Ge0.95Sn0.05 on Si avalanche photodiode with spectral response cutoff at 2.14 <i>μ</i>m

GeSn-based avalanche photodiode operating in shortwave infrared wavelength was demonstrated in this work. A separate absorption and charge multiplication structure was employed to take advantage of long wavelength absorption in GeSn and low impact ionization ratio of Si. Due to lattice mismatch between Si and GeSn that would degrade GeSn material quality if with direct growth, a 240-nm-thick Ge buffer was utilized, which simultaneously allows for the transporting photo generated electrons from the GeSn absorber to Si multiplication layer. Spectral response showed the cutoff wavelength beyond 2.1 μm at room temperature. Dark current and capacitance–voltage measurements indicated a punch-through voltage of −10 V. The measured responsivities were 0.55 and 0.34 A/W under 1.55 and 1.9 μm excitation lasers, respectively. The maximum gain was obtained as 3.44 at 77 K under 1.9 μm laser. Even at 250 K, the calculated gain was greater than unity. Simulation of the electric field distribution revealed that the GeSn is partially depleted at operating voltages, which can be improved by reducing the background doping levels in the GeSn absorber and Ge buffer layers.