Abstract
A major challenge to routine non-invasive, nanoscale magnetic imaging is the development of Hall sensors that are stable under ambient conditions and retain low minimum detectable fields down to nanoscale dimensions. To address these issues we have fabricated and characterised chemical vapour deposition (CVD) graphene Hall sensors with wire widths between 50 nm and 1500 nm, in order to exploit the high carrier mobility and tuneability of this material. The measured Hall voltage noise is in good agreement with theoretical models and we demonstrate that minimum detectable fields at fixed drive current are lowest in the vicinity of the charge neutrality point. Our best performing deep sub-micron sensors, based on a wire width of 85 nm, display the excellent room temperature resolution of 59 µT/√Hz at a dc drive current of 12 µA and measurement frequency of 531 Hz. We observe a weak increase in minimum detectable field as the active sensor area is reduced while the Hall offset field is largely independent of size. These figures-of-merit significantly surpass prior results on larger probes in competing materials systems, with considerable scope for further optimisation. Our results clearly demonstrate the feasibility of using CVD graphene to realise very high spatial resolution nanosensors for quantitative room temperature magnetic imaging.
Highlights
Magnetic field sensors are used for a very wide variety of purposes, including but not limited to; biosensing, instrumentation and process calibration as well as high precision magnetic field mapping such as Scanning Hall Probe Microscopy (SHPM) and magnetic susceptometry[1,2,3,4,5]
We have studied the influence of wire width, w, on the minimum detectable field, Bmin, and systematically investigated the impact of increasing drive currents and varying carrier density and carrier type
We have demonstrated that chemical vapour deposition (CVD) graphene can be used to fabricate nanoscale Hall sensors for state-of-the-art high spatial resolution magnetic imaging
Summary
Where l is the length and w the width of the Hall voltage contacts as illustrated in the inset of Fig. 1. In practice it is well established that the carrier mobility in CVD graphene implicitly depends on the carrier density, n27. Assuming μ~ 1/nη we find the following limiting dependencies on carrier density, current and frequency in both limits. Upon fitting transconductance curves for all our devices we find η ~ 0.6, suggesting that scattering by neutral impurities is dominant in our structures[28]. We expect the coefficient of carrier density to lie somewhere between the limiting values of ~0.1 and ~0.8, while the coefficient of Hall current will lie between 0 and −1
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