Transition metal impurities in silicon - computational search for a semiconductor qubit
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2022-08-19 |
| Journal | npj Computational Materials |
| Authors | ChengâWei Lee, Meenakshi Singh, Adele C. Tamboli, Vladan StevanoviÄ |
| Institutions | Colorado School of Mines, National Renewable Energy Laboratory |
| Citations | 8 |
| Analysis | Full AI Review Included |
Technical Documentation: Transition Metal Impurities in Silicon for Semiconductor Qubits
Section titled âTechnical Documentation: Transition Metal Impurities in Silicon for Semiconductor QubitsâThis documentation analyzes the research paper âTransition metal impurities in silicon: computational search for a semiconductor qubitâ and connects its findings to the advanced material capabilities offered by 6CCVD in the field of MPCVD diamond.
Executive Summary
Section titled âExecutive SummaryâThis research identifies promising transition metal (TM) impurities in crystalline silicon (Si) that mimic the desirable properties of Nitrogen-Vacancy (NV) centers in diamond, aiming to overcome the scalability and low operating temperature limitations of current Si-based qubits.
- Qubit Candidates: Seven TM impurities (Sc, V, Co, Cu, Zn, Zr) were computationally identified as potential optically active spin qubits in Si.
- Optical Transitions: The candidates exhibit optically allowed spin triplet-triplet transitions within the Si band gap (0.4-0.6 eV).
- Wavelength Range: The resulting photon emission falls within the Mid-Infrared (Mid-IR) range (3100-2066 nm), suitable for free-space communication applications.
- High-Temperature Potential: These defects offer a pathway toward Si-based qubits with higher operating temperatures for quantum sensing, benchmarking against the room-temperature performance of diamond NV centers.
- Methodology: State-of-the-art computational methods (HSE06(+U) DFT) were used to calculate defect formation energies, charge transition levels (CTL), and optical absorption spectra.
- Future Requirements: Experimental realization requires precise ion implantation, structural control, and custom metalization for Fermi energy tuning.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Host Material | Crystalline Si | N/A | Target semiconductor host. |
| Computational Method | HSE06(+U) DFT | N/A | Used for defect calculations and band gap correction. |
| Supercell Size | 216 | Atoms | Used for defect formation energy calculations. |
| Si Band Gap (Predicted) | 1.16 | eV | Matches experimental value (1.17 eV). |
| Triplet-Triplet Transition Energy | 0.4 - 0.6 | eV | In-gap optical transition range for candidates. |
| Triplet-Triplet Wavelength | 3100 - 2066 | nm | Mid-Infrared (Mid-IR) range. |
| Absorption Coefficient (α) | 103 - 104 | cm-1 | High transition probability for first peaks. |
| Smallest Thermal Excitation (ÎE) | 0.1 - 0.2 | eV | Energy difference in single-particle defect-level diagram. |
| Candidate Impurities (Substitutional) | CoSi, CuSi, ZnSi | N/A | Identified with stable triplet ground states. |
| Candidate Impurities (Interstitial) | Sci, Vi, Zri | N/A | Identified with stable triplet ground states. |
Key Methodologies
Section titled âKey MethodologiesâThe computational search utilized a rigorous workflow based on Density Functional Theory (DFT) to identify TM impurities suitable for spin-qubit applications.
- DFT Selection and Calibration: Calculations were performed using spin-polarized generalized hybrid DFT (HSE06) with a standard mixing parameter (α = 0.25) to accurately predict the Si band gap (1.16 eV).
- Koopmansâ Condition Correction: An occupation-dependent potential (HSE06+U) was applied to TM d-orbitals when the non-Koopmansâ energy (|ENK|) exceeded 0.2 eV, ensuring compliance with the generalized Koopmansâ condition (gKC).
- Defect Formation and Transition Levels: Thermodynamic Charge Transition Levels (CTL) and defect formation energies (ÎHD,q) were calculated using a 216-atom supercell approach to determine stable charge states.
- Spin State Screening: Defects were screened for a stable spin-triplet electronic ground state (S=1) and the existence of a spin-triplet excited state within the Si band gap.
- Localization and Symmetry: Charge density visualization confirmed localized mid-gap defect states and determined defect symmetry (e.g., C2v for CoSi-1, C3v for Sci+1 and Zri0).
- Optical Transition Verification: The one-particle optical absorption coefficient (α) was calculated to verify allowed triplet-triplet transitions and estimate high transition probability (103-104 cm-1).
6CCVD Solutions & Capabilities
Section titled â6CCVD Solutions & CapabilitiesâThe research highlights the critical need for NV-center-like defects for high-temperature quantum applications. While the study focuses on Si, 6CCVD specializes in the superior host materialâMPCVD Diamondâwhich already hosts the benchmark NV center and offers unparalleled performance for solid-state qubits.
6CCVD provides the foundational materials and engineering services necessary to replicate, extend, and accelerate research into solid-state spin qubits, whether in Si or, preferably, in diamond.
| Research Requirement / Challenge | 6CCVD Diamond Solution | Technical Advantage |
|---|---|---|
| Benchmark Qubit Host Material (NV-center analog search) | Optical Grade Single Crystal Diamond (SCD) | Diamond possesses the highest Debye temperature and lowest spin-orbit coupling, maximizing coherence time and operational temperature (NV centers operate at Room Temperature). |
| High-Fidelity Defect Creation | High Purity SCD Wafers (Ra < 1 nm) | SCD provides the most stable, low-noise host for creating localized point defects (e.g., SiV, GeV, NV) necessary for robust spin-photon coupling and sharp Zero-Phonon Lines (ZPL). |
| Ion Implantation & Substrate Robustness | Custom SCD/PCD Substrates (up to 10 mm thick) | Provides robust, high-quality substrates necessary for high-energy ion implantation (e.g., TM ions) and subsequent high-temperature annealing required to form deep centers. |
| Custom Dimensions for Scalability | Large Area Polycrystalline Diamond (PCD) Wafers (up to 125 mm) | Enables the scaling of quantum sensing arrays and integrated quantum circuits far beyond typical lab-scale samples. |
| Charge State Control (Fermi Energy Tuning) | Internal Metalization Services (Au, Pt, Pd, Ti, W, Cu) | Essential for realizing gate-controlled nanostructures and tuning the Fermi energy to achieve targeted charge states (e.g., CoSi-1, Sci+1) as required by the computational models. |
| Advanced Photonic Integration | Ultra-Low Roughness Polishing (SCD: Ra < 1 nm) | Critical for subsequent lithography and fabrication of high-Q photonic structures (waveguides, resonators) required for efficient spin-photon coupling. |
Applicable Materials
Section titled âApplicable MaterialsâTo replicate or extend this research into the most robust solid-state qubit platforms, 6CCVD recommends:
- Optical Grade Single Crystal Diamond (SCD): Ideal for fundamental research requiring ultra-low defect density, long coherence times, and the highest possible operating temperatures. Available in thicknesses from 0.1 ”m to 500 ”m, with Ra < 1 nm polishing.
- Heavy Boron Doped Diamond (BDD): Useful for experiments requiring conductive diamond layers or electrochemical applications, allowing for electrical gating to control defect charge states.
Customization Potential
Section titled âCustomization PotentialâThe paper emphasizes the need for precise control over defect location (via ion implantation) and charge state (via electrical gating). 6CCVD supports these advanced fabrication steps:
- Custom Dimensions: We provide plates and wafers up to 125 mm (PCD) and custom-cut SCD pieces, ensuring compatibility with standard semiconductor processing tools.
- Custom Metalization Stacks: Our in-house capabilities include depositing thin films of Ti, Pt, Au, W, Cu, and Pdâcritical materials for creating ohmic contacts and gate electrodes necessary for Fermi level tuning.
Engineering Support
Section titled âEngineering SupportâThe research identifies critical next steps, including calculating Zero-Field Splitting (ZFS), Intersystem Crossing (ISC) rates, and the Debye-Waller factor.
6CCVDâs in-house PhD team specializes in the material science and quantum properties of MPCVD diamond. We offer consultation and material selection assistance for similar Quantum Sensing and Spin-Photon Interface projects, ensuring researchers select the optimal diamond material specifications (purity, orientation, doping) to maximize ZFS, coherence, and optical fidelity.
For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.
View Original Abstract
Abstract Semiconductors offer a promising platform for physical implementation of qubits, but their broad adoption is presently hindered by limited scalability and/or very low operating temperatures. Learning from the nitrogen-vacancy centers in diamond, our goal is to find equivalent optically active point defect centers in crystalline silicon, which could be advantageous for their scalability and integration with classical devices. Transition metal (TM) impurities in silicon are common paramagnetic deep defects, but a comprehensive theoretical study of the whole 3 d series that considers generalized Koopmansâ condition is missing. We apply the HSE06(+U) method to examine their potential as optically active spin qubits and identify seven TM impurities that have optically allowed triplet-triplet transitions within the silicon band gap. These results provide the first step toward silicon-based qubits with higher operating temperatures for quantum sensing. Additionally, these point defects could lead to spin-photon interfaces in silicon-based qubits and devices for mid-infrared free-space communications.