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Influence of substrate orientation on surface segregation process in silicon-MBE
Thin Solid Films, 183 (1989) 315-322
315
INFLUENCE OF SUBSTRATE ORIENTATION ON SURFACE SEGREGATION PROCESS IN SILICON-MBE KIYOKAZU NAKAGAWA AND MASANOBU MIYAO
Central Research Laboratory, Hitachi Ltd., Kokubunji, Tokyo 185 (Japan) YASUHIRO SHIRAKI
Research Center for Advanced Science and Technology, Tokyo University, Meguro, Tokyo 153 (Japan) (Received May 30, 1989)
The substrate orientation dependence of the surface segregation of antimony and gallium atoms during silicon molecular beam epitaxy (MBE) was studied using the delta doping technique. During growth the dopant atoms are only partly incorporated in the growing layer and the residuals segregate on the MBE-grown surfaces. The incorporation coefficients of antimony and gallium have a temperature dependence of the thermal activation type. The activation energies for antimony and gallium are 0.75 eV and 0.45 eV respectively. The value of the incorporation coefficient of antimony is much larger than that of gallium on both Si(100) and Si(111) substrates. The incorporation coefficients on Si(111) are about ten times larger than Si(100) for both dopant atoms, although they have the same activation energy. These results are well explained by a model based on the surface migration.
1. INTRODUCTION Molecular beam epitaxy (MBE) has been recognized as a powerful tool to realize arbitrary doping profiles with abrupt transitions. This is because the growth temperature of MBE is sumciently low that the thermal diffusion of the dopant atoms can be avoided. Recent studies, however, have shown that in silicon MBE such typical dopant atoms as antimony 1'2 and gallium2"3 segregate on surfaces during epitaxial growth to form a dopant reservoir. This dopant reservoir inhibits abrupt changes in dopant profiles, even if dopant fluxes are suddenly changed. However, it has also been found that once incorporated in epitaxial layers the dopant atoms do not segregate to the epitaxial surface. That is to say, during MBE growth, the surface segregation of dopant atoms takes place only when dopant atoms exist on the growing surface, suggesting the phenomenon is due to surface effects, such as surface migration, reaction and so on. Since the density of silicon bonds and therefore the surface reactivities are different for different surface orientations, the segregation phenomenon along with MBE growth is thought to have crystal orientation dependence. This paper describes the experimental results on the surface segregation of 0040-6090/89/$3.50
© ElsevierSequoia/Printedin The Netherlands
316
K. NAKAGAWA, M. MIYAO, Y. SHIRAKI
dopant atoms on various silicon surfaces in order to clarify the mechanism of surface segregation. In these experiments, the delta doping technique, which has been demonstrated previously to be very powerful in investigating the behaviour of atoms during crystal growth, was employed. 2. EXPERIMENTAL
The silicon MBE apparatus was a Vacuum Generator's V80 system with a base pressure of about 2 × 10 11 torr. This system consists of an e-gun evaporator for silicon deposition and two effusion cells for gallium and antimony doping. Si(100) and Si(111) wafers were precleaned by chemical treatment and a protective thin oxide film was formed. Then the oxide layer was sublimated at 850 °C for 20 min. The temperature of substrates was lowered to 750°C and to obtain clean and smooth surfaces a buffer layer of about 500/~ was grown. Gallium or antimony atoms were deposited on the epitaxial silicon surfaces at temperatures where, owing to the binding energy difference between dopant-silicon and dopant-dopant atoms, only one monolayer of atoms would be automatically adsorbed. This was confirmed experimentally. Undoped silicon layers with various thickness were then overgrown on the 1 ML gallium or antimony deposited surface at various substrate temperatures. This procedure is known as delta doping. It provides atomic planar doping in epitaxial layers, if surface segregation does not occur. Using a thickness monitor, the silicon deposition rate was fixed at 1/~ s- 1. The surface concentration of dopant atoms segregated on overgrown silicon surfaces was measured using X-ray photoelectron spectroscopy (XPS). Since the escape depth of photoelectrons of Sb(3d) or Ga(2p) is small (less than about 10,~), XPS signals observed here mainly come from atoms segregated on the silicon epitaxial surface and the contribution from dopant atoms incorporated in the silicon is negligible. The crystallinity of the epitaxial silicon film was checked with reflection high energy electron diffraction (RHEED). 3.
EXPERIMENTAL RESULTS
3.1. Silicon MBE growth on Si( lO0) and Si( l l l ) substrates First, in order to understand the substrate orientation dependence of silicon MBE growth itself, undoped layers were grown on Si(100) and (111) substrates. Figure 1 shows the R H E E D patterns from surfaces on which 100 ,&silicon films were grown on cleaned substrates at various substrate temperatures. An amorphous pattern was observed from the surface grown on Si(111) below 300 °C. In contrast, on a Si(100) surface, the silicon film grown even as low as 100 °C shows a R H E E D pattern of single-crystal silicon. The fact that the epitaxial temperature on Si(100) surfaces is lower than that on Si(111) surfaces means that the surface migration of silicon is greater on Si(100) than Si(111) surfaces. 3.2. Surface segregation of antimony and gallium atoms on Si( lO0) and Si( l l l ) substrates Figure 2 shows an example of the change in XPS spectra after silicon
SURFACE SEGREGATION IN
Si MBE
317
50"C
Si(111)
Si( )
Fig. 1. RH EED patterns from 100 A silicon films grown at various temperatures on Si(100) and Si(111) without doping.
overgrowth. The signal at around 530eV mainly comes from antimony atoms segregated on the epitaxial surfaces, since the escape depth of photoelectrons is very small (less than about 10 ~). The temperature range of the experiment is so low that the thermal desorption of dopant atoms hardly occurs. The decrease in dopant surface concentration is due to the dopant incorporation in the growing silicon film. The dopant surface concentration Cs (atoms cm- 2) decreases exponentially with the silicon deposition thickness as shown in Fig. 3, and the relation can be written as C~ = Co e x p ( - K x )
(1)
318
K. NAKAGAWA, M. MIYAO, Y. SHIRAKI
Sb Id)
Sb on Si(lO0) Tsub = 300"C
Si(2s)
Si(2p) 1
,~. -I
\ z
5b&d: t~
','
A.
u')
I 0
I
I
J 500
BINDING ENERGY(eV )
Fig. 2. XPS spectra for (a) 1 M L antimony deposited on Si(100), (b) 100/~ silicon film grown at 300 =C on 1 M L antimony, (c) 200,~, silicon film grown at 300°C on 1 ML antimony and (d) 300/~ silicon grown at 3 0 0 ' C on 1 ML.
~ I0 0 E - : : 8 : 2 1 - ] 0
SUBSTRATE °-- TE~TU~:
-~*',
I-Z Ul
........ \
500'C
-o_ 400"C
\x
Z 0 o 1 0-I
\\ \\
LIJ U
\
\,
300"C m ¢.j
\
o 250"C
lo-2 0
I
I
Si DEPOSITION
I
I
I
500 THICKNESS(~)
t
Fig. 3. Relative XPS signal height for antimony as a function of silicon deposition thickness.
SURFACE SEGREGATION IN Si MBE
319
where Co (atoms cm -2) is the initial value of C~, K (cm -1) is the incorporation coefficient and x (cm) is the thickness of the silicon growth layer, i.e. the distance between the growth surface and the initial surface. Figure 4 shows the temperature dependence of the antimony and gallium incorporation coefficients on Si(100) and Si(lll) substrates, obtained from the results shown in Fig. 3. There are two processes, one in the high and the other in the low temperature ranges. It can be seen that in the high temperature range, the incorporation coefficients on Si(111), of both antimony and gallium are about ten times larger than those on Si(100). The data suggest that this orientation dependence does not depend on dopant species. However, the absolute values of the incorporation coefficients strongly depend on dopant species and those of antimony are much larger than gallium. The activation energies of gallium and antimony are about 0.45 eV and 0.75 eV respectively. It should be pointed out that these values are much smaller than those of bulk diffusion (about 3.5 eV). 600 400 I' I i
200 (%)
I
, Io
E 6
u
-,
IL 1.1_
u.I
/
8 ~10 s
./
/
sb' Lea.)
o
o
o
•
o °
1041. (
2.0
on Si(111) on Si(100) 3.0
1000/T(K-I)
Fig. 4. Temperaturedependenceof incorporationcoefficientsfor antimonyand galliumdopant atoms on Si(lO0)and Si(111). In the low temperature range, diffuse RHEED patterns were observed. This result suggests that the silicon layer becomes amorphous like at these low temperatures. The temperature dependence of the incorporation coefficients is different from that in the high temperature range and the orientation dependence tends to disappear. 3.3. Influence of dopant atoms on silicon M B E growth. To observe the influence of dopant atoms on the MBE growth, 100/~, silicon films were grown at 300 °C on 1 ML of antimony or gallium atoms, deposited on
320
K. NAKAGAWA, M. MIYAO, Y. SHIRAKI
Si(l 11) substrates. R H E E D patterns in Fig. 5 show that the films are amorphous in the antimony adsorbed case and crystalline in the gallium case. The crystal quality of the silicon film with gallium adsorption is almost the same as that of the undoped film. That is, although gallium doping does not affect the silicon epitaxial growth, antimony causes the deterioration of the epitaxial films.
b~ Fig. 5. R H E E D patterns from I(X)~ silicon films grown on (a) ] M L antimony and (b) ] M L gallium
deposited on Si(111)substrates. 600 400 _lU I
I
200 (°C) I
u
A I
E
6a 6
W
W
8
~ 10 5 < o ou z
1041.11=t'l~'''l(2.0........
3.0~1. . . . .
IO00/T(K ~) Fig. 6. Temperature dependence of incorporation coefficientsfor antimony and gallium dopant atoms on amorphous substrates.
3.4. Surface segregation on amorphous substrate. Since the segregation behaviour of dopant atoms on amorphous substrates is expected to be different from that on crystal substrates (i.e. different from the results described in the Section 3.2), amorphous silicon substrates were also prepared and
SURFACE SEGREGATION IN
Si
MBE
321
the incorporation coefficients were evaluated. First, amorphous silicon, 300/~ thick, was deposited at room temperature on previously cleaned substrates and the densification of the amorphous layer was performed by annealing at 350°C for 10min. About 1 ML of antimony or gallium atoms were evaporated on to the amorphous layers at room temperature and then at temperatures below 350 °C to prevent the deposited layers from crystallizing, silicon overgrowth was carried out. Figure 6 shows antimony and gallium incorporation coefficients on amorphous substrates as a function of inverse growth temperatures. In the low temperature range this temperature dependence is quite similar to that of the crystalline substrates. The activation energies in the antimony and gallium cases are 0.1 eV and 0,2 eV respectively. 4. DISCUSSION Dopant atoms as well as silicon atoms migrate on crystal surfaces before acquiring stable lattice sites, and hence crystal growth and doping take place at the same time. The fact that the epitaxial temperature on Si(100) is lower than that on Si(111) indicates that the surface migration on Si(100) is greater than on Si(111). It has also been found that the incorporation coefficient of dopant atoms on Si(100) is smaller than on Si(111). These results suggest that the incorporation and surface segregation phenomena of dopant atoms have a strong correlation with the surface migration process. We propose a model, based on surface migration, that the segregation of dopant atoms is caused by the climbing of dopant atoms over surface steps formed by the silicon deposition, and the probability of dopant atoms climbing over the surface steps is larger the greater the surface migration. Since the surface migration of antimony atoms is smaller than that of silicon atoms, they impede epitaxial growth of silicon as seen in Fig. 5. As a result, the surface segregation is smaller and the incorporation coefficient of antimony is larger than that of gallium atoms, which do not impede epitaxial growth. In the low temperature range, diffuse RHEED patterns are observed, indicating that the growth layer becomes amorphous like or distorted. The temperature dependence of the incorporation coefficients in this layer are similar to that on the amorphous substrates which have a low activation energy for incorporation coefficients of dopant impurities (0.1-0.2 eV). 5. CONCLUSION
The surface segregation of antimony and gallium atoms on Si(100) and Si(111) substrates was studied using the delta doping technique. The incorporation or surface segregation phenomenon of dopant atoms is explained well by a model based on the surface migration. The much smaller activation energies of the incorporation coefficients obtained here compared with those of bulk diffusion are attributed to that of the surface migration process. It is also found that there are two kinds of activation energies in impurity incorporation during MBE growth due to surface migration. One is due to the surface migration on crystal surfaces and the other is on amorphous like surfaces, which gives very small activation energies.
322
K. NAKAGAWA, M. MIYAO, Y. SHIRAKI
ACKNOWLEDGMENT
We would like to thank T. Ikezu for expert technical assistance. This work was performed under the management of the R & D Association for Future Electron Devices as a part of the R & D of Basic Technology for Future Industries sponsored by NEDO (New Energy and Industrial Technology Development Organization). REFERENCES l 2 3 4
J.C. Bean, Appl. Phys. Lett., 33 (1978) 654. K. Nakagawa, M. Miyao and Y. Shiraki, Jpn. J. Appl. Phys., 27 (I 988) L2013. T. Sakamoto and H. Kawanami, Surf. Sci., 111 (1981) 177. A.A. van Gorkum, K. Nakagawa and Y. Shiraki. Jpn. J. Appl. Phys., 26 (1987) L1933.
315
INFLUENCE OF SUBSTRATE ORIENTATION ON SURFACE SEGREGATION PROCESS IN SILICON-MBE KIYOKAZU NAKAGAWA AND MASANOBU MIYAO
Central Research Laboratory, Hitachi Ltd., Kokubunji, Tokyo 185 (Japan) YASUHIRO SHIRAKI
Research Center for Advanced Science and Technology, Tokyo University, Meguro, Tokyo 153 (Japan) (Received May 30, 1989)
The substrate orientation dependence of the surface segregation of antimony and gallium atoms during silicon molecular beam epitaxy (MBE) was studied using the delta doping technique. During growth the dopant atoms are only partly incorporated in the growing layer and the residuals segregate on the MBE-grown surfaces. The incorporation coefficients of antimony and gallium have a temperature dependence of the thermal activation type. The activation energies for antimony and gallium are 0.75 eV and 0.45 eV respectively. The value of the incorporation coefficient of antimony is much larger than that of gallium on both Si(100) and Si(111) substrates. The incorporation coefficients on Si(111) are about ten times larger than Si(100) for both dopant atoms, although they have the same activation energy. These results are well explained by a model based on the surface migration.
1. INTRODUCTION Molecular beam epitaxy (MBE) has been recognized as a powerful tool to realize arbitrary doping profiles with abrupt transitions. This is because the growth temperature of MBE is sumciently low that the thermal diffusion of the dopant atoms can be avoided. Recent studies, however, have shown that in silicon MBE such typical dopant atoms as antimony 1'2 and gallium2"3 segregate on surfaces during epitaxial growth to form a dopant reservoir. This dopant reservoir inhibits abrupt changes in dopant profiles, even if dopant fluxes are suddenly changed. However, it has also been found that once incorporated in epitaxial layers the dopant atoms do not segregate to the epitaxial surface. That is to say, during MBE growth, the surface segregation of dopant atoms takes place only when dopant atoms exist on the growing surface, suggesting the phenomenon is due to surface effects, such as surface migration, reaction and so on. Since the density of silicon bonds and therefore the surface reactivities are different for different surface orientations, the segregation phenomenon along with MBE growth is thought to have crystal orientation dependence. This paper describes the experimental results on the surface segregation of 0040-6090/89/$3.50
© ElsevierSequoia/Printedin The Netherlands
316
K. NAKAGAWA, M. MIYAO, Y. SHIRAKI
dopant atoms on various silicon surfaces in order to clarify the mechanism of surface segregation. In these experiments, the delta doping technique, which has been demonstrated previously to be very powerful in investigating the behaviour of atoms during crystal growth, was employed. 2. EXPERIMENTAL
The silicon MBE apparatus was a Vacuum Generator's V80 system with a base pressure of about 2 × 10 11 torr. This system consists of an e-gun evaporator for silicon deposition and two effusion cells for gallium and antimony doping. Si(100) and Si(111) wafers were precleaned by chemical treatment and a protective thin oxide film was formed. Then the oxide layer was sublimated at 850 °C for 20 min. The temperature of substrates was lowered to 750°C and to obtain clean and smooth surfaces a buffer layer of about 500/~ was grown. Gallium or antimony atoms were deposited on the epitaxial silicon surfaces at temperatures where, owing to the binding energy difference between dopant-silicon and dopant-dopant atoms, only one monolayer of atoms would be automatically adsorbed. This was confirmed experimentally. Undoped silicon layers with various thickness were then overgrown on the 1 ML gallium or antimony deposited surface at various substrate temperatures. This procedure is known as delta doping. It provides atomic planar doping in epitaxial layers, if surface segregation does not occur. Using a thickness monitor, the silicon deposition rate was fixed at 1/~ s- 1. The surface concentration of dopant atoms segregated on overgrown silicon surfaces was measured using X-ray photoelectron spectroscopy (XPS). Since the escape depth of photoelectrons of Sb(3d) or Ga(2p) is small (less than about 10,~), XPS signals observed here mainly come from atoms segregated on the silicon epitaxial surface and the contribution from dopant atoms incorporated in the silicon is negligible. The crystallinity of the epitaxial silicon film was checked with reflection high energy electron diffraction (RHEED). 3.
EXPERIMENTAL RESULTS
3.1. Silicon MBE growth on Si( lO0) and Si( l l l ) substrates First, in order to understand the substrate orientation dependence of silicon MBE growth itself, undoped layers were grown on Si(100) and (111) substrates. Figure 1 shows the R H E E D patterns from surfaces on which 100 ,&silicon films were grown on cleaned substrates at various substrate temperatures. An amorphous pattern was observed from the surface grown on Si(111) below 300 °C. In contrast, on a Si(100) surface, the silicon film grown even as low as 100 °C shows a R H E E D pattern of single-crystal silicon. The fact that the epitaxial temperature on Si(100) surfaces is lower than that on Si(111) surfaces means that the surface migration of silicon is greater on Si(100) than Si(111) surfaces. 3.2. Surface segregation of antimony and gallium atoms on Si( lO0) and Si( l l l ) substrates Figure 2 shows an example of the change in XPS spectra after silicon
SURFACE SEGREGATION IN
Si MBE
317
50"C
Si(111)
Si( )
Fig. 1. RH EED patterns from 100 A silicon films grown at various temperatures on Si(100) and Si(111) without doping.
overgrowth. The signal at around 530eV mainly comes from antimony atoms segregated on the epitaxial surfaces, since the escape depth of photoelectrons is very small (less than about 10 ~). The temperature range of the experiment is so low that the thermal desorption of dopant atoms hardly occurs. The decrease in dopant surface concentration is due to the dopant incorporation in the growing silicon film. The dopant surface concentration Cs (atoms cm- 2) decreases exponentially with the silicon deposition thickness as shown in Fig. 3, and the relation can be written as C~ = Co e x p ( - K x )
(1)
318
K. NAKAGAWA, M. MIYAO, Y. SHIRAKI
Sb Id)
Sb on Si(lO0) Tsub = 300"C
Si(2s)
Si(2p) 1
,~. -I
\ z
5b&d: t~
','
A.
u')
I 0
I
I
J 500
BINDING ENERGY(eV )
Fig. 2. XPS spectra for (a) 1 M L antimony deposited on Si(100), (b) 100/~ silicon film grown at 300 =C on 1 M L antimony, (c) 200,~, silicon film grown at 300°C on 1 ML antimony and (d) 300/~ silicon grown at 3 0 0 ' C on 1 ML.
~ I0 0 E - : : 8 : 2 1 - ] 0
SUBSTRATE °-- TE~TU~:
-~*',
I-Z Ul
........ \
500'C
-o_ 400"C
\x
Z 0 o 1 0-I
\\ \\
LIJ U
\
\,
300"C m ¢.j
\
o 250"C
lo-2 0
I
I
Si DEPOSITION
I
I
I
500 THICKNESS(~)
t
Fig. 3. Relative XPS signal height for antimony as a function of silicon deposition thickness.
SURFACE SEGREGATION IN Si MBE
319
where Co (atoms cm -2) is the initial value of C~, K (cm -1) is the incorporation coefficient and x (cm) is the thickness of the silicon growth layer, i.e. the distance between the growth surface and the initial surface. Figure 4 shows the temperature dependence of the antimony and gallium incorporation coefficients on Si(100) and Si(lll) substrates, obtained from the results shown in Fig. 3. There are two processes, one in the high and the other in the low temperature ranges. It can be seen that in the high temperature range, the incorporation coefficients on Si(111), of both antimony and gallium are about ten times larger than those on Si(100). The data suggest that this orientation dependence does not depend on dopant species. However, the absolute values of the incorporation coefficients strongly depend on dopant species and those of antimony are much larger than gallium. The activation energies of gallium and antimony are about 0.45 eV and 0.75 eV respectively. It should be pointed out that these values are much smaller than those of bulk diffusion (about 3.5 eV). 600 400 I' I i
200 (%)
I
, Io
E 6
u
-,
IL 1.1_
u.I
/
8 ~10 s
./
/
sb' Lea.)
o
o
o
•
o °
1041. (
2.0
on Si(111) on Si(100) 3.0
1000/T(K-I)
Fig. 4. Temperaturedependenceof incorporationcoefficientsfor antimonyand galliumdopant atoms on Si(lO0)and Si(111). In the low temperature range, diffuse RHEED patterns were observed. This result suggests that the silicon layer becomes amorphous like at these low temperatures. The temperature dependence of the incorporation coefficients is different from that in the high temperature range and the orientation dependence tends to disappear. 3.3. Influence of dopant atoms on silicon M B E growth. To observe the influence of dopant atoms on the MBE growth, 100/~, silicon films were grown at 300 °C on 1 ML of antimony or gallium atoms, deposited on
320
K. NAKAGAWA, M. MIYAO, Y. SHIRAKI
Si(l 11) substrates. R H E E D patterns in Fig. 5 show that the films are amorphous in the antimony adsorbed case and crystalline in the gallium case. The crystal quality of the silicon film with gallium adsorption is almost the same as that of the undoped film. That is, although gallium doping does not affect the silicon epitaxial growth, antimony causes the deterioration of the epitaxial films.
b~ Fig. 5. R H E E D patterns from I(X)~ silicon films grown on (a) ] M L antimony and (b) ] M L gallium
deposited on Si(111)substrates. 600 400 _lU I
I
200 (°C) I
u
A I
E
6a 6
W
W
8
~ 10 5 < o ou z
1041.11=t'l~'''l(2.0........
3.0~1. . . . .
IO00/T(K ~) Fig. 6. Temperature dependence of incorporation coefficientsfor antimony and gallium dopant atoms on amorphous substrates.
3.4. Surface segregation on amorphous substrate. Since the segregation behaviour of dopant atoms on amorphous substrates is expected to be different from that on crystal substrates (i.e. different from the results described in the Section 3.2), amorphous silicon substrates were also prepared and
SURFACE SEGREGATION IN
Si
MBE
321
the incorporation coefficients were evaluated. First, amorphous silicon, 300/~ thick, was deposited at room temperature on previously cleaned substrates and the densification of the amorphous layer was performed by annealing at 350°C for 10min. About 1 ML of antimony or gallium atoms were evaporated on to the amorphous layers at room temperature and then at temperatures below 350 °C to prevent the deposited layers from crystallizing, silicon overgrowth was carried out. Figure 6 shows antimony and gallium incorporation coefficients on amorphous substrates as a function of inverse growth temperatures. In the low temperature range this temperature dependence is quite similar to that of the crystalline substrates. The activation energies in the antimony and gallium cases are 0.1 eV and 0,2 eV respectively. 4. DISCUSSION Dopant atoms as well as silicon atoms migrate on crystal surfaces before acquiring stable lattice sites, and hence crystal growth and doping take place at the same time. The fact that the epitaxial temperature on Si(100) is lower than that on Si(111) indicates that the surface migration on Si(100) is greater than on Si(111). It has also been found that the incorporation coefficient of dopant atoms on Si(100) is smaller than on Si(111). These results suggest that the incorporation and surface segregation phenomena of dopant atoms have a strong correlation with the surface migration process. We propose a model, based on surface migration, that the segregation of dopant atoms is caused by the climbing of dopant atoms over surface steps formed by the silicon deposition, and the probability of dopant atoms climbing over the surface steps is larger the greater the surface migration. Since the surface migration of antimony atoms is smaller than that of silicon atoms, they impede epitaxial growth of silicon as seen in Fig. 5. As a result, the surface segregation is smaller and the incorporation coefficient of antimony is larger than that of gallium atoms, which do not impede epitaxial growth. In the low temperature range, diffuse RHEED patterns are observed, indicating that the growth layer becomes amorphous like or distorted. The temperature dependence of the incorporation coefficients in this layer are similar to that on the amorphous substrates which have a low activation energy for incorporation coefficients of dopant impurities (0.1-0.2 eV). 5. CONCLUSION
The surface segregation of antimony and gallium atoms on Si(100) and Si(111) substrates was studied using the delta doping technique. The incorporation or surface segregation phenomenon of dopant atoms is explained well by a model based on the surface migration. The much smaller activation energies of the incorporation coefficients obtained here compared with those of bulk diffusion are attributed to that of the surface migration process. It is also found that there are two kinds of activation energies in impurity incorporation during MBE growth due to surface migration. One is due to the surface migration on crystal surfaces and the other is on amorphous like surfaces, which gives very small activation energies.
322
K. NAKAGAWA, M. MIYAO, Y. SHIRAKI
ACKNOWLEDGMENT
We would like to thank T. Ikezu for expert technical assistance. This work was performed under the management of the R & D Association for Future Electron Devices as a part of the R & D of Basic Technology for Future Industries sponsored by NEDO (New Energy and Industrial Technology Development Organization). REFERENCES l 2 3 4
J.C. Bean, Appl. Phys. Lett., 33 (1978) 654. K. Nakagawa, M. Miyao and Y. Shiraki, Jpn. J. Appl. Phys., 27 (I 988) L2013. T. Sakamoto and H. Kawanami, Surf. Sci., 111 (1981) 177. A.A. van Gorkum, K. Nakagawa and Y. Shiraki. Jpn. J. Appl. Phys., 26 (1987) L1933.