Showing papers by "Marc Kastner published in 1978"
TL;DR: In this paper, a chemical-bond approach reveals the possibility of several unusual bonding configurations that have relatively low energy in materials with lone-pair valence electrons, and the possibility that valence-alternation centres can form intimate or bound pairs.
Abstract: A chemical-bond approach reveals the possibility of several unusual bonding configurations that have relatively low energy in materials with lone-pair valence electrons. Valence-alternation pairs (V.A.P.) are the lowest energy charged defects. The interconversion between positive and negative centres is described in detail. The nature of V.A.P.s in pnictides (group V elements) is discussed. The possibility that valence-alternation centres can form intimate or bound pairs is explored. It is pointed out that certain unusual bonding configurations (configuration distortions) arise even in an ideal lone-pair material. It is stressed that the unusual bonding configurations discussed are a result of the special bonding in lone-pair materials and are, therefore, expected to arise in crystals as well as amorphous materials.
210 citations
TL;DR: In this article, the Street-Mott model of gap states, interpreted in terms of valence alternation defects, is used to calculate the temperature dependence of the carrier concentrations and of the conductivity as a function of the concentration of charged additives in chalcogenide semiconductors.
Abstract: The Street—Mott model of gap states, interpreted in terms of valence alternation defects, is used to calculate the temperature dependence of the carrier concentrations and of the conductivity as a function of the concentration of charged additives in chalcogenide semiconductors. When the charged additives are allowed to equilibrate with the valence alternation defects at elevated temperatures we find that the conductivity activation energy remains essentially unchanged although the conductivity may be increased by a few orders of magnitude for large concentrations of additives. Very much larger increases in conductivity result when the charged additives are prevented from equilibrating with the valence alternation defects. At high additive concentrations the chalcogenide material is expected to behave like a partially compensated and nearly degenerate semiconductor in agreement with observations of Ovshinsky (1977). The gradual unpinning of the Fermi level with increasing additive concentration i...
86 citations
TL;DR: In this paper, a general approach is presented for predicting the fraction x of additive that is actually effective in changing the density of centres, which is less than unity because some of the additive has its valence requirements satisfied (in part by coordinate bonds).
Abstract: Although the addition of small concentrations of additives to chalcogenide semiconductors can change the density of valence-alternation centres, the effect may be much smaller than expected. A general approach is presented for predicting the fraction x of additive that is actually effective in changing the density of centres. This fraction is less than unity because some of the additive has its valence requirements satisfied (in part by coordinate bonds). Only additive atoms in unusual (charged) bonding configurations can influence the density of valence-alternation centres, x is enhanced by entropy effects but reduced by the presence of the intrinsic density of valence-alternation centres and by the higher energy of the charged bonding configuration.
81 citations
TL;DR: The development of models for states in the gap of amorphous semiconductors is reviewed in this paper, where it is pointed out that early models, the ideas of Anderson, and those of Street and Mott all are essentially ad hoc.
Abstract: The development of models for states in the gap of amorphous semiconductors is reviewed. It is pointed out that early models, the ideas of Anderson, and those of Street and Mott all are essentially ad hoc . On the other hand, ideas developed by Kastner, Ovshinsky, and Kastner, Adler, and Fritzsche, are based on the underlying symmetry of the system: the chemical bonding. Recent successes of the valence-alternation model for defects in lone-pair semiconductors are summarized. Although many results are easily explained, the photoluminescence process appears much more complicated than previously believed.
55 citations
TL;DR: In this article, it was shown that luminescence centres in chalcogenide glasses are not charged as is widely believed and that electron-hole pairs at such centres are tightly bound.
Abstract: New experimental results are reported which suggest that luminescence centres in chalcogenide glasses are not charged as is widely believed. Measurements of the photoluminescenee excitation quantum efficiency and the absence of any effect on the luminescence of intense magnetic or electric fields strongly suggest that the centres are neutral and that electron-hole pairs at such centres are tightly bound. A number of other recent experiments are reviewed which offer additional evidence for a dipole-centre model of luminescence. In this model, dipole centres (perhaps intimate valence-alternation centres) are excited by absorbed photons, some luminescence is emitted by the excited state in ∼ 10−8 s before it decays into a meta-stable state which lives for ∼ 10−4 s. During the latter time diffusion and energy transfer take place. The time and temperature dependence of photoluminescence fatigue is also reported. Since the temperature dependence of fatigue is so different from that of the luminescence,...
42 citations
TL;DR: The Street-Mott gap states are shifted towards lower electron energies with respect to the states of intrinsic valence alternation centres because of the larger electronegativity of the additives as discussed by the authors.
Abstract: Whereas metal additives, incorporated in lone-pair semiconductors as positively charged centres, act like ionized donors, electronegative additives which possess lone-pair electrons give rise to centres which can be in either one of three charge states, negative, neutral, and positive. As a consequence, oxygen or halogen additive may give rise to a complete set of Street-Mott gap states. These are shifted towards lower electron, energies with respect to the states of intrinsic valence alternation centres because of the larger electronegativity of the additives. The addition of oxygen or halogen may lead to an increase or a decrease of the electrical conductivity depending on the relative values of the average carrier excitation energies.
39 citations
19 citations
TL;DR: The first measurement of the temperature dependence of the diamagnetic properties of amorphous tetrahedral semiconductors was reported in this article, where it was found that the increase of diamagnetic enhancement of ammorphous Si arises from an increase of χv relative to that of the crystal.
Abstract: The first measurement of the temperature dependence of the diamagnetism in an amorphous elemental tetrahedral semiconductor is reported. This allows the separation of the valence electron susceptibility into its diagmagnetic χv and paramagnetic χp components. It is found that the diamagnetic enhancement of amorphous Si arises from an increase of χv relative to that of the crystal. However, an analysis of a large number of molecules containing Si suggests that the susceptibility of the crystal rather than than that of the amorphous phase is anomalous.
6 citations