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Microstructure and phase evolution of SrTiO3 thin films on Si prepared by the use of polymeric precursors
June 1997
Materials Letters 31 (1997) 173-178
ELSEVIER
Microstructure and phase evolution of SrTiO, thin films on Si prepared by the use of polymeric precursors SM. Zanetti a, E. Longo a, J.A. Varela b, E.R. Leite a,* a Departamento de Quimica - UFSCar, P.O. Box 676, 13560-905 Silo Carlos, SP, Brazil b Institute de Quimica - UNESP, P.O. Box 355, 14801-970 Araraquara, SP, Brazil Received 9 September
1996; revised 21 October 1996; accepted 25 October 1996
Abstract The use of polymeric precursors was employed in preparing SrTiO, thin films by dip coating using Si (111) as substrate. Crack free films were obtained after sintering at temperatures ranging from 550 to 1000°C. The microstructure, characterized by SEM, shows the development of dense polycrystalline films with smooth surface and mean grain size of 52 nm, for films sintered at 1000°C. Grazing incident angle XRD characterization of these films shows that the SrTiO, phase crystallizes from an inorganic amorphous matrix. No intermediate crystalline phase was identified. Keywords: SrTiO,;
Thin films; Pecbini technique;
Microstructure;
1. Introduction Strontium titanate (SrTiO,) is a material with perovskite structure that shows useful dielectric properties such as high dielectric constant, small temperature coefficient of capacitance and high volumetric resistivity. These characteristics make this material a good candidate for use as capacitors, dynamic random ac’cess memories (DRAMS) and superconductor devices either in bulk or thin films
[1,21. SrTiO, thin films have been prepared by several techniques such as metallorganic deposition (MOD) [3], sol-gel [4,5], sputtering [6], and laser ablation [7]. In general these methods lead to a SrTiO, phase with no intermediate phases at temperatures in the
* Corresponding
author.
00167-577X/97/$17.00 Copyright PII SO167-577X(96)00267-4
Strontium
carbonate;
Titanium citrate solutions;
Si substrates
range 500 to 700°C. The microstructure evolution during additional heat treatment has not yet been well described. Recently Tahan et al. [5] showed the microstructure evolution of Sr,,Bao,,TiO, prepared by the sol-gel method and deposited on Pt/Ti/SiO,/Si substrate. These authors observed that the film, heat treated at 5OO”C, did not show evident grain structure. However, after heat treatment at temperatures ranging from 600 to 700°C, a fine grain structure was observed. The mean grain size changes from 25 to 50 nm for temperatures increasing from 600 to 700°C. Salvaraj et al. [4] reported the variation of grain size from 50 to 100 nm for SrTiO, thin films deposited on SrTiO, single crystals by the sol-gel method and heat treated at 650°C for 1 h. SrTiO, powder with small particle sizes has been obtained at low calcining temperatures when prepared by use of polymeric precursors [8,9]. However
0 1997 Elsevier Science B.V. All rights reserved.
174
SM. Zanetti et al./h4aterials
this method has not been extensively used to obtain thin films. Liu and Wang [ 101reported the deposition of La,_,Sr,Co,_,Fe,O,_, on dense or porous substrates using polymeric precursors. They obtained 400 nm thick non-porous and uniform crack free films with a single dip. The most important parameter in controlling the deposition process, according to Liu and Wang [lo], is the citric acid/metal ratio. The citric acid/ethylene glycol ratio is not a critical parameter in obtaining dense and crack free films. The objective of this work was to study microstructural and phase evolution during processing of SrTiO, thin films on Si (111) substrates dip coated using polymeric precursors. This method was originally developed by Pechini [l 11 and is based on the metallic cation chelation by a carboxylic acid, such as citric acid, and further polymerization promoted by the addition of ethylene glycol and consequent polyester&cation reaction.
2. Experimental procedure 2.1. Synthesis and deposition Fig. 1 shows the flow chart for preparing the SrTiO, precursor solution. Strontium carbonate ( > 99%) was dissolved in the titanium citrate aqueous solution prepared from titanium isopropoxide. The molar ratio between titanium and strontium was 1.00 ([Ti]/[Sr] = 1.OO)and the citric acid/metal ratio was fixed at 1.28 (molar ratio). In this’ study the citric
Letters 31 (1997) 173-178
acid/ethylene glycol ratio was fixed at 40/60 (mass ratio). The precursor solution had high stability and no precipitation was observed for several months after preparation. Prior to coating, the Si (111) substrate was cleaned by immersion in a sulfochromic solution followed by rinsing several times in deionized water. The viscosity of the polymeric solution was adjusted by water to the solution. The dip coating was conducted by immersion of the clean Si (111) substrate in the polymeric solution followed by controlled withdrawal at a speed of 0.33 cm/min. After deposition the substrates were dried on a hot plate (= 150°C) followed by heat treatment at temperatures ranging from 550 to 1000°C during 1 and 2 h, with heating and cooling rate of l”C/min. 2.2. Characterization The (Sr, Ti) precursor solution, after partial elimination of water, was characterized by simultaneous thermal analysis, TG/DTA (STA 409, Netzsch, Germany), in synthetic air (50 cm3/min) using a constant heating rate of lO”C/min from room temperature up to 1000°C. The phase evolution was followed by XRD (D 5000, Siemens, Germany), using a grazing incident angle of 2” (8) and LiF (100) monochromator. The (111) microstructure evolution was characterized in samples coated with a gold film by using a scanning electron microscope (SEMI (DSM 940A, Zeiss, Germany). The film thickness was evaluated by using SEM. The crystallite size was determined using the (110) SrTiO, diffraction peak and the Scherrer equation, D hk, = hk/P
cos 0,
(1)
where A is the wavelength (Cu KcY,), 8 is the diffraction angle, k is a constant and p is the corrected half-width of the diffraction peak. In this study the diffraction peak profile was fitted using a pseudo-Voigt function to calculate the full width at half maximum (FWHM). The @value was determined considering the following equation: Fig. 1. Flow chart for the preparation for SrTiO,.
of the precursors
solution
P = (B,6, - b2)l?
(2)
175
SM. Zanetti et al. /Materials Letters 31 (1997) 173-I 78
where Babs is the FWHM related to the sample and b is the FWHM of the external standard (quartz (SiO,)).
3. Results and discussion 3. I. Phases formation
The thermal analysis results showed weight loss in several temperature ranges. For temperatures higher than 450°C no additional weight modification is observed. The DTA results show an endothermic reaction starting at 110°C related to water loss, and an exothermic reactEon starting at 250°C related to the pyrolysis of the organic precursor and SrTiO, formation. No reactions were observed above 450°C. Fig. 2 shows the phase evolution characterized by XRD. A diffuse XRD pattern is observed for the film heat treated at 450°C indicating that the precursor is amorphous. For temperatures above 550°C the SrTiO, is formed, as shown by the XRD pattern. Since no weight loss was observed for temperatures above 500°C SrTiO,, likely nucleated from an inorganic amorphous matrix. No intermediate phase was observed as reported for SrTiO, powder synthesized by the Pechini method [8,12]. The intermediate mixed carbonate Sr,Ti,O, . CO, was observed during the synthesis of this titanate. This carbonate phase de-
Fig. 2. XRD patterns obtained temperatures.
with grazing
composes with formation of the titanate phase according to the reaction: Sr,Ti,O, . CO, + 2SrTi0, + CO, t .
(3)
We did not detect this mixed carbonate by XRD and this could be associated with the low crystallinity of this phase [13] or due to no formation of this carbonate during the heat treatment of the film. Considering that there is a large availability of oxygen in the furnace atmosphere and low liberation of CO, from the organic material deposited on the substrate, the formation of a mixed carbonate could be inhibited. FTIR with diffuse reflectance tests are in progress to check this hypothesis. The XRD pattern of Fig. 2 shows that the crystallinity of the film increases with heat treatment for temperatures above 550°C. 3.2. Microstructure evolution Crack free thin films with areas of 1.5 cm X 1.5 cm were obtained after heat treatment at several temperatures. The secondary electron micrograph of Fig. 3a shows a crack free film, with a smooth surface and with no defined grain structure (film heat treated at 550°C for 1 h). Films with cracks were observed only for films thicker than 150 nm. Fig. 3b shows the secondary electron micrograph of a film with cracks. Typical film thickness obtained after a single dip was 110 nm.
incident angle of 2’. for films deposited
on Si (111) substrates
and heat treated at different
176
S.M. Zanetti et al. /Materials
Letters 31 (1997) 173-l 78
ment at 1000°C the film is homogeneous and practically free of pores (Fig. 4d). The grain size showed substantial modification with heat treatment temperature. At 650°C a fine grain structure is observed, with grains on the order of 24 nm. An additional increase in the heat treatment temperature to 1000°C resulted in an increase in the grain size to approximately 53 nm. 3.3. General discussion
Fig. 3. Secondary electron micrographs obtained by SEM: (a) crack free film, heat treated at 550°C for 1 h and (b) film with cracks, heat treated at 550°C for 1 h.
A well-defined grain structure for the films was observed at magnifications above 20 k X , indicating that the film has a fine microstructure. The micrographs in Fig. 4 show the crystallization microstructure evolution as a function of heat treatment temperature. Small grain domains and fissures are observed for films heat treated at 550°C for 1 h (Fig. 4a). Increasing the heat treatment temperature to 650°C results in a better definition of the granular texture (Fig. 4b). This micrograph shows that there is an elimination of fissures and formation of well-defined boundaries among the small grains. Compared with Fig. 4a a rearrangement of crystalline domains with increasing pore size is observed. With further heat treatment to 750°C for 1 h the domain boundaries are practically eliminated (Fig. 4c) and after heat treat-
The thermal analysis results as well as XRD and SEM results show that SrTiO, thin films were obtained after deposition of a polymeric solution of precursors on Si (111) substrate and heat treated at temperatures above 550°C. No intermediate phase was observed before the phase formation and very high crystallinity was observed after heat treatment at 1000°C. SEM observations indicate that, during crystallization, clusters of small crystals are formed, and after heat treatment at 550°C a microstructure formed by aggregates of nanometric grains is observed. Increasing the heat treatment these clusters or domains of small grains are sintered forming a pore and fissure free granular microstructure. This type of microstructure leads to formation of smooth surfaces as can be observed in the micrograph of Fig. 5 (transverse section of film/Si composite). Table 1 shows the estimated mean grain size, from SEM analysis and the crystallite size from XRD analysis, as a function of the heat treatment temperature. An increase in the mean grain size and the crystallite size is observed. From 850°C to lOOO”C, an increase of 21% in the mean grain size and 24% in the crystallite size was observed. These results suggest that grain and crystallite growth are a result of the same phenomenon.
Table 1 Estimated mean grain size and crystallite treatment temperature Temperature 550 650 750 850 1000
(“C)
size as a function of heat
Mean grain size (nm)
Crystallite
_
_
24 38 42 53
20 25 26 34
size (nm)
S.M. Zanetti et al./Materials Letters 31 (1997)
173-178
Fig. 4. Secondary electron micrographs obtained by SEM for films heat treated at different temperatures: h; (c) 750°C for 2 h; (d) 1000°C for 2 h.
117
(a) 550°C for 1 h; (b) 650°C for 1
Considering the results reported in this study, the following microstructural evolution for the SrTiO, thin films, prepared from polymeric precursors, can be proposed: (a) formation of clusters of nanometric crystals from an amorphous matrix; (b) densification of clusters resulting in a microstructure consisting of domains of small grains; (c) densification or grain growth of the small grain domains forming a polycrystalline microstructure with granular texture, low porosity and with a smooth surface.
4. Conclusions Fig. 5. Secondary electron micrographs obtained by SEM for films heat treated at 1000c’C for 2 h. (Transverse section of the film/silicon sample with inclination of 75”.)
The experimental results show that crack free polycrystalline SrTiO, thin films can be formed by dip coating on Si (11 I> substrate using the polymeric
S.M. Zanetti et al./hfaterials
178
precursors method. The XBD results show that no intermediate phase was detected and a single SrTiO, phase is crystallized from an amorphous matrix. After heat treatment at lOOO”C,dense, crack free, and smooth surface films with mean grain size of 53 nm were obtained.
Acknowledgements
The authors acknowledge Professor Seshu B. Desu and Dr. Carlos T.A. Suchicital of the Virginia Polytechnique Institute for discussions and donation of Si substrates. The following Brazilian financing support agencies are deeply acknowledged: FAPESP, CNPq, CAPES and FUNDACAO BANCO DO BRASIL.
References [l] M. Iwabachi 5295.
and T. Kobayashi,
J. Appl. Phys. 75 (1994)
Letters 31 (1997) 173-178
[21 U. Syamaprassad,
P.K. Galgali and B.L. Mohanty, Mater. Lett. 7 (1988) 197. [31 P.C. Joshi and S.B. Krupanidhi, J. Appl. Phys. 73 (19931 7627. [41 D.M. Tahan, A. Safari and L.C. Klein, J. Am. Ceram. Soe. 79 (1996) 1593. S. Komameni and R. Roy, 151 U. Selvaraj, A.V. Prasadarao, Mater. Lett. 23 (19951 123. I61T. Horikawa, N. Mikami, T. Makita, J. Tanimura, M. Kataoka, K. Sato and M. Nunoshita, Japan. J. Appl. Phys. 32 (1993) 4126. t71 S.B. Krupanidhi and GM. Rao, Thin Solid Films 240 (1994) 1593. [81E.R. Leite, C.M.G. Souza, E. Longo and J.A. Varela, Ceramic Intern. 21 (1995) 143. E. Longo and J.A. Varela, [91 E.R. Leite, C.A. Paskocimas, Ceramic Intern. 21 (1995) 153. [lOI M. Liu and D. Wang, J. Mater. Res. 10 (19951 3210. [ill M.P. Pechini, U.S. Patent No. 3.330.697(1967). WI S.G. Cho, P.F. Johnson and R.A. Condrate, J. Mater. Sci. 25 (1990) 4738. [I31 S. Kumar, G.L. Messing and W.B. White, J. Am. Ceram. Sot. 76 (1993) 617.
Materials Letters 31 (1997) 173-178
ELSEVIER
Microstructure and phase evolution of SrTiO, thin films on Si prepared by the use of polymeric precursors SM. Zanetti a, E. Longo a, J.A. Varela b, E.R. Leite a,* a Departamento de Quimica - UFSCar, P.O. Box 676, 13560-905 Silo Carlos, SP, Brazil b Institute de Quimica - UNESP, P.O. Box 355, 14801-970 Araraquara, SP, Brazil Received 9 September
1996; revised 21 October 1996; accepted 25 October 1996
Abstract The use of polymeric precursors was employed in preparing SrTiO, thin films by dip coating using Si (111) as substrate. Crack free films were obtained after sintering at temperatures ranging from 550 to 1000°C. The microstructure, characterized by SEM, shows the development of dense polycrystalline films with smooth surface and mean grain size of 52 nm, for films sintered at 1000°C. Grazing incident angle XRD characterization of these films shows that the SrTiO, phase crystallizes from an inorganic amorphous matrix. No intermediate crystalline phase was identified. Keywords: SrTiO,;
Thin films; Pecbini technique;
Microstructure;
1. Introduction Strontium titanate (SrTiO,) is a material with perovskite structure that shows useful dielectric properties such as high dielectric constant, small temperature coefficient of capacitance and high volumetric resistivity. These characteristics make this material a good candidate for use as capacitors, dynamic random ac’cess memories (DRAMS) and superconductor devices either in bulk or thin films
[1,21. SrTiO, thin films have been prepared by several techniques such as metallorganic deposition (MOD) [3], sol-gel [4,5], sputtering [6], and laser ablation [7]. In general these methods lead to a SrTiO, phase with no intermediate phases at temperatures in the
* Corresponding
author.
00167-577X/97/$17.00 Copyright PII SO167-577X(96)00267-4
Strontium
carbonate;
Titanium citrate solutions;
Si substrates
range 500 to 700°C. The microstructure evolution during additional heat treatment has not yet been well described. Recently Tahan et al. [5] showed the microstructure evolution of Sr,,Bao,,TiO, prepared by the sol-gel method and deposited on Pt/Ti/SiO,/Si substrate. These authors observed that the film, heat treated at 5OO”C, did not show evident grain structure. However, after heat treatment at temperatures ranging from 600 to 700°C, a fine grain structure was observed. The mean grain size changes from 25 to 50 nm for temperatures increasing from 600 to 700°C. Salvaraj et al. [4] reported the variation of grain size from 50 to 100 nm for SrTiO, thin films deposited on SrTiO, single crystals by the sol-gel method and heat treated at 650°C for 1 h. SrTiO, powder with small particle sizes has been obtained at low calcining temperatures when prepared by use of polymeric precursors [8,9]. However
0 1997 Elsevier Science B.V. All rights reserved.
174
SM. Zanetti et al./h4aterials
this method has not been extensively used to obtain thin films. Liu and Wang [ 101reported the deposition of La,_,Sr,Co,_,Fe,O,_, on dense or porous substrates using polymeric precursors. They obtained 400 nm thick non-porous and uniform crack free films with a single dip. The most important parameter in controlling the deposition process, according to Liu and Wang [lo], is the citric acid/metal ratio. The citric acid/ethylene glycol ratio is not a critical parameter in obtaining dense and crack free films. The objective of this work was to study microstructural and phase evolution during processing of SrTiO, thin films on Si (111) substrates dip coated using polymeric precursors. This method was originally developed by Pechini [l 11 and is based on the metallic cation chelation by a carboxylic acid, such as citric acid, and further polymerization promoted by the addition of ethylene glycol and consequent polyester&cation reaction.
2. Experimental procedure 2.1. Synthesis and deposition Fig. 1 shows the flow chart for preparing the SrTiO, precursor solution. Strontium carbonate ( > 99%) was dissolved in the titanium citrate aqueous solution prepared from titanium isopropoxide. The molar ratio between titanium and strontium was 1.00 ([Ti]/[Sr] = 1.OO)and the citric acid/metal ratio was fixed at 1.28 (molar ratio). In this’ study the citric
Letters 31 (1997) 173-178
acid/ethylene glycol ratio was fixed at 40/60 (mass ratio). The precursor solution had high stability and no precipitation was observed for several months after preparation. Prior to coating, the Si (111) substrate was cleaned by immersion in a sulfochromic solution followed by rinsing several times in deionized water. The viscosity of the polymeric solution was adjusted by water to the solution. The dip coating was conducted by immersion of the clean Si (111) substrate in the polymeric solution followed by controlled withdrawal at a speed of 0.33 cm/min. After deposition the substrates were dried on a hot plate (= 150°C) followed by heat treatment at temperatures ranging from 550 to 1000°C during 1 and 2 h, with heating and cooling rate of l”C/min. 2.2. Characterization The (Sr, Ti) precursor solution, after partial elimination of water, was characterized by simultaneous thermal analysis, TG/DTA (STA 409, Netzsch, Germany), in synthetic air (50 cm3/min) using a constant heating rate of lO”C/min from room temperature up to 1000°C. The phase evolution was followed by XRD (D 5000, Siemens, Germany), using a grazing incident angle of 2” (8) and LiF (100) monochromator. The (111) microstructure evolution was characterized in samples coated with a gold film by using a scanning electron microscope (SEMI (DSM 940A, Zeiss, Germany). The film thickness was evaluated by using SEM. The crystallite size was determined using the (110) SrTiO, diffraction peak and the Scherrer equation, D hk, = hk/P
cos 0,
(1)
where A is the wavelength (Cu KcY,), 8 is the diffraction angle, k is a constant and p is the corrected half-width of the diffraction peak. In this study the diffraction peak profile was fitted using a pseudo-Voigt function to calculate the full width at half maximum (FWHM). The @value was determined considering the following equation: Fig. 1. Flow chart for the preparation for SrTiO,.
of the precursors
solution
P = (B,6, - b2)l?
(2)
175
SM. Zanetti et al. /Materials Letters 31 (1997) 173-I 78
where Babs is the FWHM related to the sample and b is the FWHM of the external standard (quartz (SiO,)).
3. Results and discussion 3. I. Phases formation
The thermal analysis results showed weight loss in several temperature ranges. For temperatures higher than 450°C no additional weight modification is observed. The DTA results show an endothermic reaction starting at 110°C related to water loss, and an exothermic reactEon starting at 250°C related to the pyrolysis of the organic precursor and SrTiO, formation. No reactions were observed above 450°C. Fig. 2 shows the phase evolution characterized by XRD. A diffuse XRD pattern is observed for the film heat treated at 450°C indicating that the precursor is amorphous. For temperatures above 550°C the SrTiO, is formed, as shown by the XRD pattern. Since no weight loss was observed for temperatures above 500°C SrTiO,, likely nucleated from an inorganic amorphous matrix. No intermediate phase was observed as reported for SrTiO, powder synthesized by the Pechini method [8,12]. The intermediate mixed carbonate Sr,Ti,O, . CO, was observed during the synthesis of this titanate. This carbonate phase de-
Fig. 2. XRD patterns obtained temperatures.
with grazing
composes with formation of the titanate phase according to the reaction: Sr,Ti,O, . CO, + 2SrTi0, + CO, t .
(3)
We did not detect this mixed carbonate by XRD and this could be associated with the low crystallinity of this phase [13] or due to no formation of this carbonate during the heat treatment of the film. Considering that there is a large availability of oxygen in the furnace atmosphere and low liberation of CO, from the organic material deposited on the substrate, the formation of a mixed carbonate could be inhibited. FTIR with diffuse reflectance tests are in progress to check this hypothesis. The XRD pattern of Fig. 2 shows that the crystallinity of the film increases with heat treatment for temperatures above 550°C. 3.2. Microstructure evolution Crack free thin films with areas of 1.5 cm X 1.5 cm were obtained after heat treatment at several temperatures. The secondary electron micrograph of Fig. 3a shows a crack free film, with a smooth surface and with no defined grain structure (film heat treated at 550°C for 1 h). Films with cracks were observed only for films thicker than 150 nm. Fig. 3b shows the secondary electron micrograph of a film with cracks. Typical film thickness obtained after a single dip was 110 nm.
incident angle of 2’. for films deposited
on Si (111) substrates
and heat treated at different
176
S.M. Zanetti et al. /Materials
Letters 31 (1997) 173-l 78
ment at 1000°C the film is homogeneous and practically free of pores (Fig. 4d). The grain size showed substantial modification with heat treatment temperature. At 650°C a fine grain structure is observed, with grains on the order of 24 nm. An additional increase in the heat treatment temperature to 1000°C resulted in an increase in the grain size to approximately 53 nm. 3.3. General discussion
Fig. 3. Secondary electron micrographs obtained by SEM: (a) crack free film, heat treated at 550°C for 1 h and (b) film with cracks, heat treated at 550°C for 1 h.
A well-defined grain structure for the films was observed at magnifications above 20 k X , indicating that the film has a fine microstructure. The micrographs in Fig. 4 show the crystallization microstructure evolution as a function of heat treatment temperature. Small grain domains and fissures are observed for films heat treated at 550°C for 1 h (Fig. 4a). Increasing the heat treatment temperature to 650°C results in a better definition of the granular texture (Fig. 4b). This micrograph shows that there is an elimination of fissures and formation of well-defined boundaries among the small grains. Compared with Fig. 4a a rearrangement of crystalline domains with increasing pore size is observed. With further heat treatment to 750°C for 1 h the domain boundaries are practically eliminated (Fig. 4c) and after heat treat-
The thermal analysis results as well as XRD and SEM results show that SrTiO, thin films were obtained after deposition of a polymeric solution of precursors on Si (111) substrate and heat treated at temperatures above 550°C. No intermediate phase was observed before the phase formation and very high crystallinity was observed after heat treatment at 1000°C. SEM observations indicate that, during crystallization, clusters of small crystals are formed, and after heat treatment at 550°C a microstructure formed by aggregates of nanometric grains is observed. Increasing the heat treatment these clusters or domains of small grains are sintered forming a pore and fissure free granular microstructure. This type of microstructure leads to formation of smooth surfaces as can be observed in the micrograph of Fig. 5 (transverse section of film/Si composite). Table 1 shows the estimated mean grain size, from SEM analysis and the crystallite size from XRD analysis, as a function of the heat treatment temperature. An increase in the mean grain size and the crystallite size is observed. From 850°C to lOOO”C, an increase of 21% in the mean grain size and 24% in the crystallite size was observed. These results suggest that grain and crystallite growth are a result of the same phenomenon.
Table 1 Estimated mean grain size and crystallite treatment temperature Temperature 550 650 750 850 1000
(“C)
size as a function of heat
Mean grain size (nm)
Crystallite
_
_
24 38 42 53
20 25 26 34
size (nm)
S.M. Zanetti et al./Materials Letters 31 (1997)
173-178
Fig. 4. Secondary electron micrographs obtained by SEM for films heat treated at different temperatures: h; (c) 750°C for 2 h; (d) 1000°C for 2 h.
117
(a) 550°C for 1 h; (b) 650°C for 1
Considering the results reported in this study, the following microstructural evolution for the SrTiO, thin films, prepared from polymeric precursors, can be proposed: (a) formation of clusters of nanometric crystals from an amorphous matrix; (b) densification of clusters resulting in a microstructure consisting of domains of small grains; (c) densification or grain growth of the small grain domains forming a polycrystalline microstructure with granular texture, low porosity and with a smooth surface.
4. Conclusions Fig. 5. Secondary electron micrographs obtained by SEM for films heat treated at 1000c’C for 2 h. (Transverse section of the film/silicon sample with inclination of 75”.)
The experimental results show that crack free polycrystalline SrTiO, thin films can be formed by dip coating on Si (11 I> substrate using the polymeric
S.M. Zanetti et al./hfaterials
178
precursors method. The XBD results show that no intermediate phase was detected and a single SrTiO, phase is crystallized from an amorphous matrix. After heat treatment at lOOO”C,dense, crack free, and smooth surface films with mean grain size of 53 nm were obtained.
Acknowledgements
The authors acknowledge Professor Seshu B. Desu and Dr. Carlos T.A. Suchicital of the Virginia Polytechnique Institute for discussions and donation of Si substrates. The following Brazilian financing support agencies are deeply acknowledged: FAPESP, CNPq, CAPES and FUNDACAO BANCO DO BRASIL.
References [l] M. Iwabachi 5295.
and T. Kobayashi,
J. Appl. Phys. 75 (1994)
Letters 31 (1997) 173-178
[21 U. Syamaprassad,
P.K. Galgali and B.L. Mohanty, Mater. Lett. 7 (1988) 197. [31 P.C. Joshi and S.B. Krupanidhi, J. Appl. Phys. 73 (19931 7627. [41 D.M. Tahan, A. Safari and L.C. Klein, J. Am. Ceram. Soe. 79 (1996) 1593. S. Komameni and R. Roy, 151 U. Selvaraj, A.V. Prasadarao, Mater. Lett. 23 (19951 123. I61T. Horikawa, N. Mikami, T. Makita, J. Tanimura, M. Kataoka, K. Sato and M. Nunoshita, Japan. J. Appl. Phys. 32 (1993) 4126. t71 S.B. Krupanidhi and GM. Rao, Thin Solid Films 240 (1994) 1593. [81E.R. Leite, C.M.G. Souza, E. Longo and J.A. Varela, Ceramic Intern. 21 (1995) 143. E. Longo and J.A. Varela, [91 E.R. Leite, C.A. Paskocimas, Ceramic Intern. 21 (1995) 153. [lOI M. Liu and D. Wang, J. Mater. Res. 10 (19951 3210. [ill M.P. Pechini, U.S. Patent No. 3.330.697(1967). WI S.G. Cho, P.F. Johnson and R.A. Condrate, J. Mater. Sci. 25 (1990) 4738. [I31 S. Kumar, G.L. Messing and W.B. White, J. Am. Ceram. Sot. 76 (1993) 617.