The influence of water of hydrolysis on microstructural development in sol-gel derived LiNbO3 thin films

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Abstract
The effect of water of hydrolysis on nucleation, crystallization, and microstructural development of sol-gel derived single phase LiNbO3 thin films has been studied using transmission electron microscopy (TEM), atomic force microscopy (AFM), x-ray diffraction (XRD), and differential scanning calorimetry (DSC). A precursor solution of double ethoxides of lithium and niobium in ethanol was used for the preparation of sol. DSC results indicated that adding water to the solution for hydrolysis of the double ethoxides lowered the crystallization temperature from 500 °C (no water) to 390 °C (2 moles water per mole ethoxide). The amount of water had no effect on the short-range order in amorphous LiNbO3 gels but rendered significant microstructural variations for the crystallized films. AFM studies indicated that surface roughness of dip-coated films increased with increasing water of hydrolysis. Films on glass, heat-treated for 1 h at 400 °C, were polycrystalline and randomly oriented. Those made with a low water-to-ethoxide ratio had smaller grains and smaller pores than films prepared from sols with higher water-to-ethoxide ratios. Annealing films with a low water concentration for longer times or at higher temperatures resulted in grain growth. Higher temperatures (600 °C) resulted in grain faceting along close-packed planes. Films deposited on c-cut sapphire made with a 1:1 ethoxide-to-water ratio and heat-treated at 400 °C were epitactic with the c-axis perpendicular to the film-substrate interface. Films with higher concentrations of water of hydrolysis on sapphire had a preferred orientation but were polycrystalline. It is postulated that a high amount of water increases the concentration of amorphous LiNbO3 building blocks in the sol through hydrolysis, which subsequently promotes crystallization during heat treatment.