Broadband optical enhancement of suspended Ge-on-Si infrared absorbers by using backside etching holes and a metallic reflector

This work introduces a method for developing a suspended Ge-on-Si structure aimed at improving infrared absorption for photodetector applications. Traditional Ge photodetectors face low responsivities around the near-infrared wavelength of 1550 nm, which is a critical wavelength in optical communications, due to the low absorption coefficient of Ge. This limitation necessitates the use of a thick Ge absorption layer, which can adversely affect the speed of the photodetector. The proposed structure incorporates a backside etching hole array and an Al reflector, forming a resonant-cavity-like structure. This design allows the incident light to follow multiple absorption paths, thereby increasing overall absorbance. We combine the spin-on-glass SiO2 hard mask technique with the tetramethylammonium hydroxide (TMAH) etching process to fabricate these backside holes. The low processing temperature and compatibility of the TMAH solution with CMOS technology are notable advantages. Optical calculations using the transfer-matrix method indicate a significant increase in absorbance for this structure, corroborated by experimental results. The backside hole etching improves the absorbance from 31.42% to 43.28% at 1550 nm. When an Al reflector is added, the absorbance increases further to 59.1%. The Au metal-semiconductor-metal device, integrated with a backside etching hole and an Al reflector, exhibits a high responsivity of 0.84 A/W—twice that of the Ge-on-Si device. This design not only demonstrates broadband absorption efficiency but also holds promise for various optoelectronic device applications.