Tunable Ti³⁺-Mediated Charge Carrier Dynamics of Atomic Layer Deposition-Grown Amorphous TiO₂

Abstract

Amorphous titania (am.-TiO2) has gained wide interest in the field of photocatalysis, thanks to exceptional disorder-mediated optical and electrical properties compared to crystalline TiO2. Here, we study the effects of intrinsic Ti3+ and nitrogen defects in am.-TiO2 thin films via the atomic layer deposition (ALD) chemistry of tetrakis­(dimethylamido)­titanium­(IV) (TDMAT) and H2O precursors at growth temperatures of 100–200 °C. X-ray photoelectron spectroscopy (XPS) and computational analysis allow us to identify structural disorder-induced penta- and heptacoordinated Ti4+ ions (Ti5/7c 4+), which are related to the formation of Ti3+ defects in am.-TiO2. The Ti3+-rich ALD-grown am.-TiO2 has stoichiometric composition, which is explained by the formation of interstitial peroxo species with oxygen vacancies. The occupation of Ti3+ 3d in-gap states increases with the ALD growth temperature, inducing both visible-light absorption and electrical conductivity via the polaron hopping mechanism. At 200 °C, the in-gap states become fully occupied extending the lifetime of photoexcited charge carriers from the picosecond to the nanosecond time domain. Nitrogen traces from the TDMAT precursor had no effect on optical properties and only little on charge transfer properties. These results provide insights into the charge transfer properties of ALD-grown am.-TiO2 that are essential to the performance of protective photoelectrode coatings in photoelectrochemical solar fuel reactors.

Tuning Intermediate Bands of Protective Coatings to Reach the Bulk‐Recombination Limit of Stable Water‐Oxidation GaP Photoanodes

“…These “impurities” are intentionally introduced during ALD growth cycles, thus resembling the Ti 3+ states of the “leaky” TiO 2 . [ 17 ] Tuning of the IB energetics has been explored by varying the type of the transition metal element, the Mn‐to‐Ti atomic ratio, and the alloying microstructures that vary from single Mn‐ion dispersions to nanometer‐sized MnO x clusters, but has not been applied to energy conversion.…”

Atomic layer deposition of titanium oxide thin films using a titanium precursor with a linked amido-cyclopentadienyl ligand

“…ALD titanium oxide films prepared using CMENT at 300 °C showed wide bandgaps of 3.0 eV and 3.1 eV for O 3 and H 2 O. The bandgap energy of 3.0 eV is very close to that of bulk rutile (3.03 eV) 77 but lower than those of the ALD film prepared using TDMAT and H 2 O at 100–200 °C (3.53–3.61 eV) 78 or at 200 and 250 °C (3.29 and 3.30 eV). 79 However, the bandgap was reduced to 2.9 eV when the deposition temperature was increased to 350 °C for both O 3 and H 2 O.…”

“…ALD titanium oxide films prepared using CMENT at 300 1C showed wide bandgaps of 3.0 eV and 3.1 eV for O 3 and H 2 O. The bandgap energy of 3.0 eV is very close to that of bulk rutile (3.03 eV) 77 but lower than those of the ALD film prepared using TDMAT and H 2 O at 100-200 1C (3.53-3.61 eV) 78 or at Fig. 8 Cross-sectional TEM photographs of the ALD titanium oxide film deposited using CMENT and O 3 at 300 1C on a trench-patterned wafer.…”

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

“…12,24,30,31 Previously, we have shown that intrinsic precursor traces and oxide defects are highly sensitive to ALD growth temperature when using tetrakis-(dimethylamido)titanium(IV) and water as precursors. 10 Interestingly, the growth temperature is also shown to steer the crystallization process toward anatase or rutile TiO 2 phases, but understanding of this phenomenon in more detail has remained without comprehensive investigation. 12,18 This work shows the role of ALD growth temperaturecontrolled (100−200 °C) intrinsic precursor traces and oxide defects on TiO 2 thin film crystallization upon post deposition annealing (PDA, 50 min at 200−500 °C and 500 min at 250 °C).…”

“…The core level XP spectra were analyzed by the least-squares fitting of Gaussian−Lorentzian lineshapes and using a Shirleytype background. Ti 2p spectra were fitted as in our previous work 10 by using the Ti 2p 3/2 reference peak shape measured for crystalline TiO 2 , i.e., the six-coordinated Ti 4+ peak (Ti 6c 4+ ), and the amorphous disordered structure was represented by under-and over-coordinated Ti 4+ (Ti 5/7c 4+ ) and Ti 3+ peaks. The binding energy scale of the spectra was calibrated by fixing the O 2− peak of TiO 2 to 530.20 eV.…”