Supercycle Atomic Layer Deposition of Ternary Oxides for the Synthesis of High-Temperature Stable Photonic Crystals
Inverse opals—photonic crystals based on a periodic arrangement of spherical pores embedded in a dielectric matrix—present a selective behavior in the reflection of electromagnetic radiation while the propagation of light within a certain spectral range is prohibited. However, when such porous microstructures based on ceramics are subjected to high temperatures, the stability of the matrix material as well as of the periodic microstructure are a key prerequisite to maintain their photonic properties. A “failure” in one of these both aspects, e.g. induced by phase transitions, grain growth or mass transport, leads to a significant decrease of their optical functionality.
In the past, we have shown that low-temperature atomic layer deposition (ALD) can be utilized to prepare inverted TiO2 opals with a well-defined photonic stop gap. However, when exposed to temperatures above 600 °C in air, the anatase-to-rutile phase transformation leads to massive structural changes impairing the ordering and as a consequence the photonic properties.
To overcome this obstacle, we used a supercycle, stop-flow mode ALD approach, which combines two binary oxide processes to synthesize ternary oxides. In detail, a superposition of individual ALD half-cycles or sub-nm sized laminates of TiO2, SiO2, and Al2O3 was used to tailor the ratio of the ions within Al-doped titania and to reach the stoichiometric range for the formation of monophasic mullite—an aluminum silicate, respectively.
X-Ray diffraction, spectral ellipsometry, and UV/VIS spectroscopy were used to investigate the ternary thin films and inverse opals before and after high-temperature exposure.
Our experimental results revealed that the super cycle ALD processes successfully generated ternary oxides, later used to infiltrate the polymeric templates. In both cases, the performance of ternary oxides surpasses that of pure binary materials: First, we could use aluminum-doping of titania to shift the anatase-rutile phase transition to temperatures above 800 °C leading to increased stability of the microstructure. Second, by matching the supercycle ratio of the silica and the alumina ALD process, we achieved the stable crystalline mullite phase which allowed us to prepare inverted photonic crystals which are stable up to 1400 °C outperforming the pure alumina counterpart.