열처리에 따른 doped ZnO 물성

Abstract

Doped ZnO has been attracting much attention as an alternative transparent conducting oxide (TCO) for tin doped indium oxide (ITO) for its high transparency and low resistivity combined with non-toxicity and low cost. Most of research on doped ZnO has been focused on the group Ⅲ metal cation dopants such as Al, B, and Ga, which increase the carrier concentration by substituting Zn sites. The metallic dopants are known to act as scattering centers due to perturbation of conduction band, leading to reduction of Hall mobility. In case of anion dopants like F or Cl, however, the electrical perturbation is confined to valence band, and much less scattering than metallic dopants can be expected. In this work, the electrical, structural and optical properties of F doped ZnO (FZO) and Al doped ZnO (AZO) transparent films deposited by radio frequency (rf) magnetron sputtering, were investigated. FZO films were fabricated using pure ZnO target in controlled volume flow of Ar and CF4 gas mixture, and AZO films were prepared using ZnO target containing 2 wt.% Al2O3 in pure Ar gas atmosphere. As-deposited FZO films showed rather poor electrical characteristics. But upon annealing in vacuum at elevated temperature of 315˚C, the electrical properties of FZO films improved very much. The Hall mobility of vacuum-annealed FZO film, which was deposited in 0.4 vol.% CF4 gas mixture, reached as high as 42.6 cm2/Vsec. With increasing CF4 gas content, the transmittance and the reflectance increased. It implies the lowering of absorption loss, leading to higher figure of merit. These improvements in electrical and optical properties of FZO films upon vacuum-annealing were ascribed to mainly the removal of interstitial F. Undoped ZnO films, however, showed degraded electrical properties due to removal of interstitial Zn upon vacuum-annealing In study of thermal stability of AZO films, AZO films with 4 different film thickness were prepared, and the real-time variation of electrical properties with annealing temperature in atmosphere was monitored. With increasing film thickness, the onset temperature of rapid increase in resistance increased, while the overall extent of degradation in electrical properties decreased. The thickness dependent behavior of the resistance with temperature indicates that the degradation is affected by diffusion of oxygen, but further analysis of the exact mechanism such as separation between the in-grain diffusion and the grain boundary diffusion, chemisorption and desorption is left for future study.