Raman Studies of Stress-Induced Phase Transformations in Titania Films
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RAMAN STUDIES OF STRESS-INDUCED PHASE TRANSFORMATIONS IN TITANIA FILMS GREGORY J. EXARHOS AND NANCY J. HESS Pacific Northwest Laboratory, PO BOX 999 MS K2-44, Richland WA 99352 ABSTRACT Time-resolved micro-Raman spectroscopy is used to follow the amorphous to crystalline phase transformation in sol-gel deposited titania films induced thermally or through the action of applied hydrostatic pressure in a diamond anvil cell. Time-dependent phonon intensities intrinsic to the growing phase are related to the volume fraction of crystallite present at any time. The sigmoidally generated curves can be modeled in terms of modified Avrami ingrowth kinetics in which diffusion of the amorphous phase to the nucleation center is restricted by the morphology of the evolving phase. Phonon frequency and linewidth measurements during the course of the transformation probe changes in film stress and particle size which are used to understand the mechanistics of the transformation. Raman measurements also are used to derive a phase stability diagram for titania films. INTRODUCTION The kinetics and mechanistics of transformation phenomena in solids can be evaluated from inelastic light scattering measurements which probe not only the time dependent growth of the new phase, but also associated changes in localized chemical bonding both of which are driven by an imposed stress to the system. Previous studies of temperatureand laser-induced glass-crystal transformations in bulk oxide glasses demonstrated the utility of in situ Raman spectroscopy as a real-time diagnostic of the crystallization kinetics. [1,2] Raman measurements on thin film dielectrics provide an interesting challenge since the smaller amount of material present in the film and an increased signal from the substrate lead to greater difficulty in detecting the crystallizing phase. The availability of more sensitive detectors, and microprobe techniques which limit the probe depth of the excitation laser have made such measurements possible and allow time resolution on the order of milliseconds and spatial resolution on the order of micrometers. [3,4,5,6] Laser Raman spectroscopy has been used to study thermally induced crystallization of amorphous TiO 2 films prepared using ion, electron beam, and sol-gel techniques. [7,8] These initial studies have established that deposition parameters and solution chemistry control both the rate of film crystallization and the phase of the crystallized material. Variations in processing conditions were proposed to influence the nature of the nucleation centers formed in the amorphous film during deposition. Thus, anatase, rutile, or mixed phases were found to evolve when samples were subjected to temperatures above 575 K. Film thickness, grain size, and inherent film stress also are predicted to influence the transformation dynamics. While thickness, and particle size effects on solid-solid phase transformations in thin films have been addressed [9,10], the stress dependence on the transformation and phase stability regions in TP space a
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