Mechanistic studies of oxidation and iodization of PbSe thin film sensitization for mid-infrared detection

Abstract

With the resurgence of uncooled PbSe-based photodetectors and the demand for mid-wave infrared (MWIR) imaging systems, a need exists for a better understanding of the sensitization process used for photoconductive detection. A mechanistic study of the oxidation and iodization process is carried out to improve the understanding of the sensitization process with experimental measurement of film composition, morphology, and crystal structure. Comprehensive material characterization is performed for PbSe thin films processed under different sensitization conditions, and results are combined to construct a physical model of the sensitization process. The relative elemental concentration distribution found in cross-sectional energy-dispersive spectroscopy line-scans is correlated with the crystallographic data collected with x-ray diffraction and Raman measurements, respectively, to build a spatially accurate model. The different sensitization conditions are then correlated to the MWIR photoresponse. This study shows that a targeted PbSeO3 layer thickness of 400 nm formed during the oxidation process is necessary to (1) restrict the top PbI2 layer thickness to less than 200 nm, (2) regulate the iodization process, allowing trace amounts of iodine to diffuse along grain boundaries to recrystallize the PbSe base-layer, and (3) preserve the stoichiometric balance of the underlying PbSe layer. An optimum oxidation process window is identified whereby too thick, as well as too thin, of a PbSeO3 layer, both result in a thick PbI2 top layer and a thin PbSe base-layer. This work also shows that PbSeO3 iodizes to form PbI2 at a rate of ∼100 nm/min at 325 °C while pure PbSe and Se-poor PbSe iodizes nearly 5× faster than PbSeO3. Overall, the oxidation step is necessary for oxygen diffusion into PbSe thin film grain boundaries and to control the iodine diffusion rate. The iodization step is necessary to form a PbI2 surface passivation layer, formation of a newly identified compound within sensitized PbSe (Pb3Se2O6I2), and incorporation of iodine into PbSe thin film grain boundaries. We identify and explain the narrow oxidation process window and iodization conditions necessary to achieve high photoconductivity.