Spectral Optical Responsivity of GeSn PIN Diodes: Experiment and Theory

Homostructure GeSn pin diodes were grown on Ge-buffered Si substrates by a combination of ultra-high-vacuum chemical vapor deposition (UHV-CVD) and gas-source molecular epitaxy (GSME). Stannane (SnD₄, SnH₄) was used as the Sn source and polygermanes (Ge₃H₈, Ge₄H₁₀) as the Ge sources. In-situ doping was made possible by the use of additional precursors containing group-III and group-V atoms, such as P(GeH₃)₃ or B₂H₆. The spectral responsivity of the diodes was measured as a function of reverse bias and compared with a model that includes a numerical solution of the semiconductor equations, the effects of multiple reflections and the role of interferences in the optical generation rate, and an accurate calculation of the complex index of refraction for all layers in the structure. The theoretical responsivity values are found to be in excellent agreement with the experimental data.

The accuracy and precision of the model made it possible to investigate the main obstacles that prevent the responsivity from attaining its maximum possible values. It is found that residual background doping in the intrinsic layer is by far the main factor limiting responsivity. A systematic study was carried out to identify the structures, thicknesses, and doping levels that maximize responsivity for different Sn concentrations.