Optimization of Photogenerated Charge Carrier Lifetimes in ALD Grown TiO2 for Photonic Applications

Abstract

Titanium dioxide (TiO2) thin films are widely employed for photocatalytic and photovoltaic applications where the long lifetime of charge carriers is a paramount requirement for the device efficiency. To ensure the long lifetime, a high temperature treatment is used which restricts the applicability of TiO2 in devices incorporating organic or polymer components. In this study, we exploited low temperature (100–150 °C) atomic layer deposition (ALD) of 30 nm TiO2 thin films from tetrakis(dimethylamido)titanium. The deposition was followed by a heat treatment in air to find the minimum temperature requirements for the film fabrication without compromising the carrier lifetime. Femto-to nanosecond transient absorption spectroscopy was used to determine the lifetimes, and grazing incidence X-ray diffraction was employed for structural analysis. The optimal result was obtained for the TiO2 thin films grown at 150 °C and heat-treated at as low as 300 °C. The deposited thin films were amorphous and crystallized into anatase phase upon heat treatment at 300–500 °C. The average carrier lifetime for amorphous TiO2 is few picoseconds but increases to >400 ps upon crystallization at 500 °C. The samples deposited at 100 °C were also crystallized as anatase but the carrier lifetime was <100 ps.

Interfacial Electron Transfer through Ultrathin ALD TiOx Layers: A Comparative Study of TiO2/TiOx and SnO2/TiOx Core/Shell Nanocrystals

“…The smaller E gap observed for all a-TiO 2 /TiO x -0 samples and for a-TiO 2 /50TiO x at the different annealing temperatures is likely not the result of a true band-gap energy reduction but rather due to the presence of mid-gap states, as the broad absorption into the near-IR is associated with these states. Prior literature investigating amorphous or disordered TiO x fabricated from the ALD of TDMA-Ti and water has exhibited enhanced absorption across the visible by the presence of mid-gap states leading to blue/black films. ,, Furthermore, there is an extensive literature on “black TiO 2 ” nanocrystals whose unique electronic structure has been attributed, among other explanations, to a thin disordered TiO x shell on top of a crystalline TiO 2 core, similar to the structures herein. − We postulate that as the TiO x shell thickness increases, so do the number of mid-gap trap states, and these trap states are removed upon increasing annealing temperature or by treatment in an oxidizing atmosphere.…”

Comparison of the heat-treatment effect on carrier dynamics in TiO2 thin films deposited by different methods

“…The TiO 2 film thickness of 30 nm (636 ALD cycles) was used for all the experiments. The precursors for ALD TiO 2 and the 150 • C growth temperature were chosen based on the previous studies 19,20 .…”

Modulating charge carriers in carbon dots toward efficient solar‐to‐energy conversion

“…52 As charge carrier lifetime is typically defined as the statistical average time calculated by the recombination of electrons and holes through multiple pathways at different times, it is difficult to track individual charge carrier lifetimes. Thus, the lifetime (τ) has been inferred from PL, [53][54][55] photoconductance, 56,57 photovoltaic, 52,58 and photocatalytic 43,[59][60][61] properties, in which generation and recombination occur. The radiative (τ r −1…”