Design of Mid-Infrared Ge1–x Snx Homojunction p-i-n Photodiodes on Si Substrate

This work reports an optimal design and modeling of a high-performance normal-incidence GeSn homojunction p-i-n PDs on p-doped silicon (Si) substrate via Intrinsic-Si buffer. Specifically, we optimize the Sn concentration and the absorption layer thickness to simultaneously achieve high-speed, responsivity, detectivity, and low-noise in the mid-infrared (MIR) spectral range (2- $5 ~\mu \text{m}$ ). We also show that a reduced defect density due to the homojunction GeSn layer can lead to a suppressed dark current. The photoresponse of the designed device was studied for the range between 1500–3000 nm, and the calculated responsivity is 3.26 A/W and 1.27 A/W at 2000 nm and 2500 nm, respectively for Sn = 9%. A specific detectivity, linear dynamic range (LDR), noise-equivalent power (NEP), and the signal-to-noise ratio (SNR) of $2.5\times 10^{10}$ Jones, 105.5 dB, $40.9\times 10^{-15}\text{W}$ Hz^−0.5, and 100.9 dB, respectively, were achieved at 2500 nm for Sn = 9%. Furthermore, the impact of intrinsic layer thickness and incident optical power was also studied in detail. The calculated results show that larger absorption layer thickness enhances the photocurrent, detectivity, LDR and SNR, however, the speed of the device degrades significantly. The increased incident optical power results in an increase in SNR and LDR of the device due to an increased photocurrent. The 3dB bandwidth was also calculated for varying Sn concentration and intrinsic layer thickness. The calculated result shows a 3dB bandwidth of > 26.2 GHz at −3V. All the calculated results show that the proposed device has significant potential applications in the MIR range.