Computational modeling of exchange splitting in Si/SiO₂ double quantum dots

One and two qubit operations have been demonstrated using electrons trapped in quantum dots at the interface of isotopically purified Si/SiO₂ materials. We present a computational study of Si/SiO₂ based double quantum dot qubits which could help in scaling the device design. Exchange splitting between the lowest singlet and triplet states, which plays an important role in two qubit operations, is calculated using the full configuration interaction (FCI) method. The single electron wavefunctions used in FCI are calculated using 20-band sp³d⁵s* tight binding (TB) Hamiltonian, which accurately represents the conduction-band X valley and spin orbit coupling in quantum dots. The electrostatic potential needed in TB is calculated using self-consistent effective-mass Schrodinger-Poisson (S-P) simulations on finite element meshes resembling the actual device geometry. Energy spectrum of S-P and TB simulations shows a good match at low detuning between the dots. At high detuning close to the (1,1)-(0,2) transition, where the two qubit operations take place, higher X-valleys along x, y and z axes are found to play a role in the exchange splitting, which could impact the response to charge noise.