Youjung Kim

Formation of Co nanoclusters in epitaxial Ti0.96Co0.04O2 thin films and their ferromagnetism

Anatase Ti0.96Co0.04O2 films were grown epitaxially on SrTiO3 (001) substrates by using pulsed laser deposition with in-situ reflection high-energy electron diffraction. The oxygen partial pressure, PO2, during the growth was systematically varied. As PO2 decreased, the growth behavior was changed from a two-dimensional layer-by-layer-like growth to a three-dimensional island-like one, which resulted in an increase in the saturation magnetization. These structural and magnetic changes were explained in terms of the formation of cobalt clusters whose existence was proved by transmission-electron-microscope studies. Our work clearly indicates that the cobalt clustering will cause room-temperature ferromagnetism in the Co-doped TiO2 films.

Ferroelectric properties of SrRuO3∕BaTiO3∕SrRuO3 ultrathin film capacitors free from passive layers

Structural studies on ultrathin SrRuO3∕BaTiO3∕SrRuO3 capacitors, with BaTiO3 thicknesses of between 5nm and 30nm, show well-defined interfaces between ferroelectric BaTiO3 and electrode SrRuO3 layers. In these capacitors, we cannot observe any extrinsic electrical effects due to either the formation of an insulating interfacial passive layer or passive-layer-induced charge injection. Such high-quality interfaces result in very good fatigue endurance, even for the 5nm thick BaTiO3 capacitor.Structural studies on ultrathin SrRuO3∕BaTiO3∕SrRuO3 capacitors, with BaTiO3 thicknesses of between 5nm and 30nm, show well-defined interfaces between ferroelectric BaTiO3 and electrode SrRuO3 layers. In these capacitors, we cannot observe any extrinsic electrical effects due to either the formation of an insulating interfacial passive layer or passive-layer-induced charge injection. Such high-quality interfaces result in very good fatigue endurance, even for the 5nm thick BaTiO3 capacitor.

Influence of the quantum-well shape on the light emission characteristics of InGaN/GaN quantum-well structures and light-emitting diodes

Structural and optical properties of various shapes of quantum wells (QWs), including rectangular, triangular, trapezoidal, and polygonal ones are investigated. Photoluminescence (PL) measurements show that the highest light emission efficiency and the best reproducibility in the intensity and wavelength are obtained from trapezoidal QWs. The temperature dependence of PL spectra indicates the more localized nature of excitons in the trapezoidal QWs. A plan-view transmission electron microscopy shows that quantum dots (QDs) are formed inside the dislocation loop in trapezoidal QWs. The distribution of QDs in size and composition becomes more uniform with trapezoidal QWs than with rectangular QWs, leading to superior light-emission characteristics. It is suggested that QD engineering and dislocation control are possible, to some extent, by the modulation of the QW shape in InGaN/GaN-based light-emitting devices.

High mobility BaSnO3 films and field effect transistors on non-perovskite MgO substrate

(Ba,La)SnO3 is a wide bandgap semiconducting perovskite oxide with high electron mobility and excellent oxygen stability. The carrier modulation of (Ba,La)SnO3 channel by field effect on perovskite SrTiO3 substrates has been demonstrated in the recent reports. Here we report that (Ba,La)SnO3 on non-perovskite MgO substrate can also exhibit a high electron mobility and excellent carrier modulation by field, an important step towards scaling up for wafer-size processing. We optimized the undoped buffer layer thickness and measured the transport properties as a function of the La doping. The maximum mobility is 97.2 cm2/Vs at 2.53×1020/cm3. The transmission electron microscope images show that the films are epitaxial with about 2×1011/cm2 threading dislocation density. The field effect device based on the (Ba,La)SnO3 channel on MgO substrates is modulated with a high mobility of 43.9 cm2/Vs and Ion/Ioff of about 3.0×107.

Dislocation behavior in InGaN/GaN multi-quantum-well structure grown by metalorganic chemical vapor deposition

Propagation characteristics of dislocations were investigated in InGaN/GaN multi-quantum-well structures grown by metalorganic chemical vapor deposition. Threading dislocations with Burgers vector of b=〈1120〉, emerged from the GaN buffer region, change their glide plane from normal to parallel to the growth plane when they meet InGaN wells. Dislocations gliding on the growth planes were pinned by quantum dots leaving two possible ways of propagation, changing their glide plane back to normal to the growth plane, {1010}, or extending loop shape pinned both ends by the quantum dots. Indium-rich quantum dots were formed on the InGaN quantum-well layers with size of 30±25 nm in diameter. It was estimated that the critical size of quantum dots to pin the dislocations is 30 nm.

High-k perovskite gate oxide BaHfO3

We have investigated epitaxial BaHfO3 as a high-k perovskite dielectric. From x-ray diffraction measurement, we confirmed the epitaxial growth of BaHfO3 on BaSnO3 and MgO. We measured optical and dielectric properties of the BaHfO3 gate insulator; the optical bandgap, the dielectric constant, and the breakdown field. Furthermore, we fabricated a perovskite heterostructure field effect transistor using epitaxial BaHfO3 as a gate insulator and La-doped BaSnO3 as a channel layer on SrTiO3 substrate. To reduce the threading dislocations and enhance the electrical properties of the channel, an undoped BaSnO3 buffer layer was grown on SrTiO3 substrates before the channel layer deposition. The device exhibited a field effect mobility value of 52.7 cm2 V−1 s−1, a Ion/Ioff ratio higher than 107, and a subthreshold swing value of 0.80 V dec−1. We compare the device performances with those of other field effect transistors based on BaSnO3 channels and different gate oxides.

Substrate pit formation and surface wetting during thermal annealing of strained-Si/relaxed-Si0.78Ge0.22 heterostructure

During the elevated-temperature thermal annealing of a strained-Si/relaxed-Si0.78Ge0.22 heterostructure, deep substrate pits penetrating into the relaxed-Si0.78Ge0.22 buffer were formed and the strained-Si surface was wetted by the Si and Ge atoms originating from the deep pits. The pit formation and surface wetting are presumably due to the decrease in the strained-Si surface energy, resulting in a reduction in the total energy. The strain of the remaining strained-Si layer situated away from the deep pits was slightly increased despite the increased relaxation of the SiGe buffer, which implies that the remaining strained-Si layer was morphologically stabilized by the SiGe wetting.

LaInO3/BaSnO3 polar interface on MgO substrates

We report on a new property of the LaInO3 (LIO)/(Ba,La)SnO3 (BLSO) polar interface using MgO substrates. The growth of well-formed LIO/BLSO interface structures on non-perovskite MgO substrates was confirmed by reciprocal space mapping image and transmission electron microscopy. Subsequently, we measured electrical properties as a function of the La doping rate of the BLSO layer and found that the LIO/BLSO polar interface shows conductance enhancement after the deposition of the polar LaInO3 layer on the BLSO layer, in agreement with our earlier results on SrTiO3 (STO) substrates. However, different electrical properties of the interfaces were found on MgO from those on STO substrates; we observed conductance enhancement even at the interface with undoped BaSnO3 (BSO) on the MgO substrates. We attribute such different behavior to the difference in the Fermi levels of BSO on MgO and STO substrates, either due to the larger donor density or the smaller acceptor density in BSO on MgO. Using such a nominally undoped interface, we fabricated the field effect transistors and presented their performances with Ion/Ioff ∼ 109.