Characterization of Bi and Fe co-doped PZT capacitors for FeRAM

Abstract

Ferroelectric random access memory (FeRAM) has been in mass production for over 15 years. Higher polarization ferroelectric materials are needed for future devices which can operate above about 100 °C. With this goal in mind, co-doping of thin Pb(Zr₄₀,Ti₆₀)O₃ (PZT) films with 1 at.% Bi and 1 at.% Fe was examined in order to enhance the ferroelectric properties as well as characterize the doped material. The XRD patterns of PZT-5% BiFeO₃ (BF) and PZT 140-nm thick films showed (111) orientation on (111) platinized Si wafers and a 30 °C increase in the tetragonal to cubic phase transition temperature, often called the Curie temperature, from 350 to 380 °C with co-doping, indicating that Bi and Fe are substituting into the PZT lattice. Raman spectra revealed decreased band intensity with Bi and Fe co-doping of PZT compared to PZT. Polarization hysteresis loops show similar values of remanent polarization, but square-shaped voltage pulse-measured net polarization values of PZT-BF were higher and showed higher endurance to repeated cycling up to 10¹⁰ cycles. It is proposed that Bi and Fe are both in the +3 oxidation state and substituting into the perovskite A and B sites, respectively. Substitution of Bi and Fe into the PZT lattice likely creates defect dipoles, which increase the net polarization when measured by the short voltage pulse positive-up-negative-down (PUND) method.

Figure 1

Shadow mask patterned IrO₂/SrRuO₃ top electrodes on (a) BF-doped PZT and (b) on PZT with an average top electrode diameter of 60 μm.

Figure 2

(a) FE-SEM images of cross-sections of PZT-BF film and (b) PZT film on Pt/IrO_x coated SiO₂/Si substrate.

Figure 3

Temperature dependence of tetragonal c and a lattice parameters toward cubic phase transition, (a) PZT film, (b) PZT-BF film and (c) c/a ratio for the PZT and PZT-BF films.

Figure 4

(a) Hysteresis loops of PZT-BF capacitor as a function of annealing temperature measured using 4 V triangular voltage profiles at 1 kHz, and (b) net polarization measured using the PUND method and square 50 μs voltage pulses as a function of voltage and annealing temperature.

Figure 5

Raman spectra of the PZT and PZT-BF powders measured at room temperature.

Figure 6

SIMS elemental depth profiles of Pb, Ti, Fe, O, Sr and Ru in the IrO_x/SRO/PZT-BF/Pt capacitor.

Figure 7

Net polarization of the PZT and PZT-BF as a function of the number of switching cycles at ±3 V and 10 MHz, where the net polarization (or sometimes called Q_sw) was measured at 3 V using 100 ns pulses and the PUND method.