A. V. Khomich

Effect of high temperature annealing on optical and thermal properties of CVD diamond

Abstract Structural changes in diamond films of different qualities caused by annealing in vacuum up to 1600°C have been studied by IR and UV-visible optical absorption, Raman and photoluminescence spectroscopy. An internal degradation of the diamond films and a strong optical absorption enhancement in the whole UV-vis-IR range take place at T>1300°C, and correlate with the loss of bonded hydrogen. At least 25% of the total amount of hydrogen is found to be in the unbound state in some of the as-grown (untreated) films. The diamond darkening is ascribed to appearance of graphite-like phases presumably at grain boundaries. Activation energy of GB transformation process is much lower (250–530 kJ/mol) compared to surface graphitization of single crystal diamond. No evidence of charge transfer altering the concentration of substitutional nitrogen N0S and NA−ND (in B-doped films) upon annealing was found. Thermal conductivity measured by laser flash technique remains almost constant (20 W/cmK) even after annealing to 1575°C, then catastrophically drops because of crack development in the film.

Fabrication of CVD Diamond Optics with Antireflective Surface Structures

Polycrystalline CVD diamond is an excellent material for advanced optical applications, especially in the IR spectral range. However, one drawback of diamond is the significant reflection loss of 29%, caused by its high refraction index n = 2.4. We fabricated subwavelength, “moth-eye” antireflective structured (ARS) surfaces (two-dimensional array of pyramids) by filling with CVD diamond the inverted pyramids etched in a Si substrate, followed by the substrate removal to obtain the patterned diamond replica. An increase in IR transmission up to T = 80% was observed at wavelengths λ > 10 μm for the ARS surfaces compared to T = 71% for flat surfaces even at the non-optimized geometry of the surface relief. A further increase in transmission could be achieved by combining ARS and a single layer AR coating of amorphous carbon.

Relative Abundance of Single and Vacancy‐Bonded Substitutional Nitrogen in CVD Diamond

Relations between the concentrations of neutral (N0) and charged (N+) single-substitutional nitrogen and of nitrogen–vacancy (N–V) complexes in chemical vapour deposited diamond films of ≈0.2 mm thickness with nitrogen impurity concentration levels of 10 ppm are studied. For this purpose the films were subjected to 8 MeV electron irradiation at room temperature and subsequent annealing at 800 °C. The samples were analysed by micro-photoluminescence, visible and IR absorption, and Electron Spin Resonance techniques. It was found that the concentration of nitrogen in the (N–V) and N+ forms, in as-grown films, is less than 0.1% and 10% of the neutral substitutional nitrogen N0, respectively.

Growth of single-crystal diamonds in microwave plasma

A microwave plasma (2.45 GHz) was used for depositing single crystal diamond layers at the deposition rate up to 40 μm/h in hydrogen-methane mixtures on the substrates from natural and synthetic diamond with the (100) deposition surface and with the size up to 5 × 5 mm. The structure and the defect-impurity composition of the fabricated single crystals with the thickness up to 600 μm have been investigated using Raman spectroscopy, photoluminescence spectroscopy, cathode luminescence spectroscopy, and electron and optical microscopy. A high quality and purity of the diamond layers deposited from a plasma was confirmed.

Laser-induced structure transformations of diamonds

Results are reported on the study of phase transformations in diamonds induced by nano- and picosecond pulses of KrF excimer laser (λ=248 nm) and second harmonic of a YAP:Nd laser (λ=539 nm). Main attention in the research was paid to i) laser-induced graphitization of high-quality CVD diamond plates and ii) laser-induced structure transitions in ion-implanted diamond single crystals. For CVD diamond, the thickness of the laser-graphitized surface layers was measured and the accumulation period for graphitization to occur was found to be longer for lower laser fluences. In the experiments with ion-implanted diamonds, multipulse laser irradiation at fluences lower than the graphitization thresholds resulted in progressive annealing, i.e., in an increase of the optical transmission and surface contraction. Under certain low-intensity irradiation conditions, it was also found that, competing with the annealing process, laser etching of the ion-implanted diamond occurred at extremely low rates of 10-4-10-3 nm/pulse. A correlation between the defect concentration distribution and graphitization thresholds in partially annealed ion-implanted diamonds is discussed.

Optical properties of laser-modified diamond surface

The results of laser polishing of 350 micrometers thick free- standing diamond films are reported. The polishing was performed with a grazing beam of a copper vapor laser. It is shown that the laser polishing conditions and the resulting surface roughness are controlled by varying an angle of incidence of a scanning laser beam during polishing. The surface roughness of the as-grown films was reduced by an order of magnitude and a minimum roughness of Ra equals 0.38 micrometers was achieved as a result of the two-step polishing. Optical transmission in the UV-visible spectral range of the diamond films polished under the optimized conditions was found to be close to the optical transmission of the mechanically polished diamond film. Properties of the laser-graphitized layer at the diamond surface were studied with optical spectroscopy techniques in the process of oxidative removal of the layer with increasing temperature of the oxidation in ambient air. The optical properties and oxidation stability of the laser-modified surface layer were found to change throughout its thickness from the surface to the diamond interface, depending on the laser polishing regime.

Nanocrystalline diamond films: new material for IR optics

Thin nanocrystalline diamond films promising for IR optical applications were grown on Si substrates from methane-hydrogen gas mixture in a DC arc plasma CVD reactor. Three stages for the synthesis of the highly smooth noncrystalline diamond films are important: (i) substrate pretreatment with ultrafine diamond powder, (ii) excimer laser irradiation of seeded substrates, and (iii) two-step deposition process. A correlation between optical properties of the films and growth conditions has been established by means of Raman spectroscopy, spectroscopic ellipsometry and optical transmission spectroscopy techniques. Surface roughness, which was Ra equals 8 - 40 nm for the 1 micrometers thick films, significantly decreased the transmission in the visible because of light scattering, but it had a negligible effect in the IR range. The films are transparent in the IR and have optical constants n equals 2.34-2.36 and k equals 0.005- 0.03. The hydrogen incorporation in the films in amounts up to 1.5% have been deduced from intensity of C-H absorption band around 2900 cm(superscript -1.

Spatial distribution of thermal conductivity of diamond wafers as measured by laser flash technique

Thermal conductivity perpendicularly to diamond film surface was measured using a laser flash technique. Polycrystalline diamond wafers of up to 500 microns thickness and 2.25 inch diameter were produced in microwave plasma at different deposition conditions. Thermal conductivity (TC) was determined from heat propagation time across the wafer after short pulse of a Nd:YAG laser absorbed at sample surface. Distributions of TC along wafer radius were measured with 1 mm spatial resolution, they correlate to optical absorption, Raman diamond peak width and amorphous carbon concentration in the material. The best TC values of 18 W/cmK were measured for selected samples.

Effect of crystal structure on the tribological properties of diamond coatings on hard-alloy cutting tools

Micro- and nanocrystalline uniform diamond coatings with barrier tungsten layers for improved adhesion were deposited in a microwave plasma from methane-hydrogen mixtures on cemented carbide WC–6% Co substrates with high aspect ratios. Dynamic study of cutting forces and sliding friction has shown a significant improvement in the tribological properties of diamond-coated tools in cutting highly abrasive materials, such as A390 silumin and carbon-carbon composites. Confocal Raman spectrometry has been used to examine the features of wear mechanism in nano- and microcrystalline diamond coatings deposited in a microwave plasma.

Growth of Nano-Crystalline Diamond on Single-Crystalline Diamond by CVD Method

Nanocrystalline (NC) diamond films are grown by chemical vapor deposition on various single crystal diamond faces. Under conditions of NC diamond growth, the growing filmmorphology is reduced to two planes: {100} and {111}. The {100} planes are smooth and homoepitaxial layerby-layer growth occurs on them, whereas the NC film formed by twin crystalliteswith sizes of several tens and hundreds nanometers grows on {111} planes. Nitrogen impurity sharply increases the diamond growth rate.