Development of high-breakdown voltage power devices requires designing suitable p-type wide-bandgap oxide semiconductors.

Development of high-breakdown voltage power devices requires designing suitable p-type wide-bandgap oxide semiconductors. We investigated the growth of Nb-doped Bi2WO6 (Nb:BWO) thin films, which are promising candidates for p-type wide-bandgap oxide semiconductors, via pulsed laser deposition (PLD) and solid-phase epitaxy (SPE). While X-ray diffraction analyses confirmed the successful growth of the target Nb:BWO phase by both methods, X-ray fluorescence and photoemission spectroscopy revealed the presence of a significant Bi deficiency and defect states near the Fermi level in the PLD-grown film, in contrast to the SPE-grown film. These defects likely degrade device performance, as demonstrated by the poorer rectifying behavior of the Nb:BWO/beta-Ga2O3 heterojunctions fabricated using the PLD-grown film. By contrast, the SPE-grown film showed better stoichiometry and improved electronic properties. These results indicate that SPE is a superior method for growing high-quality p-type oxide films suitable for power device applications.

In order to understand the various functions and properties emerging at surfaces and interfaces of thin films, it is insufficient to evaluate them using a single method, but a comprehensive analysis should be performed by combining multiple probes and methods. We have been developing the beamline-crossing sample transfer system that enables the sample transfer from a sample preparation chamber to an analysis device and between multiple end stations preventing the degradation of the sample surface due to air exposure. To realize this system, we performed the standardization of the holder shape at end stations for surface science and the installation of a sample transfer vessel. The results of photoemission measurements on SnO thin films, which are easily degraded (oxidized) in air, indicate the effectiveness of the developed system for surface analysis.

Thin-film transistors based on oxide semiconductors, such as amorphous InGaZnO4 (a-IGZO), are widely used in edge devices owing to their low power consumption. However, the fundamental limitation of the subthreshold swing (SS) hinders both high-speed response and further reduction in power consumption. In this study, we focus on the concept of the tunnel field-effect transistor (TFET), which can significantly reduce SS, and explore its application to a-IGZO-based transistors. To design the a-IGZO based TFETs, we experimentally determined the band diagrams of a-IGZO/SnO and a-IGZO/Nb-doped Bi2WO6 (Nb:BWO), considering depletion layers, using photoemission spectroscopy. Both heterojunctions exhibited type II band alignment, with the energy barriers between the conduction band minimum of a-IGZO and valence band maximum of SnO or Nb:BWO being 0.57 +/- 0.05 and 1.72 +/- 0.05 eV, respectively. Simulations based on these band diagrams demonstrated the on-off switching behavior of the a-IGZO/SnO TFET only, achieving a minimum SS value of 48 mV/dec with a 3 mm-thick a-IGZO, assuming an ideal interface. These results suggest that optimizing junction properties and material selection plays a crucial role in the realization of a-IGZO-based TFETs for enhanced energy efficiency in electronic devices. (c) 2025 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC) license

The operation of superconducting qubits requires a sensitive readout circuit at cryogenic temperatures, driving the demand for cryogenic non reciprocal microwave components such as circulators. However, evaluating these components at low temperatures presents significant challenges for companies and institutions without specialized measurement systems. In the development of such cryogenic non reciprocal components, the temperature dependence of ferrite's magnetic properties is the most critical factor. Therefore, an evaluation technique for accurately assessing these properties at cryogenic temperatures is essential. In this study, we develop a measurement method to characterize low loss ferrite materials over a temperature range of 300 K to 2 K. The use of the circularly polarized resonance mode TE_11n enables the direct estimation of circular complex permeability and the determination of key material parameters, including saturation magnetization and damping constant both essential for assessing the performance of ferrite materials in circulator applications. To validate the reliability of our measurement method, we selected single crystal YIG as the test material, as its magnetic properties at cryogenic temperatures are relatively well known. This demonstration confirms that our method is effective for characterizing various low loss ferrite materials that are potential candidates for compact cryogenic non reciprocal devices.

Based on the fundamental design concept of modulating the valence band maximum of oxides and subsequent predictions through computational approaches, several lone-pair ns2-based p-type oxide semiconductors, such as Sn2+- or Bi3+-based complex oxides, have been developed. Thus far, the bandgap can be modified via tuning of the chemical composition, whereas the hole density cannot be intentionally controlled because of the poor chemical stability of Sn2+ and/or the formation of oxygen vacancies. The inability to control hole density prohibits the design and realization of emergent electronic devices based on p- and n-type oxide semiconductors. Herein, we report the control of hole density via intentional chemical doping in polycrystalline Bi2WO6. While the holes of polycrystalline Nb- or Ta-doped Bi2WO6 are strongly trapped by grain boundaries, the hole density obtained at high temperatures monotonically increases with the increase in the doping concentration. This study provides important insights into the development of practical p-type oxide semiconductors.

Abstract High energy‐conversion efficiency (ZT) of thermoelectric materials has been achieved in heavy metal chalcogenides, but the use of toxic Pb or Te is an obstacle for wide applications of thermoelectricity. Here, high ZT is demonstrated in toxic‐element free Ba3BO (B = Si and Ge) with inverse‐perovskite structure. The negatively charged B ion contributes to hole transport with long carrier life time, and their highly dispersive bands with multiple valley degeneracy realize both high p‐type electronic conductivity and high Seebeck coefficient, resulting in high power factor (PF). In addition, extremely low lattice thermal conductivities (κlat) 1.0–0.4 W m−1 K−1 at T = 300–600 K are observed in Ba3BO. Highly distorted O–Ba6 octahedral framework with weak ionic bonds between Ba with large mass and O provides low phonon velocities and strong phonon scattering in Ba3BO. As a consequence of high PF and low κlat, Ba3SiO (Ba3GeO) exhibits rather high ZT = 0.16–0.84 (0.35–0.65) at T = 300–623 K (300–523 K). Finally, based on first‐principles carrier and phonon transport calculations, maximum ZT is predicted to be 2.14 for Ba3SiO and 1.21 for Ba3GeO at T = 600 K by optimizing hole concentration. Present results propose that inverse‐perovskites would be a new platform of environmentally‐benign high‐ZT thermoelectric materials.

This study developed a high-precision sample goniometer that can be driven over a wide range of angles to examine the detailed and comprehensive electronic structure of materials via angle-resolved photoemission spectroscopy (ARPES). The sample goniometer can tilt the sample in the range of +/- 90 degrees from the normal direction of the sample. Combined with a rotary feedthrough that controls the polar angle, all the photoelectrons emitted from the sample surface can be detected. The sample goniometer can rotate the azimuth angle of the sample in the range of +/- 180 degrees, facilitating the investigation of the symmetry of the electronic structure via polarization of the incident light. The tilt rotation and the azimuth rotation could be controlled with repeatability of +/- 0.007 degrees and +/- 0.016 degrees, respectively, by correcting gear transmission errors. The ARPES measurements of Au(1 1 1), a SnO (001) film, and a SnO film after exposing to the atmosphere demonstrated the effectiveness of the developed sample goniometer.

Sn2Nb2-xTaxO7 (x = 0.0-2.0) with pyrochlore structure is a promising material for p-type oxide semiconductors. A systematic study of its Nb/Ta ratio indicated that the hole-generation efficiency of the Nb end (Sn2Nb2O7) was an order of magnitude lower than that of the Ta end (Sn2Ta2O7). Although this occurs due to differences in oxygen-vacancy formation, the origins of the hole-generation efficiencies remain unclear due to limited information on local and global crystal-structure disorders in pyrochlore Sn2Nb2O7 and Sn2Ta2O7. In this study, the crystal structures of Sn2B2O7 (B = Nb, Ta), composed of BO6 octahedra and Sn4O tetrahedra, were investigated using X-ray absorption spectroscopy and X-ray diffraction. A detailed investigation of the local and global crystal structures indicated a larger amount of disorder in the Sn4O tetrahedra in Sn2Nb2O7 compared to Sn2Ta2O7; disorder in the BO6 octahedra occurred only in Sn2Ta2O7. This study indicates that an appropriate selection of the B-site element is vital for suppressing defect and disorder formation in Sn4O tetrahedra and subsequently improving the hole-carrier-generation efficiency.

Materials exhibiting unique electronic properties arising from a characteristic crystal structure have physical properties that are sensitive to structural dimensionality. This study involves the destabilization of Sn 5s2 lone-pair states of SnO films by decreasing their structural dimensionality in the out-of-plane direction. The inherent dispersive band structure of the SnO films remained unchanged between 80 and 11 nm. Below 11 nm, their dispersive band structure disappeared, the O/Sn ratio increased, and the carrier type changed from the p type to the n type, whereas the Sn valency remained constant at +2. These unconventional changes arose from the electronic separation corresponding to the Debye length, which is proportional to permittivity, and were attributed to weakened interactions between Sn 5s2 lone-pair electrons. Therefore, designing low-permittivity materials is beneficial for reducing the crystallite size required for stabilizing lone-pair states. These results are essential for designing emergent p-type oxides and improving their semiconducting properties and performance in transparent or high-power electronics.

Metastable cubic (Sn1-xPbx)Se with x ≥ 0.5 is expected to be a high mobility semiconductor due to its Dirac-like electronic state, but it has an excessively high carrier concentration of ∼1019 cm-3 and is not suitable for semiconductor device applications such as thin film transistors and solar cells. Further, thin films of (Sn1-xPbx)Se require a complicated synthesis process because of the high vapor pressure of Pb. We herein report the direct growth of metastable cubic (Sn1-xCax)Se films alloyed with CaSe, which has a wider bandgap and lower vapor pressure than PbSe. The cubic (Sn1-xCax)Se epitaxial films with x = 0.4-0.8 are stabilized on YSZ (111) single crystalline substrates by pulsed laser deposition. (Sn1-xCax)Se has a direct-transition-type bandgap, and the bandgap energy can be varied from 1.4 eV (x = 0.4) to 2.0 eV (x = 0.8) by changing x. These films with x = 0.4-0.6 show p-type conduction with low hole carrier concentrations of ∼1017 cm-3. Hall mobility analysis suggests that the hole transport would be dominated by 180° rotational domain structures, which is specific to (111) oriented epitaxial films. However, it, in turn, clarifies that the in-grain carrier mobility in the (Sn0.6Ca0.4)Se film is as high as 322 cm2/(Vs), which is much higher than those in thermodynamically stable layered SnSe and other Sn-based layered semiconductor films at room temperature. Therefore, the present results prove the potential of high mobility (Sn1-xCax)Se films for semiconductor device applications via a simple thin-film deposition process.

The relationship between the magnetic interaction and photoinduced dynamics in antiferromagnetic perovskites is investigated in this study. In La_1/3Sr_2/3FeO_3 thin films, commensurate spin ordering is accompanied by charge disproportionation, whereas SrFeO_3 thin films show incommensurate helical antiferromagnetic spin ordering due to increased ferromagnetic coupling compared to La_1/3Sr_2/3FeO_3. To understand the photoinduced spin dynamics in these materials, we investigate the spin ordering through time-resolved resonant soft X-ray scattering. In La_1/3Sr_2/3FeO_3, ultrafast quenching of the magnetic ordering within 130 fs through a nonthermal process is observed, triggered by charge transfer between the Fe atoms. We compare this to the photoinduced dynamics of the helical magnetic ordering of SrFeO_3. We find that the change in the magnetic coupling through optically induced charge transfer can offer an even more efficient channel for spin-order manipulation.

The development of p-type oxide semiconductors is challenging owing to the localized nature of the valence band maximum (VBM), which primarily comprises O 2p orbitals. Although some Sn2+-based pyrochlore-type oxides (A2B2O7) with VBMs comprising spatially spread Sn 5s orbitals show p-type semiconducting properties, these properties cannot be tuned by an intentional chemical doping owing to the instability of their valence states. Herein, the influence of chemical doping on the crystal structure and electrical properties of Bi2Sn2O7, whose VBM is composed of valence-stable ions with Bi 6s orbitals, in the VBM is investigated. X-ray absorption spectroscopy reveals that In3+ is doped substitutionally at the Sn4+ site. Although the doped In3+ ions are expected to act as acceptors, their electrical properties always show n-type character. Simultaneously, extended X-ray absorption fine structure reveals the formation of oxygen vacancies in BiOBi bonds contributing to the VBM, whereas negligible changes are observed in the SnO6 octahedra, suggesting that the generated holes are electrically compensated by electrons because of the oxygen vacancies. P-type electric properties can be observed under oxidizing conditions, providing a critical design concept for the realization of hole conduction in acceptor-doped Bi2Sn2O7.

Actual knowledge of the intrinsic electronic characteristics of p-type oxide semiconductors should help guide the design of innovative electronic devices. The electronic characteristics of oxide semiconductors in thin-film form potentially differ from those in the bulk form owing to lattice strain. In this Letter, we report on the empirical band structure of stannous oxide (SnO) film, which has been shown to have a higher hole mobility than the theoretically expected values for SnO in the bulk form. In vacuo angle-resolved photoemission spectroscopy measurements reveal that the uppermost valence band is anisotropic between the out-of-plane and in-plane directions, and more dispersive than the theoretical predictions. Our findings unveil the underlying mechanism of the semiconductor properties of SnO films and suggest a suitable device structure based on the electronic characteristics.

We report the synthesis and optoelectronic properties of high phase-purity (>94 mol %) bulk polycrystals of KCoO2-type layered nitrides AETMN2 (AE = Sr, Ba; and TM = Ti, Zr, Hf), which are expected to exhibit unique electron transport properties originating from their natural two-dimensional (2D) electronic structure, but high-purity intrinsic samples have yet been reported. The bulks were synthesized using a solid-state reaction between AENH and TMN precursors with NaN3 to achieve high N chemical potential during the reaction. The AETMN2 bulks are n-type semiconductors with optical band gaps of 1.63 eV for SrTiN2, 1.97 eV for BaZrN2, and 2.17 eV for BaHfN2. SrTiN2 and BaZrN2 bulks show degenerated electron conduction due to the natural high-density electron doping and paramagnetic behavior in all of the temperature ranges examined, while such unintentional carrier generation is largely suppressed in BaHfN2, which exhibits nondegenerated electron conduction. The BaHfN2 sample also exhibits weak ferromagnetic behavior at temperatures lower than 35 K. Density functional theory calculations suggest that the high-density electron carriers in SrTiN2 come from oxygen impurity substitution at the N site (ON) acting as a shallow donor even if the high-N chemical potential synthesis conditions are employed. On the other hand, the formation energy of ON becomes larger in BaHfN2 because of the stronger TM-N chemical bonds. Present results demonstrate that the easiness of impurity incorporation is designed by density functional calculations to produce a more intrinsic semiconductor in wider chemical conditions, opening a way to cultivating novel functional materials that are sensitive to atmospheric impurities and defects.

We demonstrate that hole carrier density of α-SnWO 4 can be tuned by controlling the annealing process. Hole carrier density increased by two orders of magnitude and achieved a value close to 10 19 cm −3 at an optimum O 2 gas concentration.

AbstractAmbipolar transistor operation in SnO thin‐film transistors (TFTs) is a promising character for future practical application, such as in integrated logic devices based on oxide semiconductors, because of its ability to develop them using a single material. However, there are only a few reports that demonstrate the apparent ambipolar operation for SnO TFTs owing to the insufficient knowledge on the reasons for deterioration of device performance. Although a previous study controls the operation mode of SnO TFTs [A. W. Lee et al. Adv. Electron. Mater. 6, 200742 (2020)], an additional passivating layer is required; however, it hinders the benefits of SnO usage. In this study, we provide the mechanism of deterioration of the ambipolar character of bare SnO TFTs, that is, the origin of defect states near the valence band maximum (near‐VB defect). Comprehensive spectroscopic approaches including photoemission, X‐ray emission, and X‐ray absorption spectroscopy, reveal that near‐VB defect states originate from oxygen vacancies, existing at the surface, and also in the entire SnO film. This finding is useful to improve device performance for obtaining the ambipolar operation of SnO TFTs.

An unusually large thermopower (S) enhancement is induced by heterostructuring thin films of the strongly correlated electron oxide LaNiO3. The phonon-drag effect, which is not observed in bulk LaNiO3, enhances S for thin films compressively strained by LaAlO3 substrates. By a reduction in the layer thickness down to three unit cells and subsequent LaAlO3 surface termination, a 10 times S enhancement over the bulk value is observed due to large phonon drag S (Sg), and the Sg contribution to the total S occurs over a much wider temperature range up to 220 K. The Sg enhancement originates from the coupling of lattice vibration to the d electrons with large effective mass in the compressively strained ultrathin LaNiO3, and the electron-phonon interaction is largely enhanced by the phonon leakage from the LaAlO3 substrate and the capping layer. The transition-metal oxide heterostructures emerge as a new playground to manipulate electronic and phononic properties in the quest for high-performance thermoelectrics.

Divalent tin oxides have attracted considerable attention as novel p-type oxide semiconductors, which are essential for realizing future oxide electronic devices. Recently, p-type Sn2Nb2O7 and SnNb2O6 were developed; however, enhanced hole mobility by reducing defect concentrations is required for practical use. In this work, we investigate the correlation between the formation of oxygen vacancy which may reduce the hole-generation efficiency and hole mobility, and the crystal structure in Sn-Nb complex oxides. Extended X-ray absorption fine structure spectroscopy and Rietveld analysis of x-ray diffraction revealed the preferential formation of oxygen vacancy at the O site bonded to the Sn ions in both the tin niobates. Moreover, a large amount of oxygen vacancy around the Sn ions were found in the p-type Sn2Nb2O7, thereby indicating the effect of oxygen vacancy to the low hole-generation efficiency. The dependence of the formation of oxygen vacancy on the crystal structure can be elucidated from the Sn-O bond strength that is evaluated based on the bond valence sum and Debye temperature. The differences in the bond strengths of the two Sn-Nb complex oxides are correlated through the steric hindrance of Sn2+ with asymmetric electron density distribution. This suggests the importance of the material design with a focus on the local structure around the Sn ions to prevent the formation of oxygen vacancy in p-type Sn2+ oxides.

Diodes, memories, logic circuits, and most other current information technologies rely on the combined use of p- and n-type semiconductors. Although oxide semiconductors have many technologically attractive functionalities, such as transparency and high dopability to enable their use as conducting films, they typically lack bipolar conductivity. In particular, the absence of p-type semiconducting properties owing to the innate electronic structures of oxides represents a bottleneck for the development of practical devices. Here, bipolar semiconducting properties are demonstrated in α-SnWO4 within a 100 °C temperature window after appropriate thermal treatment. Comprehensive spectroscopic observations reveal that Sn4+ is present in p-type α-SnWO4 in a notably greater quantity than in n-type. This result strongly suggests that the Sn4+ substitutional defects on the W6+ sites contribute to hole-carrier generation in α-SnWO4. We also find that oxygen vacancies are initially formed in Sn-O-W bonds and migrate to W-O-W bonds with changes in semiconducting properties from p-type to n-type. These findings suggest useful strategies for exploring p-type oxide semiconductors and controlling their carrier type by utilizing the octahedral structure.