Improvement on the efficiency of up-conversion infrared photodetectors using surface microstructure

Abstract

In recent decades, infrared (IR) detection technology has been widely used in many fields such as weather monitoring, environmental protection, medical diagnostics, security protection, etc. With the progress and mature of the technologies, more attention has been paid to the imaging detections of weak IR signals. So the higher efficiency of the device is required. Moreover the next-generation IR photodetection technology focuses on large-scale, high-speed and low-dark-current imaging. The mechanical bonding between infrared detector chip and silicon readout circuit inevitably causes a thermal mismatch problem. Up-conversion IR photodetectors can solve the problem about the performance deterioration of photodetector and the thermal mismatch with silicon-based readout circuit, hence they have great advantages in realizing large-format focal plane array detection.However, the poor light extraction efficiency due to total reflection severely restricts the overall efficiency of the up-conversion device, which has become one of the bottlenecks to improve the device efficiency. In this paper, surface microstructures with micro-pillar morphology are designed and fabricated on quantum-cascade up-conversion IR photodetectors. The effect on the up-conversion efficiency is investigated by enhancing the light extraction efficiency.Firstly, by the optical ray retracing method, the influence of surface microstructure on light extraction efficiency is studied when considering different morphology parameters, and optimized surface microstructure is designed to possess a pillar base length of 150 nm, height of 105 nm and side wall angle of 75.Then based on the results of simulation, up-conversion IR photodetectors with surface microstructures are fabricated using polystyrene nanospheres as mask. The self-assembled monolayer nanospheres are first etched to a proper size by using O2 plasma, then the patterns are transferred to SiNx film, which acts as an ICP dry etching mask of the micro-pillars. Finally, the up-conversion device and a silicon detector are together loaded on a cold finger of a cryogenic dewar. The characteristics of the up-converter and up-conversion system are evaluated using a blackbody source.The experimental results show that the devices with and without surface microstructure exhibit similar IR responses and dark currents, while the emission of device with microstructure is obviously increased. Taking into consideration other factors related to external quantum efficiency, the light extraction efficiency of the device with micro-pillar structure on surface can be increased by up to 130%. Therefore it can be concluded that this method is an efficient way to improve the efficiency of up-conversion IR photodetector. The finding in this paper can also be applied to other semiconductor device with light extraction efficiency.