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Growth mechanism of beam-induced lateral epitaxy on (0 0 1) GaAs substrate in molecular beam epitaxy
ARTICLE IN PRESS
Journal of Crystal Growth 276 (2005) 64–71 www.elsevier.com/locate/jcrysgro
Growth mechanism of beam-induced lateral epitaxy on (0 0 1) GaAs substrate in molecular beam epitaxy S. Naritsukaa,b,, T. Suzukia,1, K. Saitoha, T. Maruyamaa,b, T. Nishinagaa,2 a
Faculty of Science and Technology, Meijo University, 1-501 Shiogama-guchi, Tenpaku-ku, Nagoya 468-8502, Japan b Meijo University, 21st century COE program ‘‘NANO FACTORY’’ Received 7 June 2004; accepted 28 October 2004 Communicated by Dr. S. Hiyamizu Available online 20 December 2004
Abstract The mechanism of GaAs beam-induced lateral epitaxy (BILE) on (0 0 1) GaAs substrate by molecular beam epitaxy (MBE) was investigated by systematically varying the crystal orientation of ridges on the surface. In the growth sequence of BILE, a certain number of facets were first formed on the growth front in connection with the crystal orientation of the ridges. These facets competed with each other as they grew. As a result, the formation of the dominant facet determined the main shape. The crystal orientation of the ridge largely affected the formation of facets and their development. The cross-sectional images of the grown layers observed by a scanning electron microscope (SEM) indicates that the facets formed in the following order from first to last: (1 1 1)A facets, (1 1 0) facets, (1 1 1)B facets, and (0 0 1) facets. During the growth, facet formation was largely controlled by the intersurface diffusion of adatoms, although shadowing effects also influenced the grown shapes. r 2004 Elsevier B.V. All rights reserved. PACS: 81.15.Hi; 81.05.Ea; 68.55.a Keywords: A1. Crystal morphology; A3. Molecular beam epitaxy; A3. Selective epitaxy; B1. Gallium compounds; B2. Semiconducting III–V materials
1. Introduction Corresponding author. Faculty of Science and Technology,
Meijo University, 1-501 Shiogama-guchi, Tenpaku-ku, Nagoya 468-8502, Japan. Tel.: +81 52 832 1151; fax: +81 52 832 1172. E-mail address: [email protected] (S. Naritsuka). 1 Present address: Hamamatsu photonics K.K. Inc. 2 Present address: Toyohoshi University of Technology.
Epitaxial lateral overgrowth is a very useful method to decrease the dislocation density in highly mismatched hetero-epitaxial systems for the fabrication of superior electronic and optoelectronic devices. By using this technique, dislocation-free GaAs and InP epitaxial layers even on Si
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.10.159
ARTICLE IN PRESS S. Naritsuka et al. / Journal of Crystal Growth 276 (2005) 64–71
substrate have been obtained [1–4] and new devices involving an array of AlGaAs/GaAs lasers on Si substrate have been fabricated [5]. The reduction of dislocation density has also been accomplished using LPE for InGaAs/GaAs [6,7] and metalorganic vapor phase epitaxy (MOVPE) for GaN/SiC and GaN/sapphire [8–11]. Molecular beam epitaxy (MBE) is very useful to precisely control thickness and composition of thin layers. However, lateral overgrowth by MBE should employ relatively high growth temperatures to prevent polycrystal deposits on SiO2 [12]. As a result, it is difficult to obtain a smooth surface of the layer. To overcome these difficulties, a new technique called beam-induced lateral epitaxy (BILE) was proposed, in which lateral overgrowth is accomplished by supplying molecular beam at a low angle to a substrate with a ridge structure [13]. Because selective growth is easily achieved at a relatively low temperature without a SiO2 mask, it is easier to use BILE to obtain a smooth surface by protecting the sample from thermal roughing. However, use of BILE on (0 0 1) GaAs usually showed a relatively rough upper surface [13]. To improve the technique, we have studied the growth mechanism of BILE, and so far, we have found that the formation of facets on the lateral growth front controls the shape of BILE [14]. For the formation of facets, the crystal orientation of the initial ridge is a very important parameter. Therefore, in this study, we investigated the mechanism of BILE in depth by systematically varying the crystal orientation of the ridges.
2. Experimental procedure After chemically cleaning the sample surface with a mixture of NH4OH:H2O2:H2O in the ratio 4:1:20, we used photolithography and chemical etching with a solution of H2SO4:H2O2:H2O in the ratio 5:1:1 to make truncated, parallel ridges on a (0 0 1) GaAs substrate. The height of the ridges was about 5 mm, and the top widths were 0.7, 3.9, and 8.7 mm. The separation between the ridges was kept constant at 11.5 mm. The orientations of the ridges were aligned by controlling the angle of the sample. Angles were changed in increments of 151,
65
[110] (e)
(f )
(g)
(d)
(c)
(b)
15° [110]
(a) Orientation Ridge stripe Fig. 1. Crystal orientation of ridges for orientations in increment of 151 from [0 1 1].
starting from the [1 1 0] orientation as shown in Fig. 1. For example, the as-etched side wall profile exhibits an inverted mesa in the h1 1 0i direction, a round one in the h1 0 0i direction, and a normal mesa in the h1 1¯ 0i direction, thus it is different depending on the ridge direction. Strictly speaking, these mesa shapes influence the facet-developing process. The influence, however, is reduced as the growth continues, because a long time growth causes complete evolution of the facet-developing process and wipes off the influence of the initial mesa shape. BILE was typically done at 580 1C with an As pressure of 1 105 Torr in the MBE chamber. The lateral growth front was characterized using scanning electron microscopy (SEM) on a cross-section, with the aid of periodically inserted 20-nm-thick AlAs markers. Fig. 2 shows how the beam would enter the truncated ridges and an expected shape of a lateral growth from the ridge. The incident angle (a) of the Ga and As beams were inclined by 121 from the substrate in all experiments. Due to the K-cell arrangement in the MBE chamber, the projection of each beam to the substrate plane is tilted by 101 in the opposite direction from the normal direction of the ridge. The low incident angle of the beams should lead to a shadowing effect from the neighboring ridge, and thus lateral growth is expected to be thinner as shown in the figure.
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Ga, As beams
Lateral growth
GaAs Shadow area
adatoms is much longer than that of Al adatoms. The Ga adatoms diffused far into the shadow of the neighboring ridge and produced a GaAs area without AlAs markers. The figure also shows that a small (0 0 1) facet formed on the top of the layer and grew slightly. Such a facet is very useful for making a wide, flat region of lateral growth. 3.2. 151 off ridge
Figs. 3(a)–(g) show cross-sections of the GaAs layers grown on the truncated ridges that are aligned by 0, 15, 30, 45, 60, 75, and 901 off the [1 1 0] orientation, respectively. We show the sequential change of the size of facets on the side surface of these same GaAs layers in Figs. 4(a)–(g). In the following, the growth of BILE is independently characterized in relation to the crystal orientation of the ridges.
Compared to the GaAs grown from 01 off ridge, the growth for 151 off ridge in Fig. 3(b) resulted in only ð2¯ 1 2ÞB and ð2¯ 1 0Þ facets. Initially, a ð2¯ 1 2ÞB facet formed at the upper area and then increased in size. However, once a ð2¯ 1 0Þ facet formed, it grew faster than and grew over the ð2¯ 1 2ÞB facet, as indicated in Fig. 4(b), and finally covered nearly the whole side. The formation of these facets induced the intersurface diffusion of Ga adatoms from the side surface of ð2¯ 1 0Þ to the top surface of ð2¯ 1 2ÞB; a process that also produced a triangular summit with slopes on both sides. The formation of the summit itself makes the lateral growth upwards. The bottom shape of the lateral growth was also made upwards by the shadow effect of the top of the neighboring ridge.
3.1. 01 off ridge
3.3. 301 off ridge
A cross-sectional SEM image of GaAs grown 0o off ridge is shown in Fig. 3(a). Although the growth shows a little disordered shape, probably due to the relatively low flux of As, it provides us with data to understand better the formation of facets. During the growth, small facets frequently appeared and disappeared. Each facet was specified into a typical group of facets permitting a small error in orientation. The change in the formation is shown in Fig. 4(a). Að1¯ 1 0Þ-like facet formed on the side of the ridge during the growth, but it decreased in size and became very small due to an overflow of Ga adatoms coming from the ð1¯ 1 2ÞB facet at the upper area. At the same time, the ð1¯ 1 2ÞB facet decreased its inclination and then changed to a ð1¯ 1 3ÞB facet. There exists a region without AlAs markers on the lower part of the layer as shown in Fig. 3(a). This is probably because the surface diffusion length of Ga
For the case of 301 off ridge, a ð1¯ 0 3Þ facet initially formed at the upper side of the layer, but it vanished as the lateral growth front grew upwards (Fig. 3c). A ð4¯ 1 4ÞB facet also appears adjacent to ¯ the ð1¯ 0 3Þ facet. Then, the ð4¯ 1 1ÞA facet started to grow at the side of the lateral growth region, increased its size as the lateral growth proceeded, and covered most of the side surface at the end. The widths of these facets are summarized in Fig. 4(c). Similar to the 151 off case, the intersurface diffusion of Ga adatoms pushed the growth front upwards, but its degree increased with growth. Consequently, the layer developed a warped shape. We suggest that the amount of intersurface diffusion of Ga adatoms increased with growth for the following reason. The Ga adatoms on the ¯ ð4¯ 1 1ÞA facet likely migrated into the top of the ¯ ð1¯ 0 3Þ facet via the ð4¯ 1 4ÞB facet. Therefore, the ¯ facet increased the flow enlargement of the ð4¯ 1 1ÞA
Fig. 2. BILE method. a is the incident angle of Ga and As. Truncated ridges are shown as dark regions and the region of lateral growth from the ridge is light.
3. Results and discussion
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Fig. 3. Cross-sectional SEM images of GaAs BILE layers on truncated ridges. The samples were aligned by the following angles away from [0 1 1]: 01 (a), 151 (b), 301 (c), 451 (d), 601 (e), 751 (f), and 901 (g).
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Fig. 4. Sequential change of the width of the facets on the side surface of GaAs BILE layers on truncated ridges. The samples were aligned by the following angles away from [0 1 1]: 01 (a), 151 (b), 301 (c), 451 (d), 601 (e), 751 (f), and 901 (g).
ARTICLE IN PRESS S. Naritsuka et al. / Journal of Crystal Growth 276 (2005) 64–71
of Ga adatoms, and consequently warped up the lateral growth front. 3.4. 45o off ridge Although the layer was obliquely chipped during cleavage, the following is seen from Fig. 3(d). A ð1¯ 0 1Þ facet initially dominates the side of the layer. In addition, a ð1¯ 0 2Þ facet appeared on the upper side of the growth front, and gradually enlarged with growth as shown in Fig. 4(d). The dotted part in the figure marks an uncertain shape due to the poor cleavage. We emphasize that, in contrast to other growth with different ridge orientation, there exists almost no region without AlAs markers under part of the layer. The ¯ formation of the ð2¯ 0 1Þ-like facet on the bottom of the lateral growth region might limit the outer diffusion of Ga adatoms to the face but extensive study is necessary to clarify the mechanism. 3.5. 60o off ridge In this case, a ð1¯ 4¯ 7ÞA facet was widely formed on the slanted region on the upper part as shown in Fig. 3(e). As Fig. 4(e) shows, the ð1¯ 4¯ 7ÞA facet expanded during growth, whereas a small ð1¯ 4¯ 0Þ facet exists on the side. We found that the latter facet continued to exist at the same size without being overtaken by the ð1¯ 4¯ 7ÞA facet. There also exists a region that has no AlAs markers at the bottom of the lateral growth region. The absence of AlAs markers indicates that the region was in the shadow of the neighboring ridge and that the growth occurred by Ga adatom diffusion from the adjacent facet. The Ga adatoms are thought to diffuse not only from the ð1¯ 4¯ 0Þ facet but also from the ð1¯ 4¯ 7ÞA facet. The outer diffusion of Ga adatoms from the ð1¯ 4¯ 0Þ facet to the bottom area also helps maintain the ð1¯ 4¯ 0Þ facet with the same size; otherwise the facet would be overtaken by the ð1¯ 4¯ 7ÞAfacet. 3.6. 75o off ridge The behavior of the facets in this case is similar to that for 601 off ridge, except that the specific facet types are different. Here, a ð1¯ 1¯ 4ÞA facet was
69
formed on the slant on the upper part and ð1¯ 1¯ 1ÞA and ð1¯ 2¯ 0Þ facets formed on the side. For the identification of these facets, it was considered that the upper and side surfaces of growth have a zigzag shape only in the case, which indicates the presence of microfacets. It is known from Fig. 3(f) that a ð1¯ 1¯ 4ÞA facet and a ð1¯ 1¯ 1ÞA facet covered the side first, then the former facet expanded during growth. This growth is consistent with intersurface diffusion from the former facet to the latter facet. Such intersurface diffusion would make the latter facet steeper and consequently change the ð1¯ 1¯ 1ÞA facet into a ð1¯ 2¯ 0Þ facet, which is vertical to the substrate. AlAs markers are also absent from the bottom of the lateral growth region, which is consistent with the outer diffusion of Ga adatoms described above. 3.7. 901 off ridge This case resembles the above case of 751 off ridge. At the beginning of growth, a ð1¯ 1¯ 1ÞA facet and a ð1¯ 1¯ 5ÞA facet covered the side face as shown in Fig. 3(g). The subsequent growth can be interpreted as follows. The intersurface diffusion of Ga adatoms from the ð1¯ 1¯ 5ÞA facet to the ð1¯ 1¯ 1ÞA facet made the former facet large. The formation of the ð1¯ 1¯ 0Þ facet in the bottom part simultaneously made the ð1¯ 1¯ 1ÞA facet small as shown in Fig. 4(g). The intersurface diffusion of Ga adatoms also transformed the ð1¯ 1¯ 1ÞA face into the shallower facet of ð1¯ 1¯ 3ÞA: The ð1¯ 1¯ 3ÞA facet enlarged and covered the top part of the lateral growth region in the end. Once a ð1¯ 1¯ 0Þ facet formed, the region without an AlAs maker would suddenly become smaller, which suggests a change of facet direction in the shadow area. For example, the formation of the ð1¯ 1¯ 0Þ facet on the area would increase the surface diffusion length of Al adatoms. The surface diffusion length of adatoms strongly depends on the growth temperature and the As overpressure [15]. For example, if the flat top surface is preferable for a device, BILE can be optimized by setting the As pressure so as to induce the intersurface diffusion of Ga adatoms to form a (0 0 1) facet to a (1 1 1)B facet with the use of a stripe of (0 1 1) crystal orientation. Though the
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3 (11n)A (11n)B
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surface diffusion has dependence on its growth condition, the widths of the facets after growth are summarized in Fig. 5. The height of the bar chart corresponds to the total width of the facets. Each bar consists of components with different crystal faces. Some facets with similar characteristics are collected into a group, for example, a (1 1 1)A facet and a (1 1 n)A facet are combined into the (1 1 1)A group. It was found that a (0 1 1) facet was very persistent and was likely to enlarge as it overtook other facets. For example, a (0 1 1) facet and a (0n1) facet enlarged and covered most of a side of the layer grown from the 451 off ridge. On the other hand, a (1 1 n)A facet dominated the growth by forming the upper slant of the layer in the cases of 60, 75, and 901 off ridge, whereas (1 1 1)B facet dominated growth in the same manner for the cases of 0 and 151 off ridge. For the case of 301 off, ¯ a large ð4¯ 1 1ÞA facet appeared at the downside of the BILE grown from the ridge. These results indicate that the facet formation has the following order, at least for the present growth conditions, from most dominant to the least: (1 1 1)A facet and their families4(1 1 0) facet4(1 1 1)B facet and their families 4(0 0 1) facet.
ing the crystal orientation of the ridges. The crosssectional observation of SEM shows the following. A (0 1 1) facet is very strong and tends to enlarge by overtaking other facets in the case of 451 off. On the other hand, a (1 1 n)A facet dominated the growth in the cases of 60, 75, and 901 off, whereas a (1 1 1)B facet did in the cases of 0 and 151 off. A ¯ large ð4¯ 1 1ÞA facet formed at the downside of the BILE grown from the ridge of 301 off. These results indicate the following priorities in the formation of facets, highest to lowest: (1 1 1)A facet and their families 4(1 1 0) facet and their families 4(1 1 1)B facet and their families 4(0 0 1) facet. In the growth sequence of BILE, a certain number of facets were formed according to the crystal direction of the ridges. Then the facets competed with each other as they grew. The growth shape was eventually determined by the formation of dominant facets. The crystal orientation of the ridge largely influenced the formation and development of facets, the latter of which was largely controlled by intersurface diffusion of adatoms. In addition, shadowing had a significant influence on the growth shape. For example, the shadow of the neighboring ridge determined the under shape of the BILE layer, even though the outer-diffusion of adatoms into the shadow produced some growth in the shadow region.
Acknowledgements This work was partly supported by Grant-inAid for Priority Areas (B) ‘‘Realization of dislocation-free epitaxy with the aid of nanochannel’’ No. 14350172 and the 21st century COE program from the Ministry of Education, Culture, Sports, Science and Technology.
References 4. Conclusion The mechanism of GaAs BILE on GaAs (0 0 1) substrate was investigated systematically by vary-
[1] Y. Ujiie, T. Nishinaga, Jpn. J. Appl. Phys. 28 (1989) L337. [2] S. Naritsuka, T. Nishinaga, Jpn. J. Appl. Phys. 34 (1995) L1432. [3] S. Naritsuka, T. Nishinaga, J. Crystal Growth 174 (1997) 622.
ARTICLE IN PRESS S. Naritsuka et al. / Journal of Crystal Growth 276 (2005) 64–71 [4] Y.S. Chang, S. Naritsuka, T. Nishinaga, J. Crystal Growth 192 (1998) 18. [5] S. Naritsuka, Y. Mochizuki, K. Motodohi, S. Ohya, N. Ikeda, Y. Sugimoto, K. Asakawa, W.D. Huang, T. Nishinaga, J. Res. Inst. Meijo University 1 (2002) 77. [6] Y. Hayakawa, S. Iida, T. Sakurai, Y. Yanagida, M. Kikuzawa, T. Koyama, M. Kumagawa, J. Crystal Growth 169 (1996) L613. [7] S. Iida, Y. Hayakawa, T. Koyama, M. Kumagawa, J. Crystal Growth 200 (1999) 368. [8] T. Gehrke, K.J. Linthicum, P. Rajagopal, A.D. Batchelor, R.F. Davis, MRS Internet J. Semicond. Nitride Res. 4S1 (1999) G3.1. [9] D.B. Thomson, T. Gehrke, K.J. Linthicum, P. Rajagopal, A.D. Batchelor, R.F. Davis, MRS Internet J. Semicond. Nitride Res. 4S1 (1999) G3.37.
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[10] T.S. Zheleva, S.A. Smith, D.B. Thomson, T. Gehrke, K.J. Linthicum, P. Rajagopal, E.P. Carlson, W. Ashmawi, R.F. Davis, MRS Internet J. Semicond. Nitride Res. 4S1 (1999) G3.38. [11] K.J. Linthicum, T. Gehrke, D.B. Thomson, K.M. Tracy, E.P. Carlson, S.A. Smith, T.S. Zheleva, C.A. Zorman, M. Methrgeny, R.F. Davis, MRS Internet J. Nitride Semicond. Res. 4S1 (1999) G4.9. [12] G. Bacchin, T. Nishinaga, J. Crystal Growth 208 (2000) 1. [13] T. Suzuki, S. Naritsuka, T. Maruyama, T. Nishinaga, Cryst. Res. Technol. 38 (2003) 614. [14] S. Naritsuka, K. Saitoh, T. Suzuki, T. Maruyama, Mat. Res. Soc. Symp. Proc. 799 (2003) 672. [15] A. Yamashiki, T. Nishinaga, Cryst. Res. Technol. 32 (1997) 1049.
Journal of Crystal Growth 276 (2005) 64–71 www.elsevier.com/locate/jcrysgro
Growth mechanism of beam-induced lateral epitaxy on (0 0 1) GaAs substrate in molecular beam epitaxy S. Naritsukaa,b,, T. Suzukia,1, K. Saitoha, T. Maruyamaa,b, T. Nishinagaa,2 a
Faculty of Science and Technology, Meijo University, 1-501 Shiogama-guchi, Tenpaku-ku, Nagoya 468-8502, Japan b Meijo University, 21st century COE program ‘‘NANO FACTORY’’ Received 7 June 2004; accepted 28 October 2004 Communicated by Dr. S. Hiyamizu Available online 20 December 2004
Abstract The mechanism of GaAs beam-induced lateral epitaxy (BILE) on (0 0 1) GaAs substrate by molecular beam epitaxy (MBE) was investigated by systematically varying the crystal orientation of ridges on the surface. In the growth sequence of BILE, a certain number of facets were first formed on the growth front in connection with the crystal orientation of the ridges. These facets competed with each other as they grew. As a result, the formation of the dominant facet determined the main shape. The crystal orientation of the ridge largely affected the formation of facets and their development. The cross-sectional images of the grown layers observed by a scanning electron microscope (SEM) indicates that the facets formed in the following order from first to last: (1 1 1)A facets, (1 1 0) facets, (1 1 1)B facets, and (0 0 1) facets. During the growth, facet formation was largely controlled by the intersurface diffusion of adatoms, although shadowing effects also influenced the grown shapes. r 2004 Elsevier B.V. All rights reserved. PACS: 81.15.Hi; 81.05.Ea; 68.55.a Keywords: A1. Crystal morphology; A3. Molecular beam epitaxy; A3. Selective epitaxy; B1. Gallium compounds; B2. Semiconducting III–V materials
1. Introduction Corresponding author. Faculty of Science and Technology,
Meijo University, 1-501 Shiogama-guchi, Tenpaku-ku, Nagoya 468-8502, Japan. Tel.: +81 52 832 1151; fax: +81 52 832 1172. E-mail address: [email protected] (S. Naritsuka). 1 Present address: Hamamatsu photonics K.K. Inc. 2 Present address: Toyohoshi University of Technology.
Epitaxial lateral overgrowth is a very useful method to decrease the dislocation density in highly mismatched hetero-epitaxial systems for the fabrication of superior electronic and optoelectronic devices. By using this technique, dislocation-free GaAs and InP epitaxial layers even on Si
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.10.159
ARTICLE IN PRESS S. Naritsuka et al. / Journal of Crystal Growth 276 (2005) 64–71
substrate have been obtained [1–4] and new devices involving an array of AlGaAs/GaAs lasers on Si substrate have been fabricated [5]. The reduction of dislocation density has also been accomplished using LPE for InGaAs/GaAs [6,7] and metalorganic vapor phase epitaxy (MOVPE) for GaN/SiC and GaN/sapphire [8–11]. Molecular beam epitaxy (MBE) is very useful to precisely control thickness and composition of thin layers. However, lateral overgrowth by MBE should employ relatively high growth temperatures to prevent polycrystal deposits on SiO2 [12]. As a result, it is difficult to obtain a smooth surface of the layer. To overcome these difficulties, a new technique called beam-induced lateral epitaxy (BILE) was proposed, in which lateral overgrowth is accomplished by supplying molecular beam at a low angle to a substrate with a ridge structure [13]. Because selective growth is easily achieved at a relatively low temperature without a SiO2 mask, it is easier to use BILE to obtain a smooth surface by protecting the sample from thermal roughing. However, use of BILE on (0 0 1) GaAs usually showed a relatively rough upper surface [13]. To improve the technique, we have studied the growth mechanism of BILE, and so far, we have found that the formation of facets on the lateral growth front controls the shape of BILE [14]. For the formation of facets, the crystal orientation of the initial ridge is a very important parameter. Therefore, in this study, we investigated the mechanism of BILE in depth by systematically varying the crystal orientation of the ridges.
2. Experimental procedure After chemically cleaning the sample surface with a mixture of NH4OH:H2O2:H2O in the ratio 4:1:20, we used photolithography and chemical etching with a solution of H2SO4:H2O2:H2O in the ratio 5:1:1 to make truncated, parallel ridges on a (0 0 1) GaAs substrate. The height of the ridges was about 5 mm, and the top widths were 0.7, 3.9, and 8.7 mm. The separation between the ridges was kept constant at 11.5 mm. The orientations of the ridges were aligned by controlling the angle of the sample. Angles were changed in increments of 151,
65
[110] (e)
(f )
(g)
(d)
(c)
(b)
15° [110]
(a) Orientation Ridge stripe Fig. 1. Crystal orientation of ridges for orientations in increment of 151 from [0 1 1].
starting from the [1 1 0] orientation as shown in Fig. 1. For example, the as-etched side wall profile exhibits an inverted mesa in the h1 1 0i direction, a round one in the h1 0 0i direction, and a normal mesa in the h1 1¯ 0i direction, thus it is different depending on the ridge direction. Strictly speaking, these mesa shapes influence the facet-developing process. The influence, however, is reduced as the growth continues, because a long time growth causes complete evolution of the facet-developing process and wipes off the influence of the initial mesa shape. BILE was typically done at 580 1C with an As pressure of 1 105 Torr in the MBE chamber. The lateral growth front was characterized using scanning electron microscopy (SEM) on a cross-section, with the aid of periodically inserted 20-nm-thick AlAs markers. Fig. 2 shows how the beam would enter the truncated ridges and an expected shape of a lateral growth from the ridge. The incident angle (a) of the Ga and As beams were inclined by 121 from the substrate in all experiments. Due to the K-cell arrangement in the MBE chamber, the projection of each beam to the substrate plane is tilted by 101 in the opposite direction from the normal direction of the ridge. The low incident angle of the beams should lead to a shadowing effect from the neighboring ridge, and thus lateral growth is expected to be thinner as shown in the figure.
ARTICLE IN PRESS S. Naritsuka et al. / Journal of Crystal Growth 276 (2005) 64–71
66
Ga, As beams
Lateral growth
GaAs Shadow area
adatoms is much longer than that of Al adatoms. The Ga adatoms diffused far into the shadow of the neighboring ridge and produced a GaAs area without AlAs markers. The figure also shows that a small (0 0 1) facet formed on the top of the layer and grew slightly. Such a facet is very useful for making a wide, flat region of lateral growth. 3.2. 151 off ridge
Figs. 3(a)–(g) show cross-sections of the GaAs layers grown on the truncated ridges that are aligned by 0, 15, 30, 45, 60, 75, and 901 off the [1 1 0] orientation, respectively. We show the sequential change of the size of facets on the side surface of these same GaAs layers in Figs. 4(a)–(g). In the following, the growth of BILE is independently characterized in relation to the crystal orientation of the ridges.
Compared to the GaAs grown from 01 off ridge, the growth for 151 off ridge in Fig. 3(b) resulted in only ð2¯ 1 2ÞB and ð2¯ 1 0Þ facets. Initially, a ð2¯ 1 2ÞB facet formed at the upper area and then increased in size. However, once a ð2¯ 1 0Þ facet formed, it grew faster than and grew over the ð2¯ 1 2ÞB facet, as indicated in Fig. 4(b), and finally covered nearly the whole side. The formation of these facets induced the intersurface diffusion of Ga adatoms from the side surface of ð2¯ 1 0Þ to the top surface of ð2¯ 1 2ÞB; a process that also produced a triangular summit with slopes on both sides. The formation of the summit itself makes the lateral growth upwards. The bottom shape of the lateral growth was also made upwards by the shadow effect of the top of the neighboring ridge.
3.1. 01 off ridge
3.3. 301 off ridge
A cross-sectional SEM image of GaAs grown 0o off ridge is shown in Fig. 3(a). Although the growth shows a little disordered shape, probably due to the relatively low flux of As, it provides us with data to understand better the formation of facets. During the growth, small facets frequently appeared and disappeared. Each facet was specified into a typical group of facets permitting a small error in orientation. The change in the formation is shown in Fig. 4(a). Að1¯ 1 0Þ-like facet formed on the side of the ridge during the growth, but it decreased in size and became very small due to an overflow of Ga adatoms coming from the ð1¯ 1 2ÞB facet at the upper area. At the same time, the ð1¯ 1 2ÞB facet decreased its inclination and then changed to a ð1¯ 1 3ÞB facet. There exists a region without AlAs markers on the lower part of the layer as shown in Fig. 3(a). This is probably because the surface diffusion length of Ga
For the case of 301 off ridge, a ð1¯ 0 3Þ facet initially formed at the upper side of the layer, but it vanished as the lateral growth front grew upwards (Fig. 3c). A ð4¯ 1 4ÞB facet also appears adjacent to ¯ the ð1¯ 0 3Þ facet. Then, the ð4¯ 1 1ÞA facet started to grow at the side of the lateral growth region, increased its size as the lateral growth proceeded, and covered most of the side surface at the end. The widths of these facets are summarized in Fig. 4(c). Similar to the 151 off case, the intersurface diffusion of Ga adatoms pushed the growth front upwards, but its degree increased with growth. Consequently, the layer developed a warped shape. We suggest that the amount of intersurface diffusion of Ga adatoms increased with growth for the following reason. The Ga adatoms on the ¯ ð4¯ 1 1ÞA facet likely migrated into the top of the ¯ ð1¯ 0 3Þ facet via the ð4¯ 1 4ÞB facet. Therefore, the ¯ facet increased the flow enlargement of the ð4¯ 1 1ÞA
Fig. 2. BILE method. a is the incident angle of Ga and As. Truncated ridges are shown as dark regions and the region of lateral growth from the ridge is light.
3. Results and discussion
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Fig. 3. Cross-sectional SEM images of GaAs BILE layers on truncated ridges. The samples were aligned by the following angles away from [0 1 1]: 01 (a), 151 (b), 301 (c), 451 (d), 601 (e), 751 (f), and 901 (g).
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Fig. 4. Sequential change of the width of the facets on the side surface of GaAs BILE layers on truncated ridges. The samples were aligned by the following angles away from [0 1 1]: 01 (a), 151 (b), 301 (c), 451 (d), 601 (e), 751 (f), and 901 (g).
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of Ga adatoms, and consequently warped up the lateral growth front. 3.4. 45o off ridge Although the layer was obliquely chipped during cleavage, the following is seen from Fig. 3(d). A ð1¯ 0 1Þ facet initially dominates the side of the layer. In addition, a ð1¯ 0 2Þ facet appeared on the upper side of the growth front, and gradually enlarged with growth as shown in Fig. 4(d). The dotted part in the figure marks an uncertain shape due to the poor cleavage. We emphasize that, in contrast to other growth with different ridge orientation, there exists almost no region without AlAs markers under part of the layer. The ¯ formation of the ð2¯ 0 1Þ-like facet on the bottom of the lateral growth region might limit the outer diffusion of Ga adatoms to the face but extensive study is necessary to clarify the mechanism. 3.5. 60o off ridge In this case, a ð1¯ 4¯ 7ÞA facet was widely formed on the slanted region on the upper part as shown in Fig. 3(e). As Fig. 4(e) shows, the ð1¯ 4¯ 7ÞA facet expanded during growth, whereas a small ð1¯ 4¯ 0Þ facet exists on the side. We found that the latter facet continued to exist at the same size without being overtaken by the ð1¯ 4¯ 7ÞA facet. There also exists a region that has no AlAs markers at the bottom of the lateral growth region. The absence of AlAs markers indicates that the region was in the shadow of the neighboring ridge and that the growth occurred by Ga adatom diffusion from the adjacent facet. The Ga adatoms are thought to diffuse not only from the ð1¯ 4¯ 0Þ facet but also from the ð1¯ 4¯ 7ÞA facet. The outer diffusion of Ga adatoms from the ð1¯ 4¯ 0Þ facet to the bottom area also helps maintain the ð1¯ 4¯ 0Þ facet with the same size; otherwise the facet would be overtaken by the ð1¯ 4¯ 7ÞAfacet. 3.6. 75o off ridge The behavior of the facets in this case is similar to that for 601 off ridge, except that the specific facet types are different. Here, a ð1¯ 1¯ 4ÞA facet was
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formed on the slant on the upper part and ð1¯ 1¯ 1ÞA and ð1¯ 2¯ 0Þ facets formed on the side. For the identification of these facets, it was considered that the upper and side surfaces of growth have a zigzag shape only in the case, which indicates the presence of microfacets. It is known from Fig. 3(f) that a ð1¯ 1¯ 4ÞA facet and a ð1¯ 1¯ 1ÞA facet covered the side first, then the former facet expanded during growth. This growth is consistent with intersurface diffusion from the former facet to the latter facet. Such intersurface diffusion would make the latter facet steeper and consequently change the ð1¯ 1¯ 1ÞA facet into a ð1¯ 2¯ 0Þ facet, which is vertical to the substrate. AlAs markers are also absent from the bottom of the lateral growth region, which is consistent with the outer diffusion of Ga adatoms described above. 3.7. 901 off ridge This case resembles the above case of 751 off ridge. At the beginning of growth, a ð1¯ 1¯ 1ÞA facet and a ð1¯ 1¯ 5ÞA facet covered the side face as shown in Fig. 3(g). The subsequent growth can be interpreted as follows. The intersurface diffusion of Ga adatoms from the ð1¯ 1¯ 5ÞA facet to the ð1¯ 1¯ 1ÞA facet made the former facet large. The formation of the ð1¯ 1¯ 0Þ facet in the bottom part simultaneously made the ð1¯ 1¯ 1ÞA facet small as shown in Fig. 4(g). The intersurface diffusion of Ga adatoms also transformed the ð1¯ 1¯ 1ÞA face into the shallower facet of ð1¯ 1¯ 3ÞA: The ð1¯ 1¯ 3ÞA facet enlarged and covered the top part of the lateral growth region in the end. Once a ð1¯ 1¯ 0Þ facet formed, the region without an AlAs maker would suddenly become smaller, which suggests a change of facet direction in the shadow area. For example, the formation of the ð1¯ 1¯ 0Þ facet on the area would increase the surface diffusion length of Al adatoms. The surface diffusion length of adatoms strongly depends on the growth temperature and the As overpressure [15]. For example, if the flat top surface is preferable for a device, BILE can be optimized by setting the As pressure so as to induce the intersurface diffusion of Ga adatoms to form a (0 0 1) facet to a (1 1 1)B facet with the use of a stripe of (0 1 1) crystal orientation. Though the
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surface diffusion has dependence on its growth condition, the widths of the facets after growth are summarized in Fig. 5. The height of the bar chart corresponds to the total width of the facets. Each bar consists of components with different crystal faces. Some facets with similar characteristics are collected into a group, for example, a (1 1 1)A facet and a (1 1 n)A facet are combined into the (1 1 1)A group. It was found that a (0 1 1) facet was very persistent and was likely to enlarge as it overtook other facets. For example, a (0 1 1) facet and a (0n1) facet enlarged and covered most of a side of the layer grown from the 451 off ridge. On the other hand, a (1 1 n)A facet dominated the growth by forming the upper slant of the layer in the cases of 60, 75, and 901 off ridge, whereas (1 1 1)B facet dominated growth in the same manner for the cases of 0 and 151 off ridge. For the case of 301 off, ¯ a large ð4¯ 1 1ÞA facet appeared at the downside of the BILE grown from the ridge. These results indicate that the facet formation has the following order, at least for the present growth conditions, from most dominant to the least: (1 1 1)A facet and their families4(1 1 0) facet4(1 1 1)B facet and their families 4(0 0 1) facet.
ing the crystal orientation of the ridges. The crosssectional observation of SEM shows the following. A (0 1 1) facet is very strong and tends to enlarge by overtaking other facets in the case of 451 off. On the other hand, a (1 1 n)A facet dominated the growth in the cases of 60, 75, and 901 off, whereas a (1 1 1)B facet did in the cases of 0 and 151 off. A ¯ large ð4¯ 1 1ÞA facet formed at the downside of the BILE grown from the ridge of 301 off. These results indicate the following priorities in the formation of facets, highest to lowest: (1 1 1)A facet and their families 4(1 1 0) facet and their families 4(1 1 1)B facet and their families 4(0 0 1) facet. In the growth sequence of BILE, a certain number of facets were formed according to the crystal direction of the ridges. Then the facets competed with each other as they grew. The growth shape was eventually determined by the formation of dominant facets. The crystal orientation of the ridge largely influenced the formation and development of facets, the latter of which was largely controlled by intersurface diffusion of adatoms. In addition, shadowing had a significant influence on the growth shape. For example, the shadow of the neighboring ridge determined the under shape of the BILE layer, even though the outer-diffusion of adatoms into the shadow produced some growth in the shadow region.
Acknowledgements This work was partly supported by Grant-inAid for Priority Areas (B) ‘‘Realization of dislocation-free epitaxy with the aid of nanochannel’’ No. 14350172 and the 21st century COE program from the Ministry of Education, Culture, Sports, Science and Technology.
References 4. Conclusion The mechanism of GaAs BILE on GaAs (0 0 1) substrate was investigated systematically by vary-
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