Very long laser-induced graphitic pillars buried in single-crystal CVD-diamond for 3D detectors realization

The morphology, optical, spectroscopic and electrical characterization of mm-long graphite pillars created by picosecond pulsed laser irradiation (lambda = 800 nm and 1 kHz of repetition rate), buried in single crystal CVD diamond to be employed as electrodes in a 3D diamond detector, is reported. The array of graphitized columns - 2.5 mm-long, with a diameter of approximate to 10 mu m - consisted of two rows spaced by 110 mu m with 12 pillars in each, which formed an interdigitated electrode structure embedded in the diamond crystal bulk. The presence of stressed regions along and between pillars were clearly shown with optical polarized microscopy, in a black field configuration. Confocal micro-Raman and photoluminescence analysis has been employed to scan local stresses, both generated around the graphitic wires and also developed on the pillars' plane. Defected/stressed regions with diameter of the order of 10 mu m surrounding the individual pillars was measured, and paired carbon interstitials (3H defects) were also revealed. For the investigated structure, detrimental effects induced by such structural defects, clearly produced by laser-induced diamond-graphite transition, as well as the presence of a relatively high voltage drop along the graphitized pillars related to their own geometry have been reflected on the charge carriers collection performances evaluated under MeV beta-particles. The creation of electronic active states within the diamond bandgap, as emphasized by spectral photoconductivity characterization, would play a fundamental role in lowering lifetime of generated carriers and then the detector collection efficiency. Indeed, states located in the middle of the diamond bandgap, acting as efficient recombination centers and decreasing the lifetime of generated carriers, drastically reduce the mean drift path of carriers and then the overall detector collection efficiency, as evaluated in the examined structure even at the highest applied voltages (up to 600 V).