Quantum-Engineered Germanium for Spin-Based Quantum Computing

Abstract

Quantum computers could solve certain problems exponentially faster than classical computers. Among the various physical implementations being explored, spin qubits in semiconductor quantum dots have emerged as a promising platform due to their potential scalability and compatibility with existing semiconductor manufacturing. Within semiconductor platforms, germanium has recently gained significant attention due to its strong spin-orbit coupling, absence of valley states, and compatibility with industrial processes. In this thesis, we explore several aspects key to advance germanium-based quantum computing technology. The work focuses on improving the material quality and understanding fundamental properties, developing new device architectures, and enabling hybrid superconductor-semiconductor systems. We begin by investigating the effective mass of holes in strained germanium quantum wells and its dependence on carrier density, confirming theoretical predictions of a remarkably light effective mass in compressively strained germanium. We then investigate the properties of a lightly-strained germanium quantum wells which can achieve holemobility exceeding 1×106 cm2/Vs while maintaining a remarkably low percolation density. Further, we achieve a major materials breakthrough by growing germanium quantum wells directly on germanium wafers, rather than the traditional silicon substrates. This novel approach reduces threading dislocation density of nearly an order of magnitude, resulting inmobility consistently exceeding 3×106 cm2/Vs. Building on our understanding of single quantum wells, we develop and characterize germanium double quantum well systems. Through magne to transport measurements, we demonstrate the formation of a high-mobility hole bilayer and study the coupling between the two layers. Leveraging this bilayer platform, we demonstrate the first vertical gate-defined double quantum dot in a strained germanium double quantum well, opening possibilities for three-dimensional quantum circuits. We further expand the germanium planar platformand create hybrid semiconductor superconductor systems by developing high-quality superconducting contacts to germanium. Using a germano silicide process formed through thermal reaction between platinum and the semiconductor, we achieve the first demonstration of a hard superconducting gap in germanium. This breakthrough enables the integration of quantum dots with superconducting elements for hybrid quantum devices. Finally, we present a qubit-array research platform for engineering and testing (QARPET). In a significant step toward testing scalability, we present a crossbar array approach for statistical testing of spin qubit tiles, each composed of one sensor and two quantum dots. The device with over 1000 potential spin qubits, achieves a quantum dot density of 2 million per mm2 while requiring minimal control lines and a single cooldown. This development represents a significant advance for characterizing quantum devices at scale. The results presented in this thesis establish planar germanium as a versatile platform for quantum devices, offering high-quality materials, novel three-dimensional architectures, and integration with superconductors. Altogether, these developments provide a foundation for scaling up the complexity of quantum devices.