Electrical characterization of Ta2O5 films deposited by laser reactive ablation of metallic Ta

Tantalum oxide films have been deposited by 355 nm pulsed laser ablation of metallic Ta target in O₃/O₂ ambient. The structure and the composition of as-deposited and annealed films were examined by X-ray diffraction and Fourier transform infrared spectroscopy. The measurements of the current–voltage and capacitance–voltage characteristics of the Al/Ta₂O₅/Si capacitors were performed to reveal the electrical properties of the Ta₂O₅ films. The effects of annealing temperature on the characteristics of thin films have been studied. The results suggest that the films annealed above 700°C have the structure of orthorhombic β-Ta₂O₅, thc annealing treatment at high temperature decreases the bulk trap charge, the border trap, and the interface trap densities of as-deposited films, and improves significantly the dielectric and electrical properties of Ta₂O₅ film.     

Atomic layer deposition of calcium oxide and calcium hafnium oxide films using calcium cyclopentadienyl precursor

Calcium oxide and calcium hafnium oxide thin films were grown by atomic layer deposition on borosilicate glass and silicon substrates in the temperature range of 205–300 °C. The calcium oxide films were grown from novel calcium cyclopentadienyl precursor and water. Calcium oxide films possessed refractive index 1.75–1.80. Calcium oxide films grown without Al₂O₃ capping layer occurred hygroscopic and converted to Ca(OH)₂ after exposure to air. As-deposited CaO films were (200)-oriented. CaO covered with Al₂O₃ capping layers contained relatively low amounts of hydrogen and re-oriented into (111) direction upon annealing at 900 °C. In order to examine the application of CaO in high-permittivity dielectric layers, mixtures of Ca and Hf oxides were grown by alternate CaO and HfO₂ growth cycles at 230 and 300 °C. HfCl₄ was used as a hafnium precursor. When grown at 230 °C, the films were amorphous with equal amounts of Ca and Hf constituents (15 at.%). These films crystallized upon annealing at 750 °C, showing X-ray diffraction peaks characteristic of hafnium-rich phases such as Ca₂Hf₇O₁₆ or Ca₆Hf₁₉O₄₄. At 300 °C, the relative Ca content remained below 8 at.%. The crystallized phase well matched with rhombohedral Ca₂Hf₇O₁₆. The dielectric films grown on Si(100) substrates possessed effective permittivity values in the range of 12.8–14.2.     

Comparative study on chemical stability of dielectric oxide films under HF wet and vapor etching for radiofrequency microelectromechanical system application

HF wet and vapor etching of dielectric oxide films, which were prepared by thermal atomic layer deposition (ALD) and plasma-enhanced ALD (PEALD), are examined for radiofrequency microelectromechanical system (RF MEMS) application. The chemical stability of oxide films was increased in the order of ALD–Al₂O₃ < PEALD–ZrO₂ < PEALD–TiO₂ ≈ ALD–Ta₂O₅ under wet etching in 6:1 buffered HF aqueous solution, but in a different order of Ta₂O₅ < ZrO₂ < TiO₂ ≈ Al₂O₃ under anhydrous HF/CH₃OH vapor etching at 4 kPa. The unstable films were uniformly and completely etched under the wet etching, while transformed to have increased thickness or non-uniformly etched with thicker residue under the vapor etching. Al₂O₃ and TiO₂ (Ta₂O₅ and TiO₂) can be used for RF MEMS capacitive switch fabricated by using HF vapor (wet) etching of sacrificial SiO₂.     

Fabrication and characterization of (1−x)SrBi2Ta2O9–xBi3TaTiO9 layered structure solid solution thin films for ferroelectric random access memory (FRAM) applications

We report on the properties of (1−x)SrBi₂Ta₂O₉–xBi₃TaTiO₉ solid solution thin films for ferroelectric non-volatile memory applications. The solid solution thin films fabricated by modified metalorganic solution deposition technique showed much improved properties compared to SrBi₂Ta₂O₉. A pyrochlore free crystalline phase was obtained at a low annealing temperature of 600°C and grain size was found to be considerably increased for the solid solution compositions. The film properties were found to be strongly dependent on the composition and annealing temperatures. The measured dielectric constant of the solid solution thin films was in the range 180–225 for films with 10–50% of Bi₃TaTiO₉ content in the solid solution. Ferroelectric properties of (1−x)SrBi₂Ta₂O₉–xBi₃TaTiO₉ thin films were significantly improved compared to SrBi₂Ta₂O₉. For example, the observed remanent polarization (2P_r) and coercive field (E_c) values for films with 0.7SrBi₂Ta₂O₉–0.3Bi₃TaTiO₉ composition, annealed at 650°C, were 12.4 μC/cm² and 80 kV/cm, respectively. The solid solution thin films showed less than 5% decay of the polarization charge after 10¹⁰ switching cycles and good memory retention characteristics after about 10⁶ s of memory retention. The improved microstructural and ferroelectric properties of (1−x)SrBi₂Ta₂O₉–xBi₃TaTiO₉ thin films compared to SrBi₂Ta₂O₉, especially at lower annealing temperatures, suggest their suitability for high density FRAM applications.     

Preparation of highly c-axis-oriented Bi4Ti3O12 thin films and their crystallographic, dielectric and optical properties

Bismuth titanate (Bi₄Ti₃O₁₂) thin films with a high c-axis orientation up to 99% were prepared on (100)-oriented silicon wafers by r.f. planar magnetron sputtering using a Bi₂TiO₅ ceramic target at a substrate temperature of 600 °C. From the Auger electron spectroscopy depth profile of the film, there is no evidence of interdiffusion of a specific element between the film and the substrate. Relative dielectric constant of these films depends on film thickness. The behavior was explained assuming a low-dielectric-constant interface layer. Using this assumption, the relative dielectric constant of Bi₄Ti₃O₁₂ film was estimated to be approximately 140. This value is close to that along the c axis in a bulk form. The remanent polarization and the coercive field were 0.8 μC cm⁻² and 20 kV cm⁻¹, respectively.     

纳米Al_2O_3/聚酰亚胺复合薄膜的介电与力学性能

采用原位聚合与热亚胺化的方法,成功制备了一系列不同纳米Al_2O_3粒子质量分数的纳米Al_2O_3/聚酰亚胺(PI)复合薄膜。通过SEM、TEM、XRD、FTIR、LCR数字电桥、高压电源及电子万能材料试验机对纳米Al_2O_3/PI复合薄膜的微观结构、介电性能及力学性能进行了表征和测试。结果表明:纳米Al_2O_3粒子在均匀地分散在PI基体中;当纳米Al_2O_3粒子质量分数为8%时,纳米Al_2O_3/PI复合薄膜的击穿强度和拉伸强度均达到了最大值;纳米Al_2O_3/PI复合薄膜的介电常数随纳米Al_2O_3质量分数的增加而增加。     

Nanostructured Metal Oxide Thin Films for Humidity Sensors

Capacitive humidity sensors were fabricated using countersunk interdigitated electrodes coated with amorphous nanostructured TiO₂, SiO₂, and Al₂O₃ thin films grown by glancing angle deposition. The capacitive response and response times for each sensor were measured. The sensor utilizing TiO₂ exhibited the largest change in capacitance, increasing exponentially from ~ 1 nF to ~ 1muF for an increase in relative humidity from 2% to 92%. Adsorption and desorption response times were measured using flow rates of 2.5 l/min and were between 90 ms and 300 ms for the sensors studied here. A simple model of the capacitive response of the devices has been developed and used to calculate the dielectric constant of the combined system of our films and adsorbed water. The obtained dielectric constants are found to be much higher than bulk or literature values for similar systems.     

片状纳米Al2O3/环氧树脂复合材料的制备及性能

采用水热法制备片状纳米Al₂O₃,经过偶联剂改性后与环氧树脂复合,通过溶液混合法制备了不同填充量的片状纳米Al₂O₃/环氧树脂复合材料,研究了片状纳米Al₂O₃用量对片状纳米Al₂O₃/环氧树脂复合材料介电性能和热性能的影响,利用SEM对复合材料的断口形貌进行了表征。结果表明: 片状纳米Al₂O₃在环氧树脂基体中分散良好;随着片状纳米Al₂O₃填充量的增加,复合材料的起始热分解温度升高、介电强度增大,当片状纳米Al₂O₃的填充量为7wt%时,复合材料的介电强度为 29.58 kV/mm,比纯环氧树脂的介电强度提高了30%;复合材料的介电常数(3.8~4.5)和介电损耗(0.015)比纯环氧树脂稍有增大,但仍维持在较好的介电性能范围内。     

-Fe2O3 thin films prepared by metalorganic deposition (MOD) from Fe(III) 2-ethylhexanoate

-Fe₂O₃ thin films were prepared by metalorganic deposition (MOD) using Fe(III) 2-ethylhexanoate as the metalorganic precursor. A series of experiments were conducted on the metalorganic spin-coated films and their correspondingly annealed samples by employing experimental techniques ranging from thermal gravimetric analysis, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and scanning electron microscopy (SEM) to optical property characterization. In this way a better understanding has been achieved regarding the decomposition process of the metalorganic precursor, the solid state -Fe₂O₃ film formation and crystallization process, and the relationship between the structure and the optical properties of the prepared films. The conclusions of our experiments are the following. The decomposition of Fe(III) 2-ethylhexanoate is a multistep process that is characterized by distinct transition temperatures and thermogravimetric loss rates. Amorphous -Fe₂O₃ film is formed at an annealing temperature of around 460°C, further annealing at higher temperatures induces the amorphous-to-crystalline phase transition and grain growth. FTIR, XRD and SEM data for structural characterization are correlated and in good agreement. A new FTIR absorption band, peaking at 1085 cm^-1, is assigned to the vibration of crystalline Fe-O mode, therefore this peak is useful in monitoring the amorphous-to-crystalline phase transition of -Fe₂O₃ material. Instead of columnar structure in physical vapour deposition-prepared films a granular structure is typical of MOD prepared films, the grain size is much larger near the surface of the film than near the substrate. Optical characterization shows that the refractive index and extinction coefficient of the -Fe₂O₃ thin films increase with the increase of annealing temperatures. The potential interesting applications of the MOD-prepared -Fe₂O₃ thin films include gas sensor materials, photoelectrodes and storage media.     

Epitaxial and polycrystalline BaFe12O19 thin films grown by chemical vapour deposition

Epitaxial and polycrystalline barium hexaferrite BaFe₁₂O₁₉ thin films were prepared by metalorganic chemical vapour deposition (MOCVD). Films were grown by a liquid MOCVD technique which aim is to control precisely the precursor vapour pressures. Two kinds of substrates were used: sapphire (001) and silicon thermally oxidized. On Si/SiO₂ films are polycrystalline and the magnetization is isotropic. On Al₂O₃ (001), structural studies reveal the films to be predominantly single phase, well crystallized without annealing procedure and with the c-axis perpendicular to the film plane; epitaxial relationships between the film and the substrate were determined. The magnetic parameters, deduced from vibrating sample magnetometer measurements, show a high dependence of the magnetization with the orientation of the field with respect to the surface of the film.     

Characterization of intermetallic phases and oxides formed in annealed Ni/Al multilayer structures

Two different multilayer structures composed of ten alternating Ni and Al thin films were sputter deposited on Si (111) substrates. These multilayers with individual Ni and Al thin film thicknesses of about 25 nm and 38 nm and of 25 nm and 13 nm, respectively, have the average compositions of Ni_0.50Al_0.50 and Ni_0.75Al_0.25. The samples were heat treated in a differential scanning calorimeter instrument with a constant heating rate of 40 °C min ⁻¹ in Ar from room temperature to 550 °C. The compositions of as-deposited and heat-treated samples were studied with high-resolution Auger electron spectroscopy (AES) rotational depth profiling. X-ray photoelectron spectroscopy (XPS) analyses show an excess of Ni in both annealed samples. X-ray diffraction measurements of annealed multilayers show the formation of Ni₂Al₃ and NiAl₃ phases in the Ni_0.50Al_0.50 sample and the presence of Ni₃Al and Ni A1₃ phases with some excess of Ni in the Ni_0.75Al_0.75 sample. AES and XPS investigations of the reacted layers after 15 min annealing in air at 500 °C disclose considerably different surface oxide thin films: on the Ni_0.50Al_0.50 layer the oxide thin film consists of Al₂O₃ with a small amount of NiO, whereas that on the top of the Ni_0.75Al_0.25 layer is thicker and consists of NiO on top and some Al₂O₃ below.     

Growth and characterization of AP-MOCVD iron doped titanium dioxide thin films

Atmospheric pressure metal organic chemical vapor deposition (AP-MOCVD) was used to prepare iron doped titanium dioxide thin films. Thin films, between 40 and 150 nm thick, were deposited on Si, SiO₂ and Al₂O₃ substrates using titanium tetra isopropoxide and ferrocene as metal organic precursors. TiO₂ iron doping was achieved in the range of 1–4 at.%. The film morphology and thickness, polycrystalline texture and doping content were studied using respectively scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The influence of growth temperature, deposition time, substrate type and dopant partial pressure were studied. Electrical characterizations of the films were also performed.     

氟化镁基底上HfO2中间层对Al2O3薄膜微观组织和力学性能的影响

采用电子束蒸镀技术在氟化镁基底上制备了单层Al₂O₃薄膜和含有HfO₂中间层的HfO₂/Al₂O₃双层薄膜。在空气中对所制备的薄膜进行1 h 600℃的退火处理。通过掠入角X射线衍射仪(GIXRD)、场发射扫描电镜(FE-SEM)、傅里叶变换红外光谱仪(FTIR)、纳米压痕和划痕法对薄膜的微观结构、红外透过率和力学性能进行了表征。结果表明: 退火处理后HfO₂/Al₂O₃双层薄膜中形成了一层树枝状的新层, 这种新层的硬度大于17.5 GPa。这种高硬度的新层能够保护氟化镁基底不被划伤。从GIXRD图谱中只能找到单斜相HfO₂的衍射峰, 而Al₂O₃薄膜仍然保持非晶态。从这些结果中可以推断出HfO₂从非晶态向单斜相的转变促进了这种树枝状新层的产生, 也正是这种新层提高了保护薄膜的力学性能。     

Long lasting phosphorescence of Sr4Al14O25:Eu,Dy thin films by magnetron sputtering

This paper reports the preparation process and the long lasting phosphorescence of the Sr₄Al₁₄O₂₅:Eu²⁺,Dy³⁺ thin films obtained by magnetron sputtering. Phosphorescence was achieved by annealing the films in reducing atmosphere. Sr₄Al₁₄O₂₅ thin film was obtained when the films were treated at 1200 °C, while SrAl₂O₄ was generated as the intermediate phase during the annealing process. Sr₄Al₁₄O₂₅:Eu²⁺,Dy³⁺ film generated an emission at 485 nm, and SrAl₂O₄:Eu²⁺,Dy³⁺ film showed an emission peaking at 515 nm. Afterglow characteristics were observed for both films, and Sr₄Al₁₄O₂₅:Eu²⁺,Dy³⁺ film showed a better afterglow property than the SrAl₂O₄:Eu²⁺,Dy³⁺ film due to a deeper trap level and a higher trap concentration formed in the thin films.     

Preparation and properties of electrochromic iridium oxide thin film by sol-gel process

We established a method for preparation of iridium oxide thin film by the sol-gel dip-coating process where iridium chloride was used as a starting material. The coating solution was prepared by reacting iridium chloride, ethanol and acetic acid. Iridium oxide coating was formed at 2.0 cm/min withdrawing rate. The coating films heat treated at 300°C did not contain impurities. Iridium oxide crystallized at temperatures above 450°C. Both crystalline and amorphous iridium oxide thin films showed electrochromism. The change in transmittance of the crystalline Ir₂O₃ film is larger than that of the amorphous Ir₂O₃ under the same experimental conditions. The transmittance of the crystalline thin film (film thickness 200 nm, measured at 400 nm) decreased 13.0% on application of 3 V for 1 s.     

Enhanced ferroelectric properties of multilayer SrBi2Ta2O9/SrBi2Nb2O9 thin films for NVRAM applications

Ferroelectric SrBi₂Ta₂O₉/SrBi₂Nb₂O₉ (SBT/SBN) multilayer thin films with various stacking periodicity were deposited on Pt/TiO₂/SiO₂/Si substrate by pulsed laser deposition technique. The X-ray diffraction patterns indicated that the perovskite phase was fully formed with polycrystalline structure in all the films. The Raman spectra showed the frequency of the O–Ta–O stretching mode for multilayer and single layer SrBi₂(Ta_0.5Nb_0.5)₂O₉ (SBNT) samples was 827–829 cm⁻¹, which was in between the stretching mode frequency in SBT (813 cm⁻¹) and SBN (834 cm⁻¹) thin films. The dielectric constant was increased from 300 (SBT) to 373 at 100 kHz in the double layer SBT/SBN sample with thickness of each layer being 200 nm. The remanent polarization (2P_r) for this film was obtained 41.7 μC/cm², which is much higher, compared to pure SBT film (19.2 μC/cm²). The coercive field of this double layer film (67 kV/cm) was found to be lower than SBN film (98 kV/cm).     

Growth of La2Mo2O9 films on porous Al2O3 substrates by radio frequency magnetron sputtering

Synthesis conditions of La₂Mo₂O₉ thin film by radio frequency (RF) sputtering technique on Al₂O₃ ceramic substrates are studied. It is found that the deposition temperature and oxygen partial pressure are the most important factors for obtaining pure La₂Mo₂O₉ films. Varying both parameters, Mo-rich, stoichiometric, and Mo-deficient films are obtained. With increasing the La:Mo ratio, films become denser. A crust layer is observed on top of the Mo-rich and the Mo-deficient films. The formation of the La₂Mo₂O₉ phase is discussed with respect to the sputtering mechanism.     

Electrical properties of Ta2O5 thin films deposited on Cu

The electrical and dielectric properties of reactively sputtered Ta₂O₅ thin films with Cu as the top and bottom electrodes forming a simple metal insulator metal (MIM) structure, Cu/Ta₂O₅/Cu/n-Si, were studied. Ta₂O₅ films subjected to rapid thermal annealing (RTA) at 800°C for 30 s in N₂ ambient crystallized the film, decreased the leakage current density and resulted in reliable time-dependent dielectric breakdown characteristics. The conduction mechanism at low electric fields (<100 kV/cm) is due to Ohmic conduction; however, the Schottky mechanism becomes predominant at high fields (>100 kV/cm). Present studies demonstrate the use of Cu as a potential electrode material to replace the conventional precious metal electrodes for Ta₂O₅ storage capacitors.     

Deposition and dielectric properties of CaCu3Ti4O12 thin films on Pt/Ti/SiO2/Si substrates using pulsed-laser deposition

CaCu₃Ti₄O₁₂ (CCTO) thin films were successfully deposited on Pt/Ti/SiO₂/Si(1 0 0) substrates using pulsed-laser deposition technique. The crystalline structure and the surface morphology of the CCTO thin films were greatly affected by the substrate temperature and oxygen pressure. Thin films with a (2 2 0) preferential orientation were obtained at the substrate temperature above 700 °C and oxygen pressure above 13.3 Pa. The 480-nm thin films deposited under 720 °C and 26.6 Pa have a fairly high dielectric constant of near 2000 at 10 kHz and room temperature. The values of the dielectric constant and loss and their temperature-dependence under different frequency are comparable with those obtained in the epitaxial CCTO films grown on oxide substrates.     

Growth and stability of Ga2O3 nanospheres

Gallium oxide thin films were prepared by thermal evaporation and deposition of Ga₂O₃ on NaCl(001) cleavage planes at varying substrate temperatures, oxygen pressures and deposition rates. The structure of the so-prepared thin films was checked by Transmission Electron Microscopy and Selected Area Diffraction and also characterized by X-ray Photoelectron Spectroscopy and Atomic Force Microscopy, both in the as-deposited state and after different oxidative and reductive treatments. The substrate temperature proved to be most crucial for the structure of the gallium oxide films, ranging from low-contrast amorphous structures at low substrate temperatures (298 K) to nanosphere structures at higher temperatures (580 K). The stability of the films was found to be mainly determined by the interaction of substrate temperature and deposition rate. Crystalline β-Ga₂O₃ structures were obtained after oxidative, reductive and annealing treatments at and beyond 773 K suggesting that the crystallization is mainly a thermal annealing effect.