Optical Signatures of Quantum Emitters in Suspended Hexagonal Boron Nitride

At a Glance

Metadata	Details
Publication Date	2017-03-07
Journal	ACS Nano
Authors	Annemarie L. Exarhos, David A. Hopper, Richard R. Grote, Audrius Alkauskas, Lee C. Bassett
Institutions	Kaunas University of Technology, Center for Physical Sciences and Technology
Citations	211
Analysis	Full AI Review Included

Technical Documentation and Analysis: Quantum Emitters in h-BN

Executive Summary

This research characterizes stable, visible single-photon quantum emitters (QEs) created and observed in mechanically exfoliated, suspended single-crystal hexagonal Boron Nitride (h-BN) films (approximately 150 nm thick). The study provides crucial insights into the spectral, temporal, and polarization characteristics of these defects, confirming h-BN’s potential as a two-dimensional platform for quantum engineering, largely inspired by the success of the archetypal Nitrogen-Vacancy (NV) center in diamond.

Core Achievement: Demonstrated robust, stable, visible-wavelength single-photon emission in free-standing, single-crystal h-BN membranes, eliminating substrate interaction effects.
Defect Characteristics: QEs exhibit complex multi-level dynamics (three-level systems) with long metastable lifetimes approaching 1 µs.
Vibronic Coupling: Analysis reveals strong electron-phonon coupling with Huang-Rhys factors ($S_{HR}$) around 1.0, and room-temperature Zero-Phonon Line (ZPL) linewidths around 30 meV.
Substrate Dependence: Suspended h-BN regions show significantly dimmer photoluminescence (PL) compared to supported regions (5-10 times brighter), indicating complex substrate-defect interactions during formation.
Material Benchmark: The work confirms that high-quality, wide-bandgap materials like SCD diamond remain the industry standard benchmark, particularly where narrow ZPLs and precise defect engineering are critical for advanced quantum applications.
6CCVD Advantage: 6CCVD supplies Optical Grade Single Crystal Diamond (SCD) that provides the ideal environment for superior QEs (e.g., NV centers) with inherently narrower ZPLs and established formation protocols, directly addressing the underlying physics explored in this research.

Technical Specifications

The following hard data was extracted from the characterization of single emitters (SE1-SE5) in suspended h-BN:

Parameter	Value	Unit	Context
h-BN Film Thickness	150	nm	Studied suspended membrane
Excitation Wavelength	532	nm	CW laser power: ~150 µW
Visible PL Range	550 - 700	nm	Observed emission window
Substrate Contrast Ratio (Supported/Suspended)	5 - 10	times	Supported regions were significantly brighter
SE1 ZPL Energy ($E_{ZPL}$)	2.0405 ± 0.0003	eV	Single Emitter 1
SE2 ZPL Energy ($E_{ZPL}$)	1.9269 ± 0.0003	eV	Single Emitter 2
SE1 ZPL Linewidth ($F_{ZPL}$)	30 ± 1	meV	Room temperature; Lorentzian fit
SE2 ZPL Linewidth ($F_{ZPL}$)	31 ± 2	meV	Room temperature; Lorentzian fit
SE1 Huang-Rhys Factor ($S_{HR}$)	1.0 ± 0.1	(unitless)	Quantifies electron-phonon coupling
SE2 Huang-Rhys Factor ($S_{HR}$)	1.2 ± 0.1	(unitless)	Quantifies electron-phonon coupling
Short Lifetime ($\tau_{1}$)	0.94 - 4.7 ± 0.4	ns	Range across confirmed emitters
Long Lifetime ($\tau_{2}$)	89.1 - 1100 ± 700	ns/µs	Metastable state, three-level system analysis
Substrate Etch Depth	~5	µm	Etched holes in Si/SiO₂ for suspension

Key Methodologies

The experiment relied on precise material preparation and controlled defect engineering steps:

Substrate Patterning: Si wafers were capped with a 90 nm layer of thermal oxide (SiO₂) chosen for optimal bright-field optical contrast.
Hole Creation: Contact photolithography defined patterns, followed by fluorine-based dry etching to create holes approximately 5 µm deep for free-standing membrane creation.
Mechanical Exfoliation: Single-crystal h-BN was mechanically exfoliated onto the patterned Si/SiO₂, resulting in 150 nm thick flakes extending across supported and suspended regions.
Initial Annealing & Cleaning: Samples were annealed in Argon (Ar) atmosphere at 850° C for 30 minutes, followed by an O₂ plasma clean.
Defect Creation (e-beam exposure): Isolated defects were created by electron bombardment using a Scanning Electron Microscope (SEM) imaging at 3 keV.
Final Annealing: A subsequent Argon anneal at 850° C for 30 minutes was performed after electron bombardment.
Crystallographic Verification: Electron Backscatter Diffraction (EBSD) was used to confirm single-crystallinity and determine the in-plane lattice orientation of the exfoliated h-BN flakes.
Optical Measurement: A home-built confocal scanning fluorescence microscope was used for PL imaging, spectral acquisition (using a spectrometer), and single-photon autocorrelation (using a Hanbury Brown-Twiss setup and single-photon avalanche diodes).

6CCVD Solutions & Capabilities

This research reinforces the demand for ultra-high-quality, wide-bandgap materials for solid-state quantum engineering, a field pioneered by SCD diamond. While h-BN shows promise, its QEs exhibit large ZPL linewidths ($F_{ZPL}$ ~ 30 meV) and strong substrate dependencies. 6CCVD provides the superior materials and precision engineering required to meet and exceed the demands of established quantum systems like the NV center in diamond.

Applicable Materials

6CCVD offers the ideal materials required to replicate, improve upon, or benchmark against the results presented in the h-BN study:

Optical Grade Single Crystal Diamond (SCD): Required for achieving the narrow ZPLs (often sub-meV at low temperature, significantly better than h-BN’s 30 meV FWHM) and highly coherent spin properties necessary for high-fidelity quantum sensing and computation. 6CCVD provides low-strain, high-purity SCD plates, essential for optimal NV or SiV performance.
Custom Low-Nitrogen SCD: For engineers focusing on silicon-vacancy (SiV) or germanium-vacancy (GeV) centers, 6CCVD provides ultra-low nitrogen diamond, ensuring superior material control for precise defect incorporation.
Boron-Doped Diamond (BDD): Available for projects requiring conductive, stable electrodes for electrochemical and electro-optical applications.

Customization Potential

The h-BN work utilized 150 nm thick membranes and required complex Si/SiO₂ patterning. 6CCVD provides the necessary material control for similar, high-precision engineering on robust diamond substrates:

Custom Capability	Research Requirement Addressed	6CCVD Specific Offering
Thickness Control	H-BN flake thickness was 150 nm.	SCD thickness control from 0.1 µm to 500 µm for quantum membranes or bulk integration.
Substrate Size	H-BN flakes were small (up to ~1000 µm²).	SCD and PCD wafers available in custom dimensions, up to 125 mm (inch-size PCD).
Surface Finish	Narrow ZPLs require minimal strain/roughness.	Polishing services achieving R_a < 1 nm (SCD) and R_a < 5 nm (Inch-size PCD).
Advanced Integration	Metalization is required for electrical contact or photonic device integration.	Internal Metalization: Custom deposition of Au, Pt, Pd, Ti, W, Cu.
Custom Geometry	Specific device geometry (e.g., photonic structures).	High-precision laser cutting services for custom shapes and patterns.

Engineering Support & Logistics

The h-BN study reveals the critical importance of defect formation kinetics (e.g., e-beam irradiation followed by annealing at 850° C). Successful implementation of QEs requires specialized process knowledge.

Process Consultation: 6CCVD’s in-house PhD engineering team offers expert consultation on material selection, defect creation protocols, and surface preparation required for solid-state quantum emitter projects, leveraging decades of collective experience in CVD diamond growth and engineering.
Global Supply Chain: Whether replicating this h-BN suspension experiment or transitioning to superior SCD diamond, 6CCVD ensures reliable supply with Global Shipping (DDU default, DDP available), supporting research teams worldwide.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

Hexagonal boron nitride (h-BN) is rapidly emerging as an attractive material for solid-state quantum engineering. Analogously to three-dimensional wide-band-gap semiconductors such as diamond, h-BN hosts isolated defects exhibiting visible fluorescence at room temperature, and the ability to position such quantum emitters within a two-dimensional material promises breakthrough advances in quantum sensing, photonics, and other quantum technologies. Critical to such applications is an understanding of the physics underlying h-BN’s quantum emission. We report the creation and characterization of visible single-photon sources in suspended, single-crystal, h-BN films. With substrate interactions eliminated, we study the spectral, temporal, and spatial characteristics of the defects’ optical emission. Theoretical analysis of the defects’ spectra reveals similarities in vibronic coupling to h-BN phonon modes despite widely varying fluorescence wavelengths, and a statistical analysis of the polarized emission from many emitters throughout the same single-crystal flake uncovers a weak correlation between the optical dipole orientations of some defects and h-BN’s primitive crystallographic axes, despite a clear misalignment for other dipoles. These measurements constrain possible defect models and, moreover, suggest that several classes of emitters can exist simultaneously throughout free-standing h-BN, whether they be different defects, different charge states of the same defect, or the result of strong local perturbations.

Tech Support

Original Source

DOI: https://doi.org/10.1021/acsnano.7b00665