Low temperature metal oxide film deposition and reaction kinetics in supercritical carbon dioxide

An effective method is developed for low temperature metal oxide deposition through thermal decomposition of metal diketonates in supercritical carbon dioxide (scCO₂) solvent. The rates of Al(acac)₃ (Aluminum acetyl acetonate) and Ga(acac)₃ (Gallium acetyl acetonate) thermal decomposition in scCO₂ to form conformal Al₂O₃ and Ga₂O₃ thin films on planar surfaces were investigated. The thermal decomposition reaction of Al(acac)₃ and Ga(acac)₃ was found to be initialized at  150 °C and 160 °C respectively in scCO₂ solvent, compared to  250 °C and 360 °C in analogous vacuum-based processes. By measuring the temperature dependence of the growth rates of metal oxide thin films, the apparent activation energy for the thermal decomposition of Al(acac)₃ in scCO₂ is found to be 68 ± 6 kJ/mol, in comparison with 80–100 kJ/mol observed for the corresponding vacuum-based thermal decomposition reaction. The enhanced thermal decomposition rate in scCO₂ is ascribed to the high density solvent which effectively reduces the energy of the polar transition states in the reaction pathway. Preliminary results of thin film deposition of other metal oxides including ZrO_x, FeO_x, Co₂O₃, Cr₂O₃, HfO_x from thermal decomposition of metal diketonates or fluorinated diketonates in scCO₂ are also presented.     

Atmospheric plasma discharge chemical vapor deposition of SnOx thin films using various tin precursors

A series of 0.2–0.6 μm thick SnO_x films were deposited onto borosilicate and sodalime silica glass substrates by atmospheric plasma discharge chemical vapor deposition at 80 °C. SnO_x films deposited from monobutyltin trichloride contained a large percentage of SnCl₂:2H₂O, and therefore were partially soluble in water. SnO_x coatings deposited from tetrabutyltin were not soluble in water or organic solvents, had good adhesion even at growth rates as high as 2.3 nm/s, had high transparency of  90% and electrical resistivity of 10⁷ Ω cm. As-grown tin oxide coatings were amorphous with a small concentration of SnO₂, SnO and Sn crystalline phases as determined by grazing angle X-ray diffraction and X-ray photoelectron spectroscopy measurements. Upon annealing in air at 600 °C the resistivity of SnO_x films decreased to 5–7 Ω cm. Furthermore, optical and X-ray measurements indicated that SnO_x was converted into SnO₂ (cassiterite) with a direct band gap of 3.66 eV. Annealing of as-grown SnO_x films in vacuum at 340 °C led to formation of the p-type conductor SnO/SnO_x. The indirect band gap of SnO was calculated from the optical spectra to be 0.3 eV.     

Controllable formation of Er–Yb codoped Al2O3 films by the non-aqueous sol–gel method

1 mol%Er³⁺–10 mol%Yb³⁺ codoped Al₂O₃ thin films have been prepared on thermally oxidized SiO₂/Si(110) substrates by a dip-coating process in the non-aqueous sol–gel method from the hydrolysis of aluminum isopropoxide [Al(OC₃H₇)₃] under isopropanol environment. Addition of N,N-dimethylformamide (DMF) as a drying control chemical additive (DCCA) into the sol suppresses formation of the cracks in the Er³⁺–Yb³⁺ codoped Al₂O₃ thin films when the rare-earth ion is doped with a high doping concentration. Homogeneous, smooth and crack-free Er³⁺–Yb³⁺ codoped Al₂O₃ thin films form at the conditions by a molar ratio of 1:1 for DMF:Al(OC₃H₇)₃. A strong photoluminescence spectrum with a broadband extending from 1.400 to 1.700 µm centered at 1.533 µm is obtained for the Er³⁺–Yb³⁺ codoped Al₂O₃ thin films, which is unrelated to the addition of DMF. Controllable formation of the Er³⁺–Yb³⁺ codoped Al₂O₃ thin films may be explained by the fact that the DMF assisted the deprotonation process of Al–OH at the surfaces of gel particles, resulting in enhancement of the degree of polymerization of sols and improvement of the mechanical properties of gel thin films.     

Structure and conducting properties of La0.5Sr0.5CoO3−δ films on YSZ

La_0.5Sr_0.5CoO_3−δ (LSCO) thin films were deposited on yttria stabilized zirconia (YSZ) substrates by pulsed laser deposition (PLD) for application to thin film solid oxide fuel cell electrodes. During the deposition, the substrate temperature was varied from 450 to 750°C, and the oxygen pressure in the chamber was varied from 80 to 310 mTorr. Films deposited at 650°C and an oxygen background pressure of 150 mTorr were mostly (100) oriented. Deposition at higher temperatures or under lower oxygen pressures lead to mostly (110) oriented films. Films with low electrical resistivity of 10⁻³ Ω·cm were obtained.     

Effect of negatively charged species on the growth behavior of silicon films in hot wire chemical vapor deposition

The deposition behavior of silicon in hot wire chemical vapor deposition was investigated, focusing on the generation of negatively charged species in the gas phase using a gas mixture of 20% SiH₄ and 80% H₂ at a 450 °C substrate temperature under a working pressure of 66.7 Pa. A negative current of 6–21 µA/cm² was measured on the substrate at all processing conditions, and its absolute value increased with increasing wire temperature in the range of 1400 °C–1900 °C. The surface roughness of the films deposited on the silicon wafers increased with increasing wire temperature in the range of 1510 °C–1800 °C. The film growth rate on the positively biased substrates (+ 100 V, + 200 V) was higher than that on the neutral (0 V) and negatively biased substrates (− 100 V, − 200 V, − 300 V). These results indicate that the negatively charged species are generated in the gas phase and contribute to deposition. The surface roughness evolved during deposition was attributed to the electrostatic interaction between these negatively charged species and the negatively charged growing surface.     

Atmospheric pressure chemical vapour deposition of SnSe and SnSe2 thin films on glass

Atmospheric pressure chemical vapour deposition of tin monoselenide and tin diselenide films on glass substrate was achieved by reaction of diethyl selenide with tin tetrachloride at 350–650 °C. X-ray diffraction showed that all the films were crystalline and matched the reported pattern for SnSe and/or SnSe₂. Wavelength dispersive analysis by X-rays show a variable Sn:Se ratio from 1:1 to 1:2 depending on conditions. The deposition temperature, flow rates and position on the substrate determined whether mixed SnSe–SnSe₂, pure SnSe or pure SnSe₂ thin films could be obtained. SnSe films were obtained at 650 °C with a SnCl₄ to Et₂Se ratio greater than 10. The SnSe films were silver–black in appearance and adhesive. SnSe₂ films were obtained at 600–650 °C they had a black appearance and were composed of 10 to 80 μm sized adherent crystals. Films of SnSe only 100 nm thick showed complete absorbtion at 300–1100 nm.     

Synthesizing and Physical Properties of GdPr-123 High-Tc Superconductor

Ceramic Gd_1-xPr_xBa₂Cu₃O_7-y (GdPr-123) high-T_c superconductors have been synthesized by the standard solid state reaction method and characterized by XRD, SEM, TGA, and DT techniques in the range of x, 0.0 ≤ x ≤ 1.0 Samples have orthorhombic structure with Pmmm symmetry and there is a small percentage (less than 1%) of the Ba sites occupied by Pr ions in some Pr-rich samples. In these samples a small trace of BaCuO₂ and Pr BaO₃ secondary phases persist with the main peaks at 2θ = 29.3° and 28.9° respectively. Microstructure analysis indicates a uniform grain size distribution with a mean size of 5 μm. No significant change of grain size is shown throughout the range of x studied. The valence of Pr and Cu were determined by magnetization measurements in the temperature range 100 to 250°K. These data reveal a valence of 3.86+ for Pr in all samples independant of x. The similarity of the superconducting and insulating properties in this system to those of the oxygen deficient RBa₂Cu₃O₇ (R-123) (R: Y or rare earth) system implies that the mechanism of superconductivity in high-T_c. superconductors cannot be two-dimensional, even though the superconductive current occurs in two dimensions.     

Pyroelectric properties of BST/PLT composite thick films with addition of xPbO–(1 − x)B2O3 glass

The Ba_xSr_1−xTiO₃ (BST)/Pb_1−xLa_xTiO₃ (PLT) composite thick films (20 μm) with 12 mol% amount of xPbO–(1 − x)B₂O₃ glass additives (x = 0.2, 0.35, 0.5, 0.65 and 0.8) have been prepared by screen-printing the paste onto the alumina substrates with silver bottom electrode. X-ray diffraction (XRD), scanning electron microscope (SEM) and an impedance analyzer and an electrometer were used to analyze the phase structures, morphologies and dielectric and pyroelectric properties of the composite thick films, respectively. The wetting and infiltration of the liquid phase on the particles results in the densification of the composite thick films sintered at 750 °C. Nice porous structure formed in the composite thick films with xPbO–(1 − x)B₂O₃ glass as the PbO content (x) is 0.5 ≥ x ≥ 0.35, while dense structure formed in these thick films as the PbO content (x) is 0.8 ≥ x ≥ 0.65. The volatilization of the PbO in PLT and the interdiffusion between the PLT and the glass lead to the reduction of the c-axis of the PLT phase. The operating temperature range of our composite thick films is 0–200 °C. At room temperature (20 °C), the BST/PLT composite thick films with 0.35PbO–0.65B₂O₃ glass additives provided low heat capacity and good pyroelectric figure-of-merit because of their porous structure. The pyroelectric coefficient and figure-of-merit F_D are 364 μC/(m² K) and 14.3 μPa^−1/2, respectively. These good pyroelectric properties as well as being able to produce low-cost devices make this kind of thick films a promising candidate for high-performance pyroelectric applications.     

Microstructure, crystallinity, and properties of low-pressure MOCVD-grown europium oxide films

A study of growth, structure, and properties of Eu₂O₃ thin films were carried out. Films were grown at 500–600 °C temperature range on Si(1 0 0) and fused quartz from the complex of Eu(acac)₃·Phen by low pressure metalorganic chemical vapor deposition technique which has been rarely used for Eu₂O₃ deposition. These films were polycrystalline. Depending on growth conditions and substrates employed, these films had also possessed a parasitic phase. This phase can be removed by post-deposition annealing in oxidizing ambient. Morphology of the films was characterized by well-packed spherical mounds. Optical measurements exhibited that the bandgap of pure Eu₂O₃ phase was 4.4 eV. High frequency 1 MHz capacitance–voltage (C–V) measurements showed that the dielectric constant of pure Eu₂O₃ film was about 12. Possible effects of cation and oxygen deficiency and parasitic phase on the optical and electrical properties of Eu₂O₃ films have been briefly discussed.     

Investigation on the cross sensitivity of NO2 sensors based on In2O3 thin films prepared by sol-gel and vacuum thermal evaporation

In₂O₃ thin films have been prepared from commercially available pure In₂O₃ powders by high vacuum thermal evaporation (HVTE) and from indium iso-propoxide solutions by sol-gel techniques (SG). The films have been deposited on sapphire substrates provided with platinum interdigital sputtered electrodes. The as-deposited HVTE and SG films have been annealed at 500°C for 24 and 1 h, respectively. The film morphology, crystalline phase and chemical composition have been characterised by SEM, glancing angle XRD and XPS techniques. After annealing at 500°C the films’ microstructure turns from amorphous to crystalline with the development of highly crystalline cubic In₂O_3−x (JCPDS card 6-0416). XPS characterisation has revealed the formation of stoichiometric In₂O₃ (HVTE) and nearly stoichiometric In₂O_3−x (SG) after annealing. SEM characterisation has highlighted substantial morphological differences between the SG (highly porous microstructure) and HVTE (denser) films. All the films show the highest sensitivity to NO₂ gas (0.7–7 ppm concentration range), at 250°C working temperature. At this temperature and 0.7 ppm NO₂ the calculated sensitivities (S=R_g/R_a) yield S=10 and S=7 for SG and HVTE, respectively. No cross sensitivity have been found by exposing the In₂O₃ films to CO and CH₄. Negligible H₂O cross has resulted in the 40–80% relative humidity range, as well as to 1 ppm Cl₂ and 10 ppm NO. Only 1000 ppm C₂H₅OH has resulted to have a significant cross to the NO₂ response.     

Preparation of Pt-PtOx thin films as electrode for memory capacitors

Pt-PtO_x thin films were prepared on Si(100) substrates at temperatures from 30 to 700°C by reactive r.f. magnetron sputtering with platinum target. Deposition atmosphere was varied with O₂/Ar flow ratio. The deposited films were characterized by X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscopy. Resistively of the deposited films was measured by d.c. four probe method. The films mainly consisted of amorphous PtO and Pt₃O₄ (or Pt₂O₃) below 400°C, and amorphous Pt was increased in the film as a deposition temperature increased to 600°C. When deposition temperature was thoroughly increased, (111) oriented pure Pt films were formed at 700°C. Compounds included in the films strongly depended on substrate temperature rather than O₂/Ar flow ratio. Electrical resistivity of Pt-PtO_x films was measured to be from the order of 10⁻¹ Ω cm to 10⁻⁵ Ω cm, which was related to the amount of Pt phase included in the deposited films.     

Synthesis and characterization of polyvinylpyrrolidine assisted tantalum pentoxide films

Micron thick tantalum pentoxide (Ta₂O₅) films have been proposed as thermal insulating layers in microchemical systems, but so far it has been difficult to deposit thick enough films over complex substrates. So far sol–gel films cracked upon heating whenever the film thicknesses were above 350 nm. A 350 nm thick film is too thin for effective insulation. Other techniques are not suitable for coating the complex structures associated with microchemical systems. In this paper we report sol–gel synthesis of 1.6 μm thick tantalum pentoxide (Ta₂O₅) films. The films are almost crack free, and adhere to silicon surfaces even upon flashing to 900 °C. The key to the synthesis is the addition of Polyvinylpyrrolidine (PVP) to the sol. Films grown in the absence of PVP all show cracks upon calcination to 900 °C while few cracks are seen with PVP. X-ray diffraction and Fourier transform infra red analysis show that orthorhombic Ta₂O₅ is formed in all cases. X-ray photoelectron spectroscopy shows the O:Ta ratio to be 2.8:1. This shows that sol–gel is a viable process for making the micron thick films of Ta₂O₅ needed as insulators for microchemical systems.     

Deposition of Sm2O3 doped CeO2 thin films from Ce(DPM)4 and Sm(DPM)3 (DPM=2,2,6,6-tetramethyl-3,5-heptanedionato) by aerosol-assisted metal–organic chemical vapor deposition

Samarium-doped ceria (SDC) thin films were prepared from Sm(DPM)₃ (DPM = 2,2,6,6-tetramethyl-3,5-heptanedionato) and Ce(DPM)₄ using the aerosol-assisted metal–organic chemical vapor deposition method. -Al₂O₃ and NiO-YSZ (YSZ = Y₂O₃-stabilized ZrO₂) disks were chosen as substrates in order to investigate the difference in the growth process on the two substrates. Single cubic structure could be obtained on either -Al₂O₃ or NiO-YSZ substrates at deposition temperatures above 450 °C; the similar structure between YSZ and SDC results in matching growth compared with the deposition on -Al₂O₃ substrate. A typical columnar structure could be obtained at 650 °C on -Al₂O₃ substrate and a more uniform surface was produced on NiO-YSZ substrate at 500 °C. The composition of SDC film deposited at 450 °C is close to that of precursor solution (Sm : Ce = 1 : 4), higher or lower deposition temperature will both lead to sharp deviation from this elemental ratio. The different thermal properties of Sm(DPM)₃ and Ce(DPM)₄ may be the key reason for the variation in composition with the increase of deposition temperature.     

Influence of substrate on the pulsed laser deposition growth and microwave behaviour of KTa0.6Nb0.4O3 potassium tantalate niobate ferroelectric thin films

Thin films of potassium tantalate niobate KTa_0.6Nb_0.4O₃ (KTN) were grown by pulsed laser deposition on five different substrates suitable for microwave devices: (100)MgO, (100)LaAlO₃, (1–102)sapphire (R-plane), (0001)sapphire (C-plane) and alumina. The high volatility of potassium at the film growth temperature required the addition of an excess of potassium to the ablation target. For optimized deposition conditions, Rutherford backscattering showed that the KTN films had a 1: 1 atomic ratio for K:(Nb + Ta). As grown KTN thin films were single-phase, without any particular orientation on sintered alumina, whereas an epitaxial growth with the (100) orientation was achieved on (100)MgO and (100)LaAlO₃ with a mosaicity Δω_(100)KTN close to 0.7°–1.5° and  0.4°–0.9°, respectively, attesting a high crystalline quality. In contrast, growth of KTN on R-plane sapphire results in a texture with the (100) orientation and the presence of the (110) orientation as a secondary one. The room temperature measurements carried out on Au interdigited capacitors patterned on KTN coated (100) LaAlO₃ and sapphire led at 1 GHz to an agility ΔC / C  4.6% and  7.2%, respectively, for a moderate applied field of 15 kV cm^− 1. Stubs patterned on the same systems led to an agility ΔF_r / F_r of  2.2% and 4.2%, respectively, for F_r = 7 GHz and the same applied field.     

Molecular beam epitaxial growth of SmBa2Cu3O7−δ thin films and effect of substrate temperature on the surface roughness

Stoichiometrically optimized, epitaxial SmBa₂Cu₃O_7-δ thin films with high T_{c, R = 0} and high critical current densities j_c have been prepared for the first time in a tightly controlled molecular beam epitaxy process in non-reactive molecular oxygen, followed by an in situ loading process with molecular oxygen. The surface roughness (on a submicrometre scale) of single-crystal films with their c axes perpendicular to the surface depends markedly on the surface temperature of the substrate during the deposition of the epitaxial films, within a range of only a few degrees centigrade. The calibrated optimal temperature for the preparation of epitaxial films 200 nm thick of this single orientation is found to be 680 ± 5 °C. In scanning tunnelling microscopy investigations, they show a surface roughness of less than 6 nm (five SmBa₂Cu₃O_7−δ unit cells) on a 2 μm × 2 μm scale. At deposition temperatures below this optimal deposition temperature, the well-known a-axis growth increases rapidly, whereas higher temperatures give a significantly higher surface roughness, which can be observed by scanning electron microscopy.     

Influence of praseodymium oxide/cobalt oxide ratio on microstructure and electrical properties of zinc oxide varistor ceramics

The microstructure and electrical properties of varistor ceramics, which are composed of (99.5–x–y)ZnO+xPr₆O₁₁+yCoO+0.5Dy₂O₃ system, were investigated with Pr₆O₁₁/CoO mole ratio (x/y=0.5/0.5, 0.5/1.0, 1.0/0.5, 1.0/1.0) and sintering temperature. The density of varistor ceramics with Pr₆O₁₁=1.0 was almost constant with sintering temperature, whereas it was increased noticeably in Pr₆O₁₁=0.5. Increasing Pr₆O₁₁ content enhanced the densification for any CoO content and the density was greatly affected not by CoO content but by Pr₆O₁₁ content. The varistor ceramics with Pr₆O₁₁/CoO=0.5/1.0 exhibited a higher nonlinearity than any other composition ratios. In particular, the varistor ceramics sintered at 1350 °C. exhibited the best electrical properties, with nonlinear exponent of 37.8, leakage current of 7.6 μA, and dissipation factor of 0.059. It was found that Pr₆O₁₁/CoO ratio greatly affects various characteristics of varistor ceramics.     

Transparent conducting F-doped SnO2 thin films grown by pulsed laser deposition

Transparent conducting fluorine-doped tin oxide (SnO₂:F) films have been deposited on glass substrates by pulsed laser deposition. The structural, electrical and optical properties of the SnO₂:F films have been investigated as a function of F-doping level and substrate deposition temperature. The optimum target composition for high conductivity was found to be 10 wt.% SnF₂ + 90 wt.% SnO₂. Under optimized deposition conditions (T_s = 300 °C, and 7.33 Pa of O₂), electrical resistivity of 5 × 10^− 4 Ω-cm, sheet resistance of 12.5 Ω/□, average optical transmittance of 87% in the visible range, and optical band-gap of 4.25 eV were obtained for 400 nm thick SnO₂:F films. Atomic force microscopy measurements for these SnO₂:F films indicated that their root-mean-square surface roughness ( 6 Å) was superior to that of commercially available chemical vapor deposited SnO₂:F films ( 85 Å).     

Catalytic CVD growth of Si–C and Si–C–O alloy films by using alkylsilane and related compounds

Cat-CVD method has been applied to the growth of Si–C and Si–C–O alloy thin films. Growth mechanism has been studied with emphasis on the effects of filament materials. Growth rates and alloy compositions were measured for W, Ta, Mo and Pt filaments at the filament temperatures ranging from 1300 to 2000 °C. Si_1−xC_x films with x ranging from 0.38 to 0.7 could be grown by using single molecule source Si(CH₃)₂H₂ (dimethylsilane). Si–C–O ternary alloy films was successfully prepared by using Si(OC₂H₅)₄ (tetraethoxysilane) and Si(CH₃)₂(OCH₃)₂ (dimethyldimethoxysilane) molecules.     

Hydrothermal synthesis, crystal structure and thermal stability of Ba-titanate nanotubes with layered crystal structure

BaTi₃O₇·nH₂O nanotubes have been synthesized through a hydrothermal reaction between Na-rich trititanate nanotubes and Ba(OH)₂ within the pH range 12–8.2. These nanotubes possess the same layered crystal structure of the precursor Na_2−xH_xTi₃O₇·nH₂O. They also keep the morphology of their precursor Na-trititanate nanotubes used in the synthesis, having an external diameter of 20–25 nm. The BaTi₃O₇·nH₂O nanotubes remained fully stable up to 300 °C, while nanotubular form continues to exist up to 600 °C, together with amorphous particles.     

Effects of oxygen partial pressure on lead content of PLZT thin films produced by excimer laser deposition

Excimer laser ablation has been used to produce thin films of lanthanum-modified lead zirconate titanate (PLZT), or Pb_1−xLa_x(Zr_1−yTi_y)_1−x/4O₃. PLZT is an interesting class of materials since it has a wide range of compositionally dependent electro-optical properties and strong non-linear optical characteristics. PLZT thin films of 7/0/100, 28/0/100 and 0/0/100 compositions have been deposited onto crystalline Si100 and amorphous fused silica substrates. Effects of oxygen backfill pressure on the Pb:(Ti + La) ratios were investigated. The results indicate that controlling the oxygen backfill pressure during laser deposition strongly influences the stoichiometry and crystal structure of PLZT thin films.