Atomic-scale disorder at the top interface of a Si quantum well is known to suppress valley
splitting. Such disorder may be inherited from the underlying substrate and relaxed buffer
growth, but can also arise at the top quantum well interface due to the random SiGe alloy.
Here, we perform activation energy (transport) measurements in the quantum Hall regime to
determine the source of the disorder affecting the valley splitting. We consider three Si/SiGe
heterostructures with nominally identical substrates but different barriers at the top of the …