Semiconductor gallium arsenide electrodes for the potentiometric titration of mercaptans in raw hydrocarbons

Electrochemically modified electrodes based on gallium arsenide (GaAs) were designed for the potentiometric determination of mercaptans in raw hydrocarbons. The main electrochemical characteristics (linearity range of the electrode function, limit of detection, and slope of electrode function) were studied. The argentometric titration of mercaptans in a gas condensate was performed using the GaAs electrodes prior to and after modification. The use of the electrochemically modified electrode ensures a more reliable determination of the titration end-point.     

Electrochemical sensing of membrane potential and enzyme function using gallium arsenide electrodes functionalized with supported membranes

We deposit phospholipid monolayers on highly doped p-GaAs electrodes that are precoated with methyl-mercaptobiphenyl monolayers and operate such a biofunctional electrolyte-insulator-semiconductor (EIS) setup as an analogue of a metal-oxide-semiconductor setup. Electrochemical impedance spectra measured over a wide frequency range demonstrate that the presence of a lipid monolayer remarkably slows down the diffusion of ions so that the membrane-functionalized GaAs can be subjected to electrochemical investigations for more than 3 days with no sign of degradation. The biofunctional EIS setup enables us to translate changes in the surface charge density Q and bias potentials Ubias into the change in the interface capacitance Cp. Since Cp is governed by the capacitance of semiconductor space charge region CSC, the linear relationships obtained for 1/Cp2 vs Q and 1/Cp2 vs Ubias suggests that Cp can be used to detect the surface charges with a high sensitivity (1 charge per 18 nm2). Furthermore, the kinetics of phospholipids degradation by phospholipase A2 can also be monitored by a significant decrease in diffusion coefficients through the membrane by a factor of 104. Thus, the operation of GaAs membrane composites established here allows for electrochemical sensing of surface potential and barrier capability of biological membranes in a quantitative manner.     

On the interaction of copper with defects in gallium arsenide

The behavior in gallium arsenide of copper introduced through the melt has been investigated. Measurements have been made on the electrical and photoluminescent properties of single crystals that had either been quenched from T_cryst, slowly cooled after crystallization or thermally annealed at 1100°C after completion of the growth. On quenching from T_cryst, formation of very shallow acceptors is observed. On slow cooling or annealing at 1100°C, acceptor levels are formed of the order of 0.12 and 0.02 eV. The concentration of carriers for both levels diminishes with lower temperatures of annealing and with concentration of copper added. The 0.12 eV level is assigned to the center (Cu_GaV_Ga)⁻.     

Rate constants for the reaction of atomic and molecular bromine with gallium arsenide

The reaction of the (100) face of a gallium arsenide single crystal with atomic and molecular bromine has been studied in a discharge flow system at temperatures between 100 and 225°C and pressures between 0.1 and 40 torr. The reaction with Br₂ was found to be first order in Br₂ only at pressures below 1 torr. Temperature dependence studies in the linear range gave the activation energy and preexponential factor for the rate controlling reaction in the low pressure regime. The results are summarized in the following Arrhenius equation: Deviations from linearity at high pressures are discussed in terms of two alternative mechanisms. The reaction of GaAs with atomic bromine was also studied as a function of temperature, and found to have a temperature dependence described by the following Arrhenius equation:      

Rate constants for the etching of gallium arsenide by atomic chlorine

Known chlorine atom concentrations were prepared in a discharge flow system and used to etch the (100) face of a gallium arsenide single crystal. The etch rate was monitored by mass spectrometry, laser interferometry, and surface proftlometry. In the temperature range from 90 to 160°C the reaction can be described by the rate law $$Etch rate = kP_{Cl} $$ where $$k = 9 \times 10^{(6 \pm 0.5)} \mu m min^{ - 1} Torr^{ - 1} e^{ - 9 \pm 1)kcal/RT} $$      

Elimination of equivalence point errors in the potentiometric titration of chloride, bromide and iodide mixtures with silver nitrate

When two or more halides are determined in solution by precipitation titration with silver nitrate as the titrant, significant errors can occur at the first equivalence point as a result of coprecipitation. Errors of up to 33% were found for the first equivalence point for solutions containing mixtures of halides at micromolar levels. The addition of a flocculating agent to the solution reduced coprecipitation by increasing the rate of exchange between the precipitated silver halide and the halide ion remaining in solution. A logarithmic relationship was observed between the charge of the flocculating agent and the logarithmic concentration of the agent needed to minimise coprecipitation. Although flocculating agents reduced coprecipitation, they do not, however, completely eliminate equivalence point errors. Here a new method is presented which effectively eliminates the problem of coprecipitation during precipitation titrations for solutions containing two halides. In order to decrease the possibility of coprecipitation, we used selective complexation of the precipitation ion Ag⁺ in order to control the AgX solubility. For example, in the case of CF⁻ plus X⁻ (X=Br⁻ or I⁻), we added sufficient NH₃ to form Ag(NH₃)⁺ so that the free Ag⁺ activity was reduced below that required for theoretical AgCl precipitation in the absence of the other halides. Once the titration of the less soluble halide was completed and the first equivalence point determined, the Ag(NH₃)⁺ complex was destroyed by acidification of the solution to a pH less than 6. The titration is then continued and the second equivalence point determined. Equivalence point errors were reduced to less than 1.5% with careful application of the method.     

Determination of nickel,gold and silver in gallium arsenide by radioactivation analysis based on the quantitative isotope dilution principle

A method of radioactivation analysis has been developed for the determination of Ni, Au and Ag impurities in gallium arsenide. The separation and substoichiometric extraction of these elements were studied and analytical procedures are suggested for their determination. All components are separated by suitable procedures and determined by substoichiometric methods. Ni is extracted as diethyldithiocarbamate into toluene, Au as a complex of rhodamine-B in chloroform, and Ag as dithizonate in carbon tetrachloride. The contents of Ni, Au and Ag in a gallium arsenide crystal with a carrier concentration of 1.8·10¹⁶/cm³ were 0.05–0.08, 0.006–0.008 and 0.002–0.005 ppm, respectively.     

Determination of iron in high purity gallium arsenide, gallium and arsenic as the bathophenanthroline complex

Summary A spectrophotometric method is described for the determination of iron in high purity GaAs and As or Ga, having a limit of detection of 3.5×10^–5% and 7×10^–5% Fe, resp. After sample dissolution and pre-reduction of Fe³⁺ to the bivalent state with hydroxylammonium chloride the red iron(II)-bathophenanthroline complex is formed in the presence of sodium citrate, ascorbic acid and perchlorate. It is extracted into chloroform and optical density of the resulting extract is measured at 533 nm. The effect of copper is discussed. The procedure is stated to be rapid and applicable not only to Ga, As and GaAs, but also to other materials.

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Zusammenfassung Es wird eine spektralphotometrische Methode zur Eisenbestimmung in Ga oder As und GaAs mit Nachweisgrenzen von 3,5 · 10^–5% bzw. 7,0 · 10^–5% Fe beschrieben. Nach Lösung der Probe und Vorreduktion des Eisens mit Hydroxylammoniumchlorid wird in Anwesenheit von Natriumcitrat, Ascorbinsäure und Perchlorat ein roter Komplex von Eisen(II)-bathophenanthrolin gebildet, der mit Chloroform extrahiert wird und dessen optische Dichte bei 533 nm gemessen wird. Der Einfluß des Kupfers wird diskutiert. Die vorgeschlagene Methode ist schnell durchfÜhrbar und kann nicht nur fÜr Ga, As und GaAs, sondern auch fÜr andere Materialien angewendet werden.

     

Platform atomization in the electrothermal AAS determination of magnesium in gallium arsenide

Summary An ET-AAS method using platform atomization is proposed for the determination of magnesium in gallium arsenide. The sample is decomposed with nitric and hydrochloric acids and the diluted solution is injected into the furnace. No significant interferences from the matrix up to 4.0 mg GaAs ml^–1 were observed by using the proposed procedure. The detection limit (3 ) is 0.12 g Mg g^–1 in GaAs. Improvements should be achieved by a better control of contamination sources. Results obtained by analysing undoped GaAs samples are presented.

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Plattform-Atomisierung bei der elektrothermischen AAS-Bestimmung von Magnesium in Galliumarsenid

     

Spectrophotometric extractive titrations-II Determination in gallium arsenide of the sum of metals extractable with dithizone

A procedure for the determination of the sum of metals, which are extractable with dithizone, in gallium arsenide is given. After the dissolution of the sample the impurities are extracted with several portions of 10(-4)M dithizone in carbon tetrachloride, gallium being screened by tartrate ion. The excess of the reagent is extracted into 0.1M aqueous ammonia and after stripping into carbon tetrachloride the dithizone is determined by spectrophotometric extractive titration. The effect of pH is considered. The limit of detection, based on the evaluation of twenty blanks, is 2.30 x 10(-8) mole, the equivalent of 1.50 mug of zinc.     

Trace analysis of gallium arsenide by selective evaporation of the matrix with bromine vapours

Gallium arsenide samples are analyzed by graphite furnace AAS after the selective evaporation of the matrix elements as bromides with bromine vapours in a simple apparatus. The evaporation step is carried out at 280–300 ° C and it is accomplished for 0.5 g sample within 1 h. Reproducibilities of 5 to 20% were obtained. Detection limits (3 s, in ng/g) are 20 (Fe), 5.0 (Ca), 3.0 (Mg), 2.0 (Ni, Cu), 1.0 (Al), 0.5 (Pb, Co), 0.2 (Mn, Cr).     

Use of the ion microanalyzer for the characterization of bulk and epitaxial silicon and gallium arsenide

The ion microanalyzer permits a localized mass spectrometric analysis, i.e. the qualitative and quantitative analysis of the impurities contained in a small selected volume. This procedure makes possible the analysis of very thin epitaxial layers (for example silicon and gallium arsenide). As regards qualitative analysis, the apparatus is designed for the selection of ions. After the recording and analysis of the ion spectrum, a large number of the impurities present in the sample are determined qualitatively. Quantitative analysis can be performed with the equipment, but this requires the analysis of a homogeneous standard sample previously dosed by spark-source mass spectrometry. The quantitative analysis of bulk and epitaxial silicon and gallium arsenide is described and the limits of detection of the principal impurities are given. It is also shown how the possibility of localized analysis was exploited. A correlation was established between the existing impurities and the chemically revealed crystal imperfections. A comparative analysis of the distribution of the impurities in the epitaxial layers was also carried out. The periodic analysis of epitaxial layers makes it possible to follow the deterioration by contamination, if any, under the epitaxial conditions, and to improve the sample quality.     

A silver electrode in the potentiometric titration of thiols

A silver wire immersed in a thiol solution gives a potential responsive to the thiol concentration, and is a sensitive indicator electrode in the potentiometric titration of thiols with mercury(II) chloride, p-chloromercuryphenyl sulphonate, and silver nitrate at pH 4.5-9.5, 7-9.5 and 9.5 respectively. Titrations of simple thiols such as cysteine or a protein such as albumin are equally successful, but the potential break was smaller for the protein. The end-point could be determined within an increment of titrant equal to 5 nmole of thiol. An inert atmosphere is needed for titration at pH 7.     

Spectrophotometric and polarographic methods for the determination of silicon at ng/g levels in gallium arsenide

Summary Spectrophotometric and differential pulse polarographic determinations of silicon in gallium arsenide as heteropoly acid have been established. The analysis comprises decomposition with a mixture of hydrochloric acid and bromine in a PTFE vessel, elimination of matrix elements by evaporation of arsenic as arsenic trichloride and extraction of gallium as gallium tetrachloro-complex anion by di-isopropyl ether, and finally spectrophotometric determination of silicomolybdenum blue or polarographic determination of -silicomolybdic acid. Optimization of sample pretreatment procedures and instrumental determination have been carefully elaborated. The detection limits of the developed methods were found to be 7 ppb and 5 ppb, respectively, for spectrophotometry and polarography. The proposed methods have been practically applied to the analysis of various Si-doped samples. The results obtained by the chemical methods are compared with those from the electrical measurement and the discrepancies found are discussed.

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Spektralphotometrische und polarographische Bestimmung von Silicium in Galliumarsenid im ng/g-Bereich

Zusammenfassung Die beschriebenen Verfahren beruhen auf der Si-Bestimmung als Heteropolysäure. Sie umfassen den Aufschluß der Probe mit Salzsäure und Brom im PTFE-Gefäß, Eliminierung der Matrixelemente (Verdampfung von Arsen als Arsentrichlorid und Extraktion von Ga als Tetrachlorokomplex mit Diisopropylether) sowie anschließende spektralphotometrische Bestimmung als Silicomolybdänblau oder polarographische Bestimmung als -Silicomolybdänsäure. Die Optimierung der Vorbehandlungstechnik sowie der instrumentellen Analysenparameter wird beschrieben. Die Nachweisgrenzen liegen bei 7 ppb bzw. 5 ppb. Die vorgeschlagenen Methoden wurden auf verschiedene Sidotierte Proben angewendet. Die Ergebnisse werden verglichen und Unterschiede diskutiert.

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Presented at the Colloquium Spectroscopicum Internationale XXIV, September 15–20, 1985, Garmisch-Partenkirchen, FRG     

A critical study on the determination of selenium in gallium arsenide by polarographic techniques

A simple voltammetric method is described for the determination of traces of selenium in gallium arsenide. Differential-pulse cathodic stripping voltammetry permits a direct determination of selenium without preliminary enrichment or separation processes. Selenium can be determined down to levels of 1–2 μg g^?1, with relative standard deviations of about 10%, in ? 100-mg samples of gallium arsenide. Results for gallium arsenide doped with 7–75 μg g^?1 selenium agree in most cases with those obtained by spectrophotometry based on 4-chloro-o-phenylenediamine.