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.