X-ray induced hole trapping in electroradiographic plates

Kasap, S. O.; Aiyah, Viswanath; Baillie, A.; Leiga, Algird G.

doi:10.1063/1.347648

Cited by 32 publications

(15 citation statements)

References 31 publications

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“…We note that the measurements we report herein obviously apply to a-Se films that have not been exposed to any radiation. The irradiation of stabilized a-Se photoconductors with heavy doses of X-rays leads to the reduction of both hole and electron lifetimes, but the drift mobilities are unchanged [23,24]. Both lifetimes recover with time through relaxation processes to their equilibrium values.…”

Section: Resultsmentioning

confidence: 96%

Temperature dependence of charge carrier ranges in stabilized a-Se photoconductors

FogalBud

KasapSafa

2014

Can. J. Phys.

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Using time-of-flight (TOF) and interrupted-field TOF we have measured the electron and hole drift mobilities, e and h , lifetimes, e and h , and hence carrier ranges, e e and h h , in stabilized a-Se as a function of temperature from −25 to 45°C. Above room temperature, e e and h h show a slight increase with temperature and peak around 31 and 35°C, respectively. The e e at 35°C and h h at 40°C are approximately the same as room temperature values. Below room temperature, down to −25°C, both e and h decrease with decreasing temperature with activation energies E e and E h that are 0.38 and 0.21 eV. The electron and hole lifetimes, e and h , are also thermally activated but they increase with decreasing temperature with activation energies E e and E h that are 0.31 and 0.18 eV. These are quite close to corresponding activation energies for the drift mobilities within experimental errors. The electron and hole ranges, e e and h h , therefore exhibit only a weak temperature dependence, both increase slightly with temperature. These observations are consistent with shallow trap controlled transport in the presence of a set of deep traps. The X-ray sensitivity of a photoconductor, among other factors, depends on the charge collection efficiency, which depends on electron and hole ranges, e e and h h . Because these carrier ranges do not show any significant temperature dependence, one can conclude that the X-ray sensitivity of stabilized a-Se over the temperature range from −25 to 40°C should not be affected by changes in the collection efficiency. Résumé: En utilisant des techniques de temps de vol (TOF), non et interrompu, nous mesurons les mobilités de dérive des électrons et des trous, e et h , les temps de vie, e et h , et donc la portée des porteurs de charge, e e et h h , dans du a-Se stabilisé en fonction de la température, de −25 à 45°C. Au dessus de la température de la salle, e e et h h augmentent légèrement avec la température avec un maximum à 31 et 35°C respectivement. On voit que e e à 35°C et h h à 40°C ont même valeur qu'à la température de la salle. Sous la température de la salle jusqu'à −25°C, à la fois e et h diminuent avec la température, avec des énergies d'activation E e ϭ 0.38 eV et E h ϭ 0.21 eV. Par contre, les temps de vie e et h croissent avec une baisse de la température, avec des énergies d'activation E e = 0.31 eV et E h = 0.18 eV. Aux erreurs de mesure près, ces énergies sont proches de celles pour la mobilité. Il s'ensuit que les portées e e et h h n'ont qu'une faible dépendance en température, les deux augmentant légèrement avec la température. Ces résultats sont cohérents avec ceux obtenus par transport contrôlé par pièges peu profonds en présence de pièges profonds. La sensibilité X d'un photoconducteur, parmi d'autres facteurs, dépend de l'efficacité à collecter des charges, qui, elle, dépend des portées e e et h h . Puisque ces portées ne montrent pas de dépendance significative en température, nous pouvons conclure que la sensibilité X du a-Se stabilisé ne devrait pas être affec...

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Section: Resultsmentioning

confidence: 96%

Temperature dependence of charge carrier ranges in stabilized a-Se photoconductors

FogalBud

KasapSafa

2014

Can. J. Phys.

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“…Xerographic measurements can provide very useful information on the DOS but their time scale means that one could only obtain information on deep states in the mobility gap [37,38]. Further, xerographic experiments are more difficult to set up and cannot be used at low temperatures.…”

Section: Introduction and Perspectivesmentioning

confidence: 97%

Charge transport in pure and stabilized amorphous selenium: re-examination of the density of states distribution in the mobility gap and the role of defects

Kasap

Koughia

Berashevich

et al. 2015

J Mater Sci: Mater Electron

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We re-examine electron and hole transport in pure and stabilized amorphous selenium (a-Se) and attempt to construct a DOS distribution in the mobility gap below E c and above E v based on time-of-flight (TOF) transient photoconductivity measurements. First, we review the current status of a-Se, its recent use in commercial X-ray detectors, and the scientific information that is available in terms of its density of states (DOS). We review and describe a convenient multiple trapping transport mechanism to calculate the TOF transient photocurrents in a system with a distributed DOS in the mobility gap and invoke a number of assumptions that allow the photocurrent to be calculated as an inverse Laplace transform. The assumptions behind the model are critically examined. Then, by comparing the calculated TOF photocurrent shapes directly with experimental waveforms, the DOS distribution has been extracted in a-Se in the vicinity of conduction and valence bands (below E c and above E v ). Below the conduction band, the DOS distribution seems to demonstrate three peaks at around E c -0.30 eV, E c -0.45 eV and below E c -0.55 eV. First principles atomistic modeling provided insight into the origins of these peaks and the peaks have been assigned to certain valence-alteration pairs (VAPs) appearing in different configurations. In contrast to the conduction band vicinity, the DOS distribution above the valence band seems to be almost totally featureless. Within the range of experimental uncertainties, a featureless DOS below E v down to 0.55 eV seems to be able to explain a vast range of data, that is, it can explain hole transport not only as a function of the electric field and temperature down to 123 K, but also the thickness dependence of the mobility over two orders of magnitude. At room temperature, the hole transport is nondispersive but at temperatures below *200 K, it becomes dispersive exactly along the lines argued by Pfister and Scher (Adv Phys 27:747, 1978), and the mobility evinces a clear thickness dependence. The first principles calculations yield two defect levels related to VAP type defects in this region of the mobility gap but the valence band tail is so broad that it masks these peaks. This seeming asymmetry between the conduction and valence bands may be related to their physical origins: the conduction band arises from antibonding states and valence band is due to interacting lone pairs.

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“…͑3͒ The possibility of x-ray induced structural changes to a-Se creating additional neutral trapping centers has also been discussed. 22 As a result, the first mechanism is enhanced. Elucidating the contribution of each of these processes requires a TOF method.…”

Section: Introductionmentioning

confidence: 96%

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

2005

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Amorphous selenium (a-Se) based real-time flat-panel imagers (FPIs) are finding their way into the digital radiology department because they offer the practical advantages of digital x-ray imaging combined with an image quality that equals or outperforms that of conventional systems. The temporal imaging characteristics of FPIs can be affected by ghosting (i.e., radiation-induced changes of sensitivity) when the dose to the detector is high (e.g., portal imaging and mammography) or the images are acquired at a high frame rate (e.g., fluoroscopy). In this paper, the x-ray time-of-flight (TOF) method is introduced as a tool for the investigation of ghosting in a-Se photoconductor layers. The method consists of irradiating layers of a-Se with short x-ray pulses. From the current generated in the a-Se layer, ghosting is quantified and the ghosting parameters (charge carrier generation rate and carrier lifetimes and mobilities) are assessed. The x-ray TOF method is novel in that (1) x-ray sensitivity (S) and ghosting parameters can be measured simultaneously, (2) the transport of both holes and electrons can be isolated, and (3) the method is applicable to the practical a-Se layer structure with blocking contacts used in FPIs. The x-ray TOF method was applied to an analysis of ghosting in a-Se photoconductor layers under portal imaging conditions, i.e., 1 mm thick a-Se layers, biased at 5 V/ microm, were irradiated using a 6 MV LINAC x-ray beam to a total dose (ghosting dose) of 30 Gy. The initial sensitivity (S0) of the a-Se layers was 63 +/- 2 nC cm(-2) cGy(-1). It was found that S decreases to 30% of S0 after a ghosting dose of 5 Gy and to 21% after 30 Gy at which point no further change in S occurs. At an x-ray intensity of 22 Gy/s (instantaneous dose rate during a LINAC x-ray pulse), the charge carrier generation rate was 1.25 +/- 0.1 x 10(22) ehp m(-3) s(-1) and, to a first approximation, independent of the ghosting dose. However, both hole and electron transport showed a strong dependence on the ghosting dose: hole transport decreased by 61%, electron transport by up to approximately 80%. Therefore, degradation of both hole and electron transport due to the recombination of mobile charge carriers with trapped carriers (of opposite polarity) were identified as the main cause of ghosting in this study.

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X-ray induced hole trapping in electroradiographic plates

Cited by 32 publications

References 31 publications

Temperature dependence of charge carrier ranges in stabilized a-Se photoconductors

Temperature dependence of charge carrier ranges in stabilized a-Se photoconductors

Charge transport in pure and stabilized amorphous selenium: re-examination of the density of states distribution in the mobility gap and the role of defects

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

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