Fabrication of Single-Crystalline Barium Titanate Thin Films for Electro-Optic Applications

We demonstrate the fabrication of single-crystalline barium titanate (BTO) thin films using spalling, a mechanical exfoliation process for producing large-scale thin films from bulk substrates. Recently, electro-optic thin films have emerged as an attractive platform for various technologies, including free-space optics, photonic integrated circuits, and quantum computing hardware. Thin-film lithium niobate (LN) has seen significant technical advancement; however, its electro-optic coefficient is not sufficiently high for versatile applications requiring low power consumption. Alternative materials, such as BTO, offer a promising alternative with Pockels coefficients exceeding those of LN by over 30 times. Traditionally, thin-film BTO is grown using bottom-up methods, such as molecular beam epitaxy, pulsed laser deposition, or sputtering. However, these methods often result in extrinsic defects, variable grain sizes, porosity, and deviation from stoichiometric BTO, leading to reduced Pockels coefficients [1]. Moreover, many bottom-up growth methods are slow and expensive.<br/><br/>In contrast to conventional growth techniques, we propose spalling of commercially available single crystal BTO substrates as a low-cost approach to obtain large-scale single crystalline thin films. Spalling is a commonly used technique for separating III-V semiconductor layers from their host substrates [2]. The process involves mechanically fracturing the substrate by depositing a metal stressor layer, electroplated nickel (Ni) in our case, on top. The Ni layer introduces compressive stress within the substrate. When the residual stress in the substrate reaches the critical stress, a crack is initiated upon application of an external pull force, such as exfoliation with tape. We demonstrate that by controlling the thickness of the electroplated Ni – and thus the compressive stress induced in the substrate – we can obtain single-crystal thin films of BTO with thicknesses ranging from 100 nm to several microns and areas upwards of 100 x 100 μm². We confirm our results using both (100) and (001) oriented single crystal substrates and report roughness values around 20 nm for 200 nm thick films. We observe a scaling of roughness with film thickness due to domain switching introduced through compressive stress in ferroelectric materials. Spalled thin films are transferred from tape onto desired substrates using a PPC/PDMS method, similar to what is used for transfer of 2D van der Waals materials. For spalled film thicknesses below 1 μm, optical microscope images reveal different colors in areas with varying film thicknesses due to thin film interference effects. We use transfer matrix calculations to create a large-scale spatial thickness map and identify areas with the target thin film thickness. Finally, transparent gate electrodes are deposited onto desired areas, and the electro-optic coefficients of the spalled films are measured as function of frequency using a Teng-Man setup [3]. The method outlined in this work presents a novel approach for producing high-quality, single crystal thin films. Based on the achieved film quality, we envision the integration of spalled electro-optic BTO thin films in various nanophotonic and quantum technologies including metasurfaces and integrated photonic circuits.<br/><br/>[1] Karvounis et al., Adv. Opt. Mater. 8, 2001249 (2020).<br/>[2] Chen and Packard, Solar Energy Materials and Solar Cells 225, 111018 (2021).<br/>[3] Teng and Man, Appl. Phys. Lett. 56, 1734–1736 (1990).