Simulation of tunneling effect on GeSn SWIR photodetector’s dark leakage current with fully relaxed GeSn buffers

• Chenyi Wang, Electrical Engineering, Zhejiang University

• Yuping Zeng, Electrical and Computer Engineering, University of Delaware

GeSn photodiodes exhibit significant potential for applications in the 1-3 μm infrared detection range. However, the device face challenges in mitigating residual biaxial stress due to the high lattice mismatch from the substrate. We have developed a novel epitaxial buffer to quickly release the lattice stress, and experimental results demonstrate improved stress relaxation in the upper GeSn p-i-n active layers. Despite achieving substantial stress relaxation, the GeSn photodetector grown on novel GeSn buffers still experience considerable reverse dark leakage current. To find the possible causes of the serious reverse leakage current, Technology Computer-Aided Design (TCAD) simulation is a helpful tool to use. In this work, we performed a 2-D simulation of the p-i-n GeSn photodiode’s IV characteristic incorporating both the trap-assisted tunneling and band-to-band tunneling models, which were thought to be the two possible mechanisms lead to the serious dark leakage current.

To simplify the simulation, we built an ideal p-i-n structure using Ge_0.91Sn_0.09. The doping concentration is 1×10¹⁹ cm^-3 for n-GeSn layer, 1×10¹⁸ cm^-3 for i-GeSn layer, and 1×10¹⁹ cm^-3 for p-GeSn layer. The band gap of the Ge_0.91Sn_0.09 is set to be 0.44 eV according to our calculation. Two critical tunneling models were used. One is the trap-assisted tunneling (as a part of SRH recombination), the other is the band-to-band tunneling. Deep level acceptor-type traps were defined in the i-GeSn layer. To obtain a good curve fitting, we fine-tuned the values of the following parameters: traps energy level (E_t=E_i), traps concentration (N_t​=5×10¹⁴ cm^-3), carriers capture cross-section (σ_n=5×10^-11 cm²_, σ_h=1.5×10^-10 cm²), and many other coefficients in the above tunneling models. The final simulation results generally fitted our experimental data.

This simulation study provides certain reference value for further improvement in reducing device’s dark leakage current more efficiently.