Abstract
The effects of surface defects at the sidewall of absorption and multiplication layers on the sidewall leakage current in separate-absorption-charge-multiplication Ge/Si avalanche photodiodes with a mesa structure are investigated. It is found that high-density surface defects and a strong electric field at the sidewall are the reasons for the large sidewall leakage current. In addition, the influence of the width of the guard ring on the sidewall leakage current is also studied. It is found that increasing the width of the guard ring is beneficial to the reduction of the sidewall leakage current by reducing the sidewall electric field and the reduction is not significant when the guard-ring width is greater than 2 µm.
Highlights
Ge/Si avalanche photodiodes (APDs) are widely used in nearinfrared detection due to the following advantages: the absorption band of Ge is in the range of 800–1550 nm,1 Si exhibits electron multiplication2 phenomenon and is easy to integrate with CMOS technology, Ge is inexpensive, etc
Large dark current will lead to low sensitivity in linear mode and a high dark count rate (DCR) in Geiger mode, so reducing the dark current is one of the most effective ways to improve the performance of Ge/Si APDs
The total dark current of APDs is composed of the bulk dark current depending on material quality and the surface leakage current related to surface defect density, which can be expressed as the following equation
Summary
Ge/Si avalanche photodiodes (APDs) are widely used in nearinfrared detection due to the following advantages: the absorption band of Ge is in the range of 800–1550 nm, Si exhibits electron multiplication phenomenon and is easy to integrate with CMOS technology, Ge is inexpensive, etc. The large exposed sidewall surface and the poor etching process of mesa Ge/Si APDs are responsible for large surface leakage current, and the large recombination velocity of the sidewall at Ge is one of the generation mechanisms.. Large dark current will lead to low sensitivity in linear mode and a high dark count rate (DCR) in Geiger mode, so reducing the dark current is one of the most effective ways to improve the performance of Ge/Si APDs. In the actual device manufacturing process, the passivation quality is often not as desired, and the existence of surface defects will bring surface charges and accelerate surface recombination, increasing the surface leakage current. The bandgap of Ge is narrow, so its sidewalls are more prone to surface recombination due to the existence of surface defects. the bandgap of Si is wide, the Si multiplication layer is the place where the electric field is the strongest, so the sidewalls of which tend to cause surface recombination. in this paper, the effects of the sidewall surface charges, the sidewall surface recombination velocity of the Ge absorption layer and Si multiplication layer, and the guard-ring width on the leakage current were studied, and the results were analyzed in detail through electrical characteristics
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