Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States.
ACS Nano. 2015 Apr 28;9(4):4316-27. doi: 10.1021/acsnano.5b00743. Epub 2015 Mar 16.
The trend to reduce device dimensions demands increasing attention to atomic-scale details of structure of thin films as well as to pathways to control it. This is of special importance in the systems with multiple competing interactions. We have used in situ scanning tunneling microscopy to image surfaces of La5/8Ca3/8MnO3 films grown by pulsed laser deposition. The atomically resolved imaging was combined with in situ angle-resolved X-ray photoelectron spectroscopy. We find a strong effect of the background oxygen pressure during deposition on structural and chemical features of the film surface. Deposition at 50 mTorr of O2 leads to mixed-terminated film surfaces, with B-site (MnO2) termination being structurally imperfect at the atomic scale. A relatively small reduction of the oxygen pressure to 20 mTorr results in a dramatic change of the surface structure leading to a nearly perfectly ordered B-site terminated surface with only a small fraction of A-site (La,Ca)O termination. This is accompanied, however, by surface roughening at a mesoscopic length scale. The results suggest that oxygen has a strong link to the adatom mobility during growth. The effect of the oxygen pressure on dopant surface segregation is also pronounced: Ca surface segregation is decreased with oxygen pressure reduction.
降低器件尺寸的趋势要求人们越来越关注薄膜结构的原子尺度细节以及控制这种结构的途径。在具有多种竞争相互作用的系统中，这一点尤为重要。我们使用原位扫描隧道显微镜来成像通过脉冲激光沉积生长的 La5/8Ca3/8MnO3 薄膜的表面。原子分辨成像与原位角度分辨 X 射线光电子能谱相结合。我们发现沉积过程中背景氧压对薄膜表面的结构和化学特征有很强的影响。在 50 mTorr 的 O2 下沉积会导致混合终止的薄膜表面，B 位（MnO2）终止在原子尺度上结构不完善。将氧压相对降低到 20 mTorr 会导致表面结构发生剧烈变化，导致几乎完全有序的 B 位终止表面，只有一小部分 A 位（La，Ca）O 终止。然而，这伴随着介观长度尺度的表面粗糙度增加。结果表明，氧与生长过程中吸附原子的迁移率有很强的联系。氧压对掺杂剂表面偏析的影响也很明显：随着氧压的降低，Ca 表面偏析减少。
