  We report anomalous behavior in the energy dispersion of a three-electron
double-quantum-dot hybrid qubit and argue that it is caused by atomic-scale
disorder at the quantum-well interface. By employing tight-binding simulations,
we identify potential disorder profiles that induce behavior consistent with
the experiments. The results indicate that disorder can give rise to "sweet
spots" where the decoherence caused by charge noise is suppressed, even in a
parameter regime where true sweet spots are unexpected. Conversely, "hot spots"
where the decoherence is enhanced can also occur. Our results suggest that,
under appropriate conditions, interfacial atomic structure can be used as a
tool to enhance the fidelity of Si double-dot qubits.
