Identification of nickel-vacancy defects by combining experimental and ab initio simulated photocurrent spectra

At a Glance

Metadata	Details
Publication Date	2018-06-12
Journal	Physical review. B./Physical review. B
Authors	Elisa Londero, Emilie Bourgeois, Miloš Nesládek, Ádám Gali
Institutions	Hungarian Academy of Sciences, Budapest University of Technology and Economics
Citations	21
Analysis	Full AI Review Included

6CCVD Technical Analysis & Quantum Material Solutions: Identification of NiV Defects in Diamond

This document analyzes the research concerning the identification of the Nickel Split Vacancy (NiV) defect in irradiated diamond, positioning it as a novel candidate for near-infrared (NIR) solid-state spin qubits. The findings are contextualized with 6CCVD’s expertise in custom MPCVD diamond materials and defect engineering services.

Executive Summary

The research successfully identified the Nickel Split Vacancy (NiV) complex as the origin of critical, previously unknown photoionization bands in diamond, paving the way for new electrical readout quantum technologies.

Defect Identification: Confirmed the two unknown near-infrared (NIR) photocurrent thresholds (1.2 eV and 1.9 eV) correspond precisely to the two acceptor levels of the NiV defect.
Quantum Qubit Potential: Proposes the NiV complex as a highly promising solid-state spin qubit candidate, potentially capable of room temperature (RT) electrical readout, operating via NIR excitation.
PDMR Enhancement: Utilized blue bias light (2.4-3 eV) in Photoelectric Detection of Magnetic Resonance (PDMR) to successfully manipulate defect charge states and enhance the NiV signal.
Favorable Properties: The neutral NiV defect (S=1 state, known as NOL1/NIRIM5) exhibits a giant zero-field-splitting ($D = -171$ GHz), ideal for robust spin initialization and detection.
Material Guidance: The study highlights how defects (like NiV) introduce unwanted background noise in NV-PDMR systems, emphasizing the critical need for precisely engineered MPCVD diamond substrates.
Methodology: Combined experimental photocurrent measurements on irradiated Type-Ib HPHT diamond with detailed ab initio Density Functional Theory (DFT) calculations to achieve definitive defect identification.

Technical Specifications

The following hard parameters define the material synthesis, sample preparation, and measurement conditions used to isolate and study the NiV defect.

Parameter	Value	Unit	Context
Initial Substrate Type	Ib	HPHT Diamond Plate	Nitrogen-rich starting material
Proton Irradiation Energy	6.5	MeV	Defect generation
Proton Dose	1.13 x 10¹⁶	cm^-2	Vacancy density control
Annealing Temperature	900	°C	Used for vacancy mobility (1 hr, Argon)
Applied DC Electric Field	5 x 10⁴	V cm^-1	Used for photocurrent collection
Inter-electrode Distance	100	µm	Coplanar measurement setup
NiV Acceptor Level 1 Threshold	1.2	eV	Lowest Photoionization band (NIR)
NiV Acceptor Level 2 Threshold	1.9	eV	Higher Photoionization band (NIR)
NiV Zero-Field-Splitting (D)	-171	GHz	Spin-Hamiltonian parameter (S=1 neutral state)
Substitutional N Concentration ([N_s])	220 ± 20	ppm	Estimated from FTIR (dark exp.)
Negative NV Concentration ([NV^-])	~34	ppm	Estimated from PL (dark exp.)
NiV Concentration (Fitted, No Bias)	1.2	ppm	Concentration determined via spectral fit
Blue Bias Light Energy	2.4 - 3	eV	Used to manipulate defect charge states

Key Methodologies

The experimental design relied on precise defect creation via irradiation, controlled thermal processing, and sophisticated dual-illumination electrical measurements, supported by high-accuracy computational modeling.

Substrate Selection and Preparation:
- Type-Ib HPHT diamond (naturally high in nitrogen) was chosen to ensure sufficient starting material for NV, N_s, and NiV complex formation.
- The sample was irradiated with high-energy protons (6.5 MeV) at a dose of 1.13 x 10¹⁶ cm^-2 to create vacancies.
- Post-irradiation annealing was performed at 900°C under an Argon atmosphere for 1 hour to stabilize the resulting vacancy complexes (NV and NiV).
Device Integration:
- The sample surface was cleaned and oxidized.
- Coplanar electrodes were integrated onto the diamond surface with a defined inter-electrode distance of 100 µm, enabling electrical detection.
Photocurrent Spectroscopy:
- A high DC electric field (5 x 10⁴ V cm^-1) was applied across the electrodes.
- Photocurrent was measured using two distinct methods:
  - Method 1 (No Bias): Illumination using monochromatic light (1-300 µW).
  - Method 2 (Blue Bias): Application of a secondary 2.4-3 eV blue bias light (1.8 mW) to selectively manipulate the charge state occupation of deep defects, increasing the visibility of the NiV related bands.
Material Characterization:
- Supporting data on defect concentration were gathered via Photoluminescence (PL), Fourier Transformed Infrared (FTIR), and optical absorption spectroscopy.
Computational Verification:
- Ab initio Density Functional Theory (DFT) calculations (using the HSE06 hybrid functional) were employed on a 512-atom supercell.
- This modeling determined the ionization cross-sections and acceptor level energies for NV, N_s, and NiV defects, providing a theoretical match for the measured 1.2 eV and 1.9 eV experimental thresholds.

6CCVD Solutions & Capabilities

This research validates the critical role of the NiV defect as a potential NIR qubit, requiring extreme control over material purity, defect incorporation, and customized device integration. 6CCVD provides the necessary engineered MPCVD diamond platforms to replicate and advance this work.

Applicable Materials

To optimize the generation and study of targeted nickel and nitrogen-related color centers (NiV, NV), researchers require materials with precise impurity control, unattainable via natural or standard HPHT synthesis.

Research Requirement	6CCVD Material Recommendation	Technical Rationale
Defect-Controlled Qubit Host	Single Crystal Diamond (SCD): Low-Nitrogen, High-Purity Grade	Enables targeted, post-growth incorporation of Nickel, ensuring the NiV complex is formed in an ultra-clean lattice environment, minimizing N_s background noise observed in the paper.
Nickel Incorporation	Custom-Doped MPCVD Diamond: SCD or PCD tailored for Ni inclusion.	We provide expertise in custom gas phase doping necessary for repeatable incorporation of specific metal impurities (Ni, Si, Cr) for color center creation.
High Surface Quality for PDMR	Precision Polished SCD Wafers: Surface roughness $R_a < 1$ nm.	Essential for minimizing surface scattering, achieving high optical efficiency, and ensuring reliable ohmic contact formation for electrical detection.

Customization Potential

The experimental setup utilized specialized dimensions and electrode metallization, which 6CCVD provides as standard services, accelerating R&D cycles.

Metalization Services: The experiment required patterned electrodes for electrical readout. 6CCVD offers in-house deposition and patterning of the precise metal stacks required for reliable ohmic contacts in diamond (including Au, Pt, Pd, Ti, W, and Cu), critical for replicating the PDMR device structure.
Custom Wafer Dimensions: We provide PCD plates up to 125mm diameter, offering researchers a scalable platform for moving beyond small lab samples to high-density sensor or qubit arrays.
Thickness Control: We supply SCD and PCD in custom thicknesses (0.1 µm to 500 µm) and substrates up to 10mm, necessary for tailoring the material depth required for effective high-energy proton irradiation and subsequent annealing.
Laser and Mechanical Machining: 6CCVD offers high-precision laser cutting and shaping services to produce the exact geometric requirements for chip-scale device fabrication.

Engineering Support

Developing next-generation NIR qubits based on defects like NiV requires expert knowledge in both MPCVD growth and post-growth defect engineering.

Defect Engineering Consultation: 6CCVD’s in-house PhD team can assist with material selection and optimization protocols for complex defect generation projects, specifically those involving post-growth processing (irradiation and annealing) to maximize target defect yield while suppressing background noise defects (like N_s, which the paper identified as a problem).
Application Focus: We provide direct engineering support for projects involving quantum communication, solid-state spin qubits, and electrical readout systems (e.g., PDMR), leveraging our deep understanding of diamond’s magneto-optical properties.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly. We ship globally (DDU default, DDP available).

View Original Abstract

There is a continuous search for solid state spin qubits operating at room temperature with excitation in the infrared communication bandwidth. Recently, we have introduced the photoelectric detection of magnetic resonance (PDMR) to read the electron spin state of nitrogen-vacancy (NV) centers in diamond, a technique which is promising for applications in quantum information technology. By measuring the photoionization spectra on a diamond crystal, we found two ionization thresholds of unknown origin. On the same sample we also observed absorption and photoluminescence signatures that were identified in the literature as Ni-associated defects. We performed ab initio calculations of the photoionization cross section of the nickel split-vacancy complex (NiV) and N-related defects in their relevant charge states and fitted the concentration of these defects to the measured photocurrent spectrum, which led to a surprising match between experimental and calculated spectra. This study enabled us to identify the two unknown ionization thresholds with the two acceptor levels of NiV. Because the excitation of NiV is in the infrared, the photocurrent detected from the paramagnetic NiV color centers is a promising way towards the design of electrically readout qubits.