## 2 Materials and Methods  
The Ga-free InAs/InAsSb T2 SL structure, strained balanced to GaSb substrate, must be chosen in terms of superlattice period (p) and antimony (Sb) composition to address the full MWIR domain but also to optimize the absorption coefficient, as the InAs/InAsSb heterostructure presents a type II-b band offset where electrons are confined in the binary layer (InAs) while holes are strongly confined in the InAsSb alloy [4; 11; 29]. The chosen T2 SL structure consists of InAs (4.17 nm)/InAs\({}_{0.63}\)Sb\({}_{0.37}\) (1.42 nm). Two types of T2 SL structure samples were grown on 2-inch n-type (Te-doped) GaSb (100) substrate by MBE using a 412 RIBER Machine. The goal of first structure sample (Figure 1a) is to study material properties as a function of temperature. Such samples, consisting of a 3 \(\upmu\)m thick InAs/InAsSb T2 SL layer on GaSb substrate, are dedicated to absorption and photoluminescence (PL) measurements. From PL measurements, we can extract the peak position to verify the bandgap energy of the T2 SL structure, the full-width-at-half-maximum (FWHM) and integrated intensity to evaluate the material quality, and the temperature-dependent spectra. PL measurements of these samples placed in a liquid nitrogen (LN2)-cooled JANIS cryostat, optically pumped with an 835 nm fiber-coupled GaAs diode laser, were collected using a Nicolet-870 Nexus Fourier transform infrared (FTIR) spectrometer. The absorption spectrum was deduced from the transmission measured by the FTIR spectrometer and the absorption coefficient was extracted from the ratio with the transmission measurement performed on the same GaSb substrate alone with no epilayers deposited on it. Figure 1: Schematic view of the different epilayers stacking under study. (**a**) T2 SL structure used for absorption and photoluminescence measurements. (**b**) XBn detector structure used for lifetime, dark current, capacitance–voltage and quantum efficiency measurements. The second structure of interest is the XBn T2 SL detector structure (Figure 1b). From bottom to top, the XBn structure consists of a 400 nm Te-doped (n-type) GaSb buffer layer, which is followed by a 100 nm thick n-type doped InAs (4.17 nm)/InAs\({}_{0.63}\)Sb\({}_{0.37}\) (1.42 nm) T2 SL and by non-intentionally doped (mid) 3 \(\upmu\)m thick AL (536 periods) made of the same T2 SL structure. A BL is then made from 120 nm mid AlAs\({}_{0.09}\)Sb\({}_{0.91}\), and finally, the contact layer (CL) of the structure is composed of an 80 nm thick mid T2 SL. The AL, CL and BL are undoped, and the residual doping is expected to be n-type for the two first layers and p-type for the third one [18]. Prior to device fabrication, several material characterizations were routinely performed, such as high-resolution X-ray diffraction (HR-XRD) scanning to evaluate the structural quality of the XBn sample and its lattice mismatch with the GaSb substrate, atomic force microscope (AFM) images to assess the surface roughness and morphology, additional PL spectra and time-resolved photoluminescence (TRPL) measurements to extract the minority carrier lifetime. For TRPL measurements, a 1.55 \(\upmu\)m laser pulse with a FWHM less than 1 ns was used to generate excess carriers. The power of the laser is tunable and measurements were performed at a given laser pulse fluence. The photoluminescence signal was detected with a HgCdTe photodiode from Vigo (PVMI-3 TE-8) and analyzed with a Yokogawa oscilloscope. Following the approach of [PERSON] et al. [33], minority carrier lifetime as a function of temperature was extracted. From epitaxial T2 SL structures, circular mesa photodetectors with diameters ranging from 60 to 310 \(\upmu\)m were fabricated using standard photolithography techniques. Mesa photodetectors were realized by isotropic wet chemical etching using a citric acid solution and with no use of specific surface treatment. Deep-etched mesa devices below the AL were performed and polymerized photoresist was spun after the wet etching to protect the mesa surface from ambient air.