Thermodynamic analysis of the deposition of zinc oxide and chalcogenides from aqueous solutions

The synthesis of zinc oxide and chalcogenides from aqueous solution is analyzed from a thermodynamic point of view. The study is intended to explain the results of recipes normally used for the growth of thin films of zinc oxide and chalcogenides from chemical solutions. Different reaction possibilities are compared in a solution of zinc, ammonia as complexant, chalcogenide anions, and hydrazine. The composition of the solution is analyzed by means of thermodynamic diagrams, as a function of pH, ammonia concentration, and redox potential. The reactivity in the solution is discussed, and free energy changes are compared as a function of pH for the different possible reactions. It arises that ZnO growth by an electroless-chemical process is the most favored reaction in nondeaerated solutions. This reaction consists of the reduction of naturally dissolved oxygen by the chalcogenide anion, or other added reducer (hydrazine), followed by chemical formation of ZnO. Zinc chalcogenides appear to be metastable phases in the aerated solutions. Under anoxic conditions, the chalcogenides are stable phases, and the most favored process (i.e., with highest free energy change) is found to be induced by hydrazine oxidation and proton reduction. That explains experimental observations that show that hydrazine is always necessary to obtain zinc chalcogenide thin films from solution deposition. In the absence of hydrazine, the oxidation of the chalcogenide is the preferential reaction, and a film with mixed composition is obtained. On the other hand, the pure chemical precipitation, usually considered as to be responsible for chalcogenide deposition from solution, is a less favored reaction and probably plays a minor role. As for ZnTe solution deposition, the high reducing character of telluride makes it impossible to obtain pure ZnTe films but gives either a mixture of ZnO and Te or ZnTe and Te depending on conditions in the solution.