## 2 Materials and Methods  
The material for this study was grown on a low Te-doped n-type GaSb (100) substrate, using a solid source molecular beam epitaxy (SS-MBE) reactor equipped with valved cracking sources for group-V and SUMO cells for group-III. The details about the growth for the \(nBn\) and \(pBn\) structures can be found elsewhere [19; 28]. Both structures had the same barrier and absorption MWIR region (only the top contacts were different: p-type (for \(pBn\)) and n-type (for \(nBn\))). The thickness of the top layer was adjusted accordingly to reflect the growth condition. This unipolar barrier structure design was chosen to allow a significant drop in electric field across the wide bandgap barrier to reduce generation-recombination currents. For the barrier, the SLS design was chosen to be AlAs\({}_{0.5}\)Sb\({}_{0.5}\)/InAs\({}_{0.5}\)Sb\({}_{0.5}\), with a deep electron quantum well in the conduction band, which has been demonstrated to act as an effective wide-bandgap electron barrier for both \(nBn\) and \(pBn\) structures [28; 29; 30; 31]. For the fabrication approach, a high quality 1-um silicon-oxide layer was deposited as a hard mask to protect the diode area against the implantation. The oxide layer was deposited by plasma-enhanced chemical vapor deposition (PECVD), using a plasma of SiH\({}_{4}\) and N\({}_{2}\)O gases. The thickness of the oxide layer was selected based on a simulation study using the stopping range of ions in matter (SRIM) software. Different steps of fabrication are schematically demonstrated in Figure 1. The first step was to deposit top metal contacts (a) on the as-grown surface, and then the silicon-oxide layer was deposited (b). This oxide layer was designed to act as a hard mask to protect the underneath SLS area, as well as the top metal contact. The oxide mask was then lithographically patterned on top of the wafers (c, d). The next step was to apply ion implantation on the isolation regions between diodes (e), whose top areas were covered by the hard mask. All samples were subjected to annealing for 15 s at 300 \({}^{\circ}\)C. Finally, the top metal contact was opened through the hard mask (f). The shape of the diodes was circular, with diameters ranging from 100 \(\upmu\)m to 400 \(\upmu\)m.  
For the implantation process, the dose and acceleration energy required for effective isolation were studied and then optimized using the SRIM simulation tool. The penetration depth was a function of the implanted material nature and implantation energy, where the dose can signify the total number of implanted atoms. Although, finding an accurate ion-implantation profile would be hard to estimate, given the complexities of SLS material and the lack of enough empirical data on SLS material ion-implantation. Figure 2 illustrates an example of a simulation test result from the SRIM software for Si implantation distribution in different depths of an SLS \(pBn\) target, performed at two different implantation energies (I\({}_{\text{E}}\)), 100 keV (a) and 190 keV (b). Figure 1: Schematic diagram of the process flow for the fabrication planar SLS photodetectors: (**a**) top metal contact, (**b**) deposition of the oxide mask, (**c**,**d**) patterning via photolithography, (**e**) implantation for diode isolation, and (**f**) top metal contact window opening. Figure 2: Simulation results performed using SRIM software for distribution of the Si ions in different depths of the target for various implantation energies: (**a**) I\({}_{\text{E}}\) = 100 keV, (**b**) I\({}_{\text{E}}\) = 190 keV. Based on the simulation result and the experimental consideration for \(pBn\) structure ion-implantation energies of 380, 190, and 100 KeV, and for the \(nBn\) structure the energies of 300, 190, and 100 KeV were chosen. For both cases and for each implantation energy, three ion-implantation doses of 1.0 \(\times\) 10\({}^{14}\), 5.0 \(\times\) 10\({}^{14}\), and 1.0 \(\times\) 10\({}^{15}\) cm\({}^{-2}\) were used for a total of nine permutations.