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STM and Raman study of the evolution of the surface morphology in μc-Si:H
J O U R N A L OF
I,t, , LIDS ELSEVIER
Journal of Non-Crystalline Solids 198-200 (1996) 863-866
STM and Raman study of the evolution of the surface morphology in / c-Si:H Kazuyuki Ikuta
a,b,*, Yasutake
T o y o s h i m a c Satoshi Y a m a s a k i a Akihisa Matsuda c K a z u n o b u Tanaka a,b
a Joint Research Center for Atom Technology (JRCA T), National Institute for Adt:anced lnterdisciplinao, Research, 1-1-4, Higashi, Tsukuba, Ibaraki 305, Japan b Institute of Materials Science, Unicersi~, of Tsukuba, 1-1-1, Tennohdai, Tsukuba, lbaraki 305, Japan c Electrotechnical Laborato~,, 1-1-4, Umezono, Tsukuba, lbaraki 305, Japan
Abstract The structure of /xc-Si:H films deposited on graphite substrates has been investigated, using scanning tunneling microscopy and Raman scattering spectroscopy as a function of film thickness ranging from I to 10 nm. At the very initial stage of deposition, chemically active sites are created on the surface of the graphite substrate, which is evidenced by a change in the graphite Raman bands. A change of the STM image found in the early stage of growth is interpreted in terms of a coalescence of nanostructures. The vertical surface roughness of/xc-Si:H is found to be much larger than that of a-Si:H, which can be explained by its inhomogeneous nature as well as crystallographic anisotropy of the /zc-Si:H film.
1. Introduction Although it is phenomenologically known that hydrogen atoms impinging on the growing surface are important for /~c-Si:H formation [1-3], the detailed mechanism, especially the nucleation at the initial stage o f microcrystal formation, remains unresolved. One o f the most promising approaches to identify the growth kinetics is to collect microscopic information of the film surface during the growth or just after the growth. For hydrogenated amorphous silicon (a-Si:H) film growth, a variety of in situ surface diagnoses have been performed, including
* Corresponding author. Joint Research Center for Atom Technology (JRCAT), 1-1-4, Higashi, Tsukuba, Ibaraki 305, Japan. Tel.: + 81-298-58-5424; fax: + 81-298-54-2786.
spectroscopic ellipsometry [4,5], infrared reflection absorption spectroscopy [6] and scanning probe microscopy [7-9]. In this work, we investigate the surface o f / z c - S i : H films deposited on graphite substrates using scanning tunneling microscopy (STM), and discuss the surface morphology of /xc-Si:H films in comparison to that of a-Si:H. Raman scattering was also used to estimate the microcrystallite size of Si as well as to identify the modification of the graphite surface.
2. Experimental The /~c-Si:H films were prepared by the rf glow discharge decomposition of S i l l 4 diluted by H 2 on a graphite substrate in a triode reactor at 300°C. Details of the setup were described elsewhere [8,9]. The
0022-3093/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved PII S0022-3093(96)00071-3
864
K. lkuta et al. / Journal of Non-Crystalline Solids 198-200 (1996) 863-866
deposition pressure, the flow rates, and the rf power density were maintained at 50mTorr, 1 sccm for Sill 4, 39sccm for H 2, and 0 . 1 3 W / c m 2, respectively. The /xc-Si:H films deposited on graphite were transferred to a high-vacuum chamber (10 -1° Torr) through a load-lock system, where STM (JSTM4500VT, JEOL) measurements were performed with a chemically etched tungsten tip that was cleaned by flash heating. STM images were taken in the constant current mode with a tunneling current of 80100pA and a sample bias of 2.0-3.0V. A 6328A excitation was employed for Raman scattering measurements (Ramascope 1000, Renishaw).
3. Results The early stage of /~c-Si:H growth has been investigated by Raman scattering. As shown in Fig. 1, only one feature, which characterizes the graphite, is seen before the growth. In the after-growth spectrum, there are three additional features, two of which are a peak at 1340 (denoted D in the figure) and a shoulder at 1600 cm-~ (G). These two features are similar to those already observed at a graphite surface that was exposed to the H e discharge [9], and thus indicate creation of surface sites that favor the film growth as discussed in our earlier work [9]. The /zc-nature of the deposited film is clearly evidenced by the appearance of the third feature peaked at 517 c m - J in Fig. 1. Fig. 2 shows 100nm × 100nm STM images of
before arowth I
-
graphite
Fig. 2. 1 0 0 n m × 100rim STM images of the /xc-Si:H films for different thicknesses: (a) 5 nm, (b) 40nm, and (c) 110nm.
E I
450
c'~i/I
:
550 1300 1 4 0 0 1 5 0 0 Frequency (cm-1)
[l
1600
Fig. l, Raman scattering spectra of a graphite substrate before (top) and after (bottom) growth of a 5 n m /xc-Si:H film.
the surface of /xc-Si:H films which are (a) 5 nm, (b) 40nm, and (c) l l 0 n m in thickness. The surface of the film shown in Fig. 2(a) is covered with nanostructures of lateral size in the range from several nm to around 10nm. As already shown in Fig. 1, this film shows a Raman band characteristic of microcrystalline silicon at 517cm J, which is less than
K. lkuta et a l . / Journal of Non-Cry'stalline Solids 198-200 (1996) 863 866
that of the TO peak (520cm -~) of the Si bulk crystal. According to the model proposed by Campbell and Fauchet [12], this peak shift corresponds to a microcrystallite size of around 6 nm, which is consistent with the STM observation of Fig. 2(a). As the film becomes thicker, the nanostructures grow to around 30 nm in lateral size at a thickness of l l 0 n m , as is seen in Fig. 2(c). This implies that larger nanostructures are produced as a result of the coalescence of small nanostructures on the growing surface. A similar behavior of evolution in the surface morphology has also been observed in a-Si:H [8]. Therefore, the coalescence of nanostructures may be a behavior common to hydrogenated silicon-film growth irrespective of its amorphous or microcrystalline structure.
4. Discussion To make a quantitative comparison of surface roughness between /~c- and a-Si:H films, the root mean square (rms) roughness (height fluctuation) [13] of the surface, tr, was estimated from the STM images. Fig. 3 shows the thickness dependence of ofor silicon films deposited under four different conditions; /zc-Si:H films deposited from H2-diluted Sill 4 at 300°C (open square), which are identical to those in Fig. 2, a-Si:H films from H2-diluted Sill 4 at room temperature (open circle), a-Si:H films from pure Sill 4 at 300°C (solid square), and a-Si:H films from pure Sill 4 at room temperature (solid circle), respectively.
[ ] SiH4/H2-300°C• SiH4-300°C O SiH./H2-R-T- • S i H ~ <'10 - ~ :
865
Two important trends are noticed from the figure: (1) tr increases with an increase in film thickness for both /xc- and a-Si:H in the thickness range up to 40nm, and (2) /zc-Si:H shows a larger o- than a-Si:H over the whole thickness range independent of the substrate temperature. The first trend about the relationship between tr and the film thickness can be explained in terms of the self-affine fractal [13]. Namely, recent scaling theory [13] predicts that the rms roughness should increase with a film thickness and saturate above a certain thickness as far as it is determined from a finite size of the STM image, which is 100nm X 100rim in our case. The data point of /xc-Si:H at l l 0 n m in Fig. 3 may represent an approach to saturation, and may thus reflect the nature of the self-affine fractal on the surface of /.~c-Si:H. As for the second trend of larger cr of/xc-Si:H in Fig. 3, o- is affected by not only surface morphology but also spatial fluctuation of electrical conductivity or surface electronic states, especially in a case of the surface of inhomogeneous material like /.Lc-Si:H, which consists of the mixed phase of microcrystallites and amorphous tissue. However, the results of atomic force microscopy on the same sample also show the larger tr for the surface of /xc-Si:H than for that of a-Si:H. Therefore, the larger tr in /zc-Si:H is not mainly due to the inhomogeneity of electrical conductivity or surface electronic states, but represents an actual roughness of the surface of /zc-Si:H. Then, a possible explanation for this trend is that preferential etching of amorphous tissue between crystallite grain boundaries or preferential growth of the grains of microcrystallites depending on their orientation may lead to an enhancement of the surface roughness. The detailed mechanism remains an open question and further investigation is required in the future.
5. Conclusions
m cr
10 100 Film thickness(A)
1000
Fig. 3. Rms roughness as a function of film thickness for hydrogenated silicon films prepared under different deposition conditions.
At the initial stage of/xc-Si:H growth, a chemical modification takes place on the graphite substrate surface, as evidenced by Raman measurements. As with a-Si:H growth behavior on the graphite surface, it is found,in STM observations that the /zc-Si:H film grows thicker through a coalescence of the
866
K. lkuta et al. / Journal of Non-C~stalline Solids 198-200 (1996) 863-866
nanostructures accompanied by an increase of the surface rms roughness, o-. It is also observed that ois clearly larger in /xc-Si:H, which is tentatively explained by the inhomogeneous nature of /xc-Si:H, the structure of which is characterized as a mixed phase of microcrystallites and amorphous tissue. Acknowledgements One of the Authors (K.I.) acknowledges support from the Fellowships of the Japan Society for the Promotion of Science for Japanese Junior Scientists. References [1] A. Matsuda, J. Non-Cryst. Solids 59/60 (1983) 767. [2] I. Shimizu, J. Non-Cryst. Solids 114 (1989) 145.
[3] C.C. Tsai, G.B. Anderson, R. Thompson and B. Wacker, J. Non-Cryst. Solids 114 (1989) 151. [4] A.M. Antoine and B. Drevillon, J. Non-Cryst. Solids 97/98 (1987) 1403. [5] I. An, H.V. Nguyen, N.V. Nguyen and R.W. Collins, Phys. Rev. Lett. 65 (1990) 2274. [6] Y. Toyoshima, K. Arai, A. Matsuda and K. Tanaka, J. Non-Cryst. Solids 137 and 138 (1991) 765. [7] G.C. Stutzin, R.M. Ostrom, D.M. Tanenbabaum and A. Gallagher, J. Appl. Phys. 74 (1993) 91. [8] K. Ikuta, K. Miki, S. Yamasaki, A. Matsuda and K. Tanaka, Appl. Phys. Lett. 65 (1994) 1760. [9] K. Ikuta, Y. Toyoshima, S. Yamasaki, A. Matsuda and K. Tanaka, Jpn. J. Appl. Phys. 34 (1995) L379. [10] M. Ramsteiner and J. Wagner, Appl. Phys. Lett. 51 (1987) 1355. [11] M. Balooch and D.R. Olander, J. Chem. Phys. 63 (1975) 4772. [12] I.H. Campbell and P.M. Fauchet, Solid State Commun. 58 (1986) 739. [13] F. Family, Physica A168 (1990) 561.
I,t, , LIDS ELSEVIER
Journal of Non-Crystalline Solids 198-200 (1996) 863-866
STM and Raman study of the evolution of the surface morphology in / c-Si:H Kazuyuki Ikuta
a,b,*, Yasutake
T o y o s h i m a c Satoshi Y a m a s a k i a Akihisa Matsuda c K a z u n o b u Tanaka a,b
a Joint Research Center for Atom Technology (JRCA T), National Institute for Adt:anced lnterdisciplinao, Research, 1-1-4, Higashi, Tsukuba, Ibaraki 305, Japan b Institute of Materials Science, Unicersi~, of Tsukuba, 1-1-1, Tennohdai, Tsukuba, lbaraki 305, Japan c Electrotechnical Laborato~,, 1-1-4, Umezono, Tsukuba, lbaraki 305, Japan
Abstract The structure of /xc-Si:H films deposited on graphite substrates has been investigated, using scanning tunneling microscopy and Raman scattering spectroscopy as a function of film thickness ranging from I to 10 nm. At the very initial stage of deposition, chemically active sites are created on the surface of the graphite substrate, which is evidenced by a change in the graphite Raman bands. A change of the STM image found in the early stage of growth is interpreted in terms of a coalescence of nanostructures. The vertical surface roughness of/xc-Si:H is found to be much larger than that of a-Si:H, which can be explained by its inhomogeneous nature as well as crystallographic anisotropy of the /zc-Si:H film.
1. Introduction Although it is phenomenologically known that hydrogen atoms impinging on the growing surface are important for /~c-Si:H formation [1-3], the detailed mechanism, especially the nucleation at the initial stage o f microcrystal formation, remains unresolved. One o f the most promising approaches to identify the growth kinetics is to collect microscopic information of the film surface during the growth or just after the growth. For hydrogenated amorphous silicon (a-Si:H) film growth, a variety of in situ surface diagnoses have been performed, including
* Corresponding author. Joint Research Center for Atom Technology (JRCAT), 1-1-4, Higashi, Tsukuba, Ibaraki 305, Japan. Tel.: + 81-298-58-5424; fax: + 81-298-54-2786.
spectroscopic ellipsometry [4,5], infrared reflection absorption spectroscopy [6] and scanning probe microscopy [7-9]. In this work, we investigate the surface o f / z c - S i : H films deposited on graphite substrates using scanning tunneling microscopy (STM), and discuss the surface morphology of /xc-Si:H films in comparison to that of a-Si:H. Raman scattering was also used to estimate the microcrystallite size of Si as well as to identify the modification of the graphite surface.
2. Experimental The /~c-Si:H films were prepared by the rf glow discharge decomposition of S i l l 4 diluted by H 2 on a graphite substrate in a triode reactor at 300°C. Details of the setup were described elsewhere [8,9]. The
0022-3093/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved PII S0022-3093(96)00071-3
864
K. lkuta et al. / Journal of Non-Crystalline Solids 198-200 (1996) 863-866
deposition pressure, the flow rates, and the rf power density were maintained at 50mTorr, 1 sccm for Sill 4, 39sccm for H 2, and 0 . 1 3 W / c m 2, respectively. The /xc-Si:H films deposited on graphite were transferred to a high-vacuum chamber (10 -1° Torr) through a load-lock system, where STM (JSTM4500VT, JEOL) measurements were performed with a chemically etched tungsten tip that was cleaned by flash heating. STM images were taken in the constant current mode with a tunneling current of 80100pA and a sample bias of 2.0-3.0V. A 6328A excitation was employed for Raman scattering measurements (Ramascope 1000, Renishaw).
3. Results The early stage of /~c-Si:H growth has been investigated by Raman scattering. As shown in Fig. 1, only one feature, which characterizes the graphite, is seen before the growth. In the after-growth spectrum, there are three additional features, two of which are a peak at 1340 (denoted D in the figure) and a shoulder at 1600 cm-~ (G). These two features are similar to those already observed at a graphite surface that was exposed to the H e discharge [9], and thus indicate creation of surface sites that favor the film growth as discussed in our earlier work [9]. The /zc-nature of the deposited film is clearly evidenced by the appearance of the third feature peaked at 517 c m - J in Fig. 1. Fig. 2 shows 100nm × 100nm STM images of
before arowth I
-
graphite
Fig. 2. 1 0 0 n m × 100rim STM images of the /xc-Si:H films for different thicknesses: (a) 5 nm, (b) 40nm, and (c) 110nm.
E I
450
c'~i/I
:
550 1300 1 4 0 0 1 5 0 0 Frequency (cm-1)
[l
1600
Fig. l, Raman scattering spectra of a graphite substrate before (top) and after (bottom) growth of a 5 n m /xc-Si:H film.
the surface of /xc-Si:H films which are (a) 5 nm, (b) 40nm, and (c) l l 0 n m in thickness. The surface of the film shown in Fig. 2(a) is covered with nanostructures of lateral size in the range from several nm to around 10nm. As already shown in Fig. 1, this film shows a Raman band characteristic of microcrystalline silicon at 517cm J, which is less than
K. lkuta et a l . / Journal of Non-Cry'stalline Solids 198-200 (1996) 863 866
that of the TO peak (520cm -~) of the Si bulk crystal. According to the model proposed by Campbell and Fauchet [12], this peak shift corresponds to a microcrystallite size of around 6 nm, which is consistent with the STM observation of Fig. 2(a). As the film becomes thicker, the nanostructures grow to around 30 nm in lateral size at a thickness of l l 0 n m , as is seen in Fig. 2(c). This implies that larger nanostructures are produced as a result of the coalescence of small nanostructures on the growing surface. A similar behavior of evolution in the surface morphology has also been observed in a-Si:H [8]. Therefore, the coalescence of nanostructures may be a behavior common to hydrogenated silicon-film growth irrespective of its amorphous or microcrystalline structure.
4. Discussion To make a quantitative comparison of surface roughness between /~c- and a-Si:H films, the root mean square (rms) roughness (height fluctuation) [13] of the surface, tr, was estimated from the STM images. Fig. 3 shows the thickness dependence of ofor silicon films deposited under four different conditions; /zc-Si:H films deposited from H2-diluted Sill 4 at 300°C (open square), which are identical to those in Fig. 2, a-Si:H films from H2-diluted Sill 4 at room temperature (open circle), a-Si:H films from pure Sill 4 at 300°C (solid square), and a-Si:H films from pure Sill 4 at room temperature (solid circle), respectively.
[ ] SiH4/H2-300°C• SiH4-300°C O SiH./H2-R-T- • S i H ~ <'10 - ~ :
865
Two important trends are noticed from the figure: (1) tr increases with an increase in film thickness for both /xc- and a-Si:H in the thickness range up to 40nm, and (2) /zc-Si:H shows a larger o- than a-Si:H over the whole thickness range independent of the substrate temperature. The first trend about the relationship between tr and the film thickness can be explained in terms of the self-affine fractal [13]. Namely, recent scaling theory [13] predicts that the rms roughness should increase with a film thickness and saturate above a certain thickness as far as it is determined from a finite size of the STM image, which is 100nm X 100rim in our case. The data point of /xc-Si:H at l l 0 n m in Fig. 3 may represent an approach to saturation, and may thus reflect the nature of the self-affine fractal on the surface of /.~c-Si:H. As for the second trend of larger cr of/xc-Si:H in Fig. 3, o- is affected by not only surface morphology but also spatial fluctuation of electrical conductivity or surface electronic states, especially in a case of the surface of inhomogeneous material like /.Lc-Si:H, which consists of the mixed phase of microcrystallites and amorphous tissue. However, the results of atomic force microscopy on the same sample also show the larger tr for the surface of /xc-Si:H than for that of a-Si:H. Therefore, the larger tr in /zc-Si:H is not mainly due to the inhomogeneity of electrical conductivity or surface electronic states, but represents an actual roughness of the surface of /zc-Si:H. Then, a possible explanation for this trend is that preferential etching of amorphous tissue between crystallite grain boundaries or preferential growth of the grains of microcrystallites depending on their orientation may lead to an enhancement of the surface roughness. The detailed mechanism remains an open question and further investigation is required in the future.
5. Conclusions
m cr
10 100 Film thickness(A)
1000
Fig. 3. Rms roughness as a function of film thickness for hydrogenated silicon films prepared under different deposition conditions.
At the initial stage of/xc-Si:H growth, a chemical modification takes place on the graphite substrate surface, as evidenced by Raman measurements. As with a-Si:H growth behavior on the graphite surface, it is found,in STM observations that the /zc-Si:H film grows thicker through a coalescence of the
866
K. lkuta et al. / Journal of Non-C~stalline Solids 198-200 (1996) 863-866
nanostructures accompanied by an increase of the surface rms roughness, o-. It is also observed that ois clearly larger in /xc-Si:H, which is tentatively explained by the inhomogeneous nature of /xc-Si:H, the structure of which is characterized as a mixed phase of microcrystallites and amorphous tissue. Acknowledgements One of the Authors (K.I.) acknowledges support from the Fellowships of the Japan Society for the Promotion of Science for Japanese Junior Scientists. References [1] A. Matsuda, J. Non-Cryst. Solids 59/60 (1983) 767. [2] I. Shimizu, J. Non-Cryst. Solids 114 (1989) 145.
[3] C.C. Tsai, G.B. Anderson, R. Thompson and B. Wacker, J. Non-Cryst. Solids 114 (1989) 151. [4] A.M. Antoine and B. Drevillon, J. Non-Cryst. Solids 97/98 (1987) 1403. [5] I. An, H.V. Nguyen, N.V. Nguyen and R.W. Collins, Phys. Rev. Lett. 65 (1990) 2274. [6] Y. Toyoshima, K. Arai, A. Matsuda and K. Tanaka, J. Non-Cryst. Solids 137 and 138 (1991) 765. [7] G.C. Stutzin, R.M. Ostrom, D.M. Tanenbabaum and A. Gallagher, J. Appl. Phys. 74 (1993) 91. [8] K. Ikuta, K. Miki, S. Yamasaki, A. Matsuda and K. Tanaka, Appl. Phys. Lett. 65 (1994) 1760. [9] K. Ikuta, Y. Toyoshima, S. Yamasaki, A. Matsuda and K. Tanaka, Jpn. J. Appl. Phys. 34 (1995) L379. [10] M. Ramsteiner and J. Wagner, Appl. Phys. Lett. 51 (1987) 1355. [11] M. Balooch and D.R. Olander, J. Chem. Phys. 63 (1975) 4772. [12] I.H. Campbell and P.M. Fauchet, Solid State Commun. 58 (1986) 739. [13] F. Family, Physica A168 (1990) 561.