- Email: [email protected]
Effects of Sb-soak on InAs quantum dots grown on (001) and (113)B GaAs substrates
Journal of Crystal Growth (xxxx) xxxx–xxxx
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Effects of Sb-soak on InAs quantum dots grown on (001) and (113)B GaAs substrates ⁎
Xiangmeng Lua, , Naoto Kumagaia,b, Yasuo Minamia, Takahiro Kitadaa, Toshiro Isua a b
Graduate School of Science and Technology, Tokushima University, Tokushima 770-8506, Japan National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8560, Japan1
A R T I C L E I N F O
A BS T RAC T
Communicated by Jean-Baptiste Rodriguez
We have investigated the effects of Sb-soak on InAs quantum dots (QDs) grown on (001) and (113)B GaAs substrates by molecular beam epitaxy. Surface morphologies of the QDs were characterized by atomic force microscopy. The optical properties of buried QDs were investigated by photoluminescence (PL). We showed that effects of Sb-soak on density and PL of (001) and (113)B QDs were quite different. The increased density and blue-shift of (001) QDs can be explained by the surfactant effect of Sb atoms which increase the areal density of the kinks for nucleation. On the other hand, for (113)B QDs, the incorporation effect should be responsible for the red-shift because the Sb atoms may be diffused into QDs.
Keywords: A3. Molecular beam epitaxy A3. Quantum dot B2. (113)B GaAs A1. Sb-soak
1. Introduction During the past several decades, self-assembled quantum dots (QDs) have been extensively investigated because its three-dimensional quantum carrier confinement properties can provide new opportunities for novel device applications such as QD lasers [1] and quantum information processing [2]. In order to effectively realize these devices, control of the QD structure properties and hence optical properties via a number of growth parameters are urgently required. Recently using of Sb-terminated or Sb-soak have attracted considerable attention among the many means for obtain high quality QDs. D. Guimard et al. reported that a significant decrease of the threshold current densities of InAs/Sb: GaAs QDs lasers in a 1.3 µm band resulted from better QD interface quality due to Sb surfactant effects [3]. It has also been indicated that Sb-mediated is a very promising approach toward the realization of high performance QD infrared photodetectors and other QD-based devices such as interband lasers where the gain is limited by the dot density [4]. Furthermore, high dot density of 1×1011 cm−2 on the GaSb/GaAs buffer [5] and ultrahigh density of 5×1011 cm−2 using GaAsSb buffer layer [6] have been demonstrated by Yamaguchi's group. These in-plane ultrahigh-density QDs will provide many possible device applications including intermediate-band solar cells and many physical interest in the laterally coupled QD system. However, it should be noted that all of the achievements utilizing Sb mentioned above were concentrated on InAs QDs grown on low-index (001) GaAs substrate. There has been no research about Sb-soak, as far
⁎
1
as we know, on InAs QDs grown on high-index GaAs substrates, which are essential for novel terahertz devices based on difference-frequency generation (DFG) in two-color surface lasers using a semiconductor coupled multilayer cavity proposed by our group [7–9]. To realize the proposed terahertz emitting devices, high-index GaAs substrates are essential for DFG because the effective second-order nonlinear coefficient is zero on the (001) orientation due to crystal symmetry [10]. Although we have suppressed PL from wetting layer (WL) of InAs QDs grown on high-index (113)B GaAs substrate by introducing AlAs cap [11], which tends to eliminate the WL due to phase separation [12], the intensity of PL of (113)B QDs is still weak compared with (001) QDs grown under identical growth conditions. Therefore, it would be highly advantageous if we can improve the optical quality of (113)B QDs using Sb-soak to make it suitable for application in terahertz devices. In this study, we have investigated the effects of Sb-soak on InAs (113)B and (001) QDs. Surface morphologies of QDs were characterized by atomic force microscopy (AFM). The optical properties of QDs were measured by PL. We showed that effects of Sb-soak on (001) and (113)B QDs were quite different. The increased density and blue-shift of (001) QDs can be explained by the surfactant effect of Sb atoms which increase the areal density of the kinks for nucleation. On the other hand, for (113)B QDs, the incorporation effect should be responsible for the red-shift because the Sb atoms may be diffused into QDs.
Corresponding author. E-mail address: [email protected] (X. Lu). Present address.
http://dx.doi.org/10.1016/j.jcrysgro.2017.01.024 Received 28 October 2016; Received in revised form 29 December 2016; Accepted 16 January 2017 0022-0248/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Lu, X., Journal of Crystal Growth (2017), http://dx.doi.org/10.1016/j.jcrysgro.2017.01.024
Journal of Crystal Growth (xxxx) xxxx–xxxx
X. Lu et al.
Fig. 1. (Color online) Sample structures of InAs QDs. Type A is conventional InAs QDs grown on GaAs buffer layer, type B is InAs QDs with Sb-soak on GaAs buffer layer, and type C is InAs QDs with both Sb-soak and 0.5 ML AlAs cap.
2. Experiments The samples were grown on (001) and (113)B semi-insulating GaAs substrates by a solid-source Riber Compact 21 MBE system. The (001) and (113)B GaAs substrates were mounted side by side on a 2-in. Mo substrate holder with the use of In solder. Native oxide on the GaAs substrates were removed by heating up to a substrate temperature (Ts) of 640 °C under As4 atmosphere in a growth chamber. After 300-nmthick GaAs buffer layer, 50-nm-thick Al0.3Ga0.7As, and 150-nm-thick GaAs layer were grown at Ts =660 °C. Then, Ts was decreased to Ts =550 °C for growth of 2.7 ML InAs QDs. Fig. 1 shows three types of InAs QDs; the conventional of InAs QDs on GaAs buffer layer (type A), InAs QDs with Sb-soak on GaAs buffer layer (type B), and InAs QDs with both Sb-soak and 0.5 ML AlAs cap (type C). For type B, Sb-soak prior to growth of QDs was continued for 60 s. Only the shutter of Sb was opening and the rest of shutters including As shutter were closed during Sb-soak. Sb flux was supplied by a standard Knudsen cell and the beam equivalent pressure of Sb was approximately 2×10−8 Torr. For type C, the procedure of Sb-soak was the same as type B but 0.5 ML AlAs cap was deposited on QDs. Buried and top surface QDs were grown under the same conditions. Surface morphologies of the QDs were characterized by AFM. The optical properties of buried QDs were investigated by PL using an excitation laser at a wavelength of 532 nm. The PL spectra was dispersed by a spectrometer and detected by a liquid-nitrogen-cooled InGaAs photodiode array at room temperature (RT). 3. Results and discussion Figs. 2(a~c) and (b~d) show AFM images of three types of InAs QDs grown on (001) and (113)B GaAs substrates, respectively. The sheet density of InAs QDs were summarized in Table 1. Compared type B with A, the sheet density of (001) QDs increased from 9.40×1010 to 1.63×1011 cm−2. However, the density of (113)B QDs almost unchanged by Sb-soak. Compared type C with B, the density of (001) QDs almost unchanged but that of (113)B decreased slightly. The PL spectra of (001) QDs and corresponding height distribution of QDs derived from AFM images are shown in Figs. 3(a) and (b), respectively. A blue-shift was observed for QDs of type B compared with type A. The blue-shift in QDs of type B can be explained by the reduction in height compared with type A as shown in Fig. 3(b). A blueshift was also observed clearly for QDs of type C compared with type A, although the height distribution of QDs of type C is almost same with type A. We may not precisely determine the height of type C QDs using AFM images because the AlAs layer on the top surface is easily oxidized in atmosphere and the peak of height distribution may slightly deviate from the actual position. Considering the coverage of InAs is same (2.7 ML) for all samples, it is easy to conjecture that the increasing in
Fig. 2. (Color online) AFM images of InAs QDs of types A, B, and C grown on (001) and (113)B GaAs substrate, respectively.
Table 1 Density (×1010 cm−2) of QDs derived from AFM images.
2
Substrate
Type A
Type B
Type C
(001) (113)B
9.40 7.31
16.3 7.23
17.4 4.74
Journal of Crystal Growth (xxxx) xxxx–xxxx
X. Lu et al.
Fig. 3. (Color online) The PL spectra of samples (a) and corresponding height distribution of QDs (b) grown on (001) GaAs substrate, and the PL spectra of samples (c) and corresponding height distribution of QDs (d) grown on (113)B GaAs substrate.
(113)B GaAs substrates. For the case of surfactant effect, the Sb atoms passivate the steps, then the originally straight steps will become rough and greatly increase the areal density of the kinks for nucleation. While the surfactant effect of Sb has been reported to increase the density of QDs, the incorporation of Sb has effect to reduce the density of QDs. Chen et al. have reported that incorporation of Sb can reduce the density of (001) QDs by more than two orders of magnitude and they claimed that the decrease in dot density resulted from an increase of critical thickness [15]. The incorporation of Sb might be responsible for the slight decrease in density for type C QDs grown on (113)B GaAs substrate. For the case of incorporation effect, the Sb atoms tend to diffused into the InAs QDs. Both the increment in volume and change in composition of InAs QDs will induce red-shift of PL. Let us next consider the effect of AlAs cap on InAs QDs. The AlAs cap was introduced to eliminate the WL due to phase separation [12]. The suppression of PL from WL of InAs QDs with AlAs cap grown on (113)
density will result in reduction of size and height of QDs. The PL spectra of (113)B QDs and corresponding height distribution of QDs are shown in Figs. 3(c) and (d), respectively. The peak of height distribution almost unchanged but a red-shift was observed when we compared type B with A. The red-shift was further enhanced after AlAs cap. These red-shift in PL cannot be explained by the height distribution of (113)B QDs. The results showed that effects of Sb-Soak on QDs grown on (001) and (113)B GaAs substrate were quite different. Although the detail microscopic mechanism of Sb on InAs QDs is still under debate it is generally believed that two of the typical effects of Sb are surfactant and incorporation [13]. The increased density and blue-shift of (001) QDs are consistent with previous studies [5,14] and can be explained by the surfactant effects. However, for the (113)B QDs, the incorporation effect should be considered. Fig. 4 illustrates the surfactant and incorporation effects of Sb during the growth InAs QDs on (001) and 3
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X. Lu et al.
the Sb atoms may be diffused into QDs. References [1] Y. Arakawa, H. Sakaki, Multidimensional quantum well laser and temperature dependence of its threshold current, Appl. Phys. Lett. 40 (1982) 939 http://dx.doi. org/10.1063/1.92959. [2] A. Imamoglu, D.D. Awschalom, G. Burkard, D.P. DiVincenzo, D. Loss, M. Sherwin, A. Small, Quantum information processing using quantum dot spins and cavity QED, Phys. Rev. Lett. 83 (1999) 4204–4207. http://dx.doi.org/10.1103/ PhysRevLett.83.4204. [3] D. Guimard, M. Ishida, L. Li, M. Nishioka, Y. Tanaka, H. Sudo, T. Yamamoto, H. Kondo, M. Sugawara, Y. Arakawa, Interface properties of InAs quantum dots produced by antimony surfactant-mediated growth: etching of segregated antimony and its impact on the photoluminescence and lasing characteristics, Appl. Phys. Lett. 94 (2009) 1–4 (103116) http://dx.doi.org/10.1063/1.3099902. [4] P. Aivaliotis, L.R. Wilson, E.A. Zibik, J.W. Cockburn, M.J. Steer, H.Y. Liu, Enhancing the dot density in quantum dot infrared photodetectors via the incorporation of antimony, Appl. Phys. Lett. 91 (2007) 013503 http://dx.doi.org/ 10.1063/1.2753727. [5] K. Yamaguchi, T. Kanto, Self-assembled InAs quantum dots on GaSb/GaAs(0 0 1) layers by molecular beam epitaxy, J. Cryst. Growth 275 (2005) e2269–e2273 http://dx.doi.org/10.1016/j.jcrysgro.2004.11.363. [6] E. Saputra, J. Ohta, N. Kakuda, K. Yamaguchi, Self-Formation of In-Plane Ultrahigh-Density InAs Quantum Dots on GaAsSb/GaAs(001), Appl. Phys. Express 5 (2012) 125502 http://dx.doi.org/10.1143/APEX.5.125502. [7] T. Kitada, F. Tanaka, T. Takahashi, K. Morita, T. Isu, GaAs/AlAs coupled multilayer cavity structures for terahertz emission devices, Appl. Phys. Lett. 95 (2009) 111106 http://dx.doi.org/10.1063/1.3226667. [8] H. Ota, X.M. Lu, N. Kumagai, T. Kitada, T. Isu, Fabrication of two-color surface emitting device of a coupled vertical cavity structure with InAs quantum dots formed by wafer bonding, Jpn. J. Appl. Phys. 55 (2016) 04EH09 http://dx.doi.org/ 10.7567/JJAP.55.04EH09. [9] T. Kitada, H. Ota, X.M. Lu, N. Kumagai, T. Isu, Two-color surface-emitting lasers using a semiconductor coupled multilayer cavity, Appl. Phys. E. 9 (2016) 111201 http://dx.doi.org/10.7567/APEX.9.111201. [10] N. Yamada, Y. Ichimura, S. Nakagawa, Y. Kaneko, T. Takeuchi, N. Mikoshiba, Second-harmonic generation in vertical-cavity surface-emitting laser, Jpn. J. Appl. Phys. 35 (1996) 2659–2664 http://dx.doi.org/10.1143/JJAP.35.2659. [11] X.M. Lu, S. Matsubara, Y. Nakagawa, T. Kitada, T. Isu, Suppression of photoluminescence from wetting layer of InAs quantum dots grown on (311)B GaAs with AlAs cap, J. Cryst. Growth 425 (2015) 106–109 http://dx.doi.org/10.1016/j. jcrysgro.2015.02.074. [12] V. Shchukin, N. Ledentsov, S. Rouvimov, Formation of three-dimensional islands in subcritical layer deposition in Stranski-Krastanow growth, Phys. Rev. Lett. 110 (2013) 176101. http://dx.doi.org/10.1103/PhysRevLett.110.176101. [13] M. Henini, Molecular Beam Epitaxy: From Research to Mass Production, Elsevier, 2012. [14] Y.I. Mazur, V.G. Dorogan, G.J. Salamo, G.G. Tarasov, B.L. Liang, C.J. Reyner, K. Nunna, D.L. Huffaker, Coexistence of type-I and type-II band alignments in antimony-incorporated InAsSb quantum dot nanostructures, Appl. Phys. Lett. 100 (2012) 33102 http://dx.doi.org/10.1063/1.3676274. [15] J.F. Chen, C.H. Chiang, Y.H. Wu, L. Chang, J.Y. Chi, Effect of antimony incorporation on the density, shape, and luminescence of InAs quantum dots, J. Appl. Phys. 104 (2008) 023509 http://dx.doi.org/10.1063/1.2959598. [16] F.K. Tutu, P. Lam, J. Wu, N. Miyashita, Y. Okada, K.-H. Lee, N.J. Ekins-Daukes, J. Wilson, H. Liu, InAs/GaAs quantum dot solar cell with an AlAs cap layer, Appl. Phys. Lett. 102 (2013) 163907 http://dx.doi.org/10.1063/1.4803459. [17] H. Yokota, K. Iizuka, H. Okamoto, T. Suzuki, AlAs coating for stacked structure of self-assembled InAs/GaAs quantum dots, J. Cryst. Growth 301–302 (2007) 825–827 http://dx.doi.org/10.1016/j.jcrysgro.2006.11.326.
Fig. 4. (Color online) Schematic diagram of surfactant and incorporation effects of Sbsoak on (001) and (113)B InAs QDs.
B GaAs [11] and the absence of WL of InAs QDs grown on (100) QDs have been demonstrated [16]. Yokota et al. have reported that PL peak from (001) InAs QDs with 0.08 and 0.2 ML AlAs cap were red-shift compared with that without AlAl cap because intermixing of InAs QDs with GaAs was suppressed by the ultra-thin AlAs layer [17]. A slight red-shift was also seen by comparing the PL of (001) QDs of type C with B as shown in Fig. 3(a). It is conceivable that this mechanism is still working for Sb-soaked (001) QDs in our case. However, a remarkable red-shift in (113)B QDs of type C compared with type B in Fig. 3(c) cannot be solely ascribed to this mechanism. It is thought that the decreased density in the (113)B type C QDs was the consequence of the incorporation effect by Sb-soak. It may cause a red-shift due to PL emission from the large QDs. In addition, the bandgap of the QDs should be reduced by the Sb incorporation. Thus, the observed remarkable red-shift can be considered to originate from the significant incorporation effect in the (113)B type C QDs. This implies that the incorporation effect by Sb-soak on (113)B might be further enhanced by AlAs cap. 4. Summary We have investigated the effects of Sb-soak on InAs QDs grown on (001) and (113)B GaAs substrates by MBE, respectively. Surface morphologies of the QDs were characterized by a tapping-mode AFM. The optical properties of buried QDs were investigated by PL. We showed that effects of Sb-soak on (001) and (113)B QDs were quite different. The increased density and blue-shift of (001) QDs can be explained by the surfactant effect of Sb atoms which increase the areal density of the kinks for nucleation. On the other hand, for (113)B QDs, the incorporation effect should be responsible for the red-shift because
4
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Effects of Sb-soak on InAs quantum dots grown on (001) and (113)B GaAs substrates ⁎
Xiangmeng Lua, , Naoto Kumagaia,b, Yasuo Minamia, Takahiro Kitadaa, Toshiro Isua a b
Graduate School of Science and Technology, Tokushima University, Tokushima 770-8506, Japan National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8560, Japan1
A R T I C L E I N F O
A BS T RAC T
Communicated by Jean-Baptiste Rodriguez
We have investigated the effects of Sb-soak on InAs quantum dots (QDs) grown on (001) and (113)B GaAs substrates by molecular beam epitaxy. Surface morphologies of the QDs were characterized by atomic force microscopy. The optical properties of buried QDs were investigated by photoluminescence (PL). We showed that effects of Sb-soak on density and PL of (001) and (113)B QDs were quite different. The increased density and blue-shift of (001) QDs can be explained by the surfactant effect of Sb atoms which increase the areal density of the kinks for nucleation. On the other hand, for (113)B QDs, the incorporation effect should be responsible for the red-shift because the Sb atoms may be diffused into QDs.
Keywords: A3. Molecular beam epitaxy A3. Quantum dot B2. (113)B GaAs A1. Sb-soak
1. Introduction During the past several decades, self-assembled quantum dots (QDs) have been extensively investigated because its three-dimensional quantum carrier confinement properties can provide new opportunities for novel device applications such as QD lasers [1] and quantum information processing [2]. In order to effectively realize these devices, control of the QD structure properties and hence optical properties via a number of growth parameters are urgently required. Recently using of Sb-terminated or Sb-soak have attracted considerable attention among the many means for obtain high quality QDs. D. Guimard et al. reported that a significant decrease of the threshold current densities of InAs/Sb: GaAs QDs lasers in a 1.3 µm band resulted from better QD interface quality due to Sb surfactant effects [3]. It has also been indicated that Sb-mediated is a very promising approach toward the realization of high performance QD infrared photodetectors and other QD-based devices such as interband lasers where the gain is limited by the dot density [4]. Furthermore, high dot density of 1×1011 cm−2 on the GaSb/GaAs buffer [5] and ultrahigh density of 5×1011 cm−2 using GaAsSb buffer layer [6] have been demonstrated by Yamaguchi's group. These in-plane ultrahigh-density QDs will provide many possible device applications including intermediate-band solar cells and many physical interest in the laterally coupled QD system. However, it should be noted that all of the achievements utilizing Sb mentioned above were concentrated on InAs QDs grown on low-index (001) GaAs substrate. There has been no research about Sb-soak, as far
⁎
1
as we know, on InAs QDs grown on high-index GaAs substrates, which are essential for novel terahertz devices based on difference-frequency generation (DFG) in two-color surface lasers using a semiconductor coupled multilayer cavity proposed by our group [7–9]. To realize the proposed terahertz emitting devices, high-index GaAs substrates are essential for DFG because the effective second-order nonlinear coefficient is zero on the (001) orientation due to crystal symmetry [10]. Although we have suppressed PL from wetting layer (WL) of InAs QDs grown on high-index (113)B GaAs substrate by introducing AlAs cap [11], which tends to eliminate the WL due to phase separation [12], the intensity of PL of (113)B QDs is still weak compared with (001) QDs grown under identical growth conditions. Therefore, it would be highly advantageous if we can improve the optical quality of (113)B QDs using Sb-soak to make it suitable for application in terahertz devices. In this study, we have investigated the effects of Sb-soak on InAs (113)B and (001) QDs. Surface morphologies of QDs were characterized by atomic force microscopy (AFM). The optical properties of QDs were measured by PL. We showed that effects of Sb-soak on (001) and (113)B QDs were quite different. The increased density and blue-shift of (001) QDs can be explained by the surfactant effect of Sb atoms which increase the areal density of the kinks for nucleation. On the other hand, for (113)B QDs, the incorporation effect should be responsible for the red-shift because the Sb atoms may be diffused into QDs.
Corresponding author. E-mail address: [email protected] (X. Lu). Present address.
http://dx.doi.org/10.1016/j.jcrysgro.2017.01.024 Received 28 October 2016; Received in revised form 29 December 2016; Accepted 16 January 2017 0022-0248/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Lu, X., Journal of Crystal Growth (2017), http://dx.doi.org/10.1016/j.jcrysgro.2017.01.024
Journal of Crystal Growth (xxxx) xxxx–xxxx
X. Lu et al.
Fig. 1. (Color online) Sample structures of InAs QDs. Type A is conventional InAs QDs grown on GaAs buffer layer, type B is InAs QDs with Sb-soak on GaAs buffer layer, and type C is InAs QDs with both Sb-soak and 0.5 ML AlAs cap.
2. Experiments The samples were grown on (001) and (113)B semi-insulating GaAs substrates by a solid-source Riber Compact 21 MBE system. The (001) and (113)B GaAs substrates were mounted side by side on a 2-in. Mo substrate holder with the use of In solder. Native oxide on the GaAs substrates were removed by heating up to a substrate temperature (Ts) of 640 °C under As4 atmosphere in a growth chamber. After 300-nmthick GaAs buffer layer, 50-nm-thick Al0.3Ga0.7As, and 150-nm-thick GaAs layer were grown at Ts =660 °C. Then, Ts was decreased to Ts =550 °C for growth of 2.7 ML InAs QDs. Fig. 1 shows three types of InAs QDs; the conventional of InAs QDs on GaAs buffer layer (type A), InAs QDs with Sb-soak on GaAs buffer layer (type B), and InAs QDs with both Sb-soak and 0.5 ML AlAs cap (type C). For type B, Sb-soak prior to growth of QDs was continued for 60 s. Only the shutter of Sb was opening and the rest of shutters including As shutter were closed during Sb-soak. Sb flux was supplied by a standard Knudsen cell and the beam equivalent pressure of Sb was approximately 2×10−8 Torr. For type C, the procedure of Sb-soak was the same as type B but 0.5 ML AlAs cap was deposited on QDs. Buried and top surface QDs were grown under the same conditions. Surface morphologies of the QDs were characterized by AFM. The optical properties of buried QDs were investigated by PL using an excitation laser at a wavelength of 532 nm. The PL spectra was dispersed by a spectrometer and detected by a liquid-nitrogen-cooled InGaAs photodiode array at room temperature (RT). 3. Results and discussion Figs. 2(a~c) and (b~d) show AFM images of three types of InAs QDs grown on (001) and (113)B GaAs substrates, respectively. The sheet density of InAs QDs were summarized in Table 1. Compared type B with A, the sheet density of (001) QDs increased from 9.40×1010 to 1.63×1011 cm−2. However, the density of (113)B QDs almost unchanged by Sb-soak. Compared type C with B, the density of (001) QDs almost unchanged but that of (113)B decreased slightly. The PL spectra of (001) QDs and corresponding height distribution of QDs derived from AFM images are shown in Figs. 3(a) and (b), respectively. A blue-shift was observed for QDs of type B compared with type A. The blue-shift in QDs of type B can be explained by the reduction in height compared with type A as shown in Fig. 3(b). A blueshift was also observed clearly for QDs of type C compared with type A, although the height distribution of QDs of type C is almost same with type A. We may not precisely determine the height of type C QDs using AFM images because the AlAs layer on the top surface is easily oxidized in atmosphere and the peak of height distribution may slightly deviate from the actual position. Considering the coverage of InAs is same (2.7 ML) for all samples, it is easy to conjecture that the increasing in
Fig. 2. (Color online) AFM images of InAs QDs of types A, B, and C grown on (001) and (113)B GaAs substrate, respectively.
Table 1 Density (×1010 cm−2) of QDs derived from AFM images.
2
Substrate
Type A
Type B
Type C
(001) (113)B
9.40 7.31
16.3 7.23
17.4 4.74
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Fig. 3. (Color online) The PL spectra of samples (a) and corresponding height distribution of QDs (b) grown on (001) GaAs substrate, and the PL spectra of samples (c) and corresponding height distribution of QDs (d) grown on (113)B GaAs substrate.
(113)B GaAs substrates. For the case of surfactant effect, the Sb atoms passivate the steps, then the originally straight steps will become rough and greatly increase the areal density of the kinks for nucleation. While the surfactant effect of Sb has been reported to increase the density of QDs, the incorporation of Sb has effect to reduce the density of QDs. Chen et al. have reported that incorporation of Sb can reduce the density of (001) QDs by more than two orders of magnitude and they claimed that the decrease in dot density resulted from an increase of critical thickness [15]. The incorporation of Sb might be responsible for the slight decrease in density for type C QDs grown on (113)B GaAs substrate. For the case of incorporation effect, the Sb atoms tend to diffused into the InAs QDs. Both the increment in volume and change in composition of InAs QDs will induce red-shift of PL. Let us next consider the effect of AlAs cap on InAs QDs. The AlAs cap was introduced to eliminate the WL due to phase separation [12]. The suppression of PL from WL of InAs QDs with AlAs cap grown on (113)
density will result in reduction of size and height of QDs. The PL spectra of (113)B QDs and corresponding height distribution of QDs are shown in Figs. 3(c) and (d), respectively. The peak of height distribution almost unchanged but a red-shift was observed when we compared type B with A. The red-shift was further enhanced after AlAs cap. These red-shift in PL cannot be explained by the height distribution of (113)B QDs. The results showed that effects of Sb-Soak on QDs grown on (001) and (113)B GaAs substrate were quite different. Although the detail microscopic mechanism of Sb on InAs QDs is still under debate it is generally believed that two of the typical effects of Sb are surfactant and incorporation [13]. The increased density and blue-shift of (001) QDs are consistent with previous studies [5,14] and can be explained by the surfactant effects. However, for the (113)B QDs, the incorporation effect should be considered. Fig. 4 illustrates the surfactant and incorporation effects of Sb during the growth InAs QDs on (001) and 3
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Fig. 4. (Color online) Schematic diagram of surfactant and incorporation effects of Sbsoak on (001) and (113)B InAs QDs.
B GaAs [11] and the absence of WL of InAs QDs grown on (100) QDs have been demonstrated [16]. Yokota et al. have reported that PL peak from (001) InAs QDs with 0.08 and 0.2 ML AlAs cap were red-shift compared with that without AlAl cap because intermixing of InAs QDs with GaAs was suppressed by the ultra-thin AlAs layer [17]. A slight red-shift was also seen by comparing the PL of (001) QDs of type C with B as shown in Fig. 3(a). It is conceivable that this mechanism is still working for Sb-soaked (001) QDs in our case. However, a remarkable red-shift in (113)B QDs of type C compared with type B in Fig. 3(c) cannot be solely ascribed to this mechanism. It is thought that the decreased density in the (113)B type C QDs was the consequence of the incorporation effect by Sb-soak. It may cause a red-shift due to PL emission from the large QDs. In addition, the bandgap of the QDs should be reduced by the Sb incorporation. Thus, the observed remarkable red-shift can be considered to originate from the significant incorporation effect in the (113)B type C QDs. This implies that the incorporation effect by Sb-soak on (113)B might be further enhanced by AlAs cap. 4. Summary We have investigated the effects of Sb-soak on InAs QDs grown on (001) and (113)B GaAs substrates by MBE, respectively. Surface morphologies of the QDs were characterized by a tapping-mode AFM. The optical properties of buried QDs were investigated by PL. We showed that effects of Sb-soak on (001) and (113)B QDs were quite different. The increased density and blue-shift of (001) QDs can be explained by the surfactant effect of Sb atoms which increase the areal density of the kinks for nucleation. On the other hand, for (113)B QDs, the incorporation effect should be responsible for the red-shift because
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