Low‐Temperature Mullite Formation in Ternary Oxide Coatings Deposited by ALD for High‐Temperature Applications

Atomic layer deposition (ALD) process presents thickness control in an Ångstrom scale due to its inherent surface self‐limited reactions. In this work, an ALD super cycle approach, where a superposition of nanolaminates of SiO 2 and Al 2 O 3 is generated by cycling the precursors APTES–H 2 O–O 3 and TMA–H 2 O, is used to deposit thin films with varying ratio of Al 2 O 3 :SiO 2 into silicon wafers and into inverse photonic crystals. The resulting ternary oxide films deposited at low temperature (150 °C) are amorphous. However, conversion to mullite occurs at 1000 °C, way below the conversion temperatures found into powder processing or diphasic sol–gel routes (type II) and comparable to monophasic sol–gel synthesis. By means of such a mullite coating, the structural stability of an Al 2 O 3 inverse photonic crystal is increased up to 1400 °C.