Lateral OMVPE growth of GaAs on patterned substrates

GaAs was grown on patterned 1 0 0 on- and off-axis GaAs substrates by organometallic vapor-phase epitaxy (OMVPE). Patterned mesas were observed to change shape because lateral growth rates varied by more than an order of magnitude in different crystallographic directions. For this study, misoriented GaAs (1 0 0) wafers were polished 3° toward the nearest [1 1 0] or [1 1 1] family of directions, and 320 nm high cross-shaped mesas were fabricated. OMVPE growth was performed between 550°C and 650°C for 1 h at a vertical growth rate of approximately 1.3 μm/h. Atomic force microscopy showed that three effects have a powerful influence on lateral growth initiated at mesa sidewalls. First, the symmetry of the dominant surface reconstruction has a major effect on the diffusion of Ga adatoms. Rapid Ga diffusion occurs along the 0 1 1–0−1−1 axis in OMVPE, or the perpendicular 0−1 1–0 1−1 axis in molecular beam epitaxy, and appears to be a result of the different surface reconstructions which exist in the two growth ambients. Second, misorientation of the wafer causes a growth asymmetry as Ga adatoms move preferentially from high-to-low terraces. When terrace steps descend toward a mesa wall, rapid lateral growth away from the wall is always observed. When terrace steps descend away from a mesa wall, little lateral growth occurs and even reduced vertical growth may be observed. When the misorientation and reconstruction symmetries align, the surface acts like an atomic diode and the rapid lateral growth can exceed the vertical growth rate by more than an order of magnitude. Third, on misoriented substrates, step bunching increases with increasing temperature, and this can lead to significant changes in the original shape of a mesa. A growth model is presented which relates the lateral growth rate in different crystallographic directions to the substrate misorientation, the growth temperature, and the partial pressure of As during growth. It is also shown that different surface reconstruction patterns are related to chemical species with continuously varying concentrations rather than thermodynamically distinct phases.     

Reaction mechanisms in OMVPE growth of GaAs determined using D2 labelling experiments

Reaction mechanisms in the organometallic vapor phase epitaxial (OMVPE) growth of GaAs using TMGa combined with either AsH₃ or tertiarybutylarsine (TBAs) are reviewed. The pyrolysis of each constituent in various ambients is discussed followed by a discussion of the combined systems used for actual GaAs growth.     

Uniformity analysis of temperature distribution in an industrial MOCVD reactor

Three‐dimensional models, coupling fluid flow and heat transfer, have been adopted to analyze influences of the process parameters on the temperature uniformity in an industrial MOCVD reactor. Important factors, such as the inlet gas flow, the susceptor rotation, the heater power, the distance between the heat shield and the susceptor (d₁), as well as the distance between the heater and the susceptor (d₂), have been investigated carefully. The system heating condition is characterized by temperature uniformity denoted as the standard deviation of temperature, and by thermal efficiency expressed as a combination parameter of the dissipated energy. The results reveal that decrease of the gas flow and the rotation rate, as well as increase of the distance d₁, could monotonically enhance the temperature uniformity. The results also show that decrease of the above three parameters could improve the thermal efficiency. Furthermore, increase of the distance d₂ enhances the temperature uniformity, and reduces the thermal efficiency slightly. The influences of the parameters on the uniformity vary at the different locations of the susceptor as divided into Zone A, Zone B and Zone C. The conclusions help the growth engineer optimize the system design and process conditions of the reactor.     

Photoluminescence study of AlGaInP/GaInP quantum well intermixing induced by zinc impurity diffusion

AlGaInP/GaInP quantum well intermixing phenomena induced by Zn impurity diffusion at 540 °C were studied using room-temperature photoluminescence (PL) spectroscopy. As the diffusion time increased from 40 to 120 min, PL blue shift taken on the AlGaInP/GaInP quantum well regions increased from 36.3 to 171.6 meV. Moreover, when the diffusion time was equal to or above 60 min, it was observed firstly that a PL red shift occurred with a PL blue shift on the samples. After detailed analysis, it was found that the red-shift PL spectra were measured on the Ga_0.51In_0.49P buffer layer of the samples, and the mechanism of the PL red shift and the PL blue shift were studied qualitatively.     

On the possibility of placing a universal neutron diffractometer in an inclined channel of the PIK reactor

Crystallography Reports - The possibility of placing a universal neutron diffractometer, which is designed for working with perfect crystals, in one of the inclined channels of the PIK reactor is...     

The use of an electric analogue to solve the lamellar eutectic diffusion problem

An electrical analogue technique has been developed to obtain numerical solutions to the diffusion equation in the liquid ahead of a growing lamellar eutectic. Using the solutions it has been possible to examine certain of the approximations made in earlier treatments of the problem. A modification has been made to the Jackson and Hunt model of eutectic growth which becomes important when composition differences in the interfacial liquid composition become large (i.e. at very high growth velocities). The modified treatment no longer predicts υλ² = constant although the variation of υλ² with growth velocity is not likely to be detected easily.     

Determination of thermal diffusion coefficient and diffusion coefficient

A new technique is described to determine the thermal diffusion coefficient, D', and the difusion, D, of Fe, Si and Al in liquid Mg. This technique is based on a boundary condition, formulated by including contributions of temperature and cocentration gradients in the liquid to the advancing solid-liquid interface during unidirectional solidification of a binary alloy. The determined values of D'_Fe, D'_Si and D'_Al are 5.57 × 10^-6, 7.35 × 10^-6 and 3.64 × 10^-6 cm/sec °C and D_Fe and D_Si are 1.9 × 10^-5 and 1.5 × 10^-5 cm²/sec, respectively.     

Sodium diffusion in boroaluminosilicate glasses

Understanding the fundamentals of alkali diffusion in boroaluminosilicate (BAS) glasses is of critical importance for advanced glass applications, e.g., the production of chemically strengthened glass covers for personal electronic devices. Here, we investigate the composition dependence of isothermal sodium diffusion in BAS glasses by ion exchange, inward diffusion, and tracer diffusion experiments. By varying the [SiO₂]/[Al₂O₃] ratio of the glasses, different structural regimes of sodium behavior are accessed. We show that the mobility of the sodium ions decreases with increasing [SiO₂]/[Al₂O₃] ratio, revealing that sodium is more mobile when it acts as a charge compensator to stabilize network formers than when it acts as a creator of non-bridging oxygens on tetrahedrally-coordinated silicon and trigonal boron. The impacts of both the addition of iron and its redox state on the sodium diffusivity are explored in terms of the structural role of ferric and ferrous ions. By comparing the results obtained by the three approaches, we observe that both the tracer Na diffusion and the Na–K interdiffusion are significantly faster than the Na inward diffusion. The origin of this discrepancy could be attributed to the fact that for sodium inward diffusion, the charge compensation for electron holes is a rather slow process that limits the rate of diffusion.     

Inward cationic diffusion in glass

We report that inward diffusion of network-modifying divalent cations can occur in iron-containing silicate glasses when they are treated in a reducing atmosphere near the glass transition temperature. As a result of the inward diffusion, a silica-rich nanolayer forms on the glass surface, which increases the hardness and chemical durability. The thickness of the layer can be controlled by varying the heat-treatment conditions. We clarify the mechanism of the inward diffusion and calculate the diffusion coefficient for the network-modifying divalent cations. We demonstrate that the diffusion of these ions is the rate-limiting factor for the reduction process.     

Cation diffusion in soda-lime-silicate glass melts

Cation diffusion was experimentally investigated in soda-lime-silicate glass melts (composition in mol%: 74SiO₂–16Na₂O–10CaO) at temperatures from 1000 to 1200 °C using the diffusion couple technique. One half of each diffusion couple was doped with 11 trace elements (500 ppm by weight of Rb, Cs, Sr, Zn, Cd, Nd, Eu, In, Sn, Ge and 1000 ppm by weight of Fe). Experiments were performed in an internally heated gas pressure vessel at a confining pressure of 100 MPa to avoid convective fluxes in the diffusion samples. The distribution of major elements was analyzed by electron microprobe. IR spectroscopy was used to quantify concentrations of dissolved water in the run products. Trace element diffusion profiles were measured simultaneously employing synchrotron X-ray fluorescence microanalysis. In all analyzed glasses the highest diffusion coefficients were observed for Rb whereas Nd was always the slowest element, e.g. at 1000 °C the diffusivity decreases from (1.51 ± 0.35) × 10⁻¹¹ m²/s for Rb to (1.29 ± 0.34) × 10⁻¹³ m²/s for Nd. The diffusivity of Nd is close to the chemical diffusivity of network former calculated from viscosity data using the Eyring relationship. Surprisingly, the rare earth elements Nd (3+) and Eu (mixed 2+, 3+) diffuse more slowly than the tetravalent Ge. Activation energies for diffusion increase from (132.1 ± 1.5) kJ/mol for Rb to (205 ± 16) kJ/mol for Eu. Based on the diffusion data for Eu, Sr and Nd we estimated that Eu²⁺/Eu_total ratios in soda-lime-silicate glass melts are below 0.04 both at reducing and oxidizing conditions.     

Band gap in cadmium zinc phosphate glass

The band gap in cadmium zinc phosphate glass samples with composition 35 mol% CdO-5 mol% ZnO-60 mol% P₂O₅ has been determined by absorption and photoconduction measurements. The two values of the band gap are nearly the same within the experimental error. The photoconductive band gap shows a slow decrease with an increase in the applied field. The results have been discussed in terms of the energy band model for non-crystalline solids.     

Electronic conductivity in zinc iron phosphate glasses

The electrical properties of (40−x)ZnO–xFe₂O₃–60P₂O₅ (x = 10, 20, 30 mol%) glasses were measured by impedance spectroscopy in the frequency from 0.01 Hz to 4 MHz and the temperature range from 303 to 473 K. It was shown that the dc conductivity strongly depends on the Fe₂O₃ content and Fe(II)/Fe_tot ratio. The increase in dc conductivity for these glasses is attributed to the increase in Fe₂O₃ content from 10 to 30 mol%. With increasing Fe(II) ion content from 6% to 17% the dc conductivity increases. This indicated that the conductivity arises mainly from polaron hopping between Fe(II) and Fe(III) ions suggesting an electron conduction in these glasses. By applying scaling on conductivity data measured at different temperatures, single master curve was obtained for each glass. On the other hand, deviation from the master curve at high frequencies was observed for glasses with different compositions. This deviation originates from a various mobility of charge carriers in different glass structures. Raman spectra showed the change of structure, from metaphosphate to pyrophosphate, with increasing Fe₂O₃ content from 10 to 30 mol%.     

Dielectric anomaly in copper doped zinc sulphide

Unusually high values of dielectric constant of copper doped zinc sulphide powders have been obtained. They are discussed in terms of polarization catastrophe (PC) phenomenon. Because of the dependence both the hydrogen-like donor and effective mass approximation on the static dielectric constant of pure ZnS, the critical concentration of copper is lower for the hexagonal phase than for cubic one. This fact applied to the interpretation of our results, however, is heavily suppressed due to the unknown distribution of copper inside the crystallite. From the above reasons the strong dependence of the PC threshold on the real structure of the doped zinc sulphide is predicted.     

Three-dimensional structure of porcine pancreatic carboxypeptidase B with an acetate ion and two zinc atoms in the active site

Crystals of porcine pancreatic carboxypeptidase B (CPB) were grown by the capillary counter-diffusion method in the presence of polyethylene glycol and zinc acetate. The three-dimensional structure of CPB was determined at 1.40 Å resolution using the X-ray diffraction data set collected from the crystals of the enzyme at the SPring 8 synchrotron facility and was refined to R _fact = 17.19%, R _free = 19.78%. The structure contains five zinc atoms, two of which are present in the active site of the enzyme, and an acetate ion. The arrangement of an additional zinc atom in the active site and the acetate ion is different from that reported by Yoshimoto et al.     

Electronic relaxation in zinc iron phosphate glasses

The electrical and dielectric properties of 10ZnO-30Fe₂O₃-60P₂O₅ (mol%) glasses, melted at different temperatures were measured by impedance spectroscopy in the frequency range from 0.01 Hz to 3 MHz and over the temperature range from 303 to 473 K. It was shown that the dc conductivity strongly depends on the Fe(II)/[Fe(II) + Fe(III)] ratio. With increasing Fe(II) ion content from 17% to 37% in these glasses, the dc conductivity increases. Procedure of scaling conductivity data measured at various temperatures into a single master curve is given. The conductivity of the present glasses is made of conduction and conduction-related polarization of the polaron hopping between Fe(II) and Fe(III), both governed by the same relaxation time, τ. The high frequency dispersion in electrical conductivity arises from the distribution in τ caused by the disordered glass structure. The evolution of the complex permittivity as a function of frequency and temperature was investigated. At low frequency the dispersion was investigated in terms of dielectric loss. The thermal activated relaxation mechanism dominates the observed relaxation behavior. The relationship between relaxation parameters and electrical conductivity indicates the electronic conductivity controlled by polaron hopping between iron ions.     

Scale-up synthesis of zinc borate from the reaction of zinc oxide and boric acid in aqueous medium

Synthesis of zinc borate was conducted in a laboratory and a pilot scale batch reactor to see the influence of process variables on the reaction parameters and the final product, 2ZnO·3B₂O₃·3.5H₂O. Effects of stirring speed, presence of baffles, amount of seed, particle size and purity of zinc oxide, and mole ratio of H₃BO₃:ZnO on the zinc borate formation reaction were examined at a constant temperature of 85 °C in a laboratory (4 L) and a pilot scale (85 L) reactor. Products obtained from the reaction in both reactors were characterized by chemical analysis, X-ray diffraction, particle size distribution analysis, thermal gravimetric analysis and scanning electron microscopy. The kinetic data for the zinc borate production reaction was fit by using the logistic model. The results revealed that the specific reaction rate, a model parameter, decreases with increase in particle size of zinc oxide and the presence of baffles, but increases with increase in stirring speed and purity of zinc oxide; however, it is unaffected with the changes in the amount of seed and reactants ratio. The reaction completion time is unaffected by scaling-up.     

Irradiation-induced diffusion in tracks of swift ions

Radiation-induced diffusion in tracks of swift ions caused by excitation of electrons and resulting in the formation of dilatations has been considered. The expressions for the longitudinal and radial atomic displacements are derived as well as those for the diagonal components of the strain tensor. The atomic-density distribution in the vicinity of a track is obtained in terms of the parameters of the three-dimensional energy distribution in an excited electron gas. The results obtained theoretically are compared with the known experimental data. The suggested approach provides the evaluation of the time of formation of track dilatations, the mobility of the atoms in the target irradiated with swift ions, and the energy losses for formation of wirelike dilatations in the tracks. It is shown that the ion irradiation of semiconductor crystals can be used for obtaining one-dimensional quantum objects.     

Radiation-enhanced diffusion of Na in alkaline glasses

The surface Na concentration of glass is found to increase under X-ray flux. The experimental data are well fitted by a first-order rate equation and are interpreted on the basis of radiation-enhanced diffusion mechanism of the Na with a coupling between material and defect fluxes.The Na concentration-versus-depth profiles show that the phenomenon also takes place at depth greater than the XPS sampling depth. Moreover, there is some evidence that in addition to diffusion from the bulk, surface segragation also occurs.