Interfacial photoconductivity effect of type-I and type-II Sb₂Se₃/Si heterojunctions for THz wave modulation

An in-depth understanding of the photoconductivity and photocarrier density at the interface is of great significance for improving the performance of optoelectronic devices. However, extraction of the photoconductivity and photocarrier density at the heterojunction interface remains elusive. Herein, we have obtained the photoconductivity and photocarrier density of 173 nm Sb₂Se₃/Si (type-I heterojunction) and 90 nm Sb₂Se₃/Si (type-II heterojunction) utilizing terahertz (THz) time-domain spectroscopy (THz-TDS) and a theoretical Drude model. Since type-I heterojunctions accelerate carrier recombination and type-II heterojunctions accelerate carrier separation, the photoconductivity and photocarrier density of the type-II heterojunction (21.8 × 10⁴ S⋅m⁻¹, 1.5 × 10¹⁵ cm⁻³) are higher than those of the type-I heterojunction (11.8 × 10⁴ S⋅m⁻¹, 0.8 × 10¹⁵ cm⁻³). These results demonstrate that a type-II heterojunction is superior to a type-I heterojunction for THz wave modulation. This work highlights THz-TDS as an effective tool for studying photoconductivity and photocarrier density at the heterojunction interface. In turn, the intriguing interfacial photoconductivity effect provides a way to improve the THz wave modulation performance.