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
_x
Ga
_1-x
As (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
_0.8
Ga
_0.2
As (50 nm) multilayer structure, much larger than that in the Ge and Al
_x
Ga
_1-x
As homojunctions, attributed by the reduction in α̅. The substantial difference in phonon scattering cross-sections in the Al
_x
Ga
_1-x
As 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
_x
Ga
_1-x
As 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.