Structure–property relationships in xerographic selenium-alloy films

Schottmiller, J. C.

doi:10.1116/1.568676

Cited by 32 publications

(7 citation statements)

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“…The addition of Cl in the ppm amounts to a-Se:0.5%As is sufficient to enhance hole transport and thus render a-Se:0.5%As doped with Cl as a stable and useful photoconductor composition. The control of charge transport in a-Se by compensation was one of the key factors in its long history of use as a viable photoconductor [19].…”

Section: Feature Articlementioning

confidence: 99%

Amorphous selenium and its alloys from early xeroradiography to high resolution X‐ray image detectors and ultrasensitive imaging tubes

Kasap¹,

Frey²,

Belev³

et al. 2009

Physica Status Solidi (b)

160

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We describe the progress in the science and technology of stabilized a‐Se from its early use in xerography and xeroradiography to its present use in commercial modern flat panel X‐ray imagers and ultrasensitive video tubes which utilize impact ionization of drifting holes. Both electrons and holes can drift in stabilized a‐Se, which is a distinct advantage since X‐ray photogeneration of charge carriers occurs throughout the bulk of the photoconductive layer. An a‐Se photoconductor has to be operated at high fields to ensure that the photogeneration efficiency is sufficiently large to provide reasonable X‐ray sensitivity. However, at high fields, the dark current is unacceptably large in simple metal/a‐Se/metal devices, and special multilayer device structures need to be designed. The dark current decays with time and increases with the nominal applied field. The reduction of the dark current to a tolerable level was one of the key factors that lead to the commercialization of a‐Se X‐ray detectors. We discuss the origin of the dark current, and highlight some of the current challenges in the design of next generation detectors. We also discuss the origin of impact ionization in a‐Se, and its fruitful utilization in ultrasensitive imaging devices, including the Harpicon, which are likely to lead to new high detective quantum efficiency detectors.

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Section: Feature Articlementioning

confidence: 99%

Amorphous selenium and its alloys from early xeroradiography to high resolution X‐ray image detectors and ultrasensitive imaging tubes

Kasap¹,

Frey²,

Belev³

et al. 2009

Physica Status Solidi (b)

160

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“…Amorphous chalcogenide semiconductors are used in a variety of electronic and optoelectronic technologies, from phase change memories to multibillion dollar X-ray image detector applications, and especially in mammography [1][2][3][4][5]. Of particular interest among this class of materials is stabilised amorphous selenium (a-Se): a-Se that has been alloyed with small amounts of As (less than 1%, typically 0.2-0.5%) to stabilise the structure against crystallisation and doped with a small amount of halogen (Cl) in the ppm range to enhance hole transport [3,[6][7][8]. a-Se is currently the most successful X-ray photoconductor used in commercial flat panel mammographic X-ray image detectors [9].…”

Section: Introductionmentioning

confidence: 99%

“…It belongs to a special class of non-crystalline solids called amorphous chalcogenide semiconductors [1][2][3]. While other chalcogenide compositions such as Ag-doped Ge-Te in this group have shown potential for radiation detection [1][2][3][4][5][6][7][8][9][10][11][12], a-Se is the only commercialised X-ray photoconductor. Current interest lies not only in the use of a-Se but also in polycrystalline Se both for existing technologies and for novel devices which may underpin next generation systems [13][14][15][16][17][18][19].…”

Section: Introductionmentioning

confidence: 99%

Frequency- and time-resolved photocurrents in vacuum-deposited stabilised a-Se films: the role of valence alternation defects

Jacobs

Belev

Brookfield

et al. 2020

J Mater Sci: Mater Electron

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Stabilised amorphous selenium (a-Se) is currently used in the majority of direct conversion mammographic X-ray imaging detectors due to its X-ray photoconductivity and its ability to be uniformly deposited over large area TFT substrates by conventional vacuum deposition. We report experimental results on photocurrent spectroscopy (frequency-resolved spectroscopy (FRS) and single-time transients), on vacuum-deposited a-Se films. We show that all measured photocurrents depend critically on the relative time spent by the material in the light and in the dark. We identify that the observed pronounced variation in optical response depends on the density of trapped (optically injected) charge within 200 nm of the surface and show that it is the ratio of dark and light exposure time that controls the density of such charge. Our data confirm that the localised charge radically influences the photocurrent transient shape due to the effective screening of the applied field within 200 nm of the surface. The field modification occurs over the optical extinction depth and changes both the photogeneration process and the drift of carriers. Many aspects of our data carry the signature of known properties of valence alternation pair (VAP) defects, which control many properties of a-Se. Modelling in the time domain shows that light generation of VAPs followed by optically triggered VAP defect conversion can lead to near-surface charge imbalance, demonstrating that VAP defects can account for the unusual optical response. The stabilised a-Se films were deposited above the glass transition temperature of the alloy with composition a-Se:0.3% As doped with ppm Cl. Electron paramagnetic resonance measurements at temperatures down to 5 K did not detect any spin active defects, even under photoexcitation above band gap.

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“…The small amount of As-alloying stabilizes a-Se against crystallization. During the evaporation, fractionation leads to a deposited film that is rich in As near the surface and rich in Se near the substrate [ 3 , 4 , 5 ]. The As content is therefore not uniform throughout the film thickness.…”

Section: Introduction and Objectivesmentioning

confidence: 99%

The Effect of Fractionation during the Vacuum Deposition of Stabilized Amorphous Selenium Alloy Photoconductors on the Overall Charge Collection Efficiency

Kasap

2022

Sensors

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The general fabrication process for stabilized amorphous selenium (a-Se) detectors is vacuum deposition. The evaporant alloy is typically selenium alloyed with 0.3–0.5% As to stabilize it against crystallization. During the evaporation, fractionation leads to the formation of a deposited film that is rich in As near the surface and rich in Se near the substrate. The As content is invariably not uniform across the film thickness. This paper examines the effect of non-uniform As content on the charge collection efficiency (CE). The model for the actual CE calculation is based on the generalized CE equation under small signals; it involves the integration of the reciprocal range-field product (the schubweg) and the photogeneration profile. The data for the model input were extracted from the literature on the dependence of charge carrier drift mobilities and lifetimes on the As content in a-Se1−xAsx alloys to generate the spatial variation of hole and electron ranges across the photoconductor film. This range variation is then used to calculate the actual CE in the integral equation as a function of the applied field. The carrier ranges corresponding to the average composition in the film are also used in the standard CE equation under uniform ranges to examine whether one can simply use the average As content to calculate the CE. The standard equation is also used with ranges from the spatial average and average inverse. Errors are then compared and quantified from the use of various averages. The particular choice for averaging depends on the polarity of the radiation-receiving electrode and the spatial variation of the carrier ranges.

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Structure–property relationships in xerographic selenium-alloy films

Cited by 32 publications

References 0 publications

Amorphous selenium and its alloys from early xeroradiography to high resolution X‐ray image detectors and ultrasensitive imaging tubes

Amorphous selenium and its alloys from early xeroradiography to high resolution X‐ray image detectors and ultrasensitive imaging tubes

Frequency- and time-resolved photocurrents in vacuum-deposited stabilised a-Se films: the role of valence alternation defects

The Effect of Fractionation during the Vacuum Deposition of Stabilized Amorphous Selenium Alloy Photoconductors on the Overall Charge Collection Efficiency

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