Spin Polarization and Magnetic Moment in Silicon Carbide Grown by the Method of Coordinated Substitution of Atoms

At a Glance

Metadata	Details
Publication Date	2021-09-26
Journal	Materials
Authors	С. А. Кукушкін, А. В. Осипов
Institutions	Institute of Problems of Mechanical Engineering
Citations	10
Analysis	Full AI Review Included

Executive Summary

This research introduces a novel method (MCSA) for creating stable, spin-polarized silicon carbide (3C-SiC) layers, highly relevant for spintronics and quantum technology.

Core Innovation: Silicon vacancies (V_Si) are intentionally generated in the Si substrate (low formation energy, ~3.3 eV) via high-temperature annealing, and then transferred into the growing 3C-SiC layer during the MCSA conversion process.
Stable Spin Defect: The V_Si stabilizes into a C₄V center (an almost flat cluster of four C atoms with an underlying void), which is structurally and functionally analogous to the NV center in diamond.
Magnetic Properties: The C₄V center possesses a stable solid-state spin and a magnetic moment of 1.0 Bohr magneton (µB).
Spin Polarization: Depending on the defect concentration (n_C4V), the material transitions from a nonmagnetic semiconductor to a half-metallic ferromagnet (100% spin polarization at n_C4V ≈ 4.2%) or a magnetic metal (84% spin polarization).
Process Control: The concentration of C₄V centers can be controlled by adjusting the time and temperature of the preliminary vacuum annealing of the Si substrate.
Integration Advantage: The resulting 3C-SiC layers are epitaxial and compatible with standard silicon microelectronics, facilitating scaling and integration of spin-based devices.

Technical Specifications

Parameter	Value	Unit	Context
SiC Polytype Grown	3C-SiC(111)	N/A	Cubic polytype, epitaxial growth.
Substrate Used	Si(111) doped with B	N/A	P-type silicon substrate.
Si Vacancy Formation Energy (Si)	~3.3	eV	Energy barrier in Si (low).
Si Vacancy Formation Energy (SiC)	~8	eV	Energy barrier in SiC (high).
Si Pre-Annealing Temperature	1350-1380	°C	Step for V_Si creation in Si.
Si Pre-Annealing Time (t_a)	1-40	min	Controls V_Si concentration.
SiC Synthesis Temperature (T_s)	~1350	°C	MCSA growth temperature.
CO Pressure (P_CO)	80-200	Pa	MCSA synthesis parameter.
Epitaxial Layer Thickness (H_SiC)	0.1-1	µm	Typical thickness range.
C₄V Magnetic Moment (Stable)	1.0	µB	Bohr magneton (DFT calculation).
C₄V Magnetic Moment (Metastable V_Si)	0.8	µB	Initial state V_Si.
V_Si to C₄V Transition Barrier	3.33	eV	Energy barrier height (nonmagnetic transition state).
C-C Bond Wavenumber (C₄V)	952	cm^-1	Observed Raman shift for stable V_Si defect.
Spin Polarization (n_C4V ≈ 4.2%)	100	%	Half-metallic ferromagnet state (System II).
Spin Polarization (n_C4V ≈ 6.3%)	84	%	Ferromagnetic metal state (System III).
SiC Cubic Cell Volume	83	A³	Volume after conversion from Si.
Si Cubic Cell Volume (Initial)	160	A³	Volume before conversion (twofold decrease).

Key Methodologies

The process relies on the Method of Coordinated Substitution of Atoms (MCSA) combined with a critical pre-annealing step to manage vacancy concentration.

Substrate Preparation: 3-inch Si(111) substrates (p-type, B-doped) are chemically purified using NH₄OH and NH₄F to remove oxides and passivate the surface with hydrogen.
Vacancy Pre-Creation: Substrates are annealed in a vacuum at T ~1350-1380 °C for 1 to 40 minutes. This high temperature induces the formation of silicon vacancies (V_Si) in the Si surface layer, exploiting the low V_Si formation energy in Si (~3.3 eV).
SiC Synthesis (MCSA): Carbon monoxide (CO) gas is introduced into the furnace. The Si substrate is converted into 3C-SiC(111) via the chemical reaction: 2Si (crystal) + CO (gas) = SiC (crystal) + SiO (gas) ↑.
- Parameters: T ~1350 °C, P_CO = 80-200 Pa, synthesis time (t_s) = 10-20 min.
Vacancy Transition and Stabilization: During the conversion, a portion of the pre-created V_Si transit into the SiC layer. Thermal fluctuations at T > 1200 °C force an adjacent C atom to jump into the V_Si site, stabilizing the defect into the magnetic C₄V center (1.0 µB).
Structural Characterization: Layers were analyzed using X-ray Diffraction (XRD) and Reflected High-Energy Electron Diffraction (RHEED) to confirm epitaxial quality (main 3C-SiC(111) peak half-width of 6 arcmin).
Defect Confirmation: Raman Spectroscopy (RS) detected the stable C₄V center via an additional peak at 952 cm^-1, corresponding to the C-C bond vibrations within the cluster.
Modeling of Spin Properties: Density Functional Theory (DFT) using Medea-VASP software (PBE and MBJLDA meta-GGA functionals) was employed to calculate the magnetic moment, band structure, and spin polarization of the SiC containing C₄V centers.

Commercial Applications

The ability to engineer intrinsic ferromagnetism and high spin polarization in SiC layers makes this technology highly valuable for next-generation electronic and quantum devices.

Spintronics:
- Development of highly efficient spin injectors and detectors utilizing the 100% spin polarization achieved in the half-metallic ferromagnet state (n_C4V ≈ 4.2%).
- Creation of spin-based logic and memory devices integrated directly onto silicon platforms.
Quantum Computing and Sensing:
- The C₄V center serves as a solid-state spin qubit candidate, offering an alternative to NV centers in diamond, but with superior compatibility for integration into Si microelectronics.
- Use in quantum sensors based on the robust solid-state spin properties of the C₄V defect.
Magnetic Materials Engineering:
- Fabrication of thin-film magnetic materials where the magnetic properties are controlled by defect concentration rather than traditional doping or alloying.
Integrated Microelectronics:
- Enabling the monolithic integration of magnetic and spin-based functionalities directly onto standard Si(111) wafers, reducing complexity and cost for advanced device manufacturing.

View Original Abstract

In the present work, a new method for obtaining silicon carbide of the cubic polytype 3C-SiC with silicon vacancies in a stable state is proposed theoretically and implemented experimentally. The idea of the method is that the silicon vacancies are first created by high-temperature annealing in a silicon substrate Si(111) doped with boron B, and only then is this silicon converted into 3C-SiC(111), due to a chemical reaction with carbon monoxide CO. A part of the silicon vacancies that have bypassed “chemical selection” during this transformation get into the SiC. As the process of SiC synthesis proceeds at temperatures of ~1350 °C, thermal fluctuations in the SiC force the carbon atom C adjacent to the vacancy to jump to its place. In this case, an almost flat cluster of four C atoms and an additional void right under it are formed. This stable state of the vacancy, by analogy with NV centers in diamond, is designated as a C4V center. The C4V centers in the grown 3C-SiC were detected experimentally by Raman spectroscopy and spectroscopic ellipsometry. Calculations performed by methods of density-functional theory have revealed that the C4V centers have a magnetic moment equal to the Bohr magneton μB and lead to spin polarization in the SiC if the concentration of C4V centers is sufficiently high.

Tech Support

Original Source

DOI: https://doi.org/10.3390/ma14195579

References

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