Ching-Chich Leu

Domain structure study of SrBi2Ta2O9 ferroelectric thin films by scanning capacitance microscopy

Scanning capacitance microscopy was used to image the polarization-induced microstructural patterns of sol-gel derivative SrBi2Ta2O9 (SBT) thin films. A sharp image contrast was induced between the nanosized domains owing to the various polarities, so that the domain structure in the SBT thin film was clearly revealed. As a result, the switched and unswitched regions could be unequivocally identified. This investigation also confirms that the reversal polarization process of a ferroelectric domain is much easier inside a large grain than in a small grain.

Study of thermal stability of nickel monogermanide on single- and polycrystalline germanium substrates

We have investigated the thermal stability of nickel monogermanide (NiGe) films formed by rapid thermal annealing on both single- and polycrystalline Ge substrates. We found that the NiGe phase is the only one present after nickel germanidation in the temperature range 400–700°C. A fairly uniform NiGe film formed on the single-crystalline Ge; it possessed excellent resistivity (15.6μΩcm) and was thermally stable up to 550°C, but it degraded rapidly at higher temperatures as a result of agglomeration. In contrast, the NiGe film formed on the polycrystalline Ge exhibited much poorer thermal stability, possibly because of polycrystalline Ge grain growth, which resulted in columnar NiGe grains interlaced with Ge grains that had a dramatically increased sheet resistance. As a result, we observed that the sheet resistances of NiGe lines subjected to annealing at 500°C depended strongly on the linewidth when this width was comparable with the grain size of the polycrystalline Ge.

High-performance Pt/SrBi/sub 2/Ta/sub 2/O/sub 9//HfO/sub 2//Si structure for nondestructive readout memory

Metal-ferroelectric-insulator-semiconductor (MFIS) capacitors with 390-nm-thick SrBi/sub 2/Ta/sub 2/O/sub 9/ (SBT) ferroelectric film and 8-nm-thick hafnium oxide (HfO/sub 2/) layer on silicon substrate have been fabricated and characterized. It is demonstrated for the first time that the MFIS stack exhibits a large memory window of around 1.08 V at an operation voltage of 3.5 V. Moreover, the MFIS memory structure suffers only 18% degradation in the memory window after 10/sup 9/ switching cycles. The excellent performance is attributed to the formation of well-crystallized SBT perovskite thin film on top of the HfO/sub 2/ buffer layer, as evidenced by the distinctive sharp peaks in X-ray diffraction (XRD) spectra. In addition to its relatively high /spl kappa/ value, HfO/sub 2/ also serves as a good seed layer for SBT crystallization, making the proposed Pt/SrBi/sub 2/Ta/sub 2/O/sub 9//HfO/sub 2//Si structure ideally suitable for low-voltage and high-performance ferroelectric memories.

Effects of tantalum adhesion layer on the properties of SrBi2Ta2O9 ferroelectric thin films

The effects of tantalum (Ta) adhesion layer on the ferroelectric and microstructural properties of sol-gel-derived SrBi2Ta2O9 (SBT) films are reported in this study. Compared to the traditional titanium (Ti) adhesion layer, the Ta adhesion layer results in more favorable, highly (115) textural structure of SBT films and therefore higher polarization and dielectric constant. The remnant polarization value of the SBT films crystallized at 750 °C increases from 11.1 to 14.2 μC/cm2 at 5 V, and the dielectric constant increases from 175 to 225. The observed improvement in the electrical properties of SBT films is ascribed to the superior microstructure of Pt thin film on Ta, which has been characterized by x-ray diffraction spectrum (XRD). XRD patterns clearly indicate that the Ti adhesion layer favors c-axis crystalline structure that is undesirable for ferroelectric properties. Moreover, secondary ion mass spectrometer profiles strongly indicate that Ti atoms diffuse deeply into the bulk of SBT thin films af...

Effects of titanium and tantalum adhesion layers on the properties of sol-gel derived SrBi2Ta2O9 thin films

The effects of titanium (Ti) and tantalum (Ta) adhesion layers on the ferroelectric and microstructural properties of sol-gel-derived SrBi2Ta2O9 (SBT) films are investigated. It is found that the atoms from the adhesion layer play a significant role in the resultant microstructures and physical properties of SBT films. The electron spectroscopy for chemical analysis clearly indicates that both the Ti and Ta atoms of adhesion layers have out-diffused behavior onto the surface of bottom-electrode Pt films after a thermal treatment of 750 °C, 1 min. Various out-diffused species do cause the distinct properties of SBT films, which are confirmed by the results of surface analysis and polarization-electric field (P-E) measurements. The formation of undesirable second phase compounds near the SBT/PT interface has been observed in specimens with Ti layer, and it is speculated to be the significant degradation of spontaneous polarization. On the contrary, Ta species are found to exhibit the pure bismuth-layered st...

Improved performance of nanocrystal memory for aminosilane-mediated Au–SiO2 embedded core–shell nanoparticles

We present a memory property study of floating gate structure embedded with Au–SiO2 core–shell nanoparticles (NPs). Herein, the Au–SiO2 core–shell configuration was readily fabricated by a self-assembly (SAM) layer-by-layer process using 3-aminopropyltrimethoxysilane (APTMS) as a versatile mediator. This functional APTMS served as both a binder for adsorbing colloidal Au NPs onto the substrate and to self-organize a SiO2 ultra-thin shell to cover the Au NP cores. A two-run APTMS SAM process was employed to develop the core–shell configuration. The first-run APTMS formed a well-organized monolayer on the substrate which was responsible for the obtained uniform SAM with high density of Au NPs. Next, the second-run APTMS formed an APTMS bilayer around the Au NPs. During the SAM process, a polymerization process simultaneously occurred with the reaction of neighboring silanol groups to form an Si–O–Si network structure. The polymerization was completed by a 400 °C-annealing to form a SiO2 layer within the APTMS bilayer. In addition, the annealing also resulted in the decomposition of the APTMS and released the preformed SiO2 layer from the APTMS bilayer. The ultra-thin SiO2 layer was flexible enough to cover the Au NP cores, eventually constructing the Au–SiO2 core–shell structure. Compared to Au nanocrystal (NC) memory, Au–SiO2 core–shell NCs show a larger memory window and significantly improved retention performance, which are attributed to the SiO2 shell-induced superior NP/oxide interface qualities and the characteristics of the Au NPs.

Variations of differential capacitance in SrBi2Ta2O9 ferroelectric films induced by photoperturbation

In this letter, we demonstrated the impact of illumination on the differential capacitance variation of a strontium bismuth tantalite (SBT) capacitor during scanning capacitance microscopy measurements. It was found that illumination with a stray light of laser in an atomic force microscope could perturb the dC∕dV signals of the samples. We attribute this phenomenon to the generation of free carriers by the photon absorptions via defect traps in the SBT thin film. Therefore, this present work suggests that the effect of laser illumination must be carefully taken into consideration whenever a field-sensitive technique is employed to analyze the properties of a ferroelectric material.

Effects of HfO 2 buffer layer thickness on the properties of Pt/SrBi 2 Ta 2 O 9 /HfO 2 /Si structure

We investigated structural and characteristic changes in thin HfO 2 films ( 2 Ta 2 O 9 /HfO 2 /Si metal/ferroelectric/insulator/semiconductor (MFIS) structures. HfO 2 films with different thicknesses were found to exhibit rather distinct characteristics and to profoundly affect the properties of the fabricated MFIS capacitor. We found that, when employing 3.2-nm-thick HfO 2 as the buffer layer, the MFIS capacitor showed good memory performance at low operation voltage. However, this study demonstrated that some of the HfO 2 limited its application in MFIS memory, even though it is the most promising alternative gate dielectric material.

Properties of Pt/SrBi2Ta2O9/BL/Si MFIS structures containing HfO2, SiO2, and Si3N4 buffer layers

The physical and electrical properties of Pt/SrBi 2 Ta 2 O 9 (SBT)/buffer layer (BL)/Si metal/ferroelectric/insulator/semiconductor (MFIS) structures incorporating HfO 2 , SiO 2 , and Si 3 N 4 as buffer layers were investigated. When employing HfO 2 as the buffer layer, an MFIS structure exhibiting a high memory ratio was constructed, presumably because of the SBT characteristics and the high quality of the HfO 2 layer on the Si substrate. This study demonstrates that HfO 2 is one of the best buffer-layer materials for ferroelectric memory applications.

Influence of Ultrathin Tantalum Buffer Layers on Microstructure and Ferroelectric Properties of SrBi2Ta2 O 9 Thin Films

The effects of buffer layers on the crystallization process of strontium bismuth tantalite (SBT) thin films, using a metallorganic decomposition technique, are investigated in this study. Ultrathin tantalum (Ta) buffer layers of various thicknesses were deposited onto Pt/TiO 2 /SiO 2 /Si substrates using magnetron sputtering. The crystallinity, microstructure, and electrical properties of the resulting SBT films on top of the Ta layer were found to be strongly dependent on the thickness of the buffer layer. By optimizing this buffer layer thickness, a homogeneous bismuth-layered structure with uniformly distributed fine grains, it was clearly evident that the SBT film, annealed at 750°C for 1 min, and the corresponding remanent polarization (2P r ), can be as large as 18.8 μC/cm 2 at an applied voltage of 5 V. It is suggested that these buffer layers cause the SBT composition to depart from stoichiometry to an enriched Ta condition. During the crystallization procedure in oxygen ambient, the excess Ta atoms in local regions easily reacted with O atoms to form TaO x centers. They are believed to serve as the nucleation sites that cause the reduction of the activation energy of SBT crystallization.