Signatures of Phonon and Defect-Assisted Tunneling in Planar Metal–Hexagonal Boron Nitride–Graphene Junctions
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2016-11-28 |
| Journal | Nano Letters |
| Authors | U. Chandni, Kenji Watanabe, Takashi Taniguchi, J. P. Eisenstein |
| Institutions | National Institute for Materials Science, California Institute of Technology |
| Citations | 60 |
| Analysis | Full AI Review Included |
Technical Documentation & Analysis: Diamond Platforms for Quantum Transport
Section titled “Technical Documentation & Analysis: Diamond Platforms for Quantum Transport”This document analyzes the research detailing phonon and defect-assisted tunneling in metal/hBN/graphene junctions, focusing on how 6CCVD’s expertise in MPCVD diamond growth and fabrication can serve as a critical enabling technology for extending and optimizing similar quantum transport studies.
Executive Summary
Section titled “Executive Summary”- Core Achievement: Demonstrated inelastic electron tunneling spectroscopy (IETS) in planar Metal/hBN/Graphene van der Waals heterostructures at a cryogenic temperature of 4.2 K.
- Key Phenomena Detected: Identified two dominant quantum effects: Phonon-assisted inelastic tunneling (PAT), marked by a strong enhancement in conductance above |V| ≈ 50 mV, and single-electron charging (Coulomb blockade) attributed to intrinsic nanometer-scale defects within the hBN barrier.
- Critical Phonon Signatures: Four distinct peaks in the second derivative of tunnel current (d2I/dV2) were observed, corresponding directly to known Graphene and hBN phonon modes (ZA, ZO, LO, TO).
- Material Opportunity: The requirement for ultra-flat surfaces and robust low-temperature substrates makes 6CCVD Single Crystal Diamond (SCD) an ideal platform replacement for conventional SiO2/Si, offering superior thermal management and surface purity (Ra < 1 nm).
- Fabrication Match: 6CCVD offers internal capabilities for depositing the necessary metal electrodes (e.g., Cr/Au, Ti/Pt/Au) and custom micro-machining of diamond wafers, facilitating integrated device architectures for advanced quantum systems.
- Core Value Proposition: Transitioning these complex quantum transport experiments onto high-purity MPCVD SCD substrates minimizes background noise, improves thermal stability, and opens pathways for integrating diamond-based quantum emitters (e.g., NV centers) into 2D material devices.
Technical Specifications
Section titled “Technical Specifications”Data extracted from the paper concerning device operation and physical parameters.
| Parameter | Value | Unit | Context |
|---|---|---|---|
| Operating Temperature (T) | 4.2 | K | Cryogenic quantum transport regime. |
| AC Excitation Voltage (Vac) | 0.5 | mV | Standard lock-in measurement sensitivity. |
| Tunnel Barrier Material | hBN | Material | Insulator in Metal/hBN/Graphene junction. |
| hBN Barrier Thickness | 2-5 (≤ 2) | Atomic Layers (nm) | Ultra-thin barrier thickness tested. |
| Tunnel Junction Area | 4 to 36 | µm2 | Lateral dimensions (2x2 to 6x6 µm2). |
| PAT Peak A Voltage ( | VA | ) | 36 ± 3 |
| PAT Peak B Voltage ( | VB | ) | 61 ± 2 |
| PAT Peak C Voltage ( | VC | ) | 74 ± 2 |
| PAT Peak D Voltage ( | VD | ) | 166 ± 8 |
| Substrate Dielectric | SiO2 | Material | Thickness: 285 nm, used for back gating. |
| Electrode Metal Stacks | Cr/Au (5/120), Ag (120) | nm | Top tunnel electrode composition. |
Key Methodologies
Section titled “Key Methodologies”A concise, step-by-step summary of the device fabrication and measurement protocol.
- Exfoliation and Preparation: Graphene/Graphite and hBN flakes prepared via mechanical exfoliation on separate Si/SiO2 handling wafers.
- Heterostructure Assembly: Polymer stamp dry transfer technique utilized to stack the layers sequentially (Graphene bottom electrode, then ultra-thin hBN barrier).
- Top Electrode Fabrication: Metallic contacts (Cr/Au 5 nm/120 nm or Ag 120 nm) defined lithographically and deposited to form the top tunnel electrode.
- Device Integration: Final structure assembled on SiO2/Si substrate (285 nm SiO2 on p-doped Si back gate) or on thick hBN layers mounted on SiO2/Si.
- Electrical Measurement: Transport characterized using standard lock-in techniques at T = 4.2 K.
- IETS Measurement: Differential conductance (dI/dV) measured at 13 Hz, and the second derivative (d2I/dV2) measured simultaneously using a second lock-in synchronized at 26 Hz (twice the excitation frequency).
- Gate Tuning: Back gate voltage (Vg) applied via the doped Si substrate to electrostatically tune the carrier density of the graphene electrode.
6CCVD Solutions & Capabilities
Section titled “6CCVD Solutions & Capabilities”6CCVD is positioned to provide advanced MPCVD diamond substrates and integration services critical for optimizing high-resolution quantum transport experiments and scaling van der Waals heterostructure devices.
Applicable Materials
Section titled “Applicable Materials”The study highlights the need for material purity and smooth interfaces. Diamond offers substantial advantages over conventional silicon-based substrates, particularly in extreme environments.
| Research Requirement | 6CCVD Material Recommendation | Material Specification |
|---|---|---|
| Ultra-High Surface Quality | Optical Grade Single Crystal Diamond (SCD) | Polishing: Ra < 1 nm across large areas. |
| Thermal Management | SCD or High Thermal Conductivity PCD | Thermal Conductivity: > 2000 W/mK (critical for localized cooling at 4.2 K). |
| Low Defect Density Substrate | Electronic Grade SCD | Purity minimizes charge traps and background parasitic capacitance, preserving signal integrity during IETS measurements. |
| Integrated Dielectric | Boron-Doped Diamond (BDD) | BDD can be used as a conductive bottom gate electrode, or insulating SCD can replace SiO2/Si, leveraging diamond’s 5.5 eV bandgap. |
Customization Potential
Section titled “Customization Potential”The planar junction design requires precise metal contacts and well-defined geometries. 6CCVD’s in-house fabrication capability ensures these requirements are met with high fidelity.
- Integrated Metal Electrodes: The paper utilized Cr/Au and Ag electrodes. 6CCVD offers expert, in-house metalization services, including Au, Pt, Pd, Ti, W, and Cu deposition. We specialize in producing custom multi-layer stacks optimized for adhesion, wire bonding, and cryogenic environments (minimizing thermal strain mismatch).
- Precision Diamond Shaping: The devices used small, precise lateral dimensions (µm2 scale). 6CCVD can laser cut and micromachine diamond plates and wafers up to 125 mm to exact specifications, ensuring perfect substrate preparation for micro-scale lithography and 2D material stacking.
- Custom Dimensions: We supply SCD/PCD plates from 0.1 µm up to 500 µm in thickness, allowing researchers flexibility in designing substrates for thermal, mechanical, or specific gate-geometry constraints.
Engineering Support & Sales Opportunity
Section titled “Engineering Support & Sales Opportunity”This research demonstrates sophisticated electronic measurements sensitive to phonon modes and nanoscale defects. Integrating these devices onto diamond offers a clear path toward commercializing van der Waals quantum devices, particularly those leveraging diamond’s inherent quantum sensing capabilities (e.g., NV centers).
6CCVD’s engineering team, composed of PhD experts in diamond material science, provides comprehensive support for:
- Selecting the optimal diamond grade (SCD vs. PCD) based on required purity and size.
- Designing custom metal stacks to ensure stable, low-resistance cryogenic contacts.
- Consultation on achieving ultra-low surface roughness (Ra < 1 nm) necessary for defect-free 2D material transfer.
For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.
View Original Abstract
Electron tunneling spectroscopy measurements on van der Waals heterostructures consisting of metal and graphene (or graphite) electrodes separated by atomically thin hexagonal boron nitride tunnel barriers are reported. The tunneling conductance, dI/dV, at low voltages is relatively weak, with a strong enhancement reproducibly observed to occur at around |V| ≈ 50 mV. While the weak tunneling at low energies is attributed to the absence of substantial overlap, in momentum space, of the metal and graphene Fermi surfaces, the enhancement at higher energies signals the onset of inelastic processes in which phonons in the heterostructure provide the momentum necessary to link the Fermi surfaces. Pronounced peaks in the second derivative of the tunnel current, d<sup>2</sup>I/dV<sup>2</sup>, are observed at voltages where known phonon modes in the tunnel junction have a high density of states. In addition, features in the tunneling conductance attributed to single electron charging of nanometer-scale defects in the boron nitride are also observed in these devices. The small electronic density of states of graphene allows the charging spectra of these defect states to be electrostatically tuned, leading to “Coulomb diamonds” in the tunneling conductance.
Tech Support
Section titled “Tech Support”Original Source
Section titled “Original Source”References
Section titled “References”- 1985 - Principles of Electron Tunneling Spectroscopy
- 2008 - Introduction to Scanning Tunneling Microscopy