Mathew C. Guenette

Substrate orientation effects on the nucleation and growth of the Mn+1AXn phase Ti2AlC

The Mn+1AXn (MAX) phases are ternary compounds comprising alternating layers of a transition metal carbide or nitride and a third “A-group” element. The effect of substrate orientation on the growth of Ti2AlC MAX phase films was investigated by studying pulsed cathodic arc deposited samples grown on sapphire cut along the (0001), (101¯0), and (11¯02) crystallographic planes. Characterization of these samples was by x-ray diffraction, atomic force microscopy, and cross-sectional transmission electron microscopy. On the (101¯0) substrate, tilted (101¯8) growth of Ti2AlC was found, such that the TiC octahedra of the MAX phase structure have the same orientation as a spontaneously formed epitaxial TiC sublayer, preserving the typical TiC–Ti2AlC epitaxial relationship and confirming the importance of this relationship in determining MAX phase film orientation. An additional component of Ti2AlC with tilted fiber texture was observed in this sample; tilted fiber texture, or axiotaxy, has not previously been seen...

Sculpting nanoscale precipitation patterns in nanocomposite thin films via hyperthermal ion deposition

Control of the morphology of self-organized nanostructures is the key issue in bottom-up approaches. Here, morphological transitions of precipitation patterns in C:Cu nanocomposite films are studied. The films have been grown by oblique incidence ionized physical vapor deposition. We show that the ion energy and directionality are transferred into the C–Cu phase separation process resulting in nanopattern formation and tilt. Increasing metal content induces the “tilted”-“lying” transition accompanied with Cu nanoparticle prolate-spherical-oblate shape transformations. The results allow the identification of metal subplantation as the key atomistic mechanism, and demonstrate the possibility to achieve nanoscale sculpting via energetic ion deposition.

Carbon diffusion in alumina from carbon and Ti2AlC thin films

Carbon diffusion is observed in single crystal α-Al2 O3 substrates from carbon and Ti2AlC thin films synthesized via pulsed cathodic arc deposition. Diffusion was found to occur at substrate temperatures of 570 °C and above. The diffusion coefficient of carbon in α-Al2 O3 is estimated to be of the order 3×10-13 cm2/s for deposition temperatures in the 570–770∘C range by examining elastic recoil detection analysis (ERDA) elemental depth profiles. It is suggested that an appropriate diffusion barrier may be useful when depositing carbon containing thin films on α-Al2 O3 substrates at high temperatures.

Deuterium retention and near-surface modification of ion-irradiated diamond exposed to fusion-relevant plasma

Chemical vapour deposited diamond was irradiated with 5 MeV carbon ions to simulate the damage caused by collision cascades from neutron irradiation in a fusion environment. Ion-irradiated samples were then exposed to a deuterium plasma in MAGPIE with ion flux of ~1.3 × 1021 ions m−2 s−1. Raman and near edge x-ray absorption fine structure (NEXAFS) spectroscopy were used to characterize the degree of disorder and sp2-bonding induced by the ion irradiation. The signals of sp2-bonded and disordered carbon were observed to decrease after exposure to the deuterium plasma, although sharp Raman peaks indicative of vacancy and interstitial defects induced by the MeV ions were less affected. Recovery of a diamond-like surface after plasma exposure was evident in the NEXAFS spectra. Elastic recoil detection analysis showed that the ion-damaged diamond retained more deuterium than diamond exposed only to deuterium plasma. For the case of unirradiated samples, diamond retained more deuterium than graphite. However, for the case of the ion-irradiated samples, diamond exhibited less deuterium retention than graphite.

Effect of Annealing upon Retention of He and H in Irradiated SiC

Silicon carbide (3C-β SiC) samples were irradiated with He ions of energy up to 30 keV and a fluence up to 1016/cm2, to produce damage in the near-surface region. A duplicate set of He ion irradiated SiC samples, as well as undamaged SiC, were also irradiated with H2+ ions of energy up to 20 keV and a similar fluence, to study the interaction of H species with pristine SiC and with He radiation-damaged SiC. Samples were annealed in steps of 200 K, from 473 K to 1273 K, and the retention of H and He were measured using elastic recoil detection analysis with 7.8 MeV C3+ ions, after each anneal step. Modification to the surface following irradiation is observed via Raman spectroscopy, which exhibits development of damage states such as disordered carbon and Si-Si peaks. Only minor changes in the H and He profiles were observed up to 1073 K, however after the 1273 K anneal the H and He profiles changed considerably, with a marked difference between samples irradiated only with He and those irradiated with He and H.