  We present a study on the crystallization process of undoped and Ta doped
TiO2 amorphous thin films. In particular, the effect of ultra-fast annealing
treatments in environments characterized by different oxygen concentrations is
investigated via in-situ resistance measurements. The accurate examination of
the key parameters involved in this process allows us to reduce the time needed
to obtain highly conducting and transparent polycrystalline thin films
(resistivity about $6 \times 10^{-4}$ {\Omega}cm, mean transmittance in the
visible range about $81\%$) to just 5 minutes (with respect to the 180 minutes
required for a standard vacuum annealing treatment) in nitrogen atmosphere (20
ppm oxygen concentration) at ambient pressure. Experimental evidence of
superficial oxygen incorporation in the thin films and its detrimental role for
the conductivity are obtained by employing different concentrations of
traceable 18O isotopes during ultra-fast annealing treatments. The results are
discussed in view of the possible implementation of the ultra-fast annealing
process for TiO2-based transparent conducting oxides as well as electron
selective layers in solar cell devices; taking advantage of the high control of
the ultra-fast crystallization processes which has been achieved, these two
functional layers are shown to be obtainable from the crystallization of a
single homogeneous thin film.
