Transition metal oxides (TMO) are compounds composed of oxygen atoms bound to transition metals. TMO thin films have recently shown immense promise for use in a slew of applications owing to their widely tunable electronic, magnetic, structural and optical properties. In fact, TMO thin films have featured prominently in the search for new and improved dielectric gate materials for integrated electronics. One area in which TMO thin films are rapidly gaining prominence is integrated photonics. Recently, SrTiO3/LaAlO3 (STO/LAO), a class of silicon-compatible TMO thin film systems, has gained extraordinary research attention due to the discovery of a high-mobility 2D electron gas at the interface of these two band insulators. Consequently, considerable focus has been devoted to investigating the STO/LAO interface in oxide electronics. Nonetheless, further research is required to explore alternative electro-optical capabilities.
Silicon compatibility with the STO/LAO system, together with its large conduction band offset and the ability to confine charge carriers in SrTiO3 quantum wells, makes it a potential candidate for use in a wide range of integrated photonics applications. To demonstrate the feasibility of utilizing the STO/LAO system for electro-optical devices integrated on silicon, Elliott Ortmann, Margaret Duncan and Professor Alexander Demkov, (Department of Physics at The University of Texas, Texas, USA) carried out numerical simulations of the electrical, optical, and electro-optical performance of silicon-integrated STO/LAO electro-optical devices utilizing the quantum-confined Stark effect to electro-optically tune inter sub-band absorptions occurring at near-infrared optical wavelengths. Their work is currently published in the research journal, Optical Materials Express . Recently, Professor Demkov and his research group have published extensively on multiple quantum wells built from perovskite oxides and their integration with Si [2-6]. In the span of five years, they have been able to go from theoretical calculations to growth of STO/LAO supertlattices of unprecedented crystal quality and experiential demonstration of quantum confinement in them, to integration of these exciting systems on silicon.
Generally, their approach entailed calculation of the wave functions of confined electrons in STO quantum wells (QWs) and the determination of the Stark shift for multiple QW geometries. The researchers then considered a hybrid silicon-TMO waveguide design for the confinement of an optical mode and simulation of the electrical, optical, and electro-optical performance of such a device. Overall, the calculations included the extent of optical confinement in the electro-optically active TMO layer and demonstrated the switching energy of pJ/bit for modulator devices.
The authors reported that their device was able to achieve electro-optic operation by utilizing the quantum-confined Stark effect to modulate the energy of inter sub-band transitions in the STO conduction band. Interestingly, their calculations of the switching energy in an STO/LAO electro-optic modulator integrated on silicon demonstrated that the device could be constructed using existing thin film growth and semiconductor processing techniques.
In summary, the present study provides calculations supporting the feasibility of producing integrated electro-optical devices operating at near-infrared optical wavelengths based on the STO/LAO material system. Altogether, they discuss the optimal design parameters for the fabrication of experimentally realizable devices and attempted to minimize the energy consumption of a STO/LAO electro-optic modulator while maximizing performance. In an interview with Advances in Engineering, Professor Demkov commented that their results offered an interesting avenue of investigation for the discovery of new electro-optical devices capable of operating in the near-infrared spectral range by utilizing the quantum-confined Stark effect in TMO thin film quantum wells heterostructures.
 J. Elliott Ortmann, Margaret A. Duncan, Alexander A. Demkov. “Designing near-infrared electro-optical devices from the SrTiO3/LaAlO3 materials system,” 9, 2982 Optical Materials Express (2019).
 C. Lin, A. B. Posadas, M. Choi, and A. A. Demkov, “Optical properties of transition metal oxide quantum wells,” J. Appl. Phys. 117, 034304 (2015).
 M. Choi, C. Lin, M. Butcher, C. Rodriguez, Q. He, A. B. Posadas, A. Y. Borisevich, S. Zollner, and A. A. Demkov, “Quantum confinement in transition metal oxide quantum wells,” Appl. Phys. Lett. 106, 192902 (2015).
 J. E. Ortmann, A. B. Posadas and A. A. Demkov, “The MBE growth of arbitrarily thick SrTiO3/LaAlO3 quantum well heterostructures for use in next-generation optoelectronic devices,” J. Appl. Phys. 124, 015301 (2018).
 J. E. Ortmann, N. Nookala, Q. He, L. Gao, C. Lin, A. B. Posadas, A. Y. Borisevich, M. A. Belkin, and A. A. Demkov, “Quantum confinement in oxide heterostructures: Room-temperature intersubband absorption in SrTiO3/LaAlO3 multiple quantum wells,” ACS NANO 12, 7682 (2018).
 J. E. Ortmann, S. Kwon, A. B. Posadas, M. J. Kim and A. A. Demkov, “Monolithic Integration of Transition Metal Oxide Multiple Quantum Wells on Silicon (001),” J. Appl. Phys. 125, 155302 (2019).
This research was funded by the Air Force Office of Scientific Research under Grants No. FA9550-12-10494 and FA9550-18-1-0053.