H. W. Allison

Adsorption of Strontium and of Barium on Tungsten

Sr or Ba was deposited on W ribbon receivers by evaporation from source filaments in which a chemical reaction produced the metal at least 99.7% pure. The receiving surfaces could be cleaned at will by heating to high temperatures; repeated tests were thus possible. By thermionic measurements, adsorption and desorption were observed independently under nonequilibrium conditions. By using radioactive Sr isotopes, the deposit corresponding to any thermionic activation was measured. For each ribbon, thermionic activation was apparently a reversible function of the amount adsorbed.Our experimental methods give information on kinetics, rather than equilibria. The adsorption mechanism more nearly resembles that proposed in Langmuirs later papers rather than that of his earlier theories but treatment of desorption by the theory of absolute reaction rates is considerably more satisfactory. This accounts for desorption kinetics over the experimental temperature range by only two disposable constants, the activati...

Diffusion of Aluminum, Magnesium, Silicon, and Zirconium in Nickel

The diffusion rates of aluminum, magnesium, silicon, and zirconium, alloyed as single additives in nickel, have been measured at temperatures between 800°C and 970°C. The method consisted of measuring the weight gain caused by surface oxidation of the additive in a wet hydrogen ambient. The surface oxides were identified by electron diffraction. Grain size was about 0.05 mm in Al–Ni, Si–Ni, and Mg–Ni but less than 10−3 mm in Zr–Ni. The composite volume and grain boundary diffusion coefficients are given by DAi→Ni = 1.1 exp(−59 500/RT); DMg→Ni = 2.3×10−5 exp(−31 300/RT); DZi→Ni = 10.6 exp(−64 800/RT); DZr→Ni = 1×10−5 exp(−26 700/RT). Stereo and photomicrographs indicate considerable grain boundary diffusion, particularly in Mg–Ni and Zr–Ni.

Hall‐Effect Levels Produced in Te‐Doped GaAs Crystals by Cu Diffusion

Hall‐effect measurements have been made on Te‐doped GaAs crystals diffused with known amounts of 64Cu sufficient to convert the crystals to p‐type. Energy levels are found above the valence band at 0.123±0.005 eV, 0.145±0.005 eV, 0.166±0.005 eV, 0.19±0.005 eV, and 0.43±0.01 eV. An analysis of the Hall data has been made by means of computer programs. The results may be summarized as follows: (1) Ga vacancies produced during the Cu diffusion associate with and neutralize Te as a compensating center. (2) The ionization energy of the TeCuGa pair is 0.19 eV rather than 0.166 eV as suggested previously.

Double Acceptor Behavior of Cu in Te‐Doped GaAs

Ionization energy levels introduced in Te‐doped GaAs after conversion to p type by diffusion of 64Cu have been measured by means of Hall effect. Levels at 0.145, 0.166, 0.20, and 0.44 eV are observed after various sequential heat treatments. The 0.145 and 0.44 eV levels are attributed to the two acceptor levels expected of Cu on a Ga site. The 0.20 eV level is assumed to arise from a Ga vacancy as previously reported, and the 0.166 eV level is interpreted as an ion pair between Cu on a Ga site and Te on an As site.

I. The Vaporization of Strontium Oxide

The vapor pressure of SrO was measured by studying the product evaporated from platinum filaments coated with SrO. Most of the experiments employed radiactive isotopes.* The possibility of systematic error caused by chemical reduction of the oxide or by its thermal dissociation is discussed. A value of λ0, the heat of evaporation at 0°K computed from the results, is used to evaluate precision and to derive a vapor‐pressure equation.

II. The Reduction of SrO by Tungsten in Vacuum

It is shown that in the temperature range 1150–1550°K, SrO is reduced by tungsten in vacuum. Both the rate of the reaction and its equilibrium constant can be calculated, giving values in substantial agreement with the experiments, which were performed under conditions such that both could be measured. The use of radioactive isotopes simplified the experimental work.

Defect Centers in GaAs Produced by Cu Diffusion

Evidence for defect centers having an ionization energy at Ev+0.10 eV in melt‐grown GaAs is given based on: (1) Hall‐effect measurements on Cu‐diffused crystals. (2) Residual 64Cu left in GaAs after extraction by indium. (3) Decrease in electron concentration of donor‐doped crystals after annealing. The failure to observe the defect level in photoluminescence is discussed. The results suggest that Ga vacancies are present in clusters in melt‐grown GaAs crystals.

Hall Effect Investigation on Lithium‐Diffused Gallium Arsenide

Hall effect results on GaAs crystals into which Li is introduced at 500°C, as well as at higher temperatures, and subsequently removed in Ga or in air at 500°C are reported. Four acceptor ionization energies: 0.023 and 0.11 eV (previously reported), 0.05 eV and 0.14 eV are found. The two levels 0.023 and 0.05 eV are attributed to Li self‐pairs. The 0.11‐ and 0.14‐eV levels are associated with Cu possibly already present in the GaAs crystals. The 0.023‐eV level is found in crystals produced by the floating‐zone process at all temperatures of the Li diffusion. The 0.023‐eV level is also found in the horizontal Bridgman crystals when diffusion is above ∼650°C. Below ∼500°C, however, the 0.05‐eV level predominates in Bridgman crystals and the 0.023‐eV level is not observed. The origins of the various levels are discussed.

Diffusion of Tungsten in Nickel and Reaction at Interface with SrO

Diffusion rates of tungsten in single crystal and polycrystalline nickel were measured at temperatures between 1100°C and 1275°C by using radioactive tungsten185 and a sectioning technique. Grain boundary diffusion was observed by use of radioautographs. A simple vacancy mechanism accounts for nearly all of the diffusion; some refinement is required for the transport observed at greater depths.The rate of reaction of polycrystalline 4.9% tungsten‐nickel alloy with SrO was measured by the hydrogen evolution technique at temperatures between 830°C and 1280°C. The rate indicates that the production of Sr in amounts up to 200 μg cm−2 is not limited by diffusion of tungsten in the nickel core.

Shallow acceptor level in GaAs crystals resulting from Cu diffusion

Abstract The shallow acceptor level at 0.021 eV observed by W helan [ J. Appl. Phys. 31 , 1507 (1960)] by introducing Cu at high temperatures (∼1000°C) and annealing at lower temperatures (∼ 500°C) has been investigated further. We have established that the acceptor requires the presence of Cu for its formation and then only when the Cu is introduced above ∼ 750°C. The net acceptor conc. is found to increase with the initial Cu conc. and to decrease with an increase in the final annealing temperature. Consideration of the stability properties of the acceptor indicates that Cu-donor pairs as well as the Ga-vacancy are unlikely choices for the acceptor. Calculations of the minority donor conc. suggest that changes in the net acceptor conc. are caused, at least in part, by changes in a donor impurity, possibly oxygen. In addition, the possibility is discussed that the net acceptor conc. changes, as well as the minority donor conc. changes are the result of transfer between III-V sites of C or Si impurities.