- Email: [email protected]
low-temperature Si buffer grown on Si (0 0 1)
Journal of Crystal Growth 227–228 (2001) 740–743
The formation of dislocations in the interface of GeSi/low-temperature Si buffer grown on Si (0 0 1) C.S. Peng*, Y.K. Li, Q. Huang, J.M. Zhou Institute of Physics & Center of Condensed Matter Physics, Chinese Academy of Sciences, PO Box 603, Beijing 100080, People’s Republic of China
Abstract We have studied the mechanism of strain relaxation of Ge0.3Si0.7 alloy layers grown on low-temperature (LT) Si buffers by high-resolution X-ray diffraction and transmission electron microscopy (TEM). X-ray rocking-curve analyses show that the strain relaxation of an alloy layer is quite inhomogeneous at its early stage, and becomes homogeneous when the layer approaches becomes fully relaxed. High-resolution cross-section TEM analyses indicate that stacking faults have formed in the LT buffer near the interface of LT-Si/GeSi and have separated the mismatch dislocations in the interface of GeSi/LT-Si into Shockley partials. We propose that this could be due to the aggregation of the vacant defects in the LT-Si layer. The mechanism of this aggregation is the osmotic force created by the vacancy super-saturation in the LT-Si layer and/or the tension stress propagating from the interface of GeSi/LT-Si. # 2001 Elsevier Science B.V. All rights reserved. PACS: 61.72.Ff; 61.72.Nn; 81.15.Hi Keywords: A1. Defects; A1. X-ray diffraction; A3. Molecular beam epitaxy; B1. Germanium silicon alloys; B2. Semiconducting germanium; B2. Semiconducting silicon
Due to the variety of their band structure, GeSi/ Si heterostructures are used in high speed and large power devices, such as HBT, and MOSFET. Some of the electronic active layers have to be grown epitaxially on a relaxed GeSi buffer layer. Unfortunately, 4% of lattice mismatch between Ge and Si often results in high threading dislocation (TD) density even up to 1011 cm2 in the GeSi epi-layers grown on Si substrates. These TDs can penetrate into the active layers grown on such GeSi epi-layers, [1,2] and decrease largely the *Corresponding author. Tel.: +86-10-8264-9433; fax: +8610-8264-9457. E-mail address: [email protected] (C.S. Peng).
mobility of the carriers by serving as the scattering center. Therefore, efforts to decrease the TD density have attracted great attention. Among various techniques used, the compositionally graded buffer seems to be a very useful one by which the TD density can be reduced to as low as 106 cm2 with Ge fraction up to 30% [3–5]. However, such buffers are usually very thick and the surface corrugation is serious with an amplitude of up to 1520 nm, and the Ge composition can not be higher than 0.5 if the TDs density is less than 107 cm2. We have developed a new MBE growth technique, by which, a relaxed, thin, and uniform GeSi layer with low TDs density is grown on low-temperature Si (LT-Si) or LT-GeSi buffer
0022-0248/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 0 8 1 8 - 1
C.S. Peng et al. / Journal of Crystal Growth 227–228 (2001) 740–743
layers [6–8]. In this paper, the mechanism of the relaxation will be investigated in detail. Perovic et al. [9] discovered that there were large amounts of spherical microvoids in LT-Si. The defect density sharply increases toward the epilayer surface. At low growth temperature, the migration rate of the deposited atoms in the surface is too low to form a smooth surface and micro-holes are induced. These micro-holes are not covered perfectly by the subsequently deposited atoms and vacant defects are formed and frozen in the epilayer. During the epilayer growth, the surface roughness and the number of microholes increases. Therefore, the vacant defect density increases toward the surface. The migration barrier energy of mono-vacancy and divacancy are Hm1 =0.33 eV and Hm2 =1.3 eV, respectively [10]. In Silicon, the vacancy equilibrium density is: 1 Wn C0 ¼ exp ; ð1Þ na kT where na b3 ¼ 5:661023 cm3 and Wn 0:2 mb3 ¼ 5:625 eVare the atomic volume and vacancy formation energy, respectively. b is the value of the Burgers vector and m is Young’s modulus of Si [11]. For instance, at T=573 K (3008C), C0 = 5.88 1028 cm3, while the epitaxially grown vacancy density reaches C 2 1021 cm3 near the surface when the thickness of the LT-Si layer is 150 nm [12]. Then, the vacancy supersaturator is: C 3:41048 : ð2Þ C0 The osmotic energy of the vacancy caused by the supersaturator is: C G ¼ kT ln ¼ 5:52 eV ð4Hml ; Hm2 Þ: ð3Þ C0 Therefore, the vacancy osmotic energy is much larger than the migration barrier. The vacancies are very easy to aggregate to divacancies and large complexes before annihilation near dislocations and separate them into Shockley partials to form stacking faults. Such an aggregation would lower the vacancy chemical potential given by Eq. (3). Samples were grown by MBE with a V80S system. On a CM 200’ FEG microscope, crosssection transmission electron microscopy
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(X-TEM) was used to observe the stacking faults and dislocation distribution. The strain status and alloy composition were determined from highresolution (HR) X-ray double-axis diffraction (XDD) measurements on a Philips X’Pert diffractometer. The structure of the samples is: Si (0 0 1) substrate+50 nm LT-Si+x (nm) Ge0.3Si0.7. The x value of samples ‘‘a’’–‘‘e’’ are 80, 100, 150, 300, and 500 nm, respectively. The growth condition was described in detail previously [7]. Results for the strain relaxation were summarized in Ref. [7], which shows that the relaxation is observable for Ge0.3Si0.7 layer thickness beyond 80 nm. This thickness is almost the same for Ge0.3Si0.7 grown in the same condition without the LT-Si layer. For the 80 nm thick layer, evidence of the onset of relaxation is observed. Further, the sample with 500 nm thick Ge0.3Si0.7 layer is already close to full relaxation (R=90%). Such a thickness is much smaller than that of compositionally graded buffer systems [13]. Due to the very large super-saturation of vacancies in LTSi layer, the vacancy osmotic energy is much larger than the migration barrier. Therefore, vacant defects are very easy to aggregate together to separate the 608 mismatch dislocation (MD) formed by GeSi relaxed on Si into Shockley partials and form stacking faults. Fig. 1 shows the HRXDD (224) rocking curves of those five samples; theoretically simulated (2 2 4) rocking curves for sample a have also been plotted (dotted lines). In curve (a) the main part of the peak of the GeSi layer is symmetrical and fits quite well to the simulation, suggesting that a major part of the alloy layer remains pseudomorphic. Whereas, the tail of the epilayer peak shows that a small portion of the alloy epilayer is already (partly) relaxed, the relaxation is quite inhomogeneous. With the increase of the GeSi layer thickness (curves (b) and (c)), the asymmetric part of the layer peak expands, while the symmetrical part of the peak shrinks. Further increase of the epilayer thickness makes the whole peak broader and finally more symmetric (curves (d) and (e)), suggesting that the relaxation of the thickest alloy layer turns out to be more homogeneous.
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C.S. Peng et al. / Journal of Crystal Growth 227–228 (2001) 740–743
Fig. 1. X-ray (2 2 4) rocking curves of the five samples with the Ge0.3Si0.7 epilayer thickness of 80, 100, 150, 300 and 500 nm, respectively. The dotted lines in both panels are theoretical simulations of curve (a).
Fortunately, such a result of relaxation is further confirmed by TEM measurements. Fig. 2(a) and (b) show the HRX-TEM images of samples ‘‘a’’ and ‘‘b’’, respectively. When the Si0.7Ge0.3 layer thickness reaches 80 nm, stacking faults are induced in (1 1 1) in LT-Si layer near the interface of LT-Si/Si0.7Ge0.3, and then grow into the LT-Si layer. In LT-Si layer, the density of vacancy near the interface of LT-Si/Si0.7Ge0.3 is the largest [12]. Under the osmotic force and the tensile stress, the vacancies near the interface aggregate together in (1 1 1) near the 608 MDs in the interface of LT-Si/GeSi to separate them into Shockley partials and form stacking faults. At the beginning, the number of 608 MDs is small and Shockley partials and their stacking faults are few, and a large part of the interface remains strained. Therefore, the relaxation is inhomogeneous. With further growth, the scale of 608 MDs and Shockley extend dislocations (SEDs) become larger and the distribution becomes more uniform. Finally, as the Si0.7Ge0.3 layer relaxes fully, the relaxation turns out to be homogeneous. Fig. 3 shows the TEM cross-section image of the almost fully relaxed sample ‘‘e’’. There are pairs of dislocations in the two intersect lines by (1 1 1) and the two interfaces between the LT-Si layer (GeSi/LT-Si/Si). The mismatch dislocation distribution is uniform and periodic.
Fig. 2. The HR-TEM images for samples a (a) and b (b). There are stacking faults in {1 1 1} in LT-Si layer near the GeSi/LT-Si interface. The number of them increases and they are grown deeply into LT-Si layer during the growth.
Fig. 3. The X-TEM image for sample (e). There are pairs of dislocations in the intersection of {1 1 1} and two GeSi/LT-Si/Si interfaces. These pairs arrange periodically along these two interfaces.
Due to the very large super-saturation of vacancies in LT-Si, the vacancy osmotic force becomes very large. The vacant defects are very easy to aggregate together to form SEDs and only need very small external force (such as a mismatch
C.S. Peng et al. / Journal of Crystal Growth 227–228 (2001) 740–743
force near 608 MDs). As a result, 608 MDs disappear fast and form the force to enlarge the scale of MDs. Therefore, the thickness of fully strained GeSi alloy layer grown on LT-Si is much smaller than that on normal Si substrate. In this paper, we have studied the mechanism of the nucleation of the mismatch dislocation in the interface of GeSi alloy layer and LT-Si layer. The strain relaxation is quite inhomogeneous from the beginning, and becomes homogeneous when the Ge0.3Si0.7 layer is relaxed fully. Stacking faults generated and formed the mismatch dislocations in the interface of GeSi/LT-Si. It is due to the aggregation of the vacant defects in the LT-Si layer. Because of the large vacancy density, the critical thickness of strained GeSi alloy layer grown on LT-Si is much smaller than that on normal Si substrate.
References [1] R. Hull, J.C. Bean, C. Buescher, J. Appl. Phys. 66 (1989) 5837.
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[2] L.B. Freud, J. Appl. Phys. 68 (1990) 2073. [3] E.P. Kvan, F. Namavar, Appl. Phys. Lett. 58 (1991) 2357. [4] E.A. Fitzgerald, Y.H. Xie, M.L. Green, D. Brasen, A.R. Kortan, J. Michel, Y.J. Mii, B.E. Weir, Appl. Phys. Lett. 59 (1991) 811. [5] F.K. LeGoues, B.S. Meyerson, J.F. Morar, Phys. Rev. Lett. 66 (1991) 2903. [6] C.S. Peng, Z.Y. Zhao, H. Chen, J.H. Li, Y.K. Li, L.W. Guo, D.Y. Dai, Q. Huang, J.M. Zhou, Y.H. Zhang, T.T. Sheng, C.H. Tung, Appl. Phys. Lett. 72 (1998) 3160. [7] J.H. Li, C.S. Peng, Y. Wu, D.Y. Dai, J.M. Zhou, Z.H. Mai, Appl. Phys. Lett. 71 (1997) 3132. [8] H. Chen, L.W. Guo, Q. Cui, Q. Hu, Q. Huang, J.M. Zhou, J. Appl. Phys. 79 (1996) 1167. [9] D.D. Perovic, G.C. Weatherly, J.P. Noel, D.C. Houghton, J. Vac. Sci. Technol. B 9 (1991) 2034. [10] G.D. Watkins, J.W. Corbett, Phys. Rev. 138 (1965) A543. [11] J.P. Hirth, J. Lothe, Theory of Dislocations, 2nd Edition, p. 612. [12] Cs. Szeles, P. Asoka-Kumar, K.G. Lynn, H.-J. Gossmann, F.C. Unterwald, T. Boone, Appl. Phys. Lett. 66 (1995) 2855. [13] P.M. Mooney, J.L. Jordan-Sweet, K. Ismail, J.O. Chu, R.M. Feenstra, F.K. LeGoues, Appl. Phys. Lett. 67 (1995) 2373.
The formation of dislocations in the interface of GeSi/low-temperature Si buffer grown on Si (0 0 1) C.S. Peng*, Y.K. Li, Q. Huang, J.M. Zhou Institute of Physics & Center of Condensed Matter Physics, Chinese Academy of Sciences, PO Box 603, Beijing 100080, People’s Republic of China
Abstract We have studied the mechanism of strain relaxation of Ge0.3Si0.7 alloy layers grown on low-temperature (LT) Si buffers by high-resolution X-ray diffraction and transmission electron microscopy (TEM). X-ray rocking-curve analyses show that the strain relaxation of an alloy layer is quite inhomogeneous at its early stage, and becomes homogeneous when the layer approaches becomes fully relaxed. High-resolution cross-section TEM analyses indicate that stacking faults have formed in the LT buffer near the interface of LT-Si/GeSi and have separated the mismatch dislocations in the interface of GeSi/LT-Si into Shockley partials. We propose that this could be due to the aggregation of the vacant defects in the LT-Si layer. The mechanism of this aggregation is the osmotic force created by the vacancy super-saturation in the LT-Si layer and/or the tension stress propagating from the interface of GeSi/LT-Si. # 2001 Elsevier Science B.V. All rights reserved. PACS: 61.72.Ff; 61.72.Nn; 81.15.Hi Keywords: A1. Defects; A1. X-ray diffraction; A3. Molecular beam epitaxy; B1. Germanium silicon alloys; B2. Semiconducting germanium; B2. Semiconducting silicon
Due to the variety of their band structure, GeSi/ Si heterostructures are used in high speed and large power devices, such as HBT, and MOSFET. Some of the electronic active layers have to be grown epitaxially on a relaxed GeSi buffer layer. Unfortunately, 4% of lattice mismatch between Ge and Si often results in high threading dislocation (TD) density even up to 1011 cm2 in the GeSi epi-layers grown on Si substrates. These TDs can penetrate into the active layers grown on such GeSi epi-layers, [1,2] and decrease largely the *Corresponding author. Tel.: +86-10-8264-9433; fax: +8610-8264-9457. E-mail address: [email protected] (C.S. Peng).
mobility of the carriers by serving as the scattering center. Therefore, efforts to decrease the TD density have attracted great attention. Among various techniques used, the compositionally graded buffer seems to be a very useful one by which the TD density can be reduced to as low as 106 cm2 with Ge fraction up to 30% [3–5]. However, such buffers are usually very thick and the surface corrugation is serious with an amplitude of up to 1520 nm, and the Ge composition can not be higher than 0.5 if the TDs density is less than 107 cm2. We have developed a new MBE growth technique, by which, a relaxed, thin, and uniform GeSi layer with low TDs density is grown on low-temperature Si (LT-Si) or LT-GeSi buffer
0022-0248/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 0 8 1 8 - 1
C.S. Peng et al. / Journal of Crystal Growth 227–228 (2001) 740–743
layers [6–8]. In this paper, the mechanism of the relaxation will be investigated in detail. Perovic et al. [9] discovered that there were large amounts of spherical microvoids in LT-Si. The defect density sharply increases toward the epilayer surface. At low growth temperature, the migration rate of the deposited atoms in the surface is too low to form a smooth surface and micro-holes are induced. These micro-holes are not covered perfectly by the subsequently deposited atoms and vacant defects are formed and frozen in the epilayer. During the epilayer growth, the surface roughness and the number of microholes increases. Therefore, the vacant defect density increases toward the surface. The migration barrier energy of mono-vacancy and divacancy are Hm1 =0.33 eV and Hm2 =1.3 eV, respectively [10]. In Silicon, the vacancy equilibrium density is: 1 Wn C0 ¼ exp ; ð1Þ na kT where na b3 ¼ 5:661023 cm3 and Wn 0:2 mb3 ¼ 5:625 eVare the atomic volume and vacancy formation energy, respectively. b is the value of the Burgers vector and m is Young’s modulus of Si [11]. For instance, at T=573 K (3008C), C0 = 5.88 1028 cm3, while the epitaxially grown vacancy density reaches C 2 1021 cm3 near the surface when the thickness of the LT-Si layer is 150 nm [12]. Then, the vacancy supersaturator is: C 3:41048 : ð2Þ C0 The osmotic energy of the vacancy caused by the supersaturator is: C G ¼ kT ln ¼ 5:52 eV ð4Hml ; Hm2 Þ: ð3Þ C0 Therefore, the vacancy osmotic energy is much larger than the migration barrier. The vacancies are very easy to aggregate to divacancies and large complexes before annihilation near dislocations and separate them into Shockley partials to form stacking faults. Such an aggregation would lower the vacancy chemical potential given by Eq. (3). Samples were grown by MBE with a V80S system. On a CM 200’ FEG microscope, crosssection transmission electron microscopy
741
(X-TEM) was used to observe the stacking faults and dislocation distribution. The strain status and alloy composition were determined from highresolution (HR) X-ray double-axis diffraction (XDD) measurements on a Philips X’Pert diffractometer. The structure of the samples is: Si (0 0 1) substrate+50 nm LT-Si+x (nm) Ge0.3Si0.7. The x value of samples ‘‘a’’–‘‘e’’ are 80, 100, 150, 300, and 500 nm, respectively. The growth condition was described in detail previously [7]. Results for the strain relaxation were summarized in Ref. [7], which shows that the relaxation is observable for Ge0.3Si0.7 layer thickness beyond 80 nm. This thickness is almost the same for Ge0.3Si0.7 grown in the same condition without the LT-Si layer. For the 80 nm thick layer, evidence of the onset of relaxation is observed. Further, the sample with 500 nm thick Ge0.3Si0.7 layer is already close to full relaxation (R=90%). Such a thickness is much smaller than that of compositionally graded buffer systems [13]. Due to the very large super-saturation of vacancies in LTSi layer, the vacancy osmotic energy is much larger than the migration barrier. Therefore, vacant defects are very easy to aggregate together to separate the 608 mismatch dislocation (MD) formed by GeSi relaxed on Si into Shockley partials and form stacking faults. Fig. 1 shows the HRXDD (224) rocking curves of those five samples; theoretically simulated (2 2 4) rocking curves for sample a have also been plotted (dotted lines). In curve (a) the main part of the peak of the GeSi layer is symmetrical and fits quite well to the simulation, suggesting that a major part of the alloy layer remains pseudomorphic. Whereas, the tail of the epilayer peak shows that a small portion of the alloy epilayer is already (partly) relaxed, the relaxation is quite inhomogeneous. With the increase of the GeSi layer thickness (curves (b) and (c)), the asymmetric part of the layer peak expands, while the symmetrical part of the peak shrinks. Further increase of the epilayer thickness makes the whole peak broader and finally more symmetric (curves (d) and (e)), suggesting that the relaxation of the thickest alloy layer turns out to be more homogeneous.
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C.S. Peng et al. / Journal of Crystal Growth 227–228 (2001) 740–743
Fig. 1. X-ray (2 2 4) rocking curves of the five samples with the Ge0.3Si0.7 epilayer thickness of 80, 100, 150, 300 and 500 nm, respectively. The dotted lines in both panels are theoretical simulations of curve (a).
Fortunately, such a result of relaxation is further confirmed by TEM measurements. Fig. 2(a) and (b) show the HRX-TEM images of samples ‘‘a’’ and ‘‘b’’, respectively. When the Si0.7Ge0.3 layer thickness reaches 80 nm, stacking faults are induced in (1 1 1) in LT-Si layer near the interface of LT-Si/Si0.7Ge0.3, and then grow into the LT-Si layer. In LT-Si layer, the density of vacancy near the interface of LT-Si/Si0.7Ge0.3 is the largest [12]. Under the osmotic force and the tensile stress, the vacancies near the interface aggregate together in (1 1 1) near the 608 MDs in the interface of LT-Si/GeSi to separate them into Shockley partials and form stacking faults. At the beginning, the number of 608 MDs is small and Shockley partials and their stacking faults are few, and a large part of the interface remains strained. Therefore, the relaxation is inhomogeneous. With further growth, the scale of 608 MDs and Shockley extend dislocations (SEDs) become larger and the distribution becomes more uniform. Finally, as the Si0.7Ge0.3 layer relaxes fully, the relaxation turns out to be homogeneous. Fig. 3 shows the TEM cross-section image of the almost fully relaxed sample ‘‘e’’. There are pairs of dislocations in the two intersect lines by (1 1 1) and the two interfaces between the LT-Si layer (GeSi/LT-Si/Si). The mismatch dislocation distribution is uniform and periodic.
Fig. 2. The HR-TEM images for samples a (a) and b (b). There are stacking faults in {1 1 1} in LT-Si layer near the GeSi/LT-Si interface. The number of them increases and they are grown deeply into LT-Si layer during the growth.
Fig. 3. The X-TEM image for sample (e). There are pairs of dislocations in the intersection of {1 1 1} and two GeSi/LT-Si/Si interfaces. These pairs arrange periodically along these two interfaces.
Due to the very large super-saturation of vacancies in LT-Si, the vacancy osmotic force becomes very large. The vacant defects are very easy to aggregate together to form SEDs and only need very small external force (such as a mismatch
C.S. Peng et al. / Journal of Crystal Growth 227–228 (2001) 740–743
force near 608 MDs). As a result, 608 MDs disappear fast and form the force to enlarge the scale of MDs. Therefore, the thickness of fully strained GeSi alloy layer grown on LT-Si is much smaller than that on normal Si substrate. In this paper, we have studied the mechanism of the nucleation of the mismatch dislocation in the interface of GeSi alloy layer and LT-Si layer. The strain relaxation is quite inhomogeneous from the beginning, and becomes homogeneous when the Ge0.3Si0.7 layer is relaxed fully. Stacking faults generated and formed the mismatch dislocations in the interface of GeSi/LT-Si. It is due to the aggregation of the vacant defects in the LT-Si layer. Because of the large vacancy density, the critical thickness of strained GeSi alloy layer grown on LT-Si is much smaller than that on normal Si substrate.
References [1] R. Hull, J.C. Bean, C. Buescher, J. Appl. Phys. 66 (1989) 5837.
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[2] L.B. Freud, J. Appl. Phys. 68 (1990) 2073. [3] E.P. Kvan, F. Namavar, Appl. Phys. Lett. 58 (1991) 2357. [4] E.A. Fitzgerald, Y.H. Xie, M.L. Green, D. Brasen, A.R. Kortan, J. Michel, Y.J. Mii, B.E. Weir, Appl. Phys. Lett. 59 (1991) 811. [5] F.K. LeGoues, B.S. Meyerson, J.F. Morar, Phys. Rev. Lett. 66 (1991) 2903. [6] C.S. Peng, Z.Y. Zhao, H. Chen, J.H. Li, Y.K. Li, L.W. Guo, D.Y. Dai, Q. Huang, J.M. Zhou, Y.H. Zhang, T.T. Sheng, C.H. Tung, Appl. Phys. Lett. 72 (1998) 3160. [7] J.H. Li, C.S. Peng, Y. Wu, D.Y. Dai, J.M. Zhou, Z.H. Mai, Appl. Phys. Lett. 71 (1997) 3132. [8] H. Chen, L.W. Guo, Q. Cui, Q. Hu, Q. Huang, J.M. Zhou, J. Appl. Phys. 79 (1996) 1167. [9] D.D. Perovic, G.C. Weatherly, J.P. Noel, D.C. Houghton, J. Vac. Sci. Technol. B 9 (1991) 2034. [10] G.D. Watkins, J.W. Corbett, Phys. Rev. 138 (1965) A543. [11] J.P. Hirth, J. Lothe, Theory of Dislocations, 2nd Edition, p. 612. [12] Cs. Szeles, P. Asoka-Kumar, K.G. Lynn, H.-J. Gossmann, F.C. Unterwald, T. Boone, Appl. Phys. Lett. 66 (1995) 2855. [13] P.M. Mooney, J.L. Jordan-Sweet, K. Ismail, J.O. Chu, R.M. Feenstra, F.K. LeGoues, Appl. Phys. Lett. 67 (1995) 2373.