LEC不掺杂半绝缘GaAs的热处理特性

本文对LEC不掺杂SI-GaAs的热处理特性进行了实验研究和分析。结果表明,As、Ga原子从表面挥发及缺陷的内扩散引起表面热蚀和热致导电层的形成。文章讨论了它们对注入层激活行为的影响。指出,采用本文设计的高温快速热处理结合压盖、InAs作过砷压源技术,可以获得满足注硅材料退火要求的SI-GaAs热处理特性。     

A new approach to the prediction of RP-HPLC retention times in gradient elution from isocratic data

Summary A general chromatographic model has been set up starting from a set of equations based on the concept of the velocity of a solute along the column. The composition of the mobile phase is taken into account solely as a numerical factor entering into suitable equations and totally independent of the chemical-properties of the constituents. A few isocratic experimental runs are necessary as input data, and subsequently a small amount of computational effort is sufficient to make predictions of retention times under gradient elution conditions for solutes of whatever chemical structure. The prediction errors are dependent on the steepness of the linear gradient chosen but are, in any case, acceptably low.     

Equilibria effects in the dynamic ion-exchange separation of metal ions

Summary Equilibria occurring during the dynamic ion exchange separation of metal ions were examined, and their effect on the chromatography of the metal ions was evaluated. Bonded reversed-phase silica, pure organic reversed phase, and silica columns were used to differentiate between the role of silica and the reverse phase. The results show that system peaks had the most important influence on the applicability of dynamic exchangers to quantitative metal ion determinations. The response of system peaks to sample composition (pH, ionic strength, matrix effects) was complicated and could not be predicted easily. Equilibria effects that can cause analytical errors were identified and recommendations for elimination of such errors are given.Dedicated to Professor S. R. Lipsky on the occasion of his 60th birthday.     

Process scale high-performance liquid chromatography Part I: An optimisation procedure to maximise column efficiency

Summary Scale up of analytical High-Performance Liquid Chromatography (HPLC) to the process scale offers promise as a preferred separation method for many high value fine chemicals, both for development purposes and continuous production. It's early adoption is curtailed by the widely held perception that preparative HPLC is a low efficiency high cost technique of limited application. This view has largely beeen generated by the generalised use of large particle size, low quality angular silica supports, and difficulties with packing methodologies. A systematic study of a new silica, in new axially compressed equipment using Plackett-Burman statistics, has demonstrated that very high efficiences, leading to high capacities, can be attained. This work was conducted in the Chemical Technology Unit, University of Manchester Institute of Science and Technology, P.O. Box 88, Manchester, M60 1QD, UK.     

Comparison of on-line SPE-HPLC and SPME-GC for the analysis of microcontaminants in water

Summary Solid-phase preconcentration coupled on-line with liquid chromatography, and solid-phase microextraction coupled with gas chromatography have been applied to the determination of trace-level concentrations of benzene and sulfur compounds (tetrahydrothiophene (THT),t-butyl mercaptan (TBM), andn-butyl mercaptan (nBM)) in aqueous media. The first technique uses a microcolumn packed with a styrene-divinylbenzene copolymer. Retention values for THT, TBM, and benzene on this sorbent, expressed as the capacity factors, are 1100, 1600, and 4000, respectively. It is, therefore, possible to preconcentrate them efficiently and this method enables minimum concentrations of 0.05, 0.7, and 0.8 μg L⁻¹, respectively, to be reached. In microextraction, a fine silica fiber coated with a thin layer of polydimethylsiloxane is used. The partition coefficients of the analytes between the polymeric coating and water have been determined to be 40, 115, and 110 for THT, TBM, and benzene, respectively. When sampling is performed from the liquid phase, partition equilibrium is reached within 5 to 20 minutes, or more quickly when the solution is agitated. Using a flame ionization detector, detection limits for THT, TBM, and benzene were 3, 5, and 0.2 μg L⁻¹, respectively. Extraction of volatile or semi-volatile analytes can also be performed from the headspace with very similar detection limits. In addition, the extraction time is shorter since equilibrium is reached within 1 to 1.5 minute, which can again be reduced by agitation of the sample. The advantages and drawbacks of both techniques are gathered and discussed in this paper, which also underlines their complementarity.     

A comparison between atomic concentration profiles and defect density profiles in GaAs annealed after implantation with beryllium

Semi-insulating chromium-doped GaAs was implanted with 100 keV Be ions to fluences of 5 × 10¹³ and 1 × 10¹⁵ ions/cm². Specimens were annealed at 800°C for thirty minutes. Beryllium atomic concentration profiles, as determined by secondary ion mass spectrometry (SIMS), were compared to the defect density profiles obtained from transmission electron stereomicroscopy techniques for the annealed samples. A major redistribution of Be was observed compared to the as-implanted distribution after annealing at the higher fluence, whereas only a slight redistribution of Be occurred for the lower fluence. A major difference in the defect density profiles was observed with the fluences used for this study in the region where the annealed specimens were compared. The distribution of defects throughout the implanted-annealed layer was examined in GaAs annealed after implantation with the higher fluence using sectioned specimens. The relationships between the atomic Be concentration profile, the defect density profile, and the distribution of some specific defects were compared in these sectioned layers. The distribution and size of defects appear to be directly influenced by the Be concentration and its associated implantation induced damage.     

Modified polystyrene,stationary phases for the separation of different aromatic hydrocarbon types by HPLC

Summary One possible method of characterizing chemicall-modified stationary phases is to describe their chromatographic properties. In this study we investigated several chemically-modifiedpolystyrene-divinylbenzene packings for the separation of nitroarenes, polycyclic aromatic hydrocarbons and aminosubstituted aromatic hydrocarbons. Chromatography was carried out on commercially available polymer stationary phases, for example C-18 or vinylpyridine modified polystyrene. In addition, a chemically-immobilized polymer packing was prepared by introducing nitrogroups, which were established by elemental analysis and infrared spectroscopy.     

Increased sample throughput in HPLC using sample switching

Summary Column switching methods are described which demonstrate an alternative approach to the reduction of analytical time in HPLC. Although most methods use a combination of techniques, it has been possible to group them into major categories involving column cleanup, sample cleanup, multicolumn methods and boxcar chromatography. Presented at the 15th International Symposium on Chromatography, Nürnberg, October 1984     

Influence of pressure on sorption equilibrium: HPLC column packing-mixed eluent

Summary Pronounced base line perturbations were repeatably observed in LC both in alternating recycling and in backflushing experiments when binary eluents and refractometric detectors were used. The perturbations which we call eigenzones appeared immediately after operating the recycling or the backflushing valve, and their size and shape depended on the nature of both the column filling and the mixed mobile phase as well as on the pressure drop across the column.Experimental results presented confirm that the primary causes of the eigenzones were the pressure changes in the columns which influenced the sorption equilibrium of mixed eluent components within the column packing. Some practical consequences of this effect are briefly discussed.Second part of the series discussing the role of pressure in LC. First part: see ref. [1].     

New ISFET interface circuit design with temperature compensation

An integrated and new interface circuit with temperature compensation has been developed to enhance the ISFET readout circuit stability. The bridge-type floating source circuit suitable for sensor array processing has been proposed to maintain reliable constant drain-source voltage and constant drain current (CVCC) conditions for measuring the threshold voltage variation of ISFET due to the corresponding hydrogen ion concentration in the buffer solution. The proposed circuitry applied to Si₃N₄ and Al₂O₃-gate ISFETs demonstrate a variation of the drain current less than 0.1 μA and drain-source voltage less than 1 mV for the buffer solutions with the pH value changed from 2 to 12. In addition, the scaling circuitry with the V_T temperature correction unit (extractor) and LABVIEW software are used to compensate the ISFET thermal characteristics. Experimental results show that the temperature dependence of the Si₃N₄-gate ISFET sensor improved from 8 mV/°C to less than 0.8 mV/°C.