We investigated the NO gas sensing characteristics of ZnO-carbon nanotube (ZnO-CNT) layered composites fabricated by coaxial coating of single-walled CNTs with a thin layer of 1 wt% Al-doped ZnO using rf magnetron sputtering deposition. Morphological studies clearly revealed that the ZnO appeared to form beadshaped crystalline nanoparticles with an average diameter as small as 30 nm, attaching to the surface of the nanotubes. It was found that the NO gas sensing properties of the ZnO-CNT layered composites were dramatically improved over Al-doped ZnO thin films. It is reasoned from these observations that an increase in the surface-to-volume ratio associated with the numerous ZnO “nanobeads” on the surface of the CNTs results in the enhancement of the NO gas sensing properties. The ZnO-CNT layered composite sensors exhibited a maximum sensitivity of 13.7 to 2 ppm NO gas at a temperature of 200˚C and a low NO gas detection limit of 0.2 ppm in dry air.

We investigated the effects of Co doping on the NO gas sensing characteristics of ZnO-carbon nanotube (ZnO-CNT) layered composites fabricated by coaxial coating of single-walled CNTs with ZnO using pulsed laser deposition. Structural examinations clearly confirmed a distinct nanostructure of the CNTs coated with ZnO nanoparticles of an average diameter as small as 10 nm and showed little influence of doping 1 at.% Co into ZnO on the morphology of the ZnO-CNT composites. It was found from the gas sensing measurements that 1 at.% Co doping into ZnO gave rise to a significant improvement in the response of the ZnO-CNT composite sensor to NO gas exposure. In particular, the Co-doped ZnO-CNT composite sensor shows a highly sensitive and fast response to NO gas at relatively low temperatures and even at low NO concentrations. The observed significant improvement of the NO gas sensing properties is attributed to an increase in the specific surface area and the role as a catalyst of the doped Co elements. These results suggest that Co-doped ZnOCNT composites are suitable for use as practical high-performance NO gas sensors.

This study develops a highly transparent ohmic contact scheme using indium oxide doped ZnO (IZO)as a current spreading layer for p-GaN in order to increase the optical output power of nitride-based light-emitting diodes (LEDs). IZO based contact layers of IZO, Ni/IZO, and NiO/IZO were prepared by e-beamevaporation, followed by a post-deposition annealing. The transmittances of the IZO based contact layers werein excess of 80% throughout the visible region of the spectrum. Specific contact resistances of 3.4×10−4,1.2×10−4, 9.2×0−5, and 3.6×10−5Ω·cm2 for IZO, Ni/Au, Ni/IZO, and NiO/IZO, respectively were obtained. Theforward voltage and the optical output power of GaN LED with a NiO/IZO ohmic contact was 0.15V lower andwas increased by 38.9%, respectively, at a forward current of 20mA compared to that of a standard GaN LEDwith an Ni/Au ohmic contact due to its high transparency, low contact resistance, and uniform current spreading.

This study develops a highly transparent ohmic contact using phosphorus doped ZnO with current spreading for p-GaN to increase the optical output power of nitride-based light-emitting diodes (LEDs). The phosphorus doped ZnO transparentohmic contact layer was prepared by radio frequency magnetron sputtering with post-deposition annealing. The transmittance ofthe phosphorus doped ZnO exceeds 90% in the region of 440nm to 500nm. The specific contact resistance of the phosphorusdoped ZnO on p-GaN was determined to be 7.82×10-3Ω·cm2 after annealing at 700oC. GaN LED chips with dimensions of300×300µm fabricated with the phosphorus doped ZnO transparent ohmic contact were developed and produced a 2.7Vincrease in forward voltage under a nominal forward current of 20mA compared to GaN LED with Ni/Au Ohmic contact.However, the output power increased by 25% at the injection current of 20mA compared to GaN LED with the Ni/Au contactscheme.

ZnO wire-like thin films were synthesized through thermal oxidation of sputtered Zn metal films in dry air. Their nanostructure was confirmed by SEM, revealing a wire-like structure with a width of less than 100 nm and a length of several microns. The gas sensors using ZnO wire-like films were found to exhibit excellent H2 gas sensing properties. In particular, the observed high sensitivity and fast response to H2 gas at a comparatively low temperature of 200˚C would lead to a reduction in the optimal operating temperature of ZnO-based H2 gas sensors. These features, together with the simple synthesis process, demonstrate that ZnO wire-like films are promising for fabrication of low-cost and high-performance H2 gas sensors operable at low temperatures. The relationship between the sensor sensitivity and H2 gas concentration suggests that the adsorbed oxygen species at the surface is O-.

ZnO thin film was grown on a sapphire single crystal substrate by plasma assisted molecular beamepitaxy. In addition to near band edge (NBE) emissions, both blue and green luminescences are also observedtogether. The PL intensity of the blue luminescence (BL) range from 2.7 to 2.9eV increased as the amountof activated oxygen increased, but green luminescence (GL) was weakly observed at about 2.4eV without muchchange in intensity. This result is quite unlike previous studies in which BL and GL were regarded as thetransition between shallow donor levels such as oxygen vacancy and interstitial zinc. Based on the transitionlevel and formation energy of the ZnO intrinsic defects predicted through the first principle calculation, whichemploys density functional approximation (DFA) revised by local density approximation (LDA) and the LDA+Uapproach, the green and blue luminescence are nearly coincident with the transition from the conduction bandto zinc vacancies of V2-Zn and V-Zn, respectively.

In this study we investigated the effect of the multi-step texturing process on the electrical and optical properties of hydrogenated Al-doped zinc oxide (HAZO) thin films deposited by rf magnetron sputtering. AZO films on glass were prepared by changing the H2/(Ar+H2) ratio at a low temperature of 150˚C. The prepared HAZO films showed lower resistivity and higher carrier concentration and mobility than those of non-hydrogenated AZO films. After deposition, the surface of the HAZO films was multi-step textured in diluted HCl (0.5%) for the investigation of the change in the optical properties and the surface morphology due to etching. As a result, the HAZO film fabricated under the type III condition showed excellent optical properties with a haze value of 52.3%.

Changes in the surface morphology and light scattering of textured Al doped ZnO thin films on glasssubstrates prepared by rf magnetron sputtering were investigated. As-deposited ZnO:Al films show a hightransmittance of above 80% in the visible range and a low electrical resistivity of 4.5×10-4Ω·cm. The surfacemorphology of textured ZnO:Al films are closely dependent on the deposition parameters of heater temperature,working pressure, and etching time in the etching process. The optimized surface morphology with a cratershape is obtained at a heater temperature of 350oC, working pressure of 0.5 mtorr, and etching time of 45seconds. The optical properties of light transmittance, haze, and angular distribution function (ADF) aresignificantly affected by the resulting surface morphologies of textured films. The film surfaces, havinguniformly size-distributed craters, represent good light scattering properties of high haze and ADF values.Compared with commercial Asahi U (SnO2:F) substrates, the suitability of textured ZnO:Al films as frontelectrode material for amorphous silicon thin film solar cells is also estimated with respect to electrical andoptical properties.

Inorganic-organic composite thin-film-transistors (TFTs) of ZnO nanowire/Poly(3-hexylthiophene)(P3HT) were investigated by changing the nanowire densities inside the composites. Crystalline ZnO nanowireswere synthesized via an aqueous solution method at a low temperature, and the nanowire densities inside thecomposites were controlled by changing the ultrasonifiaction time. The channel layers were prepared withcomposites by spin-coating at 2000rpm, which was followed by annealing in a vacuum at 100oC for 10 hours.Au/inorganic-organic composite layer/SiO2 structures were fabricated and the mobility, Ion/Ioff ratio, andthreshold voltage were then measured to analyze the electrical characteristics of the channel layer. Comparedwith a P3HT TFT, the electrical properties of TFT were found to be improved after increasing the nanowiredensity inside the composites. The mobility of the P3HT TFT was approximately 10-4cm2/V·s. However, themobility of the ZnO nanowire/P3HT composite TFT was increased by two orders compared to that of theP3HT TFT. In terms of the Ion/Ioff ratio, the composite device showed a two-fold increase compared to thatof the P3HT TFT.

This study investigates Bi2O3-B2O3-BaO-ZnO glass with variations of the Co3O4 content (0.25,0.5,1,and 2wt%) and the interaction between transparent dielectric and Ag electrodes heat-treated at 500-560oC for30min. The glass transition temperature, softening temperature and thermal expansion coefficient were432oC, 460oC and 81.4×10-7/oC, respectively. The transmittance of 0.25wt% Co3O4 to which dielectric wasadded was highest and was decreased due to coloration with the addition of more than 0.25wt%. However,without Co3O4, the transmittance of the transparent layer was decreased due to the formation of Ba5Bi3;however, the occurrence of the crystal phase decreased as a result of the addition of Co3O4. The amount ofCo2+ ions increased as the Co3O4 increased. With a maximum of Co3+ ions, the highest transmittance wasobserved.

Well-aligned Zinc oxide (ZnO) nanowires were synthesized on silicon substrates by a carbothermal evaporation method using a mixture of ZnO and graphite powder with Au thin film was used as a catalyst. The XRD results showed that as-prepared product is the hexagonal wurzite ZnO nanostructure and SEM images demonstrated that ZnO nanowires had been grown along the [0001] direction with hexagonal cross section. As-grown ZnO nanowires were coated with glucose oxidase (GOx) for glucose sensing. Glucose converted into gluconic acid by reaction with GOx and two electrons are generated. They transfer into ZnO nanowires due to the electric force between electrons and the positively charged ZnO nanostructures in PBS. Photoluminescence (PL) spectroscopy was employed for investigating the movements of electrons, and the peak PL intensity increased with the glucose concentration and became saturated when the glucose concentration is above 10 mM. These results demonstrate that ZnO nanostructures have potential applications in biosensors.

ZnO/ZnS core/shell nanocrystals (~5-7 nm in diameter) with a size close to the quantum confinement regime were successfully synthesized using polyol and thermolysis. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) analyses reveal that they exist in a highly crystalline wurtzite structure. The ZnO/ZnS nanocrystals show significantly enhanced UV-light emission (~384 nm) due to effective surface passivation of the ZnO core, whereas the emission of green light (~550 nm) was almost negligible. They also showed slight photoluminescence (PL) red-shift, which is possibly due to further growth of the ZnO core and/or the extension of the electron wave function to the shell. The ZnO/ZnS core/shell nanocrystals demonstrate strong potential for use as low-cost UV-light emitting devices.

The NO gas sensing properties of ZnO-carbon nanotube (ZnO-CNT) composites fabricated by the coaxial coating of single-walled CNTs with ZnO were investigated using pulsed laser deposition. Upon examination, the morphology and crystallinity of the ZnO-CNT composites showed that CNTs were uniformly coated with polycrystalline ZnO with a grain size as small as 5-10 nm. Gas sensing measurements clearly indicated a remarkable enhancement of the sensitivity of ZnO-CNT composites for NO gas compared to that of ZnO films while maintaining the strong sensing stability of the composites, properties that CNT-based sensing materials do not have. The enhanced gas sensing properties of the ZnO-CNT composites are attributed to an increase in the surface adsorption area of the ZnO layer via the coating by CNTs of a high surface-to-volume ratio structure. These results suggest that the ZnO-CNT composite is a promising template for novel solid-state semiconducting gas sensors.

ZnO nanopowders were synthesized by the sol-gel method using hydrazine reduction, and their gas responses to 6 gases (200 ppm of C2H5OH, CH3COCH3, H2, C3H8, 100 ppm of CO, and 5 ppm of NO2) were measured at 300 ~ 400˚C. The prepared ZnO nanopowders showed high gas responses to C2H5OH and CH3COCH3 at 400˚C. The sensing materials prepared at the compositions of [ZnCl2]:[N2H4]:[NaOH] = 1:1:1 and 1:2:2 showed particularly high gas responses (S = Ra/Rg, Ra : resistance in air, Rg : resistance in gas) to 200 ppm of C2H5OH(S = 102.8~160.7) and 200 ppm of CH3COCH3(S = 72.6~166.2), while they showed low gas responses to H2, C3H8, CO, and NO2. The reason for high sensitivity to these 2 gases was discussed in relation to the reaction mechanism, oxidation state, surface area, and particle morphology of the sensing materials.

In this study, the additivity factors of compositions to density and glass transition point (Tg) in a xLi2O-(1-x)[(1-y)TeO2-yZnO] (0<x<20, 0<y<20) glass system were analyzed by using mixture design, and the change of ionic conductivity with density and Tg was discussed. As a method for predicting the relation between glass structure and ionic conductivity, density was measured by the Archimedes method. The glass transition point was analyzed to predict the relation between ionic conductivity and the bonding energy between alkali ions and non-bridge oxygen (NBO). The relation equations showing the additivity factor of each composition to the two properties are as follows: Density(g/cm3) = 2.441x1 ＋ 5.559x2 ＋ 4.863x3 Tg(˚C) = 319x1 ＋ 247x2 ＋ 609x3 － 1950x1x3 (x1 : fraction of Li2O, x2 : fraction of TeO2, x3 : fraction of ZnO) The density decreased as Li2O content increased. This was attributed to change of the TeO2 structure. From this structural result, the electric conductivity of the glass samples was predicted following the ionic conduction mechanism. Finally, it is expected that electric conductivity will increase as the activation energy for ion movement decreases.

The hydrogen gas sensing properties of a zinc oxide nanowire structure were studied. Porous zinc oxide nanowire structures were fabricated by oxidizing zinc deposited on a single-wall carbon nanotube (SWNT) template. This revealed a porous ZnO-SWNT composite due to the porosity in the SWNT film. The gas sensing properties were compared with those of zinc oxide thin films deposited on SiO2/Si substrates in sensitivity and operating temperature. The composite structure showed higher sensitivity and lower operating temperature than the zinc oxide film. It showed a response even at room temperature while the film structure did not.

ZnO nanorod gas sensors were prepared by an ultrasound radiation method and their gas sensing properties were investigated for NO gas. For this procedure, 0.01, 0.005 and 0.001M of zinc nitrate hydrate [Zn(NO3)2 · 6H2O] and hexamethyleneteramine [C6H12N4] aqueous solutions were prepared and then the solution was irradiated with high intensity ultrasound for 1 h. The lengths of ZnO nanorods ranged from 200 nm to 500 nm with diameters ranging from 40 nm to 80 nm. The size of the ZnO nanorods could be controlled by the concentration of solution. The sensing characteristics of these nanostructures were investigated for three kinds of sensor. The properties of the sensors were influenced by the morphology of the nanorods.

The effects of the deposition and annealing temperature on the structural, electrical and opticalproperties of Ag doped ZnO (ZnO:Ag) thin films were investigated. All of the films were deposited with a 2wt%Ag2O-doped ZnO target using an e-beam evaporator. The substrate temperature varied from room temperature(RT) to 250oC. An undoped ZnO thin film was also fabricated at 150oC as a reference. The as-grown films wereannealed in temperatures ranging from 350 to 650oC for 5h in air. The Ag content in the film decreased asthe deposition and the post-annealing temperature increased due to the evaporation of the Ag in the film.During the annealing process, grain growth occurred, as confirmed from XRD and SEM results. The as-grownfilm deposited at RT showed n-type conduction; however, the films deposited at higher temperatures showedp-type conduction. The films fabricated at 150oC revealed the highest hole concentration of 3.98×1019cm-3 anda resistivity of 0.347Ω·cm. The RT PL spectra of the as-grown ZnO:Ag films exhibited very weak emissionintensity compared to undoped ZnO; moreover, the emission intensities became stronger as the annealingtemperature increased with two main emission bands of near band-edge UV and defect-related greenluminescence exhibited. The film deposited at 150oC and annealed at 350oC exhibited the lowest value of Ivis/Iuv of 0.05.

This study examined the effect of growth temperature on the electrical and optical properties ofhydrogenated Al-doped zinc oxide (AZO:H) thin films deposited by rf magnetron sputtering using a ceramictarget (98wt.% ZnO, 2wt.% Al2O3). Various AZO films on glass were prepared by changing the substratetemperature from room temperature to 200oC. It was shown that intentionally incorporated hydrogen plays animportant role on the electrical properties of AZO:H films by increasing free carrier concentration. As a result,in the 2% H2 addition at the growth temperature of 150oC, resistivity of 3.21×10-4Ω·cm, mobility of 21.9cm2/V−s, electric charge carrier concentration of 9.35×1020cm-3 was obtained. The AZO:H films show a hexagonalwurtzite structure preferentially oriented in the (002) crystallographic direction.