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Surfactant influence on the Ge heteroepilayer on Si(0 0 1) studied by X-ray diffraction and atomic force microscopy
j. . . . . . . . C R Y I T A L OROWTH
ELSEVIER
Journal of Crystal Growth 179 (1997) 115 119
Surfactant influence on the Ge heteroepilayer on Si(O 0 1) studied by X-ray diffraction and atomic force microscopy H a i j u n Z h u a'*, Z u i m i n J i a n g a, A m e i X u a, M i n g c h u n M a o a, D o n g z h i H u a, X i a n g j i u Z h a n g a, X i a o h a n L i u a, D a m i n g H u a n g a, X u n W a n g a, J i e l i n S u n b, M i n q i a n L i b, X i a o m i n g J i a n g c aSurface Physics Laboratory, Fundan University, Shanghai 200433, People "s Republic of China bLaboratory of Analysis Technique, Shanghai Institute of Nuclear Research, Chinese Academy of Sciences, Shanghai 201800, People's Republic of China CBSRF, Institute of High Energy Physics, Beijing 100039, People's Republic of China
Received 20 December 1996
Abstract
X-ray diffraction and atomic force microscope were used to investigate the effect of Sb atoms as a surfactant on the morphology and strain relaxation of 6 nm-thick Ge epilayers grown on Si(00 1). Without Sb atoms, Ge atoms accumulate and form fully relaxed islands. With the presence of Sb atoms, the Ge epilayer is smooth with a roughness of 0.28 nm and partially relaxed.
1. I n t r o d u c t i o n
Smooth and fully relaxed SiGe and Ge epitaxial layers grown on Si substrate have a vast potential for applications: it can be used as a template for the combination of Si and GaAs technology and as a buffer layer for Si/Ge superlattices in order to get electron wells in Si layers [1, 2]. Several groups have successfully grown a fully relaxed and graded Six-xGex layer [3, 4-1, and obtained a very high electron mobility in Si layer grown on such a buffer layer [4]. However, such a graded layer is typically
* Corresponding author.
of several microns thickness, which limits the application in practice. The other way to realize a smooth and relaxed SiGe or Ge epitaxial layer on Si substrate is by the introduction of a surfactant [5, 6]. Hoegen et al. [5] have studied the Ge epitaxial growth on Si(1 1 1) substrate, and concluded that the 20 monolayer (ML) thick Ge film completely relaxed to the bulk lattice parameters and the dislocations were localized to within a few atomic planes of the interface. Thornton et al. [6] have investigated the relaxation of the strain in Ge epilayer grown on Si(0 0 1) with the presence of surfactant of Sb. They found that at the thickness of 9-10 ML, the Ge epilayer began to relax and did not fully relax even at the thickness of 50 ML. Further study on the relaxation and morphology of
0022-0248/97/$17.00 Copyright © 1997 Elsevier ScienceB.V. All rights reserved PII S0022-0248(97)00099-7
116
Haijun Zhu et aL / Journal of Crystal Growth 179 (1997) 115-119
Ge layer under different growth conditions and thickness of Ge layer is essential to grow a smooth and fully relaxed Ge buffer layer. In this paper, large angle X-ray diffraction and Raman scatterings were used to investigate the relaxation of strain in Ge epilayer grown on Si(0 0 1) substrate with and without surfactant. In situ reflection high energy electron diffraction (RHEED), small angle X-ray reflectivity and atomic force microscope were used to observe the morphology of the surface of the samples.
2. Experimental procedure The samples were grown in a Si MBE system (Riber EVA-32), equipped with RHEED with the primary beam energy of 10 keV, an electron gun evaporator to deposit Si and Ge, a Knudsen cell to generate Sb molecules, a Sentinel III thickness monitor. The base and growth pressures of the system were about 10 -1° and 10-9Torr, respectively. P-type Si(0 0 1) substrates with resistivities of 1-5 Q cm were cleaned by Shiraki treatment to form a very thin (< 1 nm) protecting oxide on the wafer surface. After being spun dried, the sample was mounted on a molybdenum block with a quartz plate put in between to act as the sample supporting part and the heat diffuser for the purpose of lateral uniformity of heating. The samples were free of strain on the block, and could be heated by the radiation and heat conductance of a fiat tantalum foil heater through the quartz diffuser. Before being transferred into the growth chamber, the samples were inletted into the introduction chamber via a load lock and were annealed at 300°C for 10-20min there (pressure about 10-9 Torr) for degassing. The protecting oxide on the substrate surface was desorbed at 1000°C for 10 min in the growth chamber, a clear and sharp double domain 2 x 1 reconstruction RHEED pattern was observed after this desorption. Si deposition rate was kept at 0.1 nm/s. The atomic Sb flux was kept at approximately 7 x 1011 atoms/cm2 s with the cell operation temperature of 320°C. A 100 nm thick Si layer was grown prior to the growth of Sb and Ge layers at 600°C. For sample A, a monolayer of Sb was deposited before the 6 nm
thick Ge layer was deposited; sample B was without Sb and only have 6 nm Ge layer deposited on the Si surface. Both samples were grown at the same temperature of 500°C. The X-ray diffraction experiments were carried out at the 14B line in PF(KEK) in Japan. The wave length of incident X-ray is 0.100 nm, line width 200 ~tm, and divergence angle of the incident X-ray 30 arcsec. The mode of 0-20 was adopted in measurements. The morphologies of the surface of the samples were observed by atomic force microscope (Nanoscope Ili). Raman spectra were measured at room temperature in the near backscattering configuration using a Jobin-Yvon U1000 Raman spectrometer. The excitation was for the 488.0 nm line of an Ar ÷ laser. The laser power was typically 100 mW and the spot size on the sample was about 200 ~tm.
3. Results and discussion Fig. 1 is the large angle X-ray diffraction curves of samples A and B. The most intensive peak
100
10
5 ,B
u) ¢t.-
--
0.1
0.01 .d
-3
-2
-1
0
1
2
Ae(degree) Fig. 1. Large angle X-ray diffraction curves of sample A (solid line) and sample B (dotted line).
Haijun Zhu et al. / Journal of Crystal Growth 179 (1997) 115-119
at 0 position corresponds to the Bragg diffraction peak of Si(0 0 4), at the low angle sides are the diffraction peaks of Ge(0 0 4) of samples A and B. The peak positions are -1.18 ° and -0.91 ° for samples A and B, respectively. According to Bragg equation, the layer spacing in sample B is almost equal to that of bulk Ge(0 0 4), which indicates that the Ge epilayer is fully relaxed. It is in agreement with previous result that the grown Ge layer on Si began to relax at the thickness of 3-6 ML, and fully relaxed at the thickness of 10 ML. Obviously, the layer spacing of Ge epilayer in sample A is larger than that of bulk Ge(0 0 4). Two factors are responsible for the increase of the layer spacing along the growth direction in sample A: Sb atoms incorporating into epilayer as impurities can enlarge the lattice constant. However, in the process of growth, Sb atoms have strong segregation and surface accumulation at the conventional growth temperature of 500°C [7]. When the growth temperature is equal to or higher than 500°C, the content of Sb in the Si epilayer is lower than the sensitivity (1 x 10~7 cm -3) by Rutherford back-scattering measurements [8]. The layer spacing increase of Si induced by Sb atoms of content 1 x 10~9 cm -3 is calculated to be 2 x 10 -5 nm [9], which corresponds to a 0.03° shift of peak position. So the Sb atoms incorporated into epilayer cannot result in such a large increase of layer spacing, which is 1.8 × 10- a nm as observed experimentally. The second reason is that surfactant Sb can restrain the lattice relaxation of Ge epilayer, thus causing the increase of layer spacing along the growth direction. According to the position of Ge(0 0 4) in sample A, 42% of the strain in Ge epilayer has not relaxed. Thornton et al. [6] observed that the (1 1 0) plane spacing varied with the thickness of Ge epilayer with the presence of Sb atoms. They found that the lattice relaxation of Ge epilayer began at 9-10 ML, and there still existed 26% strain at the grown thickness of 42 ML. The degree of relaxation in sample A is much less than that observed by Thornton et al. The large difference could be attributed to the different growth conditions. The growth rate of Ge in our experiment is 0.1 nm/s which is much larger than 1 ML/8 min in Thornton's experiment. It is well known that the growth rate of Ge will affect the segregation of Sb,
117
thus can affect the lattice relaxation. Of course the growth temperature can also affect the lattice relaxation; however, the growth temperatures are quite same. In order to further examine the residual strain in sample A, Raman scattering experiments were carried out on both samples. As shown in Fig. 2, about 4 cm- 1 difference of peak position is observed between the two samples. According to the peak position of sample A, the residual strain in the epilayer is estimated to be 18%, which is smaller than the 42% obtained by X-ray diffraction. Such a large difference could not be explained until now. Even with the consideration of limited thickness of Ge epilayer, the residual strain is estimated to be 24%, because the peak position difference is less than 1 cm- 1 between the fully relaxed 6 nm-thick Ge layer and bulk Ge, 1 cm-~ difference of peak positions corresponds to 6% strain change [10]. A series of samples are being made in order to understand the above strain difference obtained by the two methods. As shown in the upper curve in Fig. 1, the oscillations are attributed to the interference effect. There is no such oscillation in the curve of sample B. According to the FWHM of Ge(0 0 4) peak, the
300
¢-
D. 200
C e-.
100 =
250
I
300
350
Wave number(cm1) Fig. 2. Raman spectra of sample A (solid line) and sample B (dottedline).The peak positionsare 304.5and 300.5cm- 1for samples A and B, respectively.
118
Haijun Zhu et al. / Journal of Crystal Growth 179 (1997) 115-119
average height of Ge epilayer of sample B is estimated to be 17 nm, which is much larger than the controlled thickness, indicating that islands are formed. Due to the formation of islands and different height of islands, surely the oscillation effect of interference can not be observed. The small angle X-ray reflectivities as shown in Fig. 3 are consistent with results of large angle X-ray diffraction; the oscillations can only be observed in the curve of sample A. RHEED patterns of samples A and B also confirm the above conclusion. RHEED pattern of sample A show clear 2 × 1 streaks whereas RHEED pattern of sample B contains three-dimensional transmission spots, indicating that the surface of sample A is smooth and sample B has a lot of facets. According to the oscillations in the reflectivity curve of sample A, the thickness of Ge epilayer is estimated to be 5.4 nm, which reasonably agrees with the controlled thickness of 6 nm. In order to get intuitive impression of surface morphology, the surfaces of two samples were observed by atomic force microscopy. As shown in Fig. 4, there are many islands in sample B with an average dimension of 0.16 Ixm, the surface of sample A is fiat, whose surface roughness is measured to be 0.28 nm.
Due to the presence of surfactant Sb, the growing surface of Ge epilayer will remain fiat, thus the strain in the Ge epilayer cannot be relieved by island formation but by the gradual introduction of dislocations. So the strain in the Ge epilayer is relieved gradually. As a buffer layer, Ge epilayer should have smooth surface and is fully relaxed. From the surface flatness point of view, Ge epilayer growth on both Si(1 1 1) and Si(0 0 1) with the presence of surfactant Sb can be used as a buffer layer. However, from the strain relaxation point of view, only Ge epilayer grown on Si(1 1 1) with the presence of surfactant Sb is fully relaxed at the thickness of 60 ML, and can be used as a buffer layer as pointed out by Hoegen et al. Still, a large
1000 A 100 e'-
10 < v
n,"
"'.., 0.1
°.°1o o(degree) Fig. 3. Reflectivities of X-ray with the incident angle 0.
Fig. 4. A F M pictures: (a) sample A (4 x 4 ktm); (b) sample B (1.4 x 1.4 I.tm).
Haijun Zhu et al. / Journal of Crystal Growth 179 (1997) 115-119
p o r t i o n of strain in Ge epilayer g r o w n on Si(0 0 1) with the presence of surfactant Sb has not been relaxed, the study on the dependence of strain relaxation of Ge epilayers is in progress by increasing thicknesses and temperatures in the presence of surfactant Sb.
4. Conclusions Using X-ray diffraction and atomic force microscope, we have investigated the effect of Sb atoms as a surfactant on m o r p h o l o g y and strain relaxation of 6 nm thick Ge epilayer grown on Si(0 0 1). When the surfactant Sb was not introduced, Ge epilayer on Si(0 0 1) forms fully relaxed islands. W h e n the surfactant Sb was introduced, the surface of Ge epilayer is s m o o t h with a roughness of 0.28 nm, and only 58% strain in Ge epilayer is relaxed.
Acknowledgements We acknowledge the support of P h o t o n F a c t o r y in J a p a n for the X-ray diffraction measurement
119
with the grant n u m b e r of 95G143. This research was partially supported by the N a t i o n a l N a t u r a l Science F o u n d a t i o n of China.
References [-1] Y. Ohmachi, T. Nishioka, Y. Shinoda, J. Appl. Phys. 54 (1983) 5466. [2] C.G. van de Walle, R.M. Martin, Phys. Rev. B 34 (1986) 5621. [3] N. Usami, F. Issiki, D.K. Nayak, Y. Shiraki, Appl. Phys. Lett. 67 (1995) 524. [4] Ya-Hong Xie, Eugene A. Fitzgerald, Donald Monroe, G. Patrick Watson, Paul J. Silverman, Jpn. J. Appl. Phys. 33 (1994) 2372. [5] M. Horn-yon Hoegen, Appl. Phys. A 59 (1994) 503. [6] J.M.C. Thornton, A.A. Williams, J.E. Macdonald, R.G. van Silfhout, J.F. van der Veen, M. Finney, C. Norris, J. Vac. Sci. Technol. B 9 (1991) 2146. [7] K.D. Hobart, D.J. Godbey, P.E. Thompson, D.S. Simons, Appl. Phys. Lett. 63 (1993) 1381. 1-8] M. Copel, M.C. Reuter, M. Horn von Hoegen, R.M. Tromp, Phys. Rev. B 42 (1990) 11682. 1-9] A.P. Powell, R.A. Kubiak, T.E. Whall, D.K. Bowen, J. Phys. D. Appl. Phys. 23 (1990) 1745. [10] F. Cerdeira, A. Pinczuk, J.C. Bean, B. Batlogg, B.A. Wilson, Appl. Phys. Lett. 45 (1984) 1138.
ELSEVIER
Journal of Crystal Growth 179 (1997) 115 119
Surfactant influence on the Ge heteroepilayer on Si(O 0 1) studied by X-ray diffraction and atomic force microscopy H a i j u n Z h u a'*, Z u i m i n J i a n g a, A m e i X u a, M i n g c h u n M a o a, D o n g z h i H u a, X i a n g j i u Z h a n g a, X i a o h a n L i u a, D a m i n g H u a n g a, X u n W a n g a, J i e l i n S u n b, M i n q i a n L i b, X i a o m i n g J i a n g c aSurface Physics Laboratory, Fundan University, Shanghai 200433, People "s Republic of China bLaboratory of Analysis Technique, Shanghai Institute of Nuclear Research, Chinese Academy of Sciences, Shanghai 201800, People's Republic of China CBSRF, Institute of High Energy Physics, Beijing 100039, People's Republic of China
Received 20 December 1996
Abstract
X-ray diffraction and atomic force microscope were used to investigate the effect of Sb atoms as a surfactant on the morphology and strain relaxation of 6 nm-thick Ge epilayers grown on Si(00 1). Without Sb atoms, Ge atoms accumulate and form fully relaxed islands. With the presence of Sb atoms, the Ge epilayer is smooth with a roughness of 0.28 nm and partially relaxed.
1. I n t r o d u c t i o n
Smooth and fully relaxed SiGe and Ge epitaxial layers grown on Si substrate have a vast potential for applications: it can be used as a template for the combination of Si and GaAs technology and as a buffer layer for Si/Ge superlattices in order to get electron wells in Si layers [1, 2]. Several groups have successfully grown a fully relaxed and graded Six-xGex layer [3, 4-1, and obtained a very high electron mobility in Si layer grown on such a buffer layer [4]. However, such a graded layer is typically
* Corresponding author.
of several microns thickness, which limits the application in practice. The other way to realize a smooth and relaxed SiGe or Ge epitaxial layer on Si substrate is by the introduction of a surfactant [5, 6]. Hoegen et al. [5] have studied the Ge epitaxial growth on Si(1 1 1) substrate, and concluded that the 20 monolayer (ML) thick Ge film completely relaxed to the bulk lattice parameters and the dislocations were localized to within a few atomic planes of the interface. Thornton et al. [6] have investigated the relaxation of the strain in Ge epilayer grown on Si(0 0 1) with the presence of surfactant of Sb. They found that at the thickness of 9-10 ML, the Ge epilayer began to relax and did not fully relax even at the thickness of 50 ML. Further study on the relaxation and morphology of
0022-0248/97/$17.00 Copyright © 1997 Elsevier ScienceB.V. All rights reserved PII S0022-0248(97)00099-7
116
Haijun Zhu et aL / Journal of Crystal Growth 179 (1997) 115-119
Ge layer under different growth conditions and thickness of Ge layer is essential to grow a smooth and fully relaxed Ge buffer layer. In this paper, large angle X-ray diffraction and Raman scatterings were used to investigate the relaxation of strain in Ge epilayer grown on Si(0 0 1) substrate with and without surfactant. In situ reflection high energy electron diffraction (RHEED), small angle X-ray reflectivity and atomic force microscope were used to observe the morphology of the surface of the samples.
2. Experimental procedure The samples were grown in a Si MBE system (Riber EVA-32), equipped with RHEED with the primary beam energy of 10 keV, an electron gun evaporator to deposit Si and Ge, a Knudsen cell to generate Sb molecules, a Sentinel III thickness monitor. The base and growth pressures of the system were about 10 -1° and 10-9Torr, respectively. P-type Si(0 0 1) substrates with resistivities of 1-5 Q cm were cleaned by Shiraki treatment to form a very thin (< 1 nm) protecting oxide on the wafer surface. After being spun dried, the sample was mounted on a molybdenum block with a quartz plate put in between to act as the sample supporting part and the heat diffuser for the purpose of lateral uniformity of heating. The samples were free of strain on the block, and could be heated by the radiation and heat conductance of a fiat tantalum foil heater through the quartz diffuser. Before being transferred into the growth chamber, the samples were inletted into the introduction chamber via a load lock and were annealed at 300°C for 10-20min there (pressure about 10-9 Torr) for degassing. The protecting oxide on the substrate surface was desorbed at 1000°C for 10 min in the growth chamber, a clear and sharp double domain 2 x 1 reconstruction RHEED pattern was observed after this desorption. Si deposition rate was kept at 0.1 nm/s. The atomic Sb flux was kept at approximately 7 x 1011 atoms/cm2 s with the cell operation temperature of 320°C. A 100 nm thick Si layer was grown prior to the growth of Sb and Ge layers at 600°C. For sample A, a monolayer of Sb was deposited before the 6 nm
thick Ge layer was deposited; sample B was without Sb and only have 6 nm Ge layer deposited on the Si surface. Both samples were grown at the same temperature of 500°C. The X-ray diffraction experiments were carried out at the 14B line in PF(KEK) in Japan. The wave length of incident X-ray is 0.100 nm, line width 200 ~tm, and divergence angle of the incident X-ray 30 arcsec. The mode of 0-20 was adopted in measurements. The morphologies of the surface of the samples were observed by atomic force microscope (Nanoscope Ili). Raman spectra were measured at room temperature in the near backscattering configuration using a Jobin-Yvon U1000 Raman spectrometer. The excitation was for the 488.0 nm line of an Ar ÷ laser. The laser power was typically 100 mW and the spot size on the sample was about 200 ~tm.
3. Results and discussion Fig. 1 is the large angle X-ray diffraction curves of samples A and B. The most intensive peak
100
10
5 ,B
u) ¢t.-
--
0.1
0.01 .d
-3
-2
-1
0
1
2
Ae(degree) Fig. 1. Large angle X-ray diffraction curves of sample A (solid line) and sample B (dotted line).
Haijun Zhu et al. / Journal of Crystal Growth 179 (1997) 115-119
at 0 position corresponds to the Bragg diffraction peak of Si(0 0 4), at the low angle sides are the diffraction peaks of Ge(0 0 4) of samples A and B. The peak positions are -1.18 ° and -0.91 ° for samples A and B, respectively. According to Bragg equation, the layer spacing in sample B is almost equal to that of bulk Ge(0 0 4), which indicates that the Ge epilayer is fully relaxed. It is in agreement with previous result that the grown Ge layer on Si began to relax at the thickness of 3-6 ML, and fully relaxed at the thickness of 10 ML. Obviously, the layer spacing of Ge epilayer in sample A is larger than that of bulk Ge(0 0 4). Two factors are responsible for the increase of the layer spacing along the growth direction in sample A: Sb atoms incorporating into epilayer as impurities can enlarge the lattice constant. However, in the process of growth, Sb atoms have strong segregation and surface accumulation at the conventional growth temperature of 500°C [7]. When the growth temperature is equal to or higher than 500°C, the content of Sb in the Si epilayer is lower than the sensitivity (1 x 10~7 cm -3) by Rutherford back-scattering measurements [8]. The layer spacing increase of Si induced by Sb atoms of content 1 x 10~9 cm -3 is calculated to be 2 x 10 -5 nm [9], which corresponds to a 0.03° shift of peak position. So the Sb atoms incorporated into epilayer cannot result in such a large increase of layer spacing, which is 1.8 × 10- a nm as observed experimentally. The second reason is that surfactant Sb can restrain the lattice relaxation of Ge epilayer, thus causing the increase of layer spacing along the growth direction. According to the position of Ge(0 0 4) in sample A, 42% of the strain in Ge epilayer has not relaxed. Thornton et al. [6] observed that the (1 1 0) plane spacing varied with the thickness of Ge epilayer with the presence of Sb atoms. They found that the lattice relaxation of Ge epilayer began at 9-10 ML, and there still existed 26% strain at the grown thickness of 42 ML. The degree of relaxation in sample A is much less than that observed by Thornton et al. The large difference could be attributed to the different growth conditions. The growth rate of Ge in our experiment is 0.1 nm/s which is much larger than 1 ML/8 min in Thornton's experiment. It is well known that the growth rate of Ge will affect the segregation of Sb,
117
thus can affect the lattice relaxation. Of course the growth temperature can also affect the lattice relaxation; however, the growth temperatures are quite same. In order to further examine the residual strain in sample A, Raman scattering experiments were carried out on both samples. As shown in Fig. 2, about 4 cm- 1 difference of peak position is observed between the two samples. According to the peak position of sample A, the residual strain in the epilayer is estimated to be 18%, which is smaller than the 42% obtained by X-ray diffraction. Such a large difference could not be explained until now. Even with the consideration of limited thickness of Ge epilayer, the residual strain is estimated to be 24%, because the peak position difference is less than 1 cm- 1 between the fully relaxed 6 nm-thick Ge layer and bulk Ge, 1 cm-~ difference of peak positions corresponds to 6% strain change [10]. A series of samples are being made in order to understand the above strain difference obtained by the two methods. As shown in the upper curve in Fig. 1, the oscillations are attributed to the interference effect. There is no such oscillation in the curve of sample B. According to the FWHM of Ge(0 0 4) peak, the
300
¢-
D. 200
C e-.
100 =
250
I
300
350
Wave number(cm1) Fig. 2. Raman spectra of sample A (solid line) and sample B (dottedline).The peak positionsare 304.5and 300.5cm- 1for samples A and B, respectively.
118
Haijun Zhu et al. / Journal of Crystal Growth 179 (1997) 115-119
average height of Ge epilayer of sample B is estimated to be 17 nm, which is much larger than the controlled thickness, indicating that islands are formed. Due to the formation of islands and different height of islands, surely the oscillation effect of interference can not be observed. The small angle X-ray reflectivities as shown in Fig. 3 are consistent with results of large angle X-ray diffraction; the oscillations can only be observed in the curve of sample A. RHEED patterns of samples A and B also confirm the above conclusion. RHEED pattern of sample A show clear 2 × 1 streaks whereas RHEED pattern of sample B contains three-dimensional transmission spots, indicating that the surface of sample A is smooth and sample B has a lot of facets. According to the oscillations in the reflectivity curve of sample A, the thickness of Ge epilayer is estimated to be 5.4 nm, which reasonably agrees with the controlled thickness of 6 nm. In order to get intuitive impression of surface morphology, the surfaces of two samples were observed by atomic force microscopy. As shown in Fig. 4, there are many islands in sample B with an average dimension of 0.16 Ixm, the surface of sample A is fiat, whose surface roughness is measured to be 0.28 nm.
Due to the presence of surfactant Sb, the growing surface of Ge epilayer will remain fiat, thus the strain in the Ge epilayer cannot be relieved by island formation but by the gradual introduction of dislocations. So the strain in the Ge epilayer is relieved gradually. As a buffer layer, Ge epilayer should have smooth surface and is fully relaxed. From the surface flatness point of view, Ge epilayer growth on both Si(1 1 1) and Si(0 0 1) with the presence of surfactant Sb can be used as a buffer layer. However, from the strain relaxation point of view, only Ge epilayer grown on Si(1 1 1) with the presence of surfactant Sb is fully relaxed at the thickness of 60 ML, and can be used as a buffer layer as pointed out by Hoegen et al. Still, a large
1000 A 100 e'-
10 < v
n,"
"'.., 0.1
°.°1o o(degree) Fig. 3. Reflectivities of X-ray with the incident angle 0.
Fig. 4. A F M pictures: (a) sample A (4 x 4 ktm); (b) sample B (1.4 x 1.4 I.tm).
Haijun Zhu et al. / Journal of Crystal Growth 179 (1997) 115-119
p o r t i o n of strain in Ge epilayer g r o w n on Si(0 0 1) with the presence of surfactant Sb has not been relaxed, the study on the dependence of strain relaxation of Ge epilayers is in progress by increasing thicknesses and temperatures in the presence of surfactant Sb.
4. Conclusions Using X-ray diffraction and atomic force microscope, we have investigated the effect of Sb atoms as a surfactant on m o r p h o l o g y and strain relaxation of 6 nm thick Ge epilayer grown on Si(0 0 1). When the surfactant Sb was not introduced, Ge epilayer on Si(0 0 1) forms fully relaxed islands. W h e n the surfactant Sb was introduced, the surface of Ge epilayer is s m o o t h with a roughness of 0.28 nm, and only 58% strain in Ge epilayer is relaxed.
Acknowledgements We acknowledge the support of P h o t o n F a c t o r y in J a p a n for the X-ray diffraction measurement
119
with the grant n u m b e r of 95G143. This research was partially supported by the N a t i o n a l N a t u r a l Science F o u n d a t i o n of China.
References [-1] Y. Ohmachi, T. Nishioka, Y. Shinoda, J. Appl. Phys. 54 (1983) 5466. [2] C.G. van de Walle, R.M. Martin, Phys. Rev. B 34 (1986) 5621. [3] N. Usami, F. Issiki, D.K. Nayak, Y. Shiraki, Appl. Phys. Lett. 67 (1995) 524. [4] Ya-Hong Xie, Eugene A. Fitzgerald, Donald Monroe, G. Patrick Watson, Paul J. Silverman, Jpn. J. Appl. Phys. 33 (1994) 2372. [5] M. Horn-yon Hoegen, Appl. Phys. A 59 (1994) 503. [6] J.M.C. Thornton, A.A. Williams, J.E. Macdonald, R.G. van Silfhout, J.F. van der Veen, M. Finney, C. Norris, J. Vac. Sci. Technol. B 9 (1991) 2146. [7] K.D. Hobart, D.J. Godbey, P.E. Thompson, D.S. Simons, Appl. Phys. Lett. 63 (1993) 1381. 1-8] M. Copel, M.C. Reuter, M. Horn von Hoegen, R.M. Tromp, Phys. Rev. B 42 (1990) 11682. 1-9] A.P. Powell, R.A. Kubiak, T.E. Whall, D.K. Bowen, J. Phys. D. Appl. Phys. 23 (1990) 1745. [10] F. Cerdeira, A. Pinczuk, J.C. Bean, B. Batlogg, B.A. Wilson, Appl. Phys. Lett. 45 (1984) 1138.