- Email: [email protected]
porous silicon combined treatment
Materials Science and Engineering B 178 (2013) 695–697
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Short communication
Monocrystalline silicon surface passivation by Al2 O3 /porous silicon combined treatment M. Ben Rabha a,∗ , M. Salem a , M.A. El Khakani b , B. Bessais a , M. Gaidi a,b,c a b c
Laboratoire de Photovoltaïque, Centre de Recherches et des Technologies de l’Energie, Technopole de Borj-Cédria, BP 95, 2050 Hammam-Lif, Tunisia Institut National de la Recherche Scientifique (INRS), 1650, Blvd. Lionel-Boulet, Varennes, Québec J3X 1S2, 6 Canada Emirates college of technology, Millennium Tower, Sheikh Hamdan Street, P.O. Box: 41009, Abu Dhabi, United Arab Emirates
a r t i c l e
i n f o
Article history: Received 27 June 2012 Received in revised form 2 November 2012 Accepted 25 November 2012 Available online 7 December 2012 Keywords: Porous silicon Al2 O3 Effective minority carrier lifetime Reflectivity
a b s t r a c t In this paper, we report on the effect of Al2 O3 /porous silicon combined treatment on the surface passivation of monocrystalline silicon (c-Si). Al2 O3 films with a thickness of 5, 20 and 80 nm are deposited by pulsed laser deposition (PLD). It was demonstrated that Al2 O3 coating is a very interesting low temperature solution for surface passivation. The level of surface passivation is determined by techniques based on photoconductance and FTIR. As a result, the effective minority carrier lifetime increase from 2 s to 7 s at a minority carrier density (n) of 1 × 1015 cm−3 and the reflectivity reduce from 28% to about 7% after Al2 O3 /PS coating. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Crystalline silicon contains a large amount of impurities that negatively influence the minority carrier lifetime and in consequence the cell efficiencies. Surface passivation of crystalline silicon (c-Si) is becoming increasingly important for the performance of c-Si solar cells. Surface passivation is requisite for high efficiency solar cells as the cost-driven reduction of the solar cell thickness leads to the increase of the surface-to-volume ratio. Thin films (typically 20–80 nm) of thermal silicon dioxide (SiO2 ), silicon nitride (a-SiNx :H), and amorphous silicon (a-Si:H) are currently used for surface passivation of c-Si solar cells reference. Materials interest for the passivation of c-Si is aluminum oxide (Al2 O3 ) [1–5] and porous silicon (PS) [6–11]. In the present work, we studied the passivation and ARC effect of Al2 O3 /SP on monocrystalline silicon. We discuss the effect of stain etching-based PS and PLDAl2 O3 combined treatments on the opto-electronic properties of monocrystalline silicon. Effect of Al2 O3 thickness on the optical reflectivity and on the passivation quality is also investigated.
The research was developed in order to improve the optoelectronic properties of c-Si by combining Al2 O3 and porous silicon (PS). The stain etching (SE) processes were applied to c-Si wafers with a surface area of 1.5 cm2 in a 1:3:5 HF (48 wt%)/HNO3 (65 wt%)/H2 O solutions to form PS on c-Si surface with a thickness of 0.5 m and a porosity of 40%. Pure Al2 O3 thin films were deposited on porous silicon by means of pulsed laser deposition (PLD) technique. The Al2 O3 thickness was varied between 5 and 80 nm by controlling the laser pulses number. During deposition, the substrate holder was rotated at 5 rpm to achieve high thickness uniformity over the 6-in.-diameter coated area. The deposition temperature was kept at ∼RT. The morphology of the c-Si surface before and after Al2 O3 /porous silicon combined treatment was investigated by atomic force microscopy (AFM). The reflectivity spectra of the mc-Si surface were measured using a LAMBDA 950 UV/Vis/NIR spectrophotometer equipped with an integrating sphere. The composition of as-formed PS was analyzed by means of Fourier transform infrared spectroscopy (FTIR) using a Nicolet MAGNA 560 spectrometer. Finally, the effective lifetime was measured in a photoconduction system (WCT-120).
2. Experimental 3. Results and discussion Experiments were carried out on p-type monocrystalline silicon wafer with a thickness of 330 m and a resistivity of 0.5–3 cm.
∗ Corresponding author. E-mail address: [email protected] (M. Ben Rabha). 0921-5107/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mseb.2012.11.021
Fig. 1 shows the AFM images of the surface morphology of the monocrystalline silicon wafers before and after Al2 O3 /PS surface treatments. AFM image of the sample after PS treatment is exposed in Fig. 1a. The obtained roughness (RMS) was estimated to be around 27 nm (Fig. 1a). Fig. 1b, c, and d shows Al2 O3 films
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M. Ben Rabha et al. / Materials Science and Engineering B 178 (2013) 695–697
Fig. 1. AFM views of porous silicon surface before and after different Al2 O3 thickness: (a) porous silicon, (b) Al2 O3 thickness = 5 nm, (c) Al2 O3 thickness = 20 nm and (d) Al2 O3 thickness = 80 nm.
morphology with a thickness of 5, 20 and 80 nm, respectively, as deposited by pulsed laser deposition on porous silicon. After PLD-Al2 O3 treatment, the surface showed essentially the same aspect. Specifically, the RMS (Fig. 2) was reduced to 0.3 nm. The decrease in the RMS roughness is attributed to Si pores filling by Al2 O3 as already reported in the literature [12]. Fig. 3 shows the total reflectance spectra evolution for a c-Si substrate after different treatments of the Al2 O3 /porous silicon. These spectra show the excellent antireflection properties of the Al2 O3 /porous silicon layer. In fact the total reflectivity is reduced from 27% to about 7%, while it attains only an average value of about 12% in the case of mechanically textured wafers [13]. The achievement is comparable with that obtained using SiN-based ARC [14]. The standard silicon nitride (SiN) ARC reduces the weighted reflectance on a planar surface from 35% to 10% [15] while it attains an average value of about 4% in the case of standard KOH etched c-Si solar cell with SiN ARC [15]. This effect, as already observed by the AFM analysis, is related to suitable surface morphology structure for light trapping and lower effective refractive index. These parameters
Fig. 2. RMS variation of Al2 O3 /PS as a function of Al2 O3 thickness.
M. Ben Rabha et al. / Materials Science and Engineering B 178 (2013) 695–697
Fig. 3. Total reflectivity of a c-Si before and after Al2 O3 treatment.
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Fig. 5. FTIR spectra of Al2 O3 /PS surface treatment for different Al2 O3 thickness.
4. Conclusion In this work, thin films of Al2 O3 have been synthesized by PLD on PS. We have demonstrated that the Al2 O3 /PS c-Si wafer exhibit an excellent antireflection layer and improved the minority carrier lifetime as compared to the untreated ones. The main results are a decrease of surface reflectivity from 28% to about 7%. Moreover, Al2 O3 /PS treatment increase the lifetime from 2 to 7 s. Al2 O3 /PS treatment was found to be very effective in improving the optoelectronic silicon quality. Al2 O3 /PS passivation/ARC layer in the solar cells is a very important parameter to enhance photoconversion and increase light absorption in the Vis region of the solar spectrum, which is expected to increase the efficiency of solar cells (specific type of: interdigitated back contact solar cell). References
Fig. 4. Evolution of the effective lifetime for different Al2 O3 thickness.
are the primary factors that enhance PS compared with c-Si [16]. To quantify the effect of dual treatment of Al2 O3 /porous silicon on the electronic quality of the c-Si, we measured the effective carrier lifetime before and after surface treatment. Fig. 4 shows the effective carrier lifetime of the tested samples. From the lifetime versus minority carrier density characteristic one may deduce an excellent improvement of effective lifetime from 2 s to about 7 s at a minority carrier density (n) of 1 × 1015 cm−3 , which is higher than that obtained by HF/HNO3 solution [17]. The enhancement of the minority carrier lifetime could be attributed to the Al O and Si H passivation produced by Al2 O3 –PS treatment as shown in Fig. 5 by FTIR analysis (reduction of the surface recombination). After Al2 O3 , we observed the appearance of bond related to Al O, this surface chemical species can quite well passivate the PS/Si interface. The enhancement of the minority carrier lifetime was explained by other mechanism: the aluminum diffuses along the wafer surface, forms traps for recombination-active impurities, and dissolves them. The mechanism of this effect can be partially connected with the activation of internal getters, e.g., precipitated SiO2 and with the formation of complexes FeSi2 and FeB [18].
[1] G. Agostinelli, A. Delabie, P. Vitanov, Z. Alexieva, H.F.W. Dekkers, S. De Wolf, G. Beaucarne, Solar Energy Materials and Solar Cells 90 (2006) 3438. [2] B. Hoex, J. Schmidt, R. Bock, P.P.M.C. Altermatt, M. van de Sanden, W.M.M. Kessels, Applied Physics Letters 91 (2007) 112107. [3] B. Hoex, J. Schmidt, P. Pohl, M.C.M. van de Sanden, W.M.M. Kessels, Journal of Applied Physics 104 (2008) 044903. [4] J. Benick, A. Richter, M. Hermle, S.W. Glunz, Physica Status Solidi RRL 3 (2009) 233. [5] G. Dingemans, R. Seguin, P. Engelhart, M.C.M. van de Sanden, W.M.M. Kessels, Physica Status Solidi RRL 4 (2010) 10. [6] J. Mizsei, J.A. Shrair, I. Zólomy, Applied Surface Science 235 (2004) 376. [7] O. Nichiporuk, A. Kaminski, M. Lemiti, A. Fave, S. Litvinenko, V. Skryshevsky, Thin Solid Films 511–512 (2006) 248. [8] A. Focsa, A. Slaoui, H. Charifi, J.P. Stoquert, S. Roques, Materials Science and Engineering B 159 (2009) 242. [9] M. Ben Rabha, B. Bessaıs, Solar Energy 84 (2010) 486. [10] A. Ramizya, Z. Hassana, K. Omara, Y. Al-Dourib, M.A. Mahdia, Applied Surface Science 257 (2011) 6112. [11] E. Osorio, R. Urteaga, L.N. Acquaroli, G. Garcıa-algado, H. Juarez, R.R. Koropecki, Solar Energy Materials and Solar Cells 95 (2011) 3069. [12] N.V. Deshmukh, T.M. Bhave, A.S. Ethiraj, S.R. Sainkar, V. Ganeshan, S.V. Bhorasker, S.K. Kulkarni, Nanotechnology 12 (2001) 290. [13] M. Ben Rabha, S. Belhadj Mohamed, W. Dimassi, M. Gaidi, H. Ezzaouia, B. Bessais, Physica Status Solidi C 8 (2011) 883–886. [14] M. Ben Rabha, A. Hajaji, W. Dimassi, M. Gaidi, M.F. Boujmil, M. Ali Kanzari, H. Ezzaouia, B. Bessais, Materiaux & Techniques 98 (2010) 165–169. [15] B.M. Damiani, R. Liidemann, D.S. Ruby, S.H. Zaidi, A. Rohatgi, Proceedings of the 28th IEEE Photovoltaic (2000) p.371. [16] P. Vitanov, M. Kamenova, N. Tyutyundzhiev, M. Delibasheva, E. Goranova, M. Peneva, Thin Solid Films 297 (1997) 299. [17] A. Montesdeoca-Santana, B. Gonzalez-Diaz, E. Jiménez-Rodriguez, J. Ziegler, J.J. Velazquez, S. Hohage, D. Borchert, R. Guerrero-Lemus, Materials Science and Engineering B 176 (2011) 1541–1545. [18] V.G. Litovchenko, V.M. Naseka, A.A. Evtukh, Ukrainian Journal of Physics 57 (2012) 73.
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Short communication
Monocrystalline silicon surface passivation by Al2 O3 /porous silicon combined treatment M. Ben Rabha a,∗ , M. Salem a , M.A. El Khakani b , B. Bessais a , M. Gaidi a,b,c a b c
Laboratoire de Photovoltaïque, Centre de Recherches et des Technologies de l’Energie, Technopole de Borj-Cédria, BP 95, 2050 Hammam-Lif, Tunisia Institut National de la Recherche Scientifique (INRS), 1650, Blvd. Lionel-Boulet, Varennes, Québec J3X 1S2, 6 Canada Emirates college of technology, Millennium Tower, Sheikh Hamdan Street, P.O. Box: 41009, Abu Dhabi, United Arab Emirates
a r t i c l e
i n f o
Article history: Received 27 June 2012 Received in revised form 2 November 2012 Accepted 25 November 2012 Available online 7 December 2012 Keywords: Porous silicon Al2 O3 Effective minority carrier lifetime Reflectivity
a b s t r a c t In this paper, we report on the effect of Al2 O3 /porous silicon combined treatment on the surface passivation of monocrystalline silicon (c-Si). Al2 O3 films with a thickness of 5, 20 and 80 nm are deposited by pulsed laser deposition (PLD). It was demonstrated that Al2 O3 coating is a very interesting low temperature solution for surface passivation. The level of surface passivation is determined by techniques based on photoconductance and FTIR. As a result, the effective minority carrier lifetime increase from 2 s to 7 s at a minority carrier density (n) of 1 × 1015 cm−3 and the reflectivity reduce from 28% to about 7% after Al2 O3 /PS coating. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Crystalline silicon contains a large amount of impurities that negatively influence the minority carrier lifetime and in consequence the cell efficiencies. Surface passivation of crystalline silicon (c-Si) is becoming increasingly important for the performance of c-Si solar cells. Surface passivation is requisite for high efficiency solar cells as the cost-driven reduction of the solar cell thickness leads to the increase of the surface-to-volume ratio. Thin films (typically 20–80 nm) of thermal silicon dioxide (SiO2 ), silicon nitride (a-SiNx :H), and amorphous silicon (a-Si:H) are currently used for surface passivation of c-Si solar cells reference. Materials interest for the passivation of c-Si is aluminum oxide (Al2 O3 ) [1–5] and porous silicon (PS) [6–11]. In the present work, we studied the passivation and ARC effect of Al2 O3 /SP on monocrystalline silicon. We discuss the effect of stain etching-based PS and PLDAl2 O3 combined treatments on the opto-electronic properties of monocrystalline silicon. Effect of Al2 O3 thickness on the optical reflectivity and on the passivation quality is also investigated.
The research was developed in order to improve the optoelectronic properties of c-Si by combining Al2 O3 and porous silicon (PS). The stain etching (SE) processes were applied to c-Si wafers with a surface area of 1.5 cm2 in a 1:3:5 HF (48 wt%)/HNO3 (65 wt%)/H2 O solutions to form PS on c-Si surface with a thickness of 0.5 m and a porosity of 40%. Pure Al2 O3 thin films were deposited on porous silicon by means of pulsed laser deposition (PLD) technique. The Al2 O3 thickness was varied between 5 and 80 nm by controlling the laser pulses number. During deposition, the substrate holder was rotated at 5 rpm to achieve high thickness uniformity over the 6-in.-diameter coated area. The deposition temperature was kept at ∼RT. The morphology of the c-Si surface before and after Al2 O3 /porous silicon combined treatment was investigated by atomic force microscopy (AFM). The reflectivity spectra of the mc-Si surface were measured using a LAMBDA 950 UV/Vis/NIR spectrophotometer equipped with an integrating sphere. The composition of as-formed PS was analyzed by means of Fourier transform infrared spectroscopy (FTIR) using a Nicolet MAGNA 560 spectrometer. Finally, the effective lifetime was measured in a photoconduction system (WCT-120).
2. Experimental 3. Results and discussion Experiments were carried out on p-type monocrystalline silicon wafer with a thickness of 330 m and a resistivity of 0.5–3 cm.
∗ Corresponding author. E-mail address: [email protected] (M. Ben Rabha). 0921-5107/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mseb.2012.11.021
Fig. 1 shows the AFM images of the surface morphology of the monocrystalline silicon wafers before and after Al2 O3 /PS surface treatments. AFM image of the sample after PS treatment is exposed in Fig. 1a. The obtained roughness (RMS) was estimated to be around 27 nm (Fig. 1a). Fig. 1b, c, and d shows Al2 O3 films
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M. Ben Rabha et al. / Materials Science and Engineering B 178 (2013) 695–697
Fig. 1. AFM views of porous silicon surface before and after different Al2 O3 thickness: (a) porous silicon, (b) Al2 O3 thickness = 5 nm, (c) Al2 O3 thickness = 20 nm and (d) Al2 O3 thickness = 80 nm.
morphology with a thickness of 5, 20 and 80 nm, respectively, as deposited by pulsed laser deposition on porous silicon. After PLD-Al2 O3 treatment, the surface showed essentially the same aspect. Specifically, the RMS (Fig. 2) was reduced to 0.3 nm. The decrease in the RMS roughness is attributed to Si pores filling by Al2 O3 as already reported in the literature [12]. Fig. 3 shows the total reflectance spectra evolution for a c-Si substrate after different treatments of the Al2 O3 /porous silicon. These spectra show the excellent antireflection properties of the Al2 O3 /porous silicon layer. In fact the total reflectivity is reduced from 27% to about 7%, while it attains only an average value of about 12% in the case of mechanically textured wafers [13]. The achievement is comparable with that obtained using SiN-based ARC [14]. The standard silicon nitride (SiN) ARC reduces the weighted reflectance on a planar surface from 35% to 10% [15] while it attains an average value of about 4% in the case of standard KOH etched c-Si solar cell with SiN ARC [15]. This effect, as already observed by the AFM analysis, is related to suitable surface morphology structure for light trapping and lower effective refractive index. These parameters
Fig. 2. RMS variation of Al2 O3 /PS as a function of Al2 O3 thickness.
M. Ben Rabha et al. / Materials Science and Engineering B 178 (2013) 695–697
Fig. 3. Total reflectivity of a c-Si before and after Al2 O3 treatment.
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Fig. 5. FTIR spectra of Al2 O3 /PS surface treatment for different Al2 O3 thickness.
4. Conclusion In this work, thin films of Al2 O3 have been synthesized by PLD on PS. We have demonstrated that the Al2 O3 /PS c-Si wafer exhibit an excellent antireflection layer and improved the minority carrier lifetime as compared to the untreated ones. The main results are a decrease of surface reflectivity from 28% to about 7%. Moreover, Al2 O3 /PS treatment increase the lifetime from 2 to 7 s. Al2 O3 /PS treatment was found to be very effective in improving the optoelectronic silicon quality. Al2 O3 /PS passivation/ARC layer in the solar cells is a very important parameter to enhance photoconversion and increase light absorption in the Vis region of the solar spectrum, which is expected to increase the efficiency of solar cells (specific type of: interdigitated back contact solar cell). References
Fig. 4. Evolution of the effective lifetime for different Al2 O3 thickness.
are the primary factors that enhance PS compared with c-Si [16]. To quantify the effect of dual treatment of Al2 O3 /porous silicon on the electronic quality of the c-Si, we measured the effective carrier lifetime before and after surface treatment. Fig. 4 shows the effective carrier lifetime of the tested samples. From the lifetime versus minority carrier density characteristic one may deduce an excellent improvement of effective lifetime from 2 s to about 7 s at a minority carrier density (n) of 1 × 1015 cm−3 , which is higher than that obtained by HF/HNO3 solution [17]. The enhancement of the minority carrier lifetime could be attributed to the Al O and Si H passivation produced by Al2 O3 –PS treatment as shown in Fig. 5 by FTIR analysis (reduction of the surface recombination). After Al2 O3 , we observed the appearance of bond related to Al O, this surface chemical species can quite well passivate the PS/Si interface. The enhancement of the minority carrier lifetime was explained by other mechanism: the aluminum diffuses along the wafer surface, forms traps for recombination-active impurities, and dissolves them. The mechanism of this effect can be partially connected with the activation of internal getters, e.g., precipitated SiO2 and with the formation of complexes FeSi2 and FeB [18].
[1] G. Agostinelli, A. Delabie, P. Vitanov, Z. Alexieva, H.F.W. Dekkers, S. De Wolf, G. Beaucarne, Solar Energy Materials and Solar Cells 90 (2006) 3438. [2] B. Hoex, J. Schmidt, R. Bock, P.P.M.C. Altermatt, M. van de Sanden, W.M.M. Kessels, Applied Physics Letters 91 (2007) 112107. [3] B. Hoex, J. Schmidt, P. Pohl, M.C.M. van de Sanden, W.M.M. Kessels, Journal of Applied Physics 104 (2008) 044903. [4] J. Benick, A. Richter, M. Hermle, S.W. Glunz, Physica Status Solidi RRL 3 (2009) 233. [5] G. Dingemans, R. Seguin, P. Engelhart, M.C.M. van de Sanden, W.M.M. Kessels, Physica Status Solidi RRL 4 (2010) 10. [6] J. Mizsei, J.A. Shrair, I. Zólomy, Applied Surface Science 235 (2004) 376. [7] O. Nichiporuk, A. Kaminski, M. Lemiti, A. Fave, S. Litvinenko, V. Skryshevsky, Thin Solid Films 511–512 (2006) 248. [8] A. Focsa, A. Slaoui, H. Charifi, J.P. Stoquert, S. Roques, Materials Science and Engineering B 159 (2009) 242. [9] M. Ben Rabha, B. Bessaıs, Solar Energy 84 (2010) 486. [10] A. Ramizya, Z. Hassana, K. Omara, Y. Al-Dourib, M.A. Mahdia, Applied Surface Science 257 (2011) 6112. [11] E. Osorio, R. Urteaga, L.N. Acquaroli, G. Garcıa-algado, H. Juarez, R.R. Koropecki, Solar Energy Materials and Solar Cells 95 (2011) 3069. [12] N.V. Deshmukh, T.M. Bhave, A.S. Ethiraj, S.R. Sainkar, V. Ganeshan, S.V. Bhorasker, S.K. Kulkarni, Nanotechnology 12 (2001) 290. [13] M. Ben Rabha, S. Belhadj Mohamed, W. Dimassi, M. Gaidi, H. Ezzaouia, B. Bessais, Physica Status Solidi C 8 (2011) 883–886. [14] M. Ben Rabha, A. Hajaji, W. Dimassi, M. Gaidi, M.F. Boujmil, M. Ali Kanzari, H. Ezzaouia, B. Bessais, Materiaux & Techniques 98 (2010) 165–169. [15] B.M. Damiani, R. Liidemann, D.S. Ruby, S.H. Zaidi, A. Rohatgi, Proceedings of the 28th IEEE Photovoltaic (2000) p.371. [16] P. Vitanov, M. Kamenova, N. Tyutyundzhiev, M. Delibasheva, E. Goranova, M. Peneva, Thin Solid Films 297 (1997) 299. [17] A. Montesdeoca-Santana, B. Gonzalez-Diaz, E. Jiménez-Rodriguez, J. Ziegler, J.J. Velazquez, S. Hohage, D. Borchert, R. Guerrero-Lemus, Materials Science and Engineering B 176 (2011) 1541–1545. [18] V.G. Litovchenko, V.M. Naseka, A.A. Evtukh, Ukrainian Journal of Physics 57 (2012) 73.