Andries Boelen

Thin-film strontium titanate (SrTiO3, STO) layers grown on silicon require accurate stoichiometry and single-crystalline order to exploit their functional properties optimally. Oxide molecular beam epitaxy can provide an epitaxial interface, but suffers from source oxidation and resulting flux instabilities, yielding only a narrow growth process window for cationic stoichiometry control. Here, we investigate post-growth annealing in oxygen as a pathway to drive the STO layer toward stoichiometry in intentionally Sr-rich epitaxial STO films on silicon (001). Annealing over a broad temperature range revealed two distinct Sr-segregation mechanisms. Below 800 °C, excess Sr segregates toward the surface, forming SrO outgrowths that progressively sublimate at elevated temperatures. Above 800 °C, a second mechanism dominates: Sr accumulates within the interfacial SiO2 layer formed by oxygen diffusion at the STO/Si interface. Together, these mechanisms effectively remove excess Sr from the STO lattice, yielding a more stoichiometric perovskite layer. Our results demonstrate that growing slightly Sr-rich STO templates followed by controlled annealing provides a practical route to improve crystalline quality, offering a scalable strategy for high-quality STO integration on silicon.

Materials that feature very high permittivity and very low losses at low temperatures offer interesting opportunities for improving the performance of a variety of microwave-optical devices such as phase shifters and quantum transducers. In this Letter, superconducting Al coplanar waveguide resonators between 4 and 6 GHz were used to determine the complex permittivity and loss of thin film SrTiO3 on Si at cryogenic temperatures. We report a permittivity of 1950 ± 140 and a high-power loss tangent (tan δSTO) of 3.4 ± 0.1×10−4 at millikelvin temperatures. To further investigate the potential of SrTiO3 for use in emerging quantum technology applications, the transition of tan δSTO into the single photon regime was analyzed. Our findings reveal a loss scaling that aligns closely with a two-level system loss mechanism. We observe a nearly two orders of magnitude difference in tan δSTO between high and low input power levels, consistent with that two-level system loss picture.

Pockels materials are notable for their strong electro-optic interaction and rapid response times and are therefore used extensively in optical communications. However, at cryogenic temperatures, Pockels coefficients are reduced in many materials optimized for room-temperature operation, which is a major hurdle for emerging quantum technologies. Here, we show that strontium titanate (SrTiO ₃ ) can be engineered to exhibit a Pockels coefficient of 345 picometers per volt at 20 hertz at cryogenic temperatures, a value twice as high as any other thin-film electro-optic material. By adjusting the stoichiometry, we were able to increase the Curie temperature and realize a ferroelectric phase yielding a high Pockels coefficient, so far with limited optical losses of decibels per centimeter. Our findings position SrTiO ₃ as a promising material for cryogenic quantum photonics applications.

Strontium titanate (SrTiO₃, STO) stands out as a promising material for various electronic applications thanks to its exceptional dielectric properties. Molecular beam epitaxy is one of the few techniques that allows epitaxial growth of STO directly on industry-relevant silicon substrates. However, maintaining precise stoichiometry and high crystalline quality in this process remains a significant challenge. Establishing this is essential to obtain STO with bulk-like dielectric properties and to minimize leakage current and optical absorbance. In this study, the importance of cationic stoichiometry and the effect of thickness are investigated for STO thin films epitaxially grown on silicon. We employed real-time reflection high-energy electron diffraction (RHEED) as a feedback loop mechanism to counteract Sr source oxidation and maintain a constant flux. Additionally, high-temperature postgrowth annealing treatments in O₂ were investigated to promote layer relaxation and reduce oxygen vacancy concentration, thereby improving the physical, electrical, and optical properties of stoichiometric STO. As a result, high-quality STO thin films exceeding 100 nm were successfully fabricated, featuring a bulk-like out-of-plane lattice parameter and refractive index, as well as a rocking curve full width at half maximum below 0.2°, smooth surface (R_q < 0.2 nm) and a leakage current density below 1 × 10^–7 A/cm².

In this study, we investigate the changes in the crystalline structure of MBE-deposited SrTiO₃ layers on Si with different deviations from Sr/Ti stoichiometry as deposited but also after annealing at high temperatures (>600 °C). We show that as-grown 15 nm thick non-stochiometric SrTiO₃ layers present surprisingly lower FWHM values of the (002) omega diffraction peak compared to fully stoichiometric layers of similar thickness. This can misleadingly point to superior crystalline quality of such non-stochiometric layers. However, thermal post-deposition anneals of these layers at temperatures up to 850 °C in oxygen show strong detrimental effects on the crystalline structure, surface and interface with the Si (001) substrate. On the contrary, the post-deposition anneals applied to the stoichiometric samples strongly improve the physical, optical and electrical properties of the epitaxial SrTiO₃ thin films.