Effects of Different Amounts of Nb Doping on Electrical, Optical and Structural Properties in Sputtered TiO_2−x Films

• Main Authors: Daniel Dorow-Gerspach, Dieter Mergel, Matthias Wuttig
• Format: Article
• Language: English
• Published: MDPI AG 2021-03-01
• Series: Crystals
• Subjects: Nb doping; titanium dioxide; electrical properties; anatase; reactive sputtering; transparent conducting oxide
• Online Access: https://www.mdpi.com/2073-4352/11/3/301

Highly conductive TiO₂ films with different Nb doping levels (up to 5 at%) were prepared by reactive DC magnetron sputtering under precise control of the oxygen partial pressure. They were deposited on unheated substrates, covered with a protective Si₃N₄ layer, and subsequently annealed at 300 °C. The doping efficiency of Nb is greater than 90%. Conductivity is a maximum for a partly oxidized target in the transition range. The best films exhibit a resistivity of 630 µΩ cm and a mobility of 7.6 cm²/Vs combined with a high transparency above 70%. Comparing the behavior of undoped and Nb-containing films, intrinsic limits of the conductivity in the TiO_2−x:Nb system could be observed, and a consistent model explaining these findings is presented. The conductivity is limited—by decreasing electron density due to Nb oxidation—by increasing incorporation formation of Nb₂O₅ clusters as scattering centers with increasing oxygen partial pressure and Nb concentration, by a transition from the crystalline to the amorphous state of the films below a critical oxygen partial pressure.