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

Technical Summary: In-Substrate Imaging of Diamond–hBN FET Current via Wide-Field Quantum Diamond Microscopy

Problem Statement
Hydrogen-terminated diamond field-effect transistors (FETs) utilizing a two-dimensional hole gas (2DHG) are promising for high-power and high-frequency electronics. However, the performance of these devices is heavily dependent on the quality of the gate dielectric. While hexagonal boron nitride (hBN) offers an ideal dielectric due to its atomic flatness and lack of dangling bonds, the charge transport mechanisms at the buried hBN/hydrogen-terminated diamond interface remain poorly understood. Conventional electrical measurements and scanning probe techniques cannot access this buried channel, leaving critical questions regarding interface states, charge screening, non-uniform gating, and current localization unresolved.

Methodology
The authors fabricated diamond FETs directly on a diamond substrate containing a near-surface nitrogen-vacancy (NV) center layer located approximately 1 µm beneath the surface. This proximity allows the NV centers to act as an in-situ magnetic sensor for the overlying 2DHG channel.

• Device Fabrication: Source and drain electrodes were patterned to define a 10 µm channel. Exfoliated hBN flakes were transferred onto the channel to serve as the gate dielectric.
• Imaging System: A wide-field quantum diamond microscope (QDM) was employed, utilizing continuous-wave optically detected magnetic resonance (CW-ODMR) with lock-in detection. A 532 nm laser uniformly illuminated the NV layer, and the resulting photoluminescence was imaged onto a scientific CMOS camera.
• Operational Protocol: The devices were electrically characterized under various drain-source ($V_{ds}$) and gate ($V_{gs}$) biases. Simultaneously, the system captured wide-field magnetic field maps. These maps were processed to reconstruct two-dimensional current density distributions. The study compared devices with uniform hBN flakes against those with non-uniform flakes to assess dielectric impact.

Key Results

1. Visualization of Current Pathways: The technique successfully visualized current injection at source-drain contacts, revealing "current crowding" lobes characteristic of spreading resistance. Beneath the hBN gate, the reconstructed current density maps showed generally uniform flow, confirming effective electrostatic control in high-quality devices.
2. Detection of Dielectric Inhomogeneities: In devices with non-uniform hBN flakes, the magnetic imaging revealed pronounced spatial modulations in current density. Regions with thicker or delaminated hBN showed weaker gate coupling and higher local current densities, directly linking dielectric defects to transport inhomogeneities.
3. Gate-Dependent Dynamics: By sweeping the gate voltage while imaging, the authors observed how electrostatic gating reshapes current flow in real space. They noted that current suppression occurred earlier and more strongly at the edges of the hBN flake compared to the center, indicating spatial variations in gate efficiency due to dielectric thickness or interface coupling.
4. Laser-Induced Modulation: A significant finding was the pronounced enhancement of drain current (increasing from ~600 µA to ~900 µA) and a shift in the apparent threshold voltage (by ~8.6 V) when the 532 nm laser used for NV readout was active. The authors attribute this to photo-induced electron-hole pair generation within the buried NV layer, followed by hole accumulation at the hydrogen-terminated surface, effectively increasing the 2DHG density.

Key Contributions

• First In-Substrate Imaging: This work presents the first direct magnetic imaging of current flow inside operating diamond FETs with hBN gate dielectrics.
• Correlative Analysis: The study establishes a direct link between macroscopic electrical transfer characteristics and microscopic spatial current distributions, providing a method to correlate device performance with local transport phenomena.
• Non-Invasive Dielectric Characterization: The methodology demonstrates the ability to map channel current distributions with micrometer resolution without invasive probes, specifically identifying non-uniform gating effects caused by dielectric defects.
• Optoelectronic Coupling Insight: The paper identifies and quantifies the dual role of the NV layer as both a magnetic sensor and an optically addressable carrier reservoir, revealing how optical excitation conditions can modulate carrier density in the device.

Significance and Claims
The authors claim that wide-field NV magnetometry serves as a powerful, non-invasive platform for probing charge transport in transistors and Van der Waals heterostructures. By enabling the visualization of buried interfaces, this approach provides insights into contact resistance, dielectric non-uniformity, and gate-induced inhomogeneities that are inaccessible to standard electrical measurements. The paper posits that this methodology is broadly applicable to emerging channel materials, including 2D materials and wide-bandgap channels, offering a new tool for the design, diagnostics, and optimization of next-generation nanoelectronic systems. The work also highlights the potential for hybrid optoelectronic concepts where spin defects are coupled to surface transport.