- Email: [email protected]
Structural modification of silicon during the formation process of porous silicon
Materials Science and Engineering C 25 (2005) 595 – 598 www.elsevier.com/locate/msec
Structural modification of silicon during the formation process of porous silicon R.J. Martı´n-Palma a,*, L. Pascual b, A.R. Landa-Ca´novas b, P. Herrero b, J.M. Martı´nez-Duart a b
a Departamento de Fı´sica Aplicada, Universidad Auto´noma de Madrid, 28049 Cantoblanco, Madrid, Spain Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Cientı´ficas, 28049 Cantoblanco, Madrid, Spain
Available online 19 July 2005
Abstract Direct examination of porous silicon (PS) by the use of high resolution transmission electron microscopy (HRTEM) allowed us to perform a deep insight into the formation mechanisms of this material. In particular, the structure of the PS/Si interface and that of the silicon nanocrystals that compose porous silicon were analyzed in detail. Furthermore, image processing was used to study in detail the structure of PS. The mechanism of PS formation and lattice matching between the PS layer and the Si substrate is analyzed and discussed. Finally, a formation mechanism for PS based on the experimental observations is proposed. D 2005 Elsevier B.V. All rights reserved. Keywords: Porous silicon; HRTEM; Nanocrystal; Lattice
1. Introduction Due to its particular structure and surface reactivity, porous silicon (PS) has stimulated much research and many applications in different fields were pointed out: light emitting diodes, optical sensors, biomedical applications, interference filters, waveguides, gas sensors, solar cells, etc (see for example Refs. [1,2]). However, the formation mechanisms of PS are still unclear due to the difficulty of characterizing very thin porous layers. In particular, the morphology of the porous layer and that of the PS/Si interface will play an important role on the optoelectronic behaviour of PS-based devices. Thus, the morphology of this interface will determine the optical spectrum of PS-based optical filters, waveguides, microcavities, or the electrical response of such devices as LEDs, sensors, solar cells. Accordingly, the accurate analysis of the morphology of PS is of great significance not only to achieve a precise control of its properties, but also to improve our understanding of the physical mechanisms responsible for the peculiar behavior of PS with respect to bulk silicon. We * Corresponding author. Tel.: +34 91 497 40 28; fax: +34 91 497 39 69. E-mail address: [email protected] (R.J. Martı´n-Palma). 0928-4931/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2005.06.014
have previously investigated in detail the structure of porous silicon by means of high resolution transmission electron microscopy and digital image processing [3]. It was found that PS is composed of rounded Si nanocrystals with ˚ , embedded into characteristic sizes between 20 and 80 A an amorphous matrix and with no preferential orientation. In the present work, high resolution transmission electron microscopy (HRTEM) cross-sectional analysis was used to study in detail the structure of the silicon nanocrystals that compose PS and the PS/Si interface. Additionally, image processing was used to study in detail the structure of PS. In particular, the mechanism of PS formation and lattice matching between the PS layer and the Si substrate is analyzed and discussed.
2. Experimental Porous silicon layers were formed by the electrochemical etch of boron-doped ( p-type) silicon wafers (orientation:<100>, resistivity: 0.1– 0.5 V cm). Low resistivity ohmic contacts were formed by coating the back sides of the Si wafers with Al and subsequently annealing at 400 -C for 5 min. The electrolyte consisted of a 2 : 1 HF (48
596
R.J. Martı´n-Palma et al. / Materials Science and Engineering C 25 (2005) 595 – 598
wt.%):ethanol (98 wt.%) mixture. The Si wafers were galvanostatically etched for 180 s under illumination from a 100 W halogen lamp, and the current density was 40 mA/ cm2. The electrochemical process resulted in PS structures of a few microns thick. Finally, the samples were rinsed in ethanol after the formation of the PS layers. For HRTEM examination a JEM 3000F transmission electron microscope with 300 kV acceleration voltage and a field emission gun giving 0.17 nm point resolution were used. The procedure for the preparation of the crosssectional samples for HRTEM characterization is based on the method reported by Barna [4] with some modifications already described in detail [5]. Image processing was performed by the use of the DigitalMicrograph (version 3.7.1) image acquisition and processing software [6].
Fig. 2. Detailed HRTEM image of the PS/Si interface. The dark regions observed can be associated to diffraction contrast created by strain fields along the PS/Si interface.
3. Results Fig. 1 shows a cross-sectional view of a porous silicon layer grown onto Si. From this figure, it can be verified that the ion milling process is not amorphizing the structure of PS, since both Si and PS remain crystalline, and their morphologies are similar to that of previously analyzed PS samples [3] prepared by scraping off the silicon substrate fragments of PS. Fig. 2 shows a detailed image of the interface between PS and the Si substrate. From Figs. 1 and 2 the presence of dark zones in the PS/Si interface which correspond to a rough transition can be observed. This rough transition in the PS/ Si interface was already observed by direct TEM observation by several authors [2,7 –10] and by other indirect experiments such as X-ray reflectivity [11], X-ray scattering [12] and optical measurements [13], and has been mainly associated to an asymmetric profile of porosity at the PS/Si interface. The dark regions observed can be associated to diffraction contrast created by strain fields along the PS/Si interface [14,15]. The strain fields are caused by high stress
at the PS/Si interface, as previously determined by our group using Raman spectroscopy [16]. The structure of the porous silicon layer is shown in Fig. 3, which is composed of Si nanocrystals with characteristic round shape and no preferential orientation. The nanocrystals are embedded into an amorphous matrix. These results are in agreement with previous HRTEM observations [3] of PS samples prepared by scraping off the silicon substrate fragments of PS, rather than preparing crosssectional samples as is the case of the present work. Thus, it can be concluded that the preparation method for the samples is not affecting the structure of PS. In order to perform a detailed analysis of the structure of PS, image processing techniques were used. In particular, two different zones of the PS layer were analyzed, marked as A and B in Fig. 3. Fig. 4(a) and (b), which correspond to the zones marked as A and B in Fig. 3, respectively, show the presence of dislocations in the neighborhood of the Si
Fig. 1. HRTEM image of porous silicon grown onto a Si substrate. The interface between the Si substrate and the PS layer is clearly observed.
Fig. 3. Structure of the PS layer. Si nanocrystals with characteristic round shape and no preferential orientation are observed.
R.J. Martı´n-Palma et al. / Materials Science and Engineering C 25 (2005) 595 – 598
597
structure by the removal of Si atoms generate dislocations in the Si matrix. In other words, the defects induced by the electrochemical formation process make the Si matrix to react by accommodating those defects as dislocations. These areas of the material should be more electronically active, thus making the electrochemical attack to continue through those areas. The continuation of the electrochemical formation process leads to the segregation of individual Si nanocrystals that compose PS. For this reason, the PS/Si interface (dark zone of Figs. 1 and 2) shows high density of dislocations since that is the area where the electrochemical process was stopped. The presence of a high concentration of defects in the frontiers of the Si nanocrystals suggests that PS formation and propagation occurs through these defects. Therefore, defect-free zones remain crystalline. From our experimental observations, the formation mechanism that we propose for PS can be summarized as follows: First, the electrochemical formation process creates Si vacancies (removed by H assisted by holes from the p-type semiconductor as proposed by other authors). Then the matrix reacts creating dislocations to accommodate those defects. Lately, the etch process continues through those areas that are electronically more active, leading to the segregation of the individual Si nanocrystals that compose PS.
5. Summary
Fig. 4. Detailed view of zones A (a) and B (b) from Fig. 3. Lattice distortion is indicated.
nanocrystals that compose PS, as well as a clear lattice distortion of the cubic Si unit cell. The processed images show that dislocation defects appear on the {111} Si planes giving rise to an intense strain field which distorts the Si unit cell due to the high electric fields that arise during the formation of PS. The dislocations at the PS/Si interface and at the Si nanocrystal boundaries behave as active centers which produce, in turn, the high chemical reactivity typical of the PS surface, in comparison with that of bulk Si. This has to be taken into account for the development of any PSbased device.
4. Discussion To understand the mechanism of PS formation, it has to be taken into account that the electrochemical formation process of porous silicon consists of a non-aggressive process which removes silicon atoms without greatly disturbing the surrounding crystal matrix [17] as observed by different techniques. However, from our experimental HRTEM results, we propose that the defects created in the
From high resolution transmission electron microscopy a strong strain contrast was observed in the interface between porous silicon and the silicon substrate, caused by high stress. It was determined that this stress at the PS/Si interface is caused by dislocations. Accordingly, the high stress usually found in the PS/Si interface has its origin in a high density of dislocations. This interface has to be considered for optoelectronic applications since it would lead to anomalous optical absorption or to undesirable leakage current paths. In addition, high density of dislocations was also observed in the neighborhood of the Si nanocrystals that compose porous silicon. From the experimental results, a mechanism for the formation of porous silicon was proposed.
Acknowledgements This work was funded by the Ministerio de Ciencia y Tecnologı´a (contract number TEC2004-05260-C02-0) and the Universidad Auto´noma de Madrid (Research Project reference number CF19).
References [1] A.G. Cullis, L.T. Canham, P.D. Calcott, J. Appl. Phys. 82 (1997) 909. [2] O. Bisi, S. Ossicini, L. Pavesi, Surf. Sci. Rep. 38 (2000) 1.
598
R.J. Martı´n-Palma et al. / Materials Science and Engineering C 25 (2005) 595 – 598
[3] R.J. Martı´n-Palma, L. Pascual, P. Herrero, J.M. Martı´nez-Duart, Appl. Phys. Lett. 81 (1) (2002) 25. [4] A. Barna, Mater. Res. Soc. Symp. 254 (1991). [5] R.J. Martı´n-Palma, P. Herrero, R. Guerrero-Lemus, J.D. Moreno, J.M. Martı´nez-Duart, J. Mater. Sci. Lett. 17 (1998) 845. [6] Gatan Inc., USA (1999). [7] V. Lehmann, B. Jobst, T. Muschik, A. Kux, V. Petrova-Koch, Jpn. J. Appl. Phys. (Part 1) 32 (5A) (1993) 2095. [8] R.R. Kunz, P.M. Nitishin, H.R. Clark, M. Rothschild, B. Ahern, Appl. Phys. Lett. 67 (12) (1995) 1766. [9] E. Takasuka, K. Kamei, Appl. Phys. Lett. 65 (4) (1994) 484. [10] A. Nakajima, Y. Ohshima, T. Itakura, Y. Goto, Appl. Phys. Lett. 62 (21) (1993) 2631. [11] D. Buttard, G. Dolino, D. Bellet, T. Baumbach, F. Rietord, Solid State Commun. 109 (1995) 1.
[12] V. Chamard, P. Bastie, D. Le Bolloch, G. Dolino, E. Elkaı¨m, C. Ferrero, J.-P. Lauriat, F. Rieutord, D. Thiaudie`re, Phys. Rev. B 64 (2001) 245416. [13] V. Torres-Costa, R. Gago, R.J. Martı´n-Palma, M. Vinnichenko, R. Gro¨tzschel J.M. Martı´nez-Duart, Mat. Sci. Eng. C 23 (2003) 1043 – 1046. [14] P. Hirsch, A. Howie, R.B. Nicholson, D.W. Pashley, M.J. Whelan, Electron Microscopy of Thin Crystals, Krieger, New York, 1997. [15] D.B. Williams, C.B. Carter, Transmission Electron Microscopy, Plenum Press, New York, 1996. [16] S. Manotas, F. Agullo´-Rueda, J.D. Moreno, F. Ben-Hander, J.M. Martı´nez-Duart, Thin Solid Films 401 (2001) 306. [17] D. Buttard, D. Bellet, G. Dolino, J. Appl. Phys. 79 (10) (1996) 8060.
Structural modification of silicon during the formation process of porous silicon R.J. Martı´n-Palma a,*, L. Pascual b, A.R. Landa-Ca´novas b, P. Herrero b, J.M. Martı´nez-Duart a b
a Departamento de Fı´sica Aplicada, Universidad Auto´noma de Madrid, 28049 Cantoblanco, Madrid, Spain Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Cientı´ficas, 28049 Cantoblanco, Madrid, Spain
Available online 19 July 2005
Abstract Direct examination of porous silicon (PS) by the use of high resolution transmission electron microscopy (HRTEM) allowed us to perform a deep insight into the formation mechanisms of this material. In particular, the structure of the PS/Si interface and that of the silicon nanocrystals that compose porous silicon were analyzed in detail. Furthermore, image processing was used to study in detail the structure of PS. The mechanism of PS formation and lattice matching between the PS layer and the Si substrate is analyzed and discussed. Finally, a formation mechanism for PS based on the experimental observations is proposed. D 2005 Elsevier B.V. All rights reserved. Keywords: Porous silicon; HRTEM; Nanocrystal; Lattice
1. Introduction Due to its particular structure and surface reactivity, porous silicon (PS) has stimulated much research and many applications in different fields were pointed out: light emitting diodes, optical sensors, biomedical applications, interference filters, waveguides, gas sensors, solar cells, etc (see for example Refs. [1,2]). However, the formation mechanisms of PS are still unclear due to the difficulty of characterizing very thin porous layers. In particular, the morphology of the porous layer and that of the PS/Si interface will play an important role on the optoelectronic behaviour of PS-based devices. Thus, the morphology of this interface will determine the optical spectrum of PS-based optical filters, waveguides, microcavities, or the electrical response of such devices as LEDs, sensors, solar cells. Accordingly, the accurate analysis of the morphology of PS is of great significance not only to achieve a precise control of its properties, but also to improve our understanding of the physical mechanisms responsible for the peculiar behavior of PS with respect to bulk silicon. We * Corresponding author. Tel.: +34 91 497 40 28; fax: +34 91 497 39 69. E-mail address: [email protected] (R.J. Martı´n-Palma). 0928-4931/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2005.06.014
have previously investigated in detail the structure of porous silicon by means of high resolution transmission electron microscopy and digital image processing [3]. It was found that PS is composed of rounded Si nanocrystals with ˚ , embedded into characteristic sizes between 20 and 80 A an amorphous matrix and with no preferential orientation. In the present work, high resolution transmission electron microscopy (HRTEM) cross-sectional analysis was used to study in detail the structure of the silicon nanocrystals that compose PS and the PS/Si interface. Additionally, image processing was used to study in detail the structure of PS. In particular, the mechanism of PS formation and lattice matching between the PS layer and the Si substrate is analyzed and discussed.
2. Experimental Porous silicon layers were formed by the electrochemical etch of boron-doped ( p-type) silicon wafers (orientation:<100>, resistivity: 0.1– 0.5 V cm). Low resistivity ohmic contacts were formed by coating the back sides of the Si wafers with Al and subsequently annealing at 400 -C for 5 min. The electrolyte consisted of a 2 : 1 HF (48
596
R.J. Martı´n-Palma et al. / Materials Science and Engineering C 25 (2005) 595 – 598
wt.%):ethanol (98 wt.%) mixture. The Si wafers were galvanostatically etched for 180 s under illumination from a 100 W halogen lamp, and the current density was 40 mA/ cm2. The electrochemical process resulted in PS structures of a few microns thick. Finally, the samples were rinsed in ethanol after the formation of the PS layers. For HRTEM examination a JEM 3000F transmission electron microscope with 300 kV acceleration voltage and a field emission gun giving 0.17 nm point resolution were used. The procedure for the preparation of the crosssectional samples for HRTEM characterization is based on the method reported by Barna [4] with some modifications already described in detail [5]. Image processing was performed by the use of the DigitalMicrograph (version 3.7.1) image acquisition and processing software [6].
Fig. 2. Detailed HRTEM image of the PS/Si interface. The dark regions observed can be associated to diffraction contrast created by strain fields along the PS/Si interface.
3. Results Fig. 1 shows a cross-sectional view of a porous silicon layer grown onto Si. From this figure, it can be verified that the ion milling process is not amorphizing the structure of PS, since both Si and PS remain crystalline, and their morphologies are similar to that of previously analyzed PS samples [3] prepared by scraping off the silicon substrate fragments of PS. Fig. 2 shows a detailed image of the interface between PS and the Si substrate. From Figs. 1 and 2 the presence of dark zones in the PS/Si interface which correspond to a rough transition can be observed. This rough transition in the PS/ Si interface was already observed by direct TEM observation by several authors [2,7 –10] and by other indirect experiments such as X-ray reflectivity [11], X-ray scattering [12] and optical measurements [13], and has been mainly associated to an asymmetric profile of porosity at the PS/Si interface. The dark regions observed can be associated to diffraction contrast created by strain fields along the PS/Si interface [14,15]. The strain fields are caused by high stress
at the PS/Si interface, as previously determined by our group using Raman spectroscopy [16]. The structure of the porous silicon layer is shown in Fig. 3, which is composed of Si nanocrystals with characteristic round shape and no preferential orientation. The nanocrystals are embedded into an amorphous matrix. These results are in agreement with previous HRTEM observations [3] of PS samples prepared by scraping off the silicon substrate fragments of PS, rather than preparing crosssectional samples as is the case of the present work. Thus, it can be concluded that the preparation method for the samples is not affecting the structure of PS. In order to perform a detailed analysis of the structure of PS, image processing techniques were used. In particular, two different zones of the PS layer were analyzed, marked as A and B in Fig. 3. Fig. 4(a) and (b), which correspond to the zones marked as A and B in Fig. 3, respectively, show the presence of dislocations in the neighborhood of the Si
Fig. 1. HRTEM image of porous silicon grown onto a Si substrate. The interface between the Si substrate and the PS layer is clearly observed.
Fig. 3. Structure of the PS layer. Si nanocrystals with characteristic round shape and no preferential orientation are observed.
R.J. Martı´n-Palma et al. / Materials Science and Engineering C 25 (2005) 595 – 598
597
structure by the removal of Si atoms generate dislocations in the Si matrix. In other words, the defects induced by the electrochemical formation process make the Si matrix to react by accommodating those defects as dislocations. These areas of the material should be more electronically active, thus making the electrochemical attack to continue through those areas. The continuation of the electrochemical formation process leads to the segregation of individual Si nanocrystals that compose PS. For this reason, the PS/Si interface (dark zone of Figs. 1 and 2) shows high density of dislocations since that is the area where the electrochemical process was stopped. The presence of a high concentration of defects in the frontiers of the Si nanocrystals suggests that PS formation and propagation occurs through these defects. Therefore, defect-free zones remain crystalline. From our experimental observations, the formation mechanism that we propose for PS can be summarized as follows: First, the electrochemical formation process creates Si vacancies (removed by H assisted by holes from the p-type semiconductor as proposed by other authors). Then the matrix reacts creating dislocations to accommodate those defects. Lately, the etch process continues through those areas that are electronically more active, leading to the segregation of the individual Si nanocrystals that compose PS.
5. Summary
Fig. 4. Detailed view of zones A (a) and B (b) from Fig. 3. Lattice distortion is indicated.
nanocrystals that compose PS, as well as a clear lattice distortion of the cubic Si unit cell. The processed images show that dislocation defects appear on the {111} Si planes giving rise to an intense strain field which distorts the Si unit cell due to the high electric fields that arise during the formation of PS. The dislocations at the PS/Si interface and at the Si nanocrystal boundaries behave as active centers which produce, in turn, the high chemical reactivity typical of the PS surface, in comparison with that of bulk Si. This has to be taken into account for the development of any PSbased device.
4. Discussion To understand the mechanism of PS formation, it has to be taken into account that the electrochemical formation process of porous silicon consists of a non-aggressive process which removes silicon atoms without greatly disturbing the surrounding crystal matrix [17] as observed by different techniques. However, from our experimental HRTEM results, we propose that the defects created in the
From high resolution transmission electron microscopy a strong strain contrast was observed in the interface between porous silicon and the silicon substrate, caused by high stress. It was determined that this stress at the PS/Si interface is caused by dislocations. Accordingly, the high stress usually found in the PS/Si interface has its origin in a high density of dislocations. This interface has to be considered for optoelectronic applications since it would lead to anomalous optical absorption or to undesirable leakage current paths. In addition, high density of dislocations was also observed in the neighborhood of the Si nanocrystals that compose porous silicon. From the experimental results, a mechanism for the formation of porous silicon was proposed.
Acknowledgements This work was funded by the Ministerio de Ciencia y Tecnologı´a (contract number TEC2004-05260-C02-0) and the Universidad Auto´noma de Madrid (Research Project reference number CF19).
References [1] A.G. Cullis, L.T. Canham, P.D. Calcott, J. Appl. Phys. 82 (1997) 909. [2] O. Bisi, S. Ossicini, L. Pavesi, Surf. Sci. Rep. 38 (2000) 1.
598
R.J. Martı´n-Palma et al. / Materials Science and Engineering C 25 (2005) 595 – 598
[3] R.J. Martı´n-Palma, L. Pascual, P. Herrero, J.M. Martı´nez-Duart, Appl. Phys. Lett. 81 (1) (2002) 25. [4] A. Barna, Mater. Res. Soc. Symp. 254 (1991). [5] R.J. Martı´n-Palma, P. Herrero, R. Guerrero-Lemus, J.D. Moreno, J.M. Martı´nez-Duart, J. Mater. Sci. Lett. 17 (1998) 845. [6] Gatan Inc., USA (1999). [7] V. Lehmann, B. Jobst, T. Muschik, A. Kux, V. Petrova-Koch, Jpn. J. Appl. Phys. (Part 1) 32 (5A) (1993) 2095. [8] R.R. Kunz, P.M. Nitishin, H.R. Clark, M. Rothschild, B. Ahern, Appl. Phys. Lett. 67 (12) (1995) 1766. [9] E. Takasuka, K. Kamei, Appl. Phys. Lett. 65 (4) (1994) 484. [10] A. Nakajima, Y. Ohshima, T. Itakura, Y. Goto, Appl. Phys. Lett. 62 (21) (1993) 2631. [11] D. Buttard, G. Dolino, D. Bellet, T. Baumbach, F. Rietord, Solid State Commun. 109 (1995) 1.
[12] V. Chamard, P. Bastie, D. Le Bolloch, G. Dolino, E. Elkaı¨m, C. Ferrero, J.-P. Lauriat, F. Rieutord, D. Thiaudie`re, Phys. Rev. B 64 (2001) 245416. [13] V. Torres-Costa, R. Gago, R.J. Martı´n-Palma, M. Vinnichenko, R. Gro¨tzschel J.M. Martı´nez-Duart, Mat. Sci. Eng. C 23 (2003) 1043 – 1046. [14] P. Hirsch, A. Howie, R.B. Nicholson, D.W. Pashley, M.J. Whelan, Electron Microscopy of Thin Crystals, Krieger, New York, 1997. [15] D.B. Williams, C.B. Carter, Transmission Electron Microscopy, Plenum Press, New York, 1996. [16] S. Manotas, F. Agullo´-Rueda, J.D. Moreno, F. Ben-Hander, J.M. Martı´nez-Duart, Thin Solid Films 401 (2001) 306. [17] D. Buttard, D. Bellet, G. Dolino, J. Appl. Phys. 79 (10) (1996) 8060.