dc conductivity of semiconducting glasses: The role of negative-Ucenters, and pressure-induced phenomena

Abstract

The ohmic dc conductivity, as a representative example of the electronic transport coefficients for semiconducting glasses, is theoretically characterized within the framework of the recent theory of negative-U centers. The latter are self-trapped singlet electron ͑hole͒ pairs, strongly bound and localized at ambient pressure. Pressure-induced delocalization of the negative-U centers and their single-particle excitations in semiconducting glasses is predicted at high pressures pуp c , the critical pressure being p c Ϸ10 5 bar. Thereby, the temperature and pressure dependencies of the dc conductivity, as well as its scale, are basically different at p Ӷ p c ͑in particular at ambient pressure͒ or at pϾ p c . The standard electron-hole pairs determine the thermally activated conductivity at pϽ p c , whereas the delocalized negative-U centers give rise to the quasimetallic conductivity at high pressures and not too high temperatures.

High-Pressure Effects in Low-Temperature Fundamental Optical Absorption and Photoluminescence of Glassy Semiconductors

“…It is worth adding that the predicted delocalization of the negative-U centres at such high pressures [10] hardly can change these conclusions, as the related optical Franck-Condon transition is characterized by a standard time t r % 10 À8 s while typically for the delocalized charge carriers near the transition [10] the related Brownian-like motion time t BM ! 10 À7 s. The pressure dependence of a nuc wY p near E nuc opt p is largely determined by c 2 p, for p g b p b p min at least, with the characteristic pressure p min [8] in the range 0X1p g`pmin 0X5p g .…”

“…Pressure p g marks a characteristic semiconductor p`p g ±metal p`p g transition, and typically p g q g  10 10 Pa at q g % 1 to 3.…”

Insulator–Metal Transition and Large Paramagnetic Effect at High Pressure in Glassy Semiconductors

“…The critical pressure p cn is here the solution of the equation g 2zJ n av (p cn )/D av (p cn ) = g cr % 1 in the semiconducting phase of the glass for p cn < p g . The pressures p c1 and p c2 are rather their upper limit, and at least p c2 is less than p g [1], 0.75 p c2 /p g 0.85 e 2 and 0.75 p c1 /p g 0.85 e 1 (2) at 0 e 1 0.20, and 0 e 2 0.10 so indeed, in general, p c2 < p g . Thus, the pressureinduced delocalization of the negative-U centres, at p c2 p`p g , and of the positive-U centres, at p c1 p`p g , in a GS is predicted theoretically in the mean-field like approximation applied.…”

“…Introduction The purpose of this paper is to report that even accessible moderately high pressures, p ( 10 6 bar, are able to give rise to drastic changes in electron properties, as well as in dynamical ones, of glassy semiconductors (GS), whereas this is not the case for the respective crystalline materials [1].…”

“…The delocalization phenomenon gives rise to essential transformations in transport processes so at the high pressures in question, p c2 p`p g , the dc conductivity s dc (p, T) is basically related to the non-localized negative-U centres as the main current carriers for which c 2 (p) is independent of T, at T < T g in the glass range (see [1]). Then, s dc (p, T) is of a metallic type,…”