Disorder signatures emerging at millikelvin temperatures in Si/SiGe field-effect stacks

The performance and scalability of electron spin qubits based on gate-defined quantum-dots in undoped Si/SiGe field-effect stacks remain constrained by disorder originating from the gate stack. Its coupling to the quantum well can be reduced by increasing the Si quantum well depth, while electrostatic charge history, for example through interface-trap filling, can further modify the effective disorder landscape. Although such disorder is commonly benchmarked through mobility measurements using magnetotransport and Hall bar devices, dedicated investigations at millikelvin temperatures relevant for quantum-dot operation remain limited. Here, we use temperature-dependent magnetotransport on Hall bar shaped field-effect transistors to investigate how mobility-based disorder signatures depend on quantum-well depth and charge history from \(1.5~\mathrm{K}\) down to the millikelvin regime. We show that magnetotransport characterization at \(1.5~\mathrm{K}\) captures the dominant mobility improvement associated with reduced dielectric-interface coupling, but can underestimate disorder differences that emerge at millikelvin temperatures, particularly in the low-density regime. Our results therefore highlight that millikelvin magnetotransport characterization of Hall bar devices can provide additional insight for optimizing Si/SiGe field-effect stacks, particularly in the context of gate-defined quantum dot spin qubits.