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Item Open Access Ab-initio electron transport calculations of carbon based string structures(American Physical Society, 2004) Tongay, S.; Senger, R. T.; Dag, S.; Çıracı, SalimShow more The new stable structures of carbon-based strings and their unusual electronic transport properties were discussed. Total energy and electronic structure calculations using first principles pseudopotential plane wave method within density functional theory (DFT) and supercell geometries were also carried out. It was found that carbon chains were suitable for structural and chemical functionalizations because of their flexibility. These carbon chains also form stable ring, helix, grid and network structures. The results show that the double covalent bonding of carbon atoms underlies their unusual chemical, mechanical and transport properties and carbon chains can form stable string structures with impressive physical properties.Show more Item Open Access Coulomb drag effect in parallel cylindrical quantum wires(Pergamon Press, 1996) Tanatar, BilalShow more We study the Coulomb drag rate for electrons in two parallel quantum wires. The double-quantum wire structure is modeled for a GaAs material with cylindrical wires having infinite potential barriers. The momentum transfer rate between the wires (Coulomb drag effect) is calculated as a function of temperature for several wire separation distances. We employ the full wave vector and frequency dependent random-phase approximation (RPA) at finite temperature to describe the effective interwire Coulomb interaction. We find that the drag rate at high temperatures (i.e., T ≥ EF/2) is dominated by the collective modes (plasmons) of the system similar to the case in double-well structures. Including the local-field effects in an approximate way we estimate the importance of intrawire correlations to be significant. Copyright © 1996 Published by Elsevier Science Ltd.Show more Item Open Access Dynamical screening effects in hot-electron scattering from electron-hole plasma and LO-phonon modes in quantum wires(Elsevier, 1996) Bennett, C. R.; Tanatar, Bilal; Constantinou, N. C.Show more We present a fully dynamical and finite temperature study of the hot-electron momentum relaxation rate and the power loss in a coupled system of electron-hole plasma and bulk LO-phonons in a quantum wire structure. Interactions of the scattered electron with neutral plasma components and phonons are treated on an equal footing within the random-phase approximation. Coupled mode effects substantially change the transport properties of the system at low temperatures. Particularly, the "plasmon-like" and "LO-phonon-like" excitations yield comparable rates which, as a consequence of the singular nature of the ID density of states, can be large at the threshold. This is in contrast to room temperature results where only the LO-phonon mode contributes significantly to the rate. The density and temperature dependence of the power loss reveals that dynamical screening effects are important, and energy-momentum conservation cannot be satisfied above a certain density for a given initial energy.Show more Item Open Access Electronic transport through a kink in an electron waveguide(Institute of Electrical and Electronics Engineers, 1994) Yalabik, M. C.Show more The current-voltage denendence correspondinp to electronic transport through a kink in an electronic waveguide is analyzed. No phase breaking dissipation mechanisms are considered, but the effects of the Coulomb interaction are included through a self consistent approximation. The results indicate very nonlinear transport properties, including negative differential resistance and bistability. © 1994 IEEEShow more Item Open Access Hydrodynamic approach for modelling transport in quantum well device structures(Institute of Physics Publishing Ltd., 1998) Besikci, C.; Tanatar, Bilal; Sen, O.Show more A semiclassical approach for modelling electron transport in quantum well structures is presented. The model is based on the balance equations governing the conservation of particle density, momentum and energy with Monte Carlo (MC) generated transport parameters. Three valleys of the conduction band, size quantization in the Γ valley, and the lowest two subbands in the quantum well are considered by taking the detailed intersubband dynamics into account. The transport parameters of the model are extracted from steady-state MC simulations based on an improved formulation of two-dimensional polar optical phonon scattering including screening effects. The predictions of the proposed model have been found to be in excellent agreement with those of the ensemble MC simulations under both time varying and spatially nonuniform fields. The calculated transport parameters which are of interest for device modelling are presented as a function of the electron energy for the AIGaAs/GaAs quantum well. The model serves as an accurate semiclassical alternative to costly ensemble MC simulations for studying the transport in quantum well structures and for the modelling and optimization of submicron devices based on these structures, such as modulation doped field-effect transistors (MODFETs).Show more Item Open Access Magnetic-field dependence of low-temperature mobility in quasi-one-dimensional electron systems(Institute of Physics Publishing Ltd., 1994) Tanatar, Bilal; Constantinou, N. C.Show more We study the mobility of a quasi-one-dimensional (Q1D) electron system in the presence of an axial magnetic field at low temperatures. We consider the mobility limits for remote-impurity scattering, homogeneous-background scattering, interface-roughness scattering, and alloy-disorder scattering mechanisms. For a system in which all carriers are in the lowest subband, the electron-impurity interaction is modelled for the above cases, and analytic expressions are derived. Calculations appropriate for a GaAs Q1D structure are presented for typical wire radius R, electron density N, impurity density Ni, and applied magnetic field B.Show more Item Open Access Monte Carlo simulation of electron transport in degenerate and inhomogeneous semiconductors(A I P Publishing LLC, 2007) Zebarjadi, M.; Bulutay, C.; Esfarjani, K.; Shakouri, A.Show more An algorithm is proposed to include Pauli exclusion principle in Monte Carlo simulations. This algorithm has significant advantages to implement in terms of simplicity, speed, and memory storage; therefore it is ideal for the three-dimensional device simulators. The authors show that even in moderately high applied fields, one can obtain the correct electronic distribution. They give the correct definition for electronic temperature and show that in high applied fields, the quasi-Fermi level and electronic temperature become valley dependent. The effect of including Pauli exclusion principle on the band profile, electronic temperature, and quasi-Fermi level for the inhomogeneous case of a single barrier heterostructure is illustrated.Show more Item Open Access Possibility of superconductivity of two-dimensional electrons on the surface of liquid heliuM, films(Pergamon Press, 1993) Tanatar, Bilal; Hakioǧlu T.Show more We consider the possibility of superconductivity in a system of two-dimensional electrons on the surface of liquid helium films. Taking into account of the interaction between electrons and the surface excitations of liquid helium films-ripplons, within the weak coupling BCS theory, we estimate the superconducting transition temperature for various interaction strengths, film thicknesses, and electron densities. The superconducting transition temperature Tc, under experimentally realizable conditions, is calculated to be a few mK's. © 1993.Show more Item Open Access Raman studies of doped polycrystalline silicon from laser-annealed, doped a-Si:H(Pergamon Press, 1994) Compaan, A.; Savage, M. E.; Aydınlı, Atilla; Azfar, T.Show more We have used Raman scattering to follow the progress of multiple-pulse (sub-melt-threshold) laser annealing in doped hydrogenated amorphous silicon films (a-Si:H) on glass. In phosphorous-doped a-Si:H the Raman signal shows that recrystallization begins with the first laser pulse but the multiple pulses are needed to generate the highest hole concentrations of ∼6×1020 cm-3. In boron-doped a-Si:H the electron concentration reaches ∼1×1021 cm-3 after laser anneal which produces a dip rather than a peak near the phonon line as a consequence of a negative Fano-interference parameter, q. The results show that Raman scattering can be used to obtain carrier concentrations in poly-silicon provided that wavelength-dependent Fano interference effects are properly included. © 1994.Show more Item Open Access Theoretical assessment of electronic transport in InN(Elsevier, 2004) Bulutay, C.; Ridley, B. K.Show more Among the group-III nitrides, InN displays markedly unusual electronic transport characteristics due to its smaller effective mass, high peak velocity and high background electron concentration. First, a non-local empirical pseudopotential band structure of InN is obtained in the light of recent experimental and first-principles results. This is utilized within an ensemble Monte Carlo framework to illuminate the interesting transport properties. It is observed that InN has a peak velocity which is about 75% higher than that of GaN while at higher fields its saturation velocity is lower than that of GaN. Because of the strongly degenerate regime brought about by the high background electron concentration, the electron-electron interaction is also investigated, but its effect on the steady-state and transient velocity-field characteristics is shown to be negligible. Finally, hot phonon generation due to excessive polar optical phonon production in the electron scattering and relaxation processes is accounted for. The main findings are the appreciable reduction in the saturation drift velocity and the slower recovery from the velocity overshoot regime. The time evolution of the hot phonon distribution is analysed in detail and it is observed to be extremely anisotropic, predominantly along the electric force direction.Show more Item Open Access Theory of the Pseudospin resonance in semiconductor bilayers(The American Physical Society, 2007) Abedinpour, S. H.; Polini, M.; MacDonald, A. H.; Tanatar, Bilal; Tosi, M. P.; Vignale, G.Show more The pseudospin degree of freedom in a semiconductor bilayer gives rise to a collective mode analogous to the ferromagnetic-resonance mode of a ferromagnet. We present a many-body theory of the dependence of the energy and the damping of this mode on layer separation d. Based on these results, we discuss the possibilities of realizing transport-current driven pseudospin-transfer oscillators in semiconductors, and of using the pseudospin-transfer effect as an experimental probe of intersubband plasmons.Show more Item Open Access Trends in molecular design strategies for ambient stable n-channel organic field effect transistors(Royal Society of Chemistry, 2017) Dhar, J.; Salzner, U.; Patil, S.Show more In recent years, organic semiconducting materials have enabled technological innovation in the field of flexible electronics. Substantial optimization and development of new π-conjugated materials has resulted in the demonstration of several practical devices, particularly in displays and photoreceptors. However, applications of organic semiconductors in bipolar junction devices, e.g. rectifiers and inverters, are limited due to an imbalance in charge transport. The performance of p-channel organic semiconducting materials exceeds that of electron transport. In addition, electron transport in π-conjugated materials exhibits poorer atmospheric stability and dispersive transient photocurrents due to extrinsic carrier trapping. Thus development of air stable n-channel conjugated materials is required. New classes of materials with delocalized n-doped states are under development, aiming at improvement of the electron transport properties of organic semiconductors. In this review, we highlight the basic tenets related to the stability of n-channel organic semiconductors, primarily focusing on the thermodynamic stability of anions and summarizing the recent progress in the development of air stable electron transporting organic semiconductors. Molecular design strategies are analysed with theoretical investigations.Show more