The Characteristics of Magnesium Titanate Thin Film as Buffer Layer by Electron Beam Evaporation

We have studied the application of magnesium titanate thin films as buffer layer for the improvement of adhesion of Pt films to Si substrate. Magnesium titanate films were successfully prepared on Si(100) substrate by electron beam evaporation. The crystal phase of MgTi₂O₅ and MgTiO₃ films on Si substrate were observed. These films had a very smooth and densely packed surface morphology and showed a good characteristic as the adhesion layer for Pt. Also, AES analysis showed the excellent properties as the reaction barrier layer between Pt and Si.     

Properties of PbZrxTi1−xO3/CeO2/SiO2/Si structure

Abstract

We report the crystalline quality and electrical properties of PbZr_xTi_1?xO₃ (PZT) films on n-type Si(100) substrates with CeO₂/SiO₂ dual buffer layers. PZT films and CeO₂ buffer layers were prepared by pulsed laser deposition technique, and SiO₂ buffer layers were formed by thermal dry oxidation. It was found that CeO₂/SiO₂ dual buffer layers effectively prevented Si and Pb interdiffusion between PZT and Si substrates. Furthermore, the capacitance-voltage (C-V) characteristics of the PZT/CeO₂/SiO₂/Si heterostructures demonstrated ferroelectric switching properties, showing a memory window as large as 2.7 V at 1 MHz.     

Characterization of the CeO2 thin films for insulation layer and Pt/SrBi2Ta2O9/CeO2/Si MFISFET structure

Abstract

CeO₂ and SrBi₂Ta₂O₉ (SBT) thin films for MFISFET (metal-fcrroelectrics-insulator-semiconductor field effect transistor) were deposited by rf sputtering and pulsed laser deposition method, respectively. The effects of oxygen partial pressure during deposition for CeO₂ films were investigated. The oxygen partial pressure significantly affected the preferred orientation, grain size and electrical properties of CeO₂ films. The CeO₂ thin films with a (200) preferred orientation were deposited on Si(100) substrates at 600°C. The films deposited under the oxygen partial pressure of 50 % showed the best C-V characteristics among those under various conditions. The leakage current density of films showed order of the 10^?7～10^?8 A/cm² at 100 kV/cm. The SBT thin films on CeO₂/Si substrate showed dense microstructure of polycrystalline phase. From the C-V characteristics of MFIS structure composed of the SBT film annealed at 800°C, the memory window width was 0.9 V at ±5 V. The leakage current density of Pt/SBT/CeO₂/Si structure annealed at 800°C was 4×10^?7 A/cm² at 5 V.     

Growth and characterization of UHV sputtering HfO2 film by plasma oxidation and low temperature annealing

Ultra-thin (∼4.0 nm) HfO₂ films were fabricated by plasma oxidation of sputtered metallic Hf films with post low temperature annealing. Advantage of this fabrication process is that the pre-deposition of Hf metal can suppress the formation of interfacial layer between HfO₂ film and Si substrate. The as-deposited HfO₂ films were subsequently treated by rapid thermal annealing at different temperatures in N₂ to investigate the effects of thermal annealing on the physical and electrical properties of HfO₂ film. A SiO₂-rich interface layer was observed after higher temperature rapid thermal annealing and the phase change of HfO₂ film from amorphous into crystalline occurred at about 700^∘C. As a result of higher temperature annealing, effective dielectric constant and leakage current were significantly influenced by the formation of interface layer and the crystallization of HfO₂ film.     

Preparation of Textured Growth Pb(Zr,Ti)O3 Thin Films on Si Substrate Using SrTiO3 as Buffer Layers

PZT thin films are deposited on SiO₂/Si substrate by metallo-organic decomposition (MOD) process, using SrTiO₃ (STO) as buffer layer for textured growth. The STO layers deposited on SiO₂/Si substrate by pulsed laser deposition process show (100)/(200) preferred orientation, whereas the STO buffer layer deposited on silica substrate using spin-coating technique show random orientation behavior. The use of STO as buffer layers enhanced the crystallization and the preferred orientations of the PZT films. The PZT on STO buffered SiO₂/Si substrates thus obtained possess high refractive index, (n)_PZT/STO = 2.1159, and are of good enough quality for optical waveguide applications.     

Novel diffusion control process using ultra thin buffer layer for MFIS memory

Abstract

MFIS structures having excellent clear interfaces and well-crystallized ferroelectric layer were successfully fabricated by a newly developed ultra thin metal buffer layer process on SiO₂/Si. We examined the effect of sputtered Zr or ZrO₂ ultra thin films as a buffer layer for Pb_xLa_1?xTiO₃ (PLT) growth. TEM observation revealed that the buffer layer formation process in which Zr oxidized after the metal deposition had advantages to produce MFIS structures. This method is also superior for the crystallization and the control of the orientation of PLT thin film on amorphous SiO₂. Especially, for buffer layer thicknesses below 10 nm, preferred c-axis oriented PLT thin films were grown. The I-V characteristics of MFIS-FET fabricated by the proposed method showed a clear memory window due to the remanent polarization of the ferroelectric thin film. This process is the most attractive candidate for realizing MFIS structure memory.     

(111)-Oriented BaTiO3 thin films hydrothermally formed on TiO2/Si substrate

Abstract

Barium titanate (BaTiO₃) thin films with high (111)-orientation were successfully grown on TiO₂-covered Si(111) substrate using hydrothermal method, where the TiO₂ layer was previously fabricated at room temperature by means of ion-beam-assisted deposition. This processing method provides a simple mild-chemical route for directly producing the analogous crystalline films on different substrates. The BaTiO₃ films did not reach the TiO₂/Si interface even if the hydrothermal treatment was prolonged to 24 hours. Both Rutherford backscattering and spread-resistance profiling characterizations confirmed the diffusion nature of the formed Ba-TiO₃/TiO₂/Si system.     

Hysteresis caused by defects in buffer layer of metal-ferroelectric-insulator-semiconductor (MFIS) devices

Abstract

We have investigated the roles of buffer layer in the Pt/SBT-Y₂O₃/p-Si (MFIS) capacitors. We found that the insertion of Y₂O₃ buffer layer prevents the charge injection from the Si substrate to ferroelectric layer. However, negative charges with the effective density of 3.21×10¹²/cm² were generated due to the additional process step for Y₂O₃ deposition. We suggested that the asymmetrical increase of a memory window is due to the domain pinning caused by negative charges in buffer layer. In addition, we reported that the mobile positive charges in ferroelectric layer can induce the shift of the hysteresis loops depending on the gate-bias polarity and a ramp rate during the capacitance-voltage (C-V) measurement. Since Y₂O₃ buffer layer minimize the charge injection, the shift of the hysteresis loops was asymmetrical.     

Preparation of Pb(Zr,Ti)O3 Thin Films on Silica Substrate by Metallo-Organic Decomposition

In this study, we report the deposition of crack-free transparent PZT films (up to ～859 nm) by metallo-organic decomposition (MOD) process on amorphous silica substrate. Effect of SrTiO₃ (STO) buffer layer on the growth behavior of PZT thin films deposited on SiO₂-coated silicon substrates was systematically studied. Perovskite phase, which cannot be formed directly on SiO₂/Si substrates, has been obtained when a thin STO film (～150 nm) was used as buffer layer. A SIMS examination indicates that the upward diffusion of Si-species into PZT layer is minimal, although downward diffusion of Pb-species into the SiO₂ layer is still observable.     

The Effect of La-Doped Bi4Ti3O12 Buffer Layer on Crystallinity and Ferroelectric Properties of PbZr0.58Ti0.42O3/Bi3.25La0.75Ti3O12 Multilayered Thin Films

PbZr_0.58Ti_0.42O₃ (PZT) ferroelectric thin films with Bi_3.25La_0.75Ti₃O₁₂ (BLT) buffer layer of various thickness were fabricated on Pt/TiO₂/SiO₂/p-Si(100) substrates by rf-magnetron sputtering method. The pure PZT film showed (111) preferential orientation in the XRD patterns, and the PZT/BLT films showed (110) preferential orientation with increasing thickness of the BLT layer. There were no obvious diffraction peaks for the BLT buffer layer, for its thin thickness in PZT/BLT multilayered films. There were the maximum number of largest-size grains in PZT/BLT(30 nm) film among all the samples from the surface images of FESEM. The growth direction and grain size had significant effects on ferroelectric properties of the multilayered films. The fatigue characteristics suggested that 30-nm-thick BLT was just an effective buffer layer enough to alleviate the accumulation of oxygen vacancies near the PZT/BLT interface. The comparison of these results suggests that the buffer layer with an appropriate thickness can improve the ferroelectric properties of multilayered films greatly.     

PIMOD processing and properties of a LiNbO3-based MFSFET for nonvolatile memories

Abstract

The photo-induced metallo-organic decomposition (PIMOD) process has been successfully used to deposit a lithium niobate thin film acting as the gate oxide of the conventional MFSFET structure. The use of the low-temperature PIMOD process for thin film deposition has increased the device yields of the molybdenum liftoff for small area isolation. The electronic alteration of the properties of the ferroelectric gate transistor was previously shown to be caused by charges in the semiconductor being injected into the ferroelectric film. To prevent this problem, a thin SiO₂ buffer layer was thermally grown on the silicon substrate immediately before lithium niobate deposition. The silicon-lithium niobate interface was stabilized and the charge injection effect was eliminated due to the formation of the buffer layer. The channel current was shown to be greatly altered by the application of voltage pulses between the gate of the device and the substrate. Upon switching, the change in surface conductivity of the semiconductor was the same as that expected for ferroelectric switching.     

MOCVD of Pb(Zr x Ti1-x )O3 thin films on MgTiO3/Si substrates and their electrical properties

Abstract

Pb(Zh _x , Ti_1-x )O₃(PZT) thin films were deposited on Si substrates using MgTiO₃ as the buffer layer and the electrical properties of those MFIS structures were investigated. PZT and MgTiO₃ films were made by MOCVD using ultrasonic spraying technique. Perovskite PZT films have been succesfully made at the substrate temperature of 550 to 600°C only when using MgTiO₃ buffer layer. AES depth profile analysis and RBS analysis revealed that there is no remarkable interdiffusion and no formation of reaction layer between PZT and MgTiO₃ and/or between MgTiO₃ and Si substrate. The capacitance-voltage (C-V) curves of the MFIS structure which were made with PZT and MgTiO₃buffer layer have shown the hysteresis resulted from the ferroelectric switching of the PZT films.     

Characterization of hetero-epitaxial PZT capacitor

Abstract

A hetero-epitaxial Au/PbZr_0.48Ti_0.52O₃(PZT)/SrRuO₃(SRO) capacitor was fabricated on a single crystal SrTiO₃ (STO) substrate by pulsed laser deposition. An SRO buffer layer (a nucleation layer) was formed at the SRO/STO interface to ensure the highly epitaxial growth of the PZT and SRO films. An X-ray diffraction measurement revealed that the (00l) planes of the PZT and SRO grew parallel to the substrate surface. A transition layer of ～ 5 nm thickness was observed at the SRO/STO interface by high-resolution transmission electron microscopy (HR-TEM). This transition layer corresponds to the nucleation layer intentionally grown at the interface. Remanent polarization of the capacitor was 32.1 μC/cm² due to the good epitaxial growth of the films.     

Characteristics of Textured Ce-Zr Mixed-Oxide Films as Buffer Layer for YBCO Coated Conductors

Single- and multi-layer (Ce_{1 – x} Zr _x )O₂ films (0 x 0.84) on Si (100) and polycrystalline Ni substrates were prepared using RF and DC magnetron co-sputtering. XRD of scan analysis showed that all (Ce_{1 – x} Zr _x )O₂ films were biaxially oriented with the c-axis perpendicular to the plane of the film. During sputtering, DC power to the Zr target was fixed at 200 W, while RF power to the Ce target was set at 30 W, 50 W, or 100 W. As-deposited ZrO₂ film was amorphous and was crystallized by post-annealing. However, as-deposited (Ce_{1 – x} Zr _x )O₂ films were crystalline even when grown at room temperature and the structures of films were cubic or tetragonal depending on the Ce ion incorporation. It was found that multilayered CeO₂/(Ce_{1 – x} Zr _x )O₂/CeO₂ films could be deposited with a continuous compositional gradient in a sputtering batch. This layered CeO₂/CZO/CeO₂ structure can maintain its original texture after 800°C annealing and is therefore suitable for subsequent YBCO film growth. Furthermore, Ni diffusion is effectively blocked by the buffer layers just like the YSZ currently used in coated conductor fabrication.     

Growth and characterization of non-c-axis-oriented SrBi2Ta2O9 epitaxial thin films on Si(100) substrates with SrRuO3 bottom electrodes

Abstract

We have successfully grown non-c-axis-oriented epitaxial ferroelectric SrBi₂Ta₂O₉ (SBT) films with (116) and (103) orientations on Si(100) substrates using epitaxial (110)- and (111)-oriented SrRuO₃ (SRO) bottom electrodes, respectively. The SRO orientations have been induced by coating the Si(100) substrates with epitaxial YSZ(100) and MgO(111)/ YSZ(100) buffer layers, respectively. All films were sequentially grown by pulsed laser deposition. Specific in-plane orientations of the epitaxial SBT films were found, which are in turn determined by specific in-plane orientations of the epitaxial SRO bottom electrodes. These include a diagonal rectangle-on-cube epitaxy of SRO(110) on YSZ(100) and a triangle-on-triangle epitaxy of SRO(111) on MgO(111).     

Glycolate tantalum based solutions for the sol-gel spin-coating deposition of SrBi2Ta2O9 thin films

Abstract

The reaction of tantalum ethoxide with a glycol solvent produces the interchange of the ethoxide groups with the glycol. As a result, a polymeric derivative is formed with a high resistance towards hydrolysis. Compounds of Sr(II) and Bi(II) can be added to this Ta-glycol sol, leading to strontium bismuth tantalate (SBT) precursor solutions stable in air. These solutions were spin-coated onto two substrates: Pt/TiO₂/SiO₂/(100)Si and Ti/Pt/Ti/SiO₂/(100)Si. Crystallisation of the SBT phase was carried out by a first formation of a fluorite phase that evolves to the layered perovskite at temperatures over 600°C. During crystallisation, a larger tendency to the formation of a substrate/film interface was observed in the films deposited onto Ti/Pt/Ti/SiO₂/(100)Si than onto Pt/TiO₂/SiO₂/(100)Si. A remanent polarisation of P_r5 μC/cm² and a coercive field of E_c <100 kV/cm were measured in the films on Pt/TiO₂/SiO₂/(100)Si. These films retain its remanent polarisation, P_r, up to 10⁵seconds and are fatigue-free up to 10⁹ cycles.     

Effects of nitridation treatments for SBT/Ta2O5 stack gate capacitors

Abstract

In this study, effects of ICP nitride treatments on characteristics of ferroelectric gate stack capacitor were investigated for FET type ferroelectric memory applications. Pt/SBT(200nm)/Ta₂O₅(20nm)/ Nitride/Si (MeFINS) structure capacitors show wide ΔV (memory window) of 1.06V under ±3V operation, while Pt/SBT(200nm)/ Ta2O₅(20nm)/Si (MeFIS) capacitors without nitride treatments exhibit memory window of 0.60V. At the same time, an accumulation capacitance of the MeFINS structure device is higher than that of the MeFIS structure capacitor. This result implies that the ICP nitride treatment successfully suppresses a formation of low dielectric constant interfacial SiO_x layer and alleviates a series capacitance problem.     

Electronic Conductivity and Dielectric Properties of Nanocrystalline CeO2 Films

The growth of dielectric layers on silicon substrates has attracted a great deal of recent interest given their potential applicability in the fabrication of high quality silicon-on-insulator (SOI) structures, high density capacitor devices, and stable buffer layers between silicon and other materials. In this study, nanocrystalline CeO₂ films were deposited on n-type (100) silicon substrates using pulsed laser deposition (PLD) to form a gate dielectric for a Pt/n-Si/CeO₂/Pt MOS device. XRD, AFM and FESEM measurements were used to characterize the crystal structure and grain size of the CeO₂ films. The electrical properties of the device structure were examined by capacitance-voltage (C-V) and impedance spectroscopy measurements. The CeO₂ films exhibited an activated conductivity, characterized by an activation energy E_a = 0.45 eV. An estimated room temperature electron mobility _e of 2.8 × 10^{– 7} cm²/Vs leads to a corresponding electron concentration n of 5.5 × 10¹⁷ cm^{– 3}. In contrast to conventional MOS capacitors, we find an additional capacitive contribution under strong accumulation conditions as a result of space charge effects inside the CeO₂ thin film.     

Application of fluorinated SiO2 interlayer dielectrics for ferroelectric memory

Abstract

Fluorine-doped silicon oxide (SiOF) as interlayer dielectric (ILD) was deposited over PZT capacitors by electron cyclotron resonance (ECR) chemical vapor deposition using SiF₄ and N₂O gases. In the conventional deposition of SiO₂ ILD layer using hydrogen-contained source gases, the properties of ferroelectric capacitors are known to be degraded during the formation of SiO₂ layer. In this study, we examined the degradation of electrical properties of SiOF-deposited PZT capacitors. The remnant polarization and leakage currents were not degraded after the deposition of SiOF. We observed that the fluorine atoms were not diffused into the metal electrode in both cases of the SiOF deposited PZT capacitors and post-deposition annealed capacitors. The SiOF films deposited in the high CF₄ flow rate exhibited rough columnar structure on the metal electrodes. We can successfully deposit SiOF in a smooth morphology by introducing TiO₂buffer layer or using the novel deposition method of changing the SiF₄ flow rate, namely two-layer-deposition method.     

Improvement on ferroelectric properties of metal-organic decomposited PZT thin film prepared by using prenucleation layer

Abstract

Perovskite SrRuO₃ (SRO) layer was, for the first time, been successfully synthesized by using metal-organic decomposition (MOD) process. The presence of SRO buffer layer on Pt(Si) substrates has significantly enhanced the crystallization and densification behavior of the subsequently deposited Pb(Zr_0.52Ti_0.48)O₃ films. The pyrochlore free perovskite phase can be obtained by post-annealing the PZT/SRO/Pt(Si) films at 500°C, which is 50°C lower than that needed in PZT/Pt(Si) films. The fine grain (～0.3 μm) microstructure can be attained by post-annealing at 650°C for PZT/SRO/Pt(Si) films and 700°C for PZT/Pt(Si) films. The ferroelectric hysteresis properties of the two PZT films are comparable to each other. The leakage current properties of PZT/SRO/Pt(Si) films increased pronouncedly with post-annealing temperature, resulting in inferriar leakage behavior to PZT/Pt(Si) films.