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.