# Research on Fast Detection Technology of Dark Currents in a Ge-Si Detector Array
## 1 Introduction  
The application of LiDAR is extensive, and in the future, it will have a large market worth hundreds of billions of yuan [1; 2; 3]. LiDAR is a radar system that uses lasers to accurately measure the position, velocity, and other characteristics of targets. It is an optical remote-sensing technology that can transmit distance information from the real world to computers, giving machines human depth vision and providing the function of \"eyes\" for humans to explore a wider three-dimensional world. LiDAR can be used for intelligent transportation, the real-time monitoring of vehicle characteristics (size, speed, and volume), and flow; intelligent vehicle (intelligent connected vehicles) ranging and speed measurements and obstacle avoidance navigation; intelligent robots to achieve precise positioning, gesture recognition, etc. ; unmanned aerial vehicles for land surveying and environmental monitoring; intelligent healthcare, by achieving virtual imaging and 3D printing; and digital cities, urban 3D modeling, digital maps, etc. [4; 5; 6; 7; 8; 9]. In the military, it can be used for battlefield reconnaissance, fighter telemetry, fire control tracking, underwater detection, laser communication, electronic countermeasures, and so on [10; 11; 12; 13]. Shortwave infrared is safer for the human eye and is the best choice for LiDAR light sources. Shortwave infrared generally refers to wavelengths of 0.9-1.7 \(\upmu\)m [14]. Electromagnetic waves between 0.9 \(\upmu\)m and 1.7 \(\upmu\)m cannot be seen by the human eye, but they are very similar to visible light and can be reflected on the surface of an object. The image has shadows and contrast, making the object easy to recognize. Shortwave infrared technology has a series of outstanding advantages such as high recognition, all-weather adaptation, low-light night vision, covert active imaging, and simple optical configuration, giving it special applicationprospects in fields such as aerospace remote sensing, archaeological identification, military reconnaissance, public safety, industrial testing, medical diagnoses, etc. [15; 16; 17]. Although MoO\({}_{3}\)-based ultraviolet photodetectors have an extremely high responsiveness, their operating wavelengths are usually 300-400 nm [18]. However, for automotive LiDAR applications, eye-safe shortwave infrared LiDAR will be the mainstream direction in the future. Presently, short-infrared-wavelength InGaAs avalanche photodiode detectors are starting to be applied in high-end LiDAR, but the cost is relatively high [19]. For example, the 32 \(\times\) 1-linear-array InGaAs detector array produced by Hamamatsu Photonics in Japan has a dark current of less than 2.5 nA and a response of 0.95 A/W, which belongs to the more advanced InGaAs detector array at present. However, the unit price of the three-line array is as high as USD 1000 [20]. Ge-Si photodetectors compatible with CMOS technology with lower costs have been extensively studied in the past decade and remarkable progress has been made [21; 22; 23; 24; 25; 26; 27; 28; 29; 30]. Especially in the past couple of years, at low temperatures, research has shown that the performance of Ge avalanche photodiode single-photon detectors is comparable to that of InGaAs [21]. Moreover, from an integration perspective, making Ge photodetectors on single-crystal Si is compatible with Si-based CMOS technology and can be integrated with optical circuits on a single chip. With a low cost and high integration, it will become the mainstream for LiDAR detectors. Therefore, the research on Ge-Si photodetectors has become increasingly important. Early Ge-Si photodetectors were mainly used in the field of optical communication, so more attention was paid to the gain-bandwidth product and the weak light detection limit. The gain-bandwidth product reached above 300 GHz [22; 23]. The Si waveguide made by the Institute of Electronics at the Universite Paris-Sud on a 300 nm Ge epitaxial layer was integrated with a Ge-Si photodetector on a single chip.