Transparent semiconducting Nb-doped anatase TiO2 films deposited by helicon-wave-excited-plasma sputtering

The authors report stoichiometry control and postdeposition annealing-free fabrication of Nb-doped transparent anatase TiO2 (A-TiO2: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 degrees C and O-2 partial pressure (P-O2) in the range between 5 x 10(-4) and 1 x 10(-2) Pa, enabled the deposition of a high refractive index semiconducting anatase phase. The electron concentration of the A-TiO2 : Nb films increased with increasing Nb concentration up to Ti0.907Nb0.093O2. The results indicate that the Nb5+ donor on the Ti4+ site can be activated under near-reducing atmosphere where unwanted compensating defects may be passivated. As a result, anatase Ti0.907Nb0.093O2 film deposited at 500 C and PO2 = 5 x 10(-4) Pa exhibited a resistivity of 3.4 x 10(-3) Omega cm and an optical transmittance higher than 90%. The refractive index of A-TiO2: Nb was found to be approximately 2.63 at 450 nm with spectroscopic ellipsometry, which is comparable to the InGaN alloys. (C) 2011 American Vacuum Society. [DOI: 10.1116/ 1.3525918]