Victor Ralchenko

Structure and properties of high-temperature annealed CVD diamond

Abstract Effects of high temperature, up to 1700 °C, annealing in vacuum of CVD diamond on its structure, optical and mechanical properties are investigated. Translucent polycrystalline diamond films of thickness 0.06–1.0 mm were grown by microwave plasma CVD method, and examined with transmission electron microscopy, optical absorption spectroscopy, and three-point bending technique to measure the fracture strength. A progressive darkening of the samples, with appearance of absorption features specific for graphite-like material, was observed upon annealing at temperatures above 1300 °C. The formation along grain boundaries of amorphous carbon and/or well crystallized graphite layers, 5–20 nm thick, as well as intra-granular graphite islands, was directly observed with TEM. This internal diamond-graphite transformation process can be described by two activation energies, both values being much less than those known for the surface graphitization of diamond. The fracture strength of the diamond films increases up to 50% with annealing temperature (1460–1640 °C), this being ascribed to a build up of compressive stress as a result of local diamond-graphite conversion.

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.

External-cavity diamond Raman laser performance at 1240 nm and 1485 nm wavelengths with high pulse energy

We report on an external-cavity diamond Raman laser (DRL) pumped with a Q-switched Nd:YAG and generating at 1st and 2nd Stokes (1240 nm and 1485 nm) with enhanced output energy. The slope efficiency of 54% and output energy as high as 1.2 mJ in single pulse at 1240 nm have been achieved with optimized cavity, while the pulse energy of 0.70 mJ was obtained in the eye-safe spectral region at 1485 nm. Calculations of thermal lensing effect indicate it as a possible reason for the observed decrease in conversion efficiency at the highest pump energies.

Growth of three-dimensional diamond mosaics by microwave plasma-assisted chemical vapor deposition

We realized the growth of a novel type of diamond mosaic crystal by chemical vapor deposition of a diamond layer on tightly placed oriented seed substrates, using a combination of seeds of different heights to form a three-dimensional structure. Here, a simple T-shaped mosaic is demonstrated as a proof of the principle. The 3D mosaic was epitaxially grown by microwave plasma CVD on (100)-oriented 3 × 3 × 1 mm3 type Ib HPHT diamond substrates arranged horizontally and vertically. Very straight and sharp junctions between the vertical and horizontal parts were observed and characterized with SEM. Confocal Raman spectroscopy mapping of the CVD diamond layer revealed only a narrow (∼20 μm wide) zone of enhanced defect abundance and/or non-uniform stress, if any, around the junction. The Raman and photoluminescence mapping of the mosaic’s cross-section gave further information on the spatial distribution of the epilayer with a thickness of up to 200 μm. The work demonstrates diamond growth on both sides of the vertically positioned seed plate, with a film thickness variation of less than 25%, thus doubling the diamond mass uptake achieved with the CVD process. The developed technique clears the way for the design and growth of various complex diamond shapes.

Diamond Deposition on Graphite in Hydrogen Microwave Plasma

Hydrogen plasma etching of graphite generates radicals that can be used for diamond synthesis by chemical vapor deposition (CVD). We studied the etching of polycrystalline graphite by a hydrogen microwave plasma, growth of diamond particles of the non-seeded graphite substrates, and characterized the diamond morphology, grain size distribution, growth rate, and phase purity. The graphite substrates served simultaneously as a carbon source, this being the specific feature of the process. A disorder of the graphite surface structure reduces as the result of the etching as revealed with Raman spectroscopy. The diamond growth rate of 3 – 5 μm/h was achieved, the quality of the produced diamond grains improving with growth time due to inherently nonstationary graphite etching process. Received on 29-12-2017 Accepted on 05-03-2018 Published on 16-08-2018

Optical emission spectroscopy for diagnosis of diamond growth and etching processes in microwave plasma

For a better control and understanding of growth process of CVD single crystal diamond in microwave (MW) plasma it is important to monitor a large number of the gas phase parameters (chemistry, temperature, concentrations, etc.). A relatively simple and non-perturbing method to measure the plasma parameters is the optical emission spectroscopy (OES) [1, 2]. The OES advantage is also in a fast response to variation of the plasma parameters in the reactor. Here we present the results of OES application to characterize the MW plasma (2.45 GHz) in course of diamond deposition in H2-CH4 mixtures and etching in pure H2. The emission from the plasma passed through a quartz window in the CVD reactor chamber and an image was formed by a lens onto a matted silica plate (OES configuration (I)). The emission collected by a 0.6 mm diameter quartz fiber was directed to a spectrometer (Ocean Optics HR 4000, wavelength range of 400– 800 nm, spectral resolution 0.6 nm. The fiber tip was translated across the image of the plasma to record spatially resolved emission spectra along X or Z axes, parallel and perpendicularly to the substrate, respectively, with the istance of –25 to +25 mm along the X axis (counting from the plasma center), and from zero (the substrate position) to 50 mm along Z axis. The probed volume was a cylinder with diameter of 1.5–2 mm directed along the plasma ball diameter (Y direction), thus the spatial resolution of ~1.5 mm in the OES was obtained both for X and Z axes [2]. In another OES configuration (II) a M833 spectrometer (Solar Laser System) with high spectral resolution of 0.01 nm, equipped with a double grating with 2400 lines/mm and CCD image sensor Hamamatsu S104201006, was used. The optical fiber system collected the emission from a cylinder of 12 mm diameter directed horizontally along the plasma ball diameter (Y direction). The gas temperature was evaluated from OES data measured in the central region of plasma by the analysis of the (0,0) band of C2 Swan system (d g → a u) following the procedure described in [3, 4]. A thermal equilibrium between heavy species’ translational and rotational modes, and a Boltzmann distribution of the rotational levels of the considered vibrational state are assumed. The spectroscopy of the Swan transitions of the C2 molecule ( = 516 nm) with well-resolved rotational components was used to measure the gas temperature with accuracy of ±150 °C upon wide variation, 1÷13%, in methane concentration [5]. The rotational temperature Trot only weakly depends on the absorbed MW power (Fig.1). It increase from ~3000 K at low (2%) CH4 content to 4000–4500 K at higher (7–13%) CH4 concentrations. Fig. 1. Rotational temperature Trot in the core of plasma cloud measured from C2 fine structure spectrum vs MW power at CH4-consentations: 2, 7, 4 10, 13, 10%, and pressure p = 130 Torr