Monte Carlo Simulation of Hot Carrier Transport in Heterogeneous Ge/ ${\rm Al}_{x}{\rm Ga}_{1-x}{\rm As}~(0\leq x\leq 0.8)$ Multilayer Avalanche Photodiodes

Impact ionization in novel Ge/Al_xGa_1-xAs (0 ≤ x ≤ 0.8) multilayer structures is investigated using numerical simulation. The simulated effective electron (α̅) and hole (β̅) ionization coefficients reveal that a large β̅/α̅ ratio of up to 8 can be obtained in a 25-period Ge (50 nm)/Al_0.8Ga_0.2As (50 nm) multilayer structure, much larger than that in the Ge and Al_xGa_1-xAs homojunctions, attributed by the reduction in α̅. The substantial difference in phonon scattering cross-sections in the Al_xGa_1-xAs barrier and the Ge well played an important role in determining the β̅/α̅ ratio in these structures. Breakdown voltage analysis indicates that hot carriers´ drift in the Al_xGa_1-xAs barriers and Ge wells sampled the transport properties of both materials, despite the electron and hole ionizations are dominated by that of Ge within the electric field range studied.