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Roughening of the surface of an Si layer grown on an Si(111)-(7 × 7) superlattice
surface science ELSEVIER
Surface Science 357-358 (1996) 414-417
Roughening of the surface of an Si layer grown on an Si ( 111 )-( 7 x 7) superlattice Y. S h i g e t a *, J. E n d o , H. F u j i n o , K. M a k i Faculty of Science and Graduate School of Integrated Science, Yokohama City University, Seto, Kanazawa-ku, Yokohama236, Japan Received 15 August 1995; accepted for publication 20 October 1995
Abstract The morphology change of the growing surface of an Si film with a thickness of 50 nm on an Si(111)-(7 x 7) superlattice held at 250°C was observed with reflection high-energy electron diffraction (RHEED) and scanning tunneling microscopy (STM). During the deposition the intensity oscillation of RHEED exponentially attenuates with an attenuation constant of about 1/4.7 nm-1, which corresponds to decreasing-area progressive layer-by-layer growth with a height of one bilayer (0.31 nm) of the S i ( l l l ) plane. STM observation shows that the surface area of an Si film, when the intensity oscillation is completely attenuated, is composed of distorted multilayer hillocks with a mean width of 20 nm and a mean height of 1.4 nm. In some re~ons, we also observed terraces with a uniform width of 2.3 nm, and two kinds of atomic arrangement on these terraces.
Keywords: Crystalline-amorphous interfaces; Scanning tunneling microscopy; Silicon; Surface structure, morphology, roughness, and topography
1. Introduction
In the growth of an Si film o n a n Si(111)-(7 x 7) superlattice at a temperature T~ below 300°C, the area of the layer satisfying an epitaxial relation gradually decreases with increasing film thickness d, and finally transforms into an amorphous phase when d reaches a certain value, d~. In other words, a limiting thickness d~ exists for epitaxial growth of the Si film on the Si(lll)-(7 x 7) substrate held at T~< 300°C. Such epitaxial growth was also found on an Si film on an Si(001) substrate by Eaglesham et al. [ 5 ] who called this growth "low-temperature epitaxial (LTE) growth". The value of dc increases with decreasing growth rate R and with increasing *Corresponding author. Fax: +81 45 7872316; e-mail: [email protected].
T~ [3,4], i.e. d~=6.5, 17 and 100nm at T~=170, 200 and 250°C, respectively. The present authors and Eaglesham et al. suggested that the transformation is related to surface roughening on the growing layer [ 3-5]. In order to confirm the above suggestion, observation of the surface morphology change in LTE growth on the atomic scale is necessary. The surface morphology at the initial growth stage of an Si film on an Si(111)-(7 × 7) superlattice surface held at 250°C was studied by scanning tunneling microscopy (STM) and reflection highenergy electron diffraction (RHEED) in a previous paper [6]. It was confirmed that the growth mode transforms from multilayer island growth to layerby-layer growth at a deposition of two bilayers (one bilayer (BL) is 0.31 nm thick). The surface of the growing layer is composed of small domains
0039-6028/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved
PH S0039-6028 ( 9 6 ) 00190-2
Y. Shigeta et al./Surface Science 357-358 (1996) 414-417
(< 10 nm) with metastable (2 × 2), (5 × 5), (9 x 9), c(2 ×4) and (2 × 1) structures in addition to the (7 x 7) structure, in contrast to the substrate surface which is composed of large domains (> 200 nm) of the stable (7 x 7) structure. Lateral growth of the first layer on the (7 × 7) superlattice surface, which requires a rearrangement of atoms in the stable (7 x 7) structure, is prevented along the dimer rows in the dimer-adatom stacking fault (DAS) structure [7] on the S i ( l l l ) - ( 7 × 7) surface, and secondlayer islands nucleate and grow on the first layer before the substrate is completely covered b y the first layer. On the other hand, after the substrate is covered with the first layer, lateral growth of the second layer progresses because the surface of the first layer is composed of small domains with metastable structures, and layer-by-layer growth starts when the second layer fills the space between the multilayer islands. Although we confirmed that an almost flat surface is formed in the initial growth stage (d> 2 BL) at T~=250°C [6], we need to confirm whether or not the growing surface of very thick film becomes too rough to lead to amorphous growth. In this paper, we show some preliminary restdts from R H E E D and STM observations of the surface of an Si film grown at a thickness of 50 nm on an S i ( l l l ) substrate, and make some comments on the morphology of the growing surface.
415
range 5 < d < 100 nm. We estimated the value of d below 5 nm from the linear relationship between Af and d. The R H E E D pattern from the growing surface was observed at an accelerating voltage of 10 kV. We also recorded the pattern on a video disk and monitored the intensity ! of the (0,0) spot during deposition.
3. Results and discussion Fig. l(a) shows the intensity change I during the deposition of Si on the Si(111)-(7 x 7) substrate at T~=250°C and R = 3 . 8 n m m i n -1. The amplitude (a) 0
Mean Thickness, d (rim) 5 10
0
1
15
4
(D)
2 Deposition Time
3. (min)
4
~- [112]
2. Experimental The surface of an S i ( l l l ) substrate was heated to ,,, 1150°C for a few hours and then flashed to 1200°C for a few rain by a direct electron current flow at pressures below 2 x 10 -8 Pa. Si films were deposited using an electron beam evaporation source. The temperature was measured with an infrared radiation thermometer (<800°C) and an optical pyrometer (> 800°C). The pressure during deposition at 3-4 nm min -1 in R was 7 x 10 -7 Pa and was recovered below 1.5 x 10 -s P a immediately after each deposition. We determined the value of the film thickness d with a multiple-beam Fizeau-type interferometer. The value of d was proportional to the frequency shift of a quartz oscillator Af for monitoring R in the thickness
Fig. 1. (a) Change in R H E E D intensity during growth of an Si film as a function of deposition time on an Si(111)-(7×7) substrate held at a temperature of 250°C. The corresponding mean thickness d is also shown. The electron beam is incident at a glancing angle of 1° along the [112] direction. (b) R H E E D pattern from the surface of the Si film at d = 50 nm at a glancing angle of 2 ° along the [ l i 0 ] direction. ~-order diffuse rods are observed along the [112] direction, as indicated by arrows.
Y. Shigeta et al./Surface Science 357-358 (1996) 414-417
416
of the R H E E D intensity oscillation with a period corresponding to the growth of 1 BL is exponentially attenuated with the deposition time or d. The attenuation constant was ~ 1/15 B L - 1 (1/4.7 r i m - 1). In the initial growth stage, the intensities at the first m i n i m u m and the second maxim u m are reduced by the effect of 3D island growth as described above [6]. After this stage, layer-bylayer growth starts and finally deteriorates because the amplitude of the intensity oscillation attenuates with progressing film growth. As will be shown later, the regions of layer-by-layer growth with a height of 1 BL decrease gradually with ongoing deposition. After oscillation could not be observed, the R H E E D intensity gradually decreased as deposition continued. Deposition was halted at 0.5do, at
which the intensity of low-energy electron diffraction was 10% of the substrate intensity I-4]. Fig. l(b) shows a R H E E D pattern from the Si film at d = 50 nm at the [110] incidence. This pattern is composed of some spots corresponding to the transmission case through Si hillocks, and some_~order diffuse rods are observed along the [112] direction (indicated by arrows in Fig. lb), which suggests that the surface becomes rough and some short-range ordered structure is constructed with a period three times as large as the lattice spacing ( b = 0 . 3 3 n m ) along the [112] direction of the S i ( l l l ) - ( 1 x 1) surface. The STM image from this surface, with a constant current m o d e at 0.3 nA and sample bias of 2.0V, is shown in Fig. 2a. We observed some hillocks with a width ,-~20 nm whose heights are i
(a)
(b)
h' ~ 3
'
i~
2
! ! '.-.-i . . . .
k
7b
14b
21b
28b
k'
(c)
v5 = 0
h
h'
Fig. 2. (a) STM image (45 nm x 45 nm) of the Si film at d= 50 nm. Many steps along the (110) directions are observed, as indicated by arrows. The height profile along the line h-h' is also plotted. (b) Enlarged STM image. Some steps with a uniform terrace width of 7b are observed, as shown in a height profile along the line k-k'. (c) Enlarged STM image showing diffuse atomic rows along the [01i] direction with a spacing of 3b.
Y. Shigeta et al./Surface Science 357-358 (1996) 414-417
,-~4 - 5 BL as shown in the height profile along the line h - h ' in the lower part of Fig. 2a. Such hillocks were also observed on the surface of thick GaAs films on a GaAs(001) surface [8,9]. We also observed m a n y steps along three equivalent ( 1 1 0 ) directions (indicated by arrows in Fig. 2b). The terrace width in some regions has a uniform value of 7b, as shown in Fig. 2b, which corresponds to the period of the (7 x 7) structure bordered by the dimer rows along the ( 1 1 0 ) direction. This suggests that the period of 7b bordered with the dimer rows corresponds to some stable width, because atoms at the step edge m a y also form dimers to decrease the n u m b e r of dangling bonds. Some steps are seen whose height is less than 1 BL, as shown in the height profile along the line k - k ' in the lower part of Fig. 2b. These findings suggest that the hillocks are grown on metastable surfaces whose structure is quenched and not rearranged during film growth. We believe that some metastable structure (such as the (2 x 1) structure) is preserved, which is composed of five- and sevenm e m b e r rings along the ( 1 1 0 ) directions and the top-most surface is positioned ~ 3/5 BL above the nearby adatoms in the DAS structure [ 10]. We also found that the structure on the terraces is classified into two cases: (i) on one type of terrace (indicated by "A" in Fig. 2b) the atoms are arranged randomly, (ii) zig-zag atomic rows are formed along the ( 1 1 0 ) directions, which are arranged with a spacing of 3b (indicated by arrows in Fig. 2c). Atomic rows with a spacing of 3b will result in the ½-order diffuse rods in the R H E E D pattern, as shown in Fig. l(b).
4. Summary From the above results, we conclude that the growing surface during Si deposition under LTE conditions becomes rough with a growth that leads
417
to attenuation of R H E E D intensity oscillation, and that the roughness reaches ~ 4 - 5 BL with a height of d = 50 nm. This roughening is caused because some of the arrived atoms cannot reach the correct site of the diamond structure and do not rearrange into the normal lattice site during the growth under LTE conditions, under which metastable surface structures (e.g.(2x2), ( 5 x 5 ) , ( 9 x 9 ) , c(2 x 4) and (2 x 1)) are formed, even in the initial growth stage.
Acknowledgements This work was partly supported by a Grant-in Aid for Scientific Research on Priority Areas "Free Radical Science" under Contract No. 06228225 from the Ministry of Education, Science and Culture of Japan. We are grateful to Mr. Yukio Y a m a m o t o for the development of the computer control system for STM observation.
References [1] Y. Shigeta and K. Maki, Jpn. J. Appl. Phys. 29 (1990) 2092. [2] Y. Shigeta and K. Maki, in: Epitaxial Crystal Growth, Part 2, Ed. 13. Lendvay (Trans Tech Publications, Switzerland, Germany, UK, USA, 1991) p. 13. [3] K. Maki, Y. Shigeta and T. Kuroda, J. Cryst. Growth 115 (1991) 567. [4] Y. Shigeta and K. Maki, J. Appl. Phys. 75 (1994) 5033. [5] D.J. 13aglesham, H.-J. Gossmann andM. Cerullo, Phys. Rev. Lett. 65 (1990) 1227. [6] Y. Shigeta, J. Endo and K. Maki, Phys. Rev. B 51 (1995) 2021. [7] K. Takayanagi, Y. Tanishiro, S. Takahashi and M. Takahashi, Surf. Sci. 164 (1985) 367. [8] M.D. Johnson, C. Orme, A.W. Hunt, D. Graft, J.L. Sudijono, L.M. Sander and B.G. Orr, Phys. Rev. Lett. 72 (1994) 116. [9] C. Orme, M.D. Johnson, J.L. Sudijono, K.T. Leung and B.G. Orr, Appl. Phys. Lett. 64 (1994) 860. [10] T. Yokoyama, H. Tanaka, M. Itoh, T. Yokotsuka and I. Sumita, Phys. Rev. B 49 (1994) 5703.
Surface Science 357-358 (1996) 414-417
Roughening of the surface of an Si layer grown on an Si ( 111 )-( 7 x 7) superlattice Y. S h i g e t a *, J. E n d o , H. F u j i n o , K. M a k i Faculty of Science and Graduate School of Integrated Science, Yokohama City University, Seto, Kanazawa-ku, Yokohama236, Japan Received 15 August 1995; accepted for publication 20 October 1995
Abstract The morphology change of the growing surface of an Si film with a thickness of 50 nm on an Si(111)-(7 x 7) superlattice held at 250°C was observed with reflection high-energy electron diffraction (RHEED) and scanning tunneling microscopy (STM). During the deposition the intensity oscillation of RHEED exponentially attenuates with an attenuation constant of about 1/4.7 nm-1, which corresponds to decreasing-area progressive layer-by-layer growth with a height of one bilayer (0.31 nm) of the S i ( l l l ) plane. STM observation shows that the surface area of an Si film, when the intensity oscillation is completely attenuated, is composed of distorted multilayer hillocks with a mean width of 20 nm and a mean height of 1.4 nm. In some re~ons, we also observed terraces with a uniform width of 2.3 nm, and two kinds of atomic arrangement on these terraces.
Keywords: Crystalline-amorphous interfaces; Scanning tunneling microscopy; Silicon; Surface structure, morphology, roughness, and topography
1. Introduction
In the growth of an Si film o n a n Si(111)-(7 x 7) superlattice at a temperature T~ below 300°C, the area of the layer satisfying an epitaxial relation gradually decreases with increasing film thickness d, and finally transforms into an amorphous phase when d reaches a certain value, d~. In other words, a limiting thickness d~ exists for epitaxial growth of the Si film on the Si(lll)-(7 x 7) substrate held at T~< 300°C. Such epitaxial growth was also found on an Si film on an Si(001) substrate by Eaglesham et al. [ 5 ] who called this growth "low-temperature epitaxial (LTE) growth". The value of dc increases with decreasing growth rate R and with increasing *Corresponding author. Fax: +81 45 7872316; e-mail: [email protected].
T~ [3,4], i.e. d~=6.5, 17 and 100nm at T~=170, 200 and 250°C, respectively. The present authors and Eaglesham et al. suggested that the transformation is related to surface roughening on the growing layer [ 3-5]. In order to confirm the above suggestion, observation of the surface morphology change in LTE growth on the atomic scale is necessary. The surface morphology at the initial growth stage of an Si film on an Si(111)-(7 × 7) superlattice surface held at 250°C was studied by scanning tunneling microscopy (STM) and reflection highenergy electron diffraction (RHEED) in a previous paper [6]. It was confirmed that the growth mode transforms from multilayer island growth to layerby-layer growth at a deposition of two bilayers (one bilayer (BL) is 0.31 nm thick). The surface of the growing layer is composed of small domains
0039-6028/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved
PH S0039-6028 ( 9 6 ) 00190-2
Y. Shigeta et al./Surface Science 357-358 (1996) 414-417
(< 10 nm) with metastable (2 × 2), (5 × 5), (9 x 9), c(2 ×4) and (2 × 1) structures in addition to the (7 x 7) structure, in contrast to the substrate surface which is composed of large domains (> 200 nm) of the stable (7 x 7) structure. Lateral growth of the first layer on the (7 × 7) superlattice surface, which requires a rearrangement of atoms in the stable (7 x 7) structure, is prevented along the dimer rows in the dimer-adatom stacking fault (DAS) structure [7] on the S i ( l l l ) - ( 7 × 7) surface, and secondlayer islands nucleate and grow on the first layer before the substrate is completely covered b y the first layer. On the other hand, after the substrate is covered with the first layer, lateral growth of the second layer progresses because the surface of the first layer is composed of small domains with metastable structures, and layer-by-layer growth starts when the second layer fills the space between the multilayer islands. Although we confirmed that an almost flat surface is formed in the initial growth stage (d> 2 BL) at T~=250°C [6], we need to confirm whether or not the growing surface of very thick film becomes too rough to lead to amorphous growth. In this paper, we show some preliminary restdts from R H E E D and STM observations of the surface of an Si film grown at a thickness of 50 nm on an S i ( l l l ) substrate, and make some comments on the morphology of the growing surface.
415
range 5 < d < 100 nm. We estimated the value of d below 5 nm from the linear relationship between Af and d. The R H E E D pattern from the growing surface was observed at an accelerating voltage of 10 kV. We also recorded the pattern on a video disk and monitored the intensity ! of the (0,0) spot during deposition.
3. Results and discussion Fig. l(a) shows the intensity change I during the deposition of Si on the Si(111)-(7 x 7) substrate at T~=250°C and R = 3 . 8 n m m i n -1. The amplitude (a) 0
Mean Thickness, d (rim) 5 10
0
1
15
4
(D)
2 Deposition Time
3. (min)
4
~- [112]
2. Experimental The surface of an S i ( l l l ) substrate was heated to ,,, 1150°C for a few hours and then flashed to 1200°C for a few rain by a direct electron current flow at pressures below 2 x 10 -8 Pa. Si films were deposited using an electron beam evaporation source. The temperature was measured with an infrared radiation thermometer (<800°C) and an optical pyrometer (> 800°C). The pressure during deposition at 3-4 nm min -1 in R was 7 x 10 -7 Pa and was recovered below 1.5 x 10 -s P a immediately after each deposition. We determined the value of the film thickness d with a multiple-beam Fizeau-type interferometer. The value of d was proportional to the frequency shift of a quartz oscillator Af for monitoring R in the thickness
Fig. 1. (a) Change in R H E E D intensity during growth of an Si film as a function of deposition time on an Si(111)-(7×7) substrate held at a temperature of 250°C. The corresponding mean thickness d is also shown. The electron beam is incident at a glancing angle of 1° along the [112] direction. (b) R H E E D pattern from the surface of the Si film at d = 50 nm at a glancing angle of 2 ° along the [ l i 0 ] direction. ~-order diffuse rods are observed along the [112] direction, as indicated by arrows.
Y. Shigeta et al./Surface Science 357-358 (1996) 414-417
416
of the R H E E D intensity oscillation with a period corresponding to the growth of 1 BL is exponentially attenuated with the deposition time or d. The attenuation constant was ~ 1/15 B L - 1 (1/4.7 r i m - 1). In the initial growth stage, the intensities at the first m i n i m u m and the second maxim u m are reduced by the effect of 3D island growth as described above [6]. After this stage, layer-bylayer growth starts and finally deteriorates because the amplitude of the intensity oscillation attenuates with progressing film growth. As will be shown later, the regions of layer-by-layer growth with a height of 1 BL decrease gradually with ongoing deposition. After oscillation could not be observed, the R H E E D intensity gradually decreased as deposition continued. Deposition was halted at 0.5do, at
which the intensity of low-energy electron diffraction was 10% of the substrate intensity I-4]. Fig. l(b) shows a R H E E D pattern from the Si film at d = 50 nm at the [110] incidence. This pattern is composed of some spots corresponding to the transmission case through Si hillocks, and some_~order diffuse rods are observed along the [112] direction (indicated by arrows in Fig. lb), which suggests that the surface becomes rough and some short-range ordered structure is constructed with a period three times as large as the lattice spacing ( b = 0 . 3 3 n m ) along the [112] direction of the S i ( l l l ) - ( 1 x 1) surface. The STM image from this surface, with a constant current m o d e at 0.3 nA and sample bias of 2.0V, is shown in Fig. 2a. We observed some hillocks with a width ,-~20 nm whose heights are i
(a)
(b)
h' ~ 3
'
i~
2
! ! '.-.-i . . . .
k
7b
14b
21b
28b
k'
(c)
v5 = 0
h
h'
Fig. 2. (a) STM image (45 nm x 45 nm) of the Si film at d= 50 nm. Many steps along the (110) directions are observed, as indicated by arrows. The height profile along the line h-h' is also plotted. (b) Enlarged STM image. Some steps with a uniform terrace width of 7b are observed, as shown in a height profile along the line k-k'. (c) Enlarged STM image showing diffuse atomic rows along the [01i] direction with a spacing of 3b.
Y. Shigeta et al./Surface Science 357-358 (1996) 414-417
,-~4 - 5 BL as shown in the height profile along the line h - h ' in the lower part of Fig. 2a. Such hillocks were also observed on the surface of thick GaAs films on a GaAs(001) surface [8,9]. We also observed m a n y steps along three equivalent ( 1 1 0 ) directions (indicated by arrows in Fig. 2b). The terrace width in some regions has a uniform value of 7b, as shown in Fig. 2b, which corresponds to the period of the (7 x 7) structure bordered by the dimer rows along the ( 1 1 0 ) direction. This suggests that the period of 7b bordered with the dimer rows corresponds to some stable width, because atoms at the step edge m a y also form dimers to decrease the n u m b e r of dangling bonds. Some steps are seen whose height is less than 1 BL, as shown in the height profile along the line k - k ' in the lower part of Fig. 2b. These findings suggest that the hillocks are grown on metastable surfaces whose structure is quenched and not rearranged during film growth. We believe that some metastable structure (such as the (2 x 1) structure) is preserved, which is composed of five- and sevenm e m b e r rings along the ( 1 1 0 ) directions and the top-most surface is positioned ~ 3/5 BL above the nearby adatoms in the DAS structure [ 10]. We also found that the structure on the terraces is classified into two cases: (i) on one type of terrace (indicated by "A" in Fig. 2b) the atoms are arranged randomly, (ii) zig-zag atomic rows are formed along the ( 1 1 0 ) directions, which are arranged with a spacing of 3b (indicated by arrows in Fig. 2c). Atomic rows with a spacing of 3b will result in the ½-order diffuse rods in the R H E E D pattern, as shown in Fig. l(b).
4. Summary From the above results, we conclude that the growing surface during Si deposition under LTE conditions becomes rough with a growth that leads
417
to attenuation of R H E E D intensity oscillation, and that the roughness reaches ~ 4 - 5 BL with a height of d = 50 nm. This roughening is caused because some of the arrived atoms cannot reach the correct site of the diamond structure and do not rearrange into the normal lattice site during the growth under LTE conditions, under which metastable surface structures (e.g.(2x2), ( 5 x 5 ) , ( 9 x 9 ) , c(2 x 4) and (2 x 1)) are formed, even in the initial growth stage.
Acknowledgements This work was partly supported by a Grant-in Aid for Scientific Research on Priority Areas "Free Radical Science" under Contract No. 06228225 from the Ministry of Education, Science and Culture of Japan. We are grateful to Mr. Yukio Y a m a m o t o for the development of the computer control system for STM observation.
References [1] Y. Shigeta and K. Maki, Jpn. J. Appl. Phys. 29 (1990) 2092. [2] Y. Shigeta and K. Maki, in: Epitaxial Crystal Growth, Part 2, Ed. 13. Lendvay (Trans Tech Publications, Switzerland, Germany, UK, USA, 1991) p. 13. [3] K. Maki, Y. Shigeta and T. Kuroda, J. Cryst. Growth 115 (1991) 567. [4] Y. Shigeta and K. Maki, J. Appl. Phys. 75 (1994) 5033. [5] D.J. 13aglesham, H.-J. Gossmann andM. Cerullo, Phys. Rev. Lett. 65 (1990) 1227. [6] Y. Shigeta, J. Endo and K. Maki, Phys. Rev. B 51 (1995) 2021. [7] K. Takayanagi, Y. Tanishiro, S. Takahashi and M. Takahashi, Surf. Sci. 164 (1985) 367. [8] M.D. Johnson, C. Orme, A.W. Hunt, D. Graft, J.L. Sudijono, L.M. Sander and B.G. Orr, Phys. Rev. Lett. 72 (1994) 116. [9] C. Orme, M.D. Johnson, J.L. Sudijono, K.T. Leung and B.G. Orr, Appl. Phys. Lett. 64 (1994) 860. [10] T. Yokoyama, H. Tanaka, M. Itoh, T. Yokotsuka and I. Sumita, Phys. Rev. B 49 (1994) 5703.