Structural, electronic, and elastic properties of different polytypes of GaSe lamellar materials under compressive stress: insights from a DFT study

The objective of this research is to look at how anisotropic mechanical stresses affect the structural, elastic, and electronic properties of various GaSe polytypes. Density functional theory (DFT) was employed in all calculations. Consistent and precise calculations as well as descriptions of trade-off and correlation effects were performed using the LDA-PBE approximation. The uni-axial compressive stresses in the GaSe layer planes, in addition to the perpendicularly applied uni-axial compressive stress, are taken into account (along the c-axis). The gap distance and the \(\widehat{Ga Ga Se}\) bond angle, both of which determine the thickness of crystal layers and the size of the unit cell in the basal plane, have the largest influence on changes in the lattice parameters a and c. Compression along the c-axis causes the interlayer spacing to shrink and the crystal anisotropy to decrease, whereas compression in the layer planes has no effect on gallium selenide’s property. The stability conditions reflect that the different polytypes of GaSe are mechanically stable at zero pressure and under pressure. Our results at zero pressure show that the B/G ratio is of order of 1.33, 1.44, 1.67, and 1.44 for ε-GaSe, β-GaSe, γ-GaSe, and δ-GaSe, respectively, which means that all polytypes GaSe crystal are fragile. The calculated band structure shows that the structure semiconductors ε-GaSe, β-GaSe, and δ-GaSe have a direct band gap of 0.751 eV, 0.818 eV, and 0.888 eV respectively and an indirect band gap of 0.819 eV for γ- GaSe. At bi-axial strains up to 8 GPa, calculations of the electronic band structure demonstrate a progressive growth in the band gap. Inter-band transition energy appears to be decreasing with loading under uni-axial compressive stress along the c-axis. A transition to the metallic type of conductivity may occur when the projected pressure dependences of the direct and indirect band gaps are taken into account at uni-axial pressures of roughly 10 GPa.