Precipitation phenomena in high-dose iron-implanted silica and annealing behavior
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D. L. Griscom Naval Research Laboratory, Washington, DC 20375 (Received 15 June 1987; accepted 20 July 1987) The effects of high-dose iron implantation into high-purity fused silica have been investigated by conversion-electron Mossbauer spectroscopy, transmission electron microscopy, Rutherford backscattering, and optical spectroscopy. In addition to isolated Fe 2 + ions, samples subjected to doses of 4 x 1016 and 6X 1016 ions cm" 2 were found to contain homogeneously dispersed, equidimensional, crystalline particles ~ 2 nm, similar to Fe 3 O 4 . Precipitated spherical particles of metallic a - F e ~ 4 nm were observed in samples receiving a dose of ~ 1017 ions cm" 2 ; as the dose was raised to 2.5 X 1017 ions cm" 2 the mean size of these particles reached ~ 30 nm. Annealing in air to 800 °C resulted in the growth of acicular grains of a-Fe 2 O 3 ~ 20-300 nm. The optical spectra of the implanted layers are compared with the predictions of small particle theory.
I. INTRODUCTION Ion implantation is a nonequilibrium doping technique that is of interest for modifying the physicochemical properties of materials or for synthesizing new phases having novel properties. This technique is extensively used with semiconductors for microelectronic applications and with metals for surface treatments (for hardness, corrosion resistance, etc.), but it also holds some interesting prospects for insulators. However, in this case the phenomena are complex because the solid phases that are created in the implanted layer can incorporate implanted impurities in various charge states that can associate in turn with various lattice defects, thus creating a multitude of metastable impurity-defect structures. Previous studies using metallic iron implantations in strongly or partially covalent oxides or halides (MgO, TiO2, Mg2SiO4, LiF)' have elucidated the influence of the principal parameters such as the nature of the implanted ions, the local concentration, temperature, etc. In MgO, for example, it was found that the electrical conductivity of the implanted layer was much higher than that of pure MgO, due evidently to the formation of a continuous solid solution Mg x Fe,_ x O in which Fe 2 + ions act as electron hopping centers.2 By contrast, preliminary studies of iron-implanted silica3 showed that the implanted zone remains insulating (p > 10 n il cm) even for doses as high as 1.4X 1017 ions cm" 2 . Mossbauer data indicated the presence of finegrained precipitated particles of metallic iron in samples receiving this high dose.3 Ferromagnetic resonance (FMR) studies4 of the implanted samples revealed some unusual properties and suggested the presence of 910
J. Mater. Res. 2 (6), Nov/Dec 1987
http://journals.cambridge.org
precipitated magnetite grains in samples receiving doses lower than 1X10 17 ions cm" 2 . Still, the FMR results could not be conclusively interpreted without detailed knowledge of the sizes, morphologies, and crystal structures of the precipitated phases. The present work was undertaken to clarify these issues. The f
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