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Atomistic study on solid phase epitaxy processes on Si(100) surfaces by the scanning tunneling microscope
,. . . . . . . .
ELSEVIER
CRYSTAL GROWTH
Journal of Crystal Growth 163 (1996)78-85
Atomistic study on solid phase epitaxy processes on Si(100) surfaces by the scanning tunneling microscope T. Yao a,b, *, K. Uesugi c, T. Komura d, M. Yoshimura e a Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980, Japan b Joint Research Centerfi)r Atom Technology, National Institute for Advanced Interdisciplinary Research, Tsukuba 305, Japan c Research Institute for Electronic Science, Hokkaido University, Sapporo 060, Japan d Department of Electrical Engineering, Hiroshima University, Higashi-Hiroshima 724, Japan e Toyoda Engineering Universi~, Tennshiro-ku, Nagoya 468, Japan
Abstract
We report an atomistic study on solid phase epitaxy processes of amorphous Si layers on Si(001) using a scanning tunneling microscope. The amorphous layers are prepared by vacuum evaporation, Ar+-ion sputtering, or P+-ion implantation. The surface morphology of the as-prepared surface is strongly damaged by energetic impinging particles. The solid phase epitaxy processes are strongly correlated with the surface damage: crystallization of amorphous layers by annealing at around 250°C is observed on both the vacuum deposited and Ar+-ion sputtered surfaces, while an annealing temperature higher than 730°C is needed to recover the crystallinity of the P+-ion implanted surface.
1. I n t r o d u c t i o n
Solid phase epitaxy (SPE) of amorphous Si (a-Si) has been intensively investigated [1-7]. It has been established that controlled epitaxial regrowth of amorphous layers fabricated by Si-ion implantation onto single-crystal Si substrates occurs in a low-temperature range around 600°C [1]. During thermal annealing, the reordering of a-Si is characterized by a uniform translation of the amorphous to crystalline interface towards the top surface. Amorphous Si layers have been formed by ion implantation [1,2], vacuum evaporation [3-5], and chemical vapor deposition (CVD) [6]. In the case of Si(100) and Si(110) substrates, a linear regrowth in
* Corresponding author.
time is observed for isothermal annealing, and the resultant layers are relatively defect free [7]. SPE growth processes have been investigated by transmission electron microscopy (TEM) [2,3,5,6], scanning electron microscopy (SEM) [1,3], Nomarski optical microscopy [3], transmission electron diffraction (TED) [3,6], and Rutherford backscattering (RBS) [1,2,4-6]. Very recently, we have performed a microscopic investigation of the regrowth processes of amorphous layers prepared by vacuum evaporation, Ar +ion sputtering, or P+-ion implantation on Si(001) surfaces using a scanning tunneling microscope (STM) [8-12]. The atomic scale regrowth processes during annealing have been elucidated. The purpose of this paper is to review our recent study on atomistic processes of solid phase epitaxy of Si amorphous layers on Si(001) surfaces.
0022-0248/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0022 -0248(95)01038-6
T. Yao et a l . / Journal of Crystal Growth 163 (1996) 78-85
79
2. Experimental procedure We have used two ultra-high vacuum STM systems consisting of an STM chamber, a preparation chamber equipped with a vacuum evaporation source or an Ar+-ion sputtering gun, and a load-lock chamber. Both STMs can be operated at elevated temperatures. The tip used was an electrochemically etched tungsten wire. The amorphous Si layers investigated in the present study were prepared as follows: (a) vacuum evaporation; (b) Ar+-ion sputtering; (c) P+-ion implantation. The respective sample preparations were as follows: (a) an amorphous Si layer of 0 . 0 3 - 4 monolayer (ML) thickness was deposited on the clean Si(001)-2 X 1 surface at room temperature; (b) the clean Si(001)-2 × 1 surface was amorphized by Ar+-ion sputtering to a dose of (0.2-5) X 1015 cm -2 in the preparation chamber; (c) a Si(001) surface was amorphized by P+-ion implantation at 30 keV to a dose of 1 X 1016 c m - 2 . Before being loaded into the load-lock chamber, the implanted specimen was etched in an buffered HF solution to remove the oxide layer.
3. Surface morphology of amorphous Si layers Fig. 1 shows typical STM images of the amorphous Si surfaces prepared by (a) vacuum evaporation (6.8 X 1014 Si/cm2), (b) Ar+-ion sputtering at 3 keV to a dose of 2 X 1014 cm -2, and (c) P+-ion implantation. The morphology of the vacuum evaporated surface and the Ar+-ion sputtered surface looks similar: the surfaces are covered with homogeneous circular particles. The size of the particles increased with the amount of deposition or ion dose. The average size of the Si cluster observed on the surface was 0.72 nm for 0.03 ML coverage with a dispersion of 0.153 nm. This fact suggests that most of the evaporated Si adatoms form single dimers at extremely low deposition (0.03 ML) even at room temperature. With increasing coverage, single dimers tend to coalesce and form larger clusters. For instance, the average size of the Si cluster for 1 ML coverage was 0.77 nm with a dispersion of 0.23 nm. Some of them are narrow in shape and form dimer strings, which suggests that deposited Si atoms at
Fig. 1. As-prepared amorphous Si surfaces by (ai vacuum evaporation at room temperature, (b) Ar+-ion bombardment, and (c) P+-ion implantation. The sample bias was - 2 V.
room temperature migrate on the surface to form epitaxial island. As-bombarded surfaces by Ar + ions showed essentially the same features regardless of the dose examined: the surface is amorphized down to the depth of 3 nm and consists of grains of 0.6-1.6 nm in diameter and 0.3-0.8 nm in height. It is reported that an Ar+-ion bombarded Si surface at 700 eV to a dose of 1 x 1018 i o n s / c m 2 was covered with typi-
80
T. Yao et a l . / Journal of Crystal Growth 163 (1996) 78-85
due to the higher ion impact energy as compared with other techniques used in the present experiment. In fact, the surface was amorphized by implantation to a depth of more than 0.1 /zm at 20 keV, while only 3 nm of the surface layer was amorphized in the case of Ar+-ion sputtering at 3 keV. Furthermore, there should be overlap of cascade at a dose of 1 x 1016 cm -2, which degrades the surface morphology and crystallinity [14]. Table 1 summarizes the characteristics of amorphous Si layers prepared by vacuum evaporation, Ar+-ion sputtering, and P+ion implantation [ 12]. Fig. 2. Vacuum deposited surface during annealing at 250°C. (Vs = - 2 V).
cally 5 nm diameter hillocks [13], which is about three times larger than the size of the hillocks observed in the present experiment. Since this difference should be attributed to a difference in Ar+-ion energy, it is likely that as the Ar+-ion energy increases, the size of the hillocks is reduced. Most of the grains on the surface show a bright contrast regardless of the sample bias polarity. Si particles or islands should show the same contrast at both polarities. However, we found that there were few grains which changed contrast as the polarity was changed: some of them were bright at a positive sample bias, while the others were bright at negative polarity. The density of these grains is estimated to be of the order of 5 X 1013 c m -2 for the Ar+-ion dose of 1.9 X 1014 c m - 2. The P+-ion implanted surfaces are heavily damaged, as shown in Fig. lc. The RMS value is 0.40 nm, much greater that those of vacuum evaporated (0.08 nm) and Ar+-ion sputtered (0.09 nm) surfaces. In addition, the size of hillocks on the surface is large and scattered (1.5-4.1 nm) and their shapes are distorted and no longer circular. This is obviously
4. Solid phase epitaxy process of vacuum deposited Si amorphous layers When the as-deposited Si amorphous layer is annealed, the surface migration of adatoms is enhanced to form dimer strings, which are perpendicular to the dimer row on the substrate surface. Fig. 2 shows an STM image of a 1 ML deposited Si surface during annealing at 250°C. Although most of the Si adatoms form dimer strings, there still remains Si clusters. As the annealing temperature or annealing time increases, these clusters dissolve and dimer strings develop. The formation and development of the dimer strings are due to anisotropic diffusion of Si adatoms on the 2 × 1 reconstructed surface [15]. Fig. 3 shows how the dimer string develops with annealing time and temperature. Here, (i,j) denotes a dimer string with i dimer unit (du) widths and j unit bond (au) lengths (1 du = 0.768 nm, 1 au = 0.384 nm). Short dimer strings (1, 1-3) are prominent on the surface during annealing at 250°C for 150 min, while both the length and width of the island increase with prolonged annealing (500 min). Additional high temperature annealing up to 300°C
Table 1 Characteristics of the surfaces of amorphous layers prepared by vacuum evaporation, Ar+-ion bombardment, and P+-ion implantation Energy (eV) Dose (cm -2 ) Size of hillocks (nm) Surface roughness a (nm) Vacuum evaporation Ar+-ion sputtering P+-ion implantation
0. I l 3 X 10 3 3 X 10 4
Estimated in root mean square (rms).
1.2 X i015 (0.2-5.0) X 1015 1.0 X 1016
1.0-2.4 0.6-2.0 1.5-4.1
0.08 0.09 0.40
T. Yao et al. / Journal of Crystal Growth 163 (1996) 78-85
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Structure of island Fig. 3. The evolution of island structure on a Si(001) surface by annealing. The annealing sequence was 250°C (150 min) ~ 250°C (500 min) --->300°C (150 min).
develops the islands both in length and width at the expense of short dimer strings. This is because the width becomes larger as the temperature increases, where a cluster smaller than the critical size becomes unstable. Consequently, smaller clusters dissolve to produce Si adatoms which migrate to be incorporated into larger stable clusters. However, only a small number of dimer strings are observed by further annealing up to 400 ° . Instead, we observed the development of islands and steps, which indicate that the dimer strings are dissolved into Si adatoms which are incorporated into larger clusters and steps due to enhanced migration at high temperature. By further annealing up to 500°C, the islands on the terrace disappeared and a 2 × 1 reconstructed surface developed with many A-type defects [16] on the terrace. These A-type defects fluctuated along the dimer rows. No fluctuation perpendicular to the dimer row was observed. A similar fluctuation o f A-type defects were observed in the annealing process of the Ar-sputtered surface at 500°C [11]. When a 4 M L deposited Si surface was annealed at 800-900°C, a 2 × 1 reconstructed structure was observed on the surface. However, the A-type defects are aligned perpendicular to the dimer rows as shown in Fig. 4 [14]. In other words, missing dimer rows are formed on the surface 4.4 nm apart on average, which results in a 2 × 11 reconstructed surface. The formation of the line defects almost an
equal distance apart from each other suggests that there exists repulsive interaction between A-type defects on the same dimer row, while attractive interaction occurs between A-type defects on the adjacent dimer rows. Both interactions should originate from strain interaction between A-type defects on the dimer rows [17]. It should be noted that similar line defects are observed on an Ar+-ion sputtered surface during the annealing process at 600°C. In this particular case, the missing dimer defects are separated approximately 5 nm apart, which resulted in a 2 × 13 reconstructed surface [18]. It is natural to consider that the surface lattice is more relaxed to reduce the
Fig. 4. 4 ML deposited Si surface after annealing at 800°C. (K = - 1.7 V).
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T. Yao et al. / Journal of Crystal Growth 163 (1996) 78-85
Fig. 5. Ar+-ion bombarded Si(001) surface to a dose of 1.9 × 1014 cm 2 during annealing at 245°C. (Vs = -0.9 V).
repulsive strain energy at higher temperature. Hence, the missing dimer defects develop more at higher temperature which results in a smaller distance between the missing dimer defects.
5. Solid phase epitaxy of Ar-ion sputtered amorphous layers The surface of the Ar+-ion sputtered surfaces is smoothed by annealing. Fig. 5 shows an STM image of an Ar+-ion bombarded Si(001) surface to a dose of 1.9 × 1014 c m - 2 during annealing at 245°C [10]. The as-bombarded surface consisted of 0.63-1.6 nm diameter grains and the density of the grain was about 2 × 10 ~5 cm -2. By comparing Fig. ib and Fig. 5, it is concluded that the surface became smooth by annealing, although the granular surface morphology was still observed. The grain grew in size by annealing: the typical size of the grain was 2 - 3 . 6 nm in diameter at 245°C, for instance. This is presumably due to the coalescence of fine grains during annealing. We found that prolonged annealing even at this temperature promoted crystallization of the amorphous surface. Eventually, a 2 × 1 surface reconstruction was found on relatively smooth areas on the surface as shown in Fig. 5. Although the surface consists of reconstructed regions and grains, atomic steps were noted as indicated by arrows. In this figure, dimer rows running along a [011] direction are clearly observed in the areas indicated by a square. The dimer rows are observed in other areas. Hence, that part of the surface shows a 2 × 1 recon-
struction. We note that the dimer rows on the terrace (dashed lines) are running perpendicular to the underlying dimer rows (solid lines), which indicates the onset of the growth of a monatomic layer on the terrace. The Si atoms which formed the layer were supplied from the grains, since the surface roughness was eventually reduced with annealing temperature and time. Hence, the surface reconstruction changed from 2 × 1 to 1 × 2 or vice versa, as the annealing time was prolonged. Since the formation of a dimer on the surface indicates that the underlying layer is crystallized, these facts indicate that the crystallization process initiates at the crystal/grain interface and extends towards the surface. The crystallization process proceeds inhomogeneously due to the presence of grains until the crystal/amorphous interface reaches the surface. Once a part of the surface is crystallized, the crystallization proceeds in a layerby-layer mode, in which Si atoms are supplied from grains. Thus, the surface morphology is greatly improved through annealing. As the annealing temperature is raised, reconstructed regions develop and a mixture of 2 × 1 and 1 × 2 reconstruction is observed, where the amorphous layer epitaxially crystallizes up to the topmost surface. However, the surface was defective and contains many point defects as well as line defects. Fig. 6 shows an STM image of the surface at 500°C. Although the terrace and step structures do not develop at this temperature, both S A and S B steps are clearly observed. Although the amorphous layer epitaxially crystallizes up to the topmost surface at annealing temperatures below 500°C, the surface is defective and
Fig. 6. Ar+-ion bombarded Si(001) surface to a dose of 1.9× 10j4 cm 2 during annealing at 500°C. (Vs = - 1.2 V).
T. Yao et al. / Journal of Crystal Growth 163 (1996) 78-85
83
Fig. 7. Successive STM images of Ar*-ion bombarded surface during thermal annealing at 500°C taken every 9 s. (Vs = 1.6 V).
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71 Yao et al. / Journal of Crystal Growth 163 (1996) 78-85
contains many monatomic-height islands. When the annealing temperature increases to 500°C, smoothing of step edges is observed in situ on the surface. Fig. 7 shows successive images during annealing at 500°C taken every 9 s. The S A and Sr~ stems of an island (A) are observed, and the smoothening of a kink (B), "step-flow growth", is observed in real time. The surface contains many point defects as well as line defects (C and D) which fluctuate along the dimer rows by annealing. The observed line defects consist of A-type vacancies and are perpendicular to the dimer row. The defect (E) is annealed out with time. However, the missing dimers at the antiphase boundary (F) do not move. Fig. 8 shows schematics of the movement of the defect (C) in Fig. 7. The arrows indicate the movement of the defects in the corresponding image. It is observed that A-type defects frequently fluctuate along the dimer rows, each of which is connected with A-type defects on the adjacent dimer rows. In contrast, neither B-type defects nor the triple-dimer vacancy (D) do fluctuate at all.
6. Solid phase epitaxy of P +-ion implanted amorphous layers A higher annealing temperature was needed to crystallize the P+-ion implanted amorphous layers at 30 keV to a dose of 1 × 1016 c m - 2 . The amorphous Si layer epitaxially crystallizes up to the topmost surface by thermal annealing at 730-950°C. Fig. 9 shows typical STM images of the surface after annealing at 950°C. The surface is covered with single-
and double-layer height islands and the sizes of the terrace are smaller than others. Voids, which are labeled " V " are observed on the surface. The size of the void is 66-107 nm in diameter and 1.8-4.1 nm in depth. The voids originate from structural depression or impurity segregation. In either case, the initial surface damage by ion implantation cannot be recovered completely at this temperature. The terrace did not develop over the surface until being heated up to 1200°C, which is higher compared to that needed for the surfaces prepared by evaporation and sputtering.
7. Conclusions SPE processes of amorphous Si(001) surfaces prepared by vacuum evaporation, Ar+-ion sputtering, and P+-ion implantation have been investigated in situ using STM operated at elevated temperatures. The amorphized surfaces prepared by evaporation and sputtering consists of homogeneous fine grains, while the surface prepared by ion implantation is very rough and consists of large hillocks. The recovery of crystallinity by thermal annealing is strongly dependent on the initial structure of the amorphous layers. Anisotropic island growth is observed on the vacuum evaporated Si surface during annealing at 250-300°C. On the Ar+-ion bombarded surface, coalescence of hillocks occurs upon thermal annealing at 245°C. Prolonged annealing at this temperature promotes crystallization of the amorphous layers. The amorphous Si layer formed by P+-ion implanta-
Fig. 9. P+-ion implanted surface after annealing at 950°C. (a) Vs = - 2 V, 50 X 50 nm 2, (b) Vs = 2 V, 600 X 600 nm 2.
T. Yao et al. / Journal of Crystal Growth 163 (1996) 78-85
tion e p i t a x i a l l y c r y s t a l l i z e s u p to the t o p m o s t l a y e r by t h e r m a l a n n e a l i n g at 7 3 0 - 9 5 0 ° C . T h e surface is c o v e r e d w i t h single- a n d d o u b l e - l a y e r h e i g h t islands. H o w e v e r , v o i d s are o b s e r v e d o n the surface a n d the initial d a m a g e s e e m s not to b e c o m p l e t e l y r e c o v e r e d e v e n at this t e m p e r a t u r e .
References [1] Csepregi, E.F. Kennedy, J.W. Mayer and T.W. Sigmon, J. Appl. Phys. 49 (1978) 3906. [2] S.S. Lau, S. Matteson, J.W. Mayer, P. Revesz, J. Gyulai, J. Roth, T.W. Sigmon and T. Cass, Appl. Phys. Lett. 34 (1979) 76. [3] H. lshiwara, H. Yamamoto and S. Furukawa, Appl. Phys. Len. 43 (1983) 1028. [4] S. Saitoh, T. Sugii, H. Ishiwara and S. Furukawa, Jpn. J. Appl. Phys. 20 (1981) LI30. [5] L.S. Hung, S.S. Lau, M. von Allmen, J.W. Mayer, B.M. Ullrich, J.E. Baker and P. Williams, Appl. Phys. Lett. 37 (1980) 909.
85
[6] Y. Kunii, M. Tabe and K. Kajiyama, Jpn. J. Appl. Phys. 21 (1982) 1431. [7] L. Csepregi, J.W. Mayer and T.W. Sigmon, Phys. Lett. A 54 (1975) 157. [8] K. Uesugi, T. Yao, T. Sato, T. Sueyoshi and M. lwatsuki, Appl. Phys. Lett. 62 (1993) 1600. [9] K. Uesugi, M. Yoshimura, T. Sato, T. Sueyoshi, M. Iwatsuki and T. Yao, Jpn. J. Appl. Phys. 32 (1993) 6203. [10] T. Yao, K. Uesugi, M. Yoshimura, T. Sato, T. Sueyoshi and M. lwatsuki, Appl. Phys. Sci. 75 (1994) 139. [11] K. Uesugi, M. Yoshimura, T. Yao, T. Sato, T. Sueyoshi and M. Iwatsuki, J. Vac. Sci. Technol. B 12 (1994) 2018. [12] K. Uesugi, T. Komura, M. Yoshimura and T. Yao, Appl. Surf. Sci. 82/83 (1994) 367. [13] R.M. Feenstra and G.S. Oehrlein, Appl. Phys. Lett. 47 (1985) 97. [14] I.H. Wilson, N.J. Zheng, U. Knipping and 1.S.T. Tsong, J. Vac. Sci. Technol. A 7 (1989) 2840. [15] Z. Zhang and H. Metiu, Surf. Sci. 245 (1991) 353. [16] R.J. Hamers and U.K. Koher, J. Vac. Sci. Technol. A 7 (1989) 2854. [17] A. Natori, private communication. [18] K. Uesugi, M. Yoshimura and T. Yao, private communication.
ELSEVIER
CRYSTAL GROWTH
Journal of Crystal Growth 163 (1996)78-85
Atomistic study on solid phase epitaxy processes on Si(100) surfaces by the scanning tunneling microscope T. Yao a,b, *, K. Uesugi c, T. Komura d, M. Yoshimura e a Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980, Japan b Joint Research Centerfi)r Atom Technology, National Institute for Advanced Interdisciplinary Research, Tsukuba 305, Japan c Research Institute for Electronic Science, Hokkaido University, Sapporo 060, Japan d Department of Electrical Engineering, Hiroshima University, Higashi-Hiroshima 724, Japan e Toyoda Engineering Universi~, Tennshiro-ku, Nagoya 468, Japan
Abstract
We report an atomistic study on solid phase epitaxy processes of amorphous Si layers on Si(001) using a scanning tunneling microscope. The amorphous layers are prepared by vacuum evaporation, Ar+-ion sputtering, or P+-ion implantation. The surface morphology of the as-prepared surface is strongly damaged by energetic impinging particles. The solid phase epitaxy processes are strongly correlated with the surface damage: crystallization of amorphous layers by annealing at around 250°C is observed on both the vacuum deposited and Ar+-ion sputtered surfaces, while an annealing temperature higher than 730°C is needed to recover the crystallinity of the P+-ion implanted surface.
1. I n t r o d u c t i o n
Solid phase epitaxy (SPE) of amorphous Si (a-Si) has been intensively investigated [1-7]. It has been established that controlled epitaxial regrowth of amorphous layers fabricated by Si-ion implantation onto single-crystal Si substrates occurs in a low-temperature range around 600°C [1]. During thermal annealing, the reordering of a-Si is characterized by a uniform translation of the amorphous to crystalline interface towards the top surface. Amorphous Si layers have been formed by ion implantation [1,2], vacuum evaporation [3-5], and chemical vapor deposition (CVD) [6]. In the case of Si(100) and Si(110) substrates, a linear regrowth in
* Corresponding author.
time is observed for isothermal annealing, and the resultant layers are relatively defect free [7]. SPE growth processes have been investigated by transmission electron microscopy (TEM) [2,3,5,6], scanning electron microscopy (SEM) [1,3], Nomarski optical microscopy [3], transmission electron diffraction (TED) [3,6], and Rutherford backscattering (RBS) [1,2,4-6]. Very recently, we have performed a microscopic investigation of the regrowth processes of amorphous layers prepared by vacuum evaporation, Ar +ion sputtering, or P+-ion implantation on Si(001) surfaces using a scanning tunneling microscope (STM) [8-12]. The atomic scale regrowth processes during annealing have been elucidated. The purpose of this paper is to review our recent study on atomistic processes of solid phase epitaxy of Si amorphous layers on Si(001) surfaces.
0022-0248/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0022 -0248(95)01038-6
T. Yao et a l . / Journal of Crystal Growth 163 (1996) 78-85
79
2. Experimental procedure We have used two ultra-high vacuum STM systems consisting of an STM chamber, a preparation chamber equipped with a vacuum evaporation source or an Ar+-ion sputtering gun, and a load-lock chamber. Both STMs can be operated at elevated temperatures. The tip used was an electrochemically etched tungsten wire. The amorphous Si layers investigated in the present study were prepared as follows: (a) vacuum evaporation; (b) Ar+-ion sputtering; (c) P+-ion implantation. The respective sample preparations were as follows: (a) an amorphous Si layer of 0 . 0 3 - 4 monolayer (ML) thickness was deposited on the clean Si(001)-2 X 1 surface at room temperature; (b) the clean Si(001)-2 × 1 surface was amorphized by Ar+-ion sputtering to a dose of (0.2-5) X 1015 cm -2 in the preparation chamber; (c) a Si(001) surface was amorphized by P+-ion implantation at 30 keV to a dose of 1 X 1016 c m - 2 . Before being loaded into the load-lock chamber, the implanted specimen was etched in an buffered HF solution to remove the oxide layer.
3. Surface morphology of amorphous Si layers Fig. 1 shows typical STM images of the amorphous Si surfaces prepared by (a) vacuum evaporation (6.8 X 1014 Si/cm2), (b) Ar+-ion sputtering at 3 keV to a dose of 2 X 1014 cm -2, and (c) P+-ion implantation. The morphology of the vacuum evaporated surface and the Ar+-ion sputtered surface looks similar: the surfaces are covered with homogeneous circular particles. The size of the particles increased with the amount of deposition or ion dose. The average size of the Si cluster observed on the surface was 0.72 nm for 0.03 ML coverage with a dispersion of 0.153 nm. This fact suggests that most of the evaporated Si adatoms form single dimers at extremely low deposition (0.03 ML) even at room temperature. With increasing coverage, single dimers tend to coalesce and form larger clusters. For instance, the average size of the Si cluster for 1 ML coverage was 0.77 nm with a dispersion of 0.23 nm. Some of them are narrow in shape and form dimer strings, which suggests that deposited Si atoms at
Fig. 1. As-prepared amorphous Si surfaces by (ai vacuum evaporation at room temperature, (b) Ar+-ion bombardment, and (c) P+-ion implantation. The sample bias was - 2 V.
room temperature migrate on the surface to form epitaxial island. As-bombarded surfaces by Ar + ions showed essentially the same features regardless of the dose examined: the surface is amorphized down to the depth of 3 nm and consists of grains of 0.6-1.6 nm in diameter and 0.3-0.8 nm in height. It is reported that an Ar+-ion bombarded Si surface at 700 eV to a dose of 1 x 1018 i o n s / c m 2 was covered with typi-
80
T. Yao et a l . / Journal of Crystal Growth 163 (1996) 78-85
due to the higher ion impact energy as compared with other techniques used in the present experiment. In fact, the surface was amorphized by implantation to a depth of more than 0.1 /zm at 20 keV, while only 3 nm of the surface layer was amorphized in the case of Ar+-ion sputtering at 3 keV. Furthermore, there should be overlap of cascade at a dose of 1 x 1016 cm -2, which degrades the surface morphology and crystallinity [14]. Table 1 summarizes the characteristics of amorphous Si layers prepared by vacuum evaporation, Ar+-ion sputtering, and P+ion implantation [ 12]. Fig. 2. Vacuum deposited surface during annealing at 250°C. (Vs = - 2 V).
cally 5 nm diameter hillocks [13], which is about three times larger than the size of the hillocks observed in the present experiment. Since this difference should be attributed to a difference in Ar+-ion energy, it is likely that as the Ar+-ion energy increases, the size of the hillocks is reduced. Most of the grains on the surface show a bright contrast regardless of the sample bias polarity. Si particles or islands should show the same contrast at both polarities. However, we found that there were few grains which changed contrast as the polarity was changed: some of them were bright at a positive sample bias, while the others were bright at negative polarity. The density of these grains is estimated to be of the order of 5 X 1013 c m -2 for the Ar+-ion dose of 1.9 X 1014 c m - 2. The P+-ion implanted surfaces are heavily damaged, as shown in Fig. lc. The RMS value is 0.40 nm, much greater that those of vacuum evaporated (0.08 nm) and Ar+-ion sputtered (0.09 nm) surfaces. In addition, the size of hillocks on the surface is large and scattered (1.5-4.1 nm) and their shapes are distorted and no longer circular. This is obviously
4. Solid phase epitaxy process of vacuum deposited Si amorphous layers When the as-deposited Si amorphous layer is annealed, the surface migration of adatoms is enhanced to form dimer strings, which are perpendicular to the dimer row on the substrate surface. Fig. 2 shows an STM image of a 1 ML deposited Si surface during annealing at 250°C. Although most of the Si adatoms form dimer strings, there still remains Si clusters. As the annealing temperature or annealing time increases, these clusters dissolve and dimer strings develop. The formation and development of the dimer strings are due to anisotropic diffusion of Si adatoms on the 2 × 1 reconstructed surface [15]. Fig. 3 shows how the dimer string develops with annealing time and temperature. Here, (i,j) denotes a dimer string with i dimer unit (du) widths and j unit bond (au) lengths (1 du = 0.768 nm, 1 au = 0.384 nm). Short dimer strings (1, 1-3) are prominent on the surface during annealing at 250°C for 150 min, while both the length and width of the island increase with prolonged annealing (500 min). Additional high temperature annealing up to 300°C
Table 1 Characteristics of the surfaces of amorphous layers prepared by vacuum evaporation, Ar+-ion bombardment, and P+-ion implantation Energy (eV) Dose (cm -2 ) Size of hillocks (nm) Surface roughness a (nm) Vacuum evaporation Ar+-ion sputtering P+-ion implantation
0. I l 3 X 10 3 3 X 10 4
Estimated in root mean square (rms).
1.2 X i015 (0.2-5.0) X 1015 1.0 X 1016
1.0-2.4 0.6-2.0 1.5-4.1
0.08 0.09 0.40
T. Yao et al. / Journal of Crystal Growth 163 (1996) 78-85
81
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Structure of island Fig. 3. The evolution of island structure on a Si(001) surface by annealing. The annealing sequence was 250°C (150 min) ~ 250°C (500 min) --->300°C (150 min).
develops the islands both in length and width at the expense of short dimer strings. This is because the width becomes larger as the temperature increases, where a cluster smaller than the critical size becomes unstable. Consequently, smaller clusters dissolve to produce Si adatoms which migrate to be incorporated into larger stable clusters. However, only a small number of dimer strings are observed by further annealing up to 400 ° . Instead, we observed the development of islands and steps, which indicate that the dimer strings are dissolved into Si adatoms which are incorporated into larger clusters and steps due to enhanced migration at high temperature. By further annealing up to 500°C, the islands on the terrace disappeared and a 2 × 1 reconstructed surface developed with many A-type defects [16] on the terrace. These A-type defects fluctuated along the dimer rows. No fluctuation perpendicular to the dimer row was observed. A similar fluctuation o f A-type defects were observed in the annealing process of the Ar-sputtered surface at 500°C [11]. When a 4 M L deposited Si surface was annealed at 800-900°C, a 2 × 1 reconstructed structure was observed on the surface. However, the A-type defects are aligned perpendicular to the dimer rows as shown in Fig. 4 [14]. In other words, missing dimer rows are formed on the surface 4.4 nm apart on average, which results in a 2 × 11 reconstructed surface. The formation of the line defects almost an
equal distance apart from each other suggests that there exists repulsive interaction between A-type defects on the same dimer row, while attractive interaction occurs between A-type defects on the adjacent dimer rows. Both interactions should originate from strain interaction between A-type defects on the dimer rows [17]. It should be noted that similar line defects are observed on an Ar+-ion sputtered surface during the annealing process at 600°C. In this particular case, the missing dimer defects are separated approximately 5 nm apart, which resulted in a 2 × 13 reconstructed surface [18]. It is natural to consider that the surface lattice is more relaxed to reduce the
Fig. 4. 4 ML deposited Si surface after annealing at 800°C. (K = - 1.7 V).
82
T. Yao et al. / Journal of Crystal Growth 163 (1996) 78-85
Fig. 5. Ar+-ion bombarded Si(001) surface to a dose of 1.9 × 1014 cm 2 during annealing at 245°C. (Vs = -0.9 V).
repulsive strain energy at higher temperature. Hence, the missing dimer defects develop more at higher temperature which results in a smaller distance between the missing dimer defects.
5. Solid phase epitaxy of Ar-ion sputtered amorphous layers The surface of the Ar+-ion sputtered surfaces is smoothed by annealing. Fig. 5 shows an STM image of an Ar+-ion bombarded Si(001) surface to a dose of 1.9 × 1014 c m - 2 during annealing at 245°C [10]. The as-bombarded surface consisted of 0.63-1.6 nm diameter grains and the density of the grain was about 2 × 10 ~5 cm -2. By comparing Fig. ib and Fig. 5, it is concluded that the surface became smooth by annealing, although the granular surface morphology was still observed. The grain grew in size by annealing: the typical size of the grain was 2 - 3 . 6 nm in diameter at 245°C, for instance. This is presumably due to the coalescence of fine grains during annealing. We found that prolonged annealing even at this temperature promoted crystallization of the amorphous surface. Eventually, a 2 × 1 surface reconstruction was found on relatively smooth areas on the surface as shown in Fig. 5. Although the surface consists of reconstructed regions and grains, atomic steps were noted as indicated by arrows. In this figure, dimer rows running along a [011] direction are clearly observed in the areas indicated by a square. The dimer rows are observed in other areas. Hence, that part of the surface shows a 2 × 1 recon-
struction. We note that the dimer rows on the terrace (dashed lines) are running perpendicular to the underlying dimer rows (solid lines), which indicates the onset of the growth of a monatomic layer on the terrace. The Si atoms which formed the layer were supplied from the grains, since the surface roughness was eventually reduced with annealing temperature and time. Hence, the surface reconstruction changed from 2 × 1 to 1 × 2 or vice versa, as the annealing time was prolonged. Since the formation of a dimer on the surface indicates that the underlying layer is crystallized, these facts indicate that the crystallization process initiates at the crystal/grain interface and extends towards the surface. The crystallization process proceeds inhomogeneously due to the presence of grains until the crystal/amorphous interface reaches the surface. Once a part of the surface is crystallized, the crystallization proceeds in a layerby-layer mode, in which Si atoms are supplied from grains. Thus, the surface morphology is greatly improved through annealing. As the annealing temperature is raised, reconstructed regions develop and a mixture of 2 × 1 and 1 × 2 reconstruction is observed, where the amorphous layer epitaxially crystallizes up to the topmost surface. However, the surface was defective and contains many point defects as well as line defects. Fig. 6 shows an STM image of the surface at 500°C. Although the terrace and step structures do not develop at this temperature, both S A and S B steps are clearly observed. Although the amorphous layer epitaxially crystallizes up to the topmost surface at annealing temperatures below 500°C, the surface is defective and
Fig. 6. Ar+-ion bombarded Si(001) surface to a dose of 1.9× 10j4 cm 2 during annealing at 500°C. (Vs = - 1.2 V).
T. Yao et al. / Journal of Crystal Growth 163 (1996) 78-85
83
Fig. 7. Successive STM images of Ar*-ion bombarded surface during thermal annealing at 500°C taken every 9 s. (Vs = 1.6 V).
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84
71 Yao et al. / Journal of Crystal Growth 163 (1996) 78-85
contains many monatomic-height islands. When the annealing temperature increases to 500°C, smoothing of step edges is observed in situ on the surface. Fig. 7 shows successive images during annealing at 500°C taken every 9 s. The S A and Sr~ stems of an island (A) are observed, and the smoothening of a kink (B), "step-flow growth", is observed in real time. The surface contains many point defects as well as line defects (C and D) which fluctuate along the dimer rows by annealing. The observed line defects consist of A-type vacancies and are perpendicular to the dimer row. The defect (E) is annealed out with time. However, the missing dimers at the antiphase boundary (F) do not move. Fig. 8 shows schematics of the movement of the defect (C) in Fig. 7. The arrows indicate the movement of the defects in the corresponding image. It is observed that A-type defects frequently fluctuate along the dimer rows, each of which is connected with A-type defects on the adjacent dimer rows. In contrast, neither B-type defects nor the triple-dimer vacancy (D) do fluctuate at all.
6. Solid phase epitaxy of P +-ion implanted amorphous layers A higher annealing temperature was needed to crystallize the P+-ion implanted amorphous layers at 30 keV to a dose of 1 × 1016 c m - 2 . The amorphous Si layer epitaxially crystallizes up to the topmost surface by thermal annealing at 730-950°C. Fig. 9 shows typical STM images of the surface after annealing at 950°C. The surface is covered with single-
and double-layer height islands and the sizes of the terrace are smaller than others. Voids, which are labeled " V " are observed on the surface. The size of the void is 66-107 nm in diameter and 1.8-4.1 nm in depth. The voids originate from structural depression or impurity segregation. In either case, the initial surface damage by ion implantation cannot be recovered completely at this temperature. The terrace did not develop over the surface until being heated up to 1200°C, which is higher compared to that needed for the surfaces prepared by evaporation and sputtering.
7. Conclusions SPE processes of amorphous Si(001) surfaces prepared by vacuum evaporation, Ar+-ion sputtering, and P+-ion implantation have been investigated in situ using STM operated at elevated temperatures. The amorphized surfaces prepared by evaporation and sputtering consists of homogeneous fine grains, while the surface prepared by ion implantation is very rough and consists of large hillocks. The recovery of crystallinity by thermal annealing is strongly dependent on the initial structure of the amorphous layers. Anisotropic island growth is observed on the vacuum evaporated Si surface during annealing at 250-300°C. On the Ar+-ion bombarded surface, coalescence of hillocks occurs upon thermal annealing at 245°C. Prolonged annealing at this temperature promotes crystallization of the amorphous layers. The amorphous Si layer formed by P+-ion implanta-
Fig. 9. P+-ion implanted surface after annealing at 950°C. (a) Vs = - 2 V, 50 X 50 nm 2, (b) Vs = 2 V, 600 X 600 nm 2.
T. Yao et al. / Journal of Crystal Growth 163 (1996) 78-85
tion e p i t a x i a l l y c r y s t a l l i z e s u p to the t o p m o s t l a y e r by t h e r m a l a n n e a l i n g at 7 3 0 - 9 5 0 ° C . T h e surface is c o v e r e d w i t h single- a n d d o u b l e - l a y e r h e i g h t islands. H o w e v e r , v o i d s are o b s e r v e d o n the surface a n d the initial d a m a g e s e e m s not to b e c o m p l e t e l y r e c o v e r e d e v e n at this t e m p e r a t u r e .
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[6] Y. Kunii, M. Tabe and K. Kajiyama, Jpn. J. Appl. Phys. 21 (1982) 1431. [7] L. Csepregi, J.W. Mayer and T.W. Sigmon, Phys. Lett. A 54 (1975) 157. [8] K. Uesugi, T. Yao, T. Sato, T. Sueyoshi and M. lwatsuki, Appl. Phys. Lett. 62 (1993) 1600. [9] K. Uesugi, M. Yoshimura, T. Sato, T. Sueyoshi, M. Iwatsuki and T. Yao, Jpn. J. Appl. Phys. 32 (1993) 6203. [10] T. Yao, K. Uesugi, M. Yoshimura, T. Sato, T. Sueyoshi and M. lwatsuki, Appl. Phys. Sci. 75 (1994) 139. [11] K. Uesugi, M. Yoshimura, T. Yao, T. Sato, T. Sueyoshi and M. Iwatsuki, J. Vac. Sci. Technol. B 12 (1994) 2018. [12] K. Uesugi, T. Komura, M. Yoshimura and T. Yao, Appl. Surf. Sci. 82/83 (1994) 367. [13] R.M. Feenstra and G.S. Oehrlein, Appl. Phys. Lett. 47 (1985) 97. [14] I.H. Wilson, N.J. Zheng, U. Knipping and 1.S.T. Tsong, J. Vac. Sci. Technol. A 7 (1989) 2840. [15] Z. Zhang and H. Metiu, Surf. Sci. 245 (1991) 353. [16] R.J. Hamers and U.K. Koher, J. Vac. Sci. Technol. A 7 (1989) 2854. [17] A. Natori, private communication. [18] K. Uesugi, M. Yoshimura and T. Yao, private communication.