Thickness dependence of electron transport in pure a-Se photoconductive films

Mortensen, Derek; Belev, George; Koughia, K.; Johanson, Robert E.; Kasap, S. O.

doi:10.1139/cjp-2013-0522

Cited by 8 publications

(7 citation statements)

References 22 publications

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“…11. For a-Se films of 100 m, as in the present case, we can assume that both e and e are reasonably thickness independent, and represent meaningful charge transport parameters.…”

Section: Resultsmentioning

confidence: 98%

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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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“…11. For a-Se films of 100 m, as in the present case, we can assume that both e and e are reasonably thickness independent, and represent meaningful charge transport parameters.…”

Section: Resultsmentioning

confidence: 98%

“…The hole transport in a-Se and stabilized a-Se is well known to be nondispersive [9,10] and the measured transport parameters do not depend on the film thickness. In the case of electron transport, the drift mobility, e , and the lifetime, e , are both relatively thickness independent for such thick films as shown in a paper in this issue [11].…”

Section: Methodsmentioning

confidence: 91%

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

FogalBud

KasapSafa

2014

Can. J. Phys.

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“…The latter becomes particularly striking when measured in samples with varying thicknesses at a constant electric field. In our previous paper [88] we investigated the dispersive transport of electrons in a-Se and showed how the electron mobility depends on sample thickness. The dependence became insignificant in thick samples ([100 lm).…”

Section: Dispersive Versus Non-dispersive Transportmentioning

confidence: 99%

“…The experimental TOF data used in this work have been extracted primarily from two main sources: (a) works carried out by the authors and reported previously in [41][42][43] and (b) TOF measurements carried out at Xerox by Pfister and Scher and reported in [44]. The two sets of samples are labelled S and X respectively.…”

Section: Introduction and Perspectivesmentioning

confidence: 99%

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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“…18,19 For photoconductive layers that are thicker than 50 lm, one can also simply use l e and s e to describe the electron transport. 20 These charge transport parameters in the product form l e s e and l h s h , called electron and hole ranges, control the collection efficiency of x-ray generated charges inside the photoconductor. 21,22 The charge collection efficiency, of course, is one the factors that control the x-ray sensitivity; but, nonetheless, an important factor.…”

Section: Introduction and Objectivesmentioning

confidence: 99%

X-ray irradiation induced changes in electron transport in stabilized a-Se photoconductors

Walornyj

Kasap

2013

Journal of Applied Physics

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We have examined the effect of high-dose x-ray irradiation on electron transport in stabilized amorphous selenium (a-Se) x-ray photoconductive films (of the type used in x-ray image detectors) by measuring the electron lifetime τe through interrupted-field time-of-flight experiments. X-ray induced effects have been examined through two types of experiments. In recovery experiments, the a-Se was preirradiated with and without an applied field (5 V/μm) during irradiation with sufficient dose (typically ∼20 Gy at 21 °C) to significantly reduce the electron lifetime by ∼50%, and then the recovery of the lifetime was monitored as a function of time at three different temperatures, 10 °C, 21 °C, and 35 °C. The lifetime recovery kinetics was exponential with a relaxation time τr that is thermally activated with an activation energy of 1.66 eV. τr is a few hours at 21 °C and only a few minutes at 35 °C. In experiments examining the irradiation induced effects, the a-Se film was repeatedly exposed to x-ray radiation and the changes in the drift mobility and lifetime were monitored as a function of accumulated dose D. There was no observable change in the drift mobility. At 21 °C, the concentration of x-ray induced deep traps (or capture centers), Nd, increases linearly with D (Nd ∼ D) whereas at 35 °C, the recovery process prevents a linear increase in Nd with D, and Nd saturates. In all cases, even under high dose irradiation (∼50 Gy), the lifetime was recoverable to its original equilibrium (pre-exposure) value within a few relaxation times.

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Thickness dependence of electron transport in pure a-Se photoconductive films

Cited by 8 publications

References 22 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

X-ray irradiation induced changes in electron transport in stabilized a-Se photoconductors

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