Measurement method for carrier concentration in TiO2 via the Mott–Schottky approach

In direct contrast to the way in which silicon is precisely doped for integrated circuit applications in order to optimize device performance, there is little nuanced understanding of the correlation between TiO2 doping level, charge carrier concentration, and the operation of TiO2-based photocatalysts, dye-sensitized solar cells, and sensors. The present work outlines a rigorous methodology for the determination of free carrier concentration for doped metal oxide semiconductors such as TiO2 that are not amenable to standard metrology methods. Undoped, Cr-, Mn-, and Nb-doped polycrystalline anatase TiO2 are synthesized via atomic layer deposition (ALD) using Ti(OCH(CH3)2)4, H2O, Cr(C5H7O2)3, Mn(DPM)3 (DPM = 2,2,6,6-tetramethyl-3, 5-heptanedionato), and Nb(OCH2CH3)5 as the source materials for Ti, O, Cr, Mn, and Nb, respectively. Chemical composition and crystallinity are investigated and a thorough “device-like” characterization of TiO2 Schottky diodes is carried out to justify the subsequent extraction of carrier concentration values from capacitance–voltage (C–V) measurements using the Mott–Schottky approach. The influence of factors such as substrate type, contact metal type, and surface and interface preparation are examined. Measurements of donor carrier concentration are obtained for undoped, Cr-, Mn-, and Nb-doped TiO2 synthesized by ALD. Possible causes for the obtained carrier concentrations are discussed.