Preparation and Transport Properties of Vacuum Evaporated Selenium Films

Tabak, M.; Hillegas, William J.

doi:10.1116/1.1316623

Cited by 50 publications

(16 citation statements)

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“…The measurements were on vacuum deposited films and in all cases, during evaporation, the substrates (usually oxidized Al plates) were heated to around 55-60°C, which is above the glass transition temperature, which is known to result in good hole transport [45]. The preparation of a-Se based films has been well described in the literature [32,[46][47][48], including in this journal [8,49]. In TOF experiments, a semitransparent top electrode is deposited onto the surface of a-Se, which is often gold.…”

Section: Introduction and Perspectivesmentioning

confidence: 99%

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: Introduction and Perspectivesmentioning

confidence: 99%

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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“…It was subsequently observed that this unfavourable effect could be countered through the addition of Cl in the 10-20 ppm range [5,6].…”

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confidence: 95%

Photocurrent measurements in chlorine‐doped amorphous selenium

Benkhedir

Mansour

Djefaflia

et al. 2009

Physica Status Solidi (b)

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Transient photocurrent (TPC) measurements have been carried out on evaporated a‐Se films doped with either 12.5 or 67 ppm of Cl and compared with analogous measurements on undoped a‐Se samples. The two doping levels lead to strikingly different resulting current traces. While the 12.5 ppm doping level weakens, but does not eliminate the signature of the two hole traps that are observed in the TPC signals of undoped a‐Se, no indication for those levels remains in the 67 ppm Cl‐doped samples. It is argued that suppression of the trap generated by the negative‐U D− centre agrees with the notion that they are being supplanted by doping‐induced Cl− centres. The enhanced density of D+ centres and accompanying cross‐linking of Se chains by those three‐fold coordinated sites leads in turn to the weakening and disappearance of the shallower hole trap ascribed to dihydral angle variations. Results from constant‐photocurrent and time‐of‐flight (TOF) experiments support this interpretation of the TPC observations.

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“…Table I lists the samples we have studied in this work, which included pure a-Se, sample A. B1 and B2 have the same composition (a-Se:0.2%As) but the material is from two different batches; batch to batch variation of electronic properties is well known for a-Se. 30 Samples C1 and C2 represent stabilized a-Se that has been doped with Cl and CaCl 2 in the ppm range.…”

Section: Methodsmentioning

confidence: 99%

X-ray irradiation induced changes in electron transport in stabilized a-Se photoconductors

Walornyj

Kasap

2013

Journal of Applied Physics

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We have examined the effect of high-dose x-ray irradiation on electron transport in stabilized amorphous selenium (a-Se) x-ray photoconductive films (of the type used in x-ray image detectors) by measuring the electron lifetime τe through interrupted-field time-of-flight experiments. X-ray induced effects have been examined through two types of experiments. In recovery experiments, the a-Se was preirradiated with and without an applied field (5 V/μm) during irradiation with sufficient dose (typically ∼20 Gy at 21 °C) to significantly reduce the electron lifetime by ∼50%, and then the recovery of the lifetime was monitored as a function of time at three different temperatures, 10 °C, 21 °C, and 35 °C. The lifetime recovery kinetics was exponential with a relaxation time τr that is thermally activated with an activation energy of 1.66 eV. τr is a few hours at 21 °C and only a few minutes at 35 °C. In experiments examining the irradiation induced effects, the a-Se film was repeatedly exposed to x-ray radiation and the changes in the drift mobility and lifetime were monitored as a function of accumulated dose D. There was no observable change in the drift mobility. At 21 °C, the concentration of x-ray induced deep traps (or capture centers), Nd, increases linearly with D (Nd ∼ D) whereas at 35 °C, the recovery process prevents a linear increase in Nd with D, and Nd saturates. In all cases, even under high dose irradiation (∼50 Gy), the lifetime was recoverable to its original equilibrium (pre-exposure) value within a few relaxation times.

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Preparation and Transport Properties of Vacuum Evaporated Selenium Films

Cited by 50 publications

References 0 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

Photocurrent measurements in chlorine‐doped amorphous selenium

X-ray irradiation induced changes in electron transport in stabilized a-Se photoconductors

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