MnO(001) thin films on MgO(001) grown by reactive MBE using supersonic molecular beams

MnO(001) thin films were grown on commercial MgO(001) substrates at 520 degrees C by reactive molecular beam epitaxy (MBE) using Mn vapor and O-2-seeded supersonic molecular beams (SMBs) both with and without radio frequency (RF) plasma excitation. For comparison, MnO(001) films were grown by reactive MBE using O-2 from a leak valve. X-ray photoelectron spectroscopy confirmed the Mn2+ oxidation state and 10%-15% excess oxygen near the growth surface. Reflection high-energy electron diffraction and x-ray diffraction evidenced that the films were rock salt cubic MnO with very strong (001) orientation. High-angle annular dark field scanning transmission electron microscopy with energy-dispersive x-ray spectroscopy demonstrated abrupt MnO/MgO interfaces and indicated [(001)(MnO)||(001)(MgO)] epitaxial growth. Ex situ atomic force microscopy of films deposited without RF excitation revealed smooth growth surfaces. An SMB-grown MnO(001) film was converted to Mn3O4 with strong (110) orientation by post-growth exposure to an RF-discharge (RFD) SMB source providing O atoms; the surface of the resultant film contained elongated pits aligned with the MgO110 directions. In contrast, using the RFD-SMB source for growth resulted in MnO(001) films with elongated growth pits and square pyramidal hillocks aligned along the MgO(110) and (100) directions, respectively.