Showing papers on "Amorphous solid published in 2000"

Organic materials for electronic and optoelectronic devices

Abstract: This article concentrates on our recent results on several classes of photo- and electro-active organic materials that permit thin film formation and discusses their synthesis, properties, functions and potential applications for electronic and optoelectronic devices. The materials studied include amorphous molecular materials, titanyl phthalocyanine, oligothiophenes with well-defined structures, and non-conjugated polymers containing pendant oligothiophenes or other π-electron systems. The thin films of these materials find potential applications for use in organic electroluminescent, photovoltaic, electrochromic, and other devices.

What is the true solubility advantage for amorphous pharmaceuticals

Abstract: Purpose To evaluate the magnitude of the solubility advantage foramorphous pharmaceutical materials when compared to their crystallinecounterpartsMethods The thermal properties of several drugs in their amorphousand crystalline states were determined using differential scanningcalorimetry From these properties the solubility advantage for theamorphous form was predicted as a function of temperature using a simplethermodynamic analysis These predictions were compared to theresults of experimental measurements of the aqueous solubilities of theamorphous and crystalline forms of the drugs at several temperaturesResults By treating each amorphous drug as either an equilibriumsupercooled liquid or a pseudo-equilibrium glass, the solubilityadvantage compared to the most stable crystalline form was predicted to bebetween 10 and 1600 fold The measured solubility advantage wasusually considerably less than this, and for one compound studied indetail its temperature dependence was also less than predicted It wascalculated that even for partially amorphous materials the apparentsolubility enhancement (theoretical or measured) is likely to influencein-vitro and in-vivo dissolution behaviorConclusions Amorphous pharmaceuticals are markedly more solublethan their crystalline counterparts, however, their experimental solubility advantage is typically less than that predicted from simplethermodynamic considerations This appears to be the result of difficulties indetermining the solubility of amorphous materials under trueequilibrium conditions Simple thermodynamic predictions can provide a useful indication of the theoretical maximum solubility advantage foramorphous pharmaceuticals, which directly reflects the driving forcefor their initial dissolution

Calculations of Mixing Enthalpy and Mismatch Entropy for Ternary Amorphous Alloys.

Abstract: Chemical mixing enthalpy (ΔH chem ) and mismatch entropy normalized by Boltzmann constant (S σ /k B ) corresponding to the three empirical rules for the achievement of high amorphous-forming ability (AFA) were calculated with thermodynamical functions for the gross number of 6450 alloys in 351 ternary amorphous systems. The temary amorphous alloys have ΔH chem of -86 to 25 kJ/mol and S σ /k B of 1.0 × 10 -3 to 5.7. The average values of ΔH chem and S σ /k B are calculated to be -33 kJ/mol and 0.33, respectively. The 30 alloys in 9 ternary amorphous systems including 10 alloys in Ag-Cu-Fe system have positive values of ΔH chem . Most of the ternary amorphous alloys have the values of ΔH chem and S σ /k B inside a trapezoid regicn in ΔH chem - log(S σ /k B ) chart except mainly for the H- and the C-containing alloys. Si-W-Zr system and the 32 alloys having positive values of ΔH chem . The analysis of AFA was carried out for typical five ternary amorphous systems. The following four results are derived. 1) Al-La-Ni and B-Fe-Zr alloys have high AFA in accordance with the concept of the three empirical rules. 2) The further multiplication of alloy components causes an increase in the AFA of Al-B-Fe alloys. 3) Thermodynamical factors represented by melting temperature and viscosity at the melting temperature are required for evaluation of AFA for Mg- and Pd-based amorphous alloys. 4) A tendency for log(S σ /k B ) to increase with decreasing ΔH chem is recognized in each alloys system, implying the stabilization of an amorphous phase against solid solution and intermediate phase.

Structural transformations of Ge2Sb2Te5 films studied by electrical resistance measurements

Abstract: Temperature dependent measurements of the electrical resistance have been employed to study structural changes in sputtered Ge2Sb2Te5 films. The pronounced changes of film resistance due to structural changes enable a precise determination of transition temperatures and activation energies. Furthermore the technique is sensitive enough to measure the influence of ultrathin capping layers on the transformation kinetics. With increasing temperature the Ge2Sb2Te5 films undergo a structural change from an amorphous to rock salt structure (Fm3m) around 140 °C and finally a hexagonal structure (p3m) around 310 °C. Both structural changes are accompanied by a major drop of resistance. Applying the Kissinger method [Anal. Chem. 29, 1702 (1957)] the activation energy for crystallization to the rock salt structure is determined to be 2.24±0.11 eV, and for the phase transformation to the hexagonal phase to be 3.64±0.19 eV, respectively. A thin capping layer of ZnS–SiO2 leads to an increase of the first transition temperature as well as of the corresponding activation energy (2.7±0.2 eV).

Correlation between Molecular Packing and Optical Properties in Different Crystalline Polymorphs and Amorphous Thin Films of mer-Tris(8-hydroxyquinoline)aluminum(III)

Abstract: Since 1987, high-luminance low-voltage driven devices based on tris(8-hydroxyquinoline)aluminum(III) (Alq3) opened the route to design low-cost large area displays and illuminators. Despite the large number of studies devoted to this material, very little is known about its basic structural and optical properties in the solid state. Therefore, we have investigated the structure(s) and the correlation between intermolecular interactions and optical properties in various Alq3 systems, including solution, amorphous thin films, and different crystalline forms. Two novel unsolvated polymorphs of Alq3, namely, α-Alq3 and β-Alq3, have been synthesized and their crystalline structures determined from X-ray diffraction data on powders (α) and single crystals (β). Crystals of α-Alq3 are triclinic, space group P-1, a = 6.2586(8) A, b = 12.914(2) A; c = 14.743(2) A, α = 109.66(1)°; β = 89.66(1)°, and γ = 97.68(1)°; crystals of β-Alq3 are triclinic, space group P-1, a = 8.4433(6) A, b = 10.2522(8) A; c = 13.1711(10) A...

Extraction current transients: new method of study of charge transport in microcrystalline silicon

Abstract: The transport properties of microcrystalline silicon, namely, mobility and conductivity, are investigated by a new method, for which the simple theory as well as numerical modeling is presented. The basic idea of the new method is verified on amorphous hydrogenated silicon by comparison with the widely used time-of-flight method. Contrary to time of flight, the new method can be used even for relatively conductive materials. Preliminary results on microcrystalline silicon clearly indicate the critical role of amorphouslike tissue in transport in microcrystalline silicon.

Structure of laser-crystallized Ge2Sb2+xTe5 sputtered thin films for use in optical memory

Abstract: The structure of laser-crystallized thin films of Ge2Sb2+xTe5 (0.0

Composition, nanostructure and origin of the ultrahardness in nc-TiN/a-Si3N4/a- and nc-TiSi2 nanocomposites with HV= 80 to ≥ 105 GPa

Abstract: Multiphase nanocomposite coatings (3–20 μm thick) consisting of nanocrystalline TiN, amorphous Si 3 N 4 , and amorphous and nanocrystalline TiSi 2 , nc-TiN/a-SiN x /a- and nc-TiSi 2 were deposited on steel substrates by means of plasma CVD. The load-independent Vickers microhardness from 80 to >105 GPa was measured by the load–depth sensing technique for applied loads between 30 and 200 mN and verified by measuring the size of the remaining plastic indentation using SEM. The results of a complex analysis provide a consistent picture of the nature of the grain boundaries which determines the hardness in the whole range of silicon content between approximately 3 and 22 at.%. At a high discharge current density of ≥2.5 mA/cm 2 the a-Si 3 N 4 forms the grain boundaries and the nanocomposites are superhard (40–50 GPa) as we reported earlier. At a lower current density of ≤1 mA/cm 2 a mixture of TiSi 2 and Si 3 N 4 is formed. With increasing Si-content the amount of a-TiSi 2 in the grain boundaries of the TiN nanocrystals increases, and above 10 at.% of Si approximately 3 nm small TiSi 2 nanocrystals precipitate. The hardness depends critically and in a complex way on the Si 3 N 4 content and the TiSi 2 /Si 3 N 4 ratio. The ultrahardness 1 of ≥80 GPa is achieved when the surface of the TiN nanocrystals is covered with approximately one monolayer of Si 3 N 4 . Under these conditions the ultrahardness of 80–100 GPa depends on the amount of a- and nc-TiSi 2 .

Crystallization Temperature-Dependent Crystal Orientations within Nanoscale Confined Lamellae of a Self-Assembled Crystalline−Amorphous Diblock Copolymer

Abstract: For a lamella-forming poly(ethylene oxide)-block-polystyrene (PEO-b-PS) diblock copolymer (MnPEO = 87K and MnPS = 92K), the glass transition temperature of the PS blocks is 62 °C, and the melting temperature of the PEO crystals is around 51 °C when the sample is crystallized below 40 °C The PEO blocks thus crystallize in a one-dimensionally confined lamellar space of 88 nm, as studied recently by one-dimensional small-angle X-ray scattering (SAXS) and transmission electron microscopy In this report, the crystal orientation (the c-axis of the PEO crystals) within nanoscale confined lamellae has been investigated using combined two-dimensional SAXS and wide-angle X-ray scattering experiments The c-axis orientation in the PEO crystals is observed for the first time to change from random to perpendicular, then to inclined, and finally to parallel to the lamellar surface normal, depending only on the crystallization temperature (Tc) Detailed crystallographic analyses indicate that the c-axis orientati

Molecular simulation of ultrathin polymeric films near the glass transition.

Abstract: Properties such as the glass transition temperature ( T(g)) and the diffusion coefficient of ultrathin polymeric films are shown to depend on the dimensions of the system. In this work, a hard-sphere molecular dynamics methodology has been applied to simulate such systems. We investigate the influence that substrates have on the behavior of thin polymer films; we report evidence suggesting that, depending on the strength of substrate-polymer interactions, the glass transition temperature for a thin film can be significantly lower or higher than that of the bulk.

Synthesis of Nitrogen-Rich Carbon Nitride Networks from an Energetic Molecular Azide Precursor

Abstract: Nitrogen-rich carbon nitrides are produced as amorphous, bulk solids from the slow thermal decomposition of 2,4,6-triazido-1,3,5-triazine [(C3N3)(N3)3]. This energetic molecular azide is thermally unstable and readily decomposes at 185 °C in a high-pressure reactor to produce carbon nitride materials, e.g., C3N4. Under applied nitrogen gas pressure, (C3N3)(N3)3 decomposes to yield a solid with one of the highest reported nitrogen-to-carbon ratios corresponding to C3N5. This azide precursor also decomposes upon rapid heating to 200 °C to form graphite nanoparticles without any retained nitrogen. Spectroscopic evidence (infrared, nuclear magnetic resonance, and ultraviolet−visible) demonstrates that the carbon−nitrogen solids have significant sp2 carbon bonding in a conjugated doubly bonded network. Electron microscopy reveals that these powders have a glassy microstructure with large irregular pores and voids. C3N4 and C3N5 are thermally stable up to 600 °C and sublime to produce carbon nitride thin films ...

Molecular dynamics simulation of phase transformations in silicon monocrystals due to nano-indentation

Abstract: This paper discusses the phase transformation of diamond cubic silicon under nano-indentation with the aid of molecular dynamics analysis using the Tersoff potential. By monitoring the positions of atoms within the model, the microstructural changes as silicon transforms from its diamond cubic structure to other phases were identified. The simulation showed that diamond cubic silicon transforms into a body-centred tetragonal form (β-silicon) upon loading of the indentor. The change of structure is accomplished by the flattening of the tetrahedron structure in diamond cubic silicon. Upon unloading, the body-centred tetragonal form transforms into an amorphous phase accompanied by the loss of long-range order of the silicon atoms. By performing a second indentation on the amorphous zone, it was found that the body-centred-tetragonal-to-amorphous phase transformation could be a reversible process.

Effect of film thickness on the properties of indium tin oxide thin films

Abstract: Transparent conducting indium tin oxide (ITO) thin films (40–870 nm) were grown by pulsed laser deposition on amorphous substrates and the structural, electrical, and optical properties of these films were investigated. Films were deposited using a KrF excimer laser (248 nm, 30 ns FWHM) at a fluence of 2 J/cm2, at substrate temperature of 300 °C and 10 mTorr of oxygen pressure. For ITO films (30–400 nm thickness) deposited at 300 °C in 10 mTorr of oxygen, a resistivity of 1.8–2.5×10−4 Ω cm was observed and the average transmission in the visible range (400–700 nm) was about 85%–90%. The Hall mobility and carrier density for ITO films (40–870 nm thickness) were observed to be in the range of 24–27 cm2/V s and 8–13×1020 cm−3, respectively. The ITO films have been used as the anode contact in organic light emitting diodes and the effect of ITO film thickness on the device performance has been studied. The optimum thickness of the ITO anode for the maximum device efficiency was observed to be about 60–100 nm....

First-order transition in confined water between high-density liquid and low-density amorphous phases.

Abstract: Supercooled water and amorphous ice have a rich metastable phase behaviour. In addition to transitions between high- and low-density amorphous solids1,2, and between high- and low-density liquids3,4,5,6,7,8, a fragile-to-strong liquid transition has recently been proposed9,10, and supported by evidence from the behaviour of deeply supercooled bilayer water confined in hydrophilic slit pores11. Here we report evidence from molecular dynamics simulations for another type of first-order phase transition—a liquid-to-bilayer amorphous transition—above the freezing temperature of bulk water at atmospheric pressure. This transition occurs only when water is confined in a hydrophobic slit pore12,13,14 with a width of less than one nanometre. On cooling, the confined water, which has an imperfect random hydrogen-bonded network, transforms into a bilayer amorphous phase with a perfect network (owing to the formation of various hydrogen-bonded polygons) but no long-range order. The transition shares some characteristics with those observed in tetrahedrally coordinated substances such as liquid silicon15,16, liquid carbon17 and liquid phosphorus18.

High ε gate dielectrics Gd2O3 and Y2O3 for silicon

Abstract: We report on growth and characterization of both epitaxial and amorphous films Gd2O3 of (e=14) and Y2O3(e=18) as the gate dielectrics for Si prepared by ultrahigh vacuum vapor deposition. The use of vicinal Si (100) substrates is key to the growth of (110) oriented, single-domain films in the Mn2O3 structure. Typical electrical leakage results are 10−3 A/cm2 at 1 V for single domain epitaxial Gd2O3 and Y2O3 films with an equivalent SiO2 thickness, teq of 15 A, and 10−6 A/cm2 at 1 V for smooth amorphous Y2O3 films (e=18) with a teq of only 10 A. For all the Gd2O3 films, the absence of SiO2 segregation at the interface is established from infrared absorption measurements.

Crystal nucleation and growth of indomethacin polymorphs from the amorphous state

Abstract: The effect of temperature on the overall crystallization, and the crystal nucleation and growth rates of indomethacin polymorphs from the amorphous state were determined Crystallization of amorphous indomethacin at temperatures close to or below its T g (42°C) favors the formation of the stable γ polymorphic form, while crystallization at higher temperatures favors the formation of the metastable α-crystal form Both the nucleation and growth rates for γ-indomethacin have maxima that coincide just above the T g of amorphous indomethacin The nucleation rate for α-indomethacin was found to have a maximum at 60°C, and the growth rate at 90°C Assuming a temperature dependent crystal–amorphous interface free energy, good agreement was observed between the experimental nucleation data and the predictions of the classical theory of nucleation The crystal–amorphous interface energy was higher for the γ than for the α-indomethacin Analysis of the crystal growth rates for both crystal forms showed that the mechanism of growth is by two-dimensional nucleation, but quantitative agreement with the theory was not found The interface energy for the α-crystal form, obtained from the growth data was in very good agreement with the value obtained from the nucleation data

Plasma-enhanced atomic layer deposition of Ta and Ti for interconnect diffusion barriers

Abstract: Thin films of inert, refractory materials are used in semiconductor interconnect applications as diffusion barriers, seed, and adhesion layers. A typical example is the use of a thin, conformal Ta or Ti/TiN films on the walls of a dielectric trench or via which reduces or eliminates out-diffusion of the primary conductor, usually Cu, into the dielectric. Atomic layer deposition is a known technique which is intrinsically conformal and is appropriate for this application. Plasma enhancement of the process allows deposition at significantly lower temperatures than conventional chemical vapor deposition, which is a requirement for low-k dielectrics. Tantalum films deposited at 25–400 °C using ALD with a TaCl5 precursor and atomic hydrogen as the reactive species at up to a rate of 1.67 Ang/cycle are amorphous, conformal, and show moderate or controllable levels of impurities; primarily oxygen and a small level of Cl. Similar results have been observed for Ti using TiCl4 as a precursor. The process scales to manufacturing dimensions and applications and will facilitate the extension of interconnect technology beyond (below) 100 nm dimensions.Thin films of inert, refractory materials are used in semiconductor interconnect applications as diffusion barriers, seed, and adhesion layers. A typical example is the use of a thin, conformal Ta or Ti/TiN films on the walls of a dielectric trench or via which reduces or eliminates out-diffusion of the primary conductor, usually Cu, into the dielectric. Atomic layer deposition is a known technique which is intrinsically conformal and is appropriate for this application. Plasma enhancement of the process allows deposition at significantly lower temperatures than conventional chemical vapor deposition, which is a requirement for low-k dielectrics. Tantalum films deposited at 25–400 °C using ALD with a TaCl5 precursor and atomic hydrogen as the reactive species at up to a rate of 1.67 Ang/cycle are amorphous, conformal, and show moderate or controllable levels of impurities; primarily oxygen and a small level of Cl. Similar results have been observed for Ti using TiCl4 as a precursor. The process scales to ...

Thermodynamic stability of amorphous oxide films on metals: Application to aluminum oxide films on aluminum substrates

Abstract: It has been shown on a thermodynamic basis that an amorphous structure for an oxide film on its metal substrate can be more stable than the crystalline structure. The thermodynamic stability of a thin amorphous metal-oxide film on top of its single-crystal metal substrate has been modeled as a function of growth temperature, oxide-film thickness, and crystallographic orientation of the metal substrate. To this end, expressions have been derived for the estimation of the energies of the metal-substrate amorphous-oxide film interface and the metal-substrate crystalline-oxide film interface as a function of growth temperature, and crystallographic orientation of the substrate (including the effect of strain due to the lattice mismatch). It follows that, up to a certain critical thickness of the amorphous oxide film, the higher bulk Gibbs free energy of the amorphous oxide film, as compared to the corresponding crystalline oxide film, can be compensated for by the lower sum of the surface and interfacial energies. The predicted occurrence of an amorphous aluminum-oxide film on various crystallographic faces of aluminum agrees well with previous transmission electron microscopy observations.

Structural Chemistry of Self-Assembled Monolayers of Octadecylphosphoric Acid on Tantalum Oxide Surfaces

Abstract: Octadecylphosphoric acid ester is shown to self-assemble on amorphous/nanocrystalline tantalum oxide (Ta2O5) layers deposited by physical vapor deposition onto glass substrates. Three complementary surface-analytical techniques (angle-dependent X-ray photoelectron spectroscopy, time-of-flight secondary ion mass spectrometry, and atomic force microscopy in lateral force mode), showed that a 2.2 nm thick, “tails-up”-oriented adlayer is formed, which displays local near-hexagonal order, strong P−O−Ta bonding, and the presence of (−P−O-)2Ta species. A model for the binding and the structural organization of the octadecyl phosphate molecules on the tantalum oxide surface is proposed involving direct coordination of the terminal phosphate headgroup to Ta(V) cations forming a strong complexation bond, two types of bonding of the octadecyl phosphate with both monodentate and bidentate phosphate-Ta(V) coordinative interactions, and, locally, the formation of a coincidence lattice of approximately hexagonal structu...

Self-organization in network glasses

Abstract: The continuous random network model is widely used as a realistic description of the structure of covalent glasses and amorphous solids. We point out that in real glasses and amorphous materials, there are non-random structural elements that go beyond just simple chemical ordering. We propose that the network can self-organize at its formation or fictive temperature, and examine some of the possible consequences of such self-organization. We find that the absence of small rings can cause the mechanical threshold to change from a second order to a first order transition. We show that if stressed regions are inhibited in the network, then there are two-phase transitions and an intermediate phase that is rigid but stress-free. This intermediate phase is bounded by a second order transition on one side and a first order transition on the other. Recent experiments in chalcogenide glasses give evidence for this intermediate phase.

Optical absorption and light scattering in microcrystalline silicon thin films and solar cells

Abstract: Optical characterization methods were applied to a series of microcrystalline silicon thin films and solar cells deposited by the very high frequency glow discharge technique. Bulk and surface light scattering effects were analyzed. A detailed theory for evaluation of the optical absorption coefficient α from transmittance, reflectance and absorptance (with the help of constant photocurrent method) measurements in a broad spectral region is presented for the case of surface and bulk light scattering. The spectral dependence of α is interpreted in terms of defect density, disorder, crystalline/amorphous fraction and material morphology. The enhanced light absorption in microcrystalline silicon films and solar cells is mainly due to a longer optical path as the result of an efficient diffuse light scattering at the textured film surface. This light scattering effect is a key characteristic of efficient thin-film-silicon solar cells.

Nanocomposite TiC/a–C:H hard coatings deposited by reactive PVD

Abstract: Thin films of titanium carbide and amorphous hydrogenated carbon at various compositions have been deposited by unbalanced reactive magnetron sputtering from a metallic titanium target in the presence of argon and acetylene. XRD probed the presence of nanocrystalline TiC and, at high titanium concentrations, of metallic titanium. The XPS examinations allowed one to determine the amount of TiC produced at any concentration of titanium. Raman spectroscopy proved the presence of a-C:H up to 38 at.% of titanium. The coatings have a pronounced hardness maximum of 35 GPa at a composition of approximately 80% TiC and 20% a-C:H. The hardness at 60% TiC and 40% a-C:H as well as that of 100% TiC does not exceed 18 GPa. The mean separation of the crystallites, whose diameter is approximately 4 nm, amounts to a few atomic distances. At the maximum hardness a coefficient of friction of 0.25–0.3 is obtained. The coatings thus provide, at the optimum composition, high hardness at low friction.

Ordering and Self-organization in Nanocrystalline Silicon

Abstract: The spontaneous formation of organized nanocrystals in semiconductors has been observed1,2,3,4,5 during heteroepitaxial growth and chemical synthesis. The ability to fabricate size-controlled silicon nanocrystals encapsulated by insulating SiO2 would be of significant interest to the microelectronics industry. But reproducible manufacture of such crystals is hampered by the amorphous nature of SiO2 and the differing thermal expansion coefficients of the two materials. Previous attempts6,7,8,9,10 to fabricate Si nanocrystals failed to achieve control over their shape and crystallographic orientation, the latter property being important in systems such as Si quantum dots. Here we report the self-organization of Si nanocrystals larger than 80 A into brick-shaped crystallites oriented along the 〈111〉 crystallographic direction. The nanocrystals are formed by the solid-phase crystallization of nanometre-thick layers of amorphous Si confined between SiO2 layers. The shape and orientation of the crystallites results in relatively narrow photoluminescence, whereas isotropic particles produce qualitatively different, broad light emission. Our results should aid the development of maskless, reproducible Si nanofabrication techniques.