Divergent dielectric characteristics in cascaded high-K gate stacks with reverse gradient bandgap structures

The characteristics of cascaded high-K gate stacks with reverse dielectric sequence, TiO2/ZrO2/Al2O3 and Al2O3/ZrO2/TiO2, on the Si substrate were investigated. The reverse sequence with different gradient bandgap structure gives rise to distinct conduction pathways, resulting in significant divergence of the leakage current density (J(g)) and the capacitance equivalent thickness (CET). The trapping sites in the high-permittivity TiO2 layer dominate the leakage current paths and strongly impact the conductance and the capacitance of the cascaded high-K gate stacks. Thus, a low CET of 1.05 nm and a low J(g) of similar to 5 x 10(-4) A cm(-2) were achieved due to effective suppression of the leakage current through the traps of TiO2 in the cascaded TiO2/ZrO2/Al2O3 gate stack. In addition, the TiO2 layer gets crystallized in the cascaded TiO2/ZrO2/Al2O3 structure to achieve a higher capacitance because of the intermixing between TiO2 and ZrO2 due to the different reactivity of the precursors for Ti and Zr. This study demonstrates a way to effectively incorporate the high permittivity and low-bandgap materials, such as TiO2, into high-K gate stacks, to further improve device scaling.