Surface Morphology and Spectroscopic Features of Homoepitaxial Diamond Films Prepared by MWPACVD at High CH4 Concentrations

At a Glance

Metadata	Details
Publication Date	2022-10-22
Journal	Materials
Authors	Javier Sierra Gómez, José Vieira, Mariana Amorim Fraga, E.J. Corat, Vladimir Jesús Trava-Airoldi
Institutions	National Institute for Space Research, Universidade Presbiteriana Mackenzie
Citations	4
Analysis	Full AI Review Included

Executive Summary

This study investigates the homoepitaxial growth of Single Crystal Diamond (SCD) films using Microwave Plasma-Assisted Chemical Vapor Deposition (MWPACVD) under high methane (CH₄) concentrations (6% to 12%).

High Growth Rate Achieved: A maximum SCD film growth rate of 26.6 µm/h was achieved at 12% CH₄ concentration, demonstrating significant potential for producing thick films rapidly.
Structural Quality vs. Rate Trade-off: The best structural quality, measured by the narrowest Raman FWHM (4.6 cm^-1), was obtained at 8% CH₄, indicating a balance between growth speed and crystalline perfection.
Surface Morphology Control: Surface roughness (Ra) was found to be inversely proportional to the CH₄ concentration. The smoothest surface (Ra = 171.59 nm) resulted from the 12% CH₄ growth condition.
High Crystalline Quality: High-Resolution X-ray Diffractometry (HRXRD) confirmed excellent crystalline quality, with FWHM values ranging from 0.014° to 0.028°, comparable to the commercial HPHT seed substrate.
Defect Analysis: Photoluminescence (PL) analysis identified Nitrogen-Vacancy (NV⁰ and NV^-) and Silicon-Vacancy (SiV) defect centers, primarily resulting from unintentional nitrogen incorporation from feed gas impurities and vacuum leakage.
Optimal Condition: While 12% CH₄ provided the highest rate and smoothest surface, 8% CH₄ provided the best overall structural quality (Raman FWHM), suggesting it may be the preferred condition for applications requiring minimal lattice strain.

Technical Specifications

Parameter	Value	Unit	Context
Growth Technique	MWPACVD	N/A	2.45 GHz system
Substrate Type	HPHT Type Ib	N/A	<100> oriented, 3 x 3 x 1.1 mm³
Microwave Power	3.62	kW	Constant parameter
Reactor Pressure	150	Torr	Constant parameter
Substrate Temperature	1060 ± 10	°C	Constant parameter
Total Gas Flow	200	sccm	H₂ + CH₄ mixture
CH₄ Concentration Range	6 to 12	%	Varied parameter
Maximum Growth Rate	26.6	µm/h	Achieved at 12% CH₄ (270 µm thickness in 10 h)
Optimal Raman FWHM	4.6	cm^-1	Achieved at 8% CH₄
HRXRD FWHM Range	0.014 to 0.028	°	(004) reflection, indicating high crystalline quality
Minimum Surface Roughness (Ra)	171.59	nm	Achieved at 12% CH₄
Maximum Surface Roughness (Ra)	629.13	nm	Measured at 6% CH₄
Unintentional N₂ (Leakage)	46.6	ppm	Estimated from vacuum system leakage
NV⁰ Defect Emission	575	nm	Photoluminescence (PL) peak
NV^- Defect Emission	637	nm	Photoluminescence (PL) peak
SiV Defect Emission	738	nm	Photoluminescence (PL) peak

Key Methodologies

The SCD films were grown homoepitaxially on commercial HPHT type Ib substrates using a 2.45 GHz MWPACVD reactor.

Substrate Preparation: <100> oriented HPHT diamond substrates were cleaned using effervescent aqua regia (100 °C, 60 min), followed by ultrasonic baths in acetone, isopropyl alcohol, and deionized water.
Pre-Growth Etching: Substrates were etched in H₂ plasma for 40 minutes at 1040 °C to remove surface damage prior to deposition.
Deposition Parameters: All samples were grown for 10 hours at a constant microwave power of 3.62 kW, 150 Torr pressure, and 1060 ± 10 °C temperature.
Gas Mixture Variation: The feed gas mixture consisted of H₂ and CH₄ (total flow 200 sccm). CH₄ concentration was systematically varied (6%, 8%, 10%, and 12%) to study its influence on film properties.
Post-Growth Processing: Samples were cleaned by grinding the bottom face with 3 µm diamond paste, followed by boiling in aqua regia and subsequent ultrasonic cleaning.
Structural and Morphological Analysis:
- Structural Quality: Raman spectroscopy (FWHM of 1332 cm^-1 peak) and High-Resolution X-ray Diffractometry (HRXRD) ω-scans around the (004) Bragg peak.
- Defect Analysis: Photoluminescence (PL) spectroscopy (514.5 nm laser source) to identify NV and SiV centers.
- Morphology: Scanning Electron Microscopy (SEM) and optical profilometry (Veeco WYKO NT1100) to measure surface roughness (Ra) and step-growth features.

Commercial Applications

The production of high-quality, thick SCD films at high growth rates is critical for advancing diamond-based technologies in several high-demand sectors:

High-Power Microelectronics: SCD’s exceptional electrical properties and elevated breakdown voltage make it ideal for high-power, high-frequency, and high-temperature (>400 °C) semiconductor devices.
Optoelectronics and Optics: SCD is an excellent transparent semiconductor material suitable for wide-spectrum optical communication systems, high-power laser windows, and specialized optical components.
Quantum Technology: The control and incorporation of NV and SiV centers, even if unintentional in this study, are fundamental for developing diamond-based quantum sensors and qubits.
Harsh Environment Devices: SCD’s superior thermal conductivity, hardness, and radiation resistance enable its use in extreme radiation conditions and high-voltage applications (>10 kV).
Energy and Storage: Potential use in advanced energy applications, such as beta batteries, leveraging diamond’s stability and properties.

View Original Abstract

Single crystal diamond (SCD) is a promising material to satisfy emerging requirements of high-demand fields, such as microelectronics, beta batteries and wide-spectrum optical communication systems, due to its excellent optical characteristics, elevated breakdown voltage, high hardness and superior thermal conductivity. For such applications, it is essential to study the optically active defects in as-grown diamonds, namely three-dimensional defects (such as stacking faults and dislocations) and the inherent defects arising from the cultivation method. This paper reports the growth of SCD films on a commercial HPHT single-crystal diamond seed substrate using a 2.45 GHz microwave plasma-assisted chemical vapor deposition (MWPACVD) technique by varying the methane (CH4) gas concentration from 6 to 12%, keeping the other parameters constant. The influence of the CH4 concentration on the properties, such as structural quality, morphology and thickness, of the highly oriented SCD films in the crystalline plane (004) was investigated and compared with those on the diamond substrate surface. The SCD film thickness is dependent on the CH4 concentration, and a high growth rate of up to 27 µm/h can be reached. Raman spectroscopy, high-resolution X-ray diffractometry (HRXRD), scanning electron microscopy (SEM), surface profilometry and optical microscopic analyses showed that the produced homoepitaxial SCD films are of good quality with few macroscopic defects.

Tech Support

Original Source

DOI: https://doi.org/10.3390/ma15217416

References

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