Simultaneous improvement of the dielectric constant and leakage currents of ZrO2 dielectrics by incorporating a highly valent Ta5+ element

Abstract

Dynamic random access memory (DRAM) is reaching the scaling limit owing to the requirements for a high capacitance density and low leakage current of metal–insulator–metal (MIM) capacitors. We investigated the Ta-doped ZrO₂ dielectric as a novel high-k candidate, utilizing the precise control of Ta-doping concentration using the atomic layer deposition (ALD) supercycle process. We systematically studied the chemical composition and crystal structure of ALD Ta-doped ZrO₂ and its effects on the electrical properties of MIM capacitors. It was shown that ZrO₂ becomes more stoichiometric with the introduction of Ta, which is attributed to the suppression of oxygen vacancy (V_O) formation. The change in the atomic arrangement due to the substitution of Zr with Ta and the reduction of V_O enhances the crystallinity of the cubic phase and causes a decrease in the molar volume of the ZrO₂ films. As a result of the change in the crystal structure along with the high dielectric polarizability of Ta, the dielectric constant of ALD Ta-doped ZrO₂ increases by up to 80% compared to that of undoped ZrO₂ films. Moreover, the reduction of the V_O species suppresses the emission of the carriers, which lowers the leakage current density by two orders of magnitude in the Ta-doped ZrO₂ films as compared to that in the undoped ZrO₂ films. In general, the dielectric constant and leakage currents have a trade-off relationship in a single high-k dielectric system. Consequently, proper doping of Ta into ZrO₂ using ALD is a promising solution to overcome the technical limits of conventional high-k dielectrics.