2, Yale University, New Haven, Connecticut, United States
Electrical transport in amorphous titanium dioxide films deposited by atomic-layer deposition (ALD) and their heterostructures with p+-Si substrates have been characterized by AC conductivity, temperature-dependent DC conductivity, space-charge-limited current (SCLC) spectroscopy, electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS) and current density-voltage (J-V) characteristics. The comprehensive characterization reported herein, indicates a Ti3+ defect-mediated transport model: a hopping mechanism with a defect density of 1019 cm-3, defect band edge ~0.6 eV below the conduction band and a free carrier concentration of 1016 cm-3, which agrees with previous Hall measurements. Amorphous TiO2 films that were fabricated using TiCl4 as the ALD precursor exhibited less DC conductivity than films formed using tetrakis(dimethylamido)-titanium (TDMAT) as the precursor. The DC conductance increased proportionally to the peak height of the defect states in the valence band for TiCl4-precursor TiO2 films grown at 50°C, indicating that substantial room temperature conductivity is not intrinsic to amorphous TiO2 but is dependent on the introduction of defect states during the ALD fabrication process.