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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 $1 \times 1{0^{15}}$ 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.

Three metals (Ag, Au, and Ti) and a carbide (WC) were studied to verify their suitability to act as high-temperature electric contacts for photon-enhanced thermionic emission cathodes based on an AlGaInP/GaInP heterostructure. Transfer length method measurements, performed in the temperature range 25-350°C, showed that the lowest specific contact resistivity values were found for Au (4.0 × 10-4 ? cm-2 @350°C) and Ti (1.2 × 10-3 ? cm-2 @350°C). A further selection was made on the basis of film adhesion tests performed after the thermal cycles, in which Ti demonstrated an excellent mechanical robustness.

A laser-induced periodic surface structure (LIPSS) has been fabricated on polycrystalline diamond by an ultrashort Ti:Sapphire pulsed laser source (? = 800 nm, P = 3 mJ, 100 fs) in a high vacuum chamber (

Recently, carbon spectroscopy became very important due to the growing interest in the applications of new carbon allotropes, especially of 2D structures such as graphene and nanotubes. It is well known that the hybridization of carbon electrons defines most of the properties of these materials. Among the analytical techniques used for the identification of carbon allotropes is the spectroscopy of carbon Auger line, which involves the transitions of valence band electrons and can be excited by electron beam (AES) or X-ray photons (XAES). From the shape of C KVV spectrum, it is possible not only to recognize the cases of pure sp2 and sp3 configurations but also to quantify their ratio in amorphous carbon films. In present work, an experimental study of three main allotropes of carbon: graphite, diamond, and graphene is reported. The average thickness of graphene was evaluated from the ratio of C 1s and substrate photoelectron peaks, registered at grazing angle. Carbon KVV spectra were investigated by using XAES and AES techniques. Obtained results were compared with the ones acquired for reference samples of graphite and monocrystalline diamond. In addition, some samples of hydrogenated diamond were analyzed. It was demonstrated that the comparison of C KVV spectra excited by electrons and X-rays can be used for identification of graphene. Diamond-like C KVV spectrum, observed in XAES experiments, is a very promising fingerprint of graphene. Obtained results are also compared with valence band spectra of graphene, diamonds and graphite that were acquired by using ultraviolet photoemission spectroscopy. Copyright © 2014 John Wiley & Sons, Ltd.

Nanoscale periodic texturing on polycrystalline CVD diamond surface was performed to obtain a significant increase in optical absorptance to visible and near-infrared radiation. Surface texturing, obtained by the use of fs-laser ultrashort pulses, has been demonstrated to induce a controlled periodicity of ripples of about 170 nm and length of several µm, able to drastically increase the diamond capability of interacting with solar radiation from its intrinsic visible blindness. Ultraviolet and visible Raman spectroscopy has been used to confirm the absence of non-diamond phases resulting from the process for the fs-laser-textured sample. Moreover, here we investigate the optical properties in the range 200 nm–25 µm. Absorbance of fs-laser-textured CVD diamond is considerably higher than the untreated one at every wavelength, resulting in a remarkable increase in the emittance: It points out the need for an optimization of process parameters to enhance the selective absorption capability.

A diamond-based photo-thermionic cathode for future high-temperature concentrating solar cells was designed and fabricated. A specific cathode structure is discussed in terms of band diagram, introduction of defect energy states, and nanotechnology fabrication methods to increase the capability of solar radiation absorption and transformation into useful electric charge carriers.

Optically triggered surface channel MESFETs were fabricated on commercial polycrystalline diamond to be tested as fast UV activated switches. Devices with an opaque-gate and asymmetric structure were designed in order to improve charges photogeneration within gate-drain region. The sensitivity to UV light was demonstrated by using both modulated over gap radiation and laser pulses at 193 nm, well over the diamond band gap. Linearity with the power light was demonstrated as well as the parabolic dependence of the photogenerated current on the gate-source voltage when the transistor is in saturation. The transient response to 193 nm laser pulses in the nanosecond regime shows as the photogeneration process and charges collection to the drain contact are completed in a time scale of few nanoseconds.

A high-speed optical switch based on an ultraviolet sensitive surface channel MESFET fabricated on polycrystalline diamond has been developed. Saturation response to a nanosecond pulsed excimer laser has been demonstrated. Such an opaque gate three-terminal device is suitable for power applications where electromagnetic interference on power control interfaces is not negligible.

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.

The optoelectronic properties of ultrananocrystalline diamond films (UNCD) grown using N 2 = 0 and 5% in the deposition gas mixture, are investigated by transient photocurrent measurements under nanosecond light pulses, both in planar and sandwich contact arrangements. Independent of contact configuration and N 2 % value, very similar characteristic times in the 6-7 ns range are detected in the nanosecond range, reflecting a homogeneous distribution of states responsible for such decay times. On a longer time scale, nitrogen addition appears to slow down carrier transport promoting trapping and detrapping processes responsible for single and two power law photocurrent decays in films deposited using N 2 = 5% for sandwich and planar contact arrangements, respectively. Such a result suggests a nitrogen induced transport anisotropy tentatively related to structural modifications occurring at relatively low N 2 %.

Surface texturing by fs-laser pulses has been performed in order to enhance optical absorptance of chemical vapour deposited diamond. The induced surface structures have been studied as a function of treatment dose D. Periodic structure with ripples of 170 nm has been observed for a D = 5.0 kJ cm−2, although not well defined texturing and damaged structures have been obtained for both lower and higher doses. Raman investigations point out negligible changes in crystal bulk for all investigated samples, thus suggesting that physical properties of the crystal were not changed by the treatment. Optical absorptance is strongly enhanced by fs-laser texturing and it is an increasing function of the treatment dose. The absorptance of solar spectrum saturates up to values larger than 90%. The obtained outstanding enhancement of photon absorption represents a preliminary and promising step for the exploitation of synthetic diamond in future high-efficient conversion devices for solar concentration based on photon-enhanced thermionic emission effect.

With the continuous development of low work-function materials, thermionic energy converters operating at moderate temperatures (

DC and RF performance of submicron gate-length metal–semiconductor field effect transistors (MESFETs) fabricated on hydrogen-terminated polycrystalline diamond is investigated in detail for different material electronic quality (grain size in the range 100–200 µm) and device geometry (drain-source channel length in the range 1–3 µm). DC characteristics appear almost independent of both properties, giving maximum drain-source current values in the range 120–140 mA/mm in MESFETs having same gate length (0.2 µm) and gate width (25 µm). The layer properties underneath the hydrogenated surface seem then to affect the DC behaviour to a lesser extent when the same hydrogenation procedure is used. At variance, the electronic quality of diamond layers employed for MESFETs realization largely affects the RF performance, resulting into a low oscillation frequency f max for a MESFET realized by a self-aligned process (1 µm drain-source channel length) onto low quality diamond polycrystalline film. Such a performance improves to f max = 35 GHz for devices realized onto large grain polycrystalline diamond, although fabricated without self-aligned gate procedure (3 µm drain-source channel length). These findings are discussed in terms of different roles played by surface hydrogenation, device geometry detail and electronic quality of the polycrystalline diamond substrate for MESFET realization.

Single-crystal chemical vapour deposition (scCVD) diamond is presently the reference wide bandgap semiconductor for the development of high-performance ionizing radiation detectors for medical and nuclear applications, due to its wide range of extreme properties (radiation hardness, chemical inertness and high thermal conductivity). Recently, it has been shown that the interaction with femtosecond laser pulses can induce a non-equilibrium phase transition from metastable sp3-bonded diamond to the stable sp2-bonded graphite. This makes possible to create inside the bulk material graphitic columns, through an accurate control of the laser interaction with the diamond lattice throughout the whole sample thickness. In this work, we performed a Raman investigation of fs-laser induced graphitic columns inside very high quality single-crystal diamond samples: by means of a proper choice of the experimental parameters, it is possible to improve the quality of the graphite columns in terms of reduction of diamond content and graphite degree of crystallinity.

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.

On the bases of the RF characteristics and measured small-signal parameters, an equivalent circuit model is formulated and characterized for Metal-Semiconductor Field Effect Transistors based on H-terminated polycrystalline diamond. Starting from on-wafer measurements, a bias dependent transistor behavior representation has been fully determinated. Such a equivalent circuit model is a first important step in order to realize an RF IC based on diamond.

On the basis of RF characteristics and measured small-signal parameters, an equivalent circuit model was formulated and characterized for metal-semiconductor field effect transistors based on H-terminated polycrystalline diamond. Starting from on-wafer measurements, a bias-dependent transistor behavior representation was fully determined. Such an equivalent circuit model is the first important step to realize an RF IC based on diamond. © 2009 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 2783–2786, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24734

Metal-Semiconductor field effect transistor (MESFETs) were fabricated on hydrogen-terminated polycrystalline diamond. Fabricated MESFETs typically showed high drain-source current (140 mA/mm) and large transconductance values (60 mS/mm), with a cut off frequency f T =10 GHz and a maximum oscillation frequency, f MAX , up to 35 GHz. These values suggest device microwave operation in the K- band and are obtained through the fabrication of devices with geometry and active region dimensions compatible with available microelectronic technologies. Devices were realized in order to be employed in Microwave Integrated Circuits for satellite communications and high frequency power amplification, areas where diamond promises the replacement of vacuum electronics: with this perspective, our group realized a first important step formulating an equivalent circuit (EQC) model.

Polycrystalline diamond detector prototypes suitable for x-ray spectroscopy were realized and tested. Thick diamond specimens, mechanically polished to reduce the surface roughness, were selected for prototypes development. Noble metal contacts were deposited on both faces, whereas multistrip structures were defined by photolithography on the growth face only with the aim of reducing the coupling capacitances and to allow characterization tests either in planar or sandwich configuration. Leakage currents as low as 20 pA at 500 V were measured on a 270μm thick device. The x-ray sensitivity was tested monitoring the photocurrent as a function of the applied voltage under continuous 8.05 keV Cu Kα irradiation. Studies in pulsed mode were also performed by using a commercial miniature x-ray source. Pulse height distributions were carried out with a system composed of a charge-sensitive preamplifier and a digital pulse processor multi-channel analyzer. Analysis was carried out around Ta Lα and Cu Kα characteristic lines. Realized prototypes were able to resolve such two characteristic lines only 90 eV apart. Energy resolutions better than 3% have been evaluated for one of the prototype at 8.14 keV. Such features address very good energy resolving capabilities and the suitability of polished polycrystalline diamond in x-ray spectroscopy.

New opportunities in solid-state physics are offered by SINBAD (Synchrotron INfrared Beamline At DAFNE), the infrared beamline operational at DAΦNE, the storage ring of the Laboratori Nazionali di Frascati of the INFN. During 2003 several experiments, including those supported by the European TARI program, have been successfully performed at SINBAD. In this work we present the preliminary results of high resolution far infrared reflectivity data collected in different ZnxCdyHg(1–x–y)Te quaternary alloys as a function of temperature. The first far-IR investigation of Colossal Magnetoresistance manganites at high pressures, using a diamond anvil cell is also presented. Indeed, FT-IR spectroscopy is a powerful tool for the investigation of insulating-to-metal transitions and charge ordering phenomena that may occur in transition metal oxides. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The maximum distance σ that the neutral excitations can run is measured between 0.45 ℴK and 1.7 ℴK. Agreement with the existing results at low temperature is found. A random-walk propagation is suggested for high temperatures. At lowerT the mean free path becomes big enough that the neutral excitations have an almost straight path. The model gives reasonable agreement with the experimental results although the temperature dependence of σ is found to be smaller than the theoretical one. As a consequence of this model a minimum for the life of the neutral excitations is found to be 0.1 s at 1.7 ℴK. Their destruction is confirmed to be due to pair annihilation. The role of the liquid surface is discussed. Evaporation of neutral excitations has also been experimentally observed. Pair annihilation in the liquid does not produce separable ions.

A potential barrier for the extraction of He ∗ (2 3 S) from liquid helium, and bound states outside the liquid surface are shown to exist. Such bound states may explain the enhancement of neutral excitation annihilation at the liquid surface.