Electronic density of states in amorphous selenium

Abstract

Steady-state and transient photoconductivity methods are used to investigate the electronic density of states in evaporated layers of amorphous selenium. From the temperature dependence of the steady-state photocurrent and, independently, from an analysis of the post-transit currents of time-of-flight transients, energy levels in the gap at 1.43 ± 0.02 eV and 0.40 ± 0.02 eV above the valence band have been determined for the occupied state of the negative-U centres. An absorption band around 1.50 eV is seen in the spectral distribution of the photocurrent. The distribution of tail states may-to first approximationbe described by a steep exponential with a characteristic width of ∼24 meV at the valence band and a more steeply declining functional of similar width at the conduction band.

Reduction of the dark current in stabilized a-Se based X-ray detectors

“…At the same time, cold deposition does not increase significantly the concentration of deep electron traps. Many attempts have been made to relate the electron and hole traps to various structural defects in a-Se films (e.g., [13,14]), the disorder in the amorphous structure including chain ends [15] and the generation of monoclinic nanocrystal inclusions at low substrate temperatures [12]; but the exact nature of these traps still remains unknown and published work is sometimes highly controversial [11]. The latter makes it particularly difficult to formulate an exact physical model that can explain all experimental observations reported in this work.…”

Photocurrent measurements in chlorine‐doped amorphous selenium

“…Given that measurements of a-Se photoconductivity in both steadystate and transient modes have been able to clarify the DOS structure of the undoped compound [7][8][9][10] Fig. 1.…”

Effect of As alloying on the valence band tail states in a‐Se

“…Two shallow neutral defect levels, one on either side of the gap, are located about 0.2 eV from the band edges; the remaining two are charged ones at about 0.45 eV from the band edges. While these latter ones have been shown to correspond to the thermal transitions involving the a-Se negative-U centers [8], the former ones can be linked to local distortions of the dihedral bonding angle along a Se chain [12]. In a recent study [13] it was seen that a-Se doping with 12.5 ppm of Cl did not disturb that defect structure, but that 67 ppm of Cl did reduce the density of the shallow centers below observability.…”

“…However, there is no experimental evidence for the actual existence of such extra hole trap, nor is there much information available on the precise changes in the density of localized density of states (DOS) that reduces the hole lifetime in As-doped aSe. In the last few years, the DOS in the undoped a-Se has been studied and clarified using, mainly, the photoconductivity in both steady-state and transient modes [8][9][10][11]. The bandgap of a-Se is around 2.0 eV, and the Fermi level is pinned in the middle of the gap.…”

“…In this work we will explore the changes that As doping introduces in the a-Se DOS and try to arrive at a reasoned explanation for the observed hole lifetime decrease. Further details on experimental techniques and instrumentation may be found in the references [8][9][10][11] cited above. Figure 1 shows a set of TPC traces with 230 V applied, measured at different temperatures.…”