Temperature-Dependent Growth Mechanisms and Optical Properties of MgF₂ Thin Films Synthesized by Plasma-Enhanced Atomic Layer Deposition

MgF₂ films are prepared using plasma-enhanced atomic layer deposition (PEALD). The influence of substrate temperature on the growth behavior, chemical composition, and optical properties of MgF₂ films is systematically investigated. The experimental results show that the deposition process transitions through three distinct regimes: an incomplete-reaction regime at 100 °C, a self-limiting ALD window at 125–150 °C, and a chemical vapor deposition (CVD)-like regime above 175 °C. At 100 °C, incomplete surface chemistry yields low growth-per-cycle, carbon incorporation, and an elevated refractive index. Within 125–150 °C, films are near-stoichiometric, smooth, and exhibit a low refractive index ≈ 1.37 ± 0.003 at 550 nm. Above 175 °C, precursor decomposition drives non-self-limiting growth with increased roughness. As an application-level validation, a film grown at 125 °C used as a double-sided antireflection coating on glass increases transmittance from 92 ± 0.1% (bare) to 97.2% ± 0.2% at 550 nm. The average transmittance of 96.4 ± 0.2% over 380–780 nm can be achieved. Overall, this work establishes the relationship between deposition temperature and PEALD-MgF₂ film properties and demonstrates precise, low-temperature, non-corrosive deposition suitable for advanced optical antireflection coatings.