Quantum Efficiency Scaling by Cascaded Ge Multi-Stage Tunnel Junctions

We present the growth and characterization of Ge-Zener-Emitters (Ge-ZE) with up to four cascaded multi-stage tunnel junctions (X<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TJ</sub> = 4), and demonstrate significant improvements in both external quantum efficiency (EQE) and wall-plug efficiency. Room-temperature measurements confirm a systematic increase in forward turn-on voltage per junction, while low-temperature operation reveals deviations in voltage scaling for X<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TJ</sub> = 4, impacting device efficiency. Through optoelectronic characterization, we confirm successful EQE scaling but also identify limitations arising from heat dissipation and device degradation at high current densities. Notably, we observe an emission rate exceeding theoretical expectations, indicating the potential presence of amplified spontaneous emission. For quantum photonic integrated circuit applications, which require a minimum optical power P<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">opt</sub> ≥ 200 nW, we demonstrate that cascaded multi-stage tunnel junctions enable sufficient power generation, achieving P<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">opt</sub> = 284 nW at a wavelength λ = 1424 nm under pulsed excitation.