Defects In Amorphous Semiconductors Research Articles

Defects in Amorphous Semiconductors: The Case of Amorphous Indium Gallium Zinc Oxide

Understanding the physics of defective amorphous semiconductors such as indium gallium zinc oxide ($a$-IGZO) is of paramount importance for engineering thin-film transistors for large-area electronics, like your television. This work introduces a first-principles approach for studying defects in amorphous semiconductors, rationalizing the nature of these defects and offering a coherent description of $a$-IGZO that explains its experimentally observed electrical instabilities. The methodology offers a basis for insightful device optimization.

Magnetic properties for metastable negative- U defects in amorphous semiconductors

Optically induced metastable centers are studied in As 2S 3 chalcogenide glasses using the magnetic susceptibility technique for 3.5≤ T≤300 K. The temperature dependencies of the magnetic susceptibility ( χ) exhibit spin instability of both a hole center localized at a nonbonding lone pair chalcogen orbital and an electron center formed by an As p-orbital. The χ fatigue at low temperatures is shown to result from the negative- U dissociation of paramagnetic states 2D 0→D −+D + that causes also the absence of an ESR signal before illumination. Irradiation with light whose energy corresponds to the Urbach tail of the absorption edge is found to result in a growing χ signal that is due to optically induced paramagnetic states revealed also by the EPR signals: D −+D ++ hν→2D 0, whereas subsequent irradiation with infrared light in the mid-gap bleaches both the optically induced magnetic susceptibility and ESR signal to its cold dark efficiency. Metastable properties for paramagnetic states located in the gap of As 2S 3 are discussed in the framework of the negative- U reaction that is accompanied by the restoration of the PL and the fatigue of the ESR signals and the magnetic susceptibility value.

Collective phonon interaction and annealing of metastable defects in amorphous semiconductors

Abstract In hydrogenated amorphous semiconductors, metastable defects (MSDs) are induced by recombination of non-equilibrium carriers. The isothermal annealing of MSD follows a stretched-exponential time dependence with a dispersion parameter β proportional to the temperature: β=T/T∗. In this study, we observe that the experimental T∗ values are proportional to the lattice optical phonon frequency for a wide collection of hydrogenated amorphous alloys, including undoped a-Ge : H, a-Si : H, a-SiC0.13 : H and a-SiN1.6 : H and n-doped a-Si : H. This result indicates an intrinsic origin for the metastability. We propose a new interpretation of the available data based on a multiphonon-assisted decay of the metastable defects. A large barrier energy E act requires a collective interaction of a large number n=Eact/hωo of localized optical phonons. The number ω(n) of accessible vibrational configurations for a set of n phonons within a finite medium-range interaction volume, is defined as the density of n-phonon...

Photoinduced metastable defects in amorphous semiconductors: communality between hydrogenated amorphous silicon and chalcogenides

The time dependent decrease in photocurrent I p during illumination has been measured as a function of illumination intensity G and temperature T in hydrogenated amorphous silicon (a-Si:H) and amorphous As 2S 3 (a-As 2S 3). It is found that I p can be proportional to 1/[1-γexp{−(t/τ) α}], where γ is a measure of the induced density of metastable defects, τ the effective relaxation time, and α the dispersion parameter. The behavior of these parameters for both material systems are almost the same, suggesting that common kinetics dominate light induced metastable defect (LIMD) creation. A microscopic model is presented to explain common features between two entirely different categories of materials.

Near-surface defects in amorphous semiconductors related to hydrogen incorporation

Using total yield photoelectron spectroscopy we demonstrate that the room temperature (RT) hydrogenation of amorphous germanium (a-Ge) surfaces leads to the creation of a deep defect band 0.45 eV above Ev that anneals out at ≈ 300°C. We argue that this defect is related to the high concentration of hydrogen at the growth surface and is responsible for the high surface defect density (≈ 1018 cm−3) in g.d. a-Ge:H and a-Si:H as derived from yield and PDS measurements. We show that the surface defect density in g.d. a-Si:H can be reduced through hydrogen dilution and through post-deposition anneals but not through post-hydrogenation. Drawing from the analogy with c-Si we propose that a small fraction (≈ 10−4) of hydrogen in the bond center position is responsible for the observed defect which is different from the more common dangling bond defect.

Theories of defects in amorphous semiconductors

The existence of defects in amorphous semiconductors is itself an problem of fundamental interest. Conventional approaches accept their existence only phenomenologically. We described a new approach to defining and understanding defects. The existence of defects is a natural outgrowth of our thermodynamical ensemble theory of electronic states in disordered systems. Specific properties are also predicted through properties of thermodynamic ensembles and the relationship between defect electronic properties and structural energies. Thus we obtain directly electronic properties without a need for detailed microscopic information.

The density of states and spectral function in amorphous Si obtained using the equation of motion method in k-space

The authors show how the equation of motion method for solving Schrodinger's equation numerically may be employed in k-space and present calculations of the density of states and spectral function for fully bonded models of amorphous Si containing 216 atoms. The magnitude of the 'gap' in the density of states depends sensitively on the strength of the pseudopotential and the structure. The method provides a simple method for investigation of the effect of defects in amorphous semiconductors and may also be used to look at individual energy levels. This method has considerable potential for the investigation of electron conduction in the vicinity of the gap in the density of the states and for self-consistent calculations.

Theory of defects in amorphous semiconductors

We review the electronic structure of defects in aSi:H, aGaAs and aSi 3N 4, emphasising in aSi:H the doping mechanism, the evidence that its dangling bond defect has a small electron-lattice coupling and a positive correlation energy, and possible atomic mechanisms for the Staebler-Wronski effect.

Modeling of defects in amorphous semiconductors: Threefold coordination in a-Si:H

Defects in a-Si:H are modeled by embedding a small “defect cluster” in a crystalline Si matrix. Electronic structure calculations for threefold coordinated Si, P, B, Ga and Al are carried out using the Green's function method. The electronic U for the Si dangling bond is in good agreement with the experimental data for a dangling bond related defect. The threefold coordinated P is found to be electrically inactive, while the threefold coordinated B, Ga and Al all have a level in the gap. The positions their respective levels, suggest that Ga or Al may be better acceptor dopants than B.

Defects in amorphous semiconductors

Abstract The electronic structure of defects in some canonical amorphous semiconductors, namely a-Se, a-As, a-Si: H, a-GaAs, a-Si3N4 and a-SiO2, is reviewed. The mechanism of substitutional doping in hydrogenated amorphous silicon (a-Si: H) is also critically considered.

Electronic structure of amorphous semiconductors

Abstract We review the bonding in a wide range of non-crystalline materials, such as the elemental and compound amorphous semiconductors, liquid semiconductors and, briefly, some oxide glasses and amorphous silicon dioxide. The review concentrates on the experimental determination of electronic structure from photoemission, optical spectra and core spectra and on its theoretical description, particularly using tight-binding methods. The review also describes the bonding at defects in amorphous semiconductors and in particular that of dopants and hydrogen in hydrogenated amorphous silicon (a-Si:H).

Cooperative charge disproportionation of defects in amorphous semiconductors

Defect centers in amorphous semiconductors with three possible states of charge (+, 0, and -) can be modeled with an antiferromagnetic S = 1 Ising Hamiltonian in an external field. We have studied this model on a one-dimensional lattice with first neighbor interactions. A clear charge disproportionation is obtained as a function of temperature for a wide range of parameters. The relevance of our finding to the ESR behavior of a-As, and possibly also the photoluminescence of a-Si:H, and to field effect behavior of chalcogenide glasses is pointed out.

Defects in amorphous semiconductors

The energies of low-energy defects in tetrahedrally coordinated, pnictide, and chalcogenide amorphous semiconductors are estimated. Both nonintimate and intimate charged pairs are considered. The sign and value of the effective correlation energy is estimated in each case and the variation of the Fermi energy with electronic correlation is calculated. Although the Fermi energy is storngly pinned for nonintimate pairs with a negative effective correlation energy, it unpins when the pairs are intimate.

New aspects of electronic properties of amorphous selenium and its use in xerography

Abstract This article gives a tutorial survey of the xerographic process and the relevant physical phenomena involved in the formation of the latent image. Emphasis is on experimental results on amorphous selenium, but other materials will be described in order to explain properties which are either general to the disordered state or characteristically different from those of wide-band semiconductors. The discussion includes a description of the Onsager mechanism of charge generation, transient dispersive transport and recent models of defects in amorphous semiconductors. The article ends with a description of recent results related to the control of defects in chalcogenide glasses.