Iron-Silicate Glassy Films by Sol-Gel Conversion Induced by Rapid Thermal Processing

Abstract

ABSTRACTThe sol-gel system iron triethoxide - tetraethylorthosilicate (TEOS) with the Fe/Si atomic ratio ranging from 0 to 1 has been investigated. Our study leads to the conclusion that the annealing method of the hydrolyzed precursor film determines the type of solid film formed. Annealing by Rapid Thermal Processing (RTP) with its high heating and cooling rates yields amorphous, glassy iron silicate films. These films result from polycondensation reactions between Si-OH and Fe-OH groups of the hydrolyzed precursors forming Fe-O-Si and Si-O-Si bonds. These bonds show a characteristic vibration band in FTIR spectra, which is most intense for Fe/Si = 1, and shifts for increasing Fe/Si ratio from 1073 down to 965 cm-1.The iron silicate film with Fe/Si =1 obtained by RTP displays an optical absorption band around 320 nm, which indicates that Fe3+-ions have a tetrahedral coordination similar to the Si4+-ions in Si02 and glass with tetrahedrally coordinated silicon and iron ions linked by bridging oxygen atoms. The films, typically 0.1 μm thick, annealed by RTP have a smooth, mechanically hard surface. This glassy structure can be formally presented as FeSiOx. Conventional furnace heating with low heating/cooling rates yields nanocrystalline films with predominantly Si-O-Si bonding in grains with sizes below 40 nm. More structural and chemical investigation is needed to elucidate the details of the structure and the formation mechanism of the amorphous compound, but we can conclude that the cooling rate has a direct effect on the degree of non-crystallinity of the compound. The high cooling rate of RTP will retain the Fe-O-Si bonds formed in a state of inequilibrium, and “quench” them in an amorphous, metastable glass compound, whereas slow cooling gives rise to nanocomposite film formation.