Hyeongmin Cho’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.

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.

Based on the interface polarization model, the two-dimensional electron gas (2DEG) at $LaInO_{3}$(LIO)/$BaSnO_{3}$(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 InO$_{2}^{-}$. We investigate the role of the terminating layer at the LIO/BSO interface in creating the 2DEG. Based on conductance measurements of our in-situ grown LIO/BSO heterostructures, we report in this work that the 2DEG only forms when the BSO surface is terminated with a SnO$_{2}$ layer. We controlled the terminating layer by additional SnO$_{2}$ deposition on the BSO surface. We show that the as-grown BSO surface has a mixed terminating layer of BaO and SnO$_{2}$ while the BSO surfaces prepared with additional SnO$_{2}$ deposition are terminated mainly with the SnO$_{2}$ layer. The terminating layer was confirmed by coaxial impact collision ion scattering spectroscopy (CAICISS). Our 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.

2DEG systems formed in quantum wells of semiconductor heterostructures have been instrumental in advancing science and technology for many decades. Here, we report two unique transport properties of 2DEG formed at the interface of two perovskite oxides LaInO3 (LIO) and BaSnO3 (BSO): the peculiar LIO thickness dependence of the high two-dimensional (2D) carrier density ($n_{2{\rm{D}}}$) and the very narrow width of the quantum well. We analyze, via Poisson-Schrödinger simulation, how the various materials parameters affect the 2D carrier density and its profile when using the “interface-polarization” model in which the polarization exists only near the interface. Our simulations show that the known material parameters of LIO and BSO are capable of generating a deep and narrow quantum well as suggested by the experimental transport properties and reveal some distinct features of the LIO/BSO interface from the conventional 2DEGs. Furthermore, they predict how the LIO/BSO 2DEG will evolve as the defect density decreases.

A 2-dimensional electron gas (2DEG) system with high mobility was discovered at the interface of two perovskite oxides: a polar orthorhombic perovskite LaScO3 and a nonpolar cubic perovskite BaSnO3. Upon depositing the LaScO3 film on the BaSnO3 film, we measured the conductance enhancement and the resulting 2DEG density (n2D). Comparing the results with the previously reported LaInO3/BaSnO3 polar interface, we applied the “interface polarization” model to the LaScO3/BaSnO3 system, in which the polarization exists only over four pseudocubic unit cells in LaScO3 from the interface and vanishes afterward like the LaInO3/BaSnO3 interface. Based on the calculations of the self-consistent Poisson–Schrödinger equations, the LaScO3 thickness dependence of n2D of the LaScO3/BaSnO3 heterointerface is consistent with this model. Furthermore, a single subband in the quantum well is predicted. By use of the conductive interface and the LaScO3 as a gate dielectric, a 2DEG transistor composed of only perovskite oxides with high field-effect mobility (μFE) close to 100 cm² V–1 s–1 is demonstrated.

A novel 2-dimensional electron gas (2DEG) system with high-mobility was discovered at the interface of two perovskite oxides, a polar orthorhombic perovskite LaScO3 (LSO) and a nonpolar cubic perovskite BaSnO3 (BSO). Upon depositing the LSO film on the BSO film, the conductance enhancement and the resulting 2DEG density (n2D) was measured. Comparing the results with the previously reported LaInO3/BaSnO3 (LIO/BSO) polar interface, we applied the interface polarization model to the LSO/BSO system, in which the polarization exists only over 4 pseudocubic unit cells in LSO from the interface and vanishes afterward like the LIO/BSO interface. Based on the calculations of the self-consistent Poisson-Schrodinger equations, the LSO thickness dependence of n2D of LSO/BSO heterointerface is consistent with this model. Furthermore, a single subband in the quantum well is predicted. Using the conductive interface and the LSO as a gate dielectric, a 2DEG transistor composed of only perovskite oxides with high field-effect mobility (uFE) close to 100 cm2 V-1 s-1 is demonstrated.

Large bulk LaInO3 single crystals are grown from the melt contained within iridium crucibles by the vertical gradient freeze (VGF) method. The obtained crystals are undoped or intentionally doped with Ba or Ce, and enabled wafer fabrication of size 10 × 10 mm². High melting point of LaInO3 (≈1880 °C) and thermal instability at high temperatures require specific conditions for bulk crystal growth. The crystals do not undergo any phase transition up to 1300 °C, above which a noticeable thermal decomposition takes place. The good structural quality of the crystals makes them suitable for epitaxy. The onset of strong optical absorption shows orientation‐dependent behavior due to the orthorhombic symmetry of the LaInO3 crystals. Assuming direct transitions, optical bandgaps of 4.35 and 4.39 eV are obtained for polarizations along the [010] and the [100], [001] crystallographic directions, respectively. There is an additional weak absorption in the range between 2.8 and 4 eV due to oxygen vacancies. Density‐functional‐theory calculations support the interpretation of the optical absorption data. Cathodoluminescence spectra show a broad, structured emission band peaking at ≈2.2 eV. All bulk crystals are electrically insulating. The relative static dielectric constant is determined at a value of 24.6 along the [001] direction.

Various δ-doped semiconductor heterostructures have been effectively used for devices at room temperature and for quantum phenomena at low temperatures. Here, we use BaSnO3 and investigate its δ-doped system, focusing on its band bending and surface boundary conditions. We measured the two-dimensional carrier density (n2D) of the δ-doped BaSnO3 system of various thicknesses and doping levels. We also studied the effect of the BaSnO3 capping layer thickness on n2D. We show that the δ-doped BaSnO3 system can be very well described by band bending with the aid of the Poisson–Schrödinger simulation. At the same time, the capping layer thickness dependence of n2D reveals how the boundary condition on the surface of La-doped BaSnO3 evolves as a function of its capping layer thickness.