InAsSb nBn structure plays a key role in realizing high operating temperature (HOT) MWIR detectors. However, the AlAsSb layer, functioning as a barrier layer, can be rapidly oxidized during the pixel isolation process, forming undesirable oxides that can contribute to increase the surface leakage current. In this work, electrical and optical analysis were performed to investigate the role of the post-treatment process after pixel isolation step in reducing the oxidation and inhibiting the formation of unfavorable oxides closely related to the surface leakage currents. Our study shows that H2O2 post-treatment after the dry pixel isolation process can effectively reduce the barrier oxidation and suppress the formation of Sb2O5, greatly reducing the surface leakage current. Using the proposed post-treatment process, the dark current density of less than 1 × 10−6 A/cm2 was obtained at an applied bias of −0.3 V and 150 K, which is one order of magnitude lower than that of the InAsSb nBn device recently reported.