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Ti) as optical selective solar absorber coatings
Solar Energy Materials & Solar Cells 60 (2000) 295}307
Application of titanium containing amorphous hydrogenated carbon "lms (a-C : H/Ti) as optical selective solar absorber coatings Andreas SchuK ler!,*, JuK rgen Geng!, Peter Oelhafen!, Stefan Brunold", Paul Gantenbein", Ueli Frei" !Institut fu( r Physik der Universita( t Basel, Klingelbergstrasse 82, CH - 4056 Basel, Switzerland "Institut fu( r Solartechnik SPF, Hochschule Rapperswil HSR, Oberseestr. 10, CH - 8640 Rapperswil, Switzerland
Abstract A combined PVD/PECVD process for the vacuum deposition of titanium containing amorphous hydrogenated carbon "lms is described. Elemental compositions of the deposited "lms have been determined by in situ core level photoelectron spectroscopy (XPS). The long-term stability of the plasma process has been demonstrated. Target poisening has not been observed. We have fabricated optical selective surfaces by the deposition of a-C : H/Ti multilayers onto aluminum substrates. Eventhough we have not optimized layer thicknesses and stoichiometries so far, the experimental results are promising: solar absorptance a of 0.876 and S thermal emittance e 3 of 0.061 have been achieved yielding an optical selectivity 100 C s" : a /e 3 of 14.4. Accelerated aging tests of these coatings have demonstrated their aging S 100 C stability: the service lifetime is predicted to amount to more than 25 years. Raman spectroscopy has been used to monitor changes in the structure of the aged coatings. Degradation mechanisms are being discussed. ( 2000 Elsevier Science B.V. All rights reserved. Keywords: Solar collectors; Solar selective coatings; Flat-plate collector
1. Introduction For some years e!orts have been undertaken to develop vacuum-deposited optical selective coatings for #at plate solar collectors. The goal of these investigations is to provide alternatives to the commercially available coatings which are usually * Corresponding author. Tel.: 41-612673720; fax: 41-6126-73784. E-mail address: [email protected] (A. SchuK ler) 0927-0248/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 4 8 ( 9 9 ) 0 0 0 7 4 - 4
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electroplated. Promising results have been achieved with chromium containing amorphous hydrogenated carbon "lms (a-C : H/Cr) deposited in a RF glow discharge [1}4]. For environmental and physiological reasons the substitution of the chromium by titanium is desirable. Titanium containing amorphous hydrogenated carbon "lms (a-C : H/Ti) have already been the focus of considerable attention due to their application as hard and wear resistant protective coatings. Researchers have investigated the mechanical properties of these "lms in relation to the titanium content in various tribological studies [5}8]. The use of a-C : H/Ti "lms as biocompatible coatings for medical implants has been suggested by Francz et al. [9]. In this work we present the application of titanium containing amorphous hydrogenated carbon "lms (a-C : H/Ti) as optical selective absorber coatings for #at plate solar collectors. Regarding the microstructure of a-C : H/Ti "lms, X-ray di!raction experiments have given evidence for the presence of nanometer-sized TiC crystallites being embedded in an amorphous hydrocarbon matrix [10,11]. This structure model is supported by recent in situ photoelectron spectroscopy experiments [12]. Design of multilayered coatings for optical selective solar absorbers requires a detailed knowledge of the optical constants of the considered material. The optical properties of titanium containing amorphous hydrogenated carbon "lms in dependence on the TiC content have been investigated recently [13]: experimental results for the optical constants index of refraction n and extinction coe$cient k have been compared to the predictions of e!ective medium theories. It has been shown that the optical constants vary strongly in dependence on the titanium content. Large area deposition at low cost demands large dimensions of the deposition apparatus and high input power. Under these conditions technological reasons favour a process which is operating at a lower frequency compared to radio frequency (RF) at 13.56 MHz. Therefore, medium frequency (MF) technology using sine or bipolar pulsed power is advantageous. Target poisening has been observed for the RF deposition of gold containing amorphous hydrogenated carbon "lms [3]. For a large area deposition of coatings subjected to certain quality standard the long-term stability of the fabrication process is a prerequisite.
2. Experiment The apparatus for the deposition of the a-C : H/Ti coatings can be pumped down to a base pressure below 1*10~6 mbar using a conventional pumping system in combination with a liquid-nitrogen cooling trap (Fig. 1). A water-cooled magnetron which is capped by a titanium target is driven by radio frequency (RF) power at 13.56 MHz, by medium frequency (MF) sine power at 40 kHz or by bipolar pulsed power (50}250 kHz). During thin "lm deposition the substrate faces the target at a distance of several centimeters. Substrates are heated resistively and the substrate temperature ¹ is measured by thermocouples. A DC power supply is used to set a substrate S voltage ; to the sample holder. This power supply is shielded from unwanted AC S signals by a low pass "lter. Argon and methane are fed into the chamber via two mass
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Fig. 1. Experimental setup for the deposition of titanium containing amorphous hydrogenated carbon "lms (a-C : H/Ti).
#ow controllers. By throttling the pumping system a working pressure around 5*10~3 mbar is adjusted. Under these conditions a stable glow discharge is sustained. A Kaufman-type ion source enables a substrate pretreatment by Ar sputtering. In order to achieve reproducible and stable deposition conditions, the process runs always for some minutes with the chosen parameters (gas mixture, total pressure, input power) before coating the substrate. Single-layered titanium containing amorphous hydrogenated carbon "lms (aC : H/Ti) have been deposited on sputter cleaned copper, aluminum and silicon substrates. Elemental compositions at the surfaces of these "lms have been determined by in situ photoelectron spectroscopy. For this kind of investigations the high vacuum deposition chamber is connected to an UHV electron spectrometer. Samples can be transferred from one system to the other without breaking the vacuum. The electron spectrometer is equipped with a hemispherical analyzer (Leybold EA10/100) and an X-ray source for core level spectroscopy (XPS: Mg K excitation, hl"1253.6 eV). As a reference for the electron energy calibration a gold sample with the Au4f core level 7@2 signal at 83.8 eV binding energy is used. The element concentrations of the deposited "lms have been determined by integration over the C 1s, Ti 2p, O 1s and Ar 2p core level signals after subtracting a Shirley background (for a mathematical description of the Shirley background see Refs. [14,15]). From the photoionization cross sections given by Sco"eld [16] the atomic concentrations at the "lm surfaces have been calculated, taking into account the mean electron escape depths [17] and the transmission function of the spectrometer. Photoelectron spectroscopy measurements of a-C : H/Ti "lms with low titanium contents can severely be in#uenced by charging e!ects. In order to avoid the accumulation of positive charge at the sample surface it is necessary to perform the measurements on "lms with appropriate thicknesses [12]. Therefore, "lm thicknesses have been in the range from 15 to 40 nm as measured by means of a quartz crystal monitor.
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Trilayered absorber coatings have been deposited on sputter cleaned aluminum substrates. The roughness R of the used substrates has been evaluated using a pro! "lometer and appeared to be in the order of 60 nm. The trilayered coatings have been fabricated by subsequently depositing pure Ti, a-C : H/Ti and pure a-C : H. These samples have been subjected to accelerated aging treatments in air at elevated temperatures. Before the aging procedure and after each treatment the total re#ectance of the sample has been measured in the wavelength range from 280 nm to 20 lm. These measurements have been performed by means of a Bruker FTIR spectrometer equipped with an integrating sphere. The solar absorptance a (air mass 1.5) and the S thermal emittance at 1003C e 3 have been calculated from the total re#ectance at 100 C room temperature. Raman spectroscopy has been performed on the samples as fabricated and after subjection to the heat treatments. The Raman spectra have been acquired using an argon ion laser operating at a wavelength of 514.50 nm. The backscattered light has been recorded by means of a Peltier cooled photomultiplier in combination with a SPEX 14018 double monochromator. For the calibration of the monochromator we have used the O phonon of a silicon (1 0 0) wafer (Raman shift at a wavenumber of ! 520 cm~1 [18]).
3. Results The photoelectron spectroscopic measurements have shown that the deposition of titanium containing amorphous hydrogenated carbon "lms (a-C : H/Ti) has succeeded for all kinds of AC input power used: RF at 13.56 MHz, MF sine at 40 kHz and bipolar pulsed in the range from 50 to 250 kHz. The shapes and positions of the C1s and Ti2p core lines indicate that in all cases the titanium is bound as TiC being embedded in a hydrocarbon matrix. For a detailed photoelectron spectroscopic study of MF deposited a-C : H/Ti see Ref. [12]. It turned out that with our deposition process a broad concentration range is accessible: one can vary the ratio c " : at% Ti/at% C from 0 to 1 by tuning the T*@C Ar/CH mass #ow ratio. The appearance of oxygen contamination correlates with the 4 titanium content: the reactivity of the "lm surface is higher for the titanium rich "lms than for the carbon rich ones. Depending on the process gas mixture there is also Ar present up to 2 at%. The Ar concentration can be directly related to the gas composition during thin "lm deposition. We are not able to comment on the hydrogen content of our samples since photoelectron spectroscopy is not sensitive to this element. However, for deposition on substrates at ambient temperature we would expect a hydrogen content in the order of 30 to 40 at% for pure a-C : H [19] and a reduced one for Ti-containing "lms [20]. We believe that elevated substrate temperatures result in a reduced hydrogen content as well. For a long-term stability check the process had been running continuously (200 W, 40 kHz MF sine) using a argon/methane mass #ow ratio of 8. Samples have been prepared after 5, 30 and 60 min. The elemental compositions determined from core level spectroscopy remain rather constant (Fig. 2). Obviously, a poisening of the
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299
Fig. 2. Stability check of the plasma process: elemental compositions of a-C : H/Ti "lms as determined by photoelectron spectroscopy (XPS) vs. operating time of the deposition process.
titanium target by a carbon layer as it has been observed for gold targets under comparable conditions [3] has not occurred. In order to achieve optical selectivity for solar absorber coatings based on titanium containing amorphous hydrogenated carbon "lms one has to consider the optical properties of this material. In a previous study [13] the optical constants index of refraction n and extinction coe$cient k have been determined for a-C : H/Ti "lms with varying titanium content. From these data we have calculated the absorption coe$cient a which is a measure for the penetration depth of light. The results are displayed in Fig. 3. The curves are labelled with the ratio c determined by core level T*@C spectroscopy. In all cases the absorption coe$cient a is monotonically decreasing with increasing wavelength. One possibility to create an optical selective absorber surface is to select a material which is transparent in the infrared but absorbing in the solar region of the spectrum, and deposit this onto a metal which is a good infrared mirror, like copper or aluminum. Because of the low infrared absorption the a-C : H/Ti "lms with low titanium content seem to be most promising for this purpose. In order
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Fig. 3. Absorption coe$cient a of a-C : H/Ti "lms with varying titanium content: the curves are labelled with the concentration ratio c " : at% Ti/at% C. The displayed data are based on Ref. [13]. T*@C
to fabricate a multilayered solar selective absorber coating we have therefore used an a-C : H/Ti layer with c +0.1, overcoated with a pure a-C : H layer. To ensure good T*@C adhesion, titanium has been deposited previously onto the aluminum substrate (coverage corresponds to a Ti layer thickness of about 10 nm). During thin "lm deposition (200 W RF) the substrate was set to a voltage of !150 V and heated to 2753C. The optical performance of this multilayered coating is shown in Fig. 4. The solid black line represents the total re#ectance of the absorber coating as fabricated. The ideal re#ectance which is desired for a #at plate absorber coating would be a step pro"le with zero in the range of solar radiation (j)2.5 lm) and one in the range of black body radiation at 1003C (j'2.5 lm). The re#ectance of the multilayered coating exhibits an absorption edge roughly in the right place and is therefore a good approximation to the ideal pro"le. The curve exhibits two minima. Since their positions depend on the layer thicknesses we ascribe them to interference. These data
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Fig. 4. Hemispherical re#ectance of a-C : H/Ti "lms as received and after various aging steps. Aging results in a shift of the absorption edge and the interference minima towards lower wavelengths. The solar spectrum at air mass 1.5 (AM1.5) and the spectrum of black body thermal radiation at 1003C are added.
yield a solar absorptance a of 0.876 and a thermal emittance e 3 of 0.061 resulting S 100 C in an optical selectivity s " : a /e 3 of 14.4. S 100 C A service lifetime estimation of this absorber coating has been performed according to the suggestions which have been elaborated within the former Task X &Solar Materials Research and Development' of the International Energy Agency (IEA) Solar Hating and Cooling Programme [21,22]. An absorber coating is quali"ed if it ful"lls the performance criterion PC " : !*a #*e )5%, S T during a service lifetime of at least 25 years. Based on a time-transformation function and the standard heat loads in a collector, this quali"cation condition can be transformed into an acceptable failure time at elevated temperature. This acceptable failure time depends strongly on the activation energy of the dominating degradation mechanism. The lifetime estimation is based on the assumption of an Arrhenius law:
AA
BB
/ 1 1 t "t exp ! , 2 1 R ¹ ¹ 2 1 where t is the failure time at temperature ¹ , t the failure time at temperature ¹ , / the 1 1 2 2 activation energy of dominating aging mechanism, and R the universal gas constant.
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Table 1 : !*a #0.25**e 3 of Solar absorptance a , thermal emittance e 3 and performance criterion PC " S 100 C 4 100 C multilayered a-C : H/Ti absorber coatings before and after accelerated aging treatments Sample
Aging step
a (%) S
e 3 (%) 100 C
A
As received 25 h @ 2503C 50 h @ 2503C As received 70 h @ 2503C As received 150 h @ 2203C
87.6 86.7 83.8 87.0 78.6 87.2 85.7
6.1 6.2 5.8 6.3 5.7 6.4 5.9
B C
PC (%)
0.9 3.7 8.5 1.4
Fig. 5. Evolution of the performance criterion PC " : !*a#0.25*e during accelerated aging at 2203C and 2503C. The solid lines illustrate the interpolations between data points, the dashed ones the allowed maximum value for the performance criterion (5%), and the dotted ones the estimations made in the text.
For the accelerated aging treatments several identical samples have been tempered in air at 2503C and 2203C. The evolution of the re#ectance of the samples subjected to a temperature of 2503C is added to Fig. 4. The absorption edge and the interference minima shift to lower wavelengths. However, in the infrared region (j'4 lm) the
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303
Fig. 6. Acceptable least failure times for solar absorber coatings aged at 2203C and 2503C in dependence on the activation energy of the dominant aging mechanism. The data are taken from Ref. [21]. A coating with a lifetime of 55 h at 2503C must be able to stand 150 h of aging at 2203C.
changes are small. Table 1 gives a survey of the quantities a , e 3 and PC as S 100 C determined from three identical samples before and after the various aging steps. At a temperature of 2503C the coating fails after 55 h: at that time the performance criterion reaches 5% (for the interpolation see Fig. 5). According to the data given in Ref. [21] this coating is quali"ed if the activation energy of the dominant aging mechanism is higher than 72 kJ/mol (Fig. 6). One can check this by aging the coating at a di!erent temperature. In order to ful"ll the above-mentioned requirement the lifetime of the coating at 2203C must exceed 150 h (Fig. 6). Indeed after 150 h tempering at 2203C the performance criterion has reached only 1.4%. Therefore, the coating has passed the accelerated aging test. The activation energy of the aging mechanism which is relevant for the degradation of the optical properties can be estimated by an interpolation of the data shown in Fig. 5. The performance criterion of 1.4% that is reached after 150 h tempering at 2203C equates the value that corresponds to roughly (30$3) h annealing at 2503C (Fig. 5). Under the assumption of an Arrhenius law these data yield an activation energy of (116$8) kJ/mol.
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Fig. 7. Raman spectra of the coatings as fabricated and after various aging steps. The sample annealed for 150 h at 2203C exhibits a larger line shift towards higher wavenumbers that the one annealed for 50 h at 2503C and a smaller one than the sample annealed for 70 h at 2503C.
The use of Raman spectroscopy to monitor structural changes of pure a-C and a-C : H coatings is quite common. Gampp et al. have used it to study the aging behavior of a-C : H/Cr thin "lms [4]. Raman spectra of the samples as fabricated and after the heat treatments are displayed in Fig. 7. The Raman spectra have been found to be sensitive only to the a-C : H component and show two broad maxima: one is the so-called G-peak at about 1580 cm~1, which is assumed to stem from the sp2-bonded, graphite-like regions, and the other is the so-called &disorder allowed' D-peak at about 1380 cm~1, which has been observed in graphite samples with "nite grain size [23]. Tempering at 2503C results in a narrowing of the D- and G-peaks and in a shift of both peaks to higher wavenumbers. Additionally, the D-peak increases in intensity relative to the G-peak.
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These e!ects are typical indications for the growth of graphite-like domains in the carbon component [24]. Obviously, a partial crystallization of the amorphous hydrogenated carbon "lms has occurred. As it has been observed by FTIR spectroscopy, these changes are accompanied by a release of hydrogen [25]. The same evolution is observed for 2203C even though it takes place on a longer time scale. The sample annealed for 150 h at 2203C exhibits a larger line shift towards higher wavenumbers than the one annealed for 50 h at 2503C and a smaller one than the sample annealed for 70 h at 2503C. This is especially remarkable because after 150 h annealing at 2203C the optical properties have changed only to the extent observed after roughly (30$3) h at 2503C. Since comparable degrees of graphitization have been observed for two di!erent temperatures the activation energy for the graphitization process can be estimated applying the Arrhenius law again: the observations described above yield a lower bound of 54 kJ/mol and an upper bound of 79 kJ/mol.
4. Discussion The long-term stability of the deposition process could be demonstrated even without any feed back control. The good reproducibility is a basis for a high-quality standard. The presented solar absorber coating consists of three layers. Since layer compositions and thicknesses have not been optimized so far, we believe that one can fabricate a-C : H/Ti-based absorber coatings with even better optical performance. One can add more absorber layers easily or use gradient layers. The aging stability of a-C : H/Ti-based coatings on aluminum substrates has been shown to ful"ll the requirements of the accelerated aging test by tempering as described in Ref. [21]. Even though the multilayered a-C : H/Ti absorber coating has passed the accelerated aging test, some degradation of the optical properties has been observed. For a further improvement of the service lifetime of such coatings it is interesting to identify the dominant aging mechanism. It has been shown by Raman spectroscopy that graphitization of the a-C : H matrix takes place. The activation energy for this process has been estimated to be in the range from 54 to 79 kJ/mol. From the optical data we have determined the activation energy of the aging mechanism which is relevant for the degradation of the performance of the coating to be (116$8) kJ/mol. To illustrate the contradiction in another way: the degree of graphitization of the sample annealed for 150 h at 2203C is comparable to the one observed in the range from 50 to 70 h annealing at 2503C. However, after 150 at 2203C the performance of the coating is much better than after 50 h at 2503C. Therefore, we conclude that for the degradation of the optical performance the graphitization is not the dominant mechanism. Let us consider again the re#ectance spectra of Fig. 4. Earlier we have described that annealing in air results in a shift of the absorption edge and interference minima towards lower wavelengths. These changes can be explained by oxidation of the a-C : H matrix to the gaseous carbon oxides resulting in a decrease of "lm
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thicknesses. Oxidation of the hydrocarbon matrix has also been observed in accelerated aging tests of a-C : H/Cr coatings [4]. We expect that by an optimization of the deposition parameters substrate bias voltage and temperature, a further increase in aging stability can be achieved.
5. Conclusions Coatings based on titanium containing amorphous hydrogenated carbon "lms can be deposited by a combined PVD/PECVD process using various kinds of input power (RF 13.56 kHz, MF sine 40 kHz and bipolar pulsed 50}250 kHz). The stability of the plasma process results in a high reproducibility and forms therefore a good basis for a high-quality standard. Optical selectivity has been achieved for a-C : H/Ti based multilayered solar absorber coatings: a solar absorptance a of 0.876 and a thermal emittance e 3 of 0.061 S 100 C yield an optical selectivity s " : a /e 3 of 14.4. The service lifetime of these coatings S 100 C on aluminum substrates is predicted to amount to more than 25 years. Aging mechanisms are graphitization and oxidation of the a-C : H matrix, the latter one is most presumably the dominant one for the degradation of the optical performance.
Acknowledgements The authors wish to thank Roland Steiner for the technical support. The "nancial support of the Bundesamt fuK r Energiewirtschaft, Switzerland is gratefully acknowledged.
References [1] R. Gampp et al., in: V. Wittwer, C.G. Granquist, C.M. Lampert (Eds.), Optical Materials Technology for Energy E$ciency and Solar Energy Conversion XIII, Proceedings of SPIE, Vol. 2255, 1994, pp. 92}106. [2] P. Oelhafen, P. Gantenbein, R. Gampp, in: V. Wittwer, C.G. Granquist, C.M. Lampert, (Eds.), Optical Materials Technology for Energy E$ciency and Solar Energy Conversion XIII, Proceedings of SPIE, Vol. 2255, 1994, pp. 64}78. [3] R. Gampp, in: Fortschr.-Ber. VDI Reihe 5, Vol. 446, VDI Verlag, DuK sseldorf, 1996. [4] R. Gampp, P. Oelhafen, P. Gantenbein, S. Brunold, U. Frei, Sol. Energy Mater. and Sol. Cells 54 (1998) 369. [5] M. Wang, K. Schmidt, K. Reichelt, H. Dimigen, H. Hubsch, J. Mater. Res. 7 (3) (1992) 667. [6] D.P. Monaghan, D.G. Teer, P.A. Logan, I. Efeoglu, R.D. Arnell, Surf. Coatings Technol. 60 (1}3) (1993) 525. [7] Deng Jianguo, M. Braun, Diamond Related Mater. 4 (7) (1995) 936. [8] A. Hellmich, T. Jung, A. Kielhorn, M. Rissland, Surf. Coatings Technol. 98 (1}3) (1998) 1541. [9] G. Francz, A. SchroK der, R. Hauert, Surface analysis and bioreactions of Ti and V containing a-C : H, Surf. Interface Anal. 28 (1999) 3}7. [10] M.P. Delplanke, V. Vassileris, R. Winand, J. Vac. Sci. Technol. A 13 (1995) 1104. [11] W.J. Meng, T.C. Curtis, L.E. Rehn, P.M. Baldo, J. Appl. Phys. 83 (11) (1998) 6076.
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[12] A. SchuK ler, R. Gampp, P. Oelhafen, In situ photoelectron spectroscopy of titanium containing amorphous hydrogenated carbon "lms (a-C : H/Ti), Phys. Rev. B, in press. [13] A. SchuK ler, C. Ellenberger, P. Oelhafen, Optical properties of titanium containing amorphous hydrogenated carbon "lms (a-C : H/Ti), J. Applied Physics, submitted. [14] D.A. Shirley, Phys. Rev. B 5 (1972) 4709. [15] S. HuK fner, Photoelectron Spectroscopy: Principles and Applications, 2nd Edition, Springer, Berlin, 1996, p. 144. [16] J.H. Sco"eld, J. Electron Spectrosc. Related Phenom. 8 (1976) 129. [17] M.P. Seah, W.A. Dench, Surf. Interface Anal. 1 (1979) 2. [18] M. Yoshikawa, G. Katagiri, H. Ishida, A. Ishitani, T. Akematsu, Solid State Commun. 66 (1988) 1177. [19] M. Wittmer, D. Ugolini, J. Eitle, P. Oelhafen, Appl. Phys. 48 (1989) 559. [20] M. Fryda, Untersuchungen der strukturellen und mechanisch-tribologischen Eigenschaften metallhaltiger, amorpher Kohlenwassersto!schichten, VDI-Verlag GmbH, DuK sseldorf, 1993. [21] B. Carlsson, M. KoK hl, U. Frei, Accelerated life testing of solar energy materials, case study of some selective solar absorber coating materials for DHW systems, A Report of Task X Solar Materials Research and Development SP-Report 93:13, 1994. Available from Swedish National Testing and Research Institute, P.O. Box 857, S-50115 Boras, Sweden. [22] U. Frei et al., in: C.M. Lampert, S.K. Deb, C.G. Granquist (Eds.), Optical Materials Technology for Energy E$ciency and Solar Energy Conversion XIV, Proceedings of SPIE, Vol. 2531, 1995, p. 282. [23] R.J. Nemanich, S.A. Solin, Phys. Rev. B 20 (1979) 392. [24] R.O. Dillon, J.A. Woollam, V. Katkanat, Phys. Rev. B 29 (1984) 3482. [25] A. Reyes-Mena, J. Gonzalez-Hernandez, R. Asomoza, in: P. Koidl, P. Oelhafen (Eds.), Amorphous Hydrogenated Carbon Films, Les Editions de Physique, Paris, 1987, p. 229.
Application of titanium containing amorphous hydrogenated carbon "lms (a-C : H/Ti) as optical selective solar absorber coatings Andreas SchuK ler!,*, JuK rgen Geng!, Peter Oelhafen!, Stefan Brunold", Paul Gantenbein", Ueli Frei" !Institut fu( r Physik der Universita( t Basel, Klingelbergstrasse 82, CH - 4056 Basel, Switzerland "Institut fu( r Solartechnik SPF, Hochschule Rapperswil HSR, Oberseestr. 10, CH - 8640 Rapperswil, Switzerland
Abstract A combined PVD/PECVD process for the vacuum deposition of titanium containing amorphous hydrogenated carbon "lms is described. Elemental compositions of the deposited "lms have been determined by in situ core level photoelectron spectroscopy (XPS). The long-term stability of the plasma process has been demonstrated. Target poisening has not been observed. We have fabricated optical selective surfaces by the deposition of a-C : H/Ti multilayers onto aluminum substrates. Eventhough we have not optimized layer thicknesses and stoichiometries so far, the experimental results are promising: solar absorptance a of 0.876 and S thermal emittance e 3 of 0.061 have been achieved yielding an optical selectivity 100 C s" : a /e 3 of 14.4. Accelerated aging tests of these coatings have demonstrated their aging S 100 C stability: the service lifetime is predicted to amount to more than 25 years. Raman spectroscopy has been used to monitor changes in the structure of the aged coatings. Degradation mechanisms are being discussed. ( 2000 Elsevier Science B.V. All rights reserved. Keywords: Solar collectors; Solar selective coatings; Flat-plate collector
1. Introduction For some years e!orts have been undertaken to develop vacuum-deposited optical selective coatings for #at plate solar collectors. The goal of these investigations is to provide alternatives to the commercially available coatings which are usually * Corresponding author. Tel.: 41-612673720; fax: 41-6126-73784. E-mail address: [email protected] (A. SchuK ler) 0927-0248/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 4 8 ( 9 9 ) 0 0 0 7 4 - 4
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electroplated. Promising results have been achieved with chromium containing amorphous hydrogenated carbon "lms (a-C : H/Cr) deposited in a RF glow discharge [1}4]. For environmental and physiological reasons the substitution of the chromium by titanium is desirable. Titanium containing amorphous hydrogenated carbon "lms (a-C : H/Ti) have already been the focus of considerable attention due to their application as hard and wear resistant protective coatings. Researchers have investigated the mechanical properties of these "lms in relation to the titanium content in various tribological studies [5}8]. The use of a-C : H/Ti "lms as biocompatible coatings for medical implants has been suggested by Francz et al. [9]. In this work we present the application of titanium containing amorphous hydrogenated carbon "lms (a-C : H/Ti) as optical selective absorber coatings for #at plate solar collectors. Regarding the microstructure of a-C : H/Ti "lms, X-ray di!raction experiments have given evidence for the presence of nanometer-sized TiC crystallites being embedded in an amorphous hydrocarbon matrix [10,11]. This structure model is supported by recent in situ photoelectron spectroscopy experiments [12]. Design of multilayered coatings for optical selective solar absorbers requires a detailed knowledge of the optical constants of the considered material. The optical properties of titanium containing amorphous hydrogenated carbon "lms in dependence on the TiC content have been investigated recently [13]: experimental results for the optical constants index of refraction n and extinction coe$cient k have been compared to the predictions of e!ective medium theories. It has been shown that the optical constants vary strongly in dependence on the titanium content. Large area deposition at low cost demands large dimensions of the deposition apparatus and high input power. Under these conditions technological reasons favour a process which is operating at a lower frequency compared to radio frequency (RF) at 13.56 MHz. Therefore, medium frequency (MF) technology using sine or bipolar pulsed power is advantageous. Target poisening has been observed for the RF deposition of gold containing amorphous hydrogenated carbon "lms [3]. For a large area deposition of coatings subjected to certain quality standard the long-term stability of the fabrication process is a prerequisite.
2. Experiment The apparatus for the deposition of the a-C : H/Ti coatings can be pumped down to a base pressure below 1*10~6 mbar using a conventional pumping system in combination with a liquid-nitrogen cooling trap (Fig. 1). A water-cooled magnetron which is capped by a titanium target is driven by radio frequency (RF) power at 13.56 MHz, by medium frequency (MF) sine power at 40 kHz or by bipolar pulsed power (50}250 kHz). During thin "lm deposition the substrate faces the target at a distance of several centimeters. Substrates are heated resistively and the substrate temperature ¹ is measured by thermocouples. A DC power supply is used to set a substrate S voltage ; to the sample holder. This power supply is shielded from unwanted AC S signals by a low pass "lter. Argon and methane are fed into the chamber via two mass
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297
Fig. 1. Experimental setup for the deposition of titanium containing amorphous hydrogenated carbon "lms (a-C : H/Ti).
#ow controllers. By throttling the pumping system a working pressure around 5*10~3 mbar is adjusted. Under these conditions a stable glow discharge is sustained. A Kaufman-type ion source enables a substrate pretreatment by Ar sputtering. In order to achieve reproducible and stable deposition conditions, the process runs always for some minutes with the chosen parameters (gas mixture, total pressure, input power) before coating the substrate. Single-layered titanium containing amorphous hydrogenated carbon "lms (aC : H/Ti) have been deposited on sputter cleaned copper, aluminum and silicon substrates. Elemental compositions at the surfaces of these "lms have been determined by in situ photoelectron spectroscopy. For this kind of investigations the high vacuum deposition chamber is connected to an UHV electron spectrometer. Samples can be transferred from one system to the other without breaking the vacuum. The electron spectrometer is equipped with a hemispherical analyzer (Leybold EA10/100) and an X-ray source for core level spectroscopy (XPS: Mg K excitation, hl"1253.6 eV). As a reference for the electron energy calibration a gold sample with the Au4f core level 7@2 signal at 83.8 eV binding energy is used. The element concentrations of the deposited "lms have been determined by integration over the C 1s, Ti 2p, O 1s and Ar 2p core level signals after subtracting a Shirley background (for a mathematical description of the Shirley background see Refs. [14,15]). From the photoionization cross sections given by Sco"eld [16] the atomic concentrations at the "lm surfaces have been calculated, taking into account the mean electron escape depths [17] and the transmission function of the spectrometer. Photoelectron spectroscopy measurements of a-C : H/Ti "lms with low titanium contents can severely be in#uenced by charging e!ects. In order to avoid the accumulation of positive charge at the sample surface it is necessary to perform the measurements on "lms with appropriate thicknesses [12]. Therefore, "lm thicknesses have been in the range from 15 to 40 nm as measured by means of a quartz crystal monitor.
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Trilayered absorber coatings have been deposited on sputter cleaned aluminum substrates. The roughness R of the used substrates has been evaluated using a pro! "lometer and appeared to be in the order of 60 nm. The trilayered coatings have been fabricated by subsequently depositing pure Ti, a-C : H/Ti and pure a-C : H. These samples have been subjected to accelerated aging treatments in air at elevated temperatures. Before the aging procedure and after each treatment the total re#ectance of the sample has been measured in the wavelength range from 280 nm to 20 lm. These measurements have been performed by means of a Bruker FTIR spectrometer equipped with an integrating sphere. The solar absorptance a (air mass 1.5) and the S thermal emittance at 1003C e 3 have been calculated from the total re#ectance at 100 C room temperature. Raman spectroscopy has been performed on the samples as fabricated and after subjection to the heat treatments. The Raman spectra have been acquired using an argon ion laser operating at a wavelength of 514.50 nm. The backscattered light has been recorded by means of a Peltier cooled photomultiplier in combination with a SPEX 14018 double monochromator. For the calibration of the monochromator we have used the O phonon of a silicon (1 0 0) wafer (Raman shift at a wavenumber of ! 520 cm~1 [18]).
3. Results The photoelectron spectroscopic measurements have shown that the deposition of titanium containing amorphous hydrogenated carbon "lms (a-C : H/Ti) has succeeded for all kinds of AC input power used: RF at 13.56 MHz, MF sine at 40 kHz and bipolar pulsed in the range from 50 to 250 kHz. The shapes and positions of the C1s and Ti2p core lines indicate that in all cases the titanium is bound as TiC being embedded in a hydrocarbon matrix. For a detailed photoelectron spectroscopic study of MF deposited a-C : H/Ti see Ref. [12]. It turned out that with our deposition process a broad concentration range is accessible: one can vary the ratio c " : at% Ti/at% C from 0 to 1 by tuning the T*@C Ar/CH mass #ow ratio. The appearance of oxygen contamination correlates with the 4 titanium content: the reactivity of the "lm surface is higher for the titanium rich "lms than for the carbon rich ones. Depending on the process gas mixture there is also Ar present up to 2 at%. The Ar concentration can be directly related to the gas composition during thin "lm deposition. We are not able to comment on the hydrogen content of our samples since photoelectron spectroscopy is not sensitive to this element. However, for deposition on substrates at ambient temperature we would expect a hydrogen content in the order of 30 to 40 at% for pure a-C : H [19] and a reduced one for Ti-containing "lms [20]. We believe that elevated substrate temperatures result in a reduced hydrogen content as well. For a long-term stability check the process had been running continuously (200 W, 40 kHz MF sine) using a argon/methane mass #ow ratio of 8. Samples have been prepared after 5, 30 and 60 min. The elemental compositions determined from core level spectroscopy remain rather constant (Fig. 2). Obviously, a poisening of the
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299
Fig. 2. Stability check of the plasma process: elemental compositions of a-C : H/Ti "lms as determined by photoelectron spectroscopy (XPS) vs. operating time of the deposition process.
titanium target by a carbon layer as it has been observed for gold targets under comparable conditions [3] has not occurred. In order to achieve optical selectivity for solar absorber coatings based on titanium containing amorphous hydrogenated carbon "lms one has to consider the optical properties of this material. In a previous study [13] the optical constants index of refraction n and extinction coe$cient k have been determined for a-C : H/Ti "lms with varying titanium content. From these data we have calculated the absorption coe$cient a which is a measure for the penetration depth of light. The results are displayed in Fig. 3. The curves are labelled with the ratio c determined by core level T*@C spectroscopy. In all cases the absorption coe$cient a is monotonically decreasing with increasing wavelength. One possibility to create an optical selective absorber surface is to select a material which is transparent in the infrared but absorbing in the solar region of the spectrum, and deposit this onto a metal which is a good infrared mirror, like copper or aluminum. Because of the low infrared absorption the a-C : H/Ti "lms with low titanium content seem to be most promising for this purpose. In order
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Fig. 3. Absorption coe$cient a of a-C : H/Ti "lms with varying titanium content: the curves are labelled with the concentration ratio c " : at% Ti/at% C. The displayed data are based on Ref. [13]. T*@C
to fabricate a multilayered solar selective absorber coating we have therefore used an a-C : H/Ti layer with c +0.1, overcoated with a pure a-C : H layer. To ensure good T*@C adhesion, titanium has been deposited previously onto the aluminum substrate (coverage corresponds to a Ti layer thickness of about 10 nm). During thin "lm deposition (200 W RF) the substrate was set to a voltage of !150 V and heated to 2753C. The optical performance of this multilayered coating is shown in Fig. 4. The solid black line represents the total re#ectance of the absorber coating as fabricated. The ideal re#ectance which is desired for a #at plate absorber coating would be a step pro"le with zero in the range of solar radiation (j)2.5 lm) and one in the range of black body radiation at 1003C (j'2.5 lm). The re#ectance of the multilayered coating exhibits an absorption edge roughly in the right place and is therefore a good approximation to the ideal pro"le. The curve exhibits two minima. Since their positions depend on the layer thicknesses we ascribe them to interference. These data
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301
Fig. 4. Hemispherical re#ectance of a-C : H/Ti "lms as received and after various aging steps. Aging results in a shift of the absorption edge and the interference minima towards lower wavelengths. The solar spectrum at air mass 1.5 (AM1.5) and the spectrum of black body thermal radiation at 1003C are added.
yield a solar absorptance a of 0.876 and a thermal emittance e 3 of 0.061 resulting S 100 C in an optical selectivity s " : a /e 3 of 14.4. S 100 C A service lifetime estimation of this absorber coating has been performed according to the suggestions which have been elaborated within the former Task X &Solar Materials Research and Development' of the International Energy Agency (IEA) Solar Hating and Cooling Programme [21,22]. An absorber coating is quali"ed if it ful"lls the performance criterion PC " : !*a #*e )5%, S T during a service lifetime of at least 25 years. Based on a time-transformation function and the standard heat loads in a collector, this quali"cation condition can be transformed into an acceptable failure time at elevated temperature. This acceptable failure time depends strongly on the activation energy of the dominating degradation mechanism. The lifetime estimation is based on the assumption of an Arrhenius law:
AA
BB
/ 1 1 t "t exp ! , 2 1 R ¹ ¹ 2 1 where t is the failure time at temperature ¹ , t the failure time at temperature ¹ , / the 1 1 2 2 activation energy of dominating aging mechanism, and R the universal gas constant.
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Table 1 : !*a #0.25**e 3 of Solar absorptance a , thermal emittance e 3 and performance criterion PC " S 100 C 4 100 C multilayered a-C : H/Ti absorber coatings before and after accelerated aging treatments Sample
Aging step
a (%) S
e 3 (%) 100 C
A
As received 25 h @ 2503C 50 h @ 2503C As received 70 h @ 2503C As received 150 h @ 2203C
87.6 86.7 83.8 87.0 78.6 87.2 85.7
6.1 6.2 5.8 6.3 5.7 6.4 5.9
B C
PC (%)
0.9 3.7 8.5 1.4
Fig. 5. Evolution of the performance criterion PC " : !*a#0.25*e during accelerated aging at 2203C and 2503C. The solid lines illustrate the interpolations between data points, the dashed ones the allowed maximum value for the performance criterion (5%), and the dotted ones the estimations made in the text.
For the accelerated aging treatments several identical samples have been tempered in air at 2503C and 2203C. The evolution of the re#ectance of the samples subjected to a temperature of 2503C is added to Fig. 4. The absorption edge and the interference minima shift to lower wavelengths. However, in the infrared region (j'4 lm) the
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303
Fig. 6. Acceptable least failure times for solar absorber coatings aged at 2203C and 2503C in dependence on the activation energy of the dominant aging mechanism. The data are taken from Ref. [21]. A coating with a lifetime of 55 h at 2503C must be able to stand 150 h of aging at 2203C.
changes are small. Table 1 gives a survey of the quantities a , e 3 and PC as S 100 C determined from three identical samples before and after the various aging steps. At a temperature of 2503C the coating fails after 55 h: at that time the performance criterion reaches 5% (for the interpolation see Fig. 5). According to the data given in Ref. [21] this coating is quali"ed if the activation energy of the dominant aging mechanism is higher than 72 kJ/mol (Fig. 6). One can check this by aging the coating at a di!erent temperature. In order to ful"ll the above-mentioned requirement the lifetime of the coating at 2203C must exceed 150 h (Fig. 6). Indeed after 150 h tempering at 2203C the performance criterion has reached only 1.4%. Therefore, the coating has passed the accelerated aging test. The activation energy of the aging mechanism which is relevant for the degradation of the optical properties can be estimated by an interpolation of the data shown in Fig. 5. The performance criterion of 1.4% that is reached after 150 h tempering at 2203C equates the value that corresponds to roughly (30$3) h annealing at 2503C (Fig. 5). Under the assumption of an Arrhenius law these data yield an activation energy of (116$8) kJ/mol.
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Fig. 7. Raman spectra of the coatings as fabricated and after various aging steps. The sample annealed for 150 h at 2203C exhibits a larger line shift towards higher wavenumbers that the one annealed for 50 h at 2503C and a smaller one than the sample annealed for 70 h at 2503C.
The use of Raman spectroscopy to monitor structural changes of pure a-C and a-C : H coatings is quite common. Gampp et al. have used it to study the aging behavior of a-C : H/Cr thin "lms [4]. Raman spectra of the samples as fabricated and after the heat treatments are displayed in Fig. 7. The Raman spectra have been found to be sensitive only to the a-C : H component and show two broad maxima: one is the so-called G-peak at about 1580 cm~1, which is assumed to stem from the sp2-bonded, graphite-like regions, and the other is the so-called &disorder allowed' D-peak at about 1380 cm~1, which has been observed in graphite samples with "nite grain size [23]. Tempering at 2503C results in a narrowing of the D- and G-peaks and in a shift of both peaks to higher wavenumbers. Additionally, the D-peak increases in intensity relative to the G-peak.
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These e!ects are typical indications for the growth of graphite-like domains in the carbon component [24]. Obviously, a partial crystallization of the amorphous hydrogenated carbon "lms has occurred. As it has been observed by FTIR spectroscopy, these changes are accompanied by a release of hydrogen [25]. The same evolution is observed for 2203C even though it takes place on a longer time scale. The sample annealed for 150 h at 2203C exhibits a larger line shift towards higher wavenumbers than the one annealed for 50 h at 2503C and a smaller one than the sample annealed for 70 h at 2503C. This is especially remarkable because after 150 h annealing at 2203C the optical properties have changed only to the extent observed after roughly (30$3) h at 2503C. Since comparable degrees of graphitization have been observed for two di!erent temperatures the activation energy for the graphitization process can be estimated applying the Arrhenius law again: the observations described above yield a lower bound of 54 kJ/mol and an upper bound of 79 kJ/mol.
4. Discussion The long-term stability of the deposition process could be demonstrated even without any feed back control. The good reproducibility is a basis for a high-quality standard. The presented solar absorber coating consists of three layers. Since layer compositions and thicknesses have not been optimized so far, we believe that one can fabricate a-C : H/Ti-based absorber coatings with even better optical performance. One can add more absorber layers easily or use gradient layers. The aging stability of a-C : H/Ti-based coatings on aluminum substrates has been shown to ful"ll the requirements of the accelerated aging test by tempering as described in Ref. [21]. Even though the multilayered a-C : H/Ti absorber coating has passed the accelerated aging test, some degradation of the optical properties has been observed. For a further improvement of the service lifetime of such coatings it is interesting to identify the dominant aging mechanism. It has been shown by Raman spectroscopy that graphitization of the a-C : H matrix takes place. The activation energy for this process has been estimated to be in the range from 54 to 79 kJ/mol. From the optical data we have determined the activation energy of the aging mechanism which is relevant for the degradation of the performance of the coating to be (116$8) kJ/mol. To illustrate the contradiction in another way: the degree of graphitization of the sample annealed for 150 h at 2203C is comparable to the one observed in the range from 50 to 70 h annealing at 2503C. However, after 150 at 2203C the performance of the coating is much better than after 50 h at 2503C. Therefore, we conclude that for the degradation of the optical performance the graphitization is not the dominant mechanism. Let us consider again the re#ectance spectra of Fig. 4. Earlier we have described that annealing in air results in a shift of the absorption edge and interference minima towards lower wavelengths. These changes can be explained by oxidation of the a-C : H matrix to the gaseous carbon oxides resulting in a decrease of "lm
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thicknesses. Oxidation of the hydrocarbon matrix has also been observed in accelerated aging tests of a-C : H/Cr coatings [4]. We expect that by an optimization of the deposition parameters substrate bias voltage and temperature, a further increase in aging stability can be achieved.
5. Conclusions Coatings based on titanium containing amorphous hydrogenated carbon "lms can be deposited by a combined PVD/PECVD process using various kinds of input power (RF 13.56 kHz, MF sine 40 kHz and bipolar pulsed 50}250 kHz). The stability of the plasma process results in a high reproducibility and forms therefore a good basis for a high-quality standard. Optical selectivity has been achieved for a-C : H/Ti based multilayered solar absorber coatings: a solar absorptance a of 0.876 and a thermal emittance e 3 of 0.061 S 100 C yield an optical selectivity s " : a /e 3 of 14.4. The service lifetime of these coatings S 100 C on aluminum substrates is predicted to amount to more than 25 years. Aging mechanisms are graphitization and oxidation of the a-C : H matrix, the latter one is most presumably the dominant one for the degradation of the optical performance.
Acknowledgements The authors wish to thank Roland Steiner for the technical support. The "nancial support of the Bundesamt fuK r Energiewirtschaft, Switzerland is gratefully acknowledged.
References [1] R. Gampp et al., in: V. Wittwer, C.G. Granquist, C.M. Lampert (Eds.), Optical Materials Technology for Energy E$ciency and Solar Energy Conversion XIII, Proceedings of SPIE, Vol. 2255, 1994, pp. 92}106. [2] P. Oelhafen, P. Gantenbein, R. Gampp, in: V. Wittwer, C.G. Granquist, C.M. Lampert, (Eds.), Optical Materials Technology for Energy E$ciency and Solar Energy Conversion XIII, Proceedings of SPIE, Vol. 2255, 1994, pp. 64}78. [3] R. Gampp, in: Fortschr.-Ber. VDI Reihe 5, Vol. 446, VDI Verlag, DuK sseldorf, 1996. [4] R. Gampp, P. Oelhafen, P. Gantenbein, S. Brunold, U. Frei, Sol. Energy Mater. and Sol. Cells 54 (1998) 369. [5] M. Wang, K. Schmidt, K. Reichelt, H. Dimigen, H. Hubsch, J. Mater. Res. 7 (3) (1992) 667. [6] D.P. Monaghan, D.G. Teer, P.A. Logan, I. Efeoglu, R.D. Arnell, Surf. Coatings Technol. 60 (1}3) (1993) 525. [7] Deng Jianguo, M. Braun, Diamond Related Mater. 4 (7) (1995) 936. [8] A. Hellmich, T. Jung, A. Kielhorn, M. Rissland, Surf. Coatings Technol. 98 (1}3) (1998) 1541. [9] G. Francz, A. SchroK der, R. Hauert, Surface analysis and bioreactions of Ti and V containing a-C : H, Surf. Interface Anal. 28 (1999) 3}7. [10] M.P. Delplanke, V. Vassileris, R. Winand, J. Vac. Sci. Technol. A 13 (1995) 1104. [11] W.J. Meng, T.C. Curtis, L.E. Rehn, P.M. Baldo, J. Appl. Phys. 83 (11) (1998) 6076.
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[12] A. SchuK ler, R. Gampp, P. Oelhafen, In situ photoelectron spectroscopy of titanium containing amorphous hydrogenated carbon "lms (a-C : H/Ti), Phys. Rev. B, in press. [13] A. SchuK ler, C. Ellenberger, P. Oelhafen, Optical properties of titanium containing amorphous hydrogenated carbon "lms (a-C : H/Ti), J. Applied Physics, submitted. [14] D.A. Shirley, Phys. Rev. B 5 (1972) 4709. [15] S. HuK fner, Photoelectron Spectroscopy: Principles and Applications, 2nd Edition, Springer, Berlin, 1996, p. 144. [16] J.H. Sco"eld, J. Electron Spectrosc. Related Phenom. 8 (1976) 129. [17] M.P. Seah, W.A. Dench, Surf. Interface Anal. 1 (1979) 2. [18] M. Yoshikawa, G. Katagiri, H. Ishida, A. Ishitani, T. Akematsu, Solid State Commun. 66 (1988) 1177. [19] M. Wittmer, D. Ugolini, J. Eitle, P. Oelhafen, Appl. Phys. 48 (1989) 559. [20] M. Fryda, Untersuchungen der strukturellen und mechanisch-tribologischen Eigenschaften metallhaltiger, amorpher Kohlenwassersto!schichten, VDI-Verlag GmbH, DuK sseldorf, 1993. [21] B. Carlsson, M. KoK hl, U. Frei, Accelerated life testing of solar energy materials, case study of some selective solar absorber coating materials for DHW systems, A Report of Task X Solar Materials Research and Development SP-Report 93:13, 1994. Available from Swedish National Testing and Research Institute, P.O. Box 857, S-50115 Boras, Sweden. [22] U. Frei et al., in: C.M. Lampert, S.K. Deb, C.G. Granquist (Eds.), Optical Materials Technology for Energy E$ciency and Solar Energy Conversion XIV, Proceedings of SPIE, Vol. 2531, 1995, p. 282. [23] R.J. Nemanich, S.A. Solin, Phys. Rev. B 20 (1979) 392. [24] R.O. Dillon, J.A. Woollam, V. Katkanat, Phys. Rev. B 29 (1984) 3482. [25] A. Reyes-Mena, J. Gonzalez-Hernandez, R. Asomoza, in: P. Koidl, P. Oelhafen (Eds.), Amorphous Hydrogenated Carbon Films, Les Editions de Physique, Paris, 1987, p. 229.