Showing papers on "Texture (crystalline) published in 1987"

The Effect of Low Pressure on the Structure of LPCVD Polycrystalline Silicon Films

Abstract: Structure and crystal growth of undoped silicon films prepared by low pressure chemical vapor deposition (LPCVD) have been investigated by x‐ray diffraction, and transmission and scanning electron microscopy. We show that for films deposited in silane partial‐pressure range and temperature range , the pressure is a determining factor for crystallite size, texture, and surface roughness. At a fixed temperature, the crystallite size decreases when the pressure increases. At very low pressures the films have a random orientation. At intermediate pressures the films are characterized by a dominant texture and at high pressure, by a strong preferred orientation. The surface roughness is closely related to the preferred orientation.

The kinetics and mechanism of oxide layer formation from porous silicon formed on p‐Si substrates

Abstract: The formation of oxide layers from porous silicon layers formed on P substrates which present different and well‐controlled porous textures is studied in detail. It is shown that the overall process leading to oxides with properties similar to standard thermal oxides consists of two steps: the oxidation of silicon in the porous layer and the densification of silica. The first step does not depend upon the initial texture of the layer, but is mostly determined by the porosity of the sample and the temperature of the oxidation sequence. The second step, oxide densification, which takes place at temperatures higher than 1000 °C, is very sensitive to the porous texture of the layer, the densification times increasing sharply with the average pore size. The densification process has a high activation energy, which can be related to the activation energy for the viscous flow of silica.

A comparative study of phase stability and film morphology in thin‐film M/GaAs systems (M=Co, Rh, Ir, Ni, Pd, and Pt)

Abstract: To attain reproducible and stable contacts to compound semiconductor devices, it is necessary to achieve thermodynamically stable phases after the reaction of metals with the compound semiconductor. In this study, the final phases produced by the reactions between GaAs and thin metal films of Co, Rh, Ir, Ni, Pd, and Pt have been investigated. They are identified as MGa for M=Co, Rh, Ni, Pd, and Pt, monoarsenides of Co and Ni, diarsenides of Rh, Ir, Pd, and Pt, and Ir3Ga5. These phases, if deposited directly onto GaAs, will produce thermally stable contacts. In addition to the identification of these stable phases, analyses of the products of thin‐film M/GaAs reactions by transmission electron microscopy, x‐ray diffraction, and Rutherford backscattering spectrometry reveal the distribution, grain size, and crystallographic texture of these end phases. Trends in these observations across the six metal/GaAs reactions studied are explained by considering the effects of bond strength and the relative diffusivities of the reacting species. The role of film thickness in determining the final phases and the phase distribution is also considered.

On the crystal texture of linear polyaryls (PEEK, PEK and PPS)

Abstract: Three linear ppolyaryls, polyetheretherketone (PEEK) polyetherketone (PEK) and polyphenylenesulphide (PPS) have been examined regarding crystal morphologies as obtained from solutions and molecular orientation in melt grown spherulites, the latter involving also experiments on the carbon fibre containing polymer. In addition to the individual features observed in solution grown crystals agreeing with those being reported concurrently from elsewhere, the present coordinated results on three polymers underline the common crystallization behaviour and texture for this family of polymers. These common features comprise the existence of lamellae as symptomatic of chain folding, acicular shapes indicative of uniaxial growth (b being the growth direction in all three cases), and irregular crystal edges. The sheaf development of these crystals then leads readily to a postulated radiolb-axis orientation in spherulites, confirmed directly in melt crystallized samples possessing transcrystalline textures induced by nucleating carbon fibres. Observations on the effect of carbon fibres drew our attention to the combined influence of the crystal nucleating effect and separation distance of these fibres on the resulting crystal texture of the polymer matrix: varying the relative magnitude of the two effects, can even reverse the overall crystal orientation in the sample. Such considerations should be pertinent to crystal texture development in composites with crystaallizable thermoplastic matrices in general.

Structural studies of low‐temperature low‐pressure chemical deposited polycrystalline silicon

Abstract: Structural studies were performed on Si films formed by chemical‐vapor deposition on SiO2 films using silane under pressures below 150 mTorr and temperatures below 640 °C. The mode of growth of these films was found pressure dependent. Films grown at pressures below 10 mTorr were found to have a columnar structure with a (001) preferred orientation ending at a curved surface. The radius of the crystallites increases and the radius of curvature of their free surface decreases as the pressure decreases, while the converse is true for the temperature dependence. Transition from this mode is associated with diminishing of the capillarity effects. For pressures above 10 mTorr the structure is striated with a 〈111〉 twin texture almost perpendicular to the substrate. At pressures above 100 mTorr the structure is similar to the previous one but with a tufty appearance. This structure is associated with compressive and dilatational strain. The size of initial crystallites was found also pressure dependent increasi...

Growth Characteristics of CVD Beta‐Silicon Carbide

Abstract: The films of were prepared from the methyltrichlorosilane (MTS) by low pressure chemical vapor deposition onto the graphite substrates. The results revealed that the growth rate of increased with the MTS flux; besides, the growth rate increased to a maximum and then decreased with increasing deposition temperature. The preferred orientation of the films was examined by x‐ray diffraction, and was found to exhibit a (220) texture in the films of high growth rate and/or long deposition time. The structural morphology was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Observed features indicated the twin plane reentrant edge (TPRE) mechanism as responsible for the growth. The growth features of particulate crystals (e.g., dendrites, whiskers, needles) were identified, and also could be interpreted in terms of the TPRE mechanism.

Three-dimensional texture analysis

Abstract: Texture is the orientation distribution of crystallites in polycrystalline materials. Since most of the physical properties of crystals are anisotropic, their macroscopic mean values are mainly con...

Interlaminar fracture morphology of carbon fibre/PEEK composites

Abstract: The interlaminar fracture morphology of a carbon fibre/poly(ether-ether-ketone) composite (Aromatic Polymer Composite, APC-2) has been examined. The techniques used included scanning electron microscopy on fracture surfaces and on polished and etched sections. Two types of interlaminar fracture are observed: stable and unstable fracture. Both fracture surfaces exhibit microductility but it is more extensive for stable fracture. The fracture surfaces are not planar but have surface roughness. Fibre breakage and peeling are also observed and a quantitative examination enables the fracture energy contributions from the various processes to be calculated. The use of an etching technique reveals the spherulite texture and the presence of a deformation zone which extends into the bulk of the composite from the fracture surface. The extent of this zone is greater in the stable fracture region than in the unstable region and its presence indicates that the volume of composite which can be brought into the energy absorbing process extends well beyond the interlaminar region. The size of the zone has also been calculated using the fracture energy contributions and there is moderate agreement between calculated and observed zone size. Patterns of microductility on the fracture surface are seen to be due to spherulite texture, however the spherulite boundaries do not influence the fracture path.

High coercive force and large remanent moment magnetic films with special anisotropies (invited)

Abstract: Special sputtering methods have been used to synthesize magnetic films that exhibit high intrinsic coercive forces iHc, large remanent moments, and special anisotropies. By special anisotropies we mean that the easy direction of magnetization in various films can be made to be either in or out of the film plane or in a particular direction within the film plane. To achieve high coercive forces in the films it has been necessary to synthesize the films from high iHc rare‐earth transition metal systems such as SmCo5, Nd2Fe14B, and several new compounds in the Sm‐Ti‐Fe system. Films have been synthesized with different crystal textures by varying the sputtering parameters. The magnetic properties observed have been found to be strongly dependent on the film texture. Several possible film scale geometries for electronic applications have been considered.

Epitaxial growth of Al(111)/Si(111) films using partially ionized beam deposition

Abstract: We observed the growth of epitaxial Al(111) films on Si(111) at room temperature by the partially ionized beam deposition technique. The films were deposited in a conventional vacuum condition without in situ cleaning. The beam contained 0.3% of Al self‐ions and a bias potential of 1 kV was applied to the substrate during deposition. X‐ray diffraction (pole figure) revealed that one of the two possible twin structures, with the Al〈110〉∥Si〈110〉 orientation, was preferentially grown on the Si substrate.

Melting of carbon at 50 to 300 kbar

Abstract: Diamond (∼1 μm) and graphite (1–10 μm) in NaCl were melted at 50 to 300 kbar in a diamond anvil cell using a pulsed YAG laser. The samples were removed from the cell and the structures of the quenched phases were studied by transmission electron microscopy. The melted regions of the samples were found to consist of nearly perfect spheres of carbon ranging in size from ∼1 μm down to less than a few nanometers. In the diamond sample melted at 300 kbar, the larger spherules (>0.2 μm) are polycrystalline diamond with either a granular or radial texture. The smaller spherules (<0.2 μm) give electron diffraction patterns with four diffuse rings that correspond to the 002, 100, 004 and 110 of graphite. This diffraction pattern is typical of disordered graphite randomly oriented about the c-axis. Dark field imaging, using a portion of the 002 ring, produces a “bow tie” figure in each of the smaller spherules. The orientation of the “bow tie” figure depends on the portion of the ring used to form the image, and indicates a radial orientation of the c-axis of the disordered graphite. The spacing between the 002 layers depends on the pressure at the time of melting. We interpret this to indicate that there is some sp3 bonding between layers in the disordered graphite in the smaller spherules. The smaller spherules may have the disordered graphite structure because of the effect of the size on the free energy relationship between the phases, or they may have been quenched more rapidly than the larger spherules thus preserving some of the character of the melt. If the latter explanation is correct, then our results may indicate that the diamond melt contains significant sp2 bonding. Lattice images (Fig. 12) of the internal structure of the smallest spherules observable (∼50 A) clearly show that the carbon layers are parallel to the surface of the spherules and that there is a great deal of disorder in the layers. These observations are entirely consistent with our conclusions based on the dark field images.

Effect of circumferential texture on the properties of thin film rigid recording disks

Abstract: The magnetic recording medium of C/CoNiCr/Cr was DC magnetron sputtered onto rigid disk substrates of various circumferential textures. The texture was characterized by mechanical and optical surface profilometry. Texture increased circumferential coercivity and squareness. Recording signal and signal-to-noise ratio improved while overwrite worsened. Structural inhomogeneities in the carbon overcoat along the texture lines increased with texture. Texture improved the head-disk friction characteristics and increased the head lift-off time during start-up. Too coarse a texture led to performance degradation.

Powder Processing for Microstructural Control in Ceramic Superconductors

Abstract: Along with fine grain sizes to increase the structural integrity of ceramic parts, control of the microstructure in samples of the new ceramic cuprates will be necessary to make possible experiments designed to reveal structure-property- processing relationships, i.e., relationships between microstructure, critical current density, and magnetic field. The low critical current densities of existing polycrystalline samples can originate in such properties as: (a) the critical importance of stoichiometry on a macro- and microscale, particularly with respect to stoichiometry-driven crystal defects; (b) extreme anisotropy, which necessitates percolative current paths in randomly oriented polycrystalline materials; (c) microcracking at the grain boundaries; (d) highly resistive grain boundaries, at least for some orientations; and (e) the lack of classical grain-boundary pinning phenomena, similar to that observed in lower-temperature superconductors. If stoichiometry, grain size, crystallographic texture, and, in general, ceramic microstructure can be controlled, these questions could perhaps be resolved. We believe, based on past history and present advanced ceramics processing capabilities, that the technically important properties for ceramic superconductors will be substantially improved by this approach and that, perhaps more importantly, a technology base for the manufacture of these new superconductors will be established.

Domains in KH2PO4

Abstract: A review of the domains in KH2PO4 crystal is given. This includes a description of the domains and the cell orientations correlated to the texture, the existence of quasidislocations and their role at the domain tips, and a study of lateral displacements of the walls and domain freezing are studied.

Structure and properties of oblique‐deposited magnetic thin films

Abstract: Structural characterizations of angle‐of‐incidence CoNi films using electron microscopy are reported. Both fixed‐angle‐of‐incidence and continuous‐variation‐of‐incidence films are included. The tangent rule is found to be valid at high oblique angle. The easy axis as measured from the torque curve and angular variation of coercivity in a 170‐nm film deposited at 75° is about 15° inclined to the film plane, whereas the columnar axis is about 30°. A weak c‐axis texture exists as revealed by the electron diffraction study. The c‐axis texture in the continuous‐variation‐of‐incidence films with and without oxygen is not detected by electron diffraction.

Interfacially initiated crystallization in amorphous germanium films

Abstract: The amorphous to microcrystalline phase transition of Ge in Pb/Ge multilayers has been extensively studied. During crystallization, the x‐ray diffraction peaks of the modulated structure disappear and the Pb texture improves. It is shown that the crystallization temperature decreases with decreasing amorphous Ge thickness and is strongly affected by the texture of the metallic component. These results imply that the crystallization is interfacially initiated.

Polycrystalline Si thin‐film transistors fabricated at ≤800 °C: Effects of grain size and {110} fiber texture

Abstract: The grain size and {110} fiber texture of low‐pressure chemical‐vapor‐deposited polycrystalline Si films were first modified using the ‘‘seed selection through ion channeling’’ process. These films were self‐implanted at 200 keV, at normal incidence, to various doses (5–20×1014 cm−2), and were subsequently recrystallized at 600 °C. The as‐deposited film was characterized by the smallest grain size, 0.08 μm, among all films and a weak {110} texture. The film processed with a dose of 11×1014 cm−2 had an intermediate average grain size of 1.0 μm but was characterized by the strongest {110} texture among all films. The film processed with a dose of 20×1014 cm−2, on the other hand, was characterized by the largest average grain size of 2.0 μm among all films but had no crystallographic texture. Metal‐oxide‐semiconductor thin‐film transistors were then fabricated on these three films to examine the effects of grain size and fiber texture on device performance. Both n‐ and p‐channel transistors were fabricated, ...

Magnetic properties of Er2Fe14B and Nd2Fe14B thin films

Abstract: Thin films of Nd2Fe14B and Er2Fe14B have been synthesized by dc sputtering onto single‐crystal sapphire substrates. X‐ray diffraction revealed that the films were single phase and that the texture was sensitive to the substrate temperature. Films deposited onto substrates at 600 °C showed directed crystalline growth with the c axis of the tetragonal structure predominantly perpendicular to the film plane. Magnetization measurements of the Er2Fe14B films and Kerr rotation of the Nd2Fe14B films indicate an intrinsic anisotropy consistent with the preferred orientation. Electrical transport measurements of the Nd2Fe14B film display a change of slope near T=150 K which is interpreted as a consequence of the spin reorientation.