Ge/Si avalanche photodiode (APD) has attracted much attention in optical communication, optical imaging, and security detection due to its high-speed operation, low noise, high sensitivity, and optical gain characteristics. However, a mismatch between the lattice constants of Si and Ge (i.e., a difference of 4.2%) has a challenge. This mismatch leads to a high density of threading dislocations at the Ge/Si heterojunction interface, ultimately deteriorating the performance of the device. Therefore, addressing the issue of lattice mismatch and reducing the dislocation density in Si-based Ge thin films is crucial for achieving high-performance Ge/Si APDs. To decrease the dislocation density of APDs, Si-based epitaxial Ge thin films are used as a fabrication method. However, the dislocation density continues to be relatively high. In this paper, the lattice mismatch between Ge and Si was alleviated. In addition, the theoretical underpinnings were also proposed.

In this research, a polycrystalline silicon (poly-Si) bonding layer was incorporated at the Ge/Si interface to mitigate the lattice mismatch effects. This poly-Si layer acted as a buffer, diminishing the stress and strain induced by the lattice mismatch. To assess the influence of the poly-Si bonding layer on the performance of Ge/Si APDs, a series of experiments at different doping concentrations of the poly-Si layer were carried out. The doping concentration was pivotal as it modulated the electrical and optical properties of the bonding layer, subsequently affecting the overall performance of the APD.

The charge distribution, electric field distribution, and carrier transport within the Ge/Si APD device were delineated by the Poisson equation, carrier transport equation, carrier continuity equation, and a parallel electric field dependent model. This facilitated the computation of the device's electrical characteristics. The Ge/Si APD involved electron and hole recombination, the concentration-dependent SRH model was used to characterize the spontaneous emission and Auger recombination processes of carriers. The optical radiation recombination model elucidated the photon Auger recombination process, while the trap recombination model (TRAP.AUGER) described the transition between trap states and non-trap states of carriers, along with their corresponding recombination processes. Ge/Si APD necessitated high voltage operation and substantial doping, and the band-band tunneling standard model was used to explain the carrier transport and ionization process instigated by band-band tunneling under high electric field conditions. In addition, the energy band narrowing model was used to illustrate how changes in band structure impact device performance under high doping conditions. This paper provided a theoretical analysis of Ge/Si APD device performance based on these models, and other related theories.

The results indicate that the photocurrent of APD with a bonding layer thickness of 2 nm initially increases and subsequently decreases as the doping concentration of the bonding layer escalates at 95% of the avalanche voltage. In contrast, the photocurrent of an APD with a bonding layer thickness of 5 nm diminishes as the doping concentration increases. Furthermore, the dark current of a Ge/Si APD can reach as low as 10^-10 A prior to avalanche due to the lattice mismatch buffering effect of poly-Si between Ge and Si, which is markedly lower than that of an APD based on InP. The maximum gain-bandwidth product of 63.8 GHz for the 2 nm bonding layer thickness significantly surpasses that for the 5 nm bonding layer thickness, and the bandwidth of the 2 nm bonding layer thickness also exceeds the bandwidth of the 5 nm bonding layer thickness when the bias voltage exceeds the avalanche voltage. It is recommended to select a poly-Si bonding layer with a thickness of 2 nm and a low doping concentration to achieve the optimal performance in a Ge/Si APD.

The efficacy of integrating a poly-Si bonding layer at the Ge/Si interface was under scored, thereby mitigating a threading dislocation density and augmenting the performance of Ge/Si APDs. Theoretical modeling and simulation analyses revealed that a diminished doping concentration in the poly-Si layer yielded superior outcomes. Such insights paved a way for the advancement of more dependable and efficient Ge/Si APDs, having a potential to transform domains such as optical communication, optical imaging, and security detection. The methodology delineated in this study presented a promising avenue for subsequent research within the realm of semiconductor photodiodes. For materials science and device engineering, it is anticipated that Ge/Si APDs could experience a further evolution, potentially yielding enhanced performance and expanding application potential. The incorporation of the poly-Si bonding layer signified a substantial advancement in addressing lattice mismatch challenges, which could be poised to be instrumental in the progression of next-generation photodetection technologies.