Showing papers on "Amorphous solid published in 1976"

Electronic properties of substitutionally doped amorphous Si and Ge

Abstract: It is shown that substitutional doping of an amorphous semiconductor is possible and can provide control of the electronic properties over a wide range. a-Si and Ge specimens have been prepared by the decomposition of silane (or germane) in a radio-frequency (r.f.) glow discharge. Doping is achieved by adding carefully measured amounts of phosphine or diborane, between 5 × 10−6 and 10−2 parts per volume, to obtain n- or p-type specimens. The room temperature conductivity of doped a-Si specimens can be controlled reproducibly over about 10 orders of magnitude, which corresponds to a movement of the Fermi level of 1·2 eV. Ion probe analysis on phosphorus doped specimens indicates that about half the phosphine molecules in the gaseous mixture introduce a phosphorus atom into the Si random network; it is estimated that 30–40% of these will act as substitutional donors. The results also show that the number of incorporated phosphorus atoms saturates at about 3 × 1019 cm−3, roughly equal to the number ...

Electromechanical properties of polarized polyvinylidene fluoride films as studied by the piezoelectric resonance method

Abstract: The piezoelectric resonance was observed in free vibrators of uniaxially oriented and biaxially oriented polyvinylidene fluoride films polarized at high electric fields. Using the piezoelectric resonance method, piezoelectric and elastic constants, and electromechanical coupling factors of such films were determined at high frequencies (20 kHz–30 MHz) in the temperature range −170–100 °C. For the uniaxially oriented films, the coupling factors k33 and k32 are independent of temperature, while k31 increases noticeably above the primary dispersion temperature. The value of k33 is about 0.2, one of the largest values ever reported for piezoelectric polymers. Similar results were obtained for the biaxially oriented films. The temperature dependence of elastic and piezoelectric constants of the uniaxially oriented films is interpreted by a model in which the crystalline and amorphous phases are combined in series along the stretching direction and in parallel along the perpendicular directions. The origin of t...

Electronic structure, spectra, and properties of 4:2-coordinated materials. I. Crystalline and amorphous SiO 2 and GeO 2

Abstract: A two-parameter tight-binding theory of the electronic structure of 4:2-coordinated materials is proposed. The parameters, a covalent and a polar energy, are fitted to the optical absorption spectra. The valence energy bands and density of states are calculated. In terms of these a consistent interpretation of all the observed photoemission and x-ray-emission spectra of ${\mathrm{SiO}}_{2}$ is obtained. The x-ray-absorption spectra are also analyzed. A bond-orbital approximation allows a simple calculation of the refractive index (or dielectric constant) of the various allotropic forms of silica and germania. Finally, the variation in total energy and charge distribution with local distortion is analyzed in order to study structural stability, elastic rigidity, and the effective charges (including dynamic contributions) which determine the piezoelectric constants and infrared absorption intensities.

Molecular dynamics study of an amorphous Lennard‐Jones system at low temperature

Abstract: Using molecular dynamics techniques, low temperature properties of a 500‐particle amorphous system of Lennard‐Jones particles have been studied. The system was prepared by rapid quenching of an originally liquid sample. The structural properties are similar to those of random close‐packed hard spheres. Propagating longitudinal phonons are observed even at fairly short wavelengths and well defined transverse phonons are found at the longest wavelengths available. Some preliminary results for the ’’glass transition’’ region are reported.

Small-angle x-ray and light scattering studies of the morphology of blends of poly(ϵ-caprolactone) with poly(vinyl chloride)†

Abstract: Solvent-cast films of blends of poly(ϵ-caprolactone) (PCL) with poly(vinyl chloride) (PVC) were examined by low-angle x-ray scattering and by small-angle light scattering. X-ray scattering from crystalline compositions were analyzed using the Tsvankin–Buchanan technique and led to values of the repeat period of the lamellar structure and the thickness of the crystalline and amorphous layers. With increasing content of PVC, the amorphous layer thickness increased sufficiently to accommodate the PVC, leading to values of the linear crystallinity consistent with macroscopic measurements by density and DSC techniques up to about 50% PVC by weight. Above this concentration, the lamellar structure no longer appeared to be volume filling. At high concentration of PCL, the polymer consisted of volume-filling spherulites containing the lamellar substructure. Spherulite sizes were measured by light scattering and absolute light scattering intensities were consistent with calculations based upon the degree of crystallinity and anisotropy of the spherulites. Compositions containing more than 60% PVC were amorphous. Low-angle x-ray scattering was interpreted in terms of the Debye–Bueche theory which leads to values for a correlation distance lc and the mean-square electron density fluctuation 〈η2〉 (which was also obtained from the invariant). By the method of Porod, the correlation distances were resolved into persistence lengths within the two phases, which were determined as a function of composition. The fluctuation 〈η2〉 was analyzed in terms of a two-phase model to show that its value was somewhat larger than would be obtained if the phases were composed of the pure components. It was not possible to uniquely determine their compositions. The data were consistent with the existence of a transition zone of the order of 30 A thick between phases.

Amorphous structures and their formation and stability

Abstract: The formation and stability of amorphous alloys are discussed in terms of the configurational entropy and in connection with the potential barrier, Δμ. The structure analysis of amorphous alloys using Bernal's dense random packing model (D.R.P.) shows quite good agreement with the actual observations. These results indicate that some degree of short range order (∼ 15 A) exists in amorphous metallic alloys, which is slightly different from the arrangement expected in a liquid. The model still shows a continuous structure without the formation of internal boundaries between ordered regions such as is characteristic of polycrystals. Although the D.R.P. does not completely reflect the structure of amorphous alloys, it is still an attractive model in understanding the amorphous state. These subjects are reviewed in this article.

Electrical and Optical Properties of GexSe1-x Amorphous Thin Films

Abstract: Measurements of electrical and optical properties have been made on GexSe1-x amorphous thin films of about 1 µm thickness. From the experiment of electrical conductivity at various temperatures, two processes of the d.c. conductivity in GexSe1-x films are observed in plots of ln σ against 1/T. The thermoelectric power exhibits a maximum value of p-type conduction at pure Se and a change of the sign from positive to negative at 0.4

Organic solar cells of hydroxy squarylium

Abstract: The photovoltaic properties of Schottky barrier solar cells made from thin films (100–1000 A) of the organic dye hydroxy squarylium have been studied. Amorphous and polycrystalline films, prepared by evaporation and solution casting, have strong absorption over the entire visible spectrum. The highest power conversion efficiency measured under AMO white light was 0.1% (0.14 mW/cm2 input intensity); this decreased to 0.02% at 135 mW/cm2. The decrease in η with intensity is characteristic of organic materials, but is usually much more pronounced. For 8500‐A monochromatic light the quantum efficiency was as high as 2.3% and the conversion efficiency was 0.2% (1 mW/cm2).

ESCA Study of the Passive Film on an Extremely Corrosion-Resistant Amorphous Iron Alloy

Abstract: X-ray photoelectron spectroscopy was applied to study the composition of the passive film formed on an extremely corrosion resistant amorphous Fe-10at.%Cr-13at.%P-7at.%C alloy in 1 N HCl. The passive film consists mainly of hydrated chromium oxyhydroxide which is a common major constituent of passive films on crystalline stainless steels. The extremely high corrosion resistance of the amorphous alloy can only in part be attributed to the formation of a protective hydrated chromium oxyhydroxide film.

Studies of amorphous GeSeTe alloys (I): Preparation and calorimetric observations

Abstract: We have used differential scanning calorimetry (DSC) to examine the thermally induced transformations of bulk and thin-film amorphous alloys within a large portion of the GeSeTe system. Most chalcogen-rich compositions showed a discontinuous increase of heat capacity when heated through the glass transition temperature T G . The Ge-rich compositions, which could only be prepared as sputtered amorphous films, were invariably characterized by an irreversible exothermic crystallization process on heating, beginning at the crystallization temperature T X . Values of T g and T X have been tabulated for all alloys investigated and the compositional dependence of T g has been examined in the light of recent models for viscous flow in glass-forming chalcogenide systems. In addition, a region of liquid immiscibility has been observed in the vicinity of Ge 20 Se 40 Te 40 in which a GeSe 2 -rich liquid phase segregates from a tellurium-rich liquid phase. The existence and limits of this immiscibility region have been rationalized on the basis of ionic perturbations to the covalent bonding. The segregation of a GeSe 2 -rich liquid increases the concentration of GeSe bonds which are the strongest and most ionic of the six angle-bond types which can occur in this system.

Structure and properties of polyethyleneterephthalate crystallized by annealing in the highly oriented state

Abstract: The structure of polyethyleneterephthalate bristles drawn about five times in the amorphous state and subsequently crystallized at temperatures between 100 and 260‡ C has been studied by means of small-angle X-ray scattering. In addition density, heat of fusion and wide-angle scattering behaviour were measured. For comparison, similar experiments were carried out with undrawn samples. The results showed that the degree of crystallinity of PET cannot be calculated from density data on the basis of a simple two-phase model, since the effective densitiesρc* andρa* of the crystalline and amorphous regions depend strongly on crystallization and drawing conditions. With rising crystallization temperature the size of the mosaic blocks building up the crystalline layers and their longitudinal mutual order increase whereas the volume fraction of the crystalline region is only rather slightly effected by the annealing temperature. The difference between the effective densityρc* and the “X-ray density”ρc of the crystalline layers is supposed to be caused by lattice vacancies in the boundaries of the mosaic blocks.

A dielectric study of molecular relaxation in oxidized and chlorinated polyethylenes

Abstract: A study was made of the dielectric relaxation in polyethylenes rendered dielectrically active through oxidation (0.5–1.7 carbonyls/1000 CH2) and chlorination (14–22 Cl/1000 CH2). Both linear and branched polymers were studied. All of the relaxations between the melt and −196° were studied in the frequency range 10 Hz to 10kHz (100 kHz in the chlorinated samples). In the linear samples a wide range of crystallinities was studied (55% in quenched specimens to 95% in extended-chain specimens obtained by crystallization at 5 kbar). As is consistent with its being a crystalline process, the α peak was found to discontinously disappear on melting of the samples and reappear on recrystallizing on cooling. The disappearance of the smaller crystals before the larger ones appeared to be evident in the isothermal loss versus frequency curves. The relaxation strength of the α process increases with crystallinity. The measured relaxation strength is less than that expected on the basis of direct proportionality to the crystalline fraction with full contribution of all dipoles in the crystalline material. However, the intensity is not sufficiently low for the process to be interpreted in terms of reorientation of localized conformational defects in the crystal. The variation of intensity with crystallinity is best interpreted in terms of full participation of crystalline dipoles but with selective partitioning of both carbonyls and chlorines favoring the amorphous domains. A strong correlation of the α loss peak location (Tmax at constant frequency or log fmax at constant T) with crystallinity for both carbonyl and chlorine containing polymers was found. This variation is interpreted in terms of chain rotations in the crystal where the activation free energy depends on crystal thickness. The dependence of log fmax and Tmax on lamellar thickness as well as a comparison with the loss peaks of ketones dissolved in parafins indicates that the chain rotation is not rigid and is accompanied by twisting as the rotation propagates through the crystal. In agreement with previous studies the β process is found to be strong only in the branched polymers but can be detected in the chlorinated linear polymer. The β process was resolved from the α in the branched samples by curve fitting and its activation parameters determined. The γ relaxation peak in oxidized polymers including its high asymmetry (low-temperature tail) and increasing emax with increasing frequency and temperature when plotted isochronally can be interpreted in terms of a simple nearly symmetrical relaxation time spectrum that narrows with increasing temperature. No increase in relaxation strength with temperature was found. The chlorinated polymers behave similarly but appear to have some Boltzmann enhancement (450–750 cal/mole) of relaxation strength with temperature. The dependence of relaxation strength on crystallinity indicates that the process is an amorphous one. Further, no evidence of relaxation peak shape changes with crystallinity that could be interpreted in terms of a crystalline component in addition to the amorphous one was found. The comparison of the γ relaxation strength with that expected on the basis of full participation of amorphous dipoles indicates that only a small fraction (∼10% in oxidized linear polymers) of them are involved in the relaxation. Thus it would seem that a glass–rubber transition interpretation is not indicated but rather a localized chain motion. It is suggested that the γ process, including its intensity, width, and activation parameters, can be interpreted in terms of an (unspecified) localized conformational (bond rotation) motion that is perturbed by differing local packing environments. The thermal expansion lessens the effects of variations in packing and leads to narrowing with increasing temperature. The conformational motion itself leads to increase in thermal expansion and hence a transition in the latter property. Some previously proposed localized amorphous phase conformational motions appear to be suitable candidates for the bond rotation motion. A weak relaxation peak found at temperatures below the γ and at 10 kHz may possibly be the dielectric analog of the δ cryogenic peak found previously mechanically at lower frequencies.

High Corrosion Resistance of Chromium-Bearing Amorphous Iron Alloys in Neutral and Acidic Solutions Containing Chloride

Abstract: The corrosion resistance of amorphous iron alloys, whose practical application is expected because of their outstanding mechanical properties, has been studied by total immersion tests and...

Stability of amorphous metallic alloys

Abstract: In a previous paper it was concluded that the migration of phosphorus was causing the embrittlement of amorphous Fe40Ni40P14B6 at temperatures as low as 100 °C. In this paper we compare the stability of the above alloy to amorphous Fe40Ni40B20, Fe50Ni30P14B6, and Fe50Ni30B20. The stability is evaluated from brittleness measurements after annealing at temperatures between 100 and 400 °C. The results confirm the prediction that the removal of phosphorus suppresses the embrittlement at low temperatures; temperatures above 225 °C are required to cause embrittlement in the phosphorus‐free alloys. At higher temperatures the fracture strain approaches that of the phosphorus‐containing alloy. It is concluded that the replacement of P by B inhibits the low‐activation processes involved in the embrittlement process. The changes are not related to the changes in the glass transition temperatures.

The increase in the conductivity of chalcogenide glasses by the addition of certain impurities

Abstract: It is suggested that in chalcogenide glasses, elements such as Cu or Mn can have coordination number 4 and be negatively charged, and that the compensating positive charge is provided by D+ centres. The law of mass action ensures that the concentration of D-centres is then small. Under these conditions the Fermi energy is no longer pinned and the activation energy for conduction is shown to be decreased by 1/3. Similar considerations are applied to the effect of O, Cl and K on the conductivity of amorphous Se.

Magnetic Structure of an Amorphous Rare-Earth Transition-Metal Alloy

Abstract: /sup 161/Dy Mossbauer spectra show that the dysprosium moments in amorphous DyCo/sub 3.4/ are strongly coupled to the local crystal field axes. The magnetic structure is one in which the cobalt ''sublattice'' is strongly ferromagnetic, and the dysprosium moments are almost randomly distributed in directions between antiparallel and perpendicular to the cobalt. (AIP)

Dynamics of coloration of amorphous electrochromic films of WO3 at low voltages

Abstract: Measurements of the time dependence of the coloration (formation of HxWO3) of amorphous films of WO3 are described and analyzed in terms of a barrier for current flow at the WO3‐electrolyte interface. A novel feature is that as coloration proceeds, the chemical potential of hydrogen in the HxWO3 increases, opposing further coloration.

Chemical vapour-deposited silicon nitride

Abstract: The Vickers microhardness (VMH) of amorphous and crystalline chemical vapour-deposited silicon nitride (Pyrolytic-Si3N4) prepared under various deposition conditions using SiCl4, NH3 and H2 has been measured at room temperature The apparent VMH of Py-Si3N4 increases with decreasing indenter load, following the Meyer equation For the amorphous Py-Si3N4, the VMH value at a load of 100 g ranges from 2200 to 3200 kg mm−2 depending upon deposition temperature (Tdep) and total gas pressure (Ptot) On the other hand, the VMH of the crystalline Py-Si3N4 does not depend uponTdep andPtot but is affected by the preferred orientation The hardness of the deposition surfaces with the (110), (210) and (222) orientations is 3800 kg mm−2, while that of the cross-section with the (001) orientation is 3100 kg mm−2 The hardness of the crystalline Py-Si3N4 is also affected by the grain size For the fine grained Py-Si3N4 (about 1μm), the VMH varies from 4600 to 5000 kg mm−2 The hardnesses of three types of Py-Si3N4 are discussed in comparison with those of other ceramics

Glass transition in the hard-sphere model

Abstract: Equations of state and transport data for the hard-sphere fluid suggest a quasi-thermodynamic transition from supercooled fluid to amorphous solid at a density slightly greater than the equilibrium freezing density This has been substantiated with a lengthy MD computation in which the density was increased very gradually from a low density fluid state to almost close-packing The amorphous close-packed density of 500 spheres with periodic boundary is found to be ρaσ3= 1217 ± 0004 A glass transition occurs in the density region ρgσ3= 100 ± 001; a (diffuse) thermodynamic transition of the third-order is indicated

Modified mean‐field model for rare‐earth–iron amorphous alloys

Abstract: A mean‐field model of the magnetic properties of amorphous rare‐earth–iron alloys has been developed which incorporates an Fe spin whose dependence on both the concentration and the species of rare earth is based on Mossbauer spectra. Using this model with a single fixed set of exchange constants, we have been able to calculate Curie temperatures which are in reasonable agreement with the data for amorphous rare‐earth (Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu–iron alloys containing between 50 and 100 at.% iron. Better agreement with the measured Curie temperatures is achieved if the magnitudes of the Fe‐Fe and rare‐earth–Fe exchange constants are allowed to increase linearly with rare‐earth concentration. The latter case has the additional advantage of reproducing the observed shape of the magnetization‐vs‐temperature curves for wide range of composition and rare‐earth species.

Electroluminescence in amorphous silicon

Abstract: Electroluminescence has been obtained in forward‐biased p‐i‐n diodes, and also in Schottky barrier diodes fabricated from discharge‐produced amorphous Si. The emission at 78 °K in both electroluminescence and photoluminescence peaks at 1.28±0.08 eV in a 0.2‐eV broad band with an external quantum efficiency of ∼10−3.

On state of amorphous threshold switches

Abstract: A detailed study of the on state of amorphous threshold switches has been performed. Transient and steady‐state I‐V characteristics have been investigated as functions of film thickness, pore size, and electrode material. Velocity saturation effects in amorphous/crystalline Si heterojunctions and threshold recovery curves were also studied. It is found that the on‐state resistivity is approximately 0.08 Ω cm, independent of electrode material. The on state is that of a semiconductor whose electronic band structure and carrier mobility are essentially unchanged from the off state, but in which a concentration of about 7×1018 free electron/cm3 produces the high conductivity observed. The decay of the on state is due to a combination of diffusion of the free carriers out of the filament, together with direct carrier recombination. A major result of this work is the determination of the size of the conducting filament as a function of operating current. As expected, the current density in the filament is esse...