SrTiO₃ holds significant potential for application in the field of oxide electronics as a wide band gap semiconductor displaying a plethora of effects that include metallic- like conductivity by n-type doping, high electron mobility exceeding 10⁴ 𝑐𝑚²/𝑉𝑠, superconductivity at 0.28K, large Seebeck coefficient, quantum paraelectric state under 37K, and room temperature ferroelectricity under compressive strain. Notably, its photoelectric activity in the low temperature regime, overlapping with the quantum paraelectric state, display remarkable photoelectric activity featuring anomalous photoconductivity and photoluminescence or coherent photo-electron emission.
In this study, we show that when photo-excited with above band gap energy photons, SrTiO₃ exhibits non-linear transport of photocarriers and voltage-controlled negative resistance, resulting from an intervalley transfer of photo-induced electrons. As a consequence of the negative resistance, the photocurrent becomes unstable and spontaneously gives rise to low frequency Gunn-like oscillations. These effects, coupled with the field quenching of the photoluminescence, reveal a complex band structure which deserves further in-depth investigations.

Photoferroics are photoactive ferroelectric materials that exhibit a bulk photovoltaic effect enabled by the lack of inversion symmetry and are, in contrast to conventional semiconductor photovoltaic devices, capable of achieving above band gap photovoltages​. Some ferroelectric oxide perovskites have been studied for this application, but their often large band gaps can limit applicability for new solar energy harvesting technologies. Brownmillerites are anion-deficient perovskite-related structures with ordered vacancies that form alternating layers of corner-linked BO₆ octahedra and layers of BO₄ tetrahedra. The existence of three distinct cation coordination sites allows for flexibility in accommodating a wide range of p-block and transition metal cations, which makes them suitable for designing materials with a range of properties. Cooperative rotations of the BO₄ tetrahedra can break the inversion symmetry of the ideal high-symmetry structure (of Imma symmetry)​ making them interesting candidates to design new photoferroics. A first-principles investigation into some of the impacts that composition could have on the electronic structure of Brownmillerites revealed some candidates with potentially suitable band gaps for photoferroic applications. Our experimental studies support the findings from our computational work regarding crystal structures and optical band gaps. Some of the materials investigated show properties that could make them potential candidates for designing new solar energy harvesting devices.

Classic ferroelectric perovskite oxides typically exhibit a large optical bandgap (>3 eV), which lies beyond the visible range. However, promising applications that leverage light-polarization interactions—such as the bulk photovoltaic effect and light-induced domain switching—favour reduced bandgaps (<3 eV) within the visible spectrum. First-principles calculations based on density functional theory have been guiding experimental bandgap engineering works through B-site doping. Preliminary results indicate significantly enhanced response under visible light for bandgap-engineered materials compared to their undoped counterparts. Interestingly, our recent study challenges the conventional belief that this visible-range response solely arises from chemical doping. Instead, our results suggest that physical strains within the materials may play a crucial role. In support of this, we present seemingly contradictory experimental evidence, including direct transition-like photoconductivity and strain-induced redshift of absorption edge. This talk aims to ignite a debate about the underlying mechanisms driving bandgap changes in ferroelectric ceramics. Despite the mechanism remaining unclear, the increased visible-range response has already demonstrated early signs of advanced applications. This talk will share an experimental example involving the use of bandgap-engineered ferroelectric ceramics for filterless colour sensing.

Current research has demonstrated different types of self-powered photodetectors utilizing the photovoltaic effect, pyroelectric effect, piezoelectric effect, and synergic effects, such as the piezo-phototronic and pyro-phototronic effects. Such effects have been demonstrated in standard semiconductors, in hybrid inorganic-organic halide perovskites and in all inorganic perovskites. Very recently, a novel type of self-powered photodetector exploring the coupling between the photovoltaic, the pyroelectric and the ferroelectric effects (i.e., ferro-pyro-phototronic effect) has attracted great interest, owing to the excellent photo current response achieved with this triple coupling. The ferro-pyro-phototronic effect can therefore, be an important route towards improving the performance of self-powered photodetectors. In this talk, we present recent results on self-powered photodetectors based on perovskite oxides, such as 0.5Ba(Zr0.2Ti0.8)O3–0.5(Ba0.7Ca0.3)TiO3 (BCZT) and/or binary oxides, such as ZrO2, in a metal–ferroelectric–insulator–semiconductor heterostructure. Herein, the performance of the photodetectors is understood based on the ferro-pyro-phototronic effect where the polarization-dependent interfacial coupling can greatly influence the photodetector performance. The proposed heterostructure have strong responsivities and rise and fall times of only a few microseconds. The photodetector performance is optimized in terms of the chopper frequency, power density of the laser and poling voltage. Overall, our work demonstrates a very promising ultrafast photodetectors based on ferroelectric oxide thin films.