Electrical and material characterization of tantalum pentoxide (Ta2O5) charge trapping layer memory

In this experiment, tantalum pentoxide (Ta₂O₅) was used in a metal/oxide/high-k Ta₂O₅/oxide/silicon (MOHOS) novel nanocrystal memory as a trapping layer. Post-annealing treatment, which can passivate defects and improve the material quality of the high-k dielectric, was applied to optimize device performance for a better memory window and faster P/E (program/erase) cycle. Material and electrical characterization techniques including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and electrical measurements were performed to analyze the device under different annealing conditions. The Ta₂O₅ charge trapping layer memory annealed at 900 °C had a higher window of 3.3 V in the current-voltage (C-V) hysteresis loop, and a higher charge retention capability than the samples prepared under various annealing conditions. These higher levels were due to the higher probability of deep-level charge trapping and lower leakage current.     

Electrical characteristics of Ta2O5 based capacitors with different gate electrodes

Al, W and TiN gate stacks using reactively sputtered thin (15–35 nm) Ta₂O₅ as a high-k dielectric have been investigated. It has been established that the type and the deposition technique of the gate electrode strongly affect the parameters of the structures. RF sputtered tungsten has been established as the most suitable electrode material (giving a nonreactive contact) providing a low leakage current (∼10^-8 A/cm² at 1 MV/cm) through capacitors and a high dielectric constant. The application of Al gate electrodes in the advanced DRAM devices is impeded by the chemical interaction of Al with the Ta₂O₅ films deteriorating the performance of the structures. The radiation-induced defects during the TiN deposition increase the leakage currents for TiN/Ta₂O₅/Si capacitors. A modified Poole–Frenkel conduction mechanism with a tendency for a reduction of the compensation level with increasing Ta₂O₅ thickness was found for W-gate capacitors. Schottky emission at low applied fields and modified Poole–Frenkel mechanism at high fields define the J–V characteristics of Al capacitors. The current through TiN capacitors is governed by ohmic and space charge limited conduction. The post metallization annealing in H₂ reduces the oxide charge but deteriorates both the breakdown fields and the leakage currents for all capacitors studied. The effect is stronger for Al and TiN structures and is accompanied by a reduction of the dielectric constant. PACS 72.80.Sk; 73.40.Qv; 77.55+f; 81.15 Cd     

两种不同(Ba,Sr)TiO3薄膜介电-温度特性的研究

研究并比较了两种不同(Ba_0.5,Sr_0.5)TiO₃(BSTO)薄膜介电-温度特性.采用脉冲激光沉积技术在Pt/Ti/SiO₂/Si(100)衬底上制备BSTO薄膜，发现制备条件的不同，可以得到介电性质完全不同的BSTO薄膜.在550℃和氮气氛下制备的BSTO薄膜在常温下具有很高的介电常数，在10kHz下，超过2500，并在200K温度以上介电常数基本不变.它的一些电学性质不同于在正常条件(650℃和氧气氛下)制得的BSTO薄膜，而类似于目前广泛报道的巨介电常数材料如CaCu_３Ti_４O₁₂.两种薄膜介电性质测试结果表明: 氧气氛下制备的BSTO薄膜呈现铁电-顺电相变，符合居里-外斯定律；低温氮气氛下制备的BSTO薄膜，介电弛豫时间和温度的关系符合德拜模型，是热激发弛豫.文中给出了产生这种介电特性的初步解释. 关键词： 薄膜 脉冲激光沉积 介电弛豫     

Characteristics of SrBi2Ta2O9 ferroelectric films on Si using LaAlO3 thin film as an insulator

Metal–ferroelectric–insulator–semiconductor structures using LaAlO₃ (LAO) layers as an insulating barrier have been investigated. LAO films were deposited on n-Si substrates by low-pressure metalorganic chemical vapor deposition (MOCVD). SrBi₂Ta₂O₉ (SBT) films were prepared as ferroelectric layers at a low processing temperature of 650 °C by a metalorganic decomposition method. The MOCVD-derived LAO buffer layer shows an amorphous structure, relatively high dielectric constant, and good electrical properties. Au/SBT/LAO/n-Si exhibits a larger counterclockwise C–V memory window of 3.7 V and a lower leakage-current density of 2.5×10^-8 A/cm² at an applied voltage of 10 V. It has been confirmed that the hysteresis loop is caused by ferroelectricity. The Auger electron spectrometry depth profile indicates that the introduction of the LAO buffer layer prevents the interfacial diffusion between SBT and the Si substrate effectively and improves the interface quality. PACS 77.84.Dy; 81.15.Fg     

Improved dielectric and electrical properties of (Ba,Sr)TiO3 thin films using Pt/LaNiO3 as the top-electrode material

(Ba,Sr)TiO₃ (BST) capacitors grown on a LaNiO₃ (LNO) bottom electrode, with Pt (80 nm), LNO (100 nm), and double-layer Pt/LNO (80/10 nm) top electrodes have been investigated. It was found that the dielectric behavior is improved by a decrease of the electrical properties for the BST capacitor using double-layer Pt/LNO top electrodes. The dielectric constant of 100 nm-thick BST films with a Pt electrode was only 165 at 100 kHz, while that with a double-layer electrode was about 242. Correspondingly, the tunability was greatly improved from 26% to 41% with an electric field of 600 kV/cm. These have been attributed to increased interfacial capacitance density, which resulted from an improved interface, between the films and the top electrode. The dielectric loss was also reduced by using a double-layer electrode. Furthermore, the leakage current of a capacitor with a double-layer Pt/LNO electrode was one order of magnitude lower than that with a single LNO electrode. It can be explained by the fact that the weak chemical interaction between LNO (10 nm) and BST causes a high potential barrier at the interface. PACS 81.15.-z; 81.15.Cd; 77.55.+f; 77.84.Dy; 77.22.-d     

Pulsed laser deposition of PZT/Pt composite thin films with high dielectric constants

PbZr_0.53Ti_0.47O₃ (PZT) thin films containing nanoparticles of Pt (3–10 nm) were produced using pulsed laser deposition (PLD). The Pt content can be tuned by varying the energy density of the laser beam. Phase and microstructure analysis of the thin films was performed using XRD, SEM, TEM and AFM. The electrical properties were investigated by C–V and I–V measurements. The effective dielectric constant of the composite films increased substantially through the Pt dispersion. These films are promising candidates, for instance, for high-density dynamic random access memory (DRAM) devices. PACS 77.22.Ch; 77.84.Lf; 81.15.Fg     

The effect of substrate material on the properties of tantalum oxide films

The effect of substrate material on the electrical characteristics of Ta _x O _y films produced by high-frequency magnetron sputtering of a tantalum oxide target is studied. The effect of oxygen plasma on leakage currents, dielectric permittivity, and dielectric dissipation factor of thin (300–400 nm) Ta _x O _y layers is found. It is proposed to process tantalum oxide films in oxygen plasma to control their electrical and dielectric properties.     

Ionizing radiation effects on electrical and reliability characteristics of sputtered Ta2O5/Si interface

This paper describes the effect of ionizing radiation on the interface properties of Al/Ta₂O₅/Si metal oxide semiconductor (MOS) capacitors using capacitance–voltage (C–V) and current–voltage (I–V) characteristics. The devices were irradiated with X-rays at different doses ranging from 100?rad to 1?Mrad. The leakage behavior, which is an important parameter for memory applications of Al/Ta₂O₅/Si MOS capacitors, along with interface properties such as effective oxide charges and interface trap density with and without irradiation has been investigated. Lower accumulation capacitance and shift in flat band voltage toward negative value were observed in annealed devices after exposure to radiation. The increase in interfacial oxide layer thickness after irradiation was confirmed by Rutherford Back Scattering measurement. The effect of post-deposition annealing on the electrical behavior of Ta₂O₅ MOS capacitors was also investigated. Improved electrical and interface properties were obtained for samples deposited in N₂ ambient. The density of interface trap states (D_it) at Ta₂O₅/Si interface sputtered in pure argon ambient was higher compared to samples reactively sputtered in nitrogen-containing plasma. Our results show that reactive sputtering in nitrogen-containing plasma is a promising approach to improve the radiation hardness of Ta₂O₅/Si MOS devices.     

Structural and dielectric properties of Ru-based gate/Hf-doped Ta2O5 stacks

Hf-doped Ta₂O₅ thin films are studied with respect to their composition, dielectric and electrical properties. The incorporation of Hf is performed by sputtering of a 0.7 nm thick Hf layer on top of Ta₂O₅ and subsequent annealing to stimulate diffusion of Hf into Ta₂O₅ and their intermixing. The elemental in-depth distribution of the films is investigated by the time of flight secondary ion mass spectroscopy (ToF-SIMS), which has revealed that Hf and Ta₂O₅ are intermixed throughout the whole thickness. Two sub-layers exist in all the samples - an upper homogeneous Hf-doped Ta₂O₅ sub-layer and a near interfacial region which is a mixture of Ta- and Si-oxides. The X-ray reflectivity (XRR) analysis shows existence of interfacial layer with a thickness of about 1.9-2 nm, irrespectively of the total thickness of the stacks. Metal-oxide-Si structures with Ru and RuO₂ metal electrodes have been prepared and investigated in terms of dielectric constant, effective work function (EWF) and interfacial layer parameters. The influence of post-metallization annealing steps on these parameters was also studied.     

Microstructure and dielectric properties of (Ba0.6Sr0.4)TiO3 thin films grown on super smooth glazed-Al2O3 ceramics substrate

Modified substrates with nanometer scale smooth surface were obtained via coating a layer of CaO-Al₂O₃-SiO₂ (CaAlSi) high temperature glaze with proper additives on the rough-95% Al₂O₃ ceramics substrates. (Ba_0.6Sr_0.4)TiO₃ (BST) thin films were deposited on modified Al₂O₃ substrates by radio-frequency magnetron sputtering. The microstructure, dielectric, and insulating properties of BST thin films grown on glazed-Al₂O₃ substrates were investigated by X-ray diffraction (XRD), atomic force microscope (AFM), and dielectric properties measurement. These results showed that microstructure and dielectric properties of BST thin films grown on glazed-Al₂O₃ substrates were almost consistent with that of BST thin films grown on LaAlO₃ (1 0 0) single-crystal substrates. Thus, the expensive single-crystal substrates may be substituted by extremely cheap glazed-Al₂O₃ substrates.     

The interfacial structures of (Ba, Sr)TiO3 films deposited by radio frequency magnetron sputtering

Ba_0.6Sr_0.4TiO₃ (BST) films were deposited on Pt/SiO₂/Si substrates by radio frequency magnetron sputtering. The deposited films were crystallized by conventional thermal annealing (CTA) and rapid thermal annealing (RTA). The interfacial structures of BST/Pt were studied. High-resolution transmission electron microscopy (HRTEM) observation shows that there is a transition layer at BST/Pt interface, and the layer is 4-5 nm thick for CTA and 2-3 nm for RTA. X-ray photoelectron spectroscopy (XPS) investigations show that the layer is composed of perovskited BST phase and non-perovskited BST phase. The content of the non-perovskited BST phase is most for CTA, whereas that of the perovskited BST phase is most for RTA. It is found that the transition layer thickens with the increase of annealing temperature, and CTA corresponds to faster thickening rate. XPS shows that the non-perovskited BST phase does not come from the absorbed CO₂ or CO contaminations, but from other interfacial elements. Also, it is indicated that the RTA-annealed BST film capacitor shows much better dielectric properties, with an average value of 150 higher dielectric constant and almost two orders of magnitude lower leakage current density than the CTA-annealed film capacitor. Grazing X-ray diffraction (GXRD) patterns exhibit that the RTA-annealed BST films present more compact structure. It is such a compact structure that can effectively prevent the absorbed elements further diffusing toward two sides, and cause thinner transition layer, thus result in higher dielectric constant and lower leakage current density.     

Retention characteristics of Au/Bi3.25La0.75Ti3O12/Si metal-ferro- electric-semiconductor structure

Lanthanum-substituted bismuth titanate (Bi_3.25La_0.75Ti₃O₁₂) (BLT) thin films were deposited on p-type Si(100) substrates using a chemical solution deposition process. The ferroelectric and dielectric properties of the films with an Au/BLT/Si structure were investigated. It was found that retention behaviors of the capacitors after polling with a negative and positive voltage were very different. The capacitor at an accumulation state exhibited a better retention characteristic than that at a depletion state. A rapid loss of memory for the capacitor at depletion state was found and attributed to the depolarization fields inside the ferroelectric film. It is proposed that the interaction between injected charges and ferroelectric polarization plays a role in the retention properties of the MFS capacitors.     

Polarization processes in BaBi<Subscript>2</Subscript>Nb<Subscript>2</Subscript>O<Subscript>9</Subscript> and SrBi<Subscript>2</Subscript>Ta<Subscript>2</Subscript>O<Subscript>9</Subscript> ceramics in infralow-frequency fields

The dielectric nonlinearity in BaBi₂Nb₂O₉ and SrBi₂Ta₂O₉ layered ceramics was studied by measuring their polarization loops and reverse dependences of permittivity. It was shown that the features of the dielectric response of BaBi₂Nb₂O₉ and SrBi₂Ta₂O₉ in strong fields can be explained by glass-like properties and the contribution of the domain structure of the ferroelectric material to repolarization processes, respectively.     

Temperature-dependent current conduction mechanism and charge trapping in Ta2O5 RF-sputtered on GaN

A gallium nitride (GaN) based Metal-Oxide-Semiconductor (MOS) capacitor was fabricated using radio frequency (RF)-sputtered tantalum oxide (Ta₂O₅) as the high-k gate dielectric. Electrical characteristics of this capacitor were evaluated via capacitance–voltage (C–V), current–voltage (I–V), and interface trap density (D_it) measurements with emphasis on the substrate temperature dependence ranging from 25 °C to 200 °C. Charge trapping and conduction mechanism in Ta₂O₅ were investigated. The experimental results suggested that higher substrate temperature rendered higher oxide capacitance, reduced gate leakage current, and lowered mid-gap interface trap density at the expenses of high border traps and high fixed oxide charges. The gate leakage current through Ta₂O₅ was found to obey the Ohm's conduction at lower gate bias and the Poole–Frenkel conduction at higher gate bias.     

Electrically and magnetically tunable microwave device using (Ba, Sr) TiO3/Y3Fe5O12 multilayer

Ferroelectric (Ba_0.6Sr_0.4)TiO₃ (BST) thin films have been deposited by pulsed laser deposition onto single-crystal Y₃Fe₅O₁₂ (YIG) substrates with/without a MgO buffer layer. The structure and microwave properties of the BST films have been investigated as a function of substrate orientation and O₂ deposition pressures (50-800 mTorr). The crystallographic orientation of BST film varies with the deposition conditions. The dielectric properties of the ferroelectric were measured using interdigitated capacitors deposited on top of the BST film. BST films exhibit high tunability (20-40%) and high dielectric Q=1/cos' (30-50) with a dc bias field of 67 kV/cm at 10 GHz. A coplanar waveguide transmission line was fabricated from a (001)-oriented BST film on (111)YIG which exhibited a 17° differential phase shift with an applied dc bias field of 21 kV/cm (10 GHz). An equivalent differential phase shift was achieved with a magnetic field of 160 Gauss.     

PLD and RF-PLD synthesis of Ba<Subscript>0.6</Subscript>Sr<Subscript>0.4</Subscript>TiO<Subscript>3</Subscript> ferroelectric thin films for electrically controlled devices

Ba_0.6Sr_0.4TiO₃ (BST) bulk ceramic synthesized by solid state reaction was used as target for thin films grown by pulsed laser deposition (PLD) and radiofrequency beam assisted PLD (RF-PLD). The X-ray diffraction patterns indicate that the films exhibit a polycrystalline cubic structure with a distorted unit cell. Scanning Electron Microscopy investigations showed a columnar microstructure with size of spherical grains up to 150 nm. The capacitance–voltage (C–V) characteristics of the BST films were performed by applying a DC voltage up to 5 V. A value of 280 for dielectric constant and 12.5% electrical tunability of the BST capacitor have been measured at room temperature.     

Effect of vertical deposition angle on structural and optical properties of tantalum oxide nano layers deposited by electron gun evaporation

Tantalum oxide nano layers were deposition on glass substrate with different thicknesses (30, 60, 90 and 120 nm) in vertical deposition angle and high vacuum condition at room temperature by using electron gun evaporation method. There were no specific peaks in XRD patterns because of amorphous nature of these layers. AFM results show that surface roughness is reduced by increasing the thickness of the layers. FESEM images show nucleation, growth, accession and integration as interconnected islands in the lower thickness and re-nucleation at higher thicknesses (120 nm). We studied Raman spectra of the produced Ta₂O₅ amorphous layers. The calculated optical coefficients by using Kramers-Kronig relations show that with increasing film thickness, dielectric properties, absorption coefficient and band gap energy have increased.     

Effects of O2/Ar ratio and annealing temperature on electrical properties of Ta2O5 film prepared by magnetron sputtering

The effects of the oxygen-argon ratio on electric properties of Ta2O5 film prepared by radio-frequency magnetron sputtering were investigated.The Ta2O5 partially transforms from the amorphous phase into the crystal phase when annealing temperatures are 800℃ or higher.The lattice constant of Ta2O5 decreases with the increase of the O2/Ar ratio,which indicates that oxygen gas in the working gas mixture contributes to reducing the density of oxygen vacancies during the deposition process.For the films deposited in working gas mixtures with different O2/Ar ratios and subsequently annealed at 700℃,the effective dielectric constant is increased from 14.7 to 18.4 with the increase of the O2/Ar ratio from 0 to 1.Considering the presence of an SiO2 layer between the film and the silicon substrate,the optimal dielectric constant of Ta2O5 film was estimated to be 31.Oxygen gas in the working gas mixture contributes to reducing the density of oxygen vacancies,and the oxygen vacancy density and leakage current of Ta2O5 film both decrease with the increase of the O2/Ar ratio.The leakage current decreases after annealing treatment and it is minimized at 700℃.However,when the annealing temperature is 800℃ or higher,it increases slightly,which results from the partially crystallized Ta2O5 layer containing defects such as grain boundaries and vacancies.     

Polymer thin-film transistors with high dielectric constant gate insulators

Field-effect transistors consisting of poly(3-hexylthiophene) have been fabricated with high dielectric constant SrBi₂Ta₂O₉ films working as the gate insulator. Significantly enhanced gate effects were observed in these devices compared to similar transistors with conventional SiO₂ gate dielectric. Our devices exhibited operating voltages around 10 V, as compared to about 100 V for devices employing SiO₂ as the gate dielectric. Moreover, inverters based on such polymer transistors were demonstrated with nice input–output characteristics. PACS 82.35.Cd     

Top electrode material related bipolar memory and unipolar threshold resistance switching in amorphous Ta2O5 films

Tantalum oxide (Ta₂O₅) is one of the most studied materials for its stable resistance switching and potential application in nonvolatile memory devices. Top electrode and essential switching material are two critical points dominating its switching characteristics. Here, Ta₂O₅ films of amorphous nature (a-Ta₂O₅) with tunable thicknesses were made by changing the applied voltage during anodic oxidation of Ta-metal foils. The resistance-switching behavior of an a-Ta₂O₅ film in a metal/a-Ta₂O₅/Ta configuration was investigated by using a sputtered W or Ag metal film as the top electrode. The unipolar threshold switching phenomenon was observed using W as top electrode (W_TE), while bipolar switching behaviors were achieved using active Ag metal as top electrode (Ag_TE). The thickness of the a-Ta₂O₅ film shows an obvious effect on the SET voltage in a W_TE/a-Ta₂O₅/Ta device. The interfacial redox reaction induced formation of more conductive Ta-rich suboxide and the Joule heating effect are proposed to contribute to the unipolar threshold switching behavior. It is also suggested that the bipolar switching could have resulted from the electrochemical reaction-induced dissolution and growth of Ag conducting channels inside the Ta₂O₅ films.