Defect States and Structural Disorder in a-Si.

1992 ◽  
Vol 258 ◽  
Author(s):  
B. N. Davidson ◽  
G. Lucovsky ◽  
J. Bernholc
Keyword(s):  
Standard Deviation ◽  
Nearest Neighbor ◽  
Bond Angle ◽  
Local Density ◽  
Tight Binding ◽  
Bethe Lattice ◽  
Structural Disorder ◽  
Defect States ◽  
Lattice Method ◽  
The Relationship

ABSTRACTWe have examined the distribution of the neutral dangling bond defect states, T30, as a function of the local disorder. T30 defects in a-Si play an important role in many of the current models of the metastable photoconductivity. To understand the relationship between the T30 defect and its bonding environment, the Local Density of States (LDOS) for under-coordinated Si atoms in disordered environments are calculated using the cluster-Bethe lattice method, CBLM. Our Hamiltonian employs the tight-binding parameters of an sp3 orbital basis containing both 1st and 2nd nearest-neighbor interaction terms fit to c-Si band structure. Averaged LDOS of atoms with various bond angle distortions are calculated in order to demonstrate the relationship between the standard deviation in bond angle and the width of the defect states. The CBLM is also used to determine the extent of the valence band tails as a function of the standard deviation in bond angle. In addition, the LDOS of clusters with 2 dangling bonds are examined to determine the degree that their energy levels split due to their interaction with each other.

Energy Differences Between the Si and the Ge Dangling Bond Defects in a-Si1-xGex Alloys

1992 ◽  
Vol 258 ◽  
Author(s):  
S.M. Cho ◽  
B.N. Davidson ◽  
G. Lucovsky
Keyword(s):  
Nearest Neighbor ◽  
Bond Angle ◽  
Local Density ◽  
Bethe Lattice ◽  
Structural Disorder ◽  
Energy Difference ◽  
Amorphous Semiconductors ◽  
Dangling Bond ◽  
Lattice Method ◽  
The Difference

ABSTRACTWe have investigated the difference in the electronic energies of neutral Si and Ge dangling bond states in undoped a-Si1-xGex alloys as a function of the alloy composition, x, and local bond-angle distortions. The local density of states, LDOS, in a-Si1-xGex alloys has been calculated using nearest-neighbor interactions, and employing the Cluster Bethe Lattice method. We conclude that for ideal, tetrahedrally bonded amorphous semiconductors alloys, the Ge dangling bond energy is lower than that of Si dangling bonds by ∼ 0.13 eV, independent of the specific nearest neighbors to the dangling bond (3 Si-atoms, 2 Si-atoms and 1 Ge-atom, etc.), but that the spread in dangling bond energies associated bond-angle variations of the order of 6–8 degrees can be larger than this energy difference (∼0.3 eV or greater). This means that structural disorder, rather than chemical disorder causes Si and Ge-atom dangling bond states to overlap in their energy distributions.

Effect of the Local Disorder in a-Si on the Electronic Density of States at the Band Edges.

1991 ◽  
Vol 219 ◽  
Author(s):  
B. N. Davidson ◽  
G. Lucovsky ◽  
J. Bernholc
Keyword(s):  
Density Of States ◽  
Nearest Neighbor ◽  
Bond Angle ◽  
Local Density ◽  
Tight Binding ◽  
Bethe Lattice ◽  
Neighbor Interaction ◽  
Lattice Method ◽  
Electronic Density ◽  
Interaction Terms

ABSTRACTWe have systematically investigated the formation of electronic states in the region of the conduction and valence band edges of a Si as functions of variations in the bond angle distributions. Local Density of States (LDOS) for Si atoms in disordered environments have been calculated using the cluster Bethe lattice method with a tight-binding Hamiltonian containing both first and second nearest neighbor interaction terms. LDOS for atoms with bond angle dis ortions in the nearest neighbor and second neighbor shells are compared and contrasted, both showing an influence on the LDOS near the gap. We also consider the role of the second neighbor term in the Hamiltonian by comparing the DOS for a distoned infinite Bethe lattice using Hamiltonians with and without the second neighbor interactions. It is found that in this case the second neighbor interaction terms cause greater conduction band tailing than using the nearest neighbor interaction terms alone.

INVERSE FORMULATION OF THE GREEN'S FUNCTION THEORY: EXACT SOLUTION FOR THE BETHE LATTICE

2001 ◽  
Vol 15 (21) ◽  
pp. 2935-2943 ◽  
Author(s):  
L. ŠAMAJ
Keyword(s):  
Green’S Function ◽  
Green's Function ◽  
Nearest Neighbor ◽  
Tight Binding ◽  
Bethe Lattice ◽  
Diagonal Matrix ◽  
Local Form ◽  
Matrix Elements ◽  
Factorization Property ◽  
Inverse Form

We study the tight-binding Hamiltonian H=∑j|j> ∊j<j| + ∑j,k |j>Vjk < k | defined on the Bethe lattice of an arbitrary coordination number; the hopping elements Vjk are nonzero (and constant) only for j,k being the nearest-neighbor sites and the energies ∊j are considered to be site-dependent. The Green's function (z-H)-1 problem is solved explicitly in the inverse form, with diagonal matrix elements {<j | (z-H)-1 | j >} as controlling (prescribed) variables. Namely: (i) the inverse profile relation, i.e. z-∊j versus diagonal matrix elements, is obtained in a local form; (ii) the off-diagonal matrix elements { <j | (z-H)-1 | k >} are shown to exhibit a simple factorization property in terms of the diagonal ones.

Linear AC transport in T-stub and crossed silicene nanosystems

2018 ◽  
Vol 32 (03) ◽  
pp. 1850016
Author(s):  
Yun-Lei Sun ◽  
En-Jia Ye
Keyword(s):  
Electric Field ◽  
Transport Properties ◽  
External Electric Field ◽  
Nearest Neighbor ◽  
Transport Theory ◽  
Energy Gap ◽  
Local Density ◽  
Tight Binding ◽  
Edge State ◽  
Topological Quantum

In this work, we theoretically study the linear AC transport properties in T-stub and crossed zigzag silicene nanosystems. The DC conductance and AC emittance are numerically calculated based on the tight-binding approach and AC transport theory, by considering the nearest-neighbor hopping, second-nearest-neighbor spin-orbit interaction (SOI) and external electric field. The relatively strong SOI of silicene was demonstrated to induce a topological quantum edge state in the nanosystems by the local density of states, which eliminates the AC emittance response at the Dirac point. Further investigations suggest that the SOI-induced AC transport is topologically protected from the changes of geometrical size. Moreover, the AC transport properties of these nanosystems can be tuned by the external electric field, which would open an energy gap and destroy the topological quantum state, making them trivial band insulators.

Electronic States in the Gap of a-Si from Bond Angle Variations

1990 ◽  
Vol 192 ◽  
Author(s):  
B. N. Davidson ◽  
G. Lucovsky
Keyword(s):  
Valence Band ◽  
Density Of States ◽  
Bond Angle ◽  
Bethe Lattice ◽  
Band Edge ◽  
Conduction Band Edge ◽  
Valence Band Edge ◽  
Defect States ◽  
Discrete State ◽  
Bond Angles

ABSTRACTWe investigate the formation of defect states in the gap of a-Si arising from deviations from the ideal tetrahedral bond angles. The local density of states for Si atoms in disordered environments is calculated using tight-binding parameters for the cluster-Bethe lattice method. The Hamiltonian for a-Si with bond angle distortions is taken as an average over many configurations associated with a random choice of bond angles, weighted by Gaussian distributions with standard deviations between 2°.and 10°. Bond angle deviations in this range generate a density of defect states at the valence band edge that: 1) increases as the average bond angle deviation increases; and 2) is significantly larger than the density of band tail states generated at the conduction band edge. We obtain a shift of the absorption edge from the joint density of states (DOS) as a function of bond angle deviations. In addition, a calculation of the DOS for a distorted tetrahedral cluster embedded in an idealized Bethe lattice yields a threshold bond angle distortion of ±20° for the appearance of a discrete state in the gap near the valence band edge.

Effects of Structural Disorder on the Electronic Properties of Silicon: Tight-Binding Calculations of Grain Boundaries

1993 ◽  
Vol 297 ◽  
Author(s):  
M. Kohyama ◽  
R. Yamamoto
Keyword(s):  
Band Gap ◽  
Electronic Properties ◽  
Local Density ◽  
Tight Binding ◽  
Structural Disorder ◽  
Local Density Of States ◽  
Deep State ◽  
Amorphous Si ◽  
Semi Empirical ◽  
Twist Boundaries

The atomic and electronic structures of tilt and twist boundaries in Si have been calculated by using the transferable semi-empirical tight-binding (SETB) method, and the relations between the local structural disorder and the electronic properties of Si have been obtained clearly. The odd-membered rings and the four-membered rings induce the changes of the shape of the local density of states (LDOS). The bond distortions generate the peaks at the band edges in the LDOS, and greatly distorted bonds induce the weak-bond states inside the band gap. The three-coordinated defect generates a deep state in the band gap, which is much localized at the three-coordinated atom. The five-coordinated defect generates both deep and shallow states. The deep state is localized in the neighboring atoms except the five-coordinated atom, although the shallow states exist among the five-coordinated atom and the neighboring atoms. Configurations of boundaries are very effective in order to clarify the effects of the local structural disorder in amorphous SI.

scholarly journals ELECTRONIC STATES AND LOCAL DENSITY OF STATES NEAR GRAPHENE CORNER EDGE

2012 ◽  
Vol 11 ◽  
pp. 151-156 ◽  
Author(s):  
YUJI SHIMOMURA ◽  
YOSITAKE TAKANE ◽  
KATSUNORI WAKABAYASHI
Keyword(s):  
Density Of States ◽  
Nearest Neighbor ◽  
Local Density ◽  
Tight Binding ◽  
Localized States ◽  
Destructive Interference ◽  
Edge States ◽  
Local Density Of States ◽  
Binding Model ◽  
Recursion Method

We study that stability of edge localized states in semi-infinite graphene with a corner edge of the angles 60°, 90°, 120° and 150°. We adopt a nearest-neighbor tight-binding model to calculate the local density of states (LDOS) near each corner edge using Haydock's recursion method. The results of the LDOS indicate that the edge localized states stably exist near the 60°, 90°, and 150° corner, but locally disappear near the 120° corner. By constructing wave functions for a graphene ribbon with three 120° corners, we show that the local disappearance of the LDOS is caused by destructive interference of edge states and evanescent waves.

Enhancement of the thermoelectric figure-of-merit in nanowire superlattices

2015 ◽  
Vol 1735 ◽  
Author(s):  
Chumin Wang ◽  
J. Eduardo González ◽  
Vicenta Sánchez
Keyword(s):  
Figure Of Merit ◽  
Nearest Neighbor ◽  
Chemical Potential ◽  
Tight Binding ◽  
Structural Disorder ◽  
Real Space ◽  
Thermoelectric Figure Of Merit ◽  
Thermoelectric Effects ◽  
Thermoelectric Figure ◽  
Cross Section Area

ABSTRACTBased on the Kubo-Greenwood formula, the thermoelectric effects in periodically and quasiperiodically segmented nanowires are studied by means of a real-space renormalization plus convolution method, where the electrical and lattice thermal conductivities are respectively calculated by using the tight-binding and Born models; the latter includes central and non-central interactions between nearest-neighbor atoms. The results show a significant enhancement of the thermoelectric figure-of-merit (ZT) induced by the structural disorder and/or the reduction of nanowire cross-section area. In addition, we observe a maximum ZT in both the chemical-potential and temperature spaces.

scholarly journals AC CONDUCTIVITY OF GRAPHENE: FROM TIGHT-BINDING MODEL TO 2 + 1-DIMENSIONAL QUANTUM ELECTRODYNAMICS

2007 ◽  
Vol 21 (27) ◽  
pp. 4611-4658 ◽  
Author(s):  
V. P. GUSYNIN ◽  
S. G. SHARAPOV ◽  
J. P. CARBOTTE
Keyword(s):  
Quantum Electrodynamics ◽  
Ac Conductivity ◽  
Nearest Neighbor ◽  
Tight Binding ◽  
Honeycomb Lattice ◽  
Dirac Fermions ◽  
Binding Model ◽  
Dirac Mass ◽  
The Relationship ◽  
The Continuum

We consider the relationship between the tight-binding Hamiltonian of the two-dimensional honeycomb lattice of carbon atoms with nearest neighbor hopping only and the 2 + 1 dimensional Hamiltonian of quantum electrodynamics, which follows in the continuum limit. We pay particular attention to the symmetries of the free Dirac fermions including spatial inversion, time reversal, charge conjugation and chirality. We illustrate the power of such a mapping by considering the effect of the possible symmetry breaking, which corresponds to the creation of a finite Dirac mass, on various optical properties. In particular, we consider the diagonal AC conductivity with emphasis on how the finite Dirac mass might manifest itself in experiment. The optical sum rules for the diagonal and Hall conductivities are discussed.

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