Spin Relaxation In Confined P-type Semiconductor Nanostructures

Spintronics is a newly developing subject which use the spin degree of freedom to take place of the electric degree of freedom to achieve various functions of electronic devices. Since the80,90s of the last century, dramatic developments had been made in the investigations of the spin character of electrons, mechanism of spin relaxation/dephasing has been revealed in different materials, structures, and circumstances. Nevertheless, investigations on spin relaxation/dephasing of holes in p-type semiconductors are relatively limited. Knowledge of the spin relaxation/dephasing of holes in p-type semiconductors is very important to the assesment of feasibility of hole-based spintronc devices. This is because a possible way to achieve high electronic spin injection without the conductance mismatch is to use magnetic semiconductors as spin source and most magnetic semiconductors are p-type at high temperature. Moreover, the spin characters of holes are very different from those of electrons due to the strong spin-orbit coupling. For example, in bulk material, the г-point degeneracy of the heavy hole and light hole makes the hole spin relaxation in the same order of the momentum relaxation (100fs). In this dissertation, we focus on the spin relaxation/dephasing of holes in different materials, structures, and circumstances.We first briefly introduce the band structure of the zinc-blende and wurtzite semiconductors in Chapter Ⅰ. We also introduce the origin and the representation of spin-orbit coupling in solid states. We devote a lot of paragraphs to the expressions of spin-orbit coupling and its affection to the band structure in different confined systems. After that we review the experimental and theoretical progress in nanostructures.In Chapter Ⅱ we perform a comprehensive investigation on hole spin relaxation in GaAs quantum dots (QDs) confined in quantum wells along (001) and (111) directions by exactly diagonalizing the hole Luttinger Hamiltonian. We find the direction and the diameter of the QDs can effectively influence the spin relaxation time (SRT) of holes. Moreover, unlike the case of electron spin where the SRT is mainly determined by the electron-phonon scattering due to the piezoelectric coupling, here only the hole-phonon scattering due to the deformation potential contributes to the spin relaxation. Strain changes the relative positions of energy levels of heavy hole and light hole for QD’s in (001) quantum well but introduces additional spin mixing for QD’s in (111) quantum well. The magnetic field dependence of SRT also changes when the growth direction and the strain is different. Finally we show that the hole SRT decreases with well width for QD’s which is totally opposite to thecases of electron spin in QD’s and quantum wells.In Chapter Ⅲ we have investigated the spin relaxation of holes in p-type GaAs quantum wires (QWRs). The SRT is calculated by numerically solving the fully microscopic kinetic spin Bloch equations in the helix spin space. Differing from electron’s case in n-type QWR systems where the spin-orbit coupling is weak and the collinear statistics is good enough, the helix statistics is adopted in this investigation because of the strong spin-orbit coupling for holes in QWR system. Using this approach, we have studied in detail how the hole spin relaxation is affected by the wire size, the hole densities, temperature and the spin polarization. We show that there are three mechanisms leading to spin relaxation:first, the bare spin-flip scattering; second, the spin-conserving scattering along with the inhomogeneous broadening; and third, the spin-flip scattering along with the inhomogeneous broadening. However, the bare spin-flip scattering is still the dominant spin relaxation mechanism. The QWR size influences the SRT effectively because the spin mixing and the subband structure in QWRs depend strongly on the confinement. The SRT can be changed by nearly2orders of magnitudes by changing the QWR size. In the same way, Temperature and the hole density can also influences the SRT by modulating the strength of spin mixing and the strength of inter-subband spin-flip scattering. We further show that the Hartree-Fock term increases with spin polarization and can reduce the spin relaxation.In Chapter IV we have investigated the spin relaxation of electrons in w-type InAs QWRs. The SRT is calculated by numerically solving the microscopic kinetic spin Bloch equations including multiple subbands. The inclusion of higher subbands allows us to investigate QWRs larger QWRs, and we find that the quantum-wire size influences the spin relaxation time via the internal effective magnetic field caused by spin-orbit coupling. We also studied different growth directions for QWRs. We show that one can obtain long spin relaxation time by optimizing the growth direction, quantum-wire width and the direction of the initial spin polariazation. Further, we investigate how the details of the microscopic scattering mechanisms and the spin-orbit effects in the band structure affect the spin relaxation. The population of higher subbands is found to have decisive influence on the behavior of the SRT.In Chapter V we have performed a systematic investigation of the spin dephasing of p-type GaAs quantum wells for small spin polarization by constructing a set of kinetic Bloch equations based on the nonequilibrium Green function method. We included the magnetic field, DP spin-orbital coupling and all spin conserving scattering such as hole-phonon, hole-nonmagnetic impurity and the hole-hole scattering. By numerically solving the kinetic equations, we studied the evolution of hole distribution functions and the spin coherence of spin polarized holes. The SDT was calculated from the slope of the incoherently summed spin coherence. In this way, we studied in detail how the spin dephasing are affected by various conditions, such as temperature, the Rashba coefficient, the impurity and the hole density. We compared the electrons, the light holes and the heavy holes spin dephasing. We found that the the intensity of inhomogeneous broadening in electrons states is far more smaller than in holes states. Therefore, the absolute value of electrons SDT is two orders larger than the holes SDT. And as we expected, both the electron-electron and the electron-impurity scattering reduce the spin dephasing and increase the SDT. Futhermore, we found that the intensity of inhomogeneous broadening in heavy holes states are stronger than in light holes states, which leads to a smaller SDT in heavy hole states. For that the stronger inhomogeneous broadening leads to a shorter SDT.In Chapter VI we have investigated the spin relaxation for n-type ZnO (0001) QWs by numerically solving the kinetic spin Bloch equations with all the relevant scattering explicitly included. It is shown that the electron-phonon scattering is pretty weak in ZnO QWs, while the Coulomb scattering always plays an important role. Therefore the ZnO QW is a good carrier to study the electron-electron scattering. We find there exists a peak of SRT both in the temperature dependence for a given electron density at low impurity density and in the electron density dependence at low temperature. Both these two peaks originate from the different temperature and electron density dependence of Coulomb scattering in degenerate and non-degenerate case. Compared with the same effect in III-V semiconductor this peak position can occur at the temperature as high as100K and is easier to observe in experiments due to the weak electron-phonon scattering. When the impurity density is high, the peak in the temperature dependence disappears and the SRT decreases with temperature monotonously. Moreover, the peak in the electron density dependence moves to larger electron density which is beyond the scope of our interest when the temperature is high. We also investigate the hot-electron effect and show that the SRT always increases with the electric field. It is also shown that the SRT reaches the order of nonosecond at low temperature and high impurity density.Finally in Chapter VII we have investigated the spin relaxation and the spin dephasing of electrons in w-type GaAs quantum wells and calculate T1, T2and T2*by numerically solving the kinetic spin Bloch equations. We have obtained that they have the same value in a very wide range of temperatures, electron densities and the impurity densities and we have shown that this behavior is due to the short correlation time.