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.

The rapidly growing number of qubits in semiconductor quantum computers requires a scalable control interface, including the efficient generation of dc bias voltages for gate electrodes. To avoid unrealistically complex wiring between any room-temperature electronics and the cryogenic qubits, this article presents an integrated cryogenic solution for the bias-voltage generation and distribution for large-scale semiconductor spin-qubit quantum processors. A dedicated cryogenic CMOS (cryo-CMOS) demultiplexer and a cryo-CMOS dc digital-to-analog converter (DAC) have been developed in a 22-nm fin field-effect transistor process to control a codeveloped 2-D array designed with 648 single-hole transistors. Thanks to the dissipation below 120 µ W, the whole system operates at temperatures below 70 mK in a custom-built electrical/mechanical infrastructure embedded in a standard single-pulse-tube dilution refrigerator. The bias voltages generated by the cryo-CMOS DAC are demultiplexed to sample-and-hold structures, allowing to store 96 unique bias voltages over a 3 V range with a voltage drift between 60 µ V / s and 18 mV/s. This work demonstrates a tight integration at mK temperatures of cryo-CMOS bias generation and distribution with a dedicated large-scale quantum device. This showcases how this approach simplifies the wiring to the electronics, thus facilitating the scaling up of quantum processors toward the large number of qubits required for a practical quantum computer.

The co-integration of spin, superconducting, and topological systems is emerging as an exciting pathway for scalable and high-fidelity quantum information technology. High-mobility planar germanium is a front-runner semiconductor for building quantum processors with spin-qubits, but progress with hybrid superconductor-semiconductor devices is hindered by the difficulty in obtaining a superconducting hard gap, that is, a gap free of subgap states. Here, we address this challenge by developing a low-disorder, oxide-free interface between high-mobility planar germanium and a germanosilicide parent superconductor. This superconducting contact is formed by the thermally-activated solid phase reaction between a metal, platinum, and the Ge/SiGe semiconductor heterostructure. Electrical characterization reveals near-unity transparency in Josephson junctions and, importantly, a hard induced superconducting gap in quantum point contacts. Furthermore, we demonstrate phase control of a Josephson junction and study transport in a gated two-dimensional superconductor-semiconductor array towards scalable architectures. These results expand the quantum technology toolbox in germanium and provide new avenues for exploring monolithic superconductor-semiconductor quantum circuits towards scalable quantum information processing.

We grow strained Ge/SiGe heterostructures by reduced-pressure chemical vapor deposition on 100 mm Ge wafers. The use of Ge wafers as substrates for epitaxy enables high-quality Ge-rich SiGe strain-relaxed buffers with a threading dislocation density of ( 6 ± 1 ) × 10 5 cm − 2 , nearly an order of magnitude improvement compared to control strain-relaxed buffers on Si wafers. The associated reduction in short-range scattering allows for a drastic improvement of the disorder properties of the two-dimensional hole gas, measured in several Ge/SiGe heterostructure field-effect transistors. We measure an average low percolation density of ( 1.22 ± 0.03 ) × 10 10 cm − 2 and an average maximum mobility of ( 3.4 ± 0.1 ) × 10 6 cm 2 / Vs and quantum mobility of ( 8.4 ± 0.5 ) × 10 4 cm 2 / Vs when the hole density in the quantum well is saturated to ( 1.65 ± 0.02 ) × 10 11 cm − 2 . We anticipate immediate application of these heterostructures for next-generation, higher-performance Ge spin-qubits, and their integration into larger quantum processors.

A hole bilayer in a strained germanium double quantum well is designed, fabricated, and studied. Magnetotransport characterization of double quantum well field-effect transistors as a function of gate voltage reveals the population of two hole channels with a high combined mobility of (Formula presented.) and a low percolation density of (Formula presented.). The individual population of the channels from the interference patterns of the Landau fan diagram was resolved. At a density of (Formula presented.) the system is in resonance and an anti-crossing of the first two bilayer subbands is observed and a symmetric-antisymmetric gap of (Formula presented.) is estimated, in agreement with Schrödinger-Poisson simulations.

We demonstrate that a lightly strained germanium channel (ϵ / / = - 0.41 %) in an undoped Ge/Si0.1Ge0.9 heterostructure field effect transistor supports a two-dimensional (2D) hole gas with mobility in excess of 1 × 10 6 cm2/Vs and percolation density less than 5 × 10 10 cm-2. This low disorder 2D hole system shows tunable fractional quantum Hall effects at low densities and low magnetic fields. The low-disorder and small effective mass (0.068 m e) defines lightly strained germanium as a basis to tune the strength of the spin-orbit coupling for fast and coherent quantum hardware.