Preparation and characterization of highly <Emphasis Type="Italic">c</Emphasis>-axis textured MgO buffer layer grown on Si(100) substrate by RF magnetron sputtering for use as growth template of ferroelectric thin film

Highly c-axis textured MgO thin films were grown directly on Si(100) substrates without any buffer layer by RF magnetron sputtering for use as growth template of ferroelectric film. We fixed the target-to-substrate spacing of 40 mm and then changed the substrate temperature, deposition pressure, and RF power to study the effect of deposition parameters on the growth of c-axis textured MgO thin films. The as-grown films were post-annealed by the rapid thermal annealing (RTA) and furnace annealing to improve the film quality. The experimental results show that the optimum deposition parameters are substrate temperature of 350 °C, oxygen pressure of 15 mTorr and RF power of 75 W. The full width at half maximum intensity (FWHM) of MgO(200) peak obtained from the XRD measurement was 0.8°, and it was further reduced to 0.5° and 0.27° after annealing by RTA and furnace, respectively. Highly c-axis textured PZT and BaTiO₃ films could be obtained on this template. Hysteresis loops of the BaTiO₃ films deposited on MgO(100) single crystalline substrate and MgO(200)/Si(100) template were measured for comparison. The results show that MgO/Si templates thus obtained are suitable for the synthesis of perovskite ferroelectric thin films.     

Characteristics of the precursors and their thermal decomposition during the preparation of LiNbO3 thin films by the Pechini method

Lithium niobate thin films have been deposited on Pt/Ti/SiO₂/Si(100) substrates by Pechini method. Characterization of the initial precursor solutions containing citric acid (CA), niobium and lithium ions has been performed by Fourier transform infrared spectroscopy, Raman spectroscopy and carbon nuclear magnetic resonance spectroscopy. The results indicate that citric acid coordinate to niobium ions to form a niobium-CA complex through one terminal carboxyl group, the hydroxyl group and the central carboxyl group as a tridentate ligand. The thermal decomposition of the Li-Nb precursors gel powder has been studied and the results show that LiNbO₃ phase is formed directly from the thermal decomposition of the precursor gel. By heat-treatment at 600 °C for 2 h, polycrystalline LiNbO₃ thin films with smooth and crack-free surface could be achieved.     

Influence of gamma irradiation on the refractive index of Fe-doped barium titanate thin films

Polycrystalline Fe-doped barium titanate (Fe-doped BaTiO₃) thin films were grown by thermal decomposition of the precursors deposited from a sol-gel system onto quartz substrates. The changes in the transmittance spectra induced by gamma irradiation on the Fe-doped BaTiO₃ thin films were quantified. The values for the optical energy band gap were in the range of 3.42-3.95 eV depending on the annealing time. The refractive index of the film, as measured in the 350-750 nm wavelength range was in the 2.17-1.88 range for the as prepared film, and this increased to 2.34-1.95 after gamma irradiation at 15 kGy. The extinction coefficient of the film was in the order of 10⁻² and increased after gamma irradiation. We obtained tuneable complex refractive index of the films by exposure to various gamma rays doses.     

Enhanced ferroelectric properties of Fe-doped BaTiO3 thin film deposited on LaNiO3/Si substrate by sol–gel technique

The ferroelectric thin films of Fe-doped BaTiO₃ and undoped BaTiO₃ were prepared on LaNiO₃ coating Si substrates by sol–gel technique. It was found that a small amount of Fe dopant could significantly enhance the ferroelectric properties of the BaTiO₃ thin film. The remnant polarization of Fe-doped BaTiO₃ thin film at room temperature reached to 14.9 μC/cm². The loss tangent, compared to the undoped BaTiO₃ film, was increased with frequency increasing and the dielectric constant was decreased. The possible mechanism of enhanced ferroelectric properties of Fe-doped BaTiO₃ thin film was discussed. The results show the potential role of Fe dopant in improving the ferroelectric properties of BaTiO₃ thin film.     

Single-source chemical vapor deposition of clean oriented Al2O3 thin films

Clean oriented Al₂O₃ thin film with a dominant Al₂O₃ <1 1 3> plane was deposited on Si <1 0 0> substrate at 550 °C, by single-source chemical vapor deposition (CVD) using aluminium(III) diisopropylcarbamate, Al₂(O₂CNⁱPr₂)₆. This process represents a substantial reduction in typical CVD film growth temperatures which are typically > 1000 °C. Through the studies of thermal stability of this precursor, we propose a specific β-elimination decomposition pathway to account for the low temperature of the precursor decomposition at the substrate, and for the lack of carbon impurity byproducts in the resulting alumina films that are characterized using X-ray photoelectron spectroscopy and depth profiling.     

A review of molecular beam epitaxy of ferroelectric BaTiO3 films on Si,Ge and GaAs substrates and their applications

Abstract

SrTiO₃ epitaxial growth by molecular beam epitaxy (MBE) on silicon has opened up the route to the monolithic integration of various complex oxides on the complementary metal-oxide–semiconductor silicon platform. Among functional oxides, ferroelectric perovskite oxides offer promising perspectives to improve or add functionalities on-chip. We review the growth by MBE of the ferroelectric compound BaTiO₃ on silicon (Si), germanium (Ge) and gallium arsenide (GaAs) and we discuss the film properties in terms of crystalline structure, microstructure and ferroelectricity. Finally, we review the last developments in two areas of interest for the applications of BaTiO₃ films on silicon, namely integrated photonics, which benefits from the large Pockels effect of BaTiO₃, and low power logic devices, which may benefit from the negative capacitance of the ferroelectric.     

Studies of BaTiO3 thin films on different bottom electrode

BaTiO₃ (BT) thin films were prepared on Pt/Ti/SiO₂/Si and Ru/Ti/SiO₂/Si substrates by a modified sol-gel technique. The microstructure of the films was characterized by atomic force microscopy (AFM), X-ray diffraction (XRD) and Raman spectroscopy. The results showed that BT thin films crystallized with perovskite structure. Compared to BT film on Pt/Ti/SiO₂/Si substrate, BT thin film deposited on Ru electrode has similar dielectric constant, while it has higher dielectric loss. C–E curve for BT film on Pt/Ti/SiO₂/Si was more symmetrical around zero-bias field than C–E curve for BT film on Ru/Ti/SiO₂/Si substrate. The tunability was 52.02% for BT film on Pt electrode, which was 33.42% on Ru electrode, at 275 kV/cm and room temperature. The leakage current density of BT on Pt electrode was about an order of magnitude lower than BT film on Ru electrode at the applied electrical field below 150 kV/cm. The leakage conduction mechanism was investigated.     

Structural and optical characterization of c-axis textured BaTiO<Subscript>3</Subscript> thin films on MgO fabricated by RF magnetron sputtering

In this paper, BaTiO₃ thin films were prepared by RF magnetron sputtering on MgO substrates and their properties such as the crystal structure and optical waveguide properties were investigated. The optimum deposition parameters, such as substrate temperature, deposition pressure, gas flow ratio, the RF power and the after annealing temperature, were obtained in order to get the best BaTiO₃ film quality. The XRD results show that highly c-axis textured BaTiO₃ thin films were successfully grown on MgO substrate. Films obtained under the optimum deposition parameters, substrate temperature of 650°C, RF power of 50 W, deposition pressure 18 mTorr and gas flow ratio O₂/(Ar+ O₂) of 15% namely, reaches a full width at half maximum intensity (FWHM) of BaTiO₃ (002) XRD peak of 0.25°. The FWHM of BaTiO₃ (002) XRD peak was further reduced to 0.24° via post-treatment with furnace annealing (at 800°C for 2 h) which indicates the film crystal quality is further improved. The bright and sharp TE modes measured by m-line spectroscopy of the BaTiO₃ film were observed indicating its possible application in optical waveguide.     

Surface,structural and electrical properties of BaTiO3 films grown on p-Si substrates by low pressure metal organic chemical vapour deposition

Metal organic chemical vapour deposition of BaTiO₃ using Ba(tmhd)₂, Ti(OC₃H₇)₄ and N₂O, where tmhd equals 2,2,6,6-tetramethyl-3,5-heptanedionate, via pyrolysis at relatively low temperatures (370C) was performed in order to produce BaTiO₃ insulator gates. Scanning electron microscopy showed that the surfaces of the BaTiO₃ films had very smooth morphologies. Atomic force microscopy showed that the BaTiO₃ thin film was polycrystalline. X-ray diffraction results indicated that BaTiO₃ crystalline films grew on Si(100) with [110] orientation. High resolution transmission electron microscopy measurements showed that the BaTiO₃ films were polycrystalline, and an interfacial layer in the BaTiO₃/Si interface was formed. The stoichiometry and atomic structure of the BaTiO₃ films were investigated by Auger electron spectroscopy and transmission measurements, respectively. Room temperature capacitance-voltage measurements clearly revealed metal-insulator-semiconductor behaviour for samples with BaTiO₃ insulator gates, and interface state densities at the BaTiO₃/p-Si interface were approximately high, 10¹¹ eV^–1 cm^–2, at the middle of the Si energy gap.     

Solvothermal approach for low temperature deposition of aluminium oxide thin films

At elevated pressure, stoichiometric and high quality Al₂O₃ thin films are fabricated at 65-105 °C. By using pre-organised single source precursor aluminium(III) diisopropylcarbamate, Al₂O₃ were deposited on the surface of a Si substrate in a single step in the liquid phase. Comprehensive removal of large carbamate ligands by proposed β-elimination during decomposition of precursor led to an effective delivery of enshrouded Al-O fragments. Scanning electron microscopy revealed dense and grainy surface morphology. The thicknesses of the films were measured to be 150-300 nm and independent to reaction temperatures or reaction times. Through the use of near edge X-ray absorption fine structure spectroscopy, Al absorption peaks suggest a short range crystalline formation in a film deposited at 105 °C.     

Nanostructured barium titanate thin films from nanoparticles obtained by an emulsion precipitation method

Spherical non-agglomerated BaTiO₃ precursor particles of 3–5 nm size were prepared by an emulsion precipitation method that consisted of the complexation of Ba- and Ti-precursors in separate water-in-decane emulsions, followed by mixing and controlled precipitation upon reactive decomposition of tetramethyl ammonium hydroxide. Stable clear yellow dispersions with a final particle yield of ∼60% were obtained. In situ high temperature X-ray diffraction (XRD) measurements indicated that the initially amorphous particles crystallised into single-phase tetragonal BaTiO₃ at 600 °C. Thin BaTiO₃ films of 25–350 nm thickness were prepared from the precursor dispersions by spin coating or controlled deposition on Si wafers. The average crystallite size after annealing at 800 °C was approximately 26 nm, but some larger grains of 100–150 nm size were also observed. Fresnoite (Ba₂TiSi₂O₈) was likely to be formed near the interface between the oxide layer of the Si substrate and the BaTiO₃ film.     

Piezoelectric BNT-BT<Subscript>0.11</Subscript> thin films processed by sol–gel technique

0.89(Na_0.5Bi_0.5)TiO₃–0.11BaTiO₃, (BNT-BT_0.11) thin film was fabricated by sol–gel/spin coating process, on platinized silicon wafer. Perovskite structure with random orientation of crystallites has been obtained at 700 °C. Piezoelectric activity of BNT-BT_0.11 thin film was detected using piezoresponse force microscopy (PFM). Effective piezoelectric coefficient d _33eff of such film, recorded at 5 V applied dc voltage, was ~29 pm/V, which is similar to other BNT-BT _x thin films. The complex refractive index and dielectric function of BNT-BT_0.11 thin films were also investigated. The high leakage current density significantly influences the dielectric, ferroelectric, and piezoelectric properties of the BNT-BT_0.11 films.     

Synthesis, crystal structure and thermal decomposition of [La2(CH3CH2COO)6·(H2O)3]·3.5H2O precursor for high-k La2O3 thin films deposition

Lanthanum acetylacetonate La(C₅H₇O₂)₃·xH₂O has been used in the preparation of the precursor solution for the deposition of polycrystalline La₂O₃ thin films on Si(1 1 1) single crystalline substrates. The precursor chemistry of the as-prepared coating solution, precursor powder and precursor single crystal have been investigated by Fourier Transformed Infrared Spectroscopy (FTIR), differential thermal analysis coupled with quadrupole mass spectrometry (TG-DTA-QMS) and X-ray diffraction. The FTIR and X-ray diffraction analyses have revealed the complex nature of the coating solution due to the formation of a lanthanum propionate complex. The La₂O₃ thin films deposited by spin coating on Si(1 1 1) substrate exhibit good morphological and structural properties. The films heat treated at 800 °C crystallize in a hexagonal phase with the lattice parameters a = 3,89 Å and c = 6.33 Å, while at 900 °C the films contain both the hexagonal and cubic La₂O₃ phase.     

Nanostructured ZnO network films deposited on Al2O3 substrates by chemical bath deposition

Nanostructured ZnO network films have been fabricated on Al₂O₃ substrates by the combination of chemical bath deposition and thermal decomposition process. Layered basic zinc nitrate (LBZN) network films were deposited on the Al₂O₃ substrates with LBZN crystal seeds in methanol solution of zinc nitrate hexahydrate and hexamethylenetetramine. The LBZN precursor films were then transformed into nanostructured ZnO films by heating at 260 °C in air. During the thermal decomposition process abnormal exothermic heat effect was observed at 200-210 °C and CH₃ groups were found in the as-deposited films. We propose that methanol molecules are integrated in the LBZN films forming LBZN-CH₃OH complex and that the heat effect comes from the exothermic release of the methanol.     

Anomalous dielectric properties of Ba1−xCaxTiO3 thin films near the solubility limit

Barium calcium titanium Ba_1?xCa_xTiO₃ (BCTx) is an important ferroelectric and antiferroelectric material. The BCTx (x = 0, 0.1, 0.21, 0.3) thin films were prepared on (100)-oriented LaNiO₃/Si substrates by a sol–gel process. X-ray diffraction showed that all the BCTx thin films exhibited a single perovskite phase with (100)-preferred orientation. The temperature dependent dielectric properties showed that the BCT0.21 film which is near the solubility limit of CaTiO₃ (CT) in BaTiO₃ (BT) had a large dielectric constant and the lowest temperature coefficient. Although the dielectric loss for BCT0.21 film is slightly higher than the others, the BCT0.21 thin film is still a promising candidate for the devices which demand high and stable memory capability in temperature changeable environment.     

Atomic layer deposition and post-deposition annealing of PbTiO3 thin films

Lead titanate thin films were deposited by atomic layer deposition on Si(100) using Ph₄Pb and Ti(O-i-Pr)₄ as metal precursors and O₃ and H₂O as oxygen sources. The influence of the Ti : Pb precursor pulsing ratio on the film growth, stoichiometry and quality was studied at two different temperatures, i.e. 250 and 300 °C. Uniform and stoichiometric films were obtained using a Ti : Pb precursor pulsing ratio of 1 : 10 at 250 °C or 1 : 28 at 300 °C. The as-deposited films were amorphous but the crystalline PbTiO₃ phase was obtained by rapid thermal annealing at 600-900 °C both in N₂ and O₂ ambient. Thin PbTiO₃ films were visually uniform and roughness values for as-deposited and annealed films were observed by atomic force microscopy.     

Chemical fabrication of p-type Cu2O transparent thin film using molecular precursor method

A transparent p-type Cu₂O thin film of 50 nm thickness was successfully fabricated by means of a solution-based process involving the thermal reaction of molecular precursor films spin-coated on a Na-free glass substrate. The precursor solution was prepared by the reaction of an isolated Cu²⁺ complex of ethylenediamine-N, N, N′, N′-tetraacetic acid with dibutylamine in ethanol. The Cu₂O thin films resulting from heat treatment of the precursor film at 450 °C for 10 min in Ar gas at a flow rate of 1.0 L min⁻¹ were characterized by X-ray diffraction which indicated a precise cubic lattice cell parameter of a = 0.4265(2) nm, with a crystallite size of 8(2) nm. X-ray photoelectron spectroscopy peaks, attributable to the O 1s and Cu 2p_3/2 level of the Cu₂O film were found at 532.6 eV and 932.4 eV, respectively. An average grain size of the deposited Cu₂O particles of ca. 200 nm was observed via field-emission scanning electron microscopy. The optical band edge evaluated from the absorption spectrum of the Cu₂O transparent thin film was 2.3 eV, assuming a direct-transition semiconductor. Hall Effect measurements of the thin film indicated that the single-phase Cu₂O thin film is a typical p-type semiconductor, with a hole concentration of 1.7 × 10¹⁶ cm⁻³ and hole mobility of 4.8 cm² V⁻¹ s⁻¹ at ambient temperature. The activation energy from the valence band to the acceptor level determined from an Arrhenius plot was 0.34 eV. The adhesion strength of the thin film on the Na-free glass substrate was also determined as a critical load (Lc1) of 2.0 N by means of a scratch test. The method described is the first example of fabrication and characterization of a p-type Cu₂O transparent thin film by a wet process.     

Structural, optical, and electric properties of BNT–BT0.08 thin films processed by sol–gel technique

Thin films with the composition [(Bi_0.5Na_0.5)TiO₃]_0.92–[BaTiO₃]_0.08 (hereafter BNT–BT_0.08) were deposited on Pt–Si by spin-coating from a stable sol precursor. The BNT–BT_0.08 film, crystallized on the Bi_0.5Na_0.5TiO₃ rhombohedral lattice, was obtained after annealing the film-gel at 700 °C. The films have a smooth surface (Rms = 2.76 nm) and grains with ferroelectric domains. The film showed a bandgap of 3.25 eV and a refractive index of 2.20 at a wavelength of 630 nm. The dielectric characteristics of BNT–BT_0.08 thin films were measured at room temperature and 10 kHz the dielectric constant (ε _r) was 243 and the loss tangent (tanδ) was 0.38. The remnant polarization (P _r) was 0.87 μC/cm² and the coercive field (E _c) was 220 kV/cm at 10 kHz and room temperature. The current density was approximately 2.7 × 10⁻⁵ A/cm² at low electric fields (100 kV/cm). BNT–BT_0.08 thin films shown piezoelectric properties (d _33eff = 100 pm/V) comparable to those of PZT thin films.     

Effect of processing on the properties of Bi3.15Nd0.85Ti3O12 thin films

Various crystallization parameters were studied during the fabrication of Bi_3.15Nd_0.85Ti₃O₁₂ (BNdT) thin films on Pt/Ti/SiO₂/Si (100) by metal organic solution decomposition method. The effect of crystallization processes, crystallization ambients on the properties of BNdT thin films such as orientation, ferroelectric properties were examined. By adopting different fabrication processes, it is possible to get both highly c-axis oriented as well as randomly oriented thin films. Highly c-axis oriented BNdT thin film showed a large remnant polarization (2P_r) of 70 μC/cm² at an applied voltage of 10 V and exhibited a fatigue free behavior unto 2 × 10⁹ switching cycles. The improved ferroelectric properties of BNdT thin films suggest their suitability for high density ferroelectric random access memory applications.     

Crystallization of chemically vapor deposited molybdenum and mixed tungsten/molybdenum oxide films for electrochromic application

The paper deals with investigation of Mo oxide and mixed W/Mo oxide films showing high electrochromic performance. Films are deposited on Si and conductive glass substrates using pyrolytical decomposition at 200 °C of Mo and W hexacarbonyls in Ar/O₂ atmosphere. The study is focused on structural transformation of the films in dependence on deposition and annealing process parameters. In case of conductive glass substrate (typical for electrochromic devices), the crystallization process in Mo oxide films is almost completed at 400 °C forming triclinic MoO_2.89 and orthorhombic MoO₃ crystalline phases. The structure of mixed W/Mo oxide films is triclinic crystalline phase of tungsten oxide matrix with Mo atoms as substitutes. Discussed are, as well, differences in the crystallization process for the same films, when the substrate is Si. All the films show electrochromic effect, the mixed W/Mo oxide films expressing stronger electrochromic effect with superior color efficiency and optical modulation.