Microstructure and ferroelectricity of BaTiO₃ thin films on Si for integrated photonics

Abstract

Significant progress has been made in integrating novel materials into silicon photonic structures in order to extend the functionality of photonic circuits. One of these promising optical materials is BaTiO or barium titanate (BTO) that exhibits a very large Pockels coefficient as required for high-speed light modulators. However, all previous demonstrations show a noticable reduction of the Pockels effect in BTO thin films deposited on silicon substrates compared to BTO bulk crystals. Here, we report on the strong dependence of the Pockels effect in BTO thin films on their microstructure, and provide guidelines on how to engineer thin films with strong electro-optic response. We employ several deposition methods such as molecular beam epitaxy and chemical vapor deposition to realize BTO thin films with different morphology and crystalline structure. While a linear electro-optic response is present even in porous, polycrystalline BTO thin films with an effective Pockels coefficient r = 6 pm V, it is maximized for dense, tetragonal, epitaxial BTO films (r = 140 pm V). By identifying the key structural predictors of electro-optic response in BTO/Si, we provide a roadmap to fully exploit the linear electro-optic effect in novel hybrid oxide/semiconductor nanophotonic devices.

Epitaxial, electro‐optically active barium titanate thin films on silicon by chemical solution deposition

“…This large deviation precludes confident simulation of the individual tensor components. However, the data are in qualitative agreement with the anticipated EO behavior of orthogonal, in‐plane oriented ferroelectric domains with P 4 mm symmetry . Near = 0° and 90°, where only a change in the birefringence of the film is induced, is minimal for the incident polarization angle ( = 0° relative to BTO[100]).…”

“…An average of 27 ± 4 PM/V is determined for multiple different pads at the same applied field and = 45°. For reference, this value is comparable to oriented BTO films grown by pulsed laser deposition (37 pm/V), radio‐frequency sputtering (41 pm/V), CVD (7 PM/V), and atomic layer deposition (30 pm/V) . Bulk BTO and high quality thin films (typically grown by MBE) exhibit coefficients on the order of 100‐1000 pm/V .…”

“…At = 45°, a rotation of the optical axis is also present in addition to a larger fraction of poled domains, and EO response is maximized. Furthermore, large anisotropy is observed between the responses at 0°/90° and 45° and may be attributed to the extended tetragonality ( c / a ) of 1.014 determined from the XRD lattice parameters (bulk tetragonality is 1.011) …”

“…EO measurements were performed in transmission by an experimental setup similar to other reports (Figure ) . A series of semi‐circular tungsten contacts were fabricated on the surface of the BTO film by photolithography, sputtering, and metal lift‐off.…”

“…The success of LNO as an electro‐optic material has inspired the study of similar materials, particularly those that can be seamlessly integrated with current semiconductor technology. Barium titanate (BaTiO 3 ; BTO), a renowned ferroelectric perovskite oxide, has recently been integrated with silicon with large Pockels coefficients and is a promising candidate for on‐chip optical modulators . Recently, nanoscale BTO modulators with large Pockels coefficients ( > 900 pm/V) have been developed using molecular beam epitaxy (MBE) and have demonstrated modulation speeds of 50 Gbit/s …”