this type, quantitative information about the interface fracture toughness can be obtained for intermediate interfaces while qualitative information may be obtained for weak and strong interfaces.     

The mechanics of compressive kinking in unidirectional fiber reinforced ductile matrix composites

Three distinct stages of kink band formation and propagation exist in ductile matrix composites subjected to compressive loading. These stages are called incipient kinking, transient kinking and kink band broadening. Each stage involves a different deformation mode. The mechanics governing each stage are discussed. Incipient kinking, where the peak load is attained, and kink band broadening, where the load attains a steady-state, are important in structural design. Two design philosophies are presented. References to pertinent literature are made throughout.     

Reactive Power Compensation Technologies: State-of-the-Art Review

This paper presents an overview of the state of the art in reactive power compensation technologies. The principles of operation, design characteristics and application examples of Var compensators implemented with thyristors and self-commutated converters are presented. Static Var generators are used to improve voltage regulation, stability, and power factor in ac transmission and distribution systems. Examples obtained from relevant applications describing the use of reactive power compensators implemented with new static Var technologies are also described.     

Monte Carlo Simulations of High-Performance Implant Free In0.3Ga 0.7As Nano-MOSFETs for Low-Power CMOS Applications

The potential performance of implant free heterostructure In_0.3Ga_0.7As channel MOSFETs with gate lengths of 30, 20, and 15 nm is investigated using state-of-the-art Monte Carlo (MC) device simulations. The simulations are carefully calibrated against the electron mobility and sheet density measured on fabricated III-V MOSFET structures with a high-kappa dielectric. The MC simulations show that the 30 nm gate length implant free MOSFET can deliver a drive current of 2174 muA/mum at 0.7 V supply voltage. The drive current increases to 2542 muA/mum in the 20 nm gate length device, saturating at 2535 muA/mum in the 15 nm gate length one. When quantum confinement corrections are included into MC simulations, they have a negligible effect on the drive current in the 30 and 20 nm gate length transistors but lower the 15 nm gate length device drive current at 0.7 V supply voltage by 10%. When compared to equivalent Si based MOSFETs, the implant free heterostructure MOSFETs can deliver a very high performance at low supply voltage, making them suitable for low-power high-performance CMOS applications     

Spray forming technology for military applications

Over the past five years, the Annapolis Detachment, Carderock Division, Naval Surface Warfare Center (formerly David Taylor Research Center) has established a state- of- the- art spray forming facility for the study and exploitation of this near net shape manufacturing process. Spray forming is a single step gas atomization/deposition process which yields ferrous and nonferrous, near final shape, near fully dense preforms that has proven to be a viable and cost effective alternative to conventional metalworking technology for the production of material preforms with properties surpassing those of their cast and wrought counterparts. Current programs at Carderock Divison Naval Surface Warfare Center are aimed at optimization of the process, certification of the spray formed products and industrialization of the technology.     

Chiroptical activity of holmium pyrogermanate: tetragonal Ho2Ge2O7

Chiroptical luminescence and circular dichroism measurements are reported for single crystals of Ho₂Ge₂O₇. These crystals belong to the tetragonal space group P4,2,2 (or P4₃2₁2) with Z=4. Each Ho³⁺ ion in the crystal structure is coordinated to seven oxygens to form a distorted pentagonal bipyramid. The Ho³⁺ ions exhibit luminescence from several excited multiplet levels; chiroptical luminescence spectra are reported for ⁶I₈ → ⁶F₆, ⁶S₂, and ⁶F₃ at a sample temperature of 10 K. Room-temperature absorption and circular dichroism measurements are reported for the ⁶I₈ → ⁶F₆, ⁵S₂, ⁶F₄, and ⁶F₃ transition regions.