CVD Encapsulation of Laser-Graphitized Electrodes in Diamond Electro-Optical Devices

At a Glance

Metadata	Details
Publication Date	2023-12-23
Journal	Photonics
Authors	М. С. Комленок, V. V. Kononenko, A. P. Bolshakov, Nikolay D. Kurochitskiy, Dmitrii G. Pasternak
Institutions	Prokhorov General Physics Institute
Citations	4
Analysis	Full AI Review Included

Technical Documentation & Analysis: CVD Encapsulation of Laser-Graphitized Electrodes

Executive Summary

This research demonstrates a critical advancement in high-field diamond electro-optical devices by successfully encapsulating laser-graphitized conductive electrodes using MPCVD homoepitaxial diamond growth.

Core Achievement: A two-step process (laser graphitization followed by CVD encapsulation) was realized on a single crystal diamond (SCD) substrate to bury conductive tracks.
Material Quality: The 40 µm thick epitaxial diamond layer exhibited high quality, confirmed by a narrow Raman FWHM of 2.9 cm^-1, matching high-purity Type IIa reference diamond.
Enhanced Conductivity: The high-temperature CVD process acted as an annealing step, surprisingly improving the conductivity of the buried graphitized tracks, reducing resistivity from 6.6 mΩ·cm to 4.6 mΩ·cm.
Breakdown Strength: The encapsulation dramatically increased the estimated lower bound of the bulk electrical breakdown field to 550 kV/cm (calculated across the 40 µm layer), overcoming the surface breakdown limitation (19-40 kV/cm).
Application Focus: This technique is crucial for scaling the performance of high-field diamond devices, particularly THz photoconductive emitters, by allowing significantly higher bias voltages.
6CCVD Relevance: 6CCVD is uniquely positioned to supply the high-purity SCD substrates and custom epitaxial layers required to replicate and scale this high-voltage device architecture.

Technical Specifications

The following hard data points were extracted from the analysis of the CVD encapsulation and electrical performance:

Parameter	Value	Unit	Context
Substrate Material	Type Ib HPHT SCD	2.9 x 2.7 x 0.4 mm³	(100) oriented
Epitaxial Layer Thickness	40	µm	Grown CVD layer
Diamond Growth Rate	~8	µm/hour	Monitored via interferometry
CVD Substrate Temperature	920	°C	Maintained during growth
CVD Pressure	177	Torr	MPCVD operating condition
Gas Composition	H₂(96%)/CH₄(4%)	%	Methane concentration 4%
Laser Wavelength	248	nm	KrF Excimer Laser (ns pulse)
Graphitized Groove Width	24	µm	Laser ablated structure
Graphitized Groove Depth	125	µm	Laser ablated structure
Graphitized Layer Thickness	> 250	nm	Estimated via absorption coefficient
Resistivity (Before CVD)	6.6	mΩ·cm	Graphitized track conductivity
Resistivity (After CVD)	4.6	mΩ·cm	Improved conductivity (annealing effect)
Breakdown Field (Surface Limit)	19 - 40	kV/cm	Limited by air/surface discharge
Breakdown Field (Bulk, Lower Est.)	550	kV/cm	Across 40 µm encapsulated layer
CVD Layer Quality (Raman FWHM)	2.9	cm^-1	Matches high-quality Type IIa SCD

Key Methodologies

The experiment successfully combined laser microstructuring and high-quality homoepitaxial CVD growth to achieve buried conductive structures.

Substrate Selection: A mechanically polished (100) oriented Type Ib HPHT single crystal diamond substrate (2.9 x 2.7 x 0.4 mm³) was used.
Laser Graphitization: Conductive grooves were created using a KrF excimer laser ($\lambda$ = 248 nm, $\tau$ = 20 ns) via projection lithography.
- Fluence: 27 J/cm².
- Groove Dimensions: 24 µm wide and 125 µm deep.
- Purpose: Nanosecond pulses were chosen to maximize the conductivity of the resulting sp² graphitic phase.
MPCVD Encapsulation: Epitaxial diamond growth was performed using an ARDIS-300 microwave plasma CVD system.
- Recipe Parameters: 5.1 kW microwave power, 177 Torr pressure, 920 °C substrate temperature, and a 4% methane concentration (H₂/CH₄).
- Result: A 40 µm thick SCD layer was grown, completely covering the 125 µm deep graphitized grooves.
Post-Growth Contacting: The epitaxial layer above the graphitized pads was ablated using an excimer laser to expose the buried conductive structures for electrical testing.
Electrical Testing: Current-voltage (I-V) characteristics were measured to determine resistivity, and high DC voltage was applied to measure the electrical breakdown threshold before and after encapsulation.

6CCVD Solutions & Capabilities

6CCVD provides the specialized MPCVD diamond materials and customization services necessary to replicate, optimize, and scale the high-field electro-optical devices demonstrated in this research.

Applicable Materials

To achieve the high breakdown voltage and high-quality epitaxial growth demonstrated, the following 6CCVD materials are recommended:

Electronic Grade SCD (SCD-E): Required for the substrate due to its extremely low nitrogen content and high crystalline purity, which is essential for achieving the high intrinsic breakdown field of diamond (theoretical 2-10 MV/cm).
Optical Grade SCD (SCD-O): Suitable for the final epitaxial layer, ensuring high transparency and low absorption across the UV-to-mm range, critical for THz photoconductive antenna applications.
Custom SCD Thickness: 6CCVD can supply SCD substrates up to 500 µm thick, allowing researchers to precisely control the thickness of the final encapsulated layer (40 µm in this study) or scale up to thicker layers for even higher voltage handling.

Customization Potential

The success of this technique relies on precise material dimensions and surface preparation, areas where 6CCVD excels:

Research Requirement	6CCVD Customization Service	Benefit to Replication/Scaling
Substrate Dimensions	Custom SCD plates/wafers up to 10 mm thick and PCD wafers up to 125 mm diameter.	We supply the exact (100) oriented HPHT substrates required, or larger formats for industrial scaling of THz emitters.
Surface Finish	Ultra-low roughness polishing: Ra < 1 nm (SCD).	Minimizes surface leakage and ensures optimal conditions for high-quality, defect-free homoepitaxial growth over complex graphitized patterns.
Metalization Integration	In-house metalization capabilities (Au, Pt, Pd, Ti, W, Cu).	We can apply custom contact pads directly to the exposed graphitized areas post-ablation, simplifying device integration and ensuring low-resistance ohmic contacts.
Complex Structuring	Custom laser cutting and micro-machining services.	While the researchers used their own laser, 6CCVD can provide substrates pre-cut or structured to specific geometries, accelerating the R&D cycle for complex electrode designs.

Engineering Support

6CCVD’s in-house PhD team specializes in optimizing MPCVD recipes for specific electronic and optical applications. We can assist with material selection and process optimization for similar THz Photoconductive Emitter projects, focusing on:

Optimizing methane concentration and temperature to control the quality and stress profile of the epitaxial layer, especially around the graphitized junction (where 40 µm wide stress zones were observed).
Developing thicker SCD layers (up to 500 µm) to push the bulk breakdown threshold beyond the 550 kV/cm lower estimate achieved in this study.
Integrating Boron-Doped Diamond (BDD) materials for specific semiconductor or electrochemical applications requiring high conductivity and high breakdown strength.

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

View Original Abstract

Conductive graphitized grooves on the dielectric surface of diamond have been created by KrF excimer laser radiation. The advantages of such a circuit board in high-field applications is rather limited because the crystal surface has a relatively low electrical breakdown threshold. To increase the electrical strength, a method of encapsulating surface conductive graphitized structures by chemical vapor deposition of an epitaxial diamond layer has been proposed and realized. The quality of the growth diamond is proved by Raman spectroscopy. A comparative study of the electrical resistivity of graphitized wires and the breakdown fields between them before and after diamond growth was carried out. The proposed technique is crucial for diamond-based high-field electro-optical devices, such as THz photoconductive emitters.

Tech Support

Original Source

DOI: https://doi.org/10.3390/photonics11010010

References

1989 - Growth of Device-Quality Homoepitaxial Diamond Thin Films [Crossref]
1995 - Photoconductive properties of chemical vapor deposited diamond switch under high electric field strength [Crossref]
1982 - Investigation of microplasma breakdown at a contact between a metal and a semiconducting diamond
2023 - Diamond field effect transistors—Concepts and challenges [Crossref]
2022 - 3326-V modulation-doped diamond MOSFETs [Crossref]
2023 - Electrical properties of cerium hexaboride gate hydrogen-terminated diamond field effect transistor with normally-off characteristics [Crossref]
2022 - Solution-processed tin oxide thin film for normally-off hydrogen terminated diamond field effect transistor [Crossref]
2022 - Diamond semiconductor and elastic strain engineering [Crossref]
2022 - Excess noise in high-current diamond diodes [Crossref]