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 ∼5 × 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.