### Challenges Related to (V)LWIR InAs/InAsSb T2 SLS Bulk Properties

Table 1: Conductivity effective masses for an MWIR and a VLWIR (4\(n\),\(n\))-InAs\(/\)InAs\({}_{0.5}\)Sb\({}_{0.5}\) T2 SLS. cutoff wavelength (\(\lambda_{\rm c}\)), they begin to diverge as the cutoff wavelength increases. However, in InAs/InAsSb T2 SLS with higher (\(\sim\)50%) Sb fraction, this difference between \(m_{\rm n,z}\)** and \(m_{\rm n,x}\)** can be kept reasonably small, even at \(\lambda_{\rm c}\) = 13 \(\upmu\)m (see Table 1). Therefore, electron conductivity effective mass anisotropy can be manageable. The picture is quite different for hole conductivity effective masses. As shown in Figure 1c, the difference between \(m_{\rm p,z}\)** and \(m_{\rm p,x}\)** for the InAs/InAsSb T2 SLS is already very significant in the MWIR, and continues to grow as the cutoff wavelength increases. This anisotropy is greater for InAs/InAsSb T2 SLS with lower Sb fraction alloys. However, even for the best case shown (InAs/InAsSb T2 SLS with 50% Sb fraction) in Figure 1c, the (\(m_{\rm p,z}\)**/\(m_{\rm p,x}\)**) ratio is 11.5 at \(\lambda_{\rm c}\) = 5.7 \(\upmu\)m, and increases to 86.8 at \(\lambda_{\rm c}\) = 13 \(\upmu\)m (See Table 1). The case for the (\(m\),7)-InAs/GaSb T2 SLS is better, with the (\(m_{\rm p,z}\)**/\(m_{\rm p,x}\)**) ratio remaining at a relatively constant value of \(\sim\)1 order of magnitude over the \(\lambda_{\rm c}\) = 5 \(\upmu\)m to 17 \(\upmu\)m range. The large hole conductivity effective mass anisotropy leads to hole mobility anisotropy. Having much larger in-plane than growth direction hole mobility can be very unfavorable from FPA crosstalk and modulation transfer function (MTF) considerations. This was explicitly demonstrated in the nBn FPA pixels simulations by [PERSON] [22]. We will provide additional comment on the crosstalk issue in Section 3, after describing our device architecture. Here, we summarize the findings of this section. In the (V)LWIR, the InAs/InAsSb T2 SLS has weaker absorption than the InAs/GaSb T2 SL, and therefore requires a thicker absorber layer to achieve comparable QE. The growth direction hole conductivity effective mass of the (V)LWIR InAs/InAsSb T2 SLS increases rapidly with cutoff wavelength, leading to low hole mobility and short hole diffusion length, which in turn limits the viable absorber thickness for \(n\)-type InAs/InAsSb T2 SLS. In general, the attainable QE for (V)LWIR InAs/InAsSb T2 SLS detectors with \(n\)-type absorber is limited, and also becomes progressively smaller as the cutoff wavelength increases. For the \(p\)-type superlattice absorbers, we can take advantage of the much smaller growth direction electron conductivity effective mass, and also the more favorable electron conductivity effective mass anisotropy characteristics. This suggests that we can address the (V)LWIR QE problem by using \(p\)-type absorbers, which provide higher mobility and longer diffusion lengths for minority carriers (electrons). It is worth noting that although the first published LWIR InAs/InAsSb T2 SLS detector results [23] were for an nBn detector (with \(n\)-type absorber), earlier studies on LWIR InAs/GaSb T2 SL detectors [24; 25; 26; 27] mostly use \(p\)-type absorbers because of the well-known concerns for low growth direction hole mobility. The reason that the \(n\)-type (V)LWIR InAs/InAsSb T2 SLS absorbers were considered at all was probably due to the success of the nBn detector architecture in general, and the success of the MWIR InAs/InAsSb T2 SLS nBn detector in particular.