Long Wavelength InAs/InAsSb Infrared Superlattice Challenges: A Theoretical Investigation

InAs/InAsSb type-II superlattice focal plane arrays that demonstrate high operability and uniformity with cutoffs ranging from 5 μm to 13 μm have already been demonstrated. Compared to InAs/GaSb, the InAs/InAsSb superlattice is easier to grow and has longer minority carrier lifetimes, but requires a longer superlattice period to achieve long or very long wavelength cutoffs. A longer type-II superlattice period leads to smaller absorption coefficients and larger growth-direction hole conductivity effective masses. We explore by theoretical modeling some of the ideas aimed at addressing these challenges for the long and very long wavelength InAs/InAsSb superlattice. Increasing the Sb fraction in the InAsSb alloy can reduce the InAs/InAsSb superlattice period significantly, but this benefit can be negated by Sb segregation. Thin AlAsSb barrier layers can be inserted in InAs/InAsSb to form polytype W, M, and N superlattices in order to increase electron–hole wavefunction overlap for stronger optical absorption. However, this strategy can be unfavorable since the AlAsSb barriers increase the band gap, and thereby increase the superlattice period required to reach a given cutoff wavelength. Metamorphic growth on virtual substrates with larger lattice constants than GaSb can decrease the superlattice period needed to reach a specified cutoff wavelength, but this benefit should be weighed against the need for metamorphic buffer growth and the resulting higher defect density.