Search

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.

Black diamond is obtained by a controlled nanoscale periodic texturing of CVD diamond surface, able to drastically modify the interaction with solar radiation from optical transparency up to solar absorptance values even >90%. Surface texturing, performed by the use of an ultra-short pulse laser, is demonstrated to induce an intermediate band within the diamond bandgap supporting an efficient photoelectronic conversion of sub-bandgap photons (

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.

Diamond dosimeters for intense ionizing radiation were assembled by developing Ohmic contacts on single-crystal high-purity diamond films. Leakage electrical resistivity of the order of 10 14 Ω cm was measured. Devices were tested with a low-intensity 17.48 keV X-ray beam to probe their maximum sensitivity and the minimum detectable dose-rate. Diamond detectors provided a linearity coefficient to X-ray dose-rate of 1.023±0.07, whose dependence on operative bias voltage is here analyzed.

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.

Hydrogen terminated diamond is a promising material for metal semi-conductor field effect transistor (MESFETs) fabrication and research effort is focused on diamond quality improvement. In this reported work, the focus of attention is on device layout and structure to improve RF performances, designed on the basis of diamond physical properties, taking into account the characterisation of previously fabricated devices. Significant increase of current gain cutoff frequency fT, from 3.4 to 6.7 GHz, has been obtained, while maximum oscillation frequency fMAX is raised from 10.4 to 11.8 GHz.

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.

Metal-Semiconductor Field Effect Transistors (MESFETs) were fabricated on polycrystalline diamond. Devices were realized to be employed in Microwave Integrated Circuits for satellite communications and high frequency power amplification, areas were diamond promise the replacement of vacuum electronics. Fabricated MESFETs typically showed high drain-source current (140 ma/mm) and large transconductance values (50 ms/mm), with a cut off frequency f =10 GHz and a maximum oscillation frequency, f, up to 35 GHz. These values suggest device microwave operation and are obtained through the fabrication of devices with geometry and active region dimensions (200-500 nm gate length) compatible with available microelectronic technologies. ©2009 IEEE.

Planar Al/GaN/Ni Schottky diodes were realized on GaN films deposited on sapphire substrates and characterized in the dark and under illumination. The optoelectronic characteristics of GaN photodetectors appear largely influenced by structural defects and impurities, which are clearly detected in photocurrent yield measurements. In particular, an exponential increase of the photocurrent is observed and explained in terms of a barrier lowering photoeffect, hence a light-induced shrinking of the space charge region, related to carrier trapping at defects and impurities. Trapping events are also responsible for a dispersive behavior of the AC responsivity with the light chopping frequency. Such effects point out the importance of a proper selection of bias voltage and working frequency for GaN photodetector operations.

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.

Planar Al/GaN/Ni Schottky diodes were realized on GaN films deposited on sapphire substrates and characterized in the dark and under illumination. The optoelectronic characteristics of GaN photodetectors appear largely influenced by structural defects and impurities, which are clearly detected in photocurrent yield measurements. In particular, an exponential increase of the photocurrent is observed and explained in terms of a barrier lowering photoeffect, hence a light induced shrinking of the space charge region, related to carrier trapping at defects and impurities. Trapping events are also responsible for a dispersive behavior of the AC responsivity with the light chopping frequency. Such effects point out the importance of a proper selection of bias voltage and working fre15quency for GaN photodetector operations.

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

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 (P =1.5 W/mm) and high frequency (and f=35 GHz) performances have been obtained. © 2009 IEEE.

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.

We experimentally investigate the large-signal radio frequency performances of surface-channel p-type diamond MESFETs fabricated on hydrogenated polycrystalline diamond. The devices under examination have a coplanar layout with two gate fingers, total gate periphery of 100 μm; in DC they exhibit a hole accumulation behavior with threshold voltage V t ≈ 0–0.5 V and maximum drain current density of 120 mA/mm. The best small-signal radio frequency performances (maximum cutoff or transition frequency f T and oscillation frequency f max ) were obtained close to the threshold and were of the order of 6 and 15 GHz, respectively. The power radio frequency response was characterized by driving the devices in class A at an operating frequency of 2 GHz and identifying through the active load-pull technique the optimum load for maximum power added efficiency. A power gain in linearity of 8 dB and an output power of approximately 0.2 W/mm with 22% power added efficiency were obtained on the optimum load impedance at a bias point V DS = −14 V, V GS = −1 V. To the best of our knowledge, these are the first large signal measurements ever reported for surface MESFET on polycrystalline diamond, and show the potential of such technology for the development of microwave power devices.

Epitaxial diamond films were deposited on polished single crystal Ib type HPHT diamond plates of (100) orientation by microwave CVD. The epilayers were used for the fabrication of surface channel MESFET structures having sub-micrometer gate length in the range 200-800 nm. Realized devices show maximum drain current and trasconductance values of about 190 mA/mm and 80 mS/mm, respectively, for MESFETs having 200 nm gate length. RF performance evaluation gave cut off frequency of about 14 GHz and maximum oscillation frequency of more than 26 GHz for the same device geometry.