Browsing by Subject "Phonon"

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    Anharmonic line shift and linewidth of the Raman modes in TlInS2 layered crystals
    (John Wiley & Sons Ltd., 2004) Yuksek, N. S.; Gasanly, N. M.; Aydınlı, Atilla
    The temperature dependence of the unpolarized Raman spectra from TlInS 2 layered crystal was measured between 10 and 300 K. The analysis of the experimental data showed that the temperature dependences of wavenumbers and linewidths are well described by considering the contributions from thermal expansion and lattice anharmonicity. The purely anharmonic contribution (phonon-phonon coupling) was found to be due to three-phonon processes. This work demonstrates that the two Raman modes at 280.9 and 292.3 cm-1 exhibit changes toward high wavenumbers as the temperature is raised from 10 to 300 K.
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    Electron momentum and energy relaxation rates in GaN and AlN in the high-field transport regime
    (The American Physical Society, 2003) Bulutay, C.; Ridley, B. K.; Zakhleniuk, N. A.
    Momentum and energy relaxation characteristics of electrons in the conduction band of GaN and AlN are investigated using two different theoretical approaches corresponding to two high electric-field regimes, one up to 1-2 MV/ cm values for incoherent dynamics, and the other at even higher fields for coherent dynamics where semiballistic and ballistic processes become important. For the former, ensemble Monte Carlo technique is utilized to evaluate these rates as a function of electron energy up to an electric-field value of 1 MV/cm (2 MV/cm) for GaN (AlN). Momentum and energy relaxation rates within this incoherent transport regime in the presence of all standard scattering mechanisms are computed as well as the average drift velocity as a function of the applied field. Major scattering mechanisms are identified as polar optical phonon (POP) scattering and the optical deformation potential (ODP) scattering. Roughly, up to fields where the steady-state electron velocity attains its peak value, the POP mechanism dominates, whereas at higher fields ODP mechanism takes over. Next, aiming to characterize coherent dynamics, the total out-scattering rate from a quantum state (chosen along a high-symmetry direction) due to these two scattering mechanisms are then computed using a first-principles full-band approach. In the case of POP scattering, momentum relaxation rate differs from the total out-scattering rate from that state; close to the conduction-band minimum, momentum relaxation rate is significantly lower than the scattering rate because of forward-scattering character of the intravalley POP emission., However, close to the zone boundary the difference between these two rates diminishes due to isotropic nature of intervalley scatterings. Finally, a simple estimate for the velocity-field behavior in the coherent transport regime is attempted, displaying a negative differential mobility due to the negative band effective mass along the electric-field direction.
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    Functionalization of group V monolayers and their compounds: alloying, doping and surface modification
    (2020-11) Kanlı, Muammer
    There has been growing interest during the last decade in two-dimensional (2D) materials due to their important roles in various scientific and technological applications such as detectors, lasers and light emitting diodes. In this thesis we present a theoretical investigation of a couple of such 2D materials from group V monolayers and their compounds. Firstly, ordered alloys of GaxAl1−xN hexagonal monolayer are studied and the effect of Al content on mechanical, electronic, thermal and optical properties are investigated. The optimized lattice constants and band gaps change in accordance to Vegard’s Law. Low barrier energies and favorable substitution of Ga by Al may show feasibility of fabrication. Segregation is also checked with mixing energy calculations. The dynamical stability of alloys is shown by phonon spectrum analysis and MD simulations. GaxAl1−xN alloys give lower in-plane stiffness than h-BN or graphene, but higher Poisson’s ratio than most realized 2D systems. Heat capacity of alloys delivers a decrease with Al content at low temperatures but it converges to the classical limit at high temperatures. The absorption onset of GaxAl1−xN alloys remain in the near UV range and prominent absorption peaks blue-shifts with increasing x in compliance with the variation of the band gap. The considered systems, in regard to their stability and tunable fundamental properties seem to be very promising 2D semiconductors for wide range of applications at reduced scales. Then, the interaction of alkali metal atoms (Li, Na, and K) with single layer and periodic structures of hb-As and sw-As phases are revealed by first-principles methods. Arsenene phases are considered to be used as electrodes (anode) for ion-batteries. Strong alkali-electrode binding and low diffusion energy barriers gives out better cycling stability and faster diffusion, respectively. hb-As shows better storage capacity than sw-As. However, deviations from ordered pattern and decline of formation energy with increasing doping level point out a possible structural transformation. By MD calculations, crystalline to amorphous phase transition is seen even for low concentrations level at ambient temperature. The average open-circuit voltages of 0.68-0.88 V (0.65-0.98 V) with specific capacity up to 715 mAhg−1 (358 mAhg−1) are calculated for single layer (periodic) configurations. Overall, non-crystalline phases are calculated to offer more favorable structures than crystalline configurations and they provide more coherent results when compared with experimental data. The obtained voltage profile together with low diffusion barriers and strong metal-electrode binding suggests arsenene as a promising anode material to be used in for alkali-ion battery applications. Lastly, the formation of dumbbell (DB) geometry upon adsorption of Ga, N adatoms to GaN monolayer is investigated. While Ga-N DBs are unstable, Ga-Ga and N-N DB geometries are predicted to form in an exothermic and spontaneous scheme. Cohesive energy of hexagonal GaN monolayer decreases when a DB is formed on its surface. Electronic structures for Ga-Ga DBs for 2×2, 3×3, 4×4 and 5×5 phases show spinpolarized and degenerate bands mainly contributed by p-orbitals of the atoms in impurity zone. Degenarated bands are not observed for N-N dumbbell for HDP, TDP, 2×2 and 3×3 phases. Upon DB formation, semiconductor GaN monolayer become spin-polarized semiconductor with varying band gap, where this functionalization allows electronic structure to be tuned substantionally. This would be highly desired for nanoscale electronic and optical devices. These Ga-Ga and N-N DB geometries may also be used for the synthesis of layered GaN structures.