Simulation of lattice matched InAs/GaAs0.09Sb0.91 Type II Superlattice Photodetectors

Type II superlattices (T2SL) are important as an alternative to Mercury cadmium telluride (MCT)
photodetectors. MCTs offer some of the best performance figures, but are restricted in use due to
cost, and the phasing out of Mercury containing products in the EU from 2017. T2SLs have
several advantages over MCTs including cheaper lattice matched substrates, less sensitive
structure composition and reduced auger recombination. InAs/GaAs0.09Sb0.91 T2SLs are lattice
matched, resulting in photodiodes grown with fewer defects in the crystal structure. In this thesis
the cutoff wavelength, absorption coefficient and current-density of InAs/GaAs0.09Sb0.91
superlattices were simulated using the program nextnano++. Models were used first were
validated by benchmarking against published results. Simulations of a wide range of T2SL
structures were performed to identify the document design parameters that controls the cutoff
wavelengths, absorption coefficients and dark current densities. This offers the opportunity to
choose the best structure for the desired wavelengths before growth and fabrication, which
incurred high cost.
Simulations showed, nextnano++ could produce excellent agreement to measured cutoff
wavelengths (λc) of InAs/GaAs0.09Sb0.91 T2SLs in the mid wavelength infrared (MWIR), long
wavelength Infrared (LWIR) and very long wavelength infrared (VLWIR). The
InAs/GaAs0.09Sb0.91 T2SLs simulations achieved the LWIR and VLWIR ranges with superlattice
quantum well (InAs) thickness between 12 -20 monolayers (MLs). The simulations indicated the
cutoff wavelength of the superlattice had a greater dependence on the thickness of the InAs well.
For example using a fixed 7 ML InAs as the quantum well, changing the GaAsSb barrier
thickness from 7 to 20 ML, only resulted in λc changing from 4.64 to 4.7 μm at 77 K. On the other
hand, for a fixed 7 ML of GaAsSb barrier, changing the InAs well from 7 to 20 ML shifted λc
from 4.64 to 10.5 μm at 77 K. Simulations of absorption coefficient for InAs/GaAs0.09Sb0.91
T2SLs at multiple temperatures showed that more symmetrical superlattice periods had higher
absorption coefficients at all the temperatures simulated.
The temperature dependence of λc and absorption coefficients was simulated in a wide range of
T2SL designs. The results show stronger temperature dependence of these parameters when the
thickness of InAs is changed. The change in λc is stronger in designs with a longer λc. For instance
in a VLWIR design with 20ML InAs/20 ML GaAsSb λc increases from 18.5 μm at 77 K to
35.0 μm at 300 K. In a MWIR design with 7 ML InAs/7 ML GaAsSb λc only increases marginally
from 4.6 to 5.7 μm over the same temperature range. When different designs with similar cutoff
wavelengths are compared, the temperature dependence behaviours are similar. However, T2SL
designs with a more symmetrical design shows higher absorption coefficients across all
8
temperatures simulated. A 16ML InAs/10 ML GaAsSb T2SL show higher absorption coefficients
than a 20 ML InAs/7 ML GaAsSb T2SL, although they have similar λc. The temperature
dependence of dark current was also investigated.
Finally an LWIR InAs/GaAs0.09Sb0.91 PIN structure with a superlattice i-region and bulk p and nregions to act as barriers was fabricated. Simulations suggested the sample had a cutoff
wavelength at 9.5 μm whereas the photo response from the blackbody emitter indicated that the
peak photoresponse signal is at 9-11 μm.