The creation of a universal quantum computer that can solve complex problems beyond the reach of classical computers - the so-called "quantum supremacy" - is an open challenge. For this to become a reality, a large number of qubits should be integrated in a quantum processor, similar to millions of transistors in a silicon chip. Among the available quantum computing approaches, semiconductor spin qubit technology has a unique advantage in scaling up to larger quantum systems, as it builds on well-established semiconductor transistor technology. However, creating a scalable quantum processor also involves several fundamental materials science challenges that go beyond merely multiplying a single device. This thesis presents our progress in the creation of a scalable Ge nanowire platform that could enable semiconductor spin-qubit technology.
The first part of this thesis introduces the reader to semiconductor spin-qubit technology and the importance of material development. It also covers the fundamentals of semiconductor and selective area epitaxy, as well as the principles of material characterization techniques and electronic transport experiments utilized in this thesis.
The second part presents the main results. We start by establishing selective area epitaxy as a scalable and reliable approach to obtain horizontal Ge nanowires on a Si substrate. Here, we present the underlying growth mechanism of Ge nanowires and the origin of crystal defects. SAE of Ge on Si proceeds via nucleation and coalescence of Ge islands. Upon coalescence, these islands merge to form continuous nanowires. Our results demonstrate the importance of the surface pretreatment step in controlling the lateral expansion of the initial Ge islands. This understanding enabled the faster formation of continuous, high-quality Ge nanowires on Si. After establishing the SAE of Ge nanowires, we demonstrate the design flexibility of SAE in obtaining Ge nanowires and their networks with precise control over their size, shape, in-plane orientation, and connectivity. The SAE Ge nanowires exhibits p-type conductivity. Low-temperature magnetotransport measurements performed on the Ge nanowire network revealed quantum diffusive transport phenomena, universal conductance fluctuations, and weak antilocalization. The ability to conduct coherent transport is key for the application of nanowires in quantum computing experiments. We then directed our efforts to improve the functionality of the Ge nanowire, which led to the development of the V-groove confined selective area epitaxy. By confining nanowire growth within a silicon V-groove structure the Si nanopillar, the Ge nanowire is naturally embedded in a crystalline silicon matrix. This eliminates the problematic Ge-SiO2 interface. A detailed analysis of the underlying growth mechanism, chemical composition, crystal quality, and electronic transport is presented. These efforts led to an increase in the hole mobility and carrier control.
Finally, the last part of this thesis concludes our progress by summarizing key results and providing an outlook on the envisioned future work.