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Spin qubits based on Si and Si$_{1-x}$Ge$_{x}$ quantum dot architecturesexhibit among the best coherence times of competing quantum computingtechnologies, yet they still suffer from charge noise that limit their qubitgate fidelities. Identifying the origins of these charge fluctuations istherefore a critical step toward improving Si quantum-dot-based qubits. Here weuse hybrid functional calculations to investigate possible atomistic sources ofcharge noise, focusing on charge trapping at Si and Ge dangling bonds (DBs). Weevaluate the role of global and local environment in the defect levelsassociated with DBs in Si, Ge, and \sige alloys, and consider their trappingand excitation energies within the framework of configuration coordinatediagrams. We additionally consider the influence of strain and oxidation incharge-trapping energetics by analyzing Si and Ge$_{\rm Si}$ DBs in SiO$_2$ andstrained Si layers in typical \sige quantum dot heterostructures. Our resultsidentify that Ge dangling bonds are more problematic charge-trapping centersboth in typical \sige alloys and associated oxidation layers, and they may beexacerbated by compositional inhomogeneities. These results suggest theimportance of alloy homogeneity and possible passivation schemes for DBs inSi-based quantum dot qubits and are of general relevance to mitigating possibletrap levels in other Si, Ge, and Si$_{1-x}$Ge$_{x}$-basedmetal-oxide-semiconductor stacks and related devices.

Dangling bonds as possible contributors to charge noise in silicon and silicon-germanium quantum dot qubits