ICMCTF2009 Session C2-2: Optical Thin Films for Active Devices and Microstystems
Wednesday, April 29, 2009 1:30 PM in Room Royal Palm 4-6
C2-2-1 ZnO:Al Transparent Conducting Oxide Films of Very High Quality Synthesized by Filtered Cathodic Arc Deposition
A. Anders (Lawrence Berkeley National Laboratory); S. Lim (University of Sydney, Australia); K.M. Yu (Lawrence Berkeley National Laboratory); J. Andersson (Ångstrom Laboratory, Sweden); J. Rosén (Linköping University, Sweden); M. McFarland, J. Brown (Acree Technologies Inc.)
Aluminum-doped zinc oxide, ZnO:Al, is a well-known n-type transparent conducting oxide with great potential in a number of applications currently dominated by indium tin oxide (ITO). ITO replacement projects are driven by the high cost of indium, and ZnO:Al may serve well under certain conditions. Additionally, ZnO is a material that offers the possibility of transparent electronics by synthesizing n- and p-type material. Here we focus on ZnO:Al deposited by filtered cathodic arc deposition as an interesting ITO alternative to magnetron sputtering because we can utilize ion-assisted deposition while avoiding the damage to films caused by energetic negative oxygen ions that are typical for reactive magnetron sputtering. The quality of the films strongly depends on the deposition temperature while only marginal improvements are obtained with post-deposition annealing. To date, our best films have conductivities in the low 10@super -4@ Ωcm range with a transmittance better than 85% in the visible part of the spectrum. We will report about the relationships of deposition conditions and structural, optical, and electronic properties. @paragraph@This work was supported in part by the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
C2-2-2 Influence of Thermal Annealing on Electrical and Optical Properties of Ga-Doped ZnO Thin Films
H. Makino, N. Yamamoto, A. Miyake, T. Yamada (Kochi University of Technology, Japan); Y. Hirashima, H. Iwaoka, T. Itoh (Geomatec Co., Ltd., Japan); H. Hokari, H. Aoki (Casio Computer Co., Ltd., Japan); T. Yamamoto (Kochi University of Technology, Japan)
Ga-doped ZnO (GZO) is one of potential candidates as substitute for indium tin oxide. In practical applications of GZO films as transparent electrode in liquid crystal display, thermal stability of GZO films is required. In this study, we report influence of thermal annealing on electrical and optical properties of GZO films. @paragraph@ 150 nm thick GZO films were deposited on alkali-free glass substrates by ion plating (IP), direct current magnetron sputtering (DCMS), and radio frequency power superimposed direct current magnetron sputtering (RF/DCMS) at a substrate temperature of 150 @super o@C. Electrical resistivity of as-deposited films by IP, DCMS, and RF/DCMS were 2.8, 8.0, and 3.7 µm, respectively. The films were annealed in air or in nitrogen gas atmosphere for 30 min at annealing temperatures, T@sub an@, between 200 and 450@super o@C. In the case of GZO films deposited by IP, the resistivity was stable up to 300@super o@C in air and up to 350@super o@C in nitrogen gas atmosphere, respectively. The resistivity increased with increasing the T@sub an@ beyond the critical temperature. The critical temperatures of GZO films deposited by DCMS and RF/DCMS were lower than that of IP. @paragraph@ Optical transmission and reflection spectra were measured by a spectrophotometer. The optical spectra also showed systematic change depending on the T@sub an@. The optical spectra were simulated by a conventional Drude model, and optical mobility, μ@sub opt@, and carrier concentration, N@sub opt@, were estimated from the model fitting. The N@sub opt@ showed overall good correspondence with carrier concentration characterized by Hall measurements. The μ@sub opt@ was also nearly identical with the Hall mobility, μ@sub Hall@, up to the critical temperature. However, the μ@sub opt@ and the μ@sub Hall@ showed different behavior at the T@sub an@ higher than the critical temperature. The μ@sub opt@ increased with increasing the T@sub an@, while the μ@sub Hall@ decreased with increasing the T@sub an@. The difference suggests different scattering mechanism governing the mobility. The increase in μ@sub opt@ suggests a decrease of some scattering centers inside grains. On the other hand, grain boundary scattering possibly contribute the decrease in μ@sub Hall@.
C2-2-3 Attractive Potential Substitute for ITO in Flat Panel Display
K. Utsumi (Tosoh Corporation, Japan)
Recently, markets of LCD (Liquid Crystal Display）and solar cell are expanding rapidly. In these devices, TCO (Transparent Conductive Oxide) are used. ITO (Tin doped Indium Oxide) is the most representative material of TCO. However, there are problems regarding In resources and stability of supply. From these backgrounds, demand of alternative ITO is increasing.@paragraph@ ITO films are prepared by sputtering, vacuum evaporation, spray, and CVD. Among these methods, sputtering is widely used for production line of LCD and solar cell because of its advantageous in good uniformity for large area deposition. So, material and deposition method of the alternative ITO must be used in present production line. ZnO based materials are one of candidate materials of alternative ITO because Zn is abundance, low price and poisonous less material. @paragraph@ In this report, characters of aluminum doped ZnO (AZO) film prepared by dc magnetron sputtering on glass substrate will be discussed.@paragraph@ Resisitivity of AZO film prepared by dc sputtering is over 1000 @micron@@ohm@cm because of lower crystallinity by the bombardment of high energy particle such as neutral Ar and negative ionized oxygen. The resistivity of AZO film strongly depend on the crystallinity of the film. Therefore, to improve the crystallinity of the film, optimization of Al content was performed. As the results, the resistivity of ZAO film was reduced under 500 @micron@@ohm@cm (thickness =150nm for color filter of LCD) and under 1000 @micron@@ohm@cm (thickness = 50nm for TFT of LCD). These values of resisitivity are adaptive for LCD.@paragraph@ It is well known that ZnO based materials are very week for heat and humidity. These are significant problems for reliability of the products.. By the addition of the third element, these problems were solved.
C2-2-5 Thermopower and Optical Studies on Undoped and Manganese Doped Indium Tin Oxide Films
S.R Kumar, V. Damodara Das, S. Kasiviswanathan (Indian Institute of Technology Madras, India)
Thermopower measurements in the range of 300-650 K along with optical studies were performed on indium tin oxide (ITO) and manganese doped indium tin oxide (Mn:ITO) films grown by DC reactive sputtering. The role of oxygen vacancies and manganese in determining the electrical properties and band gap of the films were examined by annealing the films in argon and oxygen and by the analysis of the thermopower and optical data. The Seebeck coefficient (S) of the films is negative and its magnitude increases linearly with temperature suggesting a near free-electron like behaviour, but decreases drastically beyond a temperature for all the films. This has been explained by considering the out-diffusion of oxygen from the films, upon argon annealing. The optical transparency of the films is found to be more than 90% in the visible region. A dielectric function model combining the Forouhi-Bloomer model for crystalline materials and Drude model was used to fit the optical transm ittance of the films. The complex dielectric function, thickness, plasma frequency and damping constant of the films were obtained from which the electrical properties were extracted. Argon annealed ITO film is found to have the highest carrier concentration (8.6 × 10@super20@ /cm@super3@) while the highest band gap (4.41 eV) is observed in Mn:ITO film annealed in argon. The results indicate that oxygen vacancies as well as presence of manganese affect the optical and electrical properties of the films. The observed changes in band gap could be explained on the basis of Burstein-Moss widening along with electron-electron and electron-impurity scattering. Comparison of results from optical and thermopower studies indicate that ionized impurity scattering is the predominant scattering process in these films.
C2-2-6 Reactively Co-Sputtering Transparent Conducting Oxide Films of Nb-Doped Titania
K.H. Hong, P.W. Lee, W.C. Hsu, H.C. Chang, M.S. Wong (National Dong Hwa University, Taiwan)
Niobium-doped titania (TNO) films were deposited on glass substrates by reactive co-sputtering of titanium and niobium targets in a mixed argon/oxygen plasma. The composition of the TNO films was modulated by the Nb target power ranging from 0 to 150 Watts, resulting Nb content from 0 to ~13.9 at.%. The XRD patterns of the as-grown TNO films revealed that all the films were amorphous in nature, and the UV-visible spectra showed a high average visible transmittance above 70%. The amorphous TNO films were found to show an insulator-like behavior with high resistivity of ~1E7 ohm-cm. After post-annealing in a hydrogen atmosphere at 600°C for 1 hour, the TNO films became anatase phase. The d-spacing of the anatase (101) peak in XRD pattern almost monotonously increased with the Nb at.% due to the larger ionic size of Nb5+ (0.78 Å) than that of Ti4+ (0.75 Å). The H2-annealed TNO film with Nb of ~10.5 at.% exhibited a high average visible transmittance of 70%. In addition, Hall measurement of the TNO film revealed an n-type semiconducting behavior with a carrier density of 6.6E21 cm@super -3@ and a mobility of 1.0 cm2V-1s-1, and a resistivity of 9.2E-4 Ω-cm, a dramatically ten order reduction in resistivity.
C2-2-7 Structural, Electrical and Optical Properties of Transparent Zn@sub 1-x@Mg@sub x@O Nanocomposite Thin Films
A. Kaushik, D. Kaur (Indian Institute of Technology, India)
We report on the growth of Zn@sub 1-x@Mg@sub x@O (ZMO) thin films on quartz substrate using ultrasonic spray pyrolysis technique. AFM images of the films deposited at optimized substrate temperature clearly reveals the formation of nanorods. The influence of varying Mg composition on structural, electrical and optical properties of Zn@sub 1-x@Mg@sub x@O films has been systematically investigated. Increase in Mg content (in the range 0.0≤x≤1.0), reflects the structural phase transition from wurtzite via mixed phase region to cubic one. The variation of the cation-anion bond length to Mg content shows that the lattice constant of the hexagonal Zn@sub 1-x@Mg@sub x@O decreases with corresponding increase in Mg content, which result in the structure gradually deviating from the wurtzite structure. Substrate temperature was also observed to have great impact on properties of these films. The optical measurements reveal a blue shift in absorption edge and increase in transmittance with increase in Mg content . Tuning of the band gap has been obtained from 3.41 eV to 6.58 eV with corresponding increase in Mg content from x = 0.0 to x = 1.0, which demonstrates that the films are useful for window layer of solar cells that improve the overall efficiency by decreasing the absorption loss. Keywords: Thin films; Zn@sub 1-x@Mg@sub x@O; Optical band gap.