Orientation Dependence of Cathodoluminescence and Photoluminescence Spectroscopy of Defects in Chemical-Vapor-Deposited Diamond Microcrystal

At a Glance

Metadata	Details
Publication Date	2020-11-29
Journal	Materials
Authors	K. Fabisiak, Szymon Łoś, K. Paprocki, Mirosław Szybowicz, Janusz Winiecki
Institutions	Poznań University of Technology, University of Bydgoszcz
Citations	11
Analysis	Full AI Review Included

Orientation Dependence of Defects in CVD Diamond Microcrystals

Executive Summary

This study investigates the critical dependence of defect concentration and type on the crystallographic orientation ((100) vs. (111) planes) in hot-filament chemical vapor deposition (HF CVD) diamond microcrystals.

Orientation Dependence: The (111) crystallographic planes were found to be significantly more defective than the (100) planes, confirming that blocking (111) facet growth is essential for high-quality diamond synthesis.
Defect Concentration: Estimated defect concentration in (111) planes (3.5 x 10¹⁸ cm^-3) is almost an order of magnitude greater than in (100) planes (8.3 x 10¹⁷ cm^-3).
Structural Defects (A-Band): A strong cathodoluminescence (CL) A-band emission (2.815 eV), attributed to lattice disorder and dislocations, was prominent in the (111) spectra but nearly absent in the (100) spectra.
Crystallinity Measure: Raman spectroscopy showed that the Full Width at Half Maximum (FWHM) for the (111) plane (13.7 cm^-1) was nearly twice as broad as that for the (100) plane (8.6 cm^-1), directly correlating FWHM with dislocation density.
Stress State: All HF CVD microcrystals exhibited high compressive residual stress, measured at approximately 3 GPa via the Raman peak shift.
Color Centers: Key nitrogen-vacancy (NV) centers, including the neutral (NV⁰ at 2.156 eV) and negatively charged (NV^- at 1.947 eV) configurations, were identified via PL and CL, confirming their presence for potential quantum applications.

Technical Specifications

Parameter	Value	Unit	Context
Growth Method	HF CVD	N/A	Hot-Filament Chemical Vapor Deposition
Filament Temperature	2100	°C	Thermal activation source
Substrate Temperature	1000	K	Growth temperature
Reactor Pressure	80	mbar	Total pressure during growth
Methane Concentration (CH₄/H₂)	1	vol.%	Working gas mixture
Total Gas Flow Rate	100	sccm	Working gas flow
Raman FWHM ((111) plane)	13.7	cm^-1	Measure of lattice disturbance
Raman FWHM ((100) plane)	8.6	cm^-1	Measure of lattice disturbance
Estimated Defect Conc. ((111))	3.5 x 10¹⁸	cm^-3	Calculated via FWHM (L^-3 relation)
Estimated Defect Conc. ((100))	8.3 x 10¹⁷	cm^-3	Calculated via FWHM (L^-3 relation)
Residual Stress (Compressive)	~3	GPa	Calculated from Raman peak shift
A-Band Emission Peak	2.815	eV	Associated with dislocations (CL)
NV⁰ Center Peak	2.156	eV	Neutral Nitrogen-Vacancy complex
NV^- Center Peak	1.947	eV	Negatively charged Nitrogen-Vacancy complex (PL only)

Key Methodologies

The investigation utilized a combination of synthesis and advanced spectroscopic characterization techniques to analyze defect structure and concentration.

Synthesis (HF CVD):
- Diamond microcrystals were grown separately on a (100) oriented silicon substrate.
- The substrate was intentionally not mechanically polished prior to growth to prevent the formation of a continuous polycrystalline layer, promoting nucleation in the gas phase.
- A tungsten filament heated to 2100 °C was used for thermal activation.
- The gas mixture consisted of 1 vol.% CH₄ in H₂, maintained at 80 mbar and 100 sccm flow rate.
Morphological Analysis (SEM):
- Scanning Electron Microscopy (Jeol JSM-6300, 20 kV) was used to study microcrystal morphology and confirm the orientation of the analyzed facets ((100) vs. (111)).
Defect Spectroscopy (CL & PL):
- Cathodoluminescence (CL): Spectra were registered at room temperature using a grating spectrometer (StellarNet’s SILVVER-Nova). Excitation was performed using a 30 kV electron beam. CL was crucial for observing the A-band (dislocations) and high-energy defects.
- Photoluminescence (PL): Used to complement CL, particularly for identifying NV centers (NV⁰ and NV^-) and the GR1 defect (vacancy in neutral charge state, 1.675 eV).
Structural Analysis (Raman Spectroscopy - RS):
- A confocal micro Raman spectrometer (InViaRaman) was used in backscattering geometry (488 nm Argon laser, 1 mW power).
- RS determined the degree of crystal lattice disturbance (FWHM) and residual stress (peak shift).
- FWHM values were used to estimate the phonon free path (L) and subsequently the defect concentration (proportional to L^-3).

Commercial Applications

The ability to control defect density and orientation, particularly the concentration of NV centers, is critical for next-generation diamond-based technologies.

Quantum Computing and Sensing: The identification and characterization of NV centers (NV⁰ and NV^-) are fundamental, as these defects serve as leading solid-state qubits and highly sensitive quantum sensors (e.g., for magnetic fields and temperature).
Optoelectronic Devices: Diamond’s wide bandgap and stable color centers make it suitable for UV photocathodes and high-efficiency light emitters, especially where defect control is necessary to manage luminescence decay times.
High-Power Electronics: Controlling structural defects (dislocations, stacking faults) in the (100) growth orientation is essential for producing high-quality diamond films required for high-power semiconductor devices (e.g., diodes and transistors) due to diamond’s superior thermal and electrical properties.
Radiation Detection: Synthetic diamond’s radiation hardness and wide bandgap make it a candidate material for particle detectors in high-energy physics experiments.
Industrial Tooling: While the study focuses on microcrystals, the underlying defect control principles are relevant for optimizing the mechanical hardness and thermal performance of diamond coatings used in drilling and cutting tools.

View Original Abstract

Point defects, impurities, and defect-impurity complexes in diamond microcrystals were studied with the cathodoluminescence (CL) spectroscopy in the scanning electron microscope, photoluminescence (PL), and Raman spectroscopy (RS). Such defects can influence the directions that microcrystals are grown. Micro-diamonds were obtained by a hot-filament chemical vapor deposition (HF CVD) technique from the methane-hydrogen gas mixture. The CL spectra of diamond microcrystals taken from (100) and (111) crystallographic planes were compared to the CL spectrum of a (100) oriented Element Six diamond monocrystal. The following color centers were identified: 2.52, 2.156, 2.055 eV attributed to a nitrogen-vacancy complex and a violet-emitting center (A-band) observed at 2.82 eV associated with dislocation line defects, whose atomic structure is still under discussion. The Raman studies showed that the planes (111) are more defective in comparison to (100) planes. What is reflected in the CL spectra as (111) shows a strong band in the UV region (2.815 eV) which is not observed in the case of the (100) plane.

Tech Support

Original Source

DOI: https://doi.org/10.3390/ma13235446

References

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