Thermally Induced Pore Formation in Epitaxial ZnO Films Grown from Low Temperature Aqueous Solution

The formation of pores has been observed in epitaxial ZnO films deposited from low temperature aqueous solution when annealed at 300 °C and higher. The effects of these pores could have a dramatic impact on the ability to utilize low temperature aqueous synthesis to deposit ZnO films for optoelectronic devices. The results of thermogravimetric analysis, evolved-gas mass-spectrometry, and secondary ion mass spectrometry analysis indicate that the formation of pores is related to the expulsion of water vapor from the ZnO crystal. We propose that the pores form via the coalescence of zinc and oxygen vacancies that result from the incorporation, and subsequent removal, of a large concentration of hydroxide ions substituting for oxygen in the ZnO lattice. Despite the large change in solid volume undergone during pore formation, the ZnO films remain single phase and epitaxial and retain good electrical conductivity.