Thermally-induced structural evolution and optical stability of Al2O3 anti-reflection thin film on 4H-SiC

Abstract

Monocrystalline 4H-SiC is a promising optical window material for extreme environments; however, its application is limited by high Fresnel reflection and a lack of understanding regarding its high-temperature optical-structural degradation in oxidizing atmospheres. In this study, we systematically investigate the evolution of optical properties, morphology, and structural integrity of single-layer Al₂O₃ anti-reflection (AR) thin films deposited on 4H-SiC via electron beam evaporation under air annealing up to 1200 °C. The double-sided Al₂O₃ films successfully suppress surface reflections, enhancing the average visible transmittance from 65% to over 90%. The Al₂O₃/4H-SiC system exhibits stable structural integrity and broadband optical tolerance up to 1000 °C. Even after 20 h of annealing, no detectable crystalline Al₂O₃ phase is observed, with the system retaining a 98.2% peak and an 84.4% broadband average transmittance across 380–2000 nm. However, a critical operational threshold was identified at 1200 °C, where the average visible transmittance severely drops by approximately 40%. Mechanistic analysis reveals that this failure is primarily driven by solid-state dewetting and thermally activated surface diffusion, which cause mass redistribution and the formation of surface depressions (100–200 nm deep). Concurrently, thermal expansion mismatch and interfacial reactions induce micro-cracking and void formation. Ultimately, these structural defects compromise the morphological integrity of the film, leading to the loss of its AR function. This work presents a feasible broadband AR approach for 4H-SiC and clarifies its high-temperature failure mechanisms, providing a reference for extreme-environment optical applications.