Photoemission studies of the interface formation of ultrathin MgO dielectric layers on the oxidised Si(111) surface

Magnesium oxide (MgO) has been proposed as a medium-k dielectric material for integration into advanced transistor fabrication. In this study, soft x-ray synchrotron radiation based photoemission and conventional x-ray photoelectron spectroscopy have been used to characterise the evolution of the MgO/Si(111) interface as an ultrathin film is been grown. The MgO film was grown by thermally depositing magnesium in an oxygen partial pressure at room temperature. The Mg2p peak profile indicates that the magnesium exists in a single chemical state throughout the deposition sequence with a binding energy indicative of MgO. The initial submonolayer deposition resulted in the Si2p peak shifting 0.3eV towards higher binding energy consistent with electron transfer between the deposited magnesium and the substrate. This is accompanied by an increase in the intensity of the Si oxidation states indicating growth of the interfacial oxide thickness. As the MgO film thickness increases, the Fermi level moves back to within 0.1eV of its original midgap position. Subsequent XPS measurements show that the saturation thickness of the interfacial silicon oxide layer is less than 0.7nm. The valence band offset has been measured and the conduction band offset has been deduced by estimating the bandgap of the thin dielectric film. The valence band offset between the thin MgO layer and the silicon substrate is 4.1eV. The MgO workfunction was 2.8eV and assuming an electron affinity value of 0.85eV, the bandgap of this film is 6.72eV, smaller than that reported for bulk MgO. The calculated conduction band offset is 1.5eV which should be a sufficiently high barrier to minimise leakage currents.