ICMCTF2009 Session C2-2: Optical Thin Films for Active Devices and Microstystems

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^-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.

This work was supported in part by the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

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.

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 °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_an, between 200 and 450°C. In the case of GZO films deposited by IP, the resistivity was stable up to 300°C in air and up to 350°C in nitrogen gas atmosphere, respectively. The resistivity increased with increasing the T_an beyond the critical temperature. The critical temperatures of GZO films deposited by DCMS and RF/DCMS were lower than that of IP.

Optical transmission and reflection spectra were measured by a spectrophotometer. The optical spectra also showed systematic change depending on the T_an. The optical spectra were simulated by a conventional Drude model, and optical mobility, μ_opt, and carrier concentration, N_opt, were estimated from the model fitting. The N_opt showed overall good correspondence with carrier concentration characterized by Hall measurements. The μ_opt was also nearly identical with the Hall mobility, μ_Hall, up to the critical temperature. However, the μ_opt and the μ_Hall showed different behavior at the T_an higher than the critical temperature. The μ_opt increased with increasing the T_an, while the μ_Hall decreased with increasing the T_an. The difference suggests different scattering mechanism governing the mobility. The increase in μ_opt suggests a decrease of some scattering centers inside grains. On the other hand, grain boundary scattering possibly contribute the decrease in μ_Hall.

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.

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.

In this report, characters of aluminum doped ZnO (AZO) film prepared by dc magnetron sputtering on glass substrate will be discussed.

Resisitivity of AZO film prepared by dc sputtering is over 1000 μmΩcm 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 μmΩcm (thickness =150nm for color filter of LCD) and under 1000 μmΩcm (thickness = 50nm for TFT of LCD). These values of resisitivity are adaptive for LCD.

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.