Dislocation nucleation mechanism and doping effect in p-type ZnSe/GaAs

Nitrogen doped ZnSe/GaAs heterostructures grown at 150 and 250°C were studied by transmission électron microscopy (TEM). The density of threading dislocations and the interfacial dislocation structure in ZnSe/GaAs heterostructures are related to the N-doping concentration. In addition, in-situ TEM heating studies show that Frank partial dislocations formed below critical thickness in N-doped ZnSe/GaAs are the sources for nucleation of a regular array of misfit dislocations at the ZnSe/GaAs interface. By the dissociation of the Frank partial dislocations and interaction reactions between the dislocations, the 60° misfit dislocations form. The Frank partial dislocations bound stacking faults which usually form in pairs at the film/substrate interface. The density of stacking faults increases with increasing N-doping concentration. Thus, at high N-doping levels, the dislocation nucleation sources are close together and not all of the Frank partial dislocations dissociate, so that a high density of threading dislocations results in samples with high N-doping concentrations. The high density of threading dislocations in the ZnSe film are found to be associated with a reduction or saturation of the net carrier density.     

Thermodynamics of heat storage in a PCM shell-and-tube heat exchanger in parallel or in series with a heat engine

This paper addresses the thermodynamics of heat storage in a PCM shell-and-tube heat exchanger, which in a solar power plant is provided to damp the pulsed behaviour of the heat source. Unequal duration of the active and passive phases of the heat source, and consequently of the heat storage and discharge, is allowed. Two alternative schemes of connection of the storage element to the heat engine, referred to as the series and the parallel set-ups, are considered. Using the general framework developed in our previous work, we reformulate the second-law efficiencies for each scheme in terms of a few dimensionless parameters, reflecting the operating conditions of the system and the relevant heat-transfer characteristics of the storage element. For the particular storage system studied in this paper, i.e. the PCM shell-and-tube heat exchanger executing periodic storage-removal cycles, the latter characteristics are determined numerically. A parametric study is conducted to compare the series and parallel schemes with regard to various design objectives.     

CAMBIO: software for modelling and simulation of bioprocesses

CAMBIO, a software package devoted to bioprocess modelling, which runs on Apollo computers, is described. This software enables bioengineers to easily and interactively design appropriate mathematical models directly from their perception of the process. CAMBIO provides the user with a set of design symbols and mnemonic icons in order to interactively design a functional diagram. This diagram has to exhibit the most relevant components with their related interactions through biological and physico-chemical reactions. Then, CAMBIO automatically generates the dynamical material balance equations of the process in the form of an algebraic-differential system by taking advantage of the knowledge involved in the functional diagram. The model may be used for control design purpose or completed by kinetics expressions with a view to simulation. CAMBIO offers facilities to generate a simulation model (for coding of kinetics, introducing auxiliary variables, etc.). This model is automatically interfaced with a specialized simulation software which allows an immediate visualization of the process dynamical behaviour under various operational conditions (possibly involving feedback control strategies). An example of an application dealing with yeast fermentation is given.     

Order and metric in the stream semantics of elemental concurrency

Summary Two denotational semantics for a language with simple concurrency are presented. The language has parallel composition in the form of the shuffle operation, in addition to the usual sequential concepts including full recursion. Two linear time models, both involving sets of finite and infinite streams, are given. The first model is order-theoretic and based on the Smyth order. The second model employs complete metric spaces. Various technical results are obtained relating the order-theoretic and metric notions. The paper culminates in the proof that the two semantics for the language considered coincide. The paper completes previous investigations of the same language, establishing the equivalence of altogether four semantic models for it.     

The Role of Adhesion Promoters on the Molecular and Mesoscopic Structure of the Interphase

New experimental results are described from Near Edge X-Ray Absorption Fine Structure (NEXAFS) spectroscopy, X-ray photoelectron spectroscopy (XPS) and Atomic Force Microscopy (AFM) on the molecular orientation, chemical composition, and topography of PMDA/ODA Polyimide (PI) deposited by the Langmuir-Blodgett (LB) technique onto Si (100) surfaces covered with a native oxide (SiO_x) and onto SiO_x substrates pre-treated with a variety of alkylsilanes. The alkylsilane monolayers used as adhesion promoters were found to be chemically and structurally stable during temperature treatments used to imidize the polymer precursor films. On both the oxidized silicon surface and on the silane-treated surfaces, we find a thickness-dependent, preferential orientation of the pyromellitimide unit in PI, but randomly-oriented oxydianiline fragments. We can not identify any specific chemical bonds between the polymer and the aminosilane film or, in the case of the untreated surface, the SiO_x substrate. However, a quantitative analysis of the NEXAFS data reveals a pronounced deficit in oxydianiline in the monolayer (ML) films, which decreases with film thickness, and a higher degree of imidization on the 3-aminopropyltrimethoxysilane (APS) surface indicating that imide linkages are formed between the polymer and the aminosilane. The AFM images show a heterogeneous surface topography with nucleation of PI polycondensates on both the SiO_x and aminosilane-treated surfaces. On the aminosilane surface additional nuclei are detected, believed to be polycondensates of aminosilane formed during imidization of the PI precursor. Nucleation of aminosilane polycondensates is not observed in the absence of polyamic acid. As suggested by a comparison of Lateral Force Microscopy (LFM) and Atomic Force Microscopy measurements, these aminosilane nuclei are covered by PI with increasing film thickness and, hence, provide mechanical connections between the polymer and substrate on a mesoscopic scale. Delamination experiments of 2 μm thick PI films spun onto ω-amino and methyl terminated alkylsilanes indicate that bond fracture always occurs in the aminosilane/PI and alkylsilane/PI and interphase. We conclude that the adhesion promotion effect of LB-deposited polyimide films by aminosilanes can be explained by chemical stabilization of the interface against decomposition and mechanical interlinking surface via polycondensates of aminosilane on a mesoscopic length scale. Our study is not conclusive as to whether covalent bonding of the polymer to the substrate via the silane is important for macroscopic adhesion.     

A high-density, four-channel, OEIC transceiver module utilizingplanar-processed optical waveguides and flip-chip, solder-bumptechnology

An optoelectronic transceiver module is described consisting of a four-channel AlGaAs integrated laser/monitor transmitter and a four-channel GaAs MESFET integrated detector/preamp receiver. The optoelectronic chips are flip-chip, solder-bump bonded to a substrate containing electrical wiring and planar-processed optical waveguides. The optical waveguide layer serves two purposes: the routing of optical signals, as well providing mechanical registrations for the optoelectronic chips and fiber-optic ribbon connector. The work described here demonstrates one approach to high-density, optoelectronic array packaging compatible with existing high-performance electronic packaging technology