Reduced optical losses in MOCVD grown lithium niobate thin films on sapphire by controlling nucleation density

Lithium niobate (LiNbO3) epitaxial thin films of high crystalline quality have been grown on c-plane sapphire substrates by the solid-source MOCVD method. For waveguiding applications, low optical propagation losses (less than 2 dB/cm) are required, and this has been difficult to achieve in films grown by various vapor growth techniques. Since scattering from rough film surfaces is a major source of optical attenuation, a knowledge of the mechanisms involved in film roughening and their relationship with the initial stage of film formation is essential. In this study, it was observed that roughening and optical losses in lithium niobate films grown on sapphire substrates increased with grain size and the depth of grain grooves formed between them. Increasing the nucleation density during the initial stage of growth can inhibit grain growth, and thereby reduce surface roughness even at high growth temperatures. Based on these results, a two-step growth process was developed in which the nucleation and growth stages were controlled separately. This process involves (1) creating a high nucleation density by using either a low deposition temperature or high source partial pressure during the early stages of film growth, and (2) enhancing crystallinity by growing the balance of the film at a higher temperature or lower source partial pressure. Using this refined growth process, rms roughness was decreased to less than 2 nm, while the crystalline quality remained equal to the best films grown by the conventional one step growth method. Optical losses were reduced to below 1.8 dB/cm for the TEo mode at a wavelength of 632.8 nm.