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Step-bunching in SiGe layers and superlattices on Si(001)
Surface Science 532–535 (2003) 721–726 www.elsevier.com/locate/susc
Step-bunching in SiGe layers and superlattices on Sið0 0 1Þ M. M€ uhlberger *, C. Schelling, G. Springholz, F. Sch€ affler Institut f€ ur Halbleiter- und Festk€orperphysik, Johannes Kepler Universit€at, Altenbergerstr. 69, 4040 Linz, Austria
Abstract The vicinal Si(0 0 1) surfaces exhibit a bunching instability of the atomic height steps. Here we present a study to investigate the effect of germanium on the kinetic instability which occurs in silicon homoepitaxy. For the growth parameters employed we find no evidence for strain-induced step-bunching neither in single Si1x Gex layers nor in Si/ Si1x Gex superlattices. The effect of germanium is to kinetically suppress the formation of step-bunches. No significant influence of the Ge-related strain on the surface morphology can be found. The effects of growth temperature and the influence of the amount of deposited pure Si are very pronounced. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Silicon; Germanium; Step formation and bunching; Molecular beam epitaxy
1. Introduction The control of the quality of semiconductor heterostructure interfaces is of crucial importance for todayÕs semiconductor industry. Mostly smooth heterointerfaces are required, as fluctuations in quantum confinement and interface roughness degrade device performance [1,2]. On the other hand the use of self-organized nanostructures offers interesting possibilities [3]. The origin of such 1D and 0D structures is usually attributed to the strain in lattice mismatched heteroepitaxy [4–6], but also growth kinetics plays an important role [7–11]. Although the Si/SiGe heterosystem is just about to gain industrial relevance there are numerous
* Corresponding author. Tel.: +43-732-2468-9607; fax: +43732-2468-8650. E-mail address: [email protected] (M. M€ uhlberger).
studies dealing with growth of low-dimensional nanostructures. We here concentrate on the stepbunching instability on vicinal Si(0 0 1) substrates. In earlier studies dealing with single Si1x Gex layers [12–14] and Si/Si1x Gex superlattices [15] the formation of a rippled surface was explained with an interplay between strain and growth kinetics. Tersoff et al. found theoretically that compressively in-plane-strained layers can minimize their free energy by a step-bunching instability [5], i.e. strain-induced step-bunching, which was often used to explain various growth phenomena on vicinal Si substrates. We found [10,11] that already in Si homoepitaxy a step-bunching instability occurs which results in a surface morphology which is indistinguishable from the ones previously reported. In this study we deal with the influence of germanium on this kinetic step-bunching mechanism. The influence of germanium on growth in the Si/SiGe heterosystem is mainly twofold: on the one hand it changes the surface kinetics with respect to
0039-6028/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0039-6028(03)00232-2
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M. M€uhlberger et al. / Surface Science 532–535 (2003) 721–726
pure Si homoepitaxy and on the other hand strain is introduced in the system via the lattice mismatch of 4.2% between Si and Ge. This paper deals with three types of samples thick Si1x Gex to investigate this problem: 1000 A layers with 0 6 x 6 0:25, 25 A thick Si1x Gex layers with x ¼ 0:5, as well as Si/Si1x Gex superlattices
(SLs) with 0 6 x 6 0:4. Unless otherwise stated, all samples were grown by MBE on substrates with a miscut of 0.66° in [1 1 0] direction. Prior to growth an HF-free RCA cleaning procedure and in situ oxide desorption at above 1000 °C were employed. AFM images of all samples were taken immediately after growth on air in contact mode.
thick Si1x Gex layers with increasing Ge content. For the AFM images the Fig. 1. Evolution of ripple height and period for 1000 A miscut direction of the substrate is indicated by the arrow.
M. M€uhlberger et al. / Surface Science 532–535 (2003) 721–726
2. 1000 A of Si1x Gex Choosing low Ge contents of 1%, 3% and 8% it thick layers and is easily possible to grow 1000 A to compare them with homoepitaxial layers grown under the same conditions. The growth rate was /s for all samples. The kinetic step-bunching 0.5 A instability in Si shows a strong temperature dependence with a pronounced maximum around 490 °C [10,11]. But already the addition of a small amount of Ge drastically decreases the ripple height and period. Fig. 1 shows a plot of ripple height and period vs. Ge content and AFM images of the samples. It can be seen that already at 3% the ripples can barely be spotted whereas at 8% the surface is practically free from any regular ripple structure due to step-bunching. In addition to growth at 490 °C samples with a Ge content of 9% were grown at lower temperatures of 450 and 400 °C to exclude a simple temperature shift of the maximum of ripple formation due to Ge-induced modifications in the growth kinetics. But at this
723
Table 1 ]/ height [A ] of the surface features in SL set 1 Mean period [A 0% 5% 25% 40%
460 °C
490 °C
520 °C
3009/14 2639/13 2497/7.3 2462/7.7
5642/29.6 5116/27 4516/20.8 4573/20.6
16412/3 )/3 )/3 )/3
The growth temperature of the superlattice is given on the top, the Ge content on the left.
composition the growth temperature obviously has no influence on the surface morphology. A /s with x ¼ 0:25 sample grown at 490 °C and 0.5 A shows a similar surface as the sample with x ¼ 0:08 and is not included in the figure. Due to critical thickness limitations it is not possible to extend this series to higher Ge contents. The suppression of step-bunches with increasing Ge content is a drastic effect and can be attributed to changes in surface reconstruction [16,17], which is important for adatom diffusion and incorporation at step edges. A higher Ge
Si/300 A Si1x Gex ] superlattices grown at various growth temperatures (indicated on the right) and Fig. 2. AFM images of 10 [30 A Ge contents (indicated on the top). The miscut direction is given by the arrow on the right side. All images are 5 5 lm2 in size.
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content leads under these kinetically limited growth conditions to smoother layers. The strain in the layer does not result in step-bunching in the temperature and strain range investigated here.
3. Si/Si1x Gex superlattices In previous studies [12,15] SLs have been investigated without taking into account that the Si interlayers exhibit kinetic step-bunching as we know now. In this paper we try to take a closer look at the kinetic effects and the strain. With our SLs we wanted to investigate two things: What is the influence of the strain in the Si1x Gex quantum well on the step-bunching and how does the ripple morphology evolve during SL growth in comparison with Si homoepitaxy? In a first series we tried to separate between the kinetic and possible strain-related contributions to the step-bunching phenomenon in superlattices. To do this we systematically varied the strain and Si1x Gex / the growth temperature for 10 [30 A Si] SLs. They were grown at 450, 490 and 300 A 520 °C with a Ge content of x ¼ 0, 0.05, 0.25 and 0.4. The total grown rate was kept constant at 0.2 /s during the whole superlattice, which was deA /s, 750 °C). posited on a flat Si-buffer layer (0.8 A Table 1 summarizes the characteristics of the surface ripples as measured by AFM. Fig. 2 compiles 5 5 lm2 AFM images of all the samples. Under all growth conditions stepbunching is most pronounced at 490 °C regardless of the Ge content. Decreasing x from 40% to 25% does not result in changes of the surface morphology. Only going to very small values of x or even x ¼ 0 (pure Si) results in somewhat more pronounced ripples. This is in agreement with the findings for the single Si1x Gex layers where also the addition of Ge reduced ripple period and height. In this case the reduction must be mostly due to the segregation of Ge from the Si1x Gex quantum well, which is near its maximum at the growth temperature employed [18]. Furthermore it is known that thin Si1x Gex layers basically replicate the structure of the underlying layer [19]. This provides strong evidence that the main influence of the germanium on the ripple morphology is of
Table 2 Sample parameters for superlattice series 2 Thickness of Si deposited @ (490 °C, /s) 0.2 A
Number of SL periods
Ripple ] height [A
Ripple ] period [A
438 870 882 1480 2730 3000 5850
3 6 3 10 7 10 15
1.7 1 42 11 5 16.8 11 13.6 5 20.8 5 21.5 5
2800 700 2810 700 2940 500 3370 500 4010 200 4570 200 5020 200
The thickness of the Si-layers was determined by fitting X-ray rocking curves.
Fig. 3. Comparison of step-bunching in Si homoepitaxy (dotted curve) and in Si/Si1x Gex superlattices (solid curve). Mean period and mean ripple height as a function of Si-layer thickness for a growth temperature of 490 °C and a growth rate of /s. 0.2 A
M. M€uhlberger et al. / Surface Science 532–535 (2003) 721–726
kinetic origin. The less Ge is present, the stronger the step-bunches appear. Strain which is associated with the pseudomorphic Si1x Gex layers has no obvious influence on the surface morphology of the topmost layer. In the second series of SL samples we stopped growth after different number of SL-periods for Si0:6 Ge0:4 /150 A Si and 30 A two kinds of SLs: 30 A Si. All these SLs were grown in Si0:6 Ge0:4 /300 A the maximum of kinetic step-bunching for Si, i.e. /s which at 490 °C and at a growth rate of 0.2 A was again kept constant throughout the SL. Detailed sample parameters are summarized in Table 2. When compared with the behaviour of pure Si, a similar thickness dependence can be found. Fig. 3 compiles results for Si-homoepitaxy taken from Ref. [20] with the findings from the superlattices. The mean period and mean ripple height are plotted against the integrated thickness of Si de/s. Since thin single posited at 490 °C and 0.2 A Si1x Gex layers do not show any tendency to step-
725
bunching [19] their contribution to the total layer thickness has been omitted. In agreement with previous results from SL series 1 the fitting curve for the SL data lies below the homoepitaxy data, but the fit can be performed using the same critical exponents: 0.5 for the development of the ripple height and 0.25 for the evolution of the ripple period. This clearly indicates that again the growth kinetics plays the important role in the formation of step-bunches. The germanium introduced by the quantum wells of the SL diminishes the formation of step-bunches via its segregation and influence on the surface kinetics of silicon. The development of step-bunches is exclusively caused by the Si layers in the SL. 4. 25 A of Si0:5 Ge0:5 While the strain in the Si1x Gex layers does not result in the formation of step-bunches it leads to the formation of hut clusters in situations closer to
thick Si0:5 Ge0:5 layers. The substrate miscut is 4.34° in [1 0 0] direction for samples pictures a, b, c Fig. 4. Surface morphology of 25 A and 0.66° in [1 1 0] for picture d. The miscut direction is indicated by the arrow. Sample (a) is as grown (Tgrowth ¼ 550 °C), sample (b) was left at Tgrowth for 1 h and samples (c) and (d) were grown at 650 °C. All images are 1 1 lm2 in size.
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thermal equilibrium. Fig. 4 shows a collection of four AFM images, each 1 1 lm2 in size, of 25 A thick Si0:5 Ge0:5 layers on samples with a miscut of 4.34° in [1 1 0] direction (Fig. 4a–c) and 0.66° in [1 1 0] direction (Fig. 4d). For the first three samples the miscut is high enough to allow ripple formation already during growth (a). But these ripples are not thermodynamically stable and disintegrate into elongated hut clusters upon leaving the sample for 1 h at the growth temperature of 550 °C (b). Growing at higher temperatures directly results in elongated hut clusters (c) with a tendency towards a square base. The same happens for sample (d) with the smaller miscut. For this sample no regular ripples can be seen in the as grown situation [19].
5. Conclusions In conclusion we find that germanium influences the step-bunching in Si due to its segregation but not due to the strain introduced because of the lattice mismatch. Step-bunching is hindered at low Ge-concentrations and completely suppressed for x larger than approximately 0.08. Also in Si/ Si1x Gex superlattices the segregation of Ge leads to a weakening of the step-bunching instability. No influence of the strain on the surface morphology can be found. If and how the vertical ordering of the step-bunches is affected is not yet clear and topic of ongoing X-ray investigations. Thin single Si0:5 Ge0:5 layers close to thermal equilibrium disintegrate into hut clusters and do not show the ripple morphology typical for stepbunching.
References [1] Y.H. Xie, D. Monroe, E.A. Fitzgerald, P.J. Silverman, F.A. Thiel, G.P. Watson, Appl. Phys. Lett. 63 (1993) 2263. [2] M.A. Sadeghzadeh, A.I. Horrell, A.O. Mironov, E.H.C. Parker, T.E. Whall, Appl. Phys. Lett. 76 (2000) 2568. [3] For a review see D. Bimberg, M. Grundmann, N.N. Ledentsov, Quantum Dot Heterostructures, Wiley, Chichester, 1999. [4] J. Zhu, K. Brunner, G. Abstreiter, O. Kienzle, F. Ernst, M. R€ uhle, Phys. Rev. B 60 (1999) 10935. [5] J. Tersoff, Y.H. Phang, Z. Zang, M.G. Lagally, Phys. Rev. Lett. 75 (1995) 2730. [6] A.J. Pidduck, D.J. Robbns, A.G. Cullis, W.Y. Leong, A.M. Pitt, Thin Solid Films 222 (1992) 78. [7] M.A. Cotta, R.A. Hamm, T.W. Stanley, S.N.G. Chu, R.L. Harriott, M.B. Panish, H. Temkin, Phys. Rev. Lett. 70 (1993) 4106. [8] B.J. Spencer, P.W. Vorhees, J. Tersoff, Phys. Rev. Lett. 84 (2000) 2449. [9] C. Duport, P. Nozieres, J. Villain, Phys. Rev. Lett. 74 (1995) 134. [10] C. Schelling, G. Springholz, F. Sch€affler, Phys Rev. Lett. 83 (1999) 995. [11] C. Schelling, G. Springholz, F. Sch€affler, Thin Solid Films 369 (2000) 1. [12] C. Teichert, Y.H. Phang, L.J. Peticolas, J.C. Bean, M.G. Lagally, in: M.C. Tringides (Ed.), Surface Diffusion: Atomistic and Collective Processes, NATO ASI Series, New York, 1997. [13] F. Liu, M.G. Lagally, Surf. Sci. 386 (1997) 169. [14] C. Teichert, J.C. Bean, M.G. Lagally, Appl. Phys. A 67 (1998) 675. [15] Y.H. Phang, C. Teichert, M.G. Lagally, L.J. Peticolas, J.C. Bean, E. Kasper, Phys. Rev. B 50 (1994) 14435. [16] F. Wu, X. Chen, Z. Zhang, M.G. Lagally, Phys. Rev. Lett. 74 (1995) 574. [17] B. Voigtl€ander, M. K€astner, Phys. Rev. B 60 (1999) R5121. [18] D.J. Godbey, J.V. Lill, J. Deppe, K.D. Hobert, Appl. Phys. Lett. 65 (1994) 711. [19] C. Schelling, M. M€ uhlberger, G. Springholz, F. Sch€affler, Phys. Rev. B 64 (2001) 041301(R). [20] M. M€ uhlberger, C. Schelling, G. Springholz, F. Sch€affler, Mater. Sci. Eng. B 89 (2002) 257.
Step-bunching in SiGe layers and superlattices on Sið0 0 1Þ M. M€ uhlberger *, C. Schelling, G. Springholz, F. Sch€ affler Institut f€ ur Halbleiter- und Festk€orperphysik, Johannes Kepler Universit€at, Altenbergerstr. 69, 4040 Linz, Austria
Abstract The vicinal Si(0 0 1) surfaces exhibit a bunching instability of the atomic height steps. Here we present a study to investigate the effect of germanium on the kinetic instability which occurs in silicon homoepitaxy. For the growth parameters employed we find no evidence for strain-induced step-bunching neither in single Si1x Gex layers nor in Si/ Si1x Gex superlattices. The effect of germanium is to kinetically suppress the formation of step-bunches. No significant influence of the Ge-related strain on the surface morphology can be found. The effects of growth temperature and the influence of the amount of deposited pure Si are very pronounced. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Silicon; Germanium; Step formation and bunching; Molecular beam epitaxy
1. Introduction The control of the quality of semiconductor heterostructure interfaces is of crucial importance for todayÕs semiconductor industry. Mostly smooth heterointerfaces are required, as fluctuations in quantum confinement and interface roughness degrade device performance [1,2]. On the other hand the use of self-organized nanostructures offers interesting possibilities [3]. The origin of such 1D and 0D structures is usually attributed to the strain in lattice mismatched heteroepitaxy [4–6], but also growth kinetics plays an important role [7–11]. Although the Si/SiGe heterosystem is just about to gain industrial relevance there are numerous
* Corresponding author. Tel.: +43-732-2468-9607; fax: +43732-2468-8650. E-mail address: [email protected] (M. M€ uhlberger).
studies dealing with growth of low-dimensional nanostructures. We here concentrate on the stepbunching instability on vicinal Si(0 0 1) substrates. In earlier studies dealing with single Si1x Gex layers [12–14] and Si/Si1x Gex superlattices [15] the formation of a rippled surface was explained with an interplay between strain and growth kinetics. Tersoff et al. found theoretically that compressively in-plane-strained layers can minimize their free energy by a step-bunching instability [5], i.e. strain-induced step-bunching, which was often used to explain various growth phenomena on vicinal Si substrates. We found [10,11] that already in Si homoepitaxy a step-bunching instability occurs which results in a surface morphology which is indistinguishable from the ones previously reported. In this study we deal with the influence of germanium on this kinetic step-bunching mechanism. The influence of germanium on growth in the Si/SiGe heterosystem is mainly twofold: on the one hand it changes the surface kinetics with respect to
0039-6028/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0039-6028(03)00232-2
722
M. M€uhlberger et al. / Surface Science 532–535 (2003) 721–726
pure Si homoepitaxy and on the other hand strain is introduced in the system via the lattice mismatch of 4.2% between Si and Ge. This paper deals with three types of samples thick Si1x Gex to investigate this problem: 1000 A layers with 0 6 x 6 0:25, 25 A thick Si1x Gex layers with x ¼ 0:5, as well as Si/Si1x Gex superlattices
(SLs) with 0 6 x 6 0:4. Unless otherwise stated, all samples were grown by MBE on substrates with a miscut of 0.66° in [1 1 0] direction. Prior to growth an HF-free RCA cleaning procedure and in situ oxide desorption at above 1000 °C were employed. AFM images of all samples were taken immediately after growth on air in contact mode.
thick Si1x Gex layers with increasing Ge content. For the AFM images the Fig. 1. Evolution of ripple height and period for 1000 A miscut direction of the substrate is indicated by the arrow.
M. M€uhlberger et al. / Surface Science 532–535 (2003) 721–726
2. 1000 A of Si1x Gex Choosing low Ge contents of 1%, 3% and 8% it thick layers and is easily possible to grow 1000 A to compare them with homoepitaxial layers grown under the same conditions. The growth rate was /s for all samples. The kinetic step-bunching 0.5 A instability in Si shows a strong temperature dependence with a pronounced maximum around 490 °C [10,11]. But already the addition of a small amount of Ge drastically decreases the ripple height and period. Fig. 1 shows a plot of ripple height and period vs. Ge content and AFM images of the samples. It can be seen that already at 3% the ripples can barely be spotted whereas at 8% the surface is practically free from any regular ripple structure due to step-bunching. In addition to growth at 490 °C samples with a Ge content of 9% were grown at lower temperatures of 450 and 400 °C to exclude a simple temperature shift of the maximum of ripple formation due to Ge-induced modifications in the growth kinetics. But at this
723
Table 1 ]/ height [A ] of the surface features in SL set 1 Mean period [A 0% 5% 25% 40%
460 °C
490 °C
520 °C
3009/14 2639/13 2497/7.3 2462/7.7
5642/29.6 5116/27 4516/20.8 4573/20.6
16412/3 )/3 )/3 )/3
The growth temperature of the superlattice is given on the top, the Ge content on the left.
composition the growth temperature obviously has no influence on the surface morphology. A /s with x ¼ 0:25 sample grown at 490 °C and 0.5 A shows a similar surface as the sample with x ¼ 0:08 and is not included in the figure. Due to critical thickness limitations it is not possible to extend this series to higher Ge contents. The suppression of step-bunches with increasing Ge content is a drastic effect and can be attributed to changes in surface reconstruction [16,17], which is important for adatom diffusion and incorporation at step edges. A higher Ge
Si/300 A Si1x Gex ] superlattices grown at various growth temperatures (indicated on the right) and Fig. 2. AFM images of 10 [30 A Ge contents (indicated on the top). The miscut direction is given by the arrow on the right side. All images are 5 5 lm2 in size.
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content leads under these kinetically limited growth conditions to smoother layers. The strain in the layer does not result in step-bunching in the temperature and strain range investigated here.
3. Si/Si1x Gex superlattices In previous studies [12,15] SLs have been investigated without taking into account that the Si interlayers exhibit kinetic step-bunching as we know now. In this paper we try to take a closer look at the kinetic effects and the strain. With our SLs we wanted to investigate two things: What is the influence of the strain in the Si1x Gex quantum well on the step-bunching and how does the ripple morphology evolve during SL growth in comparison with Si homoepitaxy? In a first series we tried to separate between the kinetic and possible strain-related contributions to the step-bunching phenomenon in superlattices. To do this we systematically varied the strain and Si1x Gex / the growth temperature for 10 [30 A Si] SLs. They were grown at 450, 490 and 300 A 520 °C with a Ge content of x ¼ 0, 0.05, 0.25 and 0.4. The total grown rate was kept constant at 0.2 /s during the whole superlattice, which was deA /s, 750 °C). posited on a flat Si-buffer layer (0.8 A Table 1 summarizes the characteristics of the surface ripples as measured by AFM. Fig. 2 compiles 5 5 lm2 AFM images of all the samples. Under all growth conditions stepbunching is most pronounced at 490 °C regardless of the Ge content. Decreasing x from 40% to 25% does not result in changes of the surface morphology. Only going to very small values of x or even x ¼ 0 (pure Si) results in somewhat more pronounced ripples. This is in agreement with the findings for the single Si1x Gex layers where also the addition of Ge reduced ripple period and height. In this case the reduction must be mostly due to the segregation of Ge from the Si1x Gex quantum well, which is near its maximum at the growth temperature employed [18]. Furthermore it is known that thin Si1x Gex layers basically replicate the structure of the underlying layer [19]. This provides strong evidence that the main influence of the germanium on the ripple morphology is of
Table 2 Sample parameters for superlattice series 2 Thickness of Si deposited @ (490 °C, /s) 0.2 A
Number of SL periods
Ripple ] height [A
Ripple ] period [A
438 870 882 1480 2730 3000 5850
3 6 3 10 7 10 15
1.7 1 42 11 5 16.8 11 13.6 5 20.8 5 21.5 5
2800 700 2810 700 2940 500 3370 500 4010 200 4570 200 5020 200
The thickness of the Si-layers was determined by fitting X-ray rocking curves.
Fig. 3. Comparison of step-bunching in Si homoepitaxy (dotted curve) and in Si/Si1x Gex superlattices (solid curve). Mean period and mean ripple height as a function of Si-layer thickness for a growth temperature of 490 °C and a growth rate of /s. 0.2 A
M. M€uhlberger et al. / Surface Science 532–535 (2003) 721–726
kinetic origin. The less Ge is present, the stronger the step-bunches appear. Strain which is associated with the pseudomorphic Si1x Gex layers has no obvious influence on the surface morphology of the topmost layer. In the second series of SL samples we stopped growth after different number of SL-periods for Si0:6 Ge0:4 /150 A Si and 30 A two kinds of SLs: 30 A Si. All these SLs were grown in Si0:6 Ge0:4 /300 A the maximum of kinetic step-bunching for Si, i.e. /s which at 490 °C and at a growth rate of 0.2 A was again kept constant throughout the SL. Detailed sample parameters are summarized in Table 2. When compared with the behaviour of pure Si, a similar thickness dependence can be found. Fig. 3 compiles results for Si-homoepitaxy taken from Ref. [20] with the findings from the superlattices. The mean period and mean ripple height are plotted against the integrated thickness of Si de/s. Since thin single posited at 490 °C and 0.2 A Si1x Gex layers do not show any tendency to step-
725
bunching [19] their contribution to the total layer thickness has been omitted. In agreement with previous results from SL series 1 the fitting curve for the SL data lies below the homoepitaxy data, but the fit can be performed using the same critical exponents: 0.5 for the development of the ripple height and 0.25 for the evolution of the ripple period. This clearly indicates that again the growth kinetics plays the important role in the formation of step-bunches. The germanium introduced by the quantum wells of the SL diminishes the formation of step-bunches via its segregation and influence on the surface kinetics of silicon. The development of step-bunches is exclusively caused by the Si layers in the SL. 4. 25 A of Si0:5 Ge0:5 While the strain in the Si1x Gex layers does not result in the formation of step-bunches it leads to the formation of hut clusters in situations closer to
thick Si0:5 Ge0:5 layers. The substrate miscut is 4.34° in [1 0 0] direction for samples pictures a, b, c Fig. 4. Surface morphology of 25 A and 0.66° in [1 1 0] for picture d. The miscut direction is indicated by the arrow. Sample (a) is as grown (Tgrowth ¼ 550 °C), sample (b) was left at Tgrowth for 1 h and samples (c) and (d) were grown at 650 °C. All images are 1 1 lm2 in size.
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thermal equilibrium. Fig. 4 shows a collection of four AFM images, each 1 1 lm2 in size, of 25 A thick Si0:5 Ge0:5 layers on samples with a miscut of 4.34° in [1 1 0] direction (Fig. 4a–c) and 0.66° in [1 1 0] direction (Fig. 4d). For the first three samples the miscut is high enough to allow ripple formation already during growth (a). But these ripples are not thermodynamically stable and disintegrate into elongated hut clusters upon leaving the sample for 1 h at the growth temperature of 550 °C (b). Growing at higher temperatures directly results in elongated hut clusters (c) with a tendency towards a square base. The same happens for sample (d) with the smaller miscut. For this sample no regular ripples can be seen in the as grown situation [19].
5. Conclusions In conclusion we find that germanium influences the step-bunching in Si due to its segregation but not due to the strain introduced because of the lattice mismatch. Step-bunching is hindered at low Ge-concentrations and completely suppressed for x larger than approximately 0.08. Also in Si/ Si1x Gex superlattices the segregation of Ge leads to a weakening of the step-bunching instability. No influence of the strain on the surface morphology can be found. If and how the vertical ordering of the step-bunches is affected is not yet clear and topic of ongoing X-ray investigations. Thin single Si0:5 Ge0:5 layers close to thermal equilibrium disintegrate into hut clusters and do not show the ripple morphology typical for stepbunching.
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