Time-zero current-voltage characteristics and time-dependent current behavior of metal-ferroelectric-metal (Pt-PZT-Pt) capacitor structures have been studied. Under constant-voltage stressing, the current density through the 1500-Å-thick lead-zirconate-titanate (PZT) film exhibits a power-law dependence on time, with the exponent (∼0.33) independent of temperature and voltage. Electrode material dependence of current density indicates that the conventional model of trap-limited single-carrier injection over nonblocking contacts is inadequate to explain the time-zero current. A change in top electrode material from Pt to In leads to the observation of work-function-driven Schottky contacts between the metal and ferroelectric. The current-voltage characteristics fit a two-carrier injection metal-semiconductor-metal model incorporating blocking contacts, with distinct low- and high-current regimes (PZT is assumed to be p-type and trap-free in this model). Temperature-dependent I-V measurements indicate a Pt-PZT barrier height of 0.6 eV and an acceptor doping level of ∼1018 cm−3 in PZT. The implications of this model on the optimization of ferroelectric capacitors for dynamic random access memory applications are discussed.

The electrical and reliability characteristics of ferroelectric capacitors fabricated using sol-gel derived 50/50 lead-zirconate-titanate (PZT) thin films have been examined for ULSI DRAM (dynamic random access memory) applications. Various electrode materials, film thicknesses (200 nm to 600 nm) and capacitor areas were used. A large stored-energy density (Q(c)) of 15 muC/cm(2) (at 125 kV/cm) was measured using different methods. The results indicate that PZT thin films exhibit material properties which might satisfy the requirements of ULSI DRAMs.

The fabrication of high dielectric constant lead zirconate titanate thin films for dynamic random access memory application involves post-deposition annealing to convert the low dielectric constant pyrochlore phase to the desired high dielectric constant perovskite phase. The results of our experiments detailing the dielectric characteristics such as degradation field (the electric field at which a sudden increase in leakage takes place), leakage current density, time-dependent dielectric breakdown characteristics, charge storage density, coercive field, and charge storage density are reported. The charge storage density was measured using both double-bipolar pulses and quasistatic capacitance-voltage techniques. The results of these two measurement techniques are discussed for oxygen and nitrogen annealed films. Additionally, a comparative study of the phase transformation kinetics when the post-deposition annealing is carried out in a nonoxidizing ambient such as nitrogen, as against an oxidizing ambient (pure oxygen), is detailed.

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.

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