Evaluating gallium-doped ZnO top electrode thickness for achieving a good switch-ability in ZnO2/ZnO bilayer transparent valence change memory

Gallium doped ZnO (GZO) top electrode thickness dependence of resistive switching characteristic of GZO/ZnO₂/ZnO/ITO transparent valence change memory device is investigated. The thickness of the GZO top electrode modulates the resistance of the pristine device. Devices made with thicker GZO layer have higher leakage current; thus, require higher current compliance. An excessively high current compliance leads to a device breakdown upon reset process. Conversely, a very low current compliance may form a tiny conducting filament and is difficult to rejuvenate after the rupture; thus, its cycle-to-cycle characteristic shows a decaying behavior. Nevertheless, transparent valence change devices with a stable endurance and sufficient memory window that operate at a moderate level of current compliance are successfully fabricated by employing an appropriate thickness of the top electrode. We suggest that a good switch-ability of transparent valence change memory devices are strongly affected by the thickness of the top electrode.

     

Resistive Switching and Current Conduction Mechanisms in Amorphous LaLuO3 Thin Films Grown by Pulsed Laser Deposition

The unipolar resistive switching characteristics of the amorphous LaLuO₃ thin films deposited by pulsed laser deposition have been studied. Reliable and repeatable nonvolatile switching of the resistance of LaLuO₃ films was obtained between two well defined states of low and high resistance with nearly constant resistance ratio ～10⁷ and non-overlapping switching voltages in the range of 0.66-0.83 V and 1.9-2.7 V respectively. The temperature dependent measurement revealed metallic and semiconducting behavior in low and high resistance states respectively. The switching between low and high resistance states was attributed to the change in the separation between oxygen vacancies in light of the correlated barrier hopping theory. The current conduction mechanism of the device in high-resistance state followed the Poole's law, whereas the conduction in low-resistance state was found to be dominated by percolation. The resistance of low and high resistance states of the film showed no obvious degradation for up to ～10⁴ seconds indicating good retention. The achieved characteristics of the resistive switching in LaLuO₃ thin films seem to be promising for futuristic nonvolatile memory applications.     

The dependence of ferroelectric and fatigue behaviors of PZT films on annealing conditions

Films of PZT about 0.2 μm thick with the composition PbZr_0.53Ti_0.47O₃ were prepared using the metalloorganic decomposition (MOD) process. The amorphous films produced by pyrolysis at 350°C were annealed at 550, 575, 600 and 650°C for 10 minutes, 1 hour or 4 hours. Films annealed at temperatures below 550°C showed no ferroelectric behavior while others annealed above 650°C showed signs of loss of ferroelectric behavior. Most films demonstrated satisfactory ferroelectric properties such as low switching voltage and high polarization values. Some PZT films also demonstrated fatigue life-time of more than 10⁹ switching reversals. The performance of the films was dependent on the annealing time and temperature. It was found that films with better initial polarization values did not necessarily demonstrate better fatigue behavior. The causes of film degradation as a result of switching based on the pinning of domains at grain boundaries triggered by the migration of pores is discussed.     

Fatigue properties of Sb doped PZT thin films deposited by dc reactive sputtering

Abstract

Sb doped reactive sputtering-derived Pb(Zr, Ti)O₃ (Zr/Ti=48/52) thin films were investigated with the intention of improving ferroelectric properties. Also, the atomic valence of Sb in PZT thin film was confirmed as trivalent cation (Sb³⁺) by x-ray photoelectron spectroscopy (XPS). According to the tolerance factor t, Sb³⁺ tends to occupy the B-site of ABO₃ perovskite structure and acts as an acceptor that generates oxygen vacancies and holes. Transmission Electron Microscope(TEM) was used to observe the structural changes of PZT thin films by Sb addition. The leakage current densities and P_r of PZT thin films increased as the Sb contents increased. 0.7at% Sb doped PZT(PZST07) thin films exhibited improved fatigue properties (about 10% degradation of the remanent polarization after 10¹⁰ switching cycles).     

Preparation of PZT Thin Films by Liquid-Source MOCVD Using of Cyclohexane Solvent

Pb(Zr, Ti)O₃ thin films were grown on 8-inch Ir(111)/SiO₂/Si substrate by a MOCVD system aiming at application utilizing high-density ferroelectric memory (FRAM). Two types of solvents, THF and cyclohexane were used for liquid source delivery. It was found that the ferroelectric properties of the MOCVD-PZT films using cyclohexane solvent were better than them using THF solvent. By choosing cyclohexane as solvent, the MOCVD-PZT thin films showed strong ?111? preferred orientation and the Pt/PZT/Ir capacitors exhibited promising ferroelectric performances, for instance, large switching charge (Qsw) of 56.4 uC/cm².     

Effect of Buffer Layer on the Properties of Laser Ablated PZT Thin Films

Lead zirconate titanate (PbZr _x Ti_1?x O₃) or PZT ceramics are a class of piezoelectric materials that are currently experiencing widespread use in industry as electromechanical devices. PtSi/ZnO/PZT thin films were deposited by pulsed laser deposition at relatively low substrate temperature. The PZT thin films on PtSi substrates and on ZnO buffer layer were deposited at substrate temperature 300°C. The composition analysis shows that the film deposited at low temperature is stoichiometric. The films exhibit ferroelectric nature with coercive field of 19.6 kV/cm for 800 nm thick film. The leakage current density of the films shows a good insulating behavior.     

Recent progress in sputtering PZT thin films for ferroelectric memories

Abstract

This paper describes amorphous Pb(Zr, Ti)O₃ (PZT) thin films deposited by cosputtering Pb(Zr_0.5 Ti_0.5)O₃ and PbO targets. By optimizing the amount of the excess Pb and the deposition temperature, PZT thin films with a single perovskite phase were obtained successfully on Ir substrates and Pt substrates at 520°C. 250-nm-thick PZT films crystallized by rapid thermal annealing (RTA) at 600°C for 20 s exhibited excellent ferroelectric properties: a coercive voltage of 1.0 V, a remanent polarization density of about 40 μC/cm², and a polarization switching endurance over 1x109 cycles. Although a heat treatment in a reductive ambient causes degradation of ferroelectric properties of PZT thin films, their degraded ferroelectric properties can be easily recovered from by a 1-min RTA in an oxygen at 400°C.     

Formation of ferroelectric nano-domains using scanning force microscopy for the future application of memory devices

Abstract

Applying voltage between the conductive tip in atomic force microscopy (AFM) and bottom electrode through Pb(Zr, Ti)O₃ (PZT) films can cause switching of ferroelectric domains. Formation and imaging of ferroelectric domains in nanometer scale could be applied to develop the future ultrahigh-density memory device. Relevant issues, i.e. bit (induced ferroelectric domains) size dependence on applied voltage and pulse width, are discussed. The bit size showed a log-linear dependence on the pulse width and a linear dependence on the pulse voltage. Using the analysis of electric field distribution, the size of the induced bits under certain pulse voltage and width was estimated.     

Measurement and simulation of partial switching in ferroelectric PZT thin-films

Abstract

In digital memory applications, metal ferroelectric metal (MFM) capacitors are typically fully switched from one polarization state to the other, with the difference in displacement current allowing determination of the cell state. However, by applying a pulse of insufficient amplitude and/or duration to fully switch the ferroelectric, such a device may be partially polarized. Here, the measurement of partial switching in sol-gel derived PZT MFM capacitors due to applied voltage pulses is reported. SPICE, a commonly used circuit simulation program, has been modified to incorporate a ferroelectric capacitor device model. The MFM device model added to SPICE is reviewed, and the simulation of partial switching is demonstrated. Simulation results modeling the PE hysteresis loops and switching transients due to applied voltage steps closely match those measured in the laboratory. We conclude with the modeling of incomplete switching due to applied pulses of insufficient amplitude to cause polarization saturation, which is attributed to the polycrystalline nature of the thin-films.     

Effect of excess-PB and prenucleation layer on properties of Pb(ZryTi1-y)O3 thin films prepared by mod process

Abstract

A modified metal-organic decomposition process, MOD has been successfully utilized to improve the ferroelectric properties of Pb(Zr_0.52Ti_0.48)O₃, PZT, thin films. Multilayer PZT/Pt(Si) films, which contain 0.12 μm layer of spin coated PbO-excess (10 mol%) precursors on top of 0.12 μm layer of stoichiometric PZT precursors, exhibit superior ferroelectric properties (P_r=14.2 μC/cm²; E_c=62 kV/cm) to the single layer PZT/Pt(Si) films of the same thickness (0.24 μm), which are either stoichiometric or 10 mol% Pb-enriched. The ferroelectric properties are further improved when the PZT films were synthesized using a thin pulsed laser deposited (PLD) prenucleation layer (0.06 μm). The subsequently MOD-prepared PZT films posses high remanent polarization (P_r=23.2–26.6 μC/cm²) and low coercive field (E_c=62.9–69.0 kV/cm).     

Ferroelectric memory FET with Ir/IrO2 electrodes

Abstract

We proposed a MFMIS structure having a floating gate as a bottom electrode between a ferroelectric thin film and the gate SiO₂. Conventional gate SiO₂ can be used and ferroelectric thin films can be grown on bottom electrodes which have a good matching with the ferroelectric materials due to adopt the MFMIS structure. Ir and IrO₂ on poly-Si were used as floating gate. When a IrO₂ layer was formed between PZT and poly-Si, a high-quality PZT thin film was obtained and the PZT films show no fatigue up to 10¹² cycles of switching pulses. From the I_D-V_G characteristics measurement for 1·2 μm P-ch MFMIS FET, the shift in V_th or the memory window for a bias sweep of ±15V was about 3·3V. The difference of I_D-V_D curves which corresponded to I_D-V_G characteristics were found between before and after a programming pulse was applied.     

Ferroelectric switching and fatigue behavior for PZT/YBCO thin film heterostructures

Abstract

The dielectric and ferroelectric properties for Au/Pb(Zr,Ti)O₃/YBa₂Cu₃O_7?x heterostructures at low temperatures are reported. The fatigue behavior and the ferroelectric switching effect for the structures are also investigated. The PZT/YBCO thin film heterostructures were deposited on MgO(100) substrates by laser ablation. The ferroelectric and dielectric properties and optical response of the oriented PZT films with different thicknesses have been studied over the temperature range from 20 K to 300 K. The dielectric loss of the structure was found to decrease by an order of magnitude when the YBCO bottom electrode became superconducting. A very low fatigue rate of the structure has also been obtained below T _c of YBCO layer.     

MFIS and MFMIS structures using Pb(Zr,Ti)O3 films for nonvolatile memory devices

Abstract

In this work, metal / ferroelectric / insulator / semiconductor (MFIS) and metal / ferroelectric / metal / insulator / semiconductor (MFMIS) structures using Pb(Zr, Ti)O₃ (PZT) films were fabricated and characterized for nonvolatile NDRO memory device. 300nm-thick PZT films were deposited by reactive RF magnetron sputtering method on ZrTiO₄(ZT)/Si and Pt/ZT/Si substrates. C-V hysteresis were measured in both MFIS and MFMIS structures. By using a small-size MFM capacitor on a large-size MIS structure, it was found that the memory window of MFMIS structure was larger than that of the MFIS structure. There is a critical area ratio (S_MIS/S_MFM) in MFMIS structure. When an area ratio in MFMIS structure is below 12, the memory window increased with increasing area ratio. We could obtain that the memory window of MFMIS structure with a S_MIS/S_MFM of 11.8 was 2.1 V and 3.2 V with an applied voltage at 3 V and 5 V.     

FERROELECTRIC GATE FET MEMORY BASED ON CONDUCTION OF FERROELECTRIC-INSULATOR INTERFACE

ABSTRACT

A new type of ferroelectric gate field effect transistor (FET) using ferroelectric-insulator interface conduction has been proposed. Drain current flows along the interface between ferroelectric and insulator layers and needs no semiconductor. This FET consists of source and drain electrodes on ferroelectric film (Pb(Zr_0.52Ti_0.48)O₃(PZT)) prepared on Pt/TiO₂/SiO₂/Si substrate, and gate electrode on HfO₂ insulator film on the PZT film between the source and the drain electrodes. Drain current flows through the interface of the ferroelectric and the insulator. Drain current versus gate voltage characteristics shows clockwise hysteresis loop similarly to the conventional p-channel FET with ferroelectric gate. The FET shows that the On/Off ratio of the conduction current is about 10⁵ and the Off state current is about 10^{? 10}A.     

Thin Film Piezoelectrics for MEMS

Thin film piezoelectric materials offer a number of advantages in microelectromechanical systems (MEMS), due to the large motions that can be generated, often with low hysteresis, the high available energy densities, as well as high sensitivity sensors with wide dynamic ranges, and low power requirements. This paper reviews the literature in this field, with an emphasis on the factors that impact the magnitude of the available piezoelectric response. For non-ferroelectric piezoelectrics such as ZnO and AlN, the importance of film orientation is discussed. The high available electrical resistivity in AlN, its compatibility with CMOS processing, and its high frequency constant make it especially attractive in resonator applications. The higher piezoelectric response available in ferroelectric films enables lower voltage operation of actuators, as well as high sensitivity sensors. Among ferroelectric films, the majority of the MEMS sensors and actuators developed have utilized lead zirconate titanate (PZT) films as the transducer. Randomly oriented PZT films show piezoelectric e _31,f coefficients of about ?7 C/m² at the morphotropic phase boundary. In PZT films, orientation, composition, grain size, defect chemistry, and mechanical boundary conditions all impact the observed piezoelectric coefficients. The highest achievable piezoelectric responses can be observed in {001} oriented rhombohedrally-distorted perovskites. For a variety of such films, e _31,f coefficients of ?12 to ?27 C/m² have been reported.     

Analysis of read-out operation in 1T2C-type ferroelectric memory cell

Abstract

A 1T2C-type ferroelectric memory cell, in which two ferroelectric capacitors with the same area are connected to the gate of an usual MOSFET with a SiO₂/Si interface, was fabricated and characterized. It was found that the memory window significantly changed by the device parameters, which means that the low voltage operation is possible if we optimize these parameters. The fabricated cell, is composed of a stacked gate structure of Pt/SBT/Pt/Ti/SiO₂/Si with the area ratio of the MOS capacitor to the ferroelectric capacitor of 6 or 10. Nonvolatile memory operation was confirmed, in which the current on/off ratio was larger than 3-order-of magnitude and the data retention time was longer than 6 × 10⁴ seconds.     

Ferroelectric Pb5Ge3O11 MFMOS capacitor for one transistor memory applications

Abstract

The basic mechanism for an one transistor memory device has been studied. Many ferroelectric materials such PbZr_xTi_1?xO₃ (PZT), SrBi₂Ta₂O₉ (SBT), Pb₅Ge₃O₁₁ (PGO) etc. were analyzed for this application. Because of its low remanent polarization and low dielectric constant, the c-oriented Pb₅Ge₃O₁₁ thin film was selected for one-transistor memory applications. Pb₅Ge₃O₁₁ thin films have been prepared using MOCVD and RTP (Rapid Thermal Process) post-annealing. Lead bis-tetramethylheptadione [Pb(thd)₂] and germanium ethoxide [Ge(OC₂H₅)₄] were used as the precursors. The Pb₅Ge₃O₁₁thin films were deposited onto Ir/Ti/SiO₂/Si wafers to measure their compositions, phase formation, microstructure and ferroelectric properties. The extremely highly c-oriented Pb₅Ge₃O₁₁ thin films showed good ferroelectric and electrical properties. A 300 nm thick PGO thin film exhibited a square and saturated hysteresis loop with 2Pr of 3.98 μC/cm², 2Ec of 128 KV/cm at an applied voltage 5V, leakage current of 5.1×10^?7 A/cm² at 100 KV/cm, and dielectric constant of close to 36. The c-axis oriented Pb₅Ge₃O₁₁ thin film also exhibited very good retention properties. The experimental results showed that a Pb₅Ge₃O₁₁ MFMOS gate stack is suitable for one-transistor memory applications.     

Advances in processing and properties of perovskite thin-films for FRAMs,DRAMs, and decoupling capacitors

Abstract

Highly oriented, dense, and crack-free ferroelectric and paraelectric thin-films on three inch diameter Pt/Ti/Si₃N₄/Si (100) substrates were obtained by polymeric sol-gel processing. Ferroelectric PZT thin-films were fabricated at temperatures as low as 550°C within 15 minutes by rapid thermal annealing. The films heat treated at 700°C for 5 minutes were single grain thick and exhibited P_r, P_sp, and E_c in the ranges of 29–32 μC/cm², 44–53 μC/cm², and 50–60 kV/cm, respectively, and high speed switching times below 5 ns on 30×30 μm² electrodes. A switching time of 2.7 ns was observed on 19×19 μm² area electrodes at a field of 200 kV/cm. Results of low and high field characterization on paraelectric PLT thin-films which were conventionally heat treated indicated that it has an excellent potential for use in ULSI DRAMs and as decoupling capacitors. These films showed a high charge storage density (15 μC/cm²) and a low leakage current (0.5 μA/cm²) at a field of 200 kV/cm. Also, the charging time for a capacitor area of 1 μm² at 200 kV/cm was estimated to be 0.10 ns.     

Current status of resistive nonvolatile memories

Several emerging nonvolatile memories (NVMs) such as ferroelectric memory, magnetoresistive rams and ovonic universal memory are being developed for possible applications. Resistive random access memory (RRAM) is another interesting competitor in the class of NVMs. The RRAM is based on a large change in electrical resistance when the memory film is exposed to voltage or current pulses, and can keep high or low resistance states without any power. The ideal RRAM should have the superior properties of reversible switching, long retention time, multilevel switching, simple structure, small size, and low operating voltage. Perovskite oxides, transition metal oxides, and molecular materials were found to have resistive memory properties. This presentation reviews the ongoing research and development activities on future resistance NVMs technologies incorporating these new memory materials. The possible basic mechanisms for their bistable resistance switching are described. The effect of processing, composition, and structure on the properties of resistive memory materials and consequently the devices are discussed.     

Properties of PbZrxTi1−xO3/CeO2/SiO2/Si structure

Abstract

We report the crystalline quality and electrical properties of PbZr_xTi_1?xO₃ (PZT) films on n-type Si(100) substrates with CeO₂/SiO₂ dual buffer layers. PZT films and CeO₂ buffer layers were prepared by pulsed laser deposition technique, and SiO₂ buffer layers were formed by thermal dry oxidation. It was found that CeO₂/SiO₂ dual buffer layers effectively prevented Si and Pb interdiffusion between PZT and Si substrates. Furthermore, the capacitance-voltage (C-V) characteristics of the PZT/CeO₂/SiO₂/Si heterostructures demonstrated ferroelectric switching properties, showing a memory window as large as 2.7 V at 1 MHz.