Micro-Raman spectroscopy has been used to monitor structural defects and stress state developing in diamond due to formation of 3D graphitic electrodes for the achievement of optimized carrier collection in ionizing radiation and particle diamond detectors. Buried graphitic pillars were fabricated in a single-crystal CVD-diamond sample by means of a 400 fs pulsed laser operating at lambda = 1030 nm. The same conditions were also used for the realization of two series of graphitic strips on the surface allowing buried pillars connections. Micro-Raman spectra of untreated regions exhibit the typical diamond peak at 1332 cm(-1) which largely changes within the laser modified regions, where a G band in the range 1580-1600 cm(-1) is also detected. Strength decrease, shifting and broadening of the diamond Raman peak are observed by crossing graphitic electrodes and along buried pillars, pointing out that phase transition from diamond to graphitic carbon is accompanied both by stress development and structural disorder in the residual diamond tissue. In these regions, Raman spectra also exhibit a broad photoluminescence background signal, whose intensity appears related to graphitization process. In particular, a splitting of the diamond Raman peak is detected around pillars on the top surfaces suggesting the occurrence of a laser-induced biaxial stress. From these results it is then tentatively suggested that conduction in the realized electrodes occurs through both conductive graphitic phases and disordered diamond paths, whereas detector performance is mainly related to charge transport within virgin diamond.

The paper reports the micron-scale investigation of an all-carbon detector based on synthetic single crystal CVD-diamond having an array of cylindrical graphitic buried-contacts, about 20 mu m in diameter each, connected at the front side by superficial graphitic strips. To induce diamond-to-graphite transformation on both detector surface and bulk volume, direct-laser-writing technique was used. Laser-treatment parameters and cell shape have been chosen to minimize the overlapping of laser-induced stressed volumes. Optical microscopy with crossed polarizers highlighted the presence of an optical anisotropy of the treated material surrounding the embedded graphitized columns, and non-uniform stress in the buried zones being confirmed with a confocal Raman spectroscopy mapping. Dark current-voltage characterization highlights the presence of a field-assisted detrapping transport mainly related to highly-stresses regions surrounding buried columns, as well as superficial graphitized strips edges, where electric field strength is more intense, too. Notwithstanding the strain and electronic-active defects, the detector demonstrated a good charge collection produced by 3.0 and 4.5 MeV protons impinging the diamond, as well as those generated by MeV beta-particles emitted by Sr-90 source. Indeed, the mapping of charge collection efficiency with Ion Beam Induced Charge technique displayed that only a few micrometers thick radial region surrounding graphitic electrodes has a reduced efficiency, while most of the device volume preserves good detection properties with a charge collection efficiency around 90% at 60 V of biasing. Moreover, a charge collection efficiency of 96% was estimated under MeV electrons irradiation, indicating the good detection activity along the buried columns depth.

A detailed characterization under Sr-90 beta-particles and 4.5 MeV protons micro-beam of a single-crystal CVD diamond-based three-dimensional detector with surface and buried graphite electrodes is presented. Pillar contacts, 300 pm long and 30 mu m diameter, were fabricated by using a femtosecond laser operating at 1030 nm wavelength and 400 fs pulse duration. Charge collected under Sr-90 beta-particles was measured in front and back irradiation conditions, pointing out that the pillars contribute to the charge collection. Charge collection efficiency (CCE) was measured to be up to 94% under proton beam irradiation. Results of a comprehensive study, including crossed-polarizers imaging, numerical simulation of the electric field distribution, and proton mapping, show that CCE is not affected from the stress induced by the pillar fabrication, and that the electric field strength is high enough to partially compensate for carrier recombination in the defected regions surrounding the pillars.

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).

The paper deals on the response of a polycrystalline diamond sensor, 500 μm thick, to particles from a 90Sr β-source. 21 × 21 nano-carbon pads, with 0.18 mm × 0.18 mm area each, were realized by ArF excimer laser irradiation on one diamond face, whereas a 7 × 7 mm2 backside contact was fabricated and used for sensor biasing during characterization of sensor under β-source irradiation. The carbon pads embrace a number of grains, which show different degrees of surface graphitization dependent on the grain orientations. Each carbon pad exhibits a linear I(V) response up to 200 V. The average number of charge carriers collected by a single pixel, as well as the distribution of pixels involved by the impinging particle tracking, is analyzed as a function of the applied voltage recording the signals acquired by 16 pixels at a time. The pulse height distribution is not affected by reversing the bias polarity. For a single pixel, the most probable collected charge value is 1.40 ± 0.02 fC whereas the mean value gives 〈Q〉coll = 1.67 ± 0.02 fC (10,430 ± 120 electrons). The charge collection distance was measured taking into account the effect induced by high-energy electrons and found to be 285 ± 3 μm, demonstrating the absence of bulk defects induced by the laser graphitization processing. Cross-talk effects between nearest-neighbor pixels have been excluded analyzing the results obtained in a batch of more than 1000 events even if the same cannot be excluded under higher energy particles.

We report on the response of metal-less CVD polycrystalline-diamond pixel sensors under β-particles irradiation. A 21×21 array of 0.18×0.18mm2 pixels was realized on one side of a 10.0×10.0×0.5mm3 polycrystalline diamond substrate by means of laser induced surface graphitization. With the same technique, a large graphite contact, used for detector biasing, was fabricated on the opposite side. A coincidence detecting method was used with two other reference polycrystalline diamond detectors for triggering, instead of commonly used scintillators, positioned in the front and on the back of the sensor-array with respect to the impinging particles trajectory. The collected charge distribution at each pixel was analyzed as a function of the applied bias. No change in the pulse height distribution was recorded by inverting the bias voltage polarity, denoting contacts ohmicity and symmetry. A fairly good pixel response uniformity was obtained: the collected charge most probable value saturates for all the pixels at an electric field strength of about ±0.6V/μm. Under saturation condition, the average collected charge was equal to =1.64±0.02fC, implying a charge collection distance of about 285µm. A similar result, within 2%, was also obtained for 400MeV electrons at beam test facility at INFN Frascati National Laboratory. Experimental results highlighted that more than 84% of impinging particles involved only one pixel, with no significant observed cross-talk effects.

The paper reviews the status of diamond detectors for UV laser monitoring and imaging. Single pixel detectors, position sensitive architectures, optically activated switches and sensor arrays for beam positioning and imaging are analyzed. The performances of natural diamond and synthetic diamond produced by chemical vapor deposition are compared to evaluate the suitability of such an outstanding material for the described applications.

The charge collection performance of a diamond-graphite detector is reported. Buried graphite pillars with high aspect ratio were formed inside a single crystal synthetic diamond slab by using a femtosecond IR laser with 200 kHz of repetition rate. Grouped in two series and connected by graphite strips on the surface, eight independent 3D electrodes were used to collect the charge carriers generated by energy deposited in the detector. Collimated Sr-90, Y beta-particles were used to test the charge collection in coincidence and self-triggering mode among pillars rows using different irradiation geometries. The charge collected by one pillar row saturates at 1.40 +/- 0.02 fC at +/- 0.67V/mu m with electrons impinging orthogonally the rows demonstrating a high charge carrier collection efficiency.

The realization and characterization of a compact 64-pixel detector based on CVD-diamond is here presented and discussed. Each pixel has been connected to a dedicated multichannel read-out electronics designed for acquisition, conditioning and elaboration of the pixel-signal, allowing a real-time beam profile reconstruction of the impinging radiation. The back-side semitransparent contact was used for diamond sensor biasing. Detector characterization was performed under a weak continuous UV light spot obtained at the output of a monochromator illuminated by a deuterium lamp. Sensor was also characterized by X-ray illumination evaluating both displacement and broadening of impinging spot. Finally, photodetector response was observed under pulsed UV laser light, demonstrating the feasibility of a real-time beam monitoring for such a kind of light source.

The charge collection performance of a three-dimensional diamond-graphite detector is reported. Buried graphite pillars with high aspect ratio were formed inside a single crystal synthetic diamond slab by using a femtosecond IR laser with 200kHz of repetition rate. Grouped in two series and connected by graphite strips on the surface, eight independent 3D electrodes were used to collect the charge carriers generated by energy deposited in the detector by 90Sr,Y β-particles. Different impinging configurations were used to test charge collection and signal dependence on voltage. Reversing the bias polarity the pulse height distribution does not changes and the charge collection saturation of any group of connected pillars was observed around ±80V (0.53V/μm). The average charge collected by one pillars row is Qav=1.60±0.02fC, with electrons impinging orthogonally the rows, in such a way demonstrating full charge collection.

A diamond detector of 3D architecture without any metallization is developed for spectroscopy of ionizing radiation and single particles detection. The carbon electrode system was fabricated using a femtosecond infrared laser (\(\lambda \) = 1,030 nm) to induce graphitization on the surface and inside 4.0 \(\times \) 4.0 \(\times \) 0.4 mm\(^{3}\) single-crystal chemical vapor deposition diamond slab, resulting in an array of 84 buried graphite pillars of 30 \(\upmu \)m diameter formed orthogonally to the surface and connected by surface graphite strips. Sensitivity to ionizing radiation with \(^{90}\)Sr \(\upbeta \)-source has been measured for the 3D detector and high charge collection efficiency is demonstrated.

1Center for Advanced Marine Core Research, Kochi University, B200 Monobe, Nankoku, Kochi 783-8502, Japan 2Instituto de Geofisica, Universidad Nacional Autonoma de México, Ciudad Universitaria S/N, 04510 Mexico D.F., Mexico 3Laboratorio Interinstitucional de Magnetismo Natural, Instituto de Geofisica, Sede Michoacan, Universidad Nacional Autonoma de Mexico, Campus Morelia, Tzintzuntzan 310, Vista Bella, 58098 Morelia, Michoacan, Mexico 4Departamento de Geologia y Mineralogia, Instituto de Investigaciones Metalurgicas, Universidad Michoacana de San Nicolas de Hidalgo

We report the Sr-90 beta response of a polycrystalline diamond pixel detector fabricated using metal-less graphitic ohmic contacts. Laser induced graphitization was used to realize multiple squared conductive contacts with 1 mm x 1 mm area, 0.2 mm apart, on one detector side while on the other side, for biasing, a 9mm x 9mm large graphite contact was realized. A proximity board was used to wire bonding nine pixels at a time and evaluate the charge collection homogeneity among the 36 detector pixels. Different configurations of biasing were experimented to test the charge collection and noise performance: connecting the pixel at the ground potential of the charge amplifier led to best results and minimum noise pedestal. The expected exponential trend typical of beta particles has been observed. Reversing the bias polarity the pulse height distribution (PHD) does not changes and signal saturation of any pixel was observed around +/- 200V (0.4 V/mu m). Reasonable pixels response uniformity has been evidenced even if smaller pitch 50 divided by 100 mu m structures need to be tested.

The response of high quality polycrystalline diamond pixel detectors to 90Sr beta particles is reported. Laser induced surface graphitization was used to realize 36 conductive contacts with 1mm×1mm area each, pitch 1.2mm, on one detector side whereas a 8mm×8mm large area graphite contact was realized on the other face for grounding or biasing. A proximity board was used to hold the matrix, the amplifiers and to bond nine pixels to test homogeneity of response among 36 detector pixels. Two configurations were used to test charge collection uniformity and signal dependence on voltage. Both configurations showed noise pedestal fitted with a Gaussian curve of 1150 equivalent electrons (1σ) and typical beta source particles spectrum. Reversing the bias polarity the pulse height distribution does not change and the saturation of most probable value of charge collection was observed around ±200V (0.4V/μm) with reasonable pixel response uniformity equal to a most probable value 1.28±0.05fC. The charge collection efficiency (CCE) measurement was implemented using coincidence mode acquisition with an external trigger made by a commercial polycrystalline diamond slab. The detector shows a CCE=0.59 estimated using the 1mm2 large graphite pixel. The information earned with this first prototype will be used to design the new board with amplifying electronics for reading all 36 pixels at a time and perform experiments with monochromatic high energy electrons.

The response of graphite-diamond pixel detectors to 90Sr β-particles is reported. Laser induced graphitization was used to realize 200 μm × 200 μm square conductive graphite pads on one detector side whereas a large area graphite contact was realized on the other face for biasing. A board with nine hybrid charge sensitive pre-amplifier channels was used to test homogeneity of response of nine pixels at a time. In the dark the current is Ohmic up to 100 V where the current increases with a power law. While the bulk pixel resistance is 2.5 × 1012 Ω, the resistance between adjacent pixels depends on voltage following a power law. Under irradiation a resolved β-spectrum well separated from the noise contribution was observed on each pixel. The most probable value of the collected charge distribution is voltage dependent and saturates around ±300 V (0.6 V/μm) with a value of 0.70±0.05 fC (4300±300 equivalent electrons charge).

A prototype sensor able to work at zero bias has been fabricated using high quality single crystal diamond and two Schottky junctions. The detector's current in the dark is 3.2±0.2×10-14 A, while the current measured under irradiation with 1.6 Gymin-1 of 6 and 15 MV x-ray photons is 7.5±0.2×10-10 A. The sensitivity is strongly dependent on the applied voltage whereas at zero bias we have measured 37±2 nCGy-1, independent of the beam quality. The sensitivity increases with a sub-linear trend up to 100V, as the sensor photocurrent dependence on voltage. Moreover, the device shows very fast response on the rise and fall transients, with the signal stability under irradiation better than 0.3%. The linearity with the dose rate is spread over three orders of magnitude from 1.3×10-3 Gymin-1 to 1.2 Gymin-1 as measured with Co-60 photons.

Optically triggered UV sensitive receivers were fabricated on polycrystalline diamond as surface channel MESFETs. Opaque gates with asymmetric structure were designed in order to improve charge photogeneration mainly within the gate–drain region. Photogenerated holes contributed to the channel charge by assistance of the local electric field, in such a way improving the current signal at the drain contact. The sensitivity to UV light is demonstrated by using 3 ns wide laser pulses at 193 nm, well over the diamond bandgap. The receiver transient response to such laser pulses shows that the photogeneration process is only limited by the pulse rise time and charge collection at the drain contact completed in a time scale of a few nanoseconds. Such opaque gate three-terminal devices are suitable for application in emerging photonic technologies, for power-management system optical receivers, where copper wires and EM shielding can be replaced by lightweight optical fibers.