Single photon emission and single spin coherence of a nitrogen vacancy center encapsulated in silicon nitride

At a Glance

Metadata	Details
Publication Date	2020-03-30
Journal	Applied Physics Letters
Authors	Joe Smith, Jorge Monroy Ruz, John G. Rarity, Krishna C. Balram, Joe Smith
Institutions	Bristol Robotics Laboratory, University of Bristol
Citations	20
Analysis	Full AI Review Included

Technical Documentation & Analysis: Single Photon Emitters in MPCVD Diamond

(Analysis of: “Single photon emission and single spin coherence of a nitrogen vacancy centre encapsulated in silicon nitride”)

Executive Summary

The research successfully demonstrates a scalable quantum photonics platform using Nitrogen Vacancy (NV) centers hosted in nanodiamonds encapsulated in nitrogen-rich amorphous silicon nitride ($\text{aSiN}_x$). This platform is key for integrating solid-state quantum emitters with large-scale, foundry-compatible silicon photonics (LSQI).

Core Achievement: The study confirms that individual NV centers preserve both single photon emission ($g^{(2)}(0) < 0.5$) and measurable electron spin coherence ($\text{T}_{\eta} = 0.29$ µs) after undergoing Plasma Enhanced Chemical Vapor Deposition (PECVD) encapsulation.
Material Innovation: Utilizing nitrogen-rich $\text{aSiN}_x$ (high $\text{NH}_3/\text{SiH}_4$ ratio) reduced background photoluminescence (PL) emission by two orders of magnitude compared to standard stoichiometric $\text{Si}_3\text{N}_4$.
Radiative Enhancement: The high refractive index ($n > 1.9$) of the encapsulating layer caused significant radiative rate enhancement, reducing the fluorescence lifetime ($\text{T}_2$) by a factor of 3 (from 19.35 ns to 6.84 ns) due to coupling to slab modes.
Scalability: This platform is compatible with mature silicon technology and opens the avenue for building integrated quantum circuits leveraging high-performance photonic components already developed in silicon nitride.
6CCVD Value Proposition: The fidelity of the single NV centers relies entirely on the purity of the source diamond material. 6CCVD provides the necessary high-purity, low-nitrogen Single Crystal Diamond (SCD) required to reliably manufacture single, high-coherence NV centers for such integrated devices.

Technical Specifications

The following critical performance metrics and process parameters were extracted directly from the research:

Parameter	Value	Unit	Context
PECVD Substrate Temperature	300	°C	Deposition temperature for $\text{aSiN}_x$ films
PECVD Chamber Pressure	1.0	Torr	Constant pressure during $\text{aSiN}_x$ growth
$\text{NH}{3}/\text{SiH}{4}$ Ratio (R)	3.0 (High N content)	(Unitless)	Optimal ratio used for low auto-fluorescence encapsulation
Encapsulation Film Thickness	100	nm	Thickness of low auto-fluorescence $\text{aSiN}_x$ layer
Nanodiamond Host Size	10-20	nm	Size range required for isolated single NV centers
Refractive Index (n)	~1.95	(Unitless)	For R=3.0 film @ 637 nm
Background PL Reduction	~Two orders of magnitude	(Factor)	Reduced by using N-rich $\text{aSiN}x$ (R=3.0) vs. $\text{Si}{3}\text{N}_{4}$
Uncapped $\text{g}^{\text{2}}(0)$ (NV D)	0.22	(Unitless)	Confirms single photon emission (Antibunching) before capping
Capped $\text{g}^{\text{2}}(0)$ (NV D)	0.43	(Unitless)	Single emitter status preserved after PECVD encapsulation
Uncapped Spin Dephasing $\text{T}_{\eta}$	0.45	µs	Coherence time on bare fused silica
Capped Spin Dephasing $\text{T}_{\eta}$	0.29	µs	Coherence time after $\text{aSiN}_x$ encapsulation
Fluorescence Lifetime Reduction	Factor of 3	(Factor)	Due to coupling to slab modes in the high-index medium

Key Methodologies

The experiment successfully characterized the optical and spin properties of the same NV centers before and after exposure to the high-index $\text{aSiN}_x$ encapsulation process.

Diamond Source Preparation: Nanodiamonds (10-20 nm) containing high-purity, low-nitrogen NV centers were spin-coated onto fused silica substrates.
Alignment and Localisation: Fiduciary markers were fabricated using electron-beam lithography (EBL) to precisely map the spatial coordinates of the NV centers, allowing for pre- and post-encapsulation measurement validation.
$\text{aSiN}_x$ Deposition: Nitrogen-rich amorphous silicon nitride films were grown via PECVD, systematically controlling the gas flow ratio ($\text{NH}{3}/\text{SiH}{4}$) to achieve optimal low auto-fluorescence (R=3.0 recipe).
Optical Characterization: Confocal microscopy and Hanbury-Brown and Twiss (HBT) setup measured the single-photon statistics ($g^{(2)}(0)$) and fluorescence decay kinetics (lifetime $\text{T}_2$).
Spin Coherence Measurement: Free Induction Decay (FID) sequence (Initialize, $\pi/2$, $\tau$, $\pi/2$, Readout) measurements were conducted using a combined 532 nm laser and microwave antenna to extract the electron spin coherence time ($\text{T}_{\eta}$) and Rabi frequency ($\Omega$).

6CCVD Solutions & Capabilities

6CCVD provides the foundational high-performance MPCVD diamond materials necessary to replicate and advance this work, ensuring the requisite purity and integration compatibility for large-scale quantum systems.

Applicable Materials

The viability of this platform hinges on maintaining the intrinsic quantum properties of the NV center, which requires the highest purity diamond available.

Research Requirement	6CCVD Solution & Material	Technical Specification Match
High Purity Source	Single Crystal Diamond (SCD)	SCD grown via MPCVD offers ultra-low nitrogen content (ppm level), maximizing spin coherence ($\text{T}_{\eta}$) required for creating robust, isolated single NV centers in nanodiamonds.
Scalability & LSQI	Polycrystalline Diamond (PCD) Wafers	We offer PCD wafers up to 125 mm, providing a pathway toward large-scale integration of the $\text{SiN}_{\text{x}}$-on-diamond platform, aligning with foundry compatibility goals.
High-Quality Interfaces	Optical Grade SCD Polishing	Our standard SCD polishing achieves surface roughness $\text{Ra} < 1$ nm. For applications requiring direct interface (like slab coupling described), this ensures minimal surface scattering losses.

Customization Potential

The integration complexity associated with the $\text{aSiN}_x$ platform suggests future requirements for highly customized diamond substrates and subsequent processing:

Custom Dimensions: While this study used small substrates, LSQI requires larger areas. 6CCVD provides SCD plates up to 10 mm thick and PCD wafers up to 125 mm in diameter, meeting future demand for scalable wafer-level processing.
Precision Fabrication: The experiment relied on EBL fiduciary markers for alignment. We offer precision laser cutting and drilling services to create custom substrate shapes or through-wafer vias required for backside illumination or RF access.
On-Chip Interfacing: The paper mentions the increase in Rabi frequency, possibly due to RF antenna proximity. 6CCVD offers in-house custom metalization (including Ti, Pt, Au, Pd, Cu, W) critical for fabricating integrated microwave antennas, ohmic contacts, and alignment features directly on the diamond surface.

Engineering Support

The observed decrease in spin coherence ($\text{T}_{\eta}$) post-encapsulation (attributed to unpassivated surface charges in the amorphous nitride matrix) highlights the need for advanced material expertise. 6CCVD’s in-house PhD team specializes in optimizing diamond material properties and surface termination for complex quantum projects. We can assist researchers in selecting materials and surface treatments to mitigate decoherence effects in near-surface NV centers for applications leveraging high-index contrast mediums.

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

View Original Abstract

Finding the right material platform for engineering efficient photonic interfaces to solid state emitters has been a long-standing bottleneck for scaling up solid state quantum systems. In this work, we demonstrate that nitrogen rich silicon nitride, with its low auto-fluorescence at visible wavelengths, is a viable quantum photonics platform by showing that nitrogen vacancy centers embedded in nanodiamonds preserve both their quantum optical and spin properties post-encapsulation. Given the variety of high-performance photonic components already demonstrated in silicon nitride, our work opens up a promising avenue for building integrated photonic platforms using solid state emitters.

Tech Support

Original Source

DOI: https://doi.org/10.1063/5.0002709

References

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