Conduction mechanisms of silicon oxide/titanium oxide MOS stack structures

During the last years, high-k dielectrics have been studied intensively looking for an alternative material to replace the SiO2 films as gate dielectric in MOS transistors. Different materials and structures have been proposed. An important concern not yet solved, is the interfacial quality between high-k materials and silicon substrate. For this reason, stack structures with SiO2 as an interfacial layer between silicon substrate and high-k film have been studied. In this contribution we analyze the main conduction mechanism observed in SiO2/TiO2 MOS stack structures obtained by room temperature plasma oxidation in different conditions and reactors. Films fabricated in a parallel-plate type reactor showed better quality with low current density where thermionic conduction mechanism is predominant. In lower quality films, for example those fabricated in a barrel type equipment, the current density is higher and the conduction mechanism observed is Poole–Frenkel. Finally we show that the presence of thermionic mechanism provides a weak thickness dependence and a strong current density reduction with respect to silicon oxide MOS structures with the same equivalent oxide thickness.