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Formation of microcrystalline silicon at low temperatures and role of hydrogen
Journal of Non-Crystalline Solids 338–340 (2004) 287–290 www.elsevier.com/locate/jnoncrysol
Section 4. Hydrogen in amorphous and micro-crystalline silicon
Formation of microcrystalline silicon at low temperatures and role of hydrogen J. Kocka a,*, T. Mates a, P. Fojtık a, M. Ledinsk y a, K. Luterov a a, H. Stuchlıkova a, a a a b b J. Stuchlık , I. Pelant , A. Fejfar , M. Ito , K. Ro , H. Uyama b a
b
Institute of Physics, Academy of Sciences of the Czech Republic (ASCR), Cukrovarnicka 10, 162 53 Prague, Czech Republic Technical Research Institute, Toppan Printing Co., 4-2-3 Takanodai-Minami, Sugito machi, Kita Katsushika, Saitama 345 8508, Japan Available online 2 April 2004
Abstract Changes in the growth of thin silicon films at very low substrate temperatures (35 °C < TS < 100 °C) near the a-Si:H/lc-Si:H transition region are illustrated using RMS roughness, crystallinity and hydrogen content measurements together with the corresponding evolution of the optoelectronic properties: dark conductivity rD , its activation energy Ea and prefactor r0 as well as the ambipolar diffusion length Ldiff . The role of hydrogen content, which increases at lower TS , is studied in relation to grain boundary formation. Ó 2004 Elsevier B.V. All rights reserved. PACS: 73.50.)h; 68.55.)a; 81.15.Gh
1. Introduction Demonstration of solar cell operation for cells prepared from amorphous hydrogenated silicon (a-Si:H) and hydrogenated microcrystalline silicon (lc-Si:H) [1] attracted a lot of attention. The possibility to use flexible (cheap) plastic substrates would open new areas for applications of these materials but imposes a limit on substrate temperature (TS ) of TS < 100 °C. It is generally accepted that lowering TS decreases the mobility of the growth precursors, hindering formation of lc-Si:H. This can be overcome by an increase of H2 to SiH4 dilution ratio, as we have recently illustrated [2] on a series of samples prepared at TS ¼ 80 °C. In this paper, we first add new results for TS ¼ 80 °C series following our model of transport in lc-Si:H [3,4] and summarize features typical for a-Si:H/lc-Si:H transition region. Then we present the results of microscopic topography, Raman evaluated crystallinity, content of H, as well as electrical properties of lc-Si:H sample series prepared at high dilution in a wide range
*
Corresponding author. Tel.: +420-2 2031 8449; fax: +420-2 3334 3184. E-mail address: [email protected] (J. Kocka). 0022-3093/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.02.086
of TS . Finally the role of H content in a formation of grain boundaries and crystallization is discussed. 2. Experimental The first series of thin silicon samples [2] has been prepared at a very high frequency of 54 MHz, TS ¼ 80 °C and with the dilution (rH ¼ [H2 ]/[SiH4 ]) changing from 26 to 168. The second series has been prepared also at 54 MHz at high dilution (rH ¼ 134) in a wide range of temperatures 35 °C < TS < 200 °C. The thickness of samples of the first series was around 1 lm and around 350 nm for the second series. For a crystalline fraction (Xc ) the Raman spectroscopy has been used. Coplanar contacts have been used for the measurement of DC dark conductivity (rD ), its activation energy (Ea ) and prefactor (r0 ) as well as for the steady-state photocarrier grating (SSPG) method, from which the ambipolar diffusion length parallel to the substrate (Ldiff ) can be evaluated. H content was determined by elastic recoil detection analysis (ERDA) [5]. For evaluation of roughness, we have used our modified atomic force microscope (AFM) [6] by which we measure in parallel surface topography and a local current map. The details of different evaluation techniques are in [4].
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J. Kocka et al. / Journal of Non-Crystalline Solids 338–340 (2004) 287–290
3. Results (a)
For a dilution series prepared at TS ¼ 80 °C [2] we have found a sharp peak in Ldiff and RMS roughness at rH ¼ 51, see Fig. 1(a). Within the same a-Si:H/lc-Si:H transition region (shadowed in Fig. 1) the sharp rise of dark conductivity above the rD 107 X1 cm1 value and crystallinity from 0% to 70% [2] have been observed. This, together with the topography illustrated fact that the c-Si grains just started to touch [2] at rH ¼ 51, indicates the presence of the percolation threshold. Having in mind our model of transport in lcSi:H [3,4] there is a question whether the percolation threshold is related to formation of ‘large grains’, for which the typical feature is the drop of conductivity prefactor r0 below 100 X1 cm1 and activation energy Ea below 0.5 eV. In Fig. 1(b) there are r0 and Ea evaluated recently as a function of rH for TS ¼ 80 °C series. Clearly large grains are formed also in a transition region. This series is a nice example of the sharp and narrow a-Si:H/lc-Si:H transition region. To test the lowest TS for sufficient crystallinity, we have prepared the series of lc-Si:H samples with TS from 35 to 200 °C. In Fig. 2 there are for three lc-Si:H samples, prepared at TS ¼ 200, 65 and 35 °C, the topography pictures measured by air AFM (a) and the topography (b) measured simultaneously with the map of local currents (c) in an UHV-AFM [6]. In Fig. 3 the dark conductivity, crystallinity and roughness are plotted as a function of TS . The corresponding activation energy and prefactor of the dark conductivity together with the diffusion length and H content are for the same samples in Fig. 4.
(b)
Fig. 1. Diffusion length Ldiff from SSPG and surface RMS roughness (a) of thin silicon films together with the values of conductivity prefactor r0 and activation energy Ea (b) as a function of the H dilution ratio rH . For the meaning of the horizontal line at r0 ¼ 100 X1 cm1 and Ea ¼ 0:5 eV see text.
4. Discussion The air AFM topography of the sample prepared at TS ¼ 35 °C, see Fig. 2(a), was a real surprise by
Fig. 2. Surface topography taken by air AFM microscope (a) and UHV-AFM topography (b) combined with the map of local currents (c) of three thin silicon samples deposited at substrate temperatures TS ¼ 200, 65 and 35 °C.
J. Kocka et al. / Journal of Non-Crystalline Solids 338–340 (2004) 287–290
Fig. 3. Room temperature dark DC conductivity rD of thin silicon films together with crystalline volume ratio XC and surface RMS roughness as a function of the substrate temperature TS .
(a)
(b)
Fig. 4. Diffusion length Ldiff from SSPG and hydrogen content from ERDA (a) of thin silicon films together with the values of conductivity prefactor r0 and activation energy Ea (b) as a function of the substrate temperature TS . For the meaning of the horizontal line at r0 ¼ 100 X1 cm1 and Ea ¼ 0:5 eV see text.
289
indicating ‘large crystallites’. Indeed (see Fig. 3) more than 50% crystallinity was later confirmed. The topography measured by UHV-AFM, see Fig. 2(b), is not influenced by the surface adsorbates and gives much better resolution: for TS ¼ 35 °C sample it illustrates that ‘large crystallites’ are actually composed of smaller ones. Very interesting results are seen from the maps of local currents, see Fig. 2(c), in which white regions represent more conductive crystalline parts and black ones much less conductive amorphous tissue [6]. It is evident that even UHV topography does not precisely describe the real microstructure. The TS ¼ 200 °C sample is formed by a lot of tiny crystallites, feature typical for high H2 dilution, for TS ¼ 65 °C sample the increased amorphous tissue is evident and in TS ¼ 35 °C sample the crystallites are mostly isolated by the amorphous tissue. This explains why for TS ¼ 35 °C sample, even with 50% crystallinity, the transport properties are controlled by amorphous tissue, see Fig. 3. With decreasing TS from 200 to 100 °C the conductivity decreases, then it starts to rise again and finally drops below typical value of rD 107 X1 cm1 at about 60 °C. Moreover, for TS < 60 °C the crystallinity drops below 70% and the roughness starts to rise – all these facts indicate that there is the a-Si:H/lc-Si:H transition region for TS < 60 °C, see Fig. 3. Usually observed additional features are the rise (peak) in diffusion length (see Fig. 1) and H content [5]. Results in Fig. 4(a) support the a-Si:H/lc-Si:H transition region for TS < 60 °C, expected rise in H content is evident and slow increase of Ldiff is also observed. In this figure there are, however, also unexpected results: surprising drop of Ldiff for TS < 100 °C and the additional peak in H content at TS 80 °C, while there is no change in topography, see RMS roughness in Fig. 3. What is the reason? In Fig. 4(b) there are the r0 and Ea shown as a function of TS . Sample prepared at TS ¼ 100 °C is the only sample outside transition region, for which values of r0 and Ea indicate no large grain boundaries (LGB). For lower (as well as higher) TS the r0 and Ea are below the limits (r0 < 100 X1 cm1 and Ea < 0:5 eV), it means that there are the LGB formed. Our hypothesis is that small decrease of crystallinity below TS 100 °C (see Fig. 3), together with the rise of H content leads to Hinduced formation of LGB for TS < 100 °C! Actually the opposite process – the annihilation of LGB – has been observed by Yoon [7]. After annealing Ea increased above 0.5 eV and r0 (evaluated from his data by us [4]) above the 100 X1 cm1 limit. Because the main observed change [7] was the decrease of H, he concluded that the grain boundaries have been modified. In agreement with [8] the important role of H has been observed for another low TS process – solid phase crystallization [9].
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J. Kocka et al. / Journal of Non-Crystalline Solids 338–340 (2004) 287–290
5. Conclusions
References
We have illustrated that the peaks in Ldiff , roughness and H content are together with the sharp change of the rD , Ea , r0 and Xc the features, typical for a-Si:H/lc-Si:H transition. We have found that even at TS ¼ 35 °C the crystallinity as high as 50% can be achieved. Dramatic changes of transport properties (Ldiff , rD ) have been observed for lc-Si:H temperature series also outside the a-Si:H/lc-Si:H transition region (60 °C < TS < 100 °C). We have proposed, in agreement with the recent independent result [7], that H-induced changes of the grain boundaries can explain these observations.
[1] J. Meier, R. Fl€ uckiger, H. Keppner, A. Shah, Appl. Phys. Lett. 65 (1994) 860. [2] T. Mates, A. Fejfar, I. Drbohlav, B. Rezek, P. Fojtık, K. Luterova, J. Kocka, Ch. Koch, M.B. Schubert, M. Ito, K. Ro, H. Uyama, J. Non-Cryst. Solids 299–302 (2002) 767. [3] J. Kocka, H. Stuchlıkova, J. Stuchlık, B. Rezek, T. Mates, V. cek, P. Fojtık, I. Pelant, A. Fejfar, J. Non-Cryst. Solids 299–302 Svr (2002) 355. [4] J. Kocka, A. Fejfar, H. Stuchlıkova, J. Stuchlık, P. Fojtık, T. cek, I. Pelant, Solar Energ. Mates, B. Rezek, K. Luterova, V. Svr Mater. Solar Cells 78 (2003) 493. [5] U. Kroll, J. Meier, A. Shah, S. Mikhailov, J. Weber, J. Appl. Phys. 80 (1996) 4971. [6] B. Rezek, J. Stuchlık, A. Fejfar, J. Kocka, Appl. Phys. Lett. 74 (1999) 1475. [7] J.H. Yoon, MRS Symp. Proc. 664 (2001) A.23.6.1. [8] A.H.M. Smets, W.M.M. Kessels, M.C.M. van de Sanden, Appl. Phys. Lett. 82 (2003) 1547. [9] P. Fojtık, K. Dohnalova, T. Mates, J. Stuchlık, I. Gregora, J. Chval, A. Fejfar, J. Kocka, I. Pelant, Philos. Mag. B 82 (2002) 1785.
Acknowledgements The authors would like to thank the support by B2949101, IAA1010316 AVOZ 1010914, GA AV CR and GA CR 202/03/0789 projects.
Section 4. Hydrogen in amorphous and micro-crystalline silicon
Formation of microcrystalline silicon at low temperatures and role of hydrogen J. Kocka a,*, T. Mates a, P. Fojtık a, M. Ledinsk y a, K. Luterov a a, H. Stuchlıkova a, a a a b b J. Stuchlık , I. Pelant , A. Fejfar , M. Ito , K. Ro , H. Uyama b a
b
Institute of Physics, Academy of Sciences of the Czech Republic (ASCR), Cukrovarnicka 10, 162 53 Prague, Czech Republic Technical Research Institute, Toppan Printing Co., 4-2-3 Takanodai-Minami, Sugito machi, Kita Katsushika, Saitama 345 8508, Japan Available online 2 April 2004
Abstract Changes in the growth of thin silicon films at very low substrate temperatures (35 °C < TS < 100 °C) near the a-Si:H/lc-Si:H transition region are illustrated using RMS roughness, crystallinity and hydrogen content measurements together with the corresponding evolution of the optoelectronic properties: dark conductivity rD , its activation energy Ea and prefactor r0 as well as the ambipolar diffusion length Ldiff . The role of hydrogen content, which increases at lower TS , is studied in relation to grain boundary formation. Ó 2004 Elsevier B.V. All rights reserved. PACS: 73.50.)h; 68.55.)a; 81.15.Gh
1. Introduction Demonstration of solar cell operation for cells prepared from amorphous hydrogenated silicon (a-Si:H) and hydrogenated microcrystalline silicon (lc-Si:H) [1] attracted a lot of attention. The possibility to use flexible (cheap) plastic substrates would open new areas for applications of these materials but imposes a limit on substrate temperature (TS ) of TS < 100 °C. It is generally accepted that lowering TS decreases the mobility of the growth precursors, hindering formation of lc-Si:H. This can be overcome by an increase of H2 to SiH4 dilution ratio, as we have recently illustrated [2] on a series of samples prepared at TS ¼ 80 °C. In this paper, we first add new results for TS ¼ 80 °C series following our model of transport in lc-Si:H [3,4] and summarize features typical for a-Si:H/lc-Si:H transition region. Then we present the results of microscopic topography, Raman evaluated crystallinity, content of H, as well as electrical properties of lc-Si:H sample series prepared at high dilution in a wide range
*
Corresponding author. Tel.: +420-2 2031 8449; fax: +420-2 3334 3184. E-mail address: [email protected] (J. Kocka). 0022-3093/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.02.086
of TS . Finally the role of H content in a formation of grain boundaries and crystallization is discussed. 2. Experimental The first series of thin silicon samples [2] has been prepared at a very high frequency of 54 MHz, TS ¼ 80 °C and with the dilution (rH ¼ [H2 ]/[SiH4 ]) changing from 26 to 168. The second series has been prepared also at 54 MHz at high dilution (rH ¼ 134) in a wide range of temperatures 35 °C < TS < 200 °C. The thickness of samples of the first series was around 1 lm and around 350 nm for the second series. For a crystalline fraction (Xc ) the Raman spectroscopy has been used. Coplanar contacts have been used for the measurement of DC dark conductivity (rD ), its activation energy (Ea ) and prefactor (r0 ) as well as for the steady-state photocarrier grating (SSPG) method, from which the ambipolar diffusion length parallel to the substrate (Ldiff ) can be evaluated. H content was determined by elastic recoil detection analysis (ERDA) [5]. For evaluation of roughness, we have used our modified atomic force microscope (AFM) [6] by which we measure in parallel surface topography and a local current map. The details of different evaluation techniques are in [4].
288
J. Kocka et al. / Journal of Non-Crystalline Solids 338–340 (2004) 287–290
3. Results (a)
For a dilution series prepared at TS ¼ 80 °C [2] we have found a sharp peak in Ldiff and RMS roughness at rH ¼ 51, see Fig. 1(a). Within the same a-Si:H/lc-Si:H transition region (shadowed in Fig. 1) the sharp rise of dark conductivity above the rD 107 X1 cm1 value and crystallinity from 0% to 70% [2] have been observed. This, together with the topography illustrated fact that the c-Si grains just started to touch [2] at rH ¼ 51, indicates the presence of the percolation threshold. Having in mind our model of transport in lcSi:H [3,4] there is a question whether the percolation threshold is related to formation of ‘large grains’, for which the typical feature is the drop of conductivity prefactor r0 below 100 X1 cm1 and activation energy Ea below 0.5 eV. In Fig. 1(b) there are r0 and Ea evaluated recently as a function of rH for TS ¼ 80 °C series. Clearly large grains are formed also in a transition region. This series is a nice example of the sharp and narrow a-Si:H/lc-Si:H transition region. To test the lowest TS for sufficient crystallinity, we have prepared the series of lc-Si:H samples with TS from 35 to 200 °C. In Fig. 2 there are for three lc-Si:H samples, prepared at TS ¼ 200, 65 and 35 °C, the topography pictures measured by air AFM (a) and the topography (b) measured simultaneously with the map of local currents (c) in an UHV-AFM [6]. In Fig. 3 the dark conductivity, crystallinity and roughness are plotted as a function of TS . The corresponding activation energy and prefactor of the dark conductivity together with the diffusion length and H content are for the same samples in Fig. 4.
(b)
Fig. 1. Diffusion length Ldiff from SSPG and surface RMS roughness (a) of thin silicon films together with the values of conductivity prefactor r0 and activation energy Ea (b) as a function of the H dilution ratio rH . For the meaning of the horizontal line at r0 ¼ 100 X1 cm1 and Ea ¼ 0:5 eV see text.
4. Discussion The air AFM topography of the sample prepared at TS ¼ 35 °C, see Fig. 2(a), was a real surprise by
Fig. 2. Surface topography taken by air AFM microscope (a) and UHV-AFM topography (b) combined with the map of local currents (c) of three thin silicon samples deposited at substrate temperatures TS ¼ 200, 65 and 35 °C.
J. Kocka et al. / Journal of Non-Crystalline Solids 338–340 (2004) 287–290
Fig. 3. Room temperature dark DC conductivity rD of thin silicon films together with crystalline volume ratio XC and surface RMS roughness as a function of the substrate temperature TS .
(a)
(b)
Fig. 4. Diffusion length Ldiff from SSPG and hydrogen content from ERDA (a) of thin silicon films together with the values of conductivity prefactor r0 and activation energy Ea (b) as a function of the substrate temperature TS . For the meaning of the horizontal line at r0 ¼ 100 X1 cm1 and Ea ¼ 0:5 eV see text.
289
indicating ‘large crystallites’. Indeed (see Fig. 3) more than 50% crystallinity was later confirmed. The topography measured by UHV-AFM, see Fig. 2(b), is not influenced by the surface adsorbates and gives much better resolution: for TS ¼ 35 °C sample it illustrates that ‘large crystallites’ are actually composed of smaller ones. Very interesting results are seen from the maps of local currents, see Fig. 2(c), in which white regions represent more conductive crystalline parts and black ones much less conductive amorphous tissue [6]. It is evident that even UHV topography does not precisely describe the real microstructure. The TS ¼ 200 °C sample is formed by a lot of tiny crystallites, feature typical for high H2 dilution, for TS ¼ 65 °C sample the increased amorphous tissue is evident and in TS ¼ 35 °C sample the crystallites are mostly isolated by the amorphous tissue. This explains why for TS ¼ 35 °C sample, even with 50% crystallinity, the transport properties are controlled by amorphous tissue, see Fig. 3. With decreasing TS from 200 to 100 °C the conductivity decreases, then it starts to rise again and finally drops below typical value of rD 107 X1 cm1 at about 60 °C. Moreover, for TS < 60 °C the crystallinity drops below 70% and the roughness starts to rise – all these facts indicate that there is the a-Si:H/lc-Si:H transition region for TS < 60 °C, see Fig. 3. Usually observed additional features are the rise (peak) in diffusion length (see Fig. 1) and H content [5]. Results in Fig. 4(a) support the a-Si:H/lc-Si:H transition region for TS < 60 °C, expected rise in H content is evident and slow increase of Ldiff is also observed. In this figure there are, however, also unexpected results: surprising drop of Ldiff for TS < 100 °C and the additional peak in H content at TS 80 °C, while there is no change in topography, see RMS roughness in Fig. 3. What is the reason? In Fig. 4(b) there are the r0 and Ea shown as a function of TS . Sample prepared at TS ¼ 100 °C is the only sample outside transition region, for which values of r0 and Ea indicate no large grain boundaries (LGB). For lower (as well as higher) TS the r0 and Ea are below the limits (r0 < 100 X1 cm1 and Ea < 0:5 eV), it means that there are the LGB formed. Our hypothesis is that small decrease of crystallinity below TS 100 °C (see Fig. 3), together with the rise of H content leads to Hinduced formation of LGB for TS < 100 °C! Actually the opposite process – the annihilation of LGB – has been observed by Yoon [7]. After annealing Ea increased above 0.5 eV and r0 (evaluated from his data by us [4]) above the 100 X1 cm1 limit. Because the main observed change [7] was the decrease of H, he concluded that the grain boundaries have been modified. In agreement with [8] the important role of H has been observed for another low TS process – solid phase crystallization [9].
290
J. Kocka et al. / Journal of Non-Crystalline Solids 338–340 (2004) 287–290
5. Conclusions
References
We have illustrated that the peaks in Ldiff , roughness and H content are together with the sharp change of the rD , Ea , r0 and Xc the features, typical for a-Si:H/lc-Si:H transition. We have found that even at TS ¼ 35 °C the crystallinity as high as 50% can be achieved. Dramatic changes of transport properties (Ldiff , rD ) have been observed for lc-Si:H temperature series also outside the a-Si:H/lc-Si:H transition region (60 °C < TS < 100 °C). We have proposed, in agreement with the recent independent result [7], that H-induced changes of the grain boundaries can explain these observations.
[1] J. Meier, R. Fl€ uckiger, H. Keppner, A. Shah, Appl. Phys. Lett. 65 (1994) 860. [2] T. Mates, A. Fejfar, I. Drbohlav, B. Rezek, P. Fojtık, K. Luterova, J. Kocka, Ch. Koch, M.B. Schubert, M. Ito, K. Ro, H. Uyama, J. Non-Cryst. Solids 299–302 (2002) 767. [3] J. Kocka, H. Stuchlıkova, J. Stuchlık, B. Rezek, T. Mates, V. cek, P. Fojtık, I. Pelant, A. Fejfar, J. Non-Cryst. Solids 299–302 Svr (2002) 355. [4] J. Kocka, A. Fejfar, H. Stuchlıkova, J. Stuchlık, P. Fojtık, T. cek, I. Pelant, Solar Energ. Mates, B. Rezek, K. Luterova, V. Svr Mater. Solar Cells 78 (2003) 493. [5] U. Kroll, J. Meier, A. Shah, S. Mikhailov, J. Weber, J. Appl. Phys. 80 (1996) 4971. [6] B. Rezek, J. Stuchlık, A. Fejfar, J. Kocka, Appl. Phys. Lett. 74 (1999) 1475. [7] J.H. Yoon, MRS Symp. Proc. 664 (2001) A.23.6.1. [8] A.H.M. Smets, W.M.M. Kessels, M.C.M. van de Sanden, Appl. Phys. Lett. 82 (2003) 1547. [9] P. Fojtık, K. Dohnalova, T. Mates, J. Stuchlık, I. Gregora, J. Chval, A. Fejfar, J. Kocka, I. Pelant, Philos. Mag. B 82 (2002) 1785.
Acknowledgements The authors would like to thank the support by B2949101, IAA1010316 AVOZ 1010914, GA AV CR and GA CR 202/03/0789 projects.