The Urbach tail in amorphous semiconductors

Kostadinov, I.Z.

doi:10.1088/0022-3719/10/9/008

Cited by 21 publications

(7 citation statements)

References 11 publications

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“…Only upon averaging over many realizations, these localized states should yield a relatively smooth spectrum, which is expected to be exponential. [7][8][9][10][11][12][13][14][15][16] This is indeed born out by the present data. In view of limited statistics and modest sample sizes, however, it is difficult to determine, based on the spectra alone, which states comprising the individual spectra in Fig.…”

Section: Salient Features Of the Density Of States: Mobility Bands An...supporting

confidence: 84%

“…The effective band edge turns out to be nearly exponential and is often called the Urbach tail. [7][8][9][10][11][12][13][14][15][16] Now in amorphous materials, there is no underlying long-range order even with regard to vibrationally-averaged atomic positions.…”

Section: Introductionmentioning

confidence: 99%

“…This is because optical excitations are much faster than nuclear motions, implying that, effectively, electrons are always subject to an aperiodic Born–Oppenheimer potential. The effective band edge turns out to be nearly exponential and is often called the Urbach tail − or Lifshitz tail . Now in amorphous materials, there is no underlying long-range order even with regard to vibrationally averaged atomic positions.…”

Section: Introductionmentioning

confidence: 99%

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Structural Origin of the Midgap Electronic States and the Urbach Tail in Pnictogen-Chalcogenide Glasses

2018

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We determine the electronic density of states for computationally generated bulk samples of amorphous chalcogenide alloys As Se. The samples were generated using a structure-building algorithm reported recently by us. Several key features of the calculated density of states are in good agreement with experiment: The trend of the mobility gap with arsenic content is reproduced. The sample-to-sample variation in the energies of states near the mobility gap is quantitatively consistent with the width of the Urbach tail in the optical edge observed in experiment. Most importantly, our samples consistently exhibit very deep-lying midgap electronic states that are delocalized significantly more than what would be expected for a deep impurity or defect state; the delocalization is highly anisotropic. These properties are consistent with those of the topological midgap electronic states that have been proposed by Zhugayevych and Lubchenko as an explanation for several puzzling optoelectronic anomalies observed in the chalcogenides, including light-induced midgap absorption and electron spin resonance signal, and anomalous photoluminescence. In a complement to the traditional view of the Urbach states as a generic consequence of disorder in atomic positions, the present results suggest these states can be also thought of as intimate pairs of topological midgap states that cannot recombine because of disorder. Finally, samples with an odd number of electrons exhibit neutral, spin 1/2 midgap states as well as polaron-like configurations that consist of a charge carrier bound to an intimate pair of midgap states; the polaron's identity, electron or hole, depends on the preparation protocol of the sample.

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Section: Salient Features Of the Density Of States: Mobility Bands An...supporting

confidence: 84%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Structural Origin of the Midgap Electronic States and the Urbach Tail in Pnictogen-Chalcogenide Glasses

2018

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“…97 According to Ref. 132, not only do these computationally generated samples appear to host both the mobility-band and the associated Urbach tail [133][134][135][136] of localised states, but they also exhibit just the type of topological states that were predicted by the ZL picture. The electronic spectrum of the sample and the wavefunction of the midgap state are exemplified in Fig.…”

Section: Collisional Transport Activated Transportmentioning

confidence: 99%

Low-temperature anomalies in disordered solids: a cold case of contested relics?

Lubchenko

2018

Advances in Physics: X

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Amorphous solids manifest puzzling effects of mysterious degrees of freedom that give rise to a heat capacity and phonon scattering in great excess over what would be expected for a solid that has a unique vibrational ground state. Of particular conceptual importance is the apparent near universality of phonon scattering in amorphous solids made by quenching a liquid. To rationalise this universality, scale-free scenarios have been proposed that either hinge on there being long-range interactions between bare structural degrees of freedom or that invoke long-range criticality stemming from the emergence of marginally stable vibrational modes. In a contrasting, local scenario, the puzzling low-temperature degrees of freedom are, instead, weakly-interacting, strongly anharmonic degrees of freedom each of which involves the motion of a few hundred particles. In this scenario, the universality of phonon scattering comes about because the characteristic energy scale of the local anharmonic resonances and the strength of their interaction with phonons are both set by the glass transition temperature Tg, while their concentration is set by the cooperativity size ξ for dynamics at Tg. The nanoscopic length ξ is manifested in vibrational excitations of the spatial boundary of the resonances, which underlie the so-called Boson peak, and very deep, topological midgap electronic states in glassy semiconductors, which are implicated in a number of strange optoelectronic phenomena in amorphous chalcogenides. I discuss the merits of the above scenarios when confronted with experimental data.

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“…Their suggested mechanism ascribes such a shape of the absorption tail to the broadening of the lowest exciton state which is caused by tunnelling of the electron away from the hole. Application of the results of this theory to experimental data on absorption in amorphous materials reveals, however, that the obtained value of the internal random field is too large [6]. Another difficulty is that the existence of excitons in amorphous solids has never been established.…”

Section: Introductionmentioning

confidence: 97%