Developing robust fabrication of silicon/silicon-germanium quantum dots with integrated RF-SET charge detectors

Solid-state approaches to quantum computing include quantum dot qubit implementations based on the Loss-DiVincenzo proposal. Prior work in GaAs two-dimensional electron gas (2DEG) materials serves as a proving ground for device designs that can be ported to Si/SiGe 2DEG systems, where the coherence time of quantum information is longer due to a combination of physical effects unique to strained Si quantum wells. In spite of the promise of Si/SiGe quantum dot qubits, several materials issues can reduce successful device yield. This work presents results from the exploration of two of these issues: the reliability of ohmic contacts to the 2DEG and the leakage current from the metallic Schottky gates used to form the quantum dots. For the ohmic contacts, growth recipes with yields approaching 100% based on two different metallizations, Au/Sb and Ag/Sb, are presented in the context of a known model for diffusion and alloying in Si. Addressing the issue of leakage currents, experiments on devices fabricated at Dartmouth strongly suggest that the major source of current leakage arises from the region near the etched mesa sidewall, where the photolithographically created metallic Schottky gate leads cover the edge of the device mesa. The solution presented here involves the deposition of SiO2 as a barrier oxide between the gate metallization and the underlying etched region to block whatever current paths might exist between the edge of the mesa and the 2DEG. Two variations on this theme are discussed: the deposition of oxide beneath the large gate leads only and the deposition of oxide immediately after etching to partially backfill the etched region with insulator. Results suggest that the latter method holds the most promise and may work even better with more robust insulators. Finally, the results of several successful devices are presented, including an radio-frequency single-electron transistor on Si/SiGe 2DEG material and a quantum dot formed in a Si/SiGe 2DEG using gates with barrier oxide underneath. This work concludes with a discussion of future directions suggested by the results obtained thus far and based on progress already made along those directions.