Alexandre Tallaire

Abstract: A direct observation of exciton diffusion in isotopically engineered C-12/C-13 diamond junctions is presented. Excitons generated under a focused electron beam in the higher-energy bandgap C-13 diamond diffuse and part of them are collected in the lower-energy bandgap C-12 diamond, where they recombine. By using cathodoluminescence spectroscopy on samples in cross-section, the recombination intensity is followed as a function of the electron-beam distance to the C-12/C-13 interface. Exciton diffusion lengths in C-13 diamond up to 7.9 mu m at 10 K are deduced. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3689783]

Abstract: In this study, 4 x 4 mm(2) freestanding boron-doped diamond single crystals with thickness up to 260 mu m have been fabricated by plasma assisted chemical vapour deposition. The boron concentrations measured by secondary ion mass spectroscopy were 10(18) to 10(20) cm(-3) which is in a good agreement with the values calculated from Fourier transform infrared spectroscopy analysis, thus indicating that almost all incorporated boron is electrically active. The dependence of lattice parameters and crystal mosaicity on boron concentrations have also been extracted from high resolution x-ray diffraction experiments on (004) planes. The widths of x-ray rocking curves have globally shown the high quality of the material despite a substantial broadening of the peak, indicating a decrease of structural quality with increasing boron doping levels. Finally, the suitability of these crystals for the development of vertical power electronic devices has been confirmed by four-point probe measurements from which electrical resistivities as low as 0.26 Omega cm have been obtained. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3697568]

Abstract: The fabrication of diamond-based electronic devices requires that several active layers with different doping concentrations are grown in different reactors. In this paper, we have investigated the effect of interrupting and resuming the epitaxial growth of a homoepitaxial diamond film using high-power plasma CVD. It was found that long lifetime blue phosphorescence which is localised on regions with a high dislocation density is generated. Such phosphorescence is related to a higher uptake of impurities at the interface between two subsequent films, due to an increased surface roughness from etching at the epitaxial growth resumption. Etching was found to occur preferentially on threading dislocations leaving typical etch-pits. Cathodoluminescence helps identify boron as the main background impurity. Besides, the formation of new dislocations was observed on the facets of these etch-pits. The continuation of epitaxy on a roughened surface thus comes with a substantial decrease in crystal quality. (C) 2011 Elsevier B.V. All rights reserved.

Abstract: Diamond films were synthesized with up to 50% argon in the gas phase using high power plasma-assisted chemical vapour deposition (PACVD). This resulted in a strong increase of the deposition rates. Polycrystalline diamond (PCD) films grown with Ar have shown a higher incorporation of sp(2) phases but with no dramatic change in crystal morphologies. Thick single crystal diamond films did not show any obvious change in crystal quality though. By using optical emission spectroscopy (OES) and plasma simulation, it was found that the gas temperature is increased by the addition of argon by at least 500 K. This in turn promotes dissociation of H(2) into H-atom as would be obtained from an increase of the plasma power density. High-power PACVD with argon addition is thus an efficient technique for the fast growth of thick single crystal films without the cost and additional problems related to an increase of the injected microwave power. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Abstract: 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 10(9). 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. (C) 2010 Elsevier B.V. All rights reserved.

Abstract: The development of diamond based devices for high power electronic applications requires the growth of thick heavily boron doped diamond. During the growth of CVD single crystals, the final form of the film depends on the different crystalline faces growth rate with respect to each others. Three parameters (alpha, beta and gamma) which correspond to the displacement speeds of the {111}, (110} and {113} faces, normalized to the {100} displacement speed can be used into a 3D crystal growth model in order to predict the final crystal morphology. This model has been applied with success to intrinsic diamond, and is extended here to the growth of boron doped diamond. It is found that the addition of diborane gas promotes the appearance of large {110} and {113} faces limiting the usable top surface area and generally leading to stress and consequently to crystal breaking-up. By adding a small amount of oxygen (0.25%) to the feed gas, the {110} faces appearance can be inhibited and the crystal integrity preserved. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Abstract: In the DiamondView instrument, blue to blue-green luminescent zones may be seen in CVD synthetic diamond when the growth run has been interrupted and resumed, a well-known practice in the production of gem-quality CVD synthetics. DiamondView, photoluminescence (PL), and cathodoluminescence (CL) imaging were applied to study the origin and nature of these luminescent regions in two samples of high-purity single-crystal CVD synthetic diamond. Diamond View and PL measurements showed a correlation with silicon-related centers. In addition, CL analysis confirmed the presence of boron. Both silicon and boron showed preferential incorporation at the interface between CVD layers, where a higher uptake of impurities lead to the observed luminescence. Although the growth interruptions cannot be detected with the naked eye, the growth history can be determined accurately using luminescence imaging and spectroscopy techniques.

Abstract: Homoepitaxially grown boron-doped diamond films have been extensively studied for many years, in particular for the development of power-electronic devices. Coplanar structures have already been fabricated and characterized but, in such structures, the current is limited by a high series resistance. A vertical component could allow overcoming this issue but this requires that thick heavily boron-doped diamond crystals with a large usable top surface are grown. In this paper we used a 3D geometrical model in order to study the evolution of the crystal shape of thick diamond crystals as a function of boron doping. The growth parameters used in the model were determined by measuring the growth rate in different crystalline orientations. It was found that the addition of boron to the gas phase promotes the appearance of large {110} and {113} crystalline faces. {110} faces have a detrimental effect on the crystal since they can generate large stress and promote crystal break-up. The results predicted by the model are consistent with that obtained for a thick boron-doped diamond single crystal.

Abstract: The fabrication of diamond-based vertical power devices which are the most suited for high current applications requires the use of thick heavily boron-doped (B-doped) diamond single crystals. Although the growth of thin B-doped diamond films is well controlled over a large concentration range, little is known about the growth conditions leading to heavily doped thick single crystals. In this paper, it was found that the microwave power densities (MWPD) coupled to the plasma used to synthesize B-doped diamond by chemical vapor deposition is one of the key parameters allowing tuning doping efficiencies over two orders of magnitude. At high MWPD (above 100 W cm(-3)) the boron doping efficiency (DE) is extremely low while further increasing the boron concentration in the gas phase is no use as this leads to plasma instability. On the other hand, when low MWPD are used (<50 W cm(-3)), DE can be strongly increased but twinning and defects formation hampers the surface morphology. The use of intermediate MWPD densities has been demonstrated as the key in obtaining thick heavily B-doped diamond crystals (>10(20) cm(-3)) with good morphologies. (c) 2010 American Institute of Physics. [doi:10.1063/1.3511449]

Abstract: Three types of growth defects commonly found epitaxial diamond films grown by chemical vapour deposition (CVD), namely unepitaxial crystals (UCs), hillocks with flat top (FHs) and pyramidal hillocks (PHs), were etched using hydrogen/oxygcn plasma to discuss their origin. UCs formed at random locations on the grown layer without any apparent relation with the substrate. Their nucleation might be due to contaminants and their development controlled by the growth conditions in the plasma. In contrast, dislocations formed from impurities segregated at the interface between the substrate and the CVD layer, were found to be the origin of the FHs and the PHs. A simple crystal model that involves micro-faceting or twinning at an intrinsic stacking fault originating from the dislocation core is proposed to explain the formation and the evolution of the growth defects. (C) 2007 Elsevier B.V. All rights reserved.

Abstract: Diamond layers were grown top a thickness of 25 to 35 mu m either on type-Ib synthetic or type-IIa natural diamond substrates by using high or low microwave-power densities. To evaluate defects and impurities depth profiles, the samples were angled by polishing and characterized by cathodoluminescence. The first important finding is that boron, nitrogen and structural defects seem to be concentrated in a 7-10-mu m-thick region near the substrate/layer interface, as evidenced by strong defect or impurity-related luminescences and a weak free-exciton peak. The diamond quality is thus much improved in the bulk or near the surface, especially when high microwave-power densities are used, due to the inhibition of unepitaxial crystals formation. The second important finding is that using type-IIa natural substrates, which contain extremely low levels of impurities, results in a strong decrease in the amount of boron and the intensity of the nitrogen-related peak both in the bulk and at the interface of the CVD layer. These results suggest that most impurities in our CVD layers could originate from the substrate itself (C) 2008 Elsevier B.V. All rights reserved.

Abstract: In the preparation of high power diamond photoswitches, thick (more than 100 pm) lightly nitrogen-doped single crystals were grown at LIMHP, for which Differential Interference Contrast Microscopy, Raman spectroscopy, photoluminescence, and cathodoluminescence have confirmed good morphology and very low but well-controlled impurity doping level. In order to evaluate the effect of nitrogen incorporation on the electronic properties of these films, photoconductivity measurements have been carried out. In an initial study, I-V and transient photocurrent measurements were conducted on several films with N-doping from 0 to 20 ppm intentionally added to the gas phase during growth, resulting into nitrogen concentrations lower than 100 ppb in the film. The results of these measurements are presented showing typical semiconductor behavior in terms of gain versus settling time, relatively high external quantum efficiency (EQE) and corresponding derived mu tau (mobility x lifetime) product. In particular, samples with no nitrogen showed EQEs of several hundreds while their settling time was quite long (tens of seconds). However, samples with small nitrogen addition were observed to have settling times decreasing below a few seconds while EQEs close to 10 showed that a compromise could be found between efficiency and response time. (c) 2007 Elsevier B.V. All rights reserved.

Abstract: In this study, homoepitaxial thick diamond films were grown by CVD at high microwave power densities for temperatures ranging from 800 degrees C to 950 degrees C and with nitrogen additions from 75 to 200 ppm relative to the total gas flow. It was observed that there is a coupled effect of these two parameters on the growth mechanisms of the CVD diamond film. For a deposition temperature close to 875 degrees C and for the lowest nitrogen concentration, the growth proceeded via a step flow mode identified by classical step bunching phenomena due to the presence of nitrogen and leading to the appearance of macro-steps. When nitrogen concentration was increased keeping the same temperature, the growth mode evolved from a step flow mode to a bidimensional nucleation mode, for which macro-steps are no longer observed. For higher growth temperatures (950 degrees C), it was found that this growth mode transition still exists but appears for much higher nitrogen concentration. These different observations, associated with the resulting growth rates, are discussed in terms of surface modification induced by the presence of nitrogen impurity. It is shown in particular that an increase of nitrogen concentration is equivalent to an increase of the surface supersaturation, this effect being compensated by an increase of the deposition temperature. (C) 2006 Elsevier B.V. All rights reserved.

Abstract: The present status of diamond-based transistors for high-frequency and high-power applications is reviewed. We have achieved the drain current density of 550 mA/mm, cut-off frequencies for current gain (f(T)) and power gain (f(MAX)) of 45 GHz and 120 GHz, respectively, and output-power density of 2.1 W/mm at 1 GHz in class-A operation of a field-effect transistor (FET) with hydrogen (H)-surface-terminated diamond. We have found that gate capacitance can be separated into depletion-layer capacitance and insulator capacitance. Concerning a stability of H-surface termination, no apparent decrease in the current for an FET without a gate contact was observed, but gate bias stress results in a slight decrease in the drain current and simultaneously an increase in the gate leakage current. (c) 2007 Elsevier B.V. All rights reserved.

Abstract: Brillouin light scattering has been used to investigate the elastic properties of high quality homoepitaxial diamond layers about 1 mm thick that have been elaborated by microwave plasma assisted chemical vapour deposition. Taking advantage of the detection of different acoustic modes, a complete elastic characterization of the crystal has been achieved. Three single crystal elastic constants, namely, c(11), (c(11)-c(12))/2 and c(44) have been selectively determined, respectively, from the frequency of the longitudinal and of the shear horizontal bulk modes travelling parallel to the film surface. These determinations are in agreement with the frequency of the observed surface modes and of the bulk waves propagating at different angles from a normal single crystal film plane and consistent with the properties of natural diamond. By adding a low amount of nitrogen ranging from 2 to 50 ppm in the gas phase, the growth rates were increased from 6 to 33 mu m/h whereas the mechanical properties of the resulting layers remained close to those of natural diamond. (c) 2006 Elsevier B.V. All rights reserved.

Abstract: Due to its unique physical properties, diamond is a very appealing material for the development of electronic devices and sensors. Its wide band gap (5.5 eV) endows diamond based devices with low thermal noise, low dark current levels and, in the case of radiation detectors, high visible-to-X-ray signal discrimination (visible blindness) as well as high sensitivity to energies greater than the band gap. Furthermore, due to its radiation hardness diamond is very interesting for applications in extreme environments, or as monitor of high fluency radiation beams. In this work the use of diamond based detectors for X-ray sensing is discussed. On purpose, some photo-conductors based on different diamond types have been tested at the DAFNE-L synchrotron radiation laboratory at Frascati. X-ray sensitivity spectra, linearity and stability of the response of these diamond devices have been measured in order to evidence the promising performance of such devices. (c) 2007 Elsevier B.V All rights reserved.

Abstract: The growth of monocrystalline diamond films of electronic quality and large thickness (> few hundreds of microns) is an important issue in particular for high-power electronics. In this paper, we will describe the different key parameters necessary to reach this objective. First, we will examine the deposition process and establish that only microwave assisted diamond deposition plasma reactors can achieve the optimal growth conditions for the efficient generation of the precursor species to diamond growth. Next, we will consider the influence of the monocrystalline diamond substrate orientation and quality on the growth of the epitaxial layer, especially when the deposited material thickness exceeds 100 mu m. The need to use a specific pre-treatment procedure of the substrate before the growth and its impact will also be discussed. Finally we will look at the growth conditions themselves and assess the influence of the process parameters, such as the substrate temperature, the methane concentration, the microwave power density and the eventual presence of nitrogen in the gas phase, on both the morphology and quality of the films on the one hand and the growth rate on the other hand. For this, we will introduce the concept of supersaturation and comment on its evolution as a function of the process parameters.

Abstract: Three thick single crystals grown at LIMHP were investigated in regard to their thermoluminescent and electric properties like glow curve, repeatability, linearity and the current under irradiation. The crystals A, B and C showed only a very short range of the linearity after irradiation with Cs-137 and Co-60 gamma rays. Next, the samples were investigated regarding to their electrical properties. The samples irradiated with a Co-60 source presented a high sensitivity to the radiation but a long saturation time. The crystal A was irradiated using a special holder at the temperature of 250 degrees C. The current recorded was three times higher than that at room temperature and quickly saturated. (C) 2007 WELEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Abstract: In this paper, the fast growth of three thick diamond single crystals using the chemical vapour deposition (CVD) method working in a pulsed mode is reported. After 48 h, a total of half a carat of uncoloured synthetic diamond was obtained. These crystals, exhibiting thicknesses of 430, 570 and 900 mu m, were then thoroughly analysed by a wide range of characterization techniques, such as Raman spectroscopy, UV and IR absorption, photoluminescence (PL) and cathodoluminescence (CL). All three samples turned out to be of relatively high quality but small differences in purity and quality could be detected. These appeared to be directly related to the slight inconsistence of the substrate temperature during growth that ranged from 800 to 900 degrees C due to non-uniformity in the radial distribution of gas temperature. A higher contamination by residual nitrogen impurities has been evidenced for the two samples that were grown with the lowest temperature as confirmed by the PL and UV absorption spectra, as well as a lower free excitonic emission in CL. Finally a 900 degrees C growth temperature was shown to be more favourable to good quality and fast growth rate. (c) 2006 Elsevier B.V. All rights reserved.

Abstract: Due to their much lower surface roughness compared to that of microcrystal line diamond, nanocrystalline diamond (NCD) films are promising candidates for tribological applications, in particular when deposited on hard ceramic materials such as silicon nitride (Si3N4). In the present work, microwave plasma-assisted chemical vapour deposition of NCD is achieved using Ar/H-2/CH4 gas mixtures on plates and ball-shaped Si3N4 specimens either by a conventional continuous mode or by a recently developed pulsed regime. The microstructure, morphology, topography and purity of the deposited films show typical NCD features for the two kinds of substrate shapes. Besides, tribological characterization of the NCD/ Si3N4 samples is carried out using self-mated pairs without lubrication in order to assess their friction and wear response. Worn surfaces were studied by SEM and AFM topography measurements in order to identify the prevalent wear mechanisms. Friction values reached a steady-state minimum of approximately 0.02 following a short running-in period where the main feature is a sharp peak which attained a maximum around 0.44. Up to the critical load of 35 N, corresponding to film delamination, the equilibrium friction values are similar, irrespective of the applied load. The calculated wear coefficient values denoted a very mild regime (K similar to 1 x 10(-8) mm(3) N-1 m(-1)) for the self-mated NCD coatings. The predominant wear mechanism was identified as self-polishing by micro-abrasion. (c) 2005 Elsevier B.V. All rights reserved.

Abstract: The progress that has been achieved over the past few years on the growth of thick and high purity homoepitaxial single crystal diamond by chemical vapour deposition (CVD) has opened a wide range of possible applications in areas such as optics or power electronics. Recently, high quality single crystals were produced at LIMHP with growth rates close to 20 mu m/h using relatively high microwave power density, both in continuous and pulsed mode [1, 2]. However, these results are conditioned by the use of an in-situ etching of the diamond substrate prior to growth using a H-2/O-2 plasma. This pre-treatment results in better and smoother morphologies of the homoepitaxial diamond layer [3] compared to growth on an untreated substrate. Moreover, the initial quality of the diamond substrate was demonstrated to be critical for the final morphology of the CVD epitaxial film. Still, the impact of surface defects on the initial diamond substrate and the role of the pre-treatment on the final morphology of the grown film remained partially unclear. Indeed, diamond homoepitaxial growth on untreated substrates leads to unexpected morphology and geometry of the crystal. To explain these observations, a 3D purely geometrical growth model has been developed, involving the growth rate of (100), (111), as well as (110) and (113) faces. The model provides a consistent explanation of the substrate pre-treatment effect on the growth and morphology of the crystal. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Abstract: Single-crystal homoepitaxial diamond has been grown by chemical vapour deposition using a high-density microwave plasma. It has been shown that the growth rate can be increased by factors of up to 2.5 by adding small concentrations (2 to 10 ppm) of nitrogen to the gas phase. Freestanding specimens up to 1.7 mm thick have been characterised using optical absorption, cathodoluminescence, photoluminescence and Raman spectroscopies, and by electron paramagnetic resonance. These techniques all demonstrate that the colourless type IIa material is of excellent quality with total defect concentrations not exceeding 200 ppb, and is ideally suited for optical and electronic applications. (c) 2006 Elsevier B.V. All rights reserved.

Abstract: High-pressure and high-temperature (HPHT) annealing effects on the chemical vapor-deposited (CVD) homoepitaxial diamond films were investigated. By the HPHT annealing, the intensity of free-exciton (FE)-related emission was increased by - 2 times and the luminescence bands from 270 to 320 min, which originate from 5RL and 2BD bands, were almost completely eliminated in the cathodoluminescence (CL) spectrum. The CL intensity of band-A emission, which is related to crystal defects in diamond, was also decreased. The hole mobility at room temperature was increased from 826 to 1030 cm(2)/Vs by HPHT annealing. These results suggest that HPHT annealing decreases the crystalline defects and improves the optical and electronic properties of homoepitaxial diamond films. (c) 2006 Elsevier B.V. All rights reserved.

Abstract: Owing to its exceptional properties, monocrystalline diamond is one possible interesting candidate for high-power electronic applications if a suitable and reproducible process allowing the deposition of high-quality thick films within a reasonable time is developed. In this paper, a Microwave Plasma-Assisted Chemical Vapour Deposition (MWPACVD) two-step process at high plasma density (microwave power 3200 W, pressure 220 mbar) has been successfully used on high-pressure high-temperature (HPHT) (100) substrates. It consisted in the etching of the diamond substrates with an O-2/H-2 plasma immediately followed by CVD diamond growth. Optimal growth conditions were determined leading to the synthesis of a 520-mu m-thick high-quality single-crystal homoepitaxial layer at growth rate of 6 mu m/h. This film exhibited a smooth surface with high optical clarity, and the photoluminescence (PL) spectrum was free from nitrogen- or silicon-related emission bands. Moreover, free exciton emission was clearly observed by cathodoluminescence (CL). Finally, the influence of small amounts of nitrogen was also investigated, which resulted in higher growth rate up to 33 mu m/h and films as thick as 1.7 mm. The relatively good quality of these films was continued by several appropriate characterizations, including Raman, photo luminescence and cathodoluminesence. The use of this process opens the way to the synthesis of thick high-quality single-crystal specimens for high-power electronic applications. (c) 2004 Elsevier B.V. All rights reserved.

Abstract: Silicon nitride (Si3N4) ceramics coated with nanocrystalline diamond (NCD) films are promising materials for tribological applications. In this work, NCD films are deposited on Si3N4 ceramic substrates in pulsed Ar/H-2/CH4 microwave discharges. Comparisons with films elaborated in continuous mode on Si3N4 and Si substrates, and on Si substrates in pulsed mode are achieved. The results give evidence for the growth of diamond films with a relatively high growth rate above 1 mu m/h. Good nanocrystalline features are obtained, with a small grain size below 20 nm and a surface roughness lower than 40 nm. The paper emphasizes that the great interest of the pulsed mode is to enable the deposition of good-quality NCD films on ball-shaped Si3N4 ceramics, by preventing the graphite formation observed in continuous regime due to a high plasma heating. Pulsed microwave discharges are then hopeful processes for treating complex-shape Si3N4 ceramics appropriate for specific tribological applications. (c) 2004 Elsevier B.V. All rights reserved.

Abstract: Diamond has been identified as a very promising material for X and ultraviolet sensing. In this Letter, a photoconductive device based on a freestanding homoepitaxial chemically vapor deposition (CVD) single-crystal diamond 500 mu m thick has been tested. Photoconductive measurements in coplanar and transverse configurations have been performed to characterize the device sensitivity in the 140-250 nm spectral range. Very high sensitivity values were achieved in both configurations. The sensitivity in the transverse configuration is at least 300 times higher than in the coplanar configuration. (c) 2005 American Institute of Physics.

Abstract: This is a detailed study of the process parameters that control the synthesis of homoepitaxial single-crystal diamond by chemical vapour deposition (CVD) using a microwave plasma-assisted reactor. The effects of substrate temperature, methane concentration and microwave power density on the surface morphology and purity of the synthesized diamond has been studied using diffraction enhanced and atomic force microscopy, Raman and photoluminescence (PL) spectroscopy. The effects of pulsing the input microwave is reported as well as the influence of adding small concentrations of nitrogen in the gas phase during growth. With no intentional nitrogen addition growth rates up to 16 mu m/h have been obtained in samples exhibiting relatively smooth surface morphologies and no indication of nitrogen contamination as measured by PL spectroscopy. Pulsing the microwave power allows ail increase of near 40% in growth rate for the same average input power with no apparent degradation in purity. The addition of nitrogen up to 10 ppm results in an increase in growth rate of 150%. These samples remain of good colour (water clear) but exhibit distinct PL emission hands corresponding to the nitrogen-vacancy impurity centres. Single-crystal diamond samples of good colour up to 1.7 mm thick have been produced. (c) 2005 Elsevier B.V. All rights reserved.

Abstract: Diamond synthetic single crystals exhibiting a high and reliable quality would find many applications in different areas such as optics, mechanics, or electronic devices. In this study, thick high quality homoepitaxial diamond films were grown on synthetic substrates using Micro-Wave Plasma Assisted Chemical Vapour Deposition. The effect of increasing the density of the hydrogen methane discharge was investigated since it is believed to allow higher deposition rates by enhancing the production of precursor species. In continuous wave mode, it was actually observed that increasing the microwave density in the plasma from 65 to 125 W/cm(3) leaded to an increase of diamond growth rates from 3 to 8.5 mu m/h and from 11 to 19 mu m/h when 4% and 7% of CH4 was added to the gas phase respectively. However, when such a high microwave power density is injected, overheating of the reactor walls, windows and wave-guides occurs leading to problematic cooling-down and higher contamination of the diamond film. In particular higher silicon incorporation was found by photoluminescence spectroscopy that probably originates from an enhanced etching of the quartz windows. Thanks to the use of a pulsed discharge with a peak power of 190 W/cm(3) the deposition of diamond at rates close to 22 mu m/h became possible while limiting this problem of overheating. The use of such pulsed discharges is thus considered to be a very promising method for the growth of high quality diamond single-crystals at high deposition rates.

Abstract: In the last decade, progress in diamond growth by chemical vapor deposition (CVD) has resulted in significant improvement in the quality of synthetic single crystals. This article reports on the gemological and spectroscopic features of six synthetic type IIa diamonds grown for research purposes at the French Laboratoire d'Ingenierie des Materiaux et des Hautes Pressions (LIMHP-CNRS), and compares their diagnostic features to CVD-grown diamonds from other producers. Three of the six samples were nitrogen doped, whereas the other three were classified as high purity. A number of characteristics that are diagnostic of CVD synthetic diamond were present in the nitrogen-doped crystals, despite an absence of defect-related absorption features in the infrared region. Identification of the high-purity samples was more complicated, but it was still possible based on features in their photoluminescence spectra, their distinctive birefringence, and characteristic luminescence images.

Abstract: Silicon nitride substrates have been diamond coated by hot-filament (home made) and microwave plasma (commercial) CVD reactors. The importance of Si3N4 as substrate relies on its favoured adhesion to the diamond film, as required in wear applications. Diamond coatings have been characterized by mu-Raman spectroscopy, X-ray diffraction, scanning electron microscopy and atomic force microscopy. The diamond film/substrate adhesion has been evaluated by performing Brale indentations at different loads. The diamond films grown by hot filament matched those grown by microwave plasma in terms of morphology and diamond quality, only loosing in uniformity. Spherical substrates have also been used to compare the films coated by both techniques.

Abstract: In this study, homoepitaxial diamond films were grown on HPHT Ib synthetic diamonds by microwave plasma assisted CVD (MWPACVD) using a two step process. Etching of the diamond substrates prior to growth was performed in oxygen-hydrogen or oxygen-argon-hydrogen plasmas for different etching times. After this step, homoepitaxial growth was performed for 20 hours. High quality monocrystalline diamond films have been synthesized as confirmed by Raman and photoluminescence spectra and growth rates close to 2.5 mum/h have been achieved. This relatively high growth rate opens the way to the synthesis of thick, high quality single crystal specimens for high-power electronic applications. Optical and Atomic Force Microscopy (AFM) were performed after each step. These analyses showed that aggressive etching with oxygen not only reveals the defects on the top of the diamonds but also leads to a better quality of the homoepitaxial film by preparing the substrate surface before the deposition. We also confirmed that the final morphology of the homoepitaxial film is highly dependent on the quality of the initial diamond substrate. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Abstract: Depending on purity, diamond could exhibit very high breakdown threshold voltages, high free carrier mobilities and relatively high free carrier lifetimes. For these reasons, diamond has been considered to be well suited for radiation induced high power switching applications in continuous operation mode. Excitation using deep UV sources has been studied but it suffers from their intrinsic low conversion efficiency (below 0.1%). The other problem encountered with deep UV excitation, especially in a bulk configuration, is the short penetration length leading to poor collection efficiencies, polarization and space charge effects. Depending on energy, electron beam excitation should be more adapted for this kind of application. In the present study we report on the conductivity modulation induced by continuous electron beam excitation in CVD diamond samples as a function of electron beam energy and current. The current gain is found to depend strongly on the energy and current of the electron beam. At low electron beam currents continuous gains up to 180 have been measured for 30 keV electrons, believed to be the highest continuous gain ever reported. (C) 2003 Elsevier B.V. All rights reserved.

Abstract: The role of the intergranular phase Of Si3N4 ceramics in CVD diamond coating is investigated. Two compositions from the SiO2-Y2O3-Al2O3 sintering aid system were selected: a more refractory one (A) inside the Y4Si3O12-mullite-alumina compatibility triangle and another (B) belonging to the Y2Si2O7-mullite pseudo-binary system. Samples with 7, 14 and 28 wt.% of these additive phases were diamond coated on a NTCVD reactor using H-2/CH4 mixtures. SEM observations after 10 min of CVD diamond deposition showed a fully covered surface by a nano-scale diamond crystallite (- 100nm; N-d - 10(10) cm(-2)) layer, regardless the intergranular phase nature. However, the emerging number of diamond micro-crystals from this primary layer decreased with the additive content. The 7 wt.% doped A substrate was almost covered by such large crystals ( similar to 3 mum; N(d)similar to6X10(6)CM(-2)) after 30 min of deposition and a continuous film was observed after 3 h, while samples with 14 and 28 wt.% of additive amounts reveal non-homogeneous films. The emerging micro-crystals after 30 min were fewer and bigger on B based samples, continuous films never being attained even for larger depositions periods. The distinct influence of the chosen intergranular phases on diamond deposition is due to the intrinsic glass properties. Glass type B, possessing lower thermal conductivity than A, leads to higher surface temperatures, increasing the Si3N4 intergranular phase sink effect with respect to carbon enrichment of the surface and subsequent diamond formation. (C) 2002 Elsevier Science B.V. All rights reserved.