Synthesis of ZnO single crystals by the flux method

Zinc oxide (ZnO) single crystals have been grown at temperatures ranging from 450–900 °C and for 1–12 h, using hydrous KOH and NaOH melts as fluxes. For a KOH flux, brown ZnO single crystals with diameter 0.5 mm × 7.5 mm were grown under conditions of 500 °C for 20 h and white crystals of diameter 0.5 mm × 7 mm were grown at 800 °C for 20 h, using a small crucible (average 50 ml). When a large crucible (average 400 ml) was used, ZnO single crystals with diameter 0.5 mm × 8 mm were formed at 900 °C for 30 h. When using a KOH + NaOH (1∶1) flux, light-brown and long crystals with diameter 1.0 mm × 18 mm could be grown. The grown ZnO single crystals were bounded with only both p- and m-faces. It seems that crystal qualities were good under conditions of 900 °C for 30 h. The following mechanisms of dissociation and formation of ZnO single crystal from KOH (or NaOH) + ZnO melt seemed to occur $$KOH(or{text{ NaOH}}){text{ }} to {rm K}^ + {text{ (or Na}}^{text{ + }} {text{) + OH}}^ - $$ $$ZnO{text{ + 2 OH}}^ - to {text{ ZnO}}_{text{2}}^{{text{2}} - } {text{ + H}}_{text{2}} {text{O,}}$$ $${text{ZnO}}_{text{2}}^{{text{2}} - } {text{ }} to {text{ ZnO + O}}^{{text{2}} - } .$$