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Motivated by theoretical predictions that spatially complex concentrationmodulations of Si and Ge can increase the valley splitting in quantum wells, wegrow and characterize Si/SiGe heterostructures with a thin, pure Ge layer atthe top of the quantum well using chemical vapor deposition. We show that theseheterostructures remain hosts for high-mobility electron gases. We measure twoquantum wells with approximately five monolayers of pure Ge at the upperbarrier, finding mobilities as high as 70,000 cm$^2$/Vs, compared to 100,000cm$^2$/Vs measured in samples with no Ge layer. Activation energy measurementsin quantum Hall states corresponding to Fermi levels in the gap betweendifferent valley states reveal energy gaps ranging from 30 to over 200 $\mu$eV,and we extract a surprisingly strong dependence of the energy gap on electrondensity. We interpret our results using tight binding theory and argue that ourresults are evidence that atomic scale disorder at the quantum well interfacedominates the behavior of the valley splittings of these modifiedheterostructures.

The critical role of substrate disorder in valley splitting in Si quantum wells