Microstructural and Optical Properties Modifications Induced by Plasma and Annealing Treatments of Lanthanum Oxide Sol-Gel Thin Films

Lanthanum oxide (La2O3) thin films have been prepared on Si(100) substrates through a sol-gel process from the lanthanum methoxyethoxide (La(OCH2CH2OCH3)(3)) precursor. To study the effects of different postdeposition treatments on film microstructure and optical properties, the as-deposited layers have been annealed at different temperatures between 200 and 700 degrees C in air or forming gas (H-2 10% in N-2) for 1 h. Low-temperature (300 degrees C remote O-2 plasma processing has also been applied to both as-deposited and previously annealed samples. Films have been fully characterized by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), spectroscopic ellipsometry (SE), glancing incidence X-ray diffraction (GIXRD), and atomic force microscopy (AFM). In particular, microstructure and optical properties correlations were accomplished by exploiting spectroscopic ellipsometric investigation. It has been found that thermal annealing at temperatures around 500 degrees C leads to subcutaneous oxidation of the Si substrate resulting in the formation of a SiO2 layer, and annealing at higher temperature (700 degrees C also results in film-substrate intermixing and formation of a lanthanum silicate layer. At variance, these interfacial reactions can be suppressed by low-temperature (300 degrees C remote O-2 plasma processing of as-deposited films, and optical transparency in the visible range can be strongly improved.