Anomalous DC dark conductivity behaviour in a-Se films

Abstract

Thin films of amorphous selenium have been prepared by thermal evaporation. DC conductivity measurements were carried out on these films in the temperature range between 208 and 322 K. Above room temperature, the dark conductivity is thermally activated with activation energy E σ = 1.05 ± 0.08 eV. For temperatures below 285 K, an increase in the dark current is observed, which is interpreted in terms of a shift of the Fermi level that makes more states available for a hopping process.

Discrete defect levels in the amorphous selenium bandgap

“…Also included in Fig. 2 is the dark current of the sample (full dots) and its maximum near 240 K that was recently reported [9]. A similar maximum is now observed at somewhat higher temperatures in the photocurrent data obtained with 470 nm illumination at intensities of I 0 = 2 · 10 13 photons cm À2 s À1 , 0.05I 0 , and 0.0038I 0 .…”

Defect levels in the band gap of amorphous selenium

“…Fig. 5 summarizes the results from our SSPC and TOF measurements, with respect to the Fermi level position as obtained from bulk samples in [19] and the value of 1.95 eV for the optical gap at room temperature as determined in [20]. The values found for the C 0 1 level with respect to the valence band mobility edge, namely (0.38 ± 0.06) eV from the SSPC activation energy DE b and (0.45 ± 0.05) eV from the TOF hole emission current, are in fair agreement, given the sizable error margins involved.…”

“…The C 0 3 level is put (0.35 ± 0.06) eV above the Fermi level by the SSPC results and (0.53 ± 0.05) eV below the conduction band mobility edge by the TOF analysis. Using the Fermi energy E F = (1.05 ± 0.08) eV from [19] and the gap value E g = (1.95 ± 0.02) eV from [20], those values translate into distances of the C 0 3 level above the valence band of (1.40 ± 0.10), respectively (1.42 ± 0.05) eV. Again the agreement between the results from the SSPC and TOF techniques is good.…”

Steady-state photoconductivity in amorphous selenium glasses

“…the levels responsible for the TOF signals, these models predict positions roughly halfway between the band edge and the Fermi level [2][3][4][5]11]. Since the optical gap of a-Se is 2.1 eV [1], and the Fermi level lies in the middle of this gap [12], the theoretical estimates place the defect levels some 0.5 eV away from the bands, in rough agreement with the values deduced from the post-transit analysis. Nevertheless, further experimental confirmation remains desirable.…”