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Influence of capping layer thickness on electronic states in self assembled MOVPE grown InAs quantum dots in GaAs
Superlattices and Microstructures 46 (2009) 324–327
Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Influence of capping layer thickness on electronic states in self assembled MOVPE grown InAs quantum dots in GaAs P. Hazdra a,∗ , J. Oswald b , V. Komarnitskyy a , K. Kuldová b , A. Hospodková b , E. Hulicius b , J. Pangrác b a
Department of Microelectronics, Faculty of Electrical Engineering, Czech Technical University in Prague, Technická 2, CZ-166 27 Prague 6, Czech Republic b
Institute of Physics of the AS CR, v. v. i., Cukrovarnická 10, 162 00 Prague 6, Czech Republic
article
info
Article history: Available online 10 January 2009 Keywords: Quantum dots Metal-organic vapor phase epitaxy Indium arsenide Gallium arsenide Photoluminescence Atomic force microscopy
abstract The influence of the capping layer on the structural and optical properties of InAs quantum dots grown on a GaAs substrate by metal-organic vapor phase epitaxy was investigated. The results show that the GaAs capping layer transforms originally elliptical lens like quantum dots into elongated structures or rhombus shaped objects with a central hole. This is accompanied by the blue shift of quantum dot luminescence maximum from 1.43 to 1.26 µm. Simulations of the electronic states in such quantum dot structures showed that this shift is caused both by the quantum dot shrinking and by the change of their band structure due to strain growth as the GaAs coverage increases. On the contrary In0.23 Ga0.77 As capping reduces the residual strain and preserves the shape of the quantum dots. As a result, the photoluminescence maximum shifts up to the desired wavelength of 1.55 µm. © 2008 Elsevier Ltd. All rights reserved.
1. Introduction Nowadays, self-assembled InAs quantum dots (QDs) grown on a GaAs substrate are intensively studied to become a base for cheap, low threshold, high output and heatsink free lasers for the 1.55 µm communication band. To be embedded into a functional structure, QDs must be covered by a capping layer (CL) providing their protection and suppressing the non-radiative recombination through surface states. During the overgrowth, residual strain in the QD layer increases and QDs
∗
Corresponding author. Tel.: +420 224 352 052; fax: +420 224 310 792. E-mail address: [email protected] (P. Hazdra).
0749-6036/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.spmi.2008.12.002
P. Hazdra et al. / Superlattices and Microstructures 46 (2009) 324–327
325
also significantly change their size and shape. Optical characteristics of the overgrown QD layers are therefore significantly influenced by the thickness and growth parameters of the CL [1]. Up to now, only a few studies [1–4] reported on the impact of the CL on the structural properties of QDs mainly grown by molecular beam epitaxy (MBE) although the overgrowth process is essential for the realization of novel optoelectronic devices. In this contribution, we report on the structural and optical changes during the process of capping of self-organized InAs QDs grown by metalorganic vapor phase epitaxy (MOVPE) which is the growth technique of choice for high volume lasers production. 2. Experimental InAs QD structures were prepared by low-pressure MOVPE on semi-insulating GaAs (001) substrates using the Stranski–Krastanow growth mode (AIXTRON 200 reactor equipped with a reflectance anisotropy spectrometer from Laytec). TMGa, TMIn, and AsH3 were used as precursors. The structures were grown at 490 ◦ C except the first GaAs buffer layer (650 ◦ C). InAs wetting layer was deposited with a growth rate of 0.06 monolayer per second, V/III ratio was 83 and a growth time of 27 s. The growth was then interrupted for 15 s to allow QDs formation. QDs were then covered by GaAs (or In0.23 Ga0.77 As) CLs with different thicknesses ranging from 0 to 42.5 nm. Atomic force microscopy (AFM) images were taken in the contact mode using the NTEGRA Prima system from NT MTD. Photoluminescence (PL) spectra were excited by semiconductor lasers (670 nm and 980 nm) and recorded by a standard lock-in detection technique. Quantum transitions were simulated by the nextnano3 [5] 3D simulator using a calibrated model of the QD structures [6]. 3. Results and discussion The 500 × 250 nm2 AFM image of uncapped sample is shown in Fig. 1a. The uncovered QDs are InAs ¯ ] direction. Their average density is 1.6 × 1010 cm−2 , lenses which are slightly elongated in the [110 ¯ lateral dimensions along [110] and [110] axes are 25±10 and 22±8 nm, resp., and their average height is 4.0 ± 1.3 nm. A significant spread of QD sizes causes the uncovered layer of QDs to exhibit a broad PL spectrum which peaks at 1.43 µm (see Fig. 2). Gradual covering by the GaAs CL reduces QDs height and density. This is evident from the AFM image of InAs QDs capped with 2.5 nm of GaAs (Fig. 1b). Density of QDs decreased to 1.3 × 1010 cm−2 (probably due to the dissolution of smaller dots) and QDs transformed either into approx. 2 nm high elongated plateaus (1) or rhombus shaped objects with a central hole (2). These undesirable, optically non-active rings with a central hole which extends up to the GaAs substrate develop preferentially from the bigger QDs. The creation mechanism of rings in partially covered QD structures is discussed, for instance, in [7,8]. The QDs shrinking is caused by the ¯ deposited Ga atoms which accumulate at QDs base (preferentially along the [110] direction) forcing In atoms to migrate from the top of QD to its base [2]. The driving mechanism of In outdiffusion and alloying weakens and vanishes with decreasing QD height (or increasing In content in the capping layer [3]). PL spectra in Fig. 2 show that the 2.5 nm GaAs CL improves homogeneity of QDs structures the PL peak significantly narrows (up to 40%), retains its intensity and spectral position. Further GaAs capping increases the PL intensity and causes a blue shift (up to 1.26 µm) and further narrowing of the PL maximum. The experimental (PL) and calculated room temperature transition energies originating from the InAs QDs covered by GaAs CLs with different thicknesses are shown in Fig. 3. The calculations were made using a 3D model, assuming an elliptical InAs QD and 0.3 nm thick InAs wetting layer embedded in GaAs. For fully overlaid QDs (15 nm CL) with narrow PL peak, the best agreement with PL data was obtained for 4 nm high QD with a 20 × 15 nm base. This structure was then used for the simulation of the effect of GaAs cap on fully overlaid structures, i.e., the CL thicker than 7 nm. Results show that the blue shift is, in this case, mainly caused by the change of the QD band structure due to increasing strain in the layer which lifts up the conduction band in the upper part of the QD structure (see the inset in Fig. 3). As stated above, higher In content in the CL could preserve the QD’s size. This is evidenced in Fig. 1c which shows the AFM image of QDs covered by 2.5 nm thick In0.23 Ga0.77 As cap. Although the density
326
(a) No cap.
P. Hazdra et al. / Superlattices and Microstructures 46 (2009) 324–327
(b) 2.5 nm GaAs.
(c) 2.5 nm In0.23 Ga0.77 As.
Fig. 1. AFM images of uncapped InAs QDs (a), QDs with 2.5 nm thick GaAs cap (b), and with 2.5 nm In0.23 Ga0.77 As cap (c). The images are 500 nm width and 250 nm height. The images are presented with different image processing: with the plane-view (upper panel) and a three-dimensional view (lower panel) to stress the three dimensional characteristics. The cross sections of typical structures along the [110] direction (dashed) are shown below each image — the vertical axis is the epitaxial growth direction [001].
Fig. 2. Normalized room temperature PL spectra of a single layer of InAs QDs covered by 0, 2.5 and 15 nm of GaAs (solid) and by 20 nm In0.23 Ga0.77 As capping layer (dashed). The fine structure at ∼0.91 eV is caused by water vapor absorption.
of QD structures also decreased to 1.3 × 1010 cm–2 , the bigger structures preferentially developed and their height was conserved. As a result, PL of In0.23 Ga0.77 As capped QD structures exhibits a red shift very close to the required wavelength of 1.55 µm (see Fig. 2). 4. Conclusions We have shown that covering of MOVPE grown InAs QDs by the GaAs cap transforms QDs into elongated structures or rhombus shaped objects. This effect is accompanied by a blue shift of QD PL from 1.43 to 1.26 µm which is caused both by the reduction of QD dimensions and by the change of
P. Hazdra et al. / Superlattices and Microstructures 46 (2009) 324–327
327
Fig. 3. Comparison of measured (PL - circles) and calculated (diamonds) room temperature transition energies originating from InAs QDs covered with GaAs capping layers with different thickness. The corresponding band diagrams (taken in the QD center along the [001] axis) are shown in the inset.
their band structure due to increasing strain. On the contrary, the In0.23 Ga0.77 As capping preserves the QD shape and reduces the residual strain. This enables to shift the PL emission maximum close to the desired 1.55 µm. Acknowledgments This work was supported by the GACR grant 202/06/0718, the research program MSM 6840770014, the GAAV grant A100100719, and the project AV0Z1-010-914. We thank K. Melichar for the MOVPE samples preparation and T. Šimeček for discussion. References [1] F. Ferdos, S. Wang, Y. Wei, A. Larsson, M. Sadeghi, Q. Zhao, Appl. Phys. Lett. 81 (2002) 1195. [2] G. Costantini, A. Rastelli, C. Manzano, P. Acosta-Diaz, R. Songmuang, G. Katsaros, O.G. Schmidt, K. Kern, Phys. Rev. Lett. 96 (2006) 226106. [3] R. Songmuang, S. Kiravittaya, O.G. Schmidt, J. Cryst. Growth 249 (2003) 416. [4] E. Hulicius, J. Oswald, J. Pangrác, J. Vyskočil, A. Hospodková, K. Kuldová, K. Melichar, T. Šimeček, J. Cryst. Growth 310 (2008) 2229. [5] http://www.wsi.tu-muenchen.de/nextnano3. [6] P. Hazdra, J. Oswald, M. Atef, K. Kuldová, A. Hospodková, E. Hulicius, J. Pangrác, Mater. Sci. Eng. B 147 (2008) 175. [7] J.M. Garcia, G. Medeiros-Ribeiro, K. Schmidt, T. Ngo, J.L. Feng, A. Lorke, J. Kotthaus, P.M. Petroff, Appl. Phys. Lett. 71 (1997) 2014. [8] A. Aierken, T. Hakkarainen, J. Riikonen, M. Sopanen, Nanotechnology 19 (2008) 245304.
Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Influence of capping layer thickness on electronic states in self assembled MOVPE grown InAs quantum dots in GaAs P. Hazdra a,∗ , J. Oswald b , V. Komarnitskyy a , K. Kuldová b , A. Hospodková b , E. Hulicius b , J. Pangrác b a
Department of Microelectronics, Faculty of Electrical Engineering, Czech Technical University in Prague, Technická 2, CZ-166 27 Prague 6, Czech Republic b
Institute of Physics of the AS CR, v. v. i., Cukrovarnická 10, 162 00 Prague 6, Czech Republic
article
info
Article history: Available online 10 January 2009 Keywords: Quantum dots Metal-organic vapor phase epitaxy Indium arsenide Gallium arsenide Photoluminescence Atomic force microscopy
abstract The influence of the capping layer on the structural and optical properties of InAs quantum dots grown on a GaAs substrate by metal-organic vapor phase epitaxy was investigated. The results show that the GaAs capping layer transforms originally elliptical lens like quantum dots into elongated structures or rhombus shaped objects with a central hole. This is accompanied by the blue shift of quantum dot luminescence maximum from 1.43 to 1.26 µm. Simulations of the electronic states in such quantum dot structures showed that this shift is caused both by the quantum dot shrinking and by the change of their band structure due to strain growth as the GaAs coverage increases. On the contrary In0.23 Ga0.77 As capping reduces the residual strain and preserves the shape of the quantum dots. As a result, the photoluminescence maximum shifts up to the desired wavelength of 1.55 µm. © 2008 Elsevier Ltd. All rights reserved.
1. Introduction Nowadays, self-assembled InAs quantum dots (QDs) grown on a GaAs substrate are intensively studied to become a base for cheap, low threshold, high output and heatsink free lasers for the 1.55 µm communication band. To be embedded into a functional structure, QDs must be covered by a capping layer (CL) providing their protection and suppressing the non-radiative recombination through surface states. During the overgrowth, residual strain in the QD layer increases and QDs
∗
Corresponding author. Tel.: +420 224 352 052; fax: +420 224 310 792. E-mail address: [email protected] (P. Hazdra).
0749-6036/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.spmi.2008.12.002
P. Hazdra et al. / Superlattices and Microstructures 46 (2009) 324–327
325
also significantly change their size and shape. Optical characteristics of the overgrown QD layers are therefore significantly influenced by the thickness and growth parameters of the CL [1]. Up to now, only a few studies [1–4] reported on the impact of the CL on the structural properties of QDs mainly grown by molecular beam epitaxy (MBE) although the overgrowth process is essential for the realization of novel optoelectronic devices. In this contribution, we report on the structural and optical changes during the process of capping of self-organized InAs QDs grown by metalorganic vapor phase epitaxy (MOVPE) which is the growth technique of choice for high volume lasers production. 2. Experimental InAs QD structures were prepared by low-pressure MOVPE on semi-insulating GaAs (001) substrates using the Stranski–Krastanow growth mode (AIXTRON 200 reactor equipped with a reflectance anisotropy spectrometer from Laytec). TMGa, TMIn, and AsH3 were used as precursors. The structures were grown at 490 ◦ C except the first GaAs buffer layer (650 ◦ C). InAs wetting layer was deposited with a growth rate of 0.06 monolayer per second, V/III ratio was 83 and a growth time of 27 s. The growth was then interrupted for 15 s to allow QDs formation. QDs were then covered by GaAs (or In0.23 Ga0.77 As) CLs with different thicknesses ranging from 0 to 42.5 nm. Atomic force microscopy (AFM) images were taken in the contact mode using the NTEGRA Prima system from NT MTD. Photoluminescence (PL) spectra were excited by semiconductor lasers (670 nm and 980 nm) and recorded by a standard lock-in detection technique. Quantum transitions were simulated by the nextnano3 [5] 3D simulator using a calibrated model of the QD structures [6]. 3. Results and discussion The 500 × 250 nm2 AFM image of uncapped sample is shown in Fig. 1a. The uncovered QDs are InAs ¯ ] direction. Their average density is 1.6 × 1010 cm−2 , lenses which are slightly elongated in the [110 ¯ lateral dimensions along [110] and [110] axes are 25±10 and 22±8 nm, resp., and their average height is 4.0 ± 1.3 nm. A significant spread of QD sizes causes the uncovered layer of QDs to exhibit a broad PL spectrum which peaks at 1.43 µm (see Fig. 2). Gradual covering by the GaAs CL reduces QDs height and density. This is evident from the AFM image of InAs QDs capped with 2.5 nm of GaAs (Fig. 1b). Density of QDs decreased to 1.3 × 1010 cm−2 (probably due to the dissolution of smaller dots) and QDs transformed either into approx. 2 nm high elongated plateaus (1) or rhombus shaped objects with a central hole (2). These undesirable, optically non-active rings with a central hole which extends up to the GaAs substrate develop preferentially from the bigger QDs. The creation mechanism of rings in partially covered QD structures is discussed, for instance, in [7,8]. The QDs shrinking is caused by the ¯ deposited Ga atoms which accumulate at QDs base (preferentially along the [110] direction) forcing In atoms to migrate from the top of QD to its base [2]. The driving mechanism of In outdiffusion and alloying weakens and vanishes with decreasing QD height (or increasing In content in the capping layer [3]). PL spectra in Fig. 2 show that the 2.5 nm GaAs CL improves homogeneity of QDs structures the PL peak significantly narrows (up to 40%), retains its intensity and spectral position. Further GaAs capping increases the PL intensity and causes a blue shift (up to 1.26 µm) and further narrowing of the PL maximum. The experimental (PL) and calculated room temperature transition energies originating from the InAs QDs covered by GaAs CLs with different thicknesses are shown in Fig. 3. The calculations were made using a 3D model, assuming an elliptical InAs QD and 0.3 nm thick InAs wetting layer embedded in GaAs. For fully overlaid QDs (15 nm CL) with narrow PL peak, the best agreement with PL data was obtained for 4 nm high QD with a 20 × 15 nm base. This structure was then used for the simulation of the effect of GaAs cap on fully overlaid structures, i.e., the CL thicker than 7 nm. Results show that the blue shift is, in this case, mainly caused by the change of the QD band structure due to increasing strain in the layer which lifts up the conduction band in the upper part of the QD structure (see the inset in Fig. 3). As stated above, higher In content in the CL could preserve the QD’s size. This is evidenced in Fig. 1c which shows the AFM image of QDs covered by 2.5 nm thick In0.23 Ga0.77 As cap. Although the density
326
(a) No cap.
P. Hazdra et al. / Superlattices and Microstructures 46 (2009) 324–327
(b) 2.5 nm GaAs.
(c) 2.5 nm In0.23 Ga0.77 As.
Fig. 1. AFM images of uncapped InAs QDs (a), QDs with 2.5 nm thick GaAs cap (b), and with 2.5 nm In0.23 Ga0.77 As cap (c). The images are 500 nm width and 250 nm height. The images are presented with different image processing: with the plane-view (upper panel) and a three-dimensional view (lower panel) to stress the three dimensional characteristics. The cross sections of typical structures along the [110] direction (dashed) are shown below each image — the vertical axis is the epitaxial growth direction [001].
Fig. 2. Normalized room temperature PL spectra of a single layer of InAs QDs covered by 0, 2.5 and 15 nm of GaAs (solid) and by 20 nm In0.23 Ga0.77 As capping layer (dashed). The fine structure at ∼0.91 eV is caused by water vapor absorption.
of QD structures also decreased to 1.3 × 1010 cm–2 , the bigger structures preferentially developed and their height was conserved. As a result, PL of In0.23 Ga0.77 As capped QD structures exhibits a red shift very close to the required wavelength of 1.55 µm (see Fig. 2). 4. Conclusions We have shown that covering of MOVPE grown InAs QDs by the GaAs cap transforms QDs into elongated structures or rhombus shaped objects. This effect is accompanied by a blue shift of QD PL from 1.43 to 1.26 µm which is caused both by the reduction of QD dimensions and by the change of
P. Hazdra et al. / Superlattices and Microstructures 46 (2009) 324–327
327
Fig. 3. Comparison of measured (PL - circles) and calculated (diamonds) room temperature transition energies originating from InAs QDs covered with GaAs capping layers with different thickness. The corresponding band diagrams (taken in the QD center along the [001] axis) are shown in the inset.
their band structure due to increasing strain. On the contrary, the In0.23 Ga0.77 As capping preserves the QD shape and reduces the residual strain. This enables to shift the PL emission maximum close to the desired 1.55 µm. Acknowledgments This work was supported by the GACR grant 202/06/0718, the research program MSM 6840770014, the GAAV grant A100100719, and the project AV0Z1-010-914. We thank K. Melichar for the MOVPE samples preparation and T. Šimeček for discussion. References [1] F. Ferdos, S. Wang, Y. Wei, A. Larsson, M. Sadeghi, Q. Zhao, Appl. Phys. Lett. 81 (2002) 1195. [2] G. Costantini, A. Rastelli, C. Manzano, P. Acosta-Diaz, R. Songmuang, G. Katsaros, O.G. Schmidt, K. Kern, Phys. Rev. Lett. 96 (2006) 226106. [3] R. Songmuang, S. Kiravittaya, O.G. Schmidt, J. Cryst. Growth 249 (2003) 416. [4] E. Hulicius, J. Oswald, J. Pangrác, J. Vyskočil, A. Hospodková, K. Kuldová, K. Melichar, T. Šimeček, J. Cryst. Growth 310 (2008) 2229. [5] http://www.wsi.tu-muenchen.de/nextnano3. [6] P. Hazdra, J. Oswald, M. Atef, K. Kuldová, A. Hospodková, E. Hulicius, J. Pangrác, Mater. Sci. Eng. B 147 (2008) 175. [7] J.M. Garcia, G. Medeiros-Ribeiro, K. Schmidt, T. Ngo, J.L. Feng, A. Lorke, J. Kotthaus, P.M. Petroff, Appl. Phys. Lett. 71 (1997) 2014. [8] A. Aierken, T. Hakkarainen, J. Riikonen, M. Sopanen, Nanotechnology 19 (2008) 245304.