Abstract
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.
Highlights
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]
We suggest that valence alternation pair (VAP) defects, and the interconversion between the two stable VAP configurations can explain our experimental data without invoking any stable populations of the various neutral overcoordinated and hypervalent defects that have been proposed to exist [31]
C30 configuration We suggest that leads to ionisation and generation this does not occur in the case of of an intimate VAPs (IVAPs) because the weakly bound anti-bonding σ* electron tahteCp30 omsiatyivbeeCs3+pacthiaalrlgye.stParboilviisdeeddbtyhethdewcelollsetimpreoxoifmtihtye of σ*
Summary
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]. Street and Mott [25] demonstrated that the same defects, labeled as dangling bonds, exhibited negative effective lone pair electron correlation energies (negative Hubbard energy U) due to lattice distortion following a change in charge state, helping to stabilise these gap states. The calculations reveal that, during optical excitation, strong hole trapping significantly modifies the electric field distribution in the near-surface region beneath the illuminated face of the solid This field modification, in turn, changes both the balance of charge exchange, photogeneration and transport processes occurring in the near-surface region significantly beyond the optical extinction depth. By taking this into account we are able to explain what at first sight appear to be “anomalous” transient shapes and FRS data “distortion” at low frequencies. We have carried out electron paramagnetic resonance (EPR) measurements down to low (4 K) cryogenic temperatures under intense laser light irradiation to test for the presence of light-induced spin active defects, but found no evidence for these
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
More From: Journal of Materials Science: Materials in Electronics
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.