Chang-In Park

Metal-organic chemical vapor deposition growth of GaN thin film on 3C-SiC/Si(111) substrate using various buffer layers

Abstract GaN films were grown on 3C-SiC/Si(111) substrate using GaN, AlN, and GaN/AlN superlattices as the buffer layer in a metal–organic chemical vapor deposition (MOCVD) system. The surface morphology and structural and optical qualities of the GaN films were greatly improved by employing the buffer layers in the growth of GaN film on the 3C-SiC/Si substrate. The use of the superlattice buffer layer produced well-oriented GaN growth along the c-axis with good surface morphology. Low-temperature photoluminescence (PL) measurements showed that peaks associated with band-edge emission and donor–accepter pair recombination (D0A0) were observed from GaN films grown with and without GaN or AlN buffer layers, whereas GaN films grown with the superlattice buffer layer exhibited a strong band-edge peak with very weak D0A0 emission. The surface morphology and structural and optical properties of the GaN films were well correlated for the evaluation of GaN crystal quality.

In-situ X-ray Absorption Fine Structure Study of TiO2 Nanoparticles under Ultraviolet Light

The local structural properties of TiO2 nanoparticles (NPs) were investigated under ultraviolet light (UV) using in-situ X-ray absorption fine structure measurements at the Ti K edge. XAFS analysis showed that the bond lengths of the atomic pairs were distorted by 0.1 A and that the Debye–Waller factors of the atomic pairs were increased substantially under UV light, compared with those of no UV light. These results suggested that the local structural changes were caused by the photocatalyst reactivity of oxygen atoms in the NPs. Unlike other techniques, in-situ XAFS is sensitive to detect local structural changes during the photocatalytic reactivity.

Structural and electrical properties of ZnO nanorods and Ti buffer layers

Vertically-well-aligned ZnO nanorods were synthesized on Ti buffer layers by a metal-organic chemical-vapor deposition process. Structural analyses demonstrated that the ZnO nanorods were well-aligned in the c-axis and ab-plane. Transmission electron microscopy (TEM) showed that the Ti buffer layer was amorphous and interdiffused into the ZnO nanorods. Energy-dispersive spectroscopy (EDS) analysis revealed the Ti buffer layers to be slightly oxide. Extended x-ray absorption fine structure confirmed the TEM and EDS results. The I-V characteristic measurements showed a 20-fold increase in current density with the Ti buffer layer, suggesting excellent electrical contact between the Ti buffer layer and ZnO nanorods.

Anomalous structural disorder and distortion in metal-to-insulator-transition Ti2O3

Mott proposed that impurity bands in corundum-symmetry Ti2O3 at high temperatures caused a collapse in the bandgap. However, the origin of the impurity bands has not yet been clarified. We examine the local structural properties of metal-to-insulator-transition Ti2O3 using in-situ x-ray absorption fine structure (XAFS) measurements at the Ti K edge in the temperature range from 288 to 739 K. The Ti2O3 powder is synthesized by using a chemical reaction method. X-ray diffraction (XRD) measurements from Ti2O3 with a Rietveld refinement demonstrate a single-phased R-3c symmetry without additional distortion. Extended-XAFS combined with XRD reveals a zigzag patterned Ti position and an anomalous structural disorder in Ti-Ti pairs, accompanied by a bond length expansion of the Ti-Ti pairs along the c-axis for T > 450 K. The local structural distortion and disorder of the Ti atoms would induce impurity levels in the band gap between the Ti 3d a1g and egπ bands, resulting in a collapse of the band gap for T > 450 K.

Linear defects and electrical properties of ZnO nanorods

Proton irradiation (17–34 MeV at flux values ranging from 1011 to 1012 cm−2) was used to assess the influences of orientation-dependent linear defects in a current passing through ZnO nanorods. Compared with the pristine ZnO nanorods, there was a significant increase in the current passing through ZnO nanorods that were irradiated with a proton beam kept in parallel with the nanorod length. The current was gradually decreased with a corresponding increase in the angle of the proton beams relative to the nanorod length. Calculations using the density functional theory demonstrated a substantial reduction and a lack of influence on the bandgap due to linear defects along the respective c- and the a-axes of the ZnO nanorods. Linear defects likely play roles as channels or traps of conduction electrons or holes in wide-bandgap materials.Proton irradiation (17–34 MeV at flux values ranging from 1011 to 1012 cm−2) was used to assess the influences of orientation-dependent linear defects in a current passing through ZnO nanorods. Compared with the pristine ZnO nanorods, there was a significant increase in the current passing through ZnO nanorods that were irradiated with a proton beam kept in parallel with the nanorod length. The current was gradually decreased with a corresponding increase in the angle of the proton beams relative to the nanorod length. Calculations using the density functional theory demonstrated a substantial reduction and a lack of influence on the bandgap due to linear defects along the respective c- and the a-axes of the ZnO nanorods. Linear defects likely play roles as channels or traps of conduction electrons or holes in wide-bandgap materials.

Polarization-dependent DANES study on vertically-aligned ZnO nanorods

The local structural and local density of states of vertically-aligned ZnO nanorods are examined by using polarization-dependent diffraction anomalous near edge structure (DANES) measurements from c-oriented ZnO nanorods at the Zn K edge at the geometry of the incident x-ray electric field parallel and perpendicular to the x-ray momentum transfer direction. Orientation-dependent local structures determined by DANES are comparable with polarization- dependent EXAFS results. Unlike other techniques, polarization-dependent DANES can uniquely describe the orientation-dependent local structural properties and the local density of states of a selected element in selected-phased crystals of compounds or mixed-phased structures.