Oriented ZnO thin films synthesis by sol–gel process for laser application

Abstract

During the last decade, a renewed interest has been devoted to ZnO-based materials owing to their applications in fields such as acoustic-wave and acousto-optic devices and short-wavelength semiconductor diode lasers. As opposed to traditional deposition by physical methods, the sol–gel process offers a simple, low-cost and highly-controlled way for ZnO film deposition. We have undertaken the systematic study of the key parameters that influence the texture and preferential orientation of films deposited by sol–gel procedures (precursor, solvent and additive nature and concentration, coating and heat treatment conditions). Zinc acetate dihydrate dissolved in ethanol is used as inorganic precursor; monoethanolamine acts at the same time as a base and a complexing agent. Thin films are obtained by dip- or spin-coating the solutions or colloidal sols (obtained by aging at 60–100 °C) on glass or quartz substrates. The as-deposited films are composed of amorphous zinc oxide-acetate species (a continuous gel, or sub-micronic particles), which are transformed into mono-oriented ZnO upon thermal treatment. Films arising from concentrated precursor solutions give rise to a ( c// n) texture; films made up from dilute colloidal sols present an unusual ( a// n) texture. We show that the preferential orientation of films depends on the nature of the solution or sol, precursor concentration and thermal conditions. We discuss the obtained orientations in terms of the interactions film–substrate and within the film. The UV absorption of these films is presented.