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Asymmetric behavior of monolayer holes after growth in GaAs molecular beam epitaxy revealed by in situ scanning electron microscopy
Journal of Crystal Growth 201/202 (1999) 141}145
Asymmetric behavior of monolayer holes after growth in GaAs molecular beam epitaxy revealed by in situ scanning electron microscopy K. Tanahashi , K. Kawamura , N. Inoue *, Y. Homma RIAST, Osaka Prefecture University, 1-2 Gakuencho, Sakai, Osaka 599-8570, Japan NTT Basic Research Laboratories, Atsugi, Kanagawa 243-0198, Japan
Abstract The behavior of monolayer holes on the GaAs (0 0 1) surface during post-growth annealing in molecular beam epitaxy (MBE) is examined in detail by in situ scanning electron microscopy (SEM). Submicron scale monolayer-deep holes are formed after small islands and holes are eliminated. Their growth and shrinkage are found to proceed asymmetrically: for example, they grow only into the right-hand side, and shrink only from the top. The mechanism is discussed in terms of environmental step structure. Ga adatoms migrate tenths of micron in several minutes. It was found that in the regrowth after annealing, three-dimensional islands are formed preferentially on the step edges. 1999 Published by Elsevier Science B.V. All rights reserved. PACS: 81.15.Hi; 81.10.!h Keywords: Molecular beam epitaxy; Electron microscopy; Asymmetric behavior; Monolayer hole; Three-dimensional island
1. Introduction It is well known that the interface roughness in the heterostructure degrades the performance of quantum well devices. Growth interruption technique at the interface is used in molecular beam
* Corresponding author. Tel.: #81 722 51 5634; fax: #81 722 54 9935; e-mail: [email protected].
epitaxy (MBE) for smoothing the grown surface [1]. As for the smoothing process, removal of islands was revealed by step-by-step STM observation [2]. Recently, we have revealed the behavior of monolayer holes on the (0 0 1) GaAs surface after growth for the "rst time by in situ scanning electron microscopy (SEM) [3]. Submicron-scale holes showed anisotropic behavior: they grew in the [1 1 0] direction but shrank in the [1 1 0] direction and became rectangles and "nally disappeared. In this paper, we report the time-dependent asymmetric
0022-0248/99/$ } see front matter 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 1 3 1 1 - 6
142
K. Tanahashi et al. / Journal of Crystal Growth 201/202 (1999) 141}145
Fig. 1. An example of asymmetric behavior of monolayer holes and illustration (without correction for the foreshortening). The substrate temperature was about 6003C. Bright lines (B) and dark lines (D) are the steps. Multiple steps (M) are located on the right-hand side. The right-hand side is higher than the left-hand side. (a) 2 40 }3 50 after growth halt, (b) 4 00 }5 10 , (c) 5 30 }6 40 . The surface is (0 0 1) and tilt by 0.23 to [1 1 0] direction. Black triangles are position markers and white triangles indicate the step motion. The ten-dot distance below the picture corresponds to 0.25 lm.
behavior and propose a possible mechanism. Three-dimensional island formation on steps during growth after annealing is also studied.
2. Experimental procedure The in situ observation was performed in the MBE-SEM hybrid system [4]. The electron beam was incident at about 53 to the sample surface. The substrate was (0 0 1) GaAs misoriented 0.23 towards the [1 1 0] direction: the left-hand side of the surface is lower than the right-hand side. A terraceand-step structure was prepared by the bu!er layer growth. Growth was performed in the 2D nucleation-and-growth mode, growth was stopped and
the sample was annealed in an As ambient at about 6003C. The wide terrace was chosen for observation so that representative hole behavior is obtained. The image was recorded on a video tape, and was corrected for the foreshortening for the quantitative analysis. Correction was also done for the sample thermal drift during observation and the accurate location of holes was determined.
3. Results and discussion 3.1. Asymmetric behavior Fig. 1 shows an example of the behavior of holes after growth. The bright horizontal lines (B) in the
K. Tanahashi et al. / Journal of Crystal Growth 201/202 (1999) 141}145
143
Fig. 2. Another example before disappearance. The substrate temperature was about 5803C. (a) From the start of annealing to 80 s, (b) from 85 to 155 s, (c) from 160 to 230 s, (d) from 235 to 305 s (before disappearance).
lower half and the dark lines (D) in the upper half are the monolayer steps. The central part is the wide terrace formed at the bottom of the valley. There are several monolayer deep holes observed immediately after growth as shown in Fig. 1a . During annealing, the smaller holes disappeared and the bigger holes changed their shape and size as illustrated in Fig. 1b and Fig. 1c . The anisotropic shape and quantitative size changes are as follows (Figures are foreshortened about "ve times in the vertical direction.) The holes were long in the vertical [1 1 0] direction initially. During annealing, they elongated in the [1 1 0] direction but shrank in the [1 1 0] direction. In the early stage, the area of holes did not change much, but in the later stage the holes shrank and disappeared. The vertical 11 1 02 steps are Ga-terminated and the horizontal 11 1 02 steps are As-terminated. In the previous analysis, we have interpreted the anisotropy by the desorption (step retreat) of Ga atoms from the Ga-terminated steps and adsorption (step advance) to the As-terminated steps [3,5]. If so, the hole shape should change symmetrically, and the hole location is not changed. In fact the asymmetric behavior of holes was found: the change is summarized in the illustration.
The left Ga-terminated steps retreated but the right one stayed unmoved, while the top As-terminated steps advanced but the bottom one stayed unmoved (marked by the open arrows in Fig. 2). It is to be noted that the dark horizontal step also advanced downward. Fig. 2 shows another example. The big terrace is also located in the center. There are two big monolayer holes in the bottom terrace of the valley. As for the horizontal direction, the time-dependent behavior was observed. In the initial stage, both left and right Gaterminated steps retreated as observed in Fig. 2a. In the later stage, only the right steps of the holes retreated as observed in Fig. 2b and Fig. 2d. The upper hole shrank from the top whereas the lower hole shrank from the bottom. Finally all the ends shrank and the holes disappeared (not shown here). The lower bright step advanced upward and retreated to the right. 3.2. Mechanism As described above, if the behavior of a hole is determined only by the nature of the marginal steps, Ga-terminated or As-terminated, the shape change should be symmetric and the hole location
144
K. Tanahashi et al. / Journal of Crystal Growth 201/202 (1999) 141}145
mental steps dominated the hole behavior. The e!ect of electron beam was excluded: the electrons were incident from the bottom to the top of the "gure, which cannot explain the horizontal asymmetry. 3.3. Three-dimensional islands preferentially formed at the steps Fig. 3. Schematic illustration of the step e!ect on the hole asymmetry.
does not change. Therefore, we have to consider another mechanism for the observed asymmetry. After the small islands and holes disappeared, steps and big holes are the source and sink of the adatoms. Therefore, the neighboring step behavior a!ects the hole behavior. The one important step features is that multiple steps were located on the right-hand side of the holes, but the left-hand side of the holes is on the open terrace in these two cases as illustrated in Fig. 3. Thus, many Ga atoms were supplied from the right-hand side, which compensate the atom detachment from the Ga-terminated steps on the right-hand side. The left edge steps do not have such a source. Another possible origin is the average tilt. The surface is low on the left-hand side and high on the right-hand side. Therefore, the asymmetry in Fig. 1 suggests that the step-down Ga atom #ow is superior than the step-up #ow. As for the case of Fig. 2, retreat on the right-hand side and the stability on the left-hand side of the upper hole are considered to occur because the step was located near on the left-hand side but far on the right-hand side. As for the lower hole, both the hole and the neighboring steps retreated right. In the vertical direction the parallel motion of environmental steps and holes was also observed. Thus, it is established that the environmental step behavior determines the hole behavior. It is to be noted that Ga adatoms migrate over tenths of micron within several minutes. The time dependent behavior observed in Fig. 2 is interpreted as follows. In the early stage, as the Ga adatoms were quickly decreasing, the detachment cancelled the attachment, resulting in the symmetric step retreat. In the later stage, macroscopic atom supply from the retreating environ-
Fig. 4 shows the initial stage of growth after annealing. In Fig. 4a, wide terraces and steps are observed. There are three islands observed as marked with the white arrows. They are located on the steps: the left one is formed on the edge of the terrace, the middle one on the straight step which is marked with the black arrows. The right one is also formed on the edge of the terrace. Fig. 4b}Fig. 4d show that these islands became three-dimensional as the growth proceeded. These results show that the three-dimensional islands are formed preferentially on the steps. It was found that during annealing most preexisting islands disappeared but some of them on the step were not completely removed. These visible and invisible traces may reform 3D islands. This e!ect may be utilized in the case of a strained system to form the ordered array of quantum dots [6].
4. Summary In summary, the behavior of monolayer holes during annealing after growth was studied in detail by in situ scanning electron microscopy. Asymmetric motion of edge steps was revealed. It was found that the steps in the center of the terrace retreated much but those near the multiple steps moved little. This was attributed to the atom detachment from the edge step being compensated by the atomic #ow from the adjacent multiple steps. Also, the holes moved in the same direction as the environmental step motion. This shows that the macroscopic atomic #ow from the environmental steps determines the hole behavior. Three-dimensional islands are formed preferentially at the step in regrowth after annealing. This is usable to the preferential formation of 3D islands in the strained system which is promising for the control of location of quantum dots.
K. Tanahashi et al. / Journal of Crystal Growth 201/202 (1999) 141}145
145
Fig. 4. Three-dimensional islands preferentially formed on the steps in regrowth after growth. The substrate temperature was about 5903C and the growth rate was 1 ML/40 s. (a) From the start of growth to 80 s, (b) from 85 to 145 s, (c) from 150 to 230 s, (d) from 235 to 315 s.
Acknowledgements The authors are grateful to T. Kawamura and E.R. Weber for their stimulating discussions and R. Shimizu for help in the manuscript preparation. This work was partially supported by the Ministry of Education, Culture and Science of Japan and by the Special Fund for Promotion in Science and Technology Agency.
References [1] H. Sakaki, M. Tanaka, J. Yoshino, Jpn. J. Appl. Phys. 24 (1985) L417.
[2] T. Ide, A. Yamashita, T. Mizutani, Phys. Rev. Lett. B 46 (1992) 1905. [3] N. Inoue, K. Morimoto, T. Araki, T. Ito, Y. Homma, J. Osaka, J. Vac. Sci. Technol. B 14 (1996) 3575. [4] Y. Homma, J. Osaka, N. Inoue, Jpn. J. Appl. Phys. 33 (1994) L563. [5] T. Shitara, J. Zhang, J.H. Neave, B.A. Joyce, J. Appl. Phys. 71 (1992) 4299. [6] J. Temmyo, E. Kuramochi, T. Nishiya, M. Sugo, R. Notzel, T. Tamamura, Electron. Lett. 31 (1995) 209.
Asymmetric behavior of monolayer holes after growth in GaAs molecular beam epitaxy revealed by in situ scanning electron microscopy K. Tanahashi , K. Kawamura , N. Inoue *, Y. Homma RIAST, Osaka Prefecture University, 1-2 Gakuencho, Sakai, Osaka 599-8570, Japan NTT Basic Research Laboratories, Atsugi, Kanagawa 243-0198, Japan
Abstract The behavior of monolayer holes on the GaAs (0 0 1) surface during post-growth annealing in molecular beam epitaxy (MBE) is examined in detail by in situ scanning electron microscopy (SEM). Submicron scale monolayer-deep holes are formed after small islands and holes are eliminated. Their growth and shrinkage are found to proceed asymmetrically: for example, they grow only into the right-hand side, and shrink only from the top. The mechanism is discussed in terms of environmental step structure. Ga adatoms migrate tenths of micron in several minutes. It was found that in the regrowth after annealing, three-dimensional islands are formed preferentially on the step edges. 1999 Published by Elsevier Science B.V. All rights reserved. PACS: 81.15.Hi; 81.10.!h Keywords: Molecular beam epitaxy; Electron microscopy; Asymmetric behavior; Monolayer hole; Three-dimensional island
1. Introduction It is well known that the interface roughness in the heterostructure degrades the performance of quantum well devices. Growth interruption technique at the interface is used in molecular beam
* Corresponding author. Tel.: #81 722 51 5634; fax: #81 722 54 9935; e-mail: [email protected].
epitaxy (MBE) for smoothing the grown surface [1]. As for the smoothing process, removal of islands was revealed by step-by-step STM observation [2]. Recently, we have revealed the behavior of monolayer holes on the (0 0 1) GaAs surface after growth for the "rst time by in situ scanning electron microscopy (SEM) [3]. Submicron-scale holes showed anisotropic behavior: they grew in the [1 1 0] direction but shrank in the [1 1 0] direction and became rectangles and "nally disappeared. In this paper, we report the time-dependent asymmetric
0022-0248/99/$ } see front matter 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 1 3 1 1 - 6
142
K. Tanahashi et al. / Journal of Crystal Growth 201/202 (1999) 141}145
Fig. 1. An example of asymmetric behavior of monolayer holes and illustration (without correction for the foreshortening). The substrate temperature was about 6003C. Bright lines (B) and dark lines (D) are the steps. Multiple steps (M) are located on the right-hand side. The right-hand side is higher than the left-hand side. (a) 2 40 }3 50 after growth halt, (b) 4 00 }5 10 , (c) 5 30 }6 40 . The surface is (0 0 1) and tilt by 0.23 to [1 1 0] direction. Black triangles are position markers and white triangles indicate the step motion. The ten-dot distance below the picture corresponds to 0.25 lm.
behavior and propose a possible mechanism. Three-dimensional island formation on steps during growth after annealing is also studied.
2. Experimental procedure The in situ observation was performed in the MBE-SEM hybrid system [4]. The electron beam was incident at about 53 to the sample surface. The substrate was (0 0 1) GaAs misoriented 0.23 towards the [1 1 0] direction: the left-hand side of the surface is lower than the right-hand side. A terraceand-step structure was prepared by the bu!er layer growth. Growth was performed in the 2D nucleation-and-growth mode, growth was stopped and
the sample was annealed in an As ambient at about 6003C. The wide terrace was chosen for observation so that representative hole behavior is obtained. The image was recorded on a video tape, and was corrected for the foreshortening for the quantitative analysis. Correction was also done for the sample thermal drift during observation and the accurate location of holes was determined.
3. Results and discussion 3.1. Asymmetric behavior Fig. 1 shows an example of the behavior of holes after growth. The bright horizontal lines (B) in the
K. Tanahashi et al. / Journal of Crystal Growth 201/202 (1999) 141}145
143
Fig. 2. Another example before disappearance. The substrate temperature was about 5803C. (a) From the start of annealing to 80 s, (b) from 85 to 155 s, (c) from 160 to 230 s, (d) from 235 to 305 s (before disappearance).
lower half and the dark lines (D) in the upper half are the monolayer steps. The central part is the wide terrace formed at the bottom of the valley. There are several monolayer deep holes observed immediately after growth as shown in Fig. 1a . During annealing, the smaller holes disappeared and the bigger holes changed their shape and size as illustrated in Fig. 1b and Fig. 1c . The anisotropic shape and quantitative size changes are as follows (Figures are foreshortened about "ve times in the vertical direction.) The holes were long in the vertical [1 1 0] direction initially. During annealing, they elongated in the [1 1 0] direction but shrank in the [1 1 0] direction. In the early stage, the area of holes did not change much, but in the later stage the holes shrank and disappeared. The vertical 11 1 02 steps are Ga-terminated and the horizontal 11 1 02 steps are As-terminated. In the previous analysis, we have interpreted the anisotropy by the desorption (step retreat) of Ga atoms from the Ga-terminated steps and adsorption (step advance) to the As-terminated steps [3,5]. If so, the hole shape should change symmetrically, and the hole location is not changed. In fact the asymmetric behavior of holes was found: the change is summarized in the illustration.
The left Ga-terminated steps retreated but the right one stayed unmoved, while the top As-terminated steps advanced but the bottom one stayed unmoved (marked by the open arrows in Fig. 2). It is to be noted that the dark horizontal step also advanced downward. Fig. 2 shows another example. The big terrace is also located in the center. There are two big monolayer holes in the bottom terrace of the valley. As for the horizontal direction, the time-dependent behavior was observed. In the initial stage, both left and right Gaterminated steps retreated as observed in Fig. 2a. In the later stage, only the right steps of the holes retreated as observed in Fig. 2b and Fig. 2d. The upper hole shrank from the top whereas the lower hole shrank from the bottom. Finally all the ends shrank and the holes disappeared (not shown here). The lower bright step advanced upward and retreated to the right. 3.2. Mechanism As described above, if the behavior of a hole is determined only by the nature of the marginal steps, Ga-terminated or As-terminated, the shape change should be symmetric and the hole location
144
K. Tanahashi et al. / Journal of Crystal Growth 201/202 (1999) 141}145
mental steps dominated the hole behavior. The e!ect of electron beam was excluded: the electrons were incident from the bottom to the top of the "gure, which cannot explain the horizontal asymmetry. 3.3. Three-dimensional islands preferentially formed at the steps Fig. 3. Schematic illustration of the step e!ect on the hole asymmetry.
does not change. Therefore, we have to consider another mechanism for the observed asymmetry. After the small islands and holes disappeared, steps and big holes are the source and sink of the adatoms. Therefore, the neighboring step behavior a!ects the hole behavior. The one important step features is that multiple steps were located on the right-hand side of the holes, but the left-hand side of the holes is on the open terrace in these two cases as illustrated in Fig. 3. Thus, many Ga atoms were supplied from the right-hand side, which compensate the atom detachment from the Ga-terminated steps on the right-hand side. The left edge steps do not have such a source. Another possible origin is the average tilt. The surface is low on the left-hand side and high on the right-hand side. Therefore, the asymmetry in Fig. 1 suggests that the step-down Ga atom #ow is superior than the step-up #ow. As for the case of Fig. 2, retreat on the right-hand side and the stability on the left-hand side of the upper hole are considered to occur because the step was located near on the left-hand side but far on the right-hand side. As for the lower hole, both the hole and the neighboring steps retreated right. In the vertical direction the parallel motion of environmental steps and holes was also observed. Thus, it is established that the environmental step behavior determines the hole behavior. It is to be noted that Ga adatoms migrate over tenths of micron within several minutes. The time dependent behavior observed in Fig. 2 is interpreted as follows. In the early stage, as the Ga adatoms were quickly decreasing, the detachment cancelled the attachment, resulting in the symmetric step retreat. In the later stage, macroscopic atom supply from the retreating environ-
Fig. 4 shows the initial stage of growth after annealing. In Fig. 4a, wide terraces and steps are observed. There are three islands observed as marked with the white arrows. They are located on the steps: the left one is formed on the edge of the terrace, the middle one on the straight step which is marked with the black arrows. The right one is also formed on the edge of the terrace. Fig. 4b}Fig. 4d show that these islands became three-dimensional as the growth proceeded. These results show that the three-dimensional islands are formed preferentially on the steps. It was found that during annealing most preexisting islands disappeared but some of them on the step were not completely removed. These visible and invisible traces may reform 3D islands. This e!ect may be utilized in the case of a strained system to form the ordered array of quantum dots [6].
4. Summary In summary, the behavior of monolayer holes during annealing after growth was studied in detail by in situ scanning electron microscopy. Asymmetric motion of edge steps was revealed. It was found that the steps in the center of the terrace retreated much but those near the multiple steps moved little. This was attributed to the atom detachment from the edge step being compensated by the atomic #ow from the adjacent multiple steps. Also, the holes moved in the same direction as the environmental step motion. This shows that the macroscopic atomic #ow from the environmental steps determines the hole behavior. Three-dimensional islands are formed preferentially at the step in regrowth after annealing. This is usable to the preferential formation of 3D islands in the strained system which is promising for the control of location of quantum dots.
K. Tanahashi et al. / Journal of Crystal Growth 201/202 (1999) 141}145
145
Fig. 4. Three-dimensional islands preferentially formed on the steps in regrowth after growth. The substrate temperature was about 5903C and the growth rate was 1 ML/40 s. (a) From the start of growth to 80 s, (b) from 85 to 145 s, (c) from 150 to 230 s, (d) from 235 to 315 s.
Acknowledgements The authors are grateful to T. Kawamura and E.R. Weber for their stimulating discussions and R. Shimizu for help in the manuscript preparation. This work was partially supported by the Ministry of Education, Culture and Science of Japan and by the Special Fund for Promotion in Science and Technology Agency.
References [1] H. Sakaki, M. Tanaka, J. Yoshino, Jpn. J. Appl. Phys. 24 (1985) L417.
[2] T. Ide, A. Yamashita, T. Mizutani, Phys. Rev. Lett. B 46 (1992) 1905. [3] N. Inoue, K. Morimoto, T. Araki, T. Ito, Y. Homma, J. Osaka, J. Vac. Sci. Technol. B 14 (1996) 3575. [4] Y. Homma, J. Osaka, N. Inoue, Jpn. J. Appl. Phys. 33 (1994) L563. [5] T. Shitara, J. Zhang, J.H. Neave, B.A. Joyce, J. Appl. Phys. 71 (1992) 4299. [6] J. Temmyo, E. Kuramochi, T. Nishiya, M. Sugo, R. Notzel, T. Tamamura, Electron. Lett. 31 (1995) 209.