arXiv subjects

D. A. Drabold

Publications and source records attributed to D. A. Drabold.

At least 55 records · Page 3Linked to original sources

arXiv2010

Phonon driven transport in amorphous semiconductors: Transition probabilities

Starting from Holstein's work on small polaron hopping, the evolution equations for localized and extended states in the presence of atomic vibrations are systematically derived for an amorphous semiconductor. The transition probabilities are obtained for transitions between all combinations of localized and extended states. For any transition process involving a localized state, the activation energy is not simply the energy difference between the final and initial states; the reorganization energy of atomic configuration is also included as an important part of the activation energy (Marcus form). The activation energy for the transitions between localized states decreases with rising temperature and leads to the Meyer-Neldel rule. The predicted Meyer-Neldel temperatures are consistent with observations in several materials. The computed field-dependence of conductivity agrees with experimental data. The present work suggests that the upper temperature limit of variable range hopping is proportional to the frequency of first peak of phonon spectrum. We have also improved the description of the photocurrent decay at low temperatures. Analysis of the transition probability from an extended state to a localized state suggests that there exists a short-lifetime belt of extended states inside the conduction band or valence band.

cond-mat.mes-hall↗

arXiv2010

Structure determination of disordered materials from diffraction data

We show that the information gained in spectroscopic experiments regarding the number and distribution of atomic environments can be used as a valuable constraint in the refinement of the atomic-scale structures of nanostructured or amorphous materials from pair distribution function (PDF) data. We illustrate the effectiveness of this approach for three paradigmatic disordered systems: molecular C60, a-Si, and a-SiO2 . Much improved atomistic models are attained in each case without any a-priori assumptions regarding coordination number or local geometry. We propose that this approach may form the basis for a generalised methodology for structure "solution" from PDF data applicable to network, nanostructured and molecular systems alike.

cond-mat.mtrl-sci↗

arXiv2009

Electronic structure of a realistic model of amorphous graphene

In this note, we calculate the electronic properties of a realistic atomistic model of amorphous graphene. The model contains odd membered rings, particularly five and seven membered rings and no coordination defects. We show that odd-membered rings increase the electronic density of states at the Fermi level relative to crystalline graphene; a honeycomb lattice with semi-metallic character. Some graphene samples contain amorphous regions, which even at small concentrations, may strongly affect many of the exotic properties of crystalline graphene, which arise because of the linear dispersion and semi-metallic character of perfectly crystalline graphene. Estimates are given for the density of states at the Fermi level using a tight-binding model for the $π$ states.

cond-mat.str-el↗

arXiv2009

Hidden structure in amorphous solids

Recent theoretical studies of amorphous silicon [Y. Pan et al. Phys. Rev. Lett. 100 206403 (2008)] have revealed subtle but significant structural correlations in network topology: the tendency for short (long) bonds to be spatially correlated with other short (long) bonds). These structures were linked to the electronic band tails in the optical gap. In this paper, we further examine these issues for amorphous silicon, and demonstrate that analogous correlations exist in amorphous SiO2, and in the organic molecule, b-carotene. We conclude with a discussion of the origin of the effects and its possible generality.

cond-mat.dis-nn↗

arXiv2009

Phonon driven transport in amorphous semiconductors: Transition probabilities

Inspired by Holstein's work on small polaron hopping, the evolution equations of localized states and extended states in presence of atomic vibrations are derived for an amorphous semiconductor. The transition probabilities are obtained for four types of transitions: from one localized state to another localized state, from a localized state to an extended state, from an extended state to a localized state, and from one extended state to another extended state. At a temperature not too low, any process involving localized state is activated. The computed mobility of the transitions between localized states agrees with the observed `hopping mobility'. We suggest that the observed `drift mobility' originates from the transitions from localized states to extended states. Analysis of the transition probability from an extended state to a localized state suggests that there exists a short-lifetime belt of extended states inside conduction band or valence band. It agrees with the fact that photoluminescence lifetime decreases with frequency in a-Si/SiO$_{2}$ quantum well while photoluminescence lifetime is not sensitive to frequency in c-Si/SiO$_{2}$ structure.

cond-mat.mtrl-sci↗

arXiv2009

An extension of the Kubo-Greenwood formula for use in molecular simulations

We discuss the foundations and extend the range of applicability of the widely used Kubo-Greenwood formula (KGF) for the electronic conductivity. The conductivity is derived from the current density, and only the probability amplitude rather than the transition probability is used. It is shown that the contribution to the conductivity from degenerate states in a low or zero frequency external electric field and the contribution from states near resonance with a finite frequency external field are finite. The improved conductivity expression does not include the familiar "energy conserving" delta function, and no artificial broadening parameter for delta function is required for the DC limit. We explored two methods of computing current density. We discuss the role of many-electron statistics in computing the conductivity in single-particle approximations, and we show that the conventional KGF is due to the contribution from single-particle excited states.

cond-mat.stat-mech↗

arXiv2008

Ab initio determination of ion-traps in silver-doped chalcogenide glass

We present a microscopic picture of silver dynamics in GeSe$_{3}$:Ag glass obtained from {\it ab initio} simulation. The dynamics of Ag is explored at two temperatures, 300K and 700K. In the relaxed network, Ag occupies bond centers between suitably separated host sites. At 700K, Ag motion proceeds via a trapping-release dynamics, between "super traps" or cages consisting of multiple bond-center sites in a small volume. Our work offers a first principles identification of trapping centers invoked in current theories, with a description of their properties. We compute the charge state of the Ag in the network, and show that it is neutral if weakly bonded and Ag$^+$ if in a trapping center.

cond-mat.dis-nn↗

arXiv2008

Intermediate Phase in Germanium-Selenide glasses: A Theoretical and Experimental Study

A theoretical and experimental study of the Intermediate Phase (IP) in GexSe1-x is presented. Based on ab initio molecular dynamics (MD) models of these glasses, a generic atomic picture of the IP is discussed thoroughly. X-Ray Absorption Near Edge Structure (XANES) measurements on Ge-Se glasses over a wide range including the IP window are presented to show the electronic signature of the IP in the form of the shift in the absorption edge with the Ge content. It is shown that MD based models clearly reproduce that shift.

cond-mat.dis-nn↗

arXiv2008

Electronic Signature of the Intermediate Phase in Ge_xSe_{1-x} Glasses: A Joint Theoretical and Experimental Study

Raman and calorimetric studies on Ge_xSe_{1-x} glasses have provided evidence for the existence of the intermediate phase (IP) in chalcogenide and other glasses. Here, we present X-Ray Absorption Near Edge Structure (XANES) measurements on germanium selenide glasses in the IP composition range, and detect an electronic signature of the IP. Ab initio molecular dynamics (MD) based models of these glasses are discussed, and an atomistic picture of the IP, based upon the models and available experiments is presented. We show that these models reproduce detailed composition-dependent structure in the XANES measurements, and otherwise appear to properly represent the structure of the GeSe glasses near the IP.

cond-mat.dis-nn↗

arXiv2008

A semi-quantitative scattering theory of amorphous materials

It is argued that topological disorder in amorphous solids can be described by local strains related to local reference crystals and local rotations. An intuitive localization criterion is formulated from this point of view. The Inverse Participation Ratio and the location of mobility edges in band tails is directly related to the character of the disorder potential in amorphous solid, the coordination number, the transition integral and the nodes of wave functions of the corresponding reference crystal. The dependence of the decay rate of band tails on temperature and static disorder are derived. \textit{Ab initio} simulations on a-Si and experiments on a-Si:H are compared to these predictions.

cond-mat.dis-nn↗

arXiv2008

Atomistic Origin of Urbach Tails in Amorphous Silicon

Exponential band edges have been observed in a variety of materials, both crystalline and amorphous. In this paper, we infer the structural origins of these tails in amorphous and defective crystalline Si by direct calculation with current {\it ab initio} methods. We find that exponential tails appear in relaxed models of diamond with suitable point defects. In amorphous silicon (a-Si), we find that structural filaments of short bonds and long bonds exist in the network, and that the tail states near the extreme edges of both band tails are are also filamentary, with much localization on the structural filaments. We connect the existence of both filament systems to structural relaxation in the presence of defects and or topological disorder.

cond-mat.dis-nn↗

arXiv2008

Inverse approach to atomistic modeling: Applications to a-Si:H and g-GeSe2

We discuss an inverse approach for atomistic modeling of glassy materials. The focus is on structural modeling and electronic properties of hydrogenated amorphous silicon and glassy GeSe2 alloy. The work is based upon a new approach "experimentally constrained molecular relaxation (ECMR)". Unlike conventional approaches (such as molecular dynamics (MD) and Monte Carlo simulations(MC), where a potential function is specified and the system evolves either deterministically (MD) or stochastically (MC), we develop a novel scheme to model structural configurations using experimental data in association with density functional calculations. We have applied this approach to model hydrogenated amorphous silicon and glassy GeSe2. The electronic and structural properties of these models are compared with experimental data and models obtained from conventional molecular dynamics simulation.

cond-mat.dis-nn↗

arXiv2007

Topological and topological-electronic correlations in amorphous silicon

In this paper, we study several structural models of amorphous silicon, and discuss structural and electronic features common to all. We note spatial correlations between short bonds, and similar correlations between long bonds. Such effects persist under a first principles relaxation of the system and at finite temperature. Next we explore the nature of the band tail states and find the states to possess a filamentary structure. We detail correlations between local geometry and the band tails.

cond-mat.mtrl-sci↗

arXiv2007

Network structure and dynamics of hydrogenated amorphous silicon

In this paper we discuss the application of current it ab initio computer simulation techniques to hydrogenated amorphous silicon (a-Si:H). We begin by discussing thermal fluctuation in the number of coordination defects in the material, and its temperature dependence. We connect this to the ``fluctuating bond center detachment" mechanism for liberating H bonded to Si atoms. Next, from extended thermal MD simulation, we illustrate various mechanisms of H motion. The dynamics of the lattice is then linked to the electrons, and we point out that the squared electron-lattice coupling (and the thermally-induced mean square variation in electron energy eigenvalues) is robustly proportional to the localization of the conjugate state, if localization is measured with inverse participation ratio. Finally we discuss the Staebler-Wronski effect using these methods, and argue that a sophisticated local heating picture (based upon reasonable calculations of the electron-lattice coupling and molecular dynamic simulation) explains significant aspects of the phenomenon.

cond-mat.mtrl-sci↗

arXiv2007

Real space information from Fluctuation electron microscopy: Applications to amorphous silicon

Ideal models of complex materials must satisfy all available information about the system. Generally, this information consists of experimental data, information implicit to sophisticated interatomic interactions and potentially other {\it a priori} information. By jointly imposing first-principles or tight-binding information in conjunction with experimental data, we have developed a method: Experimentally Constrained Molecular Relaxation (ECMR) that uses {\it all} of the information available. We apply the method to model medium range order in amorphous silicon using Fluctuation Electron microscopy (FEM) data as experimental information. The paracrystalline model of medium range order is examined, and a new model based on voids in amorphous silicon is proposed. Our work suggests that films of amorphous silicon showing medium range order (in FEM experiments) can be accurately represented by a continuous random network model with inhomogeneities consisting of ordered grains and voids dispersed in the network.

cond-mat.dis-nn↗

arXiv2007

Thermally stimulated H emission and diffusion in hydrogenated amorphous silicon

We report first principles ab initio density functional calculations of hydrogen dynam- ics in hydrogenated amorphous silicon. Thermal motion of the host Si atoms drives H diffusion, as we demonstrate by direct simulation and explain with simple models. Si-Si bond centers and Si ring centers are local energy minima as expected. We also describe a new mechanism for break- ing Si-H bonds to release free atomic H into the network: a fluctuation bond center detachment (FBCD) assisted diffusion. H dynamics in a-Si:H is dominated by structural fluctuations intrinsic to the amorphous phase not present in the crystal.

cond-mat.mtrl-sci↗

arXiv2007

Ab initio estimate of temperature dependence of electrical conductivity in a model amorphous material: hydrogenated amorphous silicon

We present an ab initio calculation of the DC conductivity of amorphous silicon and hydrogenated amorphous silicon. The Kubo-Greenwood formula is used to obtain the DC conductivity, by thermal averaging over extended dynamical simulation. Its application to disordered solids is discussed. The conductivity is computed for a wide range of temperatures and doping is explored in a naive way by shifting the Fermi level. We observed the Meyer-Neldel rule for the electrical conductivity with E_MNR = 0.06 eV and a temperature coefficient of resistance, TCR ~ -2.0% K^-1 for a-Si:H. In general, experimental trends are reproduced by these calculations, and this suggests the possible utility of the approach for modeling carrier transport in other disordered systems.

cond-mat.mtrl-sci↗

arXiv2006

First principles molecular dynamics study of amorphous Si\sub{1-x}Ge\sub{x}:H alloys

We study the structural, dynamical and electronic properties of amorphous Si\sub{1-x}Ge\sub{x}:H alloys using first principles local basis molecular dynamics simulation. The network topology and defects in the amorphous network have been analyzed. Structural changes and an increase in number of defects have been found as the Ge atomic percentage increases from x=0.1 to x=0.5. The electronic density of states exhibits a decreasing band-gap and increased mid-gap and band-tail defect states as Ge concentration increases. Investigation of the band tails of the density of states show an exponential (Urbach) behavior. The mobility gap is estimated as a function of Ge concentration.

cond-mat.mtrl-sci↗