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Critical thickness for InAs quantum dot formation on (3 1 1)B InP substrates
ARTICLE IN PRESS Journal of Crystal Growth 311 (2009) 2626–2629
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Critical thickness for InAs quantum dot formation on (3 11)B InP substrates P. Caroff, N. Bertru , W. Lu, G. Elias, O. Dehaese, A. Le´toublon, A. Le Corre FOTON, UMR 6082, INSA de Rennes, 20 avenue des Buttes de Coe¨smes, CS 14315, 35043 Rennes Cedex, France
a r t i c l e in f o
a b s t r a c t
Article history: Received 16 December 2008 Received in revised form 23 February 2009 Accepted 25 February 2009 Communicated by K.H. Ploog Available online 12 March 2009
We report on the critical thickness for InAs quantum dot (QD) formation on (3 11)B InP substrates. Firstly, critical thicknesses for InAs QD formation on InP surfaces have been measured by reflection high-energy electron diffraction. Large change of the critical thickness has been observed as a function of substrate temperature. We assume that is related to large As/P exchange on InP surface which leads to the formation of extra InAs on surface. Then, change of critical thickness during QD stacking has been investigated. When capping layers were grown continuously a large decrease of the critical thickness was observed as a function of the number of QD layers. In contrast, when capping layers were grown in two steps (double cap procedure) a nearly constant critical thickness was measured. We propose an explanation based on stress-driven mass transport and As/P exchange on InP surface to interpret such results. & 2009 Elsevier B.V. All rights reserved.
PACS: 81.15.Hi 81.16.Dn 81.07.Ta 61.14.Hg 64.75.+g Keywords: A1. Reflection high-energy electron diffraction A3. Molecular beam epitaxy A3. Quantum dots
1. Introduction Self-assembled InAs quantum dots (QDs) grown on InP substrates are attracting a great interest due to their potential applications as active regions of optoelectronics devices operating at 1.55 mm, such as lasers or semiconductor optical amplifiers [1,2]. However due to the low lattice mismatch existing between InAs and InP, the self-assembled formation of nanostructure in InAs/InP system is largely modified in comparison with the InAs/ GaAs reference system. The nanostructures formed by molecular beam epitaxy (MBE) present generally an elongated shape and are therefore referred as quantum dashes [3]. Large improvements have been reported for deposits on high-index substrates. The QDs formed on (3 11)B substrates, present an in-plane isotropic shape and a density up to 1011 cm 2 [4] .On such substrate orientation, QD lasers operating at low threshold current density at room temperature have been already achieved [5]. However basic parameters of Stransky–Krastanow (SK) transition such as critical thickness for InAs island formation are still unknown on (3 11)B InP substrates. The main reason of this fact is that critical thickness measurements on InP substrates are not as straightforCorresponding author.
E-mail address: [email protected] (N. Bertru). 0022-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2009.02.048
ward as they are in InAs/GaAs systems. For InAs QDs formed on InP surfaces, the As/P exchange leads to the formation of an excess amount of InAs on surface [6]. Similarly for InAs QDs formed on lattice-matched InGaAs layer, the control of indium amount on surface is inaccurate due to indium-rich floating layers, induced by indium segregation [7,8]. In this paper, the apparent critical thickness for InAs QD formation on InP surface was measured by reflection high-energy electron diffraction (RHEED). In a second part, the change of island formation thickness during QD stacking has been investigated. A large decrease of the InAs thickness required to form islands was measured when the capping layers were grown continuously and weak evolution when the capping layers were grown in two steps.
2. Experiments All samples were grown on InP(3 11)B substrates by gas source MBE. The growth temperature was calibrated from InSb melting at 525 1C. After oxide removal at 530 1C, a 500 nm InP buffer layer was deposited at 480 1C. Growth rate calibrations have been done by X-ray diffraction from superlattice samples grown on (0 0 1) substrates. Due to the lower (3 11)B surface atom density, one (0 0 1) monolayer (ML) contains the same number of group III
ARTICLE IN PRESS P. Caroff et al. / Journal of Crystal Growth 311 (2009) 2626–2629
atoms than 1.65, (3 11)B crystallographic plane. In consequence the growth rate of 0.096 ML/s determined on (0 0 1) substrates corresponds to 0.165 ML of equivalent (3 11)B plane per second. In the following, the thickness will be given in (3 11)B equivalent ML. RHEED patterns were recorded along the [11 0] azimuth. The monitored area was selected to correspond to brightness spots after QD formation. The measurements have been done few times for each growth runs. The critical thicknesses reported correspond to an average value. A complementary set of samples has been grown in order to perform atomic force microscopy (AFM) on uncapped QDs. A specially designed image processing software was used to extract the QD structural characteristics and the amount of InAs within ensemble of dots.
3. Results and discussion The RHEED intensity recorded during InAs deposits on InP surfaces is reported in Fig. 1. For deposits higher than critical thickness, a rapid increase of the RHEED intensity occurs, followed by intensity saturation. With temperature decrease, the slope of the RHEED intensity during QD formation decreases. Since RHEED intensity is correlated to the QD total volume [9,10], we interpret the slope change as a consequence of a slower QD formation at low temperature. The measured critical thicknesses are reported on Fig. 1 inset. They decrease from 2.9 ML at 360 1C to 0.9 ML at 520 1C. Note that the critical thickness determination is inaccurate at 360 1C due to the low slope of the RHEED intensity. Such large change of the critical thickness with temperature is unusual and has not been reported in InGaAs/GaAs system. A first explanation is to consider change of the wetting layer (WL) thicknesses with temperature. However, WL thickness is mainly governed by the lattice mismatch and by surface and interface energies. These parameters have weak temperature dependences and WL thicknesses are generally considered as a temperature independent variable [11]. Moreover in photoluminescence spectra (not shown), WL emission energy does not change noticeably with growth temperature which is a proof of stability of the WL thickness. Therefore, we assume that the large change of the critical thickness with temperature is not related to WL thickness change. On InP surfaces under As flux, the desorption of phosphorus from surface and absorption of arsenic leads to the formation of
Fig. 1. RHEED intensity as a function of InAs deposited on InP at various substrate temperature. Inset: critical thickness deduced from RHEED intensity.
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an extra InAs amount on surface [6]. The As/P exchange is a temperature dependant phenomenon. Higher is the temperature, higher is the extra InAs amount formed. If As/P exchange is effective, extra InAs is formed and thus a lower amount of InAs has to be deposited to reach the 2D–3D transition. In order to confirm this hypothesis, complementary AFM experiments have been performed. AFM images recorded after 2.6 ML InAs deposition at different temperature are reported in Fig. 2. Despite the same amount of InAs deposited, AFM images show an increase of the QD density and of the QD height with temperature. The total QD volumes were determined from AFM images, from a dedicated image processing software. It should be noted that such measurement leads to large errors in volume [12] that we estimated to be at least 0.5 ML. A dramatic increase of QD total volume from 1.2 to 5 ML for deposit at 400 and 500 1C, respectively, was measured. Because low intermixing has been reported for InAs/InP nanostructure [13], the large increase of the amount of InAs within QD ensemble is mainly related to As/P exchange on surface. Assuming low As/P exchange at 400 1C [14], a WL thickness of 1.5 ML is evaluated from the difference between the amount of InAs deposited and the amount of InAs within QDs. Note that this corresponds roughly with the critical thickness, measured at 400 1C by RHEED. Moreover assuming constant WL thickness, an extra InAs amount of 3.9 ML at 500 1C is deduced. Such large As/P exchange has been already observed. In MOCVD, QDs have been formed by annealing under arsine without indium deposition [15]. Due to the extra InAs formed by As/P exchange on InP, absolute determination of the wetting layer thickness is difficult to achieve in this material system. However, we observed that a critical thickness for island formation is reproducibly measured at a fixed temperature. We have thus measured the apparent critical thickness during QD stacking in order to determine optimal spacer layer procedure. The QD superlattices have been grown at 480 1C. Each QD plane was separated by a 20 nm InP spacer layer. Growth interrupt of 5 and 30 s under As flux will be done before and after QD formation. In the first set of experiments, the capping layers were grown continuously without growth interrupt. Fig. 3a shows the RHEED intensity observed during the InAs deposition versus stack numbers. A decrease of the critical thickness was observed when the number of QD plane was increased. For the first plane, QD formation started after 1.5 ML InAs deposition. For
Fig. 2. 1 1 mm2 AFM images recorded after the deposition of 2.6 ML InAs on InP surfaces at three different growth temperatures (400 1C–450 1C–500 1C) and QD volume deduced from AFM image analysis as a function of growth temperature.
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P. Caroff et al. / Journal of Crystal Growth 311 (2009) 2626–2629
As2
As/P exchange
In diffusion
Surface Strain
InP
Fig. 4. Schematic of As/P exchange and In mass transport process occurring during QD stacking.
Fig. 3. RHEED intensity as a function of the amount of InAs deposited and of the number of QD stack. (a) InP capping layers grown continuously. (b) InP capping layers grown in two steps (double cap procedure). (c) Critical thickness versus number of QD planes (continuous capping layer growth, modified capping layer growth: double cap procedure).
the second plane only 1.1 ML was deposited to reach 2D–3D transition and for the 4th, 0.7 ML. The RHEED intensity decreased after each plane and only partial RHEED intensity recovering was
observed during the spacer layer growth. Beyond 5th plane, the RHEED pattern remained spotty during InP spacer growth and critical thickness could not be detected. Same experiments have been done on samples for which capping layers were grown in two steps, separated by an annealing under phosphorus flux between. This capping layer growth procedure, named double cap procedure allows QD height control and leads to a large decrease of the QD volume after capping [16]. Details of this capping layer growth procedure have been reported elsewhere [17,18]. Fig. 3b shows the RHEED intensity using this process. In contrast with the previous measurement, nearly constant thickness for island formation was measured as function of the number of QD stack. The only noticeable critical thickness change was observed between the first and the second QD planes during which the critical thickness decreases from 1.5 to 1.4 ML. For the other plane, islands were observed after 1.4 ML InAs deposit. During the capping layer growth, a complete RHEED intensity recovery was observed whatever the QD plane number. We assume that the large difference as a function of the capping layer growth procedure between the amount of InAs which must be deposited before islands formation is related to the enhancement of the As/P exchange induced by the buried QD strain (see Fig. 4). In QD stacks, the buried QDs produce inhomogeneous strain fields that propagate toward the capping layer [19]. When the InP capping layer growth is stopped and InP surface is exposited to arsenic flux, the stress field induced by buried QD enhances the mass transport on the growth front and thus As/P exchange. In consequence, with the number of QD plane, stress-driven mass transport from QD periphery increases and the amount of InAs required for QD formation decreases as observed experimentally. For large QD plane stacking, the growing surface becomes very rough and QD formation cannot be still observed by RHEED. When double cap procedure is used, the volume of buried QD is considerably reduced. In consequence the magnitude of strain induced by buried QD is reduced and thus mass transport is limited. Therefore, nearly constant InAs amount has to be deposited to form QDs and constant critical thicknesses are observed. This scheme is confirmed by PL measurements (not shown) on which large redshift of the QD peaks is observed as a function of number of QD layers in continuous capping layer samples. Such result reveals an increase of the QD size during
ARTICLE IN PRESS P. Caroff et al. / Journal of Crystal Growth 311 (2009) 2626–2629
stacking for sample on which InP layers are grown continuously. In the opposite constant QD peak energies have been observed with number of QD layer for the DC samples which confirm size stability during stacking for DC samples.
4. Conclusion In summary, the critical thickness for InAs QD formation on (3 11)B InP substrates have been measured by RHEED. For QDs formed on InP surfaces, a large change of CT has been observed with temperature. We assume that the WL thickness do not change noticeably as a function of temperature and that CT changes are related to As/P exchanges occurring at InP surface. Complementary AFM measurements confirm large extra InAs amount on surface, formed by As/P exchange. In the second part of the paper, the evolution of CT during QD stacking was measured as a function of number of QD layers. Two growth procedures of InP capping layer growth have been tested. When the InP capping layers are grown continuously, large decrease of the critical thickness and roughening of the growth front have been observed during QD stack growth. In contrast, nearly constant critical thickness was measured. We considered that the critical thickness changes are related to an efficient As/P exchange on InP surfaces which is enhanced by a stress-driven mass transport from the QD periphery to QD apex. In contrast, nearly constant critical thickness was measured during QD stacking when the capping layers were grown in two steps. That confirms the interest of this modified capping layer procedure to ensure high-quality QD stacking.
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References [1] F. Lelarge, B. Dagens, J. Renaudier, R. Brenot, A. Accard, F. Van Dijk, D. Make, O. Le Gouezigou, J.G. Provost, F. Poingt, J. Landreau, O. Drisse, E. Derouin, B. Rousseau, F. Pommereau, G.H. Duan, IEEE J. Sel. Top. Quantum Electron. (2007) 111. [2] K. Kawaguchi, N. Yasuoka, M. Ekawa, H. Ebe, T. Akiyama, M. Sugawara, Y. Arakawa, Appl. Phys. Lett. 93 (2008) 121908. [3] L. Gonza´lez, J.M. Garcı´a, R. Garcı´a, F. Briones, J. Martı´nez-Pastor, C. Ballesteros, Appl. Phys. Lett. 76 (2000) 1104. [4] S. Fre´chengues, V. Drouot, N. Bertru, B. Lambert, S. Loualiche, A. Le Corre, J. Cryst. Growth (1999) 1180. [5] P. Caroff, C. Paranthoe¨n, C. Platz, O. Dehaese, H. Folliot, N. Bertru, C. Labbe´, R. Piron, E. Homeyer, A. Le Corre, S. Loualiche, Appl. Phys. Lett. 87 (2005) 243107. [6] H.R. Gutierrez, M.A. Cotta, J.R.R. Bortoleto, M.M.G. de Carvalho, J. Appl. Phys. 92 (2002) 7523. [7] T. Walther, A.G. Cullis, D.J. Norris, M. Hopkinson, Phys. Rev. Lett. 86 (2001) 002381. [8] J.M. Ge´rard, J. Cryst. Growth 127 (1993) 981. [9] H.Z. Song, T. Usuki, Y. Nakata, N. Yokoyama, H. Sasakura, S. Muto, Phys. Rev. B 73 (2006) 115327. [10] P. Muller, R. Kern, Appl. Surf. Sci. 102 (1996) 6. [11] V.G. Dubrovskii, G.E. Cirlin, Yu.G. Musikhin, Yu.B. Samsonenko, A.A. Tonkikh, N.K. Polyakov, V.A. Egorov, A.F. Tsatsul’nikov, N.A. Krizhanovskaya, V.M. Ustinov, P. Werner, J. Cryst. Growth 267 (2004) 47. [12] K. Shiramine, S. Muto, T. Shibayama, N. Sakaguchi, H. Ichinose, T. Kozaki, S. Sato, Y. Nakata, N. Yokoyama, M. Taniwaki, J. Appl. Phys. 101 (2007) 033527. [13] C. C - elebi, J.M. Ulloa, P.M. Koenraad, A. Simon, A. Letoublon, N. Bertru, Appl. Phys. Lett. 89 (2006) 023119. [14] K.Y. Suh, Hong H. Lee, E. Yoon, J. Appl. Phys. 85 (1999) 233. [15] S. Yoon, Y. Moon, T.-W. Lee, E. Yoon, Y.D. Kim, Appl. Phys. Lett. 74 (1999) 2029. [16] J.M. Ulloa, P.M. Koenraad, E. Gapihan, A. Le´toublon, N. Bertru, Appl. Phys. Lett. 91 (2007) 073106. [17] C. Paranthoen, N. Bertru, O. Dehaese, A. Le Corre, S. Loualiche, B. Lambert, G. Patriarche, Appl. Phys. Lett. 78 (2001) 1751. [18] Y. Saito, T. Okawa, M. Akaishi, K. Shimomura, J. Cryst. Growth 310 (2008) 5073. [19] D. Fuster, M. Gonza´lez, L. Gonza´lez, Y. Gonza´lez, T. Ben, A. Ponce, S. Molina, Appl. Phys. Lett. 84 (2004) 4723.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Critical thickness for InAs quantum dot formation on (3 11)B InP substrates P. Caroff, N. Bertru , W. Lu, G. Elias, O. Dehaese, A. Le´toublon, A. Le Corre FOTON, UMR 6082, INSA de Rennes, 20 avenue des Buttes de Coe¨smes, CS 14315, 35043 Rennes Cedex, France
a r t i c l e in f o
a b s t r a c t
Article history: Received 16 December 2008 Received in revised form 23 February 2009 Accepted 25 February 2009 Communicated by K.H. Ploog Available online 12 March 2009
We report on the critical thickness for InAs quantum dot (QD) formation on (3 11)B InP substrates. Firstly, critical thicknesses for InAs QD formation on InP surfaces have been measured by reflection high-energy electron diffraction. Large change of the critical thickness has been observed as a function of substrate temperature. We assume that is related to large As/P exchange on InP surface which leads to the formation of extra InAs on surface. Then, change of critical thickness during QD stacking has been investigated. When capping layers were grown continuously a large decrease of the critical thickness was observed as a function of the number of QD layers. In contrast, when capping layers were grown in two steps (double cap procedure) a nearly constant critical thickness was measured. We propose an explanation based on stress-driven mass transport and As/P exchange on InP surface to interpret such results. & 2009 Elsevier B.V. All rights reserved.
PACS: 81.15.Hi 81.16.Dn 81.07.Ta 61.14.Hg 64.75.+g Keywords: A1. Reflection high-energy electron diffraction A3. Molecular beam epitaxy A3. Quantum dots
1. Introduction Self-assembled InAs quantum dots (QDs) grown on InP substrates are attracting a great interest due to their potential applications as active regions of optoelectronics devices operating at 1.55 mm, such as lasers or semiconductor optical amplifiers [1,2]. However due to the low lattice mismatch existing between InAs and InP, the self-assembled formation of nanostructure in InAs/InP system is largely modified in comparison with the InAs/ GaAs reference system. The nanostructures formed by molecular beam epitaxy (MBE) present generally an elongated shape and are therefore referred as quantum dashes [3]. Large improvements have been reported for deposits on high-index substrates. The QDs formed on (3 11)B substrates, present an in-plane isotropic shape and a density up to 1011 cm 2 [4] .On such substrate orientation, QD lasers operating at low threshold current density at room temperature have been already achieved [5]. However basic parameters of Stransky–Krastanow (SK) transition such as critical thickness for InAs island formation are still unknown on (3 11)B InP substrates. The main reason of this fact is that critical thickness measurements on InP substrates are not as straightforCorresponding author.
E-mail address: [email protected] (N. Bertru). 0022-0248/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2009.02.048
ward as they are in InAs/GaAs systems. For InAs QDs formed on InP surfaces, the As/P exchange leads to the formation of an excess amount of InAs on surface [6]. Similarly for InAs QDs formed on lattice-matched InGaAs layer, the control of indium amount on surface is inaccurate due to indium-rich floating layers, induced by indium segregation [7,8]. In this paper, the apparent critical thickness for InAs QD formation on InP surface was measured by reflection high-energy electron diffraction (RHEED). In a second part, the change of island formation thickness during QD stacking has been investigated. A large decrease of the InAs thickness required to form islands was measured when the capping layers were grown continuously and weak evolution when the capping layers were grown in two steps.
2. Experiments All samples were grown on InP(3 11)B substrates by gas source MBE. The growth temperature was calibrated from InSb melting at 525 1C. After oxide removal at 530 1C, a 500 nm InP buffer layer was deposited at 480 1C. Growth rate calibrations have been done by X-ray diffraction from superlattice samples grown on (0 0 1) substrates. Due to the lower (3 11)B surface atom density, one (0 0 1) monolayer (ML) contains the same number of group III
ARTICLE IN PRESS P. Caroff et al. / Journal of Crystal Growth 311 (2009) 2626–2629
atoms than 1.65, (3 11)B crystallographic plane. In consequence the growth rate of 0.096 ML/s determined on (0 0 1) substrates corresponds to 0.165 ML of equivalent (3 11)B plane per second. In the following, the thickness will be given in (3 11)B equivalent ML. RHEED patterns were recorded along the [11 0] azimuth. The monitored area was selected to correspond to brightness spots after QD formation. The measurements have been done few times for each growth runs. The critical thicknesses reported correspond to an average value. A complementary set of samples has been grown in order to perform atomic force microscopy (AFM) on uncapped QDs. A specially designed image processing software was used to extract the QD structural characteristics and the amount of InAs within ensemble of dots.
3. Results and discussion The RHEED intensity recorded during InAs deposits on InP surfaces is reported in Fig. 1. For deposits higher than critical thickness, a rapid increase of the RHEED intensity occurs, followed by intensity saturation. With temperature decrease, the slope of the RHEED intensity during QD formation decreases. Since RHEED intensity is correlated to the QD total volume [9,10], we interpret the slope change as a consequence of a slower QD formation at low temperature. The measured critical thicknesses are reported on Fig. 1 inset. They decrease from 2.9 ML at 360 1C to 0.9 ML at 520 1C. Note that the critical thickness determination is inaccurate at 360 1C due to the low slope of the RHEED intensity. Such large change of the critical thickness with temperature is unusual and has not been reported in InGaAs/GaAs system. A first explanation is to consider change of the wetting layer (WL) thicknesses with temperature. However, WL thickness is mainly governed by the lattice mismatch and by surface and interface energies. These parameters have weak temperature dependences and WL thicknesses are generally considered as a temperature independent variable [11]. Moreover in photoluminescence spectra (not shown), WL emission energy does not change noticeably with growth temperature which is a proof of stability of the WL thickness. Therefore, we assume that the large change of the critical thickness with temperature is not related to WL thickness change. On InP surfaces under As flux, the desorption of phosphorus from surface and absorption of arsenic leads to the formation of
Fig. 1. RHEED intensity as a function of InAs deposited on InP at various substrate temperature. Inset: critical thickness deduced from RHEED intensity.
2627
an extra InAs amount on surface [6]. The As/P exchange is a temperature dependant phenomenon. Higher is the temperature, higher is the extra InAs amount formed. If As/P exchange is effective, extra InAs is formed and thus a lower amount of InAs has to be deposited to reach the 2D–3D transition. In order to confirm this hypothesis, complementary AFM experiments have been performed. AFM images recorded after 2.6 ML InAs deposition at different temperature are reported in Fig. 2. Despite the same amount of InAs deposited, AFM images show an increase of the QD density and of the QD height with temperature. The total QD volumes were determined from AFM images, from a dedicated image processing software. It should be noted that such measurement leads to large errors in volume [12] that we estimated to be at least 0.5 ML. A dramatic increase of QD total volume from 1.2 to 5 ML for deposit at 400 and 500 1C, respectively, was measured. Because low intermixing has been reported for InAs/InP nanostructure [13], the large increase of the amount of InAs within QD ensemble is mainly related to As/P exchange on surface. Assuming low As/P exchange at 400 1C [14], a WL thickness of 1.5 ML is evaluated from the difference between the amount of InAs deposited and the amount of InAs within QDs. Note that this corresponds roughly with the critical thickness, measured at 400 1C by RHEED. Moreover assuming constant WL thickness, an extra InAs amount of 3.9 ML at 500 1C is deduced. Such large As/P exchange has been already observed. In MOCVD, QDs have been formed by annealing under arsine without indium deposition [15]. Due to the extra InAs formed by As/P exchange on InP, absolute determination of the wetting layer thickness is difficult to achieve in this material system. However, we observed that a critical thickness for island formation is reproducibly measured at a fixed temperature. We have thus measured the apparent critical thickness during QD stacking in order to determine optimal spacer layer procedure. The QD superlattices have been grown at 480 1C. Each QD plane was separated by a 20 nm InP spacer layer. Growth interrupt of 5 and 30 s under As flux will be done before and after QD formation. In the first set of experiments, the capping layers were grown continuously without growth interrupt. Fig. 3a shows the RHEED intensity observed during the InAs deposition versus stack numbers. A decrease of the critical thickness was observed when the number of QD plane was increased. For the first plane, QD formation started after 1.5 ML InAs deposition. For
Fig. 2. 1 1 mm2 AFM images recorded after the deposition of 2.6 ML InAs on InP surfaces at three different growth temperatures (400 1C–450 1C–500 1C) and QD volume deduced from AFM image analysis as a function of growth temperature.
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As2
As/P exchange
In diffusion
Surface Strain
InP
Fig. 4. Schematic of As/P exchange and In mass transport process occurring during QD stacking.
Fig. 3. RHEED intensity as a function of the amount of InAs deposited and of the number of QD stack. (a) InP capping layers grown continuously. (b) InP capping layers grown in two steps (double cap procedure). (c) Critical thickness versus number of QD planes (continuous capping layer growth, modified capping layer growth: double cap procedure).
the second plane only 1.1 ML was deposited to reach 2D–3D transition and for the 4th, 0.7 ML. The RHEED intensity decreased after each plane and only partial RHEED intensity recovering was
observed during the spacer layer growth. Beyond 5th plane, the RHEED pattern remained spotty during InP spacer growth and critical thickness could not be detected. Same experiments have been done on samples for which capping layers were grown in two steps, separated by an annealing under phosphorus flux between. This capping layer growth procedure, named double cap procedure allows QD height control and leads to a large decrease of the QD volume after capping [16]. Details of this capping layer growth procedure have been reported elsewhere [17,18]. Fig. 3b shows the RHEED intensity using this process. In contrast with the previous measurement, nearly constant thickness for island formation was measured as function of the number of QD stack. The only noticeable critical thickness change was observed between the first and the second QD planes during which the critical thickness decreases from 1.5 to 1.4 ML. For the other plane, islands were observed after 1.4 ML InAs deposit. During the capping layer growth, a complete RHEED intensity recovery was observed whatever the QD plane number. We assume that the large difference as a function of the capping layer growth procedure between the amount of InAs which must be deposited before islands formation is related to the enhancement of the As/P exchange induced by the buried QD strain (see Fig. 4). In QD stacks, the buried QDs produce inhomogeneous strain fields that propagate toward the capping layer [19]. When the InP capping layer growth is stopped and InP surface is exposited to arsenic flux, the stress field induced by buried QD enhances the mass transport on the growth front and thus As/P exchange. In consequence, with the number of QD plane, stress-driven mass transport from QD periphery increases and the amount of InAs required for QD formation decreases as observed experimentally. For large QD plane stacking, the growing surface becomes very rough and QD formation cannot be still observed by RHEED. When double cap procedure is used, the volume of buried QD is considerably reduced. In consequence the magnitude of strain induced by buried QD is reduced and thus mass transport is limited. Therefore, nearly constant InAs amount has to be deposited to form QDs and constant critical thicknesses are observed. This scheme is confirmed by PL measurements (not shown) on which large redshift of the QD peaks is observed as a function of number of QD layers in continuous capping layer samples. Such result reveals an increase of the QD size during
ARTICLE IN PRESS P. Caroff et al. / Journal of Crystal Growth 311 (2009) 2626–2629
stacking for sample on which InP layers are grown continuously. In the opposite constant QD peak energies have been observed with number of QD layer for the DC samples which confirm size stability during stacking for DC samples.
4. Conclusion In summary, the critical thickness for InAs QD formation on (3 11)B InP substrates have been measured by RHEED. For QDs formed on InP surfaces, a large change of CT has been observed with temperature. We assume that the WL thickness do not change noticeably as a function of temperature and that CT changes are related to As/P exchanges occurring at InP surface. Complementary AFM measurements confirm large extra InAs amount on surface, formed by As/P exchange. In the second part of the paper, the evolution of CT during QD stacking was measured as a function of number of QD layers. Two growth procedures of InP capping layer growth have been tested. When the InP capping layers are grown continuously, large decrease of the critical thickness and roughening of the growth front have been observed during QD stack growth. In contrast, nearly constant critical thickness was measured. We considered that the critical thickness changes are related to an efficient As/P exchange on InP surfaces which is enhanced by a stress-driven mass transport from the QD periphery to QD apex. In contrast, nearly constant critical thickness was measured during QD stacking when the capping layers were grown in two steps. That confirms the interest of this modified capping layer procedure to ensure high-quality QD stacking.
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