Interfacial Symmetry-Breaking Effects in the Quantum Paraelectric SrTiO3

Oxides lacking a center of symmetry are highly desired, as they usually have fascinating physical prop-erties. However, there are limited numbers of noncentrosymmetric media in nature and most of them do not have multiple functionalities integrated. Here, using the bulk photovoltaic effect as the probing technique, we demonstrate that, at interfaces with a wide range of oxides, strontium titanate (SrTiO3) is polar, i.e., with broken inversion symmetry in the quantum paraelectric phase. Studies comprising conductivity and the bulk photovoltaic effect on LaAlO3/SrTiO3 thin films further show that excessive electronic band bending screens the induced polarity, revealing that appropriate band bending at the interface is the key parameter to control symmetry breaking. Inheriting high carrier mobility from SrTiO3, the polar interface under illumination at low temperatures is conductive or metallic, permitting multifunctionality coupling between the oxides and SrTiO3. Our studies show that significant photovoltaic effects can be generated at the polar interfaces, especially in the quantum-paraelectric phase of SrTiO3, thus giving practical design strategies for multifunctional devices.