Spin coherence as a function of depth for high-density ensembles of silicon vacancies in proton-irradiated 4H–SiC

At a Glance

Metadata	Details
Publication Date	2020-07-18
Journal	Solid State Communications
Authors	Peter Brereton, D. Puent, J. R. Vanhoy, E. R. Glaser, S.G. Carter
Institutions	United States Naval Research Laboratory, United States Naval Academy
Citations	9
Analysis	Full AI Review Included

Technical Documentation & Analysis: Spin Coherence in SiC Vacancies

This document analyzes the research concerning spin coherence in proton-irradiated 4H-SiC silicon vacancies (V_Si) and outlines how 6CCVD’s high-purity MPCVD diamond materials provide superior platforms for advancing solid-state quantum technologies, particularly where high coherence and high defect density are required.

Executive Summary

Research Focus: The study successfully measured the spin coherence time (T₂) of high-density silicon vacancy (V_Si) ensembles in 4H-SiC generated by 2 MeV proton irradiation as a function of implantation depth.
Key Finding (Limitation): T₂ coherence time decreases sharply from a maximum of 11.5 µs (near surface) to a minimum of 3.4 µs (near the 40 µm stopping layer).
Decoherence Mechanism: This decrease is directly correlated with increased defect density, confirming that dipole-dipole interactions between V_Si centers are the dominant decoherence mechanism, severely limiting performance in high-density SiC ensembles.
Quantum Material Pivot: While SiC is a promising host, the observed T₂ times are significantly shorter than the millisecond-scale coherence achieved by the canonical solid-state qubit, the Nitrogen Vacancy (NV) center in high-purity Single Crystal Diamond (SCD).
6CCVD Value Proposition: 6CCVD specializes in producing ultra-high purity MPCVD SCD, the established material benchmark for achieving long T₂ coherence times necessary for scalable quantum information processing and sensing applications.
Scalability Requirement: The paper notes that achieving superradiant emission requires ensembles of 10¹⁶ interacting spins; 6CCVD provides the high-quality, low-noise SCD substrates required to reach this density threshold while maintaining high coherence.

Technical Specifications

The following hard data points were extracted from the experimental results regarding the V_Si defect generation and measurement:

Parameter	Value	Unit	Context
Irradiation Particle	Proton	N/A	Defect generation method
Proton Beam Energy	2	MeV	Used for deep implantation
Proton Fluence	3.5 x 10¹⁵	proton/cm²	High-density ensemble creation
Calculated Proton Range	32	µm	Projected stopping depth
Maximum Depth Measured	~40	µm	Depth where T₂ minimum was observed
ODMR Static Magnetic Field (B)	68.5	mT	Applied parallel to c-axis
Zero Field Splitting (2D)	70	MHz	Ground state (GS) V_Si sublevel splitting
ODMR Transition Frequencies	1883, 2022	MHz	Dipole-allowed transitions (∆m_s = ±1)
Maximum Coherence Time (T₂)	11.5	µs	Measured near irradiated surface (low density)
Minimum Coherence Time (T₂)	3.4	µs	Measured near stopping layer (high density)
Excitation Wavelength	850	nm	Below-gap excitation for Photoluminescence (PL)
V2 ZPL Energy	1353	meV	Zero-phonon line for the V2 transition

Key Methodologies

The experiment utilized advanced defect generation and spin control techniques to characterize the V_Si ensemble:

Substrate Selection: High purity semi-insulating 4H-SiC substrates (CREE) were used as the host material.
Defect Creation: Defects were generated via flood irradiation using a 2 MeV proton beam, aligned parallel to the c-axis, achieving a high fluence of 3.5 x 10¹⁵ proton/cm². No post-irradiation annealing was performed.
Depth Profiling: The sample was mounted on a stepper motor stage, allowing the confocal excitation and collection optics to scan along the cleaved edge, measuring spin properties as a function of depth (from surface to ~40 µm).
Optical Excitation: Photoluminescence (PL) was excited using an 850 nm Ti:sapphire laser, and the V2 transition (1353 meV) was the focus due to its strong spin-dependence at room temperature.
Microwave Delivery: Microwave excitation was delivered via a 50 µm diameter gold wire loop shorted between the center and outer conductor of a coaxial cable.
Spin Control and Readout: Optically Detected Magnetic Resonance (ODMR) was used to measure the spin state. The single defect coherence time (T₂) was specifically measured using the Hahn echo separated pulse sequence (π/2 - T - π - T - π/2).

6CCVD Solutions & Capabilities

The research demonstrates that achieving long spin coherence in high-density ensembles is the primary challenge for SiC-based quantum devices. 6CCVD provides the necessary MPCVD diamond materials and engineering services to overcome these limitations, offering a superior platform for quantum information and sensing applications.

Applicable Materials for Quantum Research

Application Requirement	6CCVD Material Recommendation	Technical Rationale
High Coherence Qubits (T₂ >> 12 µs)	Isotopically Purified Optical Grade SCD	Diamond with extremely low nitrogen (< 1 ppm) and controlled ¹³C content minimizes the nuclear spin bath, enabling NV center T₂ times in the millisecond range—orders of magnitude better than the SiC V_Si results.
Integrated Quantum Photonics	Optical Grade SCD Plates (Ra < 1 nm)	Ultra-smooth, high-purity SCD wafers (up to 500 µm thick) are essential for fabricating low-loss photonic crystal cavities and waveguides, critical for realizing chip-scale ultra-stable lasers and detectors (Sec 4.3).
High-Density Sensing Ensembles	High-Purity Polycrystalline Diamond (PCD)	For applications requiring large area coverage (up to 125mm) where T₂ requirements are moderate, our high-quality PCD offers a cost-effective, scalable platform for ensemble sensing.
Electrochemical/BDD Applications	Boron-Doped Diamond (BDD)	While not directly used in this ODMR study, BDD is available for researchers requiring conductive diamond electrodes or substrates for integrated device architectures.

Customization Potential for Advanced Qubit Fabrication

6CCVD’s in-house engineering capabilities directly support the advanced fabrication techniques required to replicate and extend the methodologies used in this SiC study onto a diamond platform:

Custom Dimensions and Thickness: We supply SCD plates up to 500 µm thick and PCD wafers up to 125mm in diameter, accommodating large-scale implantation and device processing. We also offer custom substrates up to 10mm thick.
Precision Polishing: Our SCD material is polished to an industry-leading surface roughness (Ra < 1 nm), crucial for minimizing scattering losses and ensuring optimal coupling for the 850 nm excitation used in PL/ODMR experiments. Inch-size PCD is polished to Ra < 5 nm.
Integrated Metalization: The SiC experiment required a 50 µm gold wire loop for microwave delivery. 6CCVD offers internal metalization services (Au, Pt, Pd, Ti, W, Cu) to deposit custom microwave striplines (e.g., coplanar waveguides) directly onto the diamond surface, ensuring highly efficient and localized spin control (Rabi rotations).
Laser Cutting and Shaping: We provide precision laser cutting services to create custom geometries, trenches, or features necessary for integrating diamond into complex quantum device architectures.

Engineering Support

6CCVD’s in-house PhD team specializes in MPCVD growth and defect engineering. We can assist researchers transitioning from SiC V_Si studies to diamond-based qubits (NV, SiV, GeV centers) by providing consultation on:

Material selection based on target T₂ coherence time and required defect density.
Optimizing substrate purity and orientation for post-growth defect creation via ion implantation (e.g., proton or silicon implantation).
Designing custom metalization layouts for efficient microwave delivery and ODMR readout.

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

Tech Support

Original Source

DOI: https://doi.org/10.1016/j.ssc.2020.114014

References

2003 - Long coherence times at 300 k for nitrogen-vacancy center spins in diamond grown by chemical vapor deposition [Crossref]
2004 - Observation of coherent oscillations in a single electron spin [Crossref]
2009 - Ultralong spin coherence time in isotopically engineered diamond [Crossref]
2000 - Stable solid-state source of single photons [Crossref]
2012 - A robust scanning diamond sensor for nanoscale imaging with single nitrogen-vacancy centres [Crossref]
2013 - Optical magnetic imaging of living cells [Crossref]
2013 - Nanoscale magnetic imaging of a single electron spin under ambient conditions [Crossref]
2008 - Nanoscale magnetic sensing with an individual electronic spin in diamond [Crossref]
2014 - Subnanometre resolution in three-dimensional magnetic resonance imaging of individual dark spins [Crossref]
2010 - Quantum computing with defects [Crossref]