Chemical Vapor Deposited Diamond Films Research Articles

Effects of microstructure of films on CVD diamond X-ray detectors

Although the unique properties of chemical vapor deposition (CVD) diamond films have made it a candidate material for radiation detectors, the polycrystalline nature of the films has severely limited the development of CVD diamond detectors. In this work, three CVD diamond films with different microstructure were grown by using a hot-filament chemical vapor deposition (HFCVD) technique and were fabricated as CVD diamond detectors. The electric contact is good ohmic for bias voltage up to 150 V. 5.9 keV 55Fe X-ray was used to measure the photocurrent and the pulse height distribution (PHD). For the detector based on the best quality film, the dark-current of 16.0 nA and the net photocurrent of 15.9 nA are obtained at an electric field of 50 kV cm −1. The PHD peak is well separated from the noise pedestal, indicating a high counting efficiency and a low detection limit.

Quantitative analysis of hydrogen in chemical vapor deposited diamond films

In this investigation, diamond sheets have been deposited using chemical vapor deposition. The as-grown diamond sheets were characterized for hydrogen content using detailed infrared analysis. The deconvolution of the spectra in the three-phonon region (2700–3150 cm −1) showed a number of vibration modes corresponding to sp m CH n phase of carbon. The spectra representing (100) and (111) textured diamond sheets, grown under different conditions, was compared. The (100) textured sheets contained two dominant peaks centred at 2860 cm −1 and 2930 cm −1, which correspond to the symmetric and asymmetric stretch bands of CH 2 group. However, the sheet with (111) texture displayed multiple CH, CH 2 and CH 3 peaks on the spectra. Using a standard sample called polymethyl methacrylate with known concentration, H content associated with various modes was evaluated in the diamond sheets. IR active hydrogen in the CVD diamond sheets was also calculated by evaluating the area of the IR band. Using these measurements we suggest that the oscillator strength of the different IR modes varies depending upon the structure and H content of CVD diamond sheets.

Distribution of electrically active defects in chemical vapor deposition diamond: Model and measurement

Defects limiting the movement of charge carriers in polycrystalline chemical vapor deposition (CVD) diamond films are located within the grains or in grain boundaries. Their geometrical distribution in the sample is different and is usually unknown. We present here a method to quantitatively evaluate the concentration and distribution of in-grain and grain-boundary located active carrier traps. Since the impact of these two kinds of defects on the performance of CVD diamond based devices is different, it is possible to obtain the defect distribution by measuring the response of diamond alpha particle detectors as a function of film thickness. The Hecht theory, describing the efficiency of a semiconductor particle detector, has been modified to take into account the polycrystalline nature of CVD diamond. This extended Hecht model was then used to fit experimental data and extract quantitative information about the defect distribution.

The effect of nitrogen-implantation on the field-emission properties of CVD diamond films

Nitrogen was implanted into the chemical vapor deposition(CVD) diamond films, and the electron fieldemission properties of the nitrogenated diamond films were investigated. Raman and xray photoelectron spectroscopy(XPS) measurement revealed that nitrogenimplantation damaged the structure of diamond film and promoted the formations of sp2 C—C and sp2 C—N bonding. By increasing the implantation dose， the threshold field of the emission of the diamond film could be lowered from 18V/μm to 4V/μm. The enhancement of field emission for nitrogenimplanted CVD diamond films was attributed to the increase of the fraction of sp2 C bonds and the formation of defect bands within the bulk diamond band gap induced by nitrogenimplantation, which could alter the work function and elevate the Feimi level. Consequently, the energy barrier on diamond surface for electron tunneling was reduced.

The damage evolution of chemical vapor deposited diamond films irradiated by He+

Diamond films (DF) were prepared on P-type 〈1 0 0〉 oriented Si substrates by hot filament chemical vapor deposition (HFCVD). In order to study the lattice damage produced by ion beam doping, the films were implanted with 100 keV He+ to fluences of 5×1014 to 3×1016 cm−2, respectively. Scanning electron microscopy (SEM) and Raman spectra reflected that the DF were sequentially damaged versus radiation fluence. When the radiation fluence was smaller, the lattice damages could be partially removed by post-annealing. Increasing the radiation fluence, the DF grains decrease in size and become vague in boundaries. In addition, electrical measurements were also carried out. Post-annealing stimulated the transformation of sp3 bonded configuration to amorphous and/or micro-polycrystalline graphite.

CVD diamond photoconductive devices for detection of x-rays

Diamond is a very attractive material to realize radiation detectors due to its exceptional properties (thermal, mechanical, optical, electronic, etc). But, the detector performances are highly dependent on the film quality. In this work, two chemical vapour deposition (CVD) diamond films, differing in film quality, obtained by a hot-filament CVD technique were used to fabricate radiation detectors. The measurements of photocurrents and pulse height distributions (PHDs) of the two detectors were carried out using 5.9 keV x-rays from a 55Fe source. For the detector with a better film quality, the electrical contact is fine Ohmic for bias voltages up to 150 V, and a dark current of 23.3 nA and the photocurrent of 16.8 nA are obtained at an electrical field of 50 kV cm−1. The peak of the PHD is well separated from the noise, indicating a high counting efficiency and a high signal-to-noise ratio. After several minutes' stabilization, the time-dependent photocurrent reveals that the photocurrent increases and then levels off due to the polarization effect.

The damage evolution of chemical vapor deposited diamond films irradiated by He +

Diamond films (DF) were prepared on P-type 〈1 0 0〉 oriented Si substrates by hot filament chemical vapor deposition (HFCVD). In order to study the lattice damage produced by ion beam doping, the films were implanted with 100 keV He + to fluences of 5×10 14 to 3×10 16 cm −2, respectively. Scanning electron microscopy (SEM) and Raman spectra reflected that the DF were sequentially damaged versus radiation fluence. When the radiation fluence was smaller, the lattice damages could be partially removed by post-annealing. Increasing the radiation fluence, the DF grains decrease in size and become vague in boundaries. In addition, electrical measurements were also carried out. Post-annealing stimulated the transformation of sp 3 bonded configuration to amorphous and/or micro-polycrystalline graphite.

Analysis of traps in CVD diamond films through thermal depumping of nuclear detectors

Carrier free paths in Chemical Vapour Deposition (CVD) diamond films depend on the presence of traps, which therefore strongly affect the performance of those CVD diamond based devices which rely on the electronic properties of the material, like radiation detectors. For the same reason, these devices can in turn be used as tools to study carrier dynamics. It is well known that some traps may be saturated by pre-irradiation with ionizing radiation (e.g. β-particles), a process called “pumping” or “priming”. Not all traps behave in the same way. Due to the large bandgap of diamond, both shallow (not affected by pumping) and deep traps for electrons and holes may exist. We measured, using 5.5 MeV 241Am α-particles, the response of high quality CVD diamond based detectors after successive annealing steps performed at selected temperatures. The analisys of the decay of the detector efficency with annealing time at several temperatures allows a quantitative evaluation of the activation energy of these defects. Two main trapping centres connected to the pumping process were found, both related to holes, having activation energies of about 1.6 eV and 1.3 eV respectively. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Diamond electrodes modified by argon ion bombardment

The electrochemical properties of boron-doped chemical vapour deposited diamond films following argon ion bombardment are investigated. The argon ion treatments used produce defects at the electrode surface, but are below the amorphisation threshold for this material, and indeed can be readily reversed by sample annealing. Photoemission studies indicate downwards band-bending of about 0.8 eV as a result of the argon ion treatment, and the photoemission spectra and associated energy loss spectra are used to examine the changes occurring. The higher argon ion exposures to the surface are found to very significantly degrade the electrochemical characteristics of diamond. However for smaller ion doses, the electrochemical properties of some redox couples are not influenced by the argon ion treatments used, whereas others are strongly inhibited. The work therefore demonstrates that argon ion bombardment can be used to selectively modify the properties of diamond electrodes.

Diamond coating of coloured Si 3N 4 ceramics

The effect of Si 3N 4 secondary phases on chemical vapour deposition (CVD) diamond film growth was analyzed. Silicon nitride substrates were obtained by pressureless sintering, placing the green samples inside a powder bed of Si 3N 4/BN. Local variations in the sintering atmosphere led to samples with different grey colouration as well as chemical and physical characteristics, determined by X-ray diffraction and thermal conductivity tests, which affected the diamond film growth. A complete characterization of the films, including thickness, average crystal size, surface roughness, texture and adhesion, was done. The Si 3N 4 substrate with glassier phase gave thicker diamond films, with smaller crystal sizes and better film adhesion to the substrate than the diamond films grown on ceramic substrates with less vitreous phase.

Evaluation of Adhesive Strength of Chemical Vapor Deposition Diamond Films by Laser Spallation

The adhesive strength of polycrystalline chemical vapor deposition (CVD)-synthesized diamond films deposited on sintered SiC substrates was estimated using laser spallation. A strong pulse expansion wave was produced on the surface opposite the diamond film by the pulse laser breakdown of silicone grease confined by a silica plate and used to cause Mode-I fracture at the interface between the diamond and SiC. The amplitude of the expansion wave was monitored by a fast laser interferometer as a function of laser characteristics and the confining method. The critical laser energy for causing film spallation changed depending on the diameter of the laser beam and the thickness of the energy-absorbing layer (grease) confined by the silica plate. The driving force of the spallation was estimated to be the tensile stress of the expansion wave following the first compression wave, and it was changed significantly by the dynamics of dielectric breakdown. This was demonstrated by the waveform simulation of the out-of-plane displacement using a theoretical Green's function of the second kind for the dipole source. A pulse dipole source with short rise and decading times generates strong expansion waves.

Development of Ultra Fine Crystalline Diamond Film and Some Applications

We would like to report on the development of our new ultra fine crystalline CVD diamond film and some applications. CVD Diamond films have good wear resistance, but the crystal size is too large. Therefore, in case of application of diamond films for cutting tools, we cannot get fine surface finish on work materials. Recently we succeed in production ultra fine crystalline CVD diamond film and various cutting tools as taps, drills and end mills. Aluminum alloys have lately been consumed more than ever in airplanes and automobiles for the purpose of decreasing the weight of structural materials.Copper alloys are used for the electrode of electric discharge machines (EDM) recently. EDM electrodes are required fine surface finish because the surface quality of mold depends on accuracy of EDM electrodes. Soft materials such as aluminum and copper alloys have high viscosity and additionally activity. So during cutting aluminum and copper alloys we have sometimes adhesion between cutting tools and work pieces. Diamond film on tools is able to improve the reactivity to aluminum and copper alloys. But up to now the diamond film has a lot of course crystal and tend to supply rough surface finish on work pieces. We developed ultra fine crystalline diamond film on end mills by plasma chemical vapor deposition (CVD) method. Now our aircraft, automotive and molding customers can get smooth surface finish of work pieces.Keeping ISO14001, we recommend oil free cutting with ultra fine crystalline diamond coated cutting tools. Here I introduce background of the development and cutting examples with new fine diamond coated end mills.

The adhesion of hot-filament CVD diamond films on AISI type 316 austenitic stainless steel

Steel ball indentation and scratch adhesion testing of hot filament chemical vapour deposited diamond films onto AISI type 316 austenitic stainless steel substrates using two different interlayer systems, namely chromium nitride and borided steel, have been investigated. In order to compare the adhesion of these films with that of a well-known and strongly adherent system, detailed adhesion tests were performed on diamond films grown on molybdenum substrates as well. Scanning electron microscopy analysis of various indented regions on the diamond-coated steel and molybdenum samples in combination with the measurement of the acoustic emission signals during loading, indicate that the adhesion of the coatings on the stainless steel substrates with the CrN interlayer is comparable and only slightly less than that of the strongly adhering diamond films on the molybdenum samples. The critical load value derived from the acoustic emission signals at which partial delamination of the diamond films is first observed, is significantly lower for the diamond-coated steel with the boride interlayer than that obtained with the CrN interlayer for the present deposition conditions used. The steel ball indentation tests also show that the hardness and thickness of the interlayer have a strong effect on the penetration depths of the indents. Scratch tests performed on the diamond coated stainless steel specimens with the boride interlayer show no significant coating failure for loads up to 80 N.

Investigation of defect properties in doped diamond films

The defect properties in chemical vapor deposition diamond films doped by sulfur and boron were investigated by the Doppler broadening measurements and electron paramagnetic resonance (EPR). The results show that the defects annihilating the positrons is almost the same in various doped films, and the interactions between the dopants and the positrons are not distinct. In addition, a small amount of boron atoms can improve the quality of diamond films. The EPR signals of the diamond films arise from carbon dangling bonds.

Study the effect of O 2 addition on hydrogen incorporation in CVD diamond

The effect of a small amount of O 2 addition on film quality and hydrogen incorporation in chemical vapour deposition (CVD) diamond films was investigated and the films were grown using a 5-kW microwave plasma CVD reactor. Film quality and bonded hydrogen were characterized using micro-Raman and Fourier transform infrared (FTIR) spectroscopy, respectively. It was found that in general for films grown using CH 4/H 2 plasma both without and with O 2 addition, the hydrogen incorporation increases with increasing substrate temperature, while a small amount of O 2 addition (O 2/CH 4=0.1) into CH 4/H 2 (4%) plasma strongly suppresses the incorporation of hydrogen into the film. Raman spectra show that the added oxygen improved film quality by etching and suppressing the amorphous carbon component formed in the film. The above effect of oxygen addition on hydrogen incorporation and film quality is discussed according to the growth mechanism of CVD diamond. The CVD diamond specific hydrogen related IR vibration at 2828 cm −1 appears as a sharp and strong peak only in the FTIR spectra of poor quality films grown at high temperature both without and with O 2 addition, but it appears much stronger in the film grown without O 2 addition. This result experimentally excludes the assignment of the 2828 cm −1 peak arises from hydrogen bonded to oxygen related defect in the literature.

Growth mechanism for epitaxial cubic boron nitride films on diamond substrates by ion beam assisted deposition

Cubic boron nitride (c-BN) films were grown on highly (001)-oriented chemical vapour deposited diamond films at various substrate temperatures ( T s) ranging from 420 to 1000 °C by using ion beam assisted deposition (IBAD). The resulting c-BN films were characterized by Fourier transformed infrared spectroscopy, high-resolution transmission electron microscopy, Rutherford backscattering spectroscopy as well as X-ray diffraction. The results demonstrate that the epitaxial growth of c-BN films on diamond can be realized only at high values of T s (approx. 900 °C), while at, e.g. 420 °C, an intermediate hexagonal BN layer is formed between the substrate and the c-BN film as it is usually observed for c-BN growth on top of Si. This may be due to the amorphization or graphitization (radiation damage) of the diamond surface at low T s under the assisting ion bombardment during c-BN nucleation. The plasmon energies of the diamond substrates that were held at various temperatures and bombarded by a mixture of argon and nitrogen ions were determined from in situ low-energy electron energy loss spectra. The results indicate that at higher T s values radiation damage can be annealed out thereby maintaining an ordered substrate surface, hence allowing epitaxial growth of c-BN.

Bulk CVD diamond devices 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 XUV diamond photodetector efficiency using bulk photoconductivity instead of usual coplanar devices. For comparisons, chemical vapor deposition diamond films of 200-μm thickness have been fabricated for working either in bulk configuration (BC) or surface configuration. They have been tested under nanosecond laser irradiation in the gap region (over-gap 193 and 213 nm; sub-gap 266 nm). For each wavelength, photoconductive responses of devices have been measured as a function of bias voltage (AC and DC) and laser fluences. With BC linear responses are obtained up to 0.5 mJ/cm 2 at 193 nm and 40 mJ/cm 2 at 266 nm. However, with BC a space charge effect appears under 193- and 213-nm irradiation, reducing the sensitivity of the detector. Such drawback is overcome by using AC bias. The suitability of the devices for detecting UV laser pulses or intense XUV fast discharge lamp is discussed.

High-bandgap semiconductor dosimeters for radiotherapy applications

We present a comparison between the performance of on-line radiotherapy dosimeters made with different high-bandgap semiconductor materials. We analysed the performances of a Schottky diode made with an epitaxial n-type 4H–SiC and a Chemical Vapour Deposited (CVD) diamond film with ohmic contacts. The current response of the dosimeters has been tested under exposure to a Co60 γ-source and to 6MV photons beam from a linear accelerator. The dose range covered is 0.1–10Gy with dose rates 0.1–10Gy/min. The two devices show a charge response linear with the dose when a constant dose rate is used. The SiC diode current response increase linearly with the dose rate; for diamond a quasi-linear behaviour is observed. The epitaxial SiC device shows no priming effects and a fast velocity of response, due to the low density of lattice defects in this material. The diamond performances are affected by trapping–detrapping mechanisms at defects with energy levels at ∼1eV. To de-activate these levels the diamond sample has been pre-irradiated with fast neutrons up to a fluence of 5×1014cm−2. The sensitivity of the two devices compare favourably to those of standard silicon dosimeters.

Adhesion improvement of CVD diamond film by introducing an electro-deposited interlayer

It is proposed to use Ni–Mo alloy produced by electro-deposition as a transient layer for the adhesion improvement of chemical vapor deposition (CVD) diamond film on Cu substrate. A layer of Ni–Mo alloy is firstly electro-deposited on Cu, and then diamond film is deposited by microwave plasma CVD. Metallurgical combination was found to form in both the interface of Ni–Mo alloy/Cu and the interface of diamond film/Ni–Mo alloy. An adherent diamond film was successfully obtained on Cu because of the combined actions of the interface compounds and the reduction in the thermal stress. The process proposed in this study will provide a new approach for diamond film deposition on cemented carbides, which are cutting tool materials in common use.

Manufacture of diamond blades via microsystem technology

The application of diamond knives has steadily increased in ophthalmic surgery. However, the geometry of the blade, its thickness and the sharpness of the cutting edge are limited by the abrasive diamond polishing process, e. g. the crystalline morphology of the bulk material and the grinding powder used. A new generation of diamond blades is presented herewith allowing free choice of blade shape and thickness and possessing excellent sharpness due to a new polishing process. The new production method is based on a high-quality CVD (chemical vapour deposition) diamond film of some tenths of microns thickness, deposited on a silicon wafer as microchip technology. The mechanical properties of this synthetic diamond film are almost equal to those of a natural diamond and the surface of this film is mirror-like after deposition without requiring post-polishing. The shape of the blade can be freely defined and is transferred into the diamond film by a plasma polishing process adopted from microsystem technology. The new production method results in highly reproducible diamond blades. Concave blades and round shapes can now be realised without the restrictions of the conventional production process. The force-free fabrication method even allows realisation of miniaturized blades (e. g. width < 0.125 mm, thickness < 50 microm) far beyond the possibilities of conventional diamond blade production. Plasma polishing by means of gas atoms results in extreme sharpness with the cutting edge radius in the range of approx. 3 nm. Utilising microsystem technology we were able to manufacture reproducible artificial diamond blades. The new process offers for the first time surgeons a possibility of designing blades with a geometry close to their personal needs. Furthermore, the potential of facet-free ergonomically shaped diamond blades may stimulate further improvements towards novel surgical techniques.