Cathodoluminescence of near-surface centres in Cr-doped MgO(001) thin films probed by scanning tunnelling microscopy

Crystalline MgO films with a defined level of Cr dopants (MgOCr) are prepared by either subsequent or simultaneous deposition of Cr and Mg atoms in an oxygen ambience onto a Mo(001) support. The structural and morphological parameters of the doped films are investigated using low-energy electron diffraction and scanning tunnelling microscopy (STM). Whereas at low Cr concentration the doped oxide has similar properties to pristine MgO(001), a new Cr/Mg mixed oxide develops at higher Cr load. The nature of the Cr impurities in the MgO matrix is deduced from cathodoluminescence spectroscopy performed by electron injection from the STM tip into the oxide film. In agreement with earlier studies on MgOCr bulk crystals, the majority of Cr adopts a 3 + charge state and occupies Mg substitutional sites. The dopants give rise to sharp emission lines at about 700 nm, arising from radiative electron transitions between the Cr 3d levels split in the MgO crystal field. From the spectral evolution upon annealing the MgOCr films, we deduce a strong tendency of the dopants to accumulate in a near-surface region. Our experiments demonstrate that high-quality MgOCr films with similar optical properties as bulk samples can be prepared on conductive supports, being a first step to make doped oxide materials accessible to surface science studies. (paper)