- Email: [email protected]
InGaAs quantum-dot-in-ring structure by droplet epitaxy
Journal of Crystal Growth 378 (2013) 435–438
Contents lists available at SciVerse ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
InGaAs quantum-dot-in-ring structure by droplet epitaxy P. Boonpeng a, S. Kiravittaya b, S. Thainoi a, S. Panyakeow a, S. Ratanathammaphan a,n a
Semiconductor Device Research Laboratory (Nanotec Center of Excellence), Department of Electrical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok 10330, Thailand Department of Electrical and Computer Engineering, Faculty of Engineering, Naresuan University, Phitsanulok 65000, Thailand
b
a r t i c l e i n f o
abstract
Available online 15 January 2013
The controlled fabrication of self-assembled InGaAs nanostructures i.e., quantum ring (QR) and quantum-dot-in-ring (QDIR) by droplet epitaxy is reported. The effects of crystallization temperature (170–260 1C) on the nanostructure shape, dimension, density, and depth profile are investigated. The QRs transform to QDIRs when the crystallization temperature is increased. At transformation state, the QRs with distorted nanohole profile along the [1–10] crystallographic direction are observed. The formation mechanism can be explained by the competitive crystallizations in and around the nanodroplet and strain relaxation in the nanohole. & 2013 Elsevier B.V. All rights reserved.
Keywords: A1. Low dimensional structures A3. Droplet epitaxy A3. Molecular beam epitaxy A3. Nanohole B1. InGaAs B2. Semiconducting III–V materials
1. Introduction Self-assembled semiconductor nanostructures such as quantum dots (QDs) and quantum rings (QRs) have gained much interest due to their unique electrical and optical properties [1,2]. Realization of complex quantum nanostructures can lead to the development of novel device applications. For instance, coupled QD nanostructures named as QD molecules (QDMs) have been proposed as a building block for a quantum computation scheme [3]. In order to control the fabrication of desired novel nanostructures, detailed study on the morphology of nanostructure obtained from a multi-step fabrication process with various conditions is needed. Droplet epitaxy has been extensively investigated as a simple self-assembly technique to fabricate the semiconductor nanostructures without any requirement for lithographical techniques. This technique includes two processes. At the beginning of droplet epitaxy, only a molecular beam of group-III element is supplied to the substrate surface leading to the formation of the group-III nanodroplet. The substrate is subsequently exposed to group-V element. The crystallization of the group-III droplets into the III–V nanocrystals spontaneously occurs. This method can be used, not only in lattice-matched material system, but also in lattice-mismatched one [4–10]. By controlling the parameters during droplet deposition and the crystallization conditions, as well as the material systems, various nanostructures can be fabricated such as QDs, coupled QDs, QRs, concentric quantum double rings (CQDRs), and ringlike-QDMs [5–10]. Recently, many
n
Corresponding author. Tel.: þ66 2 218 6522; fax: þ66 2 251 8991. E-mail address: [email protected] (S. Ratanathammaphan).
0022-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2012.12.056
groups have demonstrated a combination of two different techniques in fabricating In(Ga)As QDMs by using the QR as a nanotemplate [11,12]. The first step is the growth of QRs by the droplet epitaxy technique and the second one is the conventional self-assembled growth (in Stranski–Krastanow mode) in regrowing InAs QDs on the QRs. The InAs QDs are formed in the hole and around sidewall of the QRs due to a high step density [12]. Therefore, the uniform nanohole template is favorable for the QDM fabrication. Hence, it is important to take a close look at the shape, dimension and density of obtained nanostructures in every step. In this paper, we have investigated the influence of crystallization process (i.e. the crystallization temperature Tcryst ) on the formation of InGaAs nanostructures. QD-in-rings (QDIRs) are observed when high crystallization temperature is applied. The structural characteristics of the resulting InGaAs nanostructures are reported. Explanation on the formation mechanism of QD in the middle of the ring is also given.
2. Experimental procedure All samples in our experiment are grown on GaAs (001) substrates in a RIBER 32P solid-source molecular beam epitaxy (MBE) system. Prior to the growth, surface oxide desorption is carried out under As4 flux by slowly ramping up substrate temperature until the reflection-high-energy-electron-diffraction pattern shows a clearly abrupt change. A 300-nm GaAs buffer layer is then grown at 600 1C with a growth rate of 0.5 monolayer/ second (ML/s) and an As4 flux of 8 10 6 Torr. Next, the substrate temperature is lowered to 300 1C without As4 beam to minimize excess As on the surface. Before indium (In) and gallium (Ga) deposition, the background pressure of the growth chamber is
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P. Boonpeng et al. / Journal of Crystal Growth 378 (2013) 435–438
reduced to less than 10 9 Torr to minimize the initial interaction between In, Ga and As4 during droplet formation. A 20 ML of In–Ga mixture (equivalent to the amount of In and Ga used for growing a 20 ML In0.15Ga0.85As layer) is deposited with a constant deposition rate of 1 ML/s. Droplets of In–Ga are formed during this step. Then, the substrate temperature is lowered to the desired crystallization temperature and it becomes stable within a few tens of seconds. For the crystallization process, the In–Ga droplets are exposed to As4 beam (8 10 6 Torr). Complex nanostructures spontaneously form at this state. For this study the crystallization temperature is systematically varied (170– 260 1C) while the crystallization time is kept constant (5 min). After the growth, the samples are rapidly quenched and taken out from the MBE system. The surface morphology of the InGaAs nanostructures is examined and analyzed by AFM (Seiko, SPA-400 in dynamic force mode) in air.
3. Results and discussion After the crystallization process described in the previous section, the InGaAs nanostructures are formed on GaAs (001) substrates. Different surface morphologies are observed when the crystallization temperatures Tcryst are altered. The leftmost AFM images of Fig. 1 show 2 2 mm2 AFM images of the surface decorated with complex nanostructures. For the low crystallization temperatures (170 1C and 200 1C), QRs with squarelike nanohole aligned along the [110] and [1–10] directions are presented (as shown in Fig. 1(a)–(b)). The ring height is asymmetric along [110] and [1–10] directions. Moreover, the cross-sectional profiles of QRs show a V-shape profile along [110] and a U-shape profile
along [1–10] direction. These can be attributed to the anisotropic behavior of the atomic migration and strain relaxation [10,13]. Due to the high crystallization temperature (230 1C and 260 1C), both QRs and QDIRs are observed. The formation of QDIR structure is due to the simultaneous crystallization at the edge of nanodroplet and strain relaxation in the nanohole during the crystallization process. More explanation on this formation process will be discussed later. The height of QD in QDIR is 4 nm for Tcryst ¼ 230 1C (see Fig. 1(c)), and it becomes mature at 12 nm for Tcryst ¼260 1C (see Fig. 1(d)). Simultaneously, asymmetric QRs are also found in every sample. In addition, highly distorted nanoholes are pronouncedly observed at the higher crystallization temperature (see Fig. 2). This might be due to non-uniformity in the dimension and composition of the initial nanodroplets and large diffusion at high temperature. For droplet of In, the crystallization typically induces elongated QDs/mounds along [1–10] direction [14] while it is quite different for InGa droplets. At high crystallization temperature, the nanostructure density is slightly reduced from 1.6 109 cm 2 for Tcryst ¼170 1C to 9 108 cm 2 for T cryst ¼260 1C and the QDIR density is about 23% of nanostructures density (see Fig. 3(a)). In addition, the QR dimensions are slightly changed as shown in Fig. 3(b)–(c). The outer and inner ring dimensions along the [110] direction ([1–10] direction) are 131–159 nm (134–151 nm) and 60–54 nm (36–60 nm), respectively. The evolution mechanism of QR into QDIR can be explained by the different crystallization rates in two regions of the nanodroplet and strain relaxation in the nanohole (Fig. 4). At the initial stage, under the conditions without As4 environments, the deposited In–Ga mixture forms liquid nanodroplets to minimize the system’s energy [15]. When the In–Ga droplets are exposed to
Fig. 1. Surface morphology of QR and QDIR formed by crystallizing 20-ML InGa droplet at different crystallization temperatures Tcryst of (a) 170 1C, (b) 200 1C, (c) 230 1C, and (d) 260 1C. Only at high Tcryst, the QDIRs are observed. The leftmost panel shows 2 2 mm2 AFM images. The next panel shows magnified three-dimensional (3D) AFM images of QRs ((a) and (b)) and QDIRs ((c) and (d)) marked by dashed-line squares. Cross-sectional height profiles along the [110] and [1–10] directions are shown in the next panels. Insets of (b) and (c) are schematics of ideal QR and QDIR, respectively.
P. Boonpeng et al. / Journal of Crystal Growth 378 (2013) 435–438
437
Fig. 2. Surface morphology of single QRs formed by crystallizing 20-ML InGa droplet at high crystallization temperatures Tcryst of (a) 230 1C and (b) 260 1C. The leftmost panel shows three-dimensional (3D) AFM images of QR marked by dashed-line circles in leftmost panels of Fig. 1(c) and (d). Cross-sectional height profiles along the [110] and [1–10] directions are shown in the next panels.
Fig. 4. Schematic illustration of the (a) QR and (b) QDIR formation at different crystallization temperatures. QD in the nanohole is formed due to material accumulation (by diffusion process) and strain relaxation.
Fig. 3. Average nanostructure (a) density, (b) outer and inner dimensions, and (c) height as a function of crystallization temperature.
As4 flux, two regions of molten droplets undego crystallization with different growth rates [16]. One is the border region where As4 flux is directly supplied into GaAs substrate surface to form the ring structure [17], and the other is the base region where the As4 flux has to diffuse through the liquid droplet in order to crystallize at bottom of the droplet. Nanoholes are formed by As out-diffusion from underneath GaAs layer. This process is the so-called droplet etching process [18,19]. However, the observed hole depths are lesser (3–5 nm) than those of other reports (5–20 nm) [18]. This might be due to the relatively low temperatures applied in this work. At elevated temperature, the diffusion will induce material accumulation
in the nanohole [20]. Since strained crystalline InGaAs nanostructures are forming during the crystallization, they can further relax the strain and form self-assembled InGaAs QDs in the middle of nanohole (see Fig. 4(b)). By these mechanisms and strain relaxation arguments [11], the QRs and QDIRs are formed as shown in Fig. 1. On increasing the crystallization temperature for In–Ga initial droplets, the dimension of InGa droplets increases while the nanostructure density is reduced (see Fig. 3(b) and (c)). If we assume that the size increment does not affect the energy minimization process of the structure, InGaAs QR can be easily induced by the higher crystallization temperature at the base region because the diffusion rate of As4 exponentially depends on the temperature [20]. In addition, QD can simultaneously form in the hole region of QR as shown in Fig. 1(c) and (d). Finally, we remark that the anisotropic behavior of the atomic migration on GaAs (001) results in larger, different ring heights of InGaAs QRs along [110] and [1–10] directions, as shown in Fig. 1(d).
4. Conclusion We have realized ensembles of InGaAs QRs and QDIRs on GaAs (001) substrates by droplet epitaxy. By simply changing the substrate temperature during the crystallization process, we
438
P. Boonpeng et al. / Journal of Crystal Growth 378 (2013) 435–438
investigate the shape, dimension, and density of self-assembled nanostructures. With the increase of crystallization temperature to 230–260 1C, the QDs are formed at the center of the QRs. This structure is so-called quantum-dot-in-ring and this study provides an easy and flexible method to fabricate a complex structure, which is promising for novel nanoelectronic and nano-optoelectronic applications.
Acknowledgment This work is supported by the Higher Education Research Promotion and National Research University Project of Thailand, Office of the Higher Education Commission (EN264A), Integrated Innovation Academic Center: IIAC Chulalongkorn University Centenary Academic Development Project (CU-Energy/CU56-EN02), Nanotechnology Center of Thailand (Nanotech), Thailand Research Fund (TRF; DPG5380002), and Chulalongkorn University. We would also like to thank Dr. Naraporn PANKAOW, Ms. Patchareewan PRONGJIT, and Mr. Pornchai CHANGMAUNG for technical assistance. One of the authors (Suwit KIRAVITTAYA) thanks the Naresuan University for partial financial support. References [1] D. Bimberg, M. Grundmann, N.N. Ledentsov, Quantum Dot Heterostructures, Wiley, Chichester, 1999; S. Kiravittaya, A. Rastelli, O.G. Schmidt, Reports on Progress in Physics 72 (2009) 046502.
[2] A. Lorke, R.J. Luyken, A.O. Govorov, J.P. Kotthaus, J.M. Garcia, P.M. Petroff, Physical Review Letters 84 (2000) 2223. ¨ [3] J. Peng, C. Hermannstadter, M. Witzany, M. Heldmaier, L. Wang, S. Kiravittaya, A. Rastelli, O.G. Schmidt, P. Michler, G. Bester, Physical Review B 81 (2010) 205315. [4] K.A. Sablon, M. Wang, G.J. Salamo, Nanotechnology 19 (2008) 125609. [5] T. Mano, K. Watanabe, S. Tsukamoto, H. Fujioka, M. Oshima, N. Koguchi, Journal of Crystal Growth 209 (2000) 504. [6] T. Mano, T. Noda, M. Yamagiwa, N. Koguchi, Thin Solid Films. 515 (2006) 531. [7] Ch. Heyn, A. Stemmann, A. Schramm, H. Welsch, W. Hansen, Applied Physics Letters 90 (2007) 203105. [8] Zh.M. Wang, B.L. Liang, K.A. Sablon, G.J. Salamo, Applied Physics Letters 90 (2007) 113120. [9] W. Jevasuwan, P. Boonpeng, S. Thainoi, S. Panyakeow, S. Ratanathammaphan, Journal of Crystal Growth 323 (2011) 275. [10] P. Boonpeng, W. Jevasuwan, N. nuntawong, S. Thainoi, S. Panyakeow, S. Ratanathammaphan, Journal of Crystal Growth 323 (2011) 271. [11] P. Boonpeng, W. Jevasuwan, S. Suraprapapich, S. Ratanathammaphan, S. Panyakeow, Microelectronic Engineering 86 (2009) 853. [12] N.W. Strom, M. Wang, J.H. Lee, Z.Y. AbuWaar, Yu.I. Mazur, G.J. Salamo, Nanoscale Research Letters 2 (2007) 112. [13] P. Boonpeng, W. Jevasuwan, S. Panyakeow, S. Ratanathammaphan, Japanese Journal of Applied Physics 49 (2010) 04DH09. [14] C. Zhao, Y.H. Chen, B. Xu, P. Jin, Z.G. Wang, Applied Physics Letters 91 (2007) 033112. [15] Zh.M. Wang, K. Holmes, Yu.I. Mazur, K.A. Ramsey, G.J. Salamo, Nanoscale Research Letters 1 (2006) 57. [16] T. Mano, N. Koguchi, Journal of Crystal Growth 278 (2005) 108. [17] C. Somaschini, S. Bietti, S. Sanguinetti, N. Koguchi, A. Fedorov, Nanotechonlogy 21 (2010) 125601. [18] Zh.M. Wang, B.L. Liang, K.A. Sablon, G.J. Salamo, Applied Physics Letters 90 (2007) 113120. [19] Ch. Heyn, Physical Review B 83 (2011) 165302. [20] G.S. Kar, S. Kiravittaya, M. Stoffel, O.G. Schmidt, Physical Review Letters 93 (2004) 246103.
Contents lists available at SciVerse ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
InGaAs quantum-dot-in-ring structure by droplet epitaxy P. Boonpeng a, S. Kiravittaya b, S. Thainoi a, S. Panyakeow a, S. Ratanathammaphan a,n a
Semiconductor Device Research Laboratory (Nanotec Center of Excellence), Department of Electrical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok 10330, Thailand Department of Electrical and Computer Engineering, Faculty of Engineering, Naresuan University, Phitsanulok 65000, Thailand
b
a r t i c l e i n f o
abstract
Available online 15 January 2013
The controlled fabrication of self-assembled InGaAs nanostructures i.e., quantum ring (QR) and quantum-dot-in-ring (QDIR) by droplet epitaxy is reported. The effects of crystallization temperature (170–260 1C) on the nanostructure shape, dimension, density, and depth profile are investigated. The QRs transform to QDIRs when the crystallization temperature is increased. At transformation state, the QRs with distorted nanohole profile along the [1–10] crystallographic direction are observed. The formation mechanism can be explained by the competitive crystallizations in and around the nanodroplet and strain relaxation in the nanohole. & 2013 Elsevier B.V. All rights reserved.
Keywords: A1. Low dimensional structures A3. Droplet epitaxy A3. Molecular beam epitaxy A3. Nanohole B1. InGaAs B2. Semiconducting III–V materials
1. Introduction Self-assembled semiconductor nanostructures such as quantum dots (QDs) and quantum rings (QRs) have gained much interest due to their unique electrical and optical properties [1,2]. Realization of complex quantum nanostructures can lead to the development of novel device applications. For instance, coupled QD nanostructures named as QD molecules (QDMs) have been proposed as a building block for a quantum computation scheme [3]. In order to control the fabrication of desired novel nanostructures, detailed study on the morphology of nanostructure obtained from a multi-step fabrication process with various conditions is needed. Droplet epitaxy has been extensively investigated as a simple self-assembly technique to fabricate the semiconductor nanostructures without any requirement for lithographical techniques. This technique includes two processes. At the beginning of droplet epitaxy, only a molecular beam of group-III element is supplied to the substrate surface leading to the formation of the group-III nanodroplet. The substrate is subsequently exposed to group-V element. The crystallization of the group-III droplets into the III–V nanocrystals spontaneously occurs. This method can be used, not only in lattice-matched material system, but also in lattice-mismatched one [4–10]. By controlling the parameters during droplet deposition and the crystallization conditions, as well as the material systems, various nanostructures can be fabricated such as QDs, coupled QDs, QRs, concentric quantum double rings (CQDRs), and ringlike-QDMs [5–10]. Recently, many
n
Corresponding author. Tel.: þ66 2 218 6522; fax: þ66 2 251 8991. E-mail address: [email protected] (S. Ratanathammaphan).
0022-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2012.12.056
groups have demonstrated a combination of two different techniques in fabricating In(Ga)As QDMs by using the QR as a nanotemplate [11,12]. The first step is the growth of QRs by the droplet epitaxy technique and the second one is the conventional self-assembled growth (in Stranski–Krastanow mode) in regrowing InAs QDs on the QRs. The InAs QDs are formed in the hole and around sidewall of the QRs due to a high step density [12]. Therefore, the uniform nanohole template is favorable for the QDM fabrication. Hence, it is important to take a close look at the shape, dimension and density of obtained nanostructures in every step. In this paper, we have investigated the influence of crystallization process (i.e. the crystallization temperature Tcryst ) on the formation of InGaAs nanostructures. QD-in-rings (QDIRs) are observed when high crystallization temperature is applied. The structural characteristics of the resulting InGaAs nanostructures are reported. Explanation on the formation mechanism of QD in the middle of the ring is also given.
2. Experimental procedure All samples in our experiment are grown on GaAs (001) substrates in a RIBER 32P solid-source molecular beam epitaxy (MBE) system. Prior to the growth, surface oxide desorption is carried out under As4 flux by slowly ramping up substrate temperature until the reflection-high-energy-electron-diffraction pattern shows a clearly abrupt change. A 300-nm GaAs buffer layer is then grown at 600 1C with a growth rate of 0.5 monolayer/ second (ML/s) and an As4 flux of 8 10 6 Torr. Next, the substrate temperature is lowered to 300 1C without As4 beam to minimize excess As on the surface. Before indium (In) and gallium (Ga) deposition, the background pressure of the growth chamber is
436
P. Boonpeng et al. / Journal of Crystal Growth 378 (2013) 435–438
reduced to less than 10 9 Torr to minimize the initial interaction between In, Ga and As4 during droplet formation. A 20 ML of In–Ga mixture (equivalent to the amount of In and Ga used for growing a 20 ML In0.15Ga0.85As layer) is deposited with a constant deposition rate of 1 ML/s. Droplets of In–Ga are formed during this step. Then, the substrate temperature is lowered to the desired crystallization temperature and it becomes stable within a few tens of seconds. For the crystallization process, the In–Ga droplets are exposed to As4 beam (8 10 6 Torr). Complex nanostructures spontaneously form at this state. For this study the crystallization temperature is systematically varied (170– 260 1C) while the crystallization time is kept constant (5 min). After the growth, the samples are rapidly quenched and taken out from the MBE system. The surface morphology of the InGaAs nanostructures is examined and analyzed by AFM (Seiko, SPA-400 in dynamic force mode) in air.
3. Results and discussion After the crystallization process described in the previous section, the InGaAs nanostructures are formed on GaAs (001) substrates. Different surface morphologies are observed when the crystallization temperatures Tcryst are altered. The leftmost AFM images of Fig. 1 show 2 2 mm2 AFM images of the surface decorated with complex nanostructures. For the low crystallization temperatures (170 1C and 200 1C), QRs with squarelike nanohole aligned along the [110] and [1–10] directions are presented (as shown in Fig. 1(a)–(b)). The ring height is asymmetric along [110] and [1–10] directions. Moreover, the cross-sectional profiles of QRs show a V-shape profile along [110] and a U-shape profile
along [1–10] direction. These can be attributed to the anisotropic behavior of the atomic migration and strain relaxation [10,13]. Due to the high crystallization temperature (230 1C and 260 1C), both QRs and QDIRs are observed. The formation of QDIR structure is due to the simultaneous crystallization at the edge of nanodroplet and strain relaxation in the nanohole during the crystallization process. More explanation on this formation process will be discussed later. The height of QD in QDIR is 4 nm for Tcryst ¼ 230 1C (see Fig. 1(c)), and it becomes mature at 12 nm for Tcryst ¼260 1C (see Fig. 1(d)). Simultaneously, asymmetric QRs are also found in every sample. In addition, highly distorted nanoholes are pronouncedly observed at the higher crystallization temperature (see Fig. 2). This might be due to non-uniformity in the dimension and composition of the initial nanodroplets and large diffusion at high temperature. For droplet of In, the crystallization typically induces elongated QDs/mounds along [1–10] direction [14] while it is quite different for InGa droplets. At high crystallization temperature, the nanostructure density is slightly reduced from 1.6 109 cm 2 for Tcryst ¼170 1C to 9 108 cm 2 for T cryst ¼260 1C and the QDIR density is about 23% of nanostructures density (see Fig. 3(a)). In addition, the QR dimensions are slightly changed as shown in Fig. 3(b)–(c). The outer and inner ring dimensions along the [110] direction ([1–10] direction) are 131–159 nm (134–151 nm) and 60–54 nm (36–60 nm), respectively. The evolution mechanism of QR into QDIR can be explained by the different crystallization rates in two regions of the nanodroplet and strain relaxation in the nanohole (Fig. 4). At the initial stage, under the conditions without As4 environments, the deposited In–Ga mixture forms liquid nanodroplets to minimize the system’s energy [15]. When the In–Ga droplets are exposed to
Fig. 1. Surface morphology of QR and QDIR formed by crystallizing 20-ML InGa droplet at different crystallization temperatures Tcryst of (a) 170 1C, (b) 200 1C, (c) 230 1C, and (d) 260 1C. Only at high Tcryst, the QDIRs are observed. The leftmost panel shows 2 2 mm2 AFM images. The next panel shows magnified three-dimensional (3D) AFM images of QRs ((a) and (b)) and QDIRs ((c) and (d)) marked by dashed-line squares. Cross-sectional height profiles along the [110] and [1–10] directions are shown in the next panels. Insets of (b) and (c) are schematics of ideal QR and QDIR, respectively.
P. Boonpeng et al. / Journal of Crystal Growth 378 (2013) 435–438
437
Fig. 2. Surface morphology of single QRs formed by crystallizing 20-ML InGa droplet at high crystallization temperatures Tcryst of (a) 230 1C and (b) 260 1C. The leftmost panel shows three-dimensional (3D) AFM images of QR marked by dashed-line circles in leftmost panels of Fig. 1(c) and (d). Cross-sectional height profiles along the [110] and [1–10] directions are shown in the next panels.
Fig. 4. Schematic illustration of the (a) QR and (b) QDIR formation at different crystallization temperatures. QD in the nanohole is formed due to material accumulation (by diffusion process) and strain relaxation.
Fig. 3. Average nanostructure (a) density, (b) outer and inner dimensions, and (c) height as a function of crystallization temperature.
As4 flux, two regions of molten droplets undego crystallization with different growth rates [16]. One is the border region where As4 flux is directly supplied into GaAs substrate surface to form the ring structure [17], and the other is the base region where the As4 flux has to diffuse through the liquid droplet in order to crystallize at bottom of the droplet. Nanoholes are formed by As out-diffusion from underneath GaAs layer. This process is the so-called droplet etching process [18,19]. However, the observed hole depths are lesser (3–5 nm) than those of other reports (5–20 nm) [18]. This might be due to the relatively low temperatures applied in this work. At elevated temperature, the diffusion will induce material accumulation
in the nanohole [20]. Since strained crystalline InGaAs nanostructures are forming during the crystallization, they can further relax the strain and form self-assembled InGaAs QDs in the middle of nanohole (see Fig. 4(b)). By these mechanisms and strain relaxation arguments [11], the QRs and QDIRs are formed as shown in Fig. 1. On increasing the crystallization temperature for In–Ga initial droplets, the dimension of InGa droplets increases while the nanostructure density is reduced (see Fig. 3(b) and (c)). If we assume that the size increment does not affect the energy minimization process of the structure, InGaAs QR can be easily induced by the higher crystallization temperature at the base region because the diffusion rate of As4 exponentially depends on the temperature [20]. In addition, QD can simultaneously form in the hole region of QR as shown in Fig. 1(c) and (d). Finally, we remark that the anisotropic behavior of the atomic migration on GaAs (001) results in larger, different ring heights of InGaAs QRs along [110] and [1–10] directions, as shown in Fig. 1(d).
4. Conclusion We have realized ensembles of InGaAs QRs and QDIRs on GaAs (001) substrates by droplet epitaxy. By simply changing the substrate temperature during the crystallization process, we
438
P. Boonpeng et al. / Journal of Crystal Growth 378 (2013) 435–438
investigate the shape, dimension, and density of self-assembled nanostructures. With the increase of crystallization temperature to 230–260 1C, the QDs are formed at the center of the QRs. This structure is so-called quantum-dot-in-ring and this study provides an easy and flexible method to fabricate a complex structure, which is promising for novel nanoelectronic and nano-optoelectronic applications.
Acknowledgment This work is supported by the Higher Education Research Promotion and National Research University Project of Thailand, Office of the Higher Education Commission (EN264A), Integrated Innovation Academic Center: IIAC Chulalongkorn University Centenary Academic Development Project (CU-Energy/CU56-EN02), Nanotechnology Center of Thailand (Nanotech), Thailand Research Fund (TRF; DPG5380002), and Chulalongkorn University. We would also like to thank Dr. Naraporn PANKAOW, Ms. Patchareewan PRONGJIT, and Mr. Pornchai CHANGMAUNG for technical assistance. One of the authors (Suwit KIRAVITTAYA) thanks the Naresuan University for partial financial support. References [1] D. Bimberg, M. Grundmann, N.N. Ledentsov, Quantum Dot Heterostructures, Wiley, Chichester, 1999; S. Kiravittaya, A. Rastelli, O.G. Schmidt, Reports on Progress in Physics 72 (2009) 046502.
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