A. I. Kingon

Leakage currents in Ba0.7Sr0.3TiO3 thin films for ultrahigh-density dynamic random access memories

(Ba,Sr)TiO3 (BST) thin films grown by chemical vapor deposition and with platinum (Pt) top and bottom electrodes have been characterized with respect to the leakage current as a function of temperature and applied voltage. The data can be interpreted via a thermionic emission model. The Schottky approximation accounts for superohmic behavior at higher fields, but the barrier lowering is stronger than expected from this theory. While the leakage mechanism is comparable to SrTiO3 thin films prepared by chemical solution deposition, the absolute values of the leakage current are significantly lower for the metalorganic chemical vapor deposition (MOCVD) prepared BST film. This is presumably due to a more homogeneous microstructure of the latter and may also be due to different electrode processing. The influence of the film thickness on the leakage in combination with additional findings is used to discuss the field distribution in the films under a dc voltage stress.

High temperature stability in lanthanum and zirconia-based gate dielectrics

Gate dielectrics composed primarily of lanthana and zirconia were prepared by reactive evaporation. The stability of the layers during high temperature anneals was investigated. By controlling the oxygen partial pressure during heat treatment, lanthana and zirconia films could be protected against reaction with the underlying Si substrate and against the growth of low-e interface layers. The electrical thickness of the dielectrics could be maintained after a 900u200a°C exposure. The critical oxygen pressure at 900u200a°C for low-e interface formation beneath ZrO2 and La2O3 dielectrics was ∼2e−4 Torr. The interfaces that formed beneath the ZrO2 and La2O3 layers are distinctly different. The sub-ZrO2 interface, influenced primarily by phase separation, tends towards pure SiO2, while the sub-La2O3 interface, influenced primarily by silicate formation, tends towards a La–Si–O alloy. For both materials, reducing the oxygen pressure to values below 10−7 Torr resulted in rapid degradation of the metal oxide. This dielec...

Pulsed laser ablation synthesis and characterization of layered Pt/SrBi2Ta2O9/Pt ferroelectric capacitors with practically no polarization fatigue

Pulsed laser ablation deposition was used to synthesize polycrystalline SrBi2Ta2O9 layered ferroelectric thin films on platinized silicon substrates. Top electrodes were produced by dc magnetron sputter deposition to fabricate capacitors for electrical tests. The polarization electric field hysteresis loops showed saturation in the 2–4 V range with a coercive field of 25 kV/cm. The capacitors showed practically no polarization fatigue up to 1011 switching cycles. The resistivity of the SrBi2Ta2O9 for a coercive field of 100 kV/cm was approximately 2×1011 Ωu2009cm. Retention and imprint characteristics of these capacitors showed no degradation as a function of cumulative waiting times.

Polycrystalline La0.5Sr0.5CoO3/PbZr0.53Ti0.47O3/ La0.5Sr0.5CoO3 ferroelectric capacitors on platinized silicon with no polarization fatigue

Pulsed laser ablation‐deposition is used to produce fatigue‐free La0.5Sr0.5CoO3(LSC)/ PbZr0.53Ti0.47O3 (PZT)/LSC ferroelectric capacitors on oxidized (100) Si substrates coated with a bilayer of Pt/Ti. These capacitors utilize a unique bottom electrode combination of LSC on Pt, where the LSC (a conducting oxide) acts as a template to promote the ferroelectric perovskite phase of PZT and to minimize polarization fatigue, while Pt is used for its high electrical conductivity and high temperature stability. We have used the hybrid Pt/LSC electrode discussed in this letter to integrate PZT‐based capacitors with Si substrates. X‐ray diffraction analysis shows that the PZT film is polycrystalline and is entirely perovskite phase. Devices show no significant degradation of the switchable polarization after 3×1010 switching cycles. Aging tests show that the rate of loss of switchable polarization may allow useful memory retention for times up to 1010 s.

Influence of platinum interlayers on the electrical properties of RuO2/Pb(Zr0.53Ti0.47)O3/RuO2 capacitor heterostructures

Lead zirconate titanate, Pb(ZrxTi1−x)O3 or PZT, thin films grown on RuO2 electrodes by the sol‐gel process have excellent resistance to polarization fatigue, but they generally have two drawbacks. The films have high leakage currents and large property variation. In this letter we show that the use of a thin Pt layer (100 A) deposited on the bottom RuO2 electrode to fabricate RuO2/PZT/Pt/RuO2/(100)MgO capacitors has two important effects. It reduces capacitor leakage by two to four orders of magnitude and it significantly reduces the large property variation. In addition, these capacitors retain their excellent resistance to polarization fatigue which is characteristic of the RuO2/PZT/RuO2 heterostructure.

Investigation of the ablated flux characteristics during pulsed laser ablation deposition of multicomponent oxides

The ablated flux characteristics of PbZr0.52Ti0.48O3 (PZT), La0.5Sr0.5CoO3 (LSC), and MgO ceramic targets have been studied as functions of the ablation time, the ablation energy, and the chamber gas pressure. The time dependence of the ablation rate shows an initial exponential decay, reaching a steady‐state value at longer times. The energy dependence of the ablation rate (in vacuum) reveals a distinct ablation threshold energy for MgO ablation, while for PZT and LSC no ablation threshold is evident. The differences in the ablation characteristics of these materials are explained mainly by differences in their melting points, thermal conductivities, and absorption coefficients. Upon adding O2 gas, a visual change in the color and shape of the PZT ablation plume is evident. The color change indicates a gas phase reaction of the ablated species with the O2 gas, while the shape change implies a change in the angular distribution of the ablated species. We have measured a narrowing of the ablated flux distr...

Crystallization in SiO2–metal Oxide Alloys

HfO 2 -SiO 2 and La 2 O 3 -SiO 2 amorphous alloys were prepared, and their crystallization behavior was studied. The results suggest that higher permittivities can be achieved in the La-containing system without devitrification. The crystallization mechanisms between systems are distinctly different, yet observations are consistent with bulk material. Hf-containing materials tend toward phase separation, while La-containing materials tend toward silicate formation. For Hf-containing films, negligible thickness or time dependence was observed. In La-containing films, rapid thermal anneals could improve crystallization resistance, and thickness effects related to interface reactions were observed. These behaviors are discussed in the context of phase diagrams and metastable immiscibility.

Characterization of conduction in PZT thin films produced by laser ablation deposition

Abstract Both direct current (d.c.) and alternating current (a.c.) conductivity measurements were undertaken on lead zirconate titanate (PZT) films synthesized by laser ablation deposition. Direct current (I) displayed an initial time dependence of the form I ∝ t−γ (γ ∝ 0.5–1.0). The possible reasons for this time dependence are discussed. At lower temperatures, the a.c. electrical conductivity shows a frequency dependence of the form σ ∝ ω′ which is explained as electrical charge hopping. At higher temperatures, the d.c. component of electrical conductivity becomes dominant, and is accompanied by a strong low frequency dispersion of the dielectric constant. The results are compared to published data on conductivity in SrTiO3, and discussed in terms of the latest theories for dielectric response of materials.

Studies of hydrogen-induced degradation processes in SrBi2Ta2O9 ferroelectric film-based capacitors

It is known that the forming gas (N2–H2 mixture) annealing process required for microcircuit fabrication results in an unacceptable electrical degradation of SrBi2Ta2O9 (SBT) ferroelectric capacitors due mainly to the interaction of H2 with the ferroelectric layer of the capacitor. We have found a strong relationship between changes in the surface composition of the ferroelectric layer and the electrical properties of SBT capacitors as a result of hydrogen annealing. Mass spectroscopy of recoiled ions (MSRI) analysis revealed a strong reduction in the Bi signal as a function of exposure to hydrogen at high temperatures (∼500u200a°C). The Bi signal reduction correlates with Bi depletion in the SBT surface region. Subsequent annealing in oxygen at temperatures in the range of 700–800u200a°C resulted in the recovery of the MSRI Bi signal, corresponding to the replenishment of Bi in the previously Bi-depleted surface region. X-ray diffraction (XRD) analysis (probing the whole SBT film thickness) showed little differe...

Synthesis and characterization of Pb (ZrxTi1−x)O3 thin films produced by an automated laser ablation deposition technique

Ferroelectric lead zirconium titanate [Pb (ZrxTi1−x)O3] (PZT) thin films have been synthesized by using an automated laser ablation deposition technique with a capability for layer‐by‐layer or simultaneous deposition of elemental film constituents. The technique is suitable for producing multicomponent and/or multilayered thin films with controlled stoichiometry, such as high‐temperature superconductor, ferroelectric, and electro‐optic thin films. PZT films were synthesized on MgO (100) by either sequential deposition of layers of ZrO2, TiO2, and PbO, produced by laser ablation of ZrO2, TiO2, and PbO targets, or by simultaneous deposition of all species from ablation of stoichiometric or PbO‐rich PZT targets. Films were deposited at 200u2009°C and subsequently annealed at 600u2009°C for different periods of time. The orientation, microstructure, surface topography, and composition of the films were characterized by x‐ray diffraction, transmission electron microscopy, scanning electron microscopy, and Rutherford b...