Abstract
The energy spectrum and wave functions of electrons in a single silicon quantum dot provide valuable insights into the capabilities and limitations of such a system in quantum information processing. Here we investigate the low-lying singlet and triplet configurations and spectra in a two-electron silicon quantum dot. To build toward a comprehensive understanding, we first examine the competition between Coulomb interaction and electron kinetic and confinement energy in the absence of valley-orbit coupling, as well as consequences of valley blockade in the presence of an ideal smooth interface. For realistic interfaces the variations in the magnitude and phase of valley-orbit coupling lead to inter-valley leakage, particularly when orbital splittings approach the valley splitting. In our study we particularly focus on the impact on the compositions of low-lying singlets and triplets. We find that for experimentally relevant parameter regimes the ground singlet and triplet states usually contain multiple configurations with significant weights as a result of a complicated competition among valley-orbit coupling, confinement potential, and Coulomb interaction. We further analyze the effects of an out-of-plane magnetic field on these the two-electron spectra. Our findings could have important implications for spin qubits in Si quantum dot in various contexts, such as qubit encoding and spin measurement.