Frequency- and time-resolved photocurrents in vacuum-deposited stabilised a-Se films: the role of valence alternation de
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Frequency‑ and time‑resolved photocurrents in vacuum‑deposited stabilised a‑Se films: the role of valence alternation defects Janet Jacobs1 · George Belev2 · Adam Brookfield1,3 · Floriana Tuna1,3 · Safa Kasap4 · Richard J. Curry1 Received: 8 June 2020 / Accepted: 25 July 2020 / Published online: 19 August 2020 © The Author(s) 2020
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.
1 Introduction 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 [1–5]. Of particular interest among this class of materials is stabilised amorphous selenium (a-Se): a-Se * Janet Jacobs [email protected] * Richard J. Curry [email protected] 1
Department of Electronic and Electrical Engineering, Photon Science Institute, University of Manchester, Manchester, UK
2
Saskatchewan Structural Sciences Center, University of Saskatchewan, Saskatoon, Canada
3
Department of Chemistry, University of Manchester, Manchester, UK
4
Department of Electrical and Computer Engineering, University of Saskatchewan, Saskatoon, Canada
that has been alloyed with small amounts of As (less than 1%, typi
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