ORDER AND DISORDER IN ZnGeP₂ CRYSTALS

G. Kimmel^1,2, Y. Shimony¹, O. Raz², M.P. Dariel²

¹Nuclear Research Center - Negev (NRCN), P.O.Box 9001, Beer-Sheva, 84190 Israel.
²Department of Materials Engineering, Ben-Gurion University of the Negev, P.O. Box 653, Beer-Sheva 84105 Israel.

Early studies using differential thermal analysis (DTA) and X-ray diffraction (XRD) techniques have shown that the low temperature stabilized phase of ZGP (-ZGP) undergoes at 950^oC a phase transformation to b-ZGP, the high temperature allotrope with the cubic zinc-blende structure. In a-ZGP, Zn and Ge atoms occupy different crystallographic sites, while in b-ZGP, Zn and Ge are randomly distributed on the same site. As a result, the unit cell of a-ZGP consists of two adjacent units of b-ZGP, forming a tetragonal chalcopyrite structure.

High quality ZnGeP₂ (ZGP) crystals were grown by means of the Horizontal Gradient Freezing technique and characterized by X-ray diffraction (XRD) and optical measurements. Analysis of diffraction patterns taken at room temperature, showed the presence of the tetragonal a-ZGP crystals and of a small amount of coherently oriented cubic phase having the zinc-blende structure. There was no significant increase in the amount of the cubic phase after quenching of ZGP from above the a->b transition (950^oC). However, rapid cooling of the melt resulted in the presence of high amount of a cubic phase.

The formation of a disordered cubic phase from the rapidly cooled melt can be attributed to local deviations from stoichiometric concentration and incomplete ordering of the (Zn,Ge) sublattice. The origin of a cubic phase in crystals grown slowly at high temperatures has not been determined yet. It may be associated with antiphase boundary regions generated during the growth of the single crystal.

Crystals were grown under temperature gradient covering the entire liquidus range from the highest solid ZGP down to the ZGP-Ge eutectic point. The products formed at different temperatures, above and below the a->b transition, were carefully characterized, and an amount of cubic phase was found even in crystal regions that solidified below the a->b transition. This result strengthens the interpretation of the cubic regions as being due to faults produced during solidification. The results of this study will be given in details.