Singlet-triplet splitting of two electrons in a Si/SiGe quantum dot*

We theoretically study the effects of quantum dot confinement strength on the singlet-triplet (ST) splitting of two-electron dots in Si/SiGe quantum wells. Our analysis includes valley effects and disorder at the quantum well interface by combining full configuration interaction (FCI) scheme with tight-binding (TB) calculations. While TB provides an accurate description of single-electron wave functions by taking microscopic effects like interface disorder into account, and captures the valley physics of silicon, FCI allows us to calculate multielectron energies and corresponding wave functions by including the effects of electron-electron interactions. We show that these interactions can have unexpectedly strong or unexpectedly weak effects on the ST splitting, depending on the confinement strength and anisotropy.