Origins of the Gap States in Polycrystalline Silicon: Tight-Binding Calculations of Twist Boundaries

1992 ◽  
Vol 262 ◽  
Author(s):  
M. Kohyama ◽  
S. Kose ◽  
R. Yamamoto
Keyword(s):  
Electronic Structures ◽  
Polycrystalline Silicon ◽  
Tight Binding ◽  
Electronic States ◽  
Structural Disorder ◽  
Interfacial Energies ◽  
Amorphous Si ◽  
Gap States ◽  
Twist Boundaries ◽  
Generate Characteristic

ABSTRACTThe atomic and electronic structures of the twist boundaries Σ (=3 (011), Σ=7 (111) and Σ=5 (001)) in Si have been calculated by using the transferable SETB method coupled with the supercell technique. The twist boundaries in Si contain larger structural disorder or more defects and larger interfacial energies than tilt grain boundaries. Several kinds of structural disorder or defects have been found to generate characteristic electronic states inside the gap. The present structural disorder or defects and the gap states are the candidates of the origins of the observed band-tails or mid-gap states in polycrystalline Si as well as those In amorphous Si.

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.

Structural Disorder and Localized Gap States in Silicon Grain Boundaries from a Tight-Binding Model

1997 ◽  
Vol 491 ◽  
Author(s):  
F. Cleri ◽  
P. Keblinski ◽  
L. Colombo ◽  
S. R. Phillpot ◽  
D. Wolf
Keyword(s):  
Molecular Dynamics ◽  
Grain Boundaries ◽  
Tight Binding ◽  
Structural Disorder ◽  
High Energy ◽  
Tight Binding Model ◽  
Binding Model ◽  
Dynamics Simulations ◽  
Gap States ◽  
Forbidden Gap

ABSTRACTTight-binding molecular dynamics simulations of typical high-energy grain boundaries in silicon show that the atomic structure of the interface in thermodynamic equilibrium is similar to that of bulk amorphous silicon and contains coordination defects. The corresponding electronic structure is also amorphous-like, displaying extra states in the forbidden gap mainly localized around the coordination defects, where large changes in the bond-hybridization character are observed. It is proposed that such coordination defects in disordered high-energy grain boundaries are responsible for the experimentally observed gap states in polycrystalline Si.

Morphologies and Related Electronic Properties of Carbon Nanotubes

1998 ◽  
Vol 13 (9) ◽  
pp. 2368-2379 ◽  
Author(s):  
J-C. Charlier
Keyword(s):  
Carbon Nanotubes ◽  
Solid State ◽  
Structural Optimization ◽  
Ab Initio ◽  
Electronic Properties ◽  
Graphene Sheet ◽  
Electronic Structures ◽  
Tight Binding ◽  
Electronic States

The electronic structures of different morphologies of carbon nanotubes are investigated within either tight-binding or ab initio frameworks. After a brief description of the electronic properties of the “perfect” rolled-up graphene sheet, nanotubes containing pentagon-heptagon pairs, tips (hemispherical caps), sp3-like lines responsible for polygonization, multishell and solid-state packings (bundles) are studied in order to point out the influence of such defects on the electronic states of the “perfect” cylinders. Most of the time, a structural optimization was performed on the atomic topology, prior to the calculation of the electronic properties. Connections with experimental facts are indicated as frequently as possible.

Tight-Binding Molecular Dynamics of Ceramic Nanocrystals Using Pc-Based Parallel Machines

1999 ◽  
Vol 581 ◽  
Author(s):  
Kenji Tsuruta ◽  
Hiroo Totsuji ◽  
Chieko Totsuji
Keyword(s):  
Molecular Dynamics ◽  
Electronic Structures ◽  
Parallel Machines ◽  
Tight Binding ◽  
Expansion Method ◽  
Surface Structures ◽  
Electronic Density ◽  
Pc Cluster ◽  
Fermi Operator ◽  
Gap States

ABSTRACTEvolution of atomic and electronic structures of silicon-carbide (SiC) nanocrystals during sintering is investigated by a tight-binding molecular dynamics (TBMD) method. An O(N) algorithm (the Fermi-operator expansion method) is employed for calculating electronic contributions in the energy and forces. Simulations are performed on our eight-node parallel PC cluster. In a sintering simulation of aligned (no tilt or twist) SiC nanocrystals at T = 1000K, we find that a neck is formed promptly without formation of defects. Analyses of local electronic density-of-states (DOS) and effective charges reveal that unsaturated bonds exist only in grain surfaces accompanying the gap states. In the case of tilted (<122>) nanocrystals, surface structures formed before sintering affect significantly the grainboundary formation.

Energies and Atomic Structures of Grain Boundaries in Silicon: Comparison Between Tilt and Twist Boundaries

1994 ◽  
Vol 343 ◽  
Author(s):  
M. Kohyama ◽  
R. Yamamoto ◽  
Y. Watanabe
Keyword(s):  
Tight Binding ◽  
The Other ◽  
Complex Structures ◽  
Structural Units ◽  
Atomic Structures ◽  
Interfacial Energies ◽  
Tilt Boundaries ◽  
The Stability ◽  
Twist Boundaries ◽  
The Ideal

ABSTRACTThe energies and atomic structures of tilt and twist boundaries in Si have been examined by using the tight-binding electronic theory, and the reason why twist boundaries are seldom found in polycrystalline Si has been investigated. About the frequently observed {122} Σ=9 and {255} Σ=27 tilt boundaries, the configurations without any coordination defects consistent with the electron microscopy observations have relatively small interfacial energies with small bond distortions. About the <111> Σ=7, <011> Σ=3 and <001> Σ = 5 twist boundaries, the configurations contain larger bond distortions or more coordination defects, and much larger interfacial energies than those of the tilt boundaries. The <001> twist boundaries have very complex structures as compared with the other twist boundaries, which can be explained by the morphology of the ideal surfaces. The stability of the tilt boundaries in Si can be explained by the viewpoint of the stable structural units consisting of atomic rings.

History and Cross-Disciplinary Perspective of Compositional Imaging and Mapping With Nexafs Microscopy

1998 ◽  
Vol 4 (S2) ◽  
pp. 154-155
Author(s):  
H. Ade
Keyword(s):  
Electronic Structures ◽  
Chemical Shifts ◽  
Electronic States ◽  
Inorganic Materials ◽  
Chemical Bonds ◽  
Fundamental Physics ◽  
X Ray ◽  
Disciplinary Perspective ◽  
Core Electrons ◽  
X Ray Absorption

In Near Edge X-ray Absorption Fine Structure (NEXAFS) microscopy, excitations of core electrons into unoccupied molecular orbitals or electronic states provide sensitivity to a wide variety of chemical functionalities in molecules and solids. This sensitivity complements infrared (IR) spectroscopy, although the NEXAFS spectra are not quite as specific and “rich” as IR spectra. The sensitivity of NEXAFS to distinguish chemical bonds and electronic structures covers a wide variety of samples: from metals to inorganics and organics. (There is a tendency in the community to use the term NEXAFS for soft x-ray spectroscopy of organic materials, while for inorganic materials or at higher energies X-ray Absorption Near Edge Spectroscopy (XANES) is utilized, even though the fundamental physics is the same.) The sensitivity of NEXAFS is particularly high to distinguish saturated from unsaturated bonds. NEXAFS can also detect conjugation in a molecule, as well as chemical shifts due to heteroatoms.

Metal Contacts on a-GaN

1996 ◽  
Vol 1 ◽  
Author(s):  
T. U. Kampen ◽  
W. Mönch
Keyword(s):  
Tight Binding ◽  
Image Force ◽  
Schottky Contacts ◽  
Metal Contacts ◽  
Zero Bias ◽  
Barrier Heights ◽  
Current Voltage ◽  
Optical Dielectric Constant ◽  
Optical Dielectric ◽  
Gap States

The Schottky barrier heights of silver and lead contacts on n-type GaN (0001) epilayers were determined from current-voltage characteristics. The zero-bias barrier heights and the ideality factors were found to be linearly correlated. Similar observations were previously reported for metal contacts on Si (111) and GaAs (110) surfaces. The barrier heights of ideal Schottky contacts are characterized by image force lowering of the barrier only. This gives an ideality factor of 1.01. From our data we obtain barrier heights of 0.82 eV and 0.73eV for ideal Ag and Pb contacts on GaN, respectively. The metal-induced gap states (MIGS) model predicts the barrier heights of ideal Schottky contacts on a given semiconductor to be linearly correlated with the electronegativities of the metals. The two important parameters of this MIGS-and-electronegativity model are the charge neutrality level (CNL) of the MIGS and a slope parameter. The CNL may be calculated from the dielectric band gap and using the empirical tight-binding method. The slope parameters are given by the optical dielectric constant of the respective semiconductor. The predictions of the MIGS model for metal/GaN contacts are confirmed by the results presented here and by barrier heights previously reported by others for Au, Ti, Pt, and Pd contacts on GaN.

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