To save this undefined to your undefined account, please select one or more formats and confirm that you agree to abide by our usage policies. If this is the first time you used this feature, you will be asked to authorise Cambridge Core to connect with your undefined account.
Find out more about saving content to .
To save this article to your Kindle, first ensure email@example.com is added to your Approved Personal Document E-mail List under your Personal Document Settings on the Manage Your Content and Devices page of your Amazon account. Then enter the ‘name’ part of your Kindle email address below.
Find out more about saving to your Kindle.
Note you can select to save to either the @free.kindle.com or @kindle.com variations. ‘@free.kindle.com’ emails are free but can only be saved to your device when it is connected to wi-fi. ‘@kindle.com’ emails can be delivered even when you are not connected to wi-fi, but note that service fees apply.
Laser reflection interferometry (LRI) has been shown to be a useful in situ technique for measuring growth rate of diamond during microwave plasma chemical vapor deposition (MPCVD). Current alternatives to LRI usually involve ex situ analysis such as cross-sectional SEM or profilometry. The ability to measure the growth rate in ‘real-time’ has allowed the variation of processing parameters during a single deposition and thus the extraction of much more information in a fraction of the time. In situ monitoring of growth processes also makes it possible to perform closed loop process control with better reproducibility and quality control. Unfortunately, LRI requires a relatively smooth surface to avoid surface scattering and the commensurate drop in reflected intensity. This problem was remedied by greatly enhancing the diamond particle nucleation via the deposition of an intermediate carbon layer using substrate biasing. When an unscratched silicon wafer is pretreated by biasing negatively relative to ground while in a methane-hydrogen plasma, nucleation densities much higher than those achieved on scratched silicon wafers are obtained. The enhanced nucleation allows a complete film composed of small grains to form in a relatively short time, resulting in a much smoother surface than is obtained from a film grown at lower nucleation densities.
Hot- and cold-wall chemical vapor deposition (CVD) using the volatile copper(I) compound (hfac)Cu(1,5-COD), where hfac = 1,1,1,5,5,5,-hexafluoroacetylacetonate and 1,5-COD = 1,5-cyclooctadiene, as a precursor was carried out in hot-wall and warm-wall, lamp-heated reactors using SiO2 substrates that had been patterned with Pt or W, over a temperature range 120 °C-250 °C. Deposition was observed onto Pt, W, and SiO2 over this temperature range at rates of up to 3750 Å/min to give copper films that contained no detectable impurities by Auger electron spectroscopy and gave resistivities of 1.9-5.7 μ ohm cm. The volatile by-products formed during deposition were 1,5-COD and Cu(hfac)2 and a mass balance was consistent with the quantitative disproportionation reaction: 2(hfac)Cu(1,5-COD) → Cu + Cu(hfac)2 + 2(1,5-COD). The measured activation energy for this CVD reaction was 26(2) kcal/mol. The absence of selectivity for metal surfaces in the presence of SiO2 is in contrast to CVD results for the related compounds (β-diketonate)Cu(PMe3) where β-diketonate = hfac, 1,1,1-trifluoroacetylacetonate (tfac), and acetylacetonate (acac).
The initiation and growth of α–Al2O3/metal composites by the directed oxidation of molten Al–Mg, Al–Si, and Al–Mg–Si alloys at 1373 and 1523 K were investigated. Spontaneous initiation and growth occurred only on the ternary Al–Mg–Si alloy. Growth also occurred on the Al-Mg binary alloy, but in this case initiation required the mechanical disruption of the protective oxide formed during initial heating to the growth temperature. In addition, mechanical disruption of the protective oxide scale on the Al–Mg–Si alloy enhanced growth initiation. Growth on the Al–Si alloy could not be induced under any conditions. From these observations it is concluded that, in the directed oxidation of Al–Mg–Si alloys, Mg is essential to the accelerated oxidation reaction, while Si appears to play a role in promoting the breakdown of the protective oxide layers. The most uniform initiation and growth results were obtained by providing a thin layer of SiO2 particles at the initial growth surface of the alloy.
The agglomeration of Co silicide films formed on Si substrates processed with evaporated Co film thicknesses from 9 to 28 nm was investigated by TEM and four-point-probe resistivity measurements. It was found that the upper portion of a reacting Co or Co silicide film can agglomerate independently from the main body of the silicide layer. This phenomenon is designated partial agglomeration in contrast to whole film agglomeration which generally occurs at higher temperatures. Partial agglomeration appears to develop more extensively for thinner films and poses a serious limitation for the application of thin silicide contact layers for advanced VLSI devices. The formation mechanism of partial agglomeration and the reason for its variation with film thickness are explained on the basis of a previously presented [MRS Proc. Vol. 202, p. 101 (1991)] theoretical model of grain boundary grooving and the onset of islanding in silicide films. Kirkendall voids and phase transformation induced volume changes play an important role in the process.
Classical sintering of YBa2Cu3O7−δ always leads to poor critical transport currents whatever the powder process and the thermal cycle are. Best results are obtained by synthesis of textured ceramics. Fabrication of YBa2Cu3O7−δ with well-oriented grain structure by the melt textured growth process was performed in order to understand the mechanism that controls the texture formation. The evolution of the microstructure was studied by quenching samples at different stages of the MTG cycle. The importance in the texture formation process of the presintering temperature, of the high temperature plateau, of the thermal gradient and its fluctuations, of the presence of second phase inclusions (essentially 211 phase), and of the substrate and its reactivity with Y–Ba–Cu–O is shown.
The superconducting properties of YBa2Cu3O6+x reacted with various known ratios of O2/CO2 gas mixtures during sintering at different temperatures were studied. Jc was found to decrease drastically upon reaction with CO2, becoming zero at certain CO2 activities. The stability region for the 123 superconductor, as a function of CO2 activity and temperature, was empirically formulated as follows: log pCO2 < (−45,000)/T + 33.4. The grain boundaries in sintered samples with Jc = 0 were investigated with HRTEM in conjunction with EDS. Two distinct types of grain boundaries were observed. Approximately 10% of the grain boundaries were wet by a thin layer of a second phase, deduced to be BaCuO2. The remaining boundaries were sharp grain boundaries. The grain structure near the sharp grain boundaries was tetragonal. These two types of grain boundaries are thought to be responsible for Jc being zero.
Preparation of Bi based high-Tc superconductors having Pb and Sb elements has been attempted using the sol-gel method. Production of the high-Tc phase was promoted by doping a little Sb (0.05 mol) together with Pb. The nominal composition of the system is Bi0.96Pb0.19Sb0.05Sr1.0Ca1.1Cu1.6Oy. It has been found that the reaction scheme of Sb doped samples is different from that of Sb nondoped samples.
The valencies of the Bi and Cu ions in Bi2Sr2(Ca1−x Yx)Cu2Oy were determined separately by a coulometric titration technique. The Cu valency decreased monotonically from 2.16 to 2.04 with increasing Y content over the range from 0 to 1. Superconductivity occurred for Cu valency greater than 2.1. The Bi valency was also found to decrease from +3.12 to +2.99 as the Y content increased. It leveled off at ∼ +3.0 for the Y content larger than 0.8. This change in the Bi valency was correlated with the change in the structural modulation period and with its transformation from an incommensurate to a commensurate state.
The fine structure of dislocations in lightly deformed samples of several cubic ordered alloys with composition based on Al3Ti has been examined by weak beam electron microscopy. For all the materials examined the dislocations tend to dissociate into two 1/2(110) partials separated by APB. Dislocation dissociation is not complete at very small strains and the strain required to dissociate, as well as the dissociation distance, varies from one alloy to another. Improvements in ductility achieved by alloying are directly related to the ease and extent of this dissociation.
Stress-controlled fatigue tests have been carried out on an Ni3Al–B/Cr/Zr alloy, at 600 °C and 800 °C in air and in vacuum at various test frequencies. Decreasing the test frequency and/or increasing the temperature leads to a decrease in the number of cycles to failure and a gradual disappearance of a fatigue fracture zone. This trend is shown to be related to a true creep component. Environment has a weak interacting effect on fatigue life but strongly influences the fracture path in the fatigue zones, with fracture becoming partly or entirely intergranular when the environment is changed from vacuum to air. It is suggested that most of the fatigue life is spent in initiating a crack. Comparisons are made with some creep data in terms of fracture paths and time to rupture. Fatigue life at 800 °C is shown to be entirely controlled by creep damage at the lowest test frequency of 0.2 Hz.
Aging experiments were conducted to study initial state Fe-clustering in Au–Fe alloys with Fe content from 10.7 to 33%. Information derived from experiments using transmission electron microscopy, electron diffraction, and high resolution electron microscopy suggests the coexistence of short-range-order and Fe-clusters. At early stages of aging, lobe-like and/or rod-shaped strain contrast images, identified as clusters, were revealed after specimens were further cleaned by an ion-miller. The (11/20) special point diffuse reflections were prominent in the as-quenched condition and/or early stage aging, and the intensities decrease gradually as aging proceeds. After a certain period, (11/20) diffuse reflections disappear but strain contrast images still remain similar. This suggests that the strain contrast images are not related to the (11/20) diffuse reflections. The cluster characteristics of Au–Fe alloys are differentiated from those of Al–Cu and Cu–Be based upon theoretical calculations.
Thermal conductivity measurements were performed on several amorphous rare earth transition metal thin films of varying microstructure. The thermal conductivity perpendicular to the plane of the film, measured by the thermal comparator method, was compared with the thermal conductivity value measured parallel to the plane of the film. The latter value was obtained by converting electrical conductivity values to thermal conductivity via the Wiedemann–Franz relationship. As expected, the columnar microstructure induced during the sputter deposition of the thin films causes an anisotropy in the thermal conductivity values, with the in-plane values consistently lower than the out-of-plane values. The effect is most pronounced for the more columnar films deposited at higher pressure, for which the in-plane thermal conductivity, 0.3 W/mK, is an order of magnitude lower than the out-of-plane thermal conductivity, 4.3 W/mK. The thermal conductivity out of the plane of the film decreased with increasing deposition pressure, due to the decreasing film density.
The origin of an anomalous sawtooth-shaped feature in x-ray powder diffraction of solid C60 is explained via electron diffraction analysis. Films sublimed on holey carbon crystallize with close-packed (111) planes parallel to the surface. Rods of diffuse scattering are found along the 〈111〉 axis normal to the surface but not along other 〈111〉 axes. Powder averaging of these rods, coupled with the x-ray form factor of spherical shells with 3.5 Å radius, accounts for the sawtooth feature. We attribute this phenomenon to planar defects parallel to close-packed layers, which form during the growth of solid C60 by sublimation. A possibly related consequence of the growth mode is the observation of strong macroscopic (111) preferred orientation in films sublimed on a variety of substrates.
Scanning tunneling microscopy images of the (0001) plane of highly oriented pyrolytic graphite show defect regions consisting of an extensive network of partial dislocations that form extended and contracted nodes. The partial dislocations in hexagonal graphite enclose triangular regions (∼1000 nm on a side) of faulted material comprised of rhombohedral graphite. Electronic and elastic interactions of the tip with the HOPG surface are proposed to explain the observed image contrast between hexagonal and rhombohedral graphite.
Chemical vapor deposition of aluminum films using tri-isobutyl aluminum on Si(111) wafers has been studied from the viewpoint of structural and electrical properties of Al films as a function of substrate temperature (Ts). The epitaxial relation of Al on Si is found to be very sensitive to Ts, thus changing from Al(100)))Si(111) with Al[1$\overline 1$0]))Si[11$\overline 1$] to Al(111)))Si(111) with Al[1$\overline 1$0]))Si[1$\overline 1$0] around 410 °C in the course of increasing Ts. The epitaxial relation is mainly determined at the initial stage of the deposition, but in some cases the relation changes with increasing film thickness. Above 420 °C, single-crystalline Al(111) is grown on Si(111), which has resistivity as low as the bulk value, high reflectivity, and a very flat surface.
Oxygen incorporation in silicon crystals during Czochralski growth is dependent on many factors, of which the dissolution of the silica crucible is of great importance. In this paper the reactions between vitreous silica and molten silicon have been analyzed, both in sealed ampoules and in Czochralski crucibles. It was found that the vitreous silica crystallizes to cristobalite by lateral growth. For this reaction to occur it is necessary that liquid silicon is present. The vitreous silica dissolves and the cristobalite grows with a thin layer of liquid silicon between them. Different oxygen concentrations in the melt in equilibrium with the amorphous and crystallized silica are necessary for the reaction to proceed. The oxygen flux in the melt is dependent upon the dissolution of both vitreous silica and cristobalite as well as the reaction between these phases.
We have observed the complete delamination of polycrystalline chromium films from single crystal silicon substrates during deposition due to the formation of high internal stresses. These intrinsic stresses can give rise to interfacial defects which assist in the separation of the film from the substrate. Stresses in the film are balanced by stresses in the substrate, which cause mechanical failure in the substrate near the interface. Extensive arrays of dislocations and cracking of the substrate have been observed. We find that the delamination of the films from the substrate is initiated by the formation of damage in the substrate, rather than to the film or the interface.
It is argued that interdiffusion must precede nucleation of new phases during reactions at interfaces between compositionally different phases. The relative rates at which elemental components diffuse in the reacting phases control the sequence in which phases can form, and can also strongly affect the relative nucleation rates of alloy products, especially in the transient nucleation regime. While detailed predictions of the relative nucleation rates require usually unavailable knowledge of the energies of the relevant interfaces, in some cases, knowledge of the relevant diffusivities, along with a thermodynamic analysis, can lead to predictions of likely phase formation sequences. These concepts are used to explain the association of diffusional asymmetry with systems that undergo solid state amorphization, and to specify semiquantitatively the degree of asymmetry required for solid state amorphization.
A series of molybdenum nitride films were synthesized by implanting energetic nitrogen ions into molybdenum thin films. The resulting films were characterized using x-ray diffraction to determine the effects of nitrogen ion dose (4 × 1016−4 × 1017 N+/cm2), accelerating voltage (50–200 kV), and target temperature (∼298–773 K) on their structural properties. The order of structural transformation with increased incorporation of nitrogen ions into the Mo film can be summarized as follows: Mo → γ−Mo2N → δ−MoN. Nitrogen incorporation was increased by either increasing the dose or decreasing the ion energy. At elevated target temperatures the metastable B1–MoN phase was also produced. In most cases the Mo nitride crystallites formed with the planes of highest atomic density parallel to the substrate surface. At high ion energies preferential orientation developed so that the more open crystallographic directions aligned with the ion beam direction. We tentatively attributed this behavior to ion channeling effects.
Experiments have been conducted to examine the relative importance of different gas feed geometries and molecular species in the hot filament assisted vapor phase deposition of diamond. Remote and local (relative to the substrate) gas feeds of methane and acetylene were tested and it was found that although dramatic effects on uniformity and rate of deposition can be observed using a gas feed of methane local to the substrate surface, little or no variation in growth rate or deposition uniformity was produced by feeding acetylene locally. The growth rate observed on a defined area of the substrate using a local gas feed of methane was 0.50 mgms/h versus 0.17 mgms/h for a local feed of acetylene. These results, as well as the patterns of deposition observed, suggest that the major species contributing to growth in the hot filament assisted CVD of diamond is the methyl radical. This species is readily formed in the gas phase from methane and atomic hydrogen by hydrogen abstraction. It is, however, further suggested that numerous different hydrocarbon species may be of importance, with the nature of the species contributing most to the growth dependent on the method used.