Thermal noise-limited detection of low-frequency magnetic fields using graphene Hall sensors with reduced flicker noise

The presence of flicker noise in graphene-based Hall sensors (GHSs) with high Hall sensitivity degrades the signal-to-noise ratio for the detection of low-frequency magnetic signals. We report the design and characterization of a local back-gated GHS that achieves record-low flicker noise magnitude and corner frequency. These unprecedented noise levels are enabled by a modified fabrication process flow that suppresses effective trap density near the graphene channel. We report the measured record-low bias and area-normalized noise densities at 10 Hz for the driving and sensing terminals as 4.6 × 10−13 and 7.33 × 10−14 μm2 Hz−1, respectively. This translates to flicker-noise corner frequencies calculated as 32.5 and 0.6 Hz at the driving and sensing terminals, respectively. Furthermore, using results from adjoint network analysis for a generalized two-dimensional Hall structure, we correlate the flicker noise behaviors at the sensing and driving terminals of the GHS. To understand the origin of flicker noise in our fabricated devices, we study the noise contributions separately from the channel and contact regions by independently modulating each region through the local and global back gates, respectively. This analysis identifies the channel region as the primary source of flicker noise. Within the channel, to understand the dominant fluctuation mechanism in our devices, i.e., either carrier number or mobility fluctuations, we perform systematic investigation, through variations in carrier density, magnetic field, and temperature, at both the driving and sensing terminals, the latter being of particular relevance to the Hall sensor architecture. These measurements collectively support mobility fluctuations as the dominant mechanism. Moreover, to demonstrate the thermal-noise-floor-limited detection of low-frequency magnetic fields below the flicker noise corner frequency, we report, for the first time, the integration of the spinning current technique (SCT) with GHS. We achieve a current-normalized Hall sensitivity of 847 V/A T and a thermal noise floor of 29 nV/Hz using SCT, equivalent to a magnetic field noise spectral density of 342 nT/Hz.