Fabrication of 15NV− centers in diamond using a deterministic single ion implanter
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2021-06-01 |
| Journal | New Journal of Physics |
| Authors | Karin Groot-Berning, Georg Jacob, Christian Osterkamp, Fedor Jelezko, F. Schmidt‐Kaler |
| Institutions | Alpine Quantum Technologies (Austria), Universität Ulm |
| Citations | 28 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”This research demonstrates a proof-of-principle for fabricating shallow 15NV- centers in diamond using a novel, deterministic single-ion implantation system.
- Deterministic Source: The system utilizes a linear Paul trap to isolate, laser-cool, and deterministically extract single 15N2+ molecular ions, avoiding the stochastic nature of traditional focused ion beams (FIB).
- High Resolution: Achieved a lateral implantation resolution (spot size) of 121(35) nm, meeting the requirements for coupling NV centers to optical fields (<100 nm).
- Low Energy/Shallow Implantation: Ions were implanted at a low energy (5.9 keV for 15N2+), resulting in a shallow penetration depth of 4.2 nm (for atomic N), which minimizes straggling and bulk damage.
- Maskless Operation: The tight focusing achieved through laser cooling eliminates the need for nanofabricated masks or apertures, preserving the deterministic nature of the source.
- NV Characterization: Successful creation of 15NV- centers was confirmed by pulsed Optically Detected Magnetic Resonance (ODMR), showing the characteristic 3.1 MHz hyperfine splitting.
- Efficiency and Coherence: The NV creation yield was determined to be 0.6%. Measured coherence times (T2*) reached up to 1.56 µs, typical for shallow NV centers near the diamond surface.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| Substrate Material | Type IIa Electronic Grade | Diamond | Supplier: Element Six |
| Implanted Ion Species | 15N2+ | - | Molecular nitrogen ion |
| Ion Extraction Energy | 5.9 | keV | Total energy of 15N2+ |
| Equivalent Atomic Energy | 3.0 | keV | Energy per 15N atom |
| Implantation Depth (SRIM) | 4.2 | nm | Calculated penetration depth |
| Lateral Resolution (σN2) | 121(35) | nm | Focus spot size for 15N2+ |
| NV Creation Yield | 0.6 | % | Conversion efficiency (Region F) |
| Implantation Dose Range | 1 to 20 | ions/spot | Controlled at single-ion level |
| Activation Annealing Temperature | 900 | °C | Held for 2 hours |
| Annealing Vacuum | 10-7 | mbar | Ultra-high vacuum (UHV) |
| Coherence Time (T2*, Max) | 1.56 | µs | Measured via Hahn echo |
| NV- Zero Field Splitting | 2.87 | GHz | Confirms NV- charge state |
| 15N Hyperfine Splitting | 3.1 | MHz | Confirms 15N nuclear spin (I=1/2) |
| NV- ZPL Wavelength | 637 | nm | Optically detected magnetic resonance (ODMR) |
Key Methodologies
Section titled “Key Methodologies”The fabrication process relies on a highly controlled, multi-step sequence involving ion preparation, implantation, chemical cleaning, and thermal annealing.
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Ion Source and Preparation:
- A linear Paul trap is used as an ultracold ion source.
- Single 15N2+ molecular ions are loaded via electron impact ionization from an isotopically pure 15N2 gas source.
- The 15N2+ ions are sympathetically cooled via Coulomb interaction with co-trapped, laser-cooled 40Ca+ ions, resulting in a small phase space occupation (low energy dispersion).
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Deterministic Extraction and Focusing:
- A voltage sequence is applied to prepare a crystal containing exactly one 40Ca+ and one 15N2+ ion (detected by the shift of the Ca+ fluorescence image).
- The single, identified ion is extracted at 5.9 keV. Time-of-flight (TOF) measurements are used for unambiguous mass discrimination.
- The beam is focused using an electrostatic Einzel-lens, achieving a lateral resolution of 121 nm without the need for physical apertures or masks.
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Implantation:
- The target is a Type IIa electronic grade diamond substrate.
- A 5x5 pattern is implanted with 2 µm spacing, with doses ranging from k=1 to k=20 ions per spot.
- The low energy (3 keV atomic N) ensures shallow implantation (4.2 nm depth).
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Post-Implantation Processing:
- Acid Cleaning (Pre-Anneal): The sample is cleaned in a 1:1:1 mixture of sulfuric, nitric, and perchloric acid, heated to 130 °C for 2 hours, to remove surface dirt and graphitic layers.
- Thermal Annealing (UHV): A two-step annealing process is performed under UHV (10-7 mbar) to activate NV centers:
- Step 1: 250 °C for 1 hour.
- Step 2: Ramped up to 900 °C and held for 2 hours.
- Acid Cleaning (Post-Anneal): A second acid boiling step is performed to ensure oxygen termination of the diamond surface, which is critical for preserving the NV- charge state of shallow centers.
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Characterization:
- NV centers are detected using a home-built confocal microscope (excitation 518 nm).
- NV- centers are identified by their Zero Phonon Line (ZPL) at 637 nm.
- Pulsed ODMR confirms the presence of the 15N nuclear spin via the 3.1 MHz hyperfine splitting.
- Hahn echo measurements are used to determine the electron spin coherence time (T2*).
Commercial Applications
Section titled “Commercial Applications”This deterministic, high-resolution implantation technology is foundational for next-generation quantum devices built on the solid-state NV center platform.
- Quantum Computing and Simulation: Enables the creation of scalable arrays of coupled NV qubits (e.g., 10-20 nm grid spacing) necessary for building quantum processors based on dipolar magnetic interaction.
- Nanoscale Quantum Sensing: Allows for the precise, shallow placement of NV centers required for high-sensitivity magnetic and electric field sensing near the diamond surface.
- Integrated Quantum Photonics: Provides the necessary nanometer-scale accuracy for placing NV centers directly into pre-fabricated photonic structures (waveguides, solid immersion lenses) to maximize light collection and coupling efficiency.
- Isotope Engineering of Qubits: The isotope-selective source allows for precise control over the nuclear spin environment (e.g., implanting 15N or co-implanting 13C+) to optimize coherence times (T2) and spin readout fidelity.
- Advanced Diamond Doping: The low-energy, single-ion control minimizes lattice damage compared to high-energy implantation, leading to higher quality doped diamond for electronic or quantum applications.
View Original Abstract
Abstract Nitrogen vacancy (NV) centers in diamond are a platform for several important quantum technologies, including sensing, communication and elementary quantum processors. In this letter we demonstrate the creation of NV centers by implantation using a deterministic single ion source. For this we sympathetically laser-cool single <mml:math xmlns:mml=“http://www.w3.org/1998/Math/MathML” display=“inline” overflow=“scroll”> <mml:mmultiscripts> <mml:mrow> <mml:mi mathvariant=“normal”>N</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> </mml:mrow> <mml:mprescripts/> <mml:none/> <mml:mrow> <mml:mn>15</mml:mn> </mml:mrow> </mml:mmultiscripts> </mml:math> molecular ions in a Paul trap and extract them at an energy of 5.9 keV. Subsequently the ions are focused with a lateral resolution of 121(35) nm and are implanted into a diamond substrate without any spatial filtering by apertures or masks. After high-temperature annealing, we detect the NV centers in a confocal microscope and determine a conversion efficiency of about 0.6%. The 15 NV centers are characterized by optically detected magnetic resonance on the hyperfine transition and coherence time.