- Email: [email protected]
Effect of growth kinetics on the InAsGaAs quantum dot arrays formation on vicinal surfaces
ELSEVIER
Surface Science 377-379 (1997) 895-898
Effect of growth kinetics on the InAs/GaAs quantum dot arrays formation on vicinal surfaces G.E. Cirlin a**, V.N. Petrov ‘, A.O. Golubok a, S.Ya. Tipissev ‘, V.G. Dubrovskii ‘, G.M. Guryanov a, N.N. Ledentsov b*l, D. Bimberg b a Institute for Analytical Instrumentation of Russian Academy of Sciences, Rizhsky pr. 26, 198103 St. Petersburg, Russia b Institut fir Festkiirperphysik, Technische Universitiit Berlin, Hardenbergstrak 36, Berlin D-10623, Germany
Received 1 August 1996; accepted for publication 15 October 1996
Abstract Using scanning tunneling microscopy we have studied the influence of initial stage (up to 1.5 monolayers) growth kinetics on the surface morphology of 3 ML InAs/GaAs(1OO) grown on vicinal surfaces (misoriented by 3” and 7” towards [Oil] direction) by different modifications of molecular beam epitaxy. The results presented clearly show the iuthtence of growth kinetics on the arrangement of InAs/GaAs quantum dot arrays on vicinal surfaces. Gallium arsenide; Indium arsenide; Molecular beam epitaxy; Scanning tunneling microscopy; Self assembly; Surface structure, morphology, roughness and topography; Vicinal single crystal surfaces
Keywords:
1. Introduction
Structures with reduced dimensionality (lower than two) are the subject of rapidly increasing interest in modern solid state physics as they possess unique properties that are expected to improve the characteristics of light-emitting devices [l]. One of the most promising ways to create quantum dots is the direct formation of nanostructures using strain-induced morphological transformation in highly mismatched hetroepitaxial systems. It has recently been shown that after a critical amount of strained layer is deposited on * Corresponding author. Fax: +7 812 2517038; e-mail: cirlin@&nin.snb.su r On leave from A.F. Ioffe Physical-Technical Institute, Russian Academy of Sciences, Politekhnicheskaya 26, 194021 St. Petersburg, Russia.
the substrate, the formation of coherent islands on the wetted layer (coherent Stranslc-Krastanow growth) occurs [2,3]. Due to the large lattice mismatch (7 percent), InAs/GaAs is a perfectly suitable semiconductor system for self-organized growth realization [4-71. It has recently been shown that growth kinetics during the initial stages of heteroepitaxial growth have a significant effect on the surface morphology [S]. In this paper we report an experimental study of the influence of early-stage growth kinetics on InAs/GaAs nanostructure formation using scanning tunneling microscopy (STM ) . 2. Experimental Samples are grown using an EP1203 molecular beam epitaxy (MBE) setup (Russia) on
0039-6028/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved PIZ SOO39-6028(96)01517-S
896
G.E. Cirlin et al. / Surface Science 377-379
GaAs(lOO) vicinal substrates misoriented by 3” and 7” towards [Oil] direction. After desorption of the oxide layer in the growth chamber, the buffer GaAs layer (300 nm) is MBE grown at substrate temperature T= 550°C providing (2 x 4) surface reconstruction for all samples. To simplify STM measurements, the buffer layers are slightly Be-doped and the samples are immediately quenched after the growth of In-containing layers. In all experiments the growth conditions during deposition of InAs layers (substrate temperature 470°C effective As/In flux ratio - 10, growth rate for InAs 0.1 ML s-l) are maintained the same. For nanostructure formation, we have used 3 ML InAs deposition by three growth modes: submonolayer migration enhanced epitaxy (SMEE), MBE ( 1.5 ML) + submonolayer MBE (SMBE, 1.5 ML) and SMBE (1.5 ML) + atomic layer MBE (ALMBE,l.SML). In SMEE growth mode [9], the In and As shutters are closed alternately, while in SMBE [lo] the As shutter is always open and the In shutter is closed periodically, keeping the surface under As flux during the desired growth interruption time. For ALMBE, the In shutter is always open while the surface is periodically exposed under As flux [ 111. After growth the samples are transferred to the STM operation at ambient pressure. The sample surface is protected with vacuum pump oil to avoid degradation of the samples in air, and STM experiments are carried out directly in the oil environment.
3. Results In Figs. la and lb we present STM images for 3 ML InAs/GaAs grown by SMEE on 3” and 7” misoriented surfaces, respectively. For this growth mode, quantum dots are arranged in a 2D quasi-periodic square lattice distorted by statistical fluctuations. By increasing the surface misorientation angle we have obtained a higher density of quantum dots (0.7 x 1011cm-’ and 1.3 x 1011cm-‘, respectively) while their average size decreased ( 19 run and 10 nm, respectively). We also note a small deviation from average dot size (of order of 10%) evaluated from cross-section analysis of the STM images. For the MBE +SMBE growth regime on a
(1997) 895-898
(4
Fig. 1. STM images of the surface after deposition of 3 ML InAs using SMEE on 3” (a) and 7” (b) misoriented surfaces. Scan area 60 x 60 mn for both cases. Sides of the images are parallel to [Oil] and [Oil] directions.
slightly misoriented surface, a mesoscopic structure with a low concentration of quantum dots is observed. At the higher misorientation angle, the surface structure becomes more regular with a sufficiently higher surface density of quantum dots and with no corrugation. The corresponding STM images are presented in Figs. 2a and 2b. Introduction of the ALMBE growth mode as the first 1.5 ML stage keeping the second 1.5 ML as SMBE leads to a drastic change in the resulting
G.E. Cirlin et al. / Surface Science 377-379 (1997) 895-898
@I
(b)
Fig. 2. STM images of the surface after deposition of 3 ML InAs using 1.5 ML MBE + 1.5 ML SMBE on 3” (a) and 7” (b) misoriented surfaces. Scan area 60 x 60 nm for both cases. Sides of the images are parallel to [Ol l] and [Oil] directions.
Fig. 3. STM images of the surface after deposition of 3 ML InAs using 1.5 ML ALMBE + 1.5 ML SMBE on 3” (a) and 7” (b) misoriented surfaces. Scan area 90 x 90 nm for both cases. Sides of the images are parallel to [Oil] and [OTO] directions.
surface morphology. STM images in Figs. 3a and 3b for 3” and 7” misoriented substrates, respectively, clearly indicate a coalescence of quantum dots into conglomerates irrespective of surface vicinity.
4. Discussion An analysis of the STM data shows the strong influence of growth kinetics on surface morphology
during strain-induced heteroepitaxy. For Stranski-Krastanow growth, two-dimensional layer growth occurs until the interfacial energy gain and the strain energy increase equal each other. Further growth can be driven only by a reduction in strain energy due to island formation. In our case, the lateral strain energy distribution during the early-stage (quasi two-dimensional) pseudomorphic growth varies for the growth modes applied. For MBE, SMBE and ALMBE a
898
G.E. Cirlin et al. /Surface Science 377-379 (1997) 895-898
corrugation with a height of several monolayers exists after deposition of 1.0-1.5 ML InAs [8], resulting in inhomogeneity in the surface strain energy distribution. Later, this leads to an inhomogeneous distribution of islands on the surface. Depending on the growth mode, it can produce either a mesoscopically corrugated surface, or conglomerates of islands. For the SMEE case, we expect a perfect two-dimensional growth mode during deposition of the first 1.5 ML of InAs, due to the higher surface mobility of In adatoms. This leads to a more homogeneous distribution of 2D InAs nuclei, and, consequently, of the lateral strain energy. Further decay of the surface into arrays of islands will have almost isotropic and spacehomogeneous character. An additional source for stress relaxation on vicinal surfaces is a relaxation on monoatomic steps. A higher misorientation angle corresponds to a higher density of steps on the surface and, under certain growth conditions, this “vicinityassociated energy” term in the total energy may cause a considerable change in surface evolution (e.g. in the case of the MBE+ SMBE growth mode). On the other hand, misorientation angle may serve as a tool for the tuning of quantum dot geometrical parameters (in the case of SMEE). A low dispersion of lateral size distribution makes this growth mode preferable for the application in the field of light-emitting and single-electron device creation where a uniform size distribution is required. Theoretical investigations of growth instabilties and the kinetics of nanostructure formation during MBE and related techniques in strained epitaxial systems will be presented in a forthcoming paper.
5. Conclusion In summary, we have studied surface morphology evolution during different deposition tech-
niques from molecular beams, and have shown that growth kinetics and surface vicinity not only play an important role in the mechanisms of nanostructure formation but also have a considerable influence on the characteristics of the resulting nanostructures.
Acknowledgement This work was partially supported by INTAS, Russian Foundation for Basic Researches, Russian National Program “Physics of solid state nanostructures”.
References [l] Y. Arakawa and H. Sakaki, Appl. Phys. Lett. 40 (1982) 939. [2] J. Tersoff and R.M. Tromp, Phys. Rev. Lett. 70 (1993) 2782. [3] L. Goldstein, F. Glas, J.Y. Marzin, M.N. Charasse and G. Le Roux, Appl. Phys. Lett. 47 (1985) 1099. [4] D. Leonard, M. Krishnamurthy, C.M. Reaves, S.P. Denbaars and P.M. Petroff, Appl. Phys. L&t. 63 (1993) 3203. [5] J.M. Moison, F. Houzay, F. Barthe, L. Leprince, E. Andre and 0. Vatel, Appl. Phys. Lett. 64 (1994) 196. [6] G.E. Cirlin, G.M. Guryanov, A.O. Golubok, S.Ya. Tipissev, N.N. Ledentsov, P.S. Kop’ev, M. Grundmann and D. Bimberg, Appl. Phys. Lett. 67 (1995) 97. [7] G.M. Guryanov, G.E. Cirlin, V.N. Petrov, N.K. Polyakov, A.O. Golubok, SYa. Tipissev, E.P. Musikhina, V.B. Gubanov, Yu.B. Samsonenko and N.N. Ledentsov, Surf. sci. 331-333(1995)414. [8] G.M. Guryanov, G.E. Cirlin, A.O. Golubok, S.Ya. Tipissev, N.N. Ledentsov, V.A. Shchukin, M. Grundmann, D. Bimberg and Zh.1. Alferov, Surf. Sci. 352-354 (1996) 646. [9] G.E. Cirlin, A.O. Golubok, S.Ya. Tipissev, N.N. Ledentsov and G.M. Guryanov, Fiz. Tekn. Poluprovod. 29 (1995) 1697 (in Russian) (English translation: Semiconductors 29 (1995) 884). [lo] P.D. Wang, N.N. Ledentsov, C.M. Sotomayor Torres, P.S. Kop’ev and V.M. Ustinov, Appl. Phys. Lett. 64 (1994) 526. [ 111 F. Briones, L. Gonzales and A. Ruiz. Appl. Phys. A (1989) 729.
Surface Science 377-379 (1997) 895-898
Effect of growth kinetics on the InAs/GaAs quantum dot arrays formation on vicinal surfaces G.E. Cirlin a**, V.N. Petrov ‘, A.O. Golubok a, S.Ya. Tipissev ‘, V.G. Dubrovskii ‘, G.M. Guryanov a, N.N. Ledentsov b*l, D. Bimberg b a Institute for Analytical Instrumentation of Russian Academy of Sciences, Rizhsky pr. 26, 198103 St. Petersburg, Russia b Institut fir Festkiirperphysik, Technische Universitiit Berlin, Hardenbergstrak 36, Berlin D-10623, Germany
Received 1 August 1996; accepted for publication 15 October 1996
Abstract Using scanning tunneling microscopy we have studied the influence of initial stage (up to 1.5 monolayers) growth kinetics on the surface morphology of 3 ML InAs/GaAs(1OO) grown on vicinal surfaces (misoriented by 3” and 7” towards [Oil] direction) by different modifications of molecular beam epitaxy. The results presented clearly show the iuthtence of growth kinetics on the arrangement of InAs/GaAs quantum dot arrays on vicinal surfaces. Gallium arsenide; Indium arsenide; Molecular beam epitaxy; Scanning tunneling microscopy; Self assembly; Surface structure, morphology, roughness and topography; Vicinal single crystal surfaces
Keywords:
1. Introduction
Structures with reduced dimensionality (lower than two) are the subject of rapidly increasing interest in modern solid state physics as they possess unique properties that are expected to improve the characteristics of light-emitting devices [l]. One of the most promising ways to create quantum dots is the direct formation of nanostructures using strain-induced morphological transformation in highly mismatched hetroepitaxial systems. It has recently been shown that after a critical amount of strained layer is deposited on * Corresponding author. Fax: +7 812 2517038; e-mail: cirlin@&nin.snb.su r On leave from A.F. Ioffe Physical-Technical Institute, Russian Academy of Sciences, Politekhnicheskaya 26, 194021 St. Petersburg, Russia.
the substrate, the formation of coherent islands on the wetted layer (coherent Stranslc-Krastanow growth) occurs [2,3]. Due to the large lattice mismatch (7 percent), InAs/GaAs is a perfectly suitable semiconductor system for self-organized growth realization [4-71. It has recently been shown that growth kinetics during the initial stages of heteroepitaxial growth have a significant effect on the surface morphology [S]. In this paper we report an experimental study of the influence of early-stage growth kinetics on InAs/GaAs nanostructure formation using scanning tunneling microscopy (STM ) . 2. Experimental Samples are grown using an EP1203 molecular beam epitaxy (MBE) setup (Russia) on
0039-6028/97/$17.00 Copyright 0 1997 Elsevier Science B.V. All rights reserved PIZ SOO39-6028(96)01517-S
896
G.E. Cirlin et al. / Surface Science 377-379
GaAs(lOO) vicinal substrates misoriented by 3” and 7” towards [Oil] direction. After desorption of the oxide layer in the growth chamber, the buffer GaAs layer (300 nm) is MBE grown at substrate temperature T= 550°C providing (2 x 4) surface reconstruction for all samples. To simplify STM measurements, the buffer layers are slightly Be-doped and the samples are immediately quenched after the growth of In-containing layers. In all experiments the growth conditions during deposition of InAs layers (substrate temperature 470°C effective As/In flux ratio - 10, growth rate for InAs 0.1 ML s-l) are maintained the same. For nanostructure formation, we have used 3 ML InAs deposition by three growth modes: submonolayer migration enhanced epitaxy (SMEE), MBE ( 1.5 ML) + submonolayer MBE (SMBE, 1.5 ML) and SMBE (1.5 ML) + atomic layer MBE (ALMBE,l.SML). In SMEE growth mode [9], the In and As shutters are closed alternately, while in SMBE [lo] the As shutter is always open and the In shutter is closed periodically, keeping the surface under As flux during the desired growth interruption time. For ALMBE, the In shutter is always open while the surface is periodically exposed under As flux [ 111. After growth the samples are transferred to the STM operation at ambient pressure. The sample surface is protected with vacuum pump oil to avoid degradation of the samples in air, and STM experiments are carried out directly in the oil environment.
3. Results In Figs. la and lb we present STM images for 3 ML InAs/GaAs grown by SMEE on 3” and 7” misoriented surfaces, respectively. For this growth mode, quantum dots are arranged in a 2D quasi-periodic square lattice distorted by statistical fluctuations. By increasing the surface misorientation angle we have obtained a higher density of quantum dots (0.7 x 1011cm-’ and 1.3 x 1011cm-‘, respectively) while their average size decreased ( 19 run and 10 nm, respectively). We also note a small deviation from average dot size (of order of 10%) evaluated from cross-section analysis of the STM images. For the MBE +SMBE growth regime on a
(1997) 895-898
(4
Fig. 1. STM images of the surface after deposition of 3 ML InAs using SMEE on 3” (a) and 7” (b) misoriented surfaces. Scan area 60 x 60 mn for both cases. Sides of the images are parallel to [Oil] and [Oil] directions.
slightly misoriented surface, a mesoscopic structure with a low concentration of quantum dots is observed. At the higher misorientation angle, the surface structure becomes more regular with a sufficiently higher surface density of quantum dots and with no corrugation. The corresponding STM images are presented in Figs. 2a and 2b. Introduction of the ALMBE growth mode as the first 1.5 ML stage keeping the second 1.5 ML as SMBE leads to a drastic change in the resulting
G.E. Cirlin et al. / Surface Science 377-379 (1997) 895-898
@I
(b)
Fig. 2. STM images of the surface after deposition of 3 ML InAs using 1.5 ML MBE + 1.5 ML SMBE on 3” (a) and 7” (b) misoriented surfaces. Scan area 60 x 60 nm for both cases. Sides of the images are parallel to [Ol l] and [Oil] directions.
Fig. 3. STM images of the surface after deposition of 3 ML InAs using 1.5 ML ALMBE + 1.5 ML SMBE on 3” (a) and 7” (b) misoriented surfaces. Scan area 90 x 90 nm for both cases. Sides of the images are parallel to [Oil] and [OTO] directions.
surface morphology. STM images in Figs. 3a and 3b for 3” and 7” misoriented substrates, respectively, clearly indicate a coalescence of quantum dots into conglomerates irrespective of surface vicinity.
4. Discussion An analysis of the STM data shows the strong influence of growth kinetics on surface morphology
during strain-induced heteroepitaxy. For Stranski-Krastanow growth, two-dimensional layer growth occurs until the interfacial energy gain and the strain energy increase equal each other. Further growth can be driven only by a reduction in strain energy due to island formation. In our case, the lateral strain energy distribution during the early-stage (quasi two-dimensional) pseudomorphic growth varies for the growth modes applied. For MBE, SMBE and ALMBE a
898
G.E. Cirlin et al. /Surface Science 377-379 (1997) 895-898
corrugation with a height of several monolayers exists after deposition of 1.0-1.5 ML InAs [8], resulting in inhomogeneity in the surface strain energy distribution. Later, this leads to an inhomogeneous distribution of islands on the surface. Depending on the growth mode, it can produce either a mesoscopically corrugated surface, or conglomerates of islands. For the SMEE case, we expect a perfect two-dimensional growth mode during deposition of the first 1.5 ML of InAs, due to the higher surface mobility of In adatoms. This leads to a more homogeneous distribution of 2D InAs nuclei, and, consequently, of the lateral strain energy. Further decay of the surface into arrays of islands will have almost isotropic and spacehomogeneous character. An additional source for stress relaxation on vicinal surfaces is a relaxation on monoatomic steps. A higher misorientation angle corresponds to a higher density of steps on the surface and, under certain growth conditions, this “vicinityassociated energy” term in the total energy may cause a considerable change in surface evolution (e.g. in the case of the MBE+ SMBE growth mode). On the other hand, misorientation angle may serve as a tool for the tuning of quantum dot geometrical parameters (in the case of SMEE). A low dispersion of lateral size distribution makes this growth mode preferable for the application in the field of light-emitting and single-electron device creation where a uniform size distribution is required. Theoretical investigations of growth instabilties and the kinetics of nanostructure formation during MBE and related techniques in strained epitaxial systems will be presented in a forthcoming paper.
5. Conclusion In summary, we have studied surface morphology evolution during different deposition tech-
niques from molecular beams, and have shown that growth kinetics and surface vicinity not only play an important role in the mechanisms of nanostructure formation but also have a considerable influence on the characteristics of the resulting nanostructures.
Acknowledgement This work was partially supported by INTAS, Russian Foundation for Basic Researches, Russian National Program “Physics of solid state nanostructures”.
References [l] Y. Arakawa and H. Sakaki, Appl. Phys. Lett. 40 (1982) 939. [2] J. Tersoff and R.M. Tromp, Phys. Rev. Lett. 70 (1993) 2782. [3] L. Goldstein, F. Glas, J.Y. Marzin, M.N. Charasse and G. Le Roux, Appl. Phys. Lett. 47 (1985) 1099. [4] D. Leonard, M. Krishnamurthy, C.M. Reaves, S.P. Denbaars and P.M. Petroff, Appl. Phys. L&t. 63 (1993) 3203. [5] J.M. Moison, F. Houzay, F. Barthe, L. Leprince, E. Andre and 0. Vatel, Appl. Phys. Lett. 64 (1994) 196. [6] G.E. Cirlin, G.M. Guryanov, A.O. Golubok, S.Ya. Tipissev, N.N. Ledentsov, P.S. Kop’ev, M. Grundmann and D. Bimberg, Appl. Phys. Lett. 67 (1995) 97. [7] G.M. Guryanov, G.E. Cirlin, V.N. Petrov, N.K. Polyakov, A.O. Golubok, SYa. Tipissev, E.P. Musikhina, V.B. Gubanov, Yu.B. Samsonenko and N.N. Ledentsov, Surf. sci. 331-333(1995)414. [8] G.M. Guryanov, G.E. Cirlin, A.O. Golubok, S.Ya. Tipissev, N.N. Ledentsov, V.A. Shchukin, M. Grundmann, D. Bimberg and Zh.1. Alferov, Surf. Sci. 352-354 (1996) 646. [9] G.E. Cirlin, A.O. Golubok, S.Ya. Tipissev, N.N. Ledentsov and G.M. Guryanov, Fiz. Tekn. Poluprovod. 29 (1995) 1697 (in Russian) (English translation: Semiconductors 29 (1995) 884). [lo] P.D. Wang, N.N. Ledentsov, C.M. Sotomayor Torres, P.S. Kop’ev and V.M. Ustinov, Appl. Phys. Lett. 64 (1994) 526. [ 111 F. Briones, L. Gonzales and A. Ruiz. Appl. Phys. A (1989) 729.