Transient photoconductivity measurements on halogenated a-Se1−x Tex photoconductors

Abstract

Time-of-flight (TOF) transient photoconductivity measurements, as functions of temperature and applied field, were carried out on chlorinated a-Se1−x Tex (x < 0.1) chalcogenide semiconductor films to study the nature of charge transport in these commercially important xerographic alloys. The films were prepared by conventional vacuum-deposition techniques and were typically 60–70 μm thick. The TOF hole drift mobility was found to follow a power-law field dependence of the manner μh ~ Fn with the index n increasing with Te and Cl additions as well with decreasing temperature. At the highest fields the drift mobility increased sharply with the applied field. The strong field dependence became more pronounced as the temperature was decreased. TOF drift mobility measurements, as a function of temperature at various applied fields, indicated a thermally activated hole-transport process with a well-defined but field-dependent mobility activation energy, which exhibited an approximately linear field dependence, similar to the mobility activation energy reported for doped a-As2Se3. The zero-field extrapolated activation energy, Eμ0, was about ~ 0.43 eV for the whole range of compositions investigated from x ≈ 0.02 to 0.09, and for the Cl additions in the ~ 100 at. ppm range. Two possible interpretations are discussed. The first involves Te-or Cl-introduced shallow defects that result in a distinct feature on the density of localized states peaking at around ~ 0.43 eV above Ev. It is assumed that the the conductivity mobility in this model remains to represent microscopic transport via extended states as in pure a-Se, i.e., μ0 ~ T−n with n ≈ 1. The other possibility is that increased disorder, as a result of alloying with Te or Cl addition, causes the basic microscopic mobility to become thermally activated owing to hopping in the band-tail states with a hop activation energy of ~ 0.14 eV. It is assumed that the energy location of the shallow-hole traps at ~ 0.29 eV above Ev remains reasonably unaffected, but the energy width and the concentration of native defects controlling the mobility are modified. The latter model is essentially a trap-controlled hopping transport. The present TOF measurements could not conclusively infer one model over the other. There was no observable electron transport in chlorinated a-Se1−x Tex films, whereas in unhalogenated a-Se1−x Tex films the electron-drift mobility decreased with the Te content and exhibited a stronger power-law field dependence. The TOF electron photocurrents were highly dispersive and the drift mobility – temperature data indicated a thermally activated charge-transport process with a well-defined but field-dependent mobility activation energy. The zero-field activation energy was ~ 0.49 eV, which is ~ 0.14 eV higher than that for electron transport in pure a-Se. The two models discussed for hole transport may also be applied to electron transport. It is suggested that the Te addition modifies the already present defect population leaving their energy location below Ec unchanged so that the increase in the mobility activation energy of ~ 0.14 eV corresponds to the hop activation energy of electrons in the band-tail localized states.