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Nucleation of islands in GaAs molecular beam epitaxy studied by in-situ scanning electron microscopy
,oumr..
Journal
of Crystal
Growth
150 (1995) 107-109
Nucleation of islands in GaAs molecular beam epitaxy studied by in-situ scanning electron microscopy N. Inoue a,*, J. Osaka a, Y. Homma b a NTT LSI Laboratories, 3-1, Morinosato-Wakamiya, At&, Kanagawa 243-01, Japan b NTT Interdisciplinary Laboratories, 3-9-11, Musashino, Tokyo 180, Japan
Abstract We studied two-dimensional nucleation of islands in the molecular beam epitaxy (MBE) of GaAs by in-situ scanning electron microscopy and found that the islands do not appear until about l/3 monolayer is deposited. In contrast to what is expected from the previous simple nucleation and growth model, we observed islands to form continuously.
1. Introduction
There are increasing demands for detailed understanding of growth processes, and we recently were able to use in-situ scanning electron microscopy to observe monolayer steps and islands on growing GaAs surfaces [Il. In the present paper we report observations on the delayed and continuous nucleation of islands in GaAs MBE and we discuss the mechanism of these nucleation kinetics.
2. Experimental
procedure
The experimental apparatus was a UHV scanning electron microscope (SEMI equipped with K-cells and a flux monitor [2]. The electron beam incident angle was 10” for the SEM observation
but was 1” for the reflection high-energy electron diffraction (RHEED) measurement. The resolution was about 5 nm. Although the observation rate was lo-80 s/frame, we could follow the change continuously as the change of the image resulting from the electron beam scanning from top to bottom. The substrates were (001) GaAs misoriented 0.2” to [1101 and the average terrace width expected for them was 80 nm. A buffer layer was grown to prepare a well-defined surface with terraces and steps. The growth rate was estimated by the RHEED oscillation measurement at a glancing condition and was corrected for the difference in the flux due to inclination of samples for SEM observation. The substrate temperature was measured by monitoring infrared emission from the substrate.
3. Results and discussion
* Corresponding
Fig. 1 shows the morphology change in the initial stage of growth. The substrate temperature
author.
0022-0248/95/$09.50 0 1995 Elsevier SSDI 0022-0248(94)00951-l
Science
B.V. All rights reserved
108
N. Inoue et al. /Journal
of Crystal Growth 150 (1995) 107-109
Fig. 1. Delay of nucleation of islands observed by in-situ SEM. The electron beam was scanned from top to bottom over 80 s. The surface before the start of growth appears at the top. The start of growth is indicated by the image shift introduced by the mechanical motion of the Ga shutter as marked by the open arrow. Before and just after the start of growth only terraces and steps were seen. The islands (bright dots) appeared about 10 s after the growth had started (indicated by the filled arrow). The growth rate was estimated to be 1 monolayer per 25 s, and the substrate temperature was about 580°C. The image is about ten times foreshortened in the vertical direction due to the grazing incidence of the electron beam. Marker represents 300 nm.
was 580°C and the growth rate was 1 monolayer per 2.5 s. The surface before growth was covered with an array of steps and terraces, and there were no
islands. When the growth started, there was no change and no islands appeared. Islands appeared when about l/3 monolayer had been deposited. The amount of deposition necessary for the formation of islands did not change with the change of growth conditions (substrate temperature, growth rate and V/III ratio). The time necessary for the islands to grow to an observable size is much shorter than the observed delay time. In this method, islands larger than 5 nm can be detected. It has been reported that islands apparent in step-by-step STM observation when 0.25 monolayer is deposited are smaller than the expected size, and this has been attributed to the consumption of adatoms by the step propagation growth mode [3]. The present work, however, showed that the steps did not move distinctly. This mechanism is therefore not likely. If we assume that all the atoms supplied to the surface behave as adatoms, the concentration is too high for them not to form clusters. One possible explanation of the delay is that most of these supplied atoms are consumed to change either the surface reconstruction or the surface stoichiometry. The present result does not necessarily mean that the well-known time-lag in nucleation [4] takes place, because we do not know the equilibrium saturation density of adatoms. Recently, it was suggested that a metastable state
Fig. 2. Continuous formation of islands on the terrace: (a) before growth no islands were observed, (b) 2 min after the start of the growth several islands appeared, (c) 3 min after, several islands appeared while the preexisting islands grew. The substrate temperature was 580°C and the growth rate was estimated to be 1 ML per 140 s. Marker represents 300 nm.
N. Inoue et al. /Journal
of Crystal Growth 150 (1995) 107-109
appears at the initial stage of growth from the RHEED intensity oscillation measurement [5]. This suggestion is related to the present results. Fig. 2 shows the morphology change on a terrace. The substrate temperature was about 580°C and the estimated growth rate was 1 ML per 140 s. There were no islands before the start of the growth as shown in (a). In (b), 2 min after the start of growth, several small islands appeared on the terrace. In the next frame (cl, almost ten islands are seen and some of them had become larger. Thus it was found that the number of islands on the terrace increases with time. The simple concept of 2D nucleation and growth in MBE is that the nucleation occurs at once and then growth and coalescence of islands occurs without further nucleation [6]. The present results are in marked contrast to this concept, though, and suggest that a continuous nucleation takes place in MBE, as it does in many kinds of condensed matter [4]. The nucleation rate was estimated to be 1 X 10’ cmP2 s-l, and the saturation island spacing was estimated to be 30 nm. No islands were observed on a narrower terrace, and the critical terrace width below which nucleation is suppressed is estimated to be 100 nm. This nearly agrees with the expected critical terrace width under conditions on which RHEED oscillation does not occur [7]. The continuous nucleation implies that the degree of supersaturation is increased during
109
growth even though the islands formed. This implies that the overlapping of diffusion fields does not take place easily. It seems that the saturation in nucleation occurs when the area for nucleation is lost by the growth and coalescence of the islands. The present results show that the in-situ, continuous SEM observation of individual islands and steps provides information that cannot be obtained by the previous step-by-step observation techniques.
Acknowledgements We are grateful to Takashi Mizutani Kurosawa for their encouragement.
and Ken
References [l] N. Inoue, M. Tanimoto, K. Kanisawa, S. Hirono, J. Osaka and Y. Homma, J. Crystal Growth 127(1993)956. [2] Y. Homma, J. Osaka and N. Inoue, Jap. J. Appl. Phys. 33(1994)L563. [3] J. Sudijono, M.D. Johnson, M.B. Elowitz, C.W. Snyder and B.G. Orr, Surf. Sci. 280(1993)247. [4] W.J. Dunning, in: Nucleation, Ed. A.C. Zettlemoyer (Dekker, New York, 1969) p. 273. [5] S.Y. Karpov, Y.V. Kovalchuk, V.E. Myachin and Y.V. Pogorelsky, Surf. Sci. 314(1994)79. [6] J.H. Neave, B.A. Joyce, P.J. Dobson and N. Norton, Appl. Phys. A31(1983)1. [7] N. Inoue, Advan. Mater. 5(1993)192.
Journal
of Crystal
Growth
150 (1995) 107-109
Nucleation of islands in GaAs molecular beam epitaxy studied by in-situ scanning electron microscopy N. Inoue a,*, J. Osaka a, Y. Homma b a NTT LSI Laboratories, 3-1, Morinosato-Wakamiya, At&, Kanagawa 243-01, Japan b NTT Interdisciplinary Laboratories, 3-9-11, Musashino, Tokyo 180, Japan
Abstract We studied two-dimensional nucleation of islands in the molecular beam epitaxy (MBE) of GaAs by in-situ scanning electron microscopy and found that the islands do not appear until about l/3 monolayer is deposited. In contrast to what is expected from the previous simple nucleation and growth model, we observed islands to form continuously.
1. Introduction
There are increasing demands for detailed understanding of growth processes, and we recently were able to use in-situ scanning electron microscopy to observe monolayer steps and islands on growing GaAs surfaces [Il. In the present paper we report observations on the delayed and continuous nucleation of islands in GaAs MBE and we discuss the mechanism of these nucleation kinetics.
2. Experimental
procedure
The experimental apparatus was a UHV scanning electron microscope (SEMI equipped with K-cells and a flux monitor [2]. The electron beam incident angle was 10” for the SEM observation
but was 1” for the reflection high-energy electron diffraction (RHEED) measurement. The resolution was about 5 nm. Although the observation rate was lo-80 s/frame, we could follow the change continuously as the change of the image resulting from the electron beam scanning from top to bottom. The substrates were (001) GaAs misoriented 0.2” to [1101 and the average terrace width expected for them was 80 nm. A buffer layer was grown to prepare a well-defined surface with terraces and steps. The growth rate was estimated by the RHEED oscillation measurement at a glancing condition and was corrected for the difference in the flux due to inclination of samples for SEM observation. The substrate temperature was measured by monitoring infrared emission from the substrate.
3. Results and discussion
* Corresponding
Fig. 1 shows the morphology change in the initial stage of growth. The substrate temperature
author.
0022-0248/95/$09.50 0 1995 Elsevier SSDI 0022-0248(94)00951-l
Science
B.V. All rights reserved
108
N. Inoue et al. /Journal
of Crystal Growth 150 (1995) 107-109
Fig. 1. Delay of nucleation of islands observed by in-situ SEM. The electron beam was scanned from top to bottom over 80 s. The surface before the start of growth appears at the top. The start of growth is indicated by the image shift introduced by the mechanical motion of the Ga shutter as marked by the open arrow. Before and just after the start of growth only terraces and steps were seen. The islands (bright dots) appeared about 10 s after the growth had started (indicated by the filled arrow). The growth rate was estimated to be 1 monolayer per 25 s, and the substrate temperature was about 580°C. The image is about ten times foreshortened in the vertical direction due to the grazing incidence of the electron beam. Marker represents 300 nm.
was 580°C and the growth rate was 1 monolayer per 2.5 s. The surface before growth was covered with an array of steps and terraces, and there were no
islands. When the growth started, there was no change and no islands appeared. Islands appeared when about l/3 monolayer had been deposited. The amount of deposition necessary for the formation of islands did not change with the change of growth conditions (substrate temperature, growth rate and V/III ratio). The time necessary for the islands to grow to an observable size is much shorter than the observed delay time. In this method, islands larger than 5 nm can be detected. It has been reported that islands apparent in step-by-step STM observation when 0.25 monolayer is deposited are smaller than the expected size, and this has been attributed to the consumption of adatoms by the step propagation growth mode [3]. The present work, however, showed that the steps did not move distinctly. This mechanism is therefore not likely. If we assume that all the atoms supplied to the surface behave as adatoms, the concentration is too high for them not to form clusters. One possible explanation of the delay is that most of these supplied atoms are consumed to change either the surface reconstruction or the surface stoichiometry. The present result does not necessarily mean that the well-known time-lag in nucleation [4] takes place, because we do not know the equilibrium saturation density of adatoms. Recently, it was suggested that a metastable state
Fig. 2. Continuous formation of islands on the terrace: (a) before growth no islands were observed, (b) 2 min after the start of the growth several islands appeared, (c) 3 min after, several islands appeared while the preexisting islands grew. The substrate temperature was 580°C and the growth rate was estimated to be 1 ML per 140 s. Marker represents 300 nm.
N. Inoue et al. /Journal
of Crystal Growth 150 (1995) 107-109
appears at the initial stage of growth from the RHEED intensity oscillation measurement [5]. This suggestion is related to the present results. Fig. 2 shows the morphology change on a terrace. The substrate temperature was about 580°C and the estimated growth rate was 1 ML per 140 s. There were no islands before the start of the growth as shown in (a). In (b), 2 min after the start of growth, several small islands appeared on the terrace. In the next frame (cl, almost ten islands are seen and some of them had become larger. Thus it was found that the number of islands on the terrace increases with time. The simple concept of 2D nucleation and growth in MBE is that the nucleation occurs at once and then growth and coalescence of islands occurs without further nucleation [6]. The present results are in marked contrast to this concept, though, and suggest that a continuous nucleation takes place in MBE, as it does in many kinds of condensed matter [4]. The nucleation rate was estimated to be 1 X 10’ cmP2 s-l, and the saturation island spacing was estimated to be 30 nm. No islands were observed on a narrower terrace, and the critical terrace width below which nucleation is suppressed is estimated to be 100 nm. This nearly agrees with the expected critical terrace width under conditions on which RHEED oscillation does not occur [7]. The continuous nucleation implies that the degree of supersaturation is increased during
109
growth even though the islands formed. This implies that the overlapping of diffusion fields does not take place easily. It seems that the saturation in nucleation occurs when the area for nucleation is lost by the growth and coalescence of the islands. The present results show that the in-situ, continuous SEM observation of individual islands and steps provides information that cannot be obtained by the previous step-by-step observation techniques.
Acknowledgements We are grateful to Takashi Mizutani Kurosawa for their encouragement.
and Ken
References [l] N. Inoue, M. Tanimoto, K. Kanisawa, S. Hirono, J. Osaka and Y. Homma, J. Crystal Growth 127(1993)956. [2] Y. Homma, J. Osaka and N. Inoue, Jap. J. Appl. Phys. 33(1994)L563. [3] J. Sudijono, M.D. Johnson, M.B. Elowitz, C.W. Snyder and B.G. Orr, Surf. Sci. 280(1993)247. [4] W.J. Dunning, in: Nucleation, Ed. A.C. Zettlemoyer (Dekker, New York, 1969) p. 273. [5] S.Y. Karpov, Y.V. Kovalchuk, V.E. Myachin and Y.V. Pogorelsky, Surf. Sci. 314(1994)79. [6] J.H. Neave, B.A. Joyce, P.J. Dobson and N. Norton, Appl. Phys. A31(1983)1. [7] N. Inoue, Advan. Mater. 5(1993)192.