Designing defect-based qubit candidates in wide-gap binary semiconductors for solid-state quantum technologies

At a Glance

Metadata	Details
Publication Date	2017-12-12
Journal	Physical Review Materials
Authors	Hosung Seo, He Ma, Marco Govoni, Giulia Galli, Hosung Seo
Institutions	Argonne National Laboratory, University of Chicago
Citations	57
Analysis	Full AI Review Included

Qubit Engineering in Wide-Gap Semiconductors: Large Metal Ion-Vacancy Complexes in 4H-SiC and w-AlN

Technical Analysis & Material Solutions from 6CCVD

Executive Summary

This computational study identifies novel, highly stable, defect-based qubit candidates in wide-gap binary semiconductors (4H-SiC and w-AlN) by proposing Large Metal Ion-Vacancy (LMI-V) complexes, specifically focusing on Hf-V (Hafnium-Vacancy) and Zr-V (Zirconium-Vacancy).

Novel Qubit Candidates: Neutral Hf-V and Zr-V complexes exhibit stable spin-triplet ground states (S=1), analogous to the established benchmark, the diamond Nitrogen-Vacancy (NV) center.
Enhanced Stability and Localization: The use of heavy metal ions (Hf, Zr) ensures significantly lower mobility compared to native host vacancies (C or Si vacancies), facilitating precise defect implantation and superior stability against diffusion.
Optically Addressable: Predicted Zero-Phonon Line (ZPL) and Zero-Field Splitting (ZFS) parameters are provided (e.g., ZPL near 1.7 eV in 4H-SiC), guiding future experimental identification and optical initialization/readout.
Strain Engineering Potential: The Hf-vacancy in w-AlN demonstrates a spin-pressure coupling coefficient (19.24 MHz/GPa) approximately twice that of the diamond NV center, making it highly advantageous for nano-scale quantum sensing applications.
Intrinsic Quantum Resources: The heavy isotopes of Hf and Zr (e.g., ¹⁷⁷Hf, ⁹¹Zr) possess intrinsic nuclear spins, which can be utilized as high-quality, localized quantum memories.
6CCVD Relevance: While the study focuses on SiC/AlN, 6CCVD provides the necessary high-purity Single Crystal Diamond (SCD) for direct NV-center benchmark comparisons and offers custom engineering services (polishing, metalization) critical for realizing these defects in prototype devices.

Technical Specifications

The following key physical and quantum parameters were calculated, providing targets for experimental validation of Hf-V and Zr-V spin qubits in 4H-SiC and w-AlN.

Parameter	Value	Unit	Context
Host Crystal Band Gap (Eg)	5.48	eV	Diamond (Experimental Benchmark)
Host Crystal Band Gap (Eg)	3.23	eV	4H-SiC (Experimental Benchmark)
ZFS Parameter (D)	2.88	GHz	Diamond NV Center (Experimental)
ZFS Parameter (D)	1.336	GHz	4H-SiC (hh)-divacancy (Experimental)
ZFS Parameter (D)	1.403	GHz	Hf-vacancy in 4H-SiC (Theory, ONCV)
ZFS Parameter (D)	3.053	GHz	Zr-vacancy in w-AlN (Theory, ONCV)
Estimated ZPL (Hf/Zr-V)	~1.7	eV	4H-SiC (Range: 1.96 eV - 2.13 eV hybrid calculation upper bound)
Estimated ZPL (Hf/Zr-V)	~3.0	eV	w-AlN (Range: 2.79 eV - 3.07 eV hybrid calculation upper bound)
Hf Hyperfine A_zz	-8.60	MHz	¹⁷⁷Hf (I=7/2, 18.6%) in 4H-SiC
Zr Hyperfine A_zz	17.57	MHz	⁹¹Zr (I=5/2, 11.2%) in 4H-SiC
Hf Hyperfine A_zz	10.53	MHz	¹⁷⁷Hf (I=7/2, 18.6%) in w-AlN
Hf-V Spin-Pressure Slope	19.24	MHz/GPa	w-AlN (High sensitivity for sensing)
Hf-V Spin-Pressure Slope	7.637	MHz/GPa	4H-SiC (Low sensitivity for robust qubits)
Stability Charge State	q=0	n/a	Hf-V and Zr-V complexes are stable in neutral charge state (mid-gap).

Key Methodologies

The core of this work relied on advanced first-principles computational techniques to model complex defect physics, including the energetic stability, electronic structure, and spectroscopic properties of LMI-vacancy complexes.

Density Functional Theory (DFT): Calculations utilized plane-wave basis sets with an energy cutoff of 75 Ry, employing optimized norm-conserving Vanderbilt (ONCV) pseudopotentials.
Hybrid Functionals: To accurately model the electronic structure and band gaps of wide-gap materials, dielectric-dependent hybrid (DDH) functionals (specifically for SiC, α_SiC=0.15) and PBE0 functionals (for AlN, α_AlN=0.25) were used, providing strong agreement with experimental band gaps.
Supercell Size: Defect calculations employed large supercells (480 atoms for PBE, 96 atoms for DDH/HSE06) to minimize spurious defect interactions.
Zero-Phonon Line (ZPL) Calculation: ZPL was derived from total energy differences using the ΔSCGF method on 480-atom supercells (PBE) and 240-atom supercells (Hybrid functionals).
Spin Hamiltonian Parameters: Zero-Field Splitting (D tensor) and Hyperfine parameters (A tensor) were computed using the magnetic dipole-dipole interaction model and the Gauge-Including Projector-Augmented Wave (GIPAW) method.
Simulated Strain/Pressure Testing: The stability and spin-lattice coupling (D(P)) were analyzed by modeling the defects under hydrostatic pressure, ranging up to 100 GPa for 4H-SiC and 30 GPa for w-AlN.
Defect Formation Energy: Calculations included charge correction schemes (Freysoldt, Neugebauer, and Van de Walle) to determine energetic stability across the full Fermi level range.

6CCVD Solutions & Capabilities

6CCVD stands as the essential partner for experimental validation and extension of this research. Our capabilities directly address the need for ultra-high-quality reference materials, precise engineering, and custom solutions required for building solid-state quantum devices based on defect spins.

Applicable Materials

The foundation of this research is the comparison to the industry standard: the diamond NV center. 6CCVD provides the ideal reference material and the necessary host substrates for analogous research pathways:

Benchmark Reference (NV Centers):
- Optical Grade Single Crystal Diamond (SCD): Required for achieving the long coherence times needed for quantum computing and sensing benchmarks. We offer SCD plates optimized for low nitrogen/low impurity concentration to create controlled NV centers.
Qubit Host Material Engineering (SiC/AlN Analogues):
- Polycrystalline Diamond (PCD) Substrates: Available up to 125mm, suitable for complex thin-film deposition or heterogeneous integration studies involving SiC or AlN epitaxy, allowing researchers to explore defect physics in layered or hybrid systems.
- Custom Boron-Doped Diamond (BDD): Our BDD material allows exploration of electrically controlled defect charge states, a critical consideration for quantum devices requiring mid-gap control (e.g., investigating La-V and Y-V complexes where charge transition levels are shallower).

Customization Potential

Experimental realization of LMI-vacancy complexes (Hf, Zr, etc.) requires precursor material preparation and subsequent precision engineering which 6CCVD specializes in:

Precision Sample Preparation: Researchers can order custom SCD or PCD wafers/plates in dimensions up to 125mm and thicknesses from 0.1 µm to 500 µm, ready for controlled heavy-ion implantation (Hf, Zr, Y, La).
Surface Preparation for Characterization: We offer ultra-low roughness polishing (Ra < 1 nm for SCD, Ra < 5 nm for PCD), crucial for enabling high-fidelity optical and EPR characterization, especially in high-pressure diamond anvil cell (DAC) experiments necessary for strain coupling measurements.
Custom Metalization & Contacts: For studying the electrical properties and read-out fidelity of these proposed qubits, 6CCVD provides in-house metalization services (Ti/Pt/Au, Au, Pt, Pd, W, Cu) for depositing ohmic contacts or gate structures directly onto the material surface.
Precision Fabrication: We offer laser cutting and shaping to create micro-structures or small DAC samples required for the high-pressure hydrostatic strain experiments discussed in the paper (up to 100 GPa).

Engineering Support

Leverage 6CCVD’s deep expertise in wide-gap semiconductor physics to accelerate your quantum research:

Material Consultation for Hybrid Systems: 6CCVD’s in-house PhD engineering team can assist researchers in selecting the optimal MPCVD diamond specifications (purity, orientation, growth method) for high-performance Diamond NV center benchmarks or for developing hybrid quantum systems that integrate diamond with SiC/AlN platforms.
Stress and Interface Optimization: We provide guidance on material selection and processing to optimize surface quality and interface stability, essential for robust electrical and mechanical coupling in applications like nano-scale pressure sensors utilizing high spin-pressure coupling.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

We report that the development of novel quantum bits is key to extending the scope of solid-state quantum-information science and technology. Using first-principles calculations, we propose that large metal ion-vacancy pairs are promising qubit candidates in two binary crystals: 4H-SiC and w-AlN. In particular, we found that the formation of neutral Hf- and Zr-vacancy pairs is energetically favorable in both solids; these defects have spin-triplet ground states, with electronic structures similar to those of the diamond nitrogen-vacancy center and the SiC divacancy. Interestingly, they exhibit different spin-strain coupling characteristics, and the nature of heavy metal ions may allow for easy defect implantation in desired lattice locations and ensure stability against defect diffusion. To support future experimental identification of the proposed defects, we report predictions of their optical zero-phonon line, zero-field splitting, and hyperfine parameters. Lastly, the defect design concept identified here may be generalized to other binary semiconductors to facilitate the exploration of new solid-state qubits.

Tech Support

Original Source

DOI: https://doi.org/10.1103/physrevmaterials.1.075002