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

Executive Summary

This research details a novel two-step fabrication technique for creating buried, high-strength conductive electrodes in single-crystal diamond, crucial for high-field electro-optical devices.

Problem Solved: Surface electrodes on diamond limit bias voltage due to low electrical breakdown thresholds (typically less than 20 kV/cm in air/surface).
Solution: Conductive graphitized grooves were created via nanosecond KrF laser ablation and subsequently encapsulated by a 40 µm thick epitaxial diamond layer using Chemical Vapor Deposition (CVD).
Electrical Strength Achievement: The lower estimate for the electrical breakdown threshold of the encapsulated epitaxial layer reached 550 kV/cm, a significant step toward utilizing diamond’s intrinsic bulk strength (2-10 MV/cm).
Conductivity Improvement: Surprisingly, the resistivity of the graphitized tracks improved after the high-temperature CVD process, dropping from 6.6 mΩ cm to 4.6 mΩ cm, attributed to high-temperature annealing of the nanographite phase.
Epitaxial Quality: Raman spectroscopy confirmed the high quality of the grown monocrystalline layer outside the strain region, matching reference type IIa diamond (1332.5 cm^-1, FWHM 2.9 cm^-1).
Critical Application: The technique is essential for developing high-power THz photoconductive emitters and other diamond-based high-voltage electronics.

Technical Specifications

Parameter	Value	Unit	Context
Substrate Type	(100) oriented Type Ib	N/A	HPHT single crystal diamond
Laser Type	KrF Excimer	N/A	Used for graphitization
Laser Wavelength (λ)	248	nm	KrF Excimer
Pulse Duration (τ)	20	ns	Chosen for high conductivity
Laser Fluence	27	J/cm²	Used for deep ablation
Groove Dimensions	24 wide, 125 deep	µm	Graphitized track dimensions
Initial Surface Breakdown	22 to 40	kV/cm	Measured over 550 µm to 300 µm gaps
CVD Pressure	177	Torr	Diamond deposition process
CVD Temperature	920	°C	Substrate temperature
Microwave Power	5.1	kW	Plasma generation
Gas Mixture	H₂ (96%) / CH₄ (4%)	%	Total flow rate 500 sccm
Epitaxial Growth Rate	~8	µm/hour	Monitored via interferometry
Grown Layer Thickness	40	µm	Final thickness of encapsulation layer
Initial Track Resistivity	6.6	mΩ cm	Before CVD encapsulation
Final Track Resistivity	4.6	mΩ cm	After CVD encapsulation (44% improvement)
Encapsulated Breakdown (Lower Estimate)	550	kV/cm	Calculated across the 40 µm epitaxial layer
Raman Peak Position (CVD Layer)	1332.5	cm^-1	Outside the strain region (high quality)
Raman Peak FWHM (CVD Layer)	2.9	cm^-1	Outside the strain region (high quality)

Key Methodologies

The fabrication process involved two main stages: laser microstructuring and Chemical Vapor Deposition (CVD) encapsulation, followed by electrical characterization.

Laser Graphitization (Direct Write):
- A commercial KrF excimer laser (λ = 248 nm, τ = 20 ns) was used to ablate the (100) HPHT diamond surface.
- High fluence (27 J/cm²) and high pulse count (400 pulses/spot) were used to create deep grooves (125 µm deep, 24 µm wide) containing the conductive sp² graphitic phase.
- A T-shaped conductive pattern with 100 x 100 µm² pads was created for electrical testing.
CVD Encapsulation:
- The structured substrate was placed in a microwave plasma CVD system (ARDIS-300).
- Epitaxial growth was performed for 5 hours at 920 °C, 177 Torr, using a 4% CH₄ in H₂ gas mixture (5.1 kW power).
- The process resulted in a 40 µm thick monocrystalline diamond layer that completely covered the graphitized grooves.
Post-CVD Processing and Characterization:
- The diamond layer above the contact pads was re-ablated using the excimer laser to expose the buried graphitized pads for external electrical contact.
- Raman Spectroscopy: Confocal Raman (λ = 473 nm) was used to assess the quality of the grown diamond (stress, FWHM) and confirm the preservation of the sp² graphitic phase.
- Conductivity Testing: Current-voltage (I-V) curves were measured before and after encapsulation to quantify the change in track resistivity.
- Breakdown Testing: High DC voltage was applied between pads to determine the surface breakdown threshold (initial measurement) and estimate the bulk breakdown threshold (after encapsulation).

Commercial Applications

This technology, enabling buried, high-strength electrodes in diamond, is critical for next-generation devices requiring extreme electrical and thermal performance.

High-Power THz Emitters: Directly addresses the primary limitation of current diamond photoconductive antennas (PCAs) by allowing significantly higher bias fields (up to 550 kV/cm demonstrated) for increased THz power output.
High-Voltage Switching Diodes: Provides robust internal wiring necessary for diamond switching diodes operating in the MV/cm regime.
Field Effect Transistors (FETs): Applicable in high-power RF and microwave electronics where diamond’s thermal conductivity and high breakdown voltage are leveraged.
3D Radiation Detectors: The laser direct-write technique combined with encapsulation can be used to fabricate complex, buried electrode geometries for all-carbon detectors of ionized radiation.
Extreme Environment Electronics: Useful for devices requiring high radiation resistance and operation in high-temperature or high-field environments.

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]
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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]