Type Ferroelectric Random Access Memories Research Articles

Investigating structural features of Ba and Zr co-substituted strontium bismuth tantalate thin films

Structural (crystal and microstructure), chemical and electronic states, and ferroelectric and electrical features of Ba and Zr co-substituted strontium bismuth tantalate (SBT) were probed in this study. Distinctly, Ba and Zr were substituted for Ta and Sr sites of \(\hbox {Sr}_{0.8}\hbox {Bi}_{2.2}\hbox {Ta}_{2}\hbox {O}_{9 }\) in the form of \(\hbox {Sr}_{0.8-x}\hbox {Ba}_{x}\hbox {Bi}_{2.2}\hbox {Ta}_{2-y}\hbox {Zr}_{y}\hbox {O}_{9}\). To investigate the impact of the co-substitution on the crystal structure, microstructure, ferroelectric and electrical properties, \(\hbox {Sr}_{0.8-x}\hbox {Ba}_{x}\hbox {Bi}_{2.2}\hbox {Ta}_{2-y}\hbox {Zr}_{y}\hbox {O}_{9}\) thin films were deposited on \(\hbox {Pt/Ti/}\hbox {SiO}_{2}/\hbox {Si}(100)\) wafers by sol–gel spin by coating method. Crystal structure, microstructure, chemical and electronic states, ferroelectric, capacitance and leakage current characteristics of the films were studied to investigate potential for one transistor type ferroelectric random access memories (1T-type FeRAMs). Successful substitutions up to 10 mol% lead to reduction of double remanent polarization \((2P_{\mathrm{r}})\) to \(10.26\,\upmu \hbox {C\,cm}^{-2}\), and dielectric constant \((\varepsilon _{\mathrm{r}})\) to 135. These values demonstrate that successful co-substitution of limited Ba and Zr in SBT with stable crystal structure has the ability to decrease \(P_{\mathrm{r}}\) and \(\varepsilon _{\mathrm{r}}\) values of the ferroelectric material which can be a candidate gate to be utilized in ferroelectric field-effect transistors (FeFETs) for 1T-type FeRAM applications.

Metal-Ferroelectric-Semiconductor Field Effect Transistor (MFS-FET) for 1T-type FRAM Based on Polyvinylidene Fluoride (PVDF) Thin Film

The metal-ferroelectric-semiconductor field effect transistor (MFSFET) was fabricated using polyvinylidene fluoride (PVDF) thin film as a ferroelectric layer. PVDF thin films of 4 and 6 wt% were spin-coated on Si(100) wafers. Ferroelectric hysteretic curves inducing a counterclockwise loop, which means that the deposited PVDF films were crystallized with ferroelectric β phase, was exhibited in the drain current–gate voltage (I D –V G ) characteristics of MFSFET. The memory window widths of the MFSFET were more than 1.6 V. The MFSFET that operates with the PVDF thin film used as a gate dielectric material is shown in the drain current–drain voltage (I D –V D ) characteristics. The MFSFET using PVDF as ferroelectric layer has promising potential for use in low-voltage operable one transistor (1T) type ferroelectric random access memories (FeRAM) using organic material.

Fabrication and Electrical Characteristics of Metal–Ferroelectric–Semiconductor Field Effect Transistor Based on Poly(vinylidene fluoride)

We fabricated the metal–ferroelectric–semiconductor field effect transistor (MFSFET) using poly(vinylidene fluoride) (PVDF) thin film as a ferroelectric layer. PVDF thin films were prepared by spin-coating PVDF solutions of 2–6 wt % on Si(100) wafers with source and drain diffusion regions. The drain current–gate voltage (ID–VG) characteristics of MFSFET exhibited ferroelectric hysteretic curves inducing a counterclockwise loop similar to that of other ferroelectric materials. It seems that the deposited PVDF films were crystallized with the ferroelectric β phase. The memory window widths of the MFSFET were more than 1.0 V. The drain current–drain voltage (ID–VD) characteristics show that the MFSFET operates with the PVDF thin film used as a gate dielectric material. The MFSFET using PVDF as ferroelectric layer has promising potential for use in low-voltage operable one-transistor (1T)-type ferroelectric random access memories (FeRAMs) using organic material.

Electrical Characteristics of Metal–Ferroelectric–Semiconductor Structures Based on Poly(vinylidene fluoride)

We prepared poly(vinylidene fluoride) (PVDF) films in the β phase by a sol–gel method using PVDF solutions with different weight fractions. The electrical properties of PVDF films were studied for the possibility of using them in one-transistor (1-T)-type ferroelectric random-access memories (FRAMs). In this research, the Au/PVDF/Si structure made from PVDF solution of 6 wt % concentration showed good ferroelectric and electrical properties. The memory window width and current density for the PVDF film from the 6 wt % PVDF solution were about 1.8 V and 10-6 A/cm2 for a bias voltage of 5 V, respectively. It is expected from these good ferroelectric properties that PVDF films will be suitable for low-cost 1-T-type FRAMs operating at a low voltage.

APPLICATIONS OF BISMUTH-LAYERED PEROVSKITE THIN FILMS TO FET-TYPE FERROELECTRIC MEMORIES

ABSTRACT Recent progress in FET (field-effect transistor)-type ferroelectric random access memories is reviewed. Effectiveness of a HfO2 buffer layer, which is inserted between a ferroelectric film and Si substrate for preventing interdiffusion of constituent elements, is first described. It is shown in the FET with Pt/SrBi2Ta2O9/HfO2/Si gate structure that the drain current on/off ratio is larger than 103 even after 30 days have elapsed. Then, the operation principle of a 1T2C-type memory cell, in which two ferroelectric capacitors with the same area are connected to the gate of a MOSFET, is discussed and basic characteristics of a 1 kbit array composed of 1T2C-type cells are presented.

Current Status of Ferroelectric Random-Access Memory

AbstractThe current status of ferroelectric random-access memory (FeRAM) technology is reviewed in this article. Presented first is the status of conventional FeRAM, in which the memory cells are composed of ferroelectric capacitors to store the data and cell-selection transistors to access the selected capacitors. Discussed next are recent developments in the field. Pb(Zrx, Ti1–x)O3 (PZT) and SrBi2Ta2O9 (SBT) films are being used to produce 0.13 mμ and 0.18 μm FeRAM cells, respectively, with a stacked capacitor configuration; these cells are easily embedded into logic circuits. A new class of FeRAM called 6T4C—containing static RAM (SRAM) cells composed of six transistors (6T) and four ferroelectric capacitors (4C)—has been commercially produced. This type of FeRAM features a nondestructive readout operation, unlimited read/write cycling, and a fast access time of less than 10 ns. Lastly, the status of field-effect-transistor (FET)-type FeRAM is reviewed, emphasizing that the data retention time of a ferroelectric-gate FET has been improved to more than a month in recent studies.

Operation of single transistor type ferroelectric random access memory

Verification was sought for the memory operation of a single transistor type ferroelectric random access memory (1T type FeRAM) with a circuit model for a memory cell transistor combined with a precharged capacitive decoupling sensing scheme. The wiring scheme of the 1T type FeRAM array was also proposed based on the operation of the fabricated memory cell transistor. As a result, the memory operation of 1T type FeRAM was confirmed at a low current level with high sensing speed and no reference cell, and the design and verification of the full chip were achieved.

Oxygen radical treatment applied to ferroelectric thin films

A low dielectric constant ferroelectric Sr 2(Ta 1− x ,Nb x ) 2O 7 (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 PVD and an oxygen radical treatment using microwave-excited (2.45 GHz) high-density (>10 12 cm −3) low electron temperature (<1 eV) Kr/O 2 plasma. Oxygen radical treatment can effectively oxidize ferroelectric film at 400 °C.

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.

YMnO3 and YbMnO3 Thin Films for fet type FeRam Application

AbstractWe have been proposing the use of RMnO3 (R: rare earth elements) films for metalferroelectric- semiconductor field effect transistor (MFSFET)-type ferroelectric random access memories (Ferroelectric RAMs). This report describes the progress of YMnO3 and YbMnO3 films for FET type FeRAM application. Although highly (0001)-oriented YMnO3 films are easily obtained on a MgO, ZnO/Sapphire, Pt/Sapphire and Pt/Si substrates, it was very hard to obtain the crystalline films directly on Si or on SiO2/Si substrate. A Y-Mn-O buffer layer improved the crystallinity of the YMnO3 films on Si, and we got the C-V curve with ferroelectric hysteresis. The real ferroelectric component responsible for the C-V hysteresis was calculated to be just 8.4 nC/cm2 by pulse measurements. On the other hand, Y2O3 buffer layer drastically improved the dielectric properties. The window width of the C-V hysteresis does not change by changing the sweep rate and measurement frequency.

Sr2(Ta, Nb)2O7 ferroelectric thin film for ferroelectric memory FET

Sr2Nb2O7 (SNO) family are suitable as ferroelectric materials for ferroelectric memory FETs, because they have low dielectric constant, low coercive field E c and high heat-resistance. In this study, we succeeded to prepare the Sr2(Ta, Nb)2O7 (STN) capacitors on the poly-Si. These capacitors were applied to FFRAM (Floating gate type Ferroelectric Random Access Memory) cells. The STN thin films were prepared by the sol-gel method. After several times spin coating and preannealing, these films were crystallized at about 950°C in oxygen for 30 sec by RTA (Rapid Thermal Annealing). When the Nb/(Ta+Nb) ratio was between 0.1 and 0.3, the ferroelectricities were confirmed. The maximum remanent polarization P r showed 0.4μC/cm2 when the ratio was 0.3. The FFRAM cell using STN thin films showed good memory characteristics.

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.

Effects of Stoichiometry and A-site Substitution on the Electrical Properties of Ferroelectric YMnO3

We proposed the use of RMnO3 (R: rare-earth elements) for metal-ferroelectric-insulator-semiconductor field-effect transistor (MFIS-FET) type ferroelectric random access memories (Ferroelectric RAMs). To decrease the leakage current in this material, the effects of nonstoichiometry and A-site substitution were studied. Current-voltage characteristics revealed that the carriers in the RMnO3 originate in excess oxygen and show Frenkel-Poole-type emission. Significant grain growth was observed in the Mn-rich composition. In particular, Mn-rich YbMnO3 shows the lowest leakage current and the best frequency dependence of dielectric permittivity.

Ferroelectric properties of c-oriented YMnO3 films deposited on Si substrates

We have proposed the use of RMnO3 (R: rare earth elements) films for metal–ferroelectric–semiconductor field effect transistor (MFSFET)-type ferroelectric random access memories (ferroelectric RAMs). This reports the production of YMnO3 films on Si substrates for MFSFET with confirmation of the distinct ferroelectricity by P–E hysteresis and capacitance–voltage (C–V) measurement. (0001)-oriented YMnO3 films were obtained on a (111)Si substrate using a pulsed-laser deposition method. Although the Pt/YMnO3/Si structure exhibits a very small remnant polarization of 1.2 nC/cm2, it has clear ferroelectric polarization switching type C–V characteristics with a memory window of 1.1 V. The dielectric constant and the dissipation factor were 27.8 and 0.035, respectively. The polarization switching characteristics are discussed.

Fabrication of YMnO3 Thin Films on Si Substrates by a Pulsed Laser Deposition Method

We have proposed ReMnO3 (Re: Rare earth elements) films for metal-ferroelectric-semiconductor field effect transistor-(MFSFET) type ferroelectric random access memories (Ferroelectric RAMs). For this kind of application, since ReMnO3 films must be fabricated directly on Si substrates, this wrote focuses on the fabrication of YMnO3 films on Si substrates with and without buffer layers. Although only amorphous YMnO3 films are obtained without buffer layers, crystalline films are obtained using Y–Mn–O buffer layers deposited without introducing oxygen. YMnO3 films can be epitaxially grown on the top of epitaxial Y2O3 on (111)Si. For both crystalline YMnO3 films, although hysteresis behavior in D-E measurement is observed, remanent polarizations are quite small. The role of buffer layers on the thin film growth of YMnO3 on (111)Si is discussed.