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Fabrication of ferroelectric (Pb,Ba)TiO3 thin films by sol–gel technique and their characterization
January 2002
Materials Letters 52 Ž2002. 57–61 www.elsevier.comrlocatermatlet
Fabrication of ferroelectric žPb,Ba/ TiO 3 thin films by sol–gel technique and their characterization N.V. Giridharan, R. Jayavel ) Crystal Growth Centre, Anna UniÕersity, Chennai-25, India Received 19 February 2001; accepted 20 April 2001
Abstract Ferroelectric ŽPb,Ba.TiO 3 ŽPBT. thin films have been fabricated by sol–gel technique using a barium modified lead titanate sol synthesized from lead acetate tri-hydrate, barium acetate and titanium Živ. butoxide. PBT thin films were deposited on platinum coated silicon and fused quartz substrates by spin coating. The crystal structure and morphology of the films are strongly influenced by the heat treatment performed to form crystalline PBT films. The deposited films possess good compositional homogeneity and thickness uniformity. The dielectric constant and the dissipation factor measured at 1 kHz at room temperature are found to be 118 and 0.015. The remanent polarization and coercive field values are estimated to be 12 mCrcm2 and 65 kVrcm from the hysteresis studies. The films possess a very low leakage current of ; 10y7 Arcm2 as measured from I–V characteristics. q 2002 Elsevier Science B.V. All rights reserved. Keywords: Ferroelectric thin film; Sol–gel technique; I–V characteristics
1. Introduction Ferroelectric thin films have attracted much attention during past years, owing to their possible integration into multifunctional microelectronic devices. The aim of using these materials in the microelectronic technology is to exploit their properties such as Ži. spontaneous polarization, when they are integrated into non-volatile ferroelectric random access memories ŽNVRAM., Žii. pyroelectricity for infrared sensors and Žiii. piezoelectricity in microelectromechanical systems ŽMEMS. w1,2x. Lead titanate ) Corresponding author. Present address: National Research Institute for Metals, 1-2-1, Sengen, Tsukuba, Ibaraki 305-0047, Japan. Tel.: q81-298-59-2311; fax: q81-298-59-2301. E-mail addresses: [email protected], [email protected] ŽR. Jayavel..
ŽPbTiO 3 . is a promising ferroelectric, pyroelectric and piezoelectric material, because of its large spontaneous polarization ŽPs., low dielectric constant and large electro-mechanical anisotropy. Partial substitution of lead by appropriate amounts of lanthanum w3x, calcium w4x and strontium w5x has been reported. In this paper, we report the partial substitution of lead by barium. Several techniques are available for the deposition of ferroelectric thin films such as r.f.magnetron sputtering w6x, chemical vapour deposition w7x, laser ablation w8x, and sol–gel synthesis w9x. Among all these methods, the sol–gel technique, with its merits of easier composition control, better homogeneity, non-vacuum process, low processing temperature, low cost and easier fabrication of large area thin films, has been extensively used for the fabrication of ferroelectric thin films. In the present study, the sol–gel spin-on technique has been adopted for
00167-577Xr02r$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 1 . 0 0 3 6 6 - 4
58
N.V. Giridharan, R. JayaÕelr Materials Letters 52 (2002) 57–61
the deposition of ferroelectric thin films of lead barium titanate. The crystal structure, morphology, surface and in-depth compositional uniformity, electrical and optical properties of the films have been investigated.
absorption spectra recorded in the wavelength region 200–900 nm. 3. Results and discussions 3.1. X-ray diffraction studies
2. Experimental The PBT precursor solution was synthesised with 10% of excess lead Ždue to the high volatile nature of lead. using lead acetate trihydrate, barium acetate and titanium Živ. butoxide as starting materials. Glacial acetic acid and 2-methoxy-ethanol were used as the solvent and complexing agent respectively. Initially lead acetate and barium acetate in the ratio 80:20 were dissolved in acetic acid in a refluxing condenser. After getting a clear solution, titanium Živ. butoxide was added in a proper molar ratio to obtain the precursor solution. The precursor solution was spin-coated on different substrates by a spinner of 2300 rpm for about 1 min. The substrates were platinum coated silicon Ž100. and fused quartz. After spinning onto the substrates, the films were kept in ambient atmosphere for 1 h to form gel films by hydrolysis and polymerisation. The films were subsequently heated to a temperature of 3008C for the decomposition of the residual organics and then cooled to room temperature. The above coating and preheating at 3008C were repeated several times in order to increase the thickness of the films. As-deposited films were amorphous in nature and annealing was carried out at different temperatures in order to convert them into crystalline form. The crystalline nature and the phase formation were confirmed by powder X-ray diffraction analysis using ˚ .. Surface morpholCuK a radiation Ž l s 1.5418 A ogy and microstructure of the films were observed using scanning electron microscope ŽLeica Cambridge, Stereoscan 440.. Surface and in-depth chemical composition of the films was estimated by field emission type Auger Electron Spectroscopy ŽAES, PHI 670 nanoprobe.. Capacitance–voltage properties were studied in a metal–insulator–metal configuration using a capacitance bridge and hysteresis loop measurements were carried out using a Sawyer– Tower circuit. Optical properties of the PBT films were determined from the optical transmission and
Powder X-ray diffraction pattern of the films heat-treated at 3008C indicated the amorphous nature. The crystalline behavior started at an annealing temperature of 4008C. With further increase in annealing temperature, diffraction peaks became increasingly sharp, intense with a decrease in the full width at half maximum ŽFWHM., indicating the enhanced crystallinity of the films. The XRD pattern reveals that the films are polycrystalline with no evidence of any preferred orientation or secondary phases. The structure is found to be tetragonal with a cra ratio of 1.036 for 20 mol% of barium. Fig. 1 shows the X-ray diffraction patterns of the PBT films annealed at 3008C and 6508C. 3.2. Micro-morphological studies Fig. 2a and b shows the scanning electron microscopic images of PBT thin films annealed at 4508C
Fig. 1. X-ray diffraction patterns of PBT thin films annealed at Ža. 3008C for 1 h; Žb. 6508C for 1 h.
N.V. Giridharan, R. JayaÕelr Materials Letters 52 (2002) 57–61
59
current of 2 mA. Fig. 3 shows the AES depth profile of the film annealed at 6508C. From the depth profile, it appears that the film is composed of three regions: surface, bulk film and the interface. Although a carbon peak was detected on the surface of the films as soon as the first monolayers are removed by the Ar ion bombardment, the carbon peak intensity decreased to the level of not being distinguishable from the noise. The peak is considered to be due to contamination during the sample preparation and handling. Lead is enriched at the surface of the films annealed at 3008C. This is because lead has relatively higher vapour pressure and can easily segregate into the surface in the amorphous layer w11x. However, after heat treatment at a temperature of 6508C, the lead content is normal as is expected for a stoichiometric composition. This is due to crystallisation, which causes the Pb atoms to incorporate into the crystal structure, thereby producing stoichiometric Pb 0.8 Ba 0.2TiO 3 thin films. 3.4. Electrical properties The dielectric measurements were carried out on the films fabricated on platinum coated silicon substrate. A metal–insulator–metal ŽM–I–M. structure was fabricated by evaporating circular platinum dot electrodes of 0.5 mm in diameter on the surface of Fig. 2. Scanning electron micrograph of PBT thin film annealed at Ža. 4508C and Žb. 6508C.
and 6508C, respectively. It has been found that the films annealed at 6508C possess uniform spherical grains with an average grains size of 0.1 mm. There is a significant increase in the grain size of films annealed at 6508C compared to the films annealed at 4508C. This is attributed to the increase in surface mobility with increasing temperature, thus allowing the films to lower its total energy by growth of grains and decrease of grain boundary area w10x. 3.3. Composition analysis by AES depth profile The Auger spectra were obtained in the derivative mode in the energy range of 0–1000 eV with an Argon gas pressure of 4.5 = 10y7 Torr. Sputtering was carried out with 2 kV Ar beam with a filament
Fig. 3. AES depth profiles of the elements of a PBT film annealed at 6508C. The profile of Si corresponds to the substrate.
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N.V. Giridharan, R. JayaÕelr Materials Letters 52 (2002) 57–61
the films by shadow masking. Fig. 4 shows the dielectric constant and dissipation factor measured at room temperature as a function of frequency in the range 1 kHz–1 MHz. The dielectric properties in different regions of the films vary within 2–3%, indicating the compositional homogeneity and thickness uniformity. The dielectric constant and the dissipation factor at 1 kHz are measured to be 118 and 0.015. Within the low frequency range, the dissipation factor is close to zero. With an increase in frequency, the dissipation factor slightly rises indicating the increase in conductivity of the film with measuring frequency w12x. Polarization versus voltage hysteresis loop was measured for the films deposited on PtrSi substrate. Fig. 5 illustrates a hysteresis loop obtained for the PBT film with a PbrBa atomic ratio of 80:20 using a 100 kHz sinusoidal voltage with amplitude of 3 V. The measured remanent polarization and coercive field values are 12 mCrcm2 and 65 kVrcm, respectively, which is comparable with values reported for PCT and PST thin films w4,5x. The I–V curve of PBT films annealed at 6508C as a function of applied voltage is shown in Fig. 6. As shown in the figure, the I–V curve has a range of absorption current at lower applied voltage and a range of leakage current at higher voltage values. The leakage current measured at an applied voltage of 1 V is 1 = 10y7 Arcm2 , which shows that the PBT films are suitable for device applications.
Fig. 4. Plots of dielectric constant and dissipation factor of PBT thin films as a function of frequency.
Fig. 5. Hysteresis loop of PBT film annealed at 6508C.
3.5. Optical properties Fig. 7 shows the optical transmission spectrum of Pb 0.8 Ba 0.2Ti0 3 thin film annealed at 6508C recorded in the wavelength region 200–900 nm. The substrate used was fused quartz. The film was transparent and colourless to the light with wavelength higher than
Fig. 6. Leakage current density characteristics of PBT film.
N.V. Giridharan, R. JayaÕelr Materials Letters 52 (2002) 57–61
61
Fig. 7. Optical transmission spectrum of PBT film annealed at 6508C.
400 nm. The transmission drops at 375 nm, and the cut off wavelength occurs at 300 nm. From the transmission and absorption spectra, the refractive index of the film was calculated using the formula ns
1 q R 1r2 1 y R 1r2
,
where R is the reflectance. The refractive index of the film was found to be 2.14 measured at 600 nm.
4. Conclusions Ferroelectric lead barium titanate thin films were deposited on platinum coated silicon and fused quartz substrates by sol–gel spin-on technique. Deposited PBT thin films possess tetragonal structure with better compositional homogeneity and thickness uniformity. The dielectric constant and the dissipation factor are measured to be 118 and 0.015, respectively. The remanent polarization and coercive field values of 12 mCrcm2 and 65 kVrcm, respectively confirm the ferroelectric behavior of the deposited films. Films possess a very low leakage current of
; 10y7 Arcm2 and refractive index of 2.14. The promising ferroelectric and leakage current characteristics reveal that PBT is a potential material for device applications. References w1x R. Siera, D. Leinen, E. Rodriguez-Castellon, M.L. Calzada, Chem. Mater. 11 Ž1999. 3437. w2x C.H. Peng, S.B. Desu, Appl. Phys. Lett. 61 Ž1992. 16. w3x M. Alguero, M.L. Calzada, L. Pardo, E. Snoeck, J. Mater. Res. 14 Ž1999. 4570. w4x H. Maiwa, N. Ichinose, Jpn. J. Appl. Phys. 36 Ž1997. 5825. w5x C. Chou, C. Hou, G. Chang, H. Cheng, Appl. Surf. Sci. 142 Ž1999. 413. w6x S.B. Krupanidhi, M. Sayer, J. Vac. Sci. Technol., A 2 Ž1984. 303. w7x S.G. Yoon, J.D. Park, J.H. Choi, H.G. Kim, J. Vac. Sci. Technol., A 9 Ž1991. 281. w8x R. Dat, J.K. Lee, O. Auciello, A.I. Kingon, Appl. Phys. Lett. 67 Ž1995. 572. w9x K.D. Budd, S.K. Dey, D.A. Payne, Br. Ceram. Proc. 36 Ž1985. 107. w10x D. Bao, X. Wu, L. Zhang, X. Yao, Thin Solid Films 350 Ž1999. 30. w11x Y.C. Lai, J. C Lin, C. Lee, Appl. Surf. Sci. 125 Ž1998. 51. w12x C.H. Lu, C.Y. Wen, Mater. Lett. 38 Ž1999. 278.
Materials Letters 52 Ž2002. 57–61 www.elsevier.comrlocatermatlet
Fabrication of ferroelectric žPb,Ba/ TiO 3 thin films by sol–gel technique and their characterization N.V. Giridharan, R. Jayavel ) Crystal Growth Centre, Anna UniÕersity, Chennai-25, India Received 19 February 2001; accepted 20 April 2001
Abstract Ferroelectric ŽPb,Ba.TiO 3 ŽPBT. thin films have been fabricated by sol–gel technique using a barium modified lead titanate sol synthesized from lead acetate tri-hydrate, barium acetate and titanium Živ. butoxide. PBT thin films were deposited on platinum coated silicon and fused quartz substrates by spin coating. The crystal structure and morphology of the films are strongly influenced by the heat treatment performed to form crystalline PBT films. The deposited films possess good compositional homogeneity and thickness uniformity. The dielectric constant and the dissipation factor measured at 1 kHz at room temperature are found to be 118 and 0.015. The remanent polarization and coercive field values are estimated to be 12 mCrcm2 and 65 kVrcm from the hysteresis studies. The films possess a very low leakage current of ; 10y7 Arcm2 as measured from I–V characteristics. q 2002 Elsevier Science B.V. All rights reserved. Keywords: Ferroelectric thin film; Sol–gel technique; I–V characteristics
1. Introduction Ferroelectric thin films have attracted much attention during past years, owing to their possible integration into multifunctional microelectronic devices. The aim of using these materials in the microelectronic technology is to exploit their properties such as Ži. spontaneous polarization, when they are integrated into non-volatile ferroelectric random access memories ŽNVRAM., Žii. pyroelectricity for infrared sensors and Žiii. piezoelectricity in microelectromechanical systems ŽMEMS. w1,2x. Lead titanate ) Corresponding author. Present address: National Research Institute for Metals, 1-2-1, Sengen, Tsukuba, Ibaraki 305-0047, Japan. Tel.: q81-298-59-2311; fax: q81-298-59-2301. E-mail addresses: [email protected], [email protected] ŽR. Jayavel..
ŽPbTiO 3 . is a promising ferroelectric, pyroelectric and piezoelectric material, because of its large spontaneous polarization ŽPs., low dielectric constant and large electro-mechanical anisotropy. Partial substitution of lead by appropriate amounts of lanthanum w3x, calcium w4x and strontium w5x has been reported. In this paper, we report the partial substitution of lead by barium. Several techniques are available for the deposition of ferroelectric thin films such as r.f.magnetron sputtering w6x, chemical vapour deposition w7x, laser ablation w8x, and sol–gel synthesis w9x. Among all these methods, the sol–gel technique, with its merits of easier composition control, better homogeneity, non-vacuum process, low processing temperature, low cost and easier fabrication of large area thin films, has been extensively used for the fabrication of ferroelectric thin films. In the present study, the sol–gel spin-on technique has been adopted for
00167-577Xr02r$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 1 . 0 0 3 6 6 - 4
58
N.V. Giridharan, R. JayaÕelr Materials Letters 52 (2002) 57–61
the deposition of ferroelectric thin films of lead barium titanate. The crystal structure, morphology, surface and in-depth compositional uniformity, electrical and optical properties of the films have been investigated.
absorption spectra recorded in the wavelength region 200–900 nm. 3. Results and discussions 3.1. X-ray diffraction studies
2. Experimental The PBT precursor solution was synthesised with 10% of excess lead Ždue to the high volatile nature of lead. using lead acetate trihydrate, barium acetate and titanium Živ. butoxide as starting materials. Glacial acetic acid and 2-methoxy-ethanol were used as the solvent and complexing agent respectively. Initially lead acetate and barium acetate in the ratio 80:20 were dissolved in acetic acid in a refluxing condenser. After getting a clear solution, titanium Živ. butoxide was added in a proper molar ratio to obtain the precursor solution. The precursor solution was spin-coated on different substrates by a spinner of 2300 rpm for about 1 min. The substrates were platinum coated silicon Ž100. and fused quartz. After spinning onto the substrates, the films were kept in ambient atmosphere for 1 h to form gel films by hydrolysis and polymerisation. The films were subsequently heated to a temperature of 3008C for the decomposition of the residual organics and then cooled to room temperature. The above coating and preheating at 3008C were repeated several times in order to increase the thickness of the films. As-deposited films were amorphous in nature and annealing was carried out at different temperatures in order to convert them into crystalline form. The crystalline nature and the phase formation were confirmed by powder X-ray diffraction analysis using ˚ .. Surface morpholCuK a radiation Ž l s 1.5418 A ogy and microstructure of the films were observed using scanning electron microscope ŽLeica Cambridge, Stereoscan 440.. Surface and in-depth chemical composition of the films was estimated by field emission type Auger Electron Spectroscopy ŽAES, PHI 670 nanoprobe.. Capacitance–voltage properties were studied in a metal–insulator–metal configuration using a capacitance bridge and hysteresis loop measurements were carried out using a Sawyer– Tower circuit. Optical properties of the PBT films were determined from the optical transmission and
Powder X-ray diffraction pattern of the films heat-treated at 3008C indicated the amorphous nature. The crystalline behavior started at an annealing temperature of 4008C. With further increase in annealing temperature, diffraction peaks became increasingly sharp, intense with a decrease in the full width at half maximum ŽFWHM., indicating the enhanced crystallinity of the films. The XRD pattern reveals that the films are polycrystalline with no evidence of any preferred orientation or secondary phases. The structure is found to be tetragonal with a cra ratio of 1.036 for 20 mol% of barium. Fig. 1 shows the X-ray diffraction patterns of the PBT films annealed at 3008C and 6508C. 3.2. Micro-morphological studies Fig. 2a and b shows the scanning electron microscopic images of PBT thin films annealed at 4508C
Fig. 1. X-ray diffraction patterns of PBT thin films annealed at Ža. 3008C for 1 h; Žb. 6508C for 1 h.
N.V. Giridharan, R. JayaÕelr Materials Letters 52 (2002) 57–61
59
current of 2 mA. Fig. 3 shows the AES depth profile of the film annealed at 6508C. From the depth profile, it appears that the film is composed of three regions: surface, bulk film and the interface. Although a carbon peak was detected on the surface of the films as soon as the first monolayers are removed by the Ar ion bombardment, the carbon peak intensity decreased to the level of not being distinguishable from the noise. The peak is considered to be due to contamination during the sample preparation and handling. Lead is enriched at the surface of the films annealed at 3008C. This is because lead has relatively higher vapour pressure and can easily segregate into the surface in the amorphous layer w11x. However, after heat treatment at a temperature of 6508C, the lead content is normal as is expected for a stoichiometric composition. This is due to crystallisation, which causes the Pb atoms to incorporate into the crystal structure, thereby producing stoichiometric Pb 0.8 Ba 0.2TiO 3 thin films. 3.4. Electrical properties The dielectric measurements were carried out on the films fabricated on platinum coated silicon substrate. A metal–insulator–metal ŽM–I–M. structure was fabricated by evaporating circular platinum dot electrodes of 0.5 mm in diameter on the surface of Fig. 2. Scanning electron micrograph of PBT thin film annealed at Ža. 4508C and Žb. 6508C.
and 6508C, respectively. It has been found that the films annealed at 6508C possess uniform spherical grains with an average grains size of 0.1 mm. There is a significant increase in the grain size of films annealed at 6508C compared to the films annealed at 4508C. This is attributed to the increase in surface mobility with increasing temperature, thus allowing the films to lower its total energy by growth of grains and decrease of grain boundary area w10x. 3.3. Composition analysis by AES depth profile The Auger spectra were obtained in the derivative mode in the energy range of 0–1000 eV with an Argon gas pressure of 4.5 = 10y7 Torr. Sputtering was carried out with 2 kV Ar beam with a filament
Fig. 3. AES depth profiles of the elements of a PBT film annealed at 6508C. The profile of Si corresponds to the substrate.
60
N.V. Giridharan, R. JayaÕelr Materials Letters 52 (2002) 57–61
the films by shadow masking. Fig. 4 shows the dielectric constant and dissipation factor measured at room temperature as a function of frequency in the range 1 kHz–1 MHz. The dielectric properties in different regions of the films vary within 2–3%, indicating the compositional homogeneity and thickness uniformity. The dielectric constant and the dissipation factor at 1 kHz are measured to be 118 and 0.015. Within the low frequency range, the dissipation factor is close to zero. With an increase in frequency, the dissipation factor slightly rises indicating the increase in conductivity of the film with measuring frequency w12x. Polarization versus voltage hysteresis loop was measured for the films deposited on PtrSi substrate. Fig. 5 illustrates a hysteresis loop obtained for the PBT film with a PbrBa atomic ratio of 80:20 using a 100 kHz sinusoidal voltage with amplitude of 3 V. The measured remanent polarization and coercive field values are 12 mCrcm2 and 65 kVrcm, respectively, which is comparable with values reported for PCT and PST thin films w4,5x. The I–V curve of PBT films annealed at 6508C as a function of applied voltage is shown in Fig. 6. As shown in the figure, the I–V curve has a range of absorption current at lower applied voltage and a range of leakage current at higher voltage values. The leakage current measured at an applied voltage of 1 V is 1 = 10y7 Arcm2 , which shows that the PBT films are suitable for device applications.
Fig. 4. Plots of dielectric constant and dissipation factor of PBT thin films as a function of frequency.
Fig. 5. Hysteresis loop of PBT film annealed at 6508C.
3.5. Optical properties Fig. 7 shows the optical transmission spectrum of Pb 0.8 Ba 0.2Ti0 3 thin film annealed at 6508C recorded in the wavelength region 200–900 nm. The substrate used was fused quartz. The film was transparent and colourless to the light with wavelength higher than
Fig. 6. Leakage current density characteristics of PBT film.
N.V. Giridharan, R. JayaÕelr Materials Letters 52 (2002) 57–61
61
Fig. 7. Optical transmission spectrum of PBT film annealed at 6508C.
400 nm. The transmission drops at 375 nm, and the cut off wavelength occurs at 300 nm. From the transmission and absorption spectra, the refractive index of the film was calculated using the formula ns
1 q R 1r2 1 y R 1r2
,
where R is the reflectance. The refractive index of the film was found to be 2.14 measured at 600 nm.
4. Conclusions Ferroelectric lead barium titanate thin films were deposited on platinum coated silicon and fused quartz substrates by sol–gel spin-on technique. Deposited PBT thin films possess tetragonal structure with better compositional homogeneity and thickness uniformity. The dielectric constant and the dissipation factor are measured to be 118 and 0.015, respectively. The remanent polarization and coercive field values of 12 mCrcm2 and 65 kVrcm, respectively confirm the ferroelectric behavior of the deposited films. Films possess a very low leakage current of
; 10y7 Arcm2 and refractive index of 2.14. The promising ferroelectric and leakage current characteristics reveal that PBT is a potential material for device applications. References w1x R. Siera, D. Leinen, E. Rodriguez-Castellon, M.L. Calzada, Chem. Mater. 11 Ž1999. 3437. w2x C.H. Peng, S.B. Desu, Appl. Phys. Lett. 61 Ž1992. 16. w3x M. Alguero, M.L. Calzada, L. Pardo, E. Snoeck, J. Mater. Res. 14 Ž1999. 4570. w4x H. Maiwa, N. Ichinose, Jpn. J. Appl. Phys. 36 Ž1997. 5825. w5x C. Chou, C. Hou, G. Chang, H. Cheng, Appl. Surf. Sci. 142 Ž1999. 413. w6x S.B. Krupanidhi, M. Sayer, J. Vac. Sci. Technol., A 2 Ž1984. 303. w7x S.G. Yoon, J.D. Park, J.H. Choi, H.G. Kim, J. Vac. Sci. Technol., A 9 Ž1991. 281. w8x R. Dat, J.K. Lee, O. Auciello, A.I. Kingon, Appl. Phys. Lett. 67 Ž1995. 572. w9x K.D. Budd, S.K. Dey, D.A. Payne, Br. Ceram. Proc. 36 Ž1985. 107. w10x D. Bao, X. Wu, L. Zhang, X. Yao, Thin Solid Films 350 Ž1999. 30. w11x Y.C. Lai, J. C Lin, C. Lee, Appl. Surf. Sci. 125 Ž1998. 51. w12x C.H. Lu, C.Y. Wen, Mater. Lett. 38 Ž1999. 278.