Showing papers on "Amorphous solid published in 1987"

Electron Spectroscopy Studies on Magneto-Optical Media and Plastic Substrate Interface

Abstract: The depth-profiles of amorphous TbFeCo films sputtered onto polycarbonate substrate were studied by X-ray photoelectron spectroscopy. Oxidized metals, oxides and hydroxides for example, and adsorbed impurities were found to exist mainly in the vicinity of the film surface and film/ substrate interface.

A.c. conduction in amorphous chalcogenide and pnictide semiconductors

Abstract: The various origins of a frequency-dependent conductivity in amorphous semiconductors are reviewed, stressing particularly recent advances and the influences that factors such as correlation and non-random spatial distributions of electrically active centres can have on the a.c. conductivity. A comprehensive survey is given of the experimental a.c. data for two types of amorphous semiconductor, namely chalcogenide and pnictide materials. It is concluded that the a.c. behaviour at intermediate to high temperatures is well accounted for by the correlated-barrier-hopping model, whereas the low-temperature behaviour is probably due to atomic tunnelling.

Thermal conductivity of amorphous solids above the plateau.

Abstract: We have measured the thermal conductivity of polymethylmethacrylate (PMMA), amorphous ${\mathrm{As}}_{2}$${\mathrm{S}}_{3}$, Ca-K nitrate glass, three ${\mathrm{SiO}}_{2}$-based glasses, and glycerol in the temperature range 30--300 K, using an ac technique that eliminates errors from blackbody radiation. This technique and the problems of blackbody radiation in thermal measurements are discussed in detail. Our data do not support a recent prediction of heat transport by fractons. Instead, we find that the thermal conductivity of these glasses above \ensuremath{\sim}50 K is well described by the minimum thermal conductivity suggested by Slack.

Observation of a glass transition in suspensions of spherical colloidal particles.

Abstract: Concentrated suspensions of submicron colloidal spheres were studied both by dynamic light scattering and by direct observation of their phase behavior. In agreement with recent theory and computer simulation, the measured dynamic structure factor developed an essentially nondecaying component, implying ``structural arrest,'' at almost the same concentration as that at which a long-lived amorphous or glass phase was first observed.

Relaxation behaviour of the amorphous components of wood

Abstract: The viscoelastic properties of mod were investigated using dynamic mechanical thermal analysis and differential scanning calorimetry. Under a limited set of conditions, two separate glass transitions (T g) could be identified with both techniques. These two transitions were identified as arising from the amorphous lignin and hernicellulose matrix in the wood cell wall. Moisture dramatically affected the temperature at which the two dispersions occurred and, consequently, the ability to resolve their independent responses. The relationship betweenT g and moisture for both components could be modelled with the Kwei equation, which accounts for the presence of secondary interactions. Annealing and specific interactions of a series of organic diluents were wed in an attempt to enhance the resolution of the two components values ofT g. Time-temperature superposition was shown to be applicable to wood plasticized with ethyl formamide, following Williams-Landel-Ferry behaviour over the temperature rangeT g toT g + 85° C. These observations allow certain conclusions to be drawn concerning the applicability of existing models of the wood cell wall's supermolecular morphology. Most notably, models of thein situ morphology of the three wood components can be limited to those which consider the amorphous matrix of lignin and hemicellulose to be immiscible.

Recrystallization of amorphized polycrystalline silicon films on SiO2: Temperature dependence of the crystallization parameters

Abstract: This paper presents a theoretical and experimental study of the recrystallization behavior of polycrystalline silicon films amorphized by self‐implantation. The crystallization behavior was found to be similar to the crystallization behavior of films deposited in the amorphous state, as reported in the literature; however, a transient time was observed, during which negligible crystallization occurs. The films were prepared by low‐pressure chemical vapor deposition onto thermally oxidized silicon wafers and amorphized by implantation of silicon ions. The transient time, nucleation rate, and characteristic crystallization time were determined from the crystalline fraction and density of grains in partially recrystallized samples for anneal temperatures from 580 to 640 °C. The growth velocity was calculated from the nucleation rate and crystallization time and is lower than values in the literature for films deposited in the amorphous state. The final grain size, as calculated from the crystallization param...

Phase diagram of silicon by molecular dynamics.

Abstract: Using the Stillinger-Weber potential we explored the liquid, crystal, and amorphous phase diagram of silicon by molecular dynamics. We obtain the chemical potential of the crystal by following the crystal-vapor coexistence curve from the T=0 harmonic solid up to the melting point. The liquid free energy is found by reversible expansion. The thermodynamic melting point is 1691\ifmmode\pm\else\textpm\fi{}20 K, which is very close to the experimental value of 1683 K. Contrary to experiment, the calculated supercooled liquid phase does not undergo a first-order transition to the fourfold-coordinated amorphous structure upon cooling, since the chemical potentials of these structures are almost equal over a wide range of temperatures. Diffusion coefficients, heat capacities, and expansivities are compared with experiment.

Tribological and electrical properties of metal‐containing hydrogenated carbon films

Abstract: The formation of metal‐containing amorphous hydrogenated carbon films (a‐C:H) in a reactive sputtering process is reported. The layers were prepared at room temperature using various metals of different concentrations. According to tribological measurements the layers exhibit small friction values and an extremely low abrasive wear rate. The electrical conductivity depends on the metal concentration and could be varied over many orders of magnitude.

Thin film electrical devices with amorphous carbon electrodes and method of making same

Abstract: Thin film electrical structures, such as threshold switching devices and phase change memory cells, preferably utilizing electrically stable, relatively inert, conductive electrodes including a non-single-crystal deposited film of carbon material, are disclosed. The film of carbon material, which preferably is amorphous and substantially pure, is disposed adjacent to a layer of active material such as an amorphous semiconductor, and serves to prevent undesired degradation of the active material, especially when the device is carrying appreciable current in its on-state. A method of making such structures with high quality interfaces between the semiconductor layer and the conductive carbon barrier layers adjacent thereto by successively depositing such layers in a continuously maintained partial vacuum is disclosed. The method may include a step performed in the vacuum for hermetically sealing all of, or at least the electrically switchable portion of, the active layer against subsequent contamination. Thin film structures suitable for threshold switching or memory applciations and employing insulating pores having substantially sloped side walls are also disclosed.

Resonant Raman scattering of hydrogenated amorphous carbon: Evidence for π‐bonded carbon clusters

Abstract: Resonant Raman spectroscopy has been used to study amorphous hydrogenated carbon films. For films containing both sp2 and sp3 bonded carbon a well‐defined high‐frequency shift of the main Raman peak is observed with increasing exciting photon energy. This shift is interpreted in terms of scattering from π‐bonded carbon clusters which is resonantly enhanced for incident photon energies approaching the π–π* resonance.

Amorphous Ti-Si alloy formed by interdiffusion of amorphous Si and crystalline Ti multilayers

Abstract: Reactions upon rapid thermal annealing of sputtered Ti‐Si multilayers have been studied by cross‐section and through‐foil transmission electron microscopy, glancing‐angle Rutherford backscattering, and x‐ray diffraction. The compositions of the samples are 40 at. % Ti, 60 at. % Si and 60 at. % Ti, 40 at. % Si, and the bilayer periodicity is about 10 nm. The silicon layers in the as‐deposited films are amorphous; the titanium layers are polycrystalline hcp. After a 30‐s anneal at 455 °C, significant interdiffusion occurs and we observed the formation of an amorphous Ti‐Si alloy by interfacial reaction. The metastable disilicide, C49 TiSi2, nucleated along with a small amount of TiSi in the sample with higher silicon content (60%) upon annealing at 550 °C for 10 s, but the amorphous alloy remained as the only product of reaction in the 40‐at. % Si sample.

Detailed investigation of doping in hydrogenated amorphous silicon and germanium.

Abstract: Substitutional doping of hydrogenated amorphous silicon (a-Si:H) and germanium (a-Ge:H) with phosphorus, arsenic, and boron has been investigated, with use of electron-spin-resonance techniques, optical absorption, and transport measurements. Doping-induced changes in the density of shallow states and deep defects (dangling bonds) are compared for the different doping-host systems. Hyperfine spectra of neutral donor levels are observed in spin resonance and used to deduce a microscopic picture of the underlying donor wave functions. Based on the dependence of the occupancy of deep and shallow states on doping levels and temperature, a detailed model for the electronic density of states in n-type a-Si:H and a-Ge:H below the conduction-band mobility edge is obtained. Furthermore, similar studies in nominally compensated a-Si:H are used to discuss the location of boron acceptor states in this material as well as questions concerning light-induced creation of metastable dangling bonds. Experimental evidence for the existence of exchange-coupled electron-hole pairs in compensated a-Si:H is presented. For an investigation of the doping process, the incorporation of the various dopant atoms from the deposition gas phase into the amorphous film has been studied by secondary-ion mass spectroscopy. The concentration of electronically active dopants in the deposited film is related to the totalmore » concentrations of dopants in the solid or the deposition gas phase for a calculation of the corresponding doping efficiencies.« less

Effects of radiation damage in ion‐implanted thin films of metal‐oxide superconductors

Abstract: The effects of ion implantation into thin films of the superconductor YBa2Cu3Ox have been studied. Using oxygen and arsenic ions, the superconducting transition temperature Tc, the change in room‐temperature electrical properties from conducting to insulating, and the crystalline to amorphous structural transition in the films were studied as a function of ion dose. The deposited energy required to change Tc was found to be 0.2 eV/atom, while 1–2 eV/atom was required to affect the room‐temperature conductivity, and 4 eV/atom to render the film amorphous. This hierarchy of effects is discussed in terms of the damage mechanisms involved.

Bias dependence of instability mechanisms in amorphous silicon thin‐film transistors

Abstract: We have measured the bias dependence of the threshold voltage shift in a series of amorphous silicon‐silicon nitride thin‐film transistors, where the composition of the nitride is varied. There are two distinct instability mechanisms: a slow increase in the density of metastable fast states and charge trapping in slow states. State creation dominates at low fields and charge trapping dominates at higher fields. The state creation is found to be independent of the nitride composition, whereas the charge trapping depends strongly on the nitride composition. This is taken as good evidence that state creation takes place in the hydrogenated amorphous silicon (a‐Si:H) layer, whereas the charge trapping takes place in the a‐SiN:H. The metastable states are suggested to be Si dangling bonds in the a‐Si:H, and the state creation process similar to the Staebler–Wronski effect. The confirmation of state creation in a thin‐film transistor means that states can be created simply by populating conduction‐band states i...

Formation of thin superconducting films by the laser processing method

Abstract: We have prepared thin films of Y‐Ba‐Cu‐O superconductors using a pulsed laser evaporation technique. Thin films were formed on (100) Si, (100) MgO, (1102) sapphire, (100) SrTiO3, and amorphous SiO2 substrates using a XeCl excimer laser (λ=0.308 μm, τ=45×10−9 s). The depositions were done in an ultrahigh vacuum chamber with pressure of about 10−6 Torr during thin‐film formation. The deposition by pulsed nanosecond laser irradiation results in stoichiometry close to that of the target. The thickness of the film was controlled by varying the pulse energy density and the number of pulses. The substrate temperature was kept at 470 °C during deposition. Subsequent annealing treatments were carried out at 900 and 650 °C in oxygen atmosphere to recover the superconducting properties of these thin films. The resistance of these films was measured as a function of temperature using the four‐point probe method. These thin films were analyzed using cross‐section transmission electron microscopy, Rutherford backscatt...

Resolution of amorphous silicon thin-film transistor instability mechanisms using ambipolar transistors

Abstract: Bias stress measurements on amorphous silicon‐silicon nitride ambipolar thin‐film transistors give clear evidence for the co‐existence of two distinct instability mechanisms: the metastable creation of states in the a‐Si:H layer and charge trapping in the a‐SiN:H layer. The creation of metastable states in the a‐Si:H is found to dominate at low positive bias, while charge trapping in the nitride dominates at larger positive bias and negative bias.

Magnetization reversal, coercivity, and the process of thermomagnetic recording in thin films of amorphous rare earth-transition metal alloys

Abstract: A model is proposed for the mechanism of magnetization reversal in thin films of amorphous alloys with perpendicular magnetic anisotropy. Examples of these alloys are TbFe, GdCo, DyFe, and GdTbFeCo, which are currently under investigation as storage media for erasable optical recording applications. The model exhibits the observed behavior of the media such as nucleation and growth of reverse‐magnetized domains under external magnetic fields; square hysteresis loops; temperature dependence of coercivity; formation and stability of domains under conditions of thermomagnetic recording; and incomplete erasure with insufficient applied fields.

Structure and dynamics of amorphous water ice.

Abstract: Further insight into the structure and dynamics of amorphous water ice, at low temperatures, was obtained by trapping in it Ar, Ne, H2, and D2. Ballistic water-vapor deposition results in the growth of smooth, approximately 1 x 0.2 micrometer2, ice needles. The amorphous ice seems to exist in at least two separate forms, at T < 85 K and at 85 < T < 136.8 K, and transform irreversibly from one form to the other through a series of temperature-dependent metastable states. The channels formed by the water hexagons in the ice are wide enough to allow the free penetration of H2 and D2 into the ice matrix even in the relatively compact cubic ice, resulting in H2-(D2-) to-ice ratios (by number) as high as 0.63. The larger Ar atoms can penetrate only into the wider channels of amorphous ice, and Ne is an intermediate case. Dynamic percolation behavior explains the emergence of Ar and Ne (but not H2 and D2) for the ice, upon warming, in small and big gas jets. The big jets, each containing approximately 5 x 10(10) atoms, break and propel the ice needles. Dynamic percolation also explains the collapse of the ice matrix under bombardment by Ar , at a pressure exceeding 2.6 dyn cm-2, and the burial of huge amounts of gas inside the collapsed matrix, up to an Ar-to-ice of 3.3 (by number). The experimental results could be relevant to comets, icy satellites, and icy grain mantles in dense interstellar clouds.

Structure of high‐density amorphous water. II. Neutron scattering study

Abstract: High‐density amorphous water is studied by neutron scattering in a Q range extending to 16 A−1. The low density form of amorphous water is also analyzed and compared with previous results. There are very important differences in the composite pair correlation functions of the two forms of amorphous ice, in particular beyond the first nearest‐neighbors distance. We conclude that the hydrogen bond network is strongly deformed in a manner analogous to that found in water at high temperature. This is in contrast with the behavior of the pair correlation function of low‐density amorphous water, which is closer to that of supercooled water.

Control of Grain Boundary Location By Selective Nucleation Over Amorphous Substrates

Abstract: A selective nucleation based crystal-growth-technique over amorphous substrates is originated. The method manipulates nucleation sites and periods and hence, controls the grain boundary location by modifing the substrate surface. In Si, small Si 3 N 4 nucleation sites are formed, 1–2 pm in diameter, 100 μm in period, over Sio 2 . One Si nucleus is formed exclusively in the small area of Si 3 N 4 by CVD. The highly faceted and periodically located nuclei grow over SiO 2 up to 100 μm in diameter before impingement. A MOS-FET fabricated inside the island operates comparably to the bulk Si control

An avalanche-mode amorphous Selenium photoconductive layer for use as a camera tube target

Abstract: It was found for the first time that an evaporated amorphous photoconductive layer of selenium produced the phenomenon of avalanche multiplication, and an amorphous photoconductive layer was used as the target in an experimental camera tube. The camera tube shows high sensitivity, greatly in excess of a quantum efficiency of unity, the upper limit of the sensitivity of a conventional blocking target layer. Deterioration of lag and resolution characteristics were hot observed. The noise produced by the avalanche mode was negligible. This experimental target layer is expected to prove especially suitable for high-performance camera tubes.

High-rate deposition of amorphous hydrogenated silicon: effect of plasma excitation frequency

Abstract: The effect of plasma excitation frequency on the deposition of amorphous hydrogenated silicon in a silane glow-discharge system is investigated. A large increase in the deposition rate up to 21 A/s is observed in the range between 25 and 150 MHz. Optical and electrical film parameters remain practically unchanged over this frequency range.

Ion-beam-induced crystallization and amorphization of silicon

Abstract: Thin amorphous silicon layers can be produced in crystalline silicon substrates by ion-implantation. Subsequent ion-irradiation at elevated temperatures can induce such layers to either crystallize epitaxially or increase in thickness, layer by layer. This paper examines these processes and their dependence on substrate temperature and ion-irradiation parameters. It is shown that both processes, epitaxial crystallization and layer-by-layer amorphization, are controlled by ion-beam induced defect production at, or near, the crystalline/amorphous interface. The competition between defect production (determined by the ion flux and rate of nuclear energy deposition) and dynamic defect annealing (determined by the substrate temperature) is shown to play an important role in determining whether the layer crystallizes or amorphizes. Possible models for the observed behavior are discussed.

Guiding principle for preparing highly photosensitive Si-based amorphous alloys

Abstract: A guiding principle was proposed for the preparation of highly-photosensitive hydrogenated amorphous SiGe (a-SiGe:H) as well as SiC (a-SiC:H) alloy materials via the plasma chemical-vapor-deposition (CVD), which was derived from the experimental analyses of the behavior of adsorbed precursors on the growing surface by means of a radical separation technique. According to this principle, Triode method and H2-dilution method of starting gas materials were used for the deposition of a-SiGe:H and a-SiC:H, and highly-photoconductive alloy materials were prepared in the optical-gap range between 1.3 and 2.3eV.

Steady‐state photocarrier grating technique for diffusion‐length measurement in semiconductors: Theory and experimental results for amorphous silicon and semi‐insulating GaAs

Abstract: The theory underlying the steady‐state photocarrier grating technique is presented, including the effect of surface recombination. Experimental results for amorphous hydrogenated silicon and semi‐insulating GaAs prove that diffusion lengths ranging from 200 A to 10 μm can be measured with an accuracy of better than 5%.