Two Types of Negative Capacitance of BaTiO₃/SnO₂:F Heterojunction: Origins and Ultraviolet Modulation

The negative capacitance effect in ferroelectric materials holds significant potential for reducing the energy consumption of transistors, and the experimental demonstration and mechanism analysis of these phenomena have attracted substantial attention. In this study, by depositing ferroelectric BaTiO₃ thin films with thicknesses ranging from 212 to 390 nm onto semiconducting SnO₂:F substrates, we directly measured two distinct types of negative capacitance. One type, observed under large bias (NCl), originates primarily from injection and delayed migration of carriers across the ferroelectric/semiconductor interface, while the other type, captured under small bias for the 390 nm sample with a larger initial polarization, corresponds to the intermediate state of ferroelectric polarization switching where intrinsic negative capacitance emerges (NCs). The direct measurement of the latter was enabled by a “dual-delay” mechanism established by the stepwise DC bias and oxygen vacancy migration, which effectively delayed both the ferroelectric polarization switching and the charge compensation, thereby provided a critical window for capturing the transient process of polarization switching. For the 390 nm BaTiO₃ film, the total capacitance measured at biases of 16 V and -16 V were approximately -50.69 nF and -7.92 nF (100 Hz), respectively. Furthermore, it was found that 275 nm ultraviolet illumination could modulate the negative capacitance by altering carrier transport and charge-screening behavior, resulting in the magnitude changes of the negative capacitance under large bias and the disappearance of negative capacitance under small bias. These findings contribute to a deeper understanding of the physical mechanisms underlying negative capacitance and provide valuable insights for the development of novel low-power integrated electronic devices based on ferroelectric materials.