Chulkwon Park

Large effects of dislocations on high mobility of epitaxial perovskite Ba0.96La0.04SnO3 films

We studied the relationship between the mobility and dislocation density for recently discovered high-mobility Ba0.96La0.04SnO3 thin-films and found that the carrier density and mobility of the film, as high as 4.0×1020 cm−3 and 70 cm2 V−1s−1, respectively, decreased as the dislocation density increased. We determined the values for dislocation density using transmission electron microscopy and atomic force microscopy after surface etching. We found that the effect of dislocations on the mobility was large, when compared with that for GaN with a similar dislocation density. The importance of dislocation scattering in the perovskite structure is emphasized, especially in a low-carrier-density regime.

Partially unzipped carbon nanotubes for high-rate and stable lithium–sulfur batteries

Lithium–sulfur (Li–S) batteries are attractive due to a high theoretical energy density and low sulfur cost. However, they have critical drawbacks such as drastic capacity fading during cycling, especially under high current density conditions. We report a suitable carbon matrix based on partially unzipped multi-walled carbon nanotubes (UZ.CNTs), which have favorable properties compared to multi-walled carbon nanotubes (MWCNTs) and fully unzipped nanoribbons (UZ.NRs). Partially unzipped walls of MWCNTs lead to increased surface area and pore volume with a retained electron conduction pathway. This also provides accessible inner pores as a stable reservoir for polysulfides. This reservoir is decorated with newly introduced oxygen containing functional groups, and affords a synergistic effect of shortening the depth that electrons penetrate and interacting with polysulfides for high-performance Li–S batteries. The synergistic effect is revealed by Monte Carlo simulations. The resulting partially unzipped MWCNT sulfur composite delivers 707.5 mA h g−1 at the initial discharge and retains 570.4 mA h g−1 after 200 cycles even at a high current rate of 5C (8375 mA g−1).

Dopant-site-dependent scattering by dislocations in epitaxial films of perovskite semiconductor BaSnO3

We studied the conduction mechanism in Sb-doped BaSnO3 epitaxial films, and compared its behavior with that of the mechanism of its counterpart, La-doped BaSnO3. We found that the electron mobility in BaSnO3 films was reduced by almost 7 times when the dopant was changed from La to Sb, despite little change in the effective mass of the carriers. This indicates that the scattering rate of conduction electrons in the BaSnO3 system is strongly affected by the site at which the dopants are located. More importantly, we found that electron scattering by threading dislocations also depends critically on the dopant site. We propose that the large enhancement of scattering by the threading dislocations in Sb-doped BaSnO3 films is caused by the combination effect of the change in the distribution of Sb impurities in the films, the formation of the Sb impurity clusters near the threading dislocations, and the conduction electron clustering near the Sb impurities.

All-perovskite transparent high mobility field effect using epitaxial BaSnO3 and LaInO3

We demonstrate an all-perovskite transparent heterojunction field effect transistor made of two lattice-matched perovskite oxides: BaSnO3 and LaInO3. We have developed epitaxial LaInO3 as the gate oxide on top of BaSnO3, which were recently reported to possess high thermal stability and electron mobility when doped with La. We measured the dielectric properties of the epitaxial LaInO3 films, such as the band gap, dielectric constant, and the dielectric breakdown field. Using the LaInO3 as a gate dielectric and the La-doped BaSnO3 as a channel layer, we fabricated field effect device structure. The field effect mobility of such device was higher than 90 cm2 V−1 s−1, the on/off ratio was larger than 107, and the subthreshold swing was 0.65 V dec−1. We discuss the possible origins for such device performance and the future directions for further improvement.

High-mobility BaSnO3 thin-film transistor with HfO2 gate insulator

Thin-film transistors have been fabricated using La-doped BaSnO3 as n-type channels and (In,Sn)2O3 as source, drain, and gate electrodes. HfO2 was grown as gate insulators by atomic layer deposition. The field-effect mobility, Ion/Ioff ratio, and subthreshold swing of the device are 24.9 cm2 V−1 s−1, 6.0 × 106, and 0.42 V dec−1, respectively. The interface trap density, evaluated to be higher than 1013 cm−2 eV−1, was found to be slightly lower than that of the thin-film transistor with an Al2O3 gate insulator. We attribute the much smaller subthreshold swing values to the higher dielectric constant of HfO2.

Rational design of exfoliated 1T MoS2@CNT-based bifunctional separators for lithium sulfur batteries

Lithium-sulfur (Li-S) batteries are experiencing a design shift from a closed structure to an open structure to further improve their performance, expanding the design realm from the development of nanostructured materials for the cathode to the production of functional separators. Rational guidelines for preparing a bifunctional separator with exfoliated MoS2 and CNTs are suggested to deal with two conflicting issues: guaranteeing the electron pathway while strongly trapping polysulfide species. In addition, various exfoliation methods ranging from mechanical to chemical were investigated to identify an adequate method for preparing exfoliated MoS2 based-bifunctional separators. The electrochemical exfoliation method was found to be effective in not only exfoliating high quality MoS2 in terms of the lateral size and number of layers, but also providing a favorable MoS2 phase, 1T metallic MoS2. A bifunctional separator of 1T exfoliated MoS2@CNT in a tandem configuration (layer-by-layer structure)-coated Celgard rather than a hetero-configuration delivered an excellent electrochemical performance of ∼670 mA h g−1 after 500 cycles at a high current density of 1C. In addition, the separator was highly effective in trapping polysulfide species and facilitating electron transfer to the irreversible discharge products. The rational guidelines suggested in this study will be extended to other two-dimensional transition-metal dichalcogenides, and applied to the development of other functional membranes.

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.

Thermally stable pn-junctions based on a single transparent perovskite semiconductor BaSnO3

We report p-doping of the BaSnO3 (BSO) by replacing Ba with K. The activation energy of K-dopants is estimated to be about 0.5 eV. We have fabricated pn junctions by using K-doped BSO as a p-type and La-doped BSO as an n-type semiconductor. I-V characteristics of these devices exhibit an ideal rectifying behavior of pn junctions with the ideality factor between 1 and 2, implying high integrity of the BSO materials. Moreover, the junction properties are found to be very stable after repeated high-bias and high-temperature thermal cycling, demonstrating a large potential for optoelectronic functions.

Conducting interface states at LaInO3/BaSnO3 polar interface controlled by Fermi level

We report on a new polar interface state between two band insulators: LaInO3 and BaSnO3, where the sheet conductance enhancement in the interface reaches more than the factor of 104 depending on the La doping concentration in BaSnO3 layer, by monitoring the conductance change before and after the polar interface formation as a function of La doping in BaSnO3. By eliminating the possibilities of oxygen vacancy involvement and cation diffusion, we show that the conductance enhancement is due to electronic reconstruction in the interface. Furthermore, we have found that the interfaces between BaSnO3 and the larger bandgap non-polar perovskites BaHfO3 and SrZrO3 did not show such a conductance enhancement. We discuss a model for the interface state where the Fermi level plays a critical role and the conductance enhancement is due to the existence of polarization in the polar perovskite, LaInO3.

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.