Structural, elastic and electronic properties of GaSe under biaxial and uniaxial compressive stress

• The band gap of GaSe decreases with increasing uniaxial pressure along the c axis.
• The charge neutrality level is located in the lower part of the band gap.
• The stiffness coefficients c11 and c33 become comparable at uniaxial pressure of 6 GPa.
• Transition to the metallic conductivity may occur at uniaxial pressure of ~10 GPa.

The influence of anisotropic mechanical stresses preserving symmetry of the hexagonal lattice on the structural, elastic and electronic properties of layered GaSe crystal is studied from first principles. In this work we consider the biaxial compressive stress in the layer planes of GaSe and uniaxial compressive stress applied in the perpendicular direction (along the c axis). The deformation mechanism of GaSe is analyzed in terms of the calculated atomic displacements under load. The changes in the a and c lattice parameters are found to be basically determined by changes in the Ga–Ga–Se bond angle and interlayer distance. According to the obtained stress dependences of the interband transition energies, the band gap continuously decreases with increasing uniaxial pressure, while application of a biaxial load leads to the opposite effect. Calculations of the charge neutrality level (CNL) show that CNL is located in the lower part of the band gap in all the considered cases. Therefore, the dominant p-type conductivity of undoped GaSe crystals can be explained by Fermi level pinning in the lower part of the forbidden band near CNL as a consequence of material defectiveness.