A Probe of Valence and Conduction Band Electronic Structure of Lead Oxide Films for Photodetectors

We investigate how the electronic structure of amorphous lead oxide (a-PbO) films deposited on ITO substrate is changed after annealing at various temperatures. Both experimental soft X-ray spectroscopic and density functional theory (DFT) based computational techniques are used to explore the electronic structure of this material. X-ray emission, resonant X-ray inelastic scattering, and X-ray absorption spectroscopic techniques are employed to directly probe the valence and conduction bands. We discover that the films are very stable and remain amorphous when exposed to temperatures below 300 degrees C. An amorphous-to-polycrystalline (alpha-PbO phase) transformation occurs during annealing at 400 degrees C. At 500 degrees C, an alpha to beta phase change is observed. These structural modifications are accompanied by the band gap value changing from 1.4 +/- 0.2 eV to 2.0 +/- 0.2 eV upon annealing at 400 degrees C and to 2.6 +/- 0.2 eV upon annealing at 500 degrees C. A difference between surface and bulk structural properties is found for all samples annealed at 500 degrees C and above; these samples also exhibit an unexpected suppression of O : 2p density of states (DOS) near the bottom of the conduction band, whereas additional electronic states appear well within the valence band. This study provides a significant step forward to understanding the electronic properties of two polymorphic forms of PbO needed for optimization of this material for use in X-ray sensors.