We report on the electrical material characterization of InAs/GaSb type-II superlattice (T2SL) mid-wave infrared (MWIR) photodiodes with different passivation schemes on the mesa-sidewalls with significant differences in the resulting low temperature dark currents. Devices fabricated by dry etching passivated with polymerized photoresist, show orders of magnitude lower dark currents as compared to unpassivated devices. The mesa side walls were examined using high resolution transmission electron microscopy (HRTEM) with a special focus on the interface between the superlattice material the dielectric passivation. Material analysis on nanometer scale at the mesa sidewall interface was performed using energy dispersive X-ray (EDX) spectrometry. EDX line scans were obtained from interfaces for different passivated unpassivated devices, using the highly focused electron beam in TEM, to investigate the chemical compositions. The unpassivated photoresist-passivated mesas, with different electrical properties, revealed different sidewall morphologies compositions. An oxygen containing layer was observed in photoresist-passivated devices covering the whole mesa sidewall. We think this plays a role in reducing surface leakage dark current. In HR-TEM the mesa sidewall topography reveals preferential etching of one superlattice component as previously observed. Nevertheless, the dielectric material covers the sidewall uniformly.