Min In O2 Atmosphere Research Articles

Monolayer FePt nanocrystal self-assembly embedded into atomic-layer-deposited Al2O3 films for nonvolatile memory applications

A simple approach is developed to fabricate oxide/FePt nanocrystals/oxide composite films by a combination of chemically-synthesized FePt nanocrystals’ self-assembly and atomic layer deposition for ultrahigh-density nonvolatile memory applications. A hexagonally arranged monolayer of well-monodispersed FePt nanocrystals with a grain size of 4.5nm has been assembled onto atomic-layer-deposited Al2O3 oxide by solution-based dip-coating. The lattice constant of this two-dimensional pattern is about 8nm with the high density of 1.8×1012/cm2. A fraction of the Fe is oxidized during annealing at 500°C for 5min in O2 atmosphere, and the core–shell structure is formed with fcc-Fe0.75Pt nanocrystal core and amorphous Fe2O3 shell. The metal–oxide–Si capacitors with unannealed and annealed FePt nanocrystals embedded into Al2O3 films are electrically measured, and exhibit obvious memory effects with a hysteresis memory window of 4.1 and 8.1V at the sweeping gate voltage of±8V, respectively. The enhanced memory window of samples with annealed FePt nanocrystals can be attributed to the existence of the Fe2O3 shell, which introduces additional interface and provides more trap sites for charge trapping storage.

Characterization of HfTaO films for gate oxide and metal-ferroelectric-insulator-silicon device applications

Characteristics of HfTaO thin films have been investigated from the viewpoints of their applications as insulating buffer layer in the metal-ferroelectric-insulator-semiconductor (MFIS) structure and gate oxide for next generation metal-oxide-silicon devices. HfTaO films with 2, 4, and 6nm in thickness have been deposited on Si substrates using electron beam evaporation and annealed typically at 800°C for 1min in O2 atmosphere. In fabrication of MFIS diodes, SrBi2Ta2O9 (SBT) films have been deposited on HfTaO using a sol-gel spin-coating method. It has been found from C-V (capacitance versus voltage) and I-V (current versus voltage) characteristics of Al∕HfTaO∕Si diodes with different film thicknesses that the optimum annealing temperature is 800°C and the dielectric constant of HfTaO is about 17. For MFIS devices with Pt∕SrBi2Ta2O9 (300nm)∕HfTaO(4nm)∕Si gate structures, a memory window of 0.65V between +4 and −4V gate voltage sweeping was observed. Furthermore, the high and low capacitance values biased at the center of the hysteresis loop are clearly distinguishable for over 24h, demonstrating excellent long-term retentions properties.

THE OPTIMIZATION OF EXCESS Pb CONTENT IN SOL-GEL DEPOSITED SANDWICH STRUCTURE PbTiO3/PbZr0.3Ti0.7O3/PbTiO3 THIN FILMS

ABSTRACT The PbTiO3/PbZr0.3Ti0.7O3/PbTiO3 (PT/PZT/PT) and PbZr0.3Ti0.7O3 (PZT) thin films were prepared by sol-gel method. In order to optimize the excess Pb content in PT layers, different excess Pb content (x = 0, 0.05, 0.10, 0.15, 0.20) were added to the PT precursor to prepare the PT/PZT/PT films, and the films were annealed at 550∼700 °C for 20 min in O2 atmosphere. The X-ray diffraction (XRD) results show that the pure perovskite structure PT/PZT/PT films can be obtained from the proper Pb/Ti ratio precursor solution (excess Pb content x = 0.10 and 0.15 in PT layers) and annealing temperature of 600∼700 °C. The results of the P–E hysteresis loops indicate that the most proper annealing temperature is 650 °C. Well-saturated hysteresis loops with higher remnant polarization (Pr) and lower conceive field (Ec) are obtained for the film with excess Pb content x = 0.15 in PT layers. The fatigue and leakage current properties indicate that the PT/PZT/PT thin film with excess Pb content x = 0.10 has the lowest fatigue rate and less leakage current density than other films. Considering the results of XRD, ferroelectric, fatigue and leakage current properties, the optimized excess Pb content (x) in PT layers and annealing temperature should be x = 0.10∼0.15 and 650 °C, respectively.

Ferroelectric Properties of Mo-Doped Bi4-XLaXTi3O12 Films

Mo-doped Bi3.35La0.75Ti3O12 (BLTM) films were deposited on Pt/Ti bottom electrodes by using a sol-gel method and crystallized at 700°C for 30 min in O2 atmosphere. The ferroelectric properties were greatly improved by substituting B-site ions with Mo ions and the BLTM films showed strong preferred (117) orientation. Typical remanent polarization (2Pr) and coercive field (2Ec) values were 32.0 μCċcm−2 and 158 kVċcm−1, respectively. It was also found that the leakage current densities in the BLTM films were lower than those in Bi3.35La0.75Ti3O12 (BLT) films.

Fabrication of MFIS diodes using sol-gel derived SBT films and LaAlO3 buffer layers

We demonstrate the ferroelectric behavior of Sr0.8Bi2.2Ta2O9 (SBT) films grown on Si(100) substrates by using lanthanum aluminate (LaAlO3) buffer layers. LaAlO3 films were prepared by vacuum evaporation method. Then, they were subjected to ex situ dry N2 annealing in a rapid thermal annealing (RTA) furnace. From the capacitance-voltage (C-V) measurement, the dielectric constant of LaAlO3 was estimated to be 20∼25. On these substrates, SBT films (210nm) were deposited by sol-gel method and they were characterized by XRD analysis after annealing under various conditions. It was found from C-V characteristics that the memory window of an SBT film annealed at 750°C for 30min in O2 atmosphere was about 3.0V for the voltage sweep of ±10V. It was also found from the retention measurement that the capacitance values of the SBT film annealed at 750°C did not change over 12hours. It is concluded from these results that the SBT/LaAlO3/Si(100) structure is one of the most promising structures for realizing MFISFETs (metal-ferroelectric-insulator-semiconductor field-effect-transistors).

Effect of Deposition Temperature and Composition on the Microstructure and Electrical Property of SrBi2Ta2O9 Thin Films Prepared by Metalorganic Chemical Vapor Deposition

SrBi2Ta2O9 (SBT) thin films were prepared by conventional thermal metalorganic chemical vapor deposition (MOCVD) and subsequent heat treatment. The SBT film deposited at 500°C had a smoother surface and better step coverage than that deposited at 750°C. The degree of step coverage deposited at 500°C was 0.82. An almost single phase of SBT was obtained for the film with a Bi/Ta mole ratio of 1.0 by heat treatment at 750°C for 30 min in O2 atmosphere after MOCVD deposition at 500°C. 2Pr and EC at an applied electric field of 620 kV/cm were 12.2 µC/cm2 and 87 kV/cm, respectively, when the film was deposited at 500°C followed by heat treatment at 800°C for 30 min in O2 atmosphere, and its Bi/Ta ratio was 1.2.

Charge transport and trapping in BaTiO3 thin films flash evaporated on Si and SiO2/Si

BaTiO3 (BT) thin films were prepared by flash evaporation onto p-Si and SiO2/p-Si from a Re boat at 1800°C in ultra high vacuum. The films are amorphous and remain so after a thickness reduction by 10–20% upon annealing at 500°C for 3min in O2 atmosphere. Annealing raises, also, the index of refraction by some 5% and the extinction coefficient by a factor of 2. Electron injection at the Al to BT interface of Al/BT/Si capacitors is enhanced by the Schottky effect, yielding a value of 11.2 for the dielectric constant of BT. Modeling current–time measurements yields a trap density of 1024 m−3, 0.82eV below the conduction band. Capacitance–voltage curve shifts due to bias stress on Al/BT/SiO2/Si capacitors are interpreted as caused by electron injection and trapping in the BT films. Starting deposition at 170°C or post-deposition annealing reduces the trap density and increases the capacitance–voltage curve shifts by bias stress, from 0.3 to over 14V at bias stress of −10V.

Effects of H2 Sintering and Pt Upper Electrode on Metallic Bi Content in Sr0.9Bi2.1Ta2O9 Thin Films for Ferroelectric Memories Prepared by Sol-Gel Method

The effects of Pt electrode and H2 sintering on the metallic Bi content of Sr0.9Bi2.1Ta2O9 (SBT) thin films for ferroelectric memories were studied using X-ray photoelectron spectroscopy (XPS). Oxidic Bi in SBT films is reduced to metallic Bi by H2 sintering. The degree of reduction depends on the structure of the SBT film. The SBT film with a fluorite structure is more difficult to reduce than that with a Bi-layered structure. Pt upper electrode formation also leads to an increase of metallic Bi content. Both H2 sintering and the Pt upper electrode work synergistically. In the combination of the pure Pt upper electrode and H2 sintering, over 80% of Bi on the SBT film surface was metallic. Pt films of 5 nm thickness on the SBT film recrystallized during the heat treatment and were broken up into particles. Pt electrode coverage was about 20% after the 2nd annealing at 800°C for 30 min in O2 atmosphere. Electrical properties were significantly affected by the presence of metallic Bi; in particular, SBT films with higher metallic Bi content showed higher leakage current density. The metallic Bi content must therefore be suppressed to obtain SBT films with low leakage current density.

Malleability and metallography of nickel

In order to obtain oxide films using catalytic chemical vapor deposition (Cat-CVD), oxidation experiments for catalyzers of Alumel, Al Chrom-O, Chromel, Hastelloy C-276, Kanthal, Kovar, Inconel-600, Inconel-601, Inconel X-750, Iron Chrome 30, Moleculoy, Monel, Ni, Nickel Chrome, Pt, SUS-304, SUS-316, Super Invar, and Ti have been carried out. The electric resistance measured for each metal heated at 900 °C, exposed for 30 min in O2 atmosphere, have revealed that Alumel, Al Chrom-O, Chromel, Hastelloy C-276, Kanthal, Inconel-600, Inconel-601, Inconel X-750, Iron Chrome 30, Moleculoy, Ni, Nickel Chrome, Pt, SUS-304, and SUS-316 are in resistance to oxidation.