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Morphology of Diamond Layers Grown on Different Facets of Single Crystal Diamond Substrates by a Microwave Plasma CVD in CH4-H2-N2 Gas Mixtures

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Morphology of Diamond Layers Grown on Different Facets of Single Crystal Diamond Substrates by a Microwave Plasma CVD in CH4-H2-N2 Gas Mixtures crystals Article Morphology of Diamond Layers Grown on Different Facets of Single Crystal Diamond Substrates by a Microwave Plasma CVD in CH4-H2-N2 Gas Mixtures Evgeny E. Ashkinazi 1, Roman A. Khmelnitskii 2,3,4, Vadim S. Sedov 1, Andrew A. Khomich 1,3, Alexander V. Khomich 1,2,3,* and Viktor G. Ralchenko 1,5 1 Prokhorov General Physics Institute, Russian Academy of Sciences, Vavilova Str. 38, Moscow 119991, Russia; [email protected] (E.E.A.); [email protected] (V.S.S.); [email protected] (A.A.K.); [email protected] (V.G.R.) 2 Lebedev Physical Institute, Russian Academy of Sciences, Leninskii Av. 53, Moscow 119991, Russia; [email protected] 3 Kotelnikov Institute of Radio Engineering and Electronics, Russian Academy of Sciences, Vvedenskogo Sq. 1, Fryazino 141190, Russia 4 Troitsk Institute for Innovation and Fusion Research, Pushkovykh Str. 12, Troitsk, Moscow 142190, Russia 5 Harbin Institute of Technology, 92 Xidazhi Str., Harbin 150001, China * Correspondence: [email protected] Academic Editor: Yuri N. Palyanov Received: 30 April 2017; Accepted: 31 May 2017; Published: 6 June 2017 Abstract: Epitaxial growth of diamond films on different facets of synthetic IIa-type single crystal (SC) high-pressure high temperature (HPHT) diamond substrate by a microwave plasma CVD in CH4-H2-N2 gas mixture with the high concentration (4%) of nitrogen is studied. A beveled SC diamond embraced with low-index {100}, {110}, {111}, {211}, and {311} faces was used as the substrate. Only the {100} face is found to sustain homoepitaxial growth at the present experimental parameters, while nanocrystalline diamond (NCD) films are produced on other planes. This observation is important for the choice of appropriate growth parameters, in particular, for the production of bi-layer or multilayer NCD-on-microcrystalline diamond (MCD) superhard coatings on tools when the deposition of continuous conformal NCD film on all facet is required. The development of the film morphology with growth time is examined with SEM. The structure of hillocks, with or without polycrystalline aggregates, that appear on {100} face is analyzed, and the stress field (up to 0.4 GPa) within the hillocks is evaluated based on high-resolution mapping of photoluminescence spectra of nitrogen-vacancy NV optical centers in the film. Keywords: diamond; microwave plasma CVD; epitaxy; nanocrystalline film; photoluminescence; hillocks 1. Introduction High hardness and abrasion resistance of diamond make it indispensable for machining of many materials such as composites and abrasive alloys, which leads to widespread use of diamond-based tools. Diamond coatings (DCs) produced by chemical vapor deposition (CVD) on tungsten carbide WC-Co cutting tools allow a great improvement in the tool performance by extending its lifetime, increasing the machining speed, and providing a better quality of the machined material [1]. The application of bilayered or multilayered micro- and nanocrystalline DCs further enhances the cutting tools’ functional properties [2,3]. The upper nanocrystalline layer provides low roughness and low friction coefficient, as well as high bending strength [4]. The bottom layer is typically deposited in the microcrystalline diamond growth regime since it has better adhesion to the WC-Co substrate, Crystals 2017, 7, 166; doi:10.3390/cryst7060166 www.mdpi.com/journal/crystals Crystals 2017, 7, 166 2 of 12 coupled to higher hardness [5]. Moreover, compared with single-layer DCs, bilayered DCs have higher heat conductivity and increased resistance to cracking [6]. The structure of a DC deposited in microwave plasma can be controlled by varying the composition of the gas mixture during the process run. For example, the addition of nitrogen allows a transition from microcrystalline diamond (MCD) to nanocrystalline diamond (NCD) coating deposition regimes. As a result, one can obtain smooth bilayered DCs with increased wear resistance by the CVD method for an optimal nitrogen concentration [3]. The N2 gas added to standard CH4-H2 mixtures strongly enhances the growth rate SC diamond seeds (typically of {100} orientation) [7,8], thus reducing the cost of CVD diamond production. In case of epitaxial growth of single crystals the growth rate and defect abundance depend on the diamond substrate face orientation, a less number of the defects forming on {100} facets [9]. Particularly, dislocation density can be significantly reduced by a lateral growth on {100} oriented substrate with a hole intentionally perforated in it as recently shown by Tallaire et al. [10]. The growth of nanocrystalline diamond on a microcrystalline diamond film has several differences from the more widely used growth on non-diamond substrates, including: (a) significant roughness of the microcrystalline film surface; (b) microcrystalline film is formed by crystallites with different faces, predominantly with {111} and {100} [11]; (c) no preliminary nucleation (seeding) is performed; and (d) there is no incubation period for growth [11]. This raises the question of possible differences of secondary nucleation and growth of nanocrystalline DCs on different faces of microcrystals of diamond films. We note that the deposition of ultrananocrystalline diamond (UNCD) films with grain typically, less than 10 nm, in a multicomponent gaseous environment, Ar-CH4-H2+N2 mixtures, on polished coarse-grained polycrystalline diamond plates (the grains with random orientation and size of 60–90 µm) was reported in [12]. The UNCD growth was assumed to start epitaxially, but the high rate of secondary nucleation very rapidly transformed the film to nanocrystalline structure uniformly over the substrate area. No impact of grain orientation of the substrate on the morphology of the deposited UNCD layer was observed. In the production of bilayer coatings of such type, it is important to deposit the NCD layer uniformly on all facets of the grains in underlying MCD film, independent on the facet orientation. The goal of the present study is the investigation of the nucleation and growth processes of DCs on different facets of a diamond single crystal as a model substrate, and examination of the growth defects on the {100} face with SEM and photoluminescence (PL) spectroscopy. To this end, we prepare faceted SC diamond substrates to deposit the diamond film simultaneously on facets of different orientations and compare the resulting morphologies for identical process parameters. 2. Experimental The diamond coatings were synthesized in “hydrogen/methane/nitrogen” gas mixtures on SC diamond substrates using a microwave plasma CVD system ARDIS-100 (2.45 GHz, 5 kW) [13] with the following parameters: total gas flow rate of 500 sccm (H2:460/CH4:20/N2:20), pressure of 130 Torr, and microwave power of 2.8 kW. The substrate temperature during deposition was about 800 ◦C as measured through plasma by the Mikron M770 two-color pyrometer. These deposition parameters and the nitrogen concentration in the gas mixture corresponded to deposition regime for nanocrystalline DCs on WC-Co substrates [3]. As the substrate, we used a multifaceted type IIa synthetic SC diamond produced by the high pressure—high temperature (HPHT) gradient method. The upper part of the crystal exhibited smooth {311}, {211}, {111}, and {110} growth faces, with the larger {100} face on the top. This multifaceted part was cut off along the {100} plane to remove the bottom part containing sides with negative slope. The top and bottom {100} planes were polished to roughness Ra of less than 10 nm, as measured with an optical profilometer (ZYGO NewView 5000), while the other facets remained unpolished. The roughness Ra of the as-grown faces was 90–100 nm for {111}, 120–130 nm for {211}, 160–180 nm for {311} and 100–110 nm for {110} planes. The roughness was mainly due to growth steps on those surfaces. The substrate thickness was 0.9 mm, which provided almost identical Crystals 2017, 7, 166 3 of 12 growth conditions in the CVD reactor, with a minimized temperature gradient across the substrate. At least the adjacent regions around common edges between the facets had identical temperatures during diamond deposition. A scanning electron microscope (SEM) JSM7001F (JEOL) was used to examine the coating surface morphology. The Raman and PL spectra, excited at 473 nm wavelength, were measured at room temperature with a LabRam HR840 (Horiba Jobin-Yvon) spectrometer in a confocal configuration with the spectral resolution of 0.5 cm−1 and spatial resolution of ~1 µm. The spatial mapping of the PL and Raman spectra over the coating surface area was carried out by placing the sample on a motorized table (MärzhäuserWetzlar) with positioning accuracy of about 0.5 µm. 3. Results and Discussion 3.1. Diamond Deposition on Low-Index Facets of the Single Crystal Substrate The SEM analysis shows that, under the growth conditions of nanocrystalline diamond (if a non-diamond substrate material is used), the NCD coating is formed on the {111} and {110} faces of the substrate from the very beginning. This coating is formed of grains with the size of a few tens to hundreds of nanometers. Figure1 presents SEM images of several regions of the sample after 5 min of growth. The panoramic image in Figure1a displays seven different faces on the HPHT diamond substrate. The original {311} and {211} faces did not contain regions with outcrops of {100} and {111} planes. However, clearly pronounced layers with {111} planes appear on those faces even at the initial growth stage, and, flat regions with {100} planes are formed, on the top of these layers. Figure1b shows an example of surface relief formed on the {311} face. The surface morphologies of the coating grown on the {100} and {111} planes are essentially different: the {100} planes are smooth, and homoepitaxial growth in the step bunching regime [14] occurs on these planes, whereas, on the {111} planes nanocrystallites arise and develop into a rough film surrounding the {100} domains.
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