Empirical Pseudopotential Calculation Of Spin Relaxation In Semiconductor Self-assembled Quantum Dot

Quantum Dots (QDs) is a kind of nanostructure, developed with quatnum wires. Because the size of QDs is comparable with the electron’s de Broglie wave length, QDs can reveal several novel features, such as quantum size effect, quantum tunnelling ef-fect, interface effect and Coulomb block, which differ greatly from that in the Microsys-tim, as the size goes down to nanometers. These features can be widely applied in the nonlinear optics, magnetic medium, biology, medicine synthesis and novel materials field.As the quantum information science and spintronics developed quickly, and the integrated level problem faced by the traditional chip industry. QDs have attracted peo-ple’s interests in quantum information science, single photon source and entangled pho-ton pairs. Although the QDs is widely investigated around the world, but there is still a long way to go before we commercialize the quantum communication and eventually invent quantum computer.In this thesis, we will introduce the basic investigation about the electronic, optical and spin relaxation research in the self-assembled InGaAs/GaAs QDs. This thesis is organized as follows:1.2nd-order process spin relaxation of single particle mediated by the phonon in QDsWe need to investigate the single particle’s relaxation at the low magnetic field as high field can fasten the relaxation times. At the low field, the1st-order process is greatly suppressed, and the multiphonon process becomes donimant. It is found that the spin-phonon interaction induced spin relaxation times is100-221s for electrons, and4.4-60ms for holes at4.2K. The calculated hole spin relaxation times are in good agreement with recent experiments. In the pure QDs, the electron relaxation times de-crease with both hight and base diameter. The hole relaxation times decrease with hight but increase with the base diameter. The electron in the alloy dot shows a slimilar as the one in the pure dot, but the hole’s lifetimes in the alloy don’t vary much as the size changes. We find that there are two competing factors:the energy spacing and spin mixng to determine the relaxation time. We also investigate the effect of external elec-tric field on the spin relaxation time. We find that the electric field can fasten the hole’s realxation, as it can affect the symmetry of QDs.2. The exciton spin relaxation time in the QDsWe calculate the acoustic phonon-assisted exciton spin relaxation via spin-orbit coupling in single self-assembled InGaAs/GaAs quantum dots using an atomistic em- pirical pseudopotential method. We show that the transition rate from bright to dark exciton states is zero under Hartree-Fock approximation. The exciton spin relaxation time obtained from sophisticated conguration interaction calculations is approximately15-55μs in pure InAs/GaAs QDs and even longer in alloy dots. These results are more than three orders of magnitude longer than previous theoretical and experimental results (a few ns), but agree with more recent experiments which suggest that excitons have long spin relaxation times (>1μs). In the experiments, the exciton spin lifetime measued ranges from several ps to hundreds of ns. We point out that spin relaxation times were extracted from the bright exciton decay time, in which the exciton radiative decay is much faster than the spin relaxation. Therefore, there might be very large errors in estimating the spin relaxation time using bright exciton dynamics. A more accurate method to estimate the exciton spin relaxation time is to measure the dark exciton lifetime, in which the radiative lifetime is extremely long.3. Tuning the electronic and optical properties through hydrostatic pressure in QDsOne can generate the entangled photon pairs via so called "time reordering" scheme, which requires that the biexciton binding energies to be zero. Exciton and biexciton emission energies as well as excitonic fine structure splitting (FSS) in single (In,Ga)As/GaAs quantum dots (QDs) have been continuously tuned using hydrostatic pressure up to4.4GPa. The blue shift of excitonic emission and the increase of FSS are320meV and150μeV, respectively, which are significantly greater than ones could be achieved by pre-viously reported techniques. We successfully produce a biexciton antibinding-binding transition along with a detailed polarization-resolved emission spectra, which is a pre-requisite for the generation of entangled photon pairs via a time reordering scheme. We have performed atomistic pseudopotential calculations on realistic QDs to understand the pressure-induced effects. We point out that the variation of strain-induced confine-ment potential and wavefunctions is the major reason for the change of EB(XX).