Electroluminescence and Franz–Keldysh Modulation Observed in Sn/Ge Multi‐Quantum Wells

ABSTRACT Thanks to their tunable electronic and optical properties, GeSn alloys have emerged as promising materials for next‐generation silicon‐compatible optoelectronic devices operating at extended near‐ to mid‐infrared wavelengths. In this work, we experimentally and theoretically investigate the optoelectronic properties of p‐type/intrinsic/n‐type (PIN) diodes based on ultrathin α‐Sn/Ge multiple quantum well (MQW) structures grown by molecular beam epitaxy with different Ge barrier thicknesses. Thorough structural analysis confirms the high crystalline quality of the MQW and clearly defined interfaces with Sn contents in the wells exceeding 4 at.%. Room‐temperature electroluminescence measurements reveal two direct transitions: an MQW stack independent higher‐energy emission, related to direct radiative recombination in intrinsic Ge, and a Ge barrier‐dependent lower‐energy peak, attributed to transitions involving weakly quantum‐confined states, whose energy shifts with the barrier thickness in agreement with theoretical prediction. In line with this observation, the devices exhibit clear signatures of the Franz–Keldysh effect when operated as photodetectors, in a wavelength range extending well beyond the direct gap of Ge, as captured by theoretical modeling. These results demonstrate the potential of α‐Sn/Ge MQWs as an integrable material platform for complementary metal—oxide–semiconductor (CMOS)‐compatible electro‐optical modulators spanning the near‐ to mid‐infrared range.