InGaAs materials as absorption layers and Si materials as multiplication layers are potential alternatives for achieving high-performance avalanche photodiodes (APDs). However, simple and well-performing InGaAs/Si APDs are difficult to fabricate owing to the 7.7% lattice mismatch between InGaAs and Si. Investigators have recently reported that a-Si was introduced at the InGaAs/Si APD bonding interface to inhibit the nucleation of mismatch dislocations and realize an ultra-low dark current. However, owing to the large bandgap of a-Si, the bonding interface has a large conduction band and valence band offset. This causes the gain of the device to decrease. Ge and Si are both indirect band gap semiconductors, and Ge materials have the advantages of a small gap width and a long absorption cutoff wavelength in the infrared region. Hence, in this study, a method to mitigate the effect of the InGaAs/Si lattice mismatch on APD performance from the source side is theoretically proposed. Here, a-Ge or poly-Ge bond layers are introduced into the InGaAs/Si bond interface, and the variation in the InGaAs/Si APD performance with the bond layer thickness is simulated and compared. In this work, theoretical guidance for the development of ultralow-noise and high-gain InGaAs/Si APDs will emerge.

An a-Ge or poly-Ge bond layer is introduced into the InGaAs/Si bond interface, and variations in APD performance with bond layer thickness are simulated and compared. Initially, the optical and dark currents of the APD are simulated and compared considering the thickness of the bonding layer. Subsequently, the recombination rate and carrier concentration of the APD under light conditions are simulated to understand the cause of the change in the APD optical current. To further understand the cause of the change in the electron concentration of the APD, the changes in the APD energy band under light conditions are simulated. Then, the changes in charge concentration, impact ionization rate, electric field, and other parameters with the bond layer thickness are simulated and compared. Finally, the gain and gain bandwidth products of the APD are simulated and compared to further explore the performance improvement of the device.

After introducing a-Ge or poly-Ge bond layers, the dark current of the APD before avalanche can be as low as 10^-11 A (Fig. 2). Moreover, potential barriers or wells appear in the energy band of the bonding interface (Figs. 8 and 9). Owing to the barrier effect and hole trapping effect, optical and dark current gaps appear in both (Fig. 2), and this phenomenon is more obvious in the APD with the poly-Ge bond layer. These results indicate that both the a-Ge and poly-Ge bonding layers can reduce device noise. The gain and gain-bandwidth products of the APD are simulated and compared. The results show that when a-Ge is used as the bonding layer and the thickness of the bonding layer is 0.5 nm, the gain and gain bandwidth product can reach its maximum. The maximum gain of the APD can reach 451.3 (Fig. 15), and the maximum gain bandwidth product can reach 13.7 GHz (Fig. 20). Theoretically, an InGaAs/Si APD with high gain and ultralow noise is obtained.

In this study, the effects of a-Ge and poly-Ge bonding layer thicknesses introduced at the InGaAs/Si bonding interface on the performance of InGaAs/Si APD are theoretically studied. The results show that the dark current before the avalanche can be as low as 10^-11 A when the a-Ge and poly-Ge bond layers are introduced. Furthermore, the APD with a-Ge or poly-Ge bond layers introduced after the avalanche exhibits optical and dark current bandgaps. This will help the APD achieve ultralow noise. Similarly, the gain and gain bandwidth products of the APD with an a-Ge bonding layer are much larger than those with a poly-Ge binding layer. The gain and gain bandwidth products of the APD decrease with an increase in the bond layer thickness. The APD performance is optimum when a-Ge is used as the bonding layer and the bond layer thickness is 0.5 nm. At this time, the maximum gain can reach 451.3 and the maximum gain bandwidth product can reach 13.7 GHz. However, when poly-Ge is used as the bonding layer, the maximum gain of the APD is only 7.9 and the maximum gain bandwidth product is only 598 MHz. Therefore, the use of a-Ge as the bonding layer material and the selection of a thin bonding layer are considered ideal schemes for preparing InGaAs/Si APD with improved device performances.