  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
variability, and to set constraints on the quality of materials and
fabrication. Here we model the variability of single qubit properties (Larmor
and Rabi frequencies) due to disorder at the Si/SiO$_2$ interface (roughness,
charge traps) 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 show that charge traps are
much more limiting than interface roughness, and can scatter Rabi frequencies
over one order of magnitude. We discuss the implications for the design of spin
qubits and for the choice of materials.
