John Bruley

Properties of amorphous diamond films prepared by a filtered cathodic arc

Amorphous diamond films have been prepared by filtered cathodic arc deposition of carbon. The filtered arc is well suited for the growth of amorphous diamond, as it provides carbon ions with optimum kinetic energies at practical deposition rates. These films contain no hydrogen and are therefore structurally different from diamond‐like carbon films generated by plasma chemical vapor deposition. Diamond‐type bonding of carbon is quantitatively determined by electron energy loss spectroscopy, as an sp3 content up to 83% is measured. Data on the macroscopic properties are provided by optical transmittance, ellipsometry, Rutherford backscattering, elastic recoil scattering, and resistivity measurements. The films exhibit high optical transparency and an optical gap of 2.4 eV. Trends in the optical gap and refractive index as a function of deposition energy are consistent with semiconductor theory and indicates a change in the average bond length.

Quantitative near-edge structure analysis of diamond-like carbon in the electron microscope using a two-window method

To extract quantitative electronic structure information from electron energy‐loss near‐edge structure typically necessitates the separation of spectra into linearly independent components. For this to be accomplished, certain conditions need to be met concerning the localization of chemical bonding and the linear scaling of fine structure with density of states. The dependence of the processing parameters, such as window widths and positions, of the commonly used ‘two‐window intensity‐ratio’ method to determine the distribution of π and σ bonding of diamond‐like carbon films is investigated. It is shown that, although the selection of the integration windows can lead to very large variations in the resultant bonding fractions, the placement of two small windows centred on the edge structures of interest does permit a robust and reliable estimate of bonding. It is demonstrated that the sp3 variations can be reproducibly monitored with a relative accuracy of about ± 5%, and an absolute concentration given with an accuracy of ± 13% for foils of thickness at least up to twice the inelastic mean free path.

Review of methods for calculating near edge structure

Electron energy loss inner shell near edge structures within about 30 eV of threshold can give information on charge redistribution, conduction band changes, coordination and structure changes on a local scale. A theoretical understanding is necessary to go beyond empirical rules based on fingerprinting. All approaches to near edge structure calculation in solids are derived from various band theory methods with varying degrees of approximation. There is a place for theories based on simple physical ideas as well as large sophisticated calculations. In many oxides estimates of the number of peaks and their positions can be based on an extension of single scattering EXAFS theory to include strong second order intrashell scattering. These effects will be illustrated in MgO and NiO. In other cases it is necessary to use band structure calculations to give either projected densities of states or wave functions from which matrix elements can be calculated directly. Examples of this approach will show results of calculations for Si, diamond and SiC using the Cambridge plane wave pseudopotential code (CASTEP).

Ion‐beam sputtered diamond‐like carbon with densities of 2.9 g/cc

Thin films of carbon have been deposited at temperatures ranging from 77 to 1073 K. It has been found that diamond‐like properties of the films change with the deposition temperature and the thermal conductivity of the substrate. Densities as high as 2.9 g/cc with an sp3 bond content >40% have been observed. Substrates that have been investigated include graphite, NaCl, Si, Al2O3, fused quartz, and diamond. Density measurements have been made using Rutherford backscattering spectrometry and electron‐energy‐loss spectroscopy while hydrogen profiling was conducted using forward recoil spectrometry.

Chemistry and bonding changes associated with the segregation of Bi to grain boundaries in Cu

Grain-boundary embrittlement, caused by the segregation of impurity and alloying elements, occurs in many systems and has been the focus of a large amount of research owing to its technological importance. However, the exact mechanism by which the segregating elements cause embrittlement remains unclear. In this paper the localized changes in the electronic structure in the classical embrittling system of Bi in Cu have been studied. Experimental results were obtained by examining the fine structure in the electron energy loss spectrum which was then compared to calculations using the layer Korringa-Kohn-Rostoker (LKKR) method. A change in the d density of states has been observed for the Cu atoms at the grain boundary, associated with Bi, and an electronic model to explain embrittlement is described.

Energetic carbon deposition at oblique angles

Carbon is energetically deposited by pulsed laser vaporization, cathodic arc and ion beam sputtering to provide amorphous diamond films with varying degrees of diamondlike quality. Laser and arc films prepared at normal particle incidence are high in sp3 content and electrical resistivity, and exhibit optical behavior comparable to diamond. As the incident angle of the depositing species becomes increasingly oblique, the diamondlike character is progressively degraded. This is verified by decreases in sp3 fraction, resistivity, and optical transmittance. These changes are accompanied by a roughening of the microstructure, but no apparent variation in the transmission electron diffraction pattern. For deposition at the most oblique angles, the development of biaxial stress anisotropy is observed, as the compressive stress measured along the direction of incidence decreases at a greater rate than along the perpendicular direction. The implications of these results on the mechanisms proposed for amorphous di...

Electron energy-loss near-edge structure of internal interfaces by spatial difference spectroscopy

Internal interfaces determine the macroscopic mechanical and electronic properties of materials. Their investigation is therefore essential for an understanding of the relationship between microstructure and the materials properties. Besides the atomic structure, the bonding and electronic structure on an atomic scale are of interest. This information is contained in the interface‐sensitive component of the electron energy‐loss near‐edge structure (ELNES). It can be extracted by the spatial difference method. Such spectra have already been interpreted with respect to the bonding and the local atomic environment of the atoms at the interface. Here we assess critically the application of the spatial difference for the investigation of internal interfaces. The method is subjective because a suitable scaling factor has to be chosen. As a guideline we propose to minimize the number of turning points in the difference spectrum. With this hypothesis we successfully determine the ELNES arising from atoms at two internal interfaces between a metal and a ceramic. This reveals qualitatively which type of bonding is present at the interface and quantitatively how many atoms are involved in it.

Spatially resolved electron energy-loss studies of metal–ceramic interfaces in transition metal/alumina cermets

Composites consisting of an alumina matrix and 20 vol.% transition metal (Ni or Fe) particles, prepared by hot pressing powder blends, have been studied using spatially resolved transmission electron energy‐loss spectroscopy (EELS), and, to a lesser extent, by high‐resolution electron microscopy (HREM). Particular attention was paid to the elucidation of the chemical bonding mechanisms at the metal‐ceramic interface; EELS spectra from interfacial regions being obtained via a spatial difference technique. From both qualitative and quantitative interpretation of EELS near‐edge structures, as well as observed HREM images, the data appear to be consistent with the presence of an Al‐terminated alumina at the interface and the formation of direct transition metal – aluminium bonds in Al(O3M) (M = Ni or Fe) tetrahedral units, possibly as a result of the dissolution and interfacial reprecipitation of Al during processing. These results correlate well with similar model studies on diffusion‐bonded Nb/Al2O3 interfaces and may, in the light of recent theoretical electronic structure calculations, have implications for the resultant interfacial bond strength in such materials.

Strain relaxation in single crystal SrTiO3 grown on Si (001) by molecular beam epitaxy

An epitaxial layer of SrTiO3 grown directly on Si may be used as a pseudo-substrate for the integration of perovskite oxides onto silicon. When SrTiO3 is initially grown on Si (001), it is nominally compressively strained. However, by subsequent annealing in oxygen at elevated temperature, an SiOx interlayer can be formed which alters the strain state of SrTiO3. We report a study of strain relaxation in SrTiO3 films grown on Si by molecular beam epitaxy as a function of annealing time and oxygen partial pressure. Using a combination of x-ray diffraction, reflection high energy electron diffraction, and transmission electron microscopy, we describe the process of interfacial oxidation and strain relaxation of SrTiO3 on Si (001). Understanding the process of strain relaxation of SrTiO3 on silicon will be useful for controlling the SrTiO3 lattice constant for lattice matching with functional oxide overlayers.

Chemistry of grain boundaries in calcia doped silicon nitride studied by spatially resolved electron energy-loss spectroscopy

Abstract Spatially resolved electron energy-loss spectroscopy has been used to interrogate the chemistry and bonding at grain boundaries in hot isostatically pressed (HIPed) high purity silicon nitride ceramics nominally doped with either 0, 100, 300 or 500 at. ppm calcium. Analysis of the nanometer thick glassy films between grains, shows that there is a monotonic increase in impurity segregation with dopant concentration. The predominant constituents of the grain boundary films are Si, O and N, forming a silicon oxynitride phase ranging in composition from SiO 2 to Si 2 N 2 O. The energy-loss near-edge structure indicates the structure of the glass is composed predominantly of tetrahedra but also suggests the presence of a small concentration of molecular N 2 , possibly as sub-microscopic bubbles. There is still no data on the composition gradients on a sub-nanometer level, though it is shown that in the detection limit set by counting statistics, near single atom sensitivity with a spatial resolution of 0.22 nm is achievable.