فهرست مطالب ghassem ansaripour

Radiative detectors and devices operating in the range (0.2-10) terahertz and room temperature are needed. The need is created by fast advances of the science and terahertz technologies in the domain of astronomy to biosecurity. By the advances of molecular beam epitaxy (MBE) the growth of semiconductor superlattices of the order of 100 A° is performed. This lattice constant is big and could reveal high field non-ohmic behaviors. The negative differential conductivity (NDC) arises from the grate numbers of electrons in the proximity edge of miniband boundary. The velocity of electrons approaching to the miniband is reduced, causing the NDC effect. In this article the NDC in semiconductor superlattices for sinusoidal and periodic square -well potentials was studied. The extracted maximum carrier drift velocities for a superlattice semiconductor were nearly 1.8x105 m/s and 2.6x105 m/s respectively and the obtained ratio of maximum drift velocity to the corresponding minimum was more than 3 for the two potentials. Also the NDC in semiconductor superlattices for parabolic and sinusoidal dispersion equations at different frequencies was investigated. In the parabolic dispersion equation, near the harmonic Bloch oscillations at frequencies between even and odd harmonic oscillations, the high frequency NDC was observed.

Recent astonishing progresses in crystal growth technology, i.e. molecular beam epitaxy, have provided the possibility of fabricating quantum well structures in which electrons confined to move in one or two dimensions. The motion of electrons in such semiconducting structures is confined and leads to size quantization effects. In this research, we investigated the limited mobility by ionized impurity scattering or from the uniform distribution of remote impurity for a one-dimensional semiconductor device such as n-type gallium arsenide and Indium Arsenide nanowire, which first calculated and then plotted. The effect of various relevant physical parameters such as temperature and radius and impurity density on mobility has been investigated. Numerical results show that limited mobility increases uniformly and slowly with increasing temperature due to background impurity scattering, while limited mobility increases rapidly with temperature due to scattering of remote impurities. It has also been shown that the mobility for both impurities decreases with increasing wire radius and with increasing density, mobility increases. The results obtained from this investigation are in agreement to recent experimental and theoretical data.

In this work, we aim to investigate the dominant scattering and mobility limiting in a one-dimensional semiconductor device, such as gallium arsenide nanowire. First, the interaction of phonon-electron in a one-dimensional semiconductor device is described and then, using the sheet wave function, some of the carrier scatterings of this system is investigated in terms of quantities such as the density of one-dimensional carriers and temperature. These scatterings include phonon scattering, such as the scattering of acoustic phonons, through the deformation potential coupling, the scattering of acoustic phonons by piezoelectric coupling and the scattering of polar optical phonons, which we first computed and then plotted. We have shown that the mobility limited scattering of carriers in the range of low and high temperatures are acoustic phonon deformation potential scattering and polar longitudinal optical phonon scattering respectively. By increase of linear carrier concentration and nanowire radius the mobility of carriers is enhanced in a given temperature. The dominant scattering in the temperature range (4-300K) is deformation potential and is independent of the carrier concentration and nanowire radius. For high carrier density and nanowire radius the dominant scattering in the temperature range (300-500K) is polar optical phonon scattering

In this article, the thermal conductivity in one-dimensional devices, including semiconducting nanowires of silicon and gallium arsenide is calculated and plotted. The method used is solving the Boltzmann equation for phonon scattering. In case of purely specular interface scattering, the thermal conductivity is found to be high in silicon and gallium arsenide nanowires. The thermal conductivity increases with increasing nanowire diameter. In this work two different models for thermal conducting have been investigated. The first model solves the Boltzmann equation and finds solutions with the relaxation time approximation, and the other is a self-consistent solution of the Boltzmann equation. The answers in these two solutions are combined. The results show that the thermal conductivity in Si and GaAs semiconductor nanowires is reduced nearly 0.21 and 0.19 than that of bulk devices respectively in agreement to the reported data. It is found that the thermal conductivity of gallium arsenide nanowire is lower than that of silicon nanowire and comparing to recent published data for the corresponding Si nanowires with smaller diameter is overestimated and for larger nanowires diameter is underestimated which could be due to not taking into account the optical phonons decay to acoustic phonons and the effect of surface roughness on the thermal conductivity.