Roy Clarke

High pressure properties of graphite and its intercalation compounds

Abstract The effects of applied pressure on graphite and its intercalation compounds are reviewed emphasizing the relationship between structure and transport properties. It has long been recognized that high pressure plays a crucial role in the polymorphic phase transitions of graphite, notably in the graphite-diamond transformation. More recent studies have revealed a wealth of pressure-induced phases associated with the unusual layer-stacking (‘staging’) mechanism in the intercalation compounds of graphite. The high degree of structural anisotropy associated with staging is strongly reflected in the electronic band structure and transport properties, and in the remarkable pressure dependence of the superconducting states of some of the graphite intercalation compounds. High pressure is shown to be a valuable means not only to realize new structural phases but also to improve our understanding of the fundamental behaviour of these important materials.

Semiconducting cubic boron nitride

Abstract We present new developments in the preparation of semiconducting cubic boron nitride. The thin films were grown on (100) silicon substrates using electron cyclotron resonance (ECR) ion-assisted magnetron sputtering with the kinetic energy of the incident nitrogen ions controlled by a dc substrate bias. Using this technique we have been able to grow thick (up to 2 μm) boron nitride films containing 100% of the cubic phase. We have found that the relatively high nitrogen ion energy (∼ 100 cV), required to nucleate the cubic phase, can be reduced substantially (to ∼ 60 eV) once the cubic phase is formed, leading to reduced film stress, larger grain size (∼ 1000 A) and improved adhesion. The films have p -type conductivity. The carrier activation energy is 60 meV and we have observed Hall mobilities of 500 cm 2 V −1 s −1 at n a ≈ (5 × 10 18 cm −3 ).

Reduced bias growth of pure-phase cubic boron nitride

We report results on an improved growth process for cubic boron nitride (c-BN) films. The films are deposited on a dc-biased silicon substrate using ion-assisted sputtering. First, we grow a BN template layer at a bias voltage which maximizes the sp3 content. After this template layer attains a thickness of ∼500 A, corresponding to the coalescence of the mosaiclike grain structure, we find that we can reduce the substrate bias to about 50% of its initial value while sustaining pure phase c-BN growth. The reduction in nitrogen ion energy results in a dramatic increase in the growth rate as well as significantly improved film quality.

Low energy kinetic threshold in the growth of cubic boron nitride films

We report the growth of cubic boron nitride (cBN) films by magnetron sputtering on Si (100) substrates. The films are grown in the presence of negative substrate bias voltages and a nitrogen plasma produced by an electron cyclotron resonance source. We find evidence for a sharp low‐voltage threshold in the substrate bias (−105 V) beyond which the samples are predominantly cBN. The structural quality of the cBN films is optimized in a narrow range of voltages near this threshold. We discuss the important role of energetic ions in the formation of cBN in light of recent theoretical findings.

In situ texture monitoring for growth of oriented cubic boron nitride films

We report evidence for oriented growth of pure-phase cubic boron nitride on silicon (100) substrates. The films are deposited at high temperatures (up to 1200 °C) by reduced-bias ion-assisted sputtering. The growth technique produces highly textured c-BN films with relatively large grain size (∼1000 A) and reduced residual stress as the bias voltage is decreased. We have been able to grow thick (up to 2 μm) cubic boron nitride films containing 100% of the cubic phase with the (001) crystallographic axis of c-BN oriented perpendicular to the surface of the film. We show how reflection high-energy electron diffraction applied to texture monitoring in polycrystalline films can be used as an in situ process control technique that allows texture identification and quantitative characterization of its angular spread.

Hexagonal close-packed Ni nanostructures grown on the (001) surface of MgO

We report the in situ microscopy observation of an unnatural phase of Ni, a highly strained hexagonal close-packed (hcp) form which we believe is stabilized by heteroepitaxial growth on the (001) face of MgO. We find that the nanosized hcp nickel islands transform into the normal face-centered cubic structure when the size of the islands exceeds a critical value (about 2.5 nm thick with a lateral size of ∼5nm). The structural transition proceeds via a martensitic change in the stacking sequence of the close-packed planes. The formation of hcp Ni nanostructures with an unusually large crystallographic c∕a ratio (∼6% larger than ideal hcp) is very interesting for spintronic and recording applications where large uniaxial anisotropies are desirable.

Single bacterial cell detection with nonlinear rotational frequency shifts of driven magnetic microspheres

Shifts in the nonlinear rotational frequency of magnetic microspheres, driven by an external magnetic field, offer a dynamic approach for the detection of single bacterial cells. We demonstrate this capability by optically measuring such frequency shifts when an Escherichia coli attaches to the surface of a 2.0μm magnetic microsphere, thereby affecting the drag of the system. From this change in drag, the nonlinear rotation rate was reduced, on average, by a factor of 3.8. Sequential bacterial cell attachments were also monitored.

In situ thin-film texture determination

A kinematic theory of reflection high energy electron diffraction (RHEED) is presented for textured polycrystalline thin films. RHEED patterns are calculated for arbitrary texture situations and for any general crystallographic orientation that may be encountered in thin-film growth. It is shown that the RHEED pattern can be used as a fast and convenient tool for in situ texture characterization. The approach also permits quantitative extraction of angular dispersion parameters which are useful for optimizing thin-film growth conditions.

X‐ray synchrotron studies of ultrafast crystalline dynamics

Ultrafast X-ray experiments at synchrotron sources hold tremendous promise for measuring the atomistic dynamics of materials under a wide variety of transient conditions. In particular, the marriage of synchrotron radiation and ultrafast laser technology is opening up a new frontier of materials research. Structural changes initiated by femtosecond laser pulses can be tracked in real time using time-resolved X-ray diffraction on picosecond time scales or shorter. Here, research at the Advanced Photon Source is described, illustrating the opportunities for ultrafast diffraction with some recent work on the generation of impulsive strain, coherent phonon generation and supersonic diffusion of electron-hole plasmas. The flexibility of time-resolved Bragg and Laue diffraction geometries are both utilized to illuminate the strain generation and evolution process. Time-resolved X-ray science will become increasingly important with the construction of linac-based ultrafast X-ray sources.

Evolution of structural and optical properties of ion-beam synthesized GaAsN nanostructures

We have investigated the evolution of structural and optical properties of GaAsN nanostructures synthesized by N ion implantation into epitaxial GaAs, followed by rapid thermal annealing. Transmission electron microscopy and x-ray diffraction indicate the formation of nanometer-sized crystallites with lattice parameters close to those of pure zincblende GaN. The average crystallite size increases with annealing temperature while the size distribution is self-similar and the volume fraction remains constant, suggesting a coarsening process governed by Ostwald ripening. These GaAsN nanostructures exhibit significant photoluminescence in the near infrared range. The apparent lowering of the fundamental band gap is likely due to the incorporation of a small amount of As in GaN.