Transparent semiconducting Nb-doped anatase TiO_2 films deposited by helicon-wave-excited-plasma sputtering

摘要

The authors report stoichiometry control and postdeposition annealing-free fabrication of Nb-doped transparent anatase TiO_2 (A-TiO_2:Nb) films on alkaline-free glass substrates by helicon-wave-excited-plasma sputtering. The films tended to crystallize in the stable electrically semi-insulating rutile phase. However, although the appropriate deposition condition window was narrow, precise stoichiometry control using near-reducing ambient, namely, the deposition temperature higher than 450 ℃ and O_2 partial pressure (P_(O_2) ) in the range between 5 × 10~(-4) and 1 × 10~(-2) Pa, enabled the deposition of a high refractive index semiconducting anatase phase. The electron concentration of the A-TiO_2: Nb films increased with increasing Nb concentration up to Ti_(0.907)Nb_(0.093)O_2. The results indicate that the Nb~(5+) donor on the Ti~(4+) site can be activated under near-reducing atmosphere where unwanted compensating defects may be passivated. As a result, anatase Ti_(0.907)Nb_(0.093)O_2 film deposited at 500 ℃ and P_(O_2) =5 × 10~(-1) Pa exhibited a resistivity of 3.4 × 10~(-3)Ω cm and an optical transmittance higher than 90%. The refractive index of A-TiO_2: Nb was found to be approximately 2.63 at 450 nm with spectroscopic ellipsometry, which is comparable to the InGaN alloys.