Relationship between Nanostructure and Optical Properties of ZnO Thin Films

Nanostructured ZnO thin films have been grown on quartz substrates by metal organic chemical vapor deposition. Zn(tta)2 center dot tmeda (H-tta = 2-thenoyltrifluoroacetone, tmeda = N,N,N',N'-tetramethylethylenediamine) has been used as Zn precursor. The impact of deposition temperature, in the range 400-750 degrees C, on film nanostructure and optical properties has been investigated. The X-ray diffraction patterns show that film crystallinity improves upon increasing the substrate temperature. This is consistent with morphologies observed through scanning electron microscopy (SEM) and atomic force microscopy (AFM), which indicate that grain dimensions increase ranging from 80 nm rounded grains at 400 degrees C to barlike grains of about 200-400 nm at the highest investigated temperature of 750 degrees C. The AFM investigation shows that the roughness is also a function of the substrate temperatures and parallels the grains size of ZnO films. The UV-vis transmission spectrum shows that ZnO films grown at 600 degrees C on quartz are highly transparent in the visible region. Furthermore, spectroscopic ellipsometry is used for investigating the dependence of the ZnO dielectric function and optical gap on film nanostructure.