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The detection properties of heteropitaxial (polycrystalline, pCVD) and homoepitaxial (single crystal, scCVD) diamond films grown by microwave chemical vapor deposition (CVD) in the Laboratories of Roma “Tor Vergata” University are reported. The pCVD diamond detectors were tested with α -particles from different sources and 12 C ions produced by 15 MV Tandem accelerator at Southern National Laboratories (LNS) in Catania (Italy). pCVDs were also used to monitor 14 MeV neutrons produced by the D-T plasma at Joint European Torus (JET), Culham, U.K. The limit of pCVDs is the poor energy resolution. To overcome this problem, we developed scCVD diamonds using the same reactor parameters that optimized pCVD diamonds. scCVD were grown on a low cost (1 0 0) HPHT single crystal substrate. A detector 110 μ m thick was tested under α -particles and under 14 MeV neutron irradiation. The charge collection efficiency spectrum measured under irradiation with a triple α -particle source shows three clearly resolved peaks, with an energy resolution of about 1.1 % . The measured spectra under neutron irradiation show a well separated C ( n , α 0 ) 9 Be 12 reaction peak with an energy spread of 0.5 MeV for 14.8 MeV neutrons and 0.3 MeV for 14.1 MeV neutrons, which are fully compatible with the energy spread of the incident neutron beams.

This work reports on the development and characterization of bi-dimensional deep-UV sensor arrays based on synthetic diamond to address the requirements of space-born astrophysical experiments. The material was synthesized at the University of Rome “Tor Vergata” where both heteroepitaxial polycrystalline diamond films and homoepitaxial single-crystal diamonds are grown using a tubular MWCVD reactor. The quality of chemical vapour deposited diamond was characterized by cathodoluminescence, photoluminescence, Raman spectroscopy and thermally stimulated currents. Then, suitable samples were selected and used to fabricate photoconductive single-pixel and 2D array devices by evaporating metal contacts on the growth surface. The electro-optical characterization of the devices was carried out in a wide spectral region, ranging from 120 to 2400 nm. A deuterium lamp and a 0.5 m vacuum monochromator were used to measure the detector responsivity under continuous monochromatic irradiation in the 120–250 nm spectral range, while an optical parametric oscillator tunable laser producing 5 ns pulses was used as light source from 210 up to 2400 nm. Time response, signal-to-noise ratio, responsivity and visible rejection factor were evaluated and the results are hereafter summarized.

The possibility to realize a high sensitive thermocouple by means of boron doped chemical vapour deposition (CVD) diamond was investigated. The thermoelectric power of p-type diamond, grown by plasma enhanced CVD was studied for films of electrical resistivity in the 0.2–40 Ω cm range in order to asses the dependence of thermocouple sensitivity on the doping level. The p-type diamond films were prepared by CH3OH + B2O3 vapour addition to a 1% CH4–H2 gas mixture during the growth. The conductive films were then tested tracing the I–V characteristic in order to study the conduction properties of the films. An appropriate experimental setup was built to evaluate the thermoelectric properties of the grown samples for different temperatures imposed between two ends of the samples. Firstly, the output voltage was measured maintaining a reference temperature of 273 K at one end and varying the second temperature between 275.5 and 360.5 K. A constant value of the temperature drop of 5 K was then used for an accurate evaluation of the thermoelectric properties of the diamond films for different value of the average temperature. The measurements provided values of thermoelectric power in the range 0.3–0.6 mV/K while conductivity increases. These values showed different decreasing behaviour with increasing temperature for different resistivity of the sample. In particular, more relevant changes in thermoelectric power were measured for high resistive samples.

Carrier free paths in Chemical Vapour Deposition (CVD) diamond films depend on the presence of traps, which therefore strongly affect the performance of those CVD diamond based devices which rely on the electronic properties of the material, like radiation detectors. For the same reason, these devices can in turn be used as tools to study carrier dynamics. It is well known that some traps may be saturated by pre-irradiation with ionizing radiation (e.g. β-particles), a process called pumping or priming. Not all traps behave in the same way. Due to the large bandgap of diamond, both shallow (not affected by pumping) and deep traps for electrons and holes may exist. We measured, using 5.5 MeV 241 Am α-particles, the response of high quality CVD diamond based detectors after successive annealing steps performed at selected temperatures. The analisys of the decay of the detector efficency with annealing time at several temperatures allows a quantitative evaluation of the activation energy of these defects. Two main trapping centres connected to the pumping process were found, both related to holes, having activation energies of about 1.6 eV and 1.3 eV respectively.

The deposition conditions and the detection properties of a homoepitaxial diamond film, grown by microwave Chemical Vapor Deposition (CVD) on a HPHT single crystal substrate are reported. The charge collection spectrum, measured under irradiation with a triple 239 Pu 241 Am 244 Cm source emitting 5.16, 5.48 and 5.80 MeV a-particles respectively, shows three clearly resolved peaks with an energy resolution of about 1.1%. Both the charge collection efficiency and the energy resolution reach saturation values when the applied voltage exceeds 60 V, suggesting 100% collection efficiency. The detector was also tested with 14.8 and 14.1 MeV neutrons. The obtained collection spectra show a well separated 12 C(n,a0) 9 Be reaction peak with an energy spread of 0.5 MeV for 14.8 MeV neutrons and 0.3 MeV for 14.1 MeV neutrons, which are fully compatible with the energy spread of the incident neutrons. D 2005 Elsevier B.V. All rights reserved.

The detection properties of a UV photodetector realized on a 150 μm thick CVD single crystal diamond film, grown at Roma “Tor Vergata” University on a low cost HPHT diamond substrate, are reported. The device was tested in the 210–2400 nm spectral range using pulsed laser irradiation and in the 20–250 nm range in continuous mode by both a deuterium lamp and a helium DC gas source irradiation. The detector shows more than five orders of magnitude of visible/UV rejection ratio, a very sharp signal drop of about 10 4 being observed in correspondence of the diamond energy gap. In the extreme UV range, the He II 25.6 and 30.4 nm as well as the He I 58.4 nm emission lines are clearly detected. The diamond time response is demonstrated to be considerably lower than 5 ns and 0.2 s in pulse and continuous mode, respectively. The extremely good signal to noise ratio, stability and reproducibility of the device response obtained, indicate that no persistent photoconductivity nor undesirable pumping effects are present, which represented so far the main problems preventing the use of diamond based detectors for UV applications.

Defects limiting the movement of charge carriers in polycrystalline chemical vapor deposition (CVD) diamond films are located within the grains or in grain boundaries. Their geometrical distribution in the sample is different and is usually unknown. We present here a method to quantitatively evaluate the concentration and distribution of in-grain and grain-boundary located active carrier traps. Since the impact of these two kinds of defects on the performance of CVD diamond based devices is different, it is possible to obtain the defect distribution by measuring the response of diamond alpha particle detectors as a function of film thickness. The Hecht theory, describing the efficiency of a semiconductor particle detector, has been modified to take into account the polycrystalline nature of CVD diamond. This extended Hecht model was then used to fit experimental data and extract quantitative information about the defect distribution.

The growth conditions and the detection properties of a homoepitaxial diamond film, deposited in Roma “Tor Vergata” University Laboratories by microwave chemical vapor deposition on a high-pressure high-temperature single-crystal substrate are reported. An energy resolution as low as 1.1% was achieved when irradiating the device with 5.5 MeV Am241 α-particles. The dependence of the charge collection efficiency and the energy resolution on the applied voltage are reported as well. A clear saturation plateau was observed in both curves. Preliminary results with 14.8 MeV neutron irradiation are reported, showing a well separated C12(n,α0)Be9 reaction peak.

A new thermochemical sensor was developed exploiting the unique properties of CVD-diamond. The thermal transduction step was based on the electric signal produced by a semiconductor–metal thermocouple. A thin film of boron doped polycrystalline diamond was grown by plasma enhanced CVD technique with CH 3 OH + B 2 O 3 vapour addition. The high melting point, the chemical inertness and the extreme hardness make diamond an interesting candidate for sensing applications in harsh environments. The active thermocouple was completed evaporating a 120 nm palladium film on the diamond film. The sensor was then tested for different temperature drops and showed good stability and repeatability in addition to high values of the thermoelectric power ( S > 350 μV/°C). The catalytic properties of thin palladium films allow the thermocouple to be used for gas detection. In particular, hydrogen could be detected due to the palladium Fermi level modification, induced by the absorption process. To this purpose, the diamond–palladium sensor was exposed to different hydrogen concentrations, varying from 0.5 to 2%. The signal due to hydrogen absorption increases linearly with the gas concentration and is superimposed to the one produced by temperature changes.

A method to measure independently the electron and hole mean drift distance (CCD) in CVD diamond is presented. Very high quality CVD diamond films were grown, and used as particle detectors. Their efficiency is measured as a function of the particle penetration depth under 5.5 MeV 241Am α-particles irradiation. The data are then fitted with a theoretical formula for the carrier mean drift distances derived from a properly modified Hecht model. Simultaneous fit of spectra collected for both positive and negative detector bias gives a separate evaluation of the mean drift distances of each carrier type. The α-particle penetration depth is controlled either using air as an absorbing layer or varying the particle incidence angle in the 0–80° range. The latter setup is demonstrated to provide higher accuracy. The results show that in our sample the electron and hole CCD are comparable in the as grown state. However, in the pumped state (i.e. after the detector is pre-irradiated with β-particles) the hole contribution is much greater than the electron one, showing that the pumping process is much more effective on hole traps than on electron traps.

A method is presented to selectively characterize the traps involved in the pumping procedure of diamond films. The pumping process strongly reduces the concentration of active carrier trapping centers, leading to an enhancement of electronic properties of such material, and is obtained by irradiating the diamond films with ionizing radiation. Since the improved transport properties lead to an increased efficiency when diamond films are used as radiation detectors, valuable information on this process can be obtained by analyzing the response of diamond based particle detectors. For this purpose a high-quality diamond film was grown by microwave chemical vapor deposition and a particle detector was realized. Its response to a 5.5-MeV 241Am α-particles was measured after successive annealing steps performed at different temperatures in the 180–228 °C range. Before each annealing curve at a given temperature, the detector was driven to the pumped state through β-particle irradiation. The efficiency versus a...

High-quality single-crystal diamond films were homoepitaxially grown by chemical vapor deposition onto low cost high-pressure high-temperature diamond substrates. The transport properties of the obtained samples were studied by photoresponse characterization. Fast ultraviolet (5 ns) laser pulses at 215 nm were used as a probe. The time evolution of the photoinduced current was observed to closely reproduce the laser pulse shape, thus indicating a time response lower than the adopted laser pulse duration. Very stable and reproducible response was measured, so that neither priming nor memory effects are observed. However, a minor slow component shows up in the charge-integrated sample response, whose temperature dependence was investigated in a −25–+50°C range. A systematic speed up of this slow component of the sample signal is observed, indicating the presence of shallow centers producing trapping-detrapping effects. The experimental results are discussed in the framework of a trapping-detrapping model af...

High-quality single-crystal diamond films, homoepitaxially grown by microwave chemical vapor deposition, have been used to produce diamond-based photodetectors. Such devices were tested over a very wide spectral range, from the extreme ultraviolet (UV) (20 nm) up to the near IR region (2400 nm). An optical parametric oscillator tunable laser was used to investigate the 210–2400 nm spectral range in pulse mode. In this region, the spectral response shows a UV to visible contrast of about 6 orders of magnitude. A time response shorter than 5 ns, i.e., the laser pulse duration, was observed. By integrating the pulse shape, a minor slow component was evidenced, which can be explained in terms of trapping–detrapping effects. Extreme UV gas sources and a toroidal grating vacuum monochromator were used to measure the device response down to 20 nm in continuous mode. In particular, the extreme UV He spectrum was measured and the He II m, 30.4 nmand He I 58.4 nm emission lines were clearly detected. The measured t...