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Selective growth of InAs islands on patterned GaAs (100) substrate
Superlattices and Microstructures 39 (2006) 446–453 www.elsevier.com/locate/superlattices
Selective growth of InAs islands on patterned GaAs (100) substrate C.X. Cui ∗ , Y.H. Chen, Y.Y. Ren, B. Xu, P. Jin, C. Zhao, Z.G. Wang Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, PR China Received 19 June 2005; received in revised form 26 September 2005; accepted 10 October 2005 Available online 7 December 2005
Abstract By a combination of prepatterned substrate and self-organized growth, InAs islands are grown on the stripe-patterned GaAs (100) substrate by solid-source molecular beam epitaxy. It is found that the InAs quantum dots can be formed either on the ridge or on the sidewall of the stripes near the bottom, depending on the structure of the stripes on the patterned substrate or molecular beam epitaxy growth conditions. When a Inx Ga1−x As strained layer is grown first before InAs deposition, almost all the InAs quantum dots are deposited at the edges of the top ridge. And when the InAs deposition amount is larger, a quasiquantum wire structure is found. The optical properties of the InAs dots on the patterned substrate are also investigated by photoluminescence. c 2005 Elsevier Ltd. All rights reserved. Keywords: Patterned substrate; Molecular beam epitaxy; Quantum dots; InAs; GaAs; InGaAs
1. Introduction Self-assembled quantum dots formed by Stranski–Krastanov growth mode have been intensively investigated for their promising technological applications. For the most parts, the positions of these self-assembled quantum dots (QDs) are random. This disadvantage hinders the application of the QDs in some electronic and optoelectronic devices, where one- or twodimensionally ordered islands are desired [1,2]. In most of the recent reports, a combination of pre-patterned substrate and self-organized growth is employed to achieve long-range ordering ∗ Corresponding author. Tel.: +86 1082 304 563; fax: +86 1082 305 052.
E-mail address: [email protected] (C.X. Cui). c 2005 Elsevier Ltd. All rights reserved. 0749-6036/$ - see front matter doi:10.1016/j.spmi.2005.10.004
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of the islands, in which different diffusion of the adatoms on the faceted surfaces are used to control the islands’ nucleation [3–6]. For the growth of InAs islands on GaAs (100) pre-patterned substrate, many interesting phenomena have been observed. For instance InAs self-assembled QDs can be preferentially grown either on the top terraces or at the sidewalls or at the trenches of a stripe-patterned GaAs substrate [7,8]. In this paper, the InAs islands tend to form at the foot of mesas when no strained buffer layers are used, but form the InAs quantum dots at the edges of the top ridge when a In0.2Ga0.8 As strained layer is deposited first. When the InAs deposition amount is larger, a quasi-quantum wire structure is found. Although the same phenomena have been observed by other groups [9], the quasi-quantum wire structure has not been reported. The various morphologies and structures occurring during self-assembly driven by the relaxation of mismatch strain usually provide significant information on growth kinetics and the nucleation mechanism, so further studies on the growth on a prepatterned substrate are necessary. 2. Experiment The patterned GaAs (100) substrate was fabricated by electron beam lithography (EBL) and wet chemical etching in the H3 PO4 :H2 O2 :H2 O (3:1:50) solution. Different pitches and depths of ¯ direction the stripes were formed. In addition, considering the ridge oriented along the [011] provided a more uniform structure compared to those in the [011] direction when the wet chemical etching on the GaAs (100) substrate, all the stripes in this paper were only oriented ¯ direction. The pre-patterned substrate was further cleaned by irradiation of O2 along the [011] plasma after the EB-resist was removed. And about 5 nm of GaAs was removed from the open surface of the substrate by wet etching before loading the sample into the MBE chamber. On samples A, B, C and E with the same stripe patterns (pitch: 300 nm; depth: 50 nm), a different epitaxial structure was grown by MBE. For sample A, after oxide desorption at 580 ◦ C, ˚ a 30 nm GaAs buffer layer was grown at 600 ◦ C with the growth rate of 2.78 A/s. Subsequently, 2.6 monolayers (MLs) InAs were deposited at the rate of 0.06 ML/s at 510 ◦ C with 1.7 s growth interruptions after each 0.1 ML InAs. The As4 overpressure was constant as 5 × 10−5 Torr. For sample B, the epilayer consisted of 30 nm GaAs, and two layers of 2.6 ML InAs separated by 50 nm GaAs. For samples C and E, a 20 nm In0.2 Ga0.8 As strained layer and 10 nm GaAs were grown on the 30 nm GaAs buffer layer before InAs deposition. The growth rate of In0.2Ga0.8 As and GaAs were 0.3 ML/s and 0.24 ML/s, respectively. The growth temperature was 510 ◦ C. The InAs deposition amount was 2.0 ML for sample C, while 2.6 ML for sample E. For sample D, on the stripe pattern with pitch of 200 nm and depth of 20 nm, the same epitaxial structures were grown as on sample E. After growth, the surface morphologies of samples were measured by atomic force microscopy (AFM) in contact mode. Low-temperature photoluminescence (PL) spectroscopy was measured on samples A and B. 3. Results and discussion Fig. 1(a) shows an AFM image of the epitaxial surface of sample A. The two line-scan sections (Fig. 1(b)) perpendicular to the stripes confirm the actual InAs island positions. It is apparent that most quantum dots are located at the conjunction of the sidewall and the bottom. And some islands are located on the top ridge. The morphologies of the top ridges in the line e and f are different, especially the height, the width, and local curvatures. Combining the AFM image, it is considered that the convex region on the ridge in line f is an InAs dot, while it is an
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Fig. 1. (a) The AFM image (1.0 µm × 1.0 µm) of the surface morphology of sample A; the arrows indicate the sporadic large InAs islands deposited on the top ridges. (b) Height profiles along line e and f ; the arrows here indicate the InAs islands.
InAs wetting layer on the top ridge in line e. The islands located on the top ridge are random, and the sporadic large islands are found on the ridges, as marked with arrows in Fig. 1(a). The respective average height and base width of the InAs islands on sample A are estimated to be 9.8 and 38.9 nm. The sizes are larger than those on the flat substrate for reference in this work (height: 9.1 nm, width: 28.2 nm). The larger size of the InAs QDs on the patterned GaAs substrate than on the flat one is consistent with the other observations [10]. Fig. 2 shows the AFM image of the epitaxial surface of sample B, whose epitaxial layer was overgrown on the same epilayer structure as sample A and it consists of 50 nm GaAs, and a 2.6 ML InAs layer. The figure shows that the 50 nm GaAs layer over the sample A structure reduces the height fluctuation on the stripe ridges, as compared to sample A. The curvatures on the top ridges are uniform and smaller than those on sample A. Ordering InAs islands are strikingly formed at the foot of the structures, and more than one aligned InAs island chain along the stripe pattern are observed, which are illustrated by arrows. Once the islands nucleated on the preferential positions during growth, it will give rise to stronger strain at their base, which will prevent more indium atoms from attaching to the formed islands. So more than one dot chain
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Fig. 2. The AFM image (500 nm × 500 nm) of the surface morphology of sample B; aligned InAs islands chains are observed, which are illustrated by arrows.
Fig. 3. Island size distribution on samples A, B and flat area. The solid lines are from the Gaussian fits, and the average heights of islands and the linewidth on samples are also indicated. Sample B shows the narrowest size distribution when comparing with those on sample A and flat area.
are formed at one sidewall in sample B. These islands are with height of 11.8 nm and width of 38.2 nm. Fig. 3 shows height histograms of islands from above two samples (A and B) and a flat area; A bimodal size distribution of islands is found on sample A, and a monomodal distribution is shown for sample B, which is narrower than that on the flat area. Overgrowth of a thicker GaAs buffer layer made the pattern more uniform, so a monomodal distribution is shown on sample B. On the other hand, for sample A, more than one deposition position, which governed by the different nucleation mechanisms, could widen the island size distribution.
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Fig. 4. 20 nm In0.2 Ga0.8 As strained layer and 10 nm GaAs were grown on a GaAs buffer layer before InAs island deposition. (a) The AFM image of the surface morphology of sample C with 2.0 ML InAs deposition. (b) Elongated InAs QDs are formed at the edges of stripe mesas for sample D. (c) Quasi-quantum wires are formed at the edges of the top mesas for sample E. (d) Line-scan sections perpendicular to the grating of samples A, B, C, E and the original mesa before overgrowth, demonstrate the morphological evolution of the mesas.
For samples C, D and E, a In0.2 Ga0.8 As strain layer was introduced, and different morphologies are formed. Fig. 4(a), (b) and (c) show the AFM images of the surface morphology of samples C, D and E, respectively. On sample C, ordering the InAs QDs are selectively fabricated at the edges of the stripe mesas, and sporadic QDs are found at the groove of the stripes. On sample D, with larger InAs deposition amount, it is found that almost all the InAs QDs are located at the edges of mesas as those on sample C, however, they are elongated along the ¯ direction (Fig. 4(b)). On the other hand, some grooves have appeared on some mesas, the [011] possible reason may be the complex mismatch dislocation by InGaAs strained layer, however, further studies are need. While for sample E, quasi-quantum wires are formed at the edges of the top ridges (Fig. 4(c)). Fig. 4(d) shows the morphological evolution of the mesas of samples A, B, C and E with the same patterns during MBE growth. It could be clearly seen that for sample B, overgrowth of GaAs lowers the curvature of top ridge. And for samples C and E, which introduced an InGaAs strain layer, the top terraces evolve into a shape with a lower central region and convex humps at the sides. In addition, the flat bottom of the mesa has disappeared and the sidewall becomes wider and connected with each other on some regions. From the above five samples, the diffusion of the indium adatoms affects the selective growth of InAs islands. The growth kinetics, i.e., the migration of adatoms, will be discussed as follows.
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For GaAs (100), the natural surface anisotropy is mainly determined by its corrugated (2 × 4) reconstruction, which the As-dimer rows run along the [01-1] direction. Considering the surface reconstruction on the terraces, there are two types of steps that are parallel (type-A) and ¯ direction on the sidewall of the mesa [11]. Only the steps perpendicular (type-B) to the [011] parallel to the mesa direction will contribute to the growth front evolution. In this paper, stripe ¯ direction, the sidewalls of the stripes are assumed mainly composed patterns are along the [011] of type-A steps. The adatoms can readily migrate to the edge of the top terrace at the beginning of each monolayer growth. Because the barrier for adatoms to move upward over the step is always higher than that for moving downwards [12,13], a part of these adatoms at the edge of the terraces can then migrate downward over the steps and a part will move back to the top terraces. In addition, the In adatoms diffusion are not only affected by the step types, the structure of the sidewall, but also by the strain energy relaxation. During epitaxial growth, the wetting layer of InAs on GaAs is under compressive strain because of the 7% lattice mismatch between the InAs and GaAs. The convex regions are most favorable for strain relaxation as the compressed InAs atoms can stretch out more easily. Thus, for a curved compressively strained film, the strain is partially relieved in the convex regions relative to a flat film. The degree of strain relaxation depends on local curvature. So the InAs QDs can be formed on the ridges by relaxation of the strained layer, and the In adatoms can diffuse upward to the top mesa. Because of the irregular morphologies of the top ridges for sample A, the local curvatures on some regions are so high that the InAs dots could be formed by strain relaxation, while some regions keep layer growth. It is consistent with the results of the line-scan section in Fig. 1(b). Two main growth mechanisms existed together, which drive the In adatoms diffusion in a contrary direction perpendicular to the stripes, and in which direction the In adatoms prefer to diffuse is determined by the competition between the two growth mechanisms. The strain relaxation can be modulated by the various curvatures of the top ridge or by introducing InGaAs strained layer. The InAs QDs deposition positions are also changed with it. For sample B, the effects of the vertical replication of the InAs islands could be neglected for the thicker space layer. Smaller curvature of the top ridge weakened the strain energy relaxation, so no islands are formed on the top ridges. However, when the nucleation mechanism is dominated by the strain energy, such as by introducing the strained In0.2 Ga0.8 As buffer layer, the In adatoms diffusion is dominated by the strain energy, which alters substantially the migration of the In adatoms perpendicular to the stripes. More of the In adatoms deposited in the valley will diffuse upwards to the mesas, and the adatoms deposited on the mesas can readily migrate to the edges at the beginning of the each monolayer growth. So the flat terraces evolve into a shape with a lower central region and convex humps at the sides before InAs QDs formed. And the InAs islands preferentially nucleated on these convex humps at the edges of the mesas. From the shape of single InAs QDs on sample D, elongated along the stripe pattern, it could be deduced that the In adatoms are more likely to diffuse along the edge of the stripes when the InAs deposition amount is larger. Different pitch of the stripe patterns affected the InAs deposition amount on every period pattern under the same growth conditions. For samples D and E, the pitches of the stripe are 200 nm and 300 nm, respectively. It is considered that the amount of InAs on the unit pattern of sample E is larger than those on sample D. So the hump structures on both sides of the top terraces are formed by the InAs islands coalesced with each other along the stripe pattern. In order to investigate the optical properties of the QDs on a patterned substrate, the PL measurements on the samples A, B and flat surface at low temperature of 77 K have been done.
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Fig. 5. (a) Low temperature of 77 K PL spectra of InAs QDs on a patterned substrate of samples A, B and the flat area. (b) PL spectra of InAs QDs on sample A with various excitation intensity, Iex = 13, 25, 50 and 100 mW/cm2 . (c) PL spectra of InAs QDs on sample A with Iex = 13 mW/cm2 compared with Gaussian fit.
Fig. 5(a) shows the PL measurements with the excitation intensity Iex of 13 mW/cm2 . One peak is observed on every PL spectra, which is attributed to the ground state of the InAs QDs. The linewidth (35.68 meV) of the ground state of the InAs islands on sample B is better than those on sample A (57.12 meV) and flat area (46.74 meV). In addition, the PL intensity of the QD array on sample B is much higher than those on the flat area and sample A. The peak energy (1.104 eV) of the QDs on the flat area is higher than that of the InAs quantum dot array on samples A (1.085 eV) and B (1.095 eV). This is probably due to the slightly smaller size of the islands’ array compared to the size of the islands on patterns. As Fig. 5(b) shows, for sample A with higher excitation intensity, Iex = 25, 50 and 100 mW/cm2 , the sub-peak is observed, which is attributed to the excited state of QDs. However, from Fig. 5(c), the PL spectra are somewhat different with the Gaussian fit. It may be caused by the wider size distribution on sample A. 4. Conclusion In a word, ordered self-assembled InAs QDs are grown on the stripe-patterned GaAs (100) substrate. The nucleation mechanisms on the different surfaces are demonstrated. Nucleation on a flat surface is random and the QDs density is limited by the atom diffusion. While on patterned
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substrate, the nucleation mechanism is not only affected by the specific atomic configuration, i.e., surface reconstruction, and step density of sidewall, but also strain energy, which contributed to the contrary diffusion direction for the In adatoms. The strain relaxation can be modulated by the various curvatures of top ridge or by introducing an InGaAs strained layer. When the InGaAs strained layer was introduced, ordering InAs QDs are formed at the edges of top ridges, and the mesas evolve into a shape with a lower central region and convex humps at the sides. The InAs QDs are elongated along the stripe mesas and quasi-quantum wires are formed with a larger InAs deposition amount. On a patterned substrate, the diffusion of the indium adatoms affects the formation of InAs islands. Further studies are required to investigate the ordering islands’ growth on a patterned substrate. Acknowledgments The authors would like to acknowledge Z.G. Li and S.B. Long for substrate preparation. This research work is financially supported by Special Funds for Major State Basic Research Project of China (No. G2000068303), National Natural Science Foundation of China (No. 60390071, 60390074, 90101004), and National High Technology Research and Development Program of China (No. 2002AA311070). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
P. Schittenhelm, J. Vac. Sci. Technol. B 16 (1998) 1575. T. Lundstrom, W. Schoenfeld, H. Lee, P.M. Petroff, Science 286 (1999) 2312. H.J. Kim, J. Motohisa, T. Fukui, Appl. Phys. Lett. 81 (2002) 5147. B.H. Choi, C.M. Park, S.H. Song, Appl. Phys. Lett. 78 (2001) 1403. Z. Zhong, G. Bauer, Appl. Phys. Lett. 84 (2004) 1922. Y. Nakamura, O.G. Schmidt, N.Y. Jin-Phillipp, S. Kiravittaya, C. M¨uller, K. Eberl, H. Gr¨abeldinger, H. Schweizer, J. Cryst. Growth. 242 (2002) 339. D.S.L. Mui, D. Leonard, L.A. Coldren, P.M. Petroff, Appl. Phys. Lett. 66 (1995) 1620. S. Jeppesen, M.S. Miller, D. Hessman, Appl. Phys. Lett. 68 (1996) 2228. H. Lee, J.A. Johnson, M.Y. He, J.S. Speck, P.M. Petroff, Appl. Phys. Lett. 78 (2001) 105. S. Kiravittaya, H. Heidemeyer, O.G. Schmidt, Physica E 23 (2004) 253. Q. Gong, R. N¨otzel, H.-P. Sch¨onherr, Physica E 13 (2002) 1176. Q.M. Zhang, C. Roland, P. Boguslawski, Phys. Rev. Lett. 75 (1995) 101. H.J. Kim, Y.J. Park, Y.M. Park, E.K. Kim, Appl. Phys. Lett. 78 (2001) 3253.
Selective growth of InAs islands on patterned GaAs (100) substrate C.X. Cui ∗ , Y.H. Chen, Y.Y. Ren, B. Xu, P. Jin, C. Zhao, Z.G. Wang Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, PR China Received 19 June 2005; received in revised form 26 September 2005; accepted 10 October 2005 Available online 7 December 2005
Abstract By a combination of prepatterned substrate and self-organized growth, InAs islands are grown on the stripe-patterned GaAs (100) substrate by solid-source molecular beam epitaxy. It is found that the InAs quantum dots can be formed either on the ridge or on the sidewall of the stripes near the bottom, depending on the structure of the stripes on the patterned substrate or molecular beam epitaxy growth conditions. When a Inx Ga1−x As strained layer is grown first before InAs deposition, almost all the InAs quantum dots are deposited at the edges of the top ridge. And when the InAs deposition amount is larger, a quasiquantum wire structure is found. The optical properties of the InAs dots on the patterned substrate are also investigated by photoluminescence. c 2005 Elsevier Ltd. All rights reserved. Keywords: Patterned substrate; Molecular beam epitaxy; Quantum dots; InAs; GaAs; InGaAs
1. Introduction Self-assembled quantum dots formed by Stranski–Krastanov growth mode have been intensively investigated for their promising technological applications. For the most parts, the positions of these self-assembled quantum dots (QDs) are random. This disadvantage hinders the application of the QDs in some electronic and optoelectronic devices, where one- or twodimensionally ordered islands are desired [1,2]. In most of the recent reports, a combination of pre-patterned substrate and self-organized growth is employed to achieve long-range ordering ∗ Corresponding author. Tel.: +86 1082 304 563; fax: +86 1082 305 052.
E-mail address: [email protected] (C.X. Cui). c 2005 Elsevier Ltd. All rights reserved. 0749-6036/$ - see front matter doi:10.1016/j.spmi.2005.10.004
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of the islands, in which different diffusion of the adatoms on the faceted surfaces are used to control the islands’ nucleation [3–6]. For the growth of InAs islands on GaAs (100) pre-patterned substrate, many interesting phenomena have been observed. For instance InAs self-assembled QDs can be preferentially grown either on the top terraces or at the sidewalls or at the trenches of a stripe-patterned GaAs substrate [7,8]. In this paper, the InAs islands tend to form at the foot of mesas when no strained buffer layers are used, but form the InAs quantum dots at the edges of the top ridge when a In0.2Ga0.8 As strained layer is deposited first. When the InAs deposition amount is larger, a quasi-quantum wire structure is found. Although the same phenomena have been observed by other groups [9], the quasi-quantum wire structure has not been reported. The various morphologies and structures occurring during self-assembly driven by the relaxation of mismatch strain usually provide significant information on growth kinetics and the nucleation mechanism, so further studies on the growth on a prepatterned substrate are necessary. 2. Experiment The patterned GaAs (100) substrate was fabricated by electron beam lithography (EBL) and wet chemical etching in the H3 PO4 :H2 O2 :H2 O (3:1:50) solution. Different pitches and depths of ¯ direction the stripes were formed. In addition, considering the ridge oriented along the [011] provided a more uniform structure compared to those in the [011] direction when the wet chemical etching on the GaAs (100) substrate, all the stripes in this paper were only oriented ¯ direction. The pre-patterned substrate was further cleaned by irradiation of O2 along the [011] plasma after the EB-resist was removed. And about 5 nm of GaAs was removed from the open surface of the substrate by wet etching before loading the sample into the MBE chamber. On samples A, B, C and E with the same stripe patterns (pitch: 300 nm; depth: 50 nm), a different epitaxial structure was grown by MBE. For sample A, after oxide desorption at 580 ◦ C, ˚ a 30 nm GaAs buffer layer was grown at 600 ◦ C with the growth rate of 2.78 A/s. Subsequently, 2.6 monolayers (MLs) InAs were deposited at the rate of 0.06 ML/s at 510 ◦ C with 1.7 s growth interruptions after each 0.1 ML InAs. The As4 overpressure was constant as 5 × 10−5 Torr. For sample B, the epilayer consisted of 30 nm GaAs, and two layers of 2.6 ML InAs separated by 50 nm GaAs. For samples C and E, a 20 nm In0.2 Ga0.8 As strained layer and 10 nm GaAs were grown on the 30 nm GaAs buffer layer before InAs deposition. The growth rate of In0.2Ga0.8 As and GaAs were 0.3 ML/s and 0.24 ML/s, respectively. The growth temperature was 510 ◦ C. The InAs deposition amount was 2.0 ML for sample C, while 2.6 ML for sample E. For sample D, on the stripe pattern with pitch of 200 nm and depth of 20 nm, the same epitaxial structures were grown as on sample E. After growth, the surface morphologies of samples were measured by atomic force microscopy (AFM) in contact mode. Low-temperature photoluminescence (PL) spectroscopy was measured on samples A and B. 3. Results and discussion Fig. 1(a) shows an AFM image of the epitaxial surface of sample A. The two line-scan sections (Fig. 1(b)) perpendicular to the stripes confirm the actual InAs island positions. It is apparent that most quantum dots are located at the conjunction of the sidewall and the bottom. And some islands are located on the top ridge. The morphologies of the top ridges in the line e and f are different, especially the height, the width, and local curvatures. Combining the AFM image, it is considered that the convex region on the ridge in line f is an InAs dot, while it is an
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Fig. 1. (a) The AFM image (1.0 µm × 1.0 µm) of the surface morphology of sample A; the arrows indicate the sporadic large InAs islands deposited on the top ridges. (b) Height profiles along line e and f ; the arrows here indicate the InAs islands.
InAs wetting layer on the top ridge in line e. The islands located on the top ridge are random, and the sporadic large islands are found on the ridges, as marked with arrows in Fig. 1(a). The respective average height and base width of the InAs islands on sample A are estimated to be 9.8 and 38.9 nm. The sizes are larger than those on the flat substrate for reference in this work (height: 9.1 nm, width: 28.2 nm). The larger size of the InAs QDs on the patterned GaAs substrate than on the flat one is consistent with the other observations [10]. Fig. 2 shows the AFM image of the epitaxial surface of sample B, whose epitaxial layer was overgrown on the same epilayer structure as sample A and it consists of 50 nm GaAs, and a 2.6 ML InAs layer. The figure shows that the 50 nm GaAs layer over the sample A structure reduces the height fluctuation on the stripe ridges, as compared to sample A. The curvatures on the top ridges are uniform and smaller than those on sample A. Ordering InAs islands are strikingly formed at the foot of the structures, and more than one aligned InAs island chain along the stripe pattern are observed, which are illustrated by arrows. Once the islands nucleated on the preferential positions during growth, it will give rise to stronger strain at their base, which will prevent more indium atoms from attaching to the formed islands. So more than one dot chain
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Fig. 2. The AFM image (500 nm × 500 nm) of the surface morphology of sample B; aligned InAs islands chains are observed, which are illustrated by arrows.
Fig. 3. Island size distribution on samples A, B and flat area. The solid lines are from the Gaussian fits, and the average heights of islands and the linewidth on samples are also indicated. Sample B shows the narrowest size distribution when comparing with those on sample A and flat area.
are formed at one sidewall in sample B. These islands are with height of 11.8 nm and width of 38.2 nm. Fig. 3 shows height histograms of islands from above two samples (A and B) and a flat area; A bimodal size distribution of islands is found on sample A, and a monomodal distribution is shown for sample B, which is narrower than that on the flat area. Overgrowth of a thicker GaAs buffer layer made the pattern more uniform, so a monomodal distribution is shown on sample B. On the other hand, for sample A, more than one deposition position, which governed by the different nucleation mechanisms, could widen the island size distribution.
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Fig. 4. 20 nm In0.2 Ga0.8 As strained layer and 10 nm GaAs were grown on a GaAs buffer layer before InAs island deposition. (a) The AFM image of the surface morphology of sample C with 2.0 ML InAs deposition. (b) Elongated InAs QDs are formed at the edges of stripe mesas for sample D. (c) Quasi-quantum wires are formed at the edges of the top mesas for sample E. (d) Line-scan sections perpendicular to the grating of samples A, B, C, E and the original mesa before overgrowth, demonstrate the morphological evolution of the mesas.
For samples C, D and E, a In0.2 Ga0.8 As strain layer was introduced, and different morphologies are formed. Fig. 4(a), (b) and (c) show the AFM images of the surface morphology of samples C, D and E, respectively. On sample C, ordering the InAs QDs are selectively fabricated at the edges of the stripe mesas, and sporadic QDs are found at the groove of the stripes. On sample D, with larger InAs deposition amount, it is found that almost all the InAs QDs are located at the edges of mesas as those on sample C, however, they are elongated along the ¯ direction (Fig. 4(b)). On the other hand, some grooves have appeared on some mesas, the [011] possible reason may be the complex mismatch dislocation by InGaAs strained layer, however, further studies are need. While for sample E, quasi-quantum wires are formed at the edges of the top ridges (Fig. 4(c)). Fig. 4(d) shows the morphological evolution of the mesas of samples A, B, C and E with the same patterns during MBE growth. It could be clearly seen that for sample B, overgrowth of GaAs lowers the curvature of top ridge. And for samples C and E, which introduced an InGaAs strain layer, the top terraces evolve into a shape with a lower central region and convex humps at the sides. In addition, the flat bottom of the mesa has disappeared and the sidewall becomes wider and connected with each other on some regions. From the above five samples, the diffusion of the indium adatoms affects the selective growth of InAs islands. The growth kinetics, i.e., the migration of adatoms, will be discussed as follows.
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For GaAs (100), the natural surface anisotropy is mainly determined by its corrugated (2 × 4) reconstruction, which the As-dimer rows run along the [01-1] direction. Considering the surface reconstruction on the terraces, there are two types of steps that are parallel (type-A) and ¯ direction on the sidewall of the mesa [11]. Only the steps perpendicular (type-B) to the [011] parallel to the mesa direction will contribute to the growth front evolution. In this paper, stripe ¯ direction, the sidewalls of the stripes are assumed mainly composed patterns are along the [011] of type-A steps. The adatoms can readily migrate to the edge of the top terrace at the beginning of each monolayer growth. Because the barrier for adatoms to move upward over the step is always higher than that for moving downwards [12,13], a part of these adatoms at the edge of the terraces can then migrate downward over the steps and a part will move back to the top terraces. In addition, the In adatoms diffusion are not only affected by the step types, the structure of the sidewall, but also by the strain energy relaxation. During epitaxial growth, the wetting layer of InAs on GaAs is under compressive strain because of the 7% lattice mismatch between the InAs and GaAs. The convex regions are most favorable for strain relaxation as the compressed InAs atoms can stretch out more easily. Thus, for a curved compressively strained film, the strain is partially relieved in the convex regions relative to a flat film. The degree of strain relaxation depends on local curvature. So the InAs QDs can be formed on the ridges by relaxation of the strained layer, and the In adatoms can diffuse upward to the top mesa. Because of the irregular morphologies of the top ridges for sample A, the local curvatures on some regions are so high that the InAs dots could be formed by strain relaxation, while some regions keep layer growth. It is consistent with the results of the line-scan section in Fig. 1(b). Two main growth mechanisms existed together, which drive the In adatoms diffusion in a contrary direction perpendicular to the stripes, and in which direction the In adatoms prefer to diffuse is determined by the competition between the two growth mechanisms. The strain relaxation can be modulated by the various curvatures of the top ridge or by introducing InGaAs strained layer. The InAs QDs deposition positions are also changed with it. For sample B, the effects of the vertical replication of the InAs islands could be neglected for the thicker space layer. Smaller curvature of the top ridge weakened the strain energy relaxation, so no islands are formed on the top ridges. However, when the nucleation mechanism is dominated by the strain energy, such as by introducing the strained In0.2 Ga0.8 As buffer layer, the In adatoms diffusion is dominated by the strain energy, which alters substantially the migration of the In adatoms perpendicular to the stripes. More of the In adatoms deposited in the valley will diffuse upwards to the mesas, and the adatoms deposited on the mesas can readily migrate to the edges at the beginning of the each monolayer growth. So the flat terraces evolve into a shape with a lower central region and convex humps at the sides before InAs QDs formed. And the InAs islands preferentially nucleated on these convex humps at the edges of the mesas. From the shape of single InAs QDs on sample D, elongated along the stripe pattern, it could be deduced that the In adatoms are more likely to diffuse along the edge of the stripes when the InAs deposition amount is larger. Different pitch of the stripe patterns affected the InAs deposition amount on every period pattern under the same growth conditions. For samples D and E, the pitches of the stripe are 200 nm and 300 nm, respectively. It is considered that the amount of InAs on the unit pattern of sample E is larger than those on sample D. So the hump structures on both sides of the top terraces are formed by the InAs islands coalesced with each other along the stripe pattern. In order to investigate the optical properties of the QDs on a patterned substrate, the PL measurements on the samples A, B and flat surface at low temperature of 77 K have been done.
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Fig. 5. (a) Low temperature of 77 K PL spectra of InAs QDs on a patterned substrate of samples A, B and the flat area. (b) PL spectra of InAs QDs on sample A with various excitation intensity, Iex = 13, 25, 50 and 100 mW/cm2 . (c) PL spectra of InAs QDs on sample A with Iex = 13 mW/cm2 compared with Gaussian fit.
Fig. 5(a) shows the PL measurements with the excitation intensity Iex of 13 mW/cm2 . One peak is observed on every PL spectra, which is attributed to the ground state of the InAs QDs. The linewidth (35.68 meV) of the ground state of the InAs islands on sample B is better than those on sample A (57.12 meV) and flat area (46.74 meV). In addition, the PL intensity of the QD array on sample B is much higher than those on the flat area and sample A. The peak energy (1.104 eV) of the QDs on the flat area is higher than that of the InAs quantum dot array on samples A (1.085 eV) and B (1.095 eV). This is probably due to the slightly smaller size of the islands’ array compared to the size of the islands on patterns. As Fig. 5(b) shows, for sample A with higher excitation intensity, Iex = 25, 50 and 100 mW/cm2 , the sub-peak is observed, which is attributed to the excited state of QDs. However, from Fig. 5(c), the PL spectra are somewhat different with the Gaussian fit. It may be caused by the wider size distribution on sample A. 4. Conclusion In a word, ordered self-assembled InAs QDs are grown on the stripe-patterned GaAs (100) substrate. The nucleation mechanisms on the different surfaces are demonstrated. Nucleation on a flat surface is random and the QDs density is limited by the atom diffusion. While on patterned
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substrate, the nucleation mechanism is not only affected by the specific atomic configuration, i.e., surface reconstruction, and step density of sidewall, but also strain energy, which contributed to the contrary diffusion direction for the In adatoms. The strain relaxation can be modulated by the various curvatures of top ridge or by introducing an InGaAs strained layer. When the InGaAs strained layer was introduced, ordering InAs QDs are formed at the edges of top ridges, and the mesas evolve into a shape with a lower central region and convex humps at the sides. The InAs QDs are elongated along the stripe mesas and quasi-quantum wires are formed with a larger InAs deposition amount. On a patterned substrate, the diffusion of the indium adatoms affects the formation of InAs islands. Further studies are required to investigate the ordering islands’ growth on a patterned substrate. Acknowledgments The authors would like to acknowledge Z.G. Li and S.B. Long for substrate preparation. This research work is financially supported by Special Funds for Major State Basic Research Project of China (No. G2000068303), National Natural Science Foundation of China (No. 60390071, 60390074, 90101004), and National High Technology Research and Development Program of China (No. 2002AA311070). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
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