that Cu surfaces easily oxidize, the actual amount of metallic Cu
contained in the ~2 nm-thick Cu layer would be insufficient to induce
full crystallization of the TiO2
film. Therefore, RBS results are more reliable to estimate the effective amount of Cu catalysts.
In general, the crystallization kinetics of amorphous films depends
on both temperature and annealing time. In fact, Cu-MIC occurred
even at 210 °C when the respective sample was annealed for several
hours, as shown in Fig. 5. Interestingly, Fig. 5 also shows lateral crystal
growth, where closely packed polycrystalline TiO2 grains grow over
the edges of the Cu pattern. This result indicates that the Cu bottom
layer behaves as a nucleation center, and that growth of TiO2 crystals
proceeds in both vertical and lateral directions.
One disadvantage of MIC with a Cu bottom layer is that most of the
Cu remains between thefilm and substrate, which could limit the use of
the resultant material in optical applications. We also examined the catalytic capabilities of a Cu cap layer, which can be removed using a wet
etch. We annealed an amorphous TiO2
film (45 nm) with a Cu pattern
(20 nm) cap (Fig. 6(a)) at 210 °C for 3 h (Fig. 6(b)) and then removed
the remaining Cu by immersing the sample in a Cu etchant (H2SO4:
H2
O2:H2
O = 55:5:40 vol.%) at 45 °C (Fig. 6(c)). We confirmed the
presence of fully crystallized TiO2 in the region covered with the Cu
pattern, as shown in Fig. 6(c). Fig. 6(c) also shows lateral growth of
large TiO2 grains, similar to the behavior of TiO2/Cu. We believe that