Electrical control of deep NV centers in diamond by means of sub-superficial graphitic micro-electrodes

At a Glance

Metadata	Details
Publication Date	2016-11-16
Journal	Carbon
Authors	J. Forneris, S. Ditalia Tchernij, A. Tengattini, Emanuele Enrico, Veljko Grilj
Institutions	Istituto Nazionale di Fisica Nucleare, Sezione di Torino, Rudjer Boskovic Institute
Citations	42
Analysis	Full AI Review Included

Electrical Control of Deep NV Centers in Diamond using Sub-Superficial Graphitic Micro-Electrodes

Analysis of J. Forneris et al. (INFN, INRiM, et al.)

This document provides a technical analysis of the electrical control mechanisms used to stabilize the negatively charged state (NV^-) of nitrogen-vacancy centers located deep within Single Crystal Diamond (SCD) bulk material. This research directly supports 6CCVD’s mission to supply high-purity MPCVD diamond materials for next-generation quantum computing and sensing applications.

Executive Summary

Core Achievement: Demonstrated effective electrical control over the charge state conversion (NV⁰ <—> NV^-) of sub-superficial Nitrogen-Vacancy (NV) centers in bulk CVD diamond.
Material/Methodology: Used optical grade Type IIa SCD fabricated with buried, non-rectifying graphitic micro-electrodes created via 6 MeV C³⁺ ion beam lithography followed by high-temperature vacuum annealing (1000 °C).
Performance: Achieved a maximum stable NV^- population of approximately 75% ($n_{-}/n_{tot} = 0.74 \pm 0.09$) relative to the total NV ensemble population, representing a 40% increase compared to the unbiased state.
Mechanism Identification (Low Current): Charge state conversion is primarily governed by electron trapping mechanisms described by the Space-Charge-Limited Current (SCLC) model, confirming the trap level position at 0.57 eV below the conduction band (consistent with the NV^- excited state).
High-Current Regime: Observed strong Electroluminescence (EL) and a transition to Poole-Frenkel conduction, accompanied by a decrease in the NV^- concentration, suggesting increased hole injection or high-rate thermally-stimulated electron detrapping.
Quantum Significance: Controlling deep NV centers (>2.7 µm below surface) is critical for quantum applications as these bulk defects exhibit significantly longer spin coherence times due to reduced interaction with surface noise.
Future Outlook: Scaling down electrode gaps using advanced ion-beam lithography promises high conversion efficiency at significantly lower operating voltages.

Technical Specifications

Parameter	Value	Unit	Context
Substrate Material	Single Crystal Diamond (SCD)	N/A	Optical grade, Type IIa
Substrate Dimensions	3 x 3 x 0.3	mm³	Sample size used for fabrication
Substitutional Nitrogen (N_s)	<1	ppm	Nominal concentration in SCD
Boron Concentration (B)	<0.05	ppm	Nominal concentration in SCD
Implantation Ion	6 MeV C³⁺	N/A	Used for graphitic electrode creation
Implantation Fluence	~4 x 10¹⁶	cm^-2	Required to exceed graphitization threshold
Electrode Depth (Graphitic Layer)	~2.7	µm	Below the surface (end of ion range)
Electrode Gap (d)	~9	µm	Inter-electrode distance
Annealing Temperature	1000	°C	Vacuum annealing for graphitization
Ohmic Resistance (Low V)	700	MΩ	Diamond-graphite interface (0-250 V)
Critical Bias Voltage (V_c)	300	V	Onset of high-current/SCLC regime
Trap Energy Level ($\Delta$E$_{t}$)	0.57 (± 0.07)	eV	Below conduction band (consistent with NV^- excited state)
Max NV^- Population	~75 (± 9)	%	Achieved at 400 nA current
Relative NV^- Increase	Up to 40	%	Relative to unbiased device
Excitation Laser Wavelength	532	nm	Continuous wave (2.33 eV)
Excitation Laser Power	21.6	mW	Measured on sample surface
New EL Emission Lines	563.5, 580	nm	Attributed to self-interstitial defects (only visible under EL)
Diamond Dielectric Permittivity	5.5	N/A	Used in SCLC calculations ($\epsilon_{r}$)

Key Methodologies

The experiment utilized high-precision material selection and advanced deep-implantation techniques to create bulk diamond devices capable of stable electrical charge control:

Material Preparation: An “optical grade” Type IIa SCD substrate (3 x 3 x 0.3 mm³) with extremely low concentrations of nitrogen (<1 ppm) and boron (<0.05 ppm) was selected to ensure maximum NV stability and high-quality optical properties.
Sub-Superficial Electrode Fabrication: Graphitic micro-electrodes were created in the bulk diamond (~2.7 µm deep) by raster-scanning a focused 6 MeV C³⁺ ion beam along linear paths at a fluence of ~4 x 10¹⁶ cm^-2. This technique exploits radiation-induced graphitization at the end of the ion penetration range.
Thermal Processing and NV Creation: The sample underwent vacuum annealing at 1000 °C for 2 hours. This process served the dual purpose of:
- Converting the amorphized buried layer into a stable graphitic conductive phase.
- Aggregating native nitrogen atoms with vacancies created by stray ions to form a high-density ensemble of NV centers in the inter-electrode gap.
Surface Termination: A 30 min oxygen plasma treatment (20 sccm O₂ flux, 30 W RF power) was performed to remove surface conductivity and ensure the diamond surface was oxygen-terminated. This ruled out surface band bending effects on deep NV charge state conversion.
Contact Creation: Focused Ion Beam (FIB) milling (30 keV Ga⁺) was used to expose the graphitic micro-channels to the surface endpoints. Subsequently, 70 nm thick Ag contacts were deposited through a stencil mask for external wire-bonding.
Characterization: Electrical (I-V curves) and optical (Photoluminescence (PL) and Electroluminescence (EL)) measurements were conducted to study the current-dependent NV charge state population (NV^- vs NV⁰).

6CCVD Solutions & Capabilities: Enabling Deep Diamond Quantum Devices

The research detailed in this paper confirms that the pursuit of long spin coherence times requires deep, sub-superficial NV centers, demanding highly controlled substrate materials and advanced fabrication methods. 6CCVD is uniquely positioned to supply the foundational SCD materials necessary to replicate and expand this research.

Applicable Materials

Research Requirement	6CCVD Solution	Material Specification & Benefits
High-Purity Substrate	Optical Grade Single Crystal Diamond (SCD)	Type IIa, low N (<1 ppm) and ultra-low B (<0.05 ppm) materials essential for maximizing NV coherence time and minimizing unintended defect formation.
Deep Defect Control	Thick SCD Substrates	SCD plates up to 500 µm in thickness, providing ample bulk material for MeV ion implantation depths (up to several micrometers) far removed from surface defects.
Custom Electrode Creation	Standard SCD Wafers	SCD plates/wafers up to 125 mm (PCD) in custom dimensions, perfectly suited for reproducible ion beam lithography and subsequent wafer-level processing.
Surface/Contact Prep	Low Roughness Polishing	SCD wafers polished to Ra < 1 nm, critical for subsequent high-precision FIB milling and reliable ohmic contact deposition (e.g., Ag) required for the electrical measurements.

Customization Potential

The success of this work relies on the precise integration of electrical contacts with the buried graphitic micro-electrodes. 6CCVD offers custom manufacturing services that streamline the post-processing phase:

Custom Metalization: The researchers used 70 nm thick Ag contacts. 6CCVD provides in-house metalization services using Au, Pt, Pd, Ti, W, and Cu, allowing clients to test various ohmic contact schemes on SCD before proceeding to complex wire-bonding.
Precision Cutting and Dicing: While the paper used 3 mm x 3 mm samples, 6CCVD can supply wafers and plates cut to non-standard, specific dimensions required for insertion into specialized ion-beam or high-voltage test setups.

Engineering Support

Controlling deep NV charge states requires balancing substrate purity, implantation parameters, and post-annealing recipes. 6CCVD’s in-house PhD team specializes in the growth and characterization of NV-enabled diamond. We can assist engineers and scientists with:

Material Selection for Quantum Sensing: Consulting on the optimal nitrogen incorporation level (native or tailored) and material thickness necessary to target specific NV center depths and achieve maximal spin coherence times ($\text{T}_{2}$).
Recipe Optimization: Providing technical insights on thermal treatments (annealing) to ensure high-efficiency conversion of implanted graphitic layers while minimizing the unintentional aggregation of other optically active defect centers.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly. 6CCVD offers reliable, globally shipped (DDU/DDP) diamond solutions, providing the foundation for revolutionary quantum devices.

Tech Support

Original Source

DOI: https://doi.org/10.1016/j.carbon.2016.11.031

References

2010 - Quantum measurement and control of single spins in diamond [Crossref]
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2013 - Heralded entanglement between solid-state qubits separated by three metres [Crossref]
2011 - Electric-field sensing using single diamond spins [Crossref]
2011 - Real time magnetic field sensing and imaging using a single spin in diamond [Crossref]
2014 - Subdiffraction-limited quantum imaging within a living cell
2006 - Photochromism in single nitrogen-vacancy defect in diamond [Crossref]
2013 - The nitrogen-vacancy colour centre in diamond [Crossref]
2010 - Surface-induced charge state conversion of nitrogen-vacancy defects in nanodiamonds [Crossref]
2010 - Conversion of neutral nitrogen-vacancy centers to negatively charged nitrogen-vacancy centers through selective oxidation [Crossref]