Molecular dynamics simulations have been used to investigate the nature of heat pulse propagation through a Y-junction carbon nanotube consisting of a (14,0) trunk splitting into a pair of (7,0) branches. For comparison, these simulations were also carried out on straight (7,0) and (14,0) carbon nanotubes. Simulations of the Y-junction nanotube were run in three different configurations: with the heat pulse originating in the trunk, in one of the branches, or in both of the branches simultaneously. All of the simulations were run at 0K, and the length of the pulse was 1ps. Results have shown that the heat pulse excites a variety of traveling phonon modes. It has been found that the junction impedes the propagation of these modes. Furthermore, it has been observed that traveling modes originating in the trunk pass through the junction more easily than those originating in the branches. This provides preliminary evidence for anisotropic heat flow in Y-junction nanotube structures at low temperatures. Finally, it is possible for a single phonon mode passing through the junction to generate multiple phonon modes on the other side, all with velocities less than or equal to the original mode.
The propagation of picosecond duration heat pulses in single wall carbon nanotubes has been investigated using Molecular Dynamics simulations. It is found that the picosecond heat pulse in (10,0) and (5,5) induces several waves that propagate at different propagation speeds. The leading waves move at the speed of sound corresponds to LA phonons, followed by waves moving at TW phonon modes. The heat energy content in the waves corresponding to LA phonon modes in (10,0) zigzag nanotubes is significantly larger than in (5,5) armchair nanotubes.
The ultrafast relaxation of photoexcited electrons in AlN has been investigated using ensemble Monte Carlo approach. The electrons are excited using infra-red laser pulses with energies ranging from 800 mev to 1000 meV above the conduction band edge at different excitation levels. The energy relaxation, valley population, the build-up and decay of the hot phonon distributions are examined. The strong polar optical phonon scattering rates coupled with the short lifetimes of A(LO) leads to quick decay of the hot phonon distributions. Additionally, the rapid electron-electron scattering leads to fast thermalization of the carrier distributions.
The effects of hot phonons on the ultrafast relaxation of photoexcited electrons in AlN has been investigated using ensemble Monte Carlo approach. The electrons are excited using infra-red laser pulses with different densities and energies. The build-up and decay of the hot phonon distribution at several phonon wavevectors is examined. The strong polar optical phonon scattering rates coupled with the short lifetimes of A(LO) leads to quick decay of the hot phonon distributions. Additionally, the rapid electron- electron scattering leads to fast thermalization of the carrier distributions.
We have examined the ultrafast relaxation of electrons photoexcited by infrared laser pulses in AlN using ensemble Monte Carlo approach. The effects of doping, excitation levels, energy, and pulse duration were investigated. It is found that even at excitation energies above the G-U separation, the role of intervalley scattering is weaker compared to polar optical scattering. This is more significant at longer pulse duration.
SOI MOSFET device structures with SiO2/AlN composite back insulator with wide range thermal conductivity values were investigated. The proposed structures resulted in different amount of self-heating depending on the fraction of SiO2 in the back insulator. For a 0.8 micron FD-SOI MOSFET devices, increases in temperature from 23 C to 173 C for 1 milliwatt of power dissipation were obtained through numerical two dimensional device simulation. Additionally, simulation results show strong variation in mobility and generation current in devices with different back gate oxide composition. Similar structures could be used to determine the temperature dependence of transport parameters such as velocity overshoot, impact ionization rate, and other device parameters.
The effects of X6 and X7 intervalley scattering on the energy relaxation of electrons in GaAs were investigated in GaAs for excitation energy of 4.3 eV. An initial build up of electron population in the upper valleys (X6 and X7) following the excitation leads to non-equilibrium LO phonon. The initial LO phonon spectrums in different valleys show maximum build up at different q vectors and then relax to similar distributions at longer times. The Heating of the LO phonons leads to slower transfer of electrons back to the central valleys.
Momentum relaxation of photo-excited carriers in GaAs was investigated using the Monte Carlo approach. A laser of 1.51 eV photon energy and 9 fs width was assumed. Simulations were performed for excitation densities ranging from 1016 cm-3 to 8 X 1017 cm-3. For nexc equals 8 X 1017 cm-3, the distribution of the carrier momentum was found to approximate a Maxwell-Boltzmann distribution 25 fs after laser excitation, which concurs with recent experimental data. The relaxation time was shown to increase with decreasing carrier density and to be shorter when the carrier-carrier scattering was treated dynamically rather than statically.
Ultrafast relaxation of photo-excited electrons in p-doped and intrinsic GaAs has been investigated using the Monte Carlo method. Dynamic screening of the carrier-carrier (c-c) interaction has been implemented using a momentum and frequency dependent dielectric function. Compared to the static c-c scattering model, the current approach results in faster cooling of the electron-hole plasma, due to enhanced carrier-carrier scattering rates. In p- GaAs, the energy relaxation shows that the electron-hole plasma (EHP) cools faster with increasing hole concentration. The transient luminescence intensity and the effective carrier temperature computed from luminescence spectra compare favorably with experimental data.
The ultrafast relaxation of electron-hole plasma photoexcited by a subpicosecond
laser pulse in GaAs is investigated using ont Carlo method. The photoexcited
carrier concentration is assumed to be 5x10 cm , and thephoton energy is assumed
to 1.82 eV. The interaction between the heavy-holes and hot LO phonons has a
minor effect on the cooling rates and the shape of hot phonon distribution but
leads to an increased energy loss rates through the deformation potential
interaction to compensate for the energy gained via LO phonon absorption.
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