The pressure-induced effects in the thermal equilibrium electron properties of semiconducting glasses

Abstract

Strong pressure-induced effects in the thermal equilibrium properties of semiconducting glasses are revealed and theoretically analysed. The basic property under consideration is the concentration of the negative-U centres which determine the mobilitygap spectral structure and the related electron phenomena in the materials. For accessible high pressures, 10 4 p 10 5 bar, a rapid increase of the concentration with growing pressure is predicted. This holds for ('weak') negative-U centres formed in typical, 'rigid', configurations for the vast majority of atoms, as both the mobility-gap width and the related effective magnitude of the negative pair-correlation energy decrease with pressure. However, at ambient (and low) pressure another type of centre, 'strong' negative-U centres formed in glassy atomic soft configurations, predominate, whose concentration decreases with increasing pressure. The resulting concentration of negative-U centres and some related characteristics are shown to exhibit a non-monotonic pressure dependence with a minimum. Future experimental tests of the corresponding theoretical relationships might determine the basic parameters of the phenomenon for the materials under consideration. 10 −4 c 2e (0) ≈ g 0 (E * c )wc a (0) 10 −3 (1.3)

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

“…In accordance with [1], the contribution of the standard electron±hole (e±h) pairs to E exp opt 0 is more important than that of the single-particle excitations of the negative-U centres at the typical concentration of the negative-U centres c 2 0 % 10 À3 to 10 À4 (see Section 1), but with increasing p the mentioned two kinds of contributions become of the same scale or the contribution of the negative-U centres predominates at high enough pressure, rather close to p g , at which E g p ( E g 0. Indeed, for such high pressure, as predicted [8], c 2 p rapidly increases and becomes very high…”

“…The relations (4) hold because ªweakº negative-U centres, with jU effp j E g pa2 ( 1 eV E g 0a2, are mainly formed due to the hybridization of states in the narrow mobility gap, in the standard, ªrigidº, local configurations of atomic concentration c a p % p g % 1 (in contrast, at ambient pressure ªstrongº negative-U centres are largely formed, with jU eff 0j % 1 eV, in soft configurations of much lower atomic concentration c a 0 % 10 À2 ) [4,8]. On the other hand, a typical transition matrix element for generating a single-particle excitation in a negative-U centre appears to be of the same scale as that for producing a ªboundº e±h pair of similar size, of which the concentration is much lower than the concentration of the vast majority of e±h pairs; the latter, however, are characterized by a significantly smaller typical transition matrix element (see, e.g.…”

“…On the other hand, a typical transition matrix element for generating a single-particle excitation in a negative-U centre appears to be of the same scale as that for producing a ªboundº e±h pair of similar size, of which the concentration is much lower than the concentration of the vast majority of e±h pairs; the latter, however, are characterized by a significantly smaller typical transition matrix element (see, e.g. [1,8]). One can conclude from the above said that at the high pressures under discussion the following relationship becomes relevant (see also [7]):…”

“…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 . Thus, in general, for " hw % E opt p, awY p a nuc wY p a e h wY p Y 10 and the absorption of the negative-U centres rather predominates over the standard e±h contribution, rapidly increasing as awY p a nuc wY p G c 2 p with growing p, i.e., the mechanism of the optical absorption basically changes, for high enough p`p g .…”

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

“…Eventually this is due to the observed decrease of E g (p), i.e., of U eff (p), and so to a respective increase of the effective spring constant of MRO atomic configurations determining the self-trapping, with increasing p. Then the hybridization of states is the decisive feature of the self-trapping, giving rise to the formation of the ''weak'' negative-U centers that are characterized by a much higher c 2 (p), up to c 2 (p)Ϸc 2 (0)͓c con f (p)/c a (0)͔Ϸ10 Ϫ2 Ϫ10 Ϫ1 ӷc 2 (0), for 0Ͻp g ϪpӶp g at least. 10 In this connection, the concentration c 2 (p) of the negative-U centers exhibits a rapid increase with growing pϾ p min and, due to the competition of the strong and weak negative-U centers, a nonmonotonic pressure dependence with a minimum at p ϭ p min Ϸ(0.35Ϫ0.45) p g . Then the characteristic atomicmotion-mode energy ប⍀ 0 (p) for the formation of the negative-U centers is noticeably lower than the debye energy ប d at ambient pressure, whereas it is close to ប d at high p:…”