Jang-Won Kang

Recent Advances in ZnO-Based Light-Emitting Diodes

ZnO has attracted considerable attention for optical device applications because of several potential advantages over GaN, such as commercial availability of bulk single crystals and a larger exciton binding energy (~60 meV compared with ~25 meV for GaN). Recent improvements in the control of background conductivity of ZnO and demonstrations of p-type doping have intensified interest in this material for applications in light-emitting diodes (LEDs). In this paper, we summarize recent progress in ZnO-based LEDs. Physical and electrical properties, bandgap engineering, and growth of n- and p-type ZnO thin films are also reviewed.

Surface plasmon-enhanced light-emitting diodes using silver nanoparticles embedded in p-GaN

We demonstrate the surface plasmon-enhanced blue light-emitting diodes (LEDs) using Ag nanoparticles embedded in p-GaN. A large increase in optical output power of 38% is achieved at an injection current of 20 mA due to an improved internal quantum efficiency of the LEDs. The enhancement of optical output power is dependent on the density of the Ag nanoparticles. This improvement can be attributed to an increase in the spontaneous emission rate through resonance coupling between the excitons in multiple quantum wells and localized surface plasmons in Ag nanoparticles embedded in p-GaN.

Enhanced optical power and low forward voltage of GaN-based light- emitting diodes with Ga-doped ZnO transparent conducting layer

Ga-doped ZnO (ZnO:Ga) films were grown by metalorganic chemical vapor deposition as transparent conducting layers for GaN light-emitting diodes (LEDs). The forward voltage of LEDs with ZnO:Ga was 3.3 V at 20 mA. The low forward voltage was attributed to the removal of a resistive ZnGa2O4 phase, decreased resistivity of ZnO:Ga films, and increased hole concentration in p-GaN by thermal annealing process. The light output power of LEDs with ZnO:Ga was increased by 25% at 20 mA compared to that of LEDs with Sn-doped indium oxide due to the enhanced transmittance and the increased hole concentration in p-GaN.

Microstructural properties of phosphorus-doped p-type ZnO grown by radio-frequency magnetron sputtering

Phosphorus (P)-doped ZnO thin films were grown by radio-frequency magnetron sputtering to study the microstructural properties of p-type ZnO. As-grown P-doped ZnO, a semi-insulator, was converted to p-type ZnO after being annealed at 800°C in an N2 ambient. X-ray diffraction, secondary-ion-mass spectrometry, and Hall effect measurements indicated that P2O5 phases in as-grown P-doped ZnO disappeared after thermal annealing to form a substitutional P at an O lattice site, which acts as an acceptor in P-doped ZnO. Transmission electron microscopy showed that the formation of stacking faults was facilitated to release the strain in P-doped ZnO during post-thermal annealing.

Localized surface plasmon-enhanced green quantum dot light-emitting diodes using gold nanoparticles

We develop a localized surface plasmon (LSP)-enhanced CdSe/ZnS green quantum dot (QD) light-emitting diode (LED) containing Au nanoparticles (NPs) embedded in a ZnO electron transport layer. Au NPs blended in ZnO solution are directly spin coated onto the QD emissive layer to provide strong coupling between LSPs in Au NPs and excitons in QDs, greatly enhancing the electroluminescence (EL). Photoluminescence (PL) and EL intensities are greatly enhanced by 4.12 and 4.33-fold, respectively. Maximum PL and EL enhancement ratios of 4.47 and 4.54 are observed at 535 and 532 nm, respectively, and these are similar to the LSP resonance wavelength of 536 nm for Au NPs in ZnO films. The results indicate that the EL enhancement of the QD-LED is attributed to strong resonance coupling between excitons in the QDs and LSPs in the Au NPs in ZnO films.

Electrical and structural properties of antimony-doped p-type ZnO nanorods with self-corrugated surfaces

We report on p-type conductivity in antimony (Sb)-doped ZnO (ZnO:Sb) nanorods which have self-corrugated surfaces. The p-ZnO:Sb/n-ZnO nanorod diode shows good rectification characteristics, confirming that a p-n homojunction is formed in the ZnO nanorod diode. The low-temperature photoluminescence (PL) spectra of the ZnO:Sb nanorods reveal that the p-type conductivity in p-ZnO:Sb is related to the Sb(Zn)-2V(Zn) complex acceptors. Transmission electron microscopy (TEM) analysis of the ZnO:Sb nanorods also shows that the p-type conductivity is attributed to the Sb(Zn)-2V(Zn) complex acceptors which can be easily formed near the self-corrugated surface regions of ZnO:Sb nanorods. These results suggest that the Sb(Zn)-2V(Zn) complex acceptors are mainly responsible for the p-type conductivity in ZnO:Sb nanorods which have corrugated surfaces.

MgxZn1–xO/Ag/MgxZn1–xO Multilayers As High-Performance Transparent Conductive Electrodes

We report on the optical and electrical properties of MgxZn1-xO/Ag/MgxZn1-xO transparent conductive electrodes. The transmittance and sheet resistance of MgxZn1-xO/Ag/MgxZn1-xO multilayers deposited at room temperature were strongly dependent on the thickness and surface morphology of Ag layer. The optical absorption edge of MgxZn1-xO/Ag/MgxZn1-xO showed a blue shift with increasing Mg composition due to the increased band gap of MgxZn1-xO. The Haack figure of merit value of Mg0.28Zn0.72O/Ag/Mg0.28Zn0.72O with a 14 nm-thick Ag layer, which has a sheet resistance of 6.36 Ω/sq and an average transmittance of 89.2% at wavelengths in the range from 350 to 780 nm, was 69% higher than that of a ZnO/Ag/ZnO multilayer electrode. These results indicate that MgxZn1-xO/Ag/MgxZn1-xO multilayers, which also show low surface roughness, can be used as highly conductive transparent electrodes in various optoelectronic devices operating over a wide wavelength region.

High-performance photoconductivity and electrical transport of ZnO/ZnS core/shell nanowires for multifunctional nanodevice applications.

We report the electrical and optical properties of ZnO/ZnS core/shell nanowire (NW) devices. The spatial separation of charge carriers due to their type II band structure together with passivation effect on ZnO/ZnS core/shell NWs not only enhanced their charge carrier transport characteristics by confining the electrons and reducing surface states in the ZnO channel but also increased the photocurrent under ultraviolet (UV) illumination by reducing the recombination probability of the photogenerated charge carriers. Here the efficacy of the type-II band structure and the passivation effect are demonstrated by showing the enhanced subthreshold swing (150 mV/decade) and mobility (17.2 cm2/(Vs)) of the electrical properties, as well as the high responsivity (4.4×10(6) A/W) in the optical properties of the ZnO/ZnS core/shell NWs, compared with the subthreshold swing (464 mV/decade), mobility (8.9 cm2/(Vs)) and responsivity (2.5×10(6) A/W) of ZnO NWs.

Effect of VI/II Gas Ratio on the Epitaxial Growth of ZnO Films by Metalorganic Chemical Vapor Deposition

We report the effect of the VI/II ratio on the metalorganic chemical vapor deposition (MOCVD) growth of ZnO film. The surface of ZnO film becomes very smooth as the VI/II ratio increases. Atomic force microscopy measurement shows that ZnO films grown at a VI/II ratio of 25,000 have atomically flat terraces with a root-mean-square roughness of 0.2 nm. Low-temperature photoluminescence spectra also reveal a very sharp excitonic emission comprised of a neutral donor bound exciton emission and a free exciton emission with first and second longitudinal optical (LO) phonon replicas, indicating that the ZnO epilayer is of a high optical quality.

Periodically Diameter-Modulated Semiconductor Nanowires for Enhanced Optical Absorption.

A diameter-modulated silicon nanowire array to enhance the optical absorption across broad spectral range is presented. Periodic shape engineering is achieved using conventional semiconductor processes and the unique optical properties are analyzed. The periodicity in the diameter of the silicon nanowires enables stronger and more closely spaced optical resonances, leading to broadband absorption enhancement.