3D graphitic microstructures for diamond-based PETE devices

Diamond is a very attractive material for the fabrication of devices based on thermionic emission, due to its low or even negative electron affinity (NEA): this feature is indeed related to a decrease of the workfunction, resulting in an enhanced capability for emitting electrons. Also, micro- and nanostructuring of single-crystal diamond, induced by femtosecond laser treatment on the sample surface as well as in the bulk, may significantly improve optical absorption, thus making feasible the fabrication of innovative PETE devices. We present here preliminary results on structural, optical and electrical characterization of femtosecond laser-induced graphitic columns within single-crystal diamond bulk, together with the fabrication technique used. High-quality single-crystal CVD diamond plates (4.5 × 4.5 mm2, 500 ?m thick, <110> edges), provided by Element Six Ltd., were used for the present work. A mode-locked Ti:Sapphire femtosecond laser, operating at 800 nm wavelength, was used to induce graphitization inside the diamond bulk. Before the start of the laser treatment, the beam focal plane was positioned behind the bottom surface of the diamond plate, then graphitic columns were induced by moving the sample along the beam direction: for this purpose, the sample was mounted on a numerically controlled XYZ stage. Following their fabrication, graphitic columns were structurally (Raman spectroscopy), optically (spectral photometry), and electrically (current-voltage measurements) characterized. Results show that 3D graphitic microstructures, acting as distributed electrodes, may introduce several benefits in novel diamond-based PETE devices, such as: 1) improvement of optical absorption both in near-infrared and visible ranges; 2) decrease of the total device series resistance, thus avoiding electron refilling bottlenecks; 3) a shorter drift path for the photogenerated carriers; 4) a lower probability of charge trapping; 4) no need for metal electrodes, thus overcoming the limits of using metal contacts at high operating temperatures.