Characterization of Electronic Transport through Amorphous TiO₂ Produced by Atomic Layer Deposition

Abstract

The electrical transport in amorphous titanium dioxide (a-TiO 2 ) thin films deposited by atomic-layer deposition (ALD), and across heterojunctions of p + -Si|a-TiO 2 |metal substrates that had various top metal contacts, has been characterized by AC conductivity, temperaturedependent DC conductivity, space-charge-limited current (SCLC) spectroscopy, electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS), and current density versus voltage (J-V) characteristics. Amorphous TiO 2 films were fabricated using either tetrakis(dimethylamido)-titanium (TDMAT) with a substrate temperature of 150 °C or TiCl 4 with a substrate temperature of 50, 100, or 150 °C. EPR spectroscopy of the films showed that the Ti 3+ concentration varied with the deposition conditions, and increases in the concentration of Ti 3+ in the films correlated with increases in film conductivity. Valence-band spectra for the a-TiO 2 films exhibited a defect-state peak below the conduction-band minimum (CBM), and increases in the intensity of this peak correlated with increases in the Ti 3+ concentration measured by EPR as well as with increases in film conductivity. The temperature dependent conduction data showed Arrhenius behavior at room temperature with an activation energy that decreased with decreasing temperature, suggesting that conduction did not occur primarily through either the valence or conduction bands. The data from all of the measurements are consistent with a Ti 3+ defect-mediated transport mode involving a hopping mechanism with a

Nanostructured LaFeO3 Photocathodes with Onset Potentials for the Hydrogen Evolution Reaction Over 1.4 V vs. RHE

“…This mechanism is different from the trap-mediated transport observed in TiO 2 protected photoanodes. 23 Further information about the transport of photogenerated electron across the TiO 2 layer can be obtained from assessing the effect of the number of TiO 2 coating steps. Figure S2A contrast cyclic voltammograms of Fe(CN) 6 4− at FTO electrodes modified by one to three TiO 2 coating steps.…”

Operandoelectrochemical study of charge carrier processes in water splitting photoanodes protected by atomic layer deposited TiO2

“…In Hu et al's work, the HF-etched Si wafers were subsequently immersed in H 2 O/HCl/H 2 O 2 solution (RCA-2 Treatments), which made the oxide layer regrow. 34,35 Presumably, this ultrathin SiO 2 layer either passivates surface states or unpins nSi so that larger photovoltage can be achieved. Nevertheless, this difference to previous work does not affect the following analysis.…”

Energetic Tug‐of‐War between Pt and Leaky TiO2: Positive and Negative Effects on the Function of Molecularly‐Modified p‐Si(111)|TiO2|Pt Photocathodes

“…In such a system, TiO 2 conducts holes through a defect band rather than through the energetically inaccessible TiO 2 valence band. [26][27][28][29][30][31][32] Electron and hole conduction through the TiO 2 conduction and defect bands, respectively, allows TiO 2 to be a versatile overlayer, used for both photocathodes [17,20,33] and photoanodes. [19,28,29,34] However, these modes of conduction also represent possible losses due to the permeability of the overlayer to both majority and minority carriers.…”

“…A recent report from Lewis et al extensively detailed hole-transport through "leaky" TiO 2 , in which they establish that Ti precursor choice and ALD growth conditions are the major determinants of defect density and thus the extent of current leakage. [30] Recently, our group showed that use of 'leaky' TiO 2 overlayers on a photocathode in contact with the one-electron outer-sphere redox couple methyl viologen showed substantial decreases in photovoltage. [35] Additionally, Ager and Javey et.…”

“…This defective film‐a ‘leaky dielectric’‐allows hole transport when the protective oxide strategy is applied to photoanodes where the minority carrier (holes) must be transported to the surface. In such a system, TiO 2 conducts holes through a defect band rather than through the energetically inaccessible TiO 2 valence band …”