Surface restructuring in sputter-damaged Bi2Sr2CaCu2O8+δ

The high-T-c cuprate Bi-2212 is sputtered with 500 eV Ar+ ions, and changes to the surface composition are investigated with low energy (2 keV) Na+ ion scattering. It is shown that ion bombardment leads to the development of a Bi-O overlayer, differing in structure from the underlying material and making the surface highly resistant to further sputtering-induced changes. In contrast, sputtering and ion-scattering simulations would suggest that atoms from lower layers should be present at the surface as a consequence of the kinematics of the sputtering process. It is thus concluded that the Bi-O layer forms because the surfactant effects of Bi reduce the surface energy.