Experimental and Theoretical Study of the Electronic Structure and Optical and Mechanical Properties of Th₃P₄-Type Hf₃N₄ Thin Films for Applications in Infrared Windows

Although the metastable phases of group IVB nitrides (M 3 N 4, M = Ti, Zr, or Hf) are highly desirable, since they were predicted to have unique semiconducting and exceptional mechanical properties, the challenge in their fabrication still hampered the research and applications. In this work, we have successfully synthesized Th 3 P 4 -type Hf 3 N 4 nanocrystalline thin films by high-power impulse magnetron sputtering and have conducted a joint experimental and theoretical study to investigate the electronic structure and optical and mechanical properties. We experimentally find that Th 3 P 4 -type Hf 3 N 4 has a direct bandgap of about 2.53 eV, which is consistent with the theoretical value of 2.25 eV calculated by the Heyd-Scuseria-Ernzerhof (HSE06) functional. Its unique infrared optical properties of high transmittance (>98%) and relatively low refractive index (∼2.5) enable it to serve as the antireflection film for Si infrared windows. Meanwhile, both experimental and computational results show that Th 3 P 4 -type Hf 3 N 4 possesses high hardness (∼21 GPa) and exhibits notable mechanical isotropy. Additionally, Th 3 P 4 -type Hf 3 N 4 thin films possess excellent hydrophobicity and good thermal stability. These superior properties make Th 3 P 4 -type Hf 3 N 4 a promising candidate as an antireflective and protective coating for Si infrared windows. This work paves a way for the low-cost preparation of metastable Th 3 P 4 -type Hf 3 N 4 nanocrystalline thin films and further promotes the understanding of promising Th 3 P 4 -type Hf 3 N 4 .