Pulsed laser surface modifications of diamond thin films

In view of practical applications requiring diamond films, plates and membranes with very smooth surfaces, ArF excimer laser polishing treatments were applied to thin (30 μm) diamond films grown by CVD on silicon substrates. The as-prepared diamond surfaces and the laser-treated parts of the samples were characterised by SEM analysis, Raman and micro-Raman spectroscopy. The presence on the laser-treated surface of a thin amorphous carbon layer responsible for the higher surface electrical conductivity and for the different optical reflectivity properties was evidenced. Using confocal micro-Raman spectroscopy a comparative depth profile analysis of the phase quality, below the surface in different regions of the films, was carried out. After short (10 min) treatment by H₂ plasma etching in the CVD chamber the graphitic top layer was completely removed from the samples.     

纳米PCD材料合成的技术难点

文章探讨了高温高压法制备纳米PCD材料技术的研究现状及存在的问题,针对纳米金刚石纯化技术、表面净化技术,以及高压烧结中表面石墨化、塑性变形、纳米聚晶形成与再结晶晶粒长大控制等关键技术难点进行分析,期望能为今后纳米PCD材料的应用开发提供理论上的帮助.     

Synthesis of carbon nanotubes on single crystal diamond

Vertically aligned multiwalled carbon nanotubes (MWCNTs) have been synthesised on Ni coated single crystal diamond substrates using a glow-discharge technique. A mixture of gases including CH₄, H₂ and N₂ has been used for growth. The effect of the CH₄/H₂ gas mixture and growth temperature on the structure and yield of the MWCNTs has been studied. Atomic force microscopy is used to characterise the annealed Ni film prior to growth. Scanning electron microscope studies have also been carried out to observe the yield, height and diameter of MWCNTs produced under various experimental conditions. Raman spectroscopy has been performed to provide quantitive information on the crystallinity of our as-grown MWCNTs. It has been shown that highly adherent, vertically aligned MWCNTs can be grown on type Ib diamond (100) substrates with an interface free from metal catalyst.     

High wear-resistance characteristic of boron-doped nano-polycrystalline diamond on optical glass

Boron-doped nano-polycrystalline diamond (B-NPD) uniformly containing boron atoms in the diamond lattice has been successfully produced by direct conversion sintering under ultra-high pressure and high temperature using boron-doped graphite as a starting material, and its wear properties on optical glass materials have been investigated. The chemical wear of B-NPD sliding on glass was highly suppressed under sliding conditions where undoped NPD is worn considerably by chemical reaction with glass because the frictional resistance of NPD decreased and its sliding performance was improved by adding boron. In addition, because B-NPD has electrical conductivity, tribo-microplasma damages attributed to frictional electrification were not observed. Thus, the wear resistance of B-NPD on glass materials was improved greatly in comparison with that of undoped NPD. These results indicate that B-NPD has outstanding potential as a cutting tool material for high-performance and high-precision cutting on various types of glass, nonconductive ceramics and rigid plastics which are difficult to cut by conventional diamonds because of tribo-chemical wear or tribo-electrical wear.     

A near edge X-ray absorption fine structure study of oxidized single crystal and polycrystalline diamond surfaces

The influence of oxidizing environments on single crystal diamond and polycrystalline chemical vapor deposited CVD diamond films was studied using the near edge X-ray absorption fine structure (NEXAFS) pre-edge region in both bulk and surface sensitive modes. The NEXAFS of (100) oriented single crystal diamond was measured following (i) exposure to a microwave (MW) hydrogen plasma, (ii) annealing to 1000 °C, (iii) exposure of the as annealed surface to H₂O, and (iv) exposure of the as annealed surface to O₂. From these measurements particular surface bonding configurations have been assigned to features in the pre-edge structure. The NEXAFS of microcrystalline CVD diamond films was studied following different oxidative treatments using (i) a thermal atomic oxygen (AO) environment, (ii) a hyperthermal (5 eV) AO source, and (iii) an RF oxygen plasma exposure. The nature of the surface layer was found to be different for differently oxidized surfaces. These treatments were carried out as part of a study of CVD diamond durability in the low Earth orbit space environment.     

纳米聚晶金刚石的高压高温合成

利用自行研制的二级大腔体静高压装置,通过高温超高压下石墨向金刚石的直接转变,合成出了纳米聚晶金刚石块体材料.合成压力约为17GPa,温度约为2300℃.微区X射线衍射分析表明,石墨转变成了立方相的金刚石,扫描电子显微镜及X射线全谱拟合分析显示,合成出来的金刚石晶粒尺寸约16nm.压痕法测得的样品维氏硬度为100GPa以上.     

Charge transport in high mobility single crystal diamond

Time of Flight (TOF) measurements using conventional laser TOF and α-particle TOF setups have been carried out on high quality CVD diamond samples to study the electron and drift mobility and to compare them with the mobility data for IIA diamond. The measured mobilities for all samples investigated are in the range 2000–2250 cm²/Vs for holes and 2200–2750 cm²/Vs for electrons, thus close to the theoretical prediction as well as to IIa diamond mobility values. The charge transient profile measured in the laser TOF measurements is influenced by the electric field profile in the sample, which might be changed based on the charge trapping at low electric fields applied, depending on the surface atomic termination. The temperature dependence of the drift mobility indicates that at room temperature the scattering on acoustic phonons is the main dominant scattering mechanism and the contribution of other types of carrier scattering mechanism is negligible.     

Elastic properties of single crystal diamond made by CVD

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, c₁₁, (c₁₁ − c₁₂) / 2 and c₄₄ 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 μm/h whereas the mechanical properties of the resulting layers remained close to those of natural diamond.     

Indentation hardness of nano-polycrystalline diamond prepared from graphite by direct conversion

Indentation hardness of nano-polycrystalline diamonds (consisting of fine particles of 10–30 nm size) prepared directly from graphite under high pressure and high temperature conditions were investigated. It was found that a measurable indentation with no cracking can only be formed using the Knoop indenter in a limited loading condition of 2–6 N, and a reliable and accurate measurement is obtained at a load around 4.9 N. The Knoop hardness measurement at the applied load of 4.9 N revealed that some of the nano-polycrystalline diamonds obtained at P≧15 GPa and T≧2300 °C have extremely high hardness (120–145 GPa), which is equivalent to that in the (001)〈100〉 of the synthetic high-purity (type IIa) diamond crystal (116–130 GPa).     

HPHT synthesis of large single crystal diamond doped with high nitrogen concentration

Large green single crystal diamond with high nitrogen concentration is firstly synthesized by temperature gradient method under high pressure and high temperature (HPHT). Sodium azide (NaN₃) is added as the source of nitrogen to the synthesis system of high pure graphite and kovar alloy (Fe₅₉Co₂₅Ni₁₇). The HPHT synthesis condition is 5.4 GPa and 1480 K. The maximum crystal size is 3.2 mm. The infrared absorption spectra indicate that the nitrogen concentration in the green large single crystal diamond reaches 1520 ppm in the form of single substitution, which is close to that in nature diamond, and several times higher than that in the man-made Ib large single crystal diamond previously.     

Synthesis of large single crystal diamond using combustion-flame method

A combustion flame method is used to synthesize large single crystal diamond in ambient atmosphere. The basic of this technique was originally described by Hirose and Kondo in 1988 [Hirose H, Komaki K. Eur Pat Appl 1988:EP324538]. The advantage of this method is the high growth rate of diamond films, which is about 60 μm/h [Alers P, Hanni W, Hintermann HE. A comparative study of laminar and turbulent oxygen-acetylene flames for diamond deposition. Diam Relat Mat 1992;2:393-6]. The diamond can grow on itself to achieve large single-crystal. Negative substrate-bias effects on diamond growth have been investigated. Diamonds films were characterized by scanning electron microscopy, Raman spectroscopy, and atomic force microscopy in tapping mode. For given conditions, diamond coatings with highly oriented {1 0 0} crystal facets were produced. Large singles crystals diamonds were obtained. The sizes of these crystals vary between 80 and 90 μm. These results are discussed with respect to the competing events occurring during the heteroepitaxial growth of diamond.     

Fabrication and characterisation of triangle-faced single crystal diamond micro-cantilevers

Single crystal diamond offers superior properties for MEMS applications to polycrystalline forms of this material. Here, a process based solely on focussed ion beam milling (Ga), has been used to fabricate nanometre-width diamond cantilevers we lengths of several tens of microns. The procedure results in low damage structures with little Ga incorporation, following post-fabrication annealing. The triangular profile of a cantilever produced using this technique is shown, theoretically, to have a factor of three improved defection response to a load typically encountered during chemical sensing compared to a conventional rectangular lever of similar dimensions.     

Luminescence centers in proton irradiated single crystal CVD diamond

Diamond displays a large variety of luminescence centers which define its optical properties and can be either created or modified by irradiation. The main purpose of the present work is to study the radiation hardness of several of such centers in homoepitaxial single-crystal CVD diamond by following the evolution of photoluminescence and ionoluminescence upon 2 MeV proton irradiation. Luminescence decays were observed with values of the fluence at half of the starting luminescence (F_1/2) of the order of 10¹⁴ cm^? 2. The 3H center displayed a non-monotonic behavior, with a growing behavior and a subsequent decay with a rather high F_1/2 value (in the order of few a 10¹⁶ cm^? 2), maintaining at the highest fluences an intensity significantly higher than the blue A-band. A simple model based on a double-exponential trend was defined to fit with satisfactory accuracy the evolution of the 3H center. Several PL centers (namely: 3H, TR12, 491 nm and 494 nm) exhibited clear correlations and anti-correlations in their fluence dependences, which were considered in the attempt to acquire some insight into their possible alternative attributions.     

Dissipation in single crystal diamond micromechanical annular plate resonators

We report measurements of energy dissipation in single crystal diamond annular plate resonators for temperatures ranging from 4 to 300 K. An order of magnitude reduction in dissipation is observed as the temperature is lowered from room temperature (1/Q = 5 × 10⁻⁴) to 30 K (1/Q = 5 × 10⁻⁵).     

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.     

X-ray topography studies of dislocations in single crystal CVD diamond

X-ray topography has been used to study single crystal diamond samples homoepitaxially grown by microwave plasma-assisted chemical vapour deposition (CVD) on high pressure high temperature (HPHT) and CVD synthetic diamond substrates. Clusters of dislocations in the CVD diamond layers emanated from points at or near the interface with the substrate. The Burgers vectors of observed dislocations have been determined from sets of {111} projection topographs. Dislocations have line directions close to the [001] growth direction and are either edge or 45° mixed dislocations. Where groups of dislocations originated at isolated points they tended to be of the edge variety. Where the substrate surface was deliberately damaged before growth, two sets of dislocations were observed to have propagated from each line of damage and there was a tendency for dislocations to be of the 45° mixed variety with a component of their Burgers vector parallel to the polishing direction. It is demonstrated that X-ray topography can be used to deduce the growth history of CVD synthetic diamond samples produced in multiple growth stages.     

Self-aligned fabrication of single crystal diamond gated field emitter array

This paper describes a self-aligned fabrication process for diamond gated field emitter array (FEA). Utilizing the non-conformal coverage sputtering conditions of silicon oxide, an interesting “sphere on cone” structure is formed on diamond nano tip array, which is the key point of gate hole opening process. This structure causes shadowing at certain regions of side-wall during Ti / Au gate metal deposition. Removal of “sphere” by wet etching leads to the successful fabrication of a single crystalline diamond gated FEA. Scanning electron microscope observations reveal the fabrication of a uniform emitter array with tip radius of curvature (20 nm) and gate hole (1.4 μm). We also confirmed that no noticeable physical damage exists on tip. In field emission characteristics of the fabricated single crystal diamond gated FEA, gate voltage control of field emission current is realized.     

单晶CVD金刚石作为功能材料的应用

阐述单晶CVD金刚石的发展、特性及应用.分析了单晶CVD金刚石在工业与技术应用方面的优越性.目前其重要应用领域主要包括:半导体器件;微波技术;监测器件与检测系统;光纤通讯以及光信息存储技术等.     

Anisotropic dry etching of boron doped single crystal CVD diamond

Semiconducting boron doped single-crystal CVD diamond has been patterned using aluminum masks and an inductively coupled plasma (ICP) etch system. For comparison insulating HPHT diamond samples were also patterned using the same process. Diamond etch rates above 200 nm/min were obtained with an O₂/Ar discharge for a gas pressure of 2.5 mTorr using 600 W RF power. We have accomplished the fabrication of structures with a minimum feature size of 1 μm with vertical sidewalls in both CVD and HPHT diamond. The ICP etching produced smooth surfaces with a typical root-mean-square surface roughness of 3 nm. The dependence of etch rate on bias voltage was somewhat different for the two types of diamond. However, for all samples both the etch rate and anisotropy were found to improve with increasing bias voltage.     

Radiation hardness of single crystal CVD diamond detector tested with MeV energy ions

The spectroscopic properties of a commercial high purity single crystal diamond detector (1 mm² area, 500 μm thickness) have been studied using focused ion beams (H, He and C ions) in the MeV energy range. A measured relative energy resolution of 1.3% (FWHM = 25 keV) for the detection of 2 MeV protons demonstrated a good spectroscopic performance of the CVD diamond device, which makes it useful for the detection of light ions or atoms. To test the radiation hardness of the diamond detector, it was selectively irradiated with a 6.5 MeV focused carbon beam up to a fluence of 10¹¹ ions/cm². Reliable measurement of the ion fluence was accomplished by means of the microprobe single ion technique IBIC (ion beam induced charge). After irradiations that produced selectively damaged regions in the diamond detector, low current mode IBIC microscopy has been performed to measure the degradation of the charge collection efficiency (CCE). In order to get a better understanding of the detector performance after irradiation, different ions with the end of a range smaller, equal and larger than the extend of the damaged layer were used as IBIC probes. The same experimental procedure of irradiation and IBIC microscopy has been performed on a detector grade silicon PIN diode in order to directly compare the radiation hardness of diamond and silicon. The presented results show that the single crystal CVD diamond is less radiation hard for the spectroscopy of short range heavy ions compared to the high resistivity silicon, which is contrary to the results obtained for diamond detectors exposed to the high energy particles.