Toward All‐Carbon Electronics Buried in Diamond
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2025-09-27 |
| Journal | Advanced Electronic Materials |
| Authors | Calum S. Henderson, Patrick S. Salter, Emil T. Jonasson, Richard B. Jackman |
| Institutions | United Kingdom Atomic Energy Authority, London Centre for Nanotechnology |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”This research demonstrates a paradigm-shifting approach to diamond electronics by fabricating functional, all-carbon nanocarbon networks (NCNs) buried within bulk diamond using femtosecond laser processing.
- Novel Fabrication: Femtosecond laser writing enables the creation of complex, 3D electrically active architectures (NCNs) deep inside the diamond substrate, circumventing the challenges associated with traditional substitutional doping (e.g., deep states, poor interface stability).
- Tunable Electrical Properties: By varying the laser Pulse Repetition Rate (PRR), the electrical behavior of the NCNs can be precisely tuned:
- 1 kHz PRR yields Ohmic, semi-metallic conduction (graphitic character).
- 1 MHz PRR yields highly resistive, semiconductive behavior (Ea ≈ 0.54 eV).
- 1 kHz overwritten by 1 MHz (PRR-1k1M) yields ambipolar/pseudo-diode behavior with a high rectification ratio (>5500).
- Device Stability: The laser-written pseudo-diodes demonstrated high stability, maintaining a rectification ratio greater than three orders of magnitude over 120 repeated voltage sweeps.
- Proof-of-Concept Transistor: A fully buried, all-carbon Field Effect Transistor (FET) architecture was successfully fabricated, utilizing the ambipolar NCN material as the channel and a separate NCN structure as the gate.
- Material Composition: The semiconductive and ambipolar NCNs are hypothesized to contain a ‘diaphite’ phase—an extended diamond-graphite interface—which provides a tunable bandgap under the strain of the surrounding crystalline diamond.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| PRR-1k Resistance | 76 ± 8 | kΩ | Ohmic NCN columns |
| PRR-1k Activation Energy (Ea) | 4.9 ± 0.2 | meV | Semi-metallic conduction |
| PRR-1k Resistivity (Sheet Model) | 1.3 x 10-5 | Ωm | Comparable to crystalline graphite |
| PRR-1M Resistance | >1 | TΩ | Highly resistive NCN columns |
| PRR-1M Activation Energy (Ea) | 0.54 ± 0.02 | eV | Semiconductive behavior (0V DC bias) |
| PRR-1k1M Rectification Ratio (Peak) | >5500 | Ratio | Pseudo-diode structure |
| PRR-1k1M Ea (Tunable Range) | 17 ± 3 to 600 | meV | Tunable via DC bias (-30V to +30V) |
| FET Maximum Current Density (JMax) | 52.5 | µA/µm2 | At VG = -20 V |
| FET Maximum Transconductance (gm) | 39.0(1) | nS | Proof-of-concept device |
| FET Gate-Channel Separation | 5 | µm | Intrinsic diamond layer |
| Diamond Substrate Grade | B < 1 ppb, N < 10 ppb | Concentration | Electronic grade CVD diamond |
| Contact Annealing Temperature | 600 | °C | Ti/Pt/Au Ohmic contact formation |
| PRR-1k/1M Laser Wavelength | 515 | nm | Light Conversion Pharos system |
| PRR-1k/1M Pulse Energy/Duration | 120 nJ / 170 fs | Energy / Time | Used for vertical columns |
| FET Gate Laser Wavelength | 790 | nm | Spectra Physics Solstice system |
| FET Gate Pulse Energy/Duration | 110 nJ / 250 fs | Energy / Time | Used for 3D gate structure |
Key Methodologies
Section titled “Key Methodologies”The fabrication process relies on two distinct femtosecond laser systems and specific post-processing steps:
-
Vertical NCN Column Fabrication (PRR-1k, PRR-1M, PRR-1k1M):
- Laser System: Light Conversion Pharos (Yb:KGW).
- Wavelength/Energy: 515 nm, 120 nJ pulse energy, 170 fs pulse duration.
- Focusing: Zeiss 20x objective (0.5 NA), resulting in a spot size of 0.6 µm (x,y) and 9.8 µm (z).
- Writing Process: The laser spot was drawn upwards from the seed side (bottom) to the laser side (top) at a speed of 10 µm/s.
- PRR Variation:
- PRR-1k: 1 kHz (Ohmic conductive).
- PRR-1M: 1 MHz (Semiconductive, highly resistive).
- PRR-1k1M: Initial 1 kHz write, followed by an overwrite using 1 MHz PRR (Ambipolar/Diode).
-
3D Gate Structure Fabrication (FET):
- Laser System: SpectraPhysics Solstice (Ti:Sapphire).
- Wavelength/Energy: 790 nm, 110 nJ pulse energy, 250 fs pulse duration.
- Focusing: 1.4 NA oil-immersion objective, achieving tighter axial confinement (spot size 0.3 µm (x,y) and 2 µm (z)).
- Structure: A cage-like gate structure was written, separated from the central channel by 5 µm of intrinsic diamond.
-
Post-Processing and Contacting:
- Cleaning: Substrates were cleaned in a boiling acid solution (H2SO4:(NH4)2SO4) at 200 °C for 20 minutes.
- Surface Passivation: Ozone treatment (200 °C, 50 mbar, 1 hour) was used to oxygen-terminate the surface, preventing surface transfer doping (STD) and isolating the devices.
- Ohmic Contacts: Ti/Pt/Au (20:5:200 nm) contacts were deposited on the laser-written features via electron-beam evaporation and photolithographic lift-off.
- Annealing: Contacts were annealed at 600 °C in vacuum for 1 hour to ensure reliable Ohmic behavior.
Commercial Applications
Section titled “Commercial Applications”The development of robust, buried, all-carbon electronics in diamond addresses critical needs in extreme environment and high-power applications, leveraging diamond’s superior physical properties.
- High-Power Electronics:
- Fabrication of robust, high-breakdown voltage Schottky diodes and FETs (using PRR-1k1M and PRR-1k NCNs).
- Enables true vertical device design by replacing metallic surface connections with protected, sub-surface graphitic channels, improving reliability and breakdown voltage.
- Extreme Environment Sensing and Control:
- Radiation Hardness: Ideal for nuclear facilities, space, and high-energy physics, where diamond’s inherent radiation tolerance is critical.
- High-Temperature Operation: The high activation energies observed (up to 0.58 eV) suggest suitability for electronics operating at elevated temperatures where conventional silicon or surface-doped diamond fails.
- Integrated Circuits (ICs):
- The ability to create complex, 3D buried architectures (like the FET gate) allows for the development of complete, robust, all-carbon integrated circuits entirely encapsulated within the diamond substrate.
- Advanced Detectors:
- The NCN structures can be used to create highly stable diamond-based radiation detectors, combining the sensitivity of thin active regions with the stability of thick diamond substrates.
View Original Abstract
Abstract This work investigates the use of femtosecond laser processing to fabricate various nanocarbon structures with distinct electrical behaviors within diamond substrates. Conventional approaches for achieving diamond doping have significant disadvantages, including challenging growth profiles, limited environmental stability, and sub‐optimal psuedo‐vertical structures. Here, it is demonstrated that laser‐written nanocarbon networks (NCNs) directly alleviate these issues, demonstrating the highly repeatable fabrication of robust and precise electrical architectures buried in diamond with proven stability over repeated temperature and voltage cycling. By varying the laser pulse repetition rate (PRR), a transition from Ohmic conductive to semiconductive/ambipolar behavior is achieved in the modified diamond. Furthermore, a proof‐of‐concept, all‐carbon transistor architecture buried within the bulk diamond is presented, showcasing the potential for integrated device fabrication using the laser‐writing process.
Tech Support
Section titled “Tech Support”Original Source
Section titled “Original Source”References
Section titled “References”- 2007 - Physica Status Solidi (A) Applications and Materials Science
- 2018 - Journal of Instrumentation