The x‐ray time of flight method for investigation of ghosting in amorphous selenium‐based flat panel medical x‐ray imagers

Rau, Andreas; Bakueva, L.; Rowlands, J. A.

doi:10.1118/1.2042248

Cited by 21 publications

(20 citation statements)

References 42 publications

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“…The process of carrier de-trapping from shallow traps is fast in comparison to the deep traps which tend to hold the carriers longer, thereby significantly impairing the temporal characteristics of the ma- terial. For x-ray application, the worst situation is when the carriers de-trapping becomes longer than the collection time of the X-ray signal and as such the de-trapped carriers contribute to the appearance of image artifacts known as 'ghosting' [7]. This limits the application of the PbO-based detectors in real-time imaging procedures.…”

Section: Introductionmentioning

confidence: 99%

Lead monoxide α-PbO: electronic properties and point defect formation

Berashevich

Semeniuk

Rubel

et al. 2013

J. Phys.: Condens. Matter

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The electronic properties of polycrystalline lead oxide consisting of a network of single-crystalline α-PbO platelets and the formation of the native point defects in α-PbO crystal lattice are studied using first principles calculations. The α-PbO lattice consists of coupled layers interaction between which is too low to produce high efficiency interlayer charge transfer. In practice, the polycrystalline nature of α-PbO causes the formation of lattice defects in such a high concentration that defectrelated conductivity becomes the dominant factor in the interlayer charge transition. We found that the formation energy for the O vacancies is low, such vacancies are occupied by two electrons in the zero charge state and tend to initiate the ionization interactions with the Pb vacancies. The vacancies introduce localized states in the band gap which can affect charge transport. The O vacancy forms a defect state at 1.03 eV above the valence band which can act as a deep trap for electrons, while the Pb vacancy forms a shallow trap for holes located just 0.1 eV above the valence band. Charge de-trapping from O vacancies can be accounted for the experimentally found dark current decay in ITO/PbO/Au structures.

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Section: Introductionmentioning

confidence: 99%

Lead monoxide α-PbO: electronic properties and point defect formation

Berashevich

Semeniuk

Rubel

et al. 2013

J. Phys.: Condens. Matter

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“…The large signal model is fitted to experimental data on collected charge versus charge generation rate characteristics of a-Se detectors for tomotherapy applications. 0e Ј Ϸ 310 s. 22 The normalized absorption depth ⌬ Ϸ 58, and therefore, the x-ray absorption is almost uniform across the photoconductor thickness. In Fig.…”

Section: Resultsmentioning

confidence: 99%

“…The necessary initial conditions are p͑x ,0͒ = n͑x ,0͒ =0. 22 Just after ͑or shortly after͒ x-ray exposure in infinitesimal time step ⌬t, the free hole concentration at x = 0 and the free electron concentration at x = 1 are zero since the carriers would have started drift.…”

Section: Finite Duration Pulse X-ray Excitationmentioning

confidence: 99%

The effects of large signals on charge collection in radiation detectors: Application to amorphous selenium detectors

Kabir¹,

Emelianova²,

Архипов³

et al. 2006

Journal of Applied Physics

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Analytical and numerical models for studying the effects of large signals on charge collection efficiency in radiation detectors are described by considering bimolecular recombination between drifting charge carriers, carrier trapping, and space charge effects. First, an analytical solution is obtained by assuming that the field remains relatively uniform. Then the continuity equations for both holes and electrons, and Poisson’s equation across the photoconductor for a short pulse irradiation are simultaneously solved by the finite difference method, without any assumptions. There is a very good agreement between the approximate analytical and numerical solutions for the charge collection efficiency. The numerical results are also compared with Monte Carlo simulations of carrier transport. The charge collection efficiency model is applied to amorphous selenium x-ray image detectors. The bimolecular-recombination-limited charge collection efficiency depends on the total photogenerated carrier density rather than on its spatial distribution. It is found that the recombination plays practically no role up to the total instantaneous carrier generation Q0 of 109EHPs∕cm2 at the applied electric field of 10V∕μm. The effect of recombination on charge collection increases with decreasing applied electric field strength. For high carrier generation (e.g., Q0 of 1012EHPs∕cm2 for an applied field of 10V∕μm), the electric field distributions vary widely across the photoconductor thickness during the travel of charge carriers towards the electrodes. However, the effect of bimolecular recombination on charge collection efficiency is almost independent of bias polarity and the field distribution. The numerical results are also compared with recent experimental data available in the literature.

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“…Although PbO is extensively used as a photoconductor, very little is known about the electronic properties and charge transport in this material. It is generally believed that transport in PbO is controlled by trapping in defects and that trapping is a cause of low mobility-time product [1]. However, the nature of defects is not fully understood.…”

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confidence: 99%

Self-passivation of vacancies in α -PbO

Berashevich¹,

Rowlands²,

Reznik³

2013

EPL

Self Cite

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We introduce a self-passivation of single lead (Pb) and oxygen (O) vacancies in the α-PbO compound through formation of a Pb-O vacancy pair. The preferential mechanism for pair formation involves initial development of the single Pb vacancy which, by weakening the covalent bonding, sets up the crystal lattice for an appearance of the O vacancy. Binding of the Pb and O vacancies occurs through the ionization interactions. Since no dangling bonds appear at the Pb-O pair site, this defect has a minor effect on the electronic properties. In such, vacancy self-passivation offers a practical way to improve the transport properties in thermally grown PbO layers.Polycrystalline Lead Oxide (PbO) is one of the few photoconductive materials with a long-more than 60 years -history of employment in imaging devices. Although PbO is extensively used as a photoconductor, very little is known about the electronic properties and charge transport in this material. It is generally believed that transport in PbO is controlled by trapping in defects and that trapping is a cause of low mobility-time product [1]. However, the nature of defects is not fully understood. Emerging applications of PbO in the direct conversion flat panel radiation medical imaging detectors revived the interest in studying defects in this material as defects can significantly affect imaging performance [2].Our recent modeling of the native point defects in α-PbO [3] has shown that thermally deposited PbO layers should contain a significant concentration of single vacancies due to their moderate formation energies. Single vacancies are amphoteric defects appearing in the different charge states. In the neutral charge state, the O vacancy (V O(0) )) holds two electrons and forms the deep donor level. The neutral Pb vacancy (V Pb(0) )) is filled with holes, and is a shallow acceptor. It was established that these vacancies prefer to appear doubly ionized, V Pb(2−) and V O(2+) , acting as compensating centres to each other. Indeed, two compensating vacancies have a lower formation energy than neutral ones, such asThe fact that vacancies prefer to appear in charge states suggests that we have to consider the ionization interactions between them and the formation of a neutral vacancy pair V Pb-O instead of two separate compensating vacancies. This should further decrease the formation energy, approximately speaking, the free energy required to insert a defect in a lattice is reduced by the energy liberated due to ionization of the donor and acceptor. Indeed, the formation of the defect complexes in favor over the single defects is often observed during material deposition [4]. In this work, we present our study of the formation mechanism of V Pb-O pair in PbO layers and its effect on the electronic properties. Analysis of the formation of VPb-O vacancy pair was performed using the density functional theory (DFT) available in the Wien2k package [5] which utilizes the fullpotential augmented plane-wave method. The PerdewBurke-Ernzerhof parameterization [6] of the gen...

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The x‐ray time of flight method for investigation of ghosting in amorphous selenium‐based flat panel medical x‐ray imagers

Cited by 21 publications

References 42 publications

Lead monoxide α-PbO: electronic properties and point defect formation

Lead monoxide α-PbO: electronic properties and point defect formation

The effects of large signals on charge collection in radiation detectors: Application to amorphous selenium detectors

Self-passivation of vacancies in α -PbO

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