### Challenges Related to InAs/InAsSb T2 SLS Surface Properties  
It has been known that the InAs surface Fermi level is actually above its conduction band edge, regardless of whether InAs is doped \(n\)-type or \(p\)-type [28; 29]; the InAs surface inverts to \(n\)-type even if it is doped degenerate \(p\)-type [28]. For InAs/InAsSb T2 SLS strain balanced on GaSb substrate, the typical net InAs fraction is \(\sim\)90%, and therefore the surface of InAs/InAsSb T2 SLS might be expected to show InAs-like behavior. Indeed, based on Schottky barrier height-derived band alignment data [30], and the calculated superlattice band edge positions [31], [PERSON] and co-workers concluded that both MWIR and LWIR InAs/InAsSb T2 SLS surface Fermi levels are in the conduction band also [32].  
In using \(p\)-type InAs/InAsSb T2 SLS absorber for QE enhancement, we encounter two surface-related issues due to the fact that, regardless of doping type, the surface of the InAs/InAsSb T2 SLS is always degenerate \(n\)-type. We provide a brief overview of these issues in this sub-section. Extensive discussions on surface related dark currents in antimonide-based detectors can be found in the references by [PERSON], [PERSON], and [PERSON] [32; 33].  
We turn to \(p\)-type InAs/InAsSb T2 SLS absorber for QE enhancement. The simplest \(p\)-type unipolar barrier detector structure is the pBp or the XBp [34; 35]. The pBp is the analogue of the nBn, but with a \(p\)-type top contact, a unipolar hole barrier, and a \(p\)-type absorber. The layer structure of a pBp detector, along with that of an nBn detector, are illustrated in Figure 2. The pBp has been implemented successfully for (V)LWIR InAs/GaSb T2 SL absorbers [36, 37]. For the InAs/InAsSb T2 SLs, there is an extra complication in that the surface of \(p\)-type InAs/InAsSb superlattice inverts to degenerate \(n\)-type. As illustrated in Figure 2b, the inverted surface provides a surface electron current path that is not blocked by the unipolar hole barrier, leading to electron surface leakage dark current (sidewall shunt current). We note from Figure 2a that the electron surface leakage path in the nBn is interrupted by the presence of the unipolar electron barrier, and therefore even for a fully reticulated nBn detector pixel, the sidewall shunt current is suppressed.  
Another surface issue for the \(p\)-type absorber can be seen in Figure 3, where the energy band diagrams near the sidewall surface of \(n\)-type and \(p\)-type absorbers are illustrated. Figure 3a show that near the \(n\)-type absorber surface, the accumulated surface potential repels the minority carriers (holes) from the etched absorber surface, which consequently is relatively benign. However, for the \(p\)-type absorber, the surface band bending is problematic. As illustrated in Figure 3b, the degenerate \(n\)-type surface of the \(p\)-type absorber attracts minority carriers (electrons); the inverted surface also creates a surface \(p\)-\(n\) junction with a sub-surface depletion region, and is subject to various surface dark current mechanisms [33, 38].  
The analysis in this sub-section shows that for the InAs/InAsSb T2 SLs, the nBn or XBn [39, 40] device architecture is robust against surface leakage and surface band bending; the only drawback is the limited (V)LWIR QE. On the other hand, the pBp can provide higher QE, but is subject to the electron surface leakage dark current, as well as surface-related dark current mechanisms associated with the surface \(p\)-\(n\) junction. Incidentally, these same surface problems also affect the simple \(p\)-\(n\) diode. Therefore, to make effective use of \(p\)-type, InAs/InAsSb T2 SLs absorber for enhanced QE, alternative device architecture is needed.  
Figure 3: Energy band diagram illustrating band bending near the sidewall surface of (**a**) an n-type, and (**b**) a p-type InAs/InAsSb T2 SLs. The surface is degenerate n-type in both cases.  
Figure 2: Schematic layer structure diagrams of (**a**) an nBn detector, and (**b**) a pBp detector.