Explosive Crystallization in Atomic Layer Deposited Mixed Titanium Oxides

Anatase crystals with lateral sizes of tens of micrometers are impossible to prepare as thin films on amorphous substrates by normal thin film deposition techniques. When deposited on amorphous substrates, polycrystalline anatase titanium dioxide (TiO2) thin films have typically grain and crystal sizes comparable to the film thickness. We show that when a suitable amount of Nb2O5 or Ta2O5 is mixed with TiO2 using atomic layer deposition (ALD), upon postdeposition annealing large similar to 10-100 mu m sized anatase monocrystals are formed in films thinner than 100 nm on amorphous substrates. Such an exceptionally extensive lateral grain growth appears to be a result of explosive crystallization when amorphous Ti-Nb-O and Ti-Ta-O thin films are heated, a phenomenon that has not been reported before for TiO2 thin films. The prepared films will likely have a number of useful applications where well-ordered anatase surfaces are required, such as photocatalysis, and they also can be used as a transparent conducting oxide.