Synthesis and characterization of ZrO2 thin films
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https://www.eduzhai.net American Journal of M aterials Science 2012, 2(4): 119-124 DOI: 10.5923/j.materials.20120204.04 Synthesis and Characterization of ZrO2 Thin Films L. P. Borilo*, L. N. Spivakova Tomsk State University, Tomsk, Russia Abstract Zirconia thin films with thicknesses of 40-120 n m on g lass, single-crystal silicon, quart z, polycor, and sap- phire substrates have been prepared from zirconiu m o xochloride and ethanol FFSs. The physicochemical p rocesses involved in film format ion and the phase composition and properties of the films have been studied. The films prepared on glass or quartz are a morphous; those on silicon, polycor, or sapphire have a c rystal structure. The resulting ZrO2 films have refract ive index indicator 1,86 – 2,08, are insulators, with h igh indicators of bandgap width 5,0 – 5,2 eV, absorption edge is limited by 220 nM, which allows to use it as reallot light covering. Keywords So l-Gel Technology, Thin Films, Zirconiu m Dio xide 1. Introduction Films materials play a special role in the development of civilizat ions based on high technologies. Studying optical characteristic of dielectric resulted in using thin layer as reallot light covering and interference filters. A mong d ifferent materia l classes oxides have various functional electro physical properties (electro physical, optical, mechanical etc), which allo ws to use these materials as a basis for synthesis of many film materials. At the present stage of development of photonics, optoinformatics, optotechnics and laser optics the requirements to optics properties of films e xtend[1- 2] . The most challenging issue is the absence of transparent in broad spectral zone film-forming materials with high Refractive index. Zirconia-based thin films have a high potential. Zirconia is transparent in the visible; it has high refractive index and bandgap values, good adhesion to substrates (glass, ceramics, silicon, polycor, and sapphire), thermal stability, and corrosion resistance[3-6]. Sol-ge l technology, which is a synthetic method involving chemical condensation in a liquid phase, is regarded as the most efficient and simp lest method for manufacturing nanoparticles. This method can provide not only dispersed powders but also thin films based on complex chemical systems having layer thicknesses of 10 to 200 n m and multilayer films having thicknesses up to 1 μm. The synthesis and characterization of nanocrystalline zirconia-based powders prepared by various methods are found in periodicals [7- 1 2]. To use thin films materials based on dioxide zirconiu m in optics, particularly as reallot light covering, we should study * Corresponding author: email@example.com (L.P. Borilo) Published online at https://www.eduzhai.net Copyright © 2012 Scientific & Academic Publishing. All Rights Reserved the influence of conditions of synthesis on thickness, phase composition and structure of resulting films, evaluate the influence of a substrate (glass, quartz, etc) on physicochemical p roperties of the films. Therefore, here we study the physicochemical processes involved in the preparation of ZrO2 thin films on various substrates by s ol-gel technology from film-forming solutions (FFSs) and the phase composition, structure, and properties of these films as dependent on the synthesis parameters. 2. Experimental Work Thin films were prepared by sol-gel technology from FFSs . The FFSs were prepared fro m 96% ethanol and zirconium oxochloride ZrOCl2·8H2O (pure for analysis); the solution concentration was 0.4 mo l/ L. Films were prepared on g lass, single-crystal silicon, quart z, polycor, and sapphire using centrifugation at 4000 rp m or pulling at 5 mm/s and subsequent heat treatment at 60℃ for 30 min and at 600-1000°C for 1 h. The film-forming power of solutions as a function of aging time was studied viscometrically on a glass capillary viscometer (the capillary diameter was 0.99 mm; temperature was 25℃). Reduced viscosity ηsp/c was determined as a function of zirconium o xochloride concentration by diluting the FFS with ethanol; oxochloride concentrations ranged within 0.53.3 g/dL. The concentration dependences were processed after Bogoslovskii et al.. The thermolysis of the dried FFS was studied on a Q-1500 derivatograph (25-1000℃, calcined A12O3 as reference, air, heating at 10 K/ min, Alundum crucibles). A microbalance based on a quartz piezocrystal resonator was used in the thermoanalytical experiments; the weighing accuracy was 10-8 g. IR absorption spectra were recorded for films on KBr substrates annealed at various temperatures within 120 L. P. Borilo et al.: Synthesis and Characterization of ZrO2 Thin Films 400-4000 cm4 on a Perkin-Elmer Spectrum One spectrometer. The composition of films was determined on a DRON-3M diffractometer (CuKα radiation,λ = 1.5418 nm, Ni filter). The refractive index and film thickness were determined on an LEF-3M laser diffractometer. The optical parameters were calculated using the uniform nonabsorbing layer on an isotropic substrate model. Reflection andtransmission spectra in the visible and IR were recorded on SF-20 and Perkin-Elmer Spectrum One spectrometers. Dielectric constants were calculated by Kramers-Kroning relations fro m the reflection and transmission spectra. The bandgap width was calculated from the absorption edge position. Adhesion was determined by scleroscopy on a PMT-3 microhardness tes ter. 3. Results and Discussion The solutions from wh ich thin films can be deposited by sol-gel technology should conform to several requirements. When precursors are dissolved in the solvent, some period is needed for so-called solution ripening or sol formation. This period ranges from several minutes to s everal days, depending on the precursor. At this stage, the true solution transforms to a colloidal one because of solvation, hydrolysis, complex formation, and condensation. The resulting associations are capable of being anchored to the surface when the solution is applied to the substrate. We used viscosity as a measure of the film-forming power of the solution[16-18]. So lution viscosities were plotted as function of time and possibility of film preparation fro m these solutions. Our studies showed that a fresh zirconiu m oxochloride-based ethanolic solution has not film-forming properties. The film-forming ability appears 2-3 days after the solution was prepared. The solution viscosity increases strongly during this period because of the solvation of zirconium o xochloride and the format ion of hydro xo co mple xes[Zr4(ОН)8(С2Н5ОH(Н2O)16-х]С18; as a result, a stable sol is formed in the solution. Good films with reproducible properties can be prepared if the solution viscosity reaches 2.3 × 10-3 Pa s. The processes occurring in the FFS during storage and operation change the composition of the complex: the number of molecu les of water of coordination decreases, more OH groups appear linked to zirconium, some water mo lecules are displaced fro m the inner coord ination sphere, and the hydroxo cation charge decreases: [Zr4(OH)8(C2H5OHUH2O)16-х]Cl8 → [Zr4(OH)12(C2H5OH)x(H2O)12x]Cl4 + 4HC1, [Zr4(OH)12(C2H5OH)x(H2O)12-x]Cl4 → [Zr4 (ОH)16(С2Н5ОH)x(H2O)8-x] + 4HC1, [Zr4(OH)8(H2O)16-x(C2H5OH)J8+→ [Zr4(OH)8+y(H2O)16-x-у(С2Н5ОH)x](8-y)+ +yH+. As a result, the system loses stability because of coagulation, and the solution viscosity increases dramatically and exceeds 3.5 × 10-3 Pa s. The films prepared from such solutions have nonuniform thicknesses and low refractive indices. Thus, the film-forming properties of the solution (the ability to form films) exist within a limited period time, namely, while the solution is ripening and aging. The optimal viscosities of zirconiu m o xochloride-based solutions for preparing ZrO2 films are (2.3 to 3.5) 3.5 × 10-3 Pa s. The zirconiu m o xoch loride-based FFS being a solution based on a polymeric hydroxo co mp lex, Kolen'ko et al.' approach is suitable for the quantitative description of colloid interactions in this solution. In this context, we studied the viscosity of the zirconium o xochloride-based FFS as a function of dilution and calculated the covalent and electro- static terms of the Gibbs free energy of mixing. To describe reduced viscosity ηsp/c as a function of concentration, we used a linear ext rapolation equation, namely, Huggins's equation ηsp /c =[η]a + k'[η]2с, (1) and nonlinear extrapolation equations, namely, ηsp /c = a + bexp (-dc) (2) ηsp /c = a + b1exp(-d1c) + b2exp (-d2c) (3) Here, a, bi, an di are empirical solvation factors, which depend on the dipole mo ment, donor and acceptor numbers of the solvent and have the dimension of the molar volu me. Various functions for the description of empirical reduced viscosity versus temperature dependences give numerical values of the characteristic viscosity r|. Variat ion in reduced viscosity of the zirconiu m o xochloride-based FFS (Fig. 1) can be described by Eqs. (l)-(3). For this solution, we cal- culated the Huggins constant k' and characteristic viscosity fro m these three equations (Table 1). The Huggins constant has a negative value (k ' = -2.41). Proba- bly, this is exp lained by the effect o f hydrogen bonds, which are ignored by the Huggins theory. Comparing the variance and correlation coefficients for the three equations, Eq. (3) most adequately describes the experimental dependence in question; the reliable characteristic viscosity value is 0.102 dL/g. Table 2 lists the solvation energies (in kJ/ mo l) for the tested FFSs derived fro m η according to Eq. (3). In solutions containing zirconiu m o xoch loride, there are insignificant electrostatic interactions (ΔGa = 3.5 kJ/ mol) and specific donor-acceptor interactions (ΔGb1 = -3.64 kJ/ mol, ΔGb2 = 2.60 kJ/mol). Figure 1. Reduced viscosity ηsp/c for the zirconium oxochloride-based FFS vs. zirconium oxochloride concent rat ion с (g/dL) American Journal of M aterials Science 2012, 2(4): 119-124 121 Table 1. Viscometric parameters of the zirconium oxochloride-based FFS (calculated from Eqs. (l)-(3)) Equat io n (1) (2) (3) [η], dL/g 0.059 0.137 0.102 k' -2.41 - This is due to the high propensity of zirconiu m to form bulky poly meric co mplex structures; these structures, which have high characteristic viscosities, are responsible for the good film-forming properties of alcoholic solutions of zirconium o xochloride. prepared fro m the zirconiu m o xochlo ride-based FFS is thermoly zed differently than ZrOCl2 · 8H2O (Fig. 2). ZrOCl2 · 8H2O is thermo lyzed as follows: ZrOCl2 ·8H2O 40−80°С→ ZrOCl2·5H2O 80−110°С→ → ZrOCl2 ·4H2O 110−130°С→ → Zr(OH)2·0.7H2O 400−480°С→ ZrO2(amorph) The thermolysis of the powder prepared from the FFS differs in that it contains the product of zirconiu m o xochloride thermo lysis: [Zr(OH)2(C2H5OH)x(H2O)4-x]Cl2 110−200°С→ → Zr(OH)4· 2H2O 250−350°С→ → ZrO2 ·H2O(amorph.) 450−720°С→ → ZrO2(cub.+tetrag.) 720−790°С→ ZrO2 (monocl.) ZrO2 film fo rmation is a mo re intricate process, as shown by piezocrystal weighing, IR spectroscopy, and X-ray powder diffract ion. This process has the follo wing d istinguishing feature: when the solution is applied to the substrate, chloride ions remain in the solution and are not contained in the thin FFS layer on the substrate surface. The solution is anchored to the substrate via the interaction of zirconiu m hydroxo co mplexes with the surface hydroxide groups of the substrate as a result of hydrolysis. Schematically, subsequent dehydration in a thin layer can be represented as follows: [Zr(OH)2(C2H5OH)x(H2O)4x](OH)2 25−125°С → → Zr(OH)4·H2O 125−230°С→ → Zr(OH)4 ·2H2O 230−300°С→ → Zr(OH)4·H2O 300−355°С→ → ZrO2 ·H2O 355−375°С→ Figure 2. Data of differential thermal analysis а) Zirconium oxochloride powder b) Powder prepared from film-forming solutions based on zirconium oxochloride We used a set of mutually supplementing methods to study the physicochemical processes involved in the formation of thin films and powders of zirconia fro m the FFS during heat treatment. Our studies showed that the powder → ZrO2 (amorph.) 375−400°С→ ZrO2 (cub.) The IR spectra of a freshly applied film contain peaks corresponding to zirconiu m hydro xide and physisorbed water (Table 3). Film adhesion to the substrate is lo w; the refract ive index is about 1.6, wh ich is not characteristic of zirconia (Fig. 3). The refractive index of the film systematically increases during heat treatment as temperature increases, while the film thickness decreases to reach a steady-state value at 400℃ (Fig. 3). The values of 2.0-2.1 for the refractive index mean that the film co mposition corresponds to zirconia. Table 2. Worksheet for the calculation of solvation energies ΔGa, ΔGb1 ΔG b2 a b1 b2 d1 d2 [η], dL/g ΔGcm kJ/mol kJ/mol 0.145 -0.150 0.107 3.5 -3.64 2.60 0.063 0.686 0.102 2.46 122 L. P. Borilo et al.: Synthesis and Characterization of ZrO2 Thin Films Figure 3. (1) Refractive index and (2) thickness vs. temperature for ZrO2 films In cases where a powder is formed, zirconia crystallizat ion is observed above 450 ℃ and acco mpanied by two exotherms, at 480 and 720℃. The former is due to the transition of the amorphous phase to the cubic and tetragonal phases; the latter, to the transition to the monoclinic phase (Fig. 4). In cases where a film is formed (with thicknesses up to 100 n m), X-ray powder d iffraction detects cubic ZrO2 even at 400℃. This correlates with the Ostwald phase rule, which says that the phase with the lowest thermodynamic stability is the first to crystallize. IR spectra show that the absorption bands associated with the vibrat ions of water and OH groups disappear as temperature elevates. Table 3 makes it clear that dehydration is accompanied by polymerization and the format ion of infinite -Zr-O-Zr- chains whose vibration frequency is about 610-620 cm-1. About ~400℃, the film is restructured to form a regular crystal structure. This is made clear by the disappearance of the bands associated with the vibrations of chains and the appearance, instead, of the vibrations of ZrO4+ tetrahedra at 1415 cm-1. Figure 4. Data of X-ray powder diffraction a)ZrO2 powder b) ZrO2 thin 1 – on glass; 2 – on quartz Table 3. Assignment of IR bands for films and powders annealed at various temperatures Vibrat ion s 25 Stretching vibrations of O-H 3350 Bending vibrations of O-H in H2O Bending vibrations of Zr-O-H O | OZr—O | O Zr-O in the ZrO2 lattice Vibrations in Zr-O-Zr 1625, 1635 - 1540, 1575 - - 610, 620 - *Data for the dry residue of the FFS are in italics 50 3340 1625,1635 - 1505,1540 - Annealing temperature, ℃ 100 175 3330 3350* 3340 3370 1620, 1635 1635 1635 1635 1535, 1560 1505, 1540 1575 1560 300 3330 3340 No 1610 1540 1540 - - - 870 - Not appear Not appear Not appear - - 584 584 - - - - 610 610, 620 620 615 - 625 620 630 450 No 3370 No 1625 No 1540 870 860 584 470 630 630 American Journal of M aterials Science 2012, 2(4): 119-124 123 Table 4. Kinetic parameters of stages in the formation of zirconia films and powders P aramet er Temperature range, К Relative process rat e, g/min Activation energy, kJ/mol Order of reaction Temperature range, К Relative process rat e, g/min Activation energy, kJ/mol Order of reaction Temperature range, К Relative process rat e, g/min Activation energy, kJ/mol Order of reaction Temperature range, К Relative process rat e, g/min Activation energy, kJ/mol Order of reaction Temperature range, К Relative process rat e, g/min Activation energy, kJ/mol Order of reaction Film 25-125 3.1 36 0.7 125-230 2.9 54 1.1 230-300 2.6 89 0.7 300-355 3.9 130 1.7 355-375 2.4 119 0.9 Powder 110-200 7.8 66 2.2 250-350 6.3 157 0.6 450-720 7.1 200 1.8 formation temperature. Table 5. Physicochemical properties of ZrO2 films Film property Glass Film thickness, nm Refract ive index Dielectric constant Bandgap width, eV Adhesion st ren gth , MP a 100-12 0 1.85 - 0.68 Su bst rat e Quartz Silicon Polycor 100-120 40-100 40-100 1.93 2.08 2.06 - 4.0 - - 5.1 - 0.84 0.93 0.98 Sapphire 40-100 2.08 - 0.94 At high annealing temperatures, polycrystalline films are formed on such substrates, as verified by their higher electrical conductivities and refractive indices. The dehydration character is also revealed by the activation energies and orders of reactions for these stages calculated after Met zer and Horowitz (Tab le 4). The activation energy of the initial stage does not exceed 70 kJ/ mol; such values imp ly that the products are desorbed fro m the surface. The o rder of react ion at this stage for films is less than unit; therefore, diffusion processes are dominant. The activation energies of subsequent stages (100-200 kJ/ mo l) are indicat ive of the occurrence of chemical processes; the order of reaction is about unity or higher. The examination of the data in Tab le 4 makes it clear that processes in a thin layer occur at lower temperatures. The activation energies of stages in thin films are lower than in disperse powers. The rate-controlling stage of oxide fo rmation in thin layers is also different, first, because of an insignificant diffusion retardation in thin layers and, secondly, because of a specific condition of the thin surface film and the influence of the surface energy of the substrate on kinetic parameters. Films in our experiments have good adhesion to glass, quartz, silicon, polycor, and sapphire substrates. These films are insulators with a high bandgap width. Table 5 compiles selected physicochemical properties of the films. The transmittance spectra of dio xide zirconiu m films on quartz is defined (Fig . 5). The absorption band of ZrO2 films is within 220 n m, which allows use it as UV-filters. One important feature of thin films is the dependence of their fundamental propert ies not only on the composition but also on the physicochemical parameters of the substrate materia l. The films on glass or a morphous quartz have the lowest refractive indices because they are amorphous on these substrates. On polycrystal (polycor) and single crystal (quartz) substrates, films in part contain a crystalline phase and an amorphous phase, depending on their Figure 5. Transmittance spectrum T (%) depending on wave length λ, nm 1-Quartz; 2- ZrO2 thin film on quartz 4. Conclusions Zirconia thin films with thicknesses of 40-120 n m on g lass, single-crystal silicon, quart z, polycor, and sapphire substrates have been prepared from zirconiu m o xoch loride and ethanol FFSs. The physicochemical processes involved in film format ion and the phase composition and properties of the films have been studied. The film-fo rming power of the FFS is controlled by the formation o f zirconiu m hydro xo complexes in the solution; the optimal v iscosities of FFSs for the deposition of quality films are within 2.3-3.5 × 10-3 Pa s. Thermally induced zirconia formation occurs in several stages involving removal of solvolysis products and ZrO2 crystallization. The films prepared on glass or quartz substrates are amorphous; those on silicon, polycor, or sapphire have a crystal structure depending on the annealing tem- 124 L. P. Borilo et al.: Synthesis and Characterization of ZrO2 Thin Films perature and film thickness. The resulting ZrO2 films have refract ive index indicator 1,86 – 2,08, are insulators, with high indicators of bandgap width 5,0 – 5,2 eV, absorption edge is limited by 220 nM, wh ich allows to use it as reallot light covering. ZrO2 films have high refractive indices, are insulators, and have good adhesion to substrates. REFERENCES  V. I. Vereshchagin, V. V. Kozik, L. 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