Low-Temperature Route to Direct Amorphous to Rutile Crystallization of TiO₂ Thin Films Grown by Atomic Layer Deposition

Abstract

The physicochemical properties of titanium dioxide (TiO2) depend strongly on the crystal structure. Compared to anatase, rutile TiO2 has a smaller bandgap, a higher dielectric constant, and a higher refractive index, which are desired properties for TiO2 thin films in many photonic applications. Unfortunately, the fabrication of rutile thin films usually requires temperatures that are too high (>400 °C, often even 600–800 °C) for applications involving, e.g., temperature-sensitive substrate materials. Here, we demonstrate atomic layer deposition (ALD)-based fabrication of anatase and rutile TiO2 thin films mediated by precursor traces and oxide defects, which are controlled by the ALD growth temperature when using tetrakis­(dimethylamido)­titanium­(IV) (TDMAT) and water as precursors. Nitrogen traces within amorphous titania grown at 100 °C inhibit the crystal nucleation until 375 °C and stabilize the anatase phase. In contrast, a higher growth temperature (200 °C) leads to a low nitrogen concentration, a high degree of oxide defects, and high mass density facilitating direct amorphous to rutile crystal nucleation at an exceptionally low post deposition annealing (PDA) temperature of 250 °C. The mixed-phase (rutile–brookite) TiO2 thin film with rutile as the primary phase forms upon the PDA at 250–500 °C that allows utilization in broad range of TiO2 thin film applications.

Rutile, anatase, brookite and titania thin film from Hubbard corrected and hybrid DFT

“…Furthermore, amorphous titania serves as a generic forerunner in the hydrothermal treatment of different single-crystalline phases [106][107][108][109]. Amorphous titania thin film can be easily prepared using various experimental techniques [109][110][111][112][113][114][115][116].…”

A theoretical study on the surface reaction of tetrakis(dimethylamino)titanium on titanium oxide

“…The ALD of TiO 2 films using TDMAT is generally amorphous due to low growth temperatures. 24,52 The TiO 2 -B phase has a similar density (3.64 g cm À3 ) to the amorphous film (3.57 g cm À3 ) as well as shows high structural similarity to the amorphous structure. 53 Therefore, we selected TiO 2 -B (mp-554278_TiO 2 ) 54 as the substrate material.…”

“…The E A4 was low at 0.48 eV, indicating that the reaction of the chemisorbed precursor with H 2 O would be energetically and kinetically favorable even at low temperatures, which agrees well with the experiments. [20][21][22][23][24] Fig. 8 shows the atomistic structures of the reactions.…”

“…16 In particular, a low-temperature ALD process is required for applications involving substrates that are sensitive to high temperatures, such as flexible displays, 17 or biomaterial functionalization. 18,19 Tetrakis(dimethylamino)titanium (TDMAT, Ti(NMe 2 ) 4 ) [20][21][22][23][24] is one of the most popular titanium precursors at low processing temperatures, ranging from room temperature to 225 1C. 25 The ALD reaction using TDMAT and water vapor (H 2 O) was studied to understand the reaction mechanism.…”

Soy Protein/Polyvinyl-Alcohol (PVA)-Based Packaging Films Reinforced by Nano-TiO2

“…Compared to anatase, rutile TiO 2 has a tetragonal crystal structure, and it has a smaller bandgap, a higher dielectric constant, and a higher refractive index, but rutile films are made at temperatures that are too high. Therefore, it is generally applied to the temperature-sensitive packaging materials [ 15 ]. Brookite TiO 2 has an orthorhombic crystal structure.…”