- Email: [email protected]
Si(0 0 1) quantum dots
Applied Surface Science 224 (2004) 143–147
Kinetics of Si capping process of Ge/Si(0 0 1) quantum dots V. Yama,*, V. Le Thanha, D. De´barrea, Y. Zhengb, D. Bouchiera a
b
Institut d’Electronique Fondamentale, UMR-CNRS 8622, Universite´ Paris-Sud, 91405 Orsay Cedex, France Laboratoire Mine´ralogie-Cristallographie, URA-CNRS 09, Universite´ Paris VI et VII, 4 Place Jussieu, 75252 Paris Cedex 5, France
Abstract The overgrowth of Si on Ge/Si(0 0 1) islands in a UHV chemical-vapor deposition system is investigated by reflection high energy electron diffraction (RHEED), atomic force microscopy (AFM), transmission electronic microscopy (TEM), and photoluminescence. It has been shown that the capping process comprises three stages, firstly a shape transition from dome to pyramid induced by strain relaxation due to interdiffusion, followed by a pyramid–dome shape transition, before the smoothening of the surface. This study shows the existence of a strong surface roughness even if the Si cap thickness is higher than the island height. The roughness is then a parameter that should be considered in a multilayer system to explain the mechanism of vertical alignment. # 2003 Elsevier B.V. All rights reserved. PACS: 68.55; 78.55.-m Keywords: Ge islands; Heterostructures; Capping process; Interdiffusion; Vertical alignment
During the last few years, a considerable amount of work has been devoted to the formation of quantum dots (QD’s) due to their potential interest for electronic and optoelectronic device applications. Among the different ways to produce QD’s, special attention has been paid to the self-assembled technique which takes advantage of the transition from two- (2D) to threedimensional (3D) growth mode occurring during growth in a highly lattice-mismatched heteroepitaxial system [1]. There are fewer studies that have focused on the effect of the capping layer [2–5] that is required for the passivation of the surface of quantum dots for devices applications. In multilayer structure, it has been observed in both III–V and IV–IV materials, that the capped layers smooth the island surface. A detailed *
Corresponding author. Tel.: þ33-1-69-15-78-39; fax: þ33-1-69-15-78-39. E-mail address: [email protected] (V. Yam).
understanding of the capping process appears crucial to explain the vertical alignment observed in multilayer structures. In this work, we study the kinetics of the Si capping of Ge islands. The classical image of the capping process assumes a gradual smooth of the surface with a Si preferentially growth between dots where strain is minimum, while lateral diffusion induces the formation of a (0 0 1) top at the island apex. Our studies show a more complex mechanism: a significant surface restructuring is observed; the buried islands induce a roughness at the Si cap surface, which may be responsible of the vertical alignment of island observed in multilayer structures. Experiments were carried out in an ultrahighvacuum chemical-vapor deposition (UHV-CVD) system. Pure SiH4 and hydrogen-diluted (10%) GeH4 were used as gas sources. The system has a base pressure of 1 1010 Torr, and the pressure during
0169-4332/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2003.08.041
144
V. Yam et al. / Applied Surface Science 224 (2004) 143–147
˚ Fig. 1. AFM images of 4.8 monolayers of Ge surface grown at 700 8C prior to Si cap deposition (a) and after a deposition at 700 8C of 11 A ˚ (c); 60 A ˚ (d); 74 A ˚ (e); 112 A ˚ (f) of Si cap. (b); 37 A
growth was about 5 104 Torr. The growth chamber is equipped with a differentially pumped reflection high energy electron diffraction (RHEED) system, allowing us to probe the growing surface even at high partial pressures of hydrides (up to 101 Torr). During experiments, RHEED patterns were recorded using a camera-based system. Ge deposition was carried out at a substrate temperature of 700 8C to ensure the formation of pyramidal and dome islands [6]. Fig. 1a shows the Ge islands surface prior to Si deposition. The Ge coverage is 4.8 monolayers. The surface exhibits two kinds of islands: the domed islands have an average size of 115 nm and a height of 22 nm while the pyramidal islands are 103 nm width and 12 nm height. Their density is 2:8 108 cm2 [6]. This kind of starting surface has been chosen to know if the initial island shape influences the kinetics of the capping process. The Si cap is deposited at 700 8C ˚ /min. with a rate of 15 A Fig. 1 shows a sequence of ex situ atomic force microscopy (AFM) images illustrating the morphological evolution of the surface when increasing Si coverage. The variation of the islands height and
density is illustrated on Fig. 2. These analyses, completed by in situ RHEED investigations, show the existence of three stages in the capping process. The first stage is dominated by a dome to pyramid shape transition with a drastic increase of the islands density and volume, that points out the existence of interdiffusion between Ge and Si atoms. The island shape is found to changes continuously from dome to ˚ of Si (Fig. 1b), pyramid. After a deposition of 15 A ˚ of Si, the domes and pyramids still coexist. With 37 A surface exhibits islands with pyramidal shape only, but presenting two different size populations: the majority ˚ height and 1300 A ˚ width, the smallest of them is 140 A ˚ height and 820 A ˚ width. Such a surface are 60 A restructuring might be induced by the deposition of Si atoms that modifies the equilibrium of the system. This asumption is confirmed by growth interruption experiments prior to Si cap deposition: such an increase of island volume has not been observed. The presence of Si adatoms modifies the surface and strain energies and the system tends to minimize its total energy by the main mechanism of Si–Ge intermixing. On the one hand, since the lattice
V. Yam et al. / Applied Surface Science 224 (2004) 143–147
145
Fig. 2. Evolution of islands height and density versus Si cap thickness. The statistic has been made with 10 mm 10 mm AFM images. After a deposition of 11.2 nm of Si, it becomes difficult to count islands.
parameter at the apex of Ge islands is closer to the Ge bulk parameter, the misfit is locally maximum for the Si atoms arriving at the surface. Therefore, interdiffusion leads to a partial strain relaxation. On the other hand, the Ge surface energy is lower than Si, Ge atoms tend to detach from the island and migrate to the top layer to minimize the surface energy [2]. A similar dome to pyramid shape transition has been observed for experiments undertaken at a lower Si cap growth temperature of 550 8C. It evidences that this morphological transition is not thermally activated but strain induced. The strain relaxation induced by interdiffusion is the main mechanism leading to this dome to pyramid shape transition [7]. While the islands volume increases, we observe a striking increase of islands density, as seen previously in reference [5], by a factor three from 2:8 108 cm2 prior to Si deposition to 1:1 109 cm2 after a deposi˚ of Si. This can be explained by the tion of 37 A creation of new Si-rich islands between the initial islands, due to the existence of a metastability of the 2D-Ge wetting layer (WLs) [8]. Under the silane flux perturbation, the system tends to a new morphology state that minimizes the total energy. Experiments undertaken with different Si overgrowth conditions have shown that the increase of island density is all the more important than the adatom mobility is high, e.g. at low pressure and high Si cap growth temperature (TSi). This shows that the appearance of new islands is
related to the Ge WLs metastability and to the Si–Ge alloying which increase when increasing TSi and decreasing silane flux. This is supported by photoluminescence (PL) investigations: Fig. 3 represents PL spectra of samples containing the same Ge islands, but capped at different conditions. The TOi and NPi peaks are respectively the optical transverse phonon transition and the non-phonon one [6] from islands, and the TOWL and NPWL are from WLs. When TSi is increased from 550 to 700 8C, we observe respectively a blueshift of the islands and WLs energies of 10 and 26 meV. This clearly evidences an increase of intermixing with growth temperature both in islands and WLs. When the silane flux is reduced by a factor of five, from 5 to 1 sccm, with the same growth temperature of 700 8C, a 16 meV blue-shift is observed for the islands energy, while the wetting-layers signals do not appear. Since at 5 sccm, the NPWL is only separated by 25 meV from the Si signal (SiTO), we assume that at 1 sccm, the band offset becomes so weak that carriers cannot be retained in the WLs. In the second stage of the cap process, the islands are found to get out of pyramidal shape (Fig. 1d) to change continuously to dome (Fig. 1e). Their height has decreased while the base has become larger and their volume has increased. The pyramid to dome shape transition can be explained by a minimization of the total energy versus island volume [9]. The islands density decreases and tends to stabilize at
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V. Yam et al. / Applied Surface Science 224 (2004) 143–147
Photoluminescence (a.u.)
T
PL
= 11 K
NP
Wetting Layers
WL
TO
Si
TO
WL
3D islands NP TO
i
i
TSi = 550 ˚C
(a)
NP
TO
TO
WL
i
NP
5 sccm WL
i
TSi = 700 ˚C
(b)
TO
NP
5 sccm i
i
T Si = 700 ˚C
(c)
1 sccm
0.7
0.8
0.9
1
1.1
1.2
1.3
Energy (eV) Fig. 3. Photoluminescence spectra of samples containing the same Ge islands layer grown at 700 8C, but covered at different temperature and silane flux, 550 8C and 5 sccm (a); 700 8C and 5 sccm (b); and 700 8C and 1 sccm. The interdiffusion increases when increasing the adatoms mobility.
˚ of Si. This 7:1 108 cm2, after a deposition of 74 A can be explained by a disintegration [2] of the small Si-rich islands, or by a complete covering of the smallest islands by Si. Nevertheless the density is higher than that of the surface prior to Si deposition. This result may explain the increase of island density by a factor two observed on the upper layer of multilayer structures compared to a single Ge layer as shown in reference [10]. At the last stage of the cap process, the dome height decreases when the Si coverage increases, and the smoothening of the surface begins (Fig. 1d). A strong ˚ ) is still observed by AFM surface roughness (10 A after a deposition of 15 nm of Si. This should have some consequences on multilayer structures where a well known vertical correlation is observed. Fig. 4 shows multilayer structures composed of two stacked Ge layers containing the same Ge first layer but separated by different thickness of Si spacer (dSi). AFM analyses show that the islands of the second layer are pyramidal when dSi is equal to 15 nm
(Fig. 4a), while a coexistence of pyramids and domes is observed for 90 nm (Fig. 4b). The pyramidal shape may be induced by the existence of a strong Si–Ge intermixing when the Si spacer is thin. The zoom of ˚ roughness Fig. 4a shows the existence of a 1–2 A around each island for dSi ¼ 15 nm, while it has completely disappeared for dSi ¼ 90 nm (Fig. 4b). To know if it is correlated to the mechanism of vertical alignment, we have completed this study by transmission electronic microscopy (TEM) characterizations. When dSi is under 85 nm (Fig. 4), each island of the second layer nucleates on top of a buried island. Above 85 nm, islands nucleation becomes random (Fig. 4d). These observations suggest that the surface roughness due to buried islands creates preferential nucleation sites for island, and is responsible of vertical alignment in multilayers. In conclusion, we have shown that the Si capping process of Ge islands includes three main stages of morphological modifications. A strong Si–Ge intermixing appears at the early stage of the overgrowth.
V. Yam et al. / Applied Surface Science 224 (2004) 143–147
147
Fig. 4. Two Ge/Si bilayers structures containing the same first Ge layer. The second Ge layer surface presents pyramidal islands when the Si spacer thickness (dSi) is 15 nm (a) and a coexistence of pyramids and domes for dSi ¼ 90 nm (b). The zoom of (a) shows the existence of roughness around each pyramidal island. The vertical alignment of islands, which is perfect when dSi is under 85 nm (c) and disappears when dSi is 85 nm (d), is correlated to this roughness.
The growth of multilayer structures depends directly on the overgrowth conditions: the vertical correlation is induced by surface roughness due to buried islands. References [1] D.J. Eaglesham, M. Cerullo, Phys. Rev. Lett. 64 (1990) 1943. [2] P. Sutter, M.G. Lagally, Phys. Rev. Lett. 81 (1998) 3471. [3] L. Kubler, D. Dentel, J.L. Bischoff, C. Ghica, C. UlhaqBouillet, J. Werckmann, Appl. Phys. Lett. 73 (1998) 1053. [4] O.G. Schmidt, U. Denker, K. Eberl, Appl. Phys. Lett. 77 (2000) 2509. [5] N. Usami, M. Miura, Y. Ito, Y. Araki, Y. Shiraki, Appl. Phys. Lett. 77 (2000) 217.
[6] V. Yam, V. Le Thanh, Y. Zheng, P. Boucaud, D. Bouchier, Phys. Rev B 63 (2001) 033313. [7] T.I. Kamins, G. Medieros-Ribeiro, D.A.A. Ohlberg, R. Stanley William, Appl. Phys. A 67 (1998) 727; W.L. Henstrom, C.P. Liu, J.M. Gibson, T.I. Kamins, S. Williams, Appl. Phys. Lett. 77 (2000) 1623; C.J. Huang, Y.H. Zuo, B.W. Li, B.W. Cheng, L.P. Luo, J.Z. Yu, Q.M. Wang, Appl. Phys. Lett. 78 (2001) 3881. [8] V. Yam, V. Le Thanh, P. Boucaud, D. De´ barre, D. Bouchier, J. Vac. Sci. Technol. B 20 (3) (2002) 1251. [9] F.M. Ross, J. Tersoff, R.M. Tromp, Phys. Rev. Lett. 80 (1998) 984; G. Medieros-Ribeiro, A.M. Bratkovski, T.I. Kamins, D.A.A. Ohlberg, R. Stanley William, Science 279 (1998) 353. [10] V. Le Thanh, V. Yam, P. Boucaud, F. Fortuna, C. Ulysse, D. Bouchier, L. Vervoort, J.-M. Lourtioz, Phys. Rev. B 60 (1999) 5851.
Kinetics of Si capping process of Ge/Si(0 0 1) quantum dots V. Yama,*, V. Le Thanha, D. De´barrea, Y. Zhengb, D. Bouchiera a
b
Institut d’Electronique Fondamentale, UMR-CNRS 8622, Universite´ Paris-Sud, 91405 Orsay Cedex, France Laboratoire Mine´ralogie-Cristallographie, URA-CNRS 09, Universite´ Paris VI et VII, 4 Place Jussieu, 75252 Paris Cedex 5, France
Abstract The overgrowth of Si on Ge/Si(0 0 1) islands in a UHV chemical-vapor deposition system is investigated by reflection high energy electron diffraction (RHEED), atomic force microscopy (AFM), transmission electronic microscopy (TEM), and photoluminescence. It has been shown that the capping process comprises three stages, firstly a shape transition from dome to pyramid induced by strain relaxation due to interdiffusion, followed by a pyramid–dome shape transition, before the smoothening of the surface. This study shows the existence of a strong surface roughness even if the Si cap thickness is higher than the island height. The roughness is then a parameter that should be considered in a multilayer system to explain the mechanism of vertical alignment. # 2003 Elsevier B.V. All rights reserved. PACS: 68.55; 78.55.-m Keywords: Ge islands; Heterostructures; Capping process; Interdiffusion; Vertical alignment
During the last few years, a considerable amount of work has been devoted to the formation of quantum dots (QD’s) due to their potential interest for electronic and optoelectronic device applications. Among the different ways to produce QD’s, special attention has been paid to the self-assembled technique which takes advantage of the transition from two- (2D) to threedimensional (3D) growth mode occurring during growth in a highly lattice-mismatched heteroepitaxial system [1]. There are fewer studies that have focused on the effect of the capping layer [2–5] that is required for the passivation of the surface of quantum dots for devices applications. In multilayer structure, it has been observed in both III–V and IV–IV materials, that the capped layers smooth the island surface. A detailed *
Corresponding author. Tel.: þ33-1-69-15-78-39; fax: þ33-1-69-15-78-39. E-mail address: [email protected] (V. Yam).
understanding of the capping process appears crucial to explain the vertical alignment observed in multilayer structures. In this work, we study the kinetics of the Si capping of Ge islands. The classical image of the capping process assumes a gradual smooth of the surface with a Si preferentially growth between dots where strain is minimum, while lateral diffusion induces the formation of a (0 0 1) top at the island apex. Our studies show a more complex mechanism: a significant surface restructuring is observed; the buried islands induce a roughness at the Si cap surface, which may be responsible of the vertical alignment of island observed in multilayer structures. Experiments were carried out in an ultrahighvacuum chemical-vapor deposition (UHV-CVD) system. Pure SiH4 and hydrogen-diluted (10%) GeH4 were used as gas sources. The system has a base pressure of 1 1010 Torr, and the pressure during
0169-4332/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2003.08.041
144
V. Yam et al. / Applied Surface Science 224 (2004) 143–147
˚ Fig. 1. AFM images of 4.8 monolayers of Ge surface grown at 700 8C prior to Si cap deposition (a) and after a deposition at 700 8C of 11 A ˚ (c); 60 A ˚ (d); 74 A ˚ (e); 112 A ˚ (f) of Si cap. (b); 37 A
growth was about 5 104 Torr. The growth chamber is equipped with a differentially pumped reflection high energy electron diffraction (RHEED) system, allowing us to probe the growing surface even at high partial pressures of hydrides (up to 101 Torr). During experiments, RHEED patterns were recorded using a camera-based system. Ge deposition was carried out at a substrate temperature of 700 8C to ensure the formation of pyramidal and dome islands [6]. Fig. 1a shows the Ge islands surface prior to Si deposition. The Ge coverage is 4.8 monolayers. The surface exhibits two kinds of islands: the domed islands have an average size of 115 nm and a height of 22 nm while the pyramidal islands are 103 nm width and 12 nm height. Their density is 2:8 108 cm2 [6]. This kind of starting surface has been chosen to know if the initial island shape influences the kinetics of the capping process. The Si cap is deposited at 700 8C ˚ /min. with a rate of 15 A Fig. 1 shows a sequence of ex situ atomic force microscopy (AFM) images illustrating the morphological evolution of the surface when increasing Si coverage. The variation of the islands height and
density is illustrated on Fig. 2. These analyses, completed by in situ RHEED investigations, show the existence of three stages in the capping process. The first stage is dominated by a dome to pyramid shape transition with a drastic increase of the islands density and volume, that points out the existence of interdiffusion between Ge and Si atoms. The island shape is found to changes continuously from dome to ˚ of Si (Fig. 1b), pyramid. After a deposition of 15 A ˚ of Si, the domes and pyramids still coexist. With 37 A surface exhibits islands with pyramidal shape only, but presenting two different size populations: the majority ˚ height and 1300 A ˚ width, the smallest of them is 140 A ˚ height and 820 A ˚ width. Such a surface are 60 A restructuring might be induced by the deposition of Si atoms that modifies the equilibrium of the system. This asumption is confirmed by growth interruption experiments prior to Si cap deposition: such an increase of island volume has not been observed. The presence of Si adatoms modifies the surface and strain energies and the system tends to minimize its total energy by the main mechanism of Si–Ge intermixing. On the one hand, since the lattice
V. Yam et al. / Applied Surface Science 224 (2004) 143–147
145
Fig. 2. Evolution of islands height and density versus Si cap thickness. The statistic has been made with 10 mm 10 mm AFM images. After a deposition of 11.2 nm of Si, it becomes difficult to count islands.
parameter at the apex of Ge islands is closer to the Ge bulk parameter, the misfit is locally maximum for the Si atoms arriving at the surface. Therefore, interdiffusion leads to a partial strain relaxation. On the other hand, the Ge surface energy is lower than Si, Ge atoms tend to detach from the island and migrate to the top layer to minimize the surface energy [2]. A similar dome to pyramid shape transition has been observed for experiments undertaken at a lower Si cap growth temperature of 550 8C. It evidences that this morphological transition is not thermally activated but strain induced. The strain relaxation induced by interdiffusion is the main mechanism leading to this dome to pyramid shape transition [7]. While the islands volume increases, we observe a striking increase of islands density, as seen previously in reference [5], by a factor three from 2:8 108 cm2 prior to Si deposition to 1:1 109 cm2 after a deposi˚ of Si. This can be explained by the tion of 37 A creation of new Si-rich islands between the initial islands, due to the existence of a metastability of the 2D-Ge wetting layer (WLs) [8]. Under the silane flux perturbation, the system tends to a new morphology state that minimizes the total energy. Experiments undertaken with different Si overgrowth conditions have shown that the increase of island density is all the more important than the adatom mobility is high, e.g. at low pressure and high Si cap growth temperature (TSi). This shows that the appearance of new islands is
related to the Ge WLs metastability and to the Si–Ge alloying which increase when increasing TSi and decreasing silane flux. This is supported by photoluminescence (PL) investigations: Fig. 3 represents PL spectra of samples containing the same Ge islands, but capped at different conditions. The TOi and NPi peaks are respectively the optical transverse phonon transition and the non-phonon one [6] from islands, and the TOWL and NPWL are from WLs. When TSi is increased from 550 to 700 8C, we observe respectively a blueshift of the islands and WLs energies of 10 and 26 meV. This clearly evidences an increase of intermixing with growth temperature both in islands and WLs. When the silane flux is reduced by a factor of five, from 5 to 1 sccm, with the same growth temperature of 700 8C, a 16 meV blue-shift is observed for the islands energy, while the wetting-layers signals do not appear. Since at 5 sccm, the NPWL is only separated by 25 meV from the Si signal (SiTO), we assume that at 1 sccm, the band offset becomes so weak that carriers cannot be retained in the WLs. In the second stage of the cap process, the islands are found to get out of pyramidal shape (Fig. 1d) to change continuously to dome (Fig. 1e). Their height has decreased while the base has become larger and their volume has increased. The pyramid to dome shape transition can be explained by a minimization of the total energy versus island volume [9]. The islands density decreases and tends to stabilize at
146
V. Yam et al. / Applied Surface Science 224 (2004) 143–147
Photoluminescence (a.u.)
T
PL
= 11 K
NP
Wetting Layers
WL
TO
Si
TO
WL
3D islands NP TO
i
i
TSi = 550 ˚C
(a)
NP
TO
TO
WL
i
NP
5 sccm WL
i
TSi = 700 ˚C
(b)
TO
NP
5 sccm i
i
T Si = 700 ˚C
(c)
1 sccm
0.7
0.8
0.9
1
1.1
1.2
1.3
Energy (eV) Fig. 3. Photoluminescence spectra of samples containing the same Ge islands layer grown at 700 8C, but covered at different temperature and silane flux, 550 8C and 5 sccm (a); 700 8C and 5 sccm (b); and 700 8C and 1 sccm. The interdiffusion increases when increasing the adatoms mobility.
˚ of Si. This 7:1 108 cm2, after a deposition of 74 A can be explained by a disintegration [2] of the small Si-rich islands, or by a complete covering of the smallest islands by Si. Nevertheless the density is higher than that of the surface prior to Si deposition. This result may explain the increase of island density by a factor two observed on the upper layer of multilayer structures compared to a single Ge layer as shown in reference [10]. At the last stage of the cap process, the dome height decreases when the Si coverage increases, and the smoothening of the surface begins (Fig. 1d). A strong ˚ ) is still observed by AFM surface roughness (10 A after a deposition of 15 nm of Si. This should have some consequences on multilayer structures where a well known vertical correlation is observed. Fig. 4 shows multilayer structures composed of two stacked Ge layers containing the same Ge first layer but separated by different thickness of Si spacer (dSi). AFM analyses show that the islands of the second layer are pyramidal when dSi is equal to 15 nm
(Fig. 4a), while a coexistence of pyramids and domes is observed for 90 nm (Fig. 4b). The pyramidal shape may be induced by the existence of a strong Si–Ge intermixing when the Si spacer is thin. The zoom of ˚ roughness Fig. 4a shows the existence of a 1–2 A around each island for dSi ¼ 15 nm, while it has completely disappeared for dSi ¼ 90 nm (Fig. 4b). To know if it is correlated to the mechanism of vertical alignment, we have completed this study by transmission electronic microscopy (TEM) characterizations. When dSi is under 85 nm (Fig. 4), each island of the second layer nucleates on top of a buried island. Above 85 nm, islands nucleation becomes random (Fig. 4d). These observations suggest that the surface roughness due to buried islands creates preferential nucleation sites for island, and is responsible of vertical alignment in multilayers. In conclusion, we have shown that the Si capping process of Ge islands includes three main stages of morphological modifications. A strong Si–Ge intermixing appears at the early stage of the overgrowth.
V. Yam et al. / Applied Surface Science 224 (2004) 143–147
147
Fig. 4. Two Ge/Si bilayers structures containing the same first Ge layer. The second Ge layer surface presents pyramidal islands when the Si spacer thickness (dSi) is 15 nm (a) and a coexistence of pyramids and domes for dSi ¼ 90 nm (b). The zoom of (a) shows the existence of roughness around each pyramidal island. The vertical alignment of islands, which is perfect when dSi is under 85 nm (c) and disappears when dSi is 85 nm (d), is correlated to this roughness.
The growth of multilayer structures depends directly on the overgrowth conditions: the vertical correlation is induced by surface roughness due to buried islands. References [1] D.J. Eaglesham, M. Cerullo, Phys. Rev. Lett. 64 (1990) 1943. [2] P. Sutter, M.G. Lagally, Phys. Rev. Lett. 81 (1998) 3471. [3] L. Kubler, D. Dentel, J.L. Bischoff, C. Ghica, C. UlhaqBouillet, J. Werckmann, Appl. Phys. Lett. 73 (1998) 1053. [4] O.G. Schmidt, U. Denker, K. Eberl, Appl. Phys. Lett. 77 (2000) 2509. [5] N. Usami, M. Miura, Y. Ito, Y. Araki, Y. Shiraki, Appl. Phys. Lett. 77 (2000) 217.
[6] V. Yam, V. Le Thanh, Y. Zheng, P. Boucaud, D. Bouchier, Phys. Rev B 63 (2001) 033313. [7] T.I. Kamins, G. Medieros-Ribeiro, D.A.A. Ohlberg, R. Stanley William, Appl. Phys. A 67 (1998) 727; W.L. Henstrom, C.P. Liu, J.M. Gibson, T.I. Kamins, S. Williams, Appl. Phys. Lett. 77 (2000) 1623; C.J. Huang, Y.H. Zuo, B.W. Li, B.W. Cheng, L.P. Luo, J.Z. Yu, Q.M. Wang, Appl. Phys. Lett. 78 (2001) 3881. [8] V. Yam, V. Le Thanh, P. Boucaud, D. De´ barre, D. Bouchier, J. Vac. Sci. Technol. B 20 (3) (2002) 1251. [9] F.M. Ross, J. Tersoff, R.M. Tromp, Phys. Rev. Lett. 80 (1998) 984; G. Medieros-Ribeiro, A.M. Bratkovski, T.I. Kamins, D.A.A. Ohlberg, R. Stanley William, Science 279 (1998) 353. [10] V. Le Thanh, V. Yam, P. Boucaud, F. Fortuna, C. Ulysse, D. Bouchier, L. Vervoort, J.-M. Lourtioz, Phys. Rev. B 60 (1999) 5851.