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.     

Transparent diamond electrodes for tunable micro-optical devices

Boron-doped nanocrystalline diamond (NCD:B) elastic layers combined with aluminum nitride (AlN) piezo-actuators are well suitable for ultrathin, highly transparent and mechanically stable micro-optical devices operating at high frequencies. In such unimorph structures NCD:B layers serve as advanced transparent electrodes instead of commonly used indium tin oxide. The carrier transport properties of heavily p-doped NCD thin films are characterized by temperature-dependent voltage-current and Hall measurements. In addition, the piezo-driven aspheric deformation of NCD:B/AlN micro lenses is demonstrated by means of integrated multi-sector actuators.     

Heat-spreading diamond films for GaN-based high-power transistor devices

We discuss the potential of heat-spreading films with respect to improving the performance of thermally limited high-power high-frequency GaN-FET devices and report on successful diamond deposition on GaN-FETs. Detailed conditions for process compatibility with GaN-FET technology are discussed and shown to be satisfied by the low-temperature deposition process developed.     

Requirements for synthetic diamond devices for radiotherapy dosimetry applications

CVD diamond is a remarkable material for the fabrication of radiation detectors. Radiation hardness, chemical resistance and high-temperature operation capabilities of diamond motivate its use for fabrication of devices operating in hostile environments such as that encountered in nuclear industry and high energy physics. Its potentialities for such applications have been well documented and recent studies have led to the developments of a few applications that are addressing specific industrial needs.One particular interest of diamond stands in the fact that its atomic number is close to that of human tissues. This implies that the response of a diamond device to radiation is close to that received by the human body. Its thus enables the straightforward measurement of the dose for radiotherapy applications. However, this requires high reproducibility and linearity. It is widely observed that radiation exposure is modifying the initial performances of diamond detectors and priming devices is therefore required to obtain the required linearity. However, the nature of defects in the material strongly influences the type of priming required. This paper will address this problem from the study of trapping levels and their influence on the device response. We present here the current status of the development of polycrystalline diamond for this type of application, and propose new techniques of improving the material characteristics toward the optimisation of ionisation chamber performances as well as that of thermoluminescent dosimeters for the particular field of radiotherapy applications.     

High quality homoepitaxial CVD diamond for electronic devices

Recent achievements in homoepitaxial CVD diamond films for electronic devices have been discussed. We have successfully synthesized high-quality homoepitaxial diamond films with atomically flat surface by the microwave plasma chemical vapor deposition (CVD) using a low CH₄ concentration of CH₄/H₂ gas system less than 0.15% CH₄/H₂ ratio and Ib (001) substrates with low-misorientation angle less than 1.5°. These films are atomically flat over an area as large as 4×4 mm² and have shown a strong excitonic emission of 5.27 eV line, even at room temperature, with no essential emission lines in the visible light region in the cathodoluminescence (CL) spectra. Furthermore, high-quality Schottky junctions between Al and P type high-conductivity layers near the surface of these films have been obtained. Based on this growth method, we have also successfully synthesized B-doped diamond films using trimethylboron [B(CH₃)₃,TMB] gas as a B-doping source, whose Hall mobility is 1840 cm²/Vs at 290 K. Schottky junction fabricated by the B-doped diamond also shows excellent performances, indicating that the homoepitaxial diamond films presented here have a high potentiality for electronic devices.     

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.     

Fabrication of aluminium nitride/diamond and gallium nitride/diamond SAW devices

Surface acoustic wave (SAW) devices have been fabricated from thin films of gallium nitride and aluminium nitride deposited on a range of chemical vapour deposition (CVD) diamond substrates. The growth of aluminium nitride and gallium nitride layers on diamond by chemical beam epitaxy (CBE) is reported for the first time. Triethyl gallium and ethyldimethylamine alane precursors were used in conjunction with nitrogen from an RF atom source to deposit the gallium nitride and aluminium nitride layers at substrate temperatures in the range 540 to 575 °C. These layers have been characterised by Raman spectroscopy and atomic force microscopy. The SAW structures were completed by the deposition of gold or aluminium interdigitated electrode structures on the as-deposited nitride surfaces. Preliminary testing indicates that these devices operate as bandpass filters with characteristics consistent with the propagation of acoustic waves at very high phase velocities within the nitride–diamond multi-layer substrate.     

Feasibility of CVD diamond radiation energy conversion devices

We analyzed the feasibility of CVD diamond to operate as the main component in active devices for conversion of high-energy radiation into electrical power. A self-sustained radiation dosimeter based on the electron emission effect was designed and tested under low-energy X-ray beam (Mg X-ray tube). The device operative conditions (absence of applied bias voltage) represented also the first experimental test towards the development of diamond energy conversion systems. On this basis, we designed a CVD diamond vacuum radiation energy converter and analyzed it using electron beams. An analysis of the performance was obtained letting both electron flux I₀ and kinetic energy E₀ to vary. For E₀ = 1 keV the power exploited by a load was estimated as tenths of nW and the total conversion efficiency was between 0.2 and 0.4%. This performance makes the device nominally competitive if compared to other similar solid-state converters. A discussion about the device design and possible improvements was performed in order to rationalize the conditions able to maximize the energy conversion efficiency.     

Thick boron doped diamond single crystals for high power electronics

Switch Mode Power Supply (SMPS) is now widely used for the control and conversion of electric power from one watt to several megawatts. In this context, the synthesis and use of wide bandgap semiconductor materials having physical characteristics superior to silicon is essential. Due to its outstanding physical properties (thermal conductivity, breakdown voltage, carrier mobilities…), diamond is a very promising material. However the success of its use in power electronics mostly relies on our ability to provide carriers by doping the material in a controlled manner. In particular the growth of thick heavily boron doped material is an essential requirement to develop vertical components which should allow, as it will be shown by modeling, limiting the series resistance of the devices in their on-state. Deposition conditions required to obtain high growth rate, high quality and heavily boron-doped material by plasma assisted chemical vapour deposition (MPACVD) will be described. It will be shown in particular that high growth rate, high-quality material, which is obtained at high microwave power density, comes at the expense of the boron concentration, and a compromise must be found. Preliminary results on boron doping of single crystal diamond will be presented and associated with electrical properties of pseudo-vertical Schottky Barrier Diodes (SBD). In particular, a critical electric field of 1.3 MV/cm has been demonstrated with a rectifying ratio of 10⁹. In the same time, current density close to 1500-2000 A cm^− 2 has been reached, showing the potentiality of diamond for power-electronic applications.     

SAW devices based on ZnO inclined c-axis on diamond

A thin zinc oxide (ZnO) films with inclined c-axis are deposited on the nucleation side of self-standing diamond substrates with. r.f. magnetron sputtering and a zinc-oxide target. The films are characterized by X-ray diffraction (XRD) with a χ-scan analysis. We have measured a c-axis angle of inclination of 45° in the case of ZnO/Diamond structure. The velocity and the electromechanical coupling coefficient is theoretically determined by the effective piezoelectric permittivity method for three angles of inclination (0°, 45°, 90°). Several SAW devices were developed on the ZnO(45°)/diamond structure.     

Oxides supported carbon air-electrodes for alkaline solution power devices

Porous carbon oxygen-reducing electrodes incorporated with perovskite oxide catalysts are reported. It has been possible to fabricate high-performance oxygen-reducing electrodes by introducing La_0.5Sr_0.5CoO₃ and La_0.99Sr_0.01NiO₃ with the activated coconut-shell charcoal; these electrodes could sustain load currents as high as 1 A cm⁻² without serious degradation. A model to explain oxygen-reducing activity of these oxides has been proposed.     

Basic parameters and models in simulation of CVD diamond devices

In this paper, we present development TCAD Sentaurus platform design for high voltage and high temperature CVD diamond devices. For the first time, in this work we detail some of the models and parameters used to simulate CVD diamond device in range of 300 K to 700 K. We discuss in particular temperature dependence of each model. The results are important in order to study and design CVD diamond devices operated at high temperature.     

Low loss microwave ferroelectric ceramics for high power tunable devices

In this paper we report the influence of the composition and concentration of Mg-containing additions such as Mg₂TiO₄, MgO and a mixture of Mg₂TiO₄–MgO on ceramics based on a mixture of BaTiO₃/SrTiO₃. Phase relations, crystal structure, microstructures, microwave dielectric properties (?, tan δ) and DC tunability have been studied over a wide range of frequencies. The temperature dependence of the dielectric properties has been measured as well. Among the compositions synthesized were low loss bulk ferroelectrics with dielectric constants in the range 150–800 and relatively high DC tunabilities (up to 1.49 under a DC electric field of 40 kV/cm). These materials can be used for high power tunable microwave devices.     

Device scaling of pseudo-vertical diamond power Schottky barrier diodes

Device size scaling of pseudo-vertical diamond Schottky barrier diodes (SBDs) has been characterized for high-power device applications based on the control of doping concentration and thickness of the p^? CVD diamond layer. Decreasing parasitic resistance on the p⁺ layer utilising lithography and etching realises a constant specific on-resistance of less than 20 mΩ cm² with increasing device size up to 200 µm. However, the leakage current under low reverse bias conditions is increased markedly. Due to the increase in the leakage current, the reverse operation limit is decreased from 2.4 to 1.3 MV/cm when the device size is increased from 30 to 150 µm. If defects induce an increase in leakage current under the reverse conditions, the density of the defects can be estimated to be 10⁴–10⁵/cm². This value is 5–10 times larger than the density of dislocations in single crystal diamond Ib substrate.     

Status of SiC power devices at Northrop Grumman

This paper presents an overview of SiC power devices. The progress in P–N diode development is described. It is found that in order to develop high voltage, high current diodes, it is critical to reduce dislocation density below 10³ cm⁻², and increase substrate doping of N⁺ 4H–SiC above 10¹⁹ cm⁻³. It is simply not enough to reduce the micropipe density to below 1 cm⁻². The paper introduces a new power switching device configuration, namely, JFET Controlled Thyristor (JCT). It is the most promising near term SiC switching device given its high power potential, ease of turn-off, potential for 500 °C operation and resulting reduction in cooling requirements. It is further concluded that in order to take advantage of SiC power devices, high temperature packages and components with double sided attachment need to be developed along with the SiC power devices. The progress in power MOSFET development is summarized and the phenomenon of “step-bunching” is introduced. This mechanism is responsible for causing surface roughness which results in low electron inversion layer mobility. It is found that by avoiding step-bunching, surface electron mobility of about 55 cm² V⁻¹ s⁻¹ can be obtained in 6H–SiC planar MOSFETs. The cause for low surface electron mobility in 4H–SiC is still an unknown.     

Improved microwave plasma cavity reactor for diamond synthesis at high-pressure and high power density

Microwave plasma assisted synthesis of diamond is experimentally investigated using high purity, 2–5% CH₄/H₂ input gas chemistries and operating at high pressures of 180–240 Torr. A microwave cavity plasma reactor (MCPR) was specifically modified to be experimentally adjustable and to enable operation with high input microwave plasma absorbed power densities within the high-pressure regime. The modified reactor produced intense microwave discharges with variable absorbed power densities of 150–475 W/cm³ and allowed the control of the discharge position, size, and shape thereby enabling process optimization. Uniform polycrystalline diamond films were synthesized on 2.54 cm diameter silicon substrates at substrate temperatures of 950–1150 °C. Thick, freestanding diamond films were synthesized and optical measurements indicated that high, optical-quality diamond films were produced. The deposition rates varied between 3 and 21 μm/h and increased as the operating pressure and the methane concentrations increased and were two to three times higher than deposition rates achieved with the MCPR operating with equivalent input methane concentrations and at lower pressures (≤ 140 Torr) and power densities.     

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.     

电厂补给水反渗透系统的优化运行与清洗

反渗透膜污染是电厂补给水反渗透系统应用中最大的问题之一。对反渗透膜的污染及其化学清洗方法进行了综述,主要内容包括反渗透膜污染物的种类、污染物形成的原因以及清洗时机的选取、清洗药品的选择、影响清洗效果的主要因素、清洗系统构成、清洗方法和清洗效果的评价,并提出了几种防护措施。     

Carrier generation within the surface region of hydrogenated thin film polycrystalline diamond

Low temperature Hall effect measurements made on diamond films subjected to a hydrogenation process, such that the near surface region becomes p-type without the addition of conventional dopant atoms, are reported. The carrier concentration within the temperature range 10–300 K does not change as expected for most films, actually increasing as the temperature falls. However, polished films display more conventional behaviour in that the carrier concentration falls with falling temperature. A model involving carrier transport within both valance and impurity bands can be considered to explain these observations, leading to the suggestion that the hydrogenation process is capable of creating acceptor states with an activation energy within the range of 10–40 meV.     

LNG冷能发电的优化设计

随着LNG产业的蓬勃发展,构建利用LNG冷能发电系统成为回收冷能的重要途径。通过HYSYS流程模拟软件对朗肯循环进行流程模拟,并根据模拟得到的过程参数以及输出功进行分析,针对系统进行算法优化设计,以系统火用效率最大为目标函数,得到最优的运行参数。