First-principles study of electronic, optical, and magnetic properties of Ta/Nb-doped and (Ta/Nb, VSi) Co-doped 4H-SiC.

The incorporation of substitutional transition-metal heteroatoms (Ta, Nb) and intrinsic silicon vacancies (VSi) induces pronounced modulation of the electronic, optical, and magnetic properties of 4H-SiC. In this work, first-principles density functional theory (DFT) calculations were systematically employed to investigate the thermodynamic stability, electronic band structure, optical response functions, and spin-polarized magnetic behavior of Ta-doped, Nb-doped, (Ta, VSi) co-doped, and (Nb, VSi) co-doped 4H-SiC. Results show that Ta and Nb preferentially substitute at the Si-k site, reducing the bandgap of 4H-SiC from 2.23 eV to 1.92 eV and 2.05 eV, respectively. Co-doping with VSi further narrows the bandgap to 1.74 eV in the (Ta, VSi) system and 1.71 eV in the (Nb, VSi) system. Optical analysis reveals significantly enhanced sub-gap absorption and reflectance across the infrared and visible spectral ranges, with the (Nb, VSi) co-doped system exhibiting the most redshifted absorption onset and strongest IR response. Magnetic analysis confirms that magnetism originates from p-d hybridization between C 2p and Ta 5d or Nb 4d orbitals. Critically, VSi co-doping redirects the dominant magnetic moment from the transition-metal site to neighboring carbon atoms and concurrently enhances the net spin polarization. These results establish a unified structure-property-function relationship for transition-metal/VSi-engineered 4H-SiC and provide a quantitative theoretical foundation for designing spin-active wide-bandgap semiconductors for infrared-visible optoelectronics and room-temperature spintronics.