InGaAsP/InGaAs heterojunction p-i-n detectors with low dark current and small capacitance for 1.3?1.6 ?m fibre optic systems

A new long wavelength p-i-n photodetector, consisting of an In0.53 Ga0.47 As absorbing layer and an adjacent InGaAsP p-n junction is demonstrated. These diodes exhibit dark currents as low as 0.2 nA and a capacitance < 0.5 pF at ? 10 V for a device area of 1.3 × 10?4 cm2. The external quantum efficiency is ? 60% at ? = 1.3 ?m for front illumination. A systematic study of the background doping of the quaternary layers using different InP sources is also reported.     

The generation of point defects in GaAs by electron-hole recombination at dislocations

The degradation of GaAs heterojunction lasers results in the formation of long dislocation dipoles which grow by a climb process involving point defect concentrations of the order of 10¹⁹ cm⁻³. The driving force for this climb process is not understood and it has been suggested that the material contains a supersaturation of native interstitials which condense on the dislocation during device operation. An alternative model proposed that the driving force is related to the energy released by electron-hole recombination on the dislocation which is partially dissipated by the dislocation emitting vacancies into the surrounding lattice.Gallium arsenide substrates containing greater than 2 × 10¹⁸ tellurium atoms cm⁻³ contain interstitial concentrations of the order of 10¹⁷ cm⁻³ which condense out to form small dislocation loops during an anneal at 880°C. The presence of these loops indicates that the annealed material does not contain excess interstitials in solution. This annealed material was optically pumped and examined by transmission electron microscopy. It was found that the loops developed into dipoles typical of degraded lasers. The number of point defects involved in this climb process increased with increasing pumping power and time. These results are discussed in terms of the two mechanisms listed above and it is concluded that the energy released by electron-hole recombination at the dislocation provides the driving force for the climb process.     

A comparison of the morphology of crazes formed in thin films and in bulk specimens of polystyrene

The microstructure of crazes formed in solvent-cast thin films have been studied by transmission electron microscopy. The results have been compared with the microstructure of crazes determined from the examination of replicas obtained from the fracture surfaces of bulk specimens. The structure of crazes formed in thin films and in the bulk have been shown to be similar with the exception that much finer fibril structures are observed in thin films at large deformations. A model of the microstructure of a craze is presented.     

Latex-coated polymeric monolithic ion-exchange stationary phases. 1. Anion-exchange capillary electrochromatography and in-line sample preconcentration in capillary electrophoresis

A sulfonated methacrylate monolithic polymer has been synthesized inside fused-silica capillaries of diameters 50-533-microm i.d. and coated with 65-nm-diameter fully functionalized quaternary ammonium latex particles (AS18, Dionex Corp.) to form an anion-exchange stationary phase. This stationary phase was used for ion-exchange capillary electrochromatography of inorganic anions in a 75-microm-i.d. capillary with Tris/perchlorate electrolyte and direct UV detection at 195 nm. Seven inorganic anions (bromide, nitrate, iodide, iodate, bromate, thiocyanate, chromate) could be separated over a period of 90 s, and the elution order indicated that both ion exchange and electrophoresis contributed to the separation mechanism. Separation efficiencies of up to 1.66 x 10(5) plates m(-1) were achieved, and the monoliths were stable under pressures of up to 62 MPa. Another latex-coated monolith in a 250-microm-i.d. capillary was used for in-line preconcentration by coupling it to a separation capillary in which the EOF had been reversed using a coating of either a cationic polymer or cationic latex particles. Several capillary volumes of sample were loaded onto the preconcentration monolith, and the analytes (inorganic anions) were then eluted from the monolith with a transient isotachophoretic gradient before being separated by electrophoresis in the separation capillary. Linear calibration curves were obtained for aqueous mixtures of bromide, nitrite, nitrate, and iodide. Recoveries of all analytes except iodide were reduced significantly when the sample matrix contained high levels of chloride. The preconcentration method was applied to the determination of iodide in open ocean water and provided a limit of detection of 75 pM (9.5 ng/L) calculated at a signal-to-noise ratio of 3. The relative standard deviation for migration time and peak area for iodide were 1.1 and 2.7%, respectively (n = 6). Iodide was eluted as an efficient peak, yielding a separation efficiency of 5.13 x 10(7) plates m(-1). This focusing was reproducible for repeated analyses of seawater.     

A balanced approach to process design by computer

Process designers can profit from the use of two classes of general programs at present available to them. These are: simple mass balance programs and the larger flowsheeting and simulation programs. Both have their uses as proven packages. It is important that the limitations of each type of program should be clearly understood in order that they are used to their full effectiveness.     

Water-soluble copolymers. V. Compositional determination of random copolymers of acrylamide with sulfonated comonomers by infrared spectroscopy and C13 nuclear magnetic resonance

Compositional analyses of random copolymers of acrylamide with sodium-2-sulfoethyl methacrylate and with sodium-2-acrylamido-2-methylpropane sulfonate have been performed utilizing elemental analysis, infrared spectroscopy, and C¹³ NMR. A mathematical method involving nitrogen to sulfur ratios has been developed to eliminate errors which often arise from associated water in hydrophilic copolymers. The ratios of infrared absorbances at 1043 cm^?1 and 1664 cm^?1 were used to determine the composition of AM—NaAMPS copolymers; likewise, absorbance ratios at 1040 cm^?1 and 3200 cm^?1 were utilized for determination of the AM—NaSEM copolymer composition. Carbon-13 NMR spectra of the copolymers were also obtained, and characteristic peaks were utilized to quantitate copolyer composition. The results obtained by elemental analyses, infrared spectroscopic studies, and NMR were in excellent agreement. These techniques are of great value for determining composition of hygroscopic copolymers which can crosslink or form strong intramolecular hydrogen bonds, often precluding further characterization.     

An investigation of the microstructure and mechanical properties of high density polyethylene spherulites

Scanning transmission electron microscopy and related techniques have been used to characterize the micromorphologies of high-density polyethylene spherulites produced under different crystallization conditions. The experimental techniques used in the investigation are summarized, as are some relationhips between spherulite morphology, crystallization conditions and mechanical properties. A preliminary model for the micromorphology of banded polyethylene spherulites is proposed.