ICMCTF2014 Session B2: CVD Coatings and Technologies

Niobium nitride (NbN) is a well-known superconductor and is considered to be one of the promising cryoelectronic materials. Till now, the thin films necessary for superconductive devices are mostly synthesized by physical vapor deposition methods. The aim of this study is to introduce chemical vapor deposition technique, CVD, as an alternative technique to process superconducting niobium nitride thin films.

In this work, NbN thin films have been grown by CVD from ammonia NH₃ (99.999 %) and Nb chlorides species NbCl_x, in situ produced via chlorination of high purity Nb wire (99.999%) with chlorine gaz Cl₂ (99.999%). Substrates are C-plane (0001) monocrystalline hexagonal sapphire, A-plane (11-20) monocrystalline hexagonal sapphire, (111) monocrystalline cubic silicon carbide. Deposition temperature has been varied between 900°C and 1300°C. Without any post-deposition treatments, the thin films have been characterized by means of SEM (Scanning Electron Microscopy), XRD (X-Ray Diffraction), XRR (X-Ray Reflectometry), Raman spectroscopy and EPMA (Electron Probe Micro Analysis). In this presentation, the influence of experimental parameters (temperature, composition of the gas phase, substrate) on the composition of the thin film and its structural properties (crystal structure, growth direction, lattice parameter, orientation relation between the substrate and the thin film) will be given. In particular, the stability of the cubic structure, the only structure which has the superconductive properties, will be discussed regarding the growth parameters. Finally, the potentiality of the CVD technique to synthesize high quality niobium nitride thin films and multilayers structures will be presented.

Due to the characteristic and attractive combination of the high strength and low weight (high strength-to-weight ratio), the composite materials - carbon reinforced fiber plastics (CFRP) and CFRP stacked with aluminum and titanium - are used as construction material in aerospace industry. The machining of these extreme abrasive composite and sandwich materials brings new challenges for the modern cutting tools. These are the surface quality of the workpieces without fiber delamination or uncut fibers and the rapid tool wear. It has been demonstrated that diamond coated cutting tools are the proper choice for such applications, due to their outstanding hardness and reduced tendency to stick of the workpiece material.

Functionalization of aluminium nitride grown by high temperature chemical vapor deposition

¹Science et Ingénierie des Matériaux et des Procédés, SIMAP, Grenoble INP-CNRS-UJF, BP 75, 38402 Saint Martin d’Hères, France

²SPADE, 73800 Ste Hélène du Lac, France

^*michel.pons@simap.grenoble-inp.fr

The application of AlN films in optoelectronics, sensors and high temperature coatings is strongly dependent on the nano- micro-structure of the film, impurity level and defect density. AlN epitaxial thin (0.5 – 10 µm) and thick polycrystalline (> 10 µm) films were grown on different foreign substrates (sapphire, silicon carbide, graphite) and single AlN crystals by Chemical Vapor Deposition (CVD), also called Hydride Vapor Phase Epitaxy (HVPE), at high temperature (1200-1750 °C). In the first part of this paper, polycrystalline growth of thick films (>10 µm) prepared at high growth rate (>100 µm.h^-1) was performed on graphite substrates to study the preferential orientation of the films. AlN/W multilayers were deposited on silicon carbide composites to increase their performance at high temperature in aggressive conditions. Such multilayer materials can be used for the cladding of nuclear fuel. The second part of this paper concerns the characterization of epitaxial films, including their crystalline state, surface morphology, and inherent and thermally induced stress which inevitably leads to high defect densities and even cracking. The full-width at half-maximum (FWHM) of X-Ray rocking curves of the grown AlN layers exhibited very large values (several thousand arcsec), and they became steeply deteriorated with increasing growth rate. To improve the crystalline quality of AlN layer, well-known growth techniques, such as multi-step growth using buffer layers, were used at temperatures above 1200 °C in order to lower the disorientation to 300 arcsec. The applications of such “templates” for deep UV light emitting diodes (UV LED) and surface acoustic wave sensors (SAW) are discussed.

The tandem solar cell was commonly fabricated by stack of hydrogenated micro-crystalline silicon (μc-Si:H) thin film for the bottom cell and amorphous silicon (a-Si:H) thin films for the top cell. The μc-Si:H can absorb light in higher wavelength towards the infrared region of the solar spectrum and has excellent stability against light soaking. However, the indirect optical transition of μc-Si:H is not efficiently absorbed sunlight, film thicknesses greater than 2 μm. To realize low-cost fabrication of solar cell with the plasma-enhanced chemical vapor deposition (PECVD) processes at low temperature, the high-rate growth of high-quality μc-Si:H films is required.

To date, the growth rate of approximately 2 nm/s was realized using a capacitively coupled plasma (CCP) system with a very high frequency (VHF)-power source and a high working pressure (ca. 1000 Pa) with a narrow electrode gap. In particular the relatively high working pressure, the high electron density is demanded on achievement effectively the SiH₄ depletion. Moreover, the H radicals are recognized as a key factor that influences the crystallinity of Si thin films. Sufficient supply of H radicals to a growing Si thin film surface induces crystallization.

The electron density in the SiH₄/H₂ plasma region was measured using a 35 GHz microwave interferometer. A multi-channel spectrometer was used to observe the optical emission intensity of the Si* (288 nm) and SiH* (414 nm) lines. The absolute density of H radicals was measured by vacuum ultraviolet laser absorption spectroscopy (VUVLAS).

The crystallinity factor, preferential orientation, defect density, micro-structure, and the post-deposition oxidation of deposited Si thin films were investigated by Raman spectroscopy, XRD, ESR, TEM, and FTIR, respectively. For the fabrication of Si thin films for solar cell devices to achieve selective enhancement of the H radical densities for crystallization of the films under low depletion of SiH₄. From these results, we will discuss effects of those radicals on surface reaction in μc-Si:H deposition.

The microstructure and wear mechanisms of texture controlled CVD α-Al₂O₃ layers with (0001), (01-12), and (10-10) growth textures were investigated in single point turning of AISI 1045. A wide range of cutting speed conditions and tool geometries were investigated to achieve broad combination of mechanical and thermal loads on the cutting tools that were also assessed experimentally.

The introduction of texture-controlled deposition processes is one of the most important recent developments in CVD of α-Al₂O₃. Consequently, today it is possible to deposit α-Al₂O₃ coatings with several preferred growth directions e.g. (012), (110), (001) and (100). α-Al₂O₃ layers with 001 texture have been shown to be superior to all the other textures and randomly oriented α-Al₂O₃ coatings in many metal cutting applications and several manufacturers have introduced products based on this technology.

Grain boundary engineering is a promising way to further enhance the performance of the textured α-Al₂O₃ coatings and grain boundaries in CVD a-alumina have attained growing interest during the last few years. By controlling the preferred growth direction in the α-Al₂O₃ during deposition the grain-boundary distribution and morphology can be modified and controlled.

In this paper experimental α-Al₂O₃ coatings with different grain boundary structures and textures will be presented and discussed. The experimental α-Al₂O₃ were deposited on Ti(C,N) coated cemented carbide inserts to identical thickness for comparative cutting tests. In addition to the cutting tests, the coatings were analyzed in detail by using SEM, TEM, XRD and EBSD.

Chemical Vapor Deposition (CVD) method has been used for the industrial production of wear resistant coatings on cutting tools and Al₂O₃ coatings have been widely used to maintain high hardness and excellent oxidation resistance under such a severe cutting condition. It is well known that Al₂O₃ exhibits a number of crystalline polymorphs, such as α, κ, δ, θ, etc. However, there are few reports on crystalline-amorphous phase mixed Al₂O₃ thin films as wear resistant coatings.

In this work, we investigated growth mechanism of amorphous phase mixed α-Al₂O₃ thin films deposited using gas mixtures of trimethylaluminium, as aluminum precursor, O₂, and Ar instead of conventional gas mixtures, such as AlCl₃, CO₂, and H₂. The deposited Al₂O₃ layer is composed with high aspect ratio columnar-like α-Al₂O₃ grains and the amorphous Al₂O₃. The amorphous phase mixed α-Al₂O₃ thin films were prepared with several interface layers to control orientation and morphology of the α-Al₂O₃ grains. The α-Al₂O₃ grains show anisotropic growth enhanced along a-axis and suppressed along c-axis depending on the prepared interface layers.

Thin films of boron nitride in the sp²-hybridized state (sp²-BN) is promising material for electronic and optoelectronic devices to be operated at high temperatures and in chemically demanding environments. This is due to high thermal and chemical stability, a wide band gap (~ 6 eV), low dielectric constant and the possibility of both p- and n-type doping.

A further advantage for sp²-BN is that the hexagonal basal plane exhibits the same structure as graphene, where the carbon atoms have been replaced by half nitrogen and half boron atoms. As a result the lattice mismatch between sp²-BN and graphene is only 2%. The structural similarity to graphene suggests additional applications as a dielectric substrate and gate dielectric in graphene electronics.

Despite the promising properties, sp²-BN is still the least investigated material among the III-nitrides, mainly due to limitations in depositing thin films with necessary quality.

In this study, we present results from growth of sp²-BN films by Chemical Vapor Deposition (CVD) focusing on the quality of sp²-BN films as a function of process temperature, precursor mixture as well as the effect of adding silicon during growth. .

The deposition of the films was conducted in a horizontal hot-wall CVD reactor using triethyl boron (TEB) and ammonia (NH₃) as boron and nitrogen precursors. As substrate 0001 oriented a-Al₂O₃, 0001 oriented hexagonal silicon carbide (SiC) and 111 oriented cubic SiC were used. We have previously shown that an AlN buffer layer is necessary for the growth of rhombohedral BN (r-BN) of good quality on a-Al₂O₃ [1]. Our recent results obtained on SiC substrates suggest opposite behavior, when AlN is in-situ deposited on SiC prior to the BN deposition, i.e. AlN buffer layer is not necessary but even makes it impossible to obtain high quality sp²-BN on SiC. Our results show that successful deposition of epitaxial r-BN is confined to a narrow process parameters window: T = 1500 °C, nitrogen-to-boron ratio of 600-700, hydrogen as carrier gas at a process pressure of 70-100 mbar [2]. In addition, we found that presence of a few parts per million (ppm) of Si is needed for the epitaxial growth of r-BN on sapphire [3] and that it improves quality of the sp²-BN on the SiC substrate.

[1] M. Chubarov, H. Pedersen, H. Högberg, V. Darakchieva, J. Jensen, Per O. Å. Persson, A. Henry,Phys. Stat. Sol. RRL5 (2011), 397

[2] M. Chubarov, H. Pedersen, H. Högberg, J. Jensen, A. Henry, Cryst. Growth Des.12 (2012), 3215

[3] M. Chubarov, H. Pedersen, H. Högberg, A. Henry, CrystEngComm15 (2013), 455