Effect of lanthanum doping on the electrical properties of sol-gel derived ferroelectric lead-zirconate-titanate for ultra-large-scale integration dynamic random acccess memory applications
- Univ. of Texas, Austin, TX (United States); and others
Thin films of lead-zirconate-titanate [(PZT) PbZr{sub 0.5}Ti{sub 0.5})O{sub 3}] 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{sup -6} A/cm{sup 2}). 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 (P{sub max}, bit {open_quotes}1{close_quotes}) and remanent polarization (P{sub r}, bit {open_quotes}0{close_quotes} is denoted as Q{prime}{sub c}, and is the relevant charge storage density in the DRAM cell. Q{prime}{sub c} 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 (J{sub L}) in comparison with undoped PZT, and after refresh annealing (at 450{degrees}C for 1/2 h in O{sub 2}), the film exhibits a Q{prime}{sub c} value of 7.2 {mu}C/cm{sup 2} at 3 V, which is believed to satisfy the 64 Mb DRAM requirements. J{sub L} 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{sub (1-1.5x)}La{sub x}V{sub x/2}(Zr{sub 0.5}Ti{sub 0.5})O{sub 3}, (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. 25 refs., 15 figs.
- OSTI ID:
- 161696
- Report Number(s):
- CONF-930115--
- Journal Information:
- Journal of Vacuum Science and Technology. B, Microelectronics Processing and Phenomena, Journal Name: Journal of Vacuum Science and Technology. B, Microelectronics Processing and Phenomena Journal Issue: 4 Vol. 11; ISSN JVTBD9; ISSN 0734-211X
- Country of Publication:
- United States
- Language:
- English
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