Seshu B. Desu

Electrical properties of metal-ferroelectric-insulator-semiconductor structures based on ferroelectric polyvinylidene fluoride copolymer film gate for nonvolatile random access memory application

Metal-ferroelectric–insulator-semiconductor device structures with ferroelectric vinylidene fluoride-trifluoroethylene copolymer and SiO2 buffer layer integrated gate stack over n-Si are formed, and their potential for fabricating polymeric nonvolatile random access memory devices is demonstrated. Capacitance-voltage (C–V) studies show that switchable polarization in poled polyvinylidene fluoride PVDF copolymer film changes the Si-surface potential and causes modulation of the Si-surface conductance. The (C–V) hysteresis and bidirectional flatband voltage shift at −10 to +6V, depending on the polarization field direction and remnant polarization at the ferroelectric PVDF copolymer gate, presents a memory window. The space charge at n-Si and switchable polarization both reduce the field across the ferroelectric PVDF. The observed asymmetry of the negative flatband-voltage shifts in the negatively poled ferroelectric polymer state is the result of the depletion layer formation, which reduces the field acros...

Thin films of layered-structure (1−x)SrBi2Ta2O9−xBi3Ti(Ta1−yNby)O9 solid solution for ferroelectric random access memory devices

We report on the thin films of solid–solution material (1−x)SrBi2Ta2O9−xBi3Ti(Ta1−yNby)O9 fabricated by a modified metalorganic solution deposition technique for ferroelectric random access memory devices. Using the modified technique, it was possible to obtain the pyrochlore free crystalline thin films at an annealing temperature as low as 600 °C. The solid–solution of layered perovskite materials helped us to significantly improve the ferroelectric properties, higher Pr and higher Tc, compared to SrBi2Ta2O9; a leading candidate material for memory applications. For example, the films with 0.7 SrBi2Ta2O9–0.3Bi3TiTaO9 composition and annealed in the temperature range 650–750 °C exhibited 2Pr and Ec values in the range 12.4–27.8 μC/cm2 and 68–80 kV/cm, respectively. The leakage current density was lower than 10−8 A/cm2 at an applied electric field of 200 kV/cm. The films exhibited good fatigue characteristics under bipolar stressing.

Ferroelectric properties of laser-ablated Sr1-xAxBi2Ta2O9 thin films (where A=Ba, Ca)

Bismuth-layered ferroelectric thin films of Sr1−xAxBi2Ta2O9, with composition x=0 and 0.2, were fabricated by using the pulsed-laser deposition technique. Structural characterization of the films by x-ray diffraction and atomic force microscopy, revealed that the films are polycrystalline in nature with average grain size of 180 nm. The films displayed spherical grains with a surface roughness of 12 nm. The ferroelectric measurements of Sr0.8Ba0.2Bi2Ta2O9, SrBi2Ta2O9, and Sr0.8Ca0.2Bi2Ta2O9 showed saturated hysteretic behavior with remanent polarization (2Pr) of 23.5, 17.9, 14 μC/cm2 and coercive field (Ec) of 31.06, 74.2, 86.3 kV/cm for a maximum applied electric field of 360 kV/cm. Films exhibited minimal (⩽17%) degradation of polarization for up to 1010 switching cycles. It was observed that the coercive field decreased with increase in the ionic size of partially substituted cations. The leakage current density of films were found to be of the order of ∼10−8 A/cm2 for up to a breakdown field of about ...

Low temperature processed 0.7SrBi2Ta2O9–0.3Bi3TaTiO9 thin films fabricated on multilayer electrode-barrier structure for high-density ferroelectric memories

Thin films of solid-solution material 0.7SrBi2Ta2O9–0.3Bi3TaTiO9 (0.7SBT–0.3BTT) were fabricated on n+-polycrystalline (n+-poly) Si substrates by a metalorganic solution deposition technique at a low processing temperature of 650 °C using a Pt–Rh/Pt–Rh–Ox electrode-barrier structure. The Pt–Rh/Pt–Rh–Ox structure was deposited using an in situ reactive radio frequency sputtering process. The electrodes had a smooth and fine-grained microstructure and were excellent diffusion barriers between the 0.7SBT–0.3BTT thin film and Si substrate. The ferroelectric (0.7SBT–0.3BTT) test capacitors using these electrode-barrier grown directly on Si showed good ferroelectric hysteresis properties, measured through n+-poly Si substrate, with 2Pr and Ec values of 11.5 μC/cm2 and 80 kV/cm, respectively, at an applied electric field of 200 kV/cm. The films exhibited good fatigue characteristics (<10% decay) under bipolar stressing up to 1011 switching cycles and the leakage current density was lower 10−7 A/cm2 at an applied...

Supression of size effects in ferroelectric films

Abstract It is widely reported that the dielectric permittivity of ferroelectric films decreases with decreasing film thickness, and understanding and controlling these size effects are very important for charge storage application of these films. By combining phenomenological theory with careful experimental work, we have shown that the form of the boundary condition for the polarization plays a decisive role in the manifestation of size effects in ferroelectric films. We have taken two extreme boundary conditions to prove our point. For the case normal electrodes, it is assumed that the boundary condition for the component of polarization vector at the ferroelectric/electrode interface is P = 0. This case corresponds to the presence of a strong edge field, resulting in “freezing out” of the ferroelectric polarization at the interface and thus exhibiting severe size effects. However, if one utilizes conductive oxide electrodes that are ferroelectric in nature the polarization would not vanish at the ferroelectric/electrode interface and therefore the size effects are largely suppressed. To prove our point and to eliminate grain size, stress, and compositional effects, epitaxial SrTiO3 thin films with stoichiometric composition on SrTiO3 single crystal substrates were investigated. In fact, the experimental data also indicates that the use of ferroelectric electrodes indeed suppresses the size effects.

Thickness dependence of leakage current in BaBi2Ta2O9 thin films

BaBi2Ta2O9 thin films having a layered structure were fabricated by metalorganic solution deposition technique. The films exhibited good structural, dielectric, and insulating properties. The room temperature resistivity was found to be in the range of 1012–1014 Ω cm up to 4 V corresponding to a field of 200 kV/cm across the capacitor for films annealed in the temperature range of 500–700 °C. The current-voltage (I–V) characteristics as a function of thickness for films annealed at 700 °C for 1 h, indicated bulk limited conduction and the log(I) vs V1/2 characteristics suggested a space-charge-limited conduction mechanism. The capacitance–voltage measurements on films in a metal–insulator–semiconductor configuration indicated good Si/BaBi2Ta2O9 interface characteristics and a SiO2 thickness of ∼5 nm was measured and calculated.

Improvement by surface modification of Ir electrode-barrier for Pb(Zr,Ti)O3-based high-density nonvolatile ferroelectric memories

Abstract We have tried to improve the Ir electrode-barrier for Pb(Zr,Ti)O 3 (PZT)-based non-volatile ferroelectric memories (NvFRAMs). Ir layer of 150-nm thickness was deposited on poly-Si substrates by means of the rf-sputtering method. Although Ir in itself acted as an oxygen barrier and a IrO 2 /PZT/Ir/poly-Si capacitor showed device-worth ferroelectric properties, whose remanent polarization, coercive field, fatigue loss after 10 11 switching repetitions were 25 μ C/cm 2 , 48 kV/cm and 12%, respectively, the high leakage current at the field of larger than 80 kV/cm was found, which might be caused by the reaction of PbO with bottom-Ir. Such poor leakage current behaviors could be successively improved by the insertion of vacuum-annealed thin IrO 2 (or IrO x ) layer of 20-nm thickness between PZT/Ir. We suggest that such improvements in the microstructures and the ferroelectric properties including the leakage current behaviors are attributed to the reduction of the reaction between PbO and Ir in bottom electrode by inserting the IrO x interlayer.

Studies on Laser Ablated SrBi 2 Ta 2 O 9 and Sr 0.8 Ca 0.2 Bi 2 Ta 2 O 9 Ferroelectric Thin Films

Ferroelectric thin films of SrBi 2 Ta 2 O 9 (SBT) and (Sr 0.8 Ca 0.2 )Bi 2 Ta 2 O 9 (SCBT) were grown on platinized silicon substrates by using pulsed laser deposition technique. The effect of annealing temperature on the structural and electrical properties of the films was studied. Films were grown at 200 mTorr oxygen pressure with a constant substrate temperature at 500°C and annealed at different temperatures ranging from 700-800 °C in an oxygen ambient. X-ray diffraction data showed that as-grown films were crystalline nature. Atomic force micrographs showed that the grain size and surface roughness increased with increase in annealing temperature. The SBT films annealed at 800 °C showed ferroelectric properties with remanent polarization of 9.1 w C/cm 2 and coercive field of nearly 72 kV/cm. Whereas the SCBT films showed maximum remanent polarization of 7.3 w C/cm 2 with higher coercive field of 86 kV/cm. The higher coercive field in case of SCBT is attributed to the higher electronegativity of partially substituted Ca at Sr site. The dielectric constant increased with increase in annealing temperature and was attributed to the higher grain size.

Novel Electrode Barriers for High Density Ferroelectric Nonvolatile Memories

In this paper, we propose a new electrode-barrier structure for direct integration of lead zirconate titanate (PZT) based capacitors onto polysilicon plug for high density FRAM applications. The electrode-barrier structure is based on noble metal alloys (e.g. Pt–Rh, Pt–Ir, Pt–Ru) and their oxides (e.g. PtRhOx) which satisfy the needs for an excellent diffusion barrier as well as a high conducting electrode. It has been found that the PtRhOx/PtRh/PtRhOx electrode-barrier structure acts as a very good bottom electrode on poly-silicon plug. The bottom PtRhOx layer has shown excellent diffusion barrier properties for lead, oxygen and silicon up to processing temperatures of 700 °C as established by Auger electron spectroscopy and Rutherford backscattering spectroscopy studies. The electrode barriers showed no hillock formation and PZT films deposited on this structure crystallized predominantly in the perovskite phase. The capacitors exhibited very good hysteresis properties with remnant polarization (Pr) of 16 μC/cm2, a coercive field (Ec) of 32 kV/cm, extremely low fatigue (after 1×1010 cycles) of 3–4%, imprint (after 3.3×109 cycles) of 8–12%, retention (after 1×105 s) of 2–3% and leakage current density (at 100 kV/cm) less than 10—8 A/cm2. These electrode barriers hold excellent promise for application in high density FRAM capacitor over bit line (COB) structure.

Memory window in ferroelectric PVDF copolymer gate integrated MOSFET devices for nondestructive readout memory application

Metal-Ferroelectric-Oxide-Si (MFEOS) field effect transistor (FET) with ferroelectric polyvinylidene fluoride trifluoroethylene copolymer (PVDF-TrFE) gate for nonvolatile memory application is demonstrated. Memory window ascribed to ferroelectric polarization switching has been quantified by shift of threshold voltage are ~ 4-5V. Non saturating IDS is due to free ionic polarization field. I DS -V DS characteristics of functional FET are realized after AC poling.