Impact Ionization In AlGaAsSb Avalanche Photodiodes

This work aims to demonstrate a separate absorber, charge, multiplication (SACM) avalanche photodiode (APD) with GaAsSb/AlGaAsSb grown on InP. AlAsSb shows very dissimilar ionization coefficients between electrons(α) and holes(β) and extremely low excess noise. The temperature dependence breakdown coefficient (Cbd) in AlAsSb was found to be very, small 8.5mV/K in a 1μm p-i-n diode, and the electron and hole impact ionization coefficients increase at about the same rate as the temperature decreases, significantly less so than in InP and InAlAs. However, this material suffers from oxidization and surface leakage current. This is significantly improved by employing the AlGaAsSb quaternary alloy system, enabling low dark current while maintaining low excess noise and a large α/β ratio. The extraction of ionization coefficients from avalanche multiplication measurements has clarified this material's characteristics and optimized the avalanche region thickness in SACM APD design. It is the first report of a room temperature, ultra-high gain (M=278, λ=1550 nm, V=69.5 V, T=296 K) linear mode avalanche photodiode, grown on an InP substrate using a GaAs0.5Sb0.5/Al0.85Ga0.15As0.56Sb0.44 separate absorption charge and multiplication (SACM) heterostructure. This design employs a novel GaAsSb absorber that is graded to wider bandgap charge and multiplication layers with several AlxGa1-xAsSb grading layers. This represents a ~10× gain improvement over commercial, state-of-the-art InGaAs/InP-based APDs (M ∼30) operating at 1550 nm. The excess noise factor is extremely low (F<3) at M=70 and this design gives a quantum efficiency of 5935.3% at maximum gain. A 200 µm diameter device gives a capacitance limited 3 dB bandwidth of 0.7 GHz (M=25, V=65 V). Furthermore, this SACM APD shows an extremely low-temperature-dependent breakdown coefficient (Cbd) of ~11.83 mV/K, which is ~10× lower than equivalent InGaAs/InP commercial APDs. This demonstration opens a pathway to realize high sensitivity receiver systems at eye-safe, infrared wavelengths (1400 - 1650 nm) for a variety of applications.