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Y 2 O 3 films with different microstructures were deposited through radio frequency magnetron sputtering. The optical properties and the antireflection effects of the cubic and the monoclinic Y 2 O 3 films on diamond substrates were investigated. Results showed that the cubic Y 2 O 3 film had denser and more uniform grains with larger grain size. The band gap energies of the cubic and the monoclinic Y 2 O 3 films were 5.79 and 5.88 eV, respectively. This finding was mainly attributed to the decrease in grain size from 30.1 nm to 20.6 nm. Meanwhile, the reflective indices of the cubic and the monoclinic films were 1.887 and 1.817, respectively. Furthermore, the monoclinic and the cubic Y 2 O 3 films that have the same thickness generated antireflection effects on diamond at the 8–12 μm long-wave infrared (LWIR) range. However, due to the lower reflective index of the monoclinic Y 2 O 3 film compared with that of the cubic Y 2 O 3 film, the maximum antireflection effects of the Y 2 O 3 films on diamond shifted to lower wavelengths. [ABSTRACT FROM AUTHOR]

• 21% increase of maximum transmittance of Er2O3 films deposited on diamond coatings. • Negative electrical bias induces the formation of monoclinic Er2O3 on Si(110). • Monoclinic Er2O3 effectively improve the transmittance of the diamond coating. Er 2 O 3 anti-reflection films were deposited on the surface of the chemical vapor deposited diamond films and Si(100) substrates by radio frequency magnetron sputtering. The microstructure of the Er 2 O 3 thin films and the interfacial layer were investigated by means of scanning electron microscopy, X-ray diffraction, atomic force microscopy and transmission electron microscopy. The refractive index, hardness and transmittance of the films were measured by ellipsometry, nanoindentation measurements, and Fourier transformed infrared spectroscopy. Results revealed that the bias induces the formation of a monoclinic phase and the nucleation of the Er 2 O 3 thin films is mainly observed along the (222) plane. A high-resolution TEM image revealed a 2.5-nm-thick amorphous layer at the interface between the Si substrate and Er 2 O 3 film. Notably, monoclinic Er 2 O 3 films can considerably improve the transmittance of the diamond film in the long-wavelength infrared range of 8–12 μm via the reduction of the refractive index. However, the introduction of the monoclinic phase led to the decrease of the hardness and elastic modulus of the films; hence, controlling a suitable amount of the monoclinic phase may play an extremely important role in the mechanical and optical properties of the Er 2 O 3 films. [ABSTRACT FROM AUTHOR]