Can surface-transfer doping and UV irradiation during annealing improve shallow implanted nitrogen-vacancy centers in diamond?
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2020-08-03 |
| Journal | Applied Physics Letters |
| Authors | Niklas J. Glaser, G. Braunbeck, Oliver Bienek, Ian D. Sharp, Friedemann Reinhard |
| Institutions | Technical University of Munich, Schott (Germany) |
| Citations | 3 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis study investigated noninvasive surface treatmentsâsurface transfer doping (STD) and UV irradiationâas methods to enhance the formation yield and spin coherence time (T2) of shallow, ion-implanted Nitrogen-Vacancy (NV) centers in diamond.
- Yield Improvement: Surface transfer doping using metal coatings (Nickel, DNi; Palladium, DPd) significantly increased the NV center conversion yield (ratio of formed NV centers to implanted N atoms). DNi achieved an 85% increase and DPd a 70% increase compared to the UV-irradiated (DUV) or Aluminum Oxide (DAlOx) samples.
- Maximum Yield: Metal-coated samples achieved a conversion yield of > 5%, approximately two times higher than the non-metal coated samples (DUV, DAlOx).
- Coherence Time (T2): The critical metric for quantum sensing, T2, showed minimal variation across all tested conditions, varying by less than a factor of two (median T2 ranged from 12 ”s to 17 ”s).
- Charge State Control: The increased yield and fluorescence intensity in metal-coated samples suggest that STD successfully shifted the Fermi level near the surface, promoting vacancy charge states (V0 or V+) that suppress divacancy clustering during annealing.
- Paired NV Formation: Palladium coating (DPd) showed a 60% increase in paired NV center formation compared to simulations, suggesting that V+ vacancies may be attracted by existing NV centers.
- Method Efficacy: The observed improvements are weaker than those achieved by bulk co-implantation or sacrificial layer doping, indicating that these noninvasive surface methods provide insufficient band structure modification for optimal NV formation.
- UV Irradiation: UV illumination during annealing had no measurable positive effect on either NV yield or T2 times.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Implantation Species | 15N+ | ions | Used for NV formation |
| Implantation Energy | 5 | keV | Shallow implantation |
| Implantation Fluence | 5 x 109 | ions/cm2 | Target density |
| Expected Implantation Depth | 10 | nm | Shallow NV centers |
| Annealing Temperature | 830 | °C | Vacuum annealing |
| Annealing Time (Hold) | 225 | min | Time at peak temperature |
| Max NV Conversion Yield (DNi) | 5.61 | % | Nickel surface transfer doping |
| Min NV Conversion Yield (DAlOx) | 2.89 | % | Aluminum oxide coating |
| Median T2 Coherence Time (DAlOx) | 17 | ”s | Longest median T2 observed |
| Median T2 Coherence Time (DNi) | 12 | ”s | Shortest median T2 observed |
| Max T2 Coherence Time (Top 10%) | 59 to 78 | ”s | Across all samples |
| Mean NV Fluorescence (DNi) | 81 ± 14 | kcps | Highest brightness, indicating high NV- stability |
| Ni/Pd Film Thickness | 50 | nm | Deposited by e-beam evaporation |
| Al2O3 Film Thickness | 10 | nm | Grown by Atomic Layer Deposition (ALD) |
| UV Irradiation Wavelength | 405 | nm | Used for DUV sample during annealing |
| UV Laser Power | ~ 250 | mW | Used for DUV sample |
| Schottky Barrier Height (Ni/O-Diamond) | 1.1 to 1.7 | eV | Expected range for band bending |
Key Methodologies
Section titled âKey MethodologiesâThe experiment utilized four IIa electronic grade (100) diamonds, each subjected to a common implantation step followed by varied surface treatments during annealing to induce temporary doping.
- Initial Cleaning and Implantation: Diamonds were acid cleaned (boiling H2SO4:HNO3:HClO4 mixture) and jointly implanted with 15N+ ions (5 keV, 5 x 109 ions/cm2).
- Surface Termination:
- DNi and DUV were Oxygen (O)-terminated via O-plasma.
- DPd and DAlOx were fully Hydrogen (H)-terminated via H-plasma (700 °C or 750 °C).
- Surface Transfer Doping (STD) Setup:
- DNi: 50 nm Nickel film deposited on O-terminated diamond (expected to promote V0 state).
- DPd: 50 nm Palladium film deposited on H-terminated diamond (expected to promote V+/V2+ states via hole gas).
- DAlOx: 10 nm Aluminum Oxide (Al2O3) grown by ALD on H-terminated diamond (expected to promote p-doping).
- DUV: No coating, but irradiated with a 405 nm UV laser during annealing.
- Annealing Process: All samples were annealed in vacuum (~ 10-6 mbar) at 830 °C for 225 minutes.
- Coating Removal: Post-annealing, metal coatings were removed using aqua regia. The Al2O3 layer was removed using hydrofluoric (HF) acid.
- Final Cleaning and Termination: All diamonds underwent a final acid cleaning to remove surface graphite. H-terminated samples (DPd, DAlOx) were O-terminated again via O-plasma to ensure stable NV- charge state for measurement.
- Measurement: NV properties (ODMR contrast, T2, fluorescence intensity) were measured using a home-built confocal microscope at room temperature (532 nm excitation).
Commercial Applications
Section titled âCommercial ApplicationsâThe ability to control the formation and stability of shallow NV centers is critical for advancing diamond-based quantum technologies.
- Quantum Sensing at Surfaces: Shallow NV centers are essential for high-sensitivity quantum sensing applications, including:
- Imaging magnetometry (e.g., measuring magnetic fields from 2D materials or biological samples).
- Nuclear Magnetic Resonance (NMR) spectroscopy of molecular-size samples.
- Local temperature and strain sensing.
- Defect Engineering and Processing: The study provides insight into noninvasive, scalable methods (STD) for controlling vacancy charge states during annealing, which is simpler than traditional co-implantation or sacrificial layer growth.
- High-Density Quantum Arrays: Increased conversion yield allows for the fabrication of denser, more uniform arrays of functional quantum sensors near the diamond surface.
- Diamond Electronics: Understanding the interaction between metal/oxide interfaces (Ni, Pd, Al2O3) and diamond defects is relevant for developing diamond-based power electronics and stable surface conductivity.
- Quantum Computing: While T2 improvement was modest here, maximizing the yield of high-quality NV centers is a prerequisite for building scalable quantum registers.
View Original Abstract
It has been reported that the conversion yield and coherence time of ion-implanted NV centers improve if the Fermi level is raised or lowered during the annealing step following implantation. Here, we investigate whether surface transfer doping and surface charging, by UV light, can be harnessed to induce this effect. We analyze the coherence times and the yield of NV centers created by ion implantation and annealing, applying various conditions during annealing. Specifically, we study coating diamond with nickel, palladium, or aluminum oxide, to induce positive surface transfer doping, as well as annealing under UV illumination to trigger vacancy charging. The metal-coated diamonds display a two times higher formation yield than the other samples. The coherence time T2 varies by less than a factor of two between the investigated samples. Both effects are weaker than previous reports, suggesting that stronger modifications of the band structure are necessary to find a pronounced effect. UV irradiation has no effect on the yield and T2 times.