The Effect of Surface Treatment on Structural Properties of CVD Diamond Layers with Different Grain Sizes Studied by Raman Spectroscopy

At a Glance

Metadata	Details
Publication Date	2021-03-08
Journal	Materials
Authors	Anna Dychalska, W. Koczorowski, Marek Trzciński, Lidia Mosińska, Mirosław Szybowicz
Institutions	Poznań University of Technology, Institute of Mathematics
Citations	15
Analysis	Full AI Review Included

Executive Summary

This study investigated the structural and chemical modifications induced by post-growth hydrogen plasma treatment on four types of CVD diamond layers (Microcrystalline, MCD, and Nanocrystalline, NCD) using localized, marker-based analysis.

Localized Analysis: A T-shaped marker was fabricated using Focused Ion Beam (FIB) on each sample, allowing Raman and SEM measurements to be performed on the exact same micro-area before and after hydrogen treatment, ensuring reliable comparison of subtle structural changes.
Sp² Etching Confirmed: Hydrogen treatment preferentially etched non-diamond (amorphous) carbon phases, particularly within the FIB-etched marker area, leading to an overall reduction in sp² content across all samples.
Quality Improvement: The Quality Factor Q(A) (approximated sp³ crystalline content) increased for all layers, showing up to a two-fold improvement for nanocrystalline samples (NCD0.02).
Surface Chemistry: XPS confirmed successful hydrogen termination, evidenced by an increase in CHx bonds and a corresponding decrease in sp³ phase (due to conversion/etching). The m/I_G parameter (related to H concentration) increased, confirming surface H enrichment.
Wettability Control: Contact Angle (CA) measurements increased significantly (up to 110° for MCD2.5), confirming the transition to a hydrophobic surface, which correlated linearly with the ratio of CHx bonds to oxidized carbon species (CHx/CO).
Grain Size Susceptibility: Diamond layers with smaller grain sizes (NCD) and higher initial sp² content were found to be more susceptible to structural modification (cracks, delamination) induced by the hydrogen termination process.

Technical Specifications

Parameter	Value	Unit	Context
Deposition Method	Hot Filament (HF) CVD	N/A	Diamond layer growth
Deposition Pressure	27	mbar	Working gas (H₂ + CH₄)
Filament Temperature (Deposition)	2000	K	Tungsten filament
Substrate Temperature (Deposition)	900	K	Diamond layer growth
H₂ Concentration (Working Gas)	96 - 99	%	Varied inversely with CH₄ concentration
CH₄ Concentration (MCD2)	1.0 ± 0.1	%	Microcrystalline Diamond (MCD)
CH₄ Concentration (NCD0.02)	3.8 ± 0.1	%	Nanocrystalline Diamond (NCD)
Average Grain Size (MCD)	2.0 to 2.5	µm	Microcrystalline samples
Average Grain Size (NCD)	0.06 to 0.2	µm	Nanocrystalline samples
H-Treatment Filament Temperature	2300	K	Activation of H₂ to radical atoms
H-Treatment Layer Temperature	1100	K	Post-growth surface modification
H-Treatment Pressure	30	mbar	Pure hydrogen gas
H-Treatment Duration	15	min	From reaching target pressure
Raman Excitation Wavelength	488	nm	Renishaw in via system
C1s Peak Binding Energy (sp³)	~285.5	eV	Bulk sp³ hybridized carbon
Maximum Contact Angle (MCD2.5 H)	110	degrees	After hydrogen termination (hydrophobic)

Key Methodologies

The study employed a multi-step process involving deposition, marking, treatment, and comprehensive characterization, focusing on comparing results from identical locations.

Substrate Preparation: Silicon (100) substrates were mechanically scratched using 0.2 µm diamond powder to ensure effective nucleation, followed by methanol cleaning.
CVD Deposition: Four diamond layers (MCD2, MCD2.5, NCD0.2, NCD0.02) were deposited via HF CVD, varying CH₄ concentration (1.0% to 3.8%) and deposition time to achieve different grain sizes and sp² carbon contents.
Marker Fabrication: A characteristic T-shape marker was fabricated on the surface of each layer using Focused Ion Beam (FIB) patterning (Ga⁺ ion, 30 keV, 9-21 nA). This marker defined the specific micro-area for comparative analysis.
Initial Characterization: Baseline measurements (Raman, SEM, XPS) were performed both inside and outside the FIB marker area.
Hydrogen Treatment: Post-growth treatment was carried out in a CVD chamber by exposing the layers to pure hydrogen gas (30 mbar) with a working filament (2300 K) for 15 minutes, heating the diamond layers to 1100 K to activate H₂ radicals.
Post-Treatment Characterization: Raman spectroscopy (488 nm) linear maps were collected with 5 µm steps, specifically targeting the marked areas to measure structural changes (diamond, D, G bands, PL background).
Surface Analysis: SEM was used to analyze morphological changes (etching, delamination). XPS (Al Kα) was used to determine surface chemical composition (sp³, sp², CHx, C-O-C, C=O bonds) and confirm hydrogen termination.
Wettability Assessment: Contact angle (CA) measurements were performed to quantify the change in surface wettability from oxygen-terminated (as-deposited) to hydrogen-terminated (post-treatment).

Commercial Applications

The ability to precisely control diamond surface termination and wettability is critical for advanced electronic and biomedical devices.

High-Frequency/Power Electronics: Hydrogen-terminated diamond surfaces exhibit p-type conductivity without doping, enabling the fabrication of high-performance Field Effect Transistors (FETs) and other high-power RF devices.
Cold Cathodes: H-terminated diamond layers, known for their Negative Electron Affinity (NEA), are used in cold cathode fluorescent lamps and electron emitters.
Biosensing and Delivery: Oxygen-terminated diamond surfaces show high biological stability and wettability, making them suitable for use as electrodes in electrochemistry, and as sensing or drug delivery devices.
Surface Functionalization: The ease of modifying diamond surface properties (e.g., switching between hydrophobic H-termination and hydrophilic O-termination) is crucial for developing robust chemical and biological sensors.
Amorphous Carbon Etching: The preferential etching of sp² phases by hydrogen plasma is a valuable process step for purifying CVD diamond films, particularly NCD layers, enhancing their overall crystalline quality (Q(A)).

View Original Abstract

Extensive Raman spectroscopy studies combined with scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) measurements were performed to investigate structural and chemical changes in diamond layers deposited by chemical vapour deposition (CVD) upon post-growth treatment with hydrogen. The aim of this study is to characterize the changes in micro-structural properties of diamond layers with different grain sizes and different contents of sp2 carbon phase. Hydrogenation or oxidization of diamond layer surface is often performed to modify its properties; however, it can also strongly affect the surface structure. In this study, the impact of hydrogenation on the structure of diamond layer surface and its chemical composition is investigated. Owing to their polycrystalline nature, the structural properties of CVD diamond layers can strongly differ within the same layer. Therefore, in this project, in order to compare the results before and after hydrogen treatment, the diamond layers are subjected to Raman spectroscopy studies in the vicinity of a T-shape marker fabricated on the surface of each diamond layer studied.

Tech Support

Original Source

DOI: https://doi.org/10.3390/ma14051301

References

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