Novel Si-Based/Ge-Based Near-Infrared Photodetectors And Its Imaging Applications

Near-infrared photodetectors have been widely used in both military and civilian fields,such as remote sensing communication,fire protection,medical care,security,and guidance.Near-infrared photodetectors can convert invisible infrared light into identifiable electrical signals,which are the most critical components in various near-infrared imaging systems.Crystalline silicon（Si）has the advantages of low cost,high controllability and high stability,and is the main photodetection material for near-infrared I region（780-1000 nm）.Although the detection performance of the commercialized silicon photodetectors is already good,the mainstream p-i-n structure Si photodetectors have complex structures and require high-cost fabrication processes.Herein,high-performance,Si-based near-infrared photodetector with a asymmetric Schottky structure is proposed.Furthermore,given the drawbacks of silicon to detect the near-infrared II region（1000-1700 nm）optical signals,we chose germanium（Ge）as the light absorber layer for the fabrication of germanium-based devices.Studies shows that PEDOT:PSS shows light absorption in the near-infrared II region,which can acts as bifunction layer of hole transport layer and infrared sensing layer.By constructing PEDOT:PSS/Ge heterojunction,the dark current of the device can be effectively suppressed.We have successfully demonstrated imaging applications in near-infrared region I and II regions with these two types of photodetectors.The main research results are as follows:1.A Si-based asymmetric Schottky junction is constructed,revealing the performance modulation of the device by device structure design.First,we propose a facile method to fabricate Si-based Schottky junction photodetectors without any modification.By directly evaporating metal electrodes with different work functions on the two surfaces of a silicon wafer,an asymmetric Schottky junction is constructed,which improves the separation efficiency of photogenerated carriers in the device.To reduces the light reflection of upper metal thin film electrode,we use fishbone electrodes and also increase the carrier collection efficiency and contributing to higher photocurrent of the device.Under 980 nm illumination,the optimized Au/Si/Ag photodetector exhibits ultra-low dark current（8.5 p A）,high specific detection rate(3.35×10¹³ Jones)and wide linear dynamic range（187 d B）,the device switching ratio is as high as 10⁸（@81 m W）.Finally,we also demonstrate the long-term stability of the asymmetric Schottky junction device,the photodetector retains 83%of the original photocurrent after four months in ambient conditions.Finally,by using the photodetector as sensing unit,we successfully demonstrate the imaging effect in the near-infrared I region.2.To realize imaging applications in the near-infrared II region,we construct an organic-inorganic hybrid germanium-based heterojunction photodetector,and systematically study the effect of the introduction of the PEDOT:PSS layer on the photoelectric performance of the Ge-based detector in the near-infrared II region.Based on the conclusion of Experiment 1,we also construct an asymmetric Au/Ge/Ag Schottky junction photodetector,but due to the narrow bandgap of Ge and the influence of the interfacial Fermi pinning effect,the dark current of the photodetector is too high(～10^-6 A),leading to poor performance.In order to obtain a wider linear dynamic range in the near-infrared II region,as well as lowering the dark current.PEDOT:PSS thin film are spin-coated on Ge wafers by a one-step spin coating method,and Ge/PEDOT:PSS heterojunction is designed.After introduction of the PEDOT:PSS layer,the dark current of the photodetector is reduced by 3 orders of magnitude(reaching to10^-9 A),with a linear dynamic range of 124 d B and a specific detection rate of 6.5×10¹¹Jones.The Ge/PEDOT:PSS photodetector exhibits a peak response of 260 m A/W in the near-infrared II region at without external bias.The above detection performance indicates that the Ge/PEDOT:PSS heterojunction photodetector is sufficient for near-infrared II imaging.Finally,the Ge/PEDOT:PSS near-infrared photodetector is integrated into the laser focusing transmission scanning imaging system to realize the imaging function of foreign bodies in biological tissues.The imaging depth reaches 4mm,indicating that we achieve the purpose of foreign body detection.