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.     

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.     

The annealing effect on properties of ZnO thin film transistors with Ti/Pt source-drain contact

ZnO-based thin film transistors (TFTs) with Ti/Pt contacts were fabricated on SiO₂/Si substrates. The as-deposited ZnO TFT did not work well as a TFT device but the annealed ZnO TFT showed acceptable characteristics with a mobility (μ_sat), threshold voltage (V_th), on/off ratio and subthreshold swing (SS) of 0.8 cm²/V.s, 2.5 V, over 10⁶ and 0.84 V/dec, respectively. Complete oxygen loss was observed in ZnO after annealing at 300°C under a N₂ atmosphere. The annealing process altered the crystallinity, density and composition of the ZnO active layers due to the formation of oxygen vacancies as shallow donors. This process is expected to play an important role in controlling the TFT performance of ZnO. In addition, it is expected to form the basis of the future electronic devices applications, such as transparent displays and active matrix organic lighting emitted displays (AMOLED).     

Iridium thin film as a bottom electrode for high dielectric (Ba,Sr)TiO3 capacitors

Abstract

We propose Ir thin films as new electrode materials for high dielectric BST capacitors. Ir was found to be superior to Pt in a number of aspects such as resistivity, adhesion and surface roughness. The Pt/BST/Ir/SiO₂/Si capacitors showed leakage currents as low as Pt/BST/Pt/SiO₂/Si ones, but higher capacitance resulted. For endurance properties with +5V unipolar pulse trains, the dielectric constant of BST films on Ir decreased by only 10% below its initial value after switching of 10⁹ cycles while that on Pt degraded by 30% after 10⁸ cycles. Ir bottom electrode effects on BST film properties were well explained by the formation of IrO₂ phases on the surface of Ir electrodes.     

Formation of MgAu alloy cathode by photolithography and its application to organic light‐emitting diodes and organic field effect transistors

To make an efficient electron injection electrode by photolithography, we studied MgAu alloy electrodes, which have a low work function and high resistance to humidity and oxygen. The measured work function of MgAu alloy thin film was 3.7 eV, which is comparable with that of a pure Mg layer and ～0.8 eV lower than that of a pure Au layer. This low work function was maintained even after photolithography, suggesting excellent stability for solvent treatment. Investigating organic field‐effect transistor (OFET) characteristics when MgAu alloy comb type source and drain electrodes were used, we successfully obtained n‐type FET operation. Furthermore, organic light‐emitting diodes (OLEDs) demonstrated the efficient electron injection characteristics of the MgAu alloy cathode, which were similar to those of the conventional MgAg cathode. © 2005 Wiley Periodicals, Inc. Electr Eng Jpn, 152(1): 37–42, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/ eej.20153     

Threshold voltage extraction techniques adaptable from sub‐micron CMOS to large‐area oxide TFT technologies

This paper proposed simple and accurate threshold voltage (V _TH ) extraction techniques, which can be directly adaptable to various semiconductor technologies ranging from deep sub‐micron complementary metal–oxide–semiconductor to large‐area thin‐film transistor devices. These techniques are developed using multiple circuits, namely, a dynamic source follower, an inverter with a diode‐connected load and a current mirror topology, which allow a direct determination of V _TH . As the proposed techniques are experimented with large‐area emerging technologies, which have a stable single type (n‐type) transistor, all the designs employed in this work are confined to only n‐type transistors for a fair comparison. The semiconductor technologies under consideration are standard complementary metal–oxide–semiconductor (65 and 130 nm) and oxide (indium–gallium–zinc–oxide and zinc–tin–oxide) thin‐film transistors. In order to validate the accuracy of the proposed techniques, extracted V _TH from these methods are compared against the value from linear transfer characteristics. The resulting relative error is within 5%, reinforcing proposed techniques suitability to different semiconductor technologies ranging from deep sub‐micron to large‐area transistors. Copyright © 2017 John Wiley & Sons, Ltd.     

Characteristics of lithium niobate based capacitors and transistors

Abstract

The electrical properties of thin film (<1000 Å) capacitor devices of lithium niobate grown on silicon and platinum and of thicker film metal-ferroelectric-semiconductor field effect transistors (MFSFET) with lithium niobate as the gate material were measured. Dielectric constants of the thin films on silicon were as high as 27, while those for films on platinum were as high as 49. The MFSFET structures showed good FET properties, and demonstrated a channel current modulation consistent with switching of the ferroelectric gate by pulsing.     

Microporous activated carbons from used coffee grounds for application to electric double‐layer capacitors

Electrochemical double‐layer capacitors (EDLCs) are devices that store enormous amounts of charge electrostatically when a potential is applied between electrodes of very high surface area (typically made of porous carbon) and an electrolyte. Wider commercialization of this technology has been held back by the lack of ultralow‐cost electrode materials. We demonstrate that used coffee grounds can be processed to form low‐cost electrodes. The surface and electrochemical characteristics of microporous activated carbons from used coffee grounds (CGCs) were measured. First, optimal times and temperatures for carbonization and activation were identified on the basis of Brunauer–Emmett–Teller (BET) surface area, pore volume, and pore size distribution. Second, CGCs were used as polarized electrodes in EDLCs, whose capacitances were evaluated using cyclic voltammetry. The results show that carbonization for 1 h at 600 °C with a heating rate of 300 °C/h, followed by CO₂ activation for 2 h at 1000 °C, affords the highest BET surface area (1867 m²/g) compared to other works. The produced CGCs have many micropores of less than 2 nm across, which contribute to the formation of an electric double layer. Capacitors made using these CGCs show the highest capacitance (103 F/g) in 0.8 M (C₂H₅)₄NBF₄/PC as an organic electrolyte, which is much higher than the ∼80 F/g typically used in organic‐electrolyte‐based commercial EDLCs, suggesting that coffee grounds are a useful electrode material. © 2014 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

Determination of Cross Sectional Variation of Ferroelectric Properties for Thin Film (Ca. 500 nm) PZT (30/70) via PFM

There is much speculation about the origin of fatigue in ferroelectric thin films that have been grown by sol-gel on various substrates. One of the most important substrates for growing thin film ferroelectric materials is native SiO₂. After the deposition of suitable electrode, most common are Ti-Pt, a thin film of a ferroelectric material can be grown on the substrate. Procedures to grow thin film lead zirconate titanate (PZT 30-70) have been well publicized and it is now routine that almost 100% [111] orientated PZT with a columnar structure of ca. 100 nm is grown on SiO₂ substrates. Recent studies using PFM have shown that it is possible to determine a variation in hysteresis loops with a spatial accuracy of 50 nm on the surface of a ferroelectric material. This study was to determine the variation in d₃₃ for a thin film sample of ferroelectric in terms of a depth profile. It was not suitable to just turn the sample on it's side and analyze the d₃₃ in terms of depth as the continuous back electrode would mean that the resulting vibration of the ferroelectric surface would be a combination of d₃₃ and d₃₁. The de-convolution of this signal was outside the scope of the current investigation. In order to determine the d₃₃ of the material in terms of depth the sample was machined (with an accuracy of 10 nm) to release the ferroelectric from the underlying electrode and electroded in the correct orientation for d₃₃ to be investigated in cross section. Samples of perovskite PZT(30/70) have been produced via sol-gel using a spin coating technique that are ca. 500 nm in cross sectional thickness on Pt/Ti/SiO₂ back electrodes. The PZT is shown to be highly [111] orientated and has in the past been well characterized both at the macro and nanoscale using a variety of techniques. Using focused ion beam (FIB) milling the PZT layer has been released from the SiO₂/Ti/Pt substrate and electroded in such a way as to determine the δ₃₃ properties of the film in terms of thickness of the as-deposited film. Figure 1 indicates the machining process used to generate the free standing PZT block. In Fig. 2, the results of the FIB machining process are shown. The ‘bridge’ structure has the dimensions of 500 nm in cross section and is long enough to ensure that electrode cross talk between the d₃₃ and d₃₁ modes is reduced. Hysteresis loops have been generated using the technique of piezo force microscopy (PFM) at discrete locations across the surface of the film corresponding to a depth profile of the as-deposited PZT. Variations in the shape of the hysteresis loops, calculated δ₃₃ and coercive field for the film have been related to the variation in oxygen defect density in the film. This study shows that the PZT thin film has a variety of fundamental piezoelectric constants that are associated with the variation in local crystal chemistry.     

Thick electrodes for high frequency high Q tunable ferroelectric thin film varactors

Abstract

Thin film barium strontium titanate (BST) shows great promise for voltage tunable dielectric devices for use at RF and microwave frequencies. An MOCVD process has been developed for production of BST, resulting in films with very low losses (as low as 0.002–0.004) and tunabilities over 50% at low operation voltages. With these values of BST loss, overall device quality factors at RF (100 MHz+) frequencies are primarily limited by losses in the thin metal electrodes, such as Pt, normally used for ferroelectric thin films. The bottom electrode in parallel plate capacitor structures is particularly challenging, since it must provide a good growth surface for BST and be stable at high (>600 °C) growth temperatures in an oxidizing atmosphere yet have high conductivity and compatibility with Si or SiO₂/Si substrates. These challenges have previously prevented use of Pt thicknesses over 0.1–0.2 urn. Our solution to this problem, involves combinations of adhesion layers at the Pt/SiO₂ interface and embedded stabilization layers to make functioning Pt bottom electrodes as thick as 2 μm. Devices with dielectric Q factors over 150 at 100 MHz (tan δ ～ 0.006 as measured and modeled by S-parameters) and overall device Q factors over 50 at 30 MHz are described. We have also inserted these devices into tunable filters, achieving tunabilities of 50% and low insertion losses (0.3 dB) at RF frequencies.     

Ferroelectric properties and reliability of La-Sr-Co-O/Pb-La-Zr-Ti-O/La-Sr-Co-O heterostructures on si for non-volatile memory applications

Abstract

Highly oriented La-Sr-Co-O(LSCO)/Pb-La-Zr-Ti-O(PLZT)/La-Sr-Co-O heterostructures have been successfully grown on a highly oriented Pt film which was grown on a thermally oxidized Si (SiO₂/Si) substrate. The growth of oriented Pt film on the SiO₂/Si substrate was made possible through the use of a thin bismuth titanate template layer which is c-axis oriented on the SiO₂/Si substrate. The hybrid LSCO/Pt structure effectively reduced the sheet resistance of the electrodes by at least 3–5 times compared with a single LSCO electrode. These ferroelectric PLZT capacitors on Si exhibited symmetric hysteresis loops with very desirable ferroelectric properties. The test capacitors showed reliable performance at both room and high (100°C) temperatures with respect to fatigue, retention, aging, and imprint, suggesting that they can be used as reliable, nonvolatile memory elements.     

The incorporation of SiO2 nanoparticles in poly(p-phenylenevinylene)(PPV) for PPV/SiO2 nanocomposite

The changes in surface morphology and current-voltage characteristics of poly(p-phenylenevinylene)(PPV) thin film has been studied by varying the amount of incorporated SiO₂ nanoparticles. The electronic structure of carbon atom in PPV and PPV/SiO₂ nanocomposite films was studied by using near-edge X-ray absorption fine structure. The surface morphology of PPV/SiO₂ nanocomposite film was found to be greatly dependent on the amount of incorporated SiO₂ nanoparticles. The current–voltage behavior of PPV/SiO₂ nanocomposite film was mainly dependent on the surface morphology of the film. The excess content of SiO₂ nanoparticles in PPV/SiO₂ nanocomposite film was revealed to induce an agglomeration of SiO₂ nanoparticles where blocking of electronic conduction happens.     

Fatigue and retention of Pb(Zr0.53Ti0.47)O3 thin film capacitors with Pt and RuO2 electrodes

Abstract

Pb(Zr_0.53Ti_0.47)O₃ (PZT) thin films were deposited on Pt and RuO₂ coated Si and MgO substrates using the sol-gel process. Fatigue and retention tests were performed on these samples. The films grown on RuO₂ electrodes are fatigue-free up to nearly 10¹¹ cycles. Their retention life-time extrapolates to more than 10¹⁰ seconds. The fatigue behavior of films grown on Pt electrodes depends on the PZT film orientation. Highly oriented (001) PZT films maintain 50% of their initial P?r-P?r value after 10¹¹ cycles. The randomly oriented films maintain less than 3% of the initial P?r-P?r value after 10¹¹ cycles. However, the retention life-time of both highly oriented and randomly oriented PZT films grown on Pt electrodes extrapolates to higher than 10¹¹ seconds. It appears that fatigue of films grown on RuO₂ is mainly controlled by the film/electrode interface. On the other hand, fatigue of films grown on Pt appears to depend on both the film/electrode interface as well as on bulk effects.     

Application of nickel alloy electrode to characterization of crystal oriented PbTiO3 thin films

A new electrode material made of nickel alloy has been developed that can be used in place of platinum electrodes, which have commonly been utilized in conjunction with PbTiO₃ and Pb(Zr, Ti)O₃ thin films. This alloy electrode is composed of nickel, chromium, iron and aluminium. When heated in an oxidation atmosphere, an alumina film layer is generated on the surface of the alloy that prevents the electrode from oxidizing and reacting with PbO. The remanent polarization of the PbTiO₃ film on the alloy electrode oriented in the (111) direction was 25 μC/cm², a value which is approximately equal to that of ceramics. The breakdown strength of the film reached the relatively large value of 190 kV/cm at room temperature. In addition, the as-grown film was already polarized with a degree of polarization estimated to be half that of fully polarized bulk ceramics.     

Water-Immersible Micromachined Pb(Zr, Ti)O3 Thin Film Actuators

We demonstrate a water-immersible thin film lead zirconate titanate, Pb(Zr, Ti)O₃, [PZT] actuator, without special passivation layer, towards in-vivo or in-vitro scanning probe microscope (SPM) measurements of living cells in water or biological fluids. In order to be water-immersible, the electrodes need to be electrically insulated and the piezoelectric layer needs to be protected against direct water contact. This paper describes our design solution with a simple fabrication process for a water-immersible piezoelectric device, which separates the bottom electrode from the top electrode by having a narrow ditch covered with PZT film. The PZT film is then encapsulated with the top metal electrode without insulation layer. In this structure, the PZT is sandwiched between the top and bottom metal electrodes to prevent water permeation. The device is fabricated using lift-off processing for the bottom and top electrodes, sol-gel spinning for the PZT thin film and wet etching for the PZT patterning. The piezoelectric constant, d₃₁, is about –100 pC/N. The dielectric polarization and fatigue properties of the devices were measured in air and water. The spontaneous polarization, remnant polarization, coercive field and dielectric constant are 54 C/cm², 15 C/cm², 60 kV/cm and 1200, respectively. The polarization property of the device was unchanged in either air or water up to 1 × 10⁹ continuous cycles.     

Ferroelectric La-Sr-Co-O/Pb-La-Zr-Ti-O/La-Sr-Co-O heterostructures on silicon

Abstract

Ferroelectric Pb_0.9La_0.1Zr_0.2Ti_0.8O₃ (PLZT) thin film capacitors with symmetrical La-Sr-Co-O top and bottom electrodes have been grown on [001] Si with a Yttria stabilized zirconia (YSZ) buffer layer and on SiO₂Si substrates. A layered perovskite “template” layer (300–500 Å thick), grown between the YSZ buffer layer or the SiO₂ layer and the bottom La-Sr-Co-O electrode, is critical for obtaining the required orientation of the subsequent layers. The fatigue, retention and aging characteristics of these new structures are quite desirable for nonvolatile memory operation. Preliminary studies show that this ferroelectric performance obtained in large (50–100 μm diameter) capacitors can be replicated in smaller capacitors (down to 4 μm diameter) processed by ion milling.     

Dielectric and Ferroelectric Properties of Ba(Zr0.35Ti0.65)O3 Thin Films Grown by a Sol-Gel Process

The Ba(Zr_0.35Ti_0.65)O₃ (BZT) thin films were deposited via sol-gel process on LaNiO₃-coated silicon substrates. XRD showed that the crystallinity of BZT film grown on LaNiO₃ coated silicon substrates is better than that of BZT film grown on Pt. Both films showed perovskite phase and polycrystalline structure. The temperature dependent dielectric measurements revealed that the thin films had the relaxor behavior and diffuse phase transition characteristics. The capacitor tuning was about 44% for each BZT film grown on LaNiO₃/Pt and Pt electrodes at 1 MHz. Especially, the values of dielectric loss at 1 MHz ranged from 0.02 to 0.009 in the bias range of 0 to 514 kV/cm, respectively. The leakage currents density of thin films grown on LaNiO₃/Pt and Pt electrodes at 300 kV/cm was about 8.5 × 10^–7 and 1.1 × 10^–5 A/cm², respectively. This work demonstrates a potential use of BZT films for application in tunable microwave devices.     

Fabrication and Characterization of Gate-Connected 1T2C-Type Ferroelectric Memory with Paired Bi4 − xLaxTi3O12 Capacitors

A gate-connected 1T2C-type ferroelectric memory, in which the bottom electrodes of paired ferroelectric capacitors are connected to the gate electrode of an underlying FET (field effect transistor) on the field oxide region, was fabricated using a Bi_{4 ? x}La_xTi₃O₁₂ (BLT) film and its electrical properties were characterized. The I_D-V_G (drain current-gate voltage) characteristics of a FET combined with a single ferroelectric capacitor showed that the paired capacitors had almost the same ferroelectric property. It was found in the readout operation that there existed an optimum voltage to maximize the drain current on/off ratio between datum ‘1’ and datum ‘0,’ and that the maximum ratio was as large as 6 × 10⁴. It was also found that the drain current level remained constant, even if the readout operation was repeatedly conducted. It was concluded from these results that the 1T2C-type memory was successfully fabricated using the proposed process and operated properly.     

Deterioration of device characteristics of MFSFET due to fatigue

Abstract

Change of device characteristics of the metal-ferroelectric-semiconductor FET (MFSFET) with the progress of fatigue of the ferroelectric thin film are simulated in this study. The field-dependent polarization model and the square-law FET model are employed in our simulation. C-V_G curves exhibit the accumulation, the depletion and inversion regions clearly. They also exhibit the memory window of 2V. I_D-V_D curves are composed of the triode and the saturation regions. The difference of saturation drain currents of the MFSFET device at the dual threshold voltages in I_D-V_D curve is 6mA/cm², which decreases as much as 50% after fatigue. Our model is expected to be very useful in the estimation of the behaviour of MFSFET devices with the progress of fatigue.     

Dielectric Characteristics of In-Plane Polarized Lead Zirconate Titanate Thin Films on Oxide Layers

Sol-gel derived lead zirconate titanate (PZT) films have been prepared on STO-passivated silicon substrates. The STO buffer layer of thickness about 55 nm is prepared by rf-magnetron sputtering. XRD results reveal that the PZT film has well-crystallized perovskite phase, indicating that the thin STO layer can effectively prevent reaction and inter-diffusion between the PZT film and silicon substrate. Interdigitated electrodes (IDEs) have then been deposited on the PZT film by magnetron sputtering and patterned using the standard photolithography. With the IDEs, the dielectric and ferroelectric properties of the PZT film under transverse or in-plane electric fields have been investigated. By assuming a uniform distribution of electric field (in-plane electric field model), the estimated relative permittivity of the PZT film is about 2100, while the dielectric loss is less than 1%. Good in-plane polarization hysteresis loop is observed, showing an observed remanent polarization value of 21 μC/cm², which is comparable to that of a PZT film with both top and bottom electrodes. The in-plane polarized PZT/STO/SiO₂/Si film can be used to fabricate d₃₃-mode unimorph bending transducers, which will have much better performance than the conventional bending transducers driven electromechanically through the piezoelectric d₃₁ mode.