Ge/Si avalanche photodiodes (APDs) are widely used in near-infrared detection; however, obtaining high-performance Ge/Si APD is challenging due to the 4.2% lattice mismatch between Ge and Si. Therefore, this study proposes introducing a polycrystalline silicon (poly-Si) bonding intermediate layer at the Ge/Si bonding interface to mitigate the effects of the Ge/Si lattice mismatch on APD device performance. With the introduction of poly-Si, the electric field at the bonding interface changes, causing a redistribution of the electric field inside the APD, which significantly impacts device performance. Consequently, this study focuses on regulating the doping concentrations of the Ge absorption layer and Si multiplication layer. It explores the effects of doping concentration on the electric field, recombination rate, carrier concentration, impact ionization, and other properties of Ge/Si APD. Ultimately, the aim is to design high-performance bonded Ge/Si APD. This study offers theoretical guidance for future research on Ge/Si APD with low noise and high gain.

In this study, a 2-nm thick layer of poly-Si material is introduced at the Ge/Si bonding interface, and the influence of the doping concentrations of the Ge and Si layers on the APD properties is investigated. Initially, changes in the APD optical and dark currents with doping concentration are simulated. The changes in the recombination rate and carrier concentration are then simulated to explore the reasons for the changes in the optical current. Next, to further understand the reasons for the change in electron concentration, changes in the energy band of the APD are simulated. Following this, changes in the charge concentration, impact ionization rate, electric field, and other parameters with the doping concentration are simulated. Finally, the gain, bandwidth, and gain-bandwidth product of the APD are simulated and compared with previous studies. The optimal doping concentration for APD devices is identified to improve device performance.

After introducing the polycrystalline silicon bonding layer, the dark current reaches 1×10^-10 A, which is five orders of magnitude lower than that of the currently reported Ge/Si APD (Fig. 3). As the doping concentrations of the Ge and Si layers increase, the conduction band in the Ge layer gradually flattens. When the doping concentration is high, the conduction band bends upward at the bonding interface, gradually forming a barrier at the bonding interface that obstructs the transport of charge carriers, resulting in challenges in transporting electrons in the Ge layer to the multiplication layer. As the doping concentration increases, the valence band becomes steeper, which facilitates the migration of holes. The holes in the multiplication layer can reach the absorption layer smoothly under the influence of a higher potential energy difference (Fig. 6). The electron and hole ionization coefficients at the p-Ge/i-Ge interface rise sharply with increasing doping concentration of the Ge layer, primarily due to the significant increase in the electric field with rising doping concentration (Fig. 8).

In this study, a poly-Si material is introduced at the bonding interface of Ge/Si, and the influence of the doping concentrations of Ge and Si layers on the performance of Ge/Si APD is theoretically examined. After the poly-Si layer is introduced, the dark current is found to reach an order of 1×10^-10 A. Furthermore, the gain of 12.21 is realized when the Ge layer doping concentration is set at 1×10¹² cm^-3 and the reverse bias is 28.0 V. The maximum gain of 12.14 is noted when the doping concentration of the Si layer is 1×10¹⁵ cm^-3 and the reverse bias is 28.2 V. As the doping concentrations of the Ge and Si layers are increased from 1×10¹² cm^-3 to 1×10¹⁶ cm^-3, under the same bias voltage, an overall upward trend in the 3-dB bandwidth is observed. However, a sharp drop in the bandwidth is observed when the Ge layer doping concentration exceeds 1×10¹⁶ cm^-3. The gain bandwidth product is found to reach its maximum value of 225.76 GHz when the Ge layer doping concentration is 1×10¹² cm^-3. A peak value of 215.15 GHz for the gain bandwidth product is achieved when the doping concentration of the Si layer is 1×10¹² cm^-3, and the bias is 29.5 V. Thus, an optimal gain and gain-bandwidth product in a Ge/Si APD can be obtained when lower doping concentrations of the Ge absorption layer and Si multiplication layer are chosen, ensuring that no electric field or tunneling phenomenon is encountered.