Mark R. De Guire

Review: Deposition of ceramic thin films at low temperatures from aqueous solutions

Many techniques for the synthesis of ceramic thin films from aqueous solutions at low temperatures (25–100°C) have been reported. This paper reviews non-electrochemical, non-hydrothermal, low-temperature aqueous deposition routes, with an emphasis on oxide materials for electronic applications. Originally used for sulfide and selenide thin films, such techniques have also been applied to oxides since the 1970s. Films of single oxides (e.g., transition metal oxides, In2O3, SiO2, SnO2) and multicomponent films (doped ZnO, Cd2SnO4, ZrTiO4, ZrO2-Y2O3, Li-Co-O spinel, ferrites, perovskites) have been produced. The maximum thicknesses of the films obtained have ranged from 100 to 1000 nm, and deposition rates have ranged from 2 to 20,000 nm/h. Compared to vapor-deposition techniques, liquid-deposition routes offer lower capital equipment costs, lower processing temperatures, and flexibility in the choice of substrates with respect to topography and thermal stability. Compared to sol-gel techniques, the routes reviewed here offer lower processing temperatures, lower shrinkage, and (being based on aqueous precursors) lower costs and the potential for reduced environmental impact. This review emphasizes the influence of solution chemistry and process design on the microstructures and growth rates of the films. The current understanding of the mechanisms of film formation is presented, and the advantages and limitations of these techniques are discussed.

Characterization of SnO2 thin films grown from aqueous solutions

Nanocrystalline SnO2 thin films with the cassiterite structure and grains 5–10 nm in size were deposited from precipitating aqueous solutions of SnCl4×5H2O and HCl at 80°C. The substrates were single-crystal {100} silicon wafers, either with or without sulfonate-functionalized self-assembled monolayer (SAM) coatings. Films, 60–70 nm thick, were obtained in 12 h from a medium of 2 mM SnCl4/0.4 M HCl. These films grew at a rate of approximately 9 nm h−1 during the first 4 h. Thicker films, 160 nm thick, containing cassiterite grains 5–10 nm in size were obtained by continuously flowing a solution of 2 mM SnCl4/0.2 M HCl past the substrates. The as-deposited films were uniform and adherent. They contained ∼2–3 at.% Cl and excess oxygen (2<O/Sn<3). A detailed X-ray photoelectron spectroscopy (XPS) analysis of the oxygen content of the films indicates that the additional oxygen may be present in an amorphous tin oxyhydroxide phase.

Electrical properties of TiO2 thin films formed on self-assembled organic monolayers on silicon

TiO2 thin films were synthesized onto sulfonated self-assembled organic monolayers from Ti4+ aqueous solutions at low temperature (80 °C). The deposited TiO2 thin films were characterized using Rutherford backscattering spectroscopy and transmission electron microscopy. The electrical properties of the TiO2 films were measured using capacitance–voltage (C–V) and current–voltage techniques. From C–V measurements, the dielectric constants were calculated to be in the range of 24–57 for the as-deposited films and 67–97 for films annealed at 400 °C. For the as-deposited samples, the breakdown voltage was 2.7 MV/cm, the leakage current was 4.5×10−6 A/cm2, the resistivity was 1–1.5×109 Ω cm, and the interface trap density ∼1×1012 cm−2 eV−1. The leakage current behavior was consistent with the Poole-Frenkel mechanism of conduction.

Nano/micro-patterning of anatase TiO2 thin film from an aqueous solution by site-selective elimination method

Abstract We proposed a novel method to fabricate nano/micro-scaled patterns of thin films and successfully fabricated patterns of anatase TiO2 thin films in an aqueous solution at 50 8C. The patterned self-assembled monolayer (SAM) having octadecyltrichlorosilane (OTS) regions and silanol regions was immersed in a solution containing a Ti precursor and subjected to ultrasonication for several hours. The difference in adhesion of thin films on substrates was employed for the site-selective elimination method. Heterogeneously nucleated TiO2 and homogeneously nucleated TiO2 particles adhering to the OTS–SAM could be easily eliminated from the substrate by ultrasonication, whereas those on silanol groups maintained their adhesion during the immersion period. TiO2 can form chemical bonds such as Ti–O–Si with silanol groups, but cannot form them with octadecyl groups, resulting in the difference in adhesion, which is the essence of the site-selectivity of this method. The site-selective elimination method can be applied to fabricate nano/micro-scaled patterns in the solution by the immersion of the substrate that has regions on which depositions adhere strongly and regions on which depositions adhere weakly, enabling elimination by treatment such as ultrasonication.

Boron‐modified polysilylcarbodi‐imides as precursors for Si–B–C–N ceramics: Synthesis, plastic‐forming and high‐temperature behavior

The synthesis, by two different reaction pathways, of boron-modified polysilylcarbodi-imides of general type {B[C 2 H 4 Si(R)NCN] 3 } n (R= singly bonded organic ligand) and the plastic-forming and the thermal behavior of these polymers are described. Compounds {B[C 2 H 4 Si(R)NCN] 3 } n [2a, R=H; 2b, R= CH 3 ; 2c, R=(NCN) 0.5 ] can be obtained by treatment of the vinyl-substituted polysilylcarbodi-imides [{H 2 C=CH)(R)SiNCN] n [1a, R =H; 1b, R= CH 3 ; 1c, R=(NCN) 0.5 ] with borane dimethylsulfide BH 3 .S(CH 3 ) 2 . The polysilylcarbodi-imides 1a-1c themselves are accessible via the reaction of vinyl-substituted chlorosilanes (H 2 C=CH)-(R)SiCl 2 with cyanamide H 2 N-C≡N in the presence of pyridine or by a non-oxide sol-gel process of vinylated chlorosilanes and bis(trimethylsilyl)carbodi-imide, (H 3 C) 3 SiN=C=N-Si(CH 3 ) 3 . In the second method for the synthesis of 2a-2c, hydroboration of vinyl-substituted chlorosilanes (H 2 C=CH)(R)SiCl 2 with borane dimethylsulfide, borane trimethylamide or borane triethylamide to yield the tris[(chlorosilyl)ethyl]boranes B[C 2 H 4 Si(R)Cl 2 ] 3 (3a, R=H; 3b, R=CH 3 ; 3c, R=Cl) is followed by treatment of the as-obtained compounds with bis(trimethylsilyl)carbodi-imide, which results in the formation of the hydroborated polysilylcarbodi-imides 2a-2c. The thermogravimetric behavior of the polymers 1a-1c and 2a-2c up to 2300°C is reported. It is shown that boron-modified polysilylcarbodi-imides are suitable precursors for the preparation of dense bulk ceramics. Therefore, the preparation of green bodies of the hydroborated polysilylcarbodiimides 2a-2c by plastic forming (PF) is described. A series of experiments points to the fact that the microstructure of the as-obtained ceramic monoliths obtained by subsequent thermolysis of the plastic-formed green bodies is strongly influenced by the conditions during plastic forming.

The series Bi2Sr2Can−1CunO2n+4 (1≤n≤5): Phase stability and superconducting properties

Abstract Phase relations at 850°C and 870°C, and melting transitions in air, oxygen, and helium have been studied for Bi2.1Sr1.9CuO6 and for the series Bi2Sr2Can−1CunO2n+4 for n=1, 2, 3, 4, 5 and ∞ (“CaCuO 2 ”) . Up to 870°C, the n=2 composition resides in the compatibility tetrahedron bounded by Bi2+x(Sr, Ca)3−yCu2O8, (Sr, Ca)14Cu24O41. Ca2CuO3, and a BiSrCaO phase. The n ≥ 3 compositions reside in the compatibility tetrahedron Bi2+x(Sr, Ca)3−yCu2O8(Sr, Ca)14Cu24O41 Ca2CuO3CuO up to 850°C. However, Bi2+x(Sr, Ca)4−yCu3O10 forms for n ≥ 3 after extended heating at 870°C. Bi2+xSr2−yCuO 6 (i.e. the Ca-free, very low-Tc phase) melts in air at 914°C, while the Bi2+x(Sr, Ca)3−yCu2O8 composition that coexists with (Sr, Ca)14Cu24O41 , Ca2CuO3, and CuO melts at 895°C. During melting, all of the compositions studied lose 1–2% by weight of oxygen from the reduction of copper.

Particle growth and particle–surface interactions during low-temperature deposition of ceramic thin films

Abstract The forces between substrates and ceramic particles in aqueous solution were measured using an atomic force microscope (AFM). The dependence of the measured forces on the pH and ionic strength of the solution are presented. The trends are consistent with expectations based on the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory. The kinetics of the nucleation and growth of tin oxide particles in aqueous solutions of tin(IV) chloride and hydrochloric acid were studied using dynamic laser scattering. The rate of formation of particles as a function of solution temperature, concentration, and pH agrees with thermodynamic calculations and with previous observations. Antimony-doped tin oxide films on SAM-coated glass have been found to be several orders of magnitude more conductive than identical coatings on bare glass. Apatite thin films have been deposited from simulated body fluid (SBF) on titanium metal. The porous topography of the metal, produced by prior etching in hydrochloric acid, was preserved in the deposited apatite films. The adhesion of these films to the substrate appears to have been greatly increased by the etching step, compared to films deposited on unetched titanium.

Point defect analysis and microstructural effects in pure and donor-doped ceria

Abstract Ceria (CeO 2− x ) can be made electronically conductive by doping with pentavalent or hexavalent cations. A technique has been developed, within the framework of standard Kroger-Vink defect analysis, by which the concentrations of three coexisting major defect species (pentavalent impurity, free electrons, and one of several possible point defects) can be modeled in ceria or other MO 2 -type oxides. This analysis provides diagnostic information regarding the dominant defect behavior that cannot be obtained from the standard Brouwer approximation. Comparisons of microstructures and electrical conductivity were made between Nb-doped ceria synthesized by a conventional mixed-oxide route and by hydroxide coprecipitation. The mixed oxide materials contained silicate intergranular phases that acted as sinks for the dopant. Coprecipitated materials were virtually free of intergranular contamination and exhibited electrical conductivities approximately 25% higher than the mixed oxide materials. Finally, experimental results on the electrical conductivity of Nb-, Ta-, and W-doped ceria indicate that all are electronic conductors, with the W-doped materials showing the highest overall conductivity in air in the temperature range 600–900°C.

Chemical Bath Deposition

Chemical bath deposition encompasses a variety of routes for producing functional oxide films and coatings at relatively low temperature by immersing a substrate in a liquid solution. Films of dozens of single- and multi-component oxide materials have been synthesized, mostly from aqueous precursor solutions, at temperatures below 100 °C, and on substrates that vary widely in their chemistries and topographies. This chapter focuses on three aspects of the chemical principles that govern the formation of such films. The first aspect is solution chemistry: the interrelated effects of solution temperature, pH, and concentration needed to generate the oxide material, and which influence all subsequent considerations of the film deposition process. The second aspect is the interactions between the substrate and the growing film, which can be utilized to promote or suppress film attachment and to control the microstructure and spatial distribution of the film. The third aspect addresses practical considerations of a deposition process, such as design factors (other than solution parameters) that affect growth rates and film thicknesses. The chapter concludes with a discussion of the limitations of CBD processes for oxide film synthesis, and of the most promising potential applications and areas for future research.

Spinel ferrite-silica glass obtained by splat quenching☆

Abstract The compositions (in mol%) 40 MnFe 2 O 4 , 60 SiO 2 , and 42.8 CoFe 2 O 4 , 57.1 SiO 2 have been melted and splat-quenched. The resulting materials have been analyzed using X-ray diffraction, transmission electron microscopy, scanning transmission electron microscopy, and room temperature Mossbauer spectroscopy. The quenched Mn-containing material was completely amorphous. Its Mossbauer spectrum contains two doublets, indicating Fe 2+ and Fe 3+ in distorted octahedral sites. The quenched Co-containing composition contained (Co, Fe) 2 SiO 4 (olivine and (Co, Fe) 2 O 4 (spinel) precipitates, 150–400 A in diameter, in a glassy matrix. The Mossbauer spectrum contains three doublets, indicating octahedral Fe 2+ in the olivine, distorted octahedral Fe 2+ in the glass, and distorted octahedral Fe 3+ in the glass. The spectrum also shows trace hyperfine splitting, attributed to the spinel ferrite.