Effects of Biaxial Strains and High Pressure on the Structural, Electronic, and Vibrational Properties of DC-HgM2Te4 (M=Al, In)

First-principles calculations are performed to study the effects of biaxial strains and high pressure on the structural, electronic, and lattice dynamical properties of defect chalcopyrite HgM2Te4 (M=Al, In) semiconductors. The evolutions of optimized structural parameters, band-gap energy, crystal-field splitting energy, and -point phonon frequencies with the biaxial strain and pressure have been analyzed. The optimized lattice parameters are reasonable compared with the existing experimental results. Both compounds undergo an indirect to direct band gap transition under tensile Exx, however, the semiconductors retain their indirect band gap character under pressure. The increasing pressure pushes the crystal-field-splitting hole (CH) band downwards; in contrast, the increasing biaxial strain makes the CH band shift upwards, which results in that the crystal-field splitting energies cf decrease from positive to negative. The pressure-induced softness of low-frequency modes around the X point suggests that both compounds become dynamically instable. Atomic displacement patterns show that the high frequency modes are mainly determined by the vibrations of group-III cations.