Elastic properties of GexAsySe100−x−y glasses

The elastic properties of Ge_xAs_ySe_100−x−y (0x30; 10y40) glasses have been studied. The results were analyzed in terms of the dependence on the theoretical mean coordination number (mean number of covalent bonds per atom) m (m=2+(2x+y)×0.01). Three ranges of m (2.1m2.51, 2.51<m2.78, 2.78<m3) were revealed, where different dependencies of elastic moduli (Young’s modulus, shear modulus) and Poisson’s ratio of glasses on m were observed.     

Three‐dimensional unsteady thermocapillary flow under rotating magnetic field

Under a rotating magnetic filed (RMF), the instability of thermocapillary flow and its evolution with increasing Marangoni number (Ma) for semiconductor melt (Pr = 0.01) in a floating liquid bridge model (As = 1) are investigated numerically. Under 5 mT RMF, the thermocapillary flow is steady and axisymmetric with Ma < Ma_c, and the critical Marangoni number Ma_c for convection instability is 29.5, which is obtained by the direct numerical simulation. When the Ma is a little bit beyond the Ma_c, the thermocapillary flow loses stability to become a three‐dimensional rotating oscillatory convection, and a periodic oscillation is confirmed by the fast Fourier transform analysis, the oscillatory main frequency decays with increasing Ma. Under 1 mT–6 mT RMF, the Ma_c increases roughly with the magnetic strength except the Ma_c at 4 mT, where the corresponding change of flow mode after the instability is observed. The oscillatory convection occurs with a smaller Ma in the RMF than that without magnetic field. In addition, no instability toward a three‐dimensional steady convection, which is the state of thermocapillary flow without magnetic field after the first instability, is observed under the RMF.     

不同高径比下浮区晶体生长熔体内对流不稳定性分析

本文通过数值模拟的方法,研究了零重力条件下半浮区液桥内熔体热毛细对流的演化规律。在液桥的高度L和温差ΔT保持不变的情况下,通过改变液桥的半径R来改变液桥的高径比(Ar=L/R)。随着高径比Ar的变化,液桥内的对流表现出不同的流动特征。在Ar=0.5时,热毛细对流处于三维稳态;在Ar=1时,流场和温度场从稳态模式向非稳态周期多频振荡模式转变,它们之间的频率关系满足倍频关系(f_n=nf₁);在Ar=1.25时,监测点的速度振荡频率增大,表现为较小幅度的振荡模式,且温度振荡消失。     

Raman spectroscopic study on the structure of ZnCl2---ZnX2 and ZnCl2---KX (X = Br, I) glasses

Raman spectra have been measured for ZnCl₂---ZnX₂ and ZnCl₂---KX (X = Br, I) glasses to investigate the structure of the glasses with varying composition. The assignment of each band was made, and the change of the spectra with composition was explained in terms of the bridging and non-bridging states of halide ions and the change of the tetrahedral units, ZnX_nCl_4−n²⁻ (n = 0–4), formed in the glasses. As the content of ZnX₂ in ZnCl₂---ZnX₂ glasses increases (20 → 80 mol%), the peak frequency of the Zn---Cl stretching mode increases (238 → 248 cm⁻¹ in X = I glasses, 238 → 259 cm⁻¹ in X = Br glasses) while the Zn---I and Zn---Br stretching frequencies decrease (173 → 120 cm⁻¹ for Zn---I, 196 → 157 cm⁻¹ for Zn---Br). The decrease of the Zn---I and Zn---Br band frequencies was attributed to the increase of the number n of the ZnX_nCl_4−n²⁻ tetrahedra. The increase of the Zn---Cl frequency suggests the existence of the bonding state of Cl⁻ ions which is intermediate between the bridging and the non-bridging states. In ZnCl₂---KX glasses, the Zn---Cl_non-bridging band at about 300 cm⁻¹ was observed in addition to the bands observed in ZnCl₂---ZnX₂ glasses. The addition of KX produces non-bridging anions while the tetrahedral units, ZnX_nCl_4−n²⁻ are also formed.     

B NMR investigations in xV2O5–B2O3 and xV2O5–B2O3–PbO glasses

11B (I=3/2) MAS NMR in the binary glass system xV₂O₅–B₂O₃ (x=0.053, 0.43) and the ternary glass system xV₂O₅–B₂O₃–PbO (0.1x1.5) has been investigated at room temperature. In the xV₂O₅–B₂O₃ glasses, one NMR line due to BO₃ unit was observed. Meanwhile in the xV₂O₅–B₂O₃–PbO, two NMR lines which arise from BO₃ and BO₄ units were detected, where the appearance of BO₄ units is produced by the presence of PbO. From the computer-simulation of the ¹¹B NMR central transition line (m=−1/2↔1/2), the quadrupole parameters (e²qQ/h and η) for BO₃ units in xV₂O₅–B₂O₃, and those for BO₃ and BO₄ units in xV₂O₅–B₂O₃–PbO were obtained as a function of x. As the V₂O₅ content increases in xV₂O₅–B₂O₃–PbO, the e²qQ/h and η values of the BO₃⁻ associated resonance are found to slightly decrease and increase, respectively. Meanwhile, the e²qQ/h and η values of BO₄⁻ associated resonance in xV₂O₅–B₂O₃–PbO are found to slightly increase and decrease, respectively. By comparing the intensities of the total transitions (m=−3/2↔−1/2,m=−1/2↔1/2, and 1/2↔3/2) for the ¹¹B NMR line of BO₃ and BO₄ units contained in xV₂O₅–B₂O₃–PbO with those of respective standard samples of 0.053V₂O₅–B₂O₃ and NaBH₄, the quantitative fractions of BO₃ and BO₄ in xV₂O₅–B₂O₃–PbO were obtained as a function of x.     

Peculiarities of liquid phase epitaxy in GaInPAs/InP lattice-matched heterostructures

The temperature phase stability of Ga_xIn_1−xP_yAs_1−y solid solution has been analyzed. A simple solution theory with the temperature-independent interaction parameters in solid and liquid phases has been used. The absence of miscibility gaps for all the compositions of the solid solution, lattice-matched to InP at a growth temperature of 640°C, has been demonstrated both theoretically and experimentally. The influence of the elastic deformations on the Ga_xIn_1−xP_yAs_1−y (λ_g = 1.4 μm) solid solution parameters has been observed assuming the model of the layer coherent conjugation in heterostructures.     

Study of the hydrodynamic instabilities in a differentially heated horizontal circular cylinder corresponding to a Bridgman growth configuration

We are interested in determining the origin of the instabilities occurring in a metallic liquid (Prandtl number Pr=0.026) contained in horizontal circular cylinders heated from the end-walls. Our approach by direct numerical simulation (DNS) allows the determination of the transition thresholds for different aspect ratios varying from 1.5 to 10 as well as a precise characterization of the nature and structure of the new flow regimes close to the thresholds. In order to understand the mechanisms of flow transition, fluctuating energy analyses close to the threshold have been performed. The main contributions have been determined and localized in the cavity: shear has been found as the main instability factor but the way it acts is different according to the aspect ratio.     

Measurement of flow phenomena using the ultrasonic velocity profile method in a simulated Czochralski crystal puller

An experimental investigation of flow phenomena in a simulated Czochralski (CZ) crystal puller was conducted using an ultrasound velocity profile (UVP) probe to measure fluid velocities. To isolate the various forces that influence the fluid motion, only the crystal was rotated and a water-glycerol mixture was used as the test fluid in this experiment. Measurements of the velocity components, V_z and V_r, under the initial transient from zero rotation to a set rotation rate as well as under steady-state conditions were made. The corresponding Reynolds number based on the crystal radius and rotation varied from Re_Ω,_x ≈ 660–5000 (Ω_x ≈ 110–2000 rpm). The measured velocity profiles, V_z and V_r, were processed using a simple color image scheme based on the magnitude and direction of flow with respect to the probe. These profiles were studied to postulate flow patterns in the simulated CZ device. The measurements revealed three major regions of flow: (1) one beneath the crystal, (2) one near the free surface and crucible wall and (3) one beneath the crystal and occupying the extent of the crucible. In the initial transient, spatially symmetric outward flow beneath the crystal developed until the recirculating flow from below disturbed this pattern. At this point the apparent shear between the flow beneath the crystal and that circulating upwards from the bottom of the crucible induced flow instabilities and a non-axisymmetric flow pattern. The non-axisymmety was verified by the color-coded velocity profiles and a power spectrum analysis. The analysis revealed the critical Reynolds number for the onset of the primary instability to be Re_Ωcrit ≈ 669. The linear variation in the power of the primary peak with the Reynolds number indicates that the onset of the axisymmetry-breaking instability is due to Hopf bifurcation.     

Oscillatory instability and minimum undercooling criterion in directional solidification

We show in this paper that, when small Peclet number effects are taken into account, steady state deep cells which satisfy the minimum undercooling criterion never undergo an oscillatory instability. However, if the stability criterion for the oscillatory mode found in this limit is extended to Peclet number of order unity, deep cells may become unstable if:

a criterion which can be directly tested on the existing experimental data. Here Δ is the tip undercooling, U the interface velocity, K the partition coefficient and v the control parameter, ratio between thermal and diffusive lengths.     

Facets formation of pyramidal Si nanocrystals selectively grown on Si(0 0 1) windows in ultrathin SiO2 films

We have used in situ scanning tunneling microscopy (STM) to study the facet formation in the selective growth of pyramidal Si nanocrystals on Si(0 0 1) windows in ultrathin 0.3-nm-thick SiO₂ films. Broad (0 0 1) surfaces developed as the top of the crystals, and {1, 1, (2n+1)} (n=1–6) facets formed the sidewalls. As growth continued, the slope angle of sidewall facets increased, and {1, 1, 9} and {1, 1, (2m+1)} (0 <m < 4) facets often came to coexist on the sidewalls. On well-oriented Si(0 0 1) surfaces, layer-by-layer growth in the [0 0 1] direction was dominant. On vicinal Si(0 0 1) surfaces, lateral step growth took place in the initial stage, and the layer-by-layer growth was suppressed until after a large (0 0 1) surface had formed as the top of the crystal.     

Modelling tracer diffusion and mobility of interstitials in disordered materials

A disordered system is described by a model in which interstitials can occupy sites of different energy. Tracer diffusion occurs in the absence of a concentration gradient and by jumps over energy barriers with the same saddle point energy. Using statistical mechanics and simple random walk considerations the tracer diffusion coefficient is D^* = γ⁰D⁰ where D⁰ is the known or unknown tracer diffusion coefficient of an arbitrary reference state, and γ⁰ is the activity coefficient with respect to the same reference state. The activity coefficient γ⁰ can be calculated for a given energy distribution. The results are in agreement with rigorous treatments of diffusion for the same potential trace. It is shown that Henry's law (γ⁰ → constant for c → 0) is valid for small interstitial concentrations c and for all types of energy distributions. Thus the diffusivity also becomes independent of concentration for c → 0.

Measurements of the hydrogen diffusion coefficient in liquid-quenched and vapour-quenched Pd₈₀Si₂₀ and its dependence upon temperature and hydrogen concentration are presented. These are in excellent agreement with the model if a Guassian distribution of energies is assumed. The width of the energy distribution is different for the two materials, but in each case agrees with results from solubility measurements. For alkali ions in oxide glasses two energy levels, one for sites adjacent to the immobile anions and one for sites far away from them, describes the so-called “weak electrolyte behaviour”, which determines diffusivity and conductivity at low alkali contents. With increasing alkali content the energy distribution changes, and predictions were made, which are in agreement with experimental findings in borate-silicate and GeO₂-glasses.     

Electrical and magnetic properties of rare-earth — Gold glasses

We present magnetic and electrical properties of amorphous rare-earth alloys of composition (R₇₅Au₂₅)_100−xB_x (R = Pr, Gd, Tb, Er and X = 0, 10). Low-field susceptibility, electrical resistivity and high-field magnetization measurements were made between 1.6 and 300 K. Resistivity measurements show both positive and negative temperature coefficients and in some cases maxima and minima that are associated with magnetic ordering and Kondo-like scattering respectively. For all the samples except those containing Gd, the local random anisotropy produces asperomagnetic ordering and high coercive fields. Large magnetic after-effects have been observed down to liquid helium temperatures particularly for the Tb glass. Metamagnetic effects have been found in (Er₇₅Au₂₅)_90B10.     

Influence of the nucleation time-lag on the activation energy in non-isothermal crystallization

In the present work, the influence of the nucleation time-lag on the non-isothermal glass crystallization is discussed. Differential thermal analysis (DTA) results of an iron-rich glass nucleated by Cr₂O₃ were obtained at different heating rates. The activation energy of crystallization, E_c, and the Avrami parameter, m, estimated by Kissinger's and Ozawa's equations were shown to be dependent on the heating rate. The value of E_c, obtained at 2.5, 5 and 7.5 K/min heating rates was calculated as 299 kJ/mol, while the value of E_c, obtained at 10, 15 and 20 K/min was as 499 kJ/mol. The value of m for ‘low' and ‘high' heating rates were 2.57 and 1.45, respectively. The results were interpreted on the basis of the non-steady state nature of the nucleation process. It was assumed that at high heating rates no nucleation takes place and the crystals grow on a existing fixed number of nuclei; the activation energy of crystal growth, E_g, can be estimated by applying the Kissinger equations. At low heating rates nucleation occurs and the number of nuclei formed is influenced by the heating rate; E_g can be estimated by the Matusita and Sakka equation.     

Dopant concentration profile in a Czochralski flow of liquid metal in a vertical or a horizontal magnetic field

Dopant concentration profiles are obtained for a Czochralski flow of liquid metal in a static crucible under either an axisymmetric vertical magnetic field or a horizontal uniform magnetic field. The latter magnetic field inevitably requires fully three-dimensional cylindrical coordinate model equations, which are successfully solved for the representative parameters Gr = 10⁷, Pr = 0.01, Re = 1620 and Ha 1000. Asymmetric concentration profiles are obtained. The average heat flux decreased with the Hartmann number. The circumferential rotational direction was found to be reversed in a lower regime against that of a top rotating crystal rod in a strong lateral magnetic field. In a vertical magnetic field, the concentration profile approached the pure diffusion state.     

XAS study of lanthanum coordination environments in glasses of the system K2O---SiO2---La2O3

The La L₁ and L₃ XANES and L₃ EXAFS have been investigated for the series of glasses 10K₂O---50SiO₂---x La₂O₃ (x = 1, 5, 10) and (10 − x)K₂O---40SiO₂−(x/3)La₂O₃ (x = 7.5, 5, 2.5) and model compounds La₂O₃, LaAlO₃, LaPO₄, La₂NiO₄, La₂CuO₄ and La(OH)₃. An edge resonance at 25 eV above the L₁ edge in the glass spectra is concentration-dependent, decreasing in intensity with increasing lanthanum concentration. The 2s → nd forbidden transition increases with La₂O₃ concentration, indicating a reduction in the ‘average’ site symmetry of the first coordination shell of La. Mapping _X(k) space, which is a new and promising technique, was employed to extract bond distance, coordination number and thermal parameters from the EXAFS. By this method, one calculates the complete _X(k) space a function of all physically reasonable values of the adjusted parameters in all possible combinations. The advantage in this method is the assurance of a global minimum. Bond lengths were comparable to those obtained by Fourier transforming the phase corrected EXAFS. The values are 2.42 Å (± 0.03 Å) for La---O. The coordination numbers (N ≤ 7 ± 1.5) were derived by mapping and comparison to the published structures for other La compounds. _X(k) mapping is compared with least-squares fitting the data, and the correlation between the Debye-Waller factor and coordination number is also discussed.     

Investigations of g-value correlations associated with the g = 4.3 ESR signal of Fe in glass

The g = 4.3 ESR signal of Fe³⁺ in glass is usually attributed to a “fully rhombic” distortion characterized by E = D/3, where E and D are the splitting constants belonging to the second-order rhombic and axial terms, respectively, of the crystal-field interaction. An alternate statistical approach to the origin of this signal has been given by Peterson et al. on the basis of axial symmetry and broad overlapping distribution of g_| and g that are correlated in opposition (negatively) resulting in spiking at g 4.3. The present paper reports detailed computer studies of the distribution of effective g value produced by large variations in both D and E to test whether negative g-value correlations leading to spiking at g 4.3 are generable from site dependent fluctuations in the second-order crystal-field terms. It is found that the ESR line arising from the middle Kramer's doublet in the fully rhombic limit (E → D/3, |D| > hν) uniquely generates the negative g-value correlation required for spiking at g 4.3.     

On the theory of the weighing method for automatic crystal shape control in czochralski growth

We derive the exact analytical expression for the meniscus volume in terms of the crystal radius R, the melt contact angle θ_L, and the meniscus height h for the axisymmetric planar growth interface Czochralski configuration. This relation is applied together with motion and mass balance considerations to find the general rate of change in the force experience by the weighing cell in a standard diameter control arrangement. In contrast to the classical work of Bardsley et al. (1977), which is limited to the close viscinity of stationary right cylinder growth, our results are valid for growth developing any crystal profile. In addition, they also account for the fall of melt level due to finite crucible size. The magnitudes of the capillary contributions to the force, and their implications for diameter servo control, are discussed over a wide range of profile parameters using the Tsivinskii formula for h = h (R, θ_L).     

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.     

Capture of adatoms by clusters in thin film deposition

The adatom diffusion and capture by clusters are considered as a random-walk problem. The method of solution is developed. Green's function for a square substrate lattice is obtained. This permits one to give a general solution for a steady-state adatom distribution around clusters and for Halpern's arrival probability. This solution is applicable for calculating capture numbers of some small capture centres. An interpolation formula for the capture number, as dependent on the perimeter length of the capture centre, is proposed for λ a. The adatoms fluxes to the 10 and 11 steps on the surface are calculated. The fluxes are anistropic but the anisotropy disappears with λ/a → ∞. The applicability of the steady-state approach is discussed. It is shown that this approximation is valid for low adatom density. The adatom fluxes to monatomic and diatomic immobile clusters are calculated under conditions when these clusters cannot be treated as capture centres. The conservation equations for these clusters are formulated whereby two different configurations of three-atomic clusters are taken into account.