P. Ascarelli

Diamond nucleation and growth on different cutting tool materials: influence of substrate pre-treatments

Abstract Diamond coated cutting tools seem to be one of the most promising economical and technological applications of the low-pressure diamond deposition methods. The main problem associated to this application is the nucleation and growth of a uniform diamond deposit with a suitable and reliable adhesion, withstanding thermal and mechanical stresses, originating by the severe cutting conditions. In this work we have studied the HF-CVD deposition conditions of diamond on different tool substrate materials: cer-mets (WC, Co); ceramics (SiAlON, Al2O3 + TiC) and whisker reinforced ceramics (Al2O3 + SiC and ZrO2). We have examined, by SEM-EDAX and XPS surface analysis, the influence of chemical, mechanical and physical substrate pretreatments on the substrate surface modification. Strong Co etching and reducing seems to be the best WC, Co substrate pre-treatment for a suitable diamond nucleation and adhesion; the SiAlON ceramics are very good substrates for high quality diamond growth. Raman spectroscopy have evidentiated an excellent diamond film quality, with a small energy shift (1337 cm−1), may be owing to some compressive deformation.

Wettability of HF-CVD diamond films

Wettability of HF-CVD diamond films grown in different conditions has been investigated. Wettability depends on surface tension: solids with high surface tension, as diamond, should be hydrophilic, while solids with low surface tension should be hydrophobic. In spite of these arguments, natural diamond exhibits a moderate hydrophobicity [J. Coll. Inter. Sci. 130 (1999) 35], depending on surface termination (hydrogen or oxygen terminated). In this work we find that CVD diamond films show wettability behaviours ranging from a small, up to clear hydrophobicity, probably according to surface carbon termination functionalities. Wettability does not seem to be influenced by characteristics as film structural morphology or growth orientation direction, which were analysed by SEM, while it seems dependent on surface reconstruction, as detected by Raman and XPS analysis. Moreover, in contrast with natural diamond [J. Coll. Inter. Sci. 130 (1999) 35] we found an enhancement to water wettability when CVD diamond films were treated in a hot filament activated hydrogen atmosphere. We argue that this effect may be due to the hydrogen etching of reconstructed surface layers with lower surface tension.

Relation between the HFCVD diamond growth rate, the line-width of Raman spectrum and the particle size

Abstract The microstructure of a thin diamond film determines its functionality and reliability. Thus, knowledge of the phenomena influencing the stabilisation of a particular microstructure is widely requested. Here the occurrence of a linear relation between the 1332 cm−1 Raman line-width and the growth rate of HFCVD diamond thin film is shown. A linear dependence of the Raman line-width on the reciprocal of the diamond particle size, measured by X-ray diffraction, already substantiated for MWCVD thin films, is shown to exist also for HFCVD films. Consequently the average particle size and the film growth rate are also shown to be related. The phonon confinement model has been reconsidered and a new phonon diffusion model is proposed to explain these results. According to this model the phonon energy, initially confined in a particle, is allowed to diffuse into the matrix with time. Finally, it is proposed that the relation of the film growth rate with the particle size is due to the concomitant effects of the matter diffusion process, necessary to reach and stabilize crystalline order, and the film growth that mainly depends on process deposition conditions.

Optimized spectral collection efficiency obtained in diamond-based ultraviolet detectors using a three-electrode structure

A diamond detector structure for uniform and efficient collection of photogenerated carriers in the ultraviolet spectral range is presented. The device operating principle is based on the contemporary collection of both photogenerated carriers in the bulk, and photoelectrons emitted from the diamond surface. A three-electrode device structure is used, having dual side contacts on the diamond surfaces, and a separated third electrode in the vacuum. A large improvement of the detector responsivity, as well as a compensation of the efficiency losses usually occurring for highly absorbed radiation is achieved, while keeping comparable wavelength selectivity.

Secondary electron emission from diamond: Physical modeling and application to scanning electron microscopy

Secondary electron emission from diamond films is studied as a function of the primary electron beam energy and bulk materialproperties. A formulation of a simple model of the secondary electron emission coefficient, as a function of the primary electron beam energy, has been found to be helpful in defining physical criteria able to guide the optimization of the diamond film electron emission performance. The secondary electron mean escape depth deduced from the model is indeed related to the density of defects in the material and represents the main factor in determining the low energy secondary electron yield. These results are supported by Raman spectroscopy measurements, indicating a lower graphitic content and a higher crystalline quality of the diamond phase in films showing better secondary electron and photoemission yields. We demonstrate that a diamond film, acting as a stable and proportional electron multiplier, can be used as a converter of backscattered electrons into secondary electrons in scanning electron microscopy. It will be shown that the use of a diamond film converter is suitable to improve the signal to noise ratio of images providing an enhanced compositional contrast.

Design and characterization of a tissue-equivalent CVD-diamond detector for clinical dosimetry in high-energy photon beams

New solid-state detectors, based on chemical vapour deposited (CVD) polycrystalline diamonds produced by hot-filament (HF) or microwave plasma (MW) assisted deposition methods, were constructed for radiation therapy dosimetry. Properties of diamond crystals, such as high radiation sensitivity, resistance to radiation damage and tissue-equivalence giving a low-energy dependence are very advantageous for clinical dosimetry. Therefore the encapsulation was specially designed for these detectors to have as little influence as possible on the radiation response. The prototypes were irradiated with use of a wide range of photon beam qualities ((60)Co gamma-rays, 6 and 18 MV X-rays). The radiation sensitivity varied considerably between samples deposited with HF (9 nC Gy(-1)mm(-3)) and MW (66 and 144 nC Gy(-1)mm(-3)) methods. For all detectors the leakage current was of the order of 10% of the radiation-induced current (bias voltage 100 V, dose rate 0.3 Gy/min). When irradiated with (60)Co gamma-rays, the detectors showed a dose-rate linearity with an exponential Delta parameter close to unity. However, a difference of 8% was found between Delta values for the different beam qualities. A small energy dependence was observed, for which the most probable sources are interface effects due to the silver electrodes and partly the geometry of the encapsulation which needs to be further optimized. Despite some limitations in the performance of present prototype detectors, with an improved CVD technique producing crystals of better electrical and dosimetric properties, and with a well-designed tissue-equivalent encapsulation, CVD-diamonds could serve as very good dosimeters for radiotherapy.

A preliminary dosimetric characterization of chemical vapor deposition diamond detector prototypes in photon and electron radiotherapy beams.

Three radiation detectors based on polycrystalline diamond films with different thickness and resistivity, obtained by microwave chemical vapor deposition, were tested to assess their suitability for relative dosimetry of photon and electron beams supplied by clinical linear accelerators. All samples showed a linear response as a function of the absorbed dose. The sensitivity per unit of detector sensitive volume spanned between 7 and 43 nC Gy(-1) mm(-3) with an applied electric field of 40 kV/cm. The dose rate dependence was evaluated following the Fowler theory and delta coefficient values between 0.95 and 1.00 were found for the three samples when polarized at 40 kV/cm. Percentage depth dose curves, output factors, and normalized dose profiles were determined for 6 and 10 MV photon beams and for 6 and 15 MeV electron beams. The results obtained with the diamond detectors were in good agreement with those obtained by reference detector measurements [all the data were within the experimental uncertainty of 1% (1sigma)].

CVD diamond dosimetric response evaluated by X-ray absorbers method

Abstract The DC electronic response of a small area polycrystalline diamond film irradiated by a low energy X-ray beam has been analysed. The relationship between the induced photocurrent and the X-ray flux has been investigated making use of an X-ray absorber method. Aluminium foils have been used to attenuate the direct beam and the metal–diamond–metal device response tested at several electric field values. The X-ray photocurrent shows a linear response of the device that has been observed for a large range of X-ray flux values.

Effect of nanostructure and back contact material on the field emission properties of carbon films

The emission properties of carbon films grown by pulsed laser ablation are investigated in relation to their nanostructure, which changes from amorphous to nanostructured carbon, according to the substrate temperature. In addition to an increasing number and size of six-member carbon rings, Raman scattering measurements reveal light polarisation sensitivity, reflecting a temperature-induced orientation of graphene domains. Such characteristics largely affect the electron emission properties, resulting in a close relation among threshold field values, graphene domain size and probably their average orientation. These results are interpreted suggesting that hot electron generation and transport through graphene domains is one of the main mechanisms enhancing the electron emission probability. A lowering of the threshold field strength is also observed when carbon films are deposited on titanium substrate in respect to the silicon one.

Grain boundary transport in x-ray irradiated polycrystalline diamond

The transport properties of a “thin” polycrystalline diamond film are analyzed after the sample exposure to 8.06-keV x-ray radiation. Structure and morphology of the as-grown film have been evaluated by Raman, x-ray diffraction, and scanning electron microscopy techniques. The transport properties have been investigated by measuring dark current–voltage characteristics in the temperature range of 60 to 360 K. Ohmic transport has been evidenced on the as-grown film up to 1.16×105 V/cm. After irradiation, nonlinear contributions to the dark current have been evidenced and related to field-assisted thermal ionization of traps. Below 200 K, hopping mechanisms have been observed. Correlations have been found among x-ray irradiation, density of traps involved in the transport processes, and the nonhomogeneous nature of the sample. A simple model of the grain boundary structure is proposed to explain the observations.