Thick BaTiO₃ Epitaxial Films Integrated on Si by RF Sputtering for Electro-Optic Modulators in Si Photonics

Abstract

Thick epitaxial BaTiO 3 films ranging from 120 nm to 1 μm were grown by off-axis RF magnetron sputtering on SrTiO 3 -templated silicon-on-insulator (SOI) substrates for use in electro-optic applications, where such large thicknesses are necessary. The films are of high quality, rivaling those grown by molecular beam epitaxy (MBE) in crystalline quality, but can be grown 10 times faster. Extraction of lattice parameters from geometric phase analysis of atomic-resolution scanning transmission electron microscopy images revealed how the in-plane and out-of-plane lattice spacings of sputtered BaTiO 3 changes as a function of layer position within a thick film. Our results indicate that compared to molecular beam epitaxy, sputtered films retain their out-of-plane polarization (c-axis) orientation for larger thicknesses. We also find an unusual re-transition from in-plane polarization (a-axis) to out-ofplane polarization (c-axis), along with an anomalous lattice expansion, near the surface. We also studied a method of achieving 100% a-axis-oriented films using a two-step process involving amorphous growth and recrystallization of a seed layer followed by normal high temperature growth. While this method is successful in achieving full a-axis orientation even at low thicknesses, the resulting film has a large number of voids and misoriented grains. Electro-optic measurement using a transmission setup of a sputtered BTO film grown using the optimized conditions yields an effective Pockels coefficient as high as 183 pm/V. A Mach− Zehnder modulator fabricated on such films exhibits phase shifting with an equivalent Pockels coefficient of 157 pm/V. These results demonstrate that sputtered BTO thick films can be used for integrated electro-optic modulators for Si photonics.

Si-Integrated BaTiO3 for Electro-Optic Applications: Crystalline and Polarization Orientation Control

“…We found the average peak height for the 45° degree pad to be approximately 85 ± 23 pm/V with a peak of 119 pm/V, while for the 0° pad we found an average response of about 13 ± 4 pm/V with a peak of 18 pm/V. Details explaining the fitting procedure for the effective Pockels coefficients have been reported in our previous work. − For fitting, we used 2.28 as the index of refraction for BTO and used the XRR fit value for the BTO thickness at 101 nm. Representative data of these input angle sweeps are shown in Figure c.…”

“…As we have mentioned, for TE-mode waveguides that are easier to fabricate, it is necessary to have the BTO polarization in-plane. 15,20,21 The region of the film with its c-axis perpendicular to the film surface is referred to as a dead layer. One method to ensure that the film's polarization is immediately in-plane is to add a buffer layer between BTO and STO, with a lattice constant larger than that of BTO.…”

Near-IR & Mid-IR Silicon Photonics Modulators

“…BTO modulators show a Pockels coefficient of as high as 923 pm/V [14], which is the highest efficiency demonstrated in silicon photonics for hybrid inorganic materials. In addition to growing BTO films with MBE [13,14], RF sputtering can also be used in heterogeneous BTO-SiN modulators with an equivalent Pockels coefficient of ≈160 pm/V [19]. The currently demonstrated BTO-based modulators show bandwidth up to 30 GHz [20].…”

Modulating the Combinatorial Target Power of MgSnN2 via RF Magnetron Sputtering for Enhanced Optoelectronic Performance: Mechanistic Insights from DFT Studies

“…15 The recent computational works predict that MgSnN 2 is thermodynamically stable at ambient temperature and pressure, 21 making it a promising target for both fundamental exploration and device applications, which sparked a new interest in MgSnN 2 . The highly explored synthetic techniques are radio-frequency (RF) sputtering, 13,22,23 high-pressure metathesis processes, 14 and plasmaassisted molecular beam epitaxy. 17 Mostly, the reported lattice constants of disordered wurtzite MgSnN 2 are extremely similar to that of ZnSnN 2 , and it was projected that the band energy gap (E g ) of MgSnN 2 could be altered within the range of 1.0−2.7 eV by varying the composition.…”

“…The recent computational works predict that MgSnN 2 is thermodynamically stable at ambient temperature and pressure, making it a promising target for both fundamental exploration and device applications, which sparked a new interest in MgSnN 2 . The highly explored synthetic techniques are radio-frequency (RF) sputtering, ,, high-pressure metathesis processes, and plasma-assisted molecular beam epitaxy …”