Self-Powered Wavelength-Selective Ultraviolet Bipolar Photodetector Based on Al/p-GaN/In:Ga ₂ O ₃ /SnO ₂ Structure for Secure Optical Communication

Free-space optical (FSO) communication is widely employed in critical sectors owing to its advantages such as high data rate and high reliability. However, signal attenuation and leakage during transmission pose significant information security challenges. In contrast to the visible and infrared (IR) spectral regions, which are susceptible to background light interference, solar-blind ultraviolet (UV) communication offers an inherent advantage of a high signal-to-noise ratio (SNR). However, novel cryptographic photodetectors operating in this waveband have been insufficiently explored. In this work, a self-powered ultraviolet (UV) photodetector based on an Al/p-GaN/In:Ga₂O₃/SnO₂ structure was designed and fabricated, featuring synergistic regulation by a heterojunction and a Schottky junction. The photodetector exhibits a unique wavelength-selective bipolar photoresponse, generating a negative photocurrent under 254 nm illumination and a positive photocurrent under 365 nm illumination. Under zero bias, the detector achieves responsivities of -5 mA/W and 0.52 mA/W, and specific detectivities of up to -7.2 × 10¹¹ Jones and 7.47 × 10¹⁰ Jones, for 254 and 365 nm illumination, respectively. Leveraging the unique physical properties of the photodetector, a secure optical communication (SOC) system is demonstrated. This system utilizes the positive and negative photocurrents, generated by distinct UV wavelengths, as binary signals ("1" and "0") to encrypt information, while decryption is performed at the receiving end using a preset key file. The experimental results demonstrate that the system successfully achieved the encrypted transmission and accurate decryption of the string "CUST" at zero bias. This work not only presents a new strategy for developing high-performance, multifunctional bipolar UV photodetectors but also offers an innovative and viable technological pathway for fabricating next-generation, high-security, and low-power optical communication systems.