- Email: [email protected]
Role of grains in protocrystalline silicon layers grown at very low substrate temperatures and studied by atomic force microscopy
Journal of Non-Crystalline Solids 299–302 (2002) 767–771 www.elsevier.com/locate/jnoncrysol
Role of grains in protocrystalline silicon layers grown at very low substrate temperatures and studied by atomic force microscopy T. Mates a,*, A. Fejfar a, I. Drbohlav a, B. Rezek a, P. Fojtık a, K. Luterov a a, J. Kocka a, C. Koch b, M.B. Schubert b, M. Ito c, K. Ro c, H. Uyama c a
Institute of Physical Electronics, Academy of Sciences of the Czech Republic, Cukrovarnick a 10, 162 53 Prague, Czech Republic b Institute of Physical Electronics, Stuttgart University, Pfaffenwaldring 47, 70569 Stuttgart, Germany c Technical Research Institute, Toppan Printing Co., 4-2-3 Takanodai-Minami, Sugito machi, Kita Katsushika, Saitama 345 8508, Japan
Abstract We have investigated the role of the sample thickness and silane dilution on the structure and electronic properties of protocrystalline silicon thin films deposited at very low substrate temperatures ð 80 °CÞ. Coincidence of the maxima in surface roughness and ambipolar diffusion length ðJ 100 nmÞ with formation of the network of interconnected crystalline grain aggregates was observed. While the presence of the isolated grain aggregates improves the photoconductive properties before the percolation threshold is reached, further increase in crystallinity may have opposite effect due to detrimental role of increasing concentration of the defective grain boundaries. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 68.55.-a; 81.15.Gh; 68.37.Ps; 73.50.-h
1. Introduction Hydrogenated microcrystalline silicon (lc-Si:H) prepared by plasma-enhanced chemical vapor deposition (PECVD) represents a wide class of materials. Structure and properties of the deposited films depend on many parameters: plasma discharge frequency and power, pressure and flow rates of silane and hydrogen, substrate material
*
Corresponding author. E-mail address: [email protected] (T. Mates).
and deposition temperature [1,2]. Moreover, the properties may change considerably with increasing film thickness [3–5]. Usually the film grows at first amorphous as so-called incubation layer and only then the crystalline nuclei appear and grow [6,7]. The properties of the films may evolve even after the transition to fully crystalline growth is completed [8]. In spite of an intensive research effort, only a small part of the possible parameter space for lcSi:H deposition was explored. Recently the research focussed on depositions at low substrate temperatures, below the standard 200–250 °C
0022-3093/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 0 9 8 0 - 2
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T. Mates et al. / Journal of Non-Crystalline Solids 299–302 (2002) 767–771
originating from optimization of hydrogenated amorphous silicon (a-Si:H). Two reasons motivated an exploration of low temperature depositions of lc-Si:H. Firstly, an obvious reason was the use of cheap polymer substrates for lc-Si:H based solar cells. Later it became clear that some unexpected physical phenomena may occur at low growth temperatures (around 150 °C), such as suppressed oxygen donor formation [9] and isotropic transport properties of lc-Si:H [10]. Even lower temperatures have to be used for depositing lc-Si:H on practical polymer substrates, e.g. polyethylene terephtalate. It was reported that at temperatures K 80 °C the best solar cells can be prepared using deposition conditions close to the boundary between a-Si and lc-Si growth [11,12]. The material grown close to this boundary is often called protocrystalline silicon which can be loosely defined as a lc-Si:H with an extended transition layer thickness. Close to the a=lc boundary a sharp maximum of ratio of photo/dark conductivity (the quantity often used as a figure of merit for absorbers in solar cells) was found together with a corresponding optimum in the solar cell performance [11,12]. The physical reason for this optimum is still not completely clear. One reason for this uncertainty may be that the common methods of transport characterization assume homogeneous material and so their results are difficult to understand for a material which is a complex mixture of the amorphous phase and crystalline grains and in which the grain boundaries play an important role. This led us to the use of atomic force microscope (AFM) with a conductive cantilever for probing local electronic properties of individual grains in lc-Si:H layers with sizes down to few tens of nanometer [13]. In this paper we present study of the morphology and transport properties of protocrystalline silicon deposited at very low temperatures. We have focused on two crucial parameters: film thickness and silane dilution. By variation of either of them the boundary between amorphous and microcrystalline growth can be crossed. We will show that the optimum of the transport properties occurs near the percolation threshold, i.e., when the crystalline grain aggregates create a connected network. Finally, we will show how a simple geo-
metrical model can be used to simulate the growth and properties of these films. 2. Experimental We have prepared two series of lc-Si:H samples. The thickness series was deposited at substrate temperature of 75 °C, discharge frequency of 54 MHz and hydrogen dilution ratio rH ¼ ð½SiH4 þ ½H2 Þ=½SiH4 ¼ 29, i.e., close to the a=lc boundary, with the sample thickness ranging from 40 to 1000 nm. The second series was prepared by changing the hydrogen dilution ratio rH from 26 to 168 at a fixed substrate temperature of 80 °C, discharge frequency of 54 MHz and adjusted deposition duration to achieve thickness around 1 lm. Morphology of the samples of both the series were measured by the ambient AFM (Topometrix Explorer) in a contact mode using soft, triangle shaped cantilevers. Simultaneous measurements of the morphology and local current were done in ultra-high vacuum (UHV) using AFM with conductivite cantilever. Crystallinity was determined from the ratio of crystalline to amorphous peaks in the Raman spectra. For the measurement of the optoelectronic properties, NiCr coplanar gap electrodes were deposited on the top of the samples and used to measure dark conductivity rDC , subgap absorption spectra by constant photocurrent method and ambipolar diffusion length parallel to the substrate by steady state photocarrier grating technique (SSPG). 3. Results Within both thickness and dilution series the morphology undergoes a transition from a rather smooth amorphous surface with only isolated grains to rough lc-Si:H surface composed of grain aggregates with a typical cauliflower structure. The transition from a-Si:H to lc-Si:H with increasing thickness is shown in Fig. 1. After overcoming the incubation layer the circular grains protruding above the amorphous matrix start to develop (leftmost image at the 40 nm thickness). At 100 nm thickness, grains start to collide with their neighbors, forming grain boundaries. Structure on the
T. Mates et al. / Journal of Non-Crystalline Solids 299–302 (2002) 767–771
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Fig. 1. AFM images of protocrystalline silicon films (deposited at 75 °C and rH ¼ ð½SiH4 þ ½H2 Þ=½SiH4 ¼ 29) with thicknesses d ranging from 40 to 1000 nm showing the transition from amorphous to microcrystalline growth.
surface of the grains indicates that they are actually aggregates of smaller grains [5]. At this thickness a variation of the grain diameters between 70 and 100 nm can be observed, indicating that the nucleation occurs not only at the film/substrate interface but also later during the growth. At 450 nm thickness the grains aggregates already form an interconnected network, reaching the percolation threshold. Further growth leads to surface fully covered with crystalline grain aggregates at 1000 nm thickness. For lc-Si:H grown at standard conditions ðTS ¼ 250 °CÞ the circular protrusions were identified as aggregates of Si subgrains by their higher conductivity [5,13]. The same is true for protocrystalline samples deposited at very low temperatures as can be seen in Fig. 2 where the simultaneously measured morphology and local current images are presented. Typical morphologies of the samples with the same thickness but prepared at different silane di-
lution ratios are shown in Fig. 3. We can again see the transition from a-Si:H to lc-Si:H growth similar to the one observed for thickness series in Fig. 1, only in this case caused by the variation of the deposition gas mixture. Note a striking similarity of the surface structure at low and high dilutions (leftmost and rightmost image in Fig. 3) even though the left sample is completely amorphous and the right one is fully microcrystalline. This shows that judging the crystallinity of the samples on the basis of AFM topography only could be misleading. Results comparing the structure and the transport properties for the dilution series are shown in Fig. 4. With growing rH both the film crystallinity and the dark conductivity rDC increase from aSi:H to lc-Si:H values (Fig. 4(b)). A sharp peak of the values of ambipolar diffusion length as well as of the surface roughness can be found at rH close to 50 (Fig. 4(a)).
4. Discussion
Fig. 2. Local current map (a) and topography (b) of the 100 nm thick sample from the Fig. 1 measured simultaneously by AFM with conductive cantilever. Microcrystalline grains with higher conductivity appear as bright areas in the local current image coded by gray scale with the range from 0 to 30 pA.
The features observed in the AFM morphologies can be described using a simple geometrical growth model [4] which consider grains as cones growing from nuclei at the film/substrate interface. A constant apex angle 40° of the conical grain is assumed. Viewed from the top the grain forms a circle which grows until it collides with its nearest neighbor(s). At the contact the straight grain boundary is formed. Comparison of the model predictions with the actually observed structural features is shown in Fig. 5. The simulated surface roughness exhibits
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T. Mates et al. / Journal of Non-Crystalline Solids 299–302 (2002) 767–771
Fig. 3. AFM images of dilution series of protocrystalline silicon films (deposited at 80 °C with thicknesses d around 1 lm). Contrary to the Fig. 1, here the transition from amorphous (leftmost image) to microcrystalline (rightmost image) growth is induced by silane dilution ratio rH ranging from 34 to 168.
Fig. 5. Surface roughness measured by AFM for the thickness series (Fig. 1) compared with the dependence resulting from the growth simulation.
Fig. 4. Surface roughness and diffusion length L from SSPG (a) and crystallinity together with dark conductivity rDC (b) of protocrystalline silicon films as a function of the silane dilution ratio rH ranging from 34 to 168. The lines are added as guides for the eye.
similar dependence on the thickness if we assume the nucleation density about 40 grains=lm2 . More detailed comparison of the observed morphologies with a model leads to the conclusion that the nucleation occurs not only at the film/substrate interface but also later during the film growth and that the apex angle of the grains also changes within the film thickness.
The coincidence of maxima in both surface roughness and ambipolar diffusion length at the same rH values around 50 (Fig. 4(a)) is worth further attention. For the samples deposited at TS ¼ 250 °C an increase of roughness is usually connected with formation of tightly packed large aggregates [5]. In the case of samples deposited at very low TS 80 °C the nucleation density is rather low and so the roughness quickly increases due to the presence of isolated large grain aggregates (see Fig. 3, rH ¼ 43). Although the diffusion length almost doubled with rH changing from rH ¼ 34 to 51, its values ð 100 nmÞ as well as the dark conductivity ðrDC 10 11 –10 9 X 1 cm 1 Þ are still values typical for a device quality a-Si:H. The presence of isolated crystalline grains thus improves the photoconductive properties of the protocrystalline silicon as already noted in [11,12,14].
T. Mates et al. / Journal of Non-Crystalline Solids 299–302 (2002) 767–771
The highest diffusion length L ¼ 110 nm is reached for the sample with rH ¼ 51 for which the morphology (Fig. 3) shows that crystallites already form an interconnected network, i.e., the percolation threshold was reached. The fact that crystallinity at this point is about 65% (instead of an ideal percolation threshold 33%) again indicates [8] important role of a-Si:H based grain boundaries. With further increase of rH ðJ 64Þ the high crystallinity (almost 80%) and high conductivities are obtained. However, the temperature dependence of rDC and its evaluated prefactor as well as the activation energy ðEa Þ indicate that the high conductivity is achieved due to low Ea and related change of the transport mechanism [15]. This change results also in the drop of the ambipolar diffusion length and it can be explained by increasing nucleation density resulting in smaller size of the grain aggregates and so enlarging the concentration of defective grain boundaries (see Fig. 3). Thus an increase in the crystallinity, accompanied by an increase in the concentration of grain boundaries, may actually lead to the deterioration of transport features. At the present stage the growth model cannot be used for a reliable prediction of the diffusion length and the work is in progress in this direction. Grain analysis methods which would allow to deduce information about nucleation and grain properties from the observed AFM images are being developed.
5. Conclusion The a=lc boundary in the growth of protocrystalline silicon films at very low temperatures (around 80 °C) can be crossed by changing either thickness or silane dilution ratio, as clearly seen by AFM surface images. The growth can be described by a simple geometry of conical crystalline grains growing from a nuclei with a constant apex angle. We have observed a close correlation between the structure and electronic transport properties, for example the maxima of the surface roughness ð 21 nmÞ and diffusion length ðL 110 nmÞ occurring for the structure close to the percolation threshold with crystallinity around 65%. The
771
presence of the isolated crystalline grain aggregates is responsible for improving the photoconductive properties before the percolation threshold is reached. However, once the grains become interconnected, further increase in crystallinity may actually deteriorate the photoconductive properties due to increasing concentration of defective grain boundaries. Acknowledgements This work was supported by the Grant Agency of the Academy of Sciences of the Czech Republic (Projects No. A1010809, B2949101).
References [1] E. Vallat-Sauvain, U. Kroll, J. Meier, A. Shah, J. Appl. Phys. 87 (2000) 3137. [2] R.W. Collins, J.M. Cavese, J. Appl. Phys. 60 (1986) 4169. [3] J. Kocka, A. Fejfar, V. Vorlıcek, H. Stuchlıkova, J. Stuchlık, Mater. Res. Soc. Symp. Proc. 557 (1999) 483. vrcek, P. [4] A. Fejfar, T. Mates, C. Koch, B. Rezek, V. S Fojtık, H. Stuchlıkova, J. Stuchlık, J. Kocka, MRS Proceedings, vol. 664 (2001), paper A16.1.1. [5] J. Kocka, H. Stuchlıkova, J. Stuchlık, B. Rezek, T. Mates, vrcek, P. Fojtık, I. Pelant, A. Fejfar, these Proceedings, V. S p. 355. [6] J.-H. Zhou, K. Ikuta, T. Yasuda, T. Umeda, S. Yamasaki, K. Tanaka, Appl. Phys. Lett. 71 (1997) 1534. [7] S. Koynov, S. Grebner, P. Radojkovic, E. Hartmann, R. Schwarz, L. Vasilev, R. Krankenhagen, I. Sieber, W. Henrion, M. Schmidt, J. Non-Crystal. Solids 198–200 (1996) 1012. [8] J. Kocka, H. Stuchlıkov, J. Stuchlık, B. Rezek, T. Mates, A. Fejfar, Tech. Dig. PVSEC 12 (2001). [9] Y. Nasuno, M. Kondo, A. Matsuda, Appl. Phys. Lett. 78 (2001) 2330. vrcek, A. Fejfar, P. Fojtık, T. Mates, A. Poruba, H. [10] V. S Stuchlıkova, I. Pelant, J. Kocka, Y. Nasuno, M. Kondo, A. Matsuda, these Proceedings, p. 395. vrcek, M.B. Schubert, J.H. Werner, [11] C. Koch, M. Ito, V. S in: R.W. Collins, H.M. Branz, S. Guha, H. Okamoto, M. Stutzmann (Eds.), MRS Proceedings, vol. 609, 2000, paper A15.6. vrcek, M.B. Schubert, J.H. Werner, [12] M. Ito, C. Koch, V. S Thin Solid Films 383 (2001) 129. [13] B. Rezek, J. Stuchlık, A. Fejfar, J. Kocka, Appl. Phys. Lett. 74 (1999) 1475. [14] T. Kamei, P. Stradins, A. Matsuda, Appl. Phys. Lett. 74 (1999) 1707. [15] T. Mates, M. Ito, A. Fejfar, J. Kocka, to be published.
Role of grains in protocrystalline silicon layers grown at very low substrate temperatures and studied by atomic force microscopy T. Mates a,*, A. Fejfar a, I. Drbohlav a, B. Rezek a, P. Fojtık a, K. Luterov a a, J. Kocka a, C. Koch b, M.B. Schubert b, M. Ito c, K. Ro c, H. Uyama c a
Institute of Physical Electronics, Academy of Sciences of the Czech Republic, Cukrovarnick a 10, 162 53 Prague, Czech Republic b Institute of Physical Electronics, Stuttgart University, Pfaffenwaldring 47, 70569 Stuttgart, Germany c Technical Research Institute, Toppan Printing Co., 4-2-3 Takanodai-Minami, Sugito machi, Kita Katsushika, Saitama 345 8508, Japan
Abstract We have investigated the role of the sample thickness and silane dilution on the structure and electronic properties of protocrystalline silicon thin films deposited at very low substrate temperatures ð 80 °CÞ. Coincidence of the maxima in surface roughness and ambipolar diffusion length ðJ 100 nmÞ with formation of the network of interconnected crystalline grain aggregates was observed. While the presence of the isolated grain aggregates improves the photoconductive properties before the percolation threshold is reached, further increase in crystallinity may have opposite effect due to detrimental role of increasing concentration of the defective grain boundaries. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 68.55.-a; 81.15.Gh; 68.37.Ps; 73.50.-h
1. Introduction Hydrogenated microcrystalline silicon (lc-Si:H) prepared by plasma-enhanced chemical vapor deposition (PECVD) represents a wide class of materials. Structure and properties of the deposited films depend on many parameters: plasma discharge frequency and power, pressure and flow rates of silane and hydrogen, substrate material
*
Corresponding author. E-mail address: [email protected] (T. Mates).
and deposition temperature [1,2]. Moreover, the properties may change considerably with increasing film thickness [3–5]. Usually the film grows at first amorphous as so-called incubation layer and only then the crystalline nuclei appear and grow [6,7]. The properties of the films may evolve even after the transition to fully crystalline growth is completed [8]. In spite of an intensive research effort, only a small part of the possible parameter space for lcSi:H deposition was explored. Recently the research focussed on depositions at low substrate temperatures, below the standard 200–250 °C
0022-3093/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 0 9 8 0 - 2
768
T. Mates et al. / Journal of Non-Crystalline Solids 299–302 (2002) 767–771
originating from optimization of hydrogenated amorphous silicon (a-Si:H). Two reasons motivated an exploration of low temperature depositions of lc-Si:H. Firstly, an obvious reason was the use of cheap polymer substrates for lc-Si:H based solar cells. Later it became clear that some unexpected physical phenomena may occur at low growth temperatures (around 150 °C), such as suppressed oxygen donor formation [9] and isotropic transport properties of lc-Si:H [10]. Even lower temperatures have to be used for depositing lc-Si:H on practical polymer substrates, e.g. polyethylene terephtalate. It was reported that at temperatures K 80 °C the best solar cells can be prepared using deposition conditions close to the boundary between a-Si and lc-Si growth [11,12]. The material grown close to this boundary is often called protocrystalline silicon which can be loosely defined as a lc-Si:H with an extended transition layer thickness. Close to the a=lc boundary a sharp maximum of ratio of photo/dark conductivity (the quantity often used as a figure of merit for absorbers in solar cells) was found together with a corresponding optimum in the solar cell performance [11,12]. The physical reason for this optimum is still not completely clear. One reason for this uncertainty may be that the common methods of transport characterization assume homogeneous material and so their results are difficult to understand for a material which is a complex mixture of the amorphous phase and crystalline grains and in which the grain boundaries play an important role. This led us to the use of atomic force microscope (AFM) with a conductive cantilever for probing local electronic properties of individual grains in lc-Si:H layers with sizes down to few tens of nanometer [13]. In this paper we present study of the morphology and transport properties of protocrystalline silicon deposited at very low temperatures. We have focused on two crucial parameters: film thickness and silane dilution. By variation of either of them the boundary between amorphous and microcrystalline growth can be crossed. We will show that the optimum of the transport properties occurs near the percolation threshold, i.e., when the crystalline grain aggregates create a connected network. Finally, we will show how a simple geo-
metrical model can be used to simulate the growth and properties of these films. 2. Experimental We have prepared two series of lc-Si:H samples. The thickness series was deposited at substrate temperature of 75 °C, discharge frequency of 54 MHz and hydrogen dilution ratio rH ¼ ð½SiH4 þ ½H2 Þ=½SiH4 ¼ 29, i.e., close to the a=lc boundary, with the sample thickness ranging from 40 to 1000 nm. The second series was prepared by changing the hydrogen dilution ratio rH from 26 to 168 at a fixed substrate temperature of 80 °C, discharge frequency of 54 MHz and adjusted deposition duration to achieve thickness around 1 lm. Morphology of the samples of both the series were measured by the ambient AFM (Topometrix Explorer) in a contact mode using soft, triangle shaped cantilevers. Simultaneous measurements of the morphology and local current were done in ultra-high vacuum (UHV) using AFM with conductivite cantilever. Crystallinity was determined from the ratio of crystalline to amorphous peaks in the Raman spectra. For the measurement of the optoelectronic properties, NiCr coplanar gap electrodes were deposited on the top of the samples and used to measure dark conductivity rDC , subgap absorption spectra by constant photocurrent method and ambipolar diffusion length parallel to the substrate by steady state photocarrier grating technique (SSPG). 3. Results Within both thickness and dilution series the morphology undergoes a transition from a rather smooth amorphous surface with only isolated grains to rough lc-Si:H surface composed of grain aggregates with a typical cauliflower structure. The transition from a-Si:H to lc-Si:H with increasing thickness is shown in Fig. 1. After overcoming the incubation layer the circular grains protruding above the amorphous matrix start to develop (leftmost image at the 40 nm thickness). At 100 nm thickness, grains start to collide with their neighbors, forming grain boundaries. Structure on the
T. Mates et al. / Journal of Non-Crystalline Solids 299–302 (2002) 767–771
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Fig. 1. AFM images of protocrystalline silicon films (deposited at 75 °C and rH ¼ ð½SiH4 þ ½H2 Þ=½SiH4 ¼ 29) with thicknesses d ranging from 40 to 1000 nm showing the transition from amorphous to microcrystalline growth.
surface of the grains indicates that they are actually aggregates of smaller grains [5]. At this thickness a variation of the grain diameters between 70 and 100 nm can be observed, indicating that the nucleation occurs not only at the film/substrate interface but also later during the growth. At 450 nm thickness the grains aggregates already form an interconnected network, reaching the percolation threshold. Further growth leads to surface fully covered with crystalline grain aggregates at 1000 nm thickness. For lc-Si:H grown at standard conditions ðTS ¼ 250 °CÞ the circular protrusions were identified as aggregates of Si subgrains by their higher conductivity [5,13]. The same is true for protocrystalline samples deposited at very low temperatures as can be seen in Fig. 2 where the simultaneously measured morphology and local current images are presented. Typical morphologies of the samples with the same thickness but prepared at different silane di-
lution ratios are shown in Fig. 3. We can again see the transition from a-Si:H to lc-Si:H growth similar to the one observed for thickness series in Fig. 1, only in this case caused by the variation of the deposition gas mixture. Note a striking similarity of the surface structure at low and high dilutions (leftmost and rightmost image in Fig. 3) even though the left sample is completely amorphous and the right one is fully microcrystalline. This shows that judging the crystallinity of the samples on the basis of AFM topography only could be misleading. Results comparing the structure and the transport properties for the dilution series are shown in Fig. 4. With growing rH both the film crystallinity and the dark conductivity rDC increase from aSi:H to lc-Si:H values (Fig. 4(b)). A sharp peak of the values of ambipolar diffusion length as well as of the surface roughness can be found at rH close to 50 (Fig. 4(a)).
4. Discussion
Fig. 2. Local current map (a) and topography (b) of the 100 nm thick sample from the Fig. 1 measured simultaneously by AFM with conductive cantilever. Microcrystalline grains with higher conductivity appear as bright areas in the local current image coded by gray scale with the range from 0 to 30 pA.
The features observed in the AFM morphologies can be described using a simple geometrical growth model [4] which consider grains as cones growing from nuclei at the film/substrate interface. A constant apex angle 40° of the conical grain is assumed. Viewed from the top the grain forms a circle which grows until it collides with its nearest neighbor(s). At the contact the straight grain boundary is formed. Comparison of the model predictions with the actually observed structural features is shown in Fig. 5. The simulated surface roughness exhibits
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T. Mates et al. / Journal of Non-Crystalline Solids 299–302 (2002) 767–771
Fig. 3. AFM images of dilution series of protocrystalline silicon films (deposited at 80 °C with thicknesses d around 1 lm). Contrary to the Fig. 1, here the transition from amorphous (leftmost image) to microcrystalline (rightmost image) growth is induced by silane dilution ratio rH ranging from 34 to 168.
Fig. 5. Surface roughness measured by AFM for the thickness series (Fig. 1) compared with the dependence resulting from the growth simulation.
Fig. 4. Surface roughness and diffusion length L from SSPG (a) and crystallinity together with dark conductivity rDC (b) of protocrystalline silicon films as a function of the silane dilution ratio rH ranging from 34 to 168. The lines are added as guides for the eye.
similar dependence on the thickness if we assume the nucleation density about 40 grains=lm2 . More detailed comparison of the observed morphologies with a model leads to the conclusion that the nucleation occurs not only at the film/substrate interface but also later during the film growth and that the apex angle of the grains also changes within the film thickness.
The coincidence of maxima in both surface roughness and ambipolar diffusion length at the same rH values around 50 (Fig. 4(a)) is worth further attention. For the samples deposited at TS ¼ 250 °C an increase of roughness is usually connected with formation of tightly packed large aggregates [5]. In the case of samples deposited at very low TS 80 °C the nucleation density is rather low and so the roughness quickly increases due to the presence of isolated large grain aggregates (see Fig. 3, rH ¼ 43). Although the diffusion length almost doubled with rH changing from rH ¼ 34 to 51, its values ð 100 nmÞ as well as the dark conductivity ðrDC 10 11 –10 9 X 1 cm 1 Þ are still values typical for a device quality a-Si:H. The presence of isolated crystalline grains thus improves the photoconductive properties of the protocrystalline silicon as already noted in [11,12,14].
T. Mates et al. / Journal of Non-Crystalline Solids 299–302 (2002) 767–771
The highest diffusion length L ¼ 110 nm is reached for the sample with rH ¼ 51 for which the morphology (Fig. 3) shows that crystallites already form an interconnected network, i.e., the percolation threshold was reached. The fact that crystallinity at this point is about 65% (instead of an ideal percolation threshold 33%) again indicates [8] important role of a-Si:H based grain boundaries. With further increase of rH ðJ 64Þ the high crystallinity (almost 80%) and high conductivities are obtained. However, the temperature dependence of rDC and its evaluated prefactor as well as the activation energy ðEa Þ indicate that the high conductivity is achieved due to low Ea and related change of the transport mechanism [15]. This change results also in the drop of the ambipolar diffusion length and it can be explained by increasing nucleation density resulting in smaller size of the grain aggregates and so enlarging the concentration of defective grain boundaries (see Fig. 3). Thus an increase in the crystallinity, accompanied by an increase in the concentration of grain boundaries, may actually lead to the deterioration of transport features. At the present stage the growth model cannot be used for a reliable prediction of the diffusion length and the work is in progress in this direction. Grain analysis methods which would allow to deduce information about nucleation and grain properties from the observed AFM images are being developed.
5. Conclusion The a=lc boundary in the growth of protocrystalline silicon films at very low temperatures (around 80 °C) can be crossed by changing either thickness or silane dilution ratio, as clearly seen by AFM surface images. The growth can be described by a simple geometry of conical crystalline grains growing from a nuclei with a constant apex angle. We have observed a close correlation between the structure and electronic transport properties, for example the maxima of the surface roughness ð 21 nmÞ and diffusion length ðL 110 nmÞ occurring for the structure close to the percolation threshold with crystallinity around 65%. The
771
presence of the isolated crystalline grain aggregates is responsible for improving the photoconductive properties before the percolation threshold is reached. However, once the grains become interconnected, further increase in crystallinity may actually deteriorate the photoconductive properties due to increasing concentration of defective grain boundaries. Acknowledgements This work was supported by the Grant Agency of the Academy of Sciences of the Czech Republic (Projects No. A1010809, B2949101).
References [1] E. Vallat-Sauvain, U. Kroll, J. Meier, A. Shah, J. Appl. Phys. 87 (2000) 3137. [2] R.W. Collins, J.M. Cavese, J. Appl. Phys. 60 (1986) 4169. [3] J. Kocka, A. Fejfar, V. Vorlıcek, H. Stuchlıkova, J. Stuchlık, Mater. Res. Soc. Symp. Proc. 557 (1999) 483. vrcek, P. [4] A. Fejfar, T. Mates, C. Koch, B. Rezek, V. S Fojtık, H. Stuchlıkova, J. Stuchlık, J. Kocka, MRS Proceedings, vol. 664 (2001), paper A16.1.1. [5] J. Kocka, H. Stuchlıkova, J. Stuchlık, B. Rezek, T. Mates, vrcek, P. Fojtık, I. Pelant, A. Fejfar, these Proceedings, V. S p. 355. [6] J.-H. Zhou, K. Ikuta, T. Yasuda, T. Umeda, S. Yamasaki, K. Tanaka, Appl. Phys. Lett. 71 (1997) 1534. [7] S. Koynov, S. Grebner, P. Radojkovic, E. Hartmann, R. Schwarz, L. Vasilev, R. Krankenhagen, I. Sieber, W. Henrion, M. Schmidt, J. Non-Crystal. Solids 198–200 (1996) 1012. [8] J. Kocka, H. Stuchlıkov, J. Stuchlık, B. Rezek, T. Mates, A. Fejfar, Tech. Dig. PVSEC 12 (2001). [9] Y. Nasuno, M. Kondo, A. Matsuda, Appl. Phys. Lett. 78 (2001) 2330. vrcek, A. Fejfar, P. Fojtık, T. Mates, A. Poruba, H. [10] V. S Stuchlıkova, I. Pelant, J. Kocka, Y. Nasuno, M. Kondo, A. Matsuda, these Proceedings, p. 395. vrcek, M.B. Schubert, J.H. Werner, [11] C. Koch, M. Ito, V. S in: R.W. Collins, H.M. Branz, S. Guha, H. Okamoto, M. Stutzmann (Eds.), MRS Proceedings, vol. 609, 2000, paper A15.6. vrcek, M.B. Schubert, J.H. Werner, [12] M. Ito, C. Koch, V. S Thin Solid Films 383 (2001) 129. [13] B. Rezek, J. Stuchlık, A. Fejfar, J. Kocka, Appl. Phys. Lett. 74 (1999) 1475. [14] T. Kamei, P. Stradins, A. Matsuda, Appl. Phys. Lett. 74 (1999) 1707. [15] T. Mates, M. Ito, A. Fejfar, J. Kocka, to be published.