Effect of lanthanum doping on the electrical properties of sol-gel derived ferroelectric lead–zirconate–titanate for ultra-large-scale integration dynamic random access memory applications

Abstract

Thin films of lead–zirconate–titanate [(PZT) Pb(Zr0.5Ti0.5)O3] possess demonstrably adequate charge storage densities and endurance to read/write cycling for ultra-large-scale integration dynamic random access memory (DRAM) applications. Lanthanum (donor) doping is expected to reduce the (p-type) conductivity to acceptable levels (<10−6 A/cm2). In this study, six thin films of 200 nm sol-gel derived lanthanum-doped PZT, with the [La]/([La]+[Pb]) concentration ratio varying from 0 to 0.23, have been examined for electrical and reliability properties. The difference between the maximum polarization attained (Pmax, bit ‘‘1’’) and remanent polarization (Pr, bit ‘‘0’’) is denoted as Qc′, and is the relevant charge storage density in the DRAM cell. Qc′ is seen to decrease with an increase in La concentration, contrary to predictions based on an assumption of stoichiometric compositions. A 5% La content results in more than a decade drop in leakage current density (JL) in comparison with undoped PZT, and after refresh annealing (at 450 °C for 1/2 h in O2), the film exhibits a Qc′ value of 7.2 μC/cm2 at 3 V, which is believed to satisfy the 64 Mb DRAM requirements. JL increases almost exponentially with the La concentration beyond 5%, indicating that the increasing volatility of Pb in the oxygen-rich ambient overwhelms the effect of the enhanced La doping concentration, probably due to a departure from the equilibrium composition Pb(1−1.5x)LaxVx/2(Zr0.5Ti0.5)O3, (where V represents a Pb vacancy). Constant voltage stressing indicates an operating lifetime (at room temperature) of about ten years (at 3 V) for the 5/50/50 film. Good fatigue properties are also observed.