L. Nistor

Direct observation of laser-induced crystallization of a-C : H films

The post-growth modification of diamond-like amorphous hydrogenated carbon a-C:H films by laser treatment has been studied by transmission electron microscopy and Raman spectroscopy. a-C:H films grown on Si substrates by benzene decomposition in a rf glow discharge were irradiated with 15 ns pulses of a KrF-excimer laser with fluences in the range of E=50–700 mJ/cm2. At fluences below 100 mJ/cm2 an increase in the number of graphitic clusters and in their ordering was evidenced from Raman spectra, while the film structure remained amorphous according to electron microscopy and electron diffraction observations. At higher fluences the appearance of diamond particles of 2–7 nm size, embedded into the lower crystallized graphitic matrix, was observed and simultaneously a progressive growth of graphite nanocrystals with dimensions from 2 nm to 4 nm was deduced from Raman measurements. The maximum thickness of the crystallized surface layer (≈400 nm) and the degree of laser annealing are limited by the film ablation which starts at E>250 mJ/cm2. The laser-treated areas lose their chemical inertness. In particular, chemical etching in chromium acid becomes possible, which may be used for patterning the highly inert carbon films.

Colloid size distributions in ion implanted glass

Abstract Ion implantation of silver ions at 60 keV into silicate glass produces metallic colloids. Colloid growth during implantation as recorded by in situ changes in optical reflectivity. Post implant analyses indluded optical absorption, Rutherford backscattering and transmission electron microscopy. The optical data imply there is a range of colloid sizes and the details of colloid size as a function of depth are determined by the TEM measurements of thinned samples. The factors influencing the changes in the optical data with ion dose and the depth dependence of the colloid size distribution are discussed.

Gallium colloid formation during ion implantation of glass

Abstract Data are reported on the size and depth distribution of gallium colloids formed by gallium ion implantation at energies of 50 and 60 keV, and nominal doses up to 1.1 × 1017 ions/cm2 into coverlip glass, float glass and white crown glass. Measurement techniques used to reveal colloid-induced changes include the wavelength dependence of optical reflectivity, transmission electron microscopy (TEM) and Rutherford backscattering (RBS). The reflectivity can be controlled by variations in ion dose, implant temperature and ion beam energy. The highest reflectivity is found after implants near 50°C and the level is extremely sensitive to the implant temperature. For controlled beam conditions, the reflectivity data are reproducible, despite there being variations in the colloid size and depth distributions as seen by TEM and RBS. The TEM data reveal that the depth distribution develops in two distinct regions, which at high concentration can precipitate into two layers of large colloids. Subsidiary experiments are reported to attempt to separate the effects of variations in the implant temperature and surface charging which influence the reflectivity, RBS and colloid formation.

Formation of amorphous carbon and graphite in CVD diamond upon annealing : A HREM, EELS, Raman and optical study

Polycrystalline diamond thick films were subjected to annealing in vacuum at temperatures of 1350-1450°C. The films were examined by optical absorption, Raman spectroscopy, transmission electron microscopy and electron energy loss spectroscopy. The formation of amorphous carbon and/or of well-crystallized graphite layers up to 20 nm thick was evidenced along grain boundaries. Intra-granular nanometer-sized graphite islands were also observed, sometimes as transformed micro-twin bands. The diamond-to-graphite transition occurs in such a way that three (111) diamond planes transform into two (0002) graphitic sheets. The internal graphitization causes a severe degradation of the optical quality of the diamond films.

Structure and properties of high-temperature annealed CVD diamond

Abstract Effects of high temperature, up to 1700 °C, annealing in vacuum of CVD diamond on its structure, optical and mechanical properties are investigated. Translucent polycrystalline diamond films of thickness 0.06–1.0 mm were grown by microwave plasma CVD method, and examined with transmission electron microscopy, optical absorption spectroscopy, and three-point bending technique to measure the fracture strength. A progressive darkening of the samples, with appearance of absorption features specific for graphite-like material, was observed upon annealing at temperatures above 1300 °C. The formation along grain boundaries of amorphous carbon and/or well crystallized graphite layers, 5–20 nm thick, as well as intra-granular graphite islands, was directly observed with TEM. This internal diamond-graphite transformation process can be described by two activation energies, both values being much less than those known for the surface graphitization of diamond. The fracture strength of the diamond films increases up to 50% with annealing temperature (1460–1640 °C), this being ascribed to a build up of compressive stress as a result of local diamond-graphite conversion.

The ferroelectric phase transition in tridymite type BaAL2O4 studied by electron microscopy

Abstract The paraelectric-ferroelectric (PE-FE) phase transition in stuffed tridymite BaAl2O4 was studied in situ by transmission electron microscopy. Electron diffraction revealed that the PE and FE phases have hexagonal symmetry. The PE-FE phase transition is accompanied by a doubling of the cell dimensions in the a-b plane. The transition is reversible, takes place over a wide temperature range (400–670 K.) and the interfaces related to the transition have a fluctuating character. The crystal structure of the high temperature PE phase was determined by high resolution electron microscopy. The structures of the PE phase (space group P6322, a≈ 5.22 Å, c ≈ 8.8 Å) and of the FE phase (space group P63, a= 10.4469(1)Å, c = 8.7927(1)Å) differ mainly by the configuration of the Al-O strings oriented along the c-axis. In the PE phase all the strings are equivalent whereas straight and corrugated strings alternate in an ordered manner in the FE phase resulting in doubling of the a and b cell parameters. Translation and orientation domains due to the decrease of the translation and point symmetry were frequently observed.

Structural Aspects of CVD Diamond Wafers Grown at Different Hydrogen Flow Rates

Microstructural modifications of thick diamond films grown by microwave plasma CVD at different reactant gas flow rates have been studied by transmission electron microscopy. A strong degradation of the diamond quality was observed at low hydrogen flow rates, while the layer grown at the highest hydrogen flow rate was almost defect free. The abundance of various defects observed, mostly microtwins, stacking faults, dislocations and even amorphous regions, clearly correlates with the gas flow rate. Illustrations of typical defects are given and, in particular, planar defect intersections are characterized at atomic resolution in specimens where their density was high.

The influence of the h-BN morphology and structure on the c-BN growth

The morphology and structure of hexagonal graphitic BN (h-BN) powders with graphitization indices GI < 5, used as precursors for the synthesis of cubic BN (c-BN) crystals, has been investigated by transmission electron microscopy in diffraction contrast and high resolution. We show that besides the GI, which is a general parameter for controlling the structural quality of h-BN powders, some other microstructural features strongly influence the synthesis of c-BN. In our opinion, the high reactivity of some h-BN powders results from the presence of some nucleation centers for c-BN, observed at the edges of the h-BN particles. They are formed by a rearrangement of the graphitic (0002) planes by bending back, joining in pairs and forming locally nanoarches (half nanotubes). In these particular places, the nature of bonding locally turns towards sp 3 , as in the case of c-BN.

High-resolution electron microscopy and electron spin resonance studies on cubic boron nitride crystals made by high-pressure/high-temperature synthesis

Cubic boron nitride (c-BN ) crystals synthesised at high pressures and temperatures are analysed by optical microscopy, transmission electron microscopy, electron diVraction and electron spin resonance. For various growth conditions, the results of these studies indicate that the c-BN crystals contain defects and impurities. This is the first time that dislocation cores have been revealed in c-BN at the atomic level. Atomic resolution at extended dislocations allows us to determine the stacking-fault energy in c-BN, yielding a mean value of 191±15 mJ m’2. This value, which is reported for the first time for c-BN, is of the same order of magnitude as in diamond.

In situ study of the phase transition in Bi2Ti4O11

Abstract The paraelectric ⇌ antiferroelectric phase transition of the compound Bi 2 Ti 4 O 11 is studied in situ by electron diffraction and electron microscopy. The transition is reversible and clearly second order. Above T c faint streaking at the superlattice positions persists. In the low-temperature phase, antiphase boundaries with a displacement vector R = 1/2[101] are revealed; they show a finite width, suggesting a gradual displacement of the Bi atoms at the transition. A model for the domain wall configuration is proposed, based on the influence of the lone pairs of Bi 3+ ions at the transition.