Hyojung Bae

Effect of Al pre-deposition on AlN buffer layer and GaN film grown on Si (111) substrate by MOCVD

In this study, we investigated the effect of Al pre-deposition time on GaN crystal quality. The GaN layer was grown on a Si (111) substrate by metal organic chemical vapor deposition using an AlN buffer layer. Atomic force microscopy, scanning electron microscopy and x-ray diffraction were used to evaluate film growth. Consequently, we found significant differences in the epitaxial properties of the AlN buffer and GaN layer, which were dependent on the Al pre-deposition time. When the deposition time was 0 s, the AlN buffer layer was very smooth, but the GaN layer did not have a high crystal quality. At a Al pre-deposition time of 60 s, the AlN buffer started to be transformed into a rough surface and the GaN layer had both a mirror-like surface and better crystal quality. When the Al pre-deposition time was greater than 120 s, GaN surfaces had a high roughness. In addition, the surface morphology of the GaN had not coalesced. Based on these results, the optimized Al pre-deposition time was found to be essential to achieve an appropriate surface roughness for high crystal quality of GaN.

Effect of niobium doping on the optical and electrical properties in titanium dioxide grown by pulsed laser deposition

The influence of niobium (Nb) on both the electrical and optical properties in titanium dioxide (TiO2) grown by pulsed laser deposition was investigated. Nb atoms with a critical doping ratio of 8 at. % were carefully controlled in a 100 nm-thick TiO2 layer (Nb:TiO2). The Hall effect results revealed that the electrical resistivity and electron concentration of the resulting Nb:TiO2 film were 30 times lower and 2 orders of magnitude higher than those of un-doped TiO2 (u-TiO2), respectively, leading to a slight degradation of optical transmittance in the visible region. The room temperature photoluminescence results showed that the emission intensity at the near band of Nb:TiO2 was greatly enhanced, while, that at the impurity band created by oxygen vacancy (Vo) was almost constant. Detailed investigation of the T-dependent conductivity indicated that the Nb:TiO2 film possessed an activation energy as shallow as 20.8 meV, which suggested that the electrical properties were dominantly determined by Nb shall...

The Polarity Effect on the Photoelectrochemical Properties of Ga- and N-Face Free-Standing GaN Substrate

The band-edge potential and photocurrent density of N-face GaN were experimentally determined to be more negative and greater than those of Ga-face GaN, respectively, in this photoelectrochemical experiment. These results indicate that the N-face GaN could generate much more hydrogen than Ga-face GaN. Although the time dependence of the photocurrent density of N-face GaN was almost constant, that of Ga-face GaN stabilized only after 260 min of reaction time. From these results, we conclude that N-face GaN, which has a more negative band-edge potential and a more stable photocurrent density, could be suitable for higher-photocurrent generation than Ga-face GaN.

Development of nanoscale Ni-embedded single-wall carbon nanotubes by electroless plating for transparent conductive electrodes of 375 nm AlGaN-based ultraviolet light-emitting diodes

We propose a nanoscale Ni-embedded single-wall carbon nanotube (SWCNT) composite for transparent conductive electrodes (TCEs) of AlGaN-based ultraviolet light-emitting diodes (UV LEDs). TCEs specifically for the ultraviolet region were developed using Ni selectively electroless-plated SWCNTs. The nanoscale Ni of TCEs improved electrical conductivity and formed ohmic contact with p-GaN while minimizing transmittance loss. We applied Ni-embedded SWCNTs, SWCNTs, and Ni/Au to the TCEs of 375 nm UV LEDs. UV LEDs with Ni-embedded SWCNTs showed a 32% higher output power than UV LEDs with conventional Ni/Au TCEs.

Eutectic solvent-mediated selective synthesis of Cu–Sb–S-based nanocrystals: combined experimental and theoretical studies toward highly efficient water splitting

Recently, emerging Cu–Sb–S-based compounds have been identified as an attractive candidate for photovoltaic (PV) applications because of their high natural abundance, eco-friendly features and typical phase-dependent characteristics. Herein, a simple eutectic solvent-mediated (choline chloride/ethylene glycol) synthetic approach for newly debuted Cu–Sb–S-based nanocrystals (NCs) with phase-selective properties is presented. This combination of material and preparation method may promote the exchange of carriers by avoiding a steric hindrance for a facile charge transport encountered in NCs prepared using amines, thiols, hydrazines and phosphine oxide solvents. A temperature-dependent study of an ethaline-based deep eutectic solvent (DES) is conducted to elucidate the characteristics of associated chemical shifts and vibrations and to determine changes in hydrogen bonding interactions using structural and thermal analytical techniques. The results suggest that ethaline is a strong candidate as a greener solvent for the synthesis of NCs at relatively low temperatures. The electronic structures of all four Cu–Sb–S phases—Cu3SbS4, CuSbS2, Cu3SbS3, and Cu12Sb4S13—were simulated using the Vienna ab initio Simulation (VASP) code, projector augmented-wave (PAW) potentials and the hybrid functional method (HSE 06) and using density functional theory for combined theoretical and experimental studies. Discrepancies between the experimental and theoretical bandgap values of 0.29, 0.18, 0.12 and 0.16 eV were observed for Cu3SbS4, CuSbS2, Cu3SbS3 and Cu12Sb4S13 compounds, respectively. A photoelectrochemical (PEC) water reduction system with a Mo/photoelectrode/CdS/Pt/electrolyte configuration generated a cathodic photocurrent of −1.28 and −2.72 mA cm−2 for Cu3SbS4 and CuSbS2 electrodes, respectively, at 0 V versus the reversible hydrogen electrode (VRHE) under AM 1.5 G illumination, demonstrating the great potential of NCs prepared via eutectic solvent-mediated synthesis. This is the first successful attempt to apply eutectic solvent-mediated Cu–Sb–S NCs for solar driven H2 production. These outcomes suggest that designing proper functional materials through the application of greener synthesis strategies can improve water-splitting performance and would help meet the perpetual technological need for greener methods.

A Research on Improvement of Hydrogen Generation using CdS/CIGS Photo-Electrode in Photoelectrochemical System

We investigated the photoelectrochemical (PEC) properties of CdS/CIGS thin films as the hydrogen-generating photo-electrode in Na2SO4 electrolyte. The onset potential of CIGS of thin film by itself which is a point of beginning redox reaction for hydrogen generation was -0.6 V under illumination of 200 mW. However, with the deposition of CdS layer on CIGS film, CdS/CIGS structures produced three times enhanced photocurrents at -0.1 V vs compared with only CIGS thin film. CdS layer has ideal band-gap energy and position for redox reaction under visible light irradiation. Also, CdS layer could make P-N junction with CIGS film and form stepwise band-gap structure, which help electrons and holes transfer more easily. From these results, CdS/CIGS thin films could be regarded as a promising photo-electrode cell structure for hydrogen generation.

Effect of oxygen incorporation in a-plane GaN on p-type ohmic contact property

We report on the origin of the non-ohmic behavior of Ni/Au-based p-type contacts on a nonpolar a-plane GaN layer. The contact properties of Ga-polar c-plane GaN and nonpolar a-plane GaN are compared. While the Ga-polar c-plane shows ohmic-contact properties in the Ni/Au contact after heat treatment, the nonpolar a-plane shows rectifying characteristics both before and after heat treatment. We determined the reasons why the two planes show substantial differences in contact properties using various tools. We conclude that the differences originated from the oxygen incorporation preference resulting in gallium oxide formation at the interface of nonpolar a-plane GaN.

Characteristics of MoO3 Thin Films Fabricated by Rapid Thermal Annealing of Mo for an Anode Buffer Layer for Organic Solar Cells

Acidic PEDOT:PSS anode buffer layers (ABLs) are widely used for improving efficiency in organic based solar cells. Recently, stable p-type metal oxides, such as NiO, MoO3, and V2O5, have been used to replace the commonly used acidic PEDOT:PSS ABL. Among these metal oxides, the thermally evaporated MoO3 anode buffer layer is largely used because it has appropriate optical and electrical properties. In this study, we used rapid thermal annealing (RTA) for fabricating polycrystalline MoO3 thin films. MoO3 thin films were fabricated by annealing 30 nm molybdenum in an oxygen atmosphere under 350 °C-550 °C RTA conditions at intervals of 100 °C. In particular, we suggest an optimized temperature of 450 °C for efficient ABL in organic based solar cells. The MoO3 thin film with 450 °C RTA conditions has the relatively highest RMS roughness (46.5 nm) and proper electrical resistance. The characteristics of MoO3 ABLs fabricated by the RTA process of Mo are compared with thermally evaporated MoO3 ABL. (Received September 25, 2012)

Effects of Controlling the AZO Thin Film's Optical Band Gap on AZO/MEH-PPV Devices with Buffer Layer

Organic/inorganic hybrid solar cells were fabricated incorporating aluminum-doped zinc oxide (AZO) thin films of varying optical band gap in AZO/poly(2-methoxy-5-(-ethyl-hexyloxy)-p-phenylene vinylene) structures. The band gaps were controlled by varying the flow rates of Ar and used to deposit the AZO. Devices with CdS buffer layer were also fabricated for improved efficiency. The effects of AZO optical band gap were assessed by testing the I–V characteristics of devices with structures of glass/ITO/AZO/MEH-PPV/Ag under AM1.5 illumination (100 mW/cm2). Efficiency was improved about 30 times by decreasing the AZO optical band gap, except in devices deposited without oxygen. A power conversion efficiency of 0.102% was obtained with the incorporation of a CdS buffer layer.