J. F. Scott

Quantitative measurement of space‐charge effects in lead zirconate‐titanate memories

By combining Auger data on the width of an oxygen depletion layer near the Pt electrodes with a modified Langmuir–Child law for the leakage current: I(V) = aV + bV2, we deduce parameters related to the space‐charge density and field in 210‐nm‐thick PbZr1−xTixO3 memories. The results are compared with the space charge fields inferred by Okazaki (∼10 kV/cm for PZT), which involve measuring the switching speeds ts(E) for positive and negative voltages. Differences in the voltage dependencies of the leakage current are found after fatigue and are related to specific electrochemical processes involving oxygen deposition on electrode surfaces.

FATIGUE AND SWITCHING IN FERROELECTRIC MEMORIES : THEORY AND EXPERIMENT

A theoretical model of fatigue in ferroelectric thin‐film memories based upon impact ionization (e.g., Ti+4 to Ti+3 conversion in PbZr1−xTixO3), resulting in dendritic growth of oxygen‐deficient filaments, is presented. The predictions of spontaneous polarization versus switching cycles Ps(N) are compared with both Monte Carlo simulations for a two‐dimensional Ising model and with experimental data on small‐grain (40 nm) sol‐gel PZT films. Excellent agreement between theory and experiment is obtained. In addition to modeling the Ps(N) curves, the theory developed explains the observed linear proportionality between switching time ts(N) and polarization Ps(N) during fatigue; other models of aging do not account for this. Earlier theories of switching are also extended to include finite grain sizes, surface nucleation, triangular drive pulses, and dipolar forces. Good agreement with sol‐gel PZT switching data is obtained.

Switching kinetics of lead zirconate titanate submicron thin‐film memories

We have measured coercive field and switching voltage versus thickness in PbZr0.54Ti0.46O3 thin (0.15–0.50 μm) films, together with switching times and current transient shapes versus field and temperature. The results show activation fields of order 120 kV/cm at room temperature, threshold voltages below 1.3 V, and switching speeds faster than 100 ns, demonstrating that fast, nonvolatile memories can be constructed that are compatible with standard silicon or GaAs integrated circuit voltage levels, without the need for an internal voltage pump. The displacement current transient data yield 2.5 as the dimensionality of domain growth if one‐step intial nucleation rate is assumed, and are compatible with the theory of Ishibashi, yielding imaxtmax/Ps=1.65±0.23, in comparison with the predicted 1.646. The switching time exhibits an activation field dependence upon both voltage and temperature through a single reduced parameter (TC−T)(VTC),−1 in accord with the theory of Orihara and Ishibashi.

Radiation effects on ferroelectric thin‐film memories: Retention failure mechanisms

Ferroelectric thin‐film (200–350 nm) memories were fabricated and tested. Two materials were used as memory cells: potassium nitrate (KNO3) and lead zirconate titanate (PbZr0.54Ti0.46O3, usually abbreviated as PZT). These devices were tested as arrays deposited either by thermal evaporation (in the case of KNO3) or sputtered films (PZT). Fully packaged devices were tested to determine switching speed and polarization (switched charge) as functions of temperature and applied voltage. Radiation hardness was also tested for both dose rate and total dose. The switching kinetics were investigated in considerable detail and found to confirm the theory of Ishibashi. The dimensionality of domain growth for the switching process in KNO3 lowers from 2.3 before irradiation to 1.6 after 0.5 Mrad. For PZT rad hardness exceeds 5 Mrad total dose and 2×1011 rad/s. A surprising result was that the hysteresis curves for all of the PZT samples became more symmetric after 5 Mrad irradiation; this is interpreted as a destruct...

Structure and Device Characteristics of SrBi 2 Ta 2 O 9 -Based Nonvolatile Random-Access Memories

Recently there has been a paradigm shift in nonvolatile computer memories from silicon-technology-based EEPROMs (electrically erasable, programmable read-only memories) to devices in which the stored information is coded into + and − polarizations in thin-film ferroelectric capacitors. Such devices have read and erase/rewrite speeds of the order of 1–35 ns, many orders of magnitude faster than the erase/rewrite speeds of the best EEPROMs (Table I). However, fundamental questions concerning their lifetimes had delayed full commercialization. Because ferroelectrics normally have extremely large dielectric constants, their use as nonswitching capacitors in dynamic random-access memories (DRAMs) is also rapidly evolving. The majority of studies to date have emphasized lead zirconate titanate (PZT)-based capacitors for nonvolatile ferroelectric random-access memories (NVFRAMs) and barium strontium titanate-based capacitor DRAMs (see Table II).

Switching kinetics in KNO3 ferroelectric thin-film memories

The time dependence of the current transient i(t) produced by the reversal of domains in ferroelectric potassium nitrate thin‐film memories of 75–300 nm is analyzed as a function of temperature and of thickness using the Avrami theory. For all the films the kinetics confirm the low‐dimensional nature of the system

Raman spectroscopy of Na0.5Bi0.5TiO3

Abstract We have measured the Raman spectra of bismuth sodium titanate in its rhombohedral, tetragonal, and cubic phases, with special attention paid to the phase transitions at 584K and 837K (heating). Both transitions appear to be order-disorder and strongly first order, with large thermal hysteresis. The phonon spectra at temperature slightly below the tetragonal phase are remarkably similar to BaTiO3 with A1(To) modes at 130, 269, and 541 cm−1 (compared with 170, 270, 520 cm−1 in BaTiO3) and an E(TO) at 52 cm−1.

Process optimization and characterization of device worthy sol-gel based PZT for ferroelectric memories

The purpose of this work is to develop and characterize an optimized sol-gel PZT process to be used in ferroelectric memories. A review of the sol-gel process is given, including discussions on hydrolysis under acidic and basic conditions. Application of the sol-gel process to thin films is then discussed. Topics such as removal of solvents, stresses in the thin film and how this relates to cracking are mentioned. A review of different synthesis methods of sol-gel PZT is then conducted in order to help determine a device worthy sol-gel PZT process. Methods of controlling the pore size by hydrolysis and different heat treatments at various stages during the drying and annealing cycles are then used. The quality of thin film PZT on Pt is characterized by using dispersive X-ray and X-ray diffraction analyses. Electrical results yield Pr ranging from 7.9-21.9 μC/cm2Ec ranging from 29.1 -92.3 kV/cm and switching times as fast as 56 ns, using a capacitor area of 1 × 104 μm2. However, these results depend on the...

Negative differential resistivity in ferroelectric thin-film current-voltage relationships

Abstract The current-voltage relationship I(V) for several different ferroelectric thin films often exhibits an apparent negative differential resistivity. In the present paper we show that in lead zirconate-titanate (PZT) this is a spurious artifact arising from rapid (parametric) measurement techniques and that it disappears for d.c. measurements made with measuring times ≥300 s. However, in other ferroelectric materials (selected samples with unusually high trap densities) it appears to be a genuine effect characterized by an I = aV3 current-voltage relationship at high current densities (≥100 nA/cm2), I(V) hysteresis above a “pseudo-breakdown voltage,” and distinct V 2-to-V 3 cross-over threshold.

Activation field, fatigue, and waiting‐time effects in KNO3 thin‐film memories

By analyzing current transient data i(t) from KNO3 thin‐film memories in terms of the Ishibashi–Takagi parametrization of the Avrami theory, we have been able to extract detailed information concerning the presence of an activation field (and the absence of a threshold), the relationship between characteristic switching time and spontaneous polarization when both are decreased either by increasing temperature or by repetitive read‐write cycles (fatigue), and the behavior of switching kinetics as a function of the delay after application of a dc bias field (‘‘waiting‐time’’).