- Email: [email protected]
GaAs quantum dots grown with Sb surfactant
ARTICLE IN PRESS
Physica E 32 (2006) 25–28 www.elsevier.com/locate/physe
Structure of InAs/GaAs quantum dots grown with Sb surfactant R. Timm, H. Eisele, A. Lenz, T.-Y. Kim, F. Streicher, K. Po¨tschke, U.W. Pohl, D. Bimberg, M. Da¨hne Institut fu¨r Festko¨rperphysik, PN 4-1, Technische Universita¨t Berlin, Hardenbergstr. 36, 10623 Berlin, Germany Available online 23 January 2006
Abstract InAs quantum dots in GaAs, grown under the presence of Sb by metalorganic chemical vapor deposition, were studied with crosssectional scanning tunneling microscopy. Large flat quantum dots with a truncated pyramidal shape, base lengths between 15 and 30 nm, heights of 1–3 nm, and a rather pure InAs stoichiometry were found for the case of an Sb supply during the InAs deposition. If Sb is already supplied during GaAs stabilization prior to InAs deposition, the dots become even larger and tend to get intermixed with Ga, but remain coherently strained with a reversed cone-like In distribution. Regarding the quantum dot growth Sb acts as surfactant, whereas an incorporation of individual Sb atoms was observed in the wetting layer. r 2006 Elsevier B.V. All rights reserved. PACS: 81.07.Ta; 68.65.Hb; 68.35.Dv; 68.37.Ef Keywords: InAs; GaAs; Quantum dot; Sb surfactant; MOCVD; XSTM
Self-organized semiconductor quantum dots (QDs) are currently in the center of broad interest due to their unique electronic properties and their huge potential for optoelectronic devices [1]. Especially the InAs/GaAs material system is intensively studied as it is highly suitable for laser applications [2]. Among various strategies to reach the technologically important lasing wavelength of 1:3 mm [1,3,4], the presence of Sb during the epitaxy of InAs/GaAs nanostructured devices is very promising both for quantum well [5,6] and QD [7–10] structures. Thereby, Sb is found to act as surfactant, similar to the Ge–Si system [11], improving the wetting layer quality and increasing the critical thickness for QD formation, probably by altering both the surface diffusion kinetics and the surface energy [6,7]. Thus, larger QDs are obtained with a favored redshift of the QD luminescence, as several authors have reported [7–9]. However, also the opposite behavior was observed, when an Sb exposure prior to InAs deposition resulted in a decrease of the critical thickness and a partial incorporation of Sb into the InAs QDs [12].
Corresponding author. Tel.: +49 30 314 22057; fax: +49 30 314 26181.
E-mail address: [email protected] (R. Timm). 1386-9477/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2005.12.009
Since the electronic properties of QDs crucially depend on the dot structure, a detailed knowledge about dot size, shape, and stoichiometry is essential for understanding the underlying physics of the system as well as for improving QD growth. Moreover, it has to be verified, if Sb atoms are incorporated in the structures or if they just act as surfactants. Cross-sectional scanning tunneling microscopy (XSTM) is a powerful tool and a suitable method to investigate buried semiconductor nanostructures, yielding atomically resolved data on shape, size, and local composition of QDs [13,14] as well as information on strain [15] and electronic properties [16]. In contrast to other structural characterization methods like transmission electron microscopy, XSTM has the ability to atomically reveal directly the stoichiometry even for quarternary systems containing In, Ga, As, and Sb atoms [17]. In this work, we present a detailed XSTM study on InAs QDs in GaAs with and without the presence of Sb during different growth phases. All samples have been grown by metalorganic chemical vapor deposition using trimethylindium (TMIn), trimethylgallium (TMGa), tertiarbutylarsine (TBAs), and triethylantimony (TESb) as follows: on a GaAs(0 0 1) substrate,
ARTICLE IN PRESS 26
R. Timm et al. / Physica E 32 (2006) 25–28
AlGaAs and GaAs buffer layers were grown at 625 1C, before the growth temperature was reduced to 485 1C for QD growth. After a growth interruption (GrI) of 2 min under TBAs flux for stabilizing the growth surface, 1.8 monolayers (ML) InAs were deposited at 485 1C and a TBAs/TMIn ratio of 1.5, followed by another GrI of 5 s. Subsequently, the InAs layer was covered by 12 nm GaAs at the same temperature, before the temperature was raised to 600 1C to grow further GaAs and AlGaAs layers, followed by a 1 mm thick GaAs cap layer. In the presented samples Sb was supplied during InAs deposition with a TESb/TMIn ratio of 0.175, and in one case additionally during the last 15 s of GaAs stabilization prior to InAs deposition. For the XSTM experiments the samples were cleaved in ultrahigh vacuum with a base pressure below 1 1010 mbar, resulting in a clean (1 1 0) cleavage surface, and analyzed using a home-built STM with an RHK Technology SPM 100 control unit, employing tungsten tips with tunneling currents between 80 and 100 pA. Typical QDs of different sizes are shown in Fig. 1. The lines perpendicular to the [0 0 1] growth direction represent the III–V zigzag chains at the zincblende (1 1 0) surface [16]. The wetting layer and especially the QDs show a significantly brighter contrast than the surrounding matrix due to an increased electron tunneling probability from the sample into the tip and due to a structural relaxation of the strained material out of the cleavage plane [15]. Additionally, some adatoms can be seen due to surface contaminations, arising from residual gas. All dots were grown under presence of Sb, but consist of rather pure InAs without incorporation of Sb atoms. They are relatively flat with shapes coming close to that of a truncated pyramid in most cases [like those shown in Fig. 1(b–e)], often with an In-rich center [Fig. 1(b, c, e)]. The images show large size fluctuations, which are partly related to different positions of the cleavage plane in the dot, but mostly result from actual size fluctuations of the dots. In the sample without Sb in the GrI [Fig. 1(b–d)], the QD base lengths vary between about 10 and 25 nm with heights ranging from 4 to 12 ML or 1 to 3 nm at a dot density of 6 1010 cm2 . Especially the bottom and top faces of the QDs are sharply defined. Thereby, the QD height increases with the base length in discrete ML steps, which can be described by a shell-like growth mode. For example, QDs with heights of 6, 8, and 12 ML can be seen in Fig. 1(b–d), respectively. Note that only each second atomic ML is imaged via an atomic chain at the cleavage surface [16]. Such a shell-like growth mode has already been predicted for QDs grown under similar conditions because of their ensemble luminescence showing a monolayer splitting [18]. For the sample grown with Sb supply during GaAs stabilization and subsequent InAs deposition [Fig. 1(a,e)], an even larger size variation was found with QD baselengths of about 20–35 nm and heights of about 6–16 ML
[110] [001]
adatoms
4 nm (a)
8 nm
(d)
(b)
4 nm
adatoms
(c)
4 nm
8 nm (e)
Fig. 1. Filled-state XSTM images of InAs:Sb/GaAs quantum dots of different sizes, taken at sample voltages V S ¼ 2:2 V (a), 2:7 V (b,c), 2:4 V (d), and 2:3 V (e). In the case of QDs (a) and (e), Sb has been offered already during the GaAs stabilization before InAs deposition.
(2 to 5 nm). The dot density is correspondingly reduced to 3 1010 cm2 . For the largest QDs like that shown in Fig. 1(e), an inhomogeneous composition with an In-rich center with a shape of a reversed truncated cone is apparent, which is typical for such large dots due to the increased strain [13,14]. The increased QD size can be
ARTICLE IN PRESS R. Timm et al. / Physica E 32 (2006) 25–28
related to the surfactant behavior of Sb [6–8], which results in both a decrease of surface energy and a change of surface diffusion kinetics. To study the stoichiometry of the QDs quantitatively, the local lattice constant, which is a measure of the chemical composition, was evaluated by analyzing the distance of neighboring atomic zigzag chains, as described in detail elsewhere [14,19]. Fig. 2(b) displays the variation of the chain distance for the two QDs shown in Fig. 2(a) (straight and dashed lines) and, for comparison, for the wetting layer (dotted line). The range of the increased chain distance extends over five atomic chains for the upper dot, over six chains for the lower dot, and over two chains for the wetting layer. Experimentally obtained data are compared with strain simulations based on continuum mechanics on an atomic scale. The results for InGaAs layers with different In content are indicated in Fig. 2(b). The stoichiometry of the dots is close to pure InAs, whereas the InAs content of the wetting layer reaches only about 40 10%. The slight reduction of the chain distance just underneath and above the large dot (straight line) shows the compressive strain in the GaAs matrix caused by the QD. We compared these stoichiometry results with those of a reference InAs/GaAs sample without Sb. For the present InAs:Sb sample, we found a reduced segregation of In into the overgrowth layer, especially at the wetting layer. This behavior again confirms the role of Sb as a surfactant.
0.72
InAs
atomic chain distance [nm]
0.70 0.68 0.66
In0.5Ga0.5As
0.64 0.62
(a) In0.25Ga0.75As
0.60 0.58 0.56 0.54 0.52 -4
(b)
-2 0 2 4 6 position in [001] growth direction [nm]
8
Fig. 2. Determination of the local stoichiometry: (a) filled-state XSTM image of two InAs:Sb QDs, acquired at V S ¼ 2:0 V; (b) variation of the atomic chain distance across the upper dot (dashed line) and the lower dot (straight line) in (a) and across the wetting layer far away from any QD (dotted line), obtained by averaging in [1¯ 1 0] direction within narrow stripes. The indicated stoichiometries correspond to calculated chain distances.
27
[001] growth direction
moving adatom
4 nm Fig. 3. Close-view filled-state XSTM image of an InAs wetting layer with Sb supply during GaAs stabilization and subsequent InAs deposition, taken at V S ¼ 2:3 V. Individual Sb atoms are incorporated with a GaSb content of about 25%.
On the other hand, we observed individual Sb atoms in the wetting layer for the sample with Sb supply during GaAs stabilization and subsequent InAs deposition. Fig. 3 shows the InAs wetting layer within the GaAs matrix with clear atomic resolution, where the bright individual spots mark single atoms. Because of the negative sample voltage, the filled states of the group V atoms are imaged [16]. The strain of the wetting layer and the different bond lengths of In and Ga atoms would only cause a weakly increased, smooth topographic contrast [4], so that the sharp contrast seen here must be of electronic nature and can only originate from different group V atoms. Thus single Sb atoms between As are visible. By evaluating the XSTM images we found an Sb content in this wetting layer of about 25%. This different behavior between QDs and wetting layer can be related to the strain, which is stronger at the InAs dots than at the InGaAs wetting layer, favoring the incorporation of Sb atoms in the wetting layer, while removing them from the QD regions. In conclusion, we presented XSTM results on InAs QDs grown under Sb supply during different growth stages. The dots show a large size variation in monolayer steps with base lengths from about 10 to 35 nm and heights from 1 to 5 nm. Sb acts as a surfactant by increasing the mobility of the In atoms and thus the QD size and by decreasing the segregation of In into the overgrowth layers. In the case of Sb supply already before InAs deposition individual Sb atoms are incorporated into the wetting layer. This work was supported by projects Da 408/4, Da 408/ 8, and Sonderforschungsbereich 296 of the Deutsche Forschungsgemeinschaft as well as by the European Commission in the SANDiE Network of Excellence.
ARTICLE IN PRESS 28
R. Timm et al. / Physica E 32 (2006) 25–28
References [1] V. Shchukin, N.N. Ledentsov, D. Bimberg, Epitaxy of Nanostructures, Springer, Berlin, 2003. [2] M. Grundmann (Ed.), Nano-Optoelectronics, Springer, Berlin, 2002. [3] D.L. Huffaker, G. Park, Z. Zou, O.B. Shchekin, D.G. Deppe, Appl. Phys. Lett. 73 (1998) 2564. [4] A. Lenz, H. Eisele, R. Timm, S.K. Becker, R.L. Sellin, U.W. Pohl, D. Bimberg, M. Da¨hne, Appl. Phys. Lett. 85 (2004) 3848. [5] H. Shimizu, K. Kumada, S. Uchiyama, A. Kasukawa, Electron. Lett. 36 (2000) 1379. [6] J.C. Harmand, L.H. Li, G. Patriarche, L. Travers, Appl. Phys. Lett. 84 (2004) 3981. [7] K. Po¨tschke, L. Mu¨ller-Kirsch, R. Heitz, R.L. Sellin, U.W. Pohl, D. Bimberg, N. Zakharov, P. Werner, Physica E 21 (2004) 606; U.W. Pohl, K. Po¨tschke, A. Schliwa, F. Guffarth, D. Bimberg, N.D. Zakharov, P. Werner, M.B. Lifshits, V.A. Shchukin, D.E. Jesson, Phys. Rev. B 72 (2005) 245332. [8] M. Kudo, T. Nakaoka, S. Iwamoto, Y. Arakawa, Jpn. J. Appl. Phys. 44 (2005) L45. [9] K. Akahane, N. Yamamoto, N. Ohtani, Physica E 21 (2004) 295.
[10] T. Matsuura, T. Miyamoto, T. Kageyama, M. Ohta, Y. Matsui, T. Furuhata, F. Koyama, Jpn. J. Appl. Phys. 43 (2004) L605. [11] A. Portavoce, I. Berbezier, A. Ronda, Phys. Rev. B 69 (2004) 155416 and references therein. [12] Y. Sun, S.F. Cheng, G. Chen, R.F. Hicks, J.G. Cederberg, R.M. Biefeld, J. Appl. Phys. 97 (2005) 053503. [13] N. Liu, J. Tersoff, O. Baklenov, A.L. Holmes Jr., C.K. Shih, Phys. Rev. Lett. 84 (2000) 334. [14] A. Lenz, R. Timm, H. Eisele, Ch. Hennig, S.K. Becker, R.L. Sellin, U.W. Pohl, D. Bimberg, M. Da¨hne, Appl. Phys. Lett. 81 (2002) 5150. [15] O. Flebbe, H. Eisele, T. Kalka, F. Heinrichsdorff, A. Krost, D. Bimberg, M. Da¨hne-Prietsch, J. Vac. Sci. Technol. B 17 (1999) 1639. [16] R.M. Feenstra, J.A. Stroscio, J. Tersoff, A.P. Fein, Phys. Rev. Lett. 58 (1987) 1192. [17] J. Steinshnider, M. Weimer, R. Kaspi, G.W. Turner, Phys. Rev. Lett. 85 (2000) 2953. [18] R. Heitz, F. Guffarth, K. Po¨tschke, A. Schliwa, D. Bimberg, N.D. Zakharov, P. Werner, Phys. Rev. B 71 (2005) 045325. [19] R. Timm, H. Eisele, A. Lenz, S.K. Becker, J. Grabowski, T.-Y. Kim, L. Mu¨ller-Kirsch, K. Po¨tschke, U.W. Pohl, D. Bimberg, M. Da¨hne, Appl. Phys. Lett. 85 (2004) 5890.
Physica E 32 (2006) 25–28 www.elsevier.com/locate/physe
Structure of InAs/GaAs quantum dots grown with Sb surfactant R. Timm, H. Eisele, A. Lenz, T.-Y. Kim, F. Streicher, K. Po¨tschke, U.W. Pohl, D. Bimberg, M. Da¨hne Institut fu¨r Festko¨rperphysik, PN 4-1, Technische Universita¨t Berlin, Hardenbergstr. 36, 10623 Berlin, Germany Available online 23 January 2006
Abstract InAs quantum dots in GaAs, grown under the presence of Sb by metalorganic chemical vapor deposition, were studied with crosssectional scanning tunneling microscopy. Large flat quantum dots with a truncated pyramidal shape, base lengths between 15 and 30 nm, heights of 1–3 nm, and a rather pure InAs stoichiometry were found for the case of an Sb supply during the InAs deposition. If Sb is already supplied during GaAs stabilization prior to InAs deposition, the dots become even larger and tend to get intermixed with Ga, but remain coherently strained with a reversed cone-like In distribution. Regarding the quantum dot growth Sb acts as surfactant, whereas an incorporation of individual Sb atoms was observed in the wetting layer. r 2006 Elsevier B.V. All rights reserved. PACS: 81.07.Ta; 68.65.Hb; 68.35.Dv; 68.37.Ef Keywords: InAs; GaAs; Quantum dot; Sb surfactant; MOCVD; XSTM
Self-organized semiconductor quantum dots (QDs) are currently in the center of broad interest due to their unique electronic properties and their huge potential for optoelectronic devices [1]. Especially the InAs/GaAs material system is intensively studied as it is highly suitable for laser applications [2]. Among various strategies to reach the technologically important lasing wavelength of 1:3 mm [1,3,4], the presence of Sb during the epitaxy of InAs/GaAs nanostructured devices is very promising both for quantum well [5,6] and QD [7–10] structures. Thereby, Sb is found to act as surfactant, similar to the Ge–Si system [11], improving the wetting layer quality and increasing the critical thickness for QD formation, probably by altering both the surface diffusion kinetics and the surface energy [6,7]. Thus, larger QDs are obtained with a favored redshift of the QD luminescence, as several authors have reported [7–9]. However, also the opposite behavior was observed, when an Sb exposure prior to InAs deposition resulted in a decrease of the critical thickness and a partial incorporation of Sb into the InAs QDs [12].
Corresponding author. Tel.: +49 30 314 22057; fax: +49 30 314 26181.
E-mail address: [email protected] (R. Timm). 1386-9477/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2005.12.009
Since the electronic properties of QDs crucially depend on the dot structure, a detailed knowledge about dot size, shape, and stoichiometry is essential for understanding the underlying physics of the system as well as for improving QD growth. Moreover, it has to be verified, if Sb atoms are incorporated in the structures or if they just act as surfactants. Cross-sectional scanning tunneling microscopy (XSTM) is a powerful tool and a suitable method to investigate buried semiconductor nanostructures, yielding atomically resolved data on shape, size, and local composition of QDs [13,14] as well as information on strain [15] and electronic properties [16]. In contrast to other structural characterization methods like transmission electron microscopy, XSTM has the ability to atomically reveal directly the stoichiometry even for quarternary systems containing In, Ga, As, and Sb atoms [17]. In this work, we present a detailed XSTM study on InAs QDs in GaAs with and without the presence of Sb during different growth phases. All samples have been grown by metalorganic chemical vapor deposition using trimethylindium (TMIn), trimethylgallium (TMGa), tertiarbutylarsine (TBAs), and triethylantimony (TESb) as follows: on a GaAs(0 0 1) substrate,
ARTICLE IN PRESS 26
R. Timm et al. / Physica E 32 (2006) 25–28
AlGaAs and GaAs buffer layers were grown at 625 1C, before the growth temperature was reduced to 485 1C for QD growth. After a growth interruption (GrI) of 2 min under TBAs flux for stabilizing the growth surface, 1.8 monolayers (ML) InAs were deposited at 485 1C and a TBAs/TMIn ratio of 1.5, followed by another GrI of 5 s. Subsequently, the InAs layer was covered by 12 nm GaAs at the same temperature, before the temperature was raised to 600 1C to grow further GaAs and AlGaAs layers, followed by a 1 mm thick GaAs cap layer. In the presented samples Sb was supplied during InAs deposition with a TESb/TMIn ratio of 0.175, and in one case additionally during the last 15 s of GaAs stabilization prior to InAs deposition. For the XSTM experiments the samples were cleaved in ultrahigh vacuum with a base pressure below 1 1010 mbar, resulting in a clean (1 1 0) cleavage surface, and analyzed using a home-built STM with an RHK Technology SPM 100 control unit, employing tungsten tips with tunneling currents between 80 and 100 pA. Typical QDs of different sizes are shown in Fig. 1. The lines perpendicular to the [0 0 1] growth direction represent the III–V zigzag chains at the zincblende (1 1 0) surface [16]. The wetting layer and especially the QDs show a significantly brighter contrast than the surrounding matrix due to an increased electron tunneling probability from the sample into the tip and due to a structural relaxation of the strained material out of the cleavage plane [15]. Additionally, some adatoms can be seen due to surface contaminations, arising from residual gas. All dots were grown under presence of Sb, but consist of rather pure InAs without incorporation of Sb atoms. They are relatively flat with shapes coming close to that of a truncated pyramid in most cases [like those shown in Fig. 1(b–e)], often with an In-rich center [Fig. 1(b, c, e)]. The images show large size fluctuations, which are partly related to different positions of the cleavage plane in the dot, but mostly result from actual size fluctuations of the dots. In the sample without Sb in the GrI [Fig. 1(b–d)], the QD base lengths vary between about 10 and 25 nm with heights ranging from 4 to 12 ML or 1 to 3 nm at a dot density of 6 1010 cm2 . Especially the bottom and top faces of the QDs are sharply defined. Thereby, the QD height increases with the base length in discrete ML steps, which can be described by a shell-like growth mode. For example, QDs with heights of 6, 8, and 12 ML can be seen in Fig. 1(b–d), respectively. Note that only each second atomic ML is imaged via an atomic chain at the cleavage surface [16]. Such a shell-like growth mode has already been predicted for QDs grown under similar conditions because of their ensemble luminescence showing a monolayer splitting [18]. For the sample grown with Sb supply during GaAs stabilization and subsequent InAs deposition [Fig. 1(a,e)], an even larger size variation was found with QD baselengths of about 20–35 nm and heights of about 6–16 ML
[110] [001]
adatoms
4 nm (a)
8 nm
(d)
(b)
4 nm
adatoms
(c)
4 nm
8 nm (e)
Fig. 1. Filled-state XSTM images of InAs:Sb/GaAs quantum dots of different sizes, taken at sample voltages V S ¼ 2:2 V (a), 2:7 V (b,c), 2:4 V (d), and 2:3 V (e). In the case of QDs (a) and (e), Sb has been offered already during the GaAs stabilization before InAs deposition.
(2 to 5 nm). The dot density is correspondingly reduced to 3 1010 cm2 . For the largest QDs like that shown in Fig. 1(e), an inhomogeneous composition with an In-rich center with a shape of a reversed truncated cone is apparent, which is typical for such large dots due to the increased strain [13,14]. The increased QD size can be
ARTICLE IN PRESS R. Timm et al. / Physica E 32 (2006) 25–28
related to the surfactant behavior of Sb [6–8], which results in both a decrease of surface energy and a change of surface diffusion kinetics. To study the stoichiometry of the QDs quantitatively, the local lattice constant, which is a measure of the chemical composition, was evaluated by analyzing the distance of neighboring atomic zigzag chains, as described in detail elsewhere [14,19]. Fig. 2(b) displays the variation of the chain distance for the two QDs shown in Fig. 2(a) (straight and dashed lines) and, for comparison, for the wetting layer (dotted line). The range of the increased chain distance extends over five atomic chains for the upper dot, over six chains for the lower dot, and over two chains for the wetting layer. Experimentally obtained data are compared with strain simulations based on continuum mechanics on an atomic scale. The results for InGaAs layers with different In content are indicated in Fig. 2(b). The stoichiometry of the dots is close to pure InAs, whereas the InAs content of the wetting layer reaches only about 40 10%. The slight reduction of the chain distance just underneath and above the large dot (straight line) shows the compressive strain in the GaAs matrix caused by the QD. We compared these stoichiometry results with those of a reference InAs/GaAs sample without Sb. For the present InAs:Sb sample, we found a reduced segregation of In into the overgrowth layer, especially at the wetting layer. This behavior again confirms the role of Sb as a surfactant.
0.72
InAs
atomic chain distance [nm]
0.70 0.68 0.66
In0.5Ga0.5As
0.64 0.62
(a) In0.25Ga0.75As
0.60 0.58 0.56 0.54 0.52 -4
(b)
-2 0 2 4 6 position in [001] growth direction [nm]
8
Fig. 2. Determination of the local stoichiometry: (a) filled-state XSTM image of two InAs:Sb QDs, acquired at V S ¼ 2:0 V; (b) variation of the atomic chain distance across the upper dot (dashed line) and the lower dot (straight line) in (a) and across the wetting layer far away from any QD (dotted line), obtained by averaging in [1¯ 1 0] direction within narrow stripes. The indicated stoichiometries correspond to calculated chain distances.
27
[001] growth direction
moving adatom
4 nm Fig. 3. Close-view filled-state XSTM image of an InAs wetting layer with Sb supply during GaAs stabilization and subsequent InAs deposition, taken at V S ¼ 2:3 V. Individual Sb atoms are incorporated with a GaSb content of about 25%.
On the other hand, we observed individual Sb atoms in the wetting layer for the sample with Sb supply during GaAs stabilization and subsequent InAs deposition. Fig. 3 shows the InAs wetting layer within the GaAs matrix with clear atomic resolution, where the bright individual spots mark single atoms. Because of the negative sample voltage, the filled states of the group V atoms are imaged [16]. The strain of the wetting layer and the different bond lengths of In and Ga atoms would only cause a weakly increased, smooth topographic contrast [4], so that the sharp contrast seen here must be of electronic nature and can only originate from different group V atoms. Thus single Sb atoms between As are visible. By evaluating the XSTM images we found an Sb content in this wetting layer of about 25%. This different behavior between QDs and wetting layer can be related to the strain, which is stronger at the InAs dots than at the InGaAs wetting layer, favoring the incorporation of Sb atoms in the wetting layer, while removing them from the QD regions. In conclusion, we presented XSTM results on InAs QDs grown under Sb supply during different growth stages. The dots show a large size variation in monolayer steps with base lengths from about 10 to 35 nm and heights from 1 to 5 nm. Sb acts as a surfactant by increasing the mobility of the In atoms and thus the QD size and by decreasing the segregation of In into the overgrowth layers. In the case of Sb supply already before InAs deposition individual Sb atoms are incorporated into the wetting layer. This work was supported by projects Da 408/4, Da 408/ 8, and Sonderforschungsbereich 296 of the Deutsche Forschungsgemeinschaft as well as by the European Commission in the SANDiE Network of Excellence.
ARTICLE IN PRESS 28
R. Timm et al. / Physica E 32 (2006) 25–28
References [1] V. Shchukin, N.N. Ledentsov, D. Bimberg, Epitaxy of Nanostructures, Springer, Berlin, 2003. [2] M. Grundmann (Ed.), Nano-Optoelectronics, Springer, Berlin, 2002. [3] D.L. Huffaker, G. Park, Z. Zou, O.B. Shchekin, D.G. Deppe, Appl. Phys. Lett. 73 (1998) 2564. [4] A. Lenz, H. Eisele, R. Timm, S.K. Becker, R.L. Sellin, U.W. Pohl, D. Bimberg, M. Da¨hne, Appl. Phys. Lett. 85 (2004) 3848. [5] H. Shimizu, K. Kumada, S. Uchiyama, A. Kasukawa, Electron. Lett. 36 (2000) 1379. [6] J.C. Harmand, L.H. Li, G. Patriarche, L. Travers, Appl. Phys. Lett. 84 (2004) 3981. [7] K. Po¨tschke, L. Mu¨ller-Kirsch, R. Heitz, R.L. Sellin, U.W. Pohl, D. Bimberg, N. Zakharov, P. Werner, Physica E 21 (2004) 606; U.W. Pohl, K. Po¨tschke, A. Schliwa, F. Guffarth, D. Bimberg, N.D. Zakharov, P. Werner, M.B. Lifshits, V.A. Shchukin, D.E. Jesson, Phys. Rev. B 72 (2005) 245332. [8] M. Kudo, T. Nakaoka, S. Iwamoto, Y. Arakawa, Jpn. J. Appl. Phys. 44 (2005) L45. [9] K. Akahane, N. Yamamoto, N. Ohtani, Physica E 21 (2004) 295.
[10] T. Matsuura, T. Miyamoto, T. Kageyama, M. Ohta, Y. Matsui, T. Furuhata, F. Koyama, Jpn. J. Appl. Phys. 43 (2004) L605. [11] A. Portavoce, I. Berbezier, A. Ronda, Phys. Rev. B 69 (2004) 155416 and references therein. [12] Y. Sun, S.F. Cheng, G. Chen, R.F. Hicks, J.G. Cederberg, R.M. Biefeld, J. Appl. Phys. 97 (2005) 053503. [13] N. Liu, J. Tersoff, O. Baklenov, A.L. Holmes Jr., C.K. Shih, Phys. Rev. Lett. 84 (2000) 334. [14] A. Lenz, R. Timm, H. Eisele, Ch. Hennig, S.K. Becker, R.L. Sellin, U.W. Pohl, D. Bimberg, M. Da¨hne, Appl. Phys. Lett. 81 (2002) 5150. [15] O. Flebbe, H. Eisele, T. Kalka, F. Heinrichsdorff, A. Krost, D. Bimberg, M. Da¨hne-Prietsch, J. Vac. Sci. Technol. B 17 (1999) 1639. [16] R.M. Feenstra, J.A. Stroscio, J. Tersoff, A.P. Fein, Phys. Rev. Lett. 58 (1987) 1192. [17] J. Steinshnider, M. Weimer, R. Kaspi, G.W. Turner, Phys. Rev. Lett. 85 (2000) 2953. [18] R. Heitz, F. Guffarth, K. Po¨tschke, A. Schliwa, D. Bimberg, N.D. Zakharov, P. Werner, Phys. Rev. B 71 (2005) 045325. [19] R. Timm, H. Eisele, A. Lenz, S.K. Becker, J. Grabowski, T.-Y. Kim, L. Mu¨ller-Kirsch, K. Po¨tschke, U.W. Pohl, D. Bimberg, M. Da¨hne, Appl. Phys. Lett. 85 (2004) 5890.