- Email: [email protected]
Properties of a-Si:H intrinsic films produced by HWPA-CVD technique
Thin Solid Films 451 – 452 (2004) 366–369
Properties of a-Si:H intrinsic films produced by HWPA-CVD technique ´ ´ Pereira, Elvira Fortunato, Rodrigo Martins Isabel Ferreira*, Hugo Aguas, Luıs CENIMAT, Department of Materials Science, Faculty of Science and Technology of New University of Lisbon, and CEMOPyUNINOVA, Campus da Caparica, 2829-516 Caparica, Portugal
Abstract In this paper, we investigate the optoelectronic properties and the photodegradation of amorphous silicon films produced by the hot wire plasma assisted technique (HWPA-CVD). We observed that hydrogen dilution in the gas phase plays an important role in the time dependence of the photoconductivity, which is correlated with an enhancement of defect density. We also compare the degradation of these films with those produced by plasma enhanced and by hot wire chemical vapour deposition techniques (PECVD and HW-CVD) and we found lower time dependence for the photodegradation of the films produced by HWPA-CVD technique 䊚 2003 Elsevier B.V. All rights reserved. Keywords: HWPA-CVD technique; Amorphous silicon; Optoelectronic properties
1. Introduction The degradation of the optoelectronic properties of intrinsic hydrogenated amorphous silicon (a-Si:H) films due to light is one of the great constraints limiting the wide-spread application of a-Si:H solar cells. The creation of metastable defects when the solar cell is submitted to high level light intensity causes a degradation in the photoconductivity of the undoped a-Si:H layer and, therefore in the p-i-n solar cell. The phenomenon is known as the Staebler–Wronski (SE) effect w1x. This effect has been studied for more than 20 years. Nevertheless, the source of the degradation is far from having been uniquely identified. Several causes were identified and studied such as, structural heterogeneities, microvoids, weak bonds, hydrogen and impurities, besides others. Several theories have been proposed w2x. Basically, those theories are divided into two groups. One describes the SW effect by a local breakage and rearrangement of the Si–H bonds. The other theory involves more global phenomena related to the amorphous network, which is the case of the long distance hydrogen diffusion such as the model for hydrogen collision w3x. Although several theories have been developed, the more explored one is that based on the breaking of *Corresponding author. Tel.: q351-21-294-85-64; fax: q351-212957810. E-mail address: [email protected] (I. Ferreira).
weak bonds with bimolecular recombination of carriers w2x. In this work, we present the results concerning the influence of hydrogen dilution on the optoelectronic properties of intrinsic a-Si:H films produced by HWPACVD. 2. Sample preparation and experimental procedures The a-Si:H films were obtained by HWPA-CVD technique. The system used is described elsewhere w4x. The films were produced using 5 sccm of silane gas flow, a total gas pressure of 0.5 Torr; filament temperature (Tf) of 1950 8C, rf power of 50 W and substrate temperature (Ts) of 200 8C. The silane gas was diluted with hydrogen, employing a hydrogen gas flow (fH2) up to 300 sccm. The films were analysed by Fourier transform infrared (FTIR) spectroscopy in an ATI Matheson FTIR system. The dark conductivity (sd) as a function of temperature was determined, under vacuum conditions (10y2 mbar), in films deposited on alkali free glass with Al coplanar contacts. The photoconductivity at room temperature (sph) was measured by illuminating the sample with AM1.5 100 mWycm2 light intensity. The hmt product was determined using a He– Ne laser beam with a power of 5 mW and ls632.5 nm, corresponding to a photon flux of 1.2=1017 cmy2 sy1. The Constant photocurrent method (CPM) was used to determine the density of deep defect states (Ns)
0040-6090/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2003.10.145
I. Ferreira et al. / Thin Solid Films 451 – 452 (2004) 366–369
Fig. 1. Influence of hydrogen gas flow on ASiOyASiH. The inset is a SEM image of a film produced with f H2 s100 sccm.
and the absorption coefficient (a) in the visible range was calculated through the transmittance spectra obtained in a UV–VIS–NIR Shimadzu spectrophotometer. 3. Results and discussion To evaluate the influence of hydrogen dilution on the compactness and on the properties of the films, the ratio between the areas of the peaks associated to the wagging modes of SiOx and SiHx in the IR spectra (ASiOx yASiHx), located at approximately 1050–1200 cmy1 and 630 cmy1, respectively, was determined. The data are shown in Fig. 1. ASiOx yASiHx increases abruptly for fH2 above 25 sccm and tends to saturate for fH2 higher than 300 sccm, for a-Si:H films exposed more than 30 days to atmospheric conditions. The influence of the fH2 on room dark conductivity (sd) and on the hmt product is plotted in Fig. 2. The hmt product exhibits an exponential decay from 8=10y7 to 8=10y9 cm2 Vy1 and sd decreases about one decade when fH2 increases from 0 to 300 sccm. Since the hmt decreases and ASiOx yASiHx increases with the enhancement of the fH2the behaviour referred before is related to a post-oxidation phenomenon associated to films with a structure formed by agglomerates, as is observed in the SEM image shown in the inset of Fig. 1a). Although sd decreases with the enhancement of the fH2, a decrease in the defect density and, therefore an enhancement of the hmt product was expected. However, the opposite is observed. This means that the photogenerated carriers, for some reason, are trapped. Certainly, the oxidation of the film is one of the causes for this behaviour. Another possible reason could be an enhancement of the molecular hydrogen incorporated into the film, in the form of clusters, since the hydrogen bonded to silicon as SiH decreases as fH2 increases w5x.
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In order to check this possibility Elastic Recoil Detection (ERD) was performed to determine the total hydrogen content of the film. For films produced without hydrogen dilution it was determined, from FTIR measurements that the concentration of hydrogen w6x bonded to silicon is approximately 4% and by ERD the hydrogen content is 8%. Therefore, this is an indication that with increasing fH2 the amount of hydrogen not bonded to Si can increase, enhancing the H2 in clusters, contributing to the trapping of photogenerated carriers. This could be due to the possible dissociation of H2 into two neutral interstitial H (1.74 eV) or into an Hq and H- pair (1.34 eV) w7x. The confirmation that hydrogen dilution enhances the porosity of the film and consequently enhances postoxidation was also obtained by spectroscopic ellipsometry, where the maximum of the pseudo-dielectric function N´2M as a function of photon energy (E) varies from 26, for films produced without hydrogen dilution, to 16 for fH2s300 sccm w6x. The previous results reveal that for the process conditions used (high pressure and high filament temperature), the hydrogen dilution does not contribute to the enhancement of the optoelectronic films properties. The films produced by HWPA-CVD with high hydrogen dilution show a significant post-deposition oxidation, although lower than those made by a-Si:H HW-CVD films (produced in the same conditions but without rf power) w6x. However, the obtained results are still not optimised as far as it concerns solar cells application. Thus, we have also studied the influence of the rf power on the films properties w6x, keeping the other deposition parameters constant. The obtained data indicate that the films produced at an rf power of 100 W, Tfs1950 8C and without hydrogen dilution have improved optoelectronic properties, sphs10y4 (V cm)y1, hmts 1.6=10y5 cm2 yV, Nss1016 cmy3 with Urbach energy (E0)s50 meV, hydrogen content of approximately 10% ˚ and a high growth rate f30 Ays. Even though the
Fig. 2. Influence of hydrogen gas flow on sd and on hmt product.
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I. Ferreira et al. / Thin Solid Films 451 – 452 (2004) 366–369
Fig. 3. Spectral variation of the absorption coefficient obtained by CPM measurements in HWPA-CVD films produced without hydrogen dilution, using Prfs100 W and Tfs1950 8C
optoelectronic properties are comparable to those obtained by other authors w8–11x, the growth rate of the films obtained by HWPA-CVD technique is at least one decade higher. The spectral variation of the absorption coefficient is plotted in Fig. 3, where a narrow Urbach edge and a low absorption of deep defect states are observed, in agreement with the other results obtained. Fig. 4 shows the SEM cross-section image of the corresponding film. A compact film with a smooth surface is observed. Fig. 5 shows the time dependence of sd and sph (with AM1.5 100 mWycm2 light intensity, continuously) for HWPA-CVD and PECVD polymorphous w12x films. The dark conductivity measurement was taken at atmospheric pressure under room temperature at dark conditions. The sph, at the beginning, is higher in PECVD films, but after 30 min of light exposure sph is lower
Fig. 5. Time dependence of sph obtained under AM1.5 100 mWycm2, for HWPA-CVD films produced with rf power of 50 W (a) and 100 W (b), without hydrogen dilution and a polymorphous film produced by PECVD w12x (c).
than that of HWPA-CVD films. However, HWPA-CVD films produced with an rf power of 50 W almost do not degrade but, sph is too low, which reflects the small post-oxidation observed, as shown in Fig. 1. This is related to a higher contribution of the HW-CVD component for the gas dissociation, leading to a much higher growth rate and consequently more defects in a-Si:H films w6x. Table 1 shows the influence of hydrogen dilution in the photodegradation and optoelectronic properties of HWPA-CVD films produced with rf power of 50 W under light illumination of 50 mWycm2. The data indicate that hydrogen dilution has no influence on the photosensitivity (sph y sd), approximately 1.7=103 for all the samples analysed. Nevertheless, the photodegradation (Dsph y sph) is significantly enhanced, from 0.09 to 0.76, as fH2 increases from 0 to 300 sccm. Again, the enhancement of the H2 clusters and the postoxidation due to films porosity are the main causes of this SW effect. 4. Conclusions The results obtained show the possibility to produce stable intrinsic a-Si:H films with improved electronic Table 1 Influence of hydrogen dilution in the photodegradation (AM1.5 spectra with light intensity of 50 mWycm2) of films produced with rf power of 50 W and Tfs1950 8C
Fig. 4. SEM cross-section image of the sample shown in Fig. 3.
dil. H2 (sccm)
Dsphysph
sph(V cm)y1
sd(V cm)y1
0 100 300
0.09 0.26 0.76
8.7=10y7 4.5=10y7 5.8=10y7
5.3=10y10 2.6=10y10 3.3=10y10
I. Ferreira et al. / Thin Solid Films 451 – 452 (2004) 366–369
properties and growth rate above 3 nmys by HWPACVD. Besides that the hydrogen dilution does not contribute to enhance the films properties. This fact, besides being related to atomic hydrogen, can also be attributed to molecular hydrogen, which is probably incorporated into the film, giving rise to clusters and concomitant porosity. Acknowledgments The authors would like to thank to Augusto Lopes from Aveiro University for performing SEM analysis and to Ana Rita Ramos for performing the EDR analysis at ITN Institut. This work was supported by Fundacao ¸˜ ˆ da Ciencia e Tecnologia through ’Financiamento Plurianuais’ of CENIMAT, and by the projects PRAXISy PyCTMy12094y1998 and H-Alpha Solar contract no. ERK and CT-1999-00004.
369
References w1x D.L. Staebler, C.R. Wronski, Appl. Phys. Lett. 31 (1977) 292. w2x K. Tanaka, E. Maruyama, T. Shimada, H. Okamoto, Amorphous Silicon, John Wiley and Sons, Inc, 1999. w3x H.M. Branz, Phys. Rev. B 59 (1999) 5498. w4x I. Ferreira, M.E.V. Costa, L. Pereira, E. Fortunato, R. Martins, A.R. Ramos, M.F. Silva, Appl. Surf. Sci. 184 (2001) 60. w5x I. Ferreira, P. Vilarinho, E. Fortunato, R. Martins, J. Appl. Phys. 91 (3) (2002) 1644. w6x I. Ferreira, Ph.D. Thesis, FCT-UNL, 2002. w7x C.G. Van de Walle, B. Tuttle, Mater. Res. Soc. Symp. Proc. 557 (1999) 275. w8x M. Konagai, T. Tsushima, M-K. Kim, K. Asakawa, A. Yamada, Y. Kudriasvtsev, A. Villegas, R. Asomoza, Thin Solid Films 395 (2001) 252. w9x A.H. Mahan, J. Canapella, B.P. Nelson, R.S. Crandall, J. Appl. Phys. 69 (9) (1991) 6728. w10x S.R. Jadkar, J.V. Sali, M.G. Tatwale, Sol. Energ. Mat. Sol. C. 71 (2002) 153. w11x R.E.I. Schropp, Thin Solid Films 403–404 (2002) 17. ´ w12x H. Aguas, E. Fortunato, V. Silva, L. Pereira, R. Martins, Thin Solid Films 403–404 (2002) 26.
Properties of a-Si:H intrinsic films produced by HWPA-CVD technique ´ ´ Pereira, Elvira Fortunato, Rodrigo Martins Isabel Ferreira*, Hugo Aguas, Luıs CENIMAT, Department of Materials Science, Faculty of Science and Technology of New University of Lisbon, and CEMOPyUNINOVA, Campus da Caparica, 2829-516 Caparica, Portugal
Abstract In this paper, we investigate the optoelectronic properties and the photodegradation of amorphous silicon films produced by the hot wire plasma assisted technique (HWPA-CVD). We observed that hydrogen dilution in the gas phase plays an important role in the time dependence of the photoconductivity, which is correlated with an enhancement of defect density. We also compare the degradation of these films with those produced by plasma enhanced and by hot wire chemical vapour deposition techniques (PECVD and HW-CVD) and we found lower time dependence for the photodegradation of the films produced by HWPA-CVD technique 䊚 2003 Elsevier B.V. All rights reserved. Keywords: HWPA-CVD technique; Amorphous silicon; Optoelectronic properties
1. Introduction The degradation of the optoelectronic properties of intrinsic hydrogenated amorphous silicon (a-Si:H) films due to light is one of the great constraints limiting the wide-spread application of a-Si:H solar cells. The creation of metastable defects when the solar cell is submitted to high level light intensity causes a degradation in the photoconductivity of the undoped a-Si:H layer and, therefore in the p-i-n solar cell. The phenomenon is known as the Staebler–Wronski (SE) effect w1x. This effect has been studied for more than 20 years. Nevertheless, the source of the degradation is far from having been uniquely identified. Several causes were identified and studied such as, structural heterogeneities, microvoids, weak bonds, hydrogen and impurities, besides others. Several theories have been proposed w2x. Basically, those theories are divided into two groups. One describes the SW effect by a local breakage and rearrangement of the Si–H bonds. The other theory involves more global phenomena related to the amorphous network, which is the case of the long distance hydrogen diffusion such as the model for hydrogen collision w3x. Although several theories have been developed, the more explored one is that based on the breaking of *Corresponding author. Tel.: q351-21-294-85-64; fax: q351-212957810. E-mail address: [email protected] (I. Ferreira).
weak bonds with bimolecular recombination of carriers w2x. In this work, we present the results concerning the influence of hydrogen dilution on the optoelectronic properties of intrinsic a-Si:H films produced by HWPACVD. 2. Sample preparation and experimental procedures The a-Si:H films were obtained by HWPA-CVD technique. The system used is described elsewhere w4x. The films were produced using 5 sccm of silane gas flow, a total gas pressure of 0.5 Torr; filament temperature (Tf) of 1950 8C, rf power of 50 W and substrate temperature (Ts) of 200 8C. The silane gas was diluted with hydrogen, employing a hydrogen gas flow (fH2) up to 300 sccm. The films were analysed by Fourier transform infrared (FTIR) spectroscopy in an ATI Matheson FTIR system. The dark conductivity (sd) as a function of temperature was determined, under vacuum conditions (10y2 mbar), in films deposited on alkali free glass with Al coplanar contacts. The photoconductivity at room temperature (sph) was measured by illuminating the sample with AM1.5 100 mWycm2 light intensity. The hmt product was determined using a He– Ne laser beam with a power of 5 mW and ls632.5 nm, corresponding to a photon flux of 1.2=1017 cmy2 sy1. The Constant photocurrent method (CPM) was used to determine the density of deep defect states (Ns)
0040-6090/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2003.10.145
I. Ferreira et al. / Thin Solid Films 451 – 452 (2004) 366–369
Fig. 1. Influence of hydrogen gas flow on ASiOyASiH. The inset is a SEM image of a film produced with f H2 s100 sccm.
and the absorption coefficient (a) in the visible range was calculated through the transmittance spectra obtained in a UV–VIS–NIR Shimadzu spectrophotometer. 3. Results and discussion To evaluate the influence of hydrogen dilution on the compactness and on the properties of the films, the ratio between the areas of the peaks associated to the wagging modes of SiOx and SiHx in the IR spectra (ASiOx yASiHx), located at approximately 1050–1200 cmy1 and 630 cmy1, respectively, was determined. The data are shown in Fig. 1. ASiOx yASiHx increases abruptly for fH2 above 25 sccm and tends to saturate for fH2 higher than 300 sccm, for a-Si:H films exposed more than 30 days to atmospheric conditions. The influence of the fH2 on room dark conductivity (sd) and on the hmt product is plotted in Fig. 2. The hmt product exhibits an exponential decay from 8=10y7 to 8=10y9 cm2 Vy1 and sd decreases about one decade when fH2 increases from 0 to 300 sccm. Since the hmt decreases and ASiOx yASiHx increases with the enhancement of the fH2the behaviour referred before is related to a post-oxidation phenomenon associated to films with a structure formed by agglomerates, as is observed in the SEM image shown in the inset of Fig. 1a). Although sd decreases with the enhancement of the fH2, a decrease in the defect density and, therefore an enhancement of the hmt product was expected. However, the opposite is observed. This means that the photogenerated carriers, for some reason, are trapped. Certainly, the oxidation of the film is one of the causes for this behaviour. Another possible reason could be an enhancement of the molecular hydrogen incorporated into the film, in the form of clusters, since the hydrogen bonded to silicon as SiH decreases as fH2 increases w5x.
367
In order to check this possibility Elastic Recoil Detection (ERD) was performed to determine the total hydrogen content of the film. For films produced without hydrogen dilution it was determined, from FTIR measurements that the concentration of hydrogen w6x bonded to silicon is approximately 4% and by ERD the hydrogen content is 8%. Therefore, this is an indication that with increasing fH2 the amount of hydrogen not bonded to Si can increase, enhancing the H2 in clusters, contributing to the trapping of photogenerated carriers. This could be due to the possible dissociation of H2 into two neutral interstitial H (1.74 eV) or into an Hq and H- pair (1.34 eV) w7x. The confirmation that hydrogen dilution enhances the porosity of the film and consequently enhances postoxidation was also obtained by spectroscopic ellipsometry, where the maximum of the pseudo-dielectric function N´2M as a function of photon energy (E) varies from 26, for films produced without hydrogen dilution, to 16 for fH2s300 sccm w6x. The previous results reveal that for the process conditions used (high pressure and high filament temperature), the hydrogen dilution does not contribute to the enhancement of the optoelectronic films properties. The films produced by HWPA-CVD with high hydrogen dilution show a significant post-deposition oxidation, although lower than those made by a-Si:H HW-CVD films (produced in the same conditions but without rf power) w6x. However, the obtained results are still not optimised as far as it concerns solar cells application. Thus, we have also studied the influence of the rf power on the films properties w6x, keeping the other deposition parameters constant. The obtained data indicate that the films produced at an rf power of 100 W, Tfs1950 8C and without hydrogen dilution have improved optoelectronic properties, sphs10y4 (V cm)y1, hmts 1.6=10y5 cm2 yV, Nss1016 cmy3 with Urbach energy (E0)s50 meV, hydrogen content of approximately 10% ˚ and a high growth rate f30 Ays. Even though the
Fig. 2. Influence of hydrogen gas flow on sd and on hmt product.
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I. Ferreira et al. / Thin Solid Films 451 – 452 (2004) 366–369
Fig. 3. Spectral variation of the absorption coefficient obtained by CPM measurements in HWPA-CVD films produced without hydrogen dilution, using Prfs100 W and Tfs1950 8C
optoelectronic properties are comparable to those obtained by other authors w8–11x, the growth rate of the films obtained by HWPA-CVD technique is at least one decade higher. The spectral variation of the absorption coefficient is plotted in Fig. 3, where a narrow Urbach edge and a low absorption of deep defect states are observed, in agreement with the other results obtained. Fig. 4 shows the SEM cross-section image of the corresponding film. A compact film with a smooth surface is observed. Fig. 5 shows the time dependence of sd and sph (with AM1.5 100 mWycm2 light intensity, continuously) for HWPA-CVD and PECVD polymorphous w12x films. The dark conductivity measurement was taken at atmospheric pressure under room temperature at dark conditions. The sph, at the beginning, is higher in PECVD films, but after 30 min of light exposure sph is lower
Fig. 5. Time dependence of sph obtained under AM1.5 100 mWycm2, for HWPA-CVD films produced with rf power of 50 W (a) and 100 W (b), without hydrogen dilution and a polymorphous film produced by PECVD w12x (c).
than that of HWPA-CVD films. However, HWPA-CVD films produced with an rf power of 50 W almost do not degrade but, sph is too low, which reflects the small post-oxidation observed, as shown in Fig. 1. This is related to a higher contribution of the HW-CVD component for the gas dissociation, leading to a much higher growth rate and consequently more defects in a-Si:H films w6x. Table 1 shows the influence of hydrogen dilution in the photodegradation and optoelectronic properties of HWPA-CVD films produced with rf power of 50 W under light illumination of 50 mWycm2. The data indicate that hydrogen dilution has no influence on the photosensitivity (sph y sd), approximately 1.7=103 for all the samples analysed. Nevertheless, the photodegradation (Dsph y sph) is significantly enhanced, from 0.09 to 0.76, as fH2 increases from 0 to 300 sccm. Again, the enhancement of the H2 clusters and the postoxidation due to films porosity are the main causes of this SW effect. 4. Conclusions The results obtained show the possibility to produce stable intrinsic a-Si:H films with improved electronic Table 1 Influence of hydrogen dilution in the photodegradation (AM1.5 spectra with light intensity of 50 mWycm2) of films produced with rf power of 50 W and Tfs1950 8C
Fig. 4. SEM cross-section image of the sample shown in Fig. 3.
dil. H2 (sccm)
Dsphysph
sph(V cm)y1
sd(V cm)y1
0 100 300
0.09 0.26 0.76
8.7=10y7 4.5=10y7 5.8=10y7
5.3=10y10 2.6=10y10 3.3=10y10
I. Ferreira et al. / Thin Solid Films 451 – 452 (2004) 366–369
properties and growth rate above 3 nmys by HWPACVD. Besides that the hydrogen dilution does not contribute to enhance the films properties. This fact, besides being related to atomic hydrogen, can also be attributed to molecular hydrogen, which is probably incorporated into the film, giving rise to clusters and concomitant porosity. Acknowledgments The authors would like to thank to Augusto Lopes from Aveiro University for performing SEM analysis and to Ana Rita Ramos for performing the EDR analysis at ITN Institut. This work was supported by Fundacao ¸˜ ˆ da Ciencia e Tecnologia through ’Financiamento Plurianuais’ of CENIMAT, and by the projects PRAXISy PyCTMy12094y1998 and H-Alpha Solar contract no. ERK and CT-1999-00004.
369
References w1x D.L. Staebler, C.R. Wronski, Appl. Phys. Lett. 31 (1977) 292. w2x K. Tanaka, E. Maruyama, T. Shimada, H. Okamoto, Amorphous Silicon, John Wiley and Sons, Inc, 1999. w3x H.M. Branz, Phys. Rev. B 59 (1999) 5498. w4x I. Ferreira, M.E.V. Costa, L. Pereira, E. Fortunato, R. Martins, A.R. Ramos, M.F. Silva, Appl. Surf. Sci. 184 (2001) 60. w5x I. Ferreira, P. Vilarinho, E. Fortunato, R. Martins, J. Appl. Phys. 91 (3) (2002) 1644. w6x I. Ferreira, Ph.D. Thesis, FCT-UNL, 2002. w7x C.G. Van de Walle, B. Tuttle, Mater. Res. Soc. Symp. Proc. 557 (1999) 275. w8x M. Konagai, T. Tsushima, M-K. Kim, K. Asakawa, A. Yamada, Y. Kudriasvtsev, A. Villegas, R. Asomoza, Thin Solid Films 395 (2001) 252. w9x A.H. Mahan, J. Canapella, B.P. Nelson, R.S. Crandall, J. Appl. Phys. 69 (9) (1991) 6728. w10x S.R. Jadkar, J.V. Sali, M.G. Tatwale, Sol. Energ. Mat. Sol. C. 71 (2002) 153. w11x R.E.I. Schropp, Thin Solid Films 403–404 (2002) 17. ´ w12x H. Aguas, E. Fortunato, V. Silva, L. Pereira, R. Martins, Thin Solid Films 403–404 (2002) 26.