Extended-wavelength germanium <i>pin</i> photodetector fabricated on a bonded silicon-on-quartz wafer

This study reports an extended-wavelength operation of a normal-incidence <i>pin</i> photodetector (PD) with an elemental Ge layer epitaxially grown on a bonded Si-on-quartz (SOQ) wafer, as an approach free from narrower-gap GeSn alloy. The backside-illuminated PD of a 500-nm-thick Ge layer on SOQ exhibits a high responsivity of more than 0.5 A/W in the O band (1.260-1.360 µm), more than 0.3 A/W in the S, C, and L bands (1.460-1.625 µm), and more than 0.1 A/W in the U band (1.625-1.675 µm). The high responsivity is attributed to not only the back reflection of the light by the metal electrode on the front surface but also the red shift in the fundamental absorption edge, resulting from the direct bandgap narrowing induced by the tensile lattice strain in Ge due to the thermal expansion mismatch with the SiO₂ substrate of SOQ. The in-plane tensile strain is as high as 0.37 ± 0.01%, which is significantly higher than 0.17 ± 0.01% on an ordinary Si wafer.