- Email: [email protected]
Self-organized MBE growth of Ge-rich SiGe dots on Si(100)
........ CRYSTAL QROWTH
ELSEVIER
Journal of Crystal Growth 157 (1995) 260-264
Self-organized MBE growth of Ge-rich SiGe dots on Si(100) P. Schittenhelm *, M. Gail, G. Abstreiter Walter Schottky Institut, TU Mfinchen, Am Coulombwall, D-85748 Garching, Germany
Abstract Si~ _xGe~ dots with high Ge contents ranging from x = 0.4 to x = 1 were grown on Si(100) substrates with varying layer thickness. The structural examination of the dots, using atomic force microscopy, shows SiGe dots with lateral sizes between 100 and 200 nm and heights ranging from 8 to 30 nm. A reduction of the nominal layer thickness to the critical thickness for dot formation leads to an abrupt growth-mode changeover from three-dimensional island growth to two-dimensional layer growth. Strong evidence of Ge surface diffusion playing an important role in the formation of the dots is found.
1. Introduction Recently, zero- and one-dimensional SiGe structures on Si have attracted growing interest due to the hope for improved optical and electrical properties compared to two-dimensional (2D) heterostructures [1-4]. Lateral confinement in such structures is expected to favor room-temperature applications and to increase the efficiency of radiative recombination processes. In situ growth of low-dimensional systems avoids damage of the structures induced by postgrowth processing and exposure of active regions to free surfaces, both related with defects and nonradiative recombination centers. SiGe layers with thicknesses exceeding the critical thickness lead to selforganized Stranski-Krastanov growth of SiGe dots [5-11]. As this method does neither require prestructured substrates nor masks, the technological effort is reduced to a minimum. In this communication we present photoluminescence (PL) and atomic force microscopy (AFM)
* Corresponding author: Fax: + 49 89 3206 620; E-mail: [email protected].
investigations on Ge-rich SiGe layers near the critical thickness for growth-mode changeover from 2D strained layer growth to three-dimensional (3D) island growth.
2. Experiments A series of pure Ge layers and Sil_xGe x layers with Ge contents ranging from x = 0.4 to x = 0.85 were grown on semi-insulating Si(001) substrates in a commercial solid source Riber Siva-32 MBE using electron beam evaporators for Si and Ge. Details on the growth and sample preparation have been reported elsewhere. [12] The growth temperature was 745°C and the growth rate of the SiGe layers was 0.2 A / s for all samples. For PL measurements the SiGe layers were covered by a Si cap layer of 90 nm, grown at the same temperature. Reference samples without a Si cap were prepared for geometrical analysis of island sizes using AFM. PL measurements were performed at 4 K, using an Ar+-laser with a typical power density of 0.1 W / c m 2 for excitation. The signals were detected with a single grating monochromator together with a
0022-0248/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 0 2 2 - 0 2 4 8 ( 9 5 ) 0 0 3 2 3 - 1
o
P. Schittenhelm et al. / Journal of Crystal Growth 157 (1995) 260-264
liquid nitrogen cooled Ge detector in standard lock-in technique. AFM images were recorded in contact mode with a Topometrix AFM in ambient air.
3. Results and discussion
AFM images of pure Ge layers with different thicknesses are presented in Fig. 1. A reduction of the SiGe thickness by only one monolayer (ML = 1.457 ,~) results in significant changes in the layer structure. The density of dots is reduced by more than one order of magnitude from 9.3 X 108 to 0.6 × 108 cm -2. Concomitantly, the size distribution of the dots becomes much more peaked. In the sample containing 6 ML of Ge (Fig. ld), dots of very different diameters, ranging from 100 to 200 nm, and heights, ranging from 8 to 30 nm, can be seen. In contrast, the sample containing only 5 ML of Ge (Fig. la) shows quite homogeneous dots, 200 nm in diameter and 25 nm high. The reason for this broadening of the size distribution with increasing layer thickness is not yet understood and will be subject to further investigations. It should be noted that absolute values for the size of the islands can not be deduced from the AFM images because of the unknown thickness of the natural oxide forming on the sample surface in ambient air prior to the AFM measurements. However, this will leave the size distribution unaffected. The PL spectra of pure Ge layers with various thicknesses embedded in Si cladding layers, are shown in Fig. 2. The two main lines observed for the sample with nominally 4 monolayers of pure Ge are attributed to the excitonic no-phonon (NP) and transverse optical (TO) phonon-assisted transitions of pseudomorphic Ge layers in Si [13]. With increasing layer thickness the intensity of these peaks is reduced. The initial decrease in energy is followed by an increase between 5 and 6 ML. In addition a pair of broader lines appears at significantly lower energies. These signals, which are also observed in samples with Ge-rich SiGe layers [10,11,14], are attributed to PL from islands. The islands form when the thickness of the Ge layers exceeds a critical value and the growth mode changes from 2D strained layer growth to 3D Stranski-Krastanov growth. The reduction of the PL intensity from the 2D layers can
261
be understood from Fig. 1. With increasing layer thickness the dot density increases and the 2D PL is more and more quenched since an increasing number of carriers diffuses to the dots with their smaller bandgap. At small island densities near the critical thickness, both line pairs can be observed. The two lines observed from the islands also correspond to NP and TO phonon-assisted transitions. With increasing layer thickness and therefore increasing height of the islands, the energetic separation of NP and TO line is reduced from 58 meV in the 2D layers to 41 meV in the sample containing the thickest Ge layer. This is believed to be due to a crossover from the Si phonon mode to the Ge phonon mode [13], because the excitons are more localized in the Ge layers when the thickness of the Ge layer increases. Fig. 3 shows the energetic position of the NP lines for the PL corresponding to the 2D layers as well as to the 3D islands as a function of the nominal Ge layer thickness. The dotted line represents the calculated, thickness dependent excitonic energy gap of pseudomorphic Ge quantum wells taking Ge segregation into account (see Ref. [12]). Excellent agreement between the experimental data and the calculated values is achieved for layer thicknesses of 4 ML and less. Thicker layers show a significant blueshift compared to the expectations and a constant PL energy for sufficiently thick layers. This blueshift may have different reasons. Strain relaxation of the thin Ge films leads to an increasing energy gap. We do not have any indication, however for strain reduction in the layers which should for example give rise to dislocations and related PL lines. These are not observed. We believe therefore that the main reason for the blueshift of the quantum well luminescence is a reduction of the thickness of the layers resulting in an increased confinement energy. This implies that Ge is diffusing towards the islands, resulting in a reduced thickness in between the islands. The thickness of the remaining Ge layer can be estimated from the PL energy. Comparing it with the calculated energy gap in Fig. 3, it corresponds to a strained Ge layer of a thickness of approximately 3.7 ML. The NP peak energy of the PL related to the islands also shows a blueshift for layer thicknesses increasing from 5 to 6 ML. The driving force for the
262
P. Schittenhelm et al./ Journal of Crystal Growth 157 (1995) 260-264
formation of islands in the Ge layer is the possibility to decrease the overall energy of the layer by local strain reduction [5,15]. This strain reduction results in an increase of the bandgap within the islands, which may explain the observed blueshift of the PL from the islands. One has to take into account,
however, that the size distribution of the islands is changing drastically between 5 and 6 ML, which may contribute to the blueshift although an additional confinement due to the lateral size of the islands is not expected because of the relatively large diameter of 100 to 200 nm. For a sample with a Ge
5 ML Ge
5.67 ML Ge
5.33 ML Ge
t.
5pm
:: 6 ML Ge
Fig. 1. AFM images of pure Ge layers with a nominal thickness of (a) 5 (b) 5.33, (c) 5.67 and (d) 6 ML. The scanned area is 5 /zm X 5 /.~m for each sample.
P. Schittenhelm et al. // Journal of Crystal Growth 157 (1995) 260-264 i
263
i
12 ML S i l . x G e x
x0.2 t~
:=0.4
e-
E ,_J n
--J
0.8
0.9
1.0
energy [eV] 0.75
Fig. 2. PL spectra of pure Ge layers of different thickness embedded in Si. The nominal thickness of the Ge layers is given. The weak feature at 1.033 eV is a two-phonon replica of the Si-subst~te signal.
layer thickness of 12 ML, a distinct redshift of the PL energy of the islands is observed. This redshift can be attributed to decreasing confinement in growth
1.15- "-.
d quantum well ~"
!
i
1.10.o 13.
1.051.00-
calculated E~,~,-. ca NP" -
rt~"~ 0.95
o~
], ~.~ G. . . . tent x
z 0.90 ~
J
.
0.85 2
4 6 8 10 nominalGe layer thickness [ML]
12
Fig. 3. Energies of the NP photoluminescence peaks of 2D quantum wells and 3D islands as a function of the nominal thickness of the Ge layer. The dotted line represents the calculated NP energy for thin Ge quantum wells. The inset shows the critical thickness for the formation of dots as a function of the Ge content x of the Si I _ ~Ge ~ layers.
i
i
0.80
0.85
,
i
i
i
i
0.90
0.95
1.00
1.05
Energy [eV] Fig. 4. PL spectra of Si~_ ~Ge.~ layers with a nominal layer thickness of 12 ML embedded in Si. The Ge content x is given. The weak feature at 1.033 eV is a two-phonon replica of the Si-substrate signal.
direction as the thickness of the islands increases. Samples with Ge contents of x = 0.85 and x = 0.75 show a similar behaviour, with the critical thickness for dot formation shifted to greater values. The inset in Fig. 3 shows the critical thickness for the formation of dots in dependence of the Ge content x of the Sij_xGe ~ layers. With decreasing Ge content the critical thickness increases from 5 ML for pure Ge to 6.5 ML for Si0.25Geo.75. The PL spectra of a series of samples containing 12 ML of Sil_xGe x with a Ge content x between 0.4 and 1 are shown in Fig. 4. With increasing Ge content a continuous redshift of the PL can be seen. A decreasing bandgap with increasing Ge content would be consistent with this behaviour. However, one should keep in mind that several mechanisms will influence the PL energy if dots are formed in the samples. This is observed in reference samples for AFM investigations for Ge contents x of 0.5 and more. For these samples, not only the Ge content increases, resulting in a reduction of the bandgap, but also strain and confinement in growth direction
264
P. Schittenhelm et al. / Journal of Crystal Growth 157 (1995) 260-264
may change with the Ge content. While a decrease of the confinement in the dots leads to a further reduction of the observable PL energy, a decreasing strain shifts the PL energy upwards. These effects can not be separated easily and require further investigation about size and strain of the dots.
4. Conclusion In conclusion, a very abrupt transition within one ML from 2D strained layers to 3D islands is observed for Si I _ xGex layers with Ge contents ranging from x = 0.75 to x = 1 on Si(001). The bandgap in the SiGe dots is reduced by about 150 meV compared to the 2D layers. The critical thickness for the formation of dislocation-free dots is determined for Sil_xGe x layers with Ge contents above x = 0.75. There is strong evidence from the PL data that Ge diffuses towards the 3D islands concomitant with a reduction of the thickness of the 2D Sil_xGe ~ layers.
Acknowledgements This work was in part supported by Siemens AG, Munich. The authors acknowledge J. Brunner and J.F. Ni~tzel for helpful discussions.
References [1] Y.S. Tang, C.D.W. Wilkinson, C.M. Sotomayor Tortes, D.W. Smith, T.E. Whall and E.H.C. Parker, Appl. Phys. Lett. 63 (1993) 497. [2] N. Usami, T. Mine, S. Fukatsu and Y. Shiraki, Appl. Phys. Leu. 64 (1994) 1126. [3] J. Brunner, P. Schittenhelm, J. Gondermann, B. Spangenberg, B. Hadam, T. K6ster, H.G. Roskos, H. Kurz, H. Gossner, I. Eisele and G. Abstreiter, J. Crystal Growth 150 (1995) 1050. [4] R. Apetz, L. Vescan, A. Hartmann, C. Dieker and H. Li~th, Appl. Phys. LeU. 66 (1995) 445. [5] D.J. Eaglesham and M. Cerullo, Phys. Rev. Lett. 64 (1990) 1943. [6] Y.W. Mo, D.E. Savage, B.S. Swartzentruber and M.G. Lagaily, Phys. Rev. Lett. 65 (1990) 1020. [7] P.O. Hansson, M. Albrecht, H.P. Strunk, E. Bauser and J.H. Wemer, Thin Solid Films 216 (1992) 199. [8] B. Voigtlhader and A. Zinner, Appl. Phys. Lett. 63 (1993) 3055. [9] P.O. Hansson, M. Albrecht, W. Dorsch, H.P. Strunk and E. Bauser, Phys. Rev. Lett. 73 (1994) 444. [10] H. Sunamura, Y. Shiraki and S. Fukatsu, Appl. Phys. Lett. 66 (1995) 953. [11] H. Sunamura, Y. Shiraki and S. Fukatsu, J. Crystal Growth 150 (1995) 1038. [12] J. Brunner, J. Niitzel, M. Gail, U. Menczigar and G. Abstreiter, J. Vac. Sci. Technol. B 11 (1993) 1097. [13] J. Weber and M.I. Alonso, Phys. Rev. B 40 (1989) 5683. [14] M. Gail, J. Brunner, J, Niitzel, G. Abstreiter, J. Engvall, J. Olajos and H. Grimmeiss, Semicond. Sci. Technol. 10 (1995) 319. [15] S. Christiansen, M. Albrecht, H.P. Strunk and H.J. Maier, Appl. Phys. Lett. 64 (1994) 3617.
ELSEVIER
Journal of Crystal Growth 157 (1995) 260-264
Self-organized MBE growth of Ge-rich SiGe dots on Si(100) P. Schittenhelm *, M. Gail, G. Abstreiter Walter Schottky Institut, TU Mfinchen, Am Coulombwall, D-85748 Garching, Germany
Abstract Si~ _xGe~ dots with high Ge contents ranging from x = 0.4 to x = 1 were grown on Si(100) substrates with varying layer thickness. The structural examination of the dots, using atomic force microscopy, shows SiGe dots with lateral sizes between 100 and 200 nm and heights ranging from 8 to 30 nm. A reduction of the nominal layer thickness to the critical thickness for dot formation leads to an abrupt growth-mode changeover from three-dimensional island growth to two-dimensional layer growth. Strong evidence of Ge surface diffusion playing an important role in the formation of the dots is found.
1. Introduction Recently, zero- and one-dimensional SiGe structures on Si have attracted growing interest due to the hope for improved optical and electrical properties compared to two-dimensional (2D) heterostructures [1-4]. Lateral confinement in such structures is expected to favor room-temperature applications and to increase the efficiency of radiative recombination processes. In situ growth of low-dimensional systems avoids damage of the structures induced by postgrowth processing and exposure of active regions to free surfaces, both related with defects and nonradiative recombination centers. SiGe layers with thicknesses exceeding the critical thickness lead to selforganized Stranski-Krastanov growth of SiGe dots [5-11]. As this method does neither require prestructured substrates nor masks, the technological effort is reduced to a minimum. In this communication we present photoluminescence (PL) and atomic force microscopy (AFM)
* Corresponding author: Fax: + 49 89 3206 620; E-mail: [email protected].
investigations on Ge-rich SiGe layers near the critical thickness for growth-mode changeover from 2D strained layer growth to three-dimensional (3D) island growth.
2. Experiments A series of pure Ge layers and Sil_xGe x layers with Ge contents ranging from x = 0.4 to x = 0.85 were grown on semi-insulating Si(001) substrates in a commercial solid source Riber Siva-32 MBE using electron beam evaporators for Si and Ge. Details on the growth and sample preparation have been reported elsewhere. [12] The growth temperature was 745°C and the growth rate of the SiGe layers was 0.2 A / s for all samples. For PL measurements the SiGe layers were covered by a Si cap layer of 90 nm, grown at the same temperature. Reference samples without a Si cap were prepared for geometrical analysis of island sizes using AFM. PL measurements were performed at 4 K, using an Ar+-laser with a typical power density of 0.1 W / c m 2 for excitation. The signals were detected with a single grating monochromator together with a
0022-0248/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 0 2 2 - 0 2 4 8 ( 9 5 ) 0 0 3 2 3 - 1
o
P. Schittenhelm et al. / Journal of Crystal Growth 157 (1995) 260-264
liquid nitrogen cooled Ge detector in standard lock-in technique. AFM images were recorded in contact mode with a Topometrix AFM in ambient air.
3. Results and discussion
AFM images of pure Ge layers with different thicknesses are presented in Fig. 1. A reduction of the SiGe thickness by only one monolayer (ML = 1.457 ,~) results in significant changes in the layer structure. The density of dots is reduced by more than one order of magnitude from 9.3 X 108 to 0.6 × 108 cm -2. Concomitantly, the size distribution of the dots becomes much more peaked. In the sample containing 6 ML of Ge (Fig. ld), dots of very different diameters, ranging from 100 to 200 nm, and heights, ranging from 8 to 30 nm, can be seen. In contrast, the sample containing only 5 ML of Ge (Fig. la) shows quite homogeneous dots, 200 nm in diameter and 25 nm high. The reason for this broadening of the size distribution with increasing layer thickness is not yet understood and will be subject to further investigations. It should be noted that absolute values for the size of the islands can not be deduced from the AFM images because of the unknown thickness of the natural oxide forming on the sample surface in ambient air prior to the AFM measurements. However, this will leave the size distribution unaffected. The PL spectra of pure Ge layers with various thicknesses embedded in Si cladding layers, are shown in Fig. 2. The two main lines observed for the sample with nominally 4 monolayers of pure Ge are attributed to the excitonic no-phonon (NP) and transverse optical (TO) phonon-assisted transitions of pseudomorphic Ge layers in Si [13]. With increasing layer thickness the intensity of these peaks is reduced. The initial decrease in energy is followed by an increase between 5 and 6 ML. In addition a pair of broader lines appears at significantly lower energies. These signals, which are also observed in samples with Ge-rich SiGe layers [10,11,14], are attributed to PL from islands. The islands form when the thickness of the Ge layers exceeds a critical value and the growth mode changes from 2D strained layer growth to 3D Stranski-Krastanov growth. The reduction of the PL intensity from the 2D layers can
261
be understood from Fig. 1. With increasing layer thickness the dot density increases and the 2D PL is more and more quenched since an increasing number of carriers diffuses to the dots with their smaller bandgap. At small island densities near the critical thickness, both line pairs can be observed. The two lines observed from the islands also correspond to NP and TO phonon-assisted transitions. With increasing layer thickness and therefore increasing height of the islands, the energetic separation of NP and TO line is reduced from 58 meV in the 2D layers to 41 meV in the sample containing the thickest Ge layer. This is believed to be due to a crossover from the Si phonon mode to the Ge phonon mode [13], because the excitons are more localized in the Ge layers when the thickness of the Ge layer increases. Fig. 3 shows the energetic position of the NP lines for the PL corresponding to the 2D layers as well as to the 3D islands as a function of the nominal Ge layer thickness. The dotted line represents the calculated, thickness dependent excitonic energy gap of pseudomorphic Ge quantum wells taking Ge segregation into account (see Ref. [12]). Excellent agreement between the experimental data and the calculated values is achieved for layer thicknesses of 4 ML and less. Thicker layers show a significant blueshift compared to the expectations and a constant PL energy for sufficiently thick layers. This blueshift may have different reasons. Strain relaxation of the thin Ge films leads to an increasing energy gap. We do not have any indication, however for strain reduction in the layers which should for example give rise to dislocations and related PL lines. These are not observed. We believe therefore that the main reason for the blueshift of the quantum well luminescence is a reduction of the thickness of the layers resulting in an increased confinement energy. This implies that Ge is diffusing towards the islands, resulting in a reduced thickness in between the islands. The thickness of the remaining Ge layer can be estimated from the PL energy. Comparing it with the calculated energy gap in Fig. 3, it corresponds to a strained Ge layer of a thickness of approximately 3.7 ML. The NP peak energy of the PL related to the islands also shows a blueshift for layer thicknesses increasing from 5 to 6 ML. The driving force for the
262
P. Schittenhelm et al./ Journal of Crystal Growth 157 (1995) 260-264
formation of islands in the Ge layer is the possibility to decrease the overall energy of the layer by local strain reduction [5,15]. This strain reduction results in an increase of the bandgap within the islands, which may explain the observed blueshift of the PL from the islands. One has to take into account,
however, that the size distribution of the islands is changing drastically between 5 and 6 ML, which may contribute to the blueshift although an additional confinement due to the lateral size of the islands is not expected because of the relatively large diameter of 100 to 200 nm. For a sample with a Ge
5 ML Ge
5.67 ML Ge
5.33 ML Ge
t.
5pm
:: 6 ML Ge
Fig. 1. AFM images of pure Ge layers with a nominal thickness of (a) 5 (b) 5.33, (c) 5.67 and (d) 6 ML. The scanned area is 5 /zm X 5 /.~m for each sample.
P. Schittenhelm et al. // Journal of Crystal Growth 157 (1995) 260-264 i
263
i
12 ML S i l . x G e x
x0.2 t~
:=0.4
e-
E ,_J n
--J
0.8
0.9
1.0
energy [eV] 0.75
Fig. 2. PL spectra of pure Ge layers of different thickness embedded in Si. The nominal thickness of the Ge layers is given. The weak feature at 1.033 eV is a two-phonon replica of the Si-subst~te signal.
layer thickness of 12 ML, a distinct redshift of the PL energy of the islands is observed. This redshift can be attributed to decreasing confinement in growth
1.15- "-.
d quantum well ~"
!
i
1.10.o 13.
1.051.00-
calculated E~,~,-. ca NP" -
rt~"~ 0.95
o~
], ~.~ G. . . . tent x
z 0.90 ~
J
.
0.85 2
4 6 8 10 nominalGe layer thickness [ML]
12
Fig. 3. Energies of the NP photoluminescence peaks of 2D quantum wells and 3D islands as a function of the nominal thickness of the Ge layer. The dotted line represents the calculated NP energy for thin Ge quantum wells. The inset shows the critical thickness for the formation of dots as a function of the Ge content x of the Si I _ ~Ge ~ layers.
i
i
0.80
0.85
,
i
i
i
i
0.90
0.95
1.00
1.05
Energy [eV] Fig. 4. PL spectra of Si~_ ~Ge.~ layers with a nominal layer thickness of 12 ML embedded in Si. The Ge content x is given. The weak feature at 1.033 eV is a two-phonon replica of the Si-substrate signal.
direction as the thickness of the islands increases. Samples with Ge contents of x = 0.85 and x = 0.75 show a similar behaviour, with the critical thickness for dot formation shifted to greater values. The inset in Fig. 3 shows the critical thickness for the formation of dots in dependence of the Ge content x of the Sij_xGe ~ layers. With decreasing Ge content the critical thickness increases from 5 ML for pure Ge to 6.5 ML for Si0.25Geo.75. The PL spectra of a series of samples containing 12 ML of Sil_xGe x with a Ge content x between 0.4 and 1 are shown in Fig. 4. With increasing Ge content a continuous redshift of the PL can be seen. A decreasing bandgap with increasing Ge content would be consistent with this behaviour. However, one should keep in mind that several mechanisms will influence the PL energy if dots are formed in the samples. This is observed in reference samples for AFM investigations for Ge contents x of 0.5 and more. For these samples, not only the Ge content increases, resulting in a reduction of the bandgap, but also strain and confinement in growth direction
264
P. Schittenhelm et al. / Journal of Crystal Growth 157 (1995) 260-264
may change with the Ge content. While a decrease of the confinement in the dots leads to a further reduction of the observable PL energy, a decreasing strain shifts the PL energy upwards. These effects can not be separated easily and require further investigation about size and strain of the dots.
4. Conclusion In conclusion, a very abrupt transition within one ML from 2D strained layers to 3D islands is observed for Si I _ xGex layers with Ge contents ranging from x = 0.75 to x = 1 on Si(001). The bandgap in the SiGe dots is reduced by about 150 meV compared to the 2D layers. The critical thickness for the formation of dislocation-free dots is determined for Sil_xGe x layers with Ge contents above x = 0.75. There is strong evidence from the PL data that Ge diffuses towards the 3D islands concomitant with a reduction of the thickness of the 2D Sil_xGe ~ layers.
Acknowledgements This work was in part supported by Siemens AG, Munich. The authors acknowledge J. Brunner and J.F. Ni~tzel for helpful discussions.
References [1] Y.S. Tang, C.D.W. Wilkinson, C.M. Sotomayor Tortes, D.W. Smith, T.E. Whall and E.H.C. Parker, Appl. Phys. Lett. 63 (1993) 497. [2] N. Usami, T. Mine, S. Fukatsu and Y. Shiraki, Appl. Phys. Leu. 64 (1994) 1126. [3] J. Brunner, P. Schittenhelm, J. Gondermann, B. Spangenberg, B. Hadam, T. K6ster, H.G. Roskos, H. Kurz, H. Gossner, I. Eisele and G. Abstreiter, J. Crystal Growth 150 (1995) 1050. [4] R. Apetz, L. Vescan, A. Hartmann, C. Dieker and H. Li~th, Appl. Phys. LeU. 66 (1995) 445. [5] D.J. Eaglesham and M. Cerullo, Phys. Rev. Lett. 64 (1990) 1943. [6] Y.W. Mo, D.E. Savage, B.S. Swartzentruber and M.G. Lagaily, Phys. Rev. Lett. 65 (1990) 1020. [7] P.O. Hansson, M. Albrecht, H.P. Strunk, E. Bauser and J.H. Wemer, Thin Solid Films 216 (1992) 199. [8] B. Voigtlhader and A. Zinner, Appl. Phys. Lett. 63 (1993) 3055. [9] P.O. Hansson, M. Albrecht, W. Dorsch, H.P. Strunk and E. Bauser, Phys. Rev. Lett. 73 (1994) 444. [10] H. Sunamura, Y. Shiraki and S. Fukatsu, Appl. Phys. Lett. 66 (1995) 953. [11] H. Sunamura, Y. Shiraki and S. Fukatsu, J. Crystal Growth 150 (1995) 1038. [12] J. Brunner, J. Niitzel, M. Gail, U. Menczigar and G. Abstreiter, J. Vac. Sci. Technol. B 11 (1993) 1097. [13] J. Weber and M.I. Alonso, Phys. Rev. B 40 (1989) 5683. [14] M. Gail, J. Brunner, J, Niitzel, G. Abstreiter, J. Engvall, J. Olajos and H. Grimmeiss, Semicond. Sci. Technol. 10 (1995) 319. [15] S. Christiansen, M. Albrecht, H.P. Strunk and H.J. Maier, Appl. Phys. Lett. 64 (1994) 3617.