Nonvolatile Ferroelectric Memory Applications Research Articles

Sol–gel derived BST (BaxSr1−xTiO3) thin film ferroelectrics for non-volatile memory application with metal–ferroelectric–semiconductor (MFS) structure

Thin film of Barium–Strontium–Titanate (BaxSr1−xTiO3) has been fabricated through sol–gel spin-coating process with optimized value of ‘x’ for non-volatile memory application. The Metal–Ferroelectric–Semiconductor (MFS) thin-film structures have been prepared using Barium–Strontium–Titanate (BST) with various Barium/Strontium (Ba/Sr) molar concentrations. The BST films prepared through sol–gel process are annealed at 650 °C in nitrogen ambient. The structural and electrical characterizations have been carried out for each of the samples. From the characterization, it is realized that the BST films are having significant changes in its electrical and structural properties on changing the molar concentration of Barium and Strontium. It has been observed that the crystallinity of the film, refractive index, leakage current (I−V), capacitance vs. voltage (C−V) relation, and the charge vs. voltage (Q–V) relation vary with the varying values of ‘x’ in BaxSr1−xTiO3 and have been presented graphically. The maximum memory window up to 9 V with average leakage current within the range of 100 nA and minimum of 10 pA for 0 V has been achieved for the sweep voltage of + 15 V to -15 V. Memory window and leakage current are optimized and discussed. Endurance testing through PUND (positive up negative down) pulse shows that the samples can sustain up to 1010 iteration pulses. These findings indicate the use of optimized BST film for ferroelectric non-volatile memory applications through sol–gel process.

Temperature dependent ferroelectric properties of Sm and Fe substituted BaTiO3

The material series with compositional formula Ba1-xSmxTi1-xFexO3 (x = 0.0025, 0.0075, 0.01) was chosen and investigated for temperature dependent ferroelectric properties. The samples in powder form were synthesized by solid state reaction method and compacted in the form of circular discs which were sintered at 1325 °C. The ferroelectric properties were investigated by recording the PE hysteresis loops of all the samples at different temperatures (at 50 Hz). Remanent polarization ‘Pr’ and Coercive field ‘Ec’ of all samples was noted from PE loops. These studies demonstrate the potential for use in applications at elevated temperatures such as in non-volatile ferroelectric memory applications.

Ferroelectric Gd-doped ZnO nanostructures: Enhanced dielectric, ferroelectric and piezoelectric properties

Gadolinium doped ZnO nanorods were grown by wet chemical method. Line broadening of each diffraction peak was studied to evaluate the crystallite size, lattice strain, stress and energy density. Crystallite size was estimated by Scherrer and Williamson-Hall methods. The size was found to be in good agreement with the results of transmission electron microscopy. As a result of Gadolinium doping, high remnant polarization (Pr = 0.29 μC/cm2) and coercive field (Ec = 16.41 kV/cm) were observed. The decrease in leakage current due to Gadolinium doping makes ZnO a remarkable candidate for ferroelectric capacitor. High ferroelectric phase transition temperature (Tc = 215 °C) and large piezoelectric coefficient (d33 = 45.49 pm/V) were observed which makes Gadolinium doped ZnO sample useful for high temperature non-volatile ferroelectric memory applications.

A study of the temperature dependence of the local ferroelectric properties of c-axis oriented Bi6Ti3Fe2O18 Aurivillius phase thin films: Illustrating the potential of a novel lead-free perovskite material for high density memory applications

The ability to control the growth, texture and orientation of self-nanostructured lead-free Aurivillius phase thin films can in principle, greatly improve their ferroelectric properties, since in these materials the polarization direction is dependent on crystallite orientation. Here, we report the growth of c-plane oriented Bi6Ti3Fe2O18 (B6TFO) functional oxide Aurivillius phase thin films on c-plane sapphire substrates by liquid injection chemical vapour deposition (LI-CVD). Microstructural analysis reveals that B6TFO thin films annealed at 850°C are highly crystalline, well textured (Lotgering factor of 0.962) and single phase. Typical Aurivillius plate-like morphology with an average film thickness of 110nm and roughness 24nm was observed. The potential of B6TFO for use as a material in lead-free piezoelectric and ferroelectric data storage applications was explored by investigating local electromechanical (piezoelectric) and ferroelectric properties at the nano-scale. Vertical and lateral piezoresponse force microscopy (PFM) reveals stronger in-plane polarization due to the controlled growth of the a-axis oriented grains lying in the plane of the B6TFO films. Switching spectroscopy PFM (SS-PFM) hysteresis loops obtained at higher temperatures (up to 200°C) and at room temperature reveal a clear ferroelectric signature with only minor changes in piezoresponse observed with increasing temperature. Ferroelectric domain patterns were written at 200°C using PFM lithography. Hysteresis loops generated inside the poled regions at room and higher temperatures show a significant increase in piezoresponse due to alignment of the c-axis polarization components under the external electric field. No observable change in written domain patterns was observed after 20hrs of PFM scanning at 200°C, confirming that B6TFO retains polarization over this finite period of time. These studies demonstrate the potential of B6TFO thin films for use in piezoelectric applications at elevated temperatures and for use in non-volatile ferroelectric memory applications.

The Nature of Polarization Fatigue in BiFeO3

As a room-temperature multiferroic, BiFeO3 has been intensively investigated for both magnetoelectric devices and non-volatile ferroelectric memory applications. Both magnetoelectric and ferroelectric memory devices have the same control knob: polarization switching by an applied electric field. Due to the rhombohedral symmetry of BiFeO3, there are four ferroelastic variances and three different polarization switching events: (1) 71{\deg} switching from r1- to r3+, (2) 109{\deg} switching from r1- to r2+ (or r4+), and (3) 180o switching from r1- to r1+ (the superscript + and - stand for up and down polarization, respectively). Each switching path is coupled to a different reorientation of the BiFeO3 unit cell, and hence different coupling to the magnetic order as well as different magnitudes of switchable polarization. A degradation of the ferroelectric properties of BiFeO3 will result in losing controllability of magnetic order switching in magnetoelectric devices and capacity for information storage in ferroelectric memory devices. Especially, polarization fatigue will directly restrict the reliability of the actual devices. Hence it is important to understand the intrinsic fatigue behavior of each polarization switching path in BiFeO3 thin films. In this communication, we report polarization fatigue in BiFeO3 depending on switching path, and propose a fatigue model which will broaden our understanding of the fatigue phenomenon in low-symmetry materials.

Crystallinity and electrical properties of neodymium-substituted bismuth titanate thin films

We report on the properties of Nd-substituted bismuth titanate Bi 4 − x Nd x Ti 3O 12 (BNdT) thin films for ferroelectric non-volatile memory applications. The Nd-substituted bismuth titanate thin films fabricated by modified chemical solution deposition technique showed much improved properties compared to pure bismuth titanate. A pyrochlore free crystalline phase was obtained at a low annealing temperature of 640 °C and grain size was found to be considerably increased as the annealing temperature increased. The film properties were found to be strongly dependent on the Nd content and annealing temperatures. The measured dielectric constant of BNdT thin films was in the range 172–130 for Bi 4 − x Nd x Ti 3O 12 with x = 0.0–0.75. Ferroelectric properties of Nd-substituted bismuth titanate thin films were significantly improved compared to pure bismuth titanate. For example, the observed 2 P r and E c for Bi 3.25Nd 0.75Ti 3O 12, annealed at 680 °C, were 38 μC/cm 2 and 98 kV/cm, respectively. The improved microstructural and ferroelectric properties of BNdT thin films suggest their suitability for high density ferroelectric random access memory applications.

Material Design Schemes for Single-Transistor-Type Ferroelectric Memory Cells Using Pt/(Bi,La)4Ti3O12/ONO/Si Structures

We successfully fabricated metal-ferroelectric-insulator-semiconductor (MFIS) structures using Bi3.465La0.85Ti3O12 (BLT) ferroelectric thin films and SiO2/Si3N4/SiO2 (ONO) stacked buffer layers for single-transistor-type ferroelectric nonvolatile memory applications. The BLT films were deposited on the prepared Pt/Ti/SiO2/Si and ONO/Si substrates by a sol–gel spin-coating method. The dielectric constant and the leakage current density of the prepared ONO film were measured to be 5.6 and 1.0×10-9 A/cm2 at 3 MV/cm, respectively. We found that the material and electrical properties of the BLT films were effectively modulated by the crystallographic orientation control of the thin films, which were strongly affected by the first annealing process at relatively low temperature. While the fabricated MFIS capacitors using (117)-oriented BLT films showed a charge injection phenomenon in C–V properties at high operating voltage, c-axis-oriented BLT-loaded MFIS capacitors showed a memory window of 0.6 V even at a voltage sweep of ±8 V. We conclude from these results that the c-axis-oriented BLT films formed by the newly proposed fabrication method can be successfully applied to single-transistor-type ferroelectric memory cells.

Crystallographic Orientations and Electrical Properties of Bi3.47La0.85Ti3O12 Thin Films on Pt/Ti/SiO2/Si and Pt/SiO2/Si Substrates

We report on the crystallization and electrical properties of Bi3.47La0.85Ti3O12 (BLT) thin films for possible ferroelectric non-volatile memory applications. The film properties were found to be strongly dependent on process conditions especially on the intermediate heat treatment conditions. The crystallographic orientation of the films showed sharp changes at the intermediate rapid thermal annealing (RTA) temperature of 450°C. Below 450°C, BLT thin films have <117 > orientation while they have preffered c-axis orientation above 450°C. We found that RTA conditions of the first coating layer play a major role in determining the entire crystallographic orientation of the films. The films also showed of ferroelectric hysterisis behavior strongly dependent on RTA treatment. In fact, the remanent polarization of Bi3.465La0.85Ti3O12 thin films having <001> preferred crystallographic orientation showed much smaller value compared to those having <117> orientation. The BLT films also exhibited good fatigue characteristics under bipolar stress up to 1011 cycles regardless of their intermediate thermal treatments.

Na 0.5 K 0.5 NbO 3 Thin Films for MFIS_FET Type Non-Volatile Memory Applications

Na 0.5 K 0.5 NbO 3 (NKN) thin films have been prepared on Pt 80 Ir 20 , SiO 2 /Si, and Ta 2 O 5 /Si substrates for ferroelectric non-volatile memory applications. Ferroelectric hysteresis loops for Au/NKN/Pt 80 Ir 20 vertical capacitor yielded remnant polarization of 12 w C/cm 2 and coercive field ∼20 kV/cm. Significant flat-band voltage V FB shifts with buffer layer thickness in Au/NKN/SiO 2 /Si structures have been attributed to the intermixing between Na and K alkali ions and SiO 2 layer. On the other hand, Au/NKN/Ta 2 O 5 /Si structure exhibited wide memory window without significant V FB deviations, low leakage currents, and rather long retention time at zero bias.

Na 0.5 K 0.5 NbO 3 Thin Films for MFIS_FET Type Non-Volatile Memory Applications

Na 0.5 K 0.5 NbO 3 (NKN) thin films have been prepared on Pt 80 Ir 20 , SiO 2 /Si, and Ta 2 O 5 /Si substrates for ferroelectric non-volatile memory applications. Ferroelectric hysteresis loops for Au/NKN/Pt 80 Ir 20 vertical capacitor yielded remnant polarization of 12 w C/cm 2 and coercive field ∼20 kV/cm. Significant flat-band voltage V FB shifts with buffer layer thickness in Au/NKN/SiO 2 /Si structures have been attributed to the intermixing between Na and K alkali ions and SiO 2 layer. On the other hand, Au/NKN/Ta 2 O 5 /Si structure exhibited wide memory window without significant V FB deviations, low leakage currents, and rather long retention time at zero bias.

Ferroelectric properties of epitaxial barium titanate thin film capacitors on silicon substrate

A heteroepitaxial BaTiO3 film with a thickness of 65 nm was prepared on SrRuO3/Pt/TiAlN/Si. The Ti0.91Al0.09N film was grown on Si substrate by DC sputtering in Ar/N2 ambient at 700°C. The BaTiO3, SrRuO3 and Pt films were deposited by radio-frequency magnetron sputtering at substrate temperatures ranging from 450°C to 600°C. The BaTiO3 film has a c-axis 1.6% longer (0.410 nm) than that of bulk, due to lattice misfit between SrRuO3 and BiTiO3. When positive and negative voltage pulses with an amplitude of 5 V were applied, a switching charge density, Q sw, of 40 μC/cm2 was obtained. No fatigue was observed after 1011 cycles of polarization switching even when an amplitude of 7 V was applied. Remanence of polarization for 4 hours was confirmed in a retention test. These results indicate that the heteroepitaxial BaTiO3 film capacitors on Si substrates can be used for ferroelectric nonvolatile memory applications.

Growth and characterization of epitaxial SrBi2Ta2O9 films on (110) SrTiO3 substrates

SrBi2Ta2O9 (SBT) is an attractive material for nonvolatile ferroelectric memory applications. In this paper we report on the deposition of highly epitaxial and smooth SrBi2Ta2O9 films on (110) SrTiO3substrates. The films were grown by pulsed laser deposition at temperatures ranging from 600 to 800°C and at various laser fluences from a Bi-excess SBT target. The background oxygen pressure was maintained at 28 Pa during the film deposition. Structural characterization of the films was performed by x-ray diffraction. Atomic force microscopy was used to investigate morphology and growth of the films. The films grew with preferred (115) or (116) orientation. The roughness was of the order of unit cell height. The films display a growth pattern resulting in corrugated film morphology.

Effect Of The Substrate Surface Termination On The Structure Of The Bi4Ti3O12 / SrTiO3 Interface

Bismuth titanate (Bi4Ti3O12) belongs to a family of layered perovskites (Aurivillius phases) that are important for nonvolatile ferroelectric memory applications. Recently epitaxial (001) Bi4Ti3O12 thin films have been grown on (001) SrTiO3 by reactive molecular beam epitaxy (MBE) in an adsorptioncontrolled growth regime. Bi4Ti3O12 has a monoclinic layered perovskite structure at room temperature which consists of alternating Bi2O2 and Bi4Ti3O10 perovskite layers (see the upper part of Fig. 1(b)). Due to the significant difference in crystal structure between film and substrate, the surface termination of the SrTiO3 substrate and the way in which growth is initiated might be expected to have a significant influence on the interfacial structure. In this work, we present high-resolution transmission electron microscope (HRTEM) studies of the Bi4Ti3O12 / SrTiO3 interfacial structure for Bi4Ti3O12 films grown on SrTiO3 substrates terminated with either the TiO2- or SrO-layers.Bi4Ti3O12 thin films were grown on SrO- or TiO2-layer terminated homoepitaxial SrTiO3 buffer layers grown on SrTiO3 (001) substrates by reactive MBE.

Fabrication and characterization of (1− x)SrBi 2Ta 2O 9– xBi 3TaTiO 9 layered structure solid solution thin films for ferroelectric random access memory (FRAM) applications

We report on the properties of (1− x)SrBi 2Ta 2O 9– xBi 3TaTiO 9 solid solution thin films for ferroelectric non-volatile memory applications. The solid solution thin films fabricated by modified metalorganic solution deposition technique showed much improved properties compared to SrBi 2Ta 2O 9. A pyrochlore free crystalline phase was obtained at a low annealing temperature of 600°C and grain size was found to be considerably increased for the solid solution compositions. The film properties were found to be strongly dependent on the composition and annealing temperatures. The measured dielectric constant of the solid solution thin films was in the range 180–225 for films with 10–50% of Bi 3TaTiO 9 content in the solid solution. Ferroelectric properties of (1− x)SrBi 2Ta 2O 9– xBi 3TaTiO 9 thin films were significantly improved compared to SrBi 2Ta 2O 9. For example, the observed remanent polarization (2 P r) and coercive field ( E c) values for films with 0.7SrBi 2Ta 2O 9–0.3Bi 3TaTiO 9 composition, annealed at 650°C, were 12.4 μC/cm 2 and 80 kV/cm, respectively. The solid solution thin films showed less than 5% decay of the polarization charge after 10 10 switching cycles and good memory retention characteristics after about 10 6 s of memory retention. The improved microstructural and ferroelectric properties of (1− x)SrBi 2Ta 2O 9– xBi 3TaTiO 9 thin films compared to SrBi 2Ta 2O 9, especially at lower annealing temperatures, suggest their suitability for high density FRAM applications.

Modification of ferroelectricity in heteroepitaxial (Ba, Sr)TiO3 films for non-volatile memory applications

Modification of ferroelectricity in heteroepitaxial Ba x Sr1-x TiO3(BST) films by making use of lattice misfit was investigated. Theoretical calculation suggested that application of two-dimensional compressive stress of 3 GPa to the ferroelectric crystal would result in Curie temperature increase by 270°C. This theoretical prediction was experimentally confirmed. Heteroepitaxial Ba0.6Sr0.4TiO3 films grown on SrRuO3/SrTiO3 substrates exhibited ferroelectric hysteresis at 200°C, even though the inherent Curie temperature is known to be 0°C. The possibility of heteroepitaxial BST films for ferroelectric non-volatile memory applications and their advantages were pointed out.

Low temperature growth of sol-gel SrBi2Ta2O9 thin films by low-pressure annealing

A new low-temperature processing method to prepare SrBi2Ta2O9 thin films is proposed. These thin films were prepared on Pt/Ta/SiO2/Si substrates by a sol-gel method, and their structural and electrical properties were investigated. Films were annealed before and after the top Pt electrode deposition. The first annealing was performed in a 760-Torr oxygen atmosphere at 600 °C for 30 min, and the second annealing was performed in a 5-Torr oxygen atmosphere at 600 °C for 30 min. The films were well crystallized and fine-grained after the second annealing. The electrical characteristics of the 200-nm-thick film obtained by this new process were as follows: remanent polarization, Pr = 8.5 μC/cm2; coercive field, Ec = 36 kV/cm; and leakage current density, IL = 1 × 10−7 A/cm2 (at 150 kV/cm). This process is very attractive for highly integrated ferroelectric nonvolatile memory applications. © 1997 Scripta Technica, Inc. Electr Eng Jpn, 120(2): 27–33, 1997

Performance of srbi2ta2o9 for low-voltage, non-volatile memory applications

The performance of SrBi2Ta2O9 for ferroelectric non-volatile memory applications is investigated. The temperature dependence of the hysteresis loops, small signal capacitance-voltage measurements, and fatigue resistance are reported. Increasing temperature accelerates fatigue, but excellent fatigue resistance to greater than 1012 cycles is found for temperatures of 125 °C and below. The difference between current-time curves for switched and unswitched capacitors using high-speed pulse measurements indicated the availability of 5.7 μC/cm2 for 3 V memory operation.

New low temperature processing of sol-gel SrBi2Ta2O9 thin films

A new low temperature processing method to prepare SrBi2Ta2O9 thin films is proposed. These thin films were prepared on Pt/Ta/SiO2/Si substrates by a sol-gel method, and their structural and electrical properties were investigated. Films were annealed before and after the top Pt electrode deposition, respectively. The 1st annealing was performed in a 760 Torr-oxygen ambient at 600°C for 30 min; the 2nd annealing was performed in a 5 Torr-oxygen ambient at 600 °C for 30 min. The films were well crystallized and fine-grained after the 2nd annealing. The electrical characteristics of the 200-nm-thick film obtained by this new process, i.e., remanent polarization (Pr), coercive field (Ec), and the leakage current density (II), were as follows; Pr = 8.5 μC/cm2, Ec=30 kV/cm, IL=1×10−7 A/cm2 (at 3V). This new processing is very attractive for highly integrated ferroelectric nonvolatile memory applications.

New Low Temperature Processing of Sol-Gel SrBi2Ta2O9 Thin Films

A new low temperature processing method for preparation of SrBi2Ta2O9 thin films is proposed. These thin films were prepared on Pt/Ta/ SiO2/Si substrates by a sol-gel method, and their structural and electrical properties were investigated. Films were annealed before and after the top Pt electrode deposition. The 1st annealing was performed in a 760 Torr oxygen atmosphere at 600° C for 30 min, and the 2nd annealing was performed in a 5 Torr oxygen atmosphere at 600° C for 30 min. The films were well crystallized and fine grained after the 2nd annealing. The electrical properties of the 200-nm-thick film obtained using this new processing method, i.e., the remanent polarization (P r), coercive field (E c), and leakage current density (I L), were as follows: P r=8.5 µ C/cm2, E c=30 kV/cm, and I L=1×10-7 A/cm2 (at 150 kV/cm). This new processing method is very attractive for highly integrated ferroelectric nonvolatile memory applications.

Electrical properties of SrBi2Ta2O9 thin films and their temperature dependence for ferroelectric nonvolatile memory applications

This letter addresses the temperature dependence of some of the electrical properties of 0.2-μm-thick SrBi2Ta2O9 capacitors with platinum electrodes on silicon wafers for nonvolatile memory applications. Investigations of the remanent polarization, the nonvolatile polarization and the coercive field with pulse amplitude were made at different temperatures. The endurance of the polarization as a function of temperature is reported. Up to 125 °C, the capacitors show less than a 10% reduction in polarization from their initial values following 1×1012 cycles.