OBSERVATION OF LONG-RANGE DISORDER CAUSED BY LOW-ENERGY (20-200 EV) O+ AND AR+ BEAMS, IN NDBACUO FILMS

To fully realize the potential of low energy ion beam-assisted thin fi lm growth, a thorough understanding of the interaction of the ion beam with the growing film is needed. In this study we carried out a detai led series of experiments to investigate the effect of O+ and Ar+ beam s (20-200 eV) on the crystalline order in the ''bulk'' of c-axis NdBaC uO superconducting films. From theta-2 theta X-ray diffraction (XRD) a nd Rutherford backscattering and ion channeling analysis (RBS-C) on fi lms exposed to the ions at room temperature, we found that there was n o change in the overall film thickness, but that 100 to 500 Angstrom o f the film was disordered. Also, the c-axis parameter of the remaining crystalline region increased. The extent of the disordering is greatl y reduced at 400 degrees C. The depth to which this disordering occurr ed was found to be inversely dependent on the energy of the impinging ions, with the lowest energy ions damaging the film to the greatest de pth. From thermal annealing studies, we found that recovery of the cry stalline order occurs in two steps, at < 300 degrees C and > 700 degre es C. At < 300 degrees C partial recovery occurred, with a reduction i n the c-axis length due to diffusion and ordering of oxygen. Above 700 degrees C, the c-axis returned to its original value and film became superconducting again. At this temperature the metal atoms can diffuse and reorder. This indicates that the 20-200 eV ions also caused catio n disorder in these NBCO films and is consistent with the above XRD an d RES results. These results are discussed in terms of the existence o f correlated, focusing collisions down the atomic rows in materials wi th closely spaced atomic layers, significantly extending the range of the atomic displacements caused by the impact of these low energy ions .