DC linearity and absorption efficiency enhancement by introducing photon-trapping microstructures into avalanche photodetectors

Abstract

The direct-current (DC) linearity of a photodiode is a parameter that indicates the direct proportionality between the input optical power and the output response current. This linear relationship defines the optical power range using which the performance limits of the photodiode control circuitry such as A/D conversion, trans-impedance amplifier, and quenching circuits are designed. Due to the absorption saturation and self-heating, the response current of the photodiode starts to saturate at higher optical power. A narrow absorber region in state-ofthe-art photodiodes (avalanche and PIN photodiodes) results in an early absorption saturation in addition to a reduction in absorption efficiency. We present a photon-trapping microstructure (PTMS) equipped-avalanche photodiodes (APD) to enhance the absorption efficiency and DC linearity. We have fabricated a mesa-based APD using complementary metal oxide semiconductor (CMOS)-compatible processes. We present a DC currentvoltage comparison of APDs in the dark and under the illumination of a wide wavelength range varying from 640 nm to 1100 nm. The fabricated PTMS-equipped APDs exhibit 5× increase in the external quantum efficiency as opposed to that of a flat device and a 70 unit multiplication gain. Further, the PTMS-equipped APDs demonstrate an increased linearity of 106.04 dB in comparison to 104.83 dB linearity in the flat device. The introduction of PTMS, despite the reduction of net-absorber volume, enables a uniform spread of the input illumination power by bending the light laterally and results in increased absorption efficiency and DC linearity.