In-Substrate Imaging of Diamond hBN FET Current via Widefield Quantum Diamond Microscopy

TL;DR

This work presents operando, wide-field imaging of buried current in hBN–diamond FETs using a near-surface NV ensemble, enabling noninvasive visualization of current injection, redistribution, and gating under realistic bias. By reconstructing full vector magnetic fields from multi-axis NV measurements and solving a Biot–Savart–derived forward model with ADMM regularization, the authors map two-dimensional current densities beneath a dielectric, revealing edge-driven current crowding and gate-nonuniformities. They demonstrate laser-assisted modulation of channel conductance and a threshold shift, explained by NV-mediated photoionization generating holes that accumulate at the hydrogen-terminated surface, thereby enhancing the 2DHG; a quantitative model aligns with ΔV_th ≈ 8.6 V and corresponding I_d changes. The approach provides a powerful, generalizable tool for diagnosing buried interfaces in diamond and van der Waals heterostructures, with potential applicability to 2D materials and wide-bandgap channels, and highlights the dual role of the NV layer as both sensor and carrier reservoir.

Abstract

We demonstrate widefield magnetic imaging of current flow in hydrogen terminated diamond field effect transistors (FETs) through in-substrate nitrogen vacancy (NV) centers. Hydrogen termination of the diamond surface induces a two dimensional hole gas (2DHG), while an ensemble of near surface NV centers located $ \sim 1~μm$ below the surface enables noninvasive magnetic imaging of current flow with micrometer scale spatial resolution. The FETs were electrically characterized over a range of drain source biases $V_{ds}= 0$ to $-15V$ and gate voltages,$V_{gs}= +3$ to $-9V$ followed by in situ widefield NV magnetometry during device operation. Magnetic field maps and reconstructed current density distributions directly visualize current injection at the source drain contacts and transport beneath the hBN gated channel. Magnetic field maps reveal current density variations in the channel region owing to non-uniformities or defects in the gate dielectric. In addition, we observe a pronounced enhancement of the drain current ($\sim 600-900 μA$) and a shift in the apparent threshold voltage during laser illumination, reflecting photo induced changes in channel electrostatics. By correlating gate dependent magnetic images with simultaneous electrical measurements, we directly link spatial current distributions to FET transfer characteristics, providing new insight into buried interface transport and non-uniform gating effects in the transistor channel. As the methodology is compatible with top gated FETs, it can be used to map channel current distributions with micrometer resolution in emerging channel materials, such as 2D materials and wide bandgap channels, and establish widefield NV magnetometry as a powerful platform for probing charge transport in transistors and Van der Waals dielectric heterostructures.

Figures (7)

• Figure 1: Experimental Setup and Device (a) Schematic of the wide-field quantum diamond microscope used for NV-based magnetic imaging. (Not drawn to scale) (b) PCB integrated with microwave loop antenna-mounted diamond device (inset: optical image of the fabricated devices). (c) Image of patterned source and drain electrodes defining the hydrogen-terminated channel. (d) Image of devices with hBN flake onto the channel region as the gate dielectric using dry transfer method. (e) Final hBN-gated diamond FET device with source, drain, and gate contact pads.
• Figure 2: Electrical characterization and laser-induced modulation hBN–diamond FET. (a) Transfer characteristics measured without laser illumination. (b) Corresponding characteristics under 532 nm laser excitation, showing enhanced drain current. (c) Comparison of laser-on and laser-off transfer curves and extracted threshold voltage shift, indicating photo-induced enhancement of the channel conductivity
• Figure 3: Wide-field magnetic imaging and current-density reconstruction in hBN–diamond FETs (a) Optical micrograph of Device 1 highlighting the source--drain contacts (①) and the hBN-gated channel (②); the dashed outline indicates the boundary of the hBN flake.(b,c) Magnetic field maps $B_{NV}$ and corresponding reconstructed current density $|\mathbf{J}|$ at the contact interface and gated channel, respectively, measured at $V_{ds}= -12V$,$V_{gs}= -5V$
• Figure 4: Gate-dependent magnetic field imaging correlation with electrical transport (a) Transfer characteristics of the diamond FET at $V_{ds}= -10V$ showing the increase in drain current as $V_{gs}$ becomes more negative.(b-e) Wide-field NV magnetic field maps ($B_{NV}$ )acquired at the corresponding gate voltages. The line-cut plots shown below each image are horizontal profiles, revealing how the spatial current distribution evolves with electrostatic gating.
• Figure 5: Fabrication process flow for hBN-gated diamond FETs