Optical and mechanical properties of MgO crystals implanted with lithium ions

Defect profile induced by implantation of Li+ ions with an energy of 175 keV and a fluence of 1x10(17) ions/cm(2) in MgO single crystals was characterized by Rutherford backscattering and optical absorption measurements. Several absorption bands at 5.0, 3.49, 2.16, and 1.27 eV, identical to those found in neutron irradiated crystals, were observed and have been previously associated with oxygen vacancies and higher-order point defects involving oxygen vacancies. Despite the high fluence of Li+ ions, no evidence was found for the formation of Li nanocolloids during implantation. Nanoindentation experiments demonstrated that both the hardness and Young's modulus were higher in the implanted layer than in the sample before implantation. The maximum values were H=(17.4+/-0.4) and E=(358+/-9) GPa, respectively, at a contact depth of approximate to165 nm. Thermal annealings in flowing argon at increasing temperatures improved the crystalline quality of the implanted layer. After annealing at 500 K, two extinction bands at approximate to2.75 and 3.80 eV emerged. These bands are attributed to Mie scattering from metallic lithium nanocolloids with either a face-centered- or a body-centered-cubic structure. The latter band was almost absent by 950 K. The former reached a maximum intensity after the thermal treatment at 1050 K and disappeared by 1250 K. The behavior of these bands can be satisfactorily explained by the Maxwell-Garnett theory. The decrease in hardening cannot be correlated with the thermal destruction of the absorption bands at 5.0, 3.49, 2.16, and 1.27 eV, but rather with the annihilation of both lithium and oxygen interstitials. Lithium outdiffusion from the implanted region takes place at temperatures of approximate to1100 K. It is concluded that the hardening observed in the implanted region was primarily due to the extraordinarily large concentration of both lithium and oxygen interstitials. (C) 2004 American Institute of Physics.