Surface restructuring in sputter-damaged Bi${}_2$Sr${}_2$CaCu${}_2$O${}_{8+\delta }$

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.

Figure 1

A side view of the crystal structure of Bi${}_2$Sr${}_2$CaCu${}_2$O${}_8$. The dimensions depicted are 7.33 Å × 7.33 Å × 12 Å.

Figure 2

TOF spectra for 2 keV Na${}^{+}$ scattered from Bi-2212 with increasing amounts of Ar${}^{+}$ sputtering damage given as fluence (total ion flux per area).

Figure 3

Ion-scattering spectra predicted by KALYPSO simulation of the Bi-2212 upper layers (Bi-O and Sr-O). Sr-O is shown for differing levels of oxygen vacancies in the Bi-O layer. Also shown for comparison are measured TOF spectra of the as-cleaved and sputtered surfaces.

Figure 4

Measured single-scattering intensities determined from the TOF spectra. The values are calculated by taking the area of the peaks in Fig. 3 and correcting for the scattering cross section.

Figure 5

Ratio of the intensities of the Bi and Sr backscattering peaks and the surface work function shift plotted as a function of sputtering fluence.

Figure 6

Number of sputtered atoms of O, Bi, Sr, Cu, as predicted by MARLOWE simulation, as a function of sputtering damage. One hundred trials of 100 incident ions of Ca were separately performed, and the results added afterwards. The data is histogrammed by time (number of ions incident on the surface out of 100).