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Anisotropic surface migration during MBE on stepped surfaces of vicinal Si(111)
510
Journal of Crystal Growth 99 (1990) 510—513 North-Holland
ANISOTROPIC SURFACE MIGRATION DURING MBE ON STEPPED SURFACES OF VICINAL Si(11I) Kunihiro SAKAMOTO, Kazushi MIKI and Tsunenori SAKAMOTO Electrotechrncal Laboratoiy, Umezono, Tsukuba 305, Japan
Surface migration of Si atoms during MBE on stepped surfaces of vicinal Si(111) was studied by reflection high-energy electron diffraction (RHEED). A vicinal Si(111) surface consists of (111) terraces separated by steps running along the (110) directions. There are two kinds of (110> steps on the Si(111) surface. Anisotropy in the surface migration between the directions normal to the two kinds of step edges was found.
1. Introduction It is very important for molecular beam epitaxy (MBE) of Si to understand the behavior of Si adatoms on the Si surface and their interaction with surface steps. Si(111) surfaces have attracted much attention for their interesting surface properties. At the early stage of Si MBE, Abbink et al., observed steps of Si(111) using a skillful replication technique for electron microscopy [1]. Later surface diffusion parameters have been estimated using various techniques [2—4]but they were rather indirect. Recently Ichikawa and Doi directly observed a step flow on reflection electron microscope (REM) images [5].A new direct method was demonstrated by Neave et a!. which used intensity oscillations in reflection high-energy electron diffraction (RHEED) pattern during MBE of GaAs [6]. RHEED intensity oscillation was also observed during MBE of Si(111) [7]. We applied the RHEED intensity technique to the surface migration measurement of Si(111).
2. Experimental Growth and measurement were performed in an ion-pumped MBE system (base pressure 2 X 10_8 Pa). A Si molecular beam was evaporated from a high-purity single-crystalline source (> 1000 ~Qcm) by a 4 kW electron gun. The 3 mm 0022-0248/90/$03.50 © Elsevier Science Publishers B.V. (North-Holland)
thick Si(111) 2 inch wafer was polished to a lensshape and had a radius of curvature of 200 mm. This lens-shaped substrate allows growth on all surface orientations within 6.2°of the (111) plane. The substrate was subjected to a standard cleaning process [8] and deposition of a Si buffer layer at 700 °Cand a subsequent 1000 °Canneal were repeated in order to eliminate the effect of initial roughness or carbon contamination [9]. The substrate was heated from the backside by radiation of a Ta heater. A 30 keY RHEED system was used for surface analyses. The incidence angle was determined by a space between the specular beam spot and the direct spot and was typically 15 mrad. The electron beam was diffracted from a 0.1 X 5 mm2 area of the lens-shaped surface. The misorientation of the surface where the electron beam was diffracted was derived with an error of less than ±0.3° from a horizontal deflection angle of the incident beam from the center of the lens-shaped wafer. The surface migration measurement is based on the technique first demonstrated by Neave et al. [6]. The surface misoriented from a low index plane in a specific direction breaks up into steps and low index terraces. If the surface diffusion length x is larger than the average terrace width L, most of diffusing adatoms will be caught by the step edge and there will be no two-dimensional (2D) nucleation on the terrace. RHEED intensity oscillations, which occur as the result of 2D
K Sakamoto eta!.
(1 12> step
/ Anisotropicsurface migration during MBE on stepped surfaces of Si(J1J)
3. Results and discussion
<112> step
- -
A series of growth sequences was initiated after the 1000 C anneal and the temporal_evolution of
-
—.4112)
a
[Tb]
the beam in thewas [110] azimuth was specular recorded.RHEED The [110] azimuth chosen be-
[110]
—
—
[211]
1121] /
-
[101]
-
~[0l
~
1]
~ /
--
[112].—
~
/~
1
(111)
)
-
—.41121
/
\~. ~
[0 ~1)
-
~‘~[ 101]
~
j
- -
[211]
-~
cause both edges of (112) steps and (112) steps are aligned along the (110) directions. Fig. 2 shows the RHEED intensity evolutions as a funclion of surface misorientation from the exact (111) plane maintaining substrate temperature T and growth rate constant. The_left-hand side is that from surfaces with the (112) steps (tilted toward [112]) and the right-hand side is that with the (112) steps (tilted towards [1121).A transition of the temporal responses from oscillations to a steady signal occurs with increasing tilt angle, which resulted from the decrease in terrace width L to that comparable to the diffusion length x. The tilt angle dependence of the RHEED inten-
[1 To}
sity evolutions is different in the two stepped surface with <112) steps and with <112) steps.
step step
From fig. 2, norma!totothethe<112) <112)step stepedge edgeisis less 6.2 nm while x xnormal than 2.9 nm, assuming that both steps have a monolayer height (0.314 nm). The diffusion length can be expressed by the diffusion coefficient D
-
b
511
<112) <112)
Fig. 1. Schematic illustration of lens-shaped Si(111) substrate: (a) side view showing a (112> step and a <112) step, (b) top I
.
based on the Einstein relation: nucleation on Si(111) [7], will not take place. If x < L, on the contrary, 2D nucleation will occur on the terraces and the oscillations will be observed. We can observe RHEED intensity oscillations with variations in L on the lens-shaped substrate and can estimate x. Because a Si(111) surface has a threefold rotation symmetry, there are two kinds of steps running along the <110) directions. We call them “<112) step” and “<112) step” as shown in fig. la. The former has a (111) step riser and the latter has a (100) step riser. A surface with only <112) steps or that with only <112) steps will appear if a substrate is tilted toward [112] or toward [112], respectively. On the lens-shaped substrate, because of the_threefold rotation symmetry, surfaces with <112) steps and those with <112) steps appear as shown in fig. lb. Thus we can measure the surface migration properties normal to the two kinds of step edges.
2
—
x 2Dt, where t is the time necessary for a monolayer growth. Fig. 3 shows the temperature dependence of D for the three directions. It should be noted that these values were derived on the assumption of the monolayer step height. Two questions come from the results. One is too small a value of diffusion length x and the other is an anisotropy in x. Ichikawa and Doi estimated x to be 50 nm for the growth condition of 350°C and 0.3 ML/mm [5]. Extrapolating their value to the growth condition of fig. 2 yields x to be 200 nm, which is much larger than our result. We suspect a step coalescence. Our results are based on the assumption of monolayer steps, so if multilayer steps exists, the real diffusion length is much longer than the estimated values. If the surface breaks up into ordered stepped-terraces, RHEED spots are split —
512
K Sakamoto et al.
/ Anisotropic surface migration during MBE on stepped surfaces of Si(l 11)
Sphe~cal Si (1111; a [110] mzimuth, 480T, 7ML/min (112> step
=
.d
J1W~NWM Time
2 (mm)
Sphe~caL Si (111); b [1101 azimuth, 480C, 6ML/min (11?) step
0
3 Time
2 (mm)
3
Fig. 2. The transition of RHEED intensity evolutions from oscillations to constant response as a function of niisorientation from exact (ill) with constant temperature and growth rate: (a) surfaces with <112) steps; (b) surface with <112) steps.
due to the interference between waves scattered from periodic terraces. The splitting of RHEED spots from the lens-shaped substrate were not observed, which indicated that steps coalesced to various height and the order of stepped-terraces disappeared. The observed anisotropic surface migration is closely related to the properties of the steps, be600 17
‘
500°C
V ‘ 2e’~ ‘S
“ 10-18
.
C Diffusion aLong -19
10
‘~
~
.
o rToii 1 ‘1
112
103/T (K1)
113
Fig. 3. Arrhemus plot of log D versus 1/T for diffusion along [11~], [112]and [101], assuming monolayer step height.
cause there is no crystallographic discrepancy between the diffusion on the precise Si(111) surface along the directions [112] and [112]. One explanation is that the growth speed of the <112) step is faster than that of the <112) step. To realize the difference in advance speeds, the BCF theory requests a different density of adatoms at the edges of the two types of steps, i.e. the <112) step acts as a more effective sink of adatoms than the <112) step does. IT is in a sense understandable because <112) steps have two dangling bonds per atom while <112) steps have one. Another simple speculation will lead toa contradictory model of the step shape. The <112) step, which strongly adsorbs adatoms, would be expected to have many kinks, while the <112) step would be straight. The early work using a replication technique [1] and a recent STM observation [10], however, showed the opposite behavior. We can not yet conclude whether the step advance speed is different or not. The other explanation for the migration anisotropy is that the height of <112) steps is larger than that of <112) steps and the average terrace widths are not equal for the_two surfaces with identical misorientation. A <112) step has a (111} riser and a <112) step has a (100) riser. Because the surface energy of (111) plane is smaller than
K. Sakamoto et al.
/ Anisotropic surface migration during MBE on stepped surfaces of Si(ll 1)
(100), it is likely that <112) steps are higher than <112) steps. Therefore, the difference in the terrace width is one of the possible explanations for the observed anisotropy. The existence of multilayer steps is also suggested by the too small diffusion parameters as discussed above. We can estimate the average step height difference of <112) and <112) steps on the assumption that the migration amsotropy is caused simply by the difference of terrace width, i.e. x and D are same for diffusion normal to (112) and (112) steps on identical growth conditions. Extrapolating the point of D normal to <112) steps at 480°Cin fig. 3 to 590 °C using an activation energy of 2 eV, D normal to (112) steps is 25 times larger than D normal to <112) steps at 590°C.Therefore, the step is estimated to be 5 times higher than the (112) step. The early work using a replication technique mdicated a strong tendency for (112) steps to coalesce into larger steps and for (112) steps to break into regular arrays [1]. By using scanning electron microscopy (SEM) Ishikawa et a! [11] observed steps with a height of tens of monolayers but whether they were (112) steps or (ii~) steps is not clear. Some REM works [12,13] observed only monolayer steps and other works [5,14] observed step bunching. The relation between these results and the misorientation direction has not been clear yet. We think the different properties of (112) steps and (112) steps are strongly related to these contradictory observations.
RHEED. An anisotropic property of disappearance of RHEED intensity oscillation was found. The anisotropic surface migration is closely related to the properties of the <112) steps and the <112) steps. The diffusion parameters derived from the RHEED intensity oscillations strongly suggest that steps coalescence to multilayer steps and the height of (112) steps are lower than (112) steps.
References [1] H.C. Abbink, R.M. Broudy and G.P. McCarthy, J. Appi. Phys. 39 (1968) 4673. [2] S.M. Bedair, Surface Sci. 42 (1974) 595. [3] E. Kasper, Appi. Phys. A28 (1982) 129. [4] S.S. Lyer, T.F. Heinz and M.M.T. Loy, J. Vacuum Sci. Technol. B5 (1987) 709. [5] M. Ichikawa and T. Doi, AppI. Phys. Letters 50 (1987) 1141. [6] J.H. Neave, P.J. Dobson and B.A. Joyce, Appi. Phys. Letters 47 (1985) 100. [7] T. Sakamoto, N.J. Kawai, T. Nakagawa, K. Ohta and T. Kojima, Appl. Phys. Letters 47 (1951) 617. [8] A. Ishizaka and Y. Shiraki, J. Electrochem. Soc. 133 (1986) 666. [9] T. Sakamoto, T. Kawamura, S. Nagao, G. Hashiguchi, K.
[10] [11] [12]
4. Conclusion [13]
Surface migration of Si atoms during MBE on stepped-surfaces of vicinal Si(111) was studied by
513
[14]
Sakamoto and K. Kuniyoshi, J. Crystal Growth 81 (1987) 59. H. Tokumoto, K. Miki and K. Kajimura, J. Crystal Growth 99 (1990) 1329. Y. Ishikawa, N. Ikeda, M. Kenmochi and T. Ichinokawa, Surface Sci. 159 (1985) 256. N. Osakabe, Y. Tanishiro, K. Yagi and G. Honjo, Surface Sci. 97 (1980) 393. N. Osakabe, Y. Tanishiro, K. Yagi and G. Honjo, Surface Sci. 109 (1981) 353. A.V. Latyshev, A.L. Assev, A.B. Krasilmkov and SI. Stemn, Surface Sci. 213 (1989) 157.
Journal of Crystal Growth 99 (1990) 510—513 North-Holland
ANISOTROPIC SURFACE MIGRATION DURING MBE ON STEPPED SURFACES OF VICINAL Si(11I) Kunihiro SAKAMOTO, Kazushi MIKI and Tsunenori SAKAMOTO Electrotechrncal Laboratoiy, Umezono, Tsukuba 305, Japan
Surface migration of Si atoms during MBE on stepped surfaces of vicinal Si(111) was studied by reflection high-energy electron diffraction (RHEED). A vicinal Si(111) surface consists of (111) terraces separated by steps running along the (110) directions. There are two kinds of (110> steps on the Si(111) surface. Anisotropy in the surface migration between the directions normal to the two kinds of step edges was found.
1. Introduction It is very important for molecular beam epitaxy (MBE) of Si to understand the behavior of Si adatoms on the Si surface and their interaction with surface steps. Si(111) surfaces have attracted much attention for their interesting surface properties. At the early stage of Si MBE, Abbink et al., observed steps of Si(111) using a skillful replication technique for electron microscopy [1]. Later surface diffusion parameters have been estimated using various techniques [2—4]but they were rather indirect. Recently Ichikawa and Doi directly observed a step flow on reflection electron microscope (REM) images [5].A new direct method was demonstrated by Neave et a!. which used intensity oscillations in reflection high-energy electron diffraction (RHEED) pattern during MBE of GaAs [6]. RHEED intensity oscillation was also observed during MBE of Si(111) [7]. We applied the RHEED intensity technique to the surface migration measurement of Si(111).
2. Experimental Growth and measurement were performed in an ion-pumped MBE system (base pressure 2 X 10_8 Pa). A Si molecular beam was evaporated from a high-purity single-crystalline source (> 1000 ~Qcm) by a 4 kW electron gun. The 3 mm 0022-0248/90/$03.50 © Elsevier Science Publishers B.V. (North-Holland)
thick Si(111) 2 inch wafer was polished to a lensshape and had a radius of curvature of 200 mm. This lens-shaped substrate allows growth on all surface orientations within 6.2°of the (111) plane. The substrate was subjected to a standard cleaning process [8] and deposition of a Si buffer layer at 700 °Cand a subsequent 1000 °Canneal were repeated in order to eliminate the effect of initial roughness or carbon contamination [9]. The substrate was heated from the backside by radiation of a Ta heater. A 30 keY RHEED system was used for surface analyses. The incidence angle was determined by a space between the specular beam spot and the direct spot and was typically 15 mrad. The electron beam was diffracted from a 0.1 X 5 mm2 area of the lens-shaped surface. The misorientation of the surface where the electron beam was diffracted was derived with an error of less than ±0.3° from a horizontal deflection angle of the incident beam from the center of the lens-shaped wafer. The surface migration measurement is based on the technique first demonstrated by Neave et al. [6]. The surface misoriented from a low index plane in a specific direction breaks up into steps and low index terraces. If the surface diffusion length x is larger than the average terrace width L, most of diffusing adatoms will be caught by the step edge and there will be no two-dimensional (2D) nucleation on the terrace. RHEED intensity oscillations, which occur as the result of 2D
K Sakamoto eta!.
(1 12> step
/ Anisotropicsurface migration during MBE on stepped surfaces of Si(J1J)
3. Results and discussion
<112> step
- -
A series of growth sequences was initiated after the 1000 C anneal and the temporal_evolution of
-
—.4112)
a
[Tb]
the beam in thewas [110] azimuth was specular recorded.RHEED The [110] azimuth chosen be-
[110]
—
—
[211]
1121] /
-
[101]
-
~[0l
~
1]
~ /
--
[112].—
~
/~
1
(111)
)
-
—.41121
/
\~. ~
[0 ~1)
-
~‘~[ 101]
~
j
- -
[211]
-~
cause both edges of (112) steps and (112) steps are aligned along the (110) directions. Fig. 2 shows the RHEED intensity evolutions as a funclion of surface misorientation from the exact (111) plane maintaining substrate temperature T and growth rate constant. The_left-hand side is that from surfaces with the (112) steps (tilted toward [112]) and the right-hand side is that with the (112) steps (tilted towards [1121).A transition of the temporal responses from oscillations to a steady signal occurs with increasing tilt angle, which resulted from the decrease in terrace width L to that comparable to the diffusion length x. The tilt angle dependence of the RHEED inten-
[1 To}
sity evolutions is different in the two stepped surface with <112) steps and with <112) steps.
step step
From fig. 2, norma!totothethe<112) <112)step stepedge edgeisis less 6.2 nm while x xnormal than 2.9 nm, assuming that both steps have a monolayer height (0.314 nm). The diffusion length can be expressed by the diffusion coefficient D
-
b
511
<112) <112)
Fig. 1. Schematic illustration of lens-shaped Si(111) substrate: (a) side view showing a (112> step and a <112) step, (b) top I
.
based on the Einstein relation: nucleation on Si(111) [7], will not take place. If x < L, on the contrary, 2D nucleation will occur on the terraces and the oscillations will be observed. We can observe RHEED intensity oscillations with variations in L on the lens-shaped substrate and can estimate x. Because a Si(111) surface has a threefold rotation symmetry, there are two kinds of steps running along the <110) directions. We call them “<112) step” and “<112) step” as shown in fig. la. The former has a (111) step riser and the latter has a (100) step riser. A surface with only <112) steps or that with only <112) steps will appear if a substrate is tilted toward [112] or toward [112], respectively. On the lens-shaped substrate, because of the_threefold rotation symmetry, surfaces with <112) steps and those with <112) steps appear as shown in fig. lb. Thus we can measure the surface migration properties normal to the two kinds of step edges.
2
—
x 2Dt, where t is the time necessary for a monolayer growth. Fig. 3 shows the temperature dependence of D for the three directions. It should be noted that these values were derived on the assumption of the monolayer step height. Two questions come from the results. One is too small a value of diffusion length x and the other is an anisotropy in x. Ichikawa and Doi estimated x to be 50 nm for the growth condition of 350°C and 0.3 ML/mm [5]. Extrapolating their value to the growth condition of fig. 2 yields x to be 200 nm, which is much larger than our result. We suspect a step coalescence. Our results are based on the assumption of monolayer steps, so if multilayer steps exists, the real diffusion length is much longer than the estimated values. If the surface breaks up into ordered stepped-terraces, RHEED spots are split —
512
K Sakamoto et al.
/ Anisotropic surface migration during MBE on stepped surfaces of Si(l 11)
Sphe~cal Si (1111; a [110] mzimuth, 480T, 7ML/min (112> step
=
.d
J1W~NWM Time
2 (mm)
Sphe~caL Si (111); b [1101 azimuth, 480C, 6ML/min (11?) step
0
3 Time
2 (mm)
3
Fig. 2. The transition of RHEED intensity evolutions from oscillations to constant response as a function of niisorientation from exact (ill) with constant temperature and growth rate: (a) surfaces with <112) steps; (b) surface with <112) steps.
due to the interference between waves scattered from periodic terraces. The splitting of RHEED spots from the lens-shaped substrate were not observed, which indicated that steps coalesced to various height and the order of stepped-terraces disappeared. The observed anisotropic surface migration is closely related to the properties of the steps, be600 17
‘
500°C
V ‘ 2e’~ ‘S
“ 10-18
.
C Diffusion aLong -19
10
‘~
~
.
o rToii 1 ‘1
112
103/T (K1)
113
Fig. 3. Arrhemus plot of log D versus 1/T for diffusion along [11~], [112]and [101], assuming monolayer step height.
cause there is no crystallographic discrepancy between the diffusion on the precise Si(111) surface along the directions [112] and [112]. One explanation is that the growth speed of the <112) step is faster than that of the <112) step. To realize the difference in advance speeds, the BCF theory requests a different density of adatoms at the edges of the two types of steps, i.e. the <112) step acts as a more effective sink of adatoms than the <112) step does. IT is in a sense understandable because <112) steps have two dangling bonds per atom while <112) steps have one. Another simple speculation will lead toa contradictory model of the step shape. The <112) step, which strongly adsorbs adatoms, would be expected to have many kinks, while the <112) step would be straight. The early work using a replication technique [1] and a recent STM observation [10], however, showed the opposite behavior. We can not yet conclude whether the step advance speed is different or not. The other explanation for the migration anisotropy is that the height of <112) steps is larger than that of <112) steps and the average terrace widths are not equal for the_two surfaces with identical misorientation. A <112) step has a (111} riser and a <112) step has a (100) riser. Because the surface energy of (111) plane is smaller than
K. Sakamoto et al.
/ Anisotropic surface migration during MBE on stepped surfaces of Si(ll 1)
(100), it is likely that <112) steps are higher than <112) steps. Therefore, the difference in the terrace width is one of the possible explanations for the observed anisotropy. The existence of multilayer steps is also suggested by the too small diffusion parameters as discussed above. We can estimate the average step height difference of <112) and <112) steps on the assumption that the migration amsotropy is caused simply by the difference of terrace width, i.e. x and D are same for diffusion normal to (112) and (112) steps on identical growth conditions. Extrapolating the point of D normal to <112) steps at 480°Cin fig. 3 to 590 °C using an activation energy of 2 eV, D normal to (112) steps is 25 times larger than D normal to <112) steps at 590°C.Therefore, the
RHEED. An anisotropic property of disappearance of RHEED intensity oscillation was found. The anisotropic surface migration is closely related to the properties of the <112) steps and the <112) steps. The diffusion parameters derived from the RHEED intensity oscillations strongly suggest that steps coalescence to multilayer steps and the height of (112) steps are lower than (112) steps.
References [1] H.C. Abbink, R.M. Broudy and G.P. McCarthy, J. Appi. Phys. 39 (1968) 4673. [2] S.M. Bedair, Surface Sci. 42 (1974) 595. [3] E. Kasper, Appi. Phys. A28 (1982) 129. [4] S.S. Lyer, T.F. Heinz and M.M.T. Loy, J. Vacuum Sci. Technol. B5 (1987) 709. [5] M. Ichikawa and T. Doi, AppI. Phys. Letters 50 (1987) 1141. [6] J.H. Neave, P.J. Dobson and B.A. Joyce, Appi. Phys. Letters 47 (1985) 100. [7] T. Sakamoto, N.J. Kawai, T. Nakagawa, K. Ohta and T. Kojima, Appl. Phys. Letters 47 (1951) 617. [8] A. Ishizaka and Y. Shiraki, J. Electrochem. Soc. 133 (1986) 666. [9] T. Sakamoto, T. Kawamura, S. Nagao, G. Hashiguchi, K.
[10] [11] [12]
4. Conclusion [13]
Surface migration of Si atoms during MBE on stepped-surfaces of vicinal Si(111) was studied by
513
[14]
Sakamoto and K. Kuniyoshi, J. Crystal Growth 81 (1987) 59. H. Tokumoto, K. Miki and K. Kajimura, J. Crystal Growth 99 (1990) 1329. Y. Ishikawa, N. Ikeda, M. Kenmochi and T. Ichinokawa, Surface Sci. 159 (1985) 256. N. Osakabe, Y. Tanishiro, K. Yagi and G. Honjo, Surface Sci. 97 (1980) 393. N. Osakabe, Y. Tanishiro, K. Yagi and G. Honjo, Surface Sci. 109 (1981) 353. A.V. Latyshev, A.L. Assev, A.B. Krasilmkov and SI. Stemn, Surface Sci. 213 (1989) 157.