Electron and Hole Spin Qubits Variability in Si MOS Devices*

Semiconductor spin qubits may show significant device-to-device variability in the presence of spin-orbit coupling mechanisms. Interface roughness, charge traps, layout or process inhomogeneities indeed shape the real space wave functions, hence the spin properties. It is, therefore, important to understand how reproducible the qubits can be in order to assess strategies to cope with this disorder. Here we model the variability of the Larmor/Rabi frequencies and spin lifetimes due to roughness and charge traps at the Si/SiO2 interface in metal-oxide-semiconductor devices. We consider both electron qubits (with synthetic spin-orbit coupling fields created by micro-magnets) and hole qubits (with intrinsic spin-orbit coupling). We highlight, in particular, the relations between the characteristic sizes of the disordered dots and their Larmor and Rabi frequencies. We show that the hole qubits are typically more sensitive to interface roughness than electron qubits due to the smaller hole confinement mass. Yet the main source of variability is charged traps, which can scatter the Rabi frequencies of both electron and hole qubits over one order of magnitude in realistic device layouts. We analyze the implications for the design of spin qubits and for the choice of materials.