Analytical Electron- and Atomic Force Microscopy Study of Multilayered Oxide Films Formedon Ce-DOPED Ni
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EXPERIMENTAL PROCEDURE The substrates of 99.99 % purity Ni were implanted with 2x1016 ions/cm 2 of 150 keV Ce'. This resulted in an enrichment of the surface region of the substrate, about 50 nm thick, with Ce up to a maximum concentration of 8 at. %. Oxide films were grown at 973 K in pure 135
Mat. Res. Soc. Symp. Proc. Vol. 382 01995 Materials Research Society
oxygen at a pressure of 0.005 torr. Growth surfaces of the oxides were examined by using atomic force microscopy (AFM), a Digital Instruments Ltd., Nanoscope Ill. The microstructure was characterized by transmission electron microscopy (TEM), a Philips CMI2, equipped with a Gatan electron energy-loss spectrometer (EELS). Microdistribution of Ce inside the oxide was analyzed by scanning transmission electron microscopy (STEM), a Vacuum Generator Microscopes Ltd. HB5. TEM/STEM observations were performed on thin foils parallel to the oxide-metal interface, prepared by ion thinning in a Gatan 600 duomill [3]. RESULTS AND DISCUSSION Atomic Force Microscopy analysis of oxide surface A typical AFM image of surface morphology of 175 nm thick oxide formed on Ce implanted Ni is shown in Fig. la. In general, the growth surface of Ce modified oxide is more uniform than that reported for pure NiO, with similar thickness [6]. Growth surface is covered with NiO overgrowths having a size up to 50 nm, as measured in the plane parallel to the substrate. An example of the cross-sectional profile along a 251.9 nm line is shown in Fig. lb. To characterize the vertical roughness of the growth surface, the commonly used roughness parameters R, rms, and R,, [7] were obtained using AFM software. From analysing the 250x250 nm area of the oxide, the values obtained for R., rms, and R,. were 1.9, 2.6, and 20 nrm, respectively. It is characteristic that the largest evolution of surface morphology takes place during the early stages of oxide growth. During further exposure, the growth surface was relatively stable, which suggests a change in reaction front, as discussed later.
251,93 Iý'NR Ia 'NS -") C Ra(Ir i t.fý 9 M'auýU-0 RZ 2,M- nm RZW 8
Fig. 1. AFM image of oxide surface morphology (a) and corresponding section analysis along the line indicated by arrows (b). The roughness parameters obtained from section analysis are: rms = 1.5 nm, R. =1.1 nm and R,.,. = 5.0 nm.
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Analytical Electron Microscopy analysis of oxide microstructure Depth composition analysis by RBS, AES and SIMS , published elsewhere [3,8], revealed that Ce is located mainly in the vicinity of the outer surface. Such a finding suggests that the oxide has a complex depth structure. To investigate the microstructural differences as a function of a distance from the outer surface, the thin foils for TEM were prepared at different depths, by using the technique described previously [3]. A typical TEM image of the region adjacent to the oxide-gas interface is shown in Fig. 2a. The structure is composed of extremely small grains of CeO 2 and NiO. As indicated by the SAD pattern, both phases are characterized by r
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