Bachelor's Defense: Jakob Damgaard Olsen

Title: Charge noise and stability of foundry-fabricated silicon spin-qubit devices

Abstract: 

Foundry fabrication of gate-controlled silicon devices provides a tantalizing approach to massive spin-qubit production. Unlike classical transistors operating at room temperatures, such quantum devices cannot easily be simulated, and must be characterized experimentally at sub-kelvin temperatures. In this thesis, devices fabricated by two different foundry processes were compared in terms of their gate behavior and charge noise, namely planar SiMOS devices with multiple gate layers, and single-gate-layer Si/SiGe heterostructures. From DC and lock-in transport measurements performed at temperatures below 100 millikelvin, we obtained barrier pinch-off characteristics, Coulomb blockade diamonds, as well as current noise, which we convert into effective gate-voltage noise (charge noise). Overall, the stability observed in several devices were not as good as the best previous devices implemented in foundry-fabricated silicon-nanowire devices, although more statistics would be useful to compare different platforms. Specifically, the SiGe devices showed high amount of instability and large amount of charge noise. Some of the SiMOS devices showed very good stability and charge noise as low as 1µeV/√Hz, comparable to state-of-the-art non-foundry Si/SiGe devices. While the Si/SiGe platform requires more work to improve the stability of the devices, we were able to tune up double quantum dots in the SiMOS devices, making these devices good candidates to explore spin-dependent effects next, such as Elzerman-type readout and spin relaxation measurements.