- Email: [email protected]
Investigation of surface roughness on etched glass surfaces
Thin Solid Films 519 (2011) 2903–2906
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Investigation of surface roughness on etched glass surfaces Z. Pápa a, J. Budai a,⁎, B. Farkas a, Z. Toth b a b
University of Szeged, Department of Optics and Quantum Electronics, H-6701 Szeged, P.O. Box 406, Hungary Research Group on Laser Physics of the Hungarian Academy of Sciences, H-6701 Szeged, P.O. Box 406, Hungary
a r t i c l e
i n f o
Keywords: Texturing Surface morphology Optical properties Ellipsometry Graded refractive index
a b s t r a c t Roughening the surface of solar cells is a common practice within the photovoltaic industry as it reduces reflectance, and thus enhances the performance of devices. In this work the relationship between reflectance characterized by the haze parameter, surface roughness and optical properties was investigated. To achieve this goal, model samples were prepared by hydrofluoric acid etching of glass for various times and measured by optical microscopy, spectroscopic ellipsometry, scanning electron microscopy, and atomic force microscopy. Our investigation showed that the surface reflectance was decreased not only by the roughening of the surface but also by the modification of the depth profile and lowering of the refractive index of the surface domain of the samples. © 2011 Elsevier B.V. All rights reserved.
1. Introduction One of the emerging applications of textured scattering layers is to reduce reflection losses within thin film solar cells. These textured layers are thought to provide an enhancement of light trapping inside the solar cell structure by increasing light scattering and path length [1,2]. However, it was recently shown that the improvements in quantum efficiency and short circuit current of the solar cell solely originate from the decrease of total reflectance (antireflective effect) and not from the enhancement of light scattering [3]. Direct surface roughness and more importantly, indirect haze and reflectance measurements are routinely used to characterize textured surfaces and, up to our knowledge, there are no reports within the literature of studies comparing these two methods. One of the aims of this work was to investigate the relationship between haze, reflectance and surface roughness. As the reflectance depends on the complex refractive index, this work also aims to study the change in the optical properties arising from the structural modification of the surface. To achieve these goals, model samples were prepared by hydrofluoric acid (HF) etching of glass for various times and measured by optical microscopy, spectroscopic ellipsometry (SE), scanning electron microscopy (SEM), atomic force microscopy (AFM) and goniophotometry methods.
2. Experimental Glass pieces (10 cm × 10 cm, 5 mm thickness) were placed over a Teflon beaker (diameter ~ 7 cm) containing HF (50 ml, 38%) and were ⁎ Corresponding author. Tel.: + 36 62 544 653; fax: + 36 62 544 658. E-mail address: [email protected] (J. Budai). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.12.236
etched by HF vapor for various times (80 s–1160 s). After etching, the glass pieces were dried in a fume hood and were measured by spectroscopic ellipsometry (Woollam M-2000F rotating compensator spectroscopic ellipsometer). The Ψ and Δ spectra were recorded in the 400–1000 nm wavelength range at 384 photon energies at 70° angle of incidence. Reflection from the backside of the samples was suppressed by the iris of the ellipsometer, since the thickness of the glass (~5 mm) was high enough to clearly separate the reflected spots from the upper and the lower surfaces of the glass. The details of the applied ellipsometric models will be described in the results section. The quality of the fittings, i.e. the difference between the fitted and measured curves was characterized with the error weighted Root Mean Squared Error (MSE) [4]. The surface topography of the samples was investigated with a PSIA XE-100 atomic force microscope in dynamic, non-contact mode over 2 μm × 2 μm and 10 μm × 10 μm scanning areas and the average roughness was determined by manufacturer's (XEI) software. The surface morphology and the cross section of the samples were examined with a Hitachi S-4700 high resolution cold cathode field emission scanning electron microscope. The light scattering ability of surfaces was characterized with goniophotometric measurements (Sopralab GES5E instrument, P polarized light, 45° angle of incidence) by mapping the intensity of the direct beam and scattered light in 0.5° steps. The haze parameter, defined here as the percentage of reflected light that deviates from the specularly reflected beam through scattering more than 2.5°, was calculated from the measured intensity distributions [5–7]. 3. Results and discussion The HF vapor corroded the surface, causing opalescent look of the etched samples. In addition to this, the application of longer etching times resulted in the appearance of a pale circular colored ring
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structure indicating an interference effect. The first optical microscopic examinations revealed that with increasing etching time rougher structures have been established at the interaction surface.
3.1. Spectroscopic ellipsometry Fig. 1a–c shows Ψ curves of untreated (reference) and etched glasses, respectively. The interference related oscillations observed in the case of the etched samples indicate the presence of at least one layer, of which the refractive index differs from that of the glass plates. To evaluate the measurements first the data corresponding to the reference glass were analyzed. For this Cauchy's dispersion relation extended with the exponential Urbach absorption tail was used [8]. The resulting optical properties were later used as the optical data of the substrate. To fit the data collected on etched surfaces first a one-layer approximation was applied, using again Cauchy's dispersion relation extended with the Urbach tail. This ensured relatively good fits only for short etching times (b400 s) and indicated that the refractive index of the top layer is smaller than the original glass. Therefore the model was replaced to Bruggeman Effective Medium Approximation (EMA) assuming the mixture of the original glass and void [9,10]. Again the fittings were relatively good for short etching times, but the model failed to describe the data belonging to longer etching times. To improve the quality of the fittings a gradient in the void content of the films was introduced. This allowed for describing the ellipsometric curves of the samples etched for shorter etching times (Fig. 1b), but at longer etching times further modification of the model was needed. It was necessary to change the optical properties of the glass used in the graded EMA model. To model the refractive index of this “pseudo” glass the Kramers–Kronig consistent Gaussian oscillator was used [11]. This model ensured finally good fits also for the samples etched for longer etching times (Fig. 1c) with an average MSE value of ~ 38.5. This separation in the sample series originates from the characteristics of EMA, which cannot result in higher extinction coefficients than the glass substrate if the void content is varied in the 0–100% range. The extinction coefficient of the reference glass at 632 nm is 0.012, below 400 s etching times the average extinction of the gradient layer is below 0.012, while longer etching times result in higher extinction coefficients (N0.03), which cannot be achieved with the simple mixture of void and glass.
Fig. 1. Ψ curves (grey squares) measured at 70 angle of incidence as a function of wavelength. Fitted curves are presented as well with black curves. Note the development of the interference structure in the spectra with etching time, which is indicating the formation of a film on the glass.
We have to note that even though the least number of fitting parameters were used in the case of such a complex model structure the parameters were still correlated. According to our observations similar fittings could be achieved with rather different Gaussian oscillator parameters, i.e. with a different refractive index for the pseudo glass. Certainly this caused a rather high fluctuation in the void contents, but the resulting effective refractive indices and effective refractive index depth profiles were similar. For example, in the case of the sample etched for 679 s similar fittings could be achieved with models differing in the effective refractive index less than 0.009 at 632 nm wavelength, while the pseudo glass' refractive index showed a 1.2 difference at this wavelength, accompanied by a 20% maximal difference in void contents. Therefore, instead of using the void distributions, refractive index depth profiles are presented in Fig. 2. The refractive index depth profiles differ from sample to sample but trends can be observed as follows. For low etching times the refractive indices increase from the bottom to the top of the layer. As medium etching times are reached this refractive index increase is confined to the ~ 70% of the layer, and at the top the refractive indices decrease.
3.2. Scanning electron microscopy To check these surprising refractive index profiles high resolution scanning electron microscope images were recorded from the cross section of the modified surfaces. Fig. 3 shows the cross section SEM images, taken from the broken sides of the samples. It can be clearly seen, that etching caused not only the development of a surface roughness, but also the appearance of a layer, which was indicated by the interference ring structures and by SE measurements. Larger magnification images showed that the layer consists of grains and the larger grains have a porous structure. Surprisingly, for longer etching times, cavities form at the glass–layer interface. Some of the layers leave the glass surface, leading to the peeling off of the layer in some cases. The morphology of buried cavities can be seen in SEM images taken from the peeled off regions. It looks like an engraved surface where holes are overlapping each other. The sizes of the holes are increasing with longer etching times. These observations confirm the refractive index depth profiles determined by SE, since the films close to the glass–layer interface contain more void, resulting in lower refractive indices. SEM allowed estimating the thickness of these layers. The results of the estimations are plotted in Fig. 4, along with thicknesses determined from SE measurements. The thicknesses arising from the two methods are in good accordance and increase almost linearly with etching time. This shows that longer exposures to HF vapor caused the degradation of the glass surface in larger depths.
Fig. 2. Refractive index depth profile of samples etched for different times. Refractive index depth profiles increase from the bottom to the top of the layer, which is followed by refractive index decrease in the case of the samples etched for longer times.
Z. Pápa et al. / Thin Solid Films 519 (2011) 2903–2906
Fig. 3. Cross sectional SEM images of samples etched for a) 404 s, b) 571 s and c) 960 s. Etching caused not only the development of a surface roughness, but also the appearance of a layer.
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Fig. 5. AFM images of samples etched for a) 120 s and b) 404 s. For shorter etching times a small lateral sized structure (below a few micrometers) appears on the surface, while longer etching times resulted in the formation of structures having larger characteristic lengths of few micrometers.
AFM images were recorded from the HF vapor exposed surfaces. For shorter etching times a small lateral sized structure (below a few micrometers) appears on the surface, while longer etching times resulted in the formation of structures having larger characteristic lengths of few micrometers. Fig. 5 shows the AFM images recorded
from the surface of two samples (different etching times). The localized structures' roughness increased from 10 nm to 70 nm with the etching time whilst the surface roughness of the larger structures increased with increasing etching times to the order of 100 nm. Plotting the surface roughness (Ra) determined from AFM measurements (Fig. 6) revealed a correlation between the Ra parameters and layer thicknesses. The Ra parameters increase almost linearly with
Fig. 4. Film thicknesses measured with ellipsometry (filled squares) and electron microscope (open diamonds) as a function of etching time. Note that the values arising from the two methods are in good correlation and increase almost linearly with etching time.
Fig. 6. Surface roughness measured with an atomic force microscope as a function of film thickness measured with ellipsometry. Note the correlation between the Ra parameters and the layer thicknesses: after a linear increase (below 1 micrometer) the roughness values show saturation.
3.3. Atomic force microscopy
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the consequence of both the increased scattering indicated by the increased FWHM parameters due to the increased surface roughness and the decreased refractive index difference between the ambient and the top surface of the etched glass plates. Since etching resulted in the appearance of a surface structure that behaves like a layer, and it has a higher refractive index than the ambient above but smaller than the bulk material below the reflectance of this system can be smaller than the reflectance of the original glass plates. Furthermore the gradient in the refractive index also leads to the decrease of the light reflection from the surfaces. 4. Conclusions
Fig. 7. a) Maximum of reflected intensity, b) full width at half maximum and c) haze parameter as a function of film thickness determined with ellipsometry. The development of the surface layer and roughness caused a significant decrease in the maximal intensity of reflection, and a slight increase in the FWHM. These two effects resulted in an increase in the haze parameter.
film thickness below 1 μm, while above this the roughness values show saturation.
In this study structured glass surfaces were produced by chemical etching. Relationship was found between etching times, surface roughness and haze values. At the surface deeper and rougher structures have been established as etching time was increased, resulting in an opalescent look of the samples. Ellipsometry revealed the appearance of a new thin layer, whose thickness increased with increasing etching time and which had a lower refractive index than the original glass. Correlation between refractive index depth distribution and cross sectional SEM images was found. Goniophotometry measurements showed that increasing etching time leads to wider angle scattering of reflected light followed by a significant decrease in specular reflection leading to an increase in haze parameter. Our observations show that changes in the haze parameter are not only caused by the increased surface roughness but they can also be attributed to the changes in the refractive index values and profiles of the forming films. Acknowledgements
3.4. Goniophotometry Goniophotometric measurements were performed to determine the angular distribution of the reflected light. In all cases the intensity of the light scattered from the surface of the specimen followed a maximum curve as a function of the scattering angle. These curves were characterized with their maximal value and full width at half maximum value (FWHM). Etching of the surface, i.e. the development of a surface layer and roughness changed significantly the reflectance properties of the glass plates. Since the layer thickness seems to be linearly proportional to etching time, the photometric parameters are plotted as a function of the film thickness in Fig. 7. The development of the surface layer and roughness caused a significant decrease in the maximal intensity of reflection (Fig. 7a), and a slight increase in the FWHM (Fig. 7b). These two effects resulted in an increase in the haze parameter: from 0.5% to 5% (Fig. 7c). The observation that the surface roughness values saturate with increasing thickness values, while the goniophotometric data show linear relationship with film thickness shows that scattering properties are not only determined by the surface roughness. The decrease in the maximal reflected intensity is
This work was supported by the Hungarian National Office for Research and Technology (NKTH) as part of the PVMET_08 project and by NKTH and OTKA foundation from the Research and Technology Innovation Fund (project number: CNK 78549). References [1] J. Müller, B. Rech, J. Springer, M. Vanecek, Sol. Energy 77 (2004) 917. [2] J. Springer, B. Rech, W. Reetz, J. Müller, M. Vanecek, Sol. Energ. Mat. Sol. C 85 (2005) 1. [3] A. Campa, J. Krc, J. Malmström, M. Edoff, F. Smole, M. Topic, Thin Solid Films 515 (2007) 5968. [4] J.A. Woollam, B. Johs, C.M. Herzinger, J. Hilfiker, R. Synowicki, C.L. Bungay, Proc. SPIE CR72 (1999) 3. [5] J. Krč, M. Zeman, F. Smole, M. Topič, J. Appl. Phys. 92 (2002) 749. [6] M. Zeman, R.A.C.M.M. van Swaaij, J.W. Metselaar, J. Appl. Phys. 88 (2000) 6436. [7] ASTM D 1003-00, Standard Test Methods for Haze and Luminous Transmittance of Transparent Plastics, Annual Book of ASTM Standards, Vol 08.01 (2000). [8] A.S. Ferlauto, G.M. Ferreira, J.M. Pearce, C.R. Wronski, R.W. Collins, J. Appl. Phys. 92 (2002) 2424. [9] D.E. Aspnes, Thin Solid Films 89 (1982) 249. [10] D.A.G. Bruggeman, Ann. Phys. - Leipzig 24 (1935) 636. [11] D. De Sousa Meneses, M. Malki, P. Echegut, J. Non-Cryst. Solids 352 (2006) 769.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Investigation of surface roughness on etched glass surfaces Z. Pápa a, J. Budai a,⁎, B. Farkas a, Z. Toth b a b
University of Szeged, Department of Optics and Quantum Electronics, H-6701 Szeged, P.O. Box 406, Hungary Research Group on Laser Physics of the Hungarian Academy of Sciences, H-6701 Szeged, P.O. Box 406, Hungary
a r t i c l e
i n f o
Keywords: Texturing Surface morphology Optical properties Ellipsometry Graded refractive index
a b s t r a c t Roughening the surface of solar cells is a common practice within the photovoltaic industry as it reduces reflectance, and thus enhances the performance of devices. In this work the relationship between reflectance characterized by the haze parameter, surface roughness and optical properties was investigated. To achieve this goal, model samples were prepared by hydrofluoric acid etching of glass for various times and measured by optical microscopy, spectroscopic ellipsometry, scanning electron microscopy, and atomic force microscopy. Our investigation showed that the surface reflectance was decreased not only by the roughening of the surface but also by the modification of the depth profile and lowering of the refractive index of the surface domain of the samples. © 2011 Elsevier B.V. All rights reserved.
1. Introduction One of the emerging applications of textured scattering layers is to reduce reflection losses within thin film solar cells. These textured layers are thought to provide an enhancement of light trapping inside the solar cell structure by increasing light scattering and path length [1,2]. However, it was recently shown that the improvements in quantum efficiency and short circuit current of the solar cell solely originate from the decrease of total reflectance (antireflective effect) and not from the enhancement of light scattering [3]. Direct surface roughness and more importantly, indirect haze and reflectance measurements are routinely used to characterize textured surfaces and, up to our knowledge, there are no reports within the literature of studies comparing these two methods. One of the aims of this work was to investigate the relationship between haze, reflectance and surface roughness. As the reflectance depends on the complex refractive index, this work also aims to study the change in the optical properties arising from the structural modification of the surface. To achieve these goals, model samples were prepared by hydrofluoric acid (HF) etching of glass for various times and measured by optical microscopy, spectroscopic ellipsometry (SE), scanning electron microscopy (SEM), atomic force microscopy (AFM) and goniophotometry methods.
2. Experimental Glass pieces (10 cm × 10 cm, 5 mm thickness) were placed over a Teflon beaker (diameter ~ 7 cm) containing HF (50 ml, 38%) and were ⁎ Corresponding author. Tel.: + 36 62 544 653; fax: + 36 62 544 658. E-mail address: [email protected] (J. Budai). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.12.236
etched by HF vapor for various times (80 s–1160 s). After etching, the glass pieces were dried in a fume hood and were measured by spectroscopic ellipsometry (Woollam M-2000F rotating compensator spectroscopic ellipsometer). The Ψ and Δ spectra were recorded in the 400–1000 nm wavelength range at 384 photon energies at 70° angle of incidence. Reflection from the backside of the samples was suppressed by the iris of the ellipsometer, since the thickness of the glass (~5 mm) was high enough to clearly separate the reflected spots from the upper and the lower surfaces of the glass. The details of the applied ellipsometric models will be described in the results section. The quality of the fittings, i.e. the difference between the fitted and measured curves was characterized with the error weighted Root Mean Squared Error (MSE) [4]. The surface topography of the samples was investigated with a PSIA XE-100 atomic force microscope in dynamic, non-contact mode over 2 μm × 2 μm and 10 μm × 10 μm scanning areas and the average roughness was determined by manufacturer's (XEI) software. The surface morphology and the cross section of the samples were examined with a Hitachi S-4700 high resolution cold cathode field emission scanning electron microscope. The light scattering ability of surfaces was characterized with goniophotometric measurements (Sopralab GES5E instrument, P polarized light, 45° angle of incidence) by mapping the intensity of the direct beam and scattered light in 0.5° steps. The haze parameter, defined here as the percentage of reflected light that deviates from the specularly reflected beam through scattering more than 2.5°, was calculated from the measured intensity distributions [5–7]. 3. Results and discussion The HF vapor corroded the surface, causing opalescent look of the etched samples. In addition to this, the application of longer etching times resulted in the appearance of a pale circular colored ring
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structure indicating an interference effect. The first optical microscopic examinations revealed that with increasing etching time rougher structures have been established at the interaction surface.
3.1. Spectroscopic ellipsometry Fig. 1a–c shows Ψ curves of untreated (reference) and etched glasses, respectively. The interference related oscillations observed in the case of the etched samples indicate the presence of at least one layer, of which the refractive index differs from that of the glass plates. To evaluate the measurements first the data corresponding to the reference glass were analyzed. For this Cauchy's dispersion relation extended with the exponential Urbach absorption tail was used [8]. The resulting optical properties were later used as the optical data of the substrate. To fit the data collected on etched surfaces first a one-layer approximation was applied, using again Cauchy's dispersion relation extended with the Urbach tail. This ensured relatively good fits only for short etching times (b400 s) and indicated that the refractive index of the top layer is smaller than the original glass. Therefore the model was replaced to Bruggeman Effective Medium Approximation (EMA) assuming the mixture of the original glass and void [9,10]. Again the fittings were relatively good for short etching times, but the model failed to describe the data belonging to longer etching times. To improve the quality of the fittings a gradient in the void content of the films was introduced. This allowed for describing the ellipsometric curves of the samples etched for shorter etching times (Fig. 1b), but at longer etching times further modification of the model was needed. It was necessary to change the optical properties of the glass used in the graded EMA model. To model the refractive index of this “pseudo” glass the Kramers–Kronig consistent Gaussian oscillator was used [11]. This model ensured finally good fits also for the samples etched for longer etching times (Fig. 1c) with an average MSE value of ~ 38.5. This separation in the sample series originates from the characteristics of EMA, which cannot result in higher extinction coefficients than the glass substrate if the void content is varied in the 0–100% range. The extinction coefficient of the reference glass at 632 nm is 0.012, below 400 s etching times the average extinction of the gradient layer is below 0.012, while longer etching times result in higher extinction coefficients (N0.03), which cannot be achieved with the simple mixture of void and glass.
Fig. 1. Ψ curves (grey squares) measured at 70 angle of incidence as a function of wavelength. Fitted curves are presented as well with black curves. Note the development of the interference structure in the spectra with etching time, which is indicating the formation of a film on the glass.
We have to note that even though the least number of fitting parameters were used in the case of such a complex model structure the parameters were still correlated. According to our observations similar fittings could be achieved with rather different Gaussian oscillator parameters, i.e. with a different refractive index for the pseudo glass. Certainly this caused a rather high fluctuation in the void contents, but the resulting effective refractive indices and effective refractive index depth profiles were similar. For example, in the case of the sample etched for 679 s similar fittings could be achieved with models differing in the effective refractive index less than 0.009 at 632 nm wavelength, while the pseudo glass' refractive index showed a 1.2 difference at this wavelength, accompanied by a 20% maximal difference in void contents. Therefore, instead of using the void distributions, refractive index depth profiles are presented in Fig. 2. The refractive index depth profiles differ from sample to sample but trends can be observed as follows. For low etching times the refractive indices increase from the bottom to the top of the layer. As medium etching times are reached this refractive index increase is confined to the ~ 70% of the layer, and at the top the refractive indices decrease.
3.2. Scanning electron microscopy To check these surprising refractive index profiles high resolution scanning electron microscope images were recorded from the cross section of the modified surfaces. Fig. 3 shows the cross section SEM images, taken from the broken sides of the samples. It can be clearly seen, that etching caused not only the development of a surface roughness, but also the appearance of a layer, which was indicated by the interference ring structures and by SE measurements. Larger magnification images showed that the layer consists of grains and the larger grains have a porous structure. Surprisingly, for longer etching times, cavities form at the glass–layer interface. Some of the layers leave the glass surface, leading to the peeling off of the layer in some cases. The morphology of buried cavities can be seen in SEM images taken from the peeled off regions. It looks like an engraved surface where holes are overlapping each other. The sizes of the holes are increasing with longer etching times. These observations confirm the refractive index depth profiles determined by SE, since the films close to the glass–layer interface contain more void, resulting in lower refractive indices. SEM allowed estimating the thickness of these layers. The results of the estimations are plotted in Fig. 4, along with thicknesses determined from SE measurements. The thicknesses arising from the two methods are in good accordance and increase almost linearly with etching time. This shows that longer exposures to HF vapor caused the degradation of the glass surface in larger depths.
Fig. 2. Refractive index depth profile of samples etched for different times. Refractive index depth profiles increase from the bottom to the top of the layer, which is followed by refractive index decrease in the case of the samples etched for longer times.
Z. Pápa et al. / Thin Solid Films 519 (2011) 2903–2906
Fig. 3. Cross sectional SEM images of samples etched for a) 404 s, b) 571 s and c) 960 s. Etching caused not only the development of a surface roughness, but also the appearance of a layer.
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Fig. 5. AFM images of samples etched for a) 120 s and b) 404 s. For shorter etching times a small lateral sized structure (below a few micrometers) appears on the surface, while longer etching times resulted in the formation of structures having larger characteristic lengths of few micrometers.
AFM images were recorded from the HF vapor exposed surfaces. For shorter etching times a small lateral sized structure (below a few micrometers) appears on the surface, while longer etching times resulted in the formation of structures having larger characteristic lengths of few micrometers. Fig. 5 shows the AFM images recorded
from the surface of two samples (different etching times). The localized structures' roughness increased from 10 nm to 70 nm with the etching time whilst the surface roughness of the larger structures increased with increasing etching times to the order of 100 nm. Plotting the surface roughness (Ra) determined from AFM measurements (Fig. 6) revealed a correlation between the Ra parameters and layer thicknesses. The Ra parameters increase almost linearly with
Fig. 4. Film thicknesses measured with ellipsometry (filled squares) and electron microscope (open diamonds) as a function of etching time. Note that the values arising from the two methods are in good correlation and increase almost linearly with etching time.
Fig. 6. Surface roughness measured with an atomic force microscope as a function of film thickness measured with ellipsometry. Note the correlation between the Ra parameters and the layer thicknesses: after a linear increase (below 1 micrometer) the roughness values show saturation.
3.3. Atomic force microscopy
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the consequence of both the increased scattering indicated by the increased FWHM parameters due to the increased surface roughness and the decreased refractive index difference between the ambient and the top surface of the etched glass plates. Since etching resulted in the appearance of a surface structure that behaves like a layer, and it has a higher refractive index than the ambient above but smaller than the bulk material below the reflectance of this system can be smaller than the reflectance of the original glass plates. Furthermore the gradient in the refractive index also leads to the decrease of the light reflection from the surfaces. 4. Conclusions
Fig. 7. a) Maximum of reflected intensity, b) full width at half maximum and c) haze parameter as a function of film thickness determined with ellipsometry. The development of the surface layer and roughness caused a significant decrease in the maximal intensity of reflection, and a slight increase in the FWHM. These two effects resulted in an increase in the haze parameter.
film thickness below 1 μm, while above this the roughness values show saturation.
In this study structured glass surfaces were produced by chemical etching. Relationship was found between etching times, surface roughness and haze values. At the surface deeper and rougher structures have been established as etching time was increased, resulting in an opalescent look of the samples. Ellipsometry revealed the appearance of a new thin layer, whose thickness increased with increasing etching time and which had a lower refractive index than the original glass. Correlation between refractive index depth distribution and cross sectional SEM images was found. Goniophotometry measurements showed that increasing etching time leads to wider angle scattering of reflected light followed by a significant decrease in specular reflection leading to an increase in haze parameter. Our observations show that changes in the haze parameter are not only caused by the increased surface roughness but they can also be attributed to the changes in the refractive index values and profiles of the forming films. Acknowledgements
3.4. Goniophotometry Goniophotometric measurements were performed to determine the angular distribution of the reflected light. In all cases the intensity of the light scattered from the surface of the specimen followed a maximum curve as a function of the scattering angle. These curves were characterized with their maximal value and full width at half maximum value (FWHM). Etching of the surface, i.e. the development of a surface layer and roughness changed significantly the reflectance properties of the glass plates. Since the layer thickness seems to be linearly proportional to etching time, the photometric parameters are plotted as a function of the film thickness in Fig. 7. The development of the surface layer and roughness caused a significant decrease in the maximal intensity of reflection (Fig. 7a), and a slight increase in the FWHM (Fig. 7b). These two effects resulted in an increase in the haze parameter: from 0.5% to 5% (Fig. 7c). The observation that the surface roughness values saturate with increasing thickness values, while the goniophotometric data show linear relationship with film thickness shows that scattering properties are not only determined by the surface roughness. The decrease in the maximal reflected intensity is
This work was supported by the Hungarian National Office for Research and Technology (NKTH) as part of the PVMET_08 project and by NKTH and OTKA foundation from the Research and Technology Innovation Fund (project number: CNK 78549). References [1] J. Müller, B. Rech, J. Springer, M. Vanecek, Sol. Energy 77 (2004) 917. [2] J. Springer, B. Rech, W. Reetz, J. Müller, M. Vanecek, Sol. Energ. Mat. Sol. C 85 (2005) 1. [3] A. Campa, J. Krc, J. Malmström, M. Edoff, F. Smole, M. Topic, Thin Solid Films 515 (2007) 5968. [4] J.A. Woollam, B. Johs, C.M. Herzinger, J. Hilfiker, R. Synowicki, C.L. Bungay, Proc. SPIE CR72 (1999) 3. [5] J. Krč, M. Zeman, F. Smole, M. Topič, J. Appl. Phys. 92 (2002) 749. [6] M. Zeman, R.A.C.M.M. van Swaaij, J.W. Metselaar, J. Appl. Phys. 88 (2000) 6436. [7] ASTM D 1003-00, Standard Test Methods for Haze and Luminous Transmittance of Transparent Plastics, Annual Book of ASTM Standards, Vol 08.01 (2000). [8] A.S. Ferlauto, G.M. Ferreira, J.M. Pearce, C.R. Wronski, R.W. Collins, J. Appl. Phys. 92 (2002) 2424. [9] D.E. Aspnes, Thin Solid Films 89 (1982) 249. [10] D.A.G. Bruggeman, Ann. Phys. - Leipzig 24 (1935) 636. [11] D. De Sousa Meneses, M. Malki, P. Echegut, J. Non-Cryst. Solids 352 (2006) 769.