Parametric Studies on Suppression of Secondary Phases in LiNbO 3 Thin Films Deposited by Pulsed Laser Deposition
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Parametric Studies on Suppression of Secondary Phases in LiNbO3 Thin Films Deposited by Pulsed Laser Deposition Ji-Won Son, Sergei S. Orlov, Bill Phillips, and Lambertus Hesselink Solid State Photonics Lab, Stanford University, Stanford, CA 94305, U.S.A. ABSTRACT We performed a parametric study to suppress secondary phases in lithium niobate thin films by using pulsed laser deposition (PLD). A KrF excimer laser (λ=248nm) was used as a PLD source and c-oriented sapphire was used as substrates, for waveguide applications. By reducing the growth rate and changing the ambient gas pressure respectively, we found that the main parameter suppressing a secondary phase is controlling the plume strength. To investigate the relation between film phases and the plume strength, extensive parametric studies were performed by changing the oxygen ambient pressure, the target to substrate distance, and also the Li content in the target. The surface morphologies of single phase films and Li-deficient phase mixed films are compared and related to the growth mechanism with a lattice misfit. Deposition parameters to achieve lower loss films are also discussed. INTRODUCTION LiNbO3 has been widely studied for applications in integrated optical devices such as optical switches, modulators, and SHG devices due to the large electro-optic and nonlinear optical effects [1]. Waveguide fabrication is a key technique to construct these devices. Up to now, the mainstream efforts of LiNbO3 waveguide fabrication are based on diffusion processes like Ti indiffusion and proton exchange. However, thin film waveguide fabrication is of particular interest since it has many attractive advantages such as producing a step index profile, freedom in design of new structure with novel functionality, and suitability for integration [2]. There have been several deposition methods attempted to achieve high quality LiNbO3 thin films. Among them, pulsed laser deposition (PLD) is considered as a suitable technique for growing complex oxide materials with excellent stoichiometry. However, a problem when growing LiNbO3 films with PLD is the appearance of secondary phases, such as a Li-deficient phase of LiNb3O8 [3, 4]. In this study, we performed extensive parametric studies to suppress secondary phases in LiNbO3 thin films deposited by PLD. From the results, phases of the lithium niobate thin films have shown strong dependencies on the plume strength and the relative position of the substrate to the plume. The main reason to change Li content in the film is postulated to be either through the difference of elemental distribution in the plume, or Li knock-out from films by the high energy plume. EXPERIMENTAL DETAILS A KrF excimer laser (λ=248nm) was used as an ablation source of the PLD system.
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Two different sintered targets, Li-enriched (10wt % Li2CO3 added) and LiNbO3 were used and c-cut sapphire was used as substrates. The deposition temperature was 550oC, and the ambient gas was O2. The films were in-situ annealed at the deposition temperatur
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