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We present results of ab initio calculations for the electronic and atomic structures of monovacancies and antisite defects in 4H-SiC in all possible charge states. The calculations make use of a plane-wave pseudopotential method based on density-functional theory and the local spin-density approximation. Formation energies, ionization levels, and local geometries of the relaxed structures are reported for defects at all possible cubic and hexagonal lattice sites. To correct for the electrostatic interaction between charged supercells, we use a Madelung-type correction for the formation energies, leading to good agreement with experimentally observed ionization levels. Our calculations indicate no negative-U behaviour for carbon vacancies. Hence, the singly positive charge state of the carbon vacancy VC+ is stable, as recently found in experiments. The silicon antisite SiC+ is found to be stable at low values of electron chemical potential—again in agreement with experiment.

We study the effects of electron-electron correlations and confinement potential on the far-infrared spectrum of a lateral two-electron quantum dot molecule by exact diagonalization. The calculated spectra directly reflect the lowered symmetry of the external confinement potential. Surprisingly, we find interactions to drive the spectrum towards that of a high-symmetry parabolic quantum dot. We conclude that far-infrared spectroscopy is suitable for probing effective confinement of the electrons in a quantum dot system, even if interaction effects cannot be resolved in a direct fashion.

All material supplied via Aaltodoc is protected by copyright and other intellectual property rights, and duplication or sale of all or part of any of the repository collections is not permitted, except that material may be duplicated by you for your research use or educational purposes in electronic or print form. You must obtain permission for any other use. Electronic or print copies may not be offered, whether for sale or otherwise to anyone who is not an authorised user. Structural properties of a finite number (Nϭ2Ϫ20) of point charges ͑classical electrons͒ confined laterally in a two-dimensional two-minima potential are calculated as a function of the distance ͑d͒ between the minima. The particles are confined by identical parabolic potentials and repel each other through a Coulomb potential. Both ground-state and metastable electron configurations are discussed. Discontinuous transitions from one configuration to another as a function of d are observed for Nϭ6,8,11,16,17,18,and 19. We show that the structural transitions have quantum-mechanical counterparts also in the limit of noninteracting electrons.