Effects of He+ ion implantation on optical and structural properties of MgAl2O4

Single crystals of magnesium-aluminate spinel were implanted with 170 keV Het ions to fluences ranging from 1 x 10(16) to I x 10(21) ions/m(2) at 120 K. The effects of ion implantation were studied using Optical Absorption Spectroscopy, Rutherford Backscattering Spectroscopy and Ion Channeling (RBS/C) and Transmission Electron Microscopy (TEM). In absorption spectra obtained from the implanted samples, growth of an F-center band at 5.3 eV was observed. At the fluence of 3 x 10(20) ions/m(2), the growth of this band not only ceases but the intensity suddenly decreases. This may be due to formation of a new phase at this fluence. This is partially confirmed by the fact that at this and higher ion doses, a modulated absorbance becomes apparent in the absorption spectrum of spinel. This effect is caused by formation of a buried layer with refraction index lower than that of an unimplanted sample. RBS/C and TEM measurements show that spinel is not amorphized over the fluence range examined in this study. TEM microdiffraction observations show that in the damaged region the intensities of superlattice spots decrease significantly, suggesting that ion beam irradiation induces either an order-disorder phase transition or a transformation into the so-called "metastable" phase of spinel. (C) 2000 Elsevier Science Ltd. All rights reserved.