PhD Defense: Charalampos Lampadaris

Quantum devices with group IV elements

Silicon and germanium are promising material platforms for hosting quantum processors based on spin qubits. In material synthesis, chemical vapour deposition methods have since dominated the field, producing state-of-the-art heterostructures. This work aims to explore the capabilities of molecular beam epitaxy. The study narrates the entire process, from receiving a Si(001) wafer to performing electrical measurements in quantum devices used for spin qubits.

First, the crystal growth and characterization of germanium quantum wells on virtual substrates using solid-source molecular beam epitaxy are presented. The focus then shifts to wafer preparation, employing an in-situ atomic hydrogen irradiation surface treatment technique to remove the native wafer oxide at significantly reduced temperatures. The heterostructures of this study are free from crosshatch dislocations and have the lowest root-mean-square surface roughness at approximately 500 pm. Comprehensive structural characterization by high-resolution transmission electron microscopy, X-ray diffraction reciprocal space mapping, atomic force microscopy, and Nomarski optical microscopy, along with the evaluation of threading dislocation density explore figures of merit of the heterostructures. Novel approaches with ex-situ deposition of superconducting films on the heterostructures aim to address hybrid experiments. The analysis closes with perspectives on future advancements and potential applications of the studied heterostructures.

The high-quality morphological features of the heterostructures motivate the investigation of their electrical properties. The second part of this thesis presents the fabrication methodologies of quantum devices on planar group IV substrates. In addition to the Ge/SiGe heterostructures, the Si/SiGe and SiMOS platforms, both in-house and foundry-fabricated devices, are also reviewed. In contrast to the conventional presentation of the tools and methods, this thesis addresses the troubleshooting of the failure modes that occurred in the fabrication of quantum devices.

Last, the devices were initially screened in a continuous adiabatic demagnetisation refrigerator, followed by more comprehensive characterisation in a dilution refrigerator. Hall measurements in Ge heterostructures explore the charge carrier properties in terms of mobility, indicating low percolation density. Quantum dot devices in the presented material platforms reach the few-carriers occupancy in both holes and electrons opening the path for spin-qubit experiments.

https://ucph-ku.zoom.us/j/68834532856?pwd=9qhesvaEZOEPGrpTerwvdPd2cOlifi.1

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