Melt growth of ZnO bulk crystals in Ir crucibles

The growth of ZnO bulk crystals from the melt is difficult due to the high melting point of the material (Tm = 1975 °C), requiring the use of Ir crucibles, and its thermal decomposition upon heating. A thermodynamic analysis shows that the oxygen partial pressure in the surrounding vapour phase should be as low as possible during the heating cycle at lower temperatures in order to avoid the oxidation of Ir to IrO2. Approaching the ZnO melting point, however, higher oxygen pressure is required for suppressing the ZnO decomposition, while IrO2 decomposes in the elements. The required increasing oxygen supply with increasing temperature can be accomplished by the addition of CO2 to the gas atmosphere. Experimental results prove the correctness of these results. In a Bridgman-like process ZnO boules are obtained that consist of a few large grains with cm dimensions.

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