An investigation into the temperature sensitivity of strained andunstrained multiple quantum-well, long wavelength lasers: new insightand methods of characterization

The magnitude of the temperature rate of change of the threshold current density (J_th) is examined with respect to J_th , for a variety of unstrained and strained, long wavelength multiple quantum-well (MQW) lasers. A strong correlation is found between these parameters, and a new relationship describing the J_th -T relationship for these lasers is arrived at in terms of two new essentially temperature and length independent constants. A third constant, T_max, also appears which estimates the theoretical maximum operating temperature of the laser. It is proposed that these constants may prove to be more useful in characterizing the temperature sensitivity of semiconductor lasers than the conventional parameters T ₀ and I₀ which exhibit both a length and temperature dependence. Furthermore, an expression is found which relates the magnitude of T_max to adjustable device structural and material parameters, such as: the cavity length, L; facet reflectivity, R; transparency current density, J_tr; and, the modal gain coefficient, β. It is revealed that a close examination of this relationship may provide new insight into the physics of semiconductor lasers as well as a means for optimizing device design to obtain a high maximum operating temperature in order to eliminate the need for thermoelectric coolers in device packaging. Finally, the measured T_max, versus L characteristics of six different strained and unstrained MQW laser structures are presented     

Double-heterojunction bipolar transistors in InP/GaInAs grown by metal organic chemical vapour deposition

Double-heterojunction bipolar transistor structures in InP/GaInAs have been grown by low-pressure metal organic chemical vapour deposition. Good control of the Zn dopant in the GaInAs base layer was achieved, and devices with current gains up to 300 at current densities of 1.4kA/cm2 have been demonstrated.     

Aphysiologic performance on dynamic posturography

The remarkable ability of the body to maintain balance is the result of central nervous system integration of sophisticated inputs from the vestibular, visual, and somatosensory systems. Strategies by patients with balance dysfunction are aphysiologic when their performance is relatively better on more difficult conditions of sensory conflict than on easier ones. Twenty-two aphysiologic patterns on computerized dynamic posturography were compared with age-matched normal and vestibular patterns. The aphysiologic group performed significantly better than the patients in the vestibular dysfunction group on the most difficult subtests of computerized dynamic posturography, conditions 5 and 6, yet significantly poorer on the easier subtests, conditions 1 through 4. In addition, patients in the aphysiologic group tended to show greater intertrial variability compared with patients in both normal and vestibular system dysfunction groups. A stepwise linear discriminant analysis was used to determine a set of conditions that had significant value in discriminating between the three patient groups. Case studies are presented to further illustrate the clinical usefulness of computerized dynamic posturography testing in the evaluation of patients suspected of having a functional component to their on-feet balance problems.     

The design of a user friendly engineering economy analysis package for a microcomputer

The design and features of a user friendly engineering economy analysis software package for the Apple II microcomputer is described. The software package is written in Applesoft Basic. An Apple II with 64k RAM, one or two disk drives and a line printer is required to use the software package. The package consists of five complementary programs: (1) Selection of the better of two alternatives using present-worth evaluations. (2) Conventional rate-of-return computations for a single project. (3) Selection from mutually exclusive alternatives using rate-of-return analysis. (4) Optimal capital allocation among several investment opportunities under risk. (5) Probabilistic after-tax economic analysis for a single project. All programs but (4) are based on the time value of money. Program (4) is a chance-constrained optimization model utilizing the Lagrange-multiplier technique. Program (5) implements depreciation, debt/equity financing, and taxes and performs a sequential after-tax Monte Carlo simulation over a specified life-span. The use of this software package does not require any previous computer or programming experience. A menu format is used. The software package allows the user to specify the shapes of the interest and inflation probability distributions on a year by year basis. The user has the option to specify the reinvestment rates. The package presents the results in tables and graphs. User acceptance has been excellent with this software package and this package is well suited for the use in an engineering economy class or for the practicing industrial engineer in industry.     

Performance comparison of AlGaAs,GaAs and InGaP tunnel junctions for concentrated multijunction solar cells

Four tunnel junction (TJ) designs for multijunction (MJ) solar cells under high concentration are studied to determine the peak tunnelling current and resistance change as a function of the doping concentration. These four TJ designs are: AlGaAs/AlGaAs, GaAs/GaAs, AlGaAs/InGaP and AlGaAs/GaAs. Time‐dependent and time‐average methods are used to experimentally characterize the entire current–voltage profile of TJ mesa structures. Experimentally calibrated numerical models are used to determine the minimum doping concentration required for each TJ design to operate within a MJ solar cell up to 2000‐suns concentration. The AlGaAs/GaAs TJ design is found to require the least doping concentration to reach a resistance of <10⁻⁴ Ω cm² followed by the GaAs/GaAs TJ and finally the AlGaAs/AlGaAs TJ. The AlGaAs/InGaP TJ is only able to obtain resistances of ≥5 × 10⁻⁴ Ω cm² within the range of doping concentrations studied. Copyright © 2010 John Wiley & Sons, Ltd.     

The Pyloritek test and the CLO test: accuracy and incremental cost analysis

Malpractice lawsuits affect most physicians at some point in their career. Proving that malpractice has been committed is based on substantiation of a variety of elements, including that the patient was rendered care that was "below the standard" of care. While many physicians believe that the "standard" will be judged objectively on the basis of published scientific sources and accepted conventions, the standard is established rather by the testimony of expert witness(es). It is the expert testimony that sets the standard and is proof of the standard. The testimony is open for acceptance or rejection by the judge or jury for a variety of nonscientific reasons. We review what the defendant doctor might expect regarding proof required to establish breach of the standard of care and what the prudent expert should be obliged to demonstrate.     

Characterization of transient oxide formation on CoNiCrAlY after heat treatment in vacuum and air

The application of thin film sensors on hot section components of gas turbine engines requires the formation a specific and complex layered structure that is functional, reliable, and durable. In the case of conducting substrates, such as the nickel- or cobalt-based superalloys, an electrically-insulating layer (typically aluminum oxide) is required between the sensing layer elements and the substrate. In order to achieve suitable bonding of alumina to the substrate, a MCrAlY type bond coat is commonly applied to the metallic substrate and the subsequent formation of dense, adherent oxide(s) on the MCrAlY coating surface can ultimately determine the durability of the layered structure. In this work, test specimens consisting of a high-velocity oxy-fuel (HVOF) CoNiCrAlY coated nickel-base substrate were subjected to selected vacuum and air furnace heat treatments in order to observe the effects of the heat treatments on the transient oxide formation on the CoNiCrAlY surface. The surfaces and cross-sectional features of the test specimens were examined using scanning electron microscope (SEM) and energy dispersive spectrometry analysis (EDS), together with the X-ray diffraction (XRD) analysis. Of the heat treatment conditions examined, the vacuum heat treated CoNiCrAlY specimens exhibited the formation of a denser alumina layer while the air furnace heat treatments resulted in a more ridge-like alumina formation on the surface. Increasing heat treatment temperature and time reduced the amount of transition metal oxide and encouraged more uniform alumina formation.     

Preparation of near-infrared absorbing composites comprised of conjugated macroligands on the surface of PbS nanoparticles

We report a facile macroligand strategy towards the synthesis of low-bandgap inorganic-organic composites comprised of semiconductor PbS nanoparticles and functional copolymers. For this, thiol-functional thiophene-based macroligands have been used as coligands for PbS nanoparticles. Thus, solution processable organic-inorganic hybrid materials with absorption in the near-infrared have been obtained. The resulting nanoparticle-polymer composites were characterized in detail by optical and FT-IR spectroscopy as well as TEM showing their potential as novel functional inorganic-organic hybrid materials when applied in initial proof-of-concept hybrid photovoltaic devices.     

The inverted horizontal reactor: Growth of uniform InP and GaInAs by LPMOCVD

The uniformity of InP and GalnAs lattice matched to InP has been studied for three different types of horizontal reactors. Employing the conventional approach,i.e. the susceptor is positioned in the centre or at the bottom of the reactor cell with the wafer surface facing upward, the uniformity in film thickness and ternary composition perpendicular to the direction of gas flow could be substantially improved by using an obstacle which spreads the incoming gas over the entire width of the reactor. In the direction of flow however, a compositional grading of the GalnAs films was observed. This gradient could be avoided by using a novel design in which the substrate is located at the highest and hottest point of the reaction cell and facing downwards.