Surface segregation and preferential sputtering of bismuth in rf-magnetron-sputtered iron-garnet films

Auger electron spectroscopy, x-ray photoelectron spectroscopy, and secondary-ion mass spectrometry have been used to elucidate the role of surface segregation and preferential sputtering of bismuth in rf-magnetron-sputtered bismuth--iron-garnet films. It turns out that bismuth is enriched by a factor of up to 2.4, as compared to the bulk content, within the first 1--1.5 nm beneath the advancing film surface during growth. Furthermore, Ar+-ion bombardment at projectile energies as low as 50 eV gives rise to a rather complete depletion of bismuth in the garnet surface. As neither surface segregation nor preferential sputtering of bismuth can be observed in as-polished single-crystalline Bi12SiO20 used as our reference, we conclude that the bond strength between bismuth and oxygen is weaker than that of the other garnet constituents. This is confirmed by comparing the calculated sputter yield of the element oxide. Thus, bismuth-rich crystallographically perfect epitaxial iron-garnet films can only be grown by sputtering if the growing film is protected against energetic particle bombardment