PbSe mid-IR photoconductive thin films (part I): Phase analysis of the functional layer

Abstract

Photoconductive thin films on the basis of PbSe have been used as functional materials for infrared (IR) detectors. High quality PbSe thin films were grown on glass substrates by chemical bath deposition (CBD) yielding a peak detectivity of 2.8 × 1010 cm Hz1/2 W−1 at room temperature. The photoconductive sensor function is established by a sensitization process by annealing PbSe films in oxygen and iodine rich atmospheres. By studying such sensitized PbSe layers we discovered that new, quaternary phases are formed at the top of the layered structure.We could show that after annealing the following layer structure had formed: a bottom layer of PbSe (1 μm) and a top layer of Pb-Se-O-I (400 nm), containing (i) a poly-crystalline and (ii) a nano-crystalline phase. The poly-crystalline phase was found to be a solid solution of Se and I and yielded an average Pb/I mole fraction ratio of 3.1 and Pb/Se of 3.9, respectively. It contained the largest iodine mole fraction of ∼20 at.% while the nano-crystalline phase yielded less iodine (<10 at.%). This proves the great efficiency of the reactive annealing for functionalizing the polycrystalline PbSe films. The d-spacings, the chemical composition and the plasmon energies of the phases were measured and bar code these phases. The quaternary Pb-Se-O-I phases in the top layer have large unit cells and d spacings exceeding 0.8 nm. The poly-crystalline phase yielded two sharp plasmon peaks at 17.2 eV and 23.8 eV, respectively. Therefore, it can be clearly distinguished from PbSe, which yields a plasmon energy of 15 eV and d spacings smaller than 0.353 nm.