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Dependence of annealing time on structural and morphological properties of Ca(Zr0.05Ti0.95)O3 thin films
Journal of Alloys and Compounds 453 (2008) 386–391
Dependence of annealing time on structural and morphological properties of Ca(Zr0.05Ti0.95)O3 thin films L.S. Cavalcante a,∗ , A.Z. Sim˜oes b , M.O. Orlandi a , M.R.M.C. Santos c , J.A. Varela b , E. Longo b ´ ´ Laborat´orio Interdisciplinar de Eletroquimica e Cerˆamica, Departamento de Quimica, Universidade Federal de Sao Carlos, P.O. Box 676, 13565-905 S˜ao Carlos, SP, Brazil Laborat´orio Interdisciplinar em Cerˆamica, Departamento de F´ısico-Qu´ımica, Instituto de Qu´ımica, Universidade Estadual Paulista, P.O. Box 355, 14801-907 Araraquara, SP, Brazil c Centro de Ciˆ ´ encias da Natureza, Departamento de Quimica, Universidade Federal do Piaui,´ 64049-550 Teresina, PI, Brazil a
b
Received 30 August 2006; received in revised form 16 October 2006; accepted 17 November 2006 Available online 29 December 2006
Abstract Ca(Zr0.05 Ti0.95 )O3 (CZT) thin films were prepared by the polymeric precursor method by spin-coating process. The films were deposited on Pt(1 1 1)/Ti/SiO2 /Si(1 0 0) substrates and annealed at 650 ◦ C for 2, 4, and 6 h in oxygen atmosphere. Structure and morphology of the CZT thin films were characterized by the X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), atomic force microscopy (AFM) and fieldemission scanning electron microscopy (FEG-SEM). XRD revealed that the film is free of secondary phases and crystallizes in the orthorhombic structure. The annealing time influences the grain size, lattices parameter and in the film thickness. © 2006 Elsevier B.V. All rights reserved. Keywords: Atomic force microscopy; Crystallization; Diffusion; Growth mechanism
1. Introduction The surface properties of solids in the form of bulk or thin films such as the atomic structure at the surface or interface, atomic defect structure, electronic surface structure and surface phonon play important roles in catalysis [1], ferroelectrics [2], sensors [3], photoluminescence [4] and fuel cells [5]. The influence of annealing time in the morphologic characteristics can be clearly noticed by atomic force microscopy (AFM). Calcium titanate (CaTiO3 ) is widely used in electronic ceramic materials to immobilize nuclear waste [6]. There has been considerable interest in the phase transitions of this typical perovskite oxide either by theoretical or experimental studies [7,8]. CaTiO3 orthorhombic with space group Pbnm below 1107 ◦ C belongs to another “orthorhombic” structure space group Cmcm between 1107 and 1227 ◦ C. At 1227 ◦ C, it transforms into “tetragonal” structure with space group I4/mcm. Above
∗
Corresponding author. Tel.: +55 16 3361 5215; fax: +55 16 3351 8350. E-mail address: [email protected](L.S. Cavalcante).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.11.129
¯ 1307 ◦ C, it becomes “cubic” structure with space group Pm3m [9]. However, single crystal X-ray diffraction measurements indicate that CaZrO3 crystallizes in an orthorhombic perovskite structure with space group Pbnm without exhibiting any phase transitions at 8.7 GPa [10]. Recently, perovskite calcium zirconate titanate Ca(Zrx Ti1−x )O3 was prepared via an acetic acidmodified sol–gel process and solid-state reaction. However, the stoichiometric phase was obtained only upon heating at 1200 ◦ C [11,12]. In this work, Ca(Zr0.05 Ti0.95 )O3 (CZT) thin films were obtained by the polymeric precursor method (PPM). The dependence of annealing time on the structural and microstructural characteristics of these films was discussed. 2. Experimental procedure Ca(Zr0.05 Ti0.95 )O3 (CZT) thin films were synthesized by the polymeric precursor method (PPM). Calcium carbonate (99.9% purity, Aldrich), titanium(IV) isopropoxide (97% purity, Aldrich), zirconium n-propoxide (99.9% purity, NOAH-Technologies), ethylene glycol (99% purity, J.T. Baker) and anhydrous citric acid (99.5% purity, J.T.
L.S. Cavalcante et al. / Journal of Alloys and Compounds 453 (2008) 386–391
Baker) were used as starting materials. Titanium and zirconium citrates were formed by the dissolution of titanium(IV) isopropoxide and zirconium(IV) n-propoxide in aqueous solutions of citric acid, respectively, under constant stirring. These solutions were homogenized and mixed in a 5:95 molar proportion of zirconium and titanium, respectively. The citrate solution was stirred at 80 ◦ C to obtain a clear and homogenous solution. Calcium carbonate was dissolved in water and then added to a stoichiometric amount of titanium–zirconium citrate solution. After homogenization of the calcium solution, ethylene glycol was added to promote the citrate polymerization [13]. The citric acid/ethylene glycol ratio was fixed at 60:40. The resin was annealed at 90 ◦ C under stirring to eliminate water until a appropriate viscosity of 13 mPa s. The CZT thin films were spin coated on Pt/Ti/SiO2 /Si substrates by a commercial spinner operating at 6500 rpm (spin-coater KW-4B, Chemat Technology). After deposition, the CZT films were kept at 150 ◦ C on a hot plate for 10 min to remove residual solvents. The heat treatment was carried out in two stages: initial heating at 350 ◦ C for 4 h at a heating rate of 1 ◦ C/min to promote the pyrolysis of the organic materials, and finally the films were annealed at 650 ◦ C for 2, 4, and 6 h at a heating rate of 1 ◦ C/min. The three CZT layers were obtained crystallizing each layer in a tube furnace under oxygen atmosphere until the desired thickness was reached. The crystalline phase of the films was analyzed by X-ray diffraction (XRD) patterns recorded on a (Rigaku-DMax 2000PC, Japan) with Cu K␣ radiation in the 2θ range from 20◦ to 60◦ with 0.3◦ /min. Infrared analysis was performed on (Bruker-Equinox 55, Germany) Fourier transformed infrared spectrometer (FT-IR), using a 30◦ specular reflectance accessory. The FT-IR reflectance spectra of the thin films were recorded at room temperature in the 400–1200 cm−1 range. The lattice vibration of the films deposited at the 650 ◦ C for different times was analyzed. The films microstructure were observed using atomic force atomic microscopy (AFM) (Digital Instruments, Nanoscope IIIa, U.S.A.) and the thickness was evaluated by observing the cross-section of the films using a high resolution field-emission gun scanning electron microscopy FEG-SEM (Supra 35-VP, Carl Zeiss, Germany).
3. Results and discussion Fig. 1 shows the XRD patterns of CZT thin films annealed at 650◦ C for 2, 4, and 6 h in oxygen atmosphere. As can be
Fig. 1. XRD patterns of CZT thin films annealed at 650 ◦ C for (a) 2 h, (b) 4 h, and (c) 6 h. The zoom shows the 32.25–34.25 2θ range of (2 0 0) and (1 2 1) peaks.
387
Fig. 2. Dependence of lattice parameters a, b, and c with annealing time for CZT films annealed at 650 ◦ C.
Table 1 Data obtained for CZT thin films Time (h)
˚ Average lattice parameter, a (A) ˚ Average lattice parameter, b (A) ˚ Average lattice parameter, c (A) Average grain size (nm) Average roughness (nm) Average thickness (nm)
2
4
6
5.437 7.636 5.374 36 6.7 550
5.446 7.651 5.346 47 4.3 450
5.441 7.643 5.379 50 3.4 425
seen, the increase of annealing time reduces the films thickness leading to sharper peaks of substrate. All peaks in Fig. 1(a–c) are ascribed to the orthorhombic perovskite structure. It is noted that the (2 0 0) and (1 2 1) peaks are overlapped as shown in the inset of Fig. 1(c). The characteristic peak of Pt substrate was observed at 2θ = 40◦ . The dependence of lattice parameters with annealing time is shown in Fig. 2. The formation of Ca(Zr0.05 Ti0.95 )O3 phase is confirmed by comparing XRD spectra with the respective JCPDS card (22–0153) of CaTiO3 with unit cell are
Fig. 3. FT-IR spectra of CZT thin films annealed at 650 ◦ C in oxygen atmosphere for (a) 2 h, (b) 4 h, and (c) 6 h.
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Fig. 4. Schematic summary of annealing time in the morphology of the CZT films.
Fig. 5. FEG-SEM micrographies of the cross-section for the CZT thin films. (a) Annealed at 650 ◦ C for 2 h. (b) Annealed at 650 ◦ C for 4 h. (c) Annealed at 650 ◦ C for 6 h.
L.S. Cavalcante et al. / Journal of Alloys and Compounds 453 (2008) 386–391
389
Fig. 6. AFM images of the surface and cross-sectional analysis of CZT thin films. (a) Annealed at 650 ◦ C for 2 h. (b) Annealed at 650 ◦ C for 4 h. (c) Annealed at 650 ◦ C for 6 h.
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L.S. Cavalcante et al. / Journal of Alloys and Compounds 453 (2008) 386–391
˚ b = 7.644 A, ˚ and c = 5.381 A. ˚ Since no JCPDS a = 5.440 A, card is available for this composition, an orthorhombic perovskite structure with space group Pnma was indexed. The lattice parameters were calculated using the least square refinement from the REDE93 program. Small deviations of lattice parameters were identified by the vertical bars (mean error standard). This occurs due to the substitution of Ti4+ (0.068 nm) by 5 mol% Zr4+ (0.087 nm) in the CaTiO3 perovskite [14], since lattice parameters had been calculated on basis of JCPDS card (220153). The film annealed at 650 ◦ C for 6 h is more crystalline and thus showed less variations in the a, b, and c lattice parameters. Table 1 presents the lattice parameter, grain size, roughness and thickness of the CZT films annealed at 650 ◦ C in oxygen atmosphere for different times. The increase of annealing time leads to a more crystalline film affecting slightly the lattice parameters a, b, and c (Table 1). It also results in the grain growth due to the coalescence of small grains and in the decrease of roughness and thickness due to the diffusion of external layers that suffer a strong influence during the crystallization of the CZT films. The FT-IR absorption spectra of the CZT thin films annealed at 650 ◦ C for different times are shown in Fig. 3. In the data a broad absorption peak of the BO6 stretching mode was observed at 600–840 cm−1 suggesting the formation of perovskite phase [15,16]. The increase of annealing time leads to a more crystalline CZT film causing a displacement of absorption peaks at 786, 790, and 792 cm−1 . This occurs due to the resonance with the longitudinal optic (LO) phonon modes which become sharper and narrower and shift toward higher wavenumbers. A schematic illustration of the diffusion mechanism of the layers and growth of CZT grains is shown in Fig. 4. Fig. 4 shows the schematic preparation of CZT films annealed at 650 ◦ C for 2, 4, and 6 h in constant oxygen flow atmosphere. The increase of annealing time leads to: (a) reduction in film thickness due to the transformation of amorphous phase into crystalline and (b) grain growth due to the coalescence process. The films annealed for 2 h present a homogeneous grain size distribution. At 4 h the CZT grains increase by coalescence of small grains leading to bigger grains. At 6 h, the films present a bimodal distribution due to the coalescence of one part of the grains. A decrease of CZT film thickness is observed with the increase of annealing time. This occurs during the sintering process as observed from FEG-SEM micrographies. Also, the CZT films deposited with three layers and annealed at 2 h present a clear interface (Fig. 5(a)). With the increase of annealing time for 4 h the second and third layers start to diffuse (Fig. 5(b)). The complete diffusion of these layers is evident for the film annealed for 6 h (Fig. 5(c)). Fig. 6 shows the AFM images of CZT film annealed at 650 ◦ C for 2, 4, and 6 h in oxygen atmosphere. The average grain size and surface roughness of CZT thin films were estimated using atomic force microscope. The surface morphology was obtained using an area of 500 nm × 500 nm.
Fig. 7. Dependence of annealing time as function of grain size and film thickness.
Fig. 6(a) shows a uniform and homogeneous CZT grain distribution. The rosette structure, which is typical for PZT thin films [17,18], was not observed in our analysis. The increase of annealing time leads to bigger grains evidencing that the oxygen atmosphere promotes the grains coalescence (Fig. 6(b)). After annealing at 650 ◦ C for 6 h the presence of larger and small grains were noted, indicating a bimodal distribution and the beginning of the coalescence process (Fig. 6(c)). However, annealing for longer times, more than 6 h, leads to the coalescence process followed by a uniform distribution of the grains. A similar process was reported for Pb(Zr,Ti)O3 and PbTiO3 thin films [19,20]. Fig. 7 shows the dependence of average grain size and thickness with the annealing time. A significant variation in the average grain size was evident with the increase of the annealing time. The largest variation was noted in the film annealed for 6 h due to the bimodal distribution as a result of the coalescence process. A strong decrease of CZT film thickness is noted with the increase of annealing time from 2 to 4 h. After that, a slight reduction of thickness from 450 to 425 nm was observed indicating a reduction of oxygen vacancies and film strain. 4. Conclusions Polycrystalline CZT thin films free of secondary phases and with orthorhombic structure were obtained from the polymeric precursor method. Less variations in the lattice parameters were observed in the more crystalline film. The increase of annealing time results in a stronger metal–oxygen octahedral vibrational mode. AFM data indicated that the CZT thin films present a homogeneous, smooth and crack free surface. The grain growth of CZT thin films is proportional to the increase of annealing time while the film thickness is reduced due to the minimization of oxygen vacancies and film strain. Acknowledgements The authors thank the financial support of the Brazilian research financing institutions: CAPES, FAPESP and CNPq. To the Prof. Lu´is Presley Santos-CEFET-MA and Prof. Edward Ralph Dockal-DQ-UFSCar-SP by the relevant discussions.
L.S. Cavalcante et al. / Journal of Alloys and Compounds 453 (2008) 386–391
References [1] N.L.V. Carreno, A.P. Maciel, E.R. Leite, P.N. Lisboa-Filho, E. Longo, A. Valentini, L.F.D. Probst, C.O. Paiva-Santos, W.H. Schreiner, Sens. Actuators B 86 (2002) 185. [2] I.A. Souza, A.Z. Sim˜oes, S. Cava, L.S. Cavalcante, M. Cilense, E. Longo, J.A. Varela, J. Solid State Chem. 179 (2006) 2972. [3] L. Francioso, A. Forleo, S. Capone, M. Epifani, A.M. Taurino, P. Siciliano, Sens. Actuators B 114 (2006) 646. [4] M. Anicete-Santos, L.S. Cavalcante, E. Orhan, E.C. Paris, L.G.P. Sim˜oes, M.R. Joya, I.L.V. Rosa, P.R. de Lucena, M.R.M.C. Santos, L.S. SantosJ´unior, P.S. Pizani, E.R. Leite, J.A. Varela, E. Longo, Chem. Phys. 316 (2005) 260. [5] K. Deshpande, A. Mukasyan, A. Varma, J. Power Sources 158 (2006) 60. [6] A.E. Ringwood, S.E. Kesson, K.D. Reeve, D.M. Levins, E.J. Ramm, W. Lutze, R.C. Ewing (Eds.), Radioactive Waste Forms for the Future, Elsevier, Amsterdam, 1998, p. 233. [7] X. Wu, Y. Dong, S. Qin, M. Abbas, Z. Wu, Solid State Commun. 136 (2005) 416. [8] T. Matsui, H. Shigematsu, Y. Arita, Y. Hanajiri, N. Nakamitsu, T. Nagasaki, J. Nucl. Mater. 247 (1997) 72.
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[9] B.J. Kenndey, C.J. Howard, B.C. Chakoumakos, J. Phys.: Condens. Matter 11 (1999) 1479. [10] N.L. Ross, T.D. Chaplin, J. Solid State Chem. 172 (2003) 123. [11] J. Xu, A.P. Wilkinson, S. Pattanaik, Chem. Mater. 12 (2000) 3321. [12] I. Levin, E. Cockayne, M.W. Lufaso, J.C. Woicik, J.E. Maslar, Chem. Mater. 18 (2006) 854. [13] T. Salmi, E. Paatero, P. Nyholm, Chem. Eng. Process. 43 (2004) 1487. [14] T.B. Wu, C.M. Wu, M.L. Chen, Appl. Phys. Lett. 69 (1996) 2659. [15] E.R. Leite, C.M.G. Campos, E. Longo, J.A. Varela, Ceram. Int. 21 (1995) 143. [16] J.T. Last, Phys. Rev. 105 (1997) 1740. [17] T. Atsuki, N. Soyama, G. Sasaki, T. Yonezawa, K. Ogi, K. Sameshima, K. Hoshiba, Y. Nakao, A. Kamisawa, Jpn. J. Appl. Phys. 33 (1994) 5196. [18] M. Klee, R. Eusemann, R. Waser, W. Brand, H.V. Hal, J. Appl. Phys. 72 (1992) 1566. [19] N. Wakiya, K. Kuroyanagi, Y. Xuan, K. Shinozaki, N. Mizutani, Thin Solid Films 357 (1999) 166. [20] V. Stankus, J. Dudonis, L. Pranevicius, L.L. Pranevicius, D. Milcius, C. Templier, J.-P. Riviere, Thin Solid Films 426 (2003) 78.
Dependence of annealing time on structural and morphological properties of Ca(Zr0.05Ti0.95)O3 thin films L.S. Cavalcante a,∗ , A.Z. Sim˜oes b , M.O. Orlandi a , M.R.M.C. Santos c , J.A. Varela b , E. Longo b ´ ´ Laborat´orio Interdisciplinar de Eletroquimica e Cerˆamica, Departamento de Quimica, Universidade Federal de Sao Carlos, P.O. Box 676, 13565-905 S˜ao Carlos, SP, Brazil Laborat´orio Interdisciplinar em Cerˆamica, Departamento de F´ısico-Qu´ımica, Instituto de Qu´ımica, Universidade Estadual Paulista, P.O. Box 355, 14801-907 Araraquara, SP, Brazil c Centro de Ciˆ ´ encias da Natureza, Departamento de Quimica, Universidade Federal do Piaui,´ 64049-550 Teresina, PI, Brazil a
b
Received 30 August 2006; received in revised form 16 October 2006; accepted 17 November 2006 Available online 29 December 2006
Abstract Ca(Zr0.05 Ti0.95 )O3 (CZT) thin films were prepared by the polymeric precursor method by spin-coating process. The films were deposited on Pt(1 1 1)/Ti/SiO2 /Si(1 0 0) substrates and annealed at 650 ◦ C for 2, 4, and 6 h in oxygen atmosphere. Structure and morphology of the CZT thin films were characterized by the X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), atomic force microscopy (AFM) and fieldemission scanning electron microscopy (FEG-SEM). XRD revealed that the film is free of secondary phases and crystallizes in the orthorhombic structure. The annealing time influences the grain size, lattices parameter and in the film thickness. © 2006 Elsevier B.V. All rights reserved. Keywords: Atomic force microscopy; Crystallization; Diffusion; Growth mechanism
1. Introduction The surface properties of solids in the form of bulk or thin films such as the atomic structure at the surface or interface, atomic defect structure, electronic surface structure and surface phonon play important roles in catalysis [1], ferroelectrics [2], sensors [3], photoluminescence [4] and fuel cells [5]. The influence of annealing time in the morphologic characteristics can be clearly noticed by atomic force microscopy (AFM). Calcium titanate (CaTiO3 ) is widely used in electronic ceramic materials to immobilize nuclear waste [6]. There has been considerable interest in the phase transitions of this typical perovskite oxide either by theoretical or experimental studies [7,8]. CaTiO3 orthorhombic with space group Pbnm below 1107 ◦ C belongs to another “orthorhombic” structure space group Cmcm between 1107 and 1227 ◦ C. At 1227 ◦ C, it transforms into “tetragonal” structure with space group I4/mcm. Above
∗
Corresponding author. Tel.: +55 16 3361 5215; fax: +55 16 3351 8350. E-mail address: [email protected](L.S. Cavalcante).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.11.129
¯ 1307 ◦ C, it becomes “cubic” structure with space group Pm3m [9]. However, single crystal X-ray diffraction measurements indicate that CaZrO3 crystallizes in an orthorhombic perovskite structure with space group Pbnm without exhibiting any phase transitions at 8.7 GPa [10]. Recently, perovskite calcium zirconate titanate Ca(Zrx Ti1−x )O3 was prepared via an acetic acidmodified sol–gel process and solid-state reaction. However, the stoichiometric phase was obtained only upon heating at 1200 ◦ C [11,12]. In this work, Ca(Zr0.05 Ti0.95 )O3 (CZT) thin films were obtained by the polymeric precursor method (PPM). The dependence of annealing time on the structural and microstructural characteristics of these films was discussed. 2. Experimental procedure Ca(Zr0.05 Ti0.95 )O3 (CZT) thin films were synthesized by the polymeric precursor method (PPM). Calcium carbonate (99.9% purity, Aldrich), titanium(IV) isopropoxide (97% purity, Aldrich), zirconium n-propoxide (99.9% purity, NOAH-Technologies), ethylene glycol (99% purity, J.T. Baker) and anhydrous citric acid (99.5% purity, J.T.
L.S. Cavalcante et al. / Journal of Alloys and Compounds 453 (2008) 386–391
Baker) were used as starting materials. Titanium and zirconium citrates were formed by the dissolution of titanium(IV) isopropoxide and zirconium(IV) n-propoxide in aqueous solutions of citric acid, respectively, under constant stirring. These solutions were homogenized and mixed in a 5:95 molar proportion of zirconium and titanium, respectively. The citrate solution was stirred at 80 ◦ C to obtain a clear and homogenous solution. Calcium carbonate was dissolved in water and then added to a stoichiometric amount of titanium–zirconium citrate solution. After homogenization of the calcium solution, ethylene glycol was added to promote the citrate polymerization [13]. The citric acid/ethylene glycol ratio was fixed at 60:40. The resin was annealed at 90 ◦ C under stirring to eliminate water until a appropriate viscosity of 13 mPa s. The CZT thin films were spin coated on Pt/Ti/SiO2 /Si substrates by a commercial spinner operating at 6500 rpm (spin-coater KW-4B, Chemat Technology). After deposition, the CZT films were kept at 150 ◦ C on a hot plate for 10 min to remove residual solvents. The heat treatment was carried out in two stages: initial heating at 350 ◦ C for 4 h at a heating rate of 1 ◦ C/min to promote the pyrolysis of the organic materials, and finally the films were annealed at 650 ◦ C for 2, 4, and 6 h at a heating rate of 1 ◦ C/min. The three CZT layers were obtained crystallizing each layer in a tube furnace under oxygen atmosphere until the desired thickness was reached. The crystalline phase of the films was analyzed by X-ray diffraction (XRD) patterns recorded on a (Rigaku-DMax 2000PC, Japan) with Cu K␣ radiation in the 2θ range from 20◦ to 60◦ with 0.3◦ /min. Infrared analysis was performed on (Bruker-Equinox 55, Germany) Fourier transformed infrared spectrometer (FT-IR), using a 30◦ specular reflectance accessory. The FT-IR reflectance spectra of the thin films were recorded at room temperature in the 400–1200 cm−1 range. The lattice vibration of the films deposited at the 650 ◦ C for different times was analyzed. The films microstructure were observed using atomic force atomic microscopy (AFM) (Digital Instruments, Nanoscope IIIa, U.S.A.) and the thickness was evaluated by observing the cross-section of the films using a high resolution field-emission gun scanning electron microscopy FEG-SEM (Supra 35-VP, Carl Zeiss, Germany).
3. Results and discussion Fig. 1 shows the XRD patterns of CZT thin films annealed at 650◦ C for 2, 4, and 6 h in oxygen atmosphere. As can be
Fig. 1. XRD patterns of CZT thin films annealed at 650 ◦ C for (a) 2 h, (b) 4 h, and (c) 6 h. The zoom shows the 32.25–34.25 2θ range of (2 0 0) and (1 2 1) peaks.
387
Fig. 2. Dependence of lattice parameters a, b, and c with annealing time for CZT films annealed at 650 ◦ C.
Table 1 Data obtained for CZT thin films Time (h)
˚ Average lattice parameter, a (A) ˚ Average lattice parameter, b (A) ˚ Average lattice parameter, c (A) Average grain size (nm) Average roughness (nm) Average thickness (nm)
2
4
6
5.437 7.636 5.374 36 6.7 550
5.446 7.651 5.346 47 4.3 450
5.441 7.643 5.379 50 3.4 425
seen, the increase of annealing time reduces the films thickness leading to sharper peaks of substrate. All peaks in Fig. 1(a–c) are ascribed to the orthorhombic perovskite structure. It is noted that the (2 0 0) and (1 2 1) peaks are overlapped as shown in the inset of Fig. 1(c). The characteristic peak of Pt substrate was observed at 2θ = 40◦ . The dependence of lattice parameters with annealing time is shown in Fig. 2. The formation of Ca(Zr0.05 Ti0.95 )O3 phase is confirmed by comparing XRD spectra with the respective JCPDS card (22–0153) of CaTiO3 with unit cell are
Fig. 3. FT-IR spectra of CZT thin films annealed at 650 ◦ C in oxygen atmosphere for (a) 2 h, (b) 4 h, and (c) 6 h.
388
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Fig. 4. Schematic summary of annealing time in the morphology of the CZT films.
Fig. 5. FEG-SEM micrographies of the cross-section for the CZT thin films. (a) Annealed at 650 ◦ C for 2 h. (b) Annealed at 650 ◦ C for 4 h. (c) Annealed at 650 ◦ C for 6 h.
L.S. Cavalcante et al. / Journal of Alloys and Compounds 453 (2008) 386–391
389
Fig. 6. AFM images of the surface and cross-sectional analysis of CZT thin films. (a) Annealed at 650 ◦ C for 2 h. (b) Annealed at 650 ◦ C for 4 h. (c) Annealed at 650 ◦ C for 6 h.
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L.S. Cavalcante et al. / Journal of Alloys and Compounds 453 (2008) 386–391
˚ b = 7.644 A, ˚ and c = 5.381 A. ˚ Since no JCPDS a = 5.440 A, card is available for this composition, an orthorhombic perovskite structure with space group Pnma was indexed. The lattice parameters were calculated using the least square refinement from the REDE93 program. Small deviations of lattice parameters were identified by the vertical bars (mean error standard). This occurs due to the substitution of Ti4+ (0.068 nm) by 5 mol% Zr4+ (0.087 nm) in the CaTiO3 perovskite [14], since lattice parameters had been calculated on basis of JCPDS card (220153). The film annealed at 650 ◦ C for 6 h is more crystalline and thus showed less variations in the a, b, and c lattice parameters. Table 1 presents the lattice parameter, grain size, roughness and thickness of the CZT films annealed at 650 ◦ C in oxygen atmosphere for different times. The increase of annealing time leads to a more crystalline film affecting slightly the lattice parameters a, b, and c (Table 1). It also results in the grain growth due to the coalescence of small grains and in the decrease of roughness and thickness due to the diffusion of external layers that suffer a strong influence during the crystallization of the CZT films. The FT-IR absorption spectra of the CZT thin films annealed at 650 ◦ C for different times are shown in Fig. 3. In the data a broad absorption peak of the BO6 stretching mode was observed at 600–840 cm−1 suggesting the formation of perovskite phase [15,16]. The increase of annealing time leads to a more crystalline CZT film causing a displacement of absorption peaks at 786, 790, and 792 cm−1 . This occurs due to the resonance with the longitudinal optic (LO) phonon modes which become sharper and narrower and shift toward higher wavenumbers. A schematic illustration of the diffusion mechanism of the layers and growth of CZT grains is shown in Fig. 4. Fig. 4 shows the schematic preparation of CZT films annealed at 650 ◦ C for 2, 4, and 6 h in constant oxygen flow atmosphere. The increase of annealing time leads to: (a) reduction in film thickness due to the transformation of amorphous phase into crystalline and (b) grain growth due to the coalescence process. The films annealed for 2 h present a homogeneous grain size distribution. At 4 h the CZT grains increase by coalescence of small grains leading to bigger grains. At 6 h, the films present a bimodal distribution due to the coalescence of one part of the grains. A decrease of CZT film thickness is observed with the increase of annealing time. This occurs during the sintering process as observed from FEG-SEM micrographies. Also, the CZT films deposited with three layers and annealed at 2 h present a clear interface (Fig. 5(a)). With the increase of annealing time for 4 h the second and third layers start to diffuse (Fig. 5(b)). The complete diffusion of these layers is evident for the film annealed for 6 h (Fig. 5(c)). Fig. 6 shows the AFM images of CZT film annealed at 650 ◦ C for 2, 4, and 6 h in oxygen atmosphere. The average grain size and surface roughness of CZT thin films were estimated using atomic force microscope. The surface morphology was obtained using an area of 500 nm × 500 nm.
Fig. 7. Dependence of annealing time as function of grain size and film thickness.
Fig. 6(a) shows a uniform and homogeneous CZT grain distribution. The rosette structure, which is typical for PZT thin films [17,18], was not observed in our analysis. The increase of annealing time leads to bigger grains evidencing that the oxygen atmosphere promotes the grains coalescence (Fig. 6(b)). After annealing at 650 ◦ C for 6 h the presence of larger and small grains were noted, indicating a bimodal distribution and the beginning of the coalescence process (Fig. 6(c)). However, annealing for longer times, more than 6 h, leads to the coalescence process followed by a uniform distribution of the grains. A similar process was reported for Pb(Zr,Ti)O3 and PbTiO3 thin films [19,20]. Fig. 7 shows the dependence of average grain size and thickness with the annealing time. A significant variation in the average grain size was evident with the increase of the annealing time. The largest variation was noted in the film annealed for 6 h due to the bimodal distribution as a result of the coalescence process. A strong decrease of CZT film thickness is noted with the increase of annealing time from 2 to 4 h. After that, a slight reduction of thickness from 450 to 425 nm was observed indicating a reduction of oxygen vacancies and film strain. 4. Conclusions Polycrystalline CZT thin films free of secondary phases and with orthorhombic structure were obtained from the polymeric precursor method. Less variations in the lattice parameters were observed in the more crystalline film. The increase of annealing time results in a stronger metal–oxygen octahedral vibrational mode. AFM data indicated that the CZT thin films present a homogeneous, smooth and crack free surface. The grain growth of CZT thin films is proportional to the increase of annealing time while the film thickness is reduced due to the minimization of oxygen vacancies and film strain. Acknowledgements The authors thank the financial support of the Brazilian research financing institutions: CAPES, FAPESP and CNPq. To the Prof. Lu´is Presley Santos-CEFET-MA and Prof. Edward Ralph Dockal-DQ-UFSCar-SP by the relevant discussions.
L.S. Cavalcante et al. / Journal of Alloys and Compounds 453 (2008) 386–391
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