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 film growth, a thorough understanding of the interaction of the ion beam with the growing film is needed. In this study we carried out a detailed series of experiments to investigate the effect of O + and Ar + beams (20-200 eV) on the crystalline order in the "bulk" of c-axis NdBaCuO superconducting films. From θ - 2θ X-ray diffraction (XRD) and Rutherford backscattering and ion channeling analysis (RBS-C) on films exposed to the ions at room temperature, we found that there was no change in the overall film thickness, but that 100 to 500 Å of the film was disordered. Also, the c-axis parameter of the remaining crystalline region increased. The extent of the disordering is greatly reduced at 400°C. The depth to which this disordering occurred was found to be inversely dependent on the energy of the impinging ions, with the lowest energy ions damaging the film to the greatest depth. From thermal annealing studies, we found that recovery of the crystalline order occurs in two steps, at < 300°C and > 700°C. At < 300°C partial recovery occurred, with a reduction in the c-axis length due to diffusion and ordering of oxygen. Above 700°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 cation disorder in these NBCO films and is consistent with the above XRD and RBS results. These results are discussed in terms of the existence of correlated, focusing collisions down the atomic rows in materials with closely spaced atomic layers, significantly extending the range of the atomic displacements caused by the impact of these low energy ions.