ICMCTF 2022 Session C1-WeM: Optical Materials and Thin Films

Interest in halide perovskite (HaPs) is motivated by the combination of superior optoelectronic properties and ease in synthesizing these materials with a surprisingly low density of electrically active defects. HaPs possess high chemical sensitivity, especially those having an organic cation at their A position (AMX₃). Although a direct role of the A cation in this sensitivity is unclear, and the structural and optoelectronic backbone lie within the M-X bond, the type of the A cation was shown to impact the chemical stability and, usually indirectly, affect optoelectronic properties of HaPs.

Silicon carbide (SiC) is a wide bandgap semiconductor that is currently driving a profound evolution in power electronics, thanks to a unique combination of outstanding physical properties. In addition, SiC also exhibits very promising optical properties, making it very suited for applications in photonics, such as waveguides and frequency combs. For this purpose, amorphous SiC (a-SiC) thin films are deposited at very low temperature with the main challenges being the control of bulk properties and the formation of perfects interfaces. Today, the relationships between the deposition conditions and the optical properties of the films is still not clear. This paper aims to provide a comprehensive picture of these relationships and finally to give some hints for further optimization.

Coatings of a-SiC were deposited by Physical Vapor Deposition (PVD) on different types of substrates, such as sapphire and silica on silicon wafers, and using a polycrystalline SiC target as source material.A design of experiments (DOE) methodology was implemented to identify and weight the main deposition parameters with respect to the optimization of the refractive index and attenuation coefficient of the films, measured by spectroscopic ellipsometry. In parallel, a systematic investigation of the chemical and structural properties of the films was carried out, using a combination of XRD, FTIR, XPS and TEM.

Patterned thin films can have interesting applications in functional glazing provided by their optical and electrical properties. Some examples are: Plasmonic effects of the dewetted structures, anisotropic electrical properties (polarizers) and high resistivity and good transparency, that could be used as transparent electrodes. In this context, dewetting is an interesting way of patterning thin films, including metallic ones.

Although most of the literature work on the laser induced dewetting is on uncapped metallic films (substrate / metallic film) using pulsed laser, the laser induced dewetting can also take place in metallic thin films encapsulated by dielectrics using a continuous wave laser. An example is the stack of Glass / Si₃N₄ / Ag / Si₃N₄ deposited on soda-lime glass by magnetron sputtering, depicting morphologies of lines and islands after laser annealing. According to the temperatures achieved during laser treatment, different mechanisms can take place in continuous wave laser dewetting (solid-state or liquid-state dewetting).

In this stack configuration, our work was interested in studying the different structures that could be obtained. The structuration substantially changed the optical properties of the films. A set of ex-situ characterizations has been done on the stack before and after structuration. The techniques used were: ellipsometry, spectrophotometry, Fourier-transform infrared spectroscopy, SEM, AFM and XRD.

In order to obtain more information on the mechanisms responsible for these structures in our systems, a set of in-situ techniques (fast microscopy and thermal measurements) has been developed and used to obtain the thermal and temporal dependency of the phenomena. In-situ thermal measurements were afterwards confronted to results obtained through numerical simulation. These results showed good accordance for temperatures below the structuration threshold regarding the temporal evolution and the spatial distribution of temperature. For temperatures above the structuration threshold, this is no longer the case. From this point on, the film presented changes in the morphology that affects the optical properties (emissivity and absorption).

The contribution reports our latest results [1-5] concerning energy-saving thermochromic multilayered VO₂-based coatings for smart window applications prepared by reactive magnetron sputtering. First, we show that and how reactive high-power impulse magnetron sputtering with a pulsed O₂ flow control allows reproducible preparation of crystalline VO₂ of the correct stoichiometry under exceptionally industry-friendly deposition conditions: on soda-lime glass substrates without any substrate bias or post-deposition annealing at a low temperature of around 300 °C. Second, doping of VO₂ by W is employed in order to shift the thermochromic transition temperature (68 °C for bulk, 57 °C for our thin film VO₂) toward the room temperature (40 °C for V_0.988W_0.012O₂, 20 °C for V_0.982W_0.018O₂), without concessions in terms of transmittance and its modulation. Third, we employ ZrO₂ antireflection layers both below and above the thermochromic V_1–xW_xO₂ layer, and explain an optimum design of the resulting ZrO₂/V_1–xW_xO₂/ZrO₂ coatings. While utilizing a first-order interference on ZrO₂ leads to a tradeoff between the luminous transmittance (T_lum) and the modulation of the solar energy transmittance (ΔT_sol), utilizing a second-order interference allows one to optimize both T_lum and ΔT_sol in parallel. Fourth, we discuss multilayered designs leading, without any further doping, to thermochromic coatings of optimized color (chromaticity as close to white as possible). The state-of-the-art T_lum and ΔT_sol values achieved under the aforementioned industry-friendly deposition conditions and at lowered transition temperature are in agreement with those predicted during the coating design.

[1] J. Vlcek et al., J. Phys. D Appl. Phys. 50, 38LT01 (2017), 10.1088/1361-6463/aa8356

[2] J. Houska et al., Sol. Energy Mater. Sol. Cells 191, 365 (2019), 10.1016/j.solmat.2018.12.004

[3] D. Kolenaty et al., Sci. Rep. 10, 11107 (2020), doi.org/s41598 020 68002-5

[4] J. Houska, Sol. Energy Mater. Sol. Cells 230, 111210 (2021). 10.1016/j.solmat.2021.111210