Cheol Hyoun Ahn

A comparative analysis of deep level emission in ZnO layers deposited by various methods

This study examined the origin of visible luminescence from ZnO layers deposited on p-Si substrates by various growth methods using temperature dependent photoluminescence measurements. The deep level emissions of ZnO layers are found to be strongly dependent on the growth conditions and growth methods used. For the samples grown by sputtering, the visible emission consisted of violet, green, and orange-red regions, which corresponded to zinc interstitial (Zni), oxygen vacancy (VO), and oxygen interstitial (Oi) defect levels, respectively. In contrast, the deep level emissions of metal organic chemical vapor deposition grown samples consisted of blue and green emissions and blue and orange-red emissions at low and high oxygen flow rates, respectively. The ZnO nanorods synthesized by thermal evaporation showed a dominant deep level emission at the green region, which is associated with oxygen vacancies (VO).

Highly selective spectral response with enhanced responsivity of n-ZnO/p-Si radial heterojunction nanowire photodiodes

A radial heterojunction nanowire diode (RND) array consisting of a ZnO (shell)/Si (core) structure was fabricated using conformal coating of a n-type ZnO layer that surrounded a p-type Si nanowire. In both ultraviolet (UV) and visible ranges, the photoresponsivity of the RND was larger than that of a planar thin film diode (PD) owing to the efficient carrier collection with improved light absorption. Compared to a PD, in the forward bias, a 6 μm long RND resulted in a ∼2.7 times enhancement of the UV responsivity at λ=365 nm, which could be explained based on the oxygen-related hole-trap mechanism. Under a reverse bias, UV-blind visible detection was observed while the UV response was suppressed.

p‑Channel Oxide Thin Film Transistors Using Solution-Processed Copper Oxide

Cu2O thin films were synthesized on Si (100) substrate with thermally grown 200-nm SiO2 by sol-gel spin coating method and postannealing under different oxygen partial pressure (0.04, 0.2, and 0.9 Torr). The morphology of Cu2O thin films was improved through N2 postannealing before O2 annealing. Under relatively high oxygen partial pressure of 0.9 Torr, the roughness of synthesized films was increased with the formation of CuO phase. Bottom-gated copper oxide (CuxO) thin film transistors (TFTs) were fabricated via conventional photolithography, and the electrical properties of the fabricated TFTs were measured. The resulting Cu2O TFTs exhibited p-channel operation, and field effect mobility of 0.16 cm2/(V s) and on-to-off drain current ratio of ∼1×10(2) were observed in the TFT device annealed at PO2 of 0.04 Torr. This study presented the potential of the solution-based process of the Cu2O TFT with p-channel characteristics for the first time.

Enhanced exciton-phonon interactions in photoluminescence of ZnO nanopencils

We report enhanced exciton-phonon interactions in the photoluminescence (PL) of ZnO nanopencils compared with ZnO nanorods grown on ZnO/Si templates by thermal evaporation. Although the low temperature ( 100 K) showed dominant contributions from the free exciton emissions and phonon-replicas of free excitons for nanorods and nanopencils, respectively. This discrepancy in the behaviors of excitonic emissions of the ZnO nanorods and nanopencils was related to surface defects causing different strengths of exciton-phonon coupling. The different excitonic emissions of the nanorods and nanopencils revealed a 52 meV redshift in the room temperature PL of nanopencils.

Artificial semiconductor/insulator superlattice channel structure for high-performance oxide thin-film transistors

High-performance thin-film transistors (TFTs) are the fundamental building blocks in realizing the potential applications of the next-generation displays. Atomically controlled superlattice structures are expected to induce advanced electric and optical performance due to two-dimensional electron gas system, resulting in high-electron mobility transistors. Here, we have utilized a semiconductor/insulator superlattice channel structure comprising of ZnO/Al2O3 layers to realize high-performance TFTs. The TFT with ZnO (5 nm)/Al2O3 (3.6 nm) superlattice channel structure exhibited high field effect mobility of 27.8 cm2/Vs, and threshold voltage shift of only < 0.5 V under positive/negative gate bias stress test during 2 hours. These properties showed extremely improved TFT performance, compared to ZnO TFTs. The enhanced field effect mobility and stability obtained for the superlattice TFT devices were explained on the basis of layer-by-layer growth mode, improved crystalline nature of the channel layers, and passivation effect of Al2O3 layers.

Ga-doped ZnO nanorod arrays grown by thermal evaporation and their electrical behavior

Vertically well-aligned Ga-doped ZnO nanorods with different Ga content were grown by thermal evaporation on a ZnO template. The Ga-doped ZnO nanorods synthesized with 50 wt% Ga with respect to the Zn content showed minimum compressive stress relative to the ZnO template, which led to a rapid growth rate along the c-axis due to the rapid release of stored strain energy. A further increase in the Ga content improved the conductivity of the nanorods due to the substitutional incorporation of Ga atoms in the Zn sites based on a decrease in lattice spacing. A p-n diode structure with Ga-doped ZnO nanorods as an n-type layer displayed a distinct white light luminescence from the side view of the device, showing weak ultraviolet and various deep-level emissions.

Improved Electrical Stability in the Al Doped ZnO Thin-Film-Transistors Grown by Atomic Layer Deposition

Bottom-gate oxide thin-film-transistors (TFTs) with improved electrical stability were fabricated with Al doped ZnO (AZO) channel layers grown by atomic layer deposition (ALD) at a relatively low temperature. The ALD growth at 110°C and the addition of 1-5 atom % Al dopant provided the thin films with reliable semiconducting characteristics, and the TFT devices fabricated with the 1 and 3 atom % AZO films showed a good field effect mobility and on-off current ratio. The transfer curves for the AZO channel TFTs exhibited improved hysteresis loop and positive gate bias stress results compared to those for the pure ZnO TFTs. The improved electrical stability was attributed to the coarsening of the crystal size and the preferred orientation along the nonpolar direction afforded by the addition of Al.

Enhancement of Electrical Stability in Oxide Thin-Film Transistors Using Multilayer Channels Grown by Atomic Layer Deposition

The effect of Hf addition on the electrical performance and bias stability of ZnO-based thin-film transistors (TFTs) has been investigated. All channel layers were deposited by atomic layer deposition with various Hf contents. In addition, multilayer oxide channel TFTs consisting of two or three Hf-doped ZnO (HZO) and ZnO layers were developed for the realization of adequate channel mobility and electrical stability. The subthreshold swing and bias stability were improved by the deposition of the thin-HZO layers with amorphous phase as the first and final channel layers. The use of a conductive ZnO layer enhanced the device mobility. The oxide TFTs with a multilayer channel of a-HZO/ZnO/ a-HZO exhibited relatively good stability and mobility due to the reduced interface trap density between the channel and dielectric layers, and the suppressed adsorption of negatively charged oxygen on the back channel. The origin of the stability issues and novel channel design are proposed on the basis of the electrical performance of various TFT structures.

Enhancement of band-edge emission of ZnO from one-dimensional ZnO/MgZnO core/shell nanostructures

One-dimensional MgZnO nanostructures were grown directly on p-Si substrates by thermal evaporation at a variety of synthesis temperatures. Transmission electron microscopy and energy dispersive x-ray spectroscopy analysis revealed the formation of slim ZnO nanowires with twin boundaries on low Mg content nanosheets at lower synthesis temperatures, where the slim nanowires on the nanosheet did not have any detectable Mg content. The MgZnO nanostructures at elevated synthesis temperatures showed core/shell structures consisting of h-ZnO/h-MgZnO/c-MgZnO, where the h-MgZnO layer has a Mg content up to ~9 at% and a further increase in Mg content in the outer shell induced the formation of the c-MgZnO phase. The ZnO nanorods covered with MgZnO layers showed enhanced band-edge emission due to the existence of a h-MgZnO barrier and a c-MgZnO dielectric layer.

Enhanced oxygen reduction and evolution by in situ decoration of hematite nanoparticles on carbon nanotube cathodes for high-capacity nonaqueous lithium–oxygen batteries

Lithium–oxygen (Li–O2) batteries are considered to be the next generation energy storage technology due to their extremely high theoretical energy density and the simplicity of the battery cells. However, a large energy density can be obtained only with a slow discharge–charge rate, and quickly decreases upon cycling. These drawbacks can be attributed to the large overpotential and sluggish kinetics of the oxygen reduction and evolution reactions. To overcome the current problems, recent research has focused on developing catalysts made of inexpensive metal oxides and carbonaceous materials in addition to precious metals, but the role of non-precious metal catalysts in the battery performance is still largely unexplored. Here we present kinetic studies and comparative evaluation of enhanced oxygen reduction and evolution reactions with carbon nanotube (CNT) arrays containing in situ decorated α-Fe2O3 nanoparticles as both a binder-free catalyst and a cathode for nonaqueous Li–O2 batteries. Our Fe2O3-decorated CNTs greatly helped to form Li2O involving the four-electron reduction pathway in addition to Li2O2 commonly formed via the one/two-electron reduction pathway, and thereby delivered a very large capacity of 26.5 A h g−1 at the 1st discharge and a relatively long cycling performance (48 cycles with a capacity limit of 1.5 A h g−1). Unique and branch-like nanocrystalline Li2O and Li2O2 after discharge would have lowered the overpotential for the oxygen evolution reaction. Our detailed compositional and morphological studies will be of great help to further improve the cycling performance as well as develop better non-precious metal catalysts and electrodes for Li–O2 batteries.