Showing papers on "Transmission electron microscopy published in 1990"

Capillary instabilities in thin films

Abstract: Very thin films, less than 100 nm-thick, are used in a variety of applications, including as catalysts and for thin film reactions to form patterned silicides in electronic devices. Because of their high surface to volume ratio, these very thin films are subject to cap-illary instability and can agglomerate well below their melting temperatures. In order to develop a general understanding of agglomeration in very thin films, we have studied initially continuous and patterned films of gold on fused silica substrates. Two in situ techniques were used to monitor agglomeration: 1) heating and video recording in a transmission electron microscope, and 2) measurement of the intensity of laser light transmitted through a sample heated in a furnace. Electron microscopy allowed inves-tigation of the role of the microstructure of the Au film and analysis of light transmis-sion during heating allowed determination of temperature-dependent and film-thick-ness-dependent agglomeration rates. These results will be described along with models for the agglomeration process.

Formation of arsenic precipitates in GaAs buffer layers grown by molecular beam epitaxy at low substrate temperatures

Abstract: We have grown film structures by molecular beam epitaxy which include GaAs buffer layers grown at low substrate temperatures (250 °C). The film structures have been examined using transmission electron microscopy. The layers grown at normal temperatures (600 °C) were free of defects or clusters. In contrast, the layer which was grown at low substrate temperatures contained precipitates which have been identified as hexagonal arsenic. The density of the arsenic precipitates is found to be very sensitive to the substrate temperature during growth.

Reactively sputtered TiN as a diffusion barrier between Cu and Si

Abstract: The properties of 100‐nm‐thick Ti55N45 and Ti45N55 films as diffusion barriers between silicon substrates and thin Cu films were studied by sheet resistance measurements, Rutherford backscattering spectrometry, Auger electron spectroscopy, secondary‐ion mass spectrometry, transmission electron microscopy, scanning electron microscopy, energy dispersive x‐ray spectroscopy, x‐ray diffractometry, and diode leakage current measurements. For unpatterned Si/titanium nitride/Cu samples, all the layers were intact and there was no indication of interdiffusion by conventional depth profiling techniques up to 700 °C for Ti55N45 and 900 °C for Ti45N55 after 30 s rapid thermal anneal in N2, respectively. Leakage current measurements did not show deterioration of diode junction (with junction depth of 0.25 and 0.30 μm) up to 650 °C for Ti55N45 and 800 °C for Ti45N55. The improvement in failure temperature of the N‐rich Ti45N55 diffusion barrier is a result of the lower defect density and a more stable feature furnished by nitrogen stuffed at the defects.

Crystal to amorphous transformation of NiTi induced by cold rolling

Abstract: A NiTi intermetallic compound was cold rolled at room temperature by 30% and 60% thickness reductions, and microstructures were studied by means of transmission electron microscopy (TEM). In the cold-rolled samples we observed both a phase of nanometer-sized crystals and an amorphous phase. A substantially high dislocation density, 1013 to 1014/cm2, was evident in the transition region between crystalline and amorphous phases. A simple estimate of the elastic energy arising from this dislocation density is of the same order as the crystallization energy, suggesting that dislocation accumulation is a major driving force for amorphization in cold-rolled NiTi.

A transmission electron microscopy study of low‐temperature reaction at the Co‐Si interface

Abstract: An efficient preparation method, which provides wedge‐shaped cross‐section transmission electron microscopy samples, has been developed. It was then used to investigate the structure of as‐deposited cobalt multilayers on silicon substrates by rf plasma sputtering. It was found that an extended reaction takes place between Co and Si probably during the deposition. The cobalt atoms react with the silicon substrate to form an amorphous silicide layer. When the deposited layer is <3 nm thick, it entirely reacts with the substrate and can form an amorphous silicide as large as 5 nm. Above 4–5 nm thickness, growth of Co crystallites comes in competition with the formation of the amorphous silicide and limits it to 2 nm. The composition of this amorphous silicide is estimated to be Co2Si. In Co/C multilayers, the reactivity between the two materials is negligible, and the coalescence thickness of cobalt is 2–3 nm. At 2 nm, the cobalt layers are noncontinuous and very rough, whereas at 3 nm the critical thickness...

Atom probe and transmission electron microscopy study of aging of cast duplex stainless steels

Abstract: The microstructural evolution of Mo bearing and Mo free cast duplex stainless steels, induced by long term aging in the range 350–4507deg;C has been studied using atom probe and field ion microscopy and transmission electron microscopy. The salient features of this evolution at 350 and 400°C are spinodal decomposition and G phase precipitation in the ferrite. The nucleation of the G phase is enhanced by spinodal decomposition. Other intermetallic phases observed at higher temperatures may also be formed at 400°C. In the ferrite/ austenite interface, an intermetallic film, possibly an icosahedral phase, is formed after aging for 30 000 h at 400 and 450°C in Mo bearing steels, whereas interfacial M23C6 carbides precipitate in the Mo free materials. The observed evolution of the spatial and compositional parameters of the spinodal decomposition in Mo bearing steels is described and its relationship with the evolution of mechanical properties is outlined.MST/1185

Phase Equilibria in Fe-Mn-Al-C Alloys

Abstract: Phase constitutions of Fe-(20-30)wt%Mn-(0-10)wt%Al-C alloys have been investigated by electron probe microanalysis and transmission electron microscopy. The phase relations between austenite and perovskite carbide, (Fe, Mn)3AlC in the temperature range of 900-1 200°C have been carefully examined. An L12-type ordered structure, which was reported to be formed in rapidly solidified alloys as a metastable phase has not been detected in specimens aged at temperatures above 600°C.

Nucleation‐limited phase selection during reactions in nickel/amorphous‐silicon multilayer thin films

Abstract: Isothermal and constant‐heating‐rate differential scanning calorimetry, cross‐sectional transmission electron microscopy, thin‐film x‐ray diffraction, and thermodynamic and kinetic analyses have been used to investigate silicide phase selection in nickel/amorphous‐silicon multilayer thin‐film reactions. The atomic concentration ratio of the films was two Ni atoms to one Si atom and the layer thickness ratio was one to one. During deposition, a thin layer of amorphous nickel silicide formed between the nickel and amorphous‐silicon layers. Upon heating, this amorphous nickel silicide thickened slightly, until crystalline Ni2Si formed at the nickel/amorphous‐nickel‐silicide interface. Further heating caused the simultaneous growth of both the amorphous nickel silicide and crystalline Ni2Si. Comparison of thermodynamic data to kinetic models for silicide formation and the analysis of calorimetry data suggests that nucleation barriers are responsible for the initial formation of the amorphous nickel silicide a...

Growth of AlN/GaN layered structures by gas source molecular‐beam epitaxy

Abstract: AlN/GaN layered structures with layer periods between 1.5 and 40 nm have been grown on (0001) oriented sapphire and α(6H)‐SiC substrates. The growth was performed using a modified gas source molecular‐beam epitaxy (MBE) technique. Standard effusion cells were used as sources of Al and Ga, and a small, MBE compatible, electron cyclotron resonance (ECR) plasma source was used to activate nitrogen gas prior to deposition. Chemical analysis of the layers was conducted using Auger spectrometry. X‐ray diffractometry, transmission electron microscopy (TEM), and high‐resolution electron microscopy (HREM) were employed for the structural and microstructural studies. Coherent interfaces (no relaxation by misfit dislocations) were observed for bilayer periods smaller than 6 nm. By contrast, completely relaxed individual layers of GaN and AlN with respect to each other were present for bilayer periods above 20 nm. Cathodoluminescence showed a shift in the emission peak from 3.42 eV for the sample with individual 10‐n...

The effects of dopants on surface‐energy‐driven secondary grain growth in silicon films

Abstract: The effects of phosphorus, arsenic, and boron on surface‐energy‐driven secondary grain growth (SEDSGG) in thin polycrystalline silicon films have been investigated. At concentrations at or above 5×1020 cm−3, phosphorus and arsenic were found to markedly enhance SEDSGG while boron had little effect. However, codoping with phosphorous and boron or arsenic and boron lead to compensation (reduction or elimination) of the enhancement effect. The kinetics of SEDSGG were analyzed using transmission electron microscopy. In order to identify electronic as well as segregation effects of dopants on the kinetics of SEDSGG, electron concentrations in the Si films were determined from Hall measurements and dopant segregation was directly measured using scanning transmission electron microscopy and energy‐dispersive x‐ray analysis. Analogous to normal grain growth, dopant‐induced enhancement of SEDSGG can be explained in terms of an increased grain‐boundary atomic mobility due to changes in point‐defect concentrations r...

Reaction zone microstructure in a Ti3Al + Nb/SiC composite

Abstract: A composite of Ti-25Al-13Nb (atomic percent) matrix with a continuous SiC fiber (SCS-6) reinforcement was fabricated by hot pressing powder cloths and mats of fiber. The fiber/matrix reaction zone was studied using scanning electron microscopy (SEM) and transmission electron microscopy/analytical electron microscopy (TEM/AEM) techniques. The extent of reaction was determined, phases were identified, and solute partitioning among the phases was determined. It was found that the matrix had reacted only with a portion of the carbon-rich outer layer of the SCS-6 fiber. The reaction zone contained two concentric zones which are distinguished by the presence of different carbide phases. Both zones contained a hexagonal Si-bearing phase, and one of the zones also contained some fine scattered porosity. The results are discussed with reference to available phase equilibria data.

Room‐temperature oxidation of silicon catalyzed by Cu3Si

Abstract: We demonstrate remarkably rapid oxidation of (100) silicon at room temperature catalyzed by the presence of Cu3Si. Thermal oxidation of Si is normally carried out at temperatures above 700 °C. Oxidation of many metal silicides occurs more rapidly than that of Si, but under controlled conditions results in a surface layer of SiO2. In contrast, the oxidation process described here produces a thick layer of SiO2 underneath the copper‐rich surface layer. The SiO2 layer grows spontaneously to over 1 μm in thickness in several weeks in air at room temperature. Analysis by Rutherford backscattering, Auger electron spectroscopy, cross‐sectional transmission electron microscopy, and scanning electron microscopy reveals the presence of Cu3Si at the buried SiO2/Si interface, epitaxially related to the underlying Si substrate. Catalytic action by this silicide phase appears responsible for the unusual oxidation process.

Interaction between titanium and SiC

Abstract: Interaction between titanium and SiC particles was studied by electron microprobe, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The distribution of the reaction products from the SiC to the titanium end is reported. Particular attention is given to the microstructure and phase analysis in areas near to the SiC. The implications of the observed microstructural and structural details are discussed.

High-temperature ion beam synthesis of cubic SiC

Abstract: The β‐SiC (or 3C‐SiC) synthesis through high‐dose carbon implantation into a silicon substrate heated at high temperature is studied by means of cross‐sectional transmission electron microscopy, x‐ray diffraction, infrared absorption spectroscopy, Auger electron spectroscopy, Rutherford backscattering spectrometry, channeling, and nuclear reaction analysis. It is shown that a thick buried layer of β‐SiC (about 300 nm) is directly formed after multiple implantations at 860 °C without post‐implantation annealing. This layer is not continuous but consists of small monocrystalline grains (average size ∼7 nm) in a near‐perfect epitaxial relationship with the silicon matrix. These grains are only slightly misorientated (∼3.5°) but are severely twinned on {111} planes.

Microstructure and Composition of Alumina/Aluminum Composites Made by Directed Oxidation of Aluminum

Abstract: Two Al2O3/Al composites, grown by the directed oxidation of molten Al alloys at 1400 and 1600 K, were investigated by X-ray diffraction, optical microscopy, scanning electron microscopy, transmission electron microscopy, and wet chemical analysis. The materials were found to contain a continuous network of Al2O3, which was predominantly free of grain-boundary phases and was made up of nanometer- to micrometer-sized crystallites, a continuous network of Al alloy, and isolated inclusions of Al alloy. No crystallographic orientation was observed in the metallic phase, whereas the Al2O3 was oriented with its c axis parallel to the growth direction. The higher process temperature yielded a lower metal content and less connectivity of the metallic consituent.

Direct observation of structural defects in laser-deposited superconducting y-ba-cu-o thin films.

Abstract: The defect structure of in situ pulsed, laser-deposited, thin films of the high—transition temperature superconductor Y-Ba-Cu-O has been observed directly by atomic resolution electron microscopy. In a thin film with the nominal composition YBa2Cu3O7 (123), stacking defects corresponding to the cationic stoichiometry of the 248, 247, and 224 compounds have been observed. Other defects observed include edge dislocations and antiphase boundaries. These defects, which are related to the nonequilibrium processing conditions, are likely to be responsible for the higher critical currents observed in these films as compared to single crystals.

Phase formation sequence for the reaction of multilayer thin films of Nb/Al

Abstract: We have investigated the phase formation sequence for the reaction of Nb and Al in multilayer films using cross‐sectional transmission electron microscopy and x‐ray diffraction. NbAl3 is the first intermetallic phase to form. Contrary to previous reports, we find evidence from cross‐sectional transmission electron microscopy that the sigma phase, Nb2Al, is not bypassed in the reaction sequence. Instead, its formation is concurrent with the formation of the superconducting A15 phase, Nb3Al. However, depending on the periodicity and the composition of the film, the Nb2Al phase can be consumed by the Nb3Al phase for long annealing times. The significance of this phase formation sequence to powder metallurgically processed magnet wire is discussed.

Electron microscopy of vapor phase deposited diamond

Abstract: Thin carbon films grown from a low pressure methane-hydrogen gas mixture by microwave plasma enhanced CVD have been examined by electron microscopy. Previously reported transmission electron microscopy (TEM) of the diamond films has shown that the majority of diamond crystals have a very high defect density comprised of {111} twins, {111} stacking faults, and dislocations. In this study, high resolution electron microscopy (HREM) has been utilized to lattice image individual defects in these polycrystalline diamond films. Interpretation of the images from these defects is not trivial and reported image simulations have been utilized to understand further these defects. Fivefold multiply twinned particles have also been examined and it was found that the 7.5° misfit present in such particles has been accommodated at the twin boundaries rather than by elastic deformation. This creates a twin boundary coincident with a low angle grain boundary which has been termed a “tilted twin boundary”. The density of defects in these particles is generally high; however, a dramatic reduction in the defect density near the twin boundaries was observed. This defect reduction is significant because if its origin can be determined, this information may be useful in producing higher quality diamond films.

Electron irradiation induced crystalline-to-amorphous transition in quartz single crystals

Abstract: The electron irradiation induced crystalline-to-amorphous (C-A) transition in quartz was studied as a function of incident electron energy over a range from 100 to 2250 keV, by means of high voltage electron microscopy. Special attention was paid to the C-A transition at electron energies greater than 650 keV for which there are no earlier reports. The C-A transition is induced via the ionization mechanism at least up to an electron energy of about 1500 keV. At electron energies greater than 1500 keV, however, the knock-on displacement mechanism seems to make an increasing contribution towards the C-A transition until it becomes dominant over the ionization mechanism. The behavior of the C-A transition as observed by bright field imaging is essentially the same for the transitions induced by both 2 MeV and 125 keV electron irradiation, in spite of the change in dominant mechanism for the C-A transition at around 1500 keV. Namely, in both cases the transition proceeds in such a manner that strain centers, which are amorphous inclusions embedded in the crystalline matrix, are formed at the beginning stage of irradiation and with continued irradiation grow in size until they coalesce into large amorphous inclusions covering the entire irradiated volume. Based on the results, the mechanism behind the C-A transition is discussed.

Properties of diamond membranes for x‐ray lithography

Abstract: Microwave plasma chemical vapor deposition was used to deposit diamond films on polished silicon and then the substrate was back‐etched to form taut free‐standing membranes up to 75 mm diam. Raman spectroscopy and x‐ray diffraction verify that the films are diamond. Transmission electron microscopy and x‐ray diffraction reveal that the films are fine‐grained polycrystalline diamond with an average grain size between 15 and 110 nm. The films contain microcrystalline graphite and nondiamond carbon as indicated by the broad Raman bands at 1355 and 1560 cm−1, respectively. The measured x‐ray transmission of a 3.25‐μm‐thick membrane at the Cu Kα line is 99.2% while the optical transmission at the He‐Ne laser line is 58% for a 1‐μm membrane, uncorrected for reflection losses from both surfaces, absorption, and scatter. The transmission in the ir is at the theoretical limit, 71%. The surface roughness of a typical 3‐μm‐thick membrane is about 30 nm on the side away from the silicon substrate and 20 nm on the sid...

Texture characterisation of sputtered MoS2 thin films by cross-sectional TEM analysis

Abstract: Cross sections of RF sputtered MoS2 thin films are investigated using lattice resolution transmission electron microscopy and electron diffraction. The results are compared with X-ray diffraction measurements. The films have a (001) texture (type II) in the first nanometres near the interface and a conventional (100) texture (type I) in the upper part.

Single‐crystal Sn/Ge superlattices on Ge substrates: Growth and structural properties

Abstract: Short‐period strained‐layer α‐Sn/Ge superlattices lattice matched to Ge(001) substrates have been synthesized for the first time. The thin, tetragonally distorted α‐Sn layers are stabilized by a modified molecular beam epitaxy technique with large modulation of substrate temperature during growth. Optimization of growth conditions is achieved via in situ Auger electron spectroscopy and low‐energy electron diffraction. This new kind of strained‐layer superlattice is characterized by transmission electron microscopy, x‐ray diffraction, and Raman scattering. Distinct superlattice effects are observed in the structural and phonon properties of the samples.

Fibre-matrix interactions during fabrication of Al2O3Mg metal matrix composites

Abstract: The reactions between alumina fibres and magnesium matrix in metal matrix composites at 1000 K are investigated theoretically and experimentally. The diffusivities of aluminium in layers of spinel and magnesia are estimated and found to be exceedingly low. It is thus concluded that diffusional growth of such layers in the boundary between matrix and fibres will not occur during the casting of the composite. The reaction between amorphous SiO2 binder and magnesium is further discussed and it is concluded that this may proceed at a considerable rate. Composites cast by means of liquid infiltration are studied metallographically by optical microscopy, scanning electron microscopy and transmission electron microscopy/scanning transmission electron microscopy equipped for microanalysis. The observations are in general agreement with the theoretical results, indicating that the reaction between binder and liquid magnesium is the only important one during fabrication of the composite. It is found that the binder transforms almost completely to a very-fine-grained magnesia and that silicon is dissolved in the molten matrix and precipitated as Mg2Si upon cooling.

Environmental degradation of AlxGa1−xAs‐GaAs quantum‐well heterostructures

Abstract: Data describing the deterioration of AlxGa1−xAs‐GaAs heterostructures in long‐term exposure (2–12 years) to normal room environmental conditions (∼20–25 °C, varying humidity) are presented Optical microscopy, scanning electron microscopy, transmission electron microscopy, and electron dispersion x‐ray spectroscopy are used to examine AlxGa1−xAs‐GaAs quantum‐well heterostructure material that has hydrolyzed at cleaved edges, cracks, and fissures, and at pinholes in cap layers The hydrolysis is found to be significant for thicker (>01 μm) AlxGa1−xAs layers of higher composition (x>085)