PCSI2026 Session PCSI-MoE: Oxides I

The ultimate scaling limit in ferroelectric switching has been attracting broad attention in the fields of materials science and nanoelectronics. Despite immense efforts to scale down ferroelectric features, however, only few materials have been shown to exhibit ferroelectricity at the unit-cell level.

Here we report a controllable unit-cell-scale domain in brownmillerite oxides [1] consisting of alternating octahedral/tetrahedral layers. Our machine-learning force-field (MLFF) phonon calculations reveal that the phonon modes related to oxygen octahedra are decoupled from those of the oxygen tetrahedra in brownmillerite oxides, and such localized oxygen tetrahedral phonons stabilize the sub-unit-cell-segmented ferroelectric domain (Fig.1) [2]. The discovery of unit-cell-scale ferroelectricity opens new possibilities for designing ultrahigh-density memory devices through phonon-mode engineering and interlayer decoupling [3].

[1] Y. Xing^*, I. Kim^*, K. T. Kang^* et al., Nature Chemistry 17, 66-73 (2025).

[2] J. Jang^*, Y. Jin^*, Y. Nam^* et al., Nature Materials 24, 1228 (2025).

[3] J. Hwang^*, S. Jeong^* et al., under review in Physical Review Letters (2025).

⁺Author for correspondence: jaekwangl@pusan.ac.kr

Dielectric and ferroelectric materials are fundamental to modern electronic and energy storage technologies, enabling applications ranging from high-κ gate dielectrics to capacitors and nonvolatile memories. However, the discovery and optimization of new materials through conventional trial-and-error approaches remain slow and resource-intensive. Establishing a general-purpose platform that integratesmaterials informatics, combinatorial synthesis, and high-throughput characterization can dramatically accelerate the exploration of complex composition-structure-property relationships. Such an approach enables rapid mapping of materials phase space, identification of promising candidates, and iterative feedback between data-driven prediction and experimental realization, ultimately transforming the pace of dielectric and ferroelectric materials innovation.

In this talk, I will discuss our progress on developing high-throughput thin film synthesis for ferroelectric and dielectric thin films. In the first part, I will discuss the design of (Ba_0.7Ca_0.3)TiO₃ (BCT) and Ba(Ti_0.8Zr_0.2)O₃ (BZT) superlattices via a high-throughput combinatorial approach. In the second part of the talk, I will discuss strategies to further optimize domain structures and suppress the leakage current in BZT-BCT films via a machine learning approach. The large polarization and the delayed polarization saturation led to greatly enhanced energy density of 80 J/cm³ and transfer efficiency of 85% over a wide temperature range. Such a data-driven design recipe for a slush-like polar state is generally applicable to quickly optimize functionalities of ferroelectric materials. Figure 1 shows the schematics of the combinatorial synthesis approach of growing the BCT-BZT thin films.

The GeO₂/Ge interface is known to generate positive fixed charges upon air exposure, posing challenges for device applications. Our research group has focused on the role of water molecules and their interaction with GeO₂/Ge surfaces. Using in situ X-ray photoelectron spectroscopy (XPS) with synchrotron radiated light, we systematically studied the relationship between relative humidity and the thickness of the adsorbed water layer. We also revealed substantial positive charging of GeO₂ films at humidity levels above 10^-4%, coinciding with the onset of water adsorption. While this effect was attributed to water penetration into the oxide, possible contributions from X-ray interactions complicated the interpretation.

In this talk, we present an investigation of the charging characteristics of GeO₂/Ge structures pre-exposed to controlled humidity conditions, evaluated through capacitance–voltage (C–V) measurements of metal-oxide-semiconductor (MOS) capacitors. To enable these experiments, we developed an integrated reaction chamber capable of annealing, controlled water adsorption, electrode formation via vacuum deposition, and electrical measurements on thermally oxidized GeO₂/Ge samples (550 °C) under high vacuum [1]. As part of the performance assessment of this system, MOS structures were fabricated on GeO₂/Ge structures exposed for 3 h to five different humidity levels (10^-6%, 0.01%, 0.1%, 0.9%, and 2.0 %) in the reaction chamber, as well as on an as-oxidized reference sample transported in air. The C–V curves of samples exposed to 1% RH or higher, as well as that of the as-oxidized sample, showed hysteresis and a significant negative shift. Furthermore, this hysteresis was identified as injection-type behavior. This phenomenon is attributed to the adsorption of water molecules onto the GeO₂/Ge structure at humidity higher than approximately 1% [1].

[1] S. Sano, H. Takano et al., J. Appl. Phys. 137, 155304 (2025).

In₂O₃ is an oxide semiconductor with high transparency and conductivity, widely used in transparent conducting films and channel layers of thin-film transistors (TFTs). For application to high-mobility TFTs, we have been studying the deposition of ~10 nm In₂O₃ films on SiO₂/Si substrates by the mist chemical vapor deposition (Mist CVD) method. Mist CVD is a technique in which the precursor solution is atomized by ultrasonic vibration and thermally decomposed on a heated substrate. The precursor solution is typically prepared by dissolving metal powders in an acidic solvent such as HCl. In our previous studies, it was confirmed that decreasing the HCl concentration in the solution during Mist CVD resulted in thinner In₂O₃ films [1]. On the other hand, when the concentration approaches the solubility limit, undissolved In₂O₃ powder remains, which negatively affects the film quality. To address this issue, we focused on InCl₃ as an alternative precursor. Since InCl₃ is easily soluble in deionized water, HCl is not required to prepare the solution, and it is expected that thinner In₂O₃ films can be obtained simply by reducing the InCl₃ concentration [2]. In this study, In₂O₃ thin films were deposited on SiO₂/Si substrates by Mist CVD using InCl₃ solutions with concentrations between 0.0050 and 0.20 mol/L. The solutions were used after visually confirming complete dissolution by visual inspection.

Figure 1 shows the film thickness as a function of InCl₃ concentration measured using SE and TEM. The results show that the In₂O₃ films became thinner as the InCl₃ concentration decreased.An In₂O₃ film thickness of about 10 nm was achieved at the concentration of 0.0050 mol/L. Figure 2 shows the bird’s-eye FE-SEM images of In₂O₃ films grown at the InCl₃ concentrations of 0.0050 and 0.20 mol/L. At the concentration of 0.0050 mol/L, the smoother surface was obtained with smaller crystal grain sizes, while at 0.20 mol/L, the three-dimensional surface was obtained with large crystal grain sizes. The flattening of the surface with decreasing InCl₃ concentration can be explained by the smaller grain size. In the presentation, we will comprehensively discuss the structure of the grown In₂O₃ thin films, including their electrical properties.

Quantum interference – whether observed in the form of Aharonov-Bohm effect, the Altshuler–Aronov–Spivak effect, or quantum conductance fluctuations – offers valuable insights into electron dynamics within an electromagnetic environment. These phenomena allow direct measurement of the phase coherence length, which is critically important for developing quantum technologies such as quantum sensing, quantum computing, and quantum communication.

We report quantum oscillations in magnetoresistance that are periodic in magnetic field (B), observed at the interface between (La_0.3Sr_0.7)(Al_0.65Ta_0.35)O₃ and SrTiO₃ [1]. Unlike Shubnikov–de Haas oscillations, which appear at magnetic fields > 7 T and diminish quickly as the temperature rises, these B-periodic oscillations emerge at low fields and persist up to 10 K. Their amplitude decays exponentially with both temperature and field, specifying the dephasing of quantum interference. Increasing the carrier density through electrostatic gating results in a systematic reduction in both the amplitude and frequency of the oscillations, with complete suppression beyond a certain gate voltage. We attribute these oscillations to the Altshuler–Aronov–Spivak effect, likely arising from naturally formed closed-loop paths due to the interconnected quasi-one-dimensional conduction channels along SrTiO₃ domain walls. The relatively long phase coherence length (~ 1.8 μm at 0.1 K), estimated from the oscillation amplitude, highlights the potential of complex oxide interfaces as a promising platform for exploring quantum interference effects and advancing device concepts in quantum technologies, such as mesoscopic interferometers and quantum sensors.