Defect levels in the band gap of amorphous selenium

Benkhedir, M. L.; Aida, M.S.; Adriaenssens, G.J.

doi:10.1016/j.jnoncrysol.2004.08.062

Cited by 27 publications

(25 citation statements)

References 21 publications

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“…In the next section, we discuss the possible origins of these features. It should be noted that the peak at 0.42-0.48 eV below E c in this work is not too far away from location of the peak at 0.53-0.6 eV below E c , that had been predicted from posttransit current measurements by Adriaenssens's group in 1999 [55,70]. The peak at 0.27-0.33 eV below E c , the same peak uncovered in Ref.…”

Section: Dos In the Vicinity Of Conduction Band As Derived From Tofsupporting

confidence: 82%

Charge transport in pure and stabilized amorphous selenium: re-examination of the density of states distribution in the mobility gap and the role of defects

Kasap

Koughia

Berashevich

et al. 2015

J Mater Sci: Mater Electron

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We re-examine electron and hole transport in pure and stabilized amorphous selenium (a-Se) and attempt to construct a DOS distribution in the mobility gap below E c and above E v based on time-of-flight (TOF) transient photoconductivity measurements. First, we review the current status of a-Se, its recent use in commercial X-ray detectors, and the scientific information that is available in terms of its density of states (DOS). We review and describe a convenient multiple trapping transport mechanism to calculate the TOF transient photocurrents in a system with a distributed DOS in the mobility gap and invoke a number of assumptions that allow the photocurrent to be calculated as an inverse Laplace transform. The assumptions behind the model are critically examined. Then, by comparing the calculated TOF photocurrent shapes directly with experimental waveforms, the DOS distribution has been extracted in a-Se in the vicinity of conduction and valence bands (below E c and above E v ). Below the conduction band, the DOS distribution seems to demonstrate three peaks at around E c -0.30 eV, E c -0.45 eV and below E c -0.55 eV. First principles atomistic modeling provided insight into the origins of these peaks and the peaks have been assigned to certain valence-alteration pairs (VAPs) appearing in different configurations. In contrast to the conduction band vicinity, the DOS distribution above the valence band seems to be almost totally featureless. Within the range of experimental uncertainties, a featureless DOS below E v down to 0.55 eV seems to be able to explain a vast range of data, that is, it can explain hole transport not only as a function of the electric field and temperature down to 123 K, but also the thickness dependence of the mobility over two orders of magnitude. At room temperature, the hole transport is nondispersive but at temperatures below *200 K, it becomes dispersive exactly along the lines argued by Pfister and Scher (Adv Phys 27:747, 1978), and the mobility evinces a clear thickness dependence. The first principles calculations yield two defect levels related to VAP type defects in this region of the mobility gap but the valence band tail is so broad that it masks these peaks. This seeming asymmetry between the conduction and valence bands may be related to their physical origins: the conduction band arises from antibonding states and valence band is due to interacting lone pairs.

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Section: Dos In the Vicinity Of Conduction Band As Derived From Tofsupporting

confidence: 82%

Charge transport in pure and stabilized amorphous selenium: re-examination of the density of states distribution in the mobility gap and the role of defects

Kasap

Koughia

Berashevich

et al. 2015

J Mater Sci: Mater Electron

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“…-5 eV/(V/m) 1/2 , which is in good agreement with the value deduced in [18] for the D − level in undoped a-Se. 3) The dip around 10 −4 s in the TPC currents from 0.5% As-doped samples is rather pronounced in comparison with the corresponding features in other a-Se samples.…”

Section: Analysis and Discussionsupporting

confidence: 90%

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

Benkhedir

Djefaflia

Mansour

et al. 2010

Phys. Status Solidi (c)

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Transient photocurrent (TPC) measurements have been carried out on evaporated a‐Se doped with either 0.2% or 0.5% of As and compared with analogous measurements on undoped and Cl doped a‐Se. These measurements show that As doping of a‐Se suppresses the shallow defects linked to the dihedral angle variations in a‐Se chains, and makes the valence band tail states distribution steeper. These changes in the a‐Se density of states increase the relative importance of the deep defect levels in As‐doped a‐Se and thus explain the widely reported hole lifetime shortening with As addition. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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“…1 just beyond 10 À3 s. While the signature of this center is not as clear on the I(t) traces as the one from the shallow trap, use of the (I Â t) À1 conversions leads to estimates of E Ã D À ¼ ð0:37 AE 0:02Þ eV and m D À = (2.6 ± 1.5) Â 10 11 Hz for the parameters of the D center. This E Ã D À value agrees well with the post-transit TOF or SSPC results for this level reported in earlier reports [5,6]. (The values reported on the basis of post-transit experiments are somewhat larger due to the ad-hoc use of m = 10 12 Hz in their determination.…”

Section: Transient Photocurrents and Their Analysissupporting

confidence: 90%

“…However, given that TOF involves the primary photocurrent only, the energy position of the deeper defect can be identified without difficulty through a TOF post-transit analysis. While the observation of Poole-Frenkel shifts [6] revealed the charged nature of the defects at 0.4-0.5 eV from the bands, the deduction of an attempt-to-escape frequency of 2.6 Â 10 11 Hz for the D center confirms that result. Indeed, detailed balance couples a large attempt frequency to a large capture cross-section, the latter one being characteristic for charged sites.…”

Section: Discussionmentioning

confidence: 58%

“…Since the negative-U defects are the primary recombination centers in the chalcogenides, their energy position in the gap can be deduced from the temperature dependence of the steady-state photoconductivity [5]. Moreover, analysis of the photocurrent transients in the post-transit regime of the time-of-flight (TOF) experiment has confirmed the presence and energy position of those gap states in both the valence band and the conduction band tail [6]. The charged nature of these TOF-resolved defects was evidenced by the observation of a Poole-Frenkel dependence of the resolved energy position on the applied electric field.…”

Section: Introductionmentioning

confidence: 94%

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