Nonadiabatic Kohn Anomaly in Heavily Boron-Doped Diamond.

We report evidence of a nonadiabatic Kohn anomaly in boron-doped diamond, using a joint theoretical and experimental analysis of the phonon dispersion relations. We demonstrate that standard calculations of phonons using density-functional perturbation theory are unable to reproduce the dispersion relations of the high-energy phonons measured by high-resolution inelastic x-ray scattering. On the contrary, by taking into account nonadiabatic effects within a many-body field-theoretic framework, we obtain excellent agreement with our experimental data. This result indicates a breakdown of the Born-Oppenheimer approximation in the phonon dispersion relations of boron-doped diamond.

Superconducting group-IV semiconductors.

Despite the amount of experimental and theoretical work on doping-induced superconductivity in covalent semiconductors based on group IV elements over the past four years, many open questions and puzzling results remain to be clarified. The nature of the coupling (whether mediated by electronic correlation, phonons or both), the relationship between the doping concentration and the critical temperature (T(c)), which affects the prospects for higher transition temperatures, and the influence of disorder and dopant homogeneity are debated issues that will determine the future of the field. Here, we present recent achievements and predictions, with a focus on boron-doped diamond and silicon. We also suggest that innovative superconducting devices, combining specific properties of diamond or silicon with the maturity of semiconductor-based technologies, will soon be developed.

Nanostructures made from superconducting boron-doped diamond.

We report on the transport properties of nanostructures made from boron-doped superconducting diamond. Starting from nanocrystalline superconducting boron-doped diamond thin films, grown by chemical vapour deposition, we pattern by electron-beam lithography devices with dimensions in the nanometer range. We show that even for such small devices, the superconducting properties of the material are well preserved: for wires of width less than 100 nm, we measure critical temperatures in the kelvin range and critical fields in the tesla range.

Crystalline Defects Induced during MPCVD Lateral Homoepitaxial Diamond Growth.

The development of new power devices taking full advantage of the potential of diamond has prompted the design of innovative 3D structures. This implies the overgrowth towards various crystallographic orientations. To understand the consequences of such growth geometries on the defects generation, a Transmission Electron Microscopy (TEM) study of overgrown, mesa-patterned, homoepitaxial, microwave-plasma-enhanced, chemical vapor deposition (MPCVD) diamond is presented. Samples have been grown under quite different conditions of doping and methane concentration in order to identify and distinguish the factors involved in the defects generation. TEM is used to reveal threading dislocations and planar defects. Sources of dislocation generation have been evidenced: (i) doping level versus growth plane, and (ii) methane concentration. The first source of dislocations was shown to generate <110> Burgers vector dislocations above a critical boron concentration, while the second induces <112> type Burgers vector above a critical methane/hydrogen molar ratio. The latter is attributed to partial dislocations whose origin is related to the dissociation of perfect ones by a Shockley process. This dissociation generated stacking faults that likely resulted in penetration twins, which were also observed on these samples. Lateral growth performed at low methane and boron content did not exhibit any dislocation.

Boron-Doping Proximity Effects on Dislocation Generation during Non-Planar MPCVD Homoepitaxial Diamond Growth.

Epitaxial lateral growth will be required if complex diamond-based device architecture, such as, for example, Metal-oxide-semiconductor Field-effect transistors (MOSFETs) or epitaxial lateral overgrowth (ELO) substrates, need to be developed for high-power applications. To this end, undoped and doped non-planar homoepitaxial diamond were overgrown on (001)-oriented diamond-patterned substrates. Defects induced by both the heavy boron doping and three-dimensional (3D) growth were studied by transmission electron microscopy (TEM). At high methane and boron concentrations, threading dislocations with Burgers vectors b = 1/6 ⟨211⟩, b = 1/2 ⟨110⟩, or both were observed. Their generation mechanisms were established, revealing boron proximity effects as precursors of dislocations generated in boron-doped samples and providing clues as to the different Burgers vectors. The concentration ranges of boron and methane resulting in good crystalline quality depended on the plane of growth. The microwave plasma-enhanced chemical vapour deposition (MPCVD) growth conditions and the maximum boron concentration versus plane orientation yielding a dislocation-free diamond epitaxial layer were determined.

The diamond superconducting quantum interference device.

Diamond is an electrical insulator in its natural form. However, when doped with boron above a critical level (∼0.25 atom %) it can be rendered superconducting at low temperatures with high critical fields. Here we present the realization of a micrometer-scale superconducting quantum interference device (µ-SQUID) made from nanocrystalline boron-doped diamond (BDD) films. Our results demonstrate that µ-SQUIDs made from superconducting diamond can be operated in magnetic fields as large as 4 T independent of the field direction. This is a decisive step toward the detection of quantum motion in a diamond-based nanomechanical oscillator.