Germanium MOSFETs for quantum computation

• Applicants: Professorin Mónica Benito González, Ph.D.; Professor Dr. Dominique Bougeard
• Subject Area: Experimental Condensed Matter Physics ; Theoretical Condensed Matter Physics
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 532774860

This project investigates holes in germanium (Ge) metal-oxide-semiconductor (MOS) nanostructures, which have not been considered for spin qubit integration before. Ge offers several advantages over silicon (Si), including the straightforward formation of p-type ohmic contacts, higher mobility (the highest of all known semiconductor materials), and lower effective mass of holes. These properties could make it easier to fabricate large-scale qubit systems with relaxed lithographic requirements. Compared to Ge/SiGe heterostructures that are grown on Si substrates and strain engineered virtual substrates, Ge-MOSFETs promise superior compatibility with industrial manufacturing techniques. They offer a larger thermal budget and provide improved electrostatic control since the charge carriers are closer to the gate electrodes. Additionally, not relying on heterostructures can reduce local alloy and strain fluctuations which impair device homogeneity. However, MOSFETs can face material-specific noise and disorder at the semiconductor-oxide interface. We use a two-fold strategy to address this issue: First, our hybrid Ge-oxide stacks are grown in situ by molecular beam epitaxy (MBE) on dislocation-free Ge substrates with a crystalline layer of magnesium oxide (MgO) being grown epitaxially on Ge without breaking the vacuum. Second, the Ge-MOS structures will employ a gate metal that minimizes interface strain at cryogenic temperatures (we explore TiN and W). Our project is a collaboration between academia and industry, bringing together experts from material growth, quantum theory, and measurement. All this expertise is crucial for reliably producing hole spin qubits in low charge noise Ge-MOS quantum dots (QDs) produced with industrial manufacturing processes. The project outline is to start by identifying growth conditions and materials resulting in the highest hole mobility through Hall bar measurements, study material properties using quantum point contacts, and then fabricate and characterize QD devices at cryogenic temperatures. The ultimate goal is to demonstrate the potential of the Ge-MOS platform by realizing hole spin qubits with competitive coherence times and control speeds.

• DFG Programme: Research Grants
• International Connection: France, Slovakia, Switzerland
• Partner Organisation: Agence Nationale de la Recherche / The French National Research Agency
• Cooperation Partners: Dr. Andreas Fuhrer Janett; Professor Thomas Markus Ihn, Ph.D.; Professor Andreas Kuhlmann, Ph.D.; Peter Stano, Ph.D.