Amorphous PbO Photoconductive Film Structure Revealed by Variable-Energy, Doppler-Broadened Positron Annihilation Spectroscopy
Bibliographic record
Abstract
Photoconductive amorphous selenium (a-Se) layers are utilized in flat panel X-ray imaging detectors as a direct conversion medium, converting X-ray photons directly into electric charge. Commercial a-Se direct conversion Active Matrix Flat Panel Imagers (AMFPIs) have demonstrated superior image quality in mammography, showcasing the potential of this X-ray imaging technology [1-2]. The use of a-Se is limited, however, by its low Z, resulting in low stopping of high energy X-rays [2]. This limitation is not shared by PbO thin films. Earlier PbO films consisted of small poly-crystalline platelets with low film density and suffered from the presence of both oxygen vacancies and impurity phases (PbO2). Recent advances [3-4] have yielded dense amorphous PbO (a-PbO) films with apparently uniform stoichiometry, as confirmed by X-ray photoelectron spectroscopy (XPS). More careful analysis [5] using X-ray absorption spectroscopy (XAS) indicated some tailing of the conduction band, which was attributed to suspected O-vacancies. An annealing study on a-PbO [3] indicated a transition to β-PbO around 500 C. X-ray diffraction (XRD) data of the β-PbO (annealed a-PbO) film matched that of a β-PbO reference, while XAS data did not. This was attributed to the different depths of the sample volumes probed by the two techniques. Doppler-broadened positron annihilation spectroscopy (DBPAS) was conducted on several a-PbO samples synthesized under different conditions using the McMaster Variable-Energy Positron Beam (MVEPB) and the results were modelled using VEPFIT [6]. All samples were found to have a three-layer structure, with the bulk S-parameters between 0.4725 and 0.4753. The two other layers were contained within the first 300nm of the film and varied in thickness, diffusion length and S-parameter value. This confirms the suitability of DBPAS, as a sensitive probe of vacancy-type defects and the layer structure of thin films, to guide the optimization of a-PbO synthesis for photoconductive detectors. Work is underway to produce a series of samples which vary systematically in their synthesis conditions to establish synthesis-structure relationships.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".