Improved Performance of TiO₂ based MOS Capacitor with Ultra-thin Al₂O₃ Layer

In this work, we report the improved performance of TiO₂ based MOS capacitor by inserting a thin layer of Al₂O₃ (5 nm) in between TiO₂ dielectric layer and Si substrate. Al₂O₃ is a stable dielectric material and act as an efficient barrier to reduce the down diffusion of O₂ into Si substrate and possible formation of uncontrolled SiO_x layer. This is useful in two ways, first it will provide a large conduction and valence band offset to Si, and second TiO₂ stoichiometry will be maintained. The dielectric constant of such bilayer MOS gate stack was estimated 31 from accumulation capacitance with improved leakage performance. The leakage current density was found 1.69×10^-8 A/cm² at + 1 V. The interface states density were extracted from DLTS method and found 2.9x10¹¹ eV^-1cm^-2, which is reasonably low for Si/high-k system. In addition to that, various other parameters such as flat band voltage (V_FB), change in flat band voltage during bidirectional sweep (△V_FB), oxide trapped charges (Q_ot), and equivalent oxide thickness (EOT) are found -0.48 V, 50 mV, 2.97x10¹¹/cm², and 5.66 nm, respectively. In conclusion, bilayer gate stacks based on TiO₂ and Al₂O₃ can be useful in future CMOS technology.