Structural, electronic, and optical properties of the pressure-driven novel polymorphs of gallium nitride: first-principles investigations

Exploring new polymorphs of groups III to V compounds of evolved physical properties has recently received substantial interest from researchers. Accordingly, we explored new pressure-driven polymorphs of gallium nitride (GaN) and investigated their physical properties using density functional theory (DFT)-based full-potential (FP) linearized-augmented-plus-local-orbital (L[APW + lo]) approach. Our analysis shows the transition of ground-state wurtzite (wz) structure to beryllium oxide (β-BeO)-type structure at a tensile stress of ~−6.82 GPa and to silicon carbide (SiC)-type structure by applying moderate compressive stress of magnitude 0.27 GPa. Similarly, the transition of wz-GaN to nickle arsenide (NiAs) and titanium arsenide (TiAs)-type structures has been realized at 43.78 and 45.72 GPa, respectively. These new polymorphs of GaN exhibited comparable cohesive energies with wz-structure and the phonon dispersions free of imaginary frequencies, which indicate them as stable as the ground-state wz-phase. Investigations of the electronic structures show the wz-, β-BeO-, and SiC-phases of GaN as semiconductors of direct bandgap of energy 3.10, 3.15, and 2.97 eV, whereas the NiAs- and TiAs- phases of GaN exhibited indirect bandgap of energy 2.59 and 2.82 eV. All the GaN polymorphs demonstrated transparent behavior for the incident light photon of energy less than 13 eV. They exhibited optical absorption as high as 3.02 × 10⁶ cm⁻¹, 2.23 × 10⁶ cm⁻¹, 2.62 × 10⁶ cm⁻¹, 2.67 × 10⁶ cm⁻¹, and 2.67 × 10⁶ cm⁻¹ in the case of wz-, β-BeO-, NiAs-, SiC-, and TiAs-structured GaN, respectively. These interesting features of the novel polymorphs of GaN indicate them promising for electronic and optoelectronic applications.