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Surface diffusion limited nucleation of Ge dots on the Si(001) surface
Materials Science and Engineering B89 (2002) 151– 156
www.elsevier.com/locate/mseb
Surface diffusion limited nucleation of Ge dots on the Si(001) surface Y.-H. Wu, C.-Y. Wang, A. Elfving, G.V. Hansson, W.-X. Ni * Department of Physics, Linko¨ping Uni6ersity, SE-581 83 Linko¨ping, Sweden
Abstract The formation of Ge islands during MBE growth is a spontaneous process and these islands, i.e. dots, are usually randomly arranged. In order to implement these nanoscaled islands into device applications, ordering of epitaxial dots is a crucial step. We report a study on the MBE growth of Ge islands on Si(001) substrates, containing 110-oriented square and long stripe type patterns defined by anisotropic wet etching of Si, in order to provide more understanding of how surface diffusion of Ge atoms would influence the formation of Ge islands on various types of surfaces. It has been found that there were preferential nucleation sites for Ge islands along the bottom edges of the Si ridges. The Ge islands at the edge positions were larger than those formed on the free surface and they could be regularly spaced. Due to the consumption of Ge at the bottom edges of ridge patterns, the density of Ge dots on the free surface varied between : 3 ×108 and :1× 109 cm − 2 when changing the spatial separation between two adjacent Si ridges (2–100 mm). A Ge mean diffusion length of : 7.5 mm has been determined for Ge growth at 700 °C. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Quantum dot; Nucleation; Diffusion; MBE; SEM; Ge
1. Introduction Recent research on low dimensional semiconductor structures has shown that by reducing the structural dimensions, important material and electronic properties of semiconductors, such as lattice strain, density of states, bandgap, band offset, etc., can be manipulated and tailored according to desire [1]. This is the so-called quantum engineering, which has enabled some advanced devices and is expected to have a more extensive impact on future applications in high speed electronics and optoelectronics, to achieve previously unattainable limits for device performance. One way to synthesize such low-dimensional semiconductor systems is to selfassemble 3-dimensional crystalline clusters (islands) with a very small diameter (10 – 100 nm). It is known that when the growth of a Ge layer exceeds a critical thickness (:3 ML) during molecular beam epitaxy (MBE), the growth mode will be changed
* Corresponding author. Tel.: +46-13-282-474; fax: +46-13-137568. E-mail address: [email protected] (W.-X. Ni).
from layer-by-layer (2-dimensional, 2D) to formation of Ge islands (3-dimensional, 3D) [2]. The layer growth with such a 2D/3D transition is called Stransky –Krastanov mode, which is a spontaneous process and the island size is strongly affected by the substrate temperature. These islands are usually randomly arranged. In order to implement these nanoscaled islands (dots) into device applications, ordering of epitaxial islands is a crucial step. The objective of this work was thus to provide more understanding how surface diffusion of Ge atoms would influence the formation of Ge islands on various types of surfaces, by carrying out a study of the Ge MBE growth on specially designed Si(001) substrates, containing 110-oriented square and long stripe type of patterns defined by anisotropic wet etching of Si. The separation between two adjacent Si ridges in the pattern varied between 2 and 100 mm. These samples were then characterized by high-resolution scanning electron microscopy (SEM) and atomic force microscopy (AFM). Statistics on the Ge island size and density have been made on various samples grown with different conditions, in order to extract information on the surface diffusion limited Ge islanding process.
0921-5107/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S0921-5107(01)00822-4
152
Y.-H. Wu et al. / Materials Science and Engineering B89 (2002) 151–156
2. Experimental All samples used in this study were grown using a solid-source MBE apparatus, Balzers UMS 630, which has been described in detail previously [3]. The Si and Ge fluxes were supplied by the e-beam evaporators and regulated by a microprocessor controlled feed-back loop with a cross-beam mass-spectrometer as a rate monitor. The base pressure in the growth chamber was
5 5× 10 − 11 mbar and could be increased to :1× 10 − 9 mbar during the e-gun evaporation. The substrates used for MBE growth were Si(001) wafers. In order to study the nucleation and growth kinetics of self-assembled Ge islands on Si, samples were grown using substrates containing the square and long stripe type of patterns of windows in a SiO2 layer on the Si wafer. These patterns were oriented along 110 directions with the side length (of the square
Fig. 1. SEM micrographs of self-assembled Ge dots deposited by MBE at 700 °C (a) with one Ge layer and (b) with two Ge layers spaced with a 25 nm Si spacer. Each Ge layer contains 10 ML of Ge.
Y.-H. Wu et al. / Materials Science and Engineering B89 (2002) 151–156
153
3. Results and discussion
3.1. Ge deposition on bare Si substrates
Fig. 2. A SEM micrograph of self-organized Ge dots along the bottom edge of a ridge on the 110-oriented Si stripe mesa. Some 5 ML Ge was deposited at 700 °C.
patterns) or the terrace width (of the long stripe patterns) ranging from 2 to 100 mm. The processing of the patterned substrates started with thermal oxidation of the p-type substrates forming a SiO2 layer of 200 nm. The windows were defined by the photolithographic technique, followed by reactive ion etching using CHF3 as an etchant. The remaining oxide layer then acted as a mask for the anisotropic wet chemical etching of Si in a KOH-based solution. The etching depth was :1 mm. Since the etching rate on the Si {111} planes is much slower than that on the {100} planes, the eventually formed patterns contained a flat (001) surface at the bottom and slopes oriented to the 111 directions at the surrounding edges. Prior to loading the substrates into the MBE chamber, the samples were cleaned by HF dip plus UV ozone exposure, according to the procedure described in Ref. [3]. After sublimating the thin protecting oxide at 830 °C at UHV, a sharp 2 × 1 reconstruction pattern was observed by reflection high-energy electron diffraction (RHEED), indicating an atomically clean surface. The MBE process started with a 70 nm thick Si buffer layer grown at 700 °C. Some 5 – 10 ML of Ge was subsequently deposited at 500– 700 °C. For some structures, two Ge dot layers were grown with a 25 nm thick Si spacer layer for separation. RHEED was used for monitoring the change of surface reconstruction patterns and morphology during the growth. Highresolution scanning electron microscopy (SEM), Leo 1550 equipped with the field-emission source (operating at 5 kV) and GEMINI column and atomic force microscopy (AFM), Digital Instruments nanoscope IIIa operating in air with standard Si cantilevers, were then used for ex-situ characterization of grown dot samples.
A sharp 2×1 RHEED pattern was usually observed during the MBE Si buffer layer growth. However, the intensity of 1/2 reconstructed RHEED spots was faded out gradually, when starting the growth of Ge. A Si [110] bulk-like diffraction pattern was observed after the growth of : 3 ML Ge, indicating formation of Ge 3D islands. This is consistent with previously reported results [2]. AFM studies of these MBE Ge samples, measured in the tapping mode, revealed that these Ge islands deposited on bare Si substrates were randomly distributed both in size and position. Typically, the mean size of islands was increased as the Ge coverage was increased, while the island density increased when decreasing the substrate temperature. The size distribution can be altered very dramatically for a sample with a second Ge layer deposited after capping of the first Ge layer with a 25 nm thick Si spacer layer for separation. A comparison can be seen in Fig. 1 between two samples prepared at 700 °C: (a) with one Ge layer deposition (10 ML), and (b) with two Ge layers (each with 10 ML) spaced by 25 nm Si. One can see for one layer deposition there are mainly three groups of islands, 100–160, 60–100 and B60 nm, while for the stacked structure only one size group (150–200 nm) is observed. Although the island sizes are different between the two samples, the total density of Ge islands for the sum of medium and large size islands is nearly the same value (:1× 109 cm − 2) for both samples. This is an expected result. The strain introduced by the lattice perturbation from the islands in the first Ge land acts as a seeding position for Ge islanding during deposition of the second Ge layer, i.e. the nucleation of Ge islands only occurred on places with a large enough strain [4], thus forming self-aligned dot column [4]. This process provides a better homogeneity of the Ge island size distribution. A larger island size observed in Fig. 1(b) is also expected, since the Ge wetting layer is getting thinner for the second Ge layer [5], such that more Ge materials contribute to the 3D growth.
3.2. Ge deposition on patterned Si substrates For practical device applications, it is desirable to obtain laterally ordered Ge dots in a controllable way. One way to achieve that is to introduce patterns on the substrate as nucleation sites for formation of Ge islands. As has been successfully demonstrated by Jin et al. [6], by using selective epitaxy of gas-source MBE, Ge dots with regular spacing can be perfectly aligned on the ridges of Si stripe mesas, forming a one-dimensionally ordered dot array.
154
Y.-H. Wu et al. / Materials Science and Engineering B89 (2002) 151–156
In this work, we followed the similar idea of Ref. [6] to deposit 5–10 ML of Ge at 700 °C on pre-prepared Si substrates containing both stripes and squares. As shown in Fig. 2, in an SEM image, Ge was preferentially forming islands along the bottom edge of the ridge. The size of Ge islands at the bottom edge of the ridge was larger than that of dots formed on the free surface and they could be regularly spaced. This phenomenon can be explained by enhanced Ge coverage at the edge of the Si ridge due to Ge surface diffusion. The size and spatial separation was then determined by the balance between the strain energy of the dots and the repulsive interaction of the neighboring dots. It is noted that there is a critical requirement for the sidewall smoothness of the etched Si ridge, in order to obtain uniform long range ordering of Ge dots. This has however been limited by our present lithographic capability.
Furthermore, SEM results revealed that the density of Ge dots on the free surface was a function of the spatial separation, W, between two Si ridges. In the present study, the samples were prepared with W values of both the stripe and square patterns varying from 2 to 100 mm. Fig. 3(a–c) are four examples of SEM images showing the W dependence of the Ge dots distribution on the stripe and square patterns with a growth temperature of 700 °C and 10 ML of Ge. Measurements of the Ge dot density showed that although the size of dots formed on the free surface is rather constant when depositing Ge on various sizes of the surface area, the dot density varied quite a lot. The results are summarized in Fig. 4(a,b) for the stripe and square patterns, respectively. One can see in Fig. 4(a) that the Ge dot density increased as the spatial separation between two adjacent Si ridges was increased for W B 15 mm. Thereafter,
Fig. 3. SEM micrographs of Ge dot distributions after 10 ML Ge deposition at 700 °C on a Si substrate containing stripe patterns with the terrace width of (a) 3.5 mm and (b) 6.5 mm, and square patterns with the terrace width of (c) 6.5 mm and (d) 8 mm, receptively.
Y.-H. Wu et al. / Materials Science and Engineering B89 (2002) 151–156
Fig. 4. Ge dot density as a function of spatial separation of Si ridges of (a) stripe patterns and (b) square patterns on a Si(001) substrate. The Ge deposition was made at 700 °C.
155
onset of the saturation dot density occurred at W :30 mm. This value is two times higher than that observed from the stripe patterns. The change of the Ge dot density on the free surface when changing the W between ridges has been attributed to consumption of Ge at the bottom edge of Si ridges. Since there are preferential nucleation sites along the bottom edge of Si ridges, these islands will act as Ge sinks [7]. The diffusion length of the Ge adatoms on the surface is determined by the substrate surface temperature. Less and less Ge atoms could eventually reach the bottom edges when the ridge separation was increased, such that the probability to nucleate Ge islands was increased. The fact that a saturation value was observed for W\ 15 mm, thus implies a mean Ge diffusion length of :7.5 mm at 700 °C. The surface diffusion limited Ge islanding phenomena also explains why the saturation value of the dot density was observed for W\ 30 mm for square patterns, because in this case more Ge materials were consumed by ridges in all four directions. For WB 5 mm, the dot density increased slightly when the ridge separation was further decreased. This might be due to some additional material supply because of Ge diffusion from the slope areas, when the terrace area is comparable with the slope area along ridges. Therefore, there must be a balance where all deposited Ge can be consumed at the bottom edge sites while leaving a flat surface free of Ge dots, i.e. only forming Ge dots along the bottom edge of ridges. An example of such self-ordered Ge dots is shown in Fig. 5, for the small square patterns (W= 3 mm, the bottom area is : 1 mm2).
4. Summary In summary, we reported a study on synthesis of self-assembled Ge dots on both bare and patterned Si(100) substrates during the MBE growth. Uniform size of Ge dots has been obtained through the layer stacking process. Surface diffusion-limited Ge nucleation processes have been studied for growth on patterned substrates with spatial separation between Si ridges, ranging from 2 to 100 mm. A mean Ge diffusion length of : 7.5 mm was determined when depositing Ge on a Si(001) substrate with stripe or square patterns at 700 °C. Fig. 5. SEM micrograph of self-ordered Ge dots along the bottom edge of Si ridges of 3 mm wide square patterns, with 10 ML Ge deposition at 700 °C.
Acknowledgements
the dot density approached a saturation value of : 1× 109 cm − 2. A similar trend was also observed for the Ge deposition on square patterns, but in that case, the
Y.-H. Wu thanks TSMC in Taiwan for the financial support for her study at Linko¨ ping University. The work was partly supported by the Swedish Engineering
156
Y.-H. Wu et al. / Materials Science and Engineering B89 (2002) 151–156
Science Foundation (TFR) and the Swedish Foundation for the Strategic Research (SSF).
References
[3] [4] [5] [6]
[1] P.M. Petroff, G. Mederiros-Ribeiro, MRS Bull. 21 (4) (1996) 50. [2] K. Tillmann, H. Trinkaus, W. Ja¨ ger, Properties of silicon germanium and SiGe:carbon, in: EMIS Data Review Series No.
[7]
24, INSPEC, IEEE, London, 2000, pp. 63 – 74 References therein. W.-X. Ni, J. Ekberg, K.B. Joelsson, H. Radamson, A. Henry, G.-D. Shen, G.V. Hansson, J. Cryst. Growth 157 (1995) 285. J. Tersoff, F.K. LeGoues, Phys. Rev. Lett. 72 (1994) 3570. O.G. Schmidt, O. Kienze, Y. Hao, K. Eberl, F. Ernst, Appl. Phy. Lett. 74 (1999) 1272. G. Jin, J.L. Liu, Y.H. Luo, K.L. Wang, Thin Sol. Films 369 (2000) 49. A. Hartmann, C. Dieker, R. Loo, L. Vescan, H. Luth, U. Bangery, Appl. Phys. Lett. 67 (1995) 1888.
www.elsevier.com/locate/mseb
Surface diffusion limited nucleation of Ge dots on the Si(001) surface Y.-H. Wu, C.-Y. Wang, A. Elfving, G.V. Hansson, W.-X. Ni * Department of Physics, Linko¨ping Uni6ersity, SE-581 83 Linko¨ping, Sweden
Abstract The formation of Ge islands during MBE growth is a spontaneous process and these islands, i.e. dots, are usually randomly arranged. In order to implement these nanoscaled islands into device applications, ordering of epitaxial dots is a crucial step. We report a study on the MBE growth of Ge islands on Si(001) substrates, containing 110-oriented square and long stripe type patterns defined by anisotropic wet etching of Si, in order to provide more understanding of how surface diffusion of Ge atoms would influence the formation of Ge islands on various types of surfaces. It has been found that there were preferential nucleation sites for Ge islands along the bottom edges of the Si ridges. The Ge islands at the edge positions were larger than those formed on the free surface and they could be regularly spaced. Due to the consumption of Ge at the bottom edges of ridge patterns, the density of Ge dots on the free surface varied between : 3 ×108 and :1× 109 cm − 2 when changing the spatial separation between two adjacent Si ridges (2–100 mm). A Ge mean diffusion length of : 7.5 mm has been determined for Ge growth at 700 °C. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Quantum dot; Nucleation; Diffusion; MBE; SEM; Ge
1. Introduction Recent research on low dimensional semiconductor structures has shown that by reducing the structural dimensions, important material and electronic properties of semiconductors, such as lattice strain, density of states, bandgap, band offset, etc., can be manipulated and tailored according to desire [1]. This is the so-called quantum engineering, which has enabled some advanced devices and is expected to have a more extensive impact on future applications in high speed electronics and optoelectronics, to achieve previously unattainable limits for device performance. One way to synthesize such low-dimensional semiconductor systems is to selfassemble 3-dimensional crystalline clusters (islands) with a very small diameter (10 – 100 nm). It is known that when the growth of a Ge layer exceeds a critical thickness (:3 ML) during molecular beam epitaxy (MBE), the growth mode will be changed
* Corresponding author. Tel.: +46-13-282-474; fax: +46-13-137568. E-mail address: [email protected] (W.-X. Ni).
from layer-by-layer (2-dimensional, 2D) to formation of Ge islands (3-dimensional, 3D) [2]. The layer growth with such a 2D/3D transition is called Stransky –Krastanov mode, which is a spontaneous process and the island size is strongly affected by the substrate temperature. These islands are usually randomly arranged. In order to implement these nanoscaled islands (dots) into device applications, ordering of epitaxial islands is a crucial step. The objective of this work was thus to provide more understanding how surface diffusion of Ge atoms would influence the formation of Ge islands on various types of surfaces, by carrying out a study of the Ge MBE growth on specially designed Si(001) substrates, containing 110-oriented square and long stripe type of patterns defined by anisotropic wet etching of Si. The separation between two adjacent Si ridges in the pattern varied between 2 and 100 mm. These samples were then characterized by high-resolution scanning electron microscopy (SEM) and atomic force microscopy (AFM). Statistics on the Ge island size and density have been made on various samples grown with different conditions, in order to extract information on the surface diffusion limited Ge islanding process.
0921-5107/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S0921-5107(01)00822-4
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Y.-H. Wu et al. / Materials Science and Engineering B89 (2002) 151–156
2. Experimental All samples used in this study were grown using a solid-source MBE apparatus, Balzers UMS 630, which has been described in detail previously [3]. The Si and Ge fluxes were supplied by the e-beam evaporators and regulated by a microprocessor controlled feed-back loop with a cross-beam mass-spectrometer as a rate monitor. The base pressure in the growth chamber was
5 5× 10 − 11 mbar and could be increased to :1× 10 − 9 mbar during the e-gun evaporation. The substrates used for MBE growth were Si(001) wafers. In order to study the nucleation and growth kinetics of self-assembled Ge islands on Si, samples were grown using substrates containing the square and long stripe type of patterns of windows in a SiO2 layer on the Si wafer. These patterns were oriented along 110 directions with the side length (of the square
Fig. 1. SEM micrographs of self-assembled Ge dots deposited by MBE at 700 °C (a) with one Ge layer and (b) with two Ge layers spaced with a 25 nm Si spacer. Each Ge layer contains 10 ML of Ge.
Y.-H. Wu et al. / Materials Science and Engineering B89 (2002) 151–156
153
3. Results and discussion
3.1. Ge deposition on bare Si substrates
Fig. 2. A SEM micrograph of self-organized Ge dots along the bottom edge of a ridge on the 110-oriented Si stripe mesa. Some 5 ML Ge was deposited at 700 °C.
patterns) or the terrace width (of the long stripe patterns) ranging from 2 to 100 mm. The processing of the patterned substrates started with thermal oxidation of the p-type substrates forming a SiO2 layer of 200 nm. The windows were defined by the photolithographic technique, followed by reactive ion etching using CHF3 as an etchant. The remaining oxide layer then acted as a mask for the anisotropic wet chemical etching of Si in a KOH-based solution. The etching depth was :1 mm. Since the etching rate on the Si {111} planes is much slower than that on the {100} planes, the eventually formed patterns contained a flat (001) surface at the bottom and slopes oriented to the 111 directions at the surrounding edges. Prior to loading the substrates into the MBE chamber, the samples were cleaned by HF dip plus UV ozone exposure, according to the procedure described in Ref. [3]. After sublimating the thin protecting oxide at 830 °C at UHV, a sharp 2 × 1 reconstruction pattern was observed by reflection high-energy electron diffraction (RHEED), indicating an atomically clean surface. The MBE process started with a 70 nm thick Si buffer layer grown at 700 °C. Some 5 – 10 ML of Ge was subsequently deposited at 500– 700 °C. For some structures, two Ge dot layers were grown with a 25 nm thick Si spacer layer for separation. RHEED was used for monitoring the change of surface reconstruction patterns and morphology during the growth. Highresolution scanning electron microscopy (SEM), Leo 1550 equipped with the field-emission source (operating at 5 kV) and GEMINI column and atomic force microscopy (AFM), Digital Instruments nanoscope IIIa operating in air with standard Si cantilevers, were then used for ex-situ characterization of grown dot samples.
A sharp 2×1 RHEED pattern was usually observed during the MBE Si buffer layer growth. However, the intensity of 1/2 reconstructed RHEED spots was faded out gradually, when starting the growth of Ge. A Si [110] bulk-like diffraction pattern was observed after the growth of : 3 ML Ge, indicating formation of Ge 3D islands. This is consistent with previously reported results [2]. AFM studies of these MBE Ge samples, measured in the tapping mode, revealed that these Ge islands deposited on bare Si substrates were randomly distributed both in size and position. Typically, the mean size of islands was increased as the Ge coverage was increased, while the island density increased when decreasing the substrate temperature. The size distribution can be altered very dramatically for a sample with a second Ge layer deposited after capping of the first Ge layer with a 25 nm thick Si spacer layer for separation. A comparison can be seen in Fig. 1 between two samples prepared at 700 °C: (a) with one Ge layer deposition (10 ML), and (b) with two Ge layers (each with 10 ML) spaced by 25 nm Si. One can see for one layer deposition there are mainly three groups of islands, 100–160, 60–100 and B60 nm, while for the stacked structure only one size group (150–200 nm) is observed. Although the island sizes are different between the two samples, the total density of Ge islands for the sum of medium and large size islands is nearly the same value (:1× 109 cm − 2) for both samples. This is an expected result. The strain introduced by the lattice perturbation from the islands in the first Ge land acts as a seeding position for Ge islanding during deposition of the second Ge layer, i.e. the nucleation of Ge islands only occurred on places with a large enough strain [4], thus forming self-aligned dot column [4]. This process provides a better homogeneity of the Ge island size distribution. A larger island size observed in Fig. 1(b) is also expected, since the Ge wetting layer is getting thinner for the second Ge layer [5], such that more Ge materials contribute to the 3D growth.
3.2. Ge deposition on patterned Si substrates For practical device applications, it is desirable to obtain laterally ordered Ge dots in a controllable way. One way to achieve that is to introduce patterns on the substrate as nucleation sites for formation of Ge islands. As has been successfully demonstrated by Jin et al. [6], by using selective epitaxy of gas-source MBE, Ge dots with regular spacing can be perfectly aligned on the ridges of Si stripe mesas, forming a one-dimensionally ordered dot array.
154
Y.-H. Wu et al. / Materials Science and Engineering B89 (2002) 151–156
In this work, we followed the similar idea of Ref. [6] to deposit 5–10 ML of Ge at 700 °C on pre-prepared Si substrates containing both stripes and squares. As shown in Fig. 2, in an SEM image, Ge was preferentially forming islands along the bottom edge of the ridge. The size of Ge islands at the bottom edge of the ridge was larger than that of dots formed on the free surface and they could be regularly spaced. This phenomenon can be explained by enhanced Ge coverage at the edge of the Si ridge due to Ge surface diffusion. The size and spatial separation was then determined by the balance between the strain energy of the dots and the repulsive interaction of the neighboring dots. It is noted that there is a critical requirement for the sidewall smoothness of the etched Si ridge, in order to obtain uniform long range ordering of Ge dots. This has however been limited by our present lithographic capability.
Furthermore, SEM results revealed that the density of Ge dots on the free surface was a function of the spatial separation, W, between two Si ridges. In the present study, the samples were prepared with W values of both the stripe and square patterns varying from 2 to 100 mm. Fig. 3(a–c) are four examples of SEM images showing the W dependence of the Ge dots distribution on the stripe and square patterns with a growth temperature of 700 °C and 10 ML of Ge. Measurements of the Ge dot density showed that although the size of dots formed on the free surface is rather constant when depositing Ge on various sizes of the surface area, the dot density varied quite a lot. The results are summarized in Fig. 4(a,b) for the stripe and square patterns, respectively. One can see in Fig. 4(a) that the Ge dot density increased as the spatial separation between two adjacent Si ridges was increased for W B 15 mm. Thereafter,
Fig. 3. SEM micrographs of Ge dot distributions after 10 ML Ge deposition at 700 °C on a Si substrate containing stripe patterns with the terrace width of (a) 3.5 mm and (b) 6.5 mm, and square patterns with the terrace width of (c) 6.5 mm and (d) 8 mm, receptively.
Y.-H. Wu et al. / Materials Science and Engineering B89 (2002) 151–156
Fig. 4. Ge dot density as a function of spatial separation of Si ridges of (a) stripe patterns and (b) square patterns on a Si(001) substrate. The Ge deposition was made at 700 °C.
155
onset of the saturation dot density occurred at W :30 mm. This value is two times higher than that observed from the stripe patterns. The change of the Ge dot density on the free surface when changing the W between ridges has been attributed to consumption of Ge at the bottom edge of Si ridges. Since there are preferential nucleation sites along the bottom edge of Si ridges, these islands will act as Ge sinks [7]. The diffusion length of the Ge adatoms on the surface is determined by the substrate surface temperature. Less and less Ge atoms could eventually reach the bottom edges when the ridge separation was increased, such that the probability to nucleate Ge islands was increased. The fact that a saturation value was observed for W\ 15 mm, thus implies a mean Ge diffusion length of :7.5 mm at 700 °C. The surface diffusion limited Ge islanding phenomena also explains why the saturation value of the dot density was observed for W\ 30 mm for square patterns, because in this case more Ge materials were consumed by ridges in all four directions. For WB 5 mm, the dot density increased slightly when the ridge separation was further decreased. This might be due to some additional material supply because of Ge diffusion from the slope areas, when the terrace area is comparable with the slope area along ridges. Therefore, there must be a balance where all deposited Ge can be consumed at the bottom edge sites while leaving a flat surface free of Ge dots, i.e. only forming Ge dots along the bottom edge of ridges. An example of such self-ordered Ge dots is shown in Fig. 5, for the small square patterns (W= 3 mm, the bottom area is : 1 mm2).
4. Summary In summary, we reported a study on synthesis of self-assembled Ge dots on both bare and patterned Si(100) substrates during the MBE growth. Uniform size of Ge dots has been obtained through the layer stacking process. Surface diffusion-limited Ge nucleation processes have been studied for growth on patterned substrates with spatial separation between Si ridges, ranging from 2 to 100 mm. A mean Ge diffusion length of : 7.5 mm was determined when depositing Ge on a Si(001) substrate with stripe or square patterns at 700 °C. Fig. 5. SEM micrograph of self-ordered Ge dots along the bottom edge of Si ridges of 3 mm wide square patterns, with 10 ML Ge deposition at 700 °C.
Acknowledgements
the dot density approached a saturation value of : 1× 109 cm − 2. A similar trend was also observed for the Ge deposition on square patterns, but in that case, the
Y.-H. Wu thanks TSMC in Taiwan for the financial support for her study at Linko¨ ping University. The work was partly supported by the Swedish Engineering
156
Y.-H. Wu et al. / Materials Science and Engineering B89 (2002) 151–156
Science Foundation (TFR) and the Swedish Foundation for the Strategic Research (SSF).
References
[3] [4] [5] [6]
[1] P.M. Petroff, G. Mederiros-Ribeiro, MRS Bull. 21 (4) (1996) 50. [2] K. Tillmann, H. Trinkaus, W. Ja¨ ger, Properties of silicon germanium and SiGe:carbon, in: EMIS Data Review Series No.
[7]
24, INSPEC, IEEE, London, 2000, pp. 63 – 74 References therein. W.-X. Ni, J. Ekberg, K.B. Joelsson, H. Radamson, A. Henry, G.-D. Shen, G.V. Hansson, J. Cryst. Growth 157 (1995) 285. J. Tersoff, F.K. LeGoues, Phys. Rev. Lett. 72 (1994) 3570. O.G. Schmidt, O. Kienze, Y. Hao, K. Eberl, F. Ernst, Appl. Phy. Lett. 74 (1999) 1272. G. Jin, J.L. Liu, Y.H. Luo, K.L. Wang, Thin Sol. Films 369 (2000) 49. A. Hartmann, C. Dieker, R. Loo, L. Vescan, H. Luth, U. Bangery, Appl. Phys. Lett. 67 (1995) 1888.