Characterizing interface defects in Si/SiGe heterostructures using bias-cooled ambipolar transport

Near-surface interface defects in Si/SiGe heterostructures have long been suspected to limit the performance and reliability of spin qubits in gate defined quantum dots. Characterizing the density of these defects and how processing conditions affect them is critical for improving the performance of spin qubits. In this work we develop a simple technique to quantify the interface defect density in Si/SiGe devices based on measuring how the voltage threshold for electrons depends on an applied bias during cooling. Furthermore, we also show that ambipolar transport in the same devices enables characterizing interfacial transport properties as well as bidirectional threshold adjustment. We explore the effects of acid treatment and annealing on defect densities, finding that etching through the Si cap increases defect densities and degrades interfacial transport, while annealing reduces defect densities and improves interfacial transport.