  We demonstrate monolithically grown germanium-tin (GeSn) on silicon avalanche
photodiodes (APDs) for infrared light detection. A relatively thinner Ge buffer
design was adopted to allow effective photo carriers to transport from the GeSn
absorber to the Si multiplication layer such that clear punch-through behavior
and a saturated primary responsivity of 0.3 A/W at 1550 nm were observed before
avalanche breakdown in GeSn/Si APDs for the first time. The spectral response
covers 1500 to 1700 nm. The measured punch-through and breakdown voltages are
15 and 17 V, respectively. Undisputed multiplication gain was obtained with the
maximum value of 4.5 at 77 K, and 1.4 at 250 K, directly in reference to the
saturated primary responsivity from the same device rather than a different
GeSn p-i-n photodiode in previous reports. A peak responsivity was measured as
1.12 A/W at 1550 nm and 77 K.
