Ab initio study of the s tability of high pressure phases in II-VI semiconductors

1. Ab initio study of the stability of high pressure phases in II-VI semiconductors Davor Kirin and Igor Lukačević Institut Ruđer Bošković, Zagreb, Croatia and Department of Physics, University of J. J. Strossmayer, Osijek, Croatia Introduction The study of pressure induced phase transitions in semiconductors has been rapidly expanding field of research during the past few decades and a number of new phases has been discovered experimentally [1]. In the present work the stability of the intermediate rock salt structure with respect to the high pressure Cmcm structure was investigated for several II-VI semiconductors (ZnS, ZnSe, CdS and CdSe). The lattice dynamics calculations reveal that the rock salt structure is unstable (i.e. some normal modes have imaginary frequency) with the respect to the transversal acoustic mode at the Brillouin zone boundary (X point) (Fig1.). Fig 2 Fig. 1 Fig. 2 Discusion The stable low pressure structure of all studied crystals is zinc-blende (zb) which at higher pressure undergoes a transition to the NaCl type of structure with large volume change. The NaCl structure is stable in a certain pressure range and eventually undergoes transition to the orthorhombic Cmcm structure with very small volume change. The fig 2. shows the phonon frequency close to the phase transition showing that the NaCl phase is still stable. Fig 1 Method Ab initio lattice dynamics calculations were performed using the ABINIT program package [4], which is based on density functional theory (DFT) and the use of pseudopotentials (PP). Results The present calculation gives the transition pressure more accurately then classical method, which uses the common tangent method. Since the volume change at the transition is very small or zero in all studied crystals, transition pressure were estimated from the pressure at which the frequency of zone boundary transversal acoustic mode goes to zero (Fig 1). The phase transition is of the second order displacive and is associated with the cell doubling in the high pressure Cmcm phase with frozen in deformation. The calculated (experimental) transition presures are 67 (69) GPa for ZnS, 49 (51) GPa for CdS, 29(30) GPa for ZnSe, and 26 (27) GPa for CdSe crystal. The observed pressures of the phase transitions [2,3] in structural measurements were reproduced more accurately then in theprevious calculations [5] and the difference between observed and calculated transition pressure is of the order of 3 to 6 %. Literature [1] Mujica, A. Rubio, A. Munoz and R. J. Needs, Rev. Mod. Phys. 75, 863 (2003). [2] R. J. Nelmes and M. I. McMahon, Semicond. Semimetals. 54, 145 (1998). [3] M. I. McMahon and R. J. Nelmes, Phys. Status Solidi B, 198, 389 (1996). [4] The ABINIT code is a common project of Universite Catholique de Louvain, Corning Incorporated and other contributors (www.abinit.org). [5] M. Cote, O. Zakharov, A. Rubio, and M. L. Cohen, Phys. Rev. 55, 13025 (1997).