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Nanocrystalline silicon formation in a SiH4 plasma cell
J O U R N A L OF
NON m SOL ELSEVIER
Journal of Non-Crystalline Solids 198-200 (1996) 875 878
Nanocrystalline silicon formation in
a Sill 4
plasma cell
Masanori Otobe, Tomonori Kanai, Toru Ifuku, Hiroshi Yajima, Shunri Oda * .1 Research Center fi)r Quantum Effect Electronics, Tokyo Institute o[ Technology, 2-12-1, O-okayama, Meguro-ku, Tokyo 152, Japan
Abstract Nanometer sized crystalline (nc) Si formation in the Sill 4 plasma cell with very-high-frequency (144 MHz) excitation has been investigated with the Ar and H 2 dilution method. The Sill 4 plasma cell is attached to the ultra-high-vacuum chamber. The crystalline Si is formed in the gas phase of the plasma cell by coalescence of radicals produced from Sill 4. The nc-Si is extracted out of the plasma cell through the orifice to the ultra-high-vacuum chamber. The deposition rate increases with Ar dilution and decreases with H z dilution. The average grain size decreases with H 2 dilution. The dependence of the deposition rate and the grain size on the dilution condition is discussed.
1. Introduction Recently, nanocrystalline silicon (nc-Si) with a grain size of less than 10 nm attracts considerable attention because the manifestation of a quantum dot structure is expected [1]. In order to apply nc-Si to future electron devices, it is required to establish the technology to control the size and the surface states of nc-Si. In an attempt to implement these controls, we have designed an apparatus for fabrication of nc-Si which consists of a very-high-frequency (VHF) plasma cell and an ultra-high-vacuum (UHV) chamber. The features of VHF plasma are higher efficiency of radical formation and lower self-bias of the plasma compared to radio-frequency plasma (13.56 MHz). In this paper, we describe the fabrication of nc-Si with various conditions of dilution of Sill 4 by
* Corresponding author. Tel: +81-3 5734 3048; fax: +81-3 5734 291 l ; e-mail: motobe @pe.titech.ac.jp. i Also with PRESTO, Research Development Corporation of Japan.
H 2 or Ar. Prepared samples were characterized by transmission electron microscopy (TEM) (accelerating voltage of 200 kV). The dependence of the deposition rate and the grain size on the dilution condition is also discussed. Nanocrystalline Si with an average grain size less than 10 nm fabricated with this apparatus shows red photoluminescence at room temperature after dry oxidation [2].
2. Experimental A schematic diagram of the experimental apparatus is depicted in Ref. [2]. This is a modified Si molecular beam epitaxy machine. The electrodes of the plasma cell are capacitively coupled. The stainless plate with an orifice, separating UHV chamber and a plasma cell is used as a grounded electrode. The dimension of the orifice is 6 mm in diameter and 2 mm in length. The volume of the plasma cell is 230 cm 3. The deposition rate of nc-Si can be monitored by a quartz crystal thickness sensor. The deposition rate is calculated from the mass deposited on the quartz
0022-3093/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 3 0 9 3 ( 9 6 ) 0 0 1 61-5
876
M. Otobe et al. / Journal of Non-Crystalline Solids 198-200 (1996) 875-878
crystal sensor. The pressure in the cell monitored by a capacitance manometer is controlled by the flow rate of the source gases. Nanocrystalline Si is formed in the gas phase of the plasma cell, by coalescence of radicals produced from S i l l 4. The nc-Si is extracted out of the plasma cell through the orifice to the U H V chamber. Then the nc-Si is collected on an amorphous carbon micro-grid for T E M observation. Experimental parameters were as follows: (1) substrates were not heated; (2) the V H F power density was 2.8 W / c m 2 ; (3) when S i l l 4 was fed into the plasma cell and plasma was not activated, the pressure in the cell was 0.42 Torr. This pressure corresponds to a S i l l 4 flow rate of ~ 7 ccm.
3. Results
Fig. 1 shows the deposition rate of nc-Si prepared from S i l l 4 diluted by Ar (open circles) or H 2 (filled circles), plotted against various flow rates of dilution gas. The flow rates of H e and Ar were varied from 0 ccm to 30 ccm. The flow rate and the partial pressure of S i l l 4 were kept constant. As S i l l 4 is diluted by Ar or H 2 at 24 ccm, the total pressures are 1.8 Torr and 0.71 Torr, respectively. When S i l l 4 was diluted by Ar, the deposition rate drastically increased (two orders of magnitude) with increasing dilution ratio. When S i l l 4 was diluted by H 2, the deposition rate decreased with increasing dilution ratio. Fig. 2(a), (b), and (c) are the TEM micrographs of the samples prepared with pure S i l l 4, H 2 dilution (H 2 flow rate of 24 ccm), and Ar dilution (flow rate of 24 ccm), respectively. Nanocrystalline Si adheres
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no
Z
|
o_ o a_ w o
....
0.1
0
5
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10 15 20 25 FLOWRATE (ecru)
....
30
Fig. 1. The deposition rate of nc-Si prepared from Sill 4 diluted by Ar (open circles) or H 2 (filled circles), plotted against various flow rates of dilution gas. to the edge of the amorphous carbon micro-grid. The crystallinity of nc-Si is confirmed by the transmission electron diffraction pattern and lattice image of S i ( l l l ) [1]. The grain size of nc-Si with H2-diluted S i l l 4 in Fig. 2(b), ( 4 - 1 5 rim), is smaller than that in Fig. 2(a) and (c) ( 7 - 4 0 nm).
4. Discussion
When the source gases were fed into the chamber, the pressure in the UHV chamber was ~ 1 mTorr. In the UHV chamber, a mean free path of S i l l 4 and Sill, (x=0-3) radicals is several cm, which is much shorter than the distance (30 cm) between the substrate and the orifice of the plasma cell. Therefore, S i l l x radicals extracted from the cell are scattered in the UHV chamber. Then the number of S i l l x radicals which reach the substrate and the thickness
Fig. 2. Images of TEM for the samples prepared with (a) pure Sill4, (b) H2-dilution (H 2 flow rate of 24 ccm), (c) Ar dilution (flow rate of 24 ccm).
M. Otobe et al. / Journal of Non-C~stalline Solids 198-200 (1996) 875-878
sensor is very small. Therefore, when the nucleation does not occur in the plasma cell, the deposition rate is almost zero. The deposition rate R is expressed as 4wr 3 R = --pf (1) 3 where r is the average radius of nc-Si, p is density of Si, and f is the number of nc-Si reaching the thickness sensor per unit time. Here, the distribution of r is neglected. The average grain sizes of nc-Si produced with pure Sill 4 and with H 2 (24 ccm) diluted Sill 4 are 14 nm ( q ) and 9.0 nm (r2), respectively. The ratio of the deposition rate of nc-Si produced with pure Sill 4 (1.0 n m / m i n ) and with He-dilution (0.27 n m / m i n ) is 3.7 which agrees well w i t h ( r l / r 2 ) 3 of 3.8. This means the number of nc-Si reaching the thickness sensor per unit time is constant for H 2 dilution, and hence the nucleation rate of nc-Si in the plasma cell is constant. For the sample prepared with Ar dilution, the average grain size is the same as the sample with pure Sill 4. The drastic increase of the deposition rate is due to the increase of the number of nc-Si reaching the thickness sensor per unit time. By the in-situ observation of Si particles in Sill 4 plasma, it is strongly suggested that short-lifetime radicals Sill,, (n = 0 - 2 ) mainly contribute to formation in the nucleation phase [3]. The Sill e radical, which has strong reactivity with Sill 4, is one of the candidates of a precursor for nucleation. When Sill 4 is diluted by Ar, the enhancement of Sill 2 radical production rate has been pointed out [4]. On the basis of homogeneous nucleation theory, the nucleation rate is determined by the competition between the desorption of atoms from the nuclei and the condensation to the nuclei. The nucleation rate in the gas phase is given by [5] • J=
167ruZy2pq k2TZ~(ln(p/pe))
2
× exp - 3 k 3 T g ( l n ( P I P e ) ) ?
(2)
where L, is the reciprocal of the number of Si atoms per unit volume in a crystal, 3' is the surface energy, q is the density of the precursor, k is Boltzmann constant, T is the temperature, p is the precursor
877
pressure, Pe is the saturated vapor pressure of the precursor, and m is the mass of the precursor. Here, precursor means the molecule which contributes to nucleation. It can be considered that the drastic increase of the deposition rate with Ar dilution is due to the nonlinearity of the nucleation rate to the density of precursor. Though it is difficult to determine the value of y and P / P e , we assumed 3' = 1.2 J / m s for our calculation, which is the surface energy of Si(111). When p / p e changes from 100 to 101, J increases several orders of magnitude. The drastic increase of the deposition rate with increasing plasma power or Sill 4 pressure can be expressed by the same model [1]. When nc-Si grows under the diffusion-controlled growth condition and the distance between nc-Si atoms is sufficiently longer than the diffusion length, the radius of nc-Si r is given by [6] r = V'~2Dv(C - s ) t
(3)
where D is the diffusion constant of the Sill x which contributes to the growth of nc-Si, u is the reciprocal of the number of Si atom per unit volume in crystal, C is the concentration of Sill X far from nc-Si, s is the concentration of SiHx near the surface of nc-Si, and t is the growth time. On the assumption of constant Sill x, the grain size of nc-Si (twice the radius r) is determined by the growth time t. The growth time in the cell corresponds to the residence time of the gas. The residence times of the gas in the cell for 100% Sill 4, 24 ccm H2-dilution, and 24 ccm At-dilution condition, is 1.1 s (tl), 0.42 s (t2), and 1.1 s 03)i respectively. The difference in the residence times is due to the conductance of evacuation which is proportional to the reciprocal of the square root of the molecular weight. The average grain sizes of nc-Si prepared with 100% Sill 4 and Ar dilution is constant because of the same residence time. The square root of the ratio of t 2 and t~ is 0.62. The ratio of the average grain size, r 2 / r l, is 0.64 which agrees well with ( t 2 / t t ) I/2. The average grain size is also described by Eq. (3) when the residence time of Sill 4 is changed by using a small orifice [7]. From Eq. (1) and (3), the deposition rate R is determined by the residence time t; that is: R is proportional to t 3/2. The dotted line in Fig. 1 is the calculated deposition rate using the residence time
878
M. Otobe et al. / Journal of Non-Crystalline Solids 198-200 (1996) 875-878
and proportional constant 73.2, which fits well to the experimental data.
5. Conclusion We have investigated nanometer sized crystalline silicon (nc-Si) formation in the Sill 4 plasma cell with high-frequency (144 MHz) excitation with the Ar and H 2 dilution method. The crystalline Si forms in the plasma cell and is extracted out of the cell toward the substrate. The deposition rate increases with Ar dilution and decreases with H 2 dilution. The average grain size decrease with H 2 dilution. The change of deposition rate is expressed by the nucleation rate and the grain size. The grain size of nc-Si corresponds to the residence time of the gas in the plasma cell. In order to obtain nc-Si with a small spread in grain size, it is required to control the nucleation and the growth processes independently.
Acknowledgements The authors wish to express their thanks to Professor K. Yagi and Professor N. Yamamoto for use
of the TEM apparatus and their useful suggestions. This work was supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture, by the Asahi Glass Foundation and by the Kurata Foundation. One of the authors (M.O.) was supported by the Research Fellow Program of the Japan Society for the Promotion of Science. Ultra-high-purity Sill 4 was donated by Mitsui Toatsu Chemicals, Inc.
References [1] S. Oda and M. Otobe, Mater. Res. Soc. Symp. Proc. 358 (1995) 721, and other papers therein. [2] M. Otobe, T. Kanai, and S. Oda, Mater. Res. Soc. Syrup. Proc. 377 (1995) 51. [3] Y. Watanabe and M. Shiratani, Plasma Sources Sci. Technol. 3 (1994) 286. [4] A. Kono, N. Koike, K. Okuda, and T. Goto, Jpn. J. Appl. Phys. 32 (1993) L543. [5] M. Volmer and A. Weber, Z. Phys. Chem. 119 (1926) 277. [6] A.E. Nielsen, Kinetics of Precipitation (Pergamon, Oxford, 1964). [7] M. Otobe, unpublished.
NON m SOL ELSEVIER
Journal of Non-Crystalline Solids 198-200 (1996) 875 878
Nanocrystalline silicon formation in
a Sill 4
plasma cell
Masanori Otobe, Tomonori Kanai, Toru Ifuku, Hiroshi Yajima, Shunri Oda * .1 Research Center fi)r Quantum Effect Electronics, Tokyo Institute o[ Technology, 2-12-1, O-okayama, Meguro-ku, Tokyo 152, Japan
Abstract Nanometer sized crystalline (nc) Si formation in the Sill 4 plasma cell with very-high-frequency (144 MHz) excitation has been investigated with the Ar and H 2 dilution method. The Sill 4 plasma cell is attached to the ultra-high-vacuum chamber. The crystalline Si is formed in the gas phase of the plasma cell by coalescence of radicals produced from Sill 4. The nc-Si is extracted out of the plasma cell through the orifice to the ultra-high-vacuum chamber. The deposition rate increases with Ar dilution and decreases with H z dilution. The average grain size decreases with H 2 dilution. The dependence of the deposition rate and the grain size on the dilution condition is discussed.
1. Introduction Recently, nanocrystalline silicon (nc-Si) with a grain size of less than 10 nm attracts considerable attention because the manifestation of a quantum dot structure is expected [1]. In order to apply nc-Si to future electron devices, it is required to establish the technology to control the size and the surface states of nc-Si. In an attempt to implement these controls, we have designed an apparatus for fabrication of nc-Si which consists of a very-high-frequency (VHF) plasma cell and an ultra-high-vacuum (UHV) chamber. The features of VHF plasma are higher efficiency of radical formation and lower self-bias of the plasma compared to radio-frequency plasma (13.56 MHz). In this paper, we describe the fabrication of nc-Si with various conditions of dilution of Sill 4 by
* Corresponding author. Tel: +81-3 5734 3048; fax: +81-3 5734 291 l ; e-mail: motobe @pe.titech.ac.jp. i Also with PRESTO, Research Development Corporation of Japan.
H 2 or Ar. Prepared samples were characterized by transmission electron microscopy (TEM) (accelerating voltage of 200 kV). The dependence of the deposition rate and the grain size on the dilution condition is also discussed. Nanocrystalline Si with an average grain size less than 10 nm fabricated with this apparatus shows red photoluminescence at room temperature after dry oxidation [2].
2. Experimental A schematic diagram of the experimental apparatus is depicted in Ref. [2]. This is a modified Si molecular beam epitaxy machine. The electrodes of the plasma cell are capacitively coupled. The stainless plate with an orifice, separating UHV chamber and a plasma cell is used as a grounded electrode. The dimension of the orifice is 6 mm in diameter and 2 mm in length. The volume of the plasma cell is 230 cm 3. The deposition rate of nc-Si can be monitored by a quartz crystal thickness sensor. The deposition rate is calculated from the mass deposited on the quartz
0022-3093/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 3 0 9 3 ( 9 6 ) 0 0 1 61-5
876
M. Otobe et al. / Journal of Non-Crystalline Solids 198-200 (1996) 875-878
crystal sensor. The pressure in the cell monitored by a capacitance manometer is controlled by the flow rate of the source gases. Nanocrystalline Si is formed in the gas phase of the plasma cell, by coalescence of radicals produced from S i l l 4. The nc-Si is extracted out of the plasma cell through the orifice to the U H V chamber. Then the nc-Si is collected on an amorphous carbon micro-grid for T E M observation. Experimental parameters were as follows: (1) substrates were not heated; (2) the V H F power density was 2.8 W / c m 2 ; (3) when S i l l 4 was fed into the plasma cell and plasma was not activated, the pressure in the cell was 0.42 Torr. This pressure corresponds to a S i l l 4 flow rate of ~ 7 ccm.
3. Results
Fig. 1 shows the deposition rate of nc-Si prepared from S i l l 4 diluted by Ar (open circles) or H 2 (filled circles), plotted against various flow rates of dilution gas. The flow rates of H e and Ar were varied from 0 ccm to 30 ccm. The flow rate and the partial pressure of S i l l 4 were kept constant. As S i l l 4 is diluted by Ar or H 2 at 24 ccm, the total pressures are 1.8 Torr and 0.71 Torr, respectively. When S i l l 4 was diluted by Ar, the deposition rate drastically increased (two orders of magnitude) with increasing dilution ratio. When S i l l 4 was diluted by H 2, the deposition rate decreased with increasing dilution ratio. Fig. 2(a), (b), and (c) are the TEM micrographs of the samples prepared with pure S i l l 4, H 2 dilution (H 2 flow rate of 24 ccm), and Ar dilution (flow rate of 24 ccm), respectively. Nanocrystalline Si adheres
1 O0
F
o o
E LH
10 !
<
no
Z
|
o_ o a_ w o
....
0.1
0
5
b___iir_
10 15 20 25 FLOWRATE (ecru)
....
30
Fig. 1. The deposition rate of nc-Si prepared from Sill 4 diluted by Ar (open circles) or H 2 (filled circles), plotted against various flow rates of dilution gas. to the edge of the amorphous carbon micro-grid. The crystallinity of nc-Si is confirmed by the transmission electron diffraction pattern and lattice image of S i ( l l l ) [1]. The grain size of nc-Si with H2-diluted S i l l 4 in Fig. 2(b), ( 4 - 1 5 rim), is smaller than that in Fig. 2(a) and (c) ( 7 - 4 0 nm).
4. Discussion
When the source gases were fed into the chamber, the pressure in the UHV chamber was ~ 1 mTorr. In the UHV chamber, a mean free path of S i l l 4 and Sill, (x=0-3) radicals is several cm, which is much shorter than the distance (30 cm) between the substrate and the orifice of the plasma cell. Therefore, S i l l x radicals extracted from the cell are scattered in the UHV chamber. Then the number of S i l l x radicals which reach the substrate and the thickness
Fig. 2. Images of TEM for the samples prepared with (a) pure Sill4, (b) H2-dilution (H 2 flow rate of 24 ccm), (c) Ar dilution (flow rate of 24 ccm).
M. Otobe et al. / Journal of Non-C~stalline Solids 198-200 (1996) 875-878
sensor is very small. Therefore, when the nucleation does not occur in the plasma cell, the deposition rate is almost zero. The deposition rate R is expressed as 4wr 3 R = --pf (1) 3 where r is the average radius of nc-Si, p is density of Si, and f is the number of nc-Si reaching the thickness sensor per unit time. Here, the distribution of r is neglected. The average grain sizes of nc-Si produced with pure Sill 4 and with H 2 (24 ccm) diluted Sill 4 are 14 nm ( q ) and 9.0 nm (r2), respectively. The ratio of the deposition rate of nc-Si produced with pure Sill 4 (1.0 n m / m i n ) and with He-dilution (0.27 n m / m i n ) is 3.7 which agrees well w i t h ( r l / r 2 ) 3 of 3.8. This means the number of nc-Si reaching the thickness sensor per unit time is constant for H 2 dilution, and hence the nucleation rate of nc-Si in the plasma cell is constant. For the sample prepared with Ar dilution, the average grain size is the same as the sample with pure Sill 4. The drastic increase of the deposition rate is due to the increase of the number of nc-Si reaching the thickness sensor per unit time. By the in-situ observation of Si particles in Sill 4 plasma, it is strongly suggested that short-lifetime radicals Sill,, (n = 0 - 2 ) mainly contribute to formation in the nucleation phase [3]. The Sill e radical, which has strong reactivity with Sill 4, is one of the candidates of a precursor for nucleation. When Sill 4 is diluted by Ar, the enhancement of Sill 2 radical production rate has been pointed out [4]. On the basis of homogeneous nucleation theory, the nucleation rate is determined by the competition between the desorption of atoms from the nuclei and the condensation to the nuclei. The nucleation rate in the gas phase is given by [5] • J=
167ruZy2pq k2TZ~(ln(p/pe))
2
× exp - 3 k 3 T g ( l n ( P I P e ) ) ?
(2)
where L, is the reciprocal of the number of Si atoms per unit volume in a crystal, 3' is the surface energy, q is the density of the precursor, k is Boltzmann constant, T is the temperature, p is the precursor
877
pressure, Pe is the saturated vapor pressure of the precursor, and m is the mass of the precursor. Here, precursor means the molecule which contributes to nucleation. It can be considered that the drastic increase of the deposition rate with Ar dilution is due to the nonlinearity of the nucleation rate to the density of precursor. Though it is difficult to determine the value of y and P / P e , we assumed 3' = 1.2 J / m s for our calculation, which is the surface energy of Si(111). When p / p e changes from 100 to 101, J increases several orders of magnitude. The drastic increase of the deposition rate with increasing plasma power or Sill 4 pressure can be expressed by the same model [1]. When nc-Si grows under the diffusion-controlled growth condition and the distance between nc-Si atoms is sufficiently longer than the diffusion length, the radius of nc-Si r is given by [6] r = V'~2Dv(C - s ) t
(3)
where D is the diffusion constant of the Sill x which contributes to the growth of nc-Si, u is the reciprocal of the number of Si atom per unit volume in crystal, C is the concentration of Sill X far from nc-Si, s is the concentration of SiHx near the surface of nc-Si, and t is the growth time. On the assumption of constant Sill x, the grain size of nc-Si (twice the radius r) is determined by the growth time t. The growth time in the cell corresponds to the residence time of the gas. The residence times of the gas in the cell for 100% Sill 4, 24 ccm H2-dilution, and 24 ccm At-dilution condition, is 1.1 s (tl), 0.42 s (t2), and 1.1 s 03)i respectively. The difference in the residence times is due to the conductance of evacuation which is proportional to the reciprocal of the square root of the molecular weight. The average grain sizes of nc-Si prepared with 100% Sill 4 and Ar dilution is constant because of the same residence time. The square root of the ratio of t 2 and t~ is 0.62. The ratio of the average grain size, r 2 / r l, is 0.64 which agrees well with ( t 2 / t t ) I/2. The average grain size is also described by Eq. (3) when the residence time of Sill 4 is changed by using a small orifice [7]. From Eq. (1) and (3), the deposition rate R is determined by the residence time t; that is: R is proportional to t 3/2. The dotted line in Fig. 1 is the calculated deposition rate using the residence time
878
M. Otobe et al. / Journal of Non-Crystalline Solids 198-200 (1996) 875-878
and proportional constant 73.2, which fits well to the experimental data.
5. Conclusion We have investigated nanometer sized crystalline silicon (nc-Si) formation in the Sill 4 plasma cell with high-frequency (144 MHz) excitation with the Ar and H 2 dilution method. The crystalline Si forms in the plasma cell and is extracted out of the cell toward the substrate. The deposition rate increases with Ar dilution and decreases with H 2 dilution. The average grain size decrease with H 2 dilution. The change of deposition rate is expressed by the nucleation rate and the grain size. The grain size of nc-Si corresponds to the residence time of the gas in the plasma cell. In order to obtain nc-Si with a small spread in grain size, it is required to control the nucleation and the growth processes independently.
Acknowledgements The authors wish to express their thanks to Professor K. Yagi and Professor N. Yamamoto for use
of the TEM apparatus and their useful suggestions. This work was supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture, by the Asahi Glass Foundation and by the Kurata Foundation. One of the authors (M.O.) was supported by the Research Fellow Program of the Japan Society for the Promotion of Science. Ultra-high-purity Sill 4 was donated by Mitsui Toatsu Chemicals, Inc.
References [1] S. Oda and M. Otobe, Mater. Res. Soc. Symp. Proc. 358 (1995) 721, and other papers therein. [2] M. Otobe, T. Kanai, and S. Oda, Mater. Res. Soc. Syrup. Proc. 377 (1995) 51. [3] Y. Watanabe and M. Shiratani, Plasma Sources Sci. Technol. 3 (1994) 286. [4] A. Kono, N. Koike, K. Okuda, and T. Goto, Jpn. J. Appl. Phys. 32 (1993) L543. [5] M. Volmer and A. Weber, Z. Phys. Chem. 119 (1926) 277. [6] A.E. Nielsen, Kinetics of Precipitation (Pergamon, Oxford, 1964). [7] M. Otobe, unpublished.