Showing papers on "Amorphous solid published in 2005"

Field-effect transistor

Abstract: Provided is a field-effect transistor including an active layer and a gate insulating film, wherein the active layer includes an amorphous oxide layer containing an amorphous region and a crystalline region, and the crystalline region is in the vicinity of or in contact with an interface between the amorphous oxide layer and the gate insulating film

Amorphous oxide and field effect transistor

Abstract: A novel amorphous oxide applicable, for example, to an active layer of a TFT is provided. The amorphous oxide comprises microcrystals.

Electric elements and circuits utilizing amorphous oxides

Abstract: Semiconductor devices and circuits with use of transparent oxide film are provided. The semiconductor device having a P-type region and an N-type region, wherein amorphous oxides with electron carrier concentration less than 10 18 /cm 3 is used for the N-type region.

230% room temperature magnetoresistance in CoFeB/MgO/CoFeB magnetic tunnel junctions

Abstract: The magnetoresistance ratio of 230% at room temperature is reported. This was achieved in spin-valve type magnetic tunnel junctions using MgO barrier layer and amorphous CoFeB ferromagnetic electrodes fabricated on thermally oxidized Si substrates. The amorphous CoFeB electrodes are of great advantage to the polycrystalline FeCo electrodes in achieving a high homogeneity in small 100 nm-sized MTJs.

Photo- and electroactive amorphous molecular materials—molecular design, syntheses, reactions, properties, and applications

Abstract: A new field of organic materials science that deals with amorphous molecular glasses has been opened up. In addition, amorphous molecular materials have constituted a new class of functional organic materials for use in various applications. This article is focused on the recent progress of studies on photo- and electroactive amorphous molecular materials, highlighting photochromic amorphous molecular materials, amorphous molecular resists, and amorphous molecular materials for use in devices such as organic EL devices. The molecular design concepts, syntheses, reactions, molecular and solid-state properties, functions, and device fabrication and performance are described.

High mobility transparent thin-film transistors with amorphous zinc tin oxide channel layer

Abstract: Transparent thin-film transistors (TTFTs) with an amorphous zinc tin oxide channel layer formed via rf magnetron sputter deposition are demonstrated. Field-effect mobilities of 5–15 and 20–50cm2V−1s−1 are obtained for devices post-deposition annealed at 300 and 600°C, respectively. TTFTs processed at 300 and 600°C yield devices with turn-on voltage of 0–15 and −5–5V, respectively. Under both processing conditions, a drain current on-to-off ratio greater than 107 is obtained. Zinc tin oxide is one example of a new class of high performance TTFT channel materials involving amorphous oxides composed of heavy-metal cations with (n−1)d10ns0 (n⩾4) electronic configurations.

Blueshift of optical band gap in ZnO thin films grown by metal-organic chemical-vapor deposition

Abstract: The optical band gap of ZnO thin films deposited on fused quartz by metal-organic chemical-vapor deposition was studied. The optical band gap of as-grown ZnO blueshifted from 3.13to4.06eV as the growth temperature decreased from 500to200°C. After annealing, the optical band gap shifted back to the single-crystal value. All the ZnO thin films studied show strong band-edge photoluminescence. X-ray diffraction measurements showed that samples deposited at low temperatures (<450°C) consisted of amorphous and crystalline phases. The redshift of the optical band gap back to the original position after annealing was strong evidence that the blueshift was due to an amorphous phase. The unshifted photoluminescence spectra indicated that the luminescence was due to the crystalline phase of ZnO, which was in the form of nanocrystals embedded in the amorphous phase.

Role of Alumina Coating on Li−Ni−Co−Mn−O Particles as Positive Electrode Material for Lithium-Ion Batteries

Abstract: The interface reaction between Al2O3-coated Li[Li0.05Ni0.4Co0.15Mn0.4]O2 and liquid electrolyte was investigated. The Al2O3-coated Li[Li0.05Ni0.4Co0.15Mn0.4]O2 showed no large difference in the bulk structure, comparing to bare Li[Li0.05Ni0.4Co0.15Mn0.4]O2. The coated Al2O3 was found to have an amorphous structure from X-ray diffraction study. A small amount of Al2O3 coating (0.25 wt % in the final composition) showed that a uniform mesoporous Al2O3-coating layer whose thickness is of about 5 nm covers Li[Li0.05Ni0.4Co0.15Mn0.4]O2 particles, as confirmed by transmission electron microscopy. At higher concentration (2.5 wt % in the final composition), the irregular tens of nanometer-sized Al2O3 powders were observed on the surface of the active material instead of the uniform coating layer. Despite the insulating nature of Al2O3, the thin coating was effective to improve the battery performances, depending on the thickness of the Al2O3-coating layer, and used electrolytic salt. The Al2O3 coating resulted i...

Transparent Highly Ordered TiO2 Nanotube Arrays via Anodization of Titanium Thin Films

Abstract: Titanium thin films, 400 nm to 1000 nm thick, fabricated by radio frequency (rf) sputter deposition are anodized in an electrolyte containing acetic acid and hydrofluoric acid to form optically transparent films of highly ordered titania nanotube arrays Real-time monitoring of the anodization current, at a fixed potential, is used to controllably eliminate the Ti layer underneath the titanium oxide nanotube array without disturbing the architecture Fabrication variables critical to achieving the transparent nanotube-array film include annealing temperature of the anodized, initially amorphous nanotube array and Ti-film sputter deposition variables, including rate, film thickness, and substrate temperature Structural investigations on the transparent nanotube arrays reveal only the presence of the anatase phase even after annealing at 500 °C In contrast, both rutile and anatase phases were observed in films with a metal layer underneath the nanotubes and annealed in an oxygen ambient above 430 °C Rutile growth occurs at the nanotube–metal interface as metal oxidation takes place during annealing The average refractive index of the transparent nanotube-array film is found to be 166 in the UV-vis range, with a calculated porosity of 67 %; the bandgap is determined as 334 eV, with a bandgap tail extending to 24 eV

Reproducible on-off switching of solid-state luminescence by controlling molecular packing through heat-mode interconversion.

Abstract: Organic luminescent solids are attracting increasing interest in various fields of application. Modification or alteration of the chemical structures of their component molecules is the most common approach for tuning their luminescence properties. However, for dynamic tuning or switching of solid-state luminescence with high efficiency and reproducibility successful examples are limited as chemical reactions in the solid state frequently encounter insufficient conversion, one-way reactions or loss of their luminescence properties. One promising approach is to control the luminescence properties by altering the mode of solid-state molecular packing without chemical reactions. Here, we show that 2,2':6',2''-terpyridine, practically non-luminescent in the form of amorphous solid or needle crystal, shows strong blue luminescence upon formation of a plate crystal. Efficient and reproducible on-off switching of solid-state luminescence is demonstrated by heat-mode interconversion between the plate and needle crystals. Because alteration of the mode of molecular packing does not require chemical reactions, the present findings would open the way for the development of novel organic luminescent solids that can be switched on and off by external thermal stimuli.

Investigation of the optical and electronic properties of Ge2Sb2Te5 phase change material in its amorphous, cubic, and hexagonal phases

Abstract: Ge–Sb–Te alloys are widely used for data recording based on the rapid and reversible amorphous-to-crystalline phase transformation that is accompanied by increases in the optical reflectivity and the electrical conductivity. However, uncertainties about the optical band gaps and electronic transport properties of these phases have persisted because of inappropriate interpretation of reported data and the lack of definitive analytical studies. In this paper we characterize the most widely used composition, Ge2Sb2Te5, in its amorphous, face-centered-cubic, and hexagonal phases, and explain the origins of inconsistent or unphysical results in previous reports. The optical absorption in all of these phases follows the relationship αhν∝(hν−Egopt)2, which corresponds to the optical transitions in most amorphous semiconductors as proposed by Tauc, Grigorovici, and Vancu [Tauc et al., Phys. Status Solidi 15, 627 (1966)], and to those in indirect-gap crystalline semiconductors. The optical band gaps of the amorpho...

Performance of Manganese Oxide/CNTs Composites as Electrode Materials for Electrochemical Capacitors

Abstract: Nanocrystalline metal oxides can be prepared with large surface area, electrochemical stability, and pseudocapacitive behavior, being able to be used as supercapacitor electrodes. Among the various metal oxides studied, amorphous and hydrated manganese oxide (a-MnO 2 .nH 2 O) is the most promising for supercapacitor electrodes due to the low cost of the raw material. In the present work, amorphous manganese dioxide (a-MnO 2 .nH 2 O) is prepared by chemical co-precipitation of Mn(VII) and Mn(II) in water medium, giving small particles of relatively high surface area. Carbon nanotubes (CNTs) are proposed as an alternative additive of carbon black for improving the electrical conductivity of the manganese oxide electrodes used to build capacitors. The results demonstrate that CNTs are effective for increasing the capacitance and improving the electrochemical properties of the a-MnO 2 .nH 2 O electrodes which show a better capacitive behavior than with carbon black. This enhancement is due to the high entanglement of CNTs which form a network of open mesopores, allowing the bulk of MnO 2 to be easily reached by the ions. The performance optimization requires a careful control of the electrolyte pH in order to avoid the irreversible reactions Mn(IV) to Mn(II) at the negative electrode and Mn(IV) to Mn(VII) at the positive one.

Effect of Dielectric Roughness on Performance of Pentacene TFTs and Restoration of Performance with a Polymeric Smoothing Layer

Abstract: The morphology, structure, and transport properties of pentacene thin film transistors (TFTs) are reported showing the influence of the gate dielectric surface roughness Upon roughening of the amorphous SiO2 gate dielectric prior to pentacene deposition, dramatic reductions in pentacene grain size and crystallinity were observed The TFT performance of pentacene films deposited on roughened substrates showed reduced free carrier mobility, larger transport activation energies, and larger trap distribution widths Spin coating roughened dielectrics with polystyrene produced surfaces with 2 A root-mean-square (rms) roughness The pentacene films deposited on these coated surfaces had grain sizes, crystallinities, mobilities, and trap distributions that were comparable to the range of values observed for pentacene films deposited on thermally grown SiO2 (roughness also ∼2 A rms)

On the rigidity of amorphous solids

Abstract: We poorly understand the properties of amorphous systems at small length scales, where a continuous elastic description breaks down. This is apparent when one considers their vibrational and transport properties, or the way forces propagate in these solids. Little is known about the microscopic cause of their rigidity. Recently it has been observed numerically that an assembly of elastic particles has a critical behavior near the jamming threshold where the pressure vanishes. At the transition such a system does not behave as a continuous medium at any length scales. When this system is compressed, scaling is observed for the elastic moduli, the coordination number, but also for the density of vibrational modes. In the present work, we derive theoretically these results, and show that they apply to various systems such as granular matter and silica, but also to colloidal glasses. In particular we show that: (i) these systems present a large excess of vibrational modes at low frequency in comparison with normal solids, called the "boson peak" in the glass literature. The corresponding modes are very different from plane waves, and their frequency is related to the system coordination; (ii) rigidity is a non-local property of the packing geometry, characterized by a length scale which can be large. For elastic particles this length diverges near the jamming transition; (iii) for repulsive systems the shear modulus can be much smaller than the bulk modulus. We compute the corresponding scaling laws near the jamming threshold. Finally, we discuss the implications of these results for the glass transition, the transport, and the geometry of the random close packing.

The ultrasmoothness of diamond-like carbon surfaces.

Abstract: The ultrasmoothness of diamond-like carbon coatings is explained by an atomistic/continuum multiscale model. At the atomic scale, carbon ion impacts induce downhill currents in the top layer of a growing film. At the continuum scale, these currents cause a rapid smoothing of initially rough substrates by erosion of hills into neighboring hollows. The predicted surface evolution is in excellent agreement with atomic force microscopy measurements. This mechanism is general, as shown by similar simulations for amorphous silicon. It explains the recently reported smoothing of multilayers and amorphous transition metal oxide films and underlines the general importance of impact-induced downhill currents for ion deposition, polishing, and nanopattering.

Strain localization and percolation of stable structure in amorphous solids.

Abstract: Spontaneous strain localization occurs during mechanical tests of a model amorphous solid simulated using molecular dynamics. The degree of localization depends upon the extent of structural relaxation prior to mechanical testing. In the most rapidly quenched samples higher strain rates lead to increased localization, while the more gradually quenched samples exhibit the opposite strain rate dependence. This transition coincides with the $k$-core percolation of atoms with quasi-crystal-like short range order. The authors infer the existence of a related microstructural length scale.

Amorphous metallic plastic.

Abstract: We report cerium-based bulk metallic glasses with an exceptionally low glass transition temperature Tg, similar to or lower than that of many polymers. We demonstrate that, in near-boiling water, these materials can be repeatedly shaped, and can thus be regarded as metallic plastics. Their resistance to crystallization permits extended forming times above Tg and ensures an adequate lifetime at room temperature. Such materials, combining polymerlike thermoplastic behavior with the distinctive properties of metallic glasses, are highly unusual for metallic alloys and have great potential in applications and can also facilitate studies of the supercooled liquid state.

A theory for amorphous viscoplastic materials undergoing finite deformations, with application to metallic glasses

Abstract: This study develops a finite-deformation, Coulomb–Mohr type constitutive theory for the elastic–viscoplastic response of pressure-sensitive and plastically-dilatant isotropic materials. The constitutive model has been implemented in a finite element program, and the numerical capability is used to study the deformation response of amorphous metallic glasses. Specifically, the response of an amorphous metallic glass in tension, compression, strip-bending, and indentation is studied, and it is shown that results from the numerical simulations qualitatively capture major features of corresponding results from physical experiments available in the literature.

Results on disordered materials from the GEneral Materials diffractometer, GEM, at ISIS

Abstract: The GEneral Materials diffractometer, GEM, at the ISIS Facility pulsed neutron source is the most advanced materials neutron diffractometer in the world A full description of GEM is given from the point of view of structural studies of disordered materials The key component for these studies is the highly stable detector array, which covers a very wide range in scattering angles and a very large solid angle, leading to a wide dynamic range and a high count rate The high real-space resolution of GEM, due to a high maximum momentum transfer of 55 A ˚ � 1 , is illustrated by data on bioactive phosphate glasses, where bonds from phosphorus to bridging and non-bridging oxygens (which differ in length by approximately 012 A ˚ ) are clearly resolved The low momentum transfer ability of GEM is demonstrated by a measurement on amorphous Si77D23, which yields reliable data down to 01 A ˚ � 1 The first successful isotopic substitution experiment on sol–gel materials has been performed on GEM in a study of amorphous titanium silicates, showing the suitability of the instrument for both isotopic substitution, and for studies of complex materials, even in the presence of hydrogen Studies of the structures of disordered group 11 cyanide crystals show the power of GEM to reveal the nature of disorder in crystalline systems as well as glassy materials The crystal structure of high temperature CuCN has been solved for the first time The disorder in AuCN, AgCN and high temperature CuCN arises from random displacements of the linear –M–(CRN)– chains relative to each other, and the one-dimensional negative thermal expansion is due to long wavelength cooperative motions of the chains

Nanosized CeO2-SiO2, CeO2-TiO2, and CeO2-ZrO2 mixed oxides: influence of supporting oxide on thermal stability and oxygen storage properties of ceria

Abstract: The influence of SiO2, TiO2, and ZrO2 on the structural and redox properties of CeO2 were systematically investigated by various techniques namely, X-ray diffraction (XRD), Raman spectroscopy (RS), UV–Vis diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HREM), BET surface area, and thermogravimetry methods. The effect of supporting oxides on the crystal modification of ceria was also mainly focused. The investigated oxides were obtained by soft chemical routes with ultrahigh dilute solutions and were subjected to thermal treatments from 773 to 1073 K. The XRD results suggest that the CeO2–SiO2 sample primarily consists of nanocrystalline CeO2 on the amorphous SiO2 surface. Both crystalline CeO2 and TiO2-anatase phases were noted in the case of CeO2–TiO2 sample. Formation of cubic Ce0.75Zr0.25O2 and Ce0.6Zr0.4O2 (at 1073 K) were observed in the case of CeO2–ZrO2 sample. The cell ‘a’ parameter estimations revealed an expansion of the ceria lattice in the case of CeO2–TiO2, while a contraction is noted in the case of CeO2–ZrO2. The DRS studies suggest that the supporting oxides significantly influence the band gap energy of CeO2. Raman measurements disclose the presence of oxygen vacancies, lattice defects, and displacement of oxide ions from their normal lattice positions in the case of CeO2–TiO2 and CeO2–ZrO2 samples. The XPS studies revealed the presence of silica, titania, and zirconia in their highest oxidation states, Si(IV), Ti(IV), and Zr(IV) at the surface of the materials. Cerium is present in both Ce4+ and Ce3+ oxidation states. The HREM results reveal well-dispersed CeO2 nanocrystals over the amorphous SiO2 matrix in the case of CeO2–SiO2, isolated CeO2 and TiO2 (A) nanocrystals and some overlapping regions in the case of CeO2–TiO2, and nanosized CeO2 and Ce–Zr oxides in the case of CeO2–ZrO2 sample. The exact structural features of these crystals as determined by digital diffraction analysis of HREM experimental images reveal that the CeO2 is mainly in cubic fluorite geometry. The oxygen storage capacity (OSC) as determined by thermogravimetry reveals that the OSC of mixed oxides is more than that of pure CeO2 and the CeO2–ZrO2 exhibits highest OSC.

A density-driven phase transition between semiconducting and metallic polyamorphs of silicon

Abstract: Amorphous and crystalline forms of silicon are well-known, tetrahedrally coordinated semiconductors. High-pressure studies have revealed extensive polymorphism among various metallic crystal structures containing atoms in six-, eight- and 12-fold coordination1,2. Melting silicon at ambient or high pressure results in a conducting liquid, in which the average coordination is greater than four (ref. 3). This liquid cannot normally be quenched to a glass, because of rapid crystallization to the diamond-structured semiconductor4. Solid amorphous silicon is obtained by synthesis routes such as chemical or physical vapour deposition that result in a tetrahedrally bonded semiconducting state. It has long been speculated that the amorphous solid and the liquid could represent two polymorphic forms of the amorphous state that are linked by density- or entropy-driven transformations5,6,7,8. Such polyamorphic transitions are recognized to occur among several different types of liquid and glassy systems9,10,11,12,13,14. Here we present experimental evidence for the occurrence of a density-driven polyamorphic transition between semiconducting and metallic forms of solid amorphous silicon. The experiments are combined with molecular dynamics simulations that map the behaviour of the amorphous solid on to that of the liquid state.