Lattice dynamics and elastic properties of zinc-blende MgS

Magnesium sulphide (MgS) normally crystallizes in the rocksalt structure; using a recent development in molecular beam epitaxy, it is now possible to obtain MgS in the zinc-blende structure grown on GaAs. A Raman microscope system with ultraviolet excitation at 244 nm has been used to obtain the Raman spectra of this material, revealing strong signals due to overtones of the LO and TO phonons in the backscattering geometry at room temperature. The strength of the signals implies that the excitation energy (5.08 eV) is close to resonance with the MgS band gap. Calculations of the frequency of the zone-center TO and LO phonons of this structure via ab initio methods are in excellent agreement with the experimental observations. The calculated and measured phonon frequencies were 433 and $425\hspace{0.5em}{\mathrm{cm}}^{-1}$ (LO) and 335 and $327\hspace{0.5em}{\mathrm{cm}}^{-1}$ (TO), respectively. Values for the lattice parameter and the elastic constants $c_{11}$ and $c_{12}$ of zinc-blende MgS were also obtained.