Whenever possible, bulk single crystals of any substance are grown from the melt. If no suitable crucible material can be found, alternative methods like cold crucible (skull melting), sublimation, chemical transport or solution growth can be applied. Problems arise for ZnO, as the substance has a high melting point Tf = 1975°C combined with a high vapor pressure at Tf of pZnO = 1.06 bar. The dissociation 2 ZnO → 2 Zn + O2 upon evaporation makes a partial pressure pO2 = 0.35 bar necessary in order to stabilize the ZnO bath. In case of dissociation the metallic Zn excess would destructively attack any metallic crucible.Thermodynamic equilibrium calculations allow to predicted and experiments showed that ZnO can be molten in iridium crucibles under suitable conditions (T, p, atmosphere). This invention offers the chance to grow ZnO by conventional melt growth techniques (Bridgman, Czochralski, gradient freeze) and to avoid some technical problems like the extremely high T gradients inherent to skull melting and the contamination problems inherent to hydrothermal growth. Up to now crystal slices with diameter of >30 mm and X-ray rocking curve widths better 1 arc min could be produced. |