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Events on Thursday, August 24th, 2023

Thesis Defense
Defect Identification using Kelvin Probe Force Microscopy and Optimization of Long Single-Channel One-dimensional Quantum Wires
Time: 4:00 pm - 5:30 pm
Place: 5310 Chamberlin
Speaker: Leah Tom, Department of Physics Graduate Student
Abstract: The advent of quantum computing has been hailed as the next industrial revolution because of its promise to solve problems that are beyond the reach of classical computers. In order to harness the potential of quantum computing, it is important to protect fragile qubit states from environmental disturbances. This requires designing microelectronics with a greater degree of control over their fabrication. This thesis describes two projects, the first of which addresses the need for an enhanced understanding of defects in quantum dot qubit systems, and the second of which develops long single-channel one-dimensional quantum wires for topological qubits.

Charge fluctuators in Atomic Layer Deposited (ALD) aluminum oxide represent a major source of charge noise in quantum dot qubit devices. To mitigate this charge noise, the defects that adversely affect qubit operations need to be identified so that they can eventually be eliminated from the gate oxide. The spatial distribution of the defects in the gate oxide needs to be determined to correlate the defects with the charge noise measured.

Towards this end, Kelvin Probe Force Microscopy (KPFM) measurements are performed on a layer of ALD aluminum oxide grown atop bulk silicon. KPFM measures local variations in the work function that reveal a high density of charged defects in the aluminum oxide layer. Sweeping the AFM tip-to-sample bias induces charging and discharging events near the surface, allowing us to probe the defects' different charge states. With the aid of electrostatic simulations, the charging and discharging energies are extracted as a function of the voltage bias. The sign and magnitude of a charge state can also be determined from KPFM measurements. This thesis presents a method for identifying point defect distributions down to individual defects in a sample of high defect density.

This thesis also proposes a split gate design for creating long and uniform 1D quantum wires in low disorder systems that will be used for topological quantum computation using Majorana Zero Modes. This gate design is predicted to increase the length of a channel in a single conducting mode to 60-75% of lithographic gate length for a range of quantum wire lengths. This thesis also discusses how the split gate design prevents the formation of quantum dots in the channel and how to improve the channel’s adiabaticity.
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