InGaAs MOMBE -- system drift and material quality

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29659/1/0000748.pd

Technical aspects of InGaAs MOMBE -- shutter action, system drift, and material quality

Lattice matched InxGa1 - x As films were deposited on InP substrates using metalorganic molecular beam epitaxy (MOMBE) with trimethylindium (TMIn), triethylgallium (TEGa) and a solid arsenic source. The effect of growth temperature and molecular beam composition on growth rate and crystal composition was investigated. A long term drift of the molecular beam composition and an increasing difference between temperature readings of the thermocouple and the pyrometer were observed. The corrected data show a linear dependence of crystal composition on molecular beam composition. Shutter action on TMIn and TEGa was investigated. The results show the adverse effect of solely using the shutters to control the metalorganic molecular beam, leading to inferior material quality and rough surface morphology. Material grown in the optimized process consistently showed electron mobilities of [mu]300 [approximate] 9000 cm2/V[middle dot]s and [mu]77 [approximate] 35,000 cm2/[middle dot]s at 1.2 x 1015 cm-3 n-type background concentration.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29421/1/0000497.pd

Transient growth analysis of the flow past a circular cylinder

We apply direct transient growth analysis in complex geometries to investigate its role in the primary and secondary bifurcation/transition process of the flow past a circular cylinder. The methodology is based on the singular value decomposition of the Navier-Stokes evolution operator linearized about a two-dimensional steady or periodic state which leads to the optimal growth modes. Linearly stable and unstable steady flow at Re=45 and 50 is considered first, where the analysis demonstrates that strong two-dimensional transient growth is observed with energy amplifications of order of 10(3) at U-infinity tau/D approximate to 30. Transient growth at Re=50 promotes the linear instability which ultimately saturates into the well known von-Kaacutermaacuten street. Subsequently we consider the transient growth upon the time-periodic base state corresponding to the von-Kaacutermaacuten street at Re=200 and 300. Depending upon the spanwise wavenumber the flow at these Reynolds numbers are linearly unstable due to the so-called mode A and B instabilities. Once again energy amplifications of order of 10(3) are observed over a time interval of tau/T=2, where T is the time period of the base flow shedding. In all cases the maximum energy of the optimal initial conditions are located within a diameter of the cylinder in contrast to the spatial distribution of the unstable eigenmodes which extend far into the downstream wake. It is therefore reasonable to consider the analysis as presenting an accelerator to the existing modal mechanism. The rapid amplification of the optimal growth modes highlights their importance in the transition process for flow past circular cylinder, particularly when comparing with experimental results where these types of convective instability mechanisms are likely to be activated. The spatial localization, close to the cylinder, of the optimal initial condition may be significant when considering strategies to promote or control shedding

Spin-based quantum information processing with semiconductor quantum dots and cavity QED

A quantum information processing scheme is proposed with semiconductor quantum dots located in a high-Q single mode QED cavity. The spin degrees of freedom of one excess conduction electron of the quantum dots are employed as qubits. Excitonic states, which can be produced ultrafastly with optical operation, are used as auxiliary states in the realization of quantum gates. We show how properly tailored ultrafast laser pulses and Pauli-blocking effects, can be used to achieve a universal encoded quantum computing.Comment: RevTex, 2 figure

InGaAs/InP hot electron transistors grown by chemical beam epitaxy

In this letter, we report on the dc performance of chemical beam epitaxy grown InGaAs/InP hot electron transistors (HETs). The highest observed differential β (dIC/dIB) is over 100. The HETs have Pd/Ge/Ti/Al shallow ohmic base contacts with diffusion lengths less than 300 Å. Furthermore, we also demonstrated ballistic transport of electrons in an InGaAs/InP HET by obtaining an energy distribution of electrons with ∼60 meV full width at half maximum. The measured conduction band discontinuity of InGaAs/InP is 250.3 meV, which is 39.8% of the band gap difference.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70748/2/APPLAB-61-2-189-1.pd

Pharmacokinetics and pharmacodynamics of antifungals in children and their clinical implications

Invasive fungal infections are a significant cause of morbidity and mortality in children. Successful management of these systemic infections requires identification of the causative pathogen, appropriate antifungal selection, and optimisation of its pharmacokinetic and pharmacodynamic properties to maximise its antifungal activity and minimise toxicity and the emergence of resistance. This review highlights salient scientific advancements in paediatric antifungal pharmacotherapies and focuses on pharmacokinetic and pharmacodynamic studies that underpin current clinical decision making. Four classes of drugs are widely used in the treatment of invasive fungal infections in children, including the polyenes, triazoles, pyrimidine analogues and echinocandins. Several lipidic formulations of the polyene amphotericin B have substantially reduced the toxicity associated with the traditional amphotericin B formulation. Monotherapy with the pyrimidine analogue flucytosine rapidly promotes the emergence of resistance and cannot be recommended. However, when used in combination with other antifungal agents, therapeutic drug monitoring of flucytosine has been shown to reduce high peak flucytosine concentrations, which are strongly associated with toxicity. The triazoles feature large inter-individual pharmacokinetic variability, although this pattern is less pronounced with fluconazole. In clinical trials, posaconazole was associated with fewer adverse effects than other members of the triazole family, though both posaconazole and itraconazole display erratic absorption that is influenced by gastric pH and the gastric emptying rate. Limited data suggest that the clinical response to therapy may be improved with higher plasma posaconazole and itraconazole concentrations. For voriconazole, pharmacokinetic studies among children have revealed that children require twice the recommended adult dose to achieve comparable blood concentrations. Voriconazole clearance is also affected by the cytochrome P450 (CYP) 2C19 genotype and hepatic impairment. Therapeutic drug monitoring is recommended as voriconazole pharmacokinetics are highly variable and small dose increases can result in marked changes in plasma concentrations. For the echinocandins, the primary source of pharmacokinetic variability stems from an age-dependent decrease in clearance with increasing age. Consequently, young children require larger doses per kilogram of body weight than older children and adults. Routine therapeutic drug monitoring for the echinocandins is not recommended. The effectiveness of many systemic antifungal agents has been correlated with pharmacodynamic targets in in vitro and in murine models of invasive candidiasis and aspergillosis. Further study is needed to translate these findings into optimal dosing regimens for children and to understand how these agents interact when multiple antifungal agents are used in combination

The growth of InAlP using trimethyl amine alane by chemical beam epitaxy

The growth of InAlP and related compounds such as InGaP lattice matched to GaAs has attracted a great deal of interest for optoelectronic devices emitting in the range from 638 to 700 nm and for electronic devices such as the heterojunction bipolar transistor. Although some gas source MBE work has been performed in this material system, very little CBE work has been done, largely attributable to the lack of a suitable aluminum source. This is the first report of trimethyl amine alane (TMAA) being used to grow InAlP. TMAA offers advantages of less carbon incorporation and less oxygen sensitivity compared to triethyl aluminum, tri-isobutyl aluminum, or trimethyl aluminum. Trimethyl amine alane has been used to grow AlGaAs HBTs and more recently to grow InAlAs/InGaAs HEMTs by CBE. One of the principal strengths of CBE is its ability to handle phosphorus based compounds efficiently, offering excellent interface control. InAlP films with carbon concentrations below 7 x 1017 cm-3 lattice matched with good surface morphology have been grown. Double crystal X-ray diffraction exhibits a single epi-peak with a full width at half max of 46 arc sec with multiple Pendellosung fringes. The epitaxial films are semi-insulating, completely depleted for thicknesses up to 1.6 [mu]m. Oxygen levels measured by secondary ion mass spectroscopy are comparable to levels measured in InAlAs films (~2.5 x 1018 cm-3) lattice matched to InP. The likely source of this oxygen is the hydride precursor as has been shown for the growth of InAlAs. As the substrate temperature is raised, the films become increasingly indium-rich. Breaking the growth rate down into its constituent binaries indicates an enhanced TMI incorporation rate. The quality of the films as measured by X-ray full width at half max and the surface morphology is extremely sensitive to substrate temperature. A very narrow window exists for the growth of good quality material in the range from 535 to 545[deg]C.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30971/1/0000644.pd

Strong and auxiliary forms of the semi-Lagrangian method for incompressible flows

We present a review of the semi-Lagrangian method for advection-diusion and incompressible Navier-Stokes equations discretized with high-order methods. In particular, we compare the strong form where the departure points are computed directly via backwards integration with the auxiliary form where an auxiliary advection equation is solved instead; the latter is also referred to as Operator Integration Factor Splitting (OIFS) scheme. For intermediate size of time steps the auxiliary form is preferrable but for large time steps only the strong form is stable

Making the user more efficient: Design for sustainable behaviour

User behaviour is a significant determinant of a product’s environmental impact; while engineering advances permit increased efficiency of product operation, the user’s decisions and habits ultimately have a major effect on the energy or other resources used by the product. There is thus a need to change users’ behaviour. A range of design techniques developed in diverse contexts suggest opportunities for engineers, designers and other stakeholders working in the field of sustainable innovation to affect users’ behaviour at the point of interaction with the product or system, in effect ‘making the user more efficient’. Approaches to changing users’ behaviour from a number of fields are reviewed and discussed, including: strategic design of affordances and behaviour-shaping constraints to control or affect energyor other resource-using interactions; the use of different kinds of feedback and persuasive technology techniques to encourage or guide users to reduce their environmental impact; and context-based systems which use feedback to adjust their behaviour to run at optimum efficiency and reduce the opportunity for user-affected inefficiency. Example implementations in the sustainable engineering and ecodesign field are suggested and discussed