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

Abstract

Photoconducting thin films on the basis of PbSe have been used as functional materials for infrared detectors. The photoconducting sensor function is established by annealing PbSe films in oxygen rich and iodine rich atmospheres. Under such conditions, new, chemically more complex phases are formed that yield outstanding photoconducting properties. Such thin films are investigated after annealing, the layer structure of these films consist of a bottom layer of PbSe (1 μm) and a top layer of Pb1 Sex Iy Oz about 400 nm thick.By electron diffraction, a pseudo-orthorhombic Bravais lattice could be assigned to the quaternary phase of the top layer, responsible for photoconductivity, which was about 400 nm thick. Lattice parameters were identified as a = 0.833 nm, b = 0.662 nm and c = 1.8 nm. The grain size of the top layer was 500–700 nm and yielded single-crystal like diffraction patterns for electron diffraction. Diffraction patterns were acquired in the [100], [010] and [111] poles and could be indexed. The Bravais lattice is simple orthorhombic, no centering was found. The single-grain diffraction patterns obtained here are more valuable than the poly-crystalline diffraction patterns shown in a previous paper (part I) and yield a pseudo-orthorhombic Bravais lattice for this phase. As a consequence, the X-ray diffractograms could be indexed with the assigned Bravais lattice for this phase. The chemical composition of the new phase was measured by EDX spectroscopy in the TEM. The elemental composition of this quaternary phase was determined as Pb1 Sex Iy Oz, for the mole fraction ratio of x = Se/Pb we found 0.17 ≤ x ≤ 0.5 and for y = I/Pb we found 0.2 ≤ y ≤ 0.5. Se and I can substitute each other and therefore a solid solution is formed. The mole fraction ratio of Pb/(Se + I) was found between 1.6 and 1.9. Electron energy-loss spectra revealed a double peak structure for the volume plasmon peak, energies of the quaternary phase lay between 15.7 and 16.3 eV (peak 1) and 23.1–23.2 eV (peak 2), respectively.