X-ray spectroscopic study of amorphous and polycrystalline PbO films, α-PbO, and β-PbO for direct conversion imaging

Abstract

We investigated the electronic structure of Lead Oxide (PbO) – one of the most promising photoconductor materials for direct conversion x-ray imaging detectors, using soft x-ray emission and absorption spectroscopy. Two structural configurations of thin PbO layers, namely the polycrystalline and the amorphous phase, were studied, and compared to the properties of powdered α-PbO and β-PbO samples. In addition, we performed calculations within the framework of density functional theory and found an excellent agreement between the calculated and the measured absorption and emission spectra, which indicates high accuracy of our structural models. Our work provides strong evidence that the electronic structure of PbO layers, specifically the width of the band gap and the presence of additional interband and intraband states in both conduction and valence band, depend on the deposition conditions. We tested several model structures using DFT simulations to understand what the origin of these states is. The presence of O vacancies is the most plausible explanation for these additional electronic states. Several other plausible models were ruled out including interstitial O, dislocated O and the presence of significant lattice stress in PbO.

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

“…mBJ exchange potential, proposed by Becke and Johnson, which takes PBE‐GGA calculation results as input, is used to calculate the band gap values . We have previously tested various types of local defects/modifications . Several different model structures were employed by introducing modifications or defects to a 2×2×2 supercell generated from experimentally determined crystalline structure.…”

“…This increase in gap is mainly due to suppression of peak ‘b’ in the XAS spectra discussed earlier. We have reported in an earlier study that the calculated band gap values using DFT are 1.7 eV, and 2.3 eV for α‐PbO and β‐PbO respectively. The colour changes with annealing temperature are consistent with band gap changes.…”

“…Due to lack of crystal lattice periodicity, the energy bands become more dispersive, resulting in all the features in the XAS spectra being broadened and introducing states within the otherwise forbidden energy gap. In an earlier study, we have shown that the shoulder‘a’ to the first main peak ‘b’ is due to the presence of oxygen vacancies supported by DFT calculations. Thus, we conclude that the amorphous PbO films show very strong structural stability if exposed to a temperature that is 300 °C or below.…”

“…A detailed peak deconvolution analysis is discussed later. As already mentioned, the samples labelled 25 °C, 200 °C, and 300 °C are all amorphous and we refer the reader to an earlier study by our group for detailed discussion of emission spectra for the amorphous PbO samples . Specifically, we showed in this study the existence of additional features in emission spectra well within the valence band due to presence of oxygen vacancies.…”

“…We recently reported a detailed structural characterization by Raman spectroscopy, X‐ray diffraction (XRD) and X‐ray photoelectron spectroscopy (XPS), and morphological examinations with scanning electron microscopy (SEM) of both, amorphous and poly‐PbO . Furthermore, a study on the electronic structure of amorphous and polycrystalline PbO films was recently published by us . The work presented here is a characterization the electronic properties of a‐PbO and analysis of its fundamental metastability at elevated temperatures with respect to its polycrystalline counterpart.…”