Preparation of ZnO single crystals by chemical transport reaction and determination of ZnO homogeneity range

Abstract

The homogeneity range of ZnO is very narrow and cannot be determined by conventional analytical methods. The determination was performed by oxidation of the substoichiometric samples in a gas stream with coulometric control of the gas-solid phase reaction by the aid of a solid electrolyte cell. ZnO single crystals were prepared by chemical transport. Transport of stoichiometric ZnO is possible with Cl2, Br2 and HCl going from the higher to the lower temperature, whereas HBr transports from the lower to the higher temperature. ZnO, which exhibits the lower phase boundary composition, can be transported only by HCl and HBr or their ammonium salts, respectively, from the higher to the lower temperature. The deviation of stoichiometry depends on the preparation temperature, i.e. the deposition temperature during the chemical transport. The value of the deviation amounts to 30–240 ppm in the range 800–1100°C. Measurements of the electrical conductivity and the Hall mobility yield the same temperature dependence of the phase boundary composition.