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 |
Technical Documentation: Surface Transfer Doping for Enhanced Shallow NV Center Yield
Section titled âTechnical Documentation: Surface Transfer Doping for Enhanced Shallow NV Center Yieldâ6CCVD Analysis of âCan surface-transfer doping and UV irradiation during annealing improve shallow implanted Nitrogen-Vacancy centers in diamond?â
This documentation analyzes the application of surface transfer doping via metal coatings (Ni, Pd) and UV irradiation during annealing to enhance the conversion yield ($\eta$) and spin coherence ($T_2$) of shallow Nitrogen-Vacancy (NV) centers in diamond. The findings confirm that surface engineering using specific metal contacts is a highly effective, noninvasive method for optimizing quantum sensor substrates.
Executive Summary
Section titled âExecutive Summaryâ- Core Achievement: Surface transfer doping using Nickel (Ni) and Palladium (Pd) coatings during post-implantation annealing successfully doubled the NV center conversion yield ($\eta$) compared to non-metal coated or UV-irradiated samples.
- Mechanism Confirmation: The enhanced yield is attributed to the positive surface transfer doping induced by the metal contacts, which shifts the Fermi level and stabilizes the neutral vacancy charge state ($V^{0}$), suppressing the formation of competing defect complexes (e.g., divacancies).
- Material Specificity: Nickel coating achieved the highest yield (5.61%), while Palladium coating significantly enhanced the formation of paired NV centers (60% increase over simulation), suggesting strong local electrostatic influence.
- Spin Coherence ($T_2$): The spin coherence time $T_2$, the most critical metric for quantum sensing, was minimally affected, varying by less than a factor of two across all surface treatments (median $T_2$ range: 12 ”s to 17 ”s).
- Surface Quality Impact: Aluminum Oxide (Al${2}$O${3}$) coating and Palladium coating resulted in the narrowest ODMR linewidth ($\Gamma$), indicating improved spin dephasing time ($T_{2}^{*}$), likely due to the effects of the hole gas or subsequent HF acid cleaning.
- UV Irradiation Ineffectiveness: UV irradiation during annealing showed no positive effect on either NV yield or $T_2$ times.
Technical Specifications
Section titled âTechnical SpecificationsâThe following hard data points were extracted from the experimental methodology and results:
| Parameter | Value | Unit | Context |
|---|---|---|---|
| Initial Material Grade | IIa Electronic Grade | N/A | (100) Orientation, [N] < 5 ppb |
| Substrate Dimensions | 2 x 2 x 0.5 | mm | Used for all four samples |
| Implantation Species | 15N+ | Ions | Used for NV precursor creation |
| Implantation Energy | 5 | keV | Expected implantation depth: 10 nm |
| Implantation Fluence | 5 x 109 | ions/cm2 | Low fluence for single NV centers |
| Annealing Temperature | 830 | °C | Held for 225 minutes |
| Annealing Environment | ~10-6 | mbar | High vacuum |
| Nickel (Ni) Film Thickness | 50 | nm | Deposited by e-beam evaporation |
| Palladium (Pd) Film Thickness | 50 | nm | Deposited by e-beam evaporation |
| Aluminum Oxide (Al2O3) Thickness | 10 | nm | Deposited by ALD |
| Maximum NV Conversion Yield ($\eta$) | 5.61 | % | Achieved with Nickel (DNi) coating |
| Median Coherence Time (T2) Range | 12 to 17 | ”s | Across all samples (DNi to DAlOx) |
| ODMR Resonance Frequency | 1.6 | GHz | Measured at B = 45 mT |
Key Methodologies
Section titled âKey MethodologiesâThe experiment utilized a multi-step process involving precise surface preparation, low-energy ion implantation, custom coating, and high-temperature annealing under controlled conditions:
- Initial Cleaning: Diamonds were subjected to a rigorous acid cleaning (boiling H2SO4:HNO3:HClO4 mixture) for 4 hours to remove surface contaminants.
- Implantation: Shallow NV precursors were created by implanting 15N+ ions at 5 keV (expected depth 10 nm) with a fluence of 5 x 109 ions/cm2.
- Surface Termination: Samples were initially O-terminated via oxygen plasma. DPd and DAlOx samples were subsequently H-terminated using a hydrogen plasma (700 °C or 750 °C).
- Surface Transfer Doping Coating:
- Metal Contacts: 50 nm films of Nickel (Ni) or Palladium (Pd) were deposited via electron beam evaporation onto O-terminated (Ni) or H-terminated (Pd) surfaces.
- Insulator Contact: 10 nm of Aluminum Oxide (Al2O3) was grown via Atomic Layer Deposition (ALD) onto H-terminated diamond.
- Annealing: Samples were annealed in a high vacuum (~10-6 mbar) at 830 °C for 225 minutes to mobilize vacancies and form NV centers.
- UV Irradiation (DUV Sample): The DUV sample was irradiated with a 405 nm laser (250 mW) during the 830 °C annealing step.
- Coating Removal: Metal layers were removed using aqua regia solution; Al2O3 was removed using hydrofluoric (HF) acid.
- Final Cleaning & Termination: All samples underwent a final acid cleaning. H-terminated samples (DPd, DAlOx) were re-O-terminated via O-Plasma to recover the NV- charge state.
- Characterization: Spin properties ($T_2$, ODMR contrast, $\Gamma$) and fluorescence yield ($\eta$) were measured using a 532 nm confocal microscope setup.
6CCVD Solutions & Capabilities
Section titled â6CCVD Solutions & CapabilitiesâThis research demonstrates the critical role of precise surface engineering and metalization in optimizing shallow NV center formation. 6CCVD is uniquely positioned to supply the high-purity materials and custom processing required to replicate and advance this work, particularly for scalable quantum sensing applications.
| Research Requirement | 6CCVD Solution & Capability | Technical Advantage |
|---|---|---|
| High-Purity Substrates | Optical Grade Single Crystal Diamond (SCD) | We provide high-purity, electronic-grade SCD plates (Type IIa, [N] < 5 ppb equivalent) up to 500 ”m thick, ensuring minimal bulk defects for optimal $T_2$ performance and low background fluorescence (IB). |
| Custom Metalization for Doping | In-House Metalization Services (Ni, Pd, Ti, Pt, Au) | 6CCVD offers custom deposition of Nickel (Ni) and Palladium (Pd) films, matching the 50 nm requirement, or custom thicknesses. We also provide robust multi-layer stacks (e.g., Ti/Pt/Au) for stable ohmic contacts and surface transfer doping that withstand high-temperature annealing (830 °C). |
| Precise Dimensions & Polishing | Custom Dimensions and Ultra-Low Roughness Polishing | While the paper used small 2x2 mm samples, 6CCVD supplies SCD wafers with custom dimensions up to 125 mm (PCD) and superior polishing (Ra < 1 nm for SCD), critical for uniform shallow implantation and minimizing surface noise. |
| Shallow NV Formation Support | Engineering Consultation on Surface Termination & Annealing | The paper highlights challenges in maintaining surface stability and charge state control. Our in-house PhD team assists researchers in optimizing pre- and post-annealing surface treatments (O-Plasma, H-Plasma) to maximize shallow NV yield and ensure stable NV- charge state recovery. |
| Scalability and Volume | Large-Area Polycrystalline Diamond (PCD) Plates | For scaling up quantum sensing devices, 6CCVD offers high-quality Polycrystalline Diamond (PCD) plates up to 125 mm in diameter, enabling high-throughput fabrication of NV-based sensors using similar surface doping techniques. |
Engineering Support
Section titled âEngineering Supportâ6CCVDâs expertise in MPCVD growth and post-processing allows us to deliver substrates that are pre-optimized for shallow NV creation. We eliminate the need for complex, multi-step cleaning and termination processes by providing ready-to-use, metalized, and polished diamond plates tailored for Quantum Sensing and Magnetometry projects.
For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.
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.