M-line spectroscopy for optical analysis of thick LiNbO3 layers grown on sapphire substrates by radio-frequency multistep sputtering

Abstract

Thick films of lithium niobate (LiNbO3) have been epitaxially grown on sapphire substrates by multistep radio-frequency magnetron sputtering. We have investigated the optical properties of the deposited layers by the m-line spectroscopy technique. The relationship between the film-substrate interface, the surface morphology of the LiNbO3 layer, and the optical measurements has been carefully established. Since this optical technique is sensitive to material defects, a detectable degradation of the optical data is observed when the microstructure of the LiNbO3 film changes. In particular, the angular width of the excited optical guided modes (Δθ) varies with the film thickness. This agrees perfectly with the evolution of the surface roughness as a function of the film thickness. We report a surface roughness rms of 2 nm corresponding to an angular width Δθ of 0.1° for the TE0 mode. The accuracy and the sensitivity of this technique can be advantageously compared to the classical tool of atomic force microscopy.