Density of states in pure and As doped amorphous selenium determined from transient photoconductivity using Laplace-transform method

Serdouk, Fadila; Benkhedir, M. L.

doi:10.1016/j.physb.2014.12.002

Cited by 13 publications

(12 citation statements)

References 20 publications

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“…It is possible that DOS is completely dominated by the developed exponential valence band tail which disguises the C À 1 related DOS peaks. More recently, DOS distributions with features have been based on the analysis of transient photocurrents measured on samples with coplanar electrodes [78]. However, in all cases in the latter work, the potential peaks in DOS distributions are significantly dispersed and only slightly overcome the featureless distribution similar to the model DOS shown in Fig.…”

Section: Dos In the Vicinity Of Valence-band: Tof And Modelingmentioning

confidence: 91%

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 Valence-band: Tof And Modelingmentioning

confidence: 91%

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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“…Later Serdouk and Benkhedir confirmed these results using the high resolution Laplace transform method [9]. A good agreement between the measured and the simulated currents was found.…”

Section: Introductionmentioning

confidence: 65%

“…The addition of low percentages of As makes it possible to im- * corresponding author; e-mail: iserdouk@yahoo.fr prove several properties but causes large changes in the hole lifetime. It will be important to use the treatment of TPC traces using high resolution Laplace transform method to calculate the lifetime of holes in agreement with the found DOS above the valence band, and to compare the found results with experimental data, to check the validity of our proposed DOS model in the lower half of the mobility gap [9].…”

Section: Introductionmentioning

confidence: 97%

Determination of the Lifetime in As Doped a-Se from Transient Photocurrents using Laplace Transform Technique

Serdouk

Benkhedir

2018

Acta Phys. Pol. A

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The identification, in our earlier papers, of the discrete energy levels in stabilized a-Se indicate that doping with As in the limit of 0.2-0.5% suppresses the shallow defect and increases the density of the deep one. It may be added at this point that an undesirable decrease in the hole lifetime τ was found to accompany the introduction of As in the a-Se lattice, which was attributed to a new hole trap. More analysis is clearly needed to resolve these questions. For these reasons we have developed a technique to calculate the lifetime. The obtained results confirm the image of the density of states near the valance band.

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“…Some experiments do indeed seem to support this result. For example, DOS distributions with peaks have been drerived from the analysis of transient photocurrents measured on coplanarly electroded samples . However, in all cases in the latter work, the potential peaks in DOS distributions are significantly dispersed and only slightly above the featureless distribution similar to the model DOS shown in Fig.…”

Section: Introduction: the Density Of States Of A‐sementioning

confidence: 91%

“…Some researchers derive a clear peak in the DOS states with energy into the mobility gap. The interpretation of steady state photoconductivity [15] and transient photocurrents [13] measured on samples with coplanar electrodes suggest a clear peak. However, the latter results may be influenced by holes diffusing to the surface of a-Se, which is well known to provide deep hole traps [16].…”

mentioning

confidence: 94%

Density of localized state distribution near the valence band in stabilized a‐Se using interrupted field time of flight measurements with long interruption times

Koughia

Reznik

Allen

et al. 2016

Physica Status Solidi (a)

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In the present paper, we have investigated hole transport in stabilized a‐Se films using interrupted‐field‐time‐of‐flight (IFTOF) experiments with interruption times up to 600 μs. A distinct advantage of IFTOF measurements is that one can monitor the average “free” hole concentration p(t) (= p(x,t) averaged over the thickness of the sample L) at a given location x1 in the sample inasmuch as the applied field is removed at a certain time t1 for an interruption period of ti. At time t1 + ti, the field is reapplied and the recovered photocurrent i2 at t = t1 + ti is measured with respect to the original photocurrent i1 at t = t1. The experimental results are interpreted by the comparison of experimental photocurrent transients with numerical and Monte‐Carlo simulations of multiple trapping hole transport for different DOS models; and their agreement with a featureless DOS distribution in the vicinity of valence band is confirmed. The examination of IFTOF experiments (i2/i1 as a function of ti) for very long interruption times puts the energy position of deep traps controlling the hole lifetime to be above 0.65 eV from Ev. The results are critically discussed in view of past experiments on the DOS in a‐Se and recent structural modeling work on defects in the structure. Further, the work in this article vindicates the use of shallow trap controlled mobility and a single deep trapping time in the modeling of hole transport in recently developed a‐Se based X‐ray detectors; that is we only need shallow and deep traps to model the detector performance.

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Density of states in pure and As doped amorphous selenium determined from transient photoconductivity using Laplace-transform method

Cited by 13 publications

References 20 publications

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

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

Determination of the Lifetime in As Doped a-Se from Transient Photocurrents using Laplace Transform Technique

Density of localized state distribution near the valence band in stabilized a‐Se using interrupted field time of flight measurements with long interruption times

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