Influence of Oxygen Pressure during Deposition on Atomic-Scale Surface Features of La$_{5/8}$Ca$_{3/8}$MnO$_{3}$ Films*

We have used \textit{in situ} scanning tunneling microscopy to visualize atomic-scale surface structure of a mixed-valence manganite La$_{5/8}$Ca$_{3/8}$MnO$_{3}$ films grown by pulsed laser deposition. Surface termination and chemical composition were identified \textit{in situ} with 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 leads to mixed-terminated films with atomic-scale structurally imperfect B-site (MnO$_{2})$ termination. A small reduction of the pressure from 50 mTorr to 20 mTorr results in a dramatic change of the atomic-scale surface structure. The surface is dominated by nearly perfectly ordered B-site termination. However, this was accompanied by surface roughening at a mesoscopic length scale with formation of mound-like structures. These results can be interpreted as a strong influence of oxygen on the adatom mobility during growth. The effect of the oxygen pressure on dopant surface segregation is also noticeable: Ca surface segregation is reduced with decrease of the oxygen pressure.