Dopant Segregation Inside and Outside Dislocation Cores in Perovskite BaSnO3 and Reconstruction of the Local Atomic and Electronic Structures

Engineering metal oxidation using epitaxial strain

The oxides of platinum group metals are promising for future electronics and spintronics due to the delicate interplay of spin-orbit coupling and electron correlation energies. However, their synthesis as thin films remains challenging due to their low vapour pressures and low oxidation potentials. Here we show how epitaxial strain can be used as a control knob to enhance metal oxidation. Using Ir as an example, we demonstrate the use of epitaxial strain in engineering its oxidation chemistry, enabling phase-pure Ir or IrO2 films despite using identical growth conditions. The observations are explained using a density-functional-theory-based modified formation enthalpy framework, which highlights the important role of metal-substrate epitaxial strain in governing the oxide formation enthalpy. We also validate the generality of this principle by demonstrating epitaxial strain effect on Ru oxidation. The IrO2 films studied in our work further revealed quantum oscillations, attesting to the excellent film quality. The epitaxial strain approach we present could enable growth of oxide films of hard-to-oxidize elements using strain engineering.

Significant Suppression of Cracks in Freestanding Perovskite Oxide Flexible Sheets Using a Capping Oxide Layer

Flexible and functional perovskite oxide sheets with high orientation and crystallization are the next step in the development of next-generation devices. One promising synthesis method is the lift-off and transfer method using a water-soluble sacrificial layer. However, the suppression of cracks during lift-off is a crucial problem that remains unsolved. In this study, we demonstrated that this problem can be solved by depositing amorphous Al₂O₃ capping layers on oxide sheets. Using this simple method, over 20 mm² of crack-free, deep-ultraviolet transparent electrode La:SrSnO₃ and ferroelectric Ba_0.75Sr_0.25TiO₃ flexible sheets were obtained. By contrast, the sheets without any capping layers broke. The obtained sheets showed considerable flexibility and high functionality. The La:SrSnO₃ sheet simultaneously exhibited a wide bandgap (4.4 eV) and high electrical conductivity (>10³ S/cm). The Ba_0.75Sr_0.25TiO₃ sheet exhibited clear room-temperature ferroelectricity with a remnant polarization of 17 μC/cm². Our findings provide a simple transfer method for obtaining large, crack-free, high-quality, single-crystalline sheets.

Oxide Two-Dimensional Electron Gas with High Mobility at Room-Temperature

The prospect of 2-dimensional electron gases (2DEGs) possessing high mobility at room temperature in wide-bandgap perovskite stannates is enticing for oxide electronics, particularly to realize transparent and high-electron mobility transistors. Nonetheless only a small number of studies to date report 2DEGs in BaSnO₃ -based heterostructures. Here, 2DEG formation at the LaScO₃ /BaSnO₃ (LSO/BSO) interface with a room-temperature mobility of 60 cm²  V^-1  s^-1 at a carrier concentration of 1.7 × 10¹³  cm^-2 is reported. This is an order of magnitude higher mobility at room temperature than achieved in SrTiO₃ -based 2DEGs. This is achieved by combining a thick BSO buffer layer with an ex situ high-temperature treatment, which not only reduces the dislocation density but also produces a SnO₂ -terminated atomically flat surface, followed by the growth of an overlying BSO/LSO interface. Using weak beam dark-field transmission electron microscopy imaging and in-line electron holography technique, a reduction of the threading dislocation density is revealed, and direct evidence for the spatial confinement of a 2DEG at the BSO/LSO interface is provided. This work opens a new pathway to explore the exciting physics of stannate-based 2DEGs at application-relevant temperatures for oxide nanoelectronics.

Reversible Manipulation of Photoconductivity Caused by Surface Oxygen Vacancies in Perovskite Stannates with Ultraviolet Light

Programmable optoelectronic devices call for the reversible control of the photocarrier recombination process by in-gap states in oxide semiconductors. However, previous approaches to produce oxygen vacancies as a source of in-gap states in oxide semiconductors have hampered the reversible formation of oxygen vacancies and their related phenomena. Here, a new strategy to manipulate the 2D photoconductivity from perovskite stannates is demonstrated by exploiting spatially selective photochemical reaction under ultraviolet illumination at room temperature. Remarkably, the ideal trap-free photocurrent of air-illuminated BaSnO₃ (≈200 pA) is reversibly switched into three orders of magnitude higher photocurrent of vacuum-illuminated BaSnO₃ (≈335 nA) with persistent photoconductivity depending on ambient oxygen pressure under illumination. Multiple characterizations elucidate that ultraviolet illumination of BaSnO₃  under low oxygen pressure induces surface oxygen vacancies as a result of surface photolysis combined with the low oxygen-diffusion coefficient of BaSnO₃ ; the concentrated oxygen vacancies are likely to induce a two-step transition of photocurrent response by changing the characteristics of in-gap states from the shallow level to the deep level. These results suggest a novel strategy that uses light-matter interaction in a reversible and spatially confined way to manipulate functionalities related to surface defect states, for the emerging applications using newly discovered oxide semiconductors.