ICMCTF2015 Session B2-2: CVD Coatings and Technologies

B and N atoms are closest neighbors to C in the periodic table which means that BN is isoelectronic to C and exhibits many properties similar to carbon. The atomic orbitals in BN can be sp² or sp³ hybridized and in the sp²-BN BN can form hexagonal (h-BN) or rhombohedral (r-BN) forms, with structural similarities to graphite. sp²-BN attracts significant interest due the combination of properties such as wide bandgap (~ 6 eV), possibility both p- and n-type doping, chemical and thermal stability, low refractive index of around 2 and high in-plane thermal conductivity of 390 W/m*K. Hence many applications within electronics and optoelectronics are envisioned for sp²-BN. However, a great hurdle is deposition of high quality epitaxial sp²-BN thin films. There are reports on growth of sp²-BN on different substrates but determining the crystalline structure of such films is challenging. This is due to the fact that the in-plane lattice constants are the same (2.504 Å) for both crystal structures as the spacing between basal planes which is around 3.333 Å. Thus, it is impossible to distinguish between c-axis oriented h-BN and r-BN thin films by using X-ray diffraction (XRD) in Bragg-Brentano geometry and requires atomic resolution investigation by cross-section transmission electron microscopy (TEM) or more advanced diffraction experiments.

We presented deposition of high quality r‑BN thin films on a‑Al₂O₃ and different polytypes of SiC by CVD [1,2]. Here we present a study of the nucleation, film development and crystal structure development at the early stages of sp²-BN thin films growth on (0001) a-Al₂O₃ with AlN buffer layer (AlN/α-Al₂O₃) and (0001) 6H‑SiC. In TEM we have observed formation of different crystal structures; AlN buffer layer promotes growth of h-BN to a thickness of around 4 nm after which a transition to r-BN growth occurs. On SiC , polytype pure r-BN is obtained. Formation of h-BN on AlN/α-Al₂O₃ was also observed using glancing incidence XRD (GI-XRD). The growth of h-BN is characteristic for the AlN/α-Al₂O₃ and was found to form even at a temperature of 1200 °C. For SiC substrate, r-BN was observed at the growth temperature of 1500 °C while no traces of crystalline sp²-BN formation were found below 1500 °C. The observed growth behavior is explained by the reproduction of the substrate crystal structure by the growing film. This explanation is also suitable to describe twinning of the r-BN crystal and observed suppression of twinning.

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

[2] M. Chubarov, H. Pedersen, H. Högberg, Zs. Czigany, A. Henry, CrystEngComm., 16 (2014) 5430)

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 (steel, sapphire, silicon carbide, graphite) and single AlN crystals by Chemical Vapor Deposition (CVD), also called Hydride Vapor Phase Epitaxy (HVPE), at high temperature (800-1750 °C). In the first part, the modeling and simulation of the growth process is presented. The growth of polycrystalline or single-crystalline AlN was studied as a function of local parameters (temperature, concentrations and local supersaturation) just above the substrate, then reactor-independent. Two regions were defined by comparing experimental and numerical results. In the second part, the two-step growth process resulting from previous optimization is presented. A 170 nm AlN nucleation layer is first grown on sapphire at 1200 °C (N/Al ratio of 3) before the deposition of a thicker film at 1500 °C (N/Al ratio of 1.5). The characterization of epitaxial films, including their crystalline state, surface morphology, and inherent and thermally induced stress which inevitably lead to high defect densities and even cracking reveals that (i) there is no etching at the sapphire/AlN interface, (ii) the stress in the nucleation layer is tensile and compensated by the compressive thermal stress during the cooling down of the process. The best "quality" we have obtained with the optimized two-step process is characterized (i) by Full-Width-Half-Maximun of (0002) reflexion of XRD rocking curves (disorientation of 300 arcsec, dislocation density ~10⁹ cm^-2) (ii) by the frequency shift of E2(high) phonon mode of Raman spectra (643.9 cm^-1 for the nucleation layer, 658.2 for the thick layer and 657.4 for unstressed). The resulting calculated stresses are 1.6 GPa (tensile) for the nucleation layer and -1 GPa for the final layer (compressive).

In the third part, the applications of graphite/AlN, SiC/AlN and steel/AlN stacks are briefly presented for potential nuclear and solar applications. AlN is an excellent barrier and buffer layer to avoid diffusion or oxidation of metals and carbides. The applications of sapphire/AlN templates for deep UV light emitting diodes (UV LED) and surface acoustic wave sensors (SAW) are discussed. The "quality" of the AlN layer is not sufficient to have an emission in the UV at 210 nm. However, for surface acoustic wave devices fabricated on the AlN/sapphire stack exhibit phase noise lower than -150 dc/Hz at 4.4 GHz.

Ti_1-xAl_xN coatings used for high-speed cutting applications are expected to withstand high temperatures, exceeding even 1000 °C, as well as high mechanical loads, making both good oxidation resistance and high hardness a requirement. In this contribution, properties of novel Ti_0.05Al_0.95N coatings [1] with a self-organized lamellar nano-composite microstructure synthesized using a low-pressure CVD process will be discussed. Their unique morphology comprises alternating lamellae of soft hexagonal w-AlN and hard cubic fcc-TiN with a bi-layer period of about 13 nm, confined in grains of approximately 150 nm diameter.

Coatings grown on WC-Co substrates were oxidized for 1 h in ambient air at temperatures in the range of 700 – 1200 °C and temperature-dependent changes of microstructure, phase composition, hardness and residual stresses were examined using a variety of analytical techniques, including depth-resolved nanoindentation and depth-resolved cross-sectional X-ray nanodiffraction.

The residual stresses in the as-deposited and annealed coatings are compressive at about ‑1 GPa, which is contrary to the tensile stress state observed in most of the common CVD coatings. The presented results show excellent oxidation resistance up to 1050 °C and a comparatively high hardness of about 29 GPa that remains almost constant even after high temperature treatments. Both characteristics are explained by the unique self-organized and surprisingly stable nano-scaled microstructure of the coating.

[1] J. Keckes, R. Daniel, C. Mitterer, I. Matko, B. Sartory, A. Köpf, R. Weissenbacher, R. Pitonak (2013), Self-organized periodic soft-hard nanolamellae in polycrystalline TiAlN thin films, Thin Solid Films 545, 29-32.

To improve the abrasion, scratch and chemical resistances of polycarbonate, which can be used as a replacement for glass, silicon oxycarbide films were deposited on it at low temperatures by using a SLAN ECR plasma. The SLAN ECR plasma source has a high plasma density and can thus rapidly deposit films with good mechanical properties at low temperatures. Before the film deposition, Ar and O₂ plasma treatments were performed for 1 min each in order to improve the adhesion between the film and the polycarbonate substrate, and then the silicon oxycarbide films were deposited from gas mixtures of octamethylcyclotetrasiloxane(OMCTS), O₂, and Ar gas onto polycarbonate and c-Si substrates at room temperature without heating the substrates. Pressure, gas flow rate (OMCTS, O₂, Ar), and deposition time have been fixed at 10 mTorr, 10 sccm, and 5 min, respectively. The properties of deposited thin films were observed by varying the plasma power from 1.2 to 2.0 kW. The transparent silicon oxycarbide films were deposited, and the average transmittance over the visible range exceeds of 90 % for all samples regardless of the deposition conditions. The deposition rates of the films are high, more than 500 nm/min. As the plasma power increases, the deposition rate is increased. When O₂ and Ar are used as the plasma gas at plasma power of 2.0kW, the maximum deposition rates are 713.2 nm/min and 1058.8 nm/min, respectively. The hardness of the films also increases as the plasma power increases. When O₂ and Ar are used at plasma power of 1.7kW, the maximum hardnesses are 3.06 GPa and 2.14 GPa, respectively, but for plasma power of 2.0kW, there is a slight reduction. To evaluate the abrasion resistance of each film, the change in the haze of its abrasion surface was investigated by using a Taber abrasion tester at standard abrasion test condition (500 g weights on both sides, CS 10-F wheels, 50 rpm, 100 cycles). The average value of the haze is 0.67% at plasma power of 1.7kW, and this value is similar to a change in haze of glass, 0.69%. The films, which are deposited at plasma power of 2kW, also have the average value of the haze less than 2%. These results satisfy the abrasion condition (haze < 2%), which must be satisfied in order to use plastic instead of glass for automobile. To evaluate the adhesion of deposited films, the adhesion test was examined by ASTM D3359-02. The films were attached well to the polycarbonate even the absence of the buffer layer. As a result, the polycarbonate coating technology was successfully developed, and this technology will contribute to replace for glass in the use of construction and automobile industries.