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

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.

Glassy-like Transients in Semiconductor Nanomaterials

“…During the excitation, I(t) has a concave shape, while during the decay relaxation, it becomes convex. This basic behavior of the I(t)s, which is quite typical of photoconductors [ 41 , 99 , 100 ], was previously observed in many nano- [ 42 , 58 , 72 , 85 , 99 ] and non-nanosystems [ 43 , 44 , 84 , 101 , 102 , 103 ]. Usually, the corresponding long relaxation decays are described by a stretched exponential, as given in Equation (2) [ 44 ].…”

Synergistic Interaction of MoS2 Nanoflakes on La2Zr2O7 Nanofibers for Improving Photoelectrochemical Nitrogen Reduction

“…The photocurrent versus time plot of the individual samples (LZO, MoS 2 @LZO, and MoS 2 ) shows both a rise and decay of the photocurrents, as shown in Figure d. The photocurrent responses of the prepared samples under transient light irradiation reveal higher photogenerated charge carriers for 60 wt % MoS 2 @LZO compared to bare LZO and MoS 2 . The change in the peak currents in the spike region of the curves shows the varied capacitive contribution to the space charge region.…”

“…The photocurrent responses of the prepared samples under transient light irradiation reveal higher photogenerated charge carriers for 60 wt % MoS 2 @LZO compared to bare LZO and MoS 2 . 54 The change in the peak currents in the spike region of the curves shows the varied capacitive contribution to the space charge region. However, the observed redox current maintains a constant value indicating significant charge transfer properties of the 60 wt % MoS 2 @LZO.…”

Inherent electron and hole trapping in amorphous phase-change memory materials: Ge2Sb2Te5

“…66 Such IVAPs have also been suggested experimentally to serve as electron and hole recombination centres in glassy Se. 67 The simulated trapping centres have a wide range of ionization energies, including relatively low values, suggesting that thermal release of the trapped electrons and holes would be feasible during the memory device operation, and also attainable over a long period of time. This observation, together with the calculated energy levels of the traps in the band gap of the glass (one-electron traps close to the top of the valence band; one-hole traps around midgap) supports a potential electronic mechanism for resistance drift in amorphous phase-change memory materials, where de-trapping of charge carriers and subsequent recombination is involved, following electron/hole injection during the RESET voltage pulse.…”

“…66 Such IVAPs have also been suggested experimentally to serve as electron and hole recombination centres in glassy Se. 67…”