Semiconductor Quantum Bits And Kinetic Monte Carlo Simulations

Charge and spin are two most important intrinsic properties of an electron. With use of these two degrees of freedom in semiconductor, the information processing can be achieved in semiconductor spintronics. Introducing various defects in semiconduc-tor materials, such as magnetic dopants or vacancy complexes, various electronic spin states can be produced in the materials. By means of production, injection and transport control of electron spin, semiconductor will show many exotic features.Two distinct types of physical quantities, which are the solid-state quantum bit (qubit) and the diffusion dynamics of magnetic impurities, are studied by first-principles calculations within the framework of density functional theory. For the former, we mainly focus on searching for deep center defects with similar quantum properties of a negatively charged nitrogen-vacancy (NV-1) center in diamond as qubit candidates. For the latter, we investigate the diffusion and clustering of substitutional Mn atoms (MnGa) in the diluted magnetic semiconductor Ga1-xMnxAs by using kinetic Monte Carlo (KMC) simulations. In this thesis, the main research results are as follows.(1) A theoretic scheme combining mean-field theory and molecular orbital theory is developed to study the electron spin properties of semiconductor defect center. Based on first-principles calculations, we investigate the spin-polaried defect energy levels, formation energies, configuration-coordinate diagrams, and spin coherence times of the oxygen-vacancy (OV) center in diamond. According to the molecular orbitals (MOs) theory, the MOs of diamond OV center possessing C3v symmetry are constructed by using the projection operator techniques. The features of defect energy levels of OV0 center are investigated by using these MOs. It is discovered that the OV0 center pos-sesses an S=1 triplet ground state and four spin-conserved excited states. By calcu-lating the defect formation energies of the OV0 center in different charge states, the OV0 center is stable in the p-type diamond. According to the configuration-coordinate diagrams, we can obtain the energies of the absorption, emission, and zero-phonon-line (ZPL) photons for the OV0 center between the ground and excited states. In addition, spin coherence times of the OV0 center are estimated at T= 0 K. These results demon-strate that, similar to the NV-1 center in diamond, the OV0 center is another promising candidate for spin coherent manipulation and qubit operation.(2) Similarly, we study the energetic stability and electronic structures of the CBVN and NB VN centers in hexagonal boron nitride (h-BN) monolayer with different charge states. According to the MOs theory, the MOs of the CB VN and NB VN centers possess-ing C2v symmetries are constructed by using the projection operator techniques. The features of defect energy levels of the CB VN and NBVN centers are investigated by us-ing these MOs in different charge states. It is discovered that the CBVN and NBVN+1 centers each possess an S=1 triplet ground state and a spin-conserved excited states. By calculating the defect formation energies of the CBVN and NB VN centers in differ-ent charge states, the CBVN0 and NBVN+1 centers are stable in the n- and p-type h-BN monolayer, respectively, and they are paramagnetic centers. Moreover, spin coherence times of the CBVN0 and NB VN+1 centers are also estimated at T=0 K. The obtained results indicate that the CBVN0 and NB VN+1 centers are also very suitable for achieving spin qubit.(3) The microstructure evolution of diluted magnetic semiconductor Ga1-xMnxAs during postgrowth annealing is studied by using the KMC simulations. We can ob-tain the binding energies of the first-neighbour and second-neighbour MnGa-MnGa and MnGa-VGa by using first-principles calculations. In addition, The migration barriers of the Ga and Mn atoms in GaAs are calculated by using the climbing image nudged e-lastic band method (CI-NEB). The calculated binding and migration energies are used to study structural evolution with KMC simulations. We find that the mechanism be-hind the long-term microstructure evolution is Ga vacancy mediated MnGa diffusion on the Ga sublattice, which leads to MnGa clustering. Moreover, the higher anneal- ing temperature, the faster formation of MnGa clusters. An increase of the size and a decrease of the concentration of MnGa clusters are resulted in during long-time anneal-ing at high temperatures. The large clusters may further indicate the formation of a secondary MnAs phase. The high annealing temperatures can result in an increase of the average distances between MnGa clusters, which directly leads to the weaken ferro-magnetic coupling between them. Therefore, the MnGa clustering can reduce the Cuire temperature.