The Wiggle Well: An oscillatory concentration of germanium within a silicon quantum well

Motivated by a desire to enhance the splitting between the two low-lying valley states in Si quantum dots for use as qubits, we present a new Si/SiGe heterostructure named the “Wiggle Well”. This heterostructure replaces the pure Si quantum well with an oscillating concentration of Ge (0-9%) in the direction of growth. Theoretical calculations indicate the valley splitting can be increased by matching the oscillation period to one of a set of specific relationships between the reciprocal lattice points and the valley minima wavevector k₀. The wavelength presented here is ~1.8 nm. The Wiggle Well is grown via CVD and we present STEM measurements to show the viability of this changing Ge concentration. Hall bar and quantum dot devices are fabricated on this heterostructure. We report an electron transport mobility in the range of 15,000-30,000 cm²/(V s). Valley splitting is measured in two separate quantum dots by excited state spectroscopy and we measure valley splitting in the range 0.1-0.2 meV. Electric field and confinement at the dot are modified while maintaining dot position, and we report a 25% tuning of the single electron valley splitting. We present a discussion of these experimental results in context with our theoretical basis for this heterostructure.