K. Char’s research while affiliated with Seoul National University and other places

The LaInO3/BaSnO3 heterostructure has recently emerged as a promising platform for realizing 2D electron gas (2DEG) with unique transport properties, including excellent field‐effect at room temperature. However, there is a limit to improving its mobility due to intrinsic defects including the threading dislocations occurring during film growth. In spite of such high density defects at present, as an effort to increase the mobility of the 2DEG, the 2D carrier density to 10¹⁴ cm⁻² by ionic‐liquid gating is increased and we found the resulting 2DEG mobility enhancement up to 2100 cm² V⁻¹ s⁻¹ at 10 K, which is consistent with the fact that 2‐dimensionality offers more effective screening for defects. This findings offer insights into the properties of 2DEG formed with perovskite oxide semiconductor BaSnO3 as well as highlight its future potential for applications.

Ferroelectric field-effect transistors offer a potential for its important role in integrated memory and computing systems, and research on them is actively ongoing. In this study, we investigated the ferroelectric properties of Pb(Zr0.7Ti0.3)O3, which is lattice matched with the La-doped high mobility perovskite oxide semiconductor Ba1−xLaxSnO3. Growth of the r phase in epitaxial Pb(Zr0.7Ti0.3)O3 on Ba1−xLaxSnO3 was confirmed and its basic ferroelectric and dielectric properties were studied by polarization-electric and capacitance-voltage measurement. We then studied the field effect of Pb(Zr0.7Ti0.3)O3 on the electrical properties of Ba1−xLaxSnO3 as we vary the La doping concentration. We find that the field effect of Pb(Zr0.7Ti0.3)O3 on Ba1−xLaxSnO3 is determined by competition between its ferroelectric and dielectric properties, depending on the La doping concentration. In high La doping rates, the field effect is dominated by the ferroelectric switching while in lower La doping rates, the effect is mainly by dielectric response as the depolarization field in the depleted layer weakens the ferroelectric effect. As per the width and direction of the hysteresis, it is also controlled by the competition between the counterclockwise ferroelectric response and the clockwise dielectric response due to the trapped charges near the interface. This study offers insights into optimizing the field effect by understanding the complex interplay between ferroelectric materials and low carrier density semiconductors.

Perovskite oxide semiconductor is unique for its capability to form epitaxial heterostructures with both dielectric and metallic perovskite oxides. The study underscores the potential of perovskite oxides for multi‐layer stacking, a key aspect in advancing semiconductor technology as silicon‐based devices evolve toward 3D stacked structures. Fabrication of the first double‐level double‐gate field‐effect transistors (DL DG‐FETs) is demonstrated, where each layer is epitaxially grown using all‐perovskite oxides. This resulted in improvements in subthreshold swing, current drivability, and field effect mobility. This innovation not only highlights the distinctive potential of perovskite oxides but also provides new avenues for integration with other perovskite oxides on Si for more advanced electronic functions.

An ex situ chemical etching method was developed to achieve a SnO2-terminated surface in BaSnO3 films. An SnO2-terminated surface is crucial for the formation of a (LaO)+/(SnO2)0 interface structure to form the two-dimensional electron gas (2DEG) state at the LaInO3 (LIO)/BaSnO3 (BSO) interface. By employing a 9:1 mixture of acetone and water, the etching rate of the surface barium oxide (BaO) layer could be effectively controlled, taking advantage of the solubility of BaO in water. To determine the optimal etching conditions, we investigated the relationship between the etching time and the resulting 2DEG conductance. The optimum times for maximizing the conductance of the 2DEG state were found to be 90 s on SrTiO3 substrates and 40 s on MgO substrates, generating a higher conductance than the in situ SnO2 dusting method reported earlier. The surface properties before and after the chemical etching were analyzed by angle reserved x-ray photoelectron spectroscopy.

Ultra‐wide bandgap semiconductors are gaining attention for their promising properties for UV optoelectronics and UV transparent electronics as well as high‐power applications. Among them, La‐doped SrSnO 3 exhibits excellent properties both for deep‐UV transparent oxide semiconductors and deep‐UV transparent conducting oxide. Here, the demonstration of thin film transistors (TFTs) with full deep‐UV transparency is reported, including electrodes, gate oxide, and substrate. The lightly La‐doped SrSnO 3 for the channel layer is grown on MgO (100) substrates with buffer layers by pulsed laser deposition. TFTs with a metal—insulator–semiconductor structure are fabricated using high‐k perovskite dielectric LaScO 3 as the gate oxide. A degenerately La‐doped SrSnO 3 is used as the gate, the source, and the drain electrodes to obtain good ohmic contact with the channel layer as well as UV transparency. The resultant device shows a field effect mobility value of ≈24 cm ² V ⁻¹ s ⁻¹ and an on/off ratio >10 ⁶ . The optical transmittance of the entire device (including the substrate) is found to be >75% at 300 nm in wavelength. Furthermore, the electrical characteristics of the device exhibit excellent stability under visible irradiation. This research highlights the potential of SrSnO 3 in advancing the field of UV optoelectronics and UV transparent electronics.

Abstract Reducing the leakage current through the gate oxide is becoming increasingly important for power consumption reduction as well as reliability in integrated circuits as the semiconducting devices continue to scale down. Here, this work reports on the high‐k dielectric SrHfO3 (SHO) based devices with ultralow leakage current density via pulsed laser deposition (PLD). The ultralow current density is achieved by optimizing the growth conditions and the associated structural properties. In the optimized conditions, the dielectric properties of the 50‐nm‐thick SHO capacitors are measured: high dielectric constant (κ = 32), low leakage current density ( 4 MV cm−1). The surprisingly low leakage current density of SHO is ascribed to the large bandgap (≈6 eV), the large conduction band offset (CB offset > 3 eV) with respect to the semiconductor, and the low density of defect states inside the bandgap. The optimized SHO dielectric with high dielectric constant and ultralow leakage current density is proposed for future low‐power consumption devices based on Si as well as perovskite oxide semiconductors.

The properties of the conductance at the LaInO3/BaSnO3 heterointerface are reported. The heterointerface is formed by covering the semi-insulating BaSnO3:La thin films with 10 nm LaInO3 films, which are all epitaxially grown on NdScO3 substrates. Structural properties of BaSnO3 thin films are investigated by means of X-ray diffraction and transmission electron microscopy and exhibit a threading dislocation density of 6 × 10^10 cm−2. Via capacitance–voltage (C–V) measurements, clear evidence is present for the accumulation of electrons at the interface within 2.5 nm in the BaSnO3 layer, confirming the formation of a 2D electron gas (2DEG). Additionally, temperature dependent Hall effect measurements reveal a semiconducting behavior of the electron density of the 2DEGs. The room temperature mobility of 22 cm^2 V^−1 s^−1 at an electron density of 4 × 10^13 cm−2 is found to increase as the temperature decreases to 25 K.

As the size of the semiconductor device decreases, the importance of the low resistance contacts to devices cannot be overstated. Here, we studied the contact resistance to buried nanometer thick δ-doped Ba 1-x La x SnO 3 (BLSO) layers. We have used epitaxial 4% (x = 0.04) BLSO as a contact material, which has additional advantages of forming Ohmic contacts to BaSnO 3 and providing thermal stability even at high temperatures. The contact resistance was measured by a modified transmission line method designed to eliminate the contribution from the resistance of the contact material. The upper bound for the contact resistance to a 12 nm thick δ-doped 1% BLSO conductive layer was measured to be 1.25 × 10 ⁻¹ or 2.87 × 10 ⁻⁷ Ω cm ² . Our results show that it is possible to provide low resistance epitaxial edge contacts to an embedded nanometer-thick BLSO conductive layer using an ion-milling process. Our low resistance contact method can be easily extended to a two-dimensional electron gas at the oxide interfaces such as LaInO 3 /BaSnO 3 .

Based on the interface polarization model, the 2D electron gas (2DEG) at LaInO3(LIO)/BaSnO3(BSO) interfaces is understood to originate from a polarization discontinuity at the interface and the conduction band offset between LIO and BSO. In this scenario, the direction of polarization at the interface is determined by whether the first atomic LIO layer at the interface is LaO+ or InO2−. The role of the terminating layer is investigated at the LIO/BSO interface in creating the 2DEG. Based on conductance measurements of the in situ grown LIO/BSO heterostructures, it has been reported in this work that the 2DEG only forms when the BSO surface is terminated mainly with a SnO2 layer. The terminating layer is controlled by additional SnO2 deposition on the BSO surface. It has been shown that the as‐grown BSO surface has a mixed terminating layer of BaO and SnO2 while the BSO surfaces prepared with additional SnO2 deposition are terminated mainly with the SnO2 layer. The terminating layer is confirmed by coaxial impact collision ion scattering spectroscopy. The finding is consistent with the interface polarization model for 2DEG formation at LIO/BSO interfaces, in which the direction of the interfacial polarization in LIO is determined by the terminating layer of the BSO surface. The LaInO3/BaSnO3 2D electron gas (2DEG) is understood by a polarization discontinuity at the interface. The direction of interfacial polarization is determined by the first atomic LaInO3 layer on the terminating layer of BaSnO3. In this paper, it is revealed that the LaInO3/BaSnO3 2DEG only forms when the BaSnO3 surface is terminated mainly with a SnO2 layer.