Valley splittings in Si/SiGe quantum dots with a germanium spike in the silicon well
Thomas McJunkin, E. R. MacQuarrie, Leah Tom, Samuel F. Neyens, J. P. Dodson, Brandur Thorgrimsson, J. Corrigan, Hamdi Ercan, D. E. Savage, M. G. Lagally, Robert Joynt, S. N. Coppersmith, Mark Friesen, M. A. Eriksson
Abstract
Silicon-germanium heterostructures have successfully hosted quantum dot qubits, but the intrinsic near-degeneracy of the two lowest valley states poses an obstacle to high-fidelity quantum computing. We present a modification to the Si/SiGe heterostructure by the inclusion of a spike in germanium concentration within the quantum well in order to increase the valley splitting. The heterostructure is grown by chemical vapor deposition and magnetospectroscopy is performed on gate-defined quantum dots to measure the excited state spectrum. We demonstrate a large and widely tunable valley splitting as a function of applied vertical electric field and lateral dot confinement. We further investigate the role of the germanium spike by means of tight-binding simulations in single-electron dots and show a robust doubling of the valley splitting when the spike is present, as compared to a standard (spike-free) heterostructure. This doubling effect is nearly independent of the electric field, germanium content of the spike, and spike location. This experimental evidence of a stable, tunable quantum dot, despite a drastic change to the heterostructure, provides a foundation for future heterostructure modifications.