Self-assembled germanium islands grown on (0 0 1) silicon substrates by low-pressure chemical vapor deposition

The time evolution of self-assembled Ge islands, during low-pressure chemical vapor deposition (LPCVD) of Ge on Si at 650 °C using high growth rates, has been investigated by atomic force microscopy, transmission electron microscopy, and Rutherford backscattering spectrometry. We have found three different island structures. The smallest islands are lens-shaped and characterized by a rather narrow size distribution, 4 nm high and 20 nm wide. Next to form are a distinct population of larger multifaceted dome-shaped islands, up to 25 nm high and 80–150 nm wide. Finally, the largest islands that form are square-based truncated pyramids with a very narrow size distribution, 50 nm high and 250 nm wide. The pyramidal islands normally seen in the intermediate size range (150 nm) are not observed. The small lens-shaped islands appear to be defect free, while some of the multifaceted islands as well as all the large truncated pyramids contain misfit dislocations. The existence of multifaceted islands, in the size range where pyramids have previously been reported and of truncated pyramids in the size range where multifaceted dome-shaped islands have previously been reported, is attributed to the high growth rate used. Furthermore, under the growth conditions used, the truncated-pyramid-shaped islands are characterized by a very narrow size distribution.     

The Nearly Classical Behavior of a Pure Fluid on the Critical Isochore Very Near the Critical Point Under the Influence of Gravity

Comprehensive measurements of the isothermal compressibility along the critical isochore in the critical region of pure sulfur hexafluoride (SF₆) and pure carbon dioxide (CO₂) were carried out. All measurements were performed with a multicell apparatus, especially designed for pT measurements in the critical region. The height of the measuring cells was either 30 or 11 mm. Independent of the cell height, we found for both fluids nearly classical values for the critical exponent in the limiting approach to the critical point. However, at a certain distance from the critical point {(T – T _c) 90 mK or 2.82 × 10^–4 for SF₆ and (T – T _c) 55 mK or 1.81 × 10^–4 for CO₂}, we observed a transition to values of the critical exponents which nearly meet the predictions of the renormalization-group theory. Since in the fitting range (T – T _c 1 mK) the correlation length ( 0.5 m) is very much smaller than the geometrical dimensions of the measuring cells, we conclude that the reason for the different behavior is an explicit gravity effect governing the inner critical region. The two different loci of the transition points for sulfur hexafluoride and carbon dioxide can be attributed to the different gravity impact on the fluid corresponding to the different critical densities of the two substances.     

Mechanical properties of alkaline earth-doped lanthanum gallate

Lanthanum gallate doped with alkaline earths was prepared from combustion-synthesized powders. Mechanical properties of the doped gallates were evaluated as a function of composition and temperature. The indentation fracture toughness of Sr-substituted gallates was significantly better than the Ca- and Ba-substituted materials, but the toughness of all the doped gallates was significantly lower than yttria-stabilized zirconia, a typical electrolyte material. Small improvements in room temperature toughness and strength were measured in (La_0.9Sr_0.1)_xGa_0.8Mg_0.2O_3–, (LSGM-1020) samples with significant A-site cation non-stoichiometry (x = 0.9). The flexural strength of stoichiometric LSGM-1020 decreased from 150 MPa at room temperature, to 100 MPa at higher temperatures (600–1000°C). The notched-beam fracture toughness of LSGM-1020 decreased from 2.0–2.2 MPam at room temperature, to 1.0 MPam at 600°C. The decrease in mechanical properties over this temperature range was correlated to changes in crystal structure that have been identified by neutron diffraction. These crystallographic changes were also accompanied by significant changes in the thermal expansion behavior and elastic modulus. For off-stoichiometric LSGM-1020 with A/B cation stoichiometry of 0.90, strength and toughness also decreased with temperature, but the retained toughness (1.5 MPam) at elevated temperatures was higher than the toughness of the stoichiometric LSGM material.     

Single-crystal growth and characterization of Cu2O and CuO

We have prepared a large number of crystals of cuprous oxide (Cu2O) by various procedures. Photoluminescence spectra of these crystals were studied to examine the concentration of defects, especially copper vacancy VCu to seek favourable conditions for growing Cu2O crystal. High-quality single crystals of Cu2O were prepared by the floating-zone melting method in air. Several synthetic crystals (specimens FA, FZ and GZ) and also a natural crystal were studied by X-ray analysis, inductively coupled plasma spectroscopy analysis, optical absorption, photoluminescence, photoconductivity and cyclotron resonance absorption, photoluminescence, photoconductivity and cyclotron resonance absorption to characterize their optical and electrical qualities. The best values of mobility and scattering time of photocarriers at T = 4.2 K are estimated to be h1.8 × 105 cm2 V–1 s–1 and h60 ps for positive holes, and 1.3 × 105 cm2 V–1 s–1 and 70 ps for electrons in Cu2O. Further, we report preliminary experimental results on transport property of crystals also of cupric oxide (CuO) purified by the floating-zone melting method.     

Formation and characterization of oxynitride glasses in the Si-Ca-Al-O-N and Si-Ca-Al,B-O-N systems

Oxynitride compositions in the Si-Ca-Al-O-N and Si-Ca-Al, B-O-N systems were melted and furnace-cooled in BN crucibles at temperatures from 1650 to 1850° C under dry nitrogen atmosphereS. Glass formation, phase stability and crystallization were studied by characterizing the cooled melts by X-ray diffraction, DTA, and electron microscopy. Oxynitride batches with nitrogen content up to 11 at % formed glasses in the Si-Ca-Al-O-N system. Glasses in the Si-Ca-Al, B-O-N system could be formed only when the B₂O₃ content of the batch was less than 3 wt %. Oxynitride glasses in these boron-containing systems were characteristically inhomogeneous, difficult to process, and prone to crystallization. In both the systems, glasses exhibited glass transitions beginning at 1000° C and crystallization at 1300 to 1500° C. Nitrogen-containing crystalline phases were identified in devitrified glasses via microstructural and micro-mechanical analyses.     

Magnetic field dependence of the density of states of YBa2Cu3O6.95: Implications for the vortex structure

The total specific heat of YBa₂Cu₃O_6.95 single crystals includes contributions from phonons and spin-1/2 particles, as well as electronic contributions. The electronic specific heat is described by a quadratic term T² in zero field and a linear term [(0)+(H)]T which is increased when a magnetic field H is applied perpendicular to the CuO₂ planes. In agreement with d-wave superconductivity, we find that _n/T_c and (H)_n(H/H_c2)^1/2, where _n is the coefficient of the normal-state linear term. The H^1/2 dependence of the density of states at the Fermi level was predicted by G. Volovik for lines of nodes in the gap: the quasiparticles which contribute to this density of states are close to the nodes in momentum space and are located outside the vortex core.     

Characterization of nearly stoichiometric SiC ceramic fibres

A comparative study of the chemical composition and microstructure of Hi-Nicalon, Hi-Nicalon type S, Tyranno SA, Sylramic and Carborundum fibres has been conducted. This analysis has confirmed results already published but has also evidenced some original features. The Hi-Nicalon type S fibre has a near stoichiometric composition but it still contains some oxygen (1 at. %) and free carbon (2 at. %). The expected near stoichiometric composition of both the Tyranno SA and the Sylramic fibres is only effective near the edge region, while the core of the fibres contains some amount of free carbon (e.g., up to 14 at. % and 6 at. % respectively in large diameter fibres) as well as some residual oxygen (0.5 at. %). The composition of the Carborundum fibre is very close to stoichiometric SiC except rare and localised free carbon or B₄C inclusions. The properties of the different fibres, some of them still beeing at a development stage, are discussed from a chemical and a phase composition point of view, on the basis of what is known about their respective preparation process.     

Electrical transport properties of low-stage AsF5-intercalated graphite

The in-plane resistivity of stage 1 and stage 2 AsF₅-graphite intercalation compounds was measured using a contactless r.f. eddy current technique from 1.6 to 290 K. The magnetoresistance of a stage 1 compound was similarly measured from 4.2 K to 290 K. The low temperature stage 2 resistivity data show a well-defined intermediate T ² region in addition to the usual T high temperature region, in qualitative agreement with the Kukkonen theory and indicative of a small, elongated cylindrical Fermi surface. Stage 2 resistivity data also show, for the first time in a graphite-acceptor compound, an apparent low temperature phase transition at 21 K. Magnetoresistance data were used to determine a stage 1 carrier concentration of 9×10²⁰ holes cm^–3. Resistive anomalies were observed at 200 K and 220 K for stage 1 and stage 2 compounds, respectively.     

Analysis of cold rolled steels of different reduction ratio using the magnetic Barkhausen noise technique

Magnetic Barkhausen Noise (MBN) was used to evaluate the magnetic behaviour of nuclear reactor pressure vessel steel specimens cold rolled to reduction ratios between 0% and 60%. Measurements performed at increasing reduction ratios revealed variations in the angular dependence of a parameter termed MBN_energy, concurrent with modifications in the shape of the pulse height distribution curves. The angular preference of MBN_energy present prior to cold rolling was destroyed at intermediate reduction ratios (25%), and restored with further reduction (40%). Along the rolling direction, the number of large voltage pulses was reduced at reduction ratios of 25%, increasing again at 60% reduction ratio. Results were attributed to competing effects between crystallographic texture, microscopic and macroscopic residual stresses.     

A study of electron paramagnetic resonance and optical absorption in calcium manganese phosphate glasses containing praseodymium

This paper reports on electron paramagnetic resonance (EPR) and optical absorption studies for Mn ions in praseodymium calcium manganese phosphate glasses. The EPR spectra exhibit three resonance signals at g2.0, g3.3 and g4.8. A six line hyperfine structure spectrum centered at g2.0 has been observed in the EPR spectra of all the glasses at lower concentration of Mn ions. The effects of the concentration of Mn ions and praseodymium oxide on resonance signals have been studied. The temperature dependence of EPR signals were also studied. The intensity of the resonance signals decreases with increase in temperature whereas the linewidths are found to be independent of temperature. As temperature is increased small fluctuations in hyperfine splitting are observed. From the optical absorption spectrum, the crystal field parameters and optical band gap energy were evaluated. The optical band gap energy is found to depend quite sensitively on added manganese content. The theoretical values of optical basicity were also evaluated.     

Influence of Electron–Phonon Interaction on Superconducting State Parameters of MgB2

A three-square well model is employed for the three interactions namely, electron–acoustic phonon, electron–optical phonon, and Coulomb in the calculation of superconducting transition temperature (T _c) for layered structure MgB₂. The analytical solutions for the energy gap equation allow us to understand the relative interplay of these interactions. The values of the coupling strength and of the Coulomb interaction parameter indicate that the test material is in the intermediate coupling regime. The superconducting transition temperature of MgB₂ is estimated as 41 K for _ac 0.3, _op 0.1, and * 0.07. We suggest from these results that both the acoustic and optical phonons within the framework of a three-square well scheme consistently explains the effective electron–electron interaction leading to superconductivity in layered structure MgB₂.     

Effect of Pulse Flows of Charged Particles on the Structure and Mechanical Properties of Metals

We investigate the effect of pulse flows of hydrogen, helium, and hydrogen–helium plasma of a specific power of 20–30 GW/m² on the surface structure and mechanical properties of vanadium, niobium, and Kh16N15M3B and Kh18N10T austenitic stainless steels. Plasma bunches acted for 2 sec with an average energy of particles of 2 keV. Tests of samples made of austenitic steels for tension showed that irradiation up to doses of 10¹⁸ cm^–2 strengthens them by a factor of 1.8 and decreases the relative elongation by a factor of 2.3–2.7. A layer-by-layer electron-microscopic analysis revealed that a cellular structure is formed in the surface layer 25 m in thickness as a result of irradiation, which explains the change in mechanical characteristics of the steels.     

Complexities in STEM analyses of polymer blend thin films

Polystyrene (PS)-polyether sulphone (PES) polymer blend thin films were prepared for examination in a scanning transmission electron microscope. The microstructures observed in 75 wt % PS-25 wt % PES films consisted of spherical inclusions, ranging from 0.2 to 1.2m in diameter. X-ray spectrometric analysis in the microscope revealed that the inclusions were PES-rich, while the matrix contained only PS. Attention in this paper is paid to the contrast in the annular dark-field detector (ADF) images from these thin films. This image contrast has a complicated dependence on both the angular range subtended by the dark-field detector and mass-thickness variations within the films. On microscopes with appropriate lens controls which permit the acceptance angle of the ADF detector to be varied, it becomes possible actually to reverse the contrast between the two phases.     

Triple Approach to Determination of the c-Axis Penetration Depth in BSCCO Crystals

The c-axis penetration depth _c in Bi₂Sr₂CaCu₂O₈₊ (BSCCO) single crystals as a function of temperature has been determined using three high-frequency techniques, namely (1) measurements of the ac-susceptibility at a frequency of 100 kHz for different sample alignments with respect to the ac magnetic field; (2) measurements of the surface impedance in both superconducting and normal states of BSCCO crystals at 9.4 GHz; (3) measurements of the surface barrier field H_J(T) 1/_c(T) at which Josephson vortices penetrate into the sample. Careful analysis of these measurements, including both numerical solution of the electrodynamic problem of the magnetic field distribution in an anisotropic plate at an arbitrary temperature and influence of defects in the sample has allowed us to estimate _c(0) 50 m in BSCCO crystals overdoped with oxygen (T_c 84 K) and _c(0) 150 m at the optimal doping level (T_c 90 K). The results obtained by different techniques are in reasonable agreement.     

Deformation and recrystallization textures of copper single crystals and bicrystals

Recrystallization textures in rolled copper specimens have been investigated so as to elucidate the mechanisms of cube texture formation. The specimens are single crystals with the orientations corresponding to the main components of the rolled textures, such as {112}111,{110}112, etc. and bicrystals consisting of such oriented crystals. The cube texture was not observed in any single crystal specimens, but observed in only two bicrystal specimens with {112}/111{100}001 and {110}112/{100}001 orientations. The formation of cube texture seems to require the existence of a cube oriented region in the deformed state, and the favourable oriented matrix to allow the growth of such nuclei.     

Pressure-induced lattice instability in fcc lanthanum at low temperature

Strong anomalies have been discovered in the dependence of the superconducting transition temperature T_c on pressure, the most prominent one being a sudden decrease of the slope dT_c/dP by a factor of four at 53 kbar. The electrical resistivity behaves anomalously in several respects as a function of temperature and pressure up to 200 kbar. The residual resistance shows a marked peak at 53 kbar. It is concluded that, at low temperatures, a subtle isostructural phase transformation (fcc fcc) occurs at 53 kbar. The existence of a critical point in the P-T phase diagram is expected at low temperature, not unlike the situation in cerium metal at elevated temperature. Another fcc fcc phase transformation may occur at 25 kbar and liquid helium temperature. The phase boundary will presumably also end in a critical point at higher temperature. There is hence some evidence for the P-T diagram of La being, in principle, isomorphous with the phase diagram of Ce. In the case of Ce, a second critical point might also exist, forming the endpoint of the / phase boundary at high temperature.Part of a thesis by H. Balster, Abteilung für Physik und Astronomie, Ruhr-Universität Bochum (1974).Paper presented at the 1974 Spring Meeting of the Deutsche Physikalische Gesellschaft in Freudenstadt [H. Balster and J. Wittig, Verhandl. DPG (VI)9, 817 (1974)].     

Review Positron annihilation in fine-grained materials and fine powders—an application to the sintering of metal powders

We consider the specific problem of the influence of an inhomogeneous distribution of defects in solids on positron annihilation characteristics. In detail, we investigate the effect of micro-structure, i.e. dislocations, vacancies, vacancy clusters, grain and subgrain boundaries, pores or inner surfaces, on positron lifetime spectroscopy. Only few materials show such small grain sizes that positron annihilation is affected. One example are powder compacts, made out of small and fine-grained powder, during different stages of the sintering process. All samples generically show positron trapping at grain boundaries (_GB300 ps) and at surfaces (_surf=500–600 ps). _GB=300 ps corresponds to small voids consisting of roughly eight vacancies. Different defects can lead to similar annihilation parameters. Hence, we compare the lifetime data obtained from porous and fine-grained samples to the kinetics of defect annealing after irradiation and plastic deformation, e.g. the thermal stability of dislocations or vacancy clusters. We conclude that _GB300 ps is apparently not related to vacancy clusters in the matrix, but is due to positron trapping at large-angle grain boundaries. This large open volume is in contrast to common grain boundary models. The change of porosity and grain size with temperature, i.e. during sintering, has been determined in a correlated study by metallography and X-ray line-profile analysis. The effective powder particle size ranges from 0.5 to 15 m, while the grain sizes are always smaller. The only detectable lattice defects in all samples above recrystallization temperature are grain boundaries, besides a surface component in very fine powders.     

Superconducting state in M3C60: Strong vs. weak coupling

The superconducting state in the doped fullerenes is due to strong coupling (e.g.,2.1 for Rb₃C₆₀) to low-frequency intramolecular modes _L 250 cm^–1 (^21/2). The analysis is based on an equation describingT _c for any strength of the coupling and on recent isotope effect and NMR data.     

Sintering mechanisms and microstructural development of coprecipitated mullite

Coprecipitated mullite precursor powders of the bulk compositions 78 wt% Al₂O₃+22 wt% SiO₂ (high-Al₂O₃ material) and 72 wt% Al₂O₃+28 wt% SiO₂ (low-Al₂O₃ material) have been used as starting materials. The precursor powders were calcined at 600, 950, 1000, 1250, and 1650 C, and test sintering runs were performed at 1550, 1600, 1650, 1700, and 1750 C. Homogeneous and dense ceramics were obtained from cold isostatically pressed (CIPed) powders sintered in air at 1700 C. Therefore, all further sintering experiments were carried out at 1700 C. After pressureless sintering, sample specimens were hot isostatically pressed (HIPed) at 1600 C and 200 bar argon gas pressure. Sintering densifications of low Al₂O₃ materials ranged between 94% and 95.5%. There was no clear dependency between densification and calcination temperature of the starting powders. High-Al₂O₃ compositions displayed sintering densities which increased from 97% at 600 C calcination temperature to 99% at 950 C calcination temperature. Higher calcination temperatures first caused slight lowering of the sintering density to 95.5% (calcination temperature 1250 C) but later the density strongly decreased to a value of 85% (calcination temperature 1650 C). HIPing of pressureless sintered specimens prepared from powders calcined between 600 and 1100 C yielded 100% density. At the given sintering temperature of 1700 C, the microstructure of sample specimens was influenced by Al₂O₃/SiO₂ ratios and by calcination temperatures of the starting powders. Homogeneous and dense microstructures consisting of equiaxed mullite plus some minor amount of -Al₂O₃ were produced from high-Al₂O₃ powders calcined between 600 and 1100 C. Low-Al₂O₃ sample specimens sintered from precursor powders calcined between 600 and 1100 C were less dense than high-Al₂O₃ materials. Their microstructure consisted of relatively large and elongated mullite crystals which were embedded in a fine-grained matrix of mullite plus a coexisting glass phase. The different microstructural developments of high- and low-Al₂O₃ compositions may be explained by solid-state and liquid-phase sintering, respectively. The microstructure of HIPed samples was very similar to that of pressureless sintered materials, but without any pores occurring at grain boundaries.     

The reflection of low energy phonons at the free surface of liquid4He. II. Experiment

Using a heater and bolometer as source and detector, the reflection of a pulsed beam of phonons at the free surface of liquid⁴He at 0.05 K has been measured for angles of incidence between 30 ° and 80 °. The energy distribution of the incident beam was obtained from the theory of phonon decay in the accompanying paper (I). The average incident phonon energy was 0.3 K, corresponding to an effective beam temperature of about 0.1 K. Both heater and bolometer were made from graphite resistor board with a layer of poly aniline fibers to increase the absorptivity for rotons. The angular distribution of the phonons from the heater is quite broad, approximately (cosⁿ +cos^m )/2 with n 0.65 and m 3.4. However, the receiver response has a broad component with n 2.2 and an extraordinarily narrow one with m 106. In agreement with the theory in I, the reflection appears to be specular within the accuracy of the experiment. The reflection coefficient is unity within the experimental error; the weighted mean value is 1.001±0.025. During the experiment, the free surface became contaminated with 0.034 of a monolayer of³He, but no effect from the³He impurity was observed.