| Ultraviolet(UV)detectors with single photon counting capability are key components in many applications,such as missile warning and tracking,environmental monitoring,biologicalagent detection and UV communications.4H-SiC can be used to fabricate high-performance ultraviolet avalanche photodetectors(UV APDs)due to its wide bandgap,good thermal conductivity,excellent radiation hardness,and high electron saturation drift velocity.Due to its relatively mature material growth and processing technologies,SiC APDs and arrays operating in Geiger mode have been reported with low dark current and decent single photon counting efficiency(SPDE).Currently,there are many problems to be solved for the development of SiC APDs,such as high dark count rate(DCR)and poor breakdown voltage uniformity.Some key performance parameters,such as after-pulse probability,have not yet been studied in detail.At the same time,many promising future applications,such as SiC APD based UV lidar,are still in the early stage of development.Therefore,in this thesis in order to improve the performance of SiC APDs and study the impact of their key performance parameters on practical applications,based on the 4H-SiC APDs developed by our own research group,the gated quenching circuit is constructed and optimized,the after-pulse probability of SiC APDs are studied,and a prototype UV lidar system based on SiC APDs is developed.The main contents of thesis are as follows:1.The operation principle of the gated quenching circuit is reviewed and compared with those of passive quenching circuit and active quenching circuit.Since the peak spectral response of 4H-SiC APDs is?280nm,a 280nm UV LED is used as the light source of the gated quenching circuit system.The precise synchronization method of UV light pulse and the gate pulse signals as well as the necessary firmware for the system construction are analyzed in detail.The advantage of the gated quenching circuit system for single-photon detection characterization of SiC APDs is demonstrated based on the APD devices prepared by our own research group.A high single photon counting efficiency as high as 32.5%is achieved with relatively low dark count rate.2.The after-pulse characteristics of SiC APDs are studied by using a double-gate method.The gated quenching circuit system is modified and optimized for the double-gate method characterization.The relationship between after-pulse probability and gate delay time of the SiC APD under different overbias are studied at room temperature.It is found that the afterpulse probability could initially exhibit a high value as large as 100%,but quickly drops as the delay time increases.If a threshold value of 1%is set for judging the existence of afterpulsing,the corresponding maximum signal repetition rates are 4.33,2.27 and 1.47 MHz for overbias of 2.5V,3V and 3.5V,respectively.With increasing temperatures,it is found that the after-pulse probability decreases at fixed overbias,which supports the carrier trapping/releasing mechanism of after-pulsing in SiC APDs.3.A prototype UV lidar system is built based on a 266nm pulse laser and a SiC APD.The working principle of the UV lidar and a comparative analysis on ranging accuracy and limit are discussed.The selection rules and reasons of APD devices,quenching circuit and optical accessories in the system are introduced.Based on the prototype setup,a maximum measurement distance of 150m is obtained for the system.It is believed that currently the main performance limitation factors of the system include the high dark count rate of the SiC APD and the relatively simple optical structure design.Several possible improvement methods of the UV lidar system are suggested. |
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