Incongruent Sputtering in Nio(100) and CoO(100) with 2 keV Argon Ions
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INCONGRUENT SPUTTERING IN NiO(100) AND CoO(100) WITH 2 keV ARGON IONS M.A. LANGELL AND L.S. COLBERT University of Nebraska, Department of Chemistry, Lincoln, Nebraska 68588-0304, USA ABSTRACT Although NiO(100) sputters congruently at 300 K under 2 keV argon ion flux, sputter reduction is observed above a substrate threshold of 350 K. The reduction kinetics are described quantitatively by Fickian diffusion of subsurface oxygen coupled to differential sputtering at the sample surface. Two additional surface oxygen species are detected on sputter reduced NiO by XPS and these species have not been found on either stoichiometric or chemically reduced NiO(100). CoO(100) also shows sputter reduction, although only surfaces that have been pre-annealed to 973 K show significant reduction. Under these conditions, an epitaxial layer of Co O4 forms on the CoO substrate and this layer has a threshold temperature to sputter reduction of 550 K. Reduction kinetics for the epitaxy appear similar to those of NiO(100). INTRODUCTION The interaction of ion beams with metal compound surfaces is important to a variety of surface analytical and fabrication techniques. However, many multicomponent surfaces sputter incongruently, resulting in compositional and topographical changes that are generally undesirable and, at the present time, are not quantitatively predictable by ion beam-surface collisional theories. Metal oxides represent one category of compounds known to undergo preferential sputtering and, for those oxides that do show compositional changes, it
is inevitably the more volatile
oxygen component that is lost. For this reason, qualitative predictions of incongruent sputtering have been made [1,2] based on equilibrium pressures of oxygen at 3000-4000 K, typical of thermal spikes induced by local deposition of large amounts of energy during the ion impact event [1,3]. Although recent evidence suggests that prompt thermal sputtering might play a role in the chemical sputtering of oxygen-exposed metals [4], it is more likely that oxide decomposition pressures, due to their correlation with crystal lattice stability, merely reflect surface binding effects [1,3,5] for noble gas ions below approximately 10 keV [6]. We present here evidence that threshold behavior to sputter reduction can occur for NiO(100) and CoO(100) at substrate temperatures considerably below that for which decomposition or evaporation is observed. Thermally activated lattice oxygen migration appears to be crucial to this process, which can create altered layers of 500-1000 A in thickness for sufficiently elevated substrate temperatures [7,8]. Compositional changes that take place in the altered layer are also discussed. EXPERIMENTAL The UHV system used in these studies has been described previously [8]. Experimental techniques available include Auger spectroscopy (AES), x-ray photoelectron spectrocopy (XPS) and low energy electron diffraction (LEED). In addition, a quadrupole mass spectrometer is used for residual gas analysis. Argon ion bombardment is performe
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