P. Calvani

Very Fast and Primingless Single-Crystal-Diamond X-Ray Dosimeters

X-ray dosimeters were developed by tailoring novel injecting diamond-like-carbon/Pt/Au contacts on single-crystal high-purity diamond films. A dark resistivity of (5.6 ± 0.1) × 10¹⁴ Ω · cm and no appreciable presence of deep traps in the bandgap confirmed a very low defect density in the diamond films. The dosimeters resulted to be primingless (i.e., no need of preactivation) and characterized by high sensitivity [(58.20 ± 3.26) × 10^-3 C · Gy^-1 cm^-3] and linear response to X-ray dose rate, produced by a molybdenum target. Transient X-ray modulated analysis allowed the determination of fast-trap influence and the estimation of very fast response times (~10^-3 s), at electric fields ≥ 3 × 10³ V/cm.

A diamond based neutron spectrometer for diagnostics of deuterium-tritium fusion plasmas

Single crystal Diamond Detectors (SDD) are being increasingly exploited for neutron diagnostics in high power fusion devices, given their significant radiation hardness and high energy resolution capabilities. The geometrical efficiency of SDDs is limited by the size of commercially available crystals, which is often smaller than the dimension of neutron beams along collimated lines of sight in tokamak devices. In this work, we present the design and fabrication of a 14 MeV neutron spectrometer consisting of 12 diamond pixels arranged in a matrix, so to achieve an improved geometrical efficiency. Each pixel is equipped with an independent high voltage supply and read-out electronics optimized to combine high energy resolution and fast signals (<30 ns), which are essential to enable high counting rate (>1 MHz) spectroscopy. The response function of a prototype SDD to 14 MeV neutrons has been measured at the Frascati Neutron Generator by observation of the 8.3 MeV peak from the (12)C(n, α)(9)Be reaction occurring between neutrons and (12)C nuclei in the detector. The measured energy resolution (2.5% FWHM) meets the requirements for neutron spectroscopy applications in deuterium-tritium plasmas.

Pixelated Single-crystal Diamond Detector for fast neutron measurements

Single-crystal Diamond Detectors (SDDs), due to their high radiation hardness, fast response time and small size, are good candidates as fast neutron detectors in those environments where the high neutron flux is an issue, such as spallation neutron sources and the next generation thermonuclear fusion plasmas, i.e. the ITER experiment. Neutron detection in SDDs is based on the collection of electron-hole pairs produced by charged particles generated by neutron interactions with 12C. Recent measurements have demonstrated the SDD capability of measuring the neutron flux with a good energy resolution and at high rates. In this work a novel detector based on a 12-pixels SDD matrix will be presented. Each pixel is equipped with an independent electronic chain: the fast shaping preamplifier coupled to a digitizer is able to combine the high rate capability and the good energy resolution. Two CAEN digitizers are compared and the possibility of performing good energy resolution measurements ( 1 MHz per channel) is described. Each pixel was characterized and calibrated using an 241Am source: the energy resolution was evaluated and gives a mean value of 1.73% at 5.5 MeV. The good energy resolution achieved and its uniformity between pixels are the demonstration of the capability of this novel detector as a spectrometer. This system will be installed during the next Deuterium-Tritium campaign on a collimated vertical line of sight at JET for 14 MeV neutron measurements.

Radiation-assisted Frenkel-Poole transport in single-crystal diamond

The measurement of the density of occupied states as a function of the applied electric field, performed on single-crystal chemical vapour deposition diamond by x-ray modulated photocurrent technique, is reported. Two regimes of non-linear charge transport were observed: a classical Frenkel-Poole (FP) process at high electric fields (>6800 V/cm), and a radiation-assisted transport mechanism at intermediate electric fields (2000 to 6800 V/cm), consisting of a double-step process in which the direct re-emission into the extended band occurs following multiple photo-induced FP-like hopping transitions.

First neutron spectroscopy measurements with a pixelated diamond detector at JET

A prototype Single crystal Diamond Detector (SDD) was installed at the Joint European Torus (JET) in 2013 along an oblique line of sight and demonstrated the possibility to carry out neutron spectroscopy measurements with good energy resolution and detector stability in discharges heated by neutral beam injection and radio-frequency waves. Starting from these positive results, within the Vertical Neutron Spectrometer project of the Joint European Torus, we have developed a pixelated instrument consisting of a matrix of 12 independent SDDs, called the Diamond Vertical Neutron Spectrometer (DVNS), which boosts the detection efficiency of a single SDD by an order of magnitude. In this paper we describe the main features of the DVNS, including the detector design, energy resolution, and data acquisition system for on-line processing. Preliminary spectroscopy measurements of 2.5 MeV neutrons from the present deuterium plasma at JET are finally presented.

Buried Boron Doped Layer for CVD Diamond Photo-Thermionic Cathodes

A buried boron (B) doped layer has been fabricated by ion implantation into the bulk structure of a chemical vapor deposition (CVD) diamond film engineered to act as a photo-thermionic cathode for high-temperature solar cells. The boron layer implantation is a fundamental step in order to obtain an efficient diamond-based solar cell. Implantation of boron ions has been performed by fixing the ion dose to a value of 1 × 1015 at/cm2, while varying the ion beam kinetic energy from 40 to 250 keV which localizes the doped layer at different depths (from tens to a few hundreds of nanometers) below the absorbing surface of the cathode. Characterization of the optical and photoelectronic properties of the different implanted layers has been carried out so as to evaluate their effectiveness within the cathode structure. An ion beam kinetic energy of 40 keV is found to significantly increase the quantum efficiency of the CVD diamond plate. This will be useful for the development of the diamond-based photo-thermionic cathode.

Defect engineering of diamond cathodes for high temperature solar cells

A cathode structure for photon-enhanced thermionic emission was designed for high temperature energy conversion in solar concentrating systems. Surface-hydrogenated diamond is one of the few semiconductors to show negative electron affinity and a work function as low as 1.7 eV if nitrogen-doped, that is connected to a significant thermionic emission at moderate temperatures (up to 800 °C). But diamond is transparent to solar radiation, consequently advanced techniques for preparing an efficient sunlight absorbing diamond are discussed.

RF power performance of submicron MESFET on hydrogen terminated polycrystalline diamond

Diamond is in principle the highest performance widegap semiconductor; its outstanding electronic and thermal properties make it an attractive material for high power radiofrequency (RF) and microwave electron devices. Even though the properties of synthetic diamond have been known from many years, only recently significant technology advances in the growth of single-crystal and polycrystalline diamond have fostered the research on high-performance diamond electronics. In this framework, the use of polycrystalline diamond as a substrate offers the advantage of larger areas and lower cost with respect to single-crystal diamond. Available approaches for the control of diamond conductivity rely mainly (up to now) on p-type doping, either through extrinsic doping with boron, or exploiting hydrogen (H) surface termination, which induces a quasi-2D hole channel a few nanometers below the surface. Both approaches have been pursued to develop high speed power FETs [1], with record cut-off frequency (45 GHz) achieved by H-terminated FETs on polycrystalline diamond [2]. Despite such promising small-signal performances, only a few examples of RF power measurements have been reported so far, limited at comparatively low frequency (1 GHz), and only for single-crystal diamond FETs, with record performance of 2W/mm [3]. In this paper, we present RF power measurements of submicron H-terminated FETs on polycrystalline diamond up to 2 GHz, showing the potential of such substrate for the development of microwave power devices.

K-band diamond MESFETs for RFIC technology

Diamond is one of the suitable semi-conductor for vacuum electronics replacement in high power and high frequency applications. Sub-micron gate-length (200 nm) Metal Semiconductor Field Effect Transistor (MESFETs) have been fabricated on h-terminated polycrystalline diamond and characterized in order to investigate the possibility of RFICs integration. High power (Pout=1.5 W/mm) and high frequency (and fMAX=35 GHz) performances have been obtained.

Opaque-Gate Phototransistors on H-Terminated Diamond

Opaque-gate ultraviolet sensitive transistors were fabricated on H-terminated polycrystalline diamond. Butterfly shaped structures with different geometric ratios were realized. Observed trends with the gate unbiased demonstrated behavior as p-channel normally-off transistors, switched-on by the impinging UV light. Linearity with the UV beam power was also observed with over-gap radiation. Under steady state illumination, a linear increase of the photocurrent was found when the gate is biased at different voltages in the saturation regime. The operative generation-charge transport mechanism of fabricated devices is discussed.