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Material characteristics of self-assembled mushroom-like InGaN nanocolumns
Superlattices and Microstructures xxx (2016) 1e6
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Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Material characteristics of self-assembled mushroom-like InGaN nanocolumns Q.M. Chen, C.L. Yan*, Y. Qu** State Key Laboratory on High-Power Semiconductor Lasers, Changchun University of Science and Technology, Changchun 130022, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 December 2015 Received in revised form 8 March 2016 Accepted 12 March 2016 Available online xxx
The material characteristics of self-assembled mushroom-like N-polar InGaN/GaN nanowire heterostructure have been clarified, which were achieved by different In content selfassembled InGaN nanocolumns grown on self-assembled GaN nanocolumns template on (111)-silicon-substrate under N-rich condition by plasma-assist molecular beam epitaxy (PA-MBE). The In component of the InGaN nanocolumns was determined by XRD (2q-u scans). SEM has been used to study the morphology which demonstrated that the diameter of the nanocolumns became larger with higher In content. The structural properties of the individual InGaN nanocolumn were further analyzed by HAADF image, EDX and TEM. The high-In-content (85%) single mushroom-like InGaN nanocolumn showed some cracks on the sidewall, however the GaN nanocolumns showed dislocation free. The (0002) facet of the nanocolumn show very clearly hexagonal structure. It is quite clear that the formation of the mushroom-like InGaN nanocolumns comes from that the lateral epitaxy is dominating with the high In content embedded. © 2016 Elsevier Ltd. All rights reserved.
Keywords: InGaN Mushroom-like Nanocolumns Molecular beam epitaxy
1. Introduction III-Nitride semiconductor-based structures have made a great success in optoelectronic device applications in the last 2 decades, due largely to the wide direct bandgap exhibited by GaN, which was emitting blue light and achieved the white light LED. At present, the indium gallium nitride (InxGa1 xN) compound semiconductor is considered as the most promising material system because of their superior material properties and the fact that the direct band gaps of InxGa1 xN is tunable from the near-infrared (0.64 eV, InN) [1e5]to near-UV (3.4 eV, GaN). These reports suggest that high-In-content InGaN has the potential to realize high performance optoelectronic devices at the optical communication wavelength. Very recently, several groups have demonstrated the growth of InN-based heterostructures, aiming at device application [6e8]. However, the material quality of InN and high-In-content InGaN epitaxial layers was not sufficient for the heterostructure device because of the high density of threading dislocations and residual electrons. A proposed method to overcome the issue is to use semiconductor nanocrystal, since self-organized nitride nanocolumns [9,10] are almost dislocation free. This technology may bring about an improvement for the crystalline quality of InN and high-In-content InGaN. Furthermore, the nanocolumns exhibit many important advantages including low dislocation density, high electron mobility, and large saturation velocity
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (C.L. Yan), [email protected] (Y. Qu). http://dx.doi.org/10.1016/j.spmi.2016.03.018 0749-6036/© 2016 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Q.M. Chen et al., Material characteristics of self-assembled mushroom-like InGaN nanocolumns, Superlattices and Microstructures (2016), http://dx.doi.org/10.1016/j.spmi.2016.03.018
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[11]. Anisotropic epitaxial growth of free-standing GaN [12,13] and InGaN NWs [14,15] presents a key technological advancement in material structure for light-emitter technology. The catalyst-free [16e18] and spontaneous growth of (Npolar) GaN nanowires [19] based on plasma-assistant molecular beam epitaxy (PA-MBE) eliminated the use of foreign metal, such as gold catalyst, which act as non-radiative recombination center and constitute material incompatibility with silicon foundry technology. In this work, we report the growth of high-In-content InGaN nanocolumns on Si (111) substrates by PA-MBE. The tendency of becoming mushroom-like with the In content increasing was demonstrated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). We also demonstrated the hexagonal-shaped top structure of single InGaN nanocolumn by TEM.
2. Experimental procedure N-polar self-assembled GaN nanocolumns have been grown on Si (111) substrate under N-rich condition by PA-MBE. Special Knudsen cells were used for In and Ga metal sources to obtain good beam stability and a conventional rf-plasma cell was used to excite the nitrogen gases active. The n-type Si (111) substrate with a scale of 2 inch substrate was deoxidized for 40 s in (10%) HF. Then, it was mounted In-free on a sample holder and degassed at high temperature until the appearance on the reflection high-energy electron diffraction(RHEED) pattern of a clear 7 7 reconstruction, characteristic of a clean (111) Si surface. The chamber temperature was first raised to 900 C for an in situ oxide desorption, which was keeping 30mins. The initial nitridation of the substrate surface at approximately 780 C in the growth chamber, GaN dots were grown at approximately 780 C as nucleation seeds of the nanocolumns. The V/III ratio was maintained 15 and growth rate up to 100 nm h 1 was observed. When the GaN nanocolumns were grown for 1 h, the InGaN nanocolumns were grown at approximately 550 C for 2 h under a N-rich condition. We grew four InGaN samples a, b, c, d with the In component set at 30%, 35%, 44% and 85%, respectively. Fig. 1 shows a schematic diagram of InGaN/GaN nanowire structure. The height of four InGaN samples was about 200 nm. In/Ga beam flux and the nitrogen-plasma condition for depositing the GaN and InGaN nanocolumns were precisely controlled by a shutter control method so that the effective V/III ratio was as unity as possible in terms of stoichiometry, which was already confirmed to be essential to obtain good crystalline quality. The structural properties of the self-assembled InGaN/GaN nanocolumns heterostructure were studied by XRD, SEM, EDX and TEM. For the EDX and TEM studies, the samples were prepared by transferring nanowires from the Si substrates to a Cu grid with carbon supporting film. The EDX data was collected with an Oxford Instruments plc detector and analyzed with INCA software.
3. Results and discussion The In content of these four InGaN/GaN nanowire heterostructures were confirmed by XRD (0002) 2q-u measurement which was shown in Fig. 2(a). From the curve of the four samples, they all have the peak that 2q equal to 28.1 coming from ntype Silicon substrate, which was a calibration of the In content. The peak corresponding to x-axis equal to 34.5 coming from the GaN nanocolumns is very sharp, which indicated that the GaN nanocolumns were grown well. Between the Si peak and GaN peak we can see the different InGaN peaks of the four samples. From the figure we can see the In content of InGaN nanocolumns was increasing. The In content increasing tendency of four samples accompany with the 2q was shown in Fig. 2(b), which indicated the In content of the four samples a, b, c, d was 30%, 35%, 44%, 85%, respectively. In addition, from the sample d, there was a peak at 31.5 which indicated that the In content was 85%. This is evidence that the high In content InGaN self-assembled nanowires have been realized successfully. On the right of this peak, there was another low In content peak due to the phase separation of the high-In -content InGaN nanocolumns.
Fig. 1. Schematic diagram of the self-assembled InGaN/GaN nanocolumns on silicon substrate.
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Fig. 2. (a) XRD (0002) 2q-u scans of four different In content InGaN/GaN nanowire heterostructures (samples aed). (b) The In content increasing tendency of four samples accompany with the XRD-2q.
The morphology of the four InGaN/GaN nanowire heterostructures was investigated by SEM as displayed in Fig. 3. This Fig. 3 showed the cross section image of the NW heterostructure, which demonstrated the GaN ones of the four samples were about 100 nm. Furthermore, the diameter of InGaN nanocolumns on the GaN nanocolumns became larger from 50 nm of sample a to 150 nm of sample d following the increasing of In component. For each nanocolumn the diameter of InGaN was gradually larger from the bottom to the top which looked mushroom-like. The mushroom-like nanowire morphology is directly related to the enhanced lateral growth due to the reduced substrate temperature and enhanced high indium component incorporation. For high resolution transmission electron microscopy analysis, the NWs from sample were scraped off the substrate and dispersed on a Cu microscope grid covered with a holey carbon film. They were investigated by high angle annular dark field (HAADF) scanning transmission electron microscopy image in Fig. 4(a), illustrating the atomic number contrast between GaN (darker) and InGaN (brighter) regions. From the HAADF image, we can found that the InGaN nanocolumn was core-shell structure. The brighter core was InGaN from the bottom to the top, and the shell was GaN. The elemental distribution in the nanowire was analyzed by EDX in Fig. 4(b). Variations of the In La, Ga Ka signals along the nanowire axial direction (red line in panel a) has been scanned during the single nanocolumns. Fig. 4(b) demonstrated that the core structure was InGaN, and also the In content and Ga content distributed equally in the single nanocolumn except the top of the nanocolumn. The In content increased dramatically at the top of the nanocolumn. The reason of this core/shell structure is in the case of low-In-content InGaN NWs, the formation of a GaN shell thicker than in the case of high-In-content material suggests that Ga/In adatom strain-induced segregation is occurring during growth. More precisely, in the absence of plastic strain relaxation, it is suggested that InGaN deposited on the top of GaN Please cite this article in press as: Q.M. Chen et al., Material characteristics of self-assembled mushroom-like InGaN nanocolumns, Superlattices and Microstructures (2016), http://dx.doi.org/10.1016/j.spmi.2016.03.018
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Fig. 3. SEM images of the four different In content InGaN nanocolumns(a-d).
nanocolumns is relaxing elastically by adopting a pyramidal shape similar to that of an SK quantum dot [20]. Such a pyramidal shape has been recently observed for InGaN insertions embedded in GaN nanowires [21]. It is then expected that the lattice parameter will be expanded at the top of the InGaN pyramid, as a result of elastic strain relaxation and also possibly from a vertical In gradient which could be assigned to the aforementioned lattice pulling effect. As a consequence, further incorporation of In in the alloy will be easier on the top of the InGaN pyramid than that on the sidewalls. Conversely, Ga incorporation is expected to be unfavorable on the top of the InGaN pyramid, while it should be favorable at the base, which is matched to a relaxed GaN NWs. Such a segregation-induced strain relaxation process should be self-maintained all along the growth of the InGaN section, leading to the spontaneous formation of an InGaN/GaN core/shell structure. The EDX of single high-In-content InGaN nanocolumns has also been demonstrated (not shown here). The InGaN composition is then found to be homogeneous and consistent with the expected nominal value while GaN shell formation is limited to the one which results from low Ga diffusion at the low growth temperature of InGaN alloy. The resulting strain uniformity leads to homogeneous In and Ga incorporation in our structure [22].
Fig. 4. (a) STEM HAADF image of individual InGaN nanocolumns from sample b. (b) EDX of In and Ga content distribution along the axial direction.
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Fig. 5. (aeb) TEM of individual InGaN nanocolumn from sample c and d, (c) TEM of individual GaN nanocolumn template, the inset in (b) is the (0002) facet of the nanocolumn.
The structural quality and defect properties of individual nanocolumn from sample c and d were investigated by transmission electron microscopy (TEM) in Fig. 5. (a-b). From sample c, the diameter of the low-In-content InGaN nanocolumn was about 100 nm from the bottom to the top which showed pencil like. But the diameter of high-In- content InGaN nanocolumn as shown in Fig. 5(b) became larger from the bottom (100 nm) to the top (150 nm), which showed mushroom-like. In our results, it is found that the GaN nanocolumns were dislocation free in the dislocation image of HR-TEM as shown in Fig. 5(c), which indicated that it is an appropriate way to improve the crystalline quality. However, for high nominal In composition, typically above 40%, the HR-TEM images show that there were some cracks appearing on the lateral sides of the NWs. These structures defects namely cracks are the signatures plastic strain relaxation mechanisms which allow for the accommodation of the large lattice mismatch in this composition range. Even though the lattice parameters alone cannot lead to the In content determination, since they are also affected by the strain state, this observation indicates that no In clustering occurs in these nanowires [22]. Furthermore, the NWs present a flat top surface just like the inset in Fig. 5(b) is the (0002) facet of the nanocolumn, which was shown very clearly hexagonal structure. These result demonstrated that the growth of high-In-content InGaN nanocolumns was under good control and maybe a good candidate for high In component InGaN film by coalescence overgrowth. 4. Conclusion In conclusion, four different In content InGaN/GaN NWs heterostructures have been grown on Si(111) substrate by MBE. The In content of 85% has been made successfully. It has been demonstrated that depending on In content increasing, the diameter of InGaN nanocolumns became larger resulting in the mushroom-like nanocolumns. This was related to the enhanced lateral growth due to the reduced substrate temperature and enhanced high indium component incorporation. The individual InGaN nanocolumn was core-shell structure demonstrated by HAADF and EDX. In addition, the GaN nanocolumns were demonstrated dislocation free by the TEM measurement, and there were some cracks on the sidewall of InGaN NWs, when for the In content higher than 40%. Acknowledgments This research was partially supported by NSFC. Grant: U1330136, science and technology planning project of Changchun No.13KG30, National Natural Science Foundation of China Project No.61376045, and Jilin Provincial Science and Technology Department Project No.20130521014JH. The author would like to thank professor X.Q. Wang of the State Key Laboratory of Artificial Microstructure and Mesoscopic Physics, School of Physics, Peking University for providing Si templates and MBE equipment. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
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Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Material characteristics of self-assembled mushroom-like InGaN nanocolumns Q.M. Chen, C.L. Yan*, Y. Qu** State Key Laboratory on High-Power Semiconductor Lasers, Changchun University of Science and Technology, Changchun 130022, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 December 2015 Received in revised form 8 March 2016 Accepted 12 March 2016 Available online xxx
The material characteristics of self-assembled mushroom-like N-polar InGaN/GaN nanowire heterostructure have been clarified, which were achieved by different In content selfassembled InGaN nanocolumns grown on self-assembled GaN nanocolumns template on (111)-silicon-substrate under N-rich condition by plasma-assist molecular beam epitaxy (PA-MBE). The In component of the InGaN nanocolumns was determined by XRD (2q-u scans). SEM has been used to study the morphology which demonstrated that the diameter of the nanocolumns became larger with higher In content. The structural properties of the individual InGaN nanocolumn were further analyzed by HAADF image, EDX and TEM. The high-In-content (85%) single mushroom-like InGaN nanocolumn showed some cracks on the sidewall, however the GaN nanocolumns showed dislocation free. The (0002) facet of the nanocolumn show very clearly hexagonal structure. It is quite clear that the formation of the mushroom-like InGaN nanocolumns comes from that the lateral epitaxy is dominating with the high In content embedded. © 2016 Elsevier Ltd. All rights reserved.
Keywords: InGaN Mushroom-like Nanocolumns Molecular beam epitaxy
1. Introduction III-Nitride semiconductor-based structures have made a great success in optoelectronic device applications in the last 2 decades, due largely to the wide direct bandgap exhibited by GaN, which was emitting blue light and achieved the white light LED. At present, the indium gallium nitride (InxGa1 xN) compound semiconductor is considered as the most promising material system because of their superior material properties and the fact that the direct band gaps of InxGa1 xN is tunable from the near-infrared (0.64 eV, InN) [1e5]to near-UV (3.4 eV, GaN). These reports suggest that high-In-content InGaN has the potential to realize high performance optoelectronic devices at the optical communication wavelength. Very recently, several groups have demonstrated the growth of InN-based heterostructures, aiming at device application [6e8]. However, the material quality of InN and high-In-content InGaN epitaxial layers was not sufficient for the heterostructure device because of the high density of threading dislocations and residual electrons. A proposed method to overcome the issue is to use semiconductor nanocrystal, since self-organized nitride nanocolumns [9,10] are almost dislocation free. This technology may bring about an improvement for the crystalline quality of InN and high-In-content InGaN. Furthermore, the nanocolumns exhibit many important advantages including low dislocation density, high electron mobility, and large saturation velocity
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (C.L. Yan), [email protected] (Y. Qu). http://dx.doi.org/10.1016/j.spmi.2016.03.018 0749-6036/© 2016 Elsevier Ltd. All rights reserved.
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[11]. Anisotropic epitaxial growth of free-standing GaN [12,13] and InGaN NWs [14,15] presents a key technological advancement in material structure for light-emitter technology. The catalyst-free [16e18] and spontaneous growth of (Npolar) GaN nanowires [19] based on plasma-assistant molecular beam epitaxy (PA-MBE) eliminated the use of foreign metal, such as gold catalyst, which act as non-radiative recombination center and constitute material incompatibility with silicon foundry technology. In this work, we report the growth of high-In-content InGaN nanocolumns on Si (111) substrates by PA-MBE. The tendency of becoming mushroom-like with the In content increasing was demonstrated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). We also demonstrated the hexagonal-shaped top structure of single InGaN nanocolumn by TEM.
2. Experimental procedure N-polar self-assembled GaN nanocolumns have been grown on Si (111) substrate under N-rich condition by PA-MBE. Special Knudsen cells were used for In and Ga metal sources to obtain good beam stability and a conventional rf-plasma cell was used to excite the nitrogen gases active. The n-type Si (111) substrate with a scale of 2 inch substrate was deoxidized for 40 s in (10%) HF. Then, it was mounted In-free on a sample holder and degassed at high temperature until the appearance on the reflection high-energy electron diffraction(RHEED) pattern of a clear 7 7 reconstruction, characteristic of a clean (111) Si surface. The chamber temperature was first raised to 900 C for an in situ oxide desorption, which was keeping 30mins. The initial nitridation of the substrate surface at approximately 780 C in the growth chamber, GaN dots were grown at approximately 780 C as nucleation seeds of the nanocolumns. The V/III ratio was maintained 15 and growth rate up to 100 nm h 1 was observed. When the GaN nanocolumns were grown for 1 h, the InGaN nanocolumns were grown at approximately 550 C for 2 h under a N-rich condition. We grew four InGaN samples a, b, c, d with the In component set at 30%, 35%, 44% and 85%, respectively. Fig. 1 shows a schematic diagram of InGaN/GaN nanowire structure. The height of four InGaN samples was about 200 nm. In/Ga beam flux and the nitrogen-plasma condition for depositing the GaN and InGaN nanocolumns were precisely controlled by a shutter control method so that the effective V/III ratio was as unity as possible in terms of stoichiometry, which was already confirmed to be essential to obtain good crystalline quality. The structural properties of the self-assembled InGaN/GaN nanocolumns heterostructure were studied by XRD, SEM, EDX and TEM. For the EDX and TEM studies, the samples were prepared by transferring nanowires from the Si substrates to a Cu grid with carbon supporting film. The EDX data was collected with an Oxford Instruments plc detector and analyzed with INCA software.
3. Results and discussion The In content of these four InGaN/GaN nanowire heterostructures were confirmed by XRD (0002) 2q-u measurement which was shown in Fig. 2(a). From the curve of the four samples, they all have the peak that 2q equal to 28.1 coming from ntype Silicon substrate, which was a calibration of the In content. The peak corresponding to x-axis equal to 34.5 coming from the GaN nanocolumns is very sharp, which indicated that the GaN nanocolumns were grown well. Between the Si peak and GaN peak we can see the different InGaN peaks of the four samples. From the figure we can see the In content of InGaN nanocolumns was increasing. The In content increasing tendency of four samples accompany with the 2q was shown in Fig. 2(b), which indicated the In content of the four samples a, b, c, d was 30%, 35%, 44%, 85%, respectively. In addition, from the sample d, there was a peak at 31.5 which indicated that the In content was 85%. This is evidence that the high In content InGaN self-assembled nanowires have been realized successfully. On the right of this peak, there was another low In content peak due to the phase separation of the high-In -content InGaN nanocolumns.
Fig. 1. Schematic diagram of the self-assembled InGaN/GaN nanocolumns on silicon substrate.
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Fig. 2. (a) XRD (0002) 2q-u scans of four different In content InGaN/GaN nanowire heterostructures (samples aed). (b) The In content increasing tendency of four samples accompany with the XRD-2q.
The morphology of the four InGaN/GaN nanowire heterostructures was investigated by SEM as displayed in Fig. 3. This Fig. 3 showed the cross section image of the NW heterostructure, which demonstrated the GaN ones of the four samples were about 100 nm. Furthermore, the diameter of InGaN nanocolumns on the GaN nanocolumns became larger from 50 nm of sample a to 150 nm of sample d following the increasing of In component. For each nanocolumn the diameter of InGaN was gradually larger from the bottom to the top which looked mushroom-like. The mushroom-like nanowire morphology is directly related to the enhanced lateral growth due to the reduced substrate temperature and enhanced high indium component incorporation. For high resolution transmission electron microscopy analysis, the NWs from sample were scraped off the substrate and dispersed on a Cu microscope grid covered with a holey carbon film. They were investigated by high angle annular dark field (HAADF) scanning transmission electron microscopy image in Fig. 4(a), illustrating the atomic number contrast between GaN (darker) and InGaN (brighter) regions. From the HAADF image, we can found that the InGaN nanocolumn was core-shell structure. The brighter core was InGaN from the bottom to the top, and the shell was GaN. The elemental distribution in the nanowire was analyzed by EDX in Fig. 4(b). Variations of the In La, Ga Ka signals along the nanowire axial direction (red line in panel a) has been scanned during the single nanocolumns. Fig. 4(b) demonstrated that the core structure was InGaN, and also the In content and Ga content distributed equally in the single nanocolumn except the top of the nanocolumn. The In content increased dramatically at the top of the nanocolumn. The reason of this core/shell structure is in the case of low-In-content InGaN NWs, the formation of a GaN shell thicker than in the case of high-In-content material suggests that Ga/In adatom strain-induced segregation is occurring during growth. More precisely, in the absence of plastic strain relaxation, it is suggested that InGaN deposited on the top of GaN Please cite this article in press as: Q.M. Chen et al., Material characteristics of self-assembled mushroom-like InGaN nanocolumns, Superlattices and Microstructures (2016), http://dx.doi.org/10.1016/j.spmi.2016.03.018
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Fig. 3. SEM images of the four different In content InGaN nanocolumns(a-d).
nanocolumns is relaxing elastically by adopting a pyramidal shape similar to that of an SK quantum dot [20]. Such a pyramidal shape has been recently observed for InGaN insertions embedded in GaN nanowires [21]. It is then expected that the lattice parameter will be expanded at the top of the InGaN pyramid, as a result of elastic strain relaxation and also possibly from a vertical In gradient which could be assigned to the aforementioned lattice pulling effect. As a consequence, further incorporation of In in the alloy will be easier on the top of the InGaN pyramid than that on the sidewalls. Conversely, Ga incorporation is expected to be unfavorable on the top of the InGaN pyramid, while it should be favorable at the base, which is matched to a relaxed GaN NWs. Such a segregation-induced strain relaxation process should be self-maintained all along the growth of the InGaN section, leading to the spontaneous formation of an InGaN/GaN core/shell structure. The EDX of single high-In-content InGaN nanocolumns has also been demonstrated (not shown here). The InGaN composition is then found to be homogeneous and consistent with the expected nominal value while GaN shell formation is limited to the one which results from low Ga diffusion at the low growth temperature of InGaN alloy. The resulting strain uniformity leads to homogeneous In and Ga incorporation in our structure [22].
Fig. 4. (a) STEM HAADF image of individual InGaN nanocolumns from sample b. (b) EDX of In and Ga content distribution along the axial direction.
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Fig. 5. (aeb) TEM of individual InGaN nanocolumn from sample c and d, (c) TEM of individual GaN nanocolumn template, the inset in (b) is the (0002) facet of the nanocolumn.
The structural quality and defect properties of individual nanocolumn from sample c and d were investigated by transmission electron microscopy (TEM) in Fig. 5. (a-b). From sample c, the diameter of the low-In-content InGaN nanocolumn was about 100 nm from the bottom to the top which showed pencil like. But the diameter of high-In- content InGaN nanocolumn as shown in Fig. 5(b) became larger from the bottom (100 nm) to the top (150 nm), which showed mushroom-like. In our results, it is found that the GaN nanocolumns were dislocation free in the dislocation image of HR-TEM as shown in Fig. 5(c), which indicated that it is an appropriate way to improve the crystalline quality. However, for high nominal In composition, typically above 40%, the HR-TEM images show that there were some cracks appearing on the lateral sides of the NWs. These structures defects namely cracks are the signatures plastic strain relaxation mechanisms which allow for the accommodation of the large lattice mismatch in this composition range. Even though the lattice parameters alone cannot lead to the In content determination, since they are also affected by the strain state, this observation indicates that no In clustering occurs in these nanowires [22]. Furthermore, the NWs present a flat top surface just like the inset in Fig. 5(b) is the (0002) facet of the nanocolumn, which was shown very clearly hexagonal structure. These result demonstrated that the growth of high-In-content InGaN nanocolumns was under good control and maybe a good candidate for high In component InGaN film by coalescence overgrowth. 4. Conclusion In conclusion, four different In content InGaN/GaN NWs heterostructures have been grown on Si(111) substrate by MBE. The In content of 85% has been made successfully. It has been demonstrated that depending on In content increasing, the diameter of InGaN nanocolumns became larger resulting in the mushroom-like nanocolumns. This was related to the enhanced lateral growth due to the reduced substrate temperature and enhanced high indium component incorporation. The individual InGaN nanocolumn was core-shell structure demonstrated by HAADF and EDX. In addition, the GaN nanocolumns were demonstrated dislocation free by the TEM measurement, and there were some cracks on the sidewall of InGaN NWs, when for the In content higher than 40%. Acknowledgments This research was partially supported by NSFC. Grant: U1330136, science and technology planning project of Changchun No.13KG30, National Natural Science Foundation of China Project No.61376045, and Jilin Provincial Science and Technology Department Project No.20130521014JH. The author would like to thank professor X.Q. Wang of the State Key Laboratory of Artificial Microstructure and Mesoscopic Physics, School of Physics, Peking University for providing Si templates and MBE equipment. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
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Please cite this article in press as: Q.M. Chen et al., Material characteristics of self-assembled mushroom-like InGaN nanocolumns, Superlattices and Microstructures (2016), http://dx.doi.org/10.1016/j.spmi.2016.03.018