Morphological, microstructural and optical properties supremacy of ...

N.K. Sahoo et al. / Applied Surface Science 253 (2007) 3455–3463 3461microstructural evolution through effective single oscillatormodel, we have plotted (n 2 1) versus E 2 in Fig. 4. It can benoticed that pure gadolinia film has demonstrated differentdispersion behavior in comparison to the most composite films.Also as an added advantage, the composite films are alsoassociated with better band gap values. This result is distinctlydepicted in Fig. 5. The shaded region in the plot has highlightedthis fact that under these compositions both the refractive indexand band gap supremacy can be achieved which is quitedifferent from the monotonic tailoring of the optical propertiesin conventional composite films. This is a very definite situationof the violation of Moss rule. This violation is highlighted inFig. 6 where the plot between n 4 and 1/E g has demonstrated aninteresting non-linear characteristic. Such situations are highlyuseful for developing multilayer optical coatings for the deepultraviolet region utilizing the advantages of the band gap andthe refractive index parameters. Such improved opticalproperties of composite films violating Moss rule have beenearlier observed in certain other co-deposited thin film systems[32–34]. Although earlier researchers have only predictedpossible microstructural superiority in composite films, wehave very authentically verified this observation throughatomic force microscopic measurements as well as refractiveindex modeling. In Fig. 7(a) the topography of the puregadolinia film has been presented. One can easily see a poorgrain structure distribution in the morphology. There are veryprominent localized aggregates or mound structures distributedin the morphology. The 3-D-autocorrelation function ofthis morphology is depicted in Fig. 7(b). The presence ofdominating mound structures (grain clustering) is highlightedin this measurement result. Besides, the correlation length hasa higher numerical value that supports the presence of suchcorrelated superstructures or aggregates. As a contrast, thecomposite gadolinia–silica films have shown a denser,amorphous type of grain structures in the morphologypresented in Fig. 8(a). The dense morphology looks morelike the result that is very much similar to the outcome of ionassisteddeposition. The 3-D-autocorrelation function alsosupported this observation which can be seen from Fig. 8(b).The autocorrelation function has displayed a more self-affinequality in the analysis result. 2-D analysis depicted in Fig. 9(aand b) of such autocorrelation functions distinctly highlightedsuch transition of the grain structures from mound to selfaffine.Fig. 10 depicted the height–height correlation functionalanalysis to the morphologies. Such an analysis has alsoFig. 8. (a) Morphology and (b) 3-D autocorrelation function of compositegadolinia–silica film depicting a more smooth self-affine micro grain structures.Fig. 9. Two-dimensional autocorrelation function of (a) pure gadolinia (100%)film and (b) composite gadolinia–silica (90/10) film demonstrating moundstructure and self-affine nature of the morphology, respectively.