Trapping–detrapping behavior in CVD diamond particle detectors

Diamond based particle detectors were built up using high quality diamond films grown by microwave chemical vapor deposition (CVD). The efficiency (η) and charge collection distance (CCD) of such devices were tested by a 5.5 Mev ²⁴¹Am α-particle source. Their response times were then carefully investigated both in the as-grown normal state and after irradiation with β-particles for approximately 60 h, in order to bring the detectors in the so called pumped state. A drastic change in the time evolution, the signal amplitude and the symmetry of the pulse shapes recorded with positive and negative polarization of the detector is observed as soon as the priming procedure takes place. This behavior is explained in the framework of a model in which a trapping–detrapping mechanism for CVD diamond is accounted for. Two different kinds of trapping centers for electrons and holes are proposed as the limiting factor in the diamond detection performance. A very good agreement between the simulation and the experimental pulse shapes is observed, thus allowing a better understanding of the priming procedure and the possible identification of the crystal defects limiting the efficiency of diamond based particle detectors.     

Extreme UV single crystal diamond photodetectors by chemical vapor deposition

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 10⁴ 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.     

Analysis of trapping–detrapping defects in high quality single crystal diamond films grown by Chemical Vapor Deposition

The photoresponse of high quality single crystal diamond films homoepitaxially grown by Chemical Vapor Deposition (CVD) onto low cost diamond substrates has been studied. The time evolution electrical response to the excitation by 5 ns laser pulses at 215 nm closely reproduces the laser pulse shape. The single crystal diamond response is therefore much faster than the laser pulse duration. The output signal is also very stable and reproducible, without significant priming or memory effects. Single crystal diamond films can therefore be grown by CVD having enough high quality to be used as photodetectors.However, a minor slow component shows up in the charge-integrated sample response. A systematic speed up of this slow component when increasing the detector temperature from − 25 °C to + 50 °C demonstrates its thermally activated origin. The slow component is therefore attributed to detrapping effects from shallow trapping centres.A model of the charge transport mechanism in the presence of trapping–detrapping centres can be developed and the results can be compared to the experimental ones. The activation energy of the shallow defects is accordingly determined as E_a = 0.4 eV.     

Growth of detector grade CVD diamond films and microscopic interpretation of their efficiency and charge collection distance in the normal and pumped states

Microwave CVD diamond films with very high particle detection efficiency have been obtained using a NIRIM-type reactor. The efficiency and charge collection distance (CCD) of nuclear particle detectors based on these films have been systematically studied as a function both of the methane contents in the growth gas mixture and of the film thickness. In particular, the effects of pre-irradiation with β-particles (pumping) have been thoroughly studied. The detector response to 5.5 MeV α particles was studied using a ²⁴¹Am source. Both efficiency and CCD behave in a markedly different way in the as-grown and pumped states, the effect of pumping being strongly dependent on film thickness. These behaviors are analyzed, taking into account the polycrystalline nature of CVD diamond films, in the framework of a recently proposed model [Appl. Phys. Lett. 75 (1999) 3216] discussing the role of in-grain defects and grain boundaries in determining the charge collection spectra of diamond films in the normal and pumped states. Experimental data are found to agree with what is expected from the model. The influence of pumping on the detector resolution is also discussed.     

Zero-bias operation of polycrystalline chemically vapour deposited diamond films for Intensity Modulated Radiation Therapy

A detailed investigation of the performance as a dosimeter of state-of-art polycrystalline CVD (pCVD) diamond detectors operated in photovoltaic regime for applications in clinical radiotherapy has been carried out with conventional 6-10-18 MV X-photons, as well as with a 10 MV photon Intensity Modulated Radiation Therapy (IMRT) beam from a linear accelerator. Our results show that the performances of a pCVD diamond dosimeter improves dramatically when operated in null-bias conditions. Main improvements with respect to operation with an external voltage applied are: a reduced pre-irradiation dose; an excellent time stability, characterised by standard deviations less than 0.5%; a rise-time comparable to that of commercial reference dosimeters; a linearity with dose proven over three decades; a reduced deviation from linearity of the current vs. dose-rate curve, output factors comparable to that of commercial reference dosimeters. These results represent a significant step towards clinical applications as IMRT with synthetic polycrystalline CVD diamond films.     

Deposition of diamond films on sapphire: studies of interfacial properties and patterning techniques

Polycrystalline diamond films were grown on single crystal sapphire substrates using hot filament chemical vapour deposition (CVD). Problems with poor adhesion, stress and film cracking became severe for deposited areas greater than about (100 μm)². Scanning electron microscopy analysis showed the films to be failing both at the interface and in the diamond layer itself. Transmission electron microscopy cross-sections of the interface showed that the interface was clean and free from non-diamond carbon impurities. Spallation problems in the diamond film could be reduced by introducing a barrier layer of epitaxial silicon grown on the sapphire prior to the diamond CVD step. Patterned silicon-on-sapphire wafers were then used as substrates for CVD of diamond in order to define features of linewidth more than 10 μm in the diamond films. Two methods were used: selective nucleation and lift off.     

EPR study of hydrogen-related defects in boron-doped p-type CVD homoepitaxial diamond films

Boron-doped p-type single crystalline chemical vapor deposition (CVD) homoepitaxial diamond films were investigated by electron paramagnetic resonance (EPR). Carbon dangling bond defects, which were accompanied by a nearby hydrogen atom, were observed in boron-doped p-type CVD diamond films on a IIa substrate similar to those observed in undoped diamond. This result suggested that the energy level position of the defects is located below the Fermi energy of boron-doped diamond, at around 0.3 eV above the valence-band top. The reason why the Fermi energy could be changed by the incorporation of boron atoms at low density (10¹⁶–10¹⁷/cm³) in the film in spite of the existence of the large defect density of EPR centers (10¹⁸/cm³) is thought to be that the singly occupied electron states of defects are located near the band edge. As for the thermal annealing effect of the defects, it was revealed that the concentration of the defects and the mobility of the p-type film did not change after annealing up to 1200 °C which is much higher than the temperature of boron–hydrogen pair dissociation.     

Nucleation and growth of diamond films on aluminum nitride by hot filament chemical vapor deposition

Nucleation and growth of diamond films on aluminum nitride (ALN) coatings were investigated by scanning electron microscopy, Raman spectroscopy and scratch test. ALN films were grown in a magnetron sputtering deposition. The substrates were Si(111) and tungsten carbide (WC). Chemical vapor deposition (CVD) diamond films were deposited on ALN films by hot filament CVD. The nucleation density of diamond on ALN films was found to be approximately 10⁵ cm⁻², whereas over 10¹⁰ cm⁻² after negative bias pre-treatment for 35 min was −320 V, and 250 mA. The experimental studies have shown that the stresses were greatly minimized between diamond overlay and ALN films as compared with WC substrate. The results obtained have also confirmed that the ALN, as buffer layers, can notably enhance the adhesion force of diamond films on the WC.     

On the two-phonon absorption of CVD diamond films

It is well known that the absorption coefficient of diamond in the two-phonon region is constant, for example at 2000 cm^{− 1}, the absorption coefficient is 12.3 cm^{− 1}. This means that the infrared absorbance in the two-phonon region is proportional to the thickness of the samples, which is generally used as standard to normalize the infrared absorption spectra of diamond samples according to their thickness. This is true for natural and HPHT synthetic single crystal diamond. However for polycrystalline or nanocrystalline CVD diamond films, we found that the situation may be different. For high quality thick CVD diamond films of thickness > 150 μm, the infrared absorbance in the two-phonon region is proportional to its thickness. While CVD diamond films of equal thickness but of different quality show variable absorbance in the two-phonon absorption region in terms of thickness. Our investigation on this observation primarily indicates that the grain size of CVD diamond films has influence on the two-phonon absorption. In this work, we present this new result and discuss the mechanism of this phenomenon in the light of the growth mechanism of CVD diamond.     

785 nm Raman spectroscopy of CVD diamond films

Raman spectroscopy is a powerful technique often used to study CVD diamond films, however, very little work has been reported for the Raman study of CVD diamond films using near-infrared (785 nm) excitation. Here, we report that when using 785 nm excitation with 1 µm spot size, the Raman spectra from thin polycrystalline diamond films exhibit a multitude of peaks (over 30) ranging from 400–3000 cm^− 1. These features are too sharp to be photoluminescence, and are a function of film thickness. For films > 30 µm thick, freestanding films, and for films grown in diamond substrates the Raman peaks disappear. This suggests that the laser is probing the vibrations of molecular units at the grain boundaries of the disordered crystallites present at the interface between the diamond and substrate.     

Diamond detectors for hadron physics research

The application of diamond for the detection of charged particles in atomic, nuclear and high-energy physics experiments is described. We compare the properties of three undoped diamond types, all of them produced by Chemical Vapor Deposition (CVD), in particular homoepitaxial single-crystal CVD Diamond (scCVDD), polycrystalline CVD Diamond (pcCVDD) grown on silicon, and CVD Diamond on Iridium (DoI) grown on the multi-layer substrate Ir/YSZ/Si001. The characteristics of the transient current (TC) signals generated from ²⁴¹Am-α-particles in the samples are exploited to evaluate the potential of the diamond crystals for particle timing and spectroscopy applications. The TC technique (TCT) results are correlated to the dark conductivity and the structural defects of the bulk materials as well as to the morphology and roughness of the diamond surfaces. The deterioration of the sensors performance after heavy irradiations with 26 MeV protons, 20 MeV neutrons, and 10 MeV electrons is discussed by means of charge-collection efficiency results, TC technique, and optical absorption spectroscopy (OAS). The important role of the diamond signal processing is underlined, which influences both the quality of the CVDD characterization data as well as the in-beam performance of the diamond sensors.     

Study the effect of O2 addition on hydrogen incorporation in CVD diamond

The effect of a small amount of O₂ addition on film quality and hydrogen incorporation in chemical vapour deposition (CVD) diamond films was investigated and the films were grown using a 5-kW microwave plasma CVD reactor. Film quality and bonded hydrogen were characterized using micro-Raman and Fourier transform infrared (FTIR) spectroscopy, respectively. It was found that in general for films grown using CH₄/H₂ plasma both without and with O₂ addition, the hydrogen incorporation increases with increasing substrate temperature, while a small amount of O₂ addition (O₂/CH₄=0.1) into CH₄/H₂ (4%) plasma strongly suppresses the incorporation of hydrogen into the film. Raman spectra show that the added oxygen improved film quality by etching and suppressing the amorphous carbon component formed in the film. The above effect of oxygen addition on hydrogen incorporation and film quality is discussed according to the growth mechanism of CVD diamond. The CVD diamond specific hydrogen related IR vibration at 2828 cm⁻¹ appears as a sharp and strong peak only in the FTIR spectra of poor quality films grown at high temperature both without and with O₂ addition, but it appears much stronger in the film grown without O₂ addition. This result experimentally excludes the assignment of the 2828 cm⁻¹ peak arises from hydrogen bonded to oxygen related defect in the literature.     

Nucleation and growth of diamond films on single crystal and polycrystalline tungsten substrates

Silicon has been the most widely studied substrate for the nucleation and growth of CVD diamond films. However, other substrates are of interest, and in this paper, we present the results of a study of the biased nucleation and growth of diamond films on bulk single and polycrystalline tungsten. Diamond films were nucleated and grown, using a range of bias and reactor conditions, and characterized by Raman spectroscopy and scanning electron microscopy (SEM). High-quality (100) textured films (Raman FWHM<4 cm⁻¹) could be grown on both single and polycrystalline forms of the tungsten substrate. On carefully prepared substrates, by varying the bias treatment, it was possible to determine the nucleation density over a 4–5 order range, up to ∼10⁹ cm⁻². Raman measurements indicated that the diamond films grown on bulk tungsten exhibited considerable thermal stress (∼1.1 GPa), which, together with a thin carbide layer, resulted in film delamination on cooling. The results of the study show that nucleation and growth conditions can be used to control the grain size, nucleation density, morphology and quality of CVD diamond films grown on tungsten.     

The influence of dark feature on optical and thermal property of DC Arc Plasma Jet CVD diamond films

Different grades of CVD diamond films were prepared by 100 kW DC Arc Plasma Jet system. The films were characterized using optical microscope (OM), high-resolution transmission electron microscopy (HRTEM), electron energy-loss spectroscopy (EELS), and Raman spectroscopy. The results show that dark feature mainly is inclusions in CVD diamond films, the concentration are amorphous carbon and nitrogen. As for transparent optical grade diamond film, it has very high IR transparency and high thermal conductivity. The appearance of dark feature degraded the quality of CVD diamond film, apparently influencing IR transparency and thermal conductivity. But even in optical grade diamond film, there are very strong absorption features in the 7–9 μm region, this will limit the practical applications of diamond films grown by Plasma Jet as IR windows for CO₂ lasers.     

Surface acoustic wave properties of natural smooth ultra-nanocrystalline diamond characterized by laser-induced SAW pulse technique

Diamond is one of the best SAW substrate candidates due to its highest sound velocity and thermal conductivity. But conventional diamond films usually express facet structure with large roughness. Ultra-nanocrystallined diamond (UNCD) films grown in a 2.45 GHz IPLAS microwave plasma enhanced chemical vapor deposition (MPECVD) system on Si (100) substrates in CH₄-Ar plasma possess naturally smooth surface and are advantageous for device applications. Moreover, highly C-axis textured aluminum nitride (AlN) films can be grown by DC-sputtering directly on UNCD coated Si substrate. However, properties of UNCD films are much complex than microcrystalline diamond films, that is because this is a very complex material system with large but not fixed portion of grain boundaries and sp²/sp³ bonding. Properties of UNCD films could change dramatically with similar deposition condition and with similar morphologies. A simple and quick method to characterize the properties of these UNCD films is important and valuable. Laser-induced SAW pulse method, which is a fast and accurate SAW properties measuring system, for the investigation of mechanical and structure properties of thin films without any patterning or piezoelectric layer.     

High quality,large surface area,homoepitaxial MPACVD diamond growth

The use of CVD diamond in electronics has very stringent requirements. For a CVD diamond industry to become viable it is mandatory to obtain very large growth rates (> 5 µm/h), all the while maintaining extremely high purity, a crystalline defect density as low as possible, and large usable surface areas. At the same time, one must keep the stress level within the growing crystal below acceptable limits to avoid crack formation and preserve the crystal structural integrity. These imperatives imply to work to improve both the plasma deposition process and the CVD diamond crystal growth. In this paper, we propose a three-pronged approach: (i) We use detailed plasma models to establish the influence of process parameters (in particular deposition pressure) on plasma chemistry in order to optimize film growth rate and diamond quality; (ii) We emphasize the need for careful substrate pre-treatment and selection (including choosing a single-sector face) to minimize defects in the growing films; (iii) We employ a 3D geometrical model to predict the crystal shape under given growth conditions, and exploit this knowledge to devise a growth strategy maximizing the usable film surface area while minimizing stresses inside the films.     

Improved field emission characteristics of nano-structured carbon films deposited on polycrystalline CVD diamond

Nano-structured carbon films were deposited on polycrystalline diamond films grown on Si wafers by means of high-power microwave-plasma chemical-vapour-deposition (MWPCVD) method. Scanning electron microscope images show that the deposited carbon films were composed of wrinkled graphitic nano-sheets with considerably disordered structures and carbon needles on the CVD diamond grains. Field emission (FE) characteristics obtained from such films yielded very high FE currents, being larger than 100 mA/cm² at a macroscopic electric field of 9.5 V/μm. A possible mechanism of the observed strong FEs is discussed in relation to a modified Fowler–Nordheim (F–N) equation considering field-dependent parameters. Rugged CVD diamond grains played an important role in enhancing the FE current density. These experimental results suggest that some field-dependent effect should be taken into account as well as the surface geometry effect to quantitatively explain the increases in the FE current density observed in the region where no saturation behaviour of the FE current occurred.     

CVD diamond for spintronics

The ability to minimise, control and manipulate defects in CVD diamond has grown rapidly over the last ten years. The application which best illustrates this is probably that of quantum information processing (QIP) or ‘diamond spintronics’. QIP is a rapidly growing area of research, covering diverse activities from computing and code breaking to encrypted communication. All these applications need ‘quantum bits’ or qubits where the quantum information can be maintained and controlled. Controlled defects in an otherwise high perfection diamond lattice are rapidly becoming a leading contender for qubits, and offer many advantages over alternative solutions. The most promising defect is the NV⁻ defect whose unique properties allow the state of its electron spin to be optically written to and read from. Substantial developments in the synthesis of CVD diamond have produced diamond lattices with a high degree of perfection, such that the electron spin of this centre exhibits very long room temperature decoherence times (T₂) in excess of 1 ms. This paper gives a brief review of the advantages and challenges of using CVD diamond as a qubit host. Lastly the various qubit applications being considered for diamond are discussed, highlighting the current state of development including the recent development of high sensitivity magnetometers.     

Diamond coating of coloured Si3N4 ceramics

The effect of Si₃N₄ secondary phases on chemical vapour deposition (CVD) diamond film growth was analyzed. Silicon nitride substrates were obtained by pressureless sintering, placing the green samples inside a powder bed of Si₃N₄/BN. Local variations in the sintering atmosphere led to samples with different grey colouration as well as chemical and physical characteristics, determined by X-ray diffraction and thermal conductivity tests, which affected the diamond film growth. A complete characterization of the films, including thickness, average crystal size, surface roughness, texture and adhesion, was done. The Si₃N₄ substrate with glassier phase gave thicker diamond films, with smaller crystal sizes and better film adhesion to the substrate than the diamond films grown on ceramic substrates with less vitreous phase.