Ferroelectric Field Effect Transistor Memory Research Articles

Electrical properties of Pr-doped Bi4Ti3O12 thin films on Si substrate

Bi3.15Pr0.85Ti3O12 thin films have been grown on a p-type Si(111) substrate by the method of metalorganic sol decomposition and spin coating. The optimum temperature of crystallization is 680°C. Atomic force microscopy reveals that the films have a dense, smooth surface. The electrical measurements were conducted on a metal–ferroelectric–insulator–semiconductor capacitor. The films exhibit good insulating property at room temperature. The clockwise hysteresis curves observed show that the films have a memory effect. The fixed charge density and the surface state density were also calculated. The results show that the films have promising applications as the ferroelectric field effect transistor memories.

Metal–Ferroelectric–Insulator–Semiconductor Memory FET With Long Retention and High Endurance

A ferroelectric memory field-effect transistor (FET) with a Pt-SrBi/sub 2/Ta/sub 2/O/sub 9/-Hf-Al-O-Si gate stack was fabricated and its electrical properties were characterized. The insulating and ferroelectric layers were successively deposited by a laser ablation technique. Excellent data retention characteristics were obtained. The drain currents of on- and off-states were measured as a function of time, after /spl plusmn/6 V poling voltage applied to the gate electrode. The current ratio of the on- and off-states was more than 10/sup 6/ even after 12 days. Moreover, after 10/sup 12/ cycles of /spl plusmn/8 V pulses, this ratio was also more than 10/sup 6/.

A Low-Dielectric-Constant Sr2(Ta1-x,Nbx)2O7 Thin Film Controlling the Crystal Orientation on an IrO2 Substrate for One-Transistor-Type Ferroelectric Memory Device

The compounds of the Sr2Nb2O7 (SNO) family are suitable for use as ferroelectric materials for ferroelectric memory field-effect transistors (FETs), because these substances have a low dielectric constant and high heat-resistance. A very low dielectric constant ferroelectric Sr2(Ta1-x,Nbx)2O7 (STN) film formation technology, which can be applied to a floating-gate-type ferroelectric random access memory (FFRAM), has been developed. Furthermore, a new technology that controls the orientation and the properties (a low dielectric constant and large coercive field) of STN has been developed. An MFMIS structure device with a large memory bias window (1.3 V) under ±5 V operation and a long retention time (>10 h) has successfully been fabricated.

Improvement in retention time of metal–ferroelectric–metal– insulator–semiconductor structures using MgO doped Ba0.7Sr0.3TiO3 insulator layer

We report the results of the fabrication and characterization of Pt/Bi3.35La0.85Ti3O12 (BLT)/LaNiO3 (LNO)/Ba0.7Sr0.3TiO3 (BST)/Si metal–ferroelectric–metal–insulator–semiconductor (MFMIS) structures for ferroelectric memory field effect transistor applications. The BLT films were deposited on LNO/BST/Si using the metalorganic decomposition method and annealed by rapid thermal annealing (RTA) process at 600 °C for 3 min. The ratio of remanent polarization to saturation polarization (Pr/Ps) increases with reducing area ratio, AF/AI. A large memory window of 3.1 V can be obtained for a small AF/AI ratio. By the utilization of 5 mol % MgO doped BST insulator layer, LNO bottom electrode layer for BLT, and small area ratio, AF/AI=1/12 in the MFMIS structure, large Pr/Ps ratio in BLT film and low leakage current and good capacitance matching of the ferroelectric and the insulator in the MFMIS structures have been achieved and, hence, long data retention time >106 s has been obtained in this study. Experimental results demonstrate the significant progress in increase of the retention time of these structures, which make them attractive for practical ferroelectric memory field effect transistor applications.

Ferroelectric Sr2(Ta1-x, Nbx)2O7 with a Low Dielectric Constant by Plasma Physical Vapor Deposition and Oxygen Radical Treatment

The compounds of the Sr2Nb2O7 (SNO) family are suitable for use as ferroelectric materials for ferroelectric memory field effect transistors (FETs), because these substances have a low dielectric constant and high heat-resistance. A very low dielectric constant ferroelectric Sr2(Ta1-x,Nbx)2O7 (STN) film formation technology which is applied to floating gate type ferroelectric random access memory (FFRAM) has been developed. The high ferroelectric performance of the STN capacitor has been achieved by plasma physical vapor deposition and an oxygen radical treatment using microwave-excited (2.45 GHz) high-density (>1012 cm-3) low electron temperature (<1 eV) Kr/O2 plasma.

Device modeling of ferroelectric memory field-effect transistor for the application of ferroelectric random access memory.

An improved theoretical analysis on the electrical characteristics of ferroelectric memory field-effect transistor (FeMFET) is given. First, we propose a new analytical expression for the polarization versus electric field (P-E) for the ferroelectric material. It is determined by one parameter and explicitly includes both the saturated and nonsaturated hysteresis loops. Using this expression, we then examine the operational properties for two practical devices such as the metal-ferroelectric-insulator-semiconductor field-effect transistor (MFIS-FET) and metal-ferroelectric-metal-insulator-semiconductor field-effect transistor (MFMIS-FET) as well. A double integral also has been used, in order to include the possible effects due to the nonuniform field and charge distribution along the channel of the device, to calculate the drain current of FeMFET. By using the relevant material parameters close to the (Bi, La)4Ti3O12 (BLT) system, accurate analyses on the capacitors and FeMFET's at various applied biases are made. We also address the issues of depolarization field and retention time about such a device.

Device modeling of ferroelectric memory field-effect transistor (FeMFET)

A numerical analysis of the electrical characteristics for the ferroelectric memory field-effect transistors (FeMFETs) is presented. Two important structures such as the metal-ferroelectric-insulator-semiconductor field-effect transistor (MFISFET) and metal-ferroelectric-metal-insulator-semiconductor field-effect transistor (MFMISFET) are considered. A new analytic expression for the relation of polarization versus electric field (P-E) is proposed to describe the nonsaturated hysteresis loop of the ferroelectric material. In order to provide a more accurate simulation, we incorporate the combined effects of the nonsaturated polarization of ferroelectric layers and the nonuniform distributions of electric field and charge along the channel. We also discuss the possible nonideal effects due to the fixed charges, charge injection, and short channel. The present theoretical work provides some new design rules for improving the performance of FeMFETs.

Why is nonvolatile ferroelectric memory field-effect transistor still elusive?

In principle, a memory field-effect transistor (FET) based on the metal-ferroelectric-semiconductor gate stack could be the building block of an ideal memory technology that offers random access, high speed, low power, high density and nonvolatility. In practice, however, so far none of the reported ferroelectric memory transistors has achieved a memory retention time of more than a few days, a far cry from the ten-year retention requirement for a nonvolatile memory device. This work will examine two major causes of the short retention (assuming no significant mobile ionic charge motion in the ferroelectric film): 1) depolarization field and 2) finite gate leakage current. A possible solution to the memory retention problem will be suggested, which involves the growth of single-crystal, single domain ferroelectric on Si. The use of the ferroelectric memory transistor as a capacitor-less DRAM cell will also be proposed.

Bi 3.25 La 0.75 Ti 3 O 12 thin films prepared on Si (100) by metalorganic decomposition method

Bi 3.25 La 0.75 Ti 3 O 12 (BLT) thin films have been grown on n-type Si (100) substrates by metalorganic decomposition method. The structural properties of the films were examined by x-ray diffraction. The BLT films exhibit good insulating property with room temperature resistivities in the range of 1012–1013 Ω cm. The current–voltage characteristics show ohmic conductivity in the lower voltage range and space-charge-limited conductivity in the higher voltage range. The capacitance–voltage characteristic hysteresis curves show that the metal-ferroelectric-semiconductor structure has memory effect. The fixed charge density and the surface state density were also calculated. The results obtained indicate that the present BLT films are suitable for making ferroelectric field effect transistor memories.

Disturb free programming scheme for single transistor ferroelectric memory arrays

Conventional AND and NOR non-volatile memory array circuits utilizing a single ferroelectric memory field effect transistor (FEMFET) cell structure were simulated by using a BSIM3v3 based FEMFET compact model. It is shown that the use of the common V pp /2 and V pp /3 rules for programming a transistor in a FEMFET cell-array may cause the loss of stored information in adjacent memory cells due to disturb pulses. To overcome this problem we propose to back-bias the substrate during the write cycle, which extends the depletion region of the FEMFET to higher gate-source voltages, and thus reduces the influence of a disturb pulse on the polarization of the gate ferroelectric. In addition we discuss device improvements which reduce the susceptibility to data loss without back-biasing the substrate in order to minimize cell area.

A nonvolatile single ferro fet memory concept with disturbance free operation scheme

A novel AND-type ferroelectric field effect transistor memory concept for solid state mass storage applications is described. Disturbance problems caused by disturbance pulses between adjacent memory cells are prevented by device improvements and by choosing appropriate programming and read voltages. The memory array presented here uses global source lines each of which is connected to its own sense amplifier. Disturbance free and fully functional operation of the memory concept has been demonstrated by circuit simulations. The results of the simulations yield a data access time comparable to DRAMs.

Equivalent-circuit modeling of ferroelectric switching devices

A compact equivalent-circuit model for ferroelectric switching devices is derived from a general model for local charge displacements. The general model consists of a matrix of repeat units describing local dissipationless charge displacements (electrostatic channel), as well as dissipative charge displacements (electrochemical channel), the channels being coupled due to the electrical charge of the moving species. The derived model for ferroelectric charge displacements is used to simulate both hysteresis and transient characteristics, and applied to two devices: (i) a ferroelectric capacitor and (ii) a ferroelectric memory field-effect transistor. The circuits are programmed in SPICE-derived analysis software. We find that experimental hysteresis data obtained on Pb(Zr,Ti)O3 ceramic capacitors and on thin-film transistors with a SnO2:Sb semiconductor and a Pb(Zr,Ti)O3 ferroelectric insulator can be reproduced and interpreted with the equivalent-circuit models.

Properties of Ferroelectric Memory FET Using Sr2(Ta, Nb)2O7 Thin Film

Compounds of the Sr2Nb2O7 (SNO) family are suitable for use as ferroelectric materials for ferroelectric memory field effect transistors (FETs), because these substances have a low dielectric constant, low coercive field and high heat resistance. C–V and ID–VG hysteresis curves which depend on ferroelectric polarization were obtained. These capacitors were applied to floating gate ferroelectric random access memory (FFRAM) cells. The degradation in the ferroelectricity of STN capacitors was not observed during the fabrication of FFRAM cells. We succeeded in operating FFRAM cells at a lower voltage than that required for PZT. Data retention characteristics of metal ferroelectric metal insulator semiconductor (MFMIS) structures with STN films were first measured to be up to 2 weeks. Data retention is close related to be the leakage current of the ferroelectric. The MFMIS FET has the potential to retain data for 10 years.

Growth of highly oriented Pb(Zr, Ti)O3 films on MgO-buffered oxidized Si substrates and its application to ferroelectric nonvolatile memory field-effect transistors

We have grown highly oriented lead zirconate titanate [Pb(Zr, Ti)O3 or PZT] films on oxidized silicon substrates using a thin MgO buffer layer (7–70 nm thick). Ferroelectric nonvolatile memory field-effect transistors (FETs) were successfully fabricated using the metal/PZT/MgO/SiO2/Si structure in conjunction with radio-frequency sputter deposition of PZT and MgO films. The fabricated devices show excellent performance in ferroelectric polarization switching and memory retention. The results indicate that a thin MgO buffer serves well not only as a template layer for the growth of oriented PZT films on amorphous substrates, but also as a diffusion barrier between a ferroelectric and a substrate during device fabrication, protecting the SiO2/Si interface and the FET channel region.

Application of Sr2Nb2O7 Family Ferroelectric Films for Ferroelectric Memory Field Effect Transistor

The compounds of the Sr2Nb2O7 (SNO) family are suitable for use as ferroelectric materials for ferroelectric memory field effect transistors (FETs), because these substances have a low dielectric constant, low coercive field and high heat-resistance. In this study, we succeeded in preparing Sr2(Ta,Nb)2O7 (STN) capacitors on polycrystalline silicon (poly-Si). From SIMS profiles, no interdiffusion in the STN metal ferroelectric metal insulator semiconductor (MFMIS) structure was confirmed. C–V and ID–VG hysteresis curves which were dependent on ferroelectric polarization were obtained. These capacitors were applied to floating gate type ferroelectric random access memory (FFRAM) cells. The degradation in ferroelectricity of STN capacitors was not observed during FFRAM cell fabrication process. We succeeded in operating FFRAM cells with a lower voltage than that required for PZT and confirmed the drain current difference of 1 or 2 orders at the 30 s after applying write pulses of ±5 V or ±10 V.

BaMgF4 thin film development and processing for ferroelectric FETS

Abstract A ferroelectric memory field-effect transistor (FEMFET) where a ferroelectric thin film is incorporated directly into the gate structure of the transistor is attractive, because it provides not only nonvolatility, but also nondestructive readout (NDRO). At Westinghouse, we are currently developing a FEMFET using thin film barium magnesium fluoride (BaMgF4), a ferroelectric material that was discovered in 1969, but was not fabricated in thin film form until 1989. The BaMgF4 films are grown by evaporation in an ultrahigh vacuum (UHV) chamber on clean Si(100). The natural tendency of these films to grow with the ferroelectric a-axis in the Si(100) plane has been overcome to obtain more random orientation with larger reversible polarization perpendicular to the film. A capping layer (SiO2) has been found to be essential for process integrability of these BaMgF4 films. Ti-W metallization produced only a slight reduction in the capacitance-voltage (C-V) memory window. Switching speed of these films has...

Physics of the ferroelectric nonvolatile memory field effect transistor

The operation of the ferroelectric nonvolatile memory field effect transistor is theoretically examined extensively for the first time. The ferroelectric transistor device properties are derived by combining the silicon charge-sheet model of metal-oxide-semiconductor field-effect transistor device operation with Maxwell’s first equation which describes the properties of the ferroelectric film. The model we present describes ferroelectric transistor I-V and C-V behavior when time-dependent voltages are applied which result in hysteresis due to ferroelectric switching. The theoretical results provide unique insight into the effects of geometrical and material parameters on the electrical properties of the transistor. These parameters include the ferroelectric spontaneous and remanent polarization, the coercive field, and dielectric layer thicknesses. We have found that the conventional concept of threshold voltage is no longer useful, and that increasing the spontaneous polarization has only a minor impact on memory operation due to reverse dipole switching of the ferroelectric layer. The application of the model to optimize design and fabrication parameters is illustrated with a virtual prototyping example. The model is also used to develop a practical testing methodology for this unique device.

Integration of ferroelectric thin films into nonvolatile memories

Recent advances in the development of high quality thin films of ferroelectric materials have created renewed interest in ferroelectric random access memory (FERRAM) devices. At the present time, several groups are pursuing the development of FERRAMs, where a thin film ferroelectric capacitor memory element is integrated either adjacent to or directly above the circuit elements in a standard silicon or gallium arsenide integrated circuit. However, these designs provide destructive readout (DRO) only. An alternative monolithic device concept, where the ferroelectric thin film is integrated directly into the semiconductor field effect transistor (FET) element, provides not only nonvolatility, but also nondestructive readout (NDRO). For DRO FERRAM structures, lead–zirconate–titanate is the most widely investigated material at present. At Westinghouse, we are concentrating our efforts on the NDRO ferroelectric memory FET approach, using barium magnesium fluoride (BaMgF4) and bismuth titanate (Bi4Ti3O12) thin films. In this paper, we shall review the current status of the integration of ferroelectric thin films into nonvolatile memory devices, with special emphasis on our own results using BaMgF4 and Bi4Ti3O12.

Growth and the microstructural and ferroelectric characterization of oriented BaMgF/sub 4/ thin films

The growth of ferroelectric BaMgF(4) thin films on Si(100), sapphire, and other substrates under ultrahigh vacuum (UHV) conditions is reported. Microstructural characterization of the films using transmission electron microscopy (TEM) revealed that they were oriented crystalline films, although not epitaxial. Ferroelectric hysteresis measurements yielded spontaneous polarization and coercivity values of almost 1.0 muC/cm(2) and 160 kV/cm, respectively. The discrepancy with the bulk ferroelectric values were attributed to the electrical contacts, impurities in the film, and lack of polar axis orientation. Preliminary capacitance-voltage (C-V) hysteresis measurements on a 480-nm-thick BaMgF(4) film yielded a 10.8-V threshold shift (memory window) in response to a +/-10-V programming voltage for a MIS gate structure similar to that of the ferroelectric memory field-effect transistor (FEMFET).

Growth and characterization of ferroelectric BaMgF4 films

Experimental data are presented that confirm ferroelectric thin films of BaMgF4 can be grown by ultrahigh vacuum (UHV) sublimation on semiconductor and metallized substrates at temperatures below 500 °C. Hysteresis properties of the films are shown to be qualitatively consistent with those measured on thinned single-crystal samples cut perpendicular to the ferroelectric (orthorhombic) a axis. Loops from weakly oriented films on Si(100) display a Pr/Ps ratio of approximately 0.84, with Ps =1.6 μC/cm2 and EC =150 kV/cm. These values are explained on the basis of the film orientation and the measured properties for the bulk crystals. Metal–insulator–semiconductor structures formed with fluoride layers grown on lightly doped Si substrates yielded C–V switching properties indicating their suitability for use in high-performance ferroelectric field-effect transistor memory structures. In particular, switching attributable to polarization charge reversal inside the ferroelectric layer, rather than by tunnel injection of carriers from the Si into the ferroelectric, has been achieved for the first time with this material.