Study of mechanical degradation of freestanding ALD Al2O3 by a hygrothermal environment and a facile protective method for environmentally stable Al2O3: toward highly reliable wearable OLEDs

Abstract

Al₂O₃ deposited via atomic layer deposition (ALD) has been used as an insulating and barrier film for thin-film transistors, organic electronics, and microelectromechanical systems. However, ALD Al₂O₃ films are easily degraded by hydrolysis under harsh hygrothermal conditions, owing to their poor environmental stability. In this study, the mechanical properties and water-vapor transmission rate (WVTR) of environmentally degraded Al₂O₃ films were investigated by varying the temperature and relative humidity (RH). The hygrothermal environment led to surface and pinhole-concentrated degradation based on aluminum hydroxide, which caused an increased WVTR and reduced elongation of the films in harsher environments. In particular, the elongation of the degraded Al₂O₃ films was reduced to 0.3%, which is one-third of that of as-deposited Al₂O₃, and their WVTR increased on the order of 10⁻¹ g m⁻² day⁻¹, which is more than 1000 times that of as-deposited Al₂O₃. Therefore, we introduced a functional silane-based inorganic–organic hybrid layer (silamer) onto the Al₂O₃ films to improve their environmental stability. The silamer helped preserve the characteristics of Al₂O₃ films by forming a strong and continuous aluminate phase of Al–O–Si at their interface in hygrothermal environments. Furthermore, the silamer-capped Al₂O₃ was shown to be an environmentally stable encapsulation for application in wearable organic devices.