Structure and infrared optical properties of evaporated erbium fluoride films

Abstract Erbium fluoride (ErF₃) films were deposited on germanium substrate with different substrate temperatures. The films deposited at low substrate temperatures show amorphous structure, while those deposited at high substrate temperatures show polycrystalline structure with (002) preferred orientation. The roughness of the film increases from 0.47 nm for the amorphous film to tens of nanometres for the crystalline films deposited at high substrate temperatures. Furthermore, it has been found that the surface morphology of the crystalline ErF₃ films is modulated by the tensile stress in thin films. The infrared optical constants of the films were determined by fitting the transmission spectrum in the wavelength range from 2 to 10 μm using the Lorentz oscillator model. Both the refractive indices and extinction coefficients decrease with increasing substrate temperature. The minimum refractive index and extinction coefficient of the ErF₃ film at a wavelength of 10 μm is as low as n = 1.32 and k = 0.006, respectively. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)