First-principles investigation of 6H-SiC dominated by strong covalent bonding: electronic structure, mechanical properties and optical properties

Context

Silicon carbide (SiC), a third-generation semiconductor, is renowned for its wide bandgap, exceptional thermal conductivity and high breakdown field. The unique ABCACB stacking atomic arrangement of hexagonal SiC (6H-SiC) induces direction-dependent electronic, optical, and mechanical responses, which are crucial for emerging applications. Using first-principles calculations, we comprehensively characterize these properties of 6H-SiC. Our findings reveal dual bandgaps (2.82 eV indirect and 4.16 eV direct), with dispersive band edges that are conducive to carrier transport. Further calculations indicate that the carrier effective mass along the (001) direction is smaller than (100) direction, and one key factor causing this anisotropy is the directional changes in sp³ hybridized orbitals due to the unique atomic stacking. DOS, Mulliken population and charge density studies collectively reveal the covalent-dominated bonding nature, which underpins its dispersive band edges, hard texture and brittleness. The wide direct bandgap and unique electronic structure contribute to its broad spectral transparency and low optical loss. Moreover, A strong directional dependence is observed in both the optical and mechanical properties of 6H-SiC, where the (001) direction demonstrates higher compressive stiffness and lower optical absorption and loss.

Methods

All calculations were conducted using density functional theory (DFT) as implemented in the CASTEP code, with norm-conserving pseudopotentials employed. For geometry optimization, we utilized the generalized gradient approximation with the Perdew-Burke-Ernzerhof (GGA-PBE) functional, whereas the electronic structure and optical characteristics were determined using the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional.