摘要:

The properties of small ensembles of cyclically twinned, rhombohedrally distorted face‐centered‐cubic crystals are examined and shown to be consistent with reported experimentally determined diffraction patterns and images of rapidly solidified Al‐Mn alloys exhibiting icosahedral symmetry. Limitations of previously reported refutations of twinning models are discussed. Proposals are made for definitive tests between microtwinning and quasicrystalline models of these structures.

ISSN:0021-8979    

DOI:10.1063/1.336542

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

The access of solvents and reactants to the microvoid volume in porous materials such as coal plays an important role in determining the overall chemistry which takes place during a variety of chemical transformations including oxidation, combustion, and pyrolysis. The structure and surface composition of these voids were studied using small‐angle neutron scattering techniques to examine selectively the subset of the overall void volume distribution which comprises the microvoid volume. Powdered Illinois No. 6 coal containing approximately 20% void volume was slurried in several different aqueous and cyclohexane solutions. The solutions used had various hydrogen‐to‐deuterium ratios in order to contrast match most of the open pore volume thereby making the microvoid volume visible. The microvoid volume observed is characterized as elongated voids having a fairly well‐defined diameter and surface composition. The scattering intensity from the microvoid volume shows a well‐defined Porod region, indicating that the smallest void dimension is resolved by the instrumental configuration employed. A Guinier region exhibitingQ−1behavior, which is characteristic of elongated structures, is also observed. The average radius of a circular cross section of these voids is found to be 25.4 A˚. The microvoids are found to be completely filled by aqueous solutions so that the residual neutron scattering, which is not eliminated by the contrast‐matching aqueous solution, is due to the organic matrix structure. Nonaqueous mixtures of cyclohexane cannot fill the entire microvoid volume as effectively as the aqueous mixtures. The scattering differences observed between the aqueous and nonaqueous filled coal indicates that the surface of the microvoids is predominantly aliphatic in character with the principal compositional variation being the presence or absence of acidic functionality on the surface.

ISSN:0021-8979    

DOI:10.1063/1.336543

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

This paper reports the results of studies on chargedN1centers, produced using a technique in which additively colored KCl and KBr crystals are exposed to near ultraviolet excitation at low temperatures. In undoped crystals, absorption and emission data are presented showing several new chargedN1transitions along with information relating to the nature of the defects. In addition, this work demonstrates that in sodium‐doped crystals,N1centers can be ionized through electron transfer toFAcenters. This technique has already been used in stabilizing (F+2)Acenters in laser‐active crystals and may have future application for lasing ionizedN1centers.

ISSN:0021-8979    

DOI:10.1063/1.336544

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

Using ionized Mg beams accelerated to energies from 130 to 500 eV, Mg doping was studied in molecular‐beam epitaxy of GaAs. The incorporation coefficient of Mg increases by a factor of about 100 when compared with the use of neutral Mg beams. Hole concentrations as high as about 1×1019cm−3have been achieved. Photoluminescence measurements suggest that the damage due to Mg‐ion bombardment is negligible when the ion accelerating voltage (Va) ≲130 V. For higherVa, the damage can be removed by postgrowth annealing.

ISSN:0021-8979    

DOI:10.1063/1.336545

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

Electron paramagnetic resonance has been used to study the isochronal and isothermal irreversible annealing ofE1defects induced in amorphous SiO2by implantation of He+, N+, O+, and Bi+ions. The annealing behavior observed is successfully compared to predictions based on two models for diffusion‐limited thermally activated processes: a simple model of a first‐order unimolecular recombination, and Simpson and Sosin’s model for bimolecular recombination of closely spatially correlated Frenkel pairs. In both cases it has been assumed that the diffusion process is controlled by a Gaussian distribution of the activation energy for diffusion. It is shown that both models offer a consistent interpretation of the annealing behavior observed, indicating that for the temperature range between 500–800 °C, the irreversible thermal annealing ofE’1defects in ion‐implanted amorphous SiO2is controlled by unimolecular recombination. A new enhanced annealing effect has been observed in amorphous SiO2implanted with chemically active N+and O+impurities.

ISSN:0021-8979    

DOI:10.1063/1.336546

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

The analysis of local crystal orientation with micron spatial resolution is of prime importance in understanding the dynamics of crystal growth processes with potential for device applications. Examples of such processes are laser annealing, lateral epitaxial growth over oxides, and molecular‐beam‐epitaxial growth over structured substrates. We present here a nondestructive technique for obtaining such information based on polarization selective Raman scattering from near‐diffraction limited O(1 &mgr;m) regions of the sample. Depth resolution is limited by the optical penetration depth at the laser wavelength, which is on the order of 1.0 &mgr;m for Si at visible wavelengths. By rastering the sample with respect to the probe laser beam, maps of the local crystal orientation can be obtained. Abrupt changes in crystal orientation at grain boundaries are readily apparent. This paper details the experimental apparatus and the method for extracting local crystal orientation information for the particular case of Si. Experiments on single crystals of known orientation are presented which confirm our theoretical predictions and serve to calibrate the technique. Practical examples of mapping of laser annealed silicon‐on‐oxide structures are given. The analysis shows the loss of the lateral epitaxial seed, with many grains exhibiting ⟨110⟩ surface normals, in agreement with qualitative results obtained from a destructive selective chemical‐etching procedure. Signal intensities are such that orientation information at a single point can be recorded in less than 30 s, thus providing the potential for a real timeinsitumonitor of slow growth processes.

ISSN:0021-8979    

DOI:10.1063/1.336547

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

Ion irradiation of poly(phenylene sulfide) (PPS) films with various ions produces highly conductive films. Using 5.6‐MeV fluorine ions, the conductivity of irradiated films at a dose of 1×1016ions/cm2is comparable to chemically doped films (0.77 S/cm), and higher conductivities should be possible with higher doses. Iodine ions of 50 MeV produced films with similar conductivities at a dose of 1×1014ions/cm2: the highest conductivity reported for an organic material irradiated by such a low dose. Films irradiated with 0.32‐MeV lithium ions did not change their conductivity up to 1×1015ions/cm2. We attribute the higher conductivities obtained in our iodine irradiated films to the substantially higher electronic energy deposition associated with 50‐MeV I ions. The Li‐implanted films showed no substantial change in conductivity probably because their energy deposition rates are too low.

ISSN:0021-8979    

DOI:10.1063/1.336548

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

The effects of phosphorus diffusion on the growth and shrinkage of oxidation‐induced stacking faults (OSF) has been studied in the temperature range from 950 to 1100 °C. By using Wright etch on angle‐lapped samples, OSF size dependence on depth has been obtained. OSF grow faster or shrink slower with increasing phosphorus dose, thus indicating a self‐interstitial supersaturation. Moreover, it was found that this supersaturation is not the result of phosphorus precipitation. Silicon self‐interstitial concentration normalized to its equilibrium value has been calculated from the OSF kinetics and was compared with time, phosphorus dose, and temperature. It was found that the interstitial supersaturation increases with phosphorus dose and decreases with annealing time. At the same phosphorus deposition condition, the interstitial concentration decreases with increasing temperature. Normalized self‐interstitial concentration calculated from enhanced phosphorus diffusivity at the profile tail, assuming a pure interstitialcy phosphorus diffusion mechanism, yields values approximately twice as high as those calculated from OSF kinetics. Possible reasons for this discrepancy are discussed.

ISSN:0021-8979    

DOI:10.1063/1.337019

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

The static dielectric constants &egr;aand &egr;cof ZnF2have been studied in the range 4–296 °K at zero pressure and in the pressure range 0–5 kbar at 296, 195, and 78 °K. &egr;aand &egr;cexhibit normal temperature behavior. &egr;cdecreases with pressure as is normally the case. &egr;a, on the other hand, exhibits an anomalous increase with pressure at room temperature. This anomalous increase is reduced with decreasing temperature and by 78 °K has completely disappeared. The pressure and temperature results are combined with thermal expansion and compressibility data to evaluate the pure volume and the pure temperature dependence of the dielectric constants. The dielectric constants are described by a Clausius–Mossotti relation. The Szigeti effective charge ratios have been determined for the transverse optical (TA) modes along the [100] andcaxes. These ratios are found to be rather small, being 0.44 and 0.54 for the two axes, respectively, suggesting that the bonding is partially covalent. The principal components for the Gruneisen tensor have also been determined. Below 80 °K, they are negative. This is due to the negative mode‐Gruneisen parameters of the transverse acoustic (TA) modes associated with theC44andCselastic constants. The possible correlation between the pressure anomaly of &egr;aand temperature anomaly ofC44is discussed and attributed to the instability of the chains of F−octahedra.

ISSN:0021-8979    

DOI:10.1063/1.336549

出版商:AIP    

年代:1986

数据来源: AIP

摘要:

Formulas for the scattering from an inhomogeneous sphere in a fluid‐saturated porous medium are used to construct a self‐consistent effective medium approximation for the coefficients in Biot’s equations of poroelasticity [J. Acoust. Soc. Am.28, 168 (1956)] when the material constituting the porous solid frame is not homogeneous on the microscopic scale. The discussion is restricted to porous materials exhibiting both macroscopic and microscopic isotropy. Brown and Korringa [Geophysics40, 608 (1975)] have previously found the general form of these coefficients. The present results give explicit estimates of all the coefficients in terms of the moduli of the solid constituents. The results are also shown to be completely consistent with the well‐known results of Gassmann and of Biot and Willis, as well as those of Brown and Korringa.

ISSN:0021-8979    

DOI:10.1063/1.336550

出版商:AIP    

年代:1986

数据来源: AIP