Imaging insulating oxides: scanning tunneling microscopy of ultrathin MgO films on Mo(001)

Abstract

Using scanning tunneling microscopy (STM) we have studied the structure of an insulating thin film, MgO grown on Mo(001). Although the bandgap in bulk MgO is 7.8 eV, the films are sufficiently conducting to perform STM. Stable tunneling and imaging can be obtained for MgO films up to 25 Å thick. Growth at room temperature produces uniform films with small domains of between 20 and 60 Å in diameter. The domains have random shapes with the perimeter of the domains exhibiting no preferred orientation. Films as thick as 8 layers typically have 3 atomic layers exposed. Films grown at temperatures in excess of 1000 K exhibited three-dimensional MgO islands with exposed substrate. Close inspection of these films determined that at voltages as low as 2.5 V and for films as thick as 5 ML the STM directly measures oxide film morphology.