BiFeO3 thin films prepared using metalorganic chemical vapor deposition

BiFeO₃ thin films were grown on (001) SrTiO₃ and (001) ZrO₂(Y₂O₃) substrates by single source metalorganic chemical vapor deposition in the temperature range T = 500 ÷ 800 °C using Fe(thd)₃ and Bi(C₆H₅)₃ as volatile precursors. X-ray diffraction analysis shows cube-on-cube epitaxial growth of BiFeO₃ on (001) SrTiO₃. The strongly reduced bismuth transfer into the film due to the high thermal stability of Bi(C₆H₅)₃ was counterbalanced by the increase of the total pressure as well as of the residence time of the precursor flow in the reactor; the Bi/Fe ratio in the film thus becomes close to that in the precursor mixture. Optical second harmonic generation measurements have evidenced the ferroelectric ordering in BiFeO₃ films and the apparent decrease of the Curie temperature of the strained films as compared to BiFeO₃ single crystal.     

Wet chemical route to transparent BiFeO3 films on SiO2 substrates

BiFeO₃ nanoparticles were prepared by a wet chemical synthesis method. Transparent films were deposited on glass and quartz substrates by dip and spin coating processes from the synthesized sol. We obtained thicker films (~ 2 µm) by dip coating process and thinner films (~ 200 nm) by spin coating process. Transmission electron microscopy images confirmed that the particles are nanocrystalline in size. From the optical transmittance spectra the band gap of the BiFeO₃ nanoparticles was determined in the range of ~ 3.03-2.88 eV (~ 410-430 nm). Electrical resistivity, polarization, zero-field-cooled and field-cooled magnetizations versus temperature characteristics were also studied for these films.     

Growth structure investigation of MgF2 and NdF3 films grown by molecular beam deposition on CaF2(111) substrates

The growth structure of MgF₂ and NdF₃ films grown on polished CaF₂(111) substrates deposited by molecular beam deposition has been investigated using transmission electron microscopy (TEM) of microfractographical and surface replications as well as cross-sectional TEM, atomic force microscopy, packing density, and absorption measurements. It has been shown that by taking advantage of ultrahigh vacuum environments and a special stratification property of MgF₂ and NdF₃ films, the preparation of nanocrystalline films of high packing density and low optical absorption is possible at a substrate temperature of 425 K.     

Densification of In2O3:Sn multilayered films elaborated by the dip-coating sol-gel route

Indium Tin Oxide (ITO) thin films have been deposited by the Sol-Gel Dip-Coating technique, the starting solutions being prepared from chlorides. These multilayered films were crystallized by means of a classical heat treatment at temperatures ranging from 500 to 600 °C. Five stacked layers are necessary to obtain a global electrical resistivity value of 2.9×10⁻³ Ω cm, for 500 °C annealed film. The paper focuses on the study of the structure of such multilayered deposits, and on the densification process, using transmission electron microscopy, Rutherford Back-scattering Spectrometry and electrical resistivity measurements. This analysis reveals structural inhomogeneities and different crystallite growth processes as a function of annealing temperature and number of deposited layers.     

Unipolar resistive switching behavior of BiFeO3 thin films prepared by chemical solution deposition

Bismuth ferrite (BiFeO₃, BFO) thin films were spin-coated on Pt/Ti/SiO₂/Si substrates by a chemical solution deposition method. The ferroelectric BFO films annealed at 500 °C and 550 °C were found to possess unipolar resistive switching behaviors. The resistance ratio of the high resistance state (HRS) to the low resistance state (LRS) of the unipolar resistance switching is about 10³ for the ferroelectric BFO films. The conduction mechanisms are concluded to be space charge-limited conduction for the initial state and Ohmic conduction for the LRS. As for the HRS, the Poole-Frenkel emission fits well in the whole voltage region. Traps composed of oxygen vacancies are considered to play a key role in forming conducting paths. The relaxation time of electronic carriers is much shorter than that of ionic oxygen vacancies; therefore, the resistance switching is considered more probably due to carrier injection and emission through the Poole-Frenkel model after forming.     

The influence of epitaxial strain on magnetic and electrical properties of BiFeO3 thin films

Strained epitaxial BiFeO₃ films deposited on (001) SrTiO₃ substrates by metal organic chemical vapor deposition were studied by optical second harmonic generation (SHG) and SQUID magnetometry. The observed SHG intensity vs temperature dependencies indicate that for less strained films (σ < 0.6 GPa) a strong interplay between the ferroelectric and magnetic subsystems exists, while for the films with larger σ-values strain-induced destruction of the magnetic cycloidal ordering takes place.     

Optimal growth windows of multiferroic BiFeO3 films and characteristics of ferroelectric domain structures

Multiferroic BiFeO₃ films of smooth surface and fully-saturated ferroelectric hysteresis loops have been grown by RF magnetron sputtering. The (001)-oriented epitaxial films showed a large remanent polarisation of 61 µC/cm². A strategy to grow BiFeO₃ films of good ferroelectric property was demonstrated, that was using fast growth rate to achieve accurate stoichiometry for the BiFeO₃ phase and at the same time to avoid the formation of impurity phases associated with the fast growth by accurate control of thermodynamic parameters such as oxygen partial pressure and temperature, as well as proper selection of substrates. Piezoresponse force microscopy revealed fine spontaneous domains for highly resistive epitaxial films, which were switchable under DC biases. For the polycrystalline films of increased density of free carriers, single-domain grains of about 200 nm in diameter were observed due to effective compensation of depolarisation field by free carriers and therefore allowing larger domains.     

CSD法在亲水性的FTO基板上制备BiFeO3薄膜及其电性能的研究

利用化学溶液沉积法在亲水性的FTO基板上制备BiFeO3薄膜。利用XRD、FE-SEM、XPS、Agi-lent E4980A精密LCR仪及TF-Analyzer2000等分析手段对BiFeO3薄膜进行表征。结果表明,薄膜为纯相的结晶良好的多晶BiFeO3薄膜,由100～300nm的BiFeO3晶粒紧密的堆积而成,表面均匀平整。薄膜厚度为450nm。Fe的氧化态为Fe3+,并没有Fe2+出现。在10kHz时,介电常数和损耗分别为134和0.005。薄膜的剩余极化率为0.58μC/cm2,在0～250kV/cm的测试电场下漏导电流步伐保持在10-6 A/cm2以下。     

Growth of SrTiO3 thin epitaxial films by aerosol MOCVD

SrTiO₃ thin films were prepared by aerosol metal-organic chemical vapour deposition on (001) MgO, R-plane Al₂O₃ and (001) Si single-crystal substrates. Strontium tetramethyl heptadionate and titanium n-butoxide dissolved in diethyleneglycol dimethyl ether were used as precursors. The structure of the films was investigated by X-ray diffraction and transmission electron microscopy. Epitaxial films with [001] and [111] orientation perpendicular to the substrate surface were obtained on MgO and Al₂O₃, respectively. The epitaxial films on the MgO substrate were found to be in a relaxed state with lattice parameters corresponding to the bulk values. SrTiO₃ films on the Si substrate were grown as highly textured in the [011]direction and randomly oriented in the plane parrallel to the substrate surface.     

Fabrication of titania films by sol-gel method using transparent colloidal aqueous solutions of anatase nanocrystals

The fabrication of dense or porous titania films by the sol-gel method using anatase colloidal solutions was examined. When anatase colloidal solutions without surfactants were deposited on a substrate, the anatase films thus obtained had a relatively high refractive index (~ 2.0). Porous films were fabricated using the anatase colloidal solutions with surfactants. Some films had phase separation structures on the submicron scale, such as spinodal decomposition, which consisted of anatase nanocrystal agglomerate regions and surfactant regions. When the dispersion of the anatase colloids was improved, mesoporous films could also be fabricated, although mesopores were irregular in size, shape, and arrangement.     

Fabrication of low-resistivity and gold-colored TiN films by halide chemical vapor deposition with a low [NH3]/[TiCl4] flow ratio

This work describes the preparation of titanium nitride (TiN) films on Si (111) substrates by atmospheric pressure halide chemical vapor deposition (AP-HCVD). Various TiN films were obtained by exploiting TiCl₄ + NH₃ gas chemistry with flow ratios [NH₃]/[TiCl₄] from 0.2 to 1.4, and deposition temperatures (T_d) from 600 to 900 °C. When T_d = 800 °C gold-colored films with electrical resistivities of under 100 μΩ cm were formed at almost all of the investigated [NH₃]/[TiCl₄] flow ratios. In particular, a lowest resistivity of about 23.7 μΩ cm, which is quite close to that of bulk TiN, was achieved using an [NH₃]/[TiCl₄] flow ratio of 0.3. Atomic force microscopy indicated that the root mean square surface roughness of that film was only about 5.1 nm. Under the same [NH₃]/[TiCl₄] flow ratio as above, X-ray diffraction analyses revealed the presence of a cubic TiN phase with a preferred orientation of (200) for T_d ≤ 800 °C, while additional (111) and (220) orientations emerged when the film was deposited at 900 °C. In conclusion, a low resistivity (< 100 μΩ cm) TiN film can be formed by AP-HCVD with very low [NH₃]/[TiCl₄] flow ratios 0.3-1.4.     

In-situ TEM observations of abnormal grain growth, coarsening, and substrate de-wetting in nanocrystalline Ag thin films

Abnormal grain growth is studied in nanocrystalline sputtered Ag films. Eighty nanometer thick Ag films are DC sputter deposited onto back-etched amorphous silicon nitride membranes. Specimens are annealed in a heating stage in an in-situ TEM for various temperatures and hold times. With the same specimen, we proceed to higher temperatures after the apparent halt of growth for sufficiently long hold times. The grain size distribution of the as-deposited films is bi-modal, with large abnormal grains with 100 nm diameters, embedded in a matrix of smaller grains of 15 nm diameters. Coarsening begins at temperatures of approximately 100°C, and quickly reaches a plateau. The growth process restarts only after sufficient temperature increases, and plateaus at each succeeding temperature. Using a variation of the Mullins–Von Neumann law, the activation energy for the abnormal growth is found to be 0.274 eV, consistent with the value reported for pore formation during electromigration via surface diffusion in Ag. Grain growth appears to stop above temperatures of 350°C, eventually leading to triple junction pore formation at 350°C and de-wetting of the film from the substrate at 600°C. The de-wetting is the high temperature limit of the thermal grooving which cancels the driving force for grain growth at the lower temperatures. TEM images as evidence of this effect are presented, along with observations on the pore formation that support surface diffusion as the mass transport mechanism for grooving, pore fomation, and as the limiting mass transport mechanism for the grain growth.     

Microstructure and composition of MgF2 optical coatings grown on Si substrate by PVD and IBS processes

MgF₂ is a current material for the optical applications in the UV and deep UV range. Nevertheless, modern applications still require improvement of the optical and structural quality of the deposited layers. In the present work, the composition and microstructure of MgF₂ single layers grown on Si [100] substrate by physical vapour deposition (PVD) and ion beam sputtering (IBS) processes, were analyzed and compared. Experiments were carried out using X-ray photoelectron spectroscopy (XPS) in depth profile, grazing angle X-ray diffraction (XRD) and transmission electron microscopy (TEM). Both layers exhibited a good stoichiometry and a low level of contamination. The sample grown by IBS revealed a more homogeneous and regular columnar microstructure than the other one.     

Transmission electron microscopy studies of nanophase TiO2

The chemical and microstructural properties of nanosized TiO₂, obtained by sol—gel procedure, were investigated. The samples were characterized by analytical electron microscopy combining electron diffraction, high resolution electron microscopy and atomic force microscopy. These methods were supported by X-ray diffraction and thermal analysis. The chemical and structural properties, as well as the size of grains, depended on the temperature of TiO₂ preparation. In the temperature range between 293 and 820 K the TiO₂ grains consisted of anatase, as the dominant phase, and brookite. High resolution electron microscopy showed that the sample heated at 573 K had grain sizes from 2 to 10 nm, the average being (5 ± 1) nm, and the pore sizes from 4.5 to 12 nm. The sample heated at820 K revealed larger grain sizes, from 7 to 12 nm, the average being (9 ± 1) nm, and the pore sizes from 6 to 10 nm. These values were confirmed by X-ray diffraction broadening.     

Improved dielectric and ferroelectric properties in Ti-doped BiFeO3-PbTiO3 thin films prepared by pulsed laser deposition

Ti-modified thin films of multiferroic 0.72Bi(Fe_1 − xTi_x)O₃-0.28PbTiO₃ (BFPT, x = 0 and 0.02) solid solution were prepared by pulsed laser deposition. The BFPT (x = 0 and 0.02) films possess a tetragonal structure with highly preferential (001) orientation. The effects of the ionic substitution on the properties of BFPT (x = 0 and 0.02) films have been investigated. The leakage current of the BFPT (x = 0.02) thin film is significantly reduced, and the dielectric and ferroelectric properties greatly improved by the aliovalent ionic substitution of Ti⁴⁺ for Fe³⁺. The BFPT (x = 0.02) thin film exhibits a reasonably high remnant polarization P_r with 2P_r up to 90 μC/cm² at 312 kV/cm and a switchable polarization up to 92 μC/cm² at 417 kV/cm.     

Microstructural changes induced by the pyrolysis step on epitaxial La2Zr2O7 thin films grown by metalorganic decomposition

La₂Zr₂O₇ (LZO) thin films are used as buffer layers in second generation high Tc superconductor tapes. The microstructure of LZO films grown by metalorganic decomposition is characterized by the presence of nanovoids throughout the whole thickness of the films. We introduced an out-gassing plateau under vacuum during the pyrolysis process to decrease the size of voids. The temperature of this plateau was determined by Fourier transformed infrared spectroscopy, electron back scattering diffraction and X-Ray diffraction characterizations. The dwelling time was also varied. Transmission Electron Microscopy (TEM) studies revealed that a high heating ramp in combination with a less than an hour pyrolysis plateau decreased pore size. The deposition rate during dip-coating was also decreased to enhance out-gassing at the plateau. Successive LZO layers were deposited and energy filtered TEM images at C K-edge were performed to identify the role of carbon in the nucleation mechanisms.     

Magnetic properties of nano-crystalline Li0.35Cd0.3Fe2.35O4 ferrite prepared by modified citrate precursor method

Single phase nano-crystalline lithium cadmium ferrite Li_0.35Cd_0.3Fe_2.35O₄ is synthesized by a modified citrate gel precursor technique in different pH media. The modified citrate precursor technique reduces the formation of the impurity phase α-Fe₂O₃ in the inverse spinel phase of lithium ferrite. X-ray diffraction (XRD) and transmission electron microscopy (TEM) confirmed the average crystallite size. As-prepared sample shows the paramagnetic behaviour of M–H curve measured by vibrating sample magnetometer (VSM). The coercivity (H_c) and magnetization (M) both increase with decrease in temperature from 300 K to 80 K. Temperature dependent magnetic properties below the Curie point are defined by the Bloch's law and Neel relaxation relations. The effect of annealing on magnetic properties at different temperature is studied.     

CuInS2 thin films deposited by sol-gel spin-coating method

CuInS₂ thin films were prepared using a sol-gel spin-coating method. Copper acetate monohydrate (Cu(CH₃COO)₂·H₂O) and indium acetate (In(CH₃COO)₃) were dissolved into 2-propanol and 1-propanol, respectively. The two solutions were mixed into a starting solution. The solution was dropped onto glass substrate, rotated at 1500 rpm, and dried at 300 °C for Cu-In as-grown films. The as-grown films were sulfurized inside a graphite container box. A clear chalcopyrite phase was observed without a secondary phase. Surface roughness of the films sulfurized at 500 °C was 19.1 nm. A Raman spectra measurement confirmed that no Cu-S or In-S compounds were created in the thin films.     

Effect of substrate temperature on the texture and structure of polycrystalline Si0.7Ge0.3 films deposited on SiO2 by molecular beam deposition

The evolution of microstructure and texture of molecular beam deposited Si_0.7Ge_0.3 films on SiO₂ at the deposition temperature range of 400–700°C was investigated by X-ray diffraction and transmission electron microscopy. At deposition temperatures between 400 and below 500°C, the films were directly deposited as a mixed-phase on SiO₂ and have a inversely cone-shaped structure. In this temperature range deposited as a mixed-phase, the grain size increases as the temperature increases, so that the grains not only grow up by deposition, but also laterally grow by the solid phase crystallization, furthermore, the texture is changed from a {110} texture to mixed {311} and {110} textures. At 500°C, the film was deposited as only a crystalline phase and has a columnar structure with a strong {110} texture. In the temperature range of 500–700°C, as the temperature increases, the {311} and {111} textures develop whereas the {110} texture reduces. The film deposited at 700°C has a random orientation and structure.     

Multi-orientation patterned deposition of BaTiO3 thin films using an Au buffer layer

The control of crystallographic orientation for ferroelectric oxide thin films grown on single crystal substrates has been investigated. We find that perovskite BaTiO₃ has unusual orientation distributions when deposited on (100)- and (110)-oriented SrTiO₃ single crystal substrates that have predeposited patterned Au regions. BTO areas deposited on gold islands were found to have a (111) orientation, whereas those deposited directly on STO had the same orientation as the substrate. Based on this method, we can select, locate, and pattern an oxide film to have different orientation distributions, and thereby engineer films with the complete property anisotropy in-plane.