Unveiling frequency-dependent electrical behaviour in Cu2O/TiO2 heterojunctions via capacitance and impedance spectroscopy

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Abstract
This study investigates the AC electrical properties of Cu2O/TiO2 heterojunctions deposited via reactive DC magnetron sputtering, with a focus on the role of interfacial dynamics and post-deposition annealing. Mott–Schottky analysis revealed built-in potentials ranging from 0.45 V to 1.92 V, with carrier concentrations between 1.43 × 1016 and 9.93 × 1016 cm−3, influenced by the annealing temperature. The depletion width was estimated to be roughly 395 nm in the 150 °C annealed samples, and the TiO2 layer was more depleted across all samples. Dielectric constant (ε′) values decreased from approximately 85 to about 10 as frequency increased from 1 kHz to 1 MHz, while tangent loss also diminished significantly, indicating reduced dipolar and space-charge polarization. The imaginary part of the dielectric constant (ε″) confirmed strong interfacial polarization at low frequencies. AC conductivity rose with frequency, supporting a thermally activated hopping conduction mechanism. Impedance spectroscopy, modelled with an R(CR)(QR)(CR) circuit, exhibited multiple relaxation behaviors, and incorporating a constant phase element (CPE) accounted for non-ideal dielectric effects caused by surface inhomogeneity. Rapid thermal annealing resulted in a decrease in series resistance, from 844 Ω at room temperature to 802 Ω at 150 °C. Annealing at 150 °C produced optimal outcomes, reducing interface losses and enhancing charge transport. These findings underscore the tunability and potential of Cu2O/TiO2 heterostructures for optoelectronic applications such as photodetectors and solar cells.

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