Electronic States In Amorphous Semiconductors Research Articles

Quantum Phenomena in Structural Glasses: The Intrinsic Origin of Electronic and Cryogenic Anomalies

The structural glass transition is often regarded as purely a problem of the classical theory of liquids. The dynamics of electrons enters only implicitly, through the interactions between ionic cores or molecules. Likewise, zero-point effects tied to the atomic masses hardly affect the typical barriers for liquid rearrangements. Yet, glasses do exhibit many quantum phenomena—electronic, optical, and cryogenic peculiarities that seem to have universal characteristics. These anomalies of the glassy state are uncommon or strongly system dependent in crystals and amorphous solids not made by a quasi-equilibrium quench of a melt. These clearly quantum phenomena include midgap electronic states in amorphous semiconductors, the two-level systems, and the Boson peak. Here, we discuss how these quantum phenomena found in glasses are not merely consequences of any kind of disorder but have universal characteristics stemming from the structural dynamics inherent in the glass transition itself. The quantum dynamics ...

Influence of the kinetic energy of localization on the distribution of electronic states in amorphous semiconductors

We examine how the kinetic energy of localization associated with the site disorder influences the distribution of electronic states in amorphous semiconductors. Employing an elementary potential well model, in which the disorder characteristic of amorphous semiconductors is represented by an ensemble of potential wells, we evaluate the one-electron density of states function by first determining the binding energy corresponding to each well, and then averaging over the ensemble of wells. We find that the kinetic energy of localization plays an important role in determining both the shape and the magnitude of this one-electron density of states function.

Percolative transport in amorphous semiconductors

Abstract For phonon-assisted transport between localized electronic states in amorphous semiconductors, the conductance of the medium is known to be the same as that of an equivalent resistor network. If the impedances of the resistors between adjacent sites vary over several orders of magnitude, this conductance is determined mainly by the magnitude of the critical impedance Z c such that a percolation path through the system of resistors with impedance less than Z o exists only if Z o ≥ Z c. The calculation of Z c is complicated by the dependence of the impedance between a pair of sites on the energies of the states localized around them. Various methods for calculating Z c and its dependence on the parameters of the system, are compared for a model system in which all the states have one of two discrete energies. Our main conclusion is that, in the calculation of Z c from the mean number of bonds per site, it can be very important to optimize the range of sites and bonds that are considered.

New explanation for differences between the valence bands of crystalline and amorphous silicon

The long-accepted belief that odd-membered rings in amorphous silicon cause the major differences between its valence-band density of states and that of crystalline silicon is overturned by new x-ray photoemission spectra of the two materials. Through application of a new theoretical approach for calculating the density of electronic states in amorphous semiconductors, we show that the changes actually observed are expected consequences of dihedral-angle disorder.

Defects and associated electronic states in amorphous semiconductors

The concept of point defects in amorphous materials is now well established and they are known to influence electrical and optical properties in much the same way as they do in crystalline semiconductors. The dominant defect, however, is often different from that in the crystalline phase; for example, in amorphous silicon it is a dangling bond. In chalcogenide glasses charged defect pairs seem to predominate and these are characterized by a negative effective correlation energy. The electronic states associated with the defects are known to lie in the gap but attempts to position them precisely, either by experiment or by theoretical calculations, are not straightforward. The current situation will be described with reference to results on representative materials.

Single and pair electronic states in amorphous semiconductors

Single and pair electronic states in amorphous semiconductors

Localized electronic states in amorphous semiconductors

The experimental results pertaining to the electronic structure of covalent amorphous semiconductors are briefly reviewed. It is found that three classes of materials exist, depending on the lowest-energy coordination of the predominant chemical component. In each case, the transport properties are ordinarily controlled by the localized states in the gap resulting from the minimum-energy defect sites, in which the local coordination is not optimal for certain atoms. These localized states are treated in terms of a Hubbard model, in which the effective repulsion between two electrons simultaneously present on the same center is taken as positive for tetrahedrally bonded solids and negative for chalcogenide and pnictide glasses. The electronic structure is discussed in detail. It is shown that even such a simple model can account for almost all of the experimental properties of the major classes of amorphous semiconductors.