Effect of Reactive Ion Etching on the Luminescence of GeV Color Centers in CVD Diamond Nanocrystals

At a Glance

Metadata	Details
Publication Date	2021-10-23
Journal	Nanomaterials
Authors	S. A. Grudinkin, Nikolay A. Feoktistov, Kirill Bogdanov, М. А. Баранов, В. Г. Голубев
Institutions	Ioffe Institute, ITMO University
Citations	8
Analysis	Full AI Review Included

Executive Summary

This research investigates the use of Reactive Ion Etching (RIE) in oxygen plasma as a post-synthetic processing step to improve the optical quality of Germanium-Vacancy (GeV^-) color centers embedded in nanodiamonds (NDs) grown by Hot Filament Chemical Vapor Deposition (HFCVD).

Problem Addressed: Inhomogeneous broadening of the GeV^- Zero-Phonon Line (ZPL) is caused by structural defects and local strain fields within the ND lattice, hindering quantum applications.
Solution: RIE selectively removes highly defective, strained, sp²-hybridized carbon regions, primarily located on the ND surface.
Strain Reduction Confirmed: The Full Width at Half Maximum (FWHM) of the diamond Raman band (1332 cm^-1) narrowed significantly from ~7 cm^-1 (as-grown) to ~5.5 cm^-1 (etched), indicating a reduction in strain distribution.
Optical Improvement: Etched NDs showed a marked reduction in the broad photoluminescence background and a narrowing of the ZPL linewidth at 80 K.
Defect Removal Indicator: The luminescence intensity of the neutral nitrogen-vacancy center (NV⁰, ~575 nm) increased significantly after etching, confirming the removal of non-radiative recombination defects near the color centers.
ZPL Fine Structure Analysis: High-resolution spectroscopy revealed that the ZPL doublet is a superposition of several narrow lines, attributed to GeV^- sub-ensembles experiencing distinct, localized uniaxial strain fields.
Material Outcome: The process yields smaller NDs (less than 200 nm, down from 250-350 nm) with improved crystalline quality and reduced inhomogeneous broadening, making them better suited for quantum optics.

Technical Specifications

Parameter	Value	Unit	Context
As-Grown ND Size	250-350	nm	HFCVD grown crystallites
Etched ND Size	less than 200	nm	Post-RIE processing
GeV^- ZPL Wavelength	~602	nm	Room Temperature (RT) emission peak
ZPL FWHM (As-Grown)	~6.2	nm	Ensemble measurement at RT
Diamond Raman Band (TO Phonon)	~1332	cm^-1	F_2g symmetry in diamond lattice
Raman FWHM (As-Grown)	~7	cm^-1	Indicates strain distribution
Raman FWHM (Etched)	~5.5	cm^-1	Indicates reduced strain distribution
NV⁰ ZPL Wavelength	~575	nm	Neutral nitrogen-vacancy center, observed at 80 K
Excited State Splitting (GeV^-)	4.5	meV	Due to spin-orbit coupling and Jahn-Teller interaction
Ground State Splitting (GeV^-)	0.7	meV	Not resolved at 80 K due to narrow ZPL width
Measurement Temperature	80	K	Liquid nitrogen temperature for high-resolution PL
High Spectral Resolution	0.03 (0.8)	nm (cm^-1)	Used for resolving ZPL fine structure

Key Methodologies

The fabrication and processing involved two primary stages: Hot Filament Chemical Vapor Deposition (HFCVD) for growth and Reactive Ion Etching (RIE) for purification.

1. HFCVD Nanodiamond Growth (Starting Material)

Method: Top-down fabrication using HFCVD with in situ incorporation of GeV^- centers.
Substrate Preparation: Seeded with detonation NDs.
Germanium Source: Bulk crystalline germanium placed on the substrate holder, etched by atomic hydrogen (H) to form volatile GeH_x radicals.
Reactor Parameters:
- Tungsten Coil Temperature: 2000-2200 °C
- Working Pressure: 40 Torr
- Hydrogen (H₂) Flow Rate: 500 sccm
- Methane (CH₄) Concentration: 2%

2. Reactive Ion Etching (RIE) Post-Processing

Purpose: Removal of defective, highly strained surface regions and sp²-hybridized carbon via oxidizing reaction and ion bombardment.
Gas Mixture: Oxygen/Nitrogen (O₂/N₂) at 20/80 vol.%
Reactor Parameters:
- Microwave Power: 250 W (2.45 GHz frequency)
- Substrate Temperature: 500-600 °C
- Gas Flow Rate: 100 sccm
- Working Pressure: 10 Torr
- Etching Duration: ~15 min

3. Characterization Techniques

Morphology: Scanning Electron Microscopy (SEM) (Zeiss Merlin, 10 kV, 150 pA probe current).
Optical and Strain Analysis: Micro-photoluminescence (PL) and micro-Raman spectroscopy (Renishaw “InVia” spectrometer).
Excitation: 488 nm laser focused to a ~2 µm spot (100x lens, NA = 0.9).
Temperature Control: Measurements performed at Room Temperature (300 K) and Liquid Nitrogen Temperature (80 K) using a Linkam THMS 600 cryogenic setup.

Commercial Applications

The improved optical properties and reduced strain heterogeneity of the GeV^- centers in nanodiamonds make them highly valuable for emerging quantum and biomedical technologies.

Industry/Sector	Application Focus	Technical Advantage Provided by RIE
Quantum Information Technologies	Solid-state single-photon emitters (SPEs), quantum memory, integrated photonic circuits.	Narrower ZPL and reduced inhomogeneous broadening enable the generation of indistinguishable photons from different emitters.
Quantum Sensing	Local strain imaging, magnetic field sensing, and electric field sensing.	The strain response of the ZPL fine structure allows for optical mapping of strain distribution in CVD NDs.
Biomedical Markers & Imaging	Bio-sensing, long-term cellular labeling, and high-resolution imaging.	NDs are biocompatible; bright, stable emission (high Debye-Waller factor) and short lifetime are ideal for high-speed biological tracking.
Nanothermometry	High-resolution temperature sensing at the nanoscale.	Color centers in NDs are sensitive to temperature changes, providing localized thermal measurements.
Hybrid Photonics	Integration of NDs with waveguides and plasmonic structures (pick-and-place techniques).	Small, well-faceted NDs (less than 200 nm) are easier to integrate into complex hybrid photonic devices with low light scattering.

View Original Abstract

The negatively charged germanium-vacancy GeV− color centers in diamond nanocrystals are solid-state photon emitters suited for quantum information technologies, bio-sensing, and labeling applications. Due to the small Huang-Rhys factor, the GeV−-center zero-phonon line emission is expected to be very intensive and spectrally narrow. However, structural defects and the inhomogeneous distribution of local strains in the nanodiamonds result in the essential broadening of the ZPL. Therefore, clarification and elimination of the reasons for the broadening of the GeV− center ZPL is an important problem. We report on the effect of reactive ion etching in oxygen plasma on the structure and luminescence properties of nanodiamonds grown by hot filament chemical vapor deposition. Emission of GeV− color centers ensembles at about 602 nm in as-grown and etched nanodiamonds is probed using micro-photoluminescence and micro-Raman spectroscopy at room and liquid nitrogen temperature. We show that the etching removes the nanodiamond surface sp2-induced defects resulting in a reduction in the broad luminescence background and a narrowing of the diamond Raman band. The zero-phonon luminescence band of the ensemble of the GeV− centers is a superposition of narrow lines originated most likely from the GeV− center sub-ensembles under different uniaxial local strain conditions.

Tech Support

Original Source

DOI: https://doi.org/10.3390/nano11112814

References

2019 - Quantum Nanophotonics with Group IV Defects in Diamond [Crossref]
2019 - Colour Centre Generation in Diamond for Quantum Technologies [Crossref]
2016 - Quantum Nanophotonics in Diamond [Crossref]
2019 - Synthesis, Properties, and Applications of Fluorescent Diamond Particles
2021 - Quantum Computer Based on Color Centers in Diamond [Crossref]
2018 - Fluorescent Nanodiamonds: Past, Present, and Future [Crossref]
2017 - Vacancy-Impurity Centers in Diamond: Prospects for Synthesis and Applications [Crossref]
2020 - Building Blocks for Quantum Network Based on Group-IV Split-Vacancy Centers in Diamond [Crossref]
2019 - Fiber-Optic Quantum Thermometry with Germanium-Vacancy Centers in Diamond [Crossref]
2019 - Bottom up Engineering of Single Crystal Diamond Membranes with Germanium Vacancy Color Centers [Crossref]