Thermoelectric and Optical Properties of HfSi2N4 and HfGe2N4: A First-Principles Investigation

This study explores the thermoelectric and optoelectronic properties of HfSi₂N₄ and HfGe₂N₄ monolayers (ML) through first-principles calculations. Both materials exhibit excellent structural stability, as confirmed by phonon dispersion and ab initio molecular dynamics simulations. HfSi₂N₄ demonstrates superior power factors and higher thermal conductivity, while HfGe₂N₄ achieves a remarkable thermoelectric figure of merit ($ZT$) of 0.92 at 900 K under p-type doping, surpassing many 2D materials. The inclusion of spin-orbit coupling further enhances the thermoelectric performance, especially for HfGe₂N₄. The electronic properties reveal indirect bandgaps of 2.89 eV for HfSi₂N₄ and 2.75 eV for HfGe₂N₄, with strong optical absorption peaks in the visible range, making them suitable for optoelectronic applications. The materials exhibit high carrier mobility, with HfSi₂N₄ reaching 582 cm²V⁻¹s⁻¹ and HfGe₂N₄ achieving an impressive 1870 cm²V⁻¹s⁻¹ for holes. Thermal conductivity analysis reveals that HfGe₂N₄ has significantly lower values than HfSi₂N₄, favoring thermoelectric efficiency. The synergy of high Seebeck coefficients (S), tunable thermal conductivity, and optical properties makes these monolayers promising candidates for advanced thermoelectric devices and visible-light optoelectronics. This study provides a comprehensive comparison, offering valuable insights into their applicability in next-generation energy conversion and optoelectronic technologies.