Switchable yttrium–hydride mirrors grown on CaF₂ (111): A x-ray photoelectron spectroscopy and diffraction study

The epitaxial growth of Y (hydride) films on CaF₂ (111) has been investigated using x-ray photoelectron spectroscopy, x-ray photoelectron diffraction, and low energy electron diffraction (LEED). For Y deposition at 700 °C the formation of high-quality epitaxial hcp(0001) oriented Y films is observed. Whenever the Y films showed good surface quality, the surface is rich in F. Only when the deposition temperature was chosen so low that the LEED reflexes were very broad was no fluorine detected. This is a strong indication that F acts as a surfactant. For Y deposition at room temperature under a H₂ partial pressure of 5×10^–6 mbar we observe the formation of a F-free, transparent YH_2.3 film of a red/yellow color and poor crystallinity. Hydrogen unloading is accomplished by annealing to 600 °C. The film loses its transparency, the poorly ordered fcc(111) lattice converts to a well-ordered hcp(0001) lattice, and F contamination is restored.