### Challenges Related to (V)LWIR InAs/InAsSb T2 SLS Bulk Properties  
Two intrinsic properties of bulk InAs/InAsSb T2 SLS present challenges for achieving good quantum efficiency (QE) in the (V)LWIR. First, InAs/InAsSb T2 SLS has weaker absorption than the InAs/GaSb T2 SL in the (V)LWIR [14; 15], and therefore requires a thicker absorber layer to achieve comparable QE. Second, (V)LWIR InAs/InAsSb T2 SLS has a short hole diffusion length, which affects devices that make use of \(n\)-type absorbers. We discuss these two issues in more detail below. An in-depth theoretical discussion on the challenges for (V)LWIR InAs/InAsSb T2 SLS can be found in [16]. The smaller absorption coefficients of (V)LWIR InAs/InAsSb T2 SLS can be traced to the fact that a longer superlattice period is required to achieve the same band gap (or equivalently the corresponding detector cutoff wavelength) than the InAs/GaSb T2 SL. In a type-II superlattice, the band-edge electron and hole wavefunctions are localized in different layers. In the case of InAs/InAsSb, the lowest conduction sub-band wavefunction is in the InAs layers, while the highest valence sub-band wavefunction is in the InAsSb layers. Longer superlattice period leads to smaller electron-hole wavefunction overlap and weaker absorption coefficient. Therefore, the period required to reach a given band gap (\(E_{\text{g}}\)), or the corresponding cutoff wavelength (assumed as \(\lambda_{\text{cutoff}}=hc/E_{\text{g}}\)), is a simple but often informative quantity for a type-II superlattice. We used an enhanced effective bond orbital model (EBOM) [17; 18; 19] to calculate the superlattice band gap. Figure 1a shows the calculated cutoff wavelength as a function of superlattice period for three sets of (\(m\),\(n\))-InAs/InAsSb T2 SLS (\(m\) and \(n\) respectively being the number of monolayers of InAs and InAsSb in each superlattice period) with different \(m/n\) ratios. The results for a set of (\(m\),\(n\))-InAs/GaSb T2 SL with \(n=7\) are also plotted for comparison. The cutoff wavelength is derived from the calculated superlattice band gap using the relationship \(\lambda_{\text{cutoff}}\) [\(\mu\)m] = 1.24/\(E_{\text{g}}\) [eV]. For the InAs/InAsSb T2 SLS, the InAsSb alloy composition for a family with a given \(m/n\) ratio is chosen to achieve strain balance with respect to the GaSb substrate; higher \(m/n\) ratio is associated with InAsSb alloy with higher Sb concentration (which is more compressively strained with respect to the GaSb substrate). Figure 1: (**a**) Calculated cutoff wavelengths for three families of InAs/InAsSb superlattices as functions of superlattice period in monolayers (MLs). The results for a set of InAs/GaSb superlattice is included for comparison, (**b**) electron conductivity effective masses along the growth direction z (\(m_{n,x}\)**, solid lines) and in-plane direction x (\(m_{n,x}\)**, dashed lines) plotted against cutoff wavelength for the same set of superlattices, and (**c**) the corresponding hole conductivity effective masses \(m_{\text{p},x}\)** and \(m_{\text{p},x}\)**. Figure 1a shows that InAs/InAsSb T2 SLS with higher Sb fraction (larger \(m/n\) ratio) can reach a specified cutoff wavelength with shorter superlattice period, and is thus more favorable. Overall, in the MWIR range, all four sets of superlattices have fairly comparable periods. However, as the cutoff wavelength increases, the periodicity disadvantage of the InAs/InAsSb T2 SLS with respect to the InAs/GaSb T2 SL becomes more apparent, especially with lower Sb alloy fractions. This is one of the main challenges InAs/InAsSb T2 SLS. We note that in Figure 1a, the highest InAsSb alloy Sb fraction that we used is 50%, and in principle, we can use InAs/InAsSb T2 SLS with even higher Sb fractions. In fact, it has been shown that at an Sb fraction of 74%, the InAs/InAsSb T2 SLS nearly matches the InAs/GaSb T2 SL in the cutoff wavelength vs. superlattice period characteristics [16].