Papers

ICMCTF2008 Session C2-2: Optical Thin Films for Active Devices and Microsystems

Tuesday, April 29, 2008 1:30 PM in Room Royal Palm 1-3

Tuesday Afternoon

Start	Invited?	Item
1:30 PM		C2-2-1 ZnO-Based TFT for Use in LCD T. Hirao, M. Furuta, T. Hiramatsu (Kochi University of Technology, Japan) High-mobility bottom-gate ZnO-TFTs have been developed using fully photolithography and etching processes. Field effect mobility of over 5 cm²/(Vs)^-1 and on/off current ratio of over 10⁷ were achieved. We have demonstrated 1.46" diagonal high image quality AM-LCDs addressed by bottom-gate ZnO-TFTs.
2:10 PM		C2-2-3 Effect of Sn Doped on Microstructural and Optical Properties of ZnO Thin Films Deposited by Sol-Gel Method C.-Y. Tsay, Y.-T. Tung (Feng Chia University, Taiwan); H.-C. Cheng (Industrial Technology Research Institute, Taiwan); W.-H. Tuan (National Taiwan University, Taiwan); C.-K. Lin (Feng Chia University, Taiwan) Wide bandgap (> 3 eV) transparent oxides have been used in green energy and optical-electrical applications. Among these materials, ZnO is one of the most promising candidates for the novel device applications since it possesses good crystallinity by preparing at low temperature region, high transparency and has a widely range conductivity from metallic to insulating. In this study, transparent thin films of Sn:ZnO were deposited onto alkali-free glass substrates by using sol-gel method and crystallinity, microstructural and optical properties affected by Sn⁺⁴ doping were investigated. The atomic percent of dopant in ZnO-based sols were Sn/Zn =0, 1, 2, 3, 5 and 10 at.%. The polycrystalline Sn:ZnO thin films would obtain when the as-coated films were preheated at 300 ^oC for 10 minutes and then annealed at 500 ^oC for 1 hr in air. The results show that, Sn doped ZnO thin films obviously improved the surface roughness, enhanced the transmittance in wavelength range of 400-600 nm and reduced the average crystallite size. ZnO thin films doped with 2 at.% Sn exhibited the best transmittance of 91.4% and the lowest RMS roughness value of 1.92 nm compared with others. Moreover, the resistivity and carrier concentration of Sn:ZnO thin films were also examined in this present work.
2:30 PM		C2-2-4 Transparent Conducting Oxide Thin Films Prepared Using Oxide Targets Sintered by Millimeter-Wave Heating T. Miyata (Kanazawa Institute of Technology, Japan); H. Abe (Fuji Dempa Kogyo Co. Ltd., Japan); T. Okada, Y. Hara, T. Minami (Kanazawa Institute of Technology, Japan) This paper describes the preparation of transparent conducting oxide (TCO) thin films by pulsed laser deposition (PLD) or magnetron sputtering deposition (MSD) using various oxide targets sintered with a newly developed millimeter-wave (28GHz) heating technique. For the purpose of evaluating the millimeter-wave-sintered oxide targets, the targets and TCO thin films were prepared under various preparation and film deposition conditions, respectively. For example, transparent conducting Al-doped ZnO (AZO) thin films were prepared with various thicknesses on glass substrates at a temperature of 200°C by PLD using an AZO target sintered for 30 min at a temperature of approximately 1250°C in an Ar gas atmosphere. AZO thin films prepared with a thickness of approximately 200nm exhibited a low resistivity on the order of 2X10^-4Ωcm; the obtainable resistivity of the prepared AZO films increased as the thickness was decreased. It was found that the resistivity and its thickness dependence obtainable in AZO thin films prepared using a millimeter-wave-sintered AZO target were comparable to those produced in AZO films prepared using a conventional furnace sintered AZO target. In addition, V-co-doped AZO (AZO:V) thin films were prepared with various thicknesses by PLD using an oxide target co-doped with an Al content (Al/(Al+Zn) atomic ratio) of 4 at.% and an V content (V/(V+Zn) atomic ratio) of 0.2at.% and sintered for 30 min at 1250°C in an Ar gas atmosphere by the millimeter-wave heating technique. It was found that the thickness dependence of resistivity in AZO thin films mentioned above was improved by the V-co-doping. In addition, chemical stability (etching rates in acid or alkaline solutions) and resistivity stability (for use in high temperature and/or high relative humidity atmospheres) could also be improved by V-co-doping into AZO thin films. It can be concluded that the newly developed energy saving millimeter-wave heating sintering technique is very well suited for producing sintered oxide targets.
2:50 PM		C2-2-6 Effect of Relative Position Between Arc Plasma and Substrate on Properties of GZO Thin Films Prepared by VAPE T. Miyata, Y. Honma, T. Ito, T. Minami (Kanazawa Institute of Technology, Japan) It has been recently reported that highly conductive and transparent Ga-doped ZnO (GZO) thin films were prepared by using a newly developed vacuum arc plasma evaporation (VAPE) method. However, there are few reports on the main part influence of variations in important deposition parameters on various properties of deposited films. This paper describes the effect of relative position between the flow direction of the arc plasma and the substrate surface on the resulting film thickness (deposition rate) and electrical properties in transparent conducting GZO thin films deposited by the VAPE method; the placement of the glass substrate (substrate position) was varied relative to the arc plasma. It should be noted that in GZO film preparation by VAPE, the substrate normally must rotate or travel linearly during the deposition in order to improve distributions in film thickness and electrical properties across the substrate surface. In this work, however, the substrate remained fixed at a specific position during each deposition. It was found that the deposition rate of GZO films deposited on glass substrates at 200°C at a pressure of 0.15 Pa and a substrate-evaporation source distance of about 58mm was strongly dependent on the substrate position relative to the arc plasma flow. In addition, when GZO thin films were deposited with various thicknesses in the range from 20 to 300nm, films with the same thickness exhibited an obtained resistivity and a thickness dependence of resistivity that were considerably affected by the relative position of the substrate during the deposition. The film thickness and substrate position dependencies of resistivity were related to those found in both Hall mobility and carrier concentration. A low resistivity of 3X10^-4Ωcm was obtained in highly transparent GZO thin films deposited with a thickness below 50 nm at a substrate position near the arc plasma source.
3:10 PM		C2-2-7 Influence of Hydrogen on the Properties of Al-doped ZnO Thin Films S.H. Lee, Y.D. Kim (Hanyang University, Korea); W.M. Kim (Korean Institute of Science and Technology, Korea) An investigation was made on the electrical, optical and structural characteristics of hydrogenated Al-doped ZnO (HAZO) thin films, which were prepared by radio frequency magnetron sputtering of ZnO targets containing Al₂O₃ content of 1 (HA₁ZO) and 2 (HA₂ZO) wt.% at substrate temperature of 150 °C in H₂/Ar gas mixtures. SIMS analysis showed that hydrogen (¹H^-) counts kept rising with increasing H₂ content in sputter gas. For both HAZO film series, the carrier concentration increased with increasing H₂ content in sputter gas, indicating that hydrogen acts as an n-type donor. The increase in carrier concentration due to hydrogen was also reflected by the increase of the free carrier absorption in a low energy region as well as the widening of optical band gap. The Hall mobility of HA₂ZO film series decreased monotonically with H₂ content, while those of HA₁ZO film series showed a maximum behavior. Although the lower resistivities were attained for HA₂ZO films, the higher figure of merit was observed for HA₁ZO films due to substantially lower absorption loss. Air-annealing at 300 °C resulted in large drop of the carrier concentration as well as the Hall mobility, while vacuum-annealing yielded only reduction of the carrier concentration with no appreciable change in the Hall mobility. SIMS analysis showed that the removal of hydrogen was responsible for the decreased carrier concentration observed in both the air- and vacuum-annealed films, and that the charge trapping at the grain boundaries was also crucial in the case of the air-annealed films.
3:30 PM		C2-2-8 Multiple Deposition Method of Producing Crack-Free Ga-Doped ZnO Films on Cyclo Olefin Polymer Substrates by Ion Plating A. Miyake, T. Yamada, H. Makino, N. Yamamoto, T. Yamamoto (Kochi University of Technology, Japan) Transparent conductive Ga-doped ZnO (GZO) films were deposited on cyclo olefin polymer (COP) substrate by ion plating deposition with dc arc discharge. The continuous deposition method is difficult for producing crack-free GZO films because of inadvertent increase in COP substrate temperature. The process temperature of using plastic substrate is limited by inherent heat characteristics, such as low glass transition temperature and large coefficient of thermal expansion, of the substrate. Crack initiation in GZO films on COP substrate due to compressive stresses arising from the difference in the thermal expansion coefficient between the GZO film and the COP substrate strongly depend on the deposition process including deposition time and maximum temperature reached. In this work, we develop a multiple deposition method with repeat of a deposition cycle including the deposition and cooling of GZO films. The present process is capable of producing crack-free GZO films with preferred orientation along the c-axis perpendicular to the COP substrates. As a result, the GZO films with a thickness of 94.2 nm have the resistivity of 8.1 x 10^-4 Ωcm (by Four-Point Probe), mean optical transmittance in the visible range of 86.1 % and optical transmittance at 550 nm of 84.6 %. The effect of multiple deposition on structures will be discussed during presentation.
3:50 PM		C2-2-9 Pulsed Laser Deposition of ZnO-MgO Nanocomposite A. Kaushal, P. Singh, A. Kumar, I. Deepak, D. Kaur (Indian Institute of Technology Roorkee, India) ZnO-MgO (ZMO) nanocomposite thin films were grown on quartz substrates by pulsed laser deposition using KrF excimer laser of wavelength 248nm. It was found structural, electrical and optical properties of the films can be tuned by changing the deposition parameters. The ZMO films consist of hexagonal wurtzite phase at Mg composition ranging from 0% to 50%; mixture of wurtzite and cubic phases at 60% to 80%, and single cubic phase at 100%. X-ray diffraction measurements show that the cubic ZMO films grow along the [111] direction while the hexagonal ZMO films grow along [001]. Increase in Mg concentration results in pronounced c-axis compression. From the optical transmittance spectra of the films we observed a blue shift in the absorption edge with increase in Mg content. The root mean square surface roughness found to depend on the oxygen partial pressure and substrate temperature during deposition. The electrical resistivity was found to decrease with increasing deposition temperature. With increase in substrate temperature, the Mg content increases and the average optical transmission also increases indicating band gap expansion. Transmittances of ZMO thin films were nearly equal to those of ZnO. ZnO-MgO nanocomposite thin films has a wider band gap than ZnO and can be expected to provide a usefull window layer of solar cell that improve the overall efficiency by decreasing the absorption loss.