- Email: [email protected]
Evolution of strained Ge islands grown on Si(111): a scanning probe microscopy study
PERGAMON
Solid State Communications 112 (1999) 145–149 www.elsevier.com/locate/ssc
Evolution of strained Ge islands grown on Si(111): a scanning probe microscopy study G. Capellini a,*, N. Motta b, A. Sgarlata b, R. Calarco c a
b
Dipartimento di Fisica “E. Amaldi” and Unita` I.N.F.M., Universita` di Roma TRE, V. Vasca Navale 84, 00146 Rome, Italy Dipartimento di Fisica and Unita` I.N.F.M., Universita` di Roma Tor Vergata, V. della Ricerca Scientifica 1, 00133 Rome, Italy c Istituto per l’Elettronica dello Stato Solido, CNR Via Cineto Romano 42, 00156 Rome, Italy Received 28 October 1998; received in revised form 21 June 1999; accepted 21 June 1999 by E. Molinari
Abstract We have followed the evolution of strained Ge/Si(111) Stranski–Krastanov islands by atomic force and scanning tunneling microscopies. Following the morphological evolution during the annealing of the samples we were able to recognize the key features of the relaxation process in these structures. The introduction of edge misfit dislocations after a critical thickness, and the inhomogeneous strain field inside the islands, lead to an intra-island ripening mechanism. We show that this mechanism changes the island shape from truncated tetrahedron to “atoll-like”. q 1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Semiconductors; A. Nanostructures; C. Scanning tunnelling microscopy
1. Introduction and experimental Quantum confined semiconductor heterostructures are very attractive both from the experimental and theoretical points of view [1,2]. The demonstrated capability of tailoring the optical and electronic properties of such heterostructures make them very well suited for the realization of optoelectronic devices which exploit the quantum effects. This is much more evident in the case of the group IV semiconductors which are indirect bandgap materials, exhibiting thus poor luminescence efficiency. On the path towards the achievement of the necessary optical performances, the quantum dots (QDs) are expected to give a significant increase of the quantum efficiency [3]. The self-assembly of island seems to be the most promising way to produce zero-dimensional confined systems. A possible way to achieve self-organization is to take advantage of the spontaneous formation of clusters under the Stranski–Krastanov (SK) growth conditions avoiding expensive lithographic patterning. In this way it is possible to produce QDs of good quality with a narrow distribution of sizes and with an increase of the optical quantum efficiency * Corresponding author. E-mail address: [email protected] (G. Capellini)
[4–6]. However, a complete understanding of the SK growth dynamic is still a long way off. The heteroepitaxial Ge/Si alloys seem to be very promising materials for microelectronic and optoelectronic applications [2,7]. The strain resulting from the 4.2% difference between Si and Ge lattices can be turned into an advantage when growing quantum structures for optoelectronic applications. Therefore the connection between the strain and the islands formation and their successive evolution is a key point to clarify. In this paper we study the strain relaxation and clustering of Ge grown on Si(111) via reactive deposition epitaxy by means of scanning tunneling microscopy (STM), atomic force microscopy (AFM), and reflection high energy electron diffraction (RHEED). Samples were grown in a UHV chamber (base pressure 2 × 10210 Torr) where RHEED, STM, CMA-Auger spectrometer and different evaporation sources are available [8]. The scanning tunneling microscope (TOPS system, WA Technology) is equipped with a tube scanner (maximum scanning area 2 mm). AFM measurements were performed in contact mode with a Park Scientific Instruments CP Microscope equipped with a high aspect ratio conical (808 sidewall angle) Ultralevere tip with 5 nm minimum radius. We have done standard tests and scan calibrations on freshly cleaved mica in order to obtain atomic imaging and to avoid
0038-1098/99/$ - see front matter q 1999 Elsevier Science Ltd. All rights reserved. PII: S0038-109 8(99)00316-6
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G. Capellini et al. / Solid State Communications 112 (1999) 145–149
Fig. 1. (a) 4 × 4 mm2 AFM image of 4 nm Ge deposition annealed for 5 min at 5008C. The [110] sample direction has been indicated. (b) 2 × 2 mm2 AFM image of 4 nm Ge deposition annealed for 30 min at 5008C. Total height range is 45 nm. The three labels denote strained, intermediate and relaxed. Note the rounding of the corner of the I island due to the introduction of MDs and the “atoll-like” shape of island R.
G. Capellini et al. / Solid State Communications 112 (1999) 145–149
147
tip imaging misleading information. The Si 0:6 × 1 cm2 substrates were cut from n-type r ù 0:003 V cm Si(111) wafers. A reconstructed Si(111) 7 × 7 surface was obtained by repeatedly flashing the sample at 12508C by Joule heating with a current of ù9 A. Surface reconstruction and cleanliness were monitored by RHEED. The deposition of Ge on the Si substrates kept at 5008C was made from an effusion cell with low evaporation rates (0.2 ML/min, 1 ML 0:314 nm) as determined by a well-calibrated Inficon quartz balance. The thickness determined in this way agrees within 10% with the determination carried out by measuring the island volumes by AFM in the as-grown samples and including in the calculation the wetting-layer thickness (3 ML). A new Si sample was prepared for each Ge deposition in order to minimize the uncertainties of successive evaporations. In order to study the islands relaxation mechanism at a constant amount of material deposited, we performed growth interruption keeping the samples at the deposition temperature for different times. All the deposited layers were analysed in situ by STM. After the growth, the samples were extracted from the UHV system and measured by AFM.
Fig. 2. (a) Island heights distribution of the 5 min (square) and 30 min annealed (diamond) sample displayed in Fig. 1. (b) Ratio R between the top and bottom base sizes of the islands in the 5 min annealed sample. The vertical dashed line separates the A-type from the B-type islands.
2. Results and discussion In Fig. 1(a) we display an AFM image of a Ge/Si sample obtained by depositing 4 nm Ge, followed by a 5 min growth interruption. On the sample surface 3D islands are present. The islands are regular truncated tetrahedra [10], with base bounded by three k110l equivalent directions. All
Fig. 3. 0:5 × 0:5 mm2 STM image of a relaxed island. Note that the atoll feature is only 0.6 nm thick (2 ML). This indicates that the depletion of the central part of the islands occurs via a layer-by-layer mode. Insets: (a) STM image of the central part of the island displayed in the main panel of this figure revealing a 7 × 7 reconstruction; (b) STM image acquired on the border location indicated by the arrow: c 2 × 8; c 4 × 4 and 2 × 2 reconstruction are detected; (c) tetrahedral bulk defect showing up at the top facet of a B-type island.
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the islands are oriented with a corner pointing in the (112¯) direction. We never observed islands oriented in anti-parallel direction in a same sample. As already noted [9], this anisotropy in the islands shape is due to the anisotropy of the growth rate in the k112l directions. In Fig. 1(b) we report an AFM image of a sample with the same nominal thickness as that of the sample presented in Fig. 1(a) and annealed for 30 min at 5008C. In Fig. 2(a) we report the height distribution of the island sizes of the above-mentioned samples, obtained by performing measurements on about 200 islands. In the histogram of the 5 min annealed sample we can recognize a peak (A) centred around 48 nm and a broad structure (B) centred around 20 nm. In the 30 min annealed sample we can observe a narrowing of the B band in just one peak, and a shift toward lower height of features A and B. A close scrutiny of the islands in Fig. 1(a), based on a line scan analysis, allows to connect peak positions with island shapes. The direct estimation of the facet orientation was performed by calculating the gradient in each point of the 256 × 256 mesh used to acquire the AFM topographies. The islands whose height is in the range of the peak A are {113} faceted (contact angle u 28 ^ 28) with a (111) top facet (7 × 7 reconstructed). The top facet is approximately 75% smaller than the base of the island and it is bounded by the same k110l directions. Decreasing the height of the islands (peak B) we note a reduction of the aspect ratio, facet angle and an increase of the ratio R between the top and the bottom base (Fig. 2(b)). Moreover the appearance of small facets bounded by k112l directions in the growing (111) plane rounds the island corners, see Fig. 1(b). In the thinnest B-type islands the central part of the top facets is often eroded, leading to “atoll-like” shapes (see island R in Figs. 1(b) and 3). These experimental results clarify the island growth evolution. Increasing the deposited material the islands grow via a Stranski–Krastanov self-assembling mechanism as truncated tetrahedra {113} faceted. It is worth noting that this orientation is the stable one for the Ge islanding [5], being a local minimum of the surface energy [11]. The islands grow until they reach a critical thickness hc 48 nm (the limiting value of the observed height distribution). In agreement with the Ge/Si(100) case [5], we interpret hc as the critical thickness for the introduction of misfit dislocations (MDs). In fact it has been reported that when the strain energy stored in the island is too high, misfit dislocations are inserted in the k110l directions [12]. The MDs nucleation occurs at the island edges where the stress is accumulated. The regions where a MD has been inserted are stress free and thus are preferential site for the Ge attachment [5,13]. In our case we suggest that this mechanism drives the morphological evolution of the islands changing their shape from type A to type B during the annealing. The atoms that are bonded in the top of a strained island experience a stress that decreases the energy barrier for the detachment. On the contrary the sites where the stress has been inserted have a
higher binding energy: this difference on the energy gained by the system bonding an atom (i.e. an inhomogeneous chemical potential) generates a flow of atoms [4] that depletes the top of the island and increases the material deposited at its base. In that way we can have a “damping” of the island that increases the dimension of the top facet and decreases the contact angle of the facets in a way that agrees with our observations. During the annealing procedure we can observe the evolution of this relaxation in time keeping fixed the amount of Ge atoms deposited. Referring to Fig. 2(a) we see that the islands belonging to peak A are unaffected by the annealing process except for a slight decrease in volume due to the sublimation of Ge atoms. Moreover the 7 × 7 reconstruction of the island top indicates the presence of a strain field (such reconstruction has never been found on free Ge surfaces, while it has been evidenced in strained Ge layers [15]). On the contrary, B-type islands continue the relaxing process decreasing their height. In Fig. 3 we display an STM image of a relaxed island showing that the aforementioned atoll shape is due to the selective depletion of the central part. The distribution of the strain in the top facet is not homogeneous being higher in the central part of the facet and lower at the edge: higher the strain is, faster is the depletion, higher is the gain in free energy. The strain distribution is reflected by the different reconstructions that we can observe in the central part with respect to the borders of the atoll-like island top facets. At the island center (Fig. 3 inset (a)) we detect a 7 × 7 reconstruction that we believe, in agreement with Gossmann [15], to be induced by the residual compressive strain in the Ge layer. On the contrary, on the island border we found a miscellanea of the typical Ge superstructure (see inset (b) in Fig. 3), i.e. c 2 × 8; c 2 × 4 and 2 × 2 [16]. Moreover we want to point out that the analysed atoll-like islands are usually free of tetrahedral defects of the type reported by Ko¨hler [17]. These defects eventually appear in A- and Btype islands and do not seem to be related to any particular change of the island shape neither to the change of the island top facet reconstruction (see inset (c) in Fig. 3). Thus we found a coexistence of strained and relaxed islands in agreement with recent theoretical equilibrium calculations [14]. We observed by STM that the facets of the relaxed island grow from the base upward in a ledge-by-ledge way. This fact makes difficult to recognize a well-defined crystallographic orientation. We guess that because the MDs are inserted in a direction parallel to the island side, the shape of the islands is conserved at the beginning of the relaxation stage. However, the MDs can interact (at the corners the MDs can intersect) modifying the stress field in the substrate. Thus MDs can round the corner of the relaxed island at initial stage of the annealing, subsequently destroying the triangular shape of the base (see island I in Fig. 1(b)). This mechanism is in agreement with the changes that we observe in the island top facets. In conclusion we have followed the evolution of strained Ge/Si(111) islands. We described the relaxation mechanism
G. Capellini et al. / Solid State Communications 112 (1999) 145–149
recognizing an intra-island ripening due to the insertion of MDs at the island edges that promotes an atomic current from the higher strained region, located at the centre of the island tops, toward the base of the island itself. This relaxation process results in a rounding of the island shape, a decrease of the aspect ratio and a selective depletion of the central part of the islands leading to an “atolllike” shape. References [1] [2] [3] [4] [5]
F. Capasso, S. Datta, Phys. Today (1990) 74. T.P. Pearsall, Si–Ge for Optoelectronics, 1995 ed. Quillec. T. Takahagara, K. Takeda, Phys. Rev. B 46 (1992) 15 578. A.L. Barabasi, Appl. Phys. Lett. 70 (1997) 2565. G. Capellini, L. DiGaspare, F. Evangelisti, E. Palange, Appl. Phys. Lett. 70 (1997) 493. [6] M. Goryll, L. Vescan, K. Schmidt, S. Mesters, H. Lu¨th, K. Szot, Appl. Phys. Lett. 71 (1997) 410.
149
[7] MRS Bulletin of Mater. Res. Soc. 21(4) April (1996). [8] N. Motta, A. Sgarlata, R. Calarco, Q. Nguyen, F. Patella, J. Castro Cal, A. Balzarotti, M. De Crescenzi, Surf. Sci. 406 (1998) 254. [9] H. Minoda, Y. Tanishiro, N. Yamamoto, K. Yagi, Surf. Sci. 357/358 (1996) 418. [10] H. Voigtla¨nder, M. Zinner, Appl. Phys. Lett. 63 (1993) 3055. [11] D.J. Eaglesham, F.C. Unterwald, D.C. Jacobson, Phys. Rev. Lett. 70 (1993) 966. [12] F.K. Le Goues, M.C. Reuter, M. Hammar, R.M. Tromp, Surf. Sci. 349 (1996) 249. [13] F.K. Le Goues, M.C. Reuter, J. Tersoff, M. Hammar, R.M. Tromp, Phys. Rev. Lett. 73 (1994) 300. [14] I. Daruka, A.L. Barabasi, Phys. Rev. Lett. 79 (1997) 3708. [15] H.-J. Gossmann, J.C. Bean, L.C. Feldman, E.G. McRae, I.K. Robinson, Phys. Rev. Lett. 55 (1985) 1106. [16] S.K. Theiss, D.M. Chen, J.A. Golovchenko, Appl. Phys. Lett. 66 (1995) 448. [17] U. Ko¨hler, O. Jusko, G. Pietsch, B. Mu¨ller, M. Henzler, Surf. Sci. 248 (1991) 321.
Solid State Communications 112 (1999) 145–149 www.elsevier.com/locate/ssc
Evolution of strained Ge islands grown on Si(111): a scanning probe microscopy study G. Capellini a,*, N. Motta b, A. Sgarlata b, R. Calarco c a
b
Dipartimento di Fisica “E. Amaldi” and Unita` I.N.F.M., Universita` di Roma TRE, V. Vasca Navale 84, 00146 Rome, Italy Dipartimento di Fisica and Unita` I.N.F.M., Universita` di Roma Tor Vergata, V. della Ricerca Scientifica 1, 00133 Rome, Italy c Istituto per l’Elettronica dello Stato Solido, CNR Via Cineto Romano 42, 00156 Rome, Italy Received 28 October 1998; received in revised form 21 June 1999; accepted 21 June 1999 by E. Molinari
Abstract We have followed the evolution of strained Ge/Si(111) Stranski–Krastanov islands by atomic force and scanning tunneling microscopies. Following the morphological evolution during the annealing of the samples we were able to recognize the key features of the relaxation process in these structures. The introduction of edge misfit dislocations after a critical thickness, and the inhomogeneous strain field inside the islands, lead to an intra-island ripening mechanism. We show that this mechanism changes the island shape from truncated tetrahedron to “atoll-like”. q 1999 Elsevier Science Ltd. All rights reserved. Keywords: A. Semiconductors; A. Nanostructures; C. Scanning tunnelling microscopy
1. Introduction and experimental Quantum confined semiconductor heterostructures are very attractive both from the experimental and theoretical points of view [1,2]. The demonstrated capability of tailoring the optical and electronic properties of such heterostructures make them very well suited for the realization of optoelectronic devices which exploit the quantum effects. This is much more evident in the case of the group IV semiconductors which are indirect bandgap materials, exhibiting thus poor luminescence efficiency. On the path towards the achievement of the necessary optical performances, the quantum dots (QDs) are expected to give a significant increase of the quantum efficiency [3]. The self-assembly of island seems to be the most promising way to produce zero-dimensional confined systems. A possible way to achieve self-organization is to take advantage of the spontaneous formation of clusters under the Stranski–Krastanov (SK) growth conditions avoiding expensive lithographic patterning. In this way it is possible to produce QDs of good quality with a narrow distribution of sizes and with an increase of the optical quantum efficiency * Corresponding author. E-mail address: [email protected] (G. Capellini)
[4–6]. However, a complete understanding of the SK growth dynamic is still a long way off. The heteroepitaxial Ge/Si alloys seem to be very promising materials for microelectronic and optoelectronic applications [2,7]. The strain resulting from the 4.2% difference between Si and Ge lattices can be turned into an advantage when growing quantum structures for optoelectronic applications. Therefore the connection between the strain and the islands formation and their successive evolution is a key point to clarify. In this paper we study the strain relaxation and clustering of Ge grown on Si(111) via reactive deposition epitaxy by means of scanning tunneling microscopy (STM), atomic force microscopy (AFM), and reflection high energy electron diffraction (RHEED). Samples were grown in a UHV chamber (base pressure 2 × 10210 Torr) where RHEED, STM, CMA-Auger spectrometer and different evaporation sources are available [8]. The scanning tunneling microscope (TOPS system, WA Technology) is equipped with a tube scanner (maximum scanning area 2 mm). AFM measurements were performed in contact mode with a Park Scientific Instruments CP Microscope equipped with a high aspect ratio conical (808 sidewall angle) Ultralevere tip with 5 nm minimum radius. We have done standard tests and scan calibrations on freshly cleaved mica in order to obtain atomic imaging and to avoid
0038-1098/99/$ - see front matter q 1999 Elsevier Science Ltd. All rights reserved. PII: S0038-109 8(99)00316-6
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G. Capellini et al. / Solid State Communications 112 (1999) 145–149
Fig. 1. (a) 4 × 4 mm2 AFM image of 4 nm Ge deposition annealed for 5 min at 5008C. The [110] sample direction has been indicated. (b) 2 × 2 mm2 AFM image of 4 nm Ge deposition annealed for 30 min at 5008C. Total height range is 45 nm. The three labels denote strained, intermediate and relaxed. Note the rounding of the corner of the I island due to the introduction of MDs and the “atoll-like” shape of island R.
G. Capellini et al. / Solid State Communications 112 (1999) 145–149
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tip imaging misleading information. The Si 0:6 × 1 cm2 substrates were cut from n-type r ù 0:003 V cm Si(111) wafers. A reconstructed Si(111) 7 × 7 surface was obtained by repeatedly flashing the sample at 12508C by Joule heating with a current of ù9 A. Surface reconstruction and cleanliness were monitored by RHEED. The deposition of Ge on the Si substrates kept at 5008C was made from an effusion cell with low evaporation rates (0.2 ML/min, 1 ML 0:314 nm) as determined by a well-calibrated Inficon quartz balance. The thickness determined in this way agrees within 10% with the determination carried out by measuring the island volumes by AFM in the as-grown samples and including in the calculation the wetting-layer thickness (3 ML). A new Si sample was prepared for each Ge deposition in order to minimize the uncertainties of successive evaporations. In order to study the islands relaxation mechanism at a constant amount of material deposited, we performed growth interruption keeping the samples at the deposition temperature for different times. All the deposited layers were analysed in situ by STM. After the growth, the samples were extracted from the UHV system and measured by AFM.
Fig. 2. (a) Island heights distribution of the 5 min (square) and 30 min annealed (diamond) sample displayed in Fig. 1. (b) Ratio R between the top and bottom base sizes of the islands in the 5 min annealed sample. The vertical dashed line separates the A-type from the B-type islands.
2. Results and discussion In Fig. 1(a) we display an AFM image of a Ge/Si sample obtained by depositing 4 nm Ge, followed by a 5 min growth interruption. On the sample surface 3D islands are present. The islands are regular truncated tetrahedra [10], with base bounded by three k110l equivalent directions. All
Fig. 3. 0:5 × 0:5 mm2 STM image of a relaxed island. Note that the atoll feature is only 0.6 nm thick (2 ML). This indicates that the depletion of the central part of the islands occurs via a layer-by-layer mode. Insets: (a) STM image of the central part of the island displayed in the main panel of this figure revealing a 7 × 7 reconstruction; (b) STM image acquired on the border location indicated by the arrow: c 2 × 8; c 4 × 4 and 2 × 2 reconstruction are detected; (c) tetrahedral bulk defect showing up at the top facet of a B-type island.
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the islands are oriented with a corner pointing in the (112¯) direction. We never observed islands oriented in anti-parallel direction in a same sample. As already noted [9], this anisotropy in the islands shape is due to the anisotropy of the growth rate in the k112l directions. In Fig. 1(b) we report an AFM image of a sample with the same nominal thickness as that of the sample presented in Fig. 1(a) and annealed for 30 min at 5008C. In Fig. 2(a) we report the height distribution of the island sizes of the above-mentioned samples, obtained by performing measurements on about 200 islands. In the histogram of the 5 min annealed sample we can recognize a peak (A) centred around 48 nm and a broad structure (B) centred around 20 nm. In the 30 min annealed sample we can observe a narrowing of the B band in just one peak, and a shift toward lower height of features A and B. A close scrutiny of the islands in Fig. 1(a), based on a line scan analysis, allows to connect peak positions with island shapes. The direct estimation of the facet orientation was performed by calculating the gradient in each point of the 256 × 256 mesh used to acquire the AFM topographies. The islands whose height is in the range of the peak A are {113} faceted (contact angle u 28 ^ 28) with a (111) top facet (7 × 7 reconstructed). The top facet is approximately 75% smaller than the base of the island and it is bounded by the same k110l directions. Decreasing the height of the islands (peak B) we note a reduction of the aspect ratio, facet angle and an increase of the ratio R between the top and the bottom base (Fig. 2(b)). Moreover the appearance of small facets bounded by k112l directions in the growing (111) plane rounds the island corners, see Fig. 1(b). In the thinnest B-type islands the central part of the top facets is often eroded, leading to “atoll-like” shapes (see island R in Figs. 1(b) and 3). These experimental results clarify the island growth evolution. Increasing the deposited material the islands grow via a Stranski–Krastanov self-assembling mechanism as truncated tetrahedra {113} faceted. It is worth noting that this orientation is the stable one for the Ge islanding [5], being a local minimum of the surface energy [11]. The islands grow until they reach a critical thickness hc 48 nm (the limiting value of the observed height distribution). In agreement with the Ge/Si(100) case [5], we interpret hc as the critical thickness for the introduction of misfit dislocations (MDs). In fact it has been reported that when the strain energy stored in the island is too high, misfit dislocations are inserted in the k110l directions [12]. The MDs nucleation occurs at the island edges where the stress is accumulated. The regions where a MD has been inserted are stress free and thus are preferential site for the Ge attachment [5,13]. In our case we suggest that this mechanism drives the morphological evolution of the islands changing their shape from type A to type B during the annealing. The atoms that are bonded in the top of a strained island experience a stress that decreases the energy barrier for the detachment. On the contrary the sites where the stress has been inserted have a
higher binding energy: this difference on the energy gained by the system bonding an atom (i.e. an inhomogeneous chemical potential) generates a flow of atoms [4] that depletes the top of the island and increases the material deposited at its base. In that way we can have a “damping” of the island that increases the dimension of the top facet and decreases the contact angle of the facets in a way that agrees with our observations. During the annealing procedure we can observe the evolution of this relaxation in time keeping fixed the amount of Ge atoms deposited. Referring to Fig. 2(a) we see that the islands belonging to peak A are unaffected by the annealing process except for a slight decrease in volume due to the sublimation of Ge atoms. Moreover the 7 × 7 reconstruction of the island top indicates the presence of a strain field (such reconstruction has never been found on free Ge surfaces, while it has been evidenced in strained Ge layers [15]). On the contrary, B-type islands continue the relaxing process decreasing their height. In Fig. 3 we display an STM image of a relaxed island showing that the aforementioned atoll shape is due to the selective depletion of the central part. The distribution of the strain in the top facet is not homogeneous being higher in the central part of the facet and lower at the edge: higher the strain is, faster is the depletion, higher is the gain in free energy. The strain distribution is reflected by the different reconstructions that we can observe in the central part with respect to the borders of the atoll-like island top facets. At the island center (Fig. 3 inset (a)) we detect a 7 × 7 reconstruction that we believe, in agreement with Gossmann [15], to be induced by the residual compressive strain in the Ge layer. On the contrary, on the island border we found a miscellanea of the typical Ge superstructure (see inset (b) in Fig. 3), i.e. c 2 × 8; c 2 × 4 and 2 × 2 [16]. Moreover we want to point out that the analysed atoll-like islands are usually free of tetrahedral defects of the type reported by Ko¨hler [17]. These defects eventually appear in A- and Btype islands and do not seem to be related to any particular change of the island shape neither to the change of the island top facet reconstruction (see inset (c) in Fig. 3). Thus we found a coexistence of strained and relaxed islands in agreement with recent theoretical equilibrium calculations [14]. We observed by STM that the facets of the relaxed island grow from the base upward in a ledge-by-ledge way. This fact makes difficult to recognize a well-defined crystallographic orientation. We guess that because the MDs are inserted in a direction parallel to the island side, the shape of the islands is conserved at the beginning of the relaxation stage. However, the MDs can interact (at the corners the MDs can intersect) modifying the stress field in the substrate. Thus MDs can round the corner of the relaxed island at initial stage of the annealing, subsequently destroying the triangular shape of the base (see island I in Fig. 1(b)). This mechanism is in agreement with the changes that we observe in the island top facets. In conclusion we have followed the evolution of strained Ge/Si(111) islands. We described the relaxation mechanism
G. Capellini et al. / Solid State Communications 112 (1999) 145–149
recognizing an intra-island ripening due to the insertion of MDs at the island edges that promotes an atomic current from the higher strained region, located at the centre of the island tops, toward the base of the island itself. This relaxation process results in a rounding of the island shape, a decrease of the aspect ratio and a selective depletion of the central part of the islands leading to an “atolllike” shape. References [1] [2] [3] [4] [5]
F. Capasso, S. Datta, Phys. Today (1990) 74. T.P. Pearsall, Si–Ge for Optoelectronics, 1995 ed. Quillec. T. Takahagara, K. Takeda, Phys. Rev. B 46 (1992) 15 578. A.L. Barabasi, Appl. Phys. Lett. 70 (1997) 2565. G. Capellini, L. DiGaspare, F. Evangelisti, E. Palange, Appl. Phys. Lett. 70 (1997) 493. [6] M. Goryll, L. Vescan, K. Schmidt, S. Mesters, H. Lu¨th, K. Szot, Appl. Phys. Lett. 71 (1997) 410.
149
[7] MRS Bulletin of Mater. Res. Soc. 21(4) April (1996). [8] N. Motta, A. Sgarlata, R. Calarco, Q. Nguyen, F. Patella, J. Castro Cal, A. Balzarotti, M. De Crescenzi, Surf. Sci. 406 (1998) 254. [9] H. Minoda, Y. Tanishiro, N. Yamamoto, K. Yagi, Surf. Sci. 357/358 (1996) 418. [10] H. Voigtla¨nder, M. Zinner, Appl. Phys. Lett. 63 (1993) 3055. [11] D.J. Eaglesham, F.C. Unterwald, D.C. Jacobson, Phys. Rev. Lett. 70 (1993) 966. [12] F.K. Le Goues, M.C. Reuter, M. Hammar, R.M. Tromp, Surf. Sci. 349 (1996) 249. [13] F.K. Le Goues, M.C. Reuter, J. Tersoff, M. Hammar, R.M. Tromp, Phys. Rev. Lett. 73 (1994) 300. [14] I. Daruka, A.L. Barabasi, Phys. Rev. Lett. 79 (1997) 3708. [15] H.-J. Gossmann, J.C. Bean, L.C. Feldman, E.G. McRae, I.K. Robinson, Phys. Rev. Lett. 55 (1985) 1106. [16] S.K. Theiss, D.M. Chen, J.A. Golovchenko, Appl. Phys. Lett. 66 (1995) 448. [17] U. Ko¨hler, O. Jusko, G. Pietsch, B. Mu¨ller, M. Henzler, Surf. Sci. 248 (1991) 321.