High mobility single crystal diamond detectors for dosimetry: Application to radiotherapy

Diamond exhibits properties of interest for applications in the medical field. It is a very attractive material for detector fabrication due to its intrinsic properties and particularly its soft-tissue equivalence (Z = 6 compared to Z = 7.42 for human tissue), mechanical robustness and radiation hardness. Detectors fabricated from natural diamonds are used in several hospitals as dosimetric tools for the dose measurement received by the patient during radiotherapy and for beam calibration. Natural diamond based devices are expensive and long delivery times are common. The use of synthetic single crystal diamond is a promising issue for point dosimeter. Here we report on the growth of synthetic diamond using the CVD technique to fabricate free standing single crystals. Samples were characterized from their optical and electronic properties (Raman, TOF) and mounted as solid ionisation chambers with blocking contacts, for the evaluation of their dosimetric properties. Clinical tests were conducted in a medical facility at the Institute Gustave Roussy (IGR) in France specialised in the medical treatment of tumours. The results obtained demonstrate that our single crystal diamond detectors comply with the required specifications for radiotherapy applications.     

CVD diamond for radiation detection devices

CVD diamond is a remarkable material for the fabrication of radiation detectors. Radiation hardness, chemical resistance and high temperature operation capabilities of diamond explain its use in the fabrication of devices operating in hostile environments such as that encountered in the nuclear industry and in high energy physics. For this purpose, we have investigated the growth of high quality chemically vapour deposited (CVD) polycrystalline diamond as well as specific material and device processing. CVD diamond films were grown using the microwave plasma enhanced technique. Deposition processes were optimised according to the application requirements. This includes the synthesis of films with high sensitivity, with weak optical absorption in the UV-VIS domain or with short carrier lifetime. One inherent problem with diamond is the presence of defect levels altering the detection characteristics: these may be the cause of an observed instability of the device responses. We have found, however, that it was possible to moderate these trends through the fine-tuning of the growth conditions and of the device preparation steps. Films with thicknesses ranging from 5 to 500 μm have been used for detector fabrication. The role of post-growth treatments and the contact formation procedure was also extensively studied, leading to significant improvements of the detector characteristics. We present recent developments studied at CEA for material optimisation towards its use for specific applications, including radiation hard counters; X-ray intensity, shape and beam position monitors; solar blind photo-detectors, and high dose rate gamma-meters.     

Effects of light on the ‘primed’ state of CVD diamond nuclear detectors

Diamond radiation detectors produced by Chemical Vapour Deposition (CVD) have been extensively studied as nuclear detectors both for the application as tracking detectors in high energy physics experiments and for applications in nuclear industry domain. A meaningful measurement to characterise the performance of such devices is the evaluation of the charge collection distance (CCD), i.e. the mean distance the charge carriers travel before being trapped. It is well known that CCD increases with the absorbed dose (priming or pumping process). The priming of diamond is usually explained by the saturation of active traps in the diamond bulk, which are filled by charge carriers generated by the ionisation. However, such a primed state is metastable, i.e. the exposure to light can de-pumps diamond to its initial state. The analysis of ‘pumping’ and ‘de-pumping’ processes is then useful to explain trapping mechanisms and polarisation effects, which limit the performances of diamond detectors. This paper deals with an investigation of the ‘pumping’ process on detector grade CVD diamond samples exposed to some doses of irradiation (X-rays or beta particles). Photoconductivity measurements carried out during monochromatic illumination as well as optical bleaching spectra of thermoluminescence glow curves highlight the passivation effects of some trap levels due to irradiation. A possible interpretation of these effects is presented and discussed, taking into account charge collection efficiency measurements carried out using Am-241 alpha particles.     

Diamond deep UV photodetectors: reducing charge decay times for 1-kHz operation

Diamond grown by chemical vapour deposition (CVD) methods is thought to be ideal for the fabrication of visible blind, fast deep UV photodetectors. However, careful device design and selection of high-quality CVD thin film diamond is, in itself, insufficient for the realisation of high performance devices. Post-growth device treatments are capable of transforming the optoelectronic properties of the material such that commercially interesting devices result. In the present study we have shown that sequentially applied methane–air treatments continue to modify both the gain level and speed of the device. Three such treatments give an optimal gain level, whilst more treatments than this lead to an improved turn-off speed. For the first time we have demonstrated the successful operation of a CVD diamond photoconductive device at at least 1 kHz at 193 nm, a frequency that is required for state-of-the-art excimer laser applications at this wavelength.     

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.     

A comparative study on comb electrodes devices made of MWPECVD diamond films grown on p-doped and intrinsic silicon substrate

Diamond is considered as a very promising material for the development of devices for radiation detection. Unlike other conventional photoconductive detectors diamond-based devices should provide high discrimination between UV and visible radiation. In this work we present the electro-optical properties of devices based on randomly oriented diamond films, synthesized in a microwave plasma enhanced chemical vapor deposition reactor. A comparative study on devices with coplanar interdigitated Cr/Au electrodes (with different interelectrode pitches) made of films grown simultaneously on intrinsic and p-doped silicon (100) substrates has been performed. The chemical-structural, morphological, electrical and optical properties of ROD films have been studied. In particular, the optical response has been measured in air using a Xe flash lamp coupled with an optical quartz fiber and a properly tailored front-end electronics based on a charge sensitive amplifier. Experimental results gave indications on how the device performances are dependent on the two types of employed substrates.     

Edge termination techniques for p-type diamond Schottky barrier diodes

The superior material properties of diamond power semiconductor devices make them a crucial technology. For high-voltage applications, optimized structures such as electric field edge termination are required for devices. In this paper, we have investigated the optimization of electric field relaxation techniques in oxygen-terminated p-type diamond Schottky barrier diodes and made comparisons with regard to electric field crowding and breakdown in oxides. Due to the low dielectric constant of diamond, Al₂O₃ is appropriate for the fabrication of field plate structures in diamond power devices.     

High-speed diamond photoconductors: a solution for high rep-rate deep-UV laser applications

Beam monitoring of excimer lasers operating at high powers in the deep ultra-violet (DUV) is becoming increasingly important, due to the rapid proliferation of these systems in micromachining, photolithography, and other areas of industrial interest. This task requires radiation-hard detectors able to operate effectively for extended periods at high laser rep-rates. DUV-visible-blind photoconductors can be fabricated on polycrystalline CVD diamond, a material that is intrinsically radiation-hard and visible-blind. However, the performance of detectors fabricated on as-grown material is insufficient to meet the requirements of many excimer laser applications. In this paper, we show that sequentially applied post-growth treatments can progressively change both the gain and speed of these devices. Charge-sensitive deep-level transient spectroscopy (Q-DLTS) and transient photoconductivity (TPC) has been used to study the effect of these treatments on the defect structure of our thin-film diamond detector material. For the first time, we report the successful operation of a diamond photoconductive device with linear bias and fluence-response characteristics at more than 1 kHz at 193 nm.     

Bulk photoconductivity of CVD diamond films for UV and XUV detection

Owing to its semiconducting properties (wide band gap, high electron and hole mobility), diamond is an interesting material for UV and XUV photodetection. In the present study, we have characterized UV and evaluated XUV diamond photodetector efficiency using volume photoconductivity instead of usual surface interdigited devices. The detectors have been tested under over-gap (13 and 193 nm) as well as sub-gap nanosecond laser irradiation (266 nm). For each wavelength, electrical characteristics of the devices have been measured as a function of bias voltage and laser fluences. The particular sandwich configuration of the detectors has shown a charge effect under over-gap irradiation. This appears by the amplitude reduction of successive pulses, and also from the different response for AC and DC bias. The suitability of these devices is discussed, the final aim being to validate bulk structures for wide band imaging devices.     

Diamond photoconductors: operational lifetime and radiation hardness under deep-UV excimer laser irradiation

The first study of long term pulse exposure and fluence level on the performance of CVD diamond photodetectors subjected to 193 nm excimer laser radiation has been performed. Whilst diamond is considered ‘radiation hard’ it is shown that damage to detector performance can be provoked at laser fluence levels considerably below that required for graphitisation or ablation. However, the application of defect passivation treatments prior to device use acts to considerably reduce the damaging effect of the radiation, such that devices suitable for stable laser monioring applications can be realised.     

Homoepitaxial Diamond Devices

Diamond has been proposed as an excellent material for high-temperature, high-power, and high-frequency applications. The interest in diamond electronics is due to its large electric breakdown field, high-saturated current velocity and high-thermal conductivity. As silicon and gallium arsenide devices begin to reach their performance limits, there is a need to develop new, better performing materials such as diamond. Significant progress in the development of diamond as a semiconducting material has been made and diamond has been implemented into numerous conventional and novel device designs. In this work homoepitaxial diamond material properties and device performance are reviewed. In summary, the large activation energy of boron-doped p-type diamond and phosphorus-doped n-type diamond severely limits diamond's use in conventional semiconductor device designs. The large activation energy reduces the number of charge carriers, which limits the current handling capability and produces temperature-dependent device performance. To overcome diamond's limitations, novel devices, such as enhancement mode field effect transistors (FETs) that use a hydrogenated surface conducting layer or pulsed doped devices with almost complete ionization, have been investigated. These devices require further development. The initial results show promise for high-temperature, high-frequency, and high-power applications.     

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.     

Nanodiamond lateral field emitter devices on thick insulator substrates for reliable high power applications

Nanocrystalline diamond field emitter array devices on a thick insulator substrate are being developed for high power applications. These monolithic lateral emitter diodes in comb array configurations demonstrate potential for high emission current applications. A 640 μm-thick aluminium nitride insulating substrate has been integrated with nanodiamond for device electrode isolation. The fabrication process and preliminary field emission characterization results are discussed. The nanodiamond lateral vacuum device may be a superior way to achieve reliable high-speed and high-power electronics.     

Extreme UV single crystal diamond Schottky photodiode in planar and transverse configuration

We report on the study of the performances of two extreme ultraviolet (EUV) photovoltaic single crystal diamond Schottky diodes based on metal/intrinsic/p-type diamond junction developed at the University of Rome “Tor Vergata” and having different contact geometries. One detector operates in transverse configuration with a semitransparent metallic contact evaporated on the intrinsic diamond surface, while the second one operates in planar configuration with an interdigitated contact structure on the growth surface of the intrinsic diamond layer. Both devices can work in an unbiased mode by using the built-in potential arising from the electrode–diamond interface and show excellent rectifying properties with a rectification ratio of about 10⁸.The devices have been characterized in the EUV spectral region by using He–Ne DC gas discharge radiation source and a toroidal grating vacuum monochromator, with a 5 Å wavelength resolution. The extremely good signal-to-noise ratio, the reproducibility of the device response, the absence of persistent photoconductivity and undesirable pumping effects suggest the high quality of our CVD diamond for UV applications. The external quantum efficiency (EQE) as well as the responsivity have been measured in the spectral range from 20 to 120 nm and opposite behaviours for the two different geometries proposed have been observed.     

Ultrananocrystalline diamond thin films for MEMS and moving mechanical assembly devices

MEMS devices are currently fabricated primarily in silicon because of the available surface machining technology. A major problem with the Si-based MEMS technology is that Si has poor mechanical and tribological properties [J.J. Sniegowski, in: B. Bushan (Ed.), Tribology Issues and Opportunities in MEMS, Kluwer Academic Publisher, The Netherlands, 1998, p. 325; A.P. Lee, A.P. Pisano, M.G. Lim, Mater. Res. Soc. Symp. Proc. 276 (1992) 67.], and practical MEMS devices are currently limited primarily to applications involving only bending and flexural motion, such as cantilever accelerometers and vibration sensors. However, because of the poor flexural strength and fracture toughness of Si, and the tendency of Si to adhere to hydrophilic surfaces, even these simple devices have limited dynamic range. Future MEMS applications that involve significant rolling or sliding contact will require the use of new materials with significantly improved mechanical and tribological properties, and the ability to perform well in harsh environments, Diamond is a superhard material of high mechanical strength, exceptional chemical inertness, and outstanding thermal stability. The brittle fracture strength is 23 times that of Si, and the projected wear life of diamond MEMS moving mechanical assemblies (MEMS MMAs) is 10 000 times greater than that of Si MMAs. However, as the hardest known material, diamond is notoriously difficult to fabricate. Conventional CVD thin film deposition methods offer an approach to the fabrication of ultra-small diamond structures, but the films have large grain size, high internal stress, poor intergranular adhesion, and very rough surfaces, and are consequently ill-suited for MEMS MMA applications. Diamond-like films are also being investigated for application to MEMS devices. However, they involve mainly physical vapor deposition methods that are not suitable for good conformal deposition on high aspect ratio features, and generally they do not exhibit the outstanding mechanical properties of diamond. We demonstrate here the application of a novel microwave plasma technique using a unique C₆₀/Ar or CH₄/Ar chemistry that produces phase-pure ultrananocrystalline diamond (UNCD) coatings with morphological and mechanical properties that are ideally suited for MEMS applications in general, and MMA use in particular. We have developed lithographic techniques for the fabrication of UNCD–MEMS components, including cantilevers and multi-level devices, acting as precursors to microbearings and gears, making UNCD a promising material for the development of high performance MEMS devices.     

X-ray micro beam analysis of the photoresponse of an enlarged CVD diamond single crystal

Diamond is one of the most promising materials for developing innovative electronic devices. Chemical vapour deposition (CVD) homoepitaxial growth allows the synthesis of high quality single crystal diamond plates. However, the use of these crystals for electronic applications is hampered by their small area (typically of the order of 10 mm²). Large areas are desired to ensure efficient particle or radiation detection with pixelated devices. By growing a thick CVD layer it is possible to enlarge the initial area of the substrate by a factor of 2 since growth also occurs laterally from the substrate.In this work, by using an X-ray collimated synchrotron radiation beam, the detection and charge collection properties of an enlarged CVD single-crystal diamond are used as a point-to-point probe to study the material quality. It was found that stress and dislocation density are correlated with the detection properties of the enlarged regions. The sensitivity of the device is affected by the vertical-to-lateral growth interface and the enlarged material quality seems to be correlated with the distance from this interface.     

Geometrical non-uniformities in the sensitivity of polycrystalline diamond radiation detectors

Chemical vapour deposited (CVD) diamond is a remarkable material for the fabrication of photon and particle detectors. However, little is known about the perturbations induced by the polycrystalline nature of this material. For this purpose, we have used a micrometer size X-ray beam generated from a synchrotron light source to induce photocurrents in a CVD diamond-based detector. By comparing the measured currents in the device as the beam interaction position is moved on the sample with the topographical image of the surface observed using a scanning electron microscope (SEM), a significant non-uniformity has been observed that could be correlated with the grain structure.     

Comparison of diamond and silicon ultraviolet photodetectors

Measurements have been made on a commercially produced diamond detector, and on detectors made in-house, all based on polycrystalline chemical vapour deposited (CVD) diamond. The uniformity of response over the sensitive area of each detector has been mapped with 50 or 100 μm resolution and the signal-to-noise ratios have been measured at 220 nm (which is near the peak of the response for the diamond detectors). We find huge variations in sensitivity as the position of the spot on the diamond detector is moved. In the most sensitive regions the photocurrent, with a bias of 0.3 V μm⁻¹, can be much larger than that obtained from a silicon photodiode operated in the short-circuit mode; however, the signal-to-noise ratio and stability of the signal are superior for the silicon detector. All the diamond detectors respond to visible light to a certain extent, particularly after illumination with ultraviolet (UV) radiation, and the UV response is modified if the detector is simultaneously illuminated with visible light. These phenomena indicate the presence of trapping centres in the material.     

Synthetic single crystal diamond dosimeters for conformal radiation therapy application

A synthetic single crystal diamond based dosimeter in a p-type/intrinsic/metal structure, operating in photovoltaic regime, is proposed for application in highly conformed radiotherapy dosimetry. The device was characterized by using 6 and 10 MV Bremsstrahlung X-ray beams and electron beams from 6 MeV up to 18 MeV, obtained by a CLINAC DHX Varian accelerator. All measurements were performed in a water phantom and commercial ionization chambers were used for calibration and comparison. Results showed a very good agreement of the diamond device response, as compared with the reference dosimeters, fast response times and high spatial resolution. One of such diamond dosimeters was then tested using a real Intensity Modulated Radiation Therapy (IMRT) prostate cancer treatment plan and its performance was compared with the ones from ionization chambers and a 2D diode array. The obtained results clearly assess the suitability of synthetic single crystal diamond for dose measurements in highly conformed radiotherapy and particularly in IMRT applications.     

Diamond components with integrated abrasion sensor for tribological applications

Tribological components made from CVD diamond are commonly used for protection against abrasion in rough environments. Such components can be used e.g. in textile industry as thread guiding devices provided that surface roughness and resultant friction are low. In this work we report on the fabrication of CVD diamond components of non-planar shape. These devices were fabricated in a negative replication approach on mechanically structured substrates. Using this technique cylindrically shaped diamond devices with smooth surfaces were produced by using the nucleation side of the diamond layers as the exposed surface, thus making subsequent polishing steps unnecessary. Applying an improved two-step nucleation method further reduced surface roughness. Sensing elements in the form of resistors made from boron-doped CVD diamond were integrated into the device surface. Device performance was characterized with respect to the temperature dependence of the resistors and the suitability as abrasion sensor.