Fabrication of fully suspended Hall crosses in GaAs/AlGaAs heterostructures and scaling of submicron AuGeNi ohmic contacts

In this work, we demonstrate the fabrication of a device to investigate electron–phonon coupling in quantum systems on a suspended island, together with a systematic study of submicrometer AuGeNi ohmic contacts required for its implementation. The island is separated from the substrate but electrically connected to the rest of the surrounding structure with four micrometer- or submicrometer-sized ohmic contacts and metal leads. The contacts are characterized on planar test structures and subsequently integrated into suspended devices. Square contacts with areas ranging from 1600 down to 0.0056 μm2 are fabricated in both surface-deposited and recessed geometries contacting a 20 nm GaAs quantum well, 50 nm below the surface, in a 120 nm thick GaAs/AlGaAs heterostructure. Two-point DC current–voltage (I–V) measurements at 77 and 3.8 K show that double annealing the contacts reliably produces ohmic behavior across all footprint sizes. Analysis of the area dependence of resistance reveals a critical area of (0.80 ± 0.01) μm2, corresponding to a transfer length (0.894 ± 0.013) μm and specific contact resistivities in the range of (0.5–2.3) × 10−6 Ω cm2, demonstrating that robust ohmic performance can be maintained at deep submicrometer dimensions. We further examine electrochemical etching effects in GaAs/AlGaAs and develop a fabrication pathway that avoids electrochemical damage, enabling the realization of a fully suspended Hall cross isolated from the phonon bath. This provides a platform for future studies of electron–phonon interactions and many-body-localization effects in semiconductor heterostructures.