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Author: Denis Nica Degree: doctor of physics and mathematics Speciality:  01.04.02 - Theoretical and mathematical physics Year: 2006 Scientific adviser: Evghenii Pocatilov doctor habilitat, professor, Moldova State University Institution: Scientific council:

Status The thesis was presented on the 26 April, 2006 Approved by NCAA on the 29 June, 2006 Abstract – 1.37 Mb / in romanian – 1.50 Mb / in russian Thesis CZU 539.21 7.88 Mb / in russian 120 pages

Keywords

Summary

Dissertation is dedicated to the complex investigation of acoustic and kinetic properties of the flat heterostructures with internal GaN layer as well as rectangular and cylindrical GaN wires, covered with shell possessing higher or lower sound velocity than GaN.

In the frame of the continual model with account of the anisotropy of the wurtzite GaN (AlN) materials for the first time there were obtained the equations of motion for elastic oscillations in the flat heterostructures as well as for rectangular and cylindrical quantum heterowires. Solution for these equations was obtained through the finite differences method. There were calculated energy spectra and group velocities of acoustic phonons in three-layered (AlN/GaN/AlN, plastic/GaN/plastic) and five-layered (plastic/AlN/GaN/AlN/plastic) heterostructures and in quantum GaN wires with rectangular and cylindrical cross-sections covered with acoustically fast AlN and acoustically slow plastic shells. It was shown that shells with lower sound velocity compress acoustic phonons spectrum and considerably (2-4 times for the considered nanostructures) decrease average acoustic phonons group velocity in comparison with the velocity in homogeneous slabs and wires. The shells with higher sound velocity ( acoustically fast ) extend the energy spectrum of acoustic phonons and increase their average group velocity (1.5-2 times for the considered nanostructures) in comparison with velocity in homogeneous slabs and wires. It was established that in the nanostructures consisting of the layers with different sound velocities the redistribution of the displacement vector components takes place. This fact leads to the appearance of the effects of phonon depletion and accumulation in the heterostructures and heterowires. The physical origin of these phenomena consists in the effect of phonon modes pressing out from the layers with higher sound velocity into the layers with lower sound velocity.

For the first time there were calculated the scattering rates of electrons by acoustic phonons in AlN/GaN/AlN heterostructures. Deformation and piezoelectric mechanisms of the electron interaction with acoustic phonons had been considered. Hamiltonians, which describe these interactions, were obtained. It was shown that in three-layered AlN/GaN/AlN heterostructures high phonon modes actively interact with electron.

For the obtaining of the transport scattering time the kinetic Boltzmann equation (obtained for the isotropic law of electron energy dispersion) had been solved. At that, in given equation, the dispersion of optical phonons had been accounted for the first time along with inelasticity of the electron-phonon interaction and acoustic phonon dispersion.

It was shown that acoustically slow shells (1) decrease electron scattering rate in the internal GaN layer (channel) of heterostructures and increase electron mobility (due to the effect of phonon depletion); (2) decrease lattice heat conductivity in the heterostructures due to the effects of spectrum compressing and decreasing of the average acoustic phonon group velocity. Simultaneous increasing of the electron mobility and decreasing of the lattice heat conductivity lead to the increasing of the figure of merit ZT, i.e. to the improving of thermoelectrical properties of heterostructures.

Oficial Reviewers

• Elerlanj Sineavschii
  doctor habilitat, professor
• Igor Belousov
  doctor habilitat, professor, Institute of Applied Physics of the

 Theses