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.

Highlights

  • There is growing research interest in the wide band gap photoconductors used as x-ray-to-charge transducers in direct conversion medical imaging detectors[1,2]

  • The technological transfer from thin films used in optical imaging to thick layers needed for x-ray imaging is less successful: the first prototype of a PbO flat panel detector for radiography showed a very high spatial resolution but suffered from incomplete charge collection and image lag, which is caused by the presence of residual current after the exposure[1,8,9,10,11]

  • We report on the electronic structure of amorphous PbO studied with synchrotron-based soft x-ray emission (XES) and absorption spectroscopy (XAS) and compare structures of differently synthesized PbO samples including the conventional polycrystalline PbO

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Summary

Results and Discussion

The spectra calculated for these model modifications are compared to the measured spectra to determine the validity of such models for the short-range order in the amorphous sample For such a model to be reasonable, it should reproduce the measured trends, which are the presence of the additional spectral weight below the main absorption feature in XAS and the shift of the CB edge to lower energy. This model explains the extra feature at 524.28 eV in the experimental RXES spectra for the amorphous samples. Technology optimization must target reducing the density of states (DOS) in the vicinity of the conduction band that in turn requires further study the electronic structure to provide a better insight on the exact DOS distribution

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