Effect of crystallographic orientation on the potential barrier and conductivity of Bessel written graphitic electrodes in diamond

At a Glance

Metadata	Details
Publication Date	2023-12-26
Journal	Diamond and Related Materials
Authors	Akhil Kuriakose, Andrea Chiappini, Pietro Aprà, Ottavia Jedrkiewicz
Institutions	University of Insubria, Istituto di Fotonica e Nanotecnologie
Citations	6
Analysis	Full AI Review Included

Technical Documentation: Crystallographic Control for High-Conductivity 3D Diamond Electrodes

This document analyzes the research on ultrafast laser graphitization in monocrystalline CVD diamond, focusing on the critical role of crystallographic orientation in eliminating potential barriers and achieving ultra-low resistivity for embedded microelectrodes.

Executive Summary

This research validates the critical role of diamond crystallographic orientation in optimizing in-bulk graphitic electrodes for advanced applications like 3D radiation detectors and quantum sensing platforms.

Orientation Breakthrough: The use of (110) oriented Single Crystal Diamond (SCD) completely eliminates the potential barrier observed in current-voltage (IV) measurements, a major limitation in (100) oriented samples fabricated in the femtosecond (fs) regime.
Ultra-Low Resistivity: Resistivity values as low as 0.013 Ω cm were achieved in (110) SCD, representing one of the lowest reported values for in-bulk graphitic micro-electrodes written perpendicular to the surface using Bessel beams.
Morphological Improvement: Graphitic wires fabricated in (110) SCD were significantly thinner (≈ 1 µm) and exhibited smoother morphology compared to those in (100) SCD (≈ 2.5 µm), leading to higher quality, continuous conductive paths.
Process Optimization: The absence of a potential barrier in (110) SCD is attributed to enhanced graphitization efficiency resulting from secondary heating mechanisms linked to the directionality of cracking along the (111) planes.
Thermal Stability: Thermal annealing (950° C) consistently reduced resistivity across all samples but had no effect on the height of the potential barrier, confirming that the barrier is a structural feature of the laser-induced transformation.
Application Relevance: These findings enable the fast fabrication of high-quality, high-conductivity 3D electrodes necessary for next-generation diamond detectors and integrated photonic/microfluidic chips.

Technical Specifications

The following table summarizes the key material properties and performance metrics extracted from the electrical and structural characterization of the graphitic microelectrodes.

Parameter	Value	Unit	Context
Diamond Material	Monocrystalline Type IIa CVD	N/A	Substrate for in-bulk graphitization
Sample Thickness	500	µm	Electrode length (through-wire)
Optimal Orientation	(110)	N/A	Eliminates potential barrier in fs regime
Lowest Resistivity (Post-Anneal)	0.013	Ω cm	Achieved in (110) cut, 6 µJ, 200 fs
Resistivity (100) Cut, 3.5 µJ	0.41	Ω cm	Significantly higher than (110) counterpart
Potential Barrier (100) Cut, 3.5 µJ	45	V	Observed barrier height (constant pre/post-anneal)
Potential Barrier (110) Cut	0	V	Absent across all tested fs/ps parameters
(110) Wire Transverse Size	≈ 1	µm	Thinner, smoother morphology
(100) Wire Transverse Size	≈ 2.5	µm	Wider, more pronounced cracking
Laser Pulse Duration (fs)	200	fs	Regime where orientation effect is most pronounced
Laser Pulse Duration (ps)	10	ps	Regime showing no barrier in either orientation

Key Methodologies

The conductive graphitic micro-electrodes were fabricated using ultrafast laser micromachining combined with high-temperature post-processing.

Laser System: 20-Hz Ti:Sapphire amplified laser system (Amplitude) delivering 40-fs transform-limited pulses at 790 nm wavelength.
Beam Shaping: A Gaussian beam was converted into a Bessel beam (BB) using a fused silica axicon (178° apex angle) and a telescopic system.
Bessel Beam Parameters:
- Central Core Size: ≈ 2.7 µm
- Total Bessel Zone Length: 700 µm (exceeding the 500 µm sample thickness)
- Cone Angle: 12°
Micromachining Regime: Transverse configuration, orthogonal injection to the surface, performed in multiple shot regime (9000 pulses used for final electrodes).
Pulse Parameters Tested:
- Pulse Durations: 200 fs (femtosecond) and 10 ps (picosecond).
- Pulse Energies: Varied from 1 µJ to 10 µJ (micro Joule range).
Post-Processing (Thermal Annealing): Samples were annealed in ultra-high vacuum (UHV) at 950° C for 1 hour to reduce resistivity.
Characterization:
- Electrical: 2-probe current-voltage (IV) measurements (KeithleyTM 6487) after silver metal deposition (400 nm thickness) on top and bottom surfaces.
- Structural: Micro-Raman spectroscopy (532 nm DPSS laser) to confirm sp³ to sp² carbon conversion (graphitization).

6CCVD Solutions & Capabilities

The successful fabrication of high-quality, barrier-free 3D electrodes hinges entirely on the quality and precise crystallographic orientation of the SCD substrate. 6CCVD is uniquely positioned to supply the materials and customization required to replicate and advance this critical research.

Applicable Materials

The research demonstrates that (110) oriented Single Crystal Diamond (SCD) is the superior platform for laser-written graphitic electrodes, offering optimal conductivity and structural uniformity.

6CCVD Material Recommendation	Specification Match	Technical Advantage
Optical Grade SCD (Type IIa)	Matches the high-purity monocrystalline material used in the study.	Low defect density ensures predictable laser-matter interaction and graphitization wave propagation.
Custom (110) Orientation	Directly addresses the key finding that (110) orientation eliminates the potential barrier.	Provides the necessary crystallographic alignment for enhanced heating and efficient sp³ to sp² conversion.
Custom Thickness Substrates	The paper used 500 µm thick plates. 6CCVD offers SCD substrates from 0.1 µm up to 500 µm, and bulk substrates up to 10 mm.	Enables scaling of 3D detector designs to greater depths or integration into thin-film devices.

Customization Potential

6CCVD’s in-house fabrication capabilities directly support the complex requirements of 3D electrode integration:

Precision Orientation: We provide SCD wafers and plates cut and polished precisely to the (110) orientation required for barrier-free electrode fabrication, ensuring reproducibility of the 0.013 Ω cm resistivity results.
Custom Dimensions: While the paper used 5 mm × 5 mm samples, 6CCVD can supply SCD plates up to 125 mm (PCD) and custom-sized SCD wafers, facilitating scale-up for industrial or large-area detector prototypes.
Integrated Metalization: The experiment required silver deposition for electrical contact. 6CCVD offers internal metalization services, including Au, Pt, Pd, Ti, W, and Cu layers, applied via sputtering or evaporation, allowing researchers to integrate contact pads directly onto the diamond surface prior to delivery.
Surface Finish: We guarantee ultra-smooth polishing (Ra < 1 nm for SCD), which is crucial for minimizing surface scattering effects during Bessel beam injection and ensuring high-quality optical access to the bulk material.

Engineering Support

The successful implementation of laser-written electrodes for applications such as 3D Diamond Detectors for Nuclear Physics and Medical Dosimetry requires precise material selection.

6CCVD’s in-house PhD team specializes in CVD diamond growth and characterization. We offer consultation services to assist researchers in selecting the optimal material parameters (orientation, nitrogen concentration, thickness, and surface finish) to maximize graphitization efficiency and minimize electrical resistance for similar Ultrafast Laser Micromachining projects.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

Tech Support

Original Source

DOI: https://doi.org/10.1016/j.diamond.2023.110760

References

2000 - Diamond detectors in particle physics [Crossref]
2019 - Femtosecond laser written photonic and microfluidic circuits in diamond
2012 - Laser in micro and nanoprocessing of diamond materials [Crossref]
2016 - Diamond photonics platform enabled by femtosecond laser writing [Crossref]
2020 - A single-crystal diamond X-ray pixel detector with embedded graphitic electrodes [Crossref]
2017 - Planar diamond-based multiarrays to monitor neurotransmitter release and action potential firing: new perspectives in cellular neuroscience [Crossref]
1962 - The graphitization of diamond [Crossref]
2016 - Softening the ultra-stiff: controlled variation of Young’s modulus in single-crystal diamond by ion implantation [Crossref]
2012 - Fabrication and electrical characterization of three-dimensional graphitic microchannels in single crystal diamond [Crossref]