高耐压和低暗计数SiC紫外雪崩光电二极管

SiC UV Avalanche Photodiode with High Voltage Withstanding Capability and Low Dark Count Rate

碳化硅(SiC)雪崩光电二极管(APD)是一种独具优势的微弱紫外光探测器,其过偏压承受能力是确保器件可靠工作的一个重要因素。本工作设计并制备了穿通型SiC吸收层电荷控制层雪崩倍增层分离(SACM)APD。基于这种结构,器件电场从雪崩倍增层向吸收层扩展,从而减小了雪崩倍增层内电场强度变化率,最终将器件过偏压承受能力提高到10 V;得益于吸收层的分压,雪崩倍增层的电场强度得到有效降低,载流子隧穿可能性减小,这能够有效降低器件暗计数,从而有利于提高器件探测灵敏度;此外,设计的SiC SACM APD倾斜台面仅刻蚀到雪崩倍增层上表面,这能够让器件填充因子提高至约60%,显著改善了深刻蚀导致的传统SACM结构有效光敏区域减小的问题。

Objective As a weak ultraviolet(UV)detector with unique advantages,SiC avalanche photodiodes(APDs)are imperative in many key fields,such as environmental monitoring,corona detection,missile plume detection,deep space detection,and ultraviolet communication.A SiC APD is highly susceptible to irreversible thermal breakdown as its current is extremely sensitive to the bias voltage when it works under the condition of a critical electric field.Therefore,the overbias voltage withstanding capability of a SiC APD is a key issue affecting the working stability of the APD.In addition,the dark count rate is an important parameter that determines the detection sensitivity of the APD in weak UV detection.However,the reported SiC APDs exhibit low overbias voltage withstanding capabilities and high dark count rates.SiC APDs with high overbias voltage withstanding capabilities and low dark count rate have been designed and fabricated in this study.Methods In this study,SiC separated-absorption-charge-multiplication(SACM)APDs have been designed and fabricated.The SiC APDs are fabricated on n+type 4HSiC substrates(Fig.1).The epitaxial structure of the SiC APDs consists of a 10-µm p type contact layer,a 0.65-µm n-type multiplication layer,a 0.15-µm n type charge control layer,a 0.6-µm n-type absorption layer,and a 0.2-µm n type contact layer from bottom to top.The fabrication process starts with mesa etching down to the multiplication layer(to an etching depth of 1.05µm)by inductively coupled plasma etching.The photoresist reflow technique is employed to obtain a positive beveled mesa(with a small slope angle of about 5°)and thereby prevent mesa edge breakdown.Then,the epitaxial wafer is etched to the bottom contact layer.Subsequently,the APD surface is passivated by a thermal oxidation layer and then by a SiO_(2) layer deposited by plasmaenhanced chemical vapor deposition.Both the ntype and ptype Ohmic contact electrodes adopt Ni/Ti/Al/Au(35 nm/50 nm/100 nm/100 nm)layers deposited by ebeam evaporation.Finally,the epitaxial structure is annealed by rapid thermal annealing at 850℃for 3 min in N2 atmosphere.Results and Discussions During the operation of APDs,the uniform distribution of the electric field in the depletion region is a key factor affecting the stability and reliability of the APDs.Therefore,the regulation of electric field distribution by the structure of APDs needs to be considered during APD fabrication.The electric field distribution of the APD at avalanche state is simulated by Silvaco to verify the suppression effect of the SiC SACM APD with a beveled partial mesa structure on the edge electric field(Fig.2).The results show that partial mesa etching can effectively suppress the peak edge electric field of SiC SACM APDs,and the SACM APD fabricated in this study is a reachthrough SACM APD as a high electric field punches through all the active layers.The SACM APD with a partial mesa structure achieves a fill factor of about 58%,which is 1.6 times that of the conventional SACM APD.The currentvoltage curves of the reachthrough SiC SACM APD and the SiC positiveintrinsicnegative(PIN)APD(Fig.3)show that the avalanche current of the reachthrough SiC SACM APD increases slower than that of the SiC PIN APD.The reverse bias voltage applied to the device does not completely act on the multiplication layer.A small change rate of the electric field intensity at the multiplication layer results in a slow increase in the avalanche current with the voltage,which is beneficial to improving the voltage withstanding performance of the device.In addition,the dark count rate of the reachthrough SiC SACM APD is only 0.5 Hz/µm^(2) when the overbias voltage is 4 V,and the singlephoton detection efficiency of the device reaches 8.4%when the dark count rate is 1 Hz/µm^(2)(Fig.5).The low carrier tunneling probability and high photon avalanche probability of the device lead to the low dark count rate and high singlephoton detection efficiency of the punchthrough SiC SACM APD,and they all contribute to the extension of the electric field to the absorption layer.Conclusions In this study,a reachthrough SiC SACM APD is designed and fabricated.When the device undergoes avalanche breakdown,the electric field extends from the multiplication layer to the absorption layer and the charge control layer.The change rate of the electric field at the multiplication layer decreases,and the avalanche current exhibits a smaller slope accordingly,which is conducive to improving the overbias voltage withstanding capability of APDs.Moreover,APDs with a smallslope avalanche current can alleviate the breakdown voltage fluctuation among the pixels in the UV imaging array,which is of great significance for highquality weak UV imaging.In addition,partial mesa etching adopted for the SiC SACM APD designed in this study not only ensures the reliable operation of the device but also increases the fill factor of the device to about 60%,which is beneficial for improving the integration level of imaging array chips.