Fast and Efficient Sub-Band Gap Photodetection in Al:ZnO/Si Heterojunction by Enhanced Photoexcited Hole Transport via Interfacial Defect States

This study focuses on sub-band gap photodetection in n+-Al:ZnO/n-Si isotype heterojunction-based photodiodes via interfacial defects induced by metal oxide films. Through comparative studies on the photoresponse of Schottky junction-based photodiodes with the modified electronic band structure by controlling the structural and electrical properties of Al:ZnO films, as well as the Si substrate's doping level, we investigate the underlying mechanisms of interfacial defect states for sub-band gap photodetection in Si. Our analysis suggests that these interfacial defects not only act as additional sources for photoexcited carrier generations but also serve as pathways for photogenerated holes in the Si valence band, enabling their flow into the Al:ZnO film and improving the operating speed. Time-resolved photocurrent measurements under near-infrared illumination illustrate an enhancement in photocurrent with lower oxygen partial pressures (0 mTorr) attributed to alterations in the energy band structure caused by interfacial defect states. Significantly, the Al:ZnO/Si photodiode fabricated under optimized conditions exhibits a photoresponse of 2.48 mA/W at 1310 nm with fast rise/fall times of 5.5/5.25 μs at 1 kHz and a 3 dB bandwidth of approximately 150 kHz, without introducing additional bulk trap states in Si. In light of these findings, the combination of simple fabrication and excellent switching speed of interfacial defect-mediated Si photodiodes has the potential to significantly impact the technologies of Si photonics and advanced Si-based photoelectric devices.