Structural, Raman and photoluminescence studies on nanocrystalline diamond films - Effects of ammonia in feedstock

At a Glance

Metadata	Details
Publication Date	2020-04-17
Journal	Diamond and Related Materials
Authors	K. Ganesan, P.K. Ajikumar, S.K. Srivastava, P. Magudapathy
Institutions	Indira Gandhi Centre for Atomic Research
Citations	20
Analysis	Full AI Review Included

Executive Summary

This study investigates the effects of ammonia (NH₃) feedstock concentration on the structural quality and photoluminescence (PL) properties of nanocrystalline diamond (NCD) films grown via Hot Filament Chemical Vapor Deposition (HFCVD).

Optimal Doping Achieved: An optimal nitrogen doping level was identified at a nominal N/C ratio of 0.35 in the feedstock, resulting in significantly improved structural quality and enhanced optical emission.
Structural Improvement: The film grown at N/C = 0.35 exhibited the largest average crystallite size (16.3 nm) and the lowest full width at half maximum (FWHM) of the (111) XRD peak, indicating superior crystallinity compared to both undoped and higher-doped films.
Enhanced PL Emission: The optimal film showed a drastic enhancement in room temperature PL emission, peaking at ~505 nm (H3 center, N-V-N defect) under 355 nm excitation and ~700 nm (N-aggregate defects) under 532 nm excitation.
Strain Analysis: Raman spectroscopy confirmed that compressive strain in the diamond lattice monotonically increases with the N/C ratio up to 0.50, manifesting as a blue shift and broadening of the diamond band.
Defect Confirmation: A unique Raman mode at 1195 cm^-1 was observed in optimally doped films, corresponding to C=N-H vibrations, confirming significant nitrogen incorporation, particularly at grain boundaries (GBs).
Application Potential: The combination of enhanced PL and improved structural quality in these radiation-hard materials makes them highly suitable for luminescence-based radiation detectors designed for extreme environments.

Technical Specifications

Parameter	Value	Unit	Context
Growth Technique	HFCVD	N/A	Synthesis method
Substrate Material	Si (111)	N/A	Substrate used
Substrate Temperature	800	°C	Constant growth temperature
Operating Pressure	30	mbar	Reactor pressure
Growth Duration	6	hours	Total synthesis time
Optimal N/C Ratio (N35)	0.35	N/A	For best structural quality and PL
Optimal Crystallite Size (N35)	16.3	nm	Calculated via Scherrer formula
Film Thickness Range	1.1 - 2.7	µm	Range across all samples (N75 to N13)
Unique Raman Mode	1195	cm^-1	Corresponds to C=N-H vibrations
Optimal PL Emission 1 (H3 Center)	~505	nm	Under 355 nm laser excitation
Optimal PL Emission 2 (N-Aggregates)	~700	nm	Under 532 nm laser excitation
N Doping Efficiency (Estimate)	~10^-4	N/A	Very low efficiency of N incorporation into the diamond lattice
Compressive Strain Indicator	Blue Shift	cm^-1	Observed in Raman peak position with increasing N/C ratio

Key Methodologies

Synthesis: Nanocrystalline diamond (NCD) films were grown on chemo-mechanically polished Si (111) substrates using a custom Hot Filament Chemical Vapor Deposition (HFCVD) system.
Feedstock Composition: Feedstock gases consisted of CH₄, H₂, and NH₃. The CH₄ (2 sccm) and H₂ (100 sccm) flow rates were kept constant, while NH₃ flow was varied (0 to 1.50 sccm) to achieve nominal N/C ratios (0, 0.13, 0.35, 0.50, and 0.75).
Growth Conditions: All films were grown under identical conditions: 30 mbar operating pressure, 800 °C substrate temperature, and a 6-hour growth duration.
Structural Analysis (XRD): X-ray Diffraction (XRD) was used to determine the crystallite size using the Scherrer formula and to analyze the full width at half maximum (FWHM) of the (111) diamond peak.
Morphological Analysis (SEM): Scanning Electron Microscopy (SEM) was employed to study the nano-crystalline microstructure and measure the variation in film thickness as a function of the N/C ratio.
Vibrational Spectroscopy (Raman): Micro-Raman spectroscopy (using 532 nm and 355 nm lasers) was performed to assess structural quality, graphitic content (I_D/I_G ratio), and lattice strain. The presence of the C=N-H bond was confirmed by a mode at 1195 cm^-1.
Optical Characterization (PL): Visible and UV Photoluminescence (PL) spectroscopy (using 532 nm and 355 nm diode lasers) identified N-related color centers (e.g., H3, N2, NV^-) and measured the enhancement of emission intensity.

Commercial Applications

The enhanced structural quality and controlled optical emission properties resulting from optimal nitrogen doping position these NCD films for several high-performance engineering applications:

Radiation Detection: The primary application highlighted is in luminescence-based radiation detectors. Diamond’s radiation hardness, combined with the drastically enhanced PL emission (H3 and N-aggregate centers), allows these devices to operate reliably in extreme, high-radiation environments.
Quantum Sensing and Emitters: The precise control and enhancement of N-related color centers (like H3 and NV^-) are fundamental for developing single-photon emitters and solid-state quantum sensors used in quantum information technology.
High-Frequency Electronics: Diamond is a wide bandgap semiconductor. N doping in NCD enhances electrical conductivity (acting as an n-type semiconductor), making it suitable for high-power, high-frequency electronic devices.
Electrochemical Applications: Conductive N-doped diamond electrodes are utilized in electrochemistry due to their wide potential window, stability, and resistance to fouling.
Micro-electromechanical Systems (MEMS): NCD films offer superior mechanical properties and wear resistance, making them valuable for robust MEMS components.

Tech Support

Original Source

DOI: https://doi.org/10.1016/j.diamond.2020.107872

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