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Improving the composition uniformity of Au-catalyzed InGaAs nanowires on silicon
Journal of Crystal Growth 372 (2013) 15–18
Contents lists available at SciVerse ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Improving the composition uniformity of Au-catalyzed InGaAs nanowires on silicon Jae Cheol Shin a, Do Yang Kim b,c, Ari Lee a, Hyo Jin Kim a, Jae Hun Kim c, Won Jun Choi c, Hyun-Seok Kim d, Kyoung Jin Choi b,n a
Korea Photonics Technology Institute, Gwangju 500-779, South Korea School of Mechanical and Advanced Materials Engineering, Ulsan National Institute of Science & Technology (UNIST), Ulsan 689-805, South Korea c Korea Institute of Science and Technology (KIST), Seoul 136-791, South Korea d Electronics and Electrical Engineering, Dongguk University, Seoul 100-715, South Korea b
a r t i c l e i n f o
abstract
Article history: Received 26 December 2012 Received in revised form 19 February 2013 Accepted 23 February 2013 Communicated by K. Deppert Available online 4 March 2013
Spatial distribution of indium (In) atoms in ternary InxGa1 xAs nanowires (NWs) was investigated by the energy-dispersive X-ray spectroscopy, which were grown on Si (111) by metal-organic chemical vapor deposition. The NWs have a tapered morphology with thicker diameter and higher In composition in the bottom of NWs. However, decreasing growth temperature and V/III ratio resulted in straight NWs with constant In composition throughout the NWs. This was attributed to enhanced deposition on the sidewall of the NW with higher In composition through the vapor–solid mode, leading to a core-shell structure consisting of low and high In-content layers. & 2013 Elsevier B.V. All rights reserved.
Keywords: A1. Nanostructures A3. Metalorganic vapor phase epitaxy B1. Nanomaterials B2. Semiconducting III–V materials
1. Introduction One-dimensional (1-D) nanostructures such as nanorods, nanowires (NWs), and nanobelts have been successfully synthesized using a wide range of semiconductors and demonstrated new design concepts for novel electronic and optoelectronic devices. Especially, hetero-structured 1-D nanostructures, including coaxial core-shell, axially-modulated, and alloyed NWs, have attracted much attention because of their controlled morphologies and multi-functional optoelectronic properties [1–4]. Among them, alloyed semiconductor NWs can offer more unique properties than the corresponding elemental or binary ones by engineering the bandgap energy, which is one of the most important parameters of a semiconductor and determines its electronic and optical properties [5–8]. III–V compound semiconductor NWs based on binary materials (e.g., GaAs, InP, GaN) have been fabricated for optical devices such as solar-cells and lightemitting-diodes [9,10]. For example, the ternary InxGa1 xAs can cover the wavelength range from near- to mid-infrared (0.87– 387–3.5 mm) by adjusting indium (In) composition, as demonstrated by the heterojunction solar cells [8] and light-emitting diodes (LEDs) for medical applications [11]. Several recent works had reported the successful growths of high quality III-V
n
Corresponding author. E-mail address: [email protected] (K.J. Choi).
0022-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2013.02.025
semiconductor NWs by using selective-area epitaxy (SAE) in metal-organic chemical vapor deposition (MOCVD) reactor [12–14]. In the SAE-grown NWs, the growths were defined by the openings formed by various lithography methods resulting in highly-order NWs [12–14]. The fabrication of these patterns had been reported by using electron-beam lithography, and other self-assembled lithography methods (i.e. 2-D close-packed monolayer colloidal deposition [15,16], or diblock-copolymer methods [17,18]). Vapor–liquid–solid (VLS) method, which facilitates onedimensional (1-D) crystallization using metal catalyst, is also well-known method to synthesize semiconductor NWs [19,20]. The ternary NWs grown via VLS method, however, are suffered from large variation of the alloy composition along the NW height [4,21]. For example, the In composition of the ternary InxGa1 xAs NW varies from 0.2 to 0.6 with NW position [4]. Formation of ternary InxGa1 xAs NW as a result of gallium (Ga) diffusion from GaAs substrate have shown relatively uniform alloy composition along the NW height [22]. However, the tunable range of the alloy composition is very limited (i.e., x¼0.81–1). In this paper, we have investigated the distribution of In atoms in the ternary InxGa1 xAs NWs grown at different process parameters such as growth temperature and V/III ratio, aiming to minimize the composition variation of the NW. In addition to the VLS, we have found that vapor–solid (VS) growth mechanism played important role for the composition variation along the NW heights. The VS growth mechanism is nearly deactivated with the decrease of growth temperature and V/III ratio, resulting in
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J.C. Shin et al. / Journal of Crystal Growth 372 (2013) 15–18
single-crystalline, ternary InxGa1 xAs NWs possessing very uniform alloy composition along the NW height.
2. Experimental methods Metal-organic chemical vapor deposition (MOCVD, Aixtron inc.) with horizontal reactor has been used for the growth of InxGa1 xAs NWs. For the growth, p-type Si (111) wafer were immersed in polyL-lysine (PLL) solution (Sigma-Aldrich inc.) for 2 min, rinsed in deionized water for 10 s and dried with N2 gun. Thin PLL layer which is positively charged promotes the uniform distribution of Au nanoparticles (NPs) on Si surface by preventing the agglomeration of negatively charged Au NPs [23]. Afterwards, Au NP colloid (Ted pella inc.) containing Au NPs of 20 nm diameter was dropped on the Si surface and blown by N2 gun after 30 s. Then, the Si substrate was immediately loaded into the MOCVD chamber and annealed at 620 1C for 10 min in the H2 ambient to form eutectic alloy at the interface between Au colloids and Si. After that, the reactor temperature was reduced to the growth temperature (i.e., 430–500 1C). Once temperature was stabilized, Trimethylindium [(CH3)3In, TMIn] and trimethylgallium [(CH3)3Ga, TMGa] and AsH3 sources were simultaneously supplied into the reactor. The growth time was fixed to 10 min for the growth of the nanowires in this experiment. The molar flow rate (mol/min) was 1.3 10 5, 2.4 10 5 and 4 10 4 for TMIn, TMGa, and AsH3, respectively. The In molar ratio [i.e., TMIn/ (TMInþTMGa)] was 0.36, and the V/III [i.e., AsH3/(TMInþTMGa)] was 10. The molar flow rate of AsH3 increases to 1.1 10 3 mol/ min to change the V/III ratio to 30. Structural properties of the InxGa1 xAs NWs were examined by scanning electron microscope (SEM, Hitachi inc.) and transmission electron microscope (TEM, FEI inc.). The compositional variation along the NW height was investigated by energy-dispersive X-ray spectroscopy (EDX) equipped in TEM. The error range of alloy composition is less than 10% and the Xray spot size for EDX was 0.1 nm. In composition, x of the InxGa1 xAs NW was calculated from the atomic ratio of EDX spectra.
3. Results and discussion Fig. 1(a–c) shows the SEM images of ternary InxGa1 xAs NWs on Si (111) substrate grown at 440, 470, and 500 1C, respectively. One can see that most InxGa1 xAs NWs were vertically grown on the substrate. The number density of the NWs is in the range of 1–5 107/cm2 while that of the Au nanoparticles is 2 109/cm2. The surface of Si is known to be easily oxidized upon exposure to air. The de-oxidation process performed by a thermal annealing at 620 1C under H2 ambient was found to be an important step to have dense and vertically aligned InxGa1 xAs NWs on Si substrates. Without thermal annealing process, the number density of the NWs significantly decreases. Insets in Fig. 1 are high-magnification SEM images of the representative NWs. The InxGa1 xAs NWs
grown at 440 1C have straight morphology, which is one of typical characteristics of VLS mechanism (i.e., 1-D growth). However, increase of growth temperature causes InxGa1 xAs NWs to taper off towards the tips with a simultaneous decrease of the NW length. The growth rate of the nanowires in height is 0.46, 0.36, and 0.22 mm/min at the growth temperature of 440, 470, and 500 1C, respectively. The tapering-off of InxGa1 xAs NWs is a result of crystallization of In, Ga, and arsenic (As) species on the sidewall of NWs via so-called vapor–solid (VS) mechanism [24]. Paiano et al. reported that the GaAs NW growth in the axial direction via VLS mechanism is thermally activated with an activation energy of 1771.0 kcal/mol [25], which is slightly lower than 20.773.2 kcal/ mol for low-temperature planar growth of GaAs via VS mechanism from TMGa and AsH3 [26]. In other words, the lateral growth of GaAs via VS mechanism becomes dominant at higher temperature region because of higher activation energy. Similar tapering-off has been observed in both elemental Si [24,27] and binary NWs (e.g., InAs or GaAs) grown at higher growth temperature [26,28]. In these tapered-off elemental and binary NWs, the composition along the axial direction should be constant irrespectively of different mechanisms for the axial and lateral NW growths. In alloyed ternary/quaternary NWs, however, this tapering phenomenon might have a critical impact on the uniformity of constituent elements of NWs and thus their corresponding bandgap energies. Thus, we characterize atomic distribution of InxGa1 xAs NWs using EDX analysis. The NWs were grown at different temperatures while other growth parameters are fixed (i.e., V/III ratio¼10). InxGa1 xAs NWs are divided into three different areas for EDX measurements (Fig. 2a); base (area 1), center (area 2), and tip (area 3). EDX measurements were performed more than 10 points in each area to get averaged In composition along the NW height. As seen in the Fig. 2(b), the In composition varies from 0.6 to 0.3 along the NW height for the NW grown at 500 1C. Similarly, In composition changes from 0.6 to 0.4 for the NW grown at 470 1C. In contrast, the composition variation is less than 10% (i.e., from 0.67 to 0.6) for the NW grown at 440 1C. The NW has very uniform diameter along the NW height, indicating that the sidewall growth via VS mode is nearly minimized at the growth temperature of 440 1C. The EDX analysis shows that the composition variation is strongly related to the tapered shape of the NW. Taking into account activation energies needed for the lateral growth of InAs and GaAs, it were reported that the activation for the growth of InAs thin film is approximately 10 kcal/mol, which is much lower than 19.7 kcal/mol for GaAs [29,30]. As a result, more In atoms are incorporated into the sidewall of NWs, leading to higher In composition at the NW shell than in the NW core. It is interesting to note that the In composition of the nanowires is much higher than the molar ratio of the gas phase [i.e., TMIn/(TMInþTMGa)] during the growth. This can be explained that the pyrolysis efficiency of TMIn is much higher than that of the TMGa in the growth temperature range of 400–500 1C [31]. We also have investigated the effects of the V/III ratio on the change of NW morphologies as shown in Fig. 3. The NWs were
Fig. 1. SEM images of ternary InxGa1 xAs NWs grown at: (a) 440, (b) 470, and (c) 500 1C, respectively. Insets in the figure are the high magnification SEM image of the NWs with 500 nm of scale bar. The NWs are more tapered as the growth temperature increases indicating that more sidewall growth is activated.
J.C. Shin et al. / Journal of Crystal Growth 372 (2013) 15–18
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Fig. 2. (a) Low magnification TEM image of InxGa1 xAs NWs grown at 440, 470, and 500 1C with fixed V/III ratio. The area 1, 2, and 3 indicate where EDX analysis has been performed. The atomic concentrations of In and Ga have been measured more than 10 points in each area and then the averaged In composition from atomic ratio is plotted in (b). The scale bar in (a) represents 0.5 mm.
Fig. 3. (a) Low resolution TEM image of the ternary InxGa1 xAs NWs. The V/III ratio for the NW is changed as labeled while the growth temperature of the NWs is fixed value of 470 1C. The area 1, 2, and 3 indicate where EDX analysis has been performed. The atomic ratio of In and Ga has been measured more than 10 points in each area and the calculated In composition from atomic ratio is plotted in (b). The scale bar in (a) represents 0.5 mm.
grown at different V/III ratios (i.e., V/III ¼10 and 30) but the growth temperature was fixed to 440 1C. The tapering angle of the ternary InxGa1 xAs NW increases with V/III ratio as seen in Fig. 3(a). The average height of the NW array decreases and the diameter of the NW increases for the higher V/III ratio (i.e., V/ III ¼30), indicating more sidewall growth for the higher V/III ratio. The increase of group V may hinder the migration of the In and Ga species into the NW tip, thus can facilitate the crystallization of the NW sidewall. The In composition of the NW grown at V/III ratio of 30 changes from 0.6 to 0.3. Similar to the results shown in Fig. 1, more tapered NW have larger composition variation along the NW height. For further investigation about the relation between tapering shape and composition, we have measured the In composition along the lateral direction of the NW. The NW shown in Fig. 4 has been grown at 440 1C with V/III ratio of 30. The NWs are divided into the three different areas for the EDX measurement (Fig. 4a); shell (area 1), core (area 2), and shell (area 3). The EDX measurements have been performed more than 3 points in each area and calculated the average values. The In composition near the NW sidewall (i.e., shell) is higher than that in the core region (i.e., core: x ¼0.54, shell: x¼0.65). The results clearly demonstrate that the NW shell grown via VS mode has more In incorporation than NW core grown via VLS mode. Base on all the observations and characterization, the growth of the Au catalyzed ternary InxGa1 xAs NWs is illustrated in Fig. 5. VLS are well known mechanism which facilitates the onedimensional (1-D) crystal growth of the semiconductor materials [19]. Metal catalyst (e.g., Au) on a semiconductor substrate forms liquid alloy at a temperature above eutectic point. The catalytic liquid alloy adsorbs reactants and reaches supersaturation point. Subsequently, crystal growth occurs at the metal-semiconductor interface towards o1114 where possesses the lowest surface
energy for most semiconductor materials [32]. Fig. 5 shows the schematic illustration of the crystal growth for the ternary InxGa1 xAs NW on Si (111) substrate. VLS mode are facilitated by reactants which are diffused from the surface or directly adsorbed onto the Au nano-particle from vapor near metal catalyst [33]. In addition to the VLS, VS growth mechanism is illustrated, which is strongly related to the tapering shape and composition variation of the ternary NWs. VS growth is a crystallization occurring through direct adsorption of a gas phase onto a solid surface [24]. The schematic illustration in Fig. 5 shows that Ga, In, and As adatoms are incorporated directly on the NW sidewall before reaching metal-semiconductor interface. As the growth temperature and V/III ratio increases, more adsorption occurs at the NW sidewalls leading to tapering-off of NWs. Low temperature growth, however, can increase an incorporation of native defects and foreign impurities, resulting in degradation of optical properties. A recent study presents that stronger photoluminescence peak intensity is observed from the VLS-grown GaAs NWs grown at 500 1C than from those grown at lower (i.e., 410–470 1C) or higher temperature (i.e., 530–580 1C) [34]. Therefore, an optimized growth temperature is necessary for a trade-off between raising crystalline quality and minimizing sidewall growth.
4. Conclusion In summary, we have investigated the effects of growth temperature and V/III ratio on the morphology and group-III composition of ternary InxGa1 xAs NWs by using EDX and TEM analysis. The VS growth mode is strongly related to the In composition variation along the NW height as well as tapered shape of the NWs. By optimizing the growth parameters, we have demonstrated the single crystalline ternary InxGa1 xAs NWs via
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Fig. 4. (a) Low resolution TEM image of the ternary InxGa1 xAs NWs grown at 440 1C with V/III ratio of 30. (b) The number 1, 2, and 3 indicate where EDX analysis has been performed. The atomic composition of In and Ga was measured more than 3 times at each points, from which the In composition was determined and plotted in (b).
Fig. 5. Schematic illustration of the ternary InxGa1 xAs growth using MOCVD. The growth rate of the sidewall is related to the VS mechanism which is activated by the growth temperature. The amount of adatoms is created by decomposition of the MO precursors near NW sidewall. The higher decomposition rate of In than Ga leads to low In composition at the sidewall.
VLS growth mode. The In composition of the NWs is very uniform along the NW height confirmed by EDX analysis. The establishment of tunable InxGa1 xAs NW array in energy could be applicable to the advanced optical devices in the near- and mid-infrared region.
Acknowledgment This research was supported by Future-based Technology Development Program (Nano Fields, grant number: 2010-0029300) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology. Rerefences [1] B. Tian, X. Zheng, T.J. Kempa, Y. Fang, N. Yu, G. Yu, J. Huang, C.M. Lieber, Nature 449 (2007) 885–889. [2] K. Haraguchi, T. Katsuyama, K. Hiruma, K. Ogawa, Applied Physics Letters 60 (1992) 745–747. [3] J.N. Shapiro, A. Lin, P.S. Wong, A.C. Scofield, C. Tu, P.N. Senanayake, G. Mariani, B.L. Liang, D.L. Huffaker, Applied Physics Letters 97 (2010) 243102.
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Contents lists available at SciVerse ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Improving the composition uniformity of Au-catalyzed InGaAs nanowires on silicon Jae Cheol Shin a, Do Yang Kim b,c, Ari Lee a, Hyo Jin Kim a, Jae Hun Kim c, Won Jun Choi c, Hyun-Seok Kim d, Kyoung Jin Choi b,n a
Korea Photonics Technology Institute, Gwangju 500-779, South Korea School of Mechanical and Advanced Materials Engineering, Ulsan National Institute of Science & Technology (UNIST), Ulsan 689-805, South Korea c Korea Institute of Science and Technology (KIST), Seoul 136-791, South Korea d Electronics and Electrical Engineering, Dongguk University, Seoul 100-715, South Korea b
a r t i c l e i n f o
abstract
Article history: Received 26 December 2012 Received in revised form 19 February 2013 Accepted 23 February 2013 Communicated by K. Deppert Available online 4 March 2013
Spatial distribution of indium (In) atoms in ternary InxGa1 xAs nanowires (NWs) was investigated by the energy-dispersive X-ray spectroscopy, which were grown on Si (111) by metal-organic chemical vapor deposition. The NWs have a tapered morphology with thicker diameter and higher In composition in the bottom of NWs. However, decreasing growth temperature and V/III ratio resulted in straight NWs with constant In composition throughout the NWs. This was attributed to enhanced deposition on the sidewall of the NW with higher In composition through the vapor–solid mode, leading to a core-shell structure consisting of low and high In-content layers. & 2013 Elsevier B.V. All rights reserved.
Keywords: A1. Nanostructures A3. Metalorganic vapor phase epitaxy B1. Nanomaterials B2. Semiconducting III–V materials
1. Introduction One-dimensional (1-D) nanostructures such as nanorods, nanowires (NWs), and nanobelts have been successfully synthesized using a wide range of semiconductors and demonstrated new design concepts for novel electronic and optoelectronic devices. Especially, hetero-structured 1-D nanostructures, including coaxial core-shell, axially-modulated, and alloyed NWs, have attracted much attention because of their controlled morphologies and multi-functional optoelectronic properties [1–4]. Among them, alloyed semiconductor NWs can offer more unique properties than the corresponding elemental or binary ones by engineering the bandgap energy, which is one of the most important parameters of a semiconductor and determines its electronic and optical properties [5–8]. III–V compound semiconductor NWs based on binary materials (e.g., GaAs, InP, GaN) have been fabricated for optical devices such as solar-cells and lightemitting-diodes [9,10]. For example, the ternary InxGa1 xAs can cover the wavelength range from near- to mid-infrared (0.87– 387–3.5 mm) by adjusting indium (In) composition, as demonstrated by the heterojunction solar cells [8] and light-emitting diodes (LEDs) for medical applications [11]. Several recent works had reported the successful growths of high quality III-V
n
Corresponding author. E-mail address: [email protected] (K.J. Choi).
0022-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2013.02.025
semiconductor NWs by using selective-area epitaxy (SAE) in metal-organic chemical vapor deposition (MOCVD) reactor [12–14]. In the SAE-grown NWs, the growths were defined by the openings formed by various lithography methods resulting in highly-order NWs [12–14]. The fabrication of these patterns had been reported by using electron-beam lithography, and other self-assembled lithography methods (i.e. 2-D close-packed monolayer colloidal deposition [15,16], or diblock-copolymer methods [17,18]). Vapor–liquid–solid (VLS) method, which facilitates onedimensional (1-D) crystallization using metal catalyst, is also well-known method to synthesize semiconductor NWs [19,20]. The ternary NWs grown via VLS method, however, are suffered from large variation of the alloy composition along the NW height [4,21]. For example, the In composition of the ternary InxGa1 xAs NW varies from 0.2 to 0.6 with NW position [4]. Formation of ternary InxGa1 xAs NW as a result of gallium (Ga) diffusion from GaAs substrate have shown relatively uniform alloy composition along the NW height [22]. However, the tunable range of the alloy composition is very limited (i.e., x¼0.81–1). In this paper, we have investigated the distribution of In atoms in the ternary InxGa1 xAs NWs grown at different process parameters such as growth temperature and V/III ratio, aiming to minimize the composition variation of the NW. In addition to the VLS, we have found that vapor–solid (VS) growth mechanism played important role for the composition variation along the NW heights. The VS growth mechanism is nearly deactivated with the decrease of growth temperature and V/III ratio, resulting in
16
J.C. Shin et al. / Journal of Crystal Growth 372 (2013) 15–18
single-crystalline, ternary InxGa1 xAs NWs possessing very uniform alloy composition along the NW height.
2. Experimental methods Metal-organic chemical vapor deposition (MOCVD, Aixtron inc.) with horizontal reactor has been used for the growth of InxGa1 xAs NWs. For the growth, p-type Si (111) wafer were immersed in polyL-lysine (PLL) solution (Sigma-Aldrich inc.) for 2 min, rinsed in deionized water for 10 s and dried with N2 gun. Thin PLL layer which is positively charged promotes the uniform distribution of Au nanoparticles (NPs) on Si surface by preventing the agglomeration of negatively charged Au NPs [23]. Afterwards, Au NP colloid (Ted pella inc.) containing Au NPs of 20 nm diameter was dropped on the Si surface and blown by N2 gun after 30 s. Then, the Si substrate was immediately loaded into the MOCVD chamber and annealed at 620 1C for 10 min in the H2 ambient to form eutectic alloy at the interface between Au colloids and Si. After that, the reactor temperature was reduced to the growth temperature (i.e., 430–500 1C). Once temperature was stabilized, Trimethylindium [(CH3)3In, TMIn] and trimethylgallium [(CH3)3Ga, TMGa] and AsH3 sources were simultaneously supplied into the reactor. The growth time was fixed to 10 min for the growth of the nanowires in this experiment. The molar flow rate (mol/min) was 1.3 10 5, 2.4 10 5 and 4 10 4 for TMIn, TMGa, and AsH3, respectively. The In molar ratio [i.e., TMIn/ (TMInþTMGa)] was 0.36, and the V/III [i.e., AsH3/(TMInþTMGa)] was 10. The molar flow rate of AsH3 increases to 1.1 10 3 mol/ min to change the V/III ratio to 30. Structural properties of the InxGa1 xAs NWs were examined by scanning electron microscope (SEM, Hitachi inc.) and transmission electron microscope (TEM, FEI inc.). The compositional variation along the NW height was investigated by energy-dispersive X-ray spectroscopy (EDX) equipped in TEM. The error range of alloy composition is less than 10% and the Xray spot size for EDX was 0.1 nm. In composition, x of the InxGa1 xAs NW was calculated from the atomic ratio of EDX spectra.
3. Results and discussion Fig. 1(a–c) shows the SEM images of ternary InxGa1 xAs NWs on Si (111) substrate grown at 440, 470, and 500 1C, respectively. One can see that most InxGa1 xAs NWs were vertically grown on the substrate. The number density of the NWs is in the range of 1–5 107/cm2 while that of the Au nanoparticles is 2 109/cm2. The surface of Si is known to be easily oxidized upon exposure to air. The de-oxidation process performed by a thermal annealing at 620 1C under H2 ambient was found to be an important step to have dense and vertically aligned InxGa1 xAs NWs on Si substrates. Without thermal annealing process, the number density of the NWs significantly decreases. Insets in Fig. 1 are high-magnification SEM images of the representative NWs. The InxGa1 xAs NWs
grown at 440 1C have straight morphology, which is one of typical characteristics of VLS mechanism (i.e., 1-D growth). However, increase of growth temperature causes InxGa1 xAs NWs to taper off towards the tips with a simultaneous decrease of the NW length. The growth rate of the nanowires in height is 0.46, 0.36, and 0.22 mm/min at the growth temperature of 440, 470, and 500 1C, respectively. The tapering-off of InxGa1 xAs NWs is a result of crystallization of In, Ga, and arsenic (As) species on the sidewall of NWs via so-called vapor–solid (VS) mechanism [24]. Paiano et al. reported that the GaAs NW growth in the axial direction via VLS mechanism is thermally activated with an activation energy of 1771.0 kcal/mol [25], which is slightly lower than 20.773.2 kcal/ mol for low-temperature planar growth of GaAs via VS mechanism from TMGa and AsH3 [26]. In other words, the lateral growth of GaAs via VS mechanism becomes dominant at higher temperature region because of higher activation energy. Similar tapering-off has been observed in both elemental Si [24,27] and binary NWs (e.g., InAs or GaAs) grown at higher growth temperature [26,28]. In these tapered-off elemental and binary NWs, the composition along the axial direction should be constant irrespectively of different mechanisms for the axial and lateral NW growths. In alloyed ternary/quaternary NWs, however, this tapering phenomenon might have a critical impact on the uniformity of constituent elements of NWs and thus their corresponding bandgap energies. Thus, we characterize atomic distribution of InxGa1 xAs NWs using EDX analysis. The NWs were grown at different temperatures while other growth parameters are fixed (i.e., V/III ratio¼10). InxGa1 xAs NWs are divided into three different areas for EDX measurements (Fig. 2a); base (area 1), center (area 2), and tip (area 3). EDX measurements were performed more than 10 points in each area to get averaged In composition along the NW height. As seen in the Fig. 2(b), the In composition varies from 0.6 to 0.3 along the NW height for the NW grown at 500 1C. Similarly, In composition changes from 0.6 to 0.4 for the NW grown at 470 1C. In contrast, the composition variation is less than 10% (i.e., from 0.67 to 0.6) for the NW grown at 440 1C. The NW has very uniform diameter along the NW height, indicating that the sidewall growth via VS mode is nearly minimized at the growth temperature of 440 1C. The EDX analysis shows that the composition variation is strongly related to the tapered shape of the NW. Taking into account activation energies needed for the lateral growth of InAs and GaAs, it were reported that the activation for the growth of InAs thin film is approximately 10 kcal/mol, which is much lower than 19.7 kcal/mol for GaAs [29,30]. As a result, more In atoms are incorporated into the sidewall of NWs, leading to higher In composition at the NW shell than in the NW core. It is interesting to note that the In composition of the nanowires is much higher than the molar ratio of the gas phase [i.e., TMIn/(TMInþTMGa)] during the growth. This can be explained that the pyrolysis efficiency of TMIn is much higher than that of the TMGa in the growth temperature range of 400–500 1C [31]. We also have investigated the effects of the V/III ratio on the change of NW morphologies as shown in Fig. 3. The NWs were
Fig. 1. SEM images of ternary InxGa1 xAs NWs grown at: (a) 440, (b) 470, and (c) 500 1C, respectively. Insets in the figure are the high magnification SEM image of the NWs with 500 nm of scale bar. The NWs are more tapered as the growth temperature increases indicating that more sidewall growth is activated.
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Fig. 2. (a) Low magnification TEM image of InxGa1 xAs NWs grown at 440, 470, and 500 1C with fixed V/III ratio. The area 1, 2, and 3 indicate where EDX analysis has been performed. The atomic concentrations of In and Ga have been measured more than 10 points in each area and then the averaged In composition from atomic ratio is plotted in (b). The scale bar in (a) represents 0.5 mm.
Fig. 3. (a) Low resolution TEM image of the ternary InxGa1 xAs NWs. The V/III ratio for the NW is changed as labeled while the growth temperature of the NWs is fixed value of 470 1C. The area 1, 2, and 3 indicate where EDX analysis has been performed. The atomic ratio of In and Ga has been measured more than 10 points in each area and the calculated In composition from atomic ratio is plotted in (b). The scale bar in (a) represents 0.5 mm.
grown at different V/III ratios (i.e., V/III ¼10 and 30) but the growth temperature was fixed to 440 1C. The tapering angle of the ternary InxGa1 xAs NW increases with V/III ratio as seen in Fig. 3(a). The average height of the NW array decreases and the diameter of the NW increases for the higher V/III ratio (i.e., V/ III ¼30), indicating more sidewall growth for the higher V/III ratio. The increase of group V may hinder the migration of the In and Ga species into the NW tip, thus can facilitate the crystallization of the NW sidewall. The In composition of the NW grown at V/III ratio of 30 changes from 0.6 to 0.3. Similar to the results shown in Fig. 1, more tapered NW have larger composition variation along the NW height. For further investigation about the relation between tapering shape and composition, we have measured the In composition along the lateral direction of the NW. The NW shown in Fig. 4 has been grown at 440 1C with V/III ratio of 30. The NWs are divided into the three different areas for the EDX measurement (Fig. 4a); shell (area 1), core (area 2), and shell (area 3). The EDX measurements have been performed more than 3 points in each area and calculated the average values. The In composition near the NW sidewall (i.e., shell) is higher than that in the core region (i.e., core: x ¼0.54, shell: x¼0.65). The results clearly demonstrate that the NW shell grown via VS mode has more In incorporation than NW core grown via VLS mode. Base on all the observations and characterization, the growth of the Au catalyzed ternary InxGa1 xAs NWs is illustrated in Fig. 5. VLS are well known mechanism which facilitates the onedimensional (1-D) crystal growth of the semiconductor materials [19]. Metal catalyst (e.g., Au) on a semiconductor substrate forms liquid alloy at a temperature above eutectic point. The catalytic liquid alloy adsorbs reactants and reaches supersaturation point. Subsequently, crystal growth occurs at the metal-semiconductor interface towards o1114 where possesses the lowest surface
energy for most semiconductor materials [32]. Fig. 5 shows the schematic illustration of the crystal growth for the ternary InxGa1 xAs NW on Si (111) substrate. VLS mode are facilitated by reactants which are diffused from the surface or directly adsorbed onto the Au nano-particle from vapor near metal catalyst [33]. In addition to the VLS, VS growth mechanism is illustrated, which is strongly related to the tapering shape and composition variation of the ternary NWs. VS growth is a crystallization occurring through direct adsorption of a gas phase onto a solid surface [24]. The schematic illustration in Fig. 5 shows that Ga, In, and As adatoms are incorporated directly on the NW sidewall before reaching metal-semiconductor interface. As the growth temperature and V/III ratio increases, more adsorption occurs at the NW sidewalls leading to tapering-off of NWs. Low temperature growth, however, can increase an incorporation of native defects and foreign impurities, resulting in degradation of optical properties. A recent study presents that stronger photoluminescence peak intensity is observed from the VLS-grown GaAs NWs grown at 500 1C than from those grown at lower (i.e., 410–470 1C) or higher temperature (i.e., 530–580 1C) [34]. Therefore, an optimized growth temperature is necessary for a trade-off between raising crystalline quality and minimizing sidewall growth.
4. Conclusion In summary, we have investigated the effects of growth temperature and V/III ratio on the morphology and group-III composition of ternary InxGa1 xAs NWs by using EDX and TEM analysis. The VS growth mode is strongly related to the In composition variation along the NW height as well as tapered shape of the NWs. By optimizing the growth parameters, we have demonstrated the single crystalline ternary InxGa1 xAs NWs via
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Fig. 4. (a) Low resolution TEM image of the ternary InxGa1 xAs NWs grown at 440 1C with V/III ratio of 30. (b) The number 1, 2, and 3 indicate where EDX analysis has been performed. The atomic composition of In and Ga was measured more than 3 times at each points, from which the In composition was determined and plotted in (b).
Fig. 5. Schematic illustration of the ternary InxGa1 xAs growth using MOCVD. The growth rate of the sidewall is related to the VS mechanism which is activated by the growth temperature. The amount of adatoms is created by decomposition of the MO precursors near NW sidewall. The higher decomposition rate of In than Ga leads to low In composition at the sidewall.
VLS growth mode. The In composition of the NWs is very uniform along the NW height confirmed by EDX analysis. The establishment of tunable InxGa1 xAs NW array in energy could be applicable to the advanced optical devices in the near- and mid-infrared region.
Acknowledgment This research was supported by Future-based Technology Development Program (Nano Fields, grant number: 2010-0029300) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology. Rerefences [1] B. Tian, X. Zheng, T.J. Kempa, Y. Fang, N. Yu, G. Yu, J. Huang, C.M. Lieber, Nature 449 (2007) 885–889. [2] K. Haraguchi, T. Katsuyama, K. Hiruma, K. Ogawa, Applied Physics Letters 60 (1992) 745–747. [3] J.N. Shapiro, A. Lin, P.S. Wong, A.C. Scofield, C. Tu, P.N. Senanayake, G. Mariani, B.L. Liang, D.L. Huffaker, Applied Physics Letters 97 (2010) 243102.
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