Perovskite Stannate Research Articles

Perovskite Stannate Heterojunctions for Self‐Powered Ultraviolet Photodiodes Operated in Extreme Environments

AbstractHigh‐performance UV photodetectors call for sensitive and energy‐efficient signal detection in extreme environments. To satisfy the requirement of a UV detection without an external power consumption, self‐powered UV photodetectors must be realized by an optimal combination of heterostructure with maximum built‐in potential using novel wide‐bandgap materials. Here, self‐powered UV photodiodes are designed via the band engineering of a wide‐bandgap Sr(Sn,Ni)O3/BaSnO3 heterojunction for the first time. Based on the theoretical concept of acceptor doping by Ni substitution in SrSnO3, remarkably, this heterojunction with a conduction band offset of 0.94 eV shows strong nonlinear electrical characteristics with extremely low Idark (≈100 fA) owing to the spatial gradient of the potential barrier across the interfaces, outstanding photo‐to‐dark current ratio (>107 at 25 °C and > 104 at 300 °C), and high stability under various extreme conditions upon UV illumination even without external bias (V = 0 V). This study suggests a novel strategy that utilizes band engineering to maximize sensitivity and minimize energy consumption in harsh environments for UV imaging using the newly discovered wide‐bandgap semiconductors.

Accelerating metal nanoparticle exsolution by exploiting tolerance factor of perovskite stannate.

The octahedral symmetry in ionic crystals can play a critical role in atomic nucleation and migration during solid-solid phase transformation. Similarly, octahedron distortion, which is characterized by Goldschmidt tolerance factor, strongly influences the exsolution kinetics in the perovskite lattice framework during high-temperature annealing. However, a fundamental study on manipulating the exsolution process by octahedron distortion is still lacking. In this study, we accelerate Ni metal exsolution on the surface of perovskite stannates by increasing the [BO6] octahedron distortion in the lattices. Decreasing the A-site ionic radius (rBa2+ = 161 pm → rSr2+ = 144 pm → rCa2+ = 134 pm) increased the density of exsolved Ni nanoparticles by up to 640% (i.e., 47 particles μm-2 of Ba(Sn, Ni)O3 → 304 particles μm-2 of Ca(Sn, Ni)O3) after the identical exsolution process. Based on the theoretical calculation and experimental characterization, the decrease in crystal symmetry by octahedral distortion promoted the Ni exsolution owing to the boosted Ni migration by weakening the bond strength and generating domain boundaries. The findings highlight the importance of octahedral distortion to control atomic migration through the perovskite lattice framework and provide a strategy to tailor the density of uniformly populated nanoparticles in nanocomposite oxides for multifunctional material design.

Perovskite stannate La-doped BaSnO3 films for near- and mid-infrared plasmonic applications

La-doped BaSnO3 (LBSO) epitaxial thin films were grown on (001) MgO substrates by pulsed laser deposition using a La0.04Ba0.96SnO3 target. The structural, electrical, and optical properties of the LBSO films were investigated as a function of oxygen pressure during deposition. The carrier mobility can be tuned from 13 cm2V−1s−1 to 44 cm2V−1s−1 by varying oxygen partial pressure from 2 Pa to 12 Pa, respectively. This improved mobility is attributed to the reduced film strain via reducing oxygen defect concentration or reducing off-stoichiometry in the film. The optical permittivity of LBSO films can also be modified as a function of the oxygen pressure during deposition, allowing tuning of their epsilon-near-zero wavelength from 1.9 µm to 5.6 µm. These results demonstrate that LBSO thin films grown on MgO can be used for plasmonic devices at mid-wave infrared wavelengths.

Deep-UV Transparent Conducting Oxide La-Doped SrSnO3 with a High Figure of Merit.

Perovskite stannate SrSnO3 (SSO) is attracting attention as ultraviolet transparent conducting oxides (UV TCOs) due to its ultrawide band gap and high conductivity. Here, we investigate in detail the thickness-dependent electrical, structural, and optical properties of sequentially strain-relaxed La-doped SrSnO3 (SLSO) epitaxial thin films. We find that the SLSO films grow as an orthorhombic Pnma phase with a–a–c+ in the c+ direction under the tensile strain. With the strain relaxation, as the films become thicker, vertical grain boundaries are created and the orthorhombic phase becomes reoriented to all three possible orientations. Simultaneously, the conductance starts to deviate from the linear behavior with increasing film thickness. Through the analysis of thickness fringes in optical transmittance, we found that a 120 nm thick nominally 4% La-doped SrSnO3 film has a figure of merit (φTC = 2.65 × 10–3 Ω–1) at λ = 300 nm in the deep-UV region, which is the highest value among the well-known candidates for UV TCOs reported to date.

Synthesis of Hydride-Doped Perovskite Stannate with Visible Light Absorption Capability.

Narrow-gap semiconductors with visible light absorption capability have attracted attention as photofunctional materials. H--doped BaSn0.7Y0.3O3-δ containing Sn(II) species was recently reported to absorb visible light up to 600 nm, which represents the first demonstration of oxyhydride-based visible-light-absorbers. In the present study, a more detailed investigation was made to obtain information on the synthesis and properties of H--doped perovskite-type stannate with respect to the A-site cation of the material and the preparation conditions. H--doped ASn0.7Y0.3O3-δ (A = Ba, Ba0.5Sr0.5, and Sr) obtained by the reaction of ASn0.7Y0.3O3-δ precursors with CaH2 at 773 K under vacuum conditions was shown to have almost the same bandgap (ca. 2.1 eV), regardless of the A-site cation. Physicochemical measurements and theoretical calculations revealed that the identical bandgaps of H--doped ASn0.7Y0.3O3-δ are due to the simultaneous shift of the midgap states composed of Sn2+ with the conduction band minimum. Experimental results also indicated that the appropriate preparation conditions with respect to Y3+-substitution and the temperature for the synthesis of the ASn0.7Y0.3O3-δ precursors were essential to obtain H--doped products that have a low density of defects.

Crystal chemistry, DFT calculation and optical properties of the double perovskite stannate Sr2Sn0.8In0.8W0.4O6

Sr2Sn0.8In0.8W0.4O6 double perovskite powder was, first, prepared following an experimental study on the effects of substitutions of In and W in the high transparent strontium stannate SrSnO3 (SSO). The high temperature solid state reaction method was adopted for the preparation of the desired phase. Further, the W and In substitution's effects on the variation of the electronic structure was investigated theoretically by density functional theory (DFT) calculations. Rietveld's refinement of the X-ray diffraction pattern announces a tetragonal symmetry of the crystal lattice (I4/m), characterized by a reduction in octahedral inclinations due to double substitution. IR absorption spectroscopy announced the presence of carbonates and water molecules due to moisture-swing sorption and confirmed by TG analysis. The optical properties studied by UV-Vis spectroscopy have shown that the material is transparent in the visible region of the spectrum. A band gap of 3.46 eV was calculated using Tauc plot, indicating a possible semiconducting behavior.

Structural Phase Transitions and Quantum Dots Regulation of Perovskite Stannates

Structural Phase Transitions and Quantum Dots Regulation of Perovskite Stannates

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 (Adv. Mater. 5/2022)

2D Photoconductivity Light–matter interaction under different oxygen pressures can reversibly manipulate photoconductivity from surface oxygen vacancies of perovskite stannate semiconductors. In article number 2107650, utilizing a surface-selective photochemical reaction, Junwoo Son and co-workers report that ultraviolet-excited electron–hole pairs of a perovskite compound under low oxygen pressure accelerate the unprecedented formation of spatially confined and optically programmable in-gap states induced by oxygen vacancies at room temperature.

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 spatiallyselective photochemical reaction under ultraviolet illumination at room temperature. Remarkably, the ideal trap-free photocurrent of air-illuminated BaSnO3 (≈200pA) is reversibly switched into three orders of magnitude higher photocurrent of vacuum-illuminated BaSnO3 (≈335nA) 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 spatiallyconfined way to manipulate functionalities related to surface defect states, for the emerging applications using newly discovered oxide semiconductors.

Tailoring of band gap to tune the optical and thermoelectric properties of Sr1-xBaxSnO3 stannates for clean energy; probed by DFT

• Tuning of band gap from ultraviolet to visible region. • Absorption in visible region increase their potential for solar cells. • Figure of merit is maxima at 50% doping of Ba. • The studied Stannates are tuned for optical and thermoelectric characteristics. The perovskite stannates Sr 1-x Ba x SnO 3 (x = 0.0, 0.25, 0.50, 0.75, 1.0) are studied theoreticaly to explore them for energy applications. The doping of Ba changes the structural behavior of studied compounds. The studied compound has direct band gap at x = 0.25 and 0.75 while indirect band gap at x = 0.0, 0.50 and 1.0. The band gap tuned from ultraviolet to visible region (4. eV to 1.7 eV) but the direct nature x = 0.25 and 0.75 increases their potential solar cell applications at specific composition. The reported optical properties show the maximum absorption of light and refractive index is favourable for solar cells. Finally, the transport properties have been elaborated by solving the Boltzmann transport equation. The doping composition Sr 0.50 Ba 0.50 SnO 3 increases the figure of merit up to 0.74 which is suitable for thermoelectric applications.

Harnessing Selective Exsolution of Sn Metal to Enhance Electrical Conductivity in Oxygen‐Deficient Perovskite Stannates

AbstractThe versatile application of newly discovered oxide semiconductors calls for developing a simple process to generate conducting carriers. High‐temperature reduction treatment leads to electrical conduction in perovskite stannate semiconductors, but carrier concentration is poorly controlled and inconsistently reported in BaSnO3−δ films after the reduction process so far. Here, a new strategy to enhance the electrical conductivity of BaSnO3−δ films is demonstrated by exploiting selective exsolution of Sn metals in the perovskite framework. Due to strong dependence of conductivity on initial Sn/Ba cation ratio in the reduced BaSnO3−δ films, interestingly, only Sn‐excess BaSnO3−δ films show a dramatic increase of carrier concentration (∆n3D = 5–7 × 1019 cm−3) after high‐temperature reduction; exceptionally high electrical conductivity (σ ≈ 6000 S cm−1) is achieved in reduced Sn‐excess (La, Ba)SnO3−δ films, which exceed full activation of La dopants in untreated (La, Ba)SnO3. By multiple characterizations combined with theoretical calculation, it is disclosed that a small fraction of segregated β‐Sn nanoparticles is likely to contribute the additional source of n3D in the BaSnO3−δ matrix as a result of spontaneous charge transfer from the segregated β‐Sn metallic phase to BaSnO3−δ. These original results propose a simple strategy to further increase electrical conductivity in perovskite oxide semiconductors by non‐stoichiometry‐driven metal exsolution.

Optical and electronic properties in amorphous BaSnO3 thin films

Wide-bandgap perovskite stannates are of interest for the emergent all-oxide transparent electronic devices due to their unparalleled room temperature electron mobility. Considering the advantage of amorphous material in integrating with non-semiconductor platforms, we herein reported the optical and electronic properties in the prototypical stannate, amorphous barium stannate (BaSnO3) thin films, which were deposited at room temperature and annealed at various temperatures. Despite remaining amorphous status, with increasing the annealing temperature, the defect level within amorphous BaSnO3 thin films could be suppressed. Accordingly, the optical band gap, the leakage current, and the breakdown electric field could be improved from 2.81 eV, 3.94 × 10−6 A cm−2 at 400 kV/cm, 1.43 MV/cm for the as-prepared BaSnO3 thin films to 2.90 eV, 6.17 × 10−8 A cm−2 at 400 kV/cm, 1.93 MV/cm for the 400 °C-annealed BaSnO3 thin films.

Origins of infrared transparency in highly conductive perovskite stannate BaSnO3

Near-infrared absorption in transparent conducting oxides (TCOs) is usually caused by electronic intraband transition at high doping levels. Improved infrared transparency is commonly explained by enhanced drift mobility in these TCOs. Here, an alternative cause behind the high infrared transparency of La-doped barium stannate (LBSO) transparent electrodes is presented. Following the Drude model formalism, we reconstructed spectrally resolved dielectric permittivity for a set of thin films with different free electron concentrations. A comparison of optical properties of LBSO with the tin-doped indium oxide thin films with identical carrier concentrations suggests that the redshift of the screened plasma wavelength for LBSO originates from its large high-frequency dielectric constant of 4.4, one of the highest reported for the s-orbital-based TCOs. Moreover, our measurements confirm an optical mobility significantly higher (>300 cm2/V s) than the drift mobility, effectively suppressing the free carrier absorption. These factors enable high infrared transparency of LBSO films and motivate further exploration of LBSO as broadband TCOs for solar cells and nanophotonics.

Metal Nanoparticle Exsolution on a Perovskite Stannate Support with High Electrical Conductivity.

In situ exsolution of metal nanoparticles (NPs) is emerging as an alternative technique to deliver thermally stable and evenly dispersed metal NPs, which exhibit excellent adhesion with conducting perovskite oxide supports. Here we provide the first demonstration that Ni metal NPs with high areal density (∼175 μm-2) and fine size (∼38.65 nm) are exsolved from an A-site-deficient perovskite stannate support (La0.2Ba0.7Sn0.9Ni0.1O3-δ (LBSNO)). The NPs are strongly anchored and impart coking resistance, and the Ni-exsolved stannates show exceptionally high electrical conductivity (∼700 S·cm-1). The excellent conductivity is attributed to conduction between delocalized Sn 5s orbitals along with structural improvement toward ABO3 stoichiometry in the stannate support. We also reveal that experimental conditions with strong interaction must be optimized to obtain Ni exsolution without degrading the perovskite stannate framework. Our finding suggests a unique process to induce the formation of metal NPs embedded in stannate with excellent electrical properties.

Composition dependent intrinsic defect structures in ASnO3 (A = Ca, Sr, Ba)

Perovskite stannates are promising semiconductors that are widely used in optoelectronic devices. Here, the composition dependent intrinsic point defects of stannate perovskites ASnO3 (A = Ca, Sr, Ba) are studied by first-principles calculations. The preferences of defects under stoichiometric and nonstoichiometric conditions are unsealed, meanwhile the charge states of each intrinsic defect varying with the change of electron Fermi energy are clarified. For stoichiometric BaSnO3 and SrSnO3, the Schottky defect complexes are predicted as the most stable defect structure, while the antisite defect complexes are the most stable one in CaSnO3. In nonstoichiometric ASnO3, excessive AO is beneficial to the formation of oxygen vacancies and A-Sn antisite defects in all ASnO3; while the Ca interstitial is another major defect existing in CaSnO3. In the case of SnO2 excess, the predominant defects are the Sn-A antisite defects and A vacancies. Since the functionality of these perovskite oxides is closely related to the types and concentrations of their point defects, the present results are expected to guide the future experiments to optimize the function of the perovskite oxides by tailoring the intrinsic defects through controlling the composition of AO and SnO2.

First-principles study of antisite defects in perovskite stannates

The perovskite stannates BaSnO3 and SrSnO3 are being actively explored for applications as transparent conductors, in power or high-frequency electronics, and as channel materials in epitaxial integration with functional perovskites. Realizing these applications requires controlled n-type doping, i.e., avoiding the formation of compensating acceptor-type defects. Here, we use density-functional theory to examine the formation of cation antisite defects. Our results indicate that antisites are not a problem in BaSnO3; however, in SrSnO3, SrSn antisites may act as compensating centers.

Metal dopants adjusted perovskite stannates: Conductivity and optical properties

Perovskite-type transparent conductive oxides have attracted continuous attentions due to their applications in optoelectronic devices, but the optimization of their transmittance and conductivity is under way. In this work, the energetics, electronic and optical properties of the perovskite stannates (ASnO3, A = Ca, Sr, Ba) with diverse dopants La, Sb and Nb are studied using density functional theory. The energetically preferred substitution positions are predicted as A site for La but Sn site for Sb and Nb. Therefore, the doped perovskite stannates are all identified as n-type semiconductors due to the extra electron from the dopants. The calculated absorption coefficient and reflectivity in La, Sb and Nb doped perovskite stannates decrease but their transmittance in the visible light range increases as a result of the adjusted band structures. In addition, the carrier concentration dependent electric conductivity of the doped perovskites is predicted. The uncovered mechanism of improved electronic and optical properties for doped perovskite stannates is expected to benefit the design of high performance perovskite transparent conductive materials.

(Invited) High-Mobility Complex Oxide Films By MBE

Functional perovskite oxides may enable entirely new electronic device concepts, ranging from negative capacitance to charge amplification in phase change devices. A major challenge is the intrinsically poor charge carrier mobility of most perovskite oxides, typically no better than 1 - 10 cm2V-1s-1 at room temperature. Recent reports of room temperature mobilities of ~300 cm2 V-1s-1 in single crystals of La-doped BaSnO3 have therefore generated significant excitement. In addition, BaSnO3has a wide band gap (~ 3 eV), which makes it of interest as a new transparent conducting oxide and as a wide band gap semiconductor for power electronics. Furthermore, it would allow for integration of functional perovskite oxides on a lattice- and symmetry-matched, high-mobility, semiconducting channel. Electronic devices require the growth of high-quality films with low defect densities. High-mobility BaSnO3 thin films have been challenging to grow. Molecular beam epitaxy (MBE) is a low energetic deposition technique and routinely produces the highest mobility semiconductor and functional oxide films. However, stoichiometry control in MBE of perovskite oxides can be challenging. We show that in the MBE of perovskite stannates using a Sn metal source, volatile SnO consumes all active oxygen in the growth environment, leaving behind only Sn droplets at low growth temperatures. No film growth occurs at high temperatures. We discuss a modified MBE approach, which supplies pre-oxidized SnO x . This technique addresses issues in the stoichiometry controls in MBE of ternary stannates related to volatile SnO formation and enables growth of epitaxial, stoichiometric BaSnO3. A second challenge is the lack of lattice-matched substrates. We discuss the effect of lattice mismatch on the growth and transport properties of BaSnO3 by comparing the properties of films grown on SrTiO3 and PrScO3 substrates. We show that the carrier mobility doubles as the lattice mismatch is reduced by a factor of two, indicating that high misfit dislocation densities are the main limiting factor in further improving carrier mobilities. We demonstrate room temperature electron mobilities of 150 cm2 V-1s-1 at carrier densities of 7×1019 cm-3 in BaSnO3 films grown on PrScO3 using the modified MBE technique. We will discuss the prospects for further improving the carrier mobilities and reducing the carrier densities.

Strain-induced ferroelectricity and lattice coupling in BaSnO3 and SrSnO3.

Perovskite stannates such as BaSnO3 and SrSnO3 exhibit promising photovoltaic properties, and hold promise for application in solar cell devices. However, the lack of ferroelectricity and the wide band gap in these materials limit their potential for photovoltaic applications. Here, by first-principles calculations, we demonstrate the realization of a primary ferroelectric polarization in non-ferroelectric BaSnO3 and SrSnO3 through strain engineering. In addition to the appearance of polarization, the band gaps of the materials are greatly narrowed when the paraelectric to ferroelectric phase transition takes place under compressive strain. Furthermore, an intriguing Q2 mode triggered by lattice coupling with the polar mode is found in the stannates subjected to a sufficient tensile strain and this mode has a significant effect on the band gap, which suggests another pathway to narrow the band gap through the electric field control of the Q2 mode. The fruitful electronic, structural, and energetic properties are discussed in detail to achieve a fundamental understanding of the strain-induced ferroelectricity, tunable band gap, and lattice couplings between the Q2 mode and different polar/rotational distortions in the perovskite stannates.