An X-ray photoelectron spectroscopy study on the

Thin Solid Films 640 (2017) 38–44

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An X-ray photoelectron spectroscopy study on the annealing effects for Al/glass Interface during aluminum induced texturing process Mustafa Ünal a,b,⁎, Aydın Tankut b, İlker Yıldız c,d, İlkay Sökmen e, Raşit Turan a,b a

Department of Micro and Nanotechnology, Middle East Technical University, Dumlupinar Blvrd no: 1, 06800 Ankara, Turkey The Center for Solar Energy Research and Applications (GÜNAM), Middle East Technical University, Dumlupinar Blvrd no: 1, 06800 Ankara, Turkey Central Laboratory of Middle East Technical University, Dumlupinar Bulvrd No:1, 06800 Ankara, Turkey d Department of Physics, Dumlupınar University, Kütahya, Turkey e Şişecam Science and Technology Center, Şişecam Str., No:2 Çayırova, Kocaeli, Turkey b c

a r t i c l e

i n f o

Article history: Received 18 January 2017 Received in revised form 27 August 2017 Accepted 28 August 2017 Available online 30 August 2017 Keywords: Depth profile X-ray photoelectron spectroscopy Aluminum Float glass Thermal annealing Aluminum induced texturing

a b s t r a c t The aluminum induced texturing method offers an effective light trapping scheme by random texture that is formed by U-shaped craters on the glass surface. The texture is mainly shaped by the reaction between Al and SiO2. However, the reaction mechanism is not totally understood. Besides, the influence of other components present in the glass such as Na2O, CaO, and MgO. is neglected. In this study, the evolution of Al films on soda-lime glass during annealing has been inspected by depth resolved X-ray photoelectron spectroscopy. The elemental distribution of Si, Al and O have been investigated for different annealing durations and compositional analysis has been conducted for Na, Ca and Mg in addition to Si, Al and O. According to results, a relevant evolution model for annealing process has been constructed. © 2017 Elsevier B.V. All rights reserved.

1. Introduction It is known that a significant portion of the total cost for Si-wafer solar cells is due to raw material usage [1]. As such, thin film solar cells offer an important cost advantage, since only a few micrometers of material is necessary for solar electricity production [2–7]. An additional benefit of thin film solar cells is simple production methods suitable for large area applications [8]. Despite these advantages, thin film technologies have yet to gain cost competitiveness against the conventional Si-wafer solar cells, due to their relatively low efficiencies or usage of rare and toxic materials. Maximizing light absorption for thin film solar cells is crucial in order to enhance photovoltaic conversion efficiency while reducing the absorber layer thickness. To address this issue, light trapping schemes are considered as viable options. Increasing the optical path length by scattering the light through textured interfaces or by reflectors causes an increase in the absorption of solar cells. Such an increase in the light absorption efficiency would enable a reduction in cost of produced solar electricity by either increasing the efficiency of the module, or alternatively decreasing the necessary thickness of the absorber layer (the latter of which could be important due to usage of critical materials in thin film solar cells like Cd, Te, In ⁎ Corresponding author at: Department of Micro and Nanotechnology, Middle East Technical University, Dumlupinar Blvrd no: 1, 06800 Ankara, Turkey. E-mail address: [email protected] (M. Ünal).

http://dx.doi.org/10.1016/j.tsf.2017.08.046 0040-6090/© 2017 Elsevier B.V. All rights reserved.

and Se [9–11]). There are different techniques used for light trapping such as texturing of aluminum-doped zinc oxide [12], the usage of back reflectors [13] and texturing the glass substrate [14–18]. Among different glass texturing methods, aluminum induced glass texturing (AIT) is a newly developed process that has recently gained attention [19]. AIT glasses have random surface structures consisting of sub-micron and micron sized U-shaped craters, which scatter the light. In addition, texture of the glass is transferred to the subsequent layers such as transparent conductive oxides and back reflectors [20–22], which eliminates the necessity of additional texturing processes for these layers. The AIT process is mainly based on a random redox reaction, in which SiO2 is reduced to Si and Al oxides at temperatures above 500 °C [3,15,19, 23–25]. Al diffuses into glass and forms Al2O3 nodules randomly, while the reduced Si diffuses through interface and dissolve into Al, forming an Al-Si alloy. Above the Al-Si eutectic temperature, the increase in Si concentration with respect to Al results in the formation of a liquid phase which increases the redox reaction rate dramatically. The Si concentration rapidly exceeds the solubility limit, wherein the Si precipitates form preferentially around the existing Al2O3 regions. This mechanism has been explained in a previous communication by our research group [26]. However, depth information was missing in this study. Additionally, even though reaction is explained solely through the Al-SiO2 reaction, it is known that the composition of the glass influences the final texture [1], suggesting other elements found in glass could be chemically involved in the process. It is, therefore, worthwhile to investigate the

M. Ünal et al. / Thin Solid Films 640 (2017) 38–44

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chemical changes in the Al film during annealing and monitor the behavior of the additional, non-SiO2 components present in the glass, in order to develop a comprehensive understanding of the AIT process. 2. Experiment Annealing experiments were carried out on 4 mm soda-lime float glass samples (Türkiye Şişe ve Cam Fabrikaları A.Ş). The chemical composition of this glass provided by manufacturing factory is presented in Table 1. All glass samples were cleaned by a chemical detergent at 50 °C for 15 min and then rinsed with deionized water at 55 °C for 15 min, followed by cold rinsing at 25 °C for 15 min. Following the cleaning process, samples were coated with 130 nm Al by thermal evaporation system under 3 × 10−5 Torr pressure with average 2 nm/s deposition rate. A classical tube furnace (10 cm diameter) under 4 slm N2 flow was used for annealing. In this step, all samples were annealed at 600 °C, which is above the Al-Si eutectic temperature, at incremental durations that includes temperature rising time. The morphology of the samples was investigated by optical microscope (Nade metallurgical microscope - NMM-800TRF). The structural and compositional distribution was studied with PHI Versaprobe 5000 Scanning X-Ray Photoelectron Spectrometer (XPS) with Al Kα as an X-ray source by depth profiling. 24.9 W X-ray power used for analysis with 100 μm X-ray spot size and 45° take-off angle. For each region, 58.70 eV pass energy used for high resolution scan. Each layer was removed by Ar ion bombardment at 3 kV energy and 3 min dwell time. C1s peak is pinned to 283 eV for survey scans. In addition, survey scan of annealed samples were obtained after Ar ion bombardment for 5 times to reduce the contribution of surface oxides. The binding energy shifts related to surface charging during data collection was corrected using SiO2 or Al peak positions, where applicable. 3. Results & discussion The evolution of the film during annealing was monitored by optical microscopy, as shown in Fig. 1. After 5 min of annealing, there are small circular regions that transmits the light as examined under reflected and transmitted light. The surface morphology was dramatically changed after 7 min of annealing; elongated features that resemble peanut shells with a high contrast can be seen in the reflection images, while the density of the bright spots on the transmission image has greatly increased. After 10 min of annealing, a more uniform contrast is seen for the reflection image, whereas areas with high brightness can be observed in the transmission image. The dark features seen in 5 min annealed sample with reflected light (Fig. 1a) are representative of Al2O3 formed during early stage of annealing as shown in the previous study [26]. On the other hand, in sample annealed for 7 min (Fig. 1b), bright regions observed by reflected light are c-Si clusters formed around Al2O3. c-Si acts as a diffusion barrier to Al and hinders its interaction with the oxide layer. Accordingly, Al on top of c-Si remains metallic and yield high brightness in the reflection image. Optical image of 7 min annealed sample that is taken by transmitted light reveals a drastic intensity increase in the number of bright Al2O3 spots, which points to the increase in the redox reaction rate by the emergence of the liquid Al-Si phase. In addition, several areas where the Al2O3 nodules have accumulated can be identified. We speculate that these areas are preferential nucleation sites for Al2O3 (such as cracks and/or other morphological defects), which promotes a reduction in activation energy. When we look at the sample annealed for 10 min (Fig. Table 1 Chemical composition (wt%, approx.) of soda-lime float glass. SiO2

Al2O3

CaO

MgO

Na2O

Trace

71.8

1.1

8.2

4.4

14.3

0.2

Fig. 1. Optical microscopy images of samples annealed for (a) 5 min, (b) 7 min and (c) 10 min at 600 °C recorded by using reflected light (top) and transmitted light (bottom).

1c), bright regions seen by reflected light in the sample annealed for 7 min (Fig. 1b) are disappeared due to oxidation of Al at later stage of annealing. By looking at low reflectance and high transmittance of this sample, it can be said that most of the Al is oxidized. The compositional distribution of non-annealed and 10 min annealed sample were investigated by XPS. Fig. 2 shows composition distribution of non-annealed Al film and soda-lime glass. In Fig. 2a, it is seen that the film is metallic Al as expected. The plasmon peaks of metallic Al can also be noticed in Fig. 2a. Peaks present in survey scan of non-annealed film are identified as Al2p (72 eV) and Al2s (117 eV) with plasmon satellite peaks as summarized in Table 2. The constituents of the soda-lime float glass can be observed in Fig. 2b, which appears to be in accord with Table 1. It is observed that there is Al present in the glass appears at 75 eV which belongs to Al2p in Al2O3 structure. In addition, oxide peaks of other components of the glass are identified as Na2O, CaO, MgO and SiO2, which are given with peak positions in Table 3. Following 10 min of annealing, the XPS spectra suggests an intermixing of components, as shown in Fig. 2(c). Photoelectron peaks of Al, Si, O, Na, Ca and Mg can all be identified in a single spectrum. There are double peaks for the binding energies of Si and Al as presented in Table 4. Si2p peak appears at 99 eV and 104 eV, the former belongs to elemental forms of Si while the latter represents Si binding energy in SiO2 form. In addition, Al2p peak arises at 72 eV and 75 eV. The 3 eV binding energy difference corresponds to oxidation of Al in the form of Al2O3. However, in other components of the glass, which are CaO, MgO, Na2O, any change in the oxidation states could not be identified in survey scan due to low intensity of the signal. In 10 min annealed sample, Ca2p (2p3/2) peak appears at 349 eV, MgKLL peak at 308 and NaKLL peak at 499 eV. In order to acquire a deeper understanding of the chemical changes that occur during annealing, high resolution XPS spectra from each element was collected. In Fig. 3, photoelectron spectra of Al for different depths and different annealing durations are presented. In non-annealed sample (Fig. 3a), the film is elemental Al, in which Al2p peak appears at 72 eV binding energy. There is a broad peak around 75 eV binding energy at the 20th and 30th layers that are belongs to the soda-lime glass. This peak represents the binding energy of Al in the form of Al2O3, which already exist in the soda-lime glass. After 7 min of annealing (Fig. 3b),

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Fig. 2. Survey scan of (a) deposited Al film and (b) soda-lime glass and (c) 10 min annealed sample by XPS.

Table 2 Identification of peaks in deposited Al film presented in Fig. 2(a) with related references and identified peak positions in the literature. Peak P. (eV)

Element & spec. line

Identification

Reported peak position (eV)

72 88 103 117 133 148 165 180

Al2p Al2p Al2p Al2s Al2s Al2s A2s Al2s

Al 1st plasmon satellite peak of metallic Al2p 2nd plasmon satellite peak of metallic Al2p Al 1st plasmon satellite peak of metallic Al2s 2nd plasmon satellite peak of metallic Al2s 3rd plasmon satellite peak of metallic Al2s 4th plasmon satellite peak of metallic Al2s

72.2 [27], 73.0 [28] 15 eV [29] or 15.5 eV [30] lower than observed Al2p binding energy 30 eV [29] lower than observed Al2p binding energy 117.9 [31], 117.7 [32], 118.0 [33,28] 16 eV [28,34] lower than observed Al2s binding energy 32 eV [34] lower than observed Al2s binding energy 49 eV [34] lower than observed Al2s binding energy 64 eV [34] lower than observed Al2s binding energy

there is significant amount of elemental Al present with a broad Al2O3 peak. When annealing time is increased to 15 min (Fig. 3c), on the other hand, it is observed that the film is composing of Al2O3 mostly.

After 40 min of annealing (Fig. 3d), the reaction is completed. There is a small amount of elemental Al in this sample but there is no further change is observed when annealing time is increased to 60 min.

Table 3 Identification of peaks in glass presented in Fig. 2(b) with related references and identified peak positions in the literature. Peak position (eV)

Element & spectral line

Identification


27 75 104 155 308 349 441 499 533

Ca3p Al2p Si2p Si2s MgKLL Ca2p Ca2s NaKLL O1s

CaO Al2O3 SiO2 SiO2 MgO CaO CaO Na2O Al2O3 SiO2 Na2O CaO MgO

27 eV [35], 26.2 eV [36] 74.4 eV [37], 73.8 eV [38], 74.7 eV [39], 75.6 eV [40] 103 eV [41], 103.5 eV [42], 103.6 eV [28] 155 eV [28], 154.8 eV [43], 155.3 eV [44] 307.5 eV [45] 347.6 eV [46], 347.1 eV [36], 349.30 eV [47] 440 [28] 496.9 eV [48], 497 eV [28] 531 eV [28], 531.3 eV [37] 533 eV [42], 533,7 eV [44] 529.7 eV [28] 531.3 eV [28], 529.9 eV [36] 530.0 eV, 531.2 eV, 532.1 eV [28]


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Table 4 Identification of peaks in 10 min annealed sample given in Fig. 2(c) with related references and identified peak positions in the literature. Peak position (eV)

Element & spectral line

Identification


72 75 99 104 118 121 151 155

Al2p Al2p Si2p Si2p Al2s Al2s Si2s Si2s

Al Al2O3 Si SiO2 Al Al2O3 Si SiO2

72.2 [27], 73.0 [28] 74.4 eV [37], 73.8 eV [38], 74.7 eV [39], 75.6 eV [40] 99.0 eV [28] 103 eV [41], 103.5 eV [42], 103.6 eV [28] 117.9 [31], 117.7 [32], 118.0 [33,28] 119.3 eV [49], 121.20 eV [40] 151 eV [28] 155 eV [28], 154.8 eV [43], 155.3 eV [44]

Fig. 3. Binding energy of Al from different depths for samples; (a) non-annelaed, (b) annealed for 7 min, (c) annealed for 15 min and (d) annealed for 40 min. Number for each graph represents layer which the scan is taken.

When we look at binding energies of Si from different depths, a scenario parallel with the case of Al can be seen as presented in Fig. 4. In Fig. 4a, the signal appears at 103 eV for 5th and 10th layers belongs to second plasmon satellite peak of elemental Al coming from Al film. After 7 min of annealing (Fig. 4b), elemental Si is formed inside the film. It can be clearly seen in 15 min annealed sample (Fig. 4c) that both elemental Si and SiO2 can be present in the same layer. This indicates non-uniform reaction rate coming from solid phase and liquid phase difference. As

mentioned, the reaction rate increased drastically due to the formation of the liquid phase Al-Si. The Si binding energy distribution in 40 min annealed sample can be seen in Fig. 4d. As the redox reaction progresses, more c-Si is formed near the surface, as evidenced by the distinct elemental Si peaks on 5th and 20th layers. When other components of soda-lime glass are investigated, it is seen that Na, Ca and Mg were diffused through the film and especially Na was accumulated on the surface after 40 min of annealing as shown in Fig. 5.

Fig. 4. Binding energy of Si2p after different number of sputtering cycles for samples (a) non-annelaed, (b) annealed for 7 min, (c) annealed for 15 min and (d) annealed for 40 min. Number for each graph represents layer which the scan is taken.

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Fig. 5. XPS depth profiles of non-annealed; (a) Na1s, (b) Mg2p, (c) Ca2p; and fully annealed (40 min) samples; (d) Na1s, (e) Mg2p, (f) Ca2p.

For this analysis, binding energies for Ca2p, Mg2p and Na1s were investigated. Binding energies of Ca and Mg appears at 348.10 eV and 50.80 eV for Ca2p (2p3/2) and Mg2p peak correspondingly, which are belongs to binding energy of Ca in CaO [36,46,47] and Mg in MgO [47]. There is no chemical reduction of Ca and Mg as suggested in the literature [50]. However, it is observed that significant amount of Na has diffused to the surface and binding energy is shifted to lower energy on the surficial layers. The binding energy of Na in the glass for Na1s peak is measured as 1073.10 eV. On the surface of the annealed film, on the other hand, peak position of Na1s shifts to 1071.50 eV that is defined as elemental Na [28]. The reason of the shift towards lower binding energy for Na can be explained by comparing oxidation potentials of Al and Na, which favors reduction of Na [51,52]. The accumulation seen in the XPS graphs, presented in Fig. 5, of the elemental Na is also observed by SEM and

EDX as given in Fig. 6. Na-rich regions appear on top of Si rich regions. However, when annealing time is increased more such as up to 240 min, the accumulation of Na on non-Si rich regions can be observed (Fig. 7). Even though redox reaction finalized, the alkaline and earth alkaline metals from the glass continue to diffuse. 4. Conclusion In this study, depth profile of annealed Al coated soda-lime glass samples were studied by XPS. It is seen that formation of elemental Si and oxidation of Al to Al2O3 is induced at 600 °C, and the majority of the surface texturing occurred within 10 min. In addition, diffusion of Na, Ca, and Mg from the glass into the over layer was observed. Na, which appears to be reduced by Al, accumulated on/near the c-Si clusters, the diffusion of

Fig. 6. SEM image (×2500 mag.) showing Na reach regions with EDX measurements.


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Fig. 7. Comparison of (a) EDX Map of Na and (b) SEM image showing accumulation of Na on the surface for 240 min of annealing. Yellow, green and blue circles indicates accumulated Na both on EDX Map and SEM image. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

those alkaline metals are governed by Si crystals. The reduction of SiO2 is significantly hindered by the reaction by-products (i.e., Al2O3 and Si), which act as a diffusion barrier for Al. As such, a small amount of unreacted Al “trapped” in the film is detected even after 40 min of annealing. Acknowledgements This works was financially supported by the Ministry of Science and Technology of Turkey under SAN-TEZ program with the project number 0392.STZ.2013-2. The authors are grateful to Türkiye Şişe ve Cam Fabrikaları A.Ş. for their support on this work and Dr. Tahir Çolakoğlu of METU for his helps. References [1] M. Ünal, H. Nasser, M. Günöven, A. Tankut, İ. Sökmen, R. Turan, Aluminum induced glass texturing (ait) on soda-lime, borosilicate, alkali-free and fused silica glass for thin film solar cell applications, 31st Eur. Photovolt. Sol. Energy Conf. Exhib 2015, pp. 230–234. [2] J. Wang, S. Venkataraj, C. Battaglia, P. Vayalakkara, A.G. Aberle, Analysis of optical and morphological properties of aluminium induced texture glass superstrates, Jpn. J. Appl. Phys. 51 (2012) 8–11, http://dx.doi.org/10.1143/JJAP.51.10NB08. [3] Y. Huang, F. Law, P.I. Widenborg, A.G. Aberle, Crystalline silicon growth in the aluminium-induced glass texturing process, J. Cryst. Growth 361 (2012) 121–128, http://dx.doi.org/10.1016/j.jcrysgro.2012.09.015. [4] N.A.S. Ahraei, M.A.P. Eters, S.E.V. Enkataraj, A.R.G.A. Berle, S.O.C. Alnan, S.V.E.N.R. Ing, B.E.S. Tannowski, R.U.S. Chlatmann, R.O.L.F.S. Tangl, Thin-film a-Si: H solar cells processed on aluminum-induced texture (AIT) glass superstrates: prediction of light absorption enhancement, Appl. Opt. 54 (2015) 4366–4373, http://dx.doi.org/ 10.1364/AO.54.004366. [5] N. Sahraei, S. Venkataraj, P. Vayalakkara, A.G. Aberle, Optical absorption enhancement in amorphous silicon films and solar cell precursors using the aluminum-induced glass texturing method, Int. J. Photoenergy. 2014 (2014) http://dx.doi.org/ 10.1155/2014/842891. [6] G. Jin, P.I. Widenborg, P. Campbell, S. Varlamov, Lambertian matched absorption enhancement in PECVD poly-Si thin film on aluminum induced textured glass superstrates for solar cell applications, Prog. Photovolt. Res. Appl. 18 (2010) 582–589, http://dx.doi.org/10.1002/pip.981. [7] A.G. Aberle, Thin-film solar cells, Thin Solid Films 517 (2009) 4706–4710, http://dx. doi.org/10.1016/j.tsf.2009.03.056. [8] K.L. Chopra, P.D. Paulson, V. Dutta, Thin-film solar cells: an overview, Prog. Photovolt. Res. Appl. 12 (2004) 69–92, http://dx.doi.org/10.1002/pip.541. [9] L. Friberg, M. Piscator, G.F. Nordberg, T. Kjellström, Cadmium in the Environment, 2nd ed. CRC Press, Inc., Ohio, 1974. [10] S. Ahmed, K.B. Reuter, O. Gunawan, L. Guo, L.T. Romankiw, H. Deligianni, A high efficiency electrodeposited Cu2ZnSnS4 solar cell, Adv. Energy Mater. 2 (2012) 253–259, http://dx.doi.org/10.1002/aenm.201100526. [11] V. Zepf, J. Simmons, A. Reller, M. Ashfield, C. Rennie, Materials Critical to the Energy Industry - An Introduction, 2nd ed. British Petrol, London, 2014. [12] H. Nasser, Z.M. Saleh, E. Özkol, A. Bek, R. Turan, Advanced light trapping interface for aSi:H thin film, Phys. Status Solidi 12 (2015) 1206–1210, http://dx.doi.org/10.1002/ pssc.201510097. [13] J. Springer, A. Pruba, L. Müllerove, M. Vanecek, O. Kluth, B. Rech, Absorption loss at nanorough silver back reflector of thin-film silicon solar cells, J. Appl. Phys. 95 (2004) 1427, http://dx.doi.org/10.1063/1.1633652.

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