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Suppressing the lateral growth of gallium nitride nanowires by introducing hydrogen plasma
Thin Solid Films 529 (2013) 133–137
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Suppressing the lateral growth of gallium nitride nanowires by introducing hydrogen plasma Tung-Hsien Wu a, Franklin Chau-Nan Hong a, b, c, d,⁎ a
Department of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 70101, Taiwan Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan 70101, Taiwan d NCKU Research Center for Energy Technology and Strategy, National Cheng Kung University, Tainan 70101, Taiwan b c
a r t i c l e
i n f o
Available online 26 March 2012 Keywords: Plasma enhanced chemical vapor deposition Gallium nitride Nanowires Hydrogen plasma Polarity
a b s t r a c t In this study, we have found that the lateral homoepitaxial growth on GaN nanowires is suppressed by introducing hydrogen gas into the plasma-enhanced chemical vapor deposition (PECVD) apparatus for the growth of GaN nanowires. The formation of GaHx (x = 2, 3) species due to the reaction between gallium atoms and hydrogen plasma is shown to decrease the amount of excess gallium atoms adsorbed on GaN nanowire surfaces, which results in the elimination of nucleation on the nanowire surface and thus improves the surface smoothness of the nanowire. The stacked-cone nanostructures appear under low hydrogen or hydrogen-less conditions, but completely disappear under high hydrogen conditions in the PECVD system. The mechanism of the elimination of lateral growth on the nanowire surface is further proposed. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Gallium nitride is one of the most widely used materials for electronics and optoelectronics applications due to its direct energy bandgap, high electron mobility, and tunable bandgap. Gallium nitride nanowires are excellent building blocks for the applications in nanophotonics [1,2]. Since the synthesis of GaN nanowires using the reaction confined by carbon nanotubes at 1997 [3], various kinds of GaN nanostructures have been reported, such as quantum dots [4,5], nanobelts [6,7], stacked-cones [7–9], zigzag [10], and hexagonal prism-like islands [6], etc. The formation of such nanostructures is understood in terms of the formation energies of different facets and the diffusivity of adatoms on different crystal planes [6,7]. The chemical reaction for the growth of GaN nanowire is shown below: Ga þ N→GaN One would expect that the reaction should be influenced by availability of nitrogen and Ga species on the growth surface. The presence of excess Ga in the gas phase results in the homoepitaxial growth on GaN nanowires [7]. Additionally, Ga-rich conditions result in the improvement of lateral growth rate, while N-rich conditions suppress the lateral growth. In other words, the presence of excess Ga adatoms on nanowire surfaces is one of the necessary conditions to develop
the laterally grown nanostructures. Consequently, we can control the lateral homoepitaxial growth rate by tuning the concentration of Ga adatoms on the nanowire surface. The presence of hydrogen in the gas phase during the growth reduces the wettability of Ga with GaN surface, and thus reduces the Ga diffusion on the GaN nanowire surface and restricts the formation of GaN nuclei, which might lead to 1-dimensinoal growth [6]. The reaction between active hydrogen atoms and Ga atoms tends to form gallium hydrides, and the free energy of gallium hydride is lower than that of elemental gallium in a temperature range between 830 and 1030 °C (the well-known temperature range adopted for the growth of GaN). In other words, the gallium species prefer to form gallium hydride (GaHx, x = 2, 3) rather than in the form of elemental gallium within this temperature range [11]. Hou and Hong also reported that the presence of small amount of hydrogen results in the formation of gallium hydride species and suppresses the surface diffusion of Ga adatoms on substrate surfaces [12]. In this study, the hydrogen plasma is introduced into the growth system to react with excess gallium atoms adsorbed on GaN nanowires, thus forming gallium hydride in the gas phase to suppress the lateral homoepitaxial growth. This reports the controlling of the amount and Ga adatoms on GaN nanowires during the growth period by introducing hydrogen plasma into the plasma-enhanced chemical vapor deposition (PECVD) apparatus. 2. Experimental details
⁎ Corresponding author at: Department of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan. Tel./fax: + 886 6 2757575 62681 218. E-mail address: [email protected] (F.C.-N. Hong). 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2012.03.052
GaN nanowires were grown in a 38 mm inner-diameter quartztube inserted in a three-temperature-zone furnace. The nitrogen
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and hydrogen plasma were generated by means of dielectric-barrier discharge using a 25 kHz high-frequency power supply (Creating Nanotechnology Inc.) [13]. The metallic Ga (99.999%, 0.148 g) placed in an alumina crucible was positioned in the central zone of the quartz tube as the Ga source, and the Si(100) substrate was placed in the downstream of the tube. Prior to the growth of GaN nanowires, a droplet of 0.01 M ethanol solution of nickel nitrate hexahydrate (Ni(NO3)2·6H2O) was dropped onto the substrate to be dried out as the catalyst for the vapor–solid–solid (VSS) growth. In order to ensure the high vaporization rate of gallium droplets, gallium was always sealed in flowing argon inside a 15 mm diameter quartz tube to prevent the nitridation of Ga droplets by active nitrogen species in N2 plasma. Gallium vapor carried by an argon flow of 250 sccm (standard cubic centimeters per minute) was introduced into the tube reactor to mix with N2 (99.9999%). The base pressure and the growth pressure were 0.13 Pa and 266.64 Pa, respectively. To investigate the effect of hydrogen on the growth of GaN nanowires, 5%, 10%, 15%, and 20% of H2 in N2 was used with a total flow rate maintained at 200 sccm. The substrate temperature was increased at a rate of 30 °C min − 1 from room temperature to the reaction temperature, 900 °C. To investigate the effect of Ga vapor pressure on the growth of GaN nanowires, the Ga temperature was increased at a rate of 30 °C min − 1 from room temperature to 850 and 900 °C. Initiation of the GaN nanowire growth is always introduced by turning the N2 Plasma on after Ga source reaching the preset temperature with a growth period fixed at 60 min. The average growth rate of the nanowires, not affected by varying the hydrogen content in the gas during growth, was about 8 μm/h. After the completion of growth, the furnace was cooled down to room temperature by ambient air at an average ramping rate of 7.5 °C/min. The samples were characterized with a variety of techniques, including scanning electron microscopy (SEM: JEOL JSM-6700 F) and transmission electronic microscopy (TEM: FEI E.O Tecnai F20 G2) with the selected area electron diffraction (SAED). SEM and TEM were operated at 10 kV and 200 kV respectively. For TEM analysis, the specimen was prepared by dispersing the GaN nanowires in
ethyl ethanol by ultrasonic agitation, and then dropping them onto the carbon film on a copper grid. Photoluminescence spectra of GaN nanowires were measured at room temperature by a spectrometer (HORIBA Jobin Yvon Triax 550) equipped with confocal microscope using a 325 nm line of a He–Cd laser as the excitation source. 3. Results Fig. 1 shows the 45°-tilting SEM images of the GaN micro-/nanostructures grown using various contents of hydrogen in the gas phase with the Ga source temperature fixed at 900 °C. Many lateral growth GaN structures were observed on the substrate, including stacked-cones, zigzag, sub-micrometer (sub-μm) rods, and nanowires. The rod-like clusters on the substrate were resulted from the aggregation of nickel nitrate during the temperature-ramping process. It is obvious that the amount of stacked-cones, zigzag, and sub-μm rods decreases as the hydrogen content in the gas phase increases. Since the density of lateral-grown GaN nanowires decreased as the active hydrogen content increases, the introduction of hydrogen plasma resulted in decrease of lateral growth on the GaN nanowires surfaces. The mechanism for the elimination of lateral growth on nanowire surface will be discussed in the following section. The 45°-tilting SEM images in Fig. 2 show the GaN nanowires grown under different contents of hydrogen in the gas phase with the Ga source temperature kept at 850 °C. When the hydrogen content was below 10%, as shown in Fig. 2-(a), a few stacked-cone nanostructures appeared on the substrate, while smooth and straight GaN nanowires were obtained as the hydrogen content was higher than 10% (Fig. 2-(b) and (d)). In addition, the low Ga vapor pressure in the gas phase, controlled by lowering the Ga source temperature, would result in straight GaN nanowires without lateral growths. The average growth rate of nanowires, not varied with the hydrogen content in the gas phase, was around 8 μm/h. Fig. 3 shows the crosssectional SEM image of the interface between Si(100) substrate and a GaN nanowires grown under 5% H2. The well bonded interface between substrate and nanowires is utterly important for device
Fig. 1. SEM images of the GaN nanowires grown under various contents of hydrogen in the gas phase with the Ga source temperature fixed at 900 °C. The hydrogen contents in the gas phase were (a) 5%, (b) 10%, (c) 15% and (d) 20%, respectively.
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Fig. 2. The 45°-tilting SEM images of GaN nanowires grown under various contents of hydrogen in the gas phase with the Ga source temperature fixed at 850 °C. The hydrogen contents in the gas phase were (a) 5%, (b) 10%, (c) 15%, and (d) 20%, respectively. Stacked–cone nanostructures appeared in (a) and (b), and smooth and straight GaN nanowires appeared in (c) and 2(d).
fabrication. Fig. 4 shows the room-temperature continuous-wave photoluminescence spectrum of GaN nanowires grown under 20% H2. The photoluminescence spectra of GaN nanowires grown under other hydrogen contents also showed similar result. The main peak at 360 nm was attributed to the near-band-edge emission [21]. The peak at 650 nm and around 720 nm indicated the second order emission of He–Cd CW laser and GaN, respectively. Besides, the broad yellow luminescence around 550 nm was not obvious at all in our PL spectra, indicating the high crystal quality of GaN nanowires. Fig. 5 presents the TEM images and the corresponding SAED patterns of two GaN nanostructures. The insets at the upper and the lower right corners in Fig. 5 show the low-magnification TEM images and the corresponding SAED patterns, respectively. High-resolution TEM (HRTEM) image and the corresponding selected-area electron diffraction pattern shown in Fig. 5(a) confirmed that the straight nanowires
observed in the HRTEM images, indicating the high crystal quality of nanowires. The adjacent lattice distance was ~0.276 nm along the growth axis, which corresponded to the d-spacing between the neighboring 1010 planes of wurtzite GaN, and further confirmed the b1010 > crystallographic growth direction of the nanowires. HRTEM image and the corresponding SAED patterns shown in Fig. 5 (b) confirmed that the nanowires with stacked–cone structure were single-crystalline wurtzite structure grown along the b0001> orientation. The adjacent lattice distance was ~ 0.517 nm along the growth axis, which corresponded to the d-spacing between neighboring (0001) planes of wurtzite GaN, and further confirmed the b0001> crystallographic growth direction of the nanowires. Besides, the pyran o midal structure was enclosed by {0001} basal planes and six 1122
were single-crystalline wurtzite structure grown along b1010 > orientation. No defects, stacking faults and amorphous shells were
Fig. 3. The cross-sectional SEM image of the interface between Si(100) substrate and a GaN nanowires grown under 5% H2.
Fig. 4. Room-temperature continuous-wave photoluminescence spectrum of GaN nanowire grown under 20% H2.
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Fig. 5. The bright-field HRTEM images and the corresponding SAED patterns of two GaN nanostructures. (a) The inset at the upper right corner shows the low-magnification TEM image of a straight GaN nanowire. The inset at the lower right corner is the corresponding SAED pattern of the nanowire recorded along [0001]zone axis. (b) The inset at theh upperi right corner shows the low-magnification TEM image of a GaN nanowire with stacked–cone structure close to the tip of the nanowire. The SAED pattern is recorded, along 0110 zone axis, at the red circle marked in the low-magnification image.
sidewalls. Low-magnification TEM images show that each GaN nanostructure was terminated with a faceted nickel nanoparticle at the tip, indicating the growth of GaN nanowires via catalyst-induced VSS mechanism [14,15]. The results that stacked–cone nanowires terminated with a faceted nickel nanoparticle at the tip are different from those reported by Cai et al. [7], as the authors pointed out that the stacked–cone nanowires grew out from the catalytic droplet on the substrate. The GaN nanowires grown along b1010 > and b0001> directions n o tend to have {0001} and 1120 planes as sidefaces, respectively. The mobility of Ga adatoms on different crystallographic planes is affected by the difference in binding strength [16]. According to the density of n o dangling bonds of {0001} and 1120 planes, 11.4 and 14.0 nm − 2, respectively [17], the mobility of Ga adatoms on {0001} planes is n o higher than that on 1120 planes. This phenomenon makes the nun o cleation on 1120 facets easier than that on {0001} facets, and leads n o to the lateral growth structure and the smooth surface on the 1120 and the {0001} facets, respectively. Hou and Hong reported that the introduction of a small amount of hydrogen into the growth system resulted in the formation of gallium
hydride species (GaHx , x = 2, 3) which did not prefer to adsorb on the substrate surface and tended to desorb from the surface [12]. Additionally, the free energy of gallium hydride is lower than that of elemental gallium at 850 and 900 °C in our PECVD system [11]. Therefore, the gallium species prefer to form gallium hydride rather than elemental gallium in this temperature range. Fig. 6 shows the mechanism we proposed for explaining the elimination of lateral growth of GaN nanowires by introducing hydrogen into the PECVD apparatus. In summary, we suggest that the active hydrogen species may react with the Ga atoms in the gas phase and the Ga atoms adsorbed on the GaN nanowire surface forming gallium hydride species. Such reactions lead to the decrease of the concentration of Ga adatoms on the nanowire surface, thus eliminating the lateral growth. With the hydrogen content below 10%, the lateral growth starts to occur due to the excess Ga atoms on the nanowire surface, more likely for a high Ga vapor pressure. With the hydrogen content above 10%, the lateral growth of nanowires will be eliminated particularly at a low Ga vapor pressure. Several groups have reported that hydrogen can etch GaN via Ga– H and N–H formation at elevated temperature [18,19]. Compared with GaN low index crystallographic planes with a low density of dangling bonds, the high index crystallographic planes with a high density of dangling bonds are easier to react with hydrogen [20]. n o Since the density of dangling bonds on 1122 planes (17.8 nm − 2) n o is higher than that on 1120 planes (14 nm − 2), hydrogen atoms n o react with the surface Ga atoms more easily on the 1122 planes at a high temperature, thus converting them into the thermodynamn o ically more stable 1120 planes, which are the surfaces of smooth GaN nanowires. The lateral growth nanostructures on the nanowire can thus be etched away by hydrogen revealing the smooth nanowire n o surfaces, 1120 .
Fig. 6. The growth model of GaN nanowire under hydrogen plasma: Hydrogen atoms react with the adsorbed Ga atoms on the GaN nanowire surface to form GaHx (x = 2, 3) species (purple arrow), which tend to desorb from the surface (red arrow).
To investigate the effect of hydrogen addition in the plasma on the lateral growth during the growth of GaN nanowires, we grew GaN nanowires under 20% hydrogen content in the first 30 min and under 0% hydrogen for the next 30 min by maintaining the Ga source temperature at 850 °C. During growth period, the total flow rate was fixed at 200 sccm. The 45°-tilting SEM images in Fig. 7 show that the nanostructures due to lateral growth appeared only appeared close to the tips of GaN nanowires.
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Fig. 7. FE-SEM images obtained by tilting the samples 45° with respect to the normal of sample showing the morphologies of the GaN nanowires on Si(100) substrates grown using 20% H2 in the first 30 min and 0% H2 in the next 30 min.
4. Conclusions In summary, we controlled the behavior of the lateral growth on the GaN nanowire surface by introducing hydrogen into the PECVD apparatus. Due to different properties of various crystallographic planes, GaN nanowires grown along b0001 > and b1010 > directions developed the stacked–cone and straight nanowires structures, respectively. Moreover, the stacked–cone structures appeared under low hydrogen or hydrogen-less conditions but completely disappeared under high hydrogen concentrations in the PECVD system. The mechanism for the elimination of lateral growth by hydrogen addition is due to the formation of gallium hydrides (GaHx, x = 2, 3) for decreasing the excess Ga adatoms on the nanowire surface as well as the etching of high index planes into the thermodynamically stable low index planes, which are the surface of smooth nanowires. Acknowledgments The authors gratefully acknowledge the financial support from the National Science Council of Taiwan under grant NSC-100-2221-E006-147 and NSC-99-2221-E-006-197-MY3, the Aim for the Top University Project from NCKU and the Central Taiwan Science Park under grant number: 302202501. References [1] W. Guo, M. Zhang, A. Banerjee, P. Bhattacharya, Nano Lett. 10 (2010) 3355. [2] R. Armitage, K. Tsubaki, Nanotechnology 21 (2010) 7. [3] W.Q. Han, S.S. Fan, Q.Q. Li, Y.D. Hu, Science 277 (1997) 1287.
[4] M. Zhang, P. Bhattacharya, W. Guo, Appl. Phys. Lett. 97 (2010) 011103. [5] M. Zhang, A. Banerjee, C.-S. Lee, J.M. Hinckley, P. Bhattacharya, Appl. Phys. Lett. 98 (2011) 221104. [6] H.W. Li, A.H. Chin, M.K. Sunkara, Adv. Mater. 18 (2006) 216. [7] X.M. Cai, A.B. Djurisic, M.H. Xie, C.S. Chiu, S. Gwo, Appl. Phys. Lett. 87 (2005) 183103. [8] H.Y. Peng, N. Wang, X.T. Zhou, Y.F. Zheng, C.S. Lee, S.T. Lee, Chem. Phys. Lett. 359 (2002) 241. [9] S.Y. Bae, H.W. Seo, D.S. Han, M.S. Park, W.S. Jang, C.W. Na, J. Park, C.S. Park, J. Cryst. Growth 258 (2003) 296. [10] X.T. Zhou, T.K. Sham, Y.Y. Shan, X.F. Duan, S.T. Lee, R.A. Rosenberg, J. Appl. Phys. 97 (2005) 104315. [11] F. Kawamura, M. Imade, M. Yoshimura, Y. Mori, T. Sasaki, Jpn. J. Appl. Phys. 44 (2005) L1. [12] W.C. Hou, F.C.-N. Hong, Nanotechnology 20 (2009) 055606. [13] W.C. Hou, L.Y. Chen, F.C.N. Hong, Diam. Relat. Mater. 17 (2008) 1780. [14] W.-C. Hou, L.-Y. Chen, W.-C. Tang, F.C.N. Hong, Cryst. Growth Des. 11 (2011) 990. [15] X. Weng, R.A. Burke, J.M. Redwing, Nanotechnology 20 (2009) 085610. [16] P. Ghekiere, S. Mahieu, G. De Winter, R. De Gryse, D. Depla, Thin Solid Films 493 (2005) 129. [17] K. Hiramatsu, K. Nishiyama, A. Motogaito, H. Miyake, Y. Iyechika, T. Maeda, Phys. Status Solidi A 176 (1999) 535. [18] E.V. Yakovlev, R.A. Talalaev, A.S. Segal, A.V. Lobanova, W.V. Lundin, E.E. Zavarin, M.A. Sinitsyn, A.F. Tsatsulnikov, A.E. Nikolaev, J. Cryst. Growth 310 (2008) 4862. [19] A. Koukitu, M. Mayumi, Y. Kumagai, J. Cryst. Growth 246 (2002) 230. [20] S.F. Li, S. Fuendling, X. Wang, S. Merzsch, M.A.M. Al-Suleiman, J.D. Wei, H.H. Wehmann, A. Waag, W. Bergbauer, M. Strassburg, Cryst. Growth Des. 11 (2011) 1573. [21] A. Kuramata, K. Horino, K. Domen, K. Shinohara, T. Tanahashi, Appl. Phys. Lett. 67 (1995) 2521.
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Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Suppressing the lateral growth of gallium nitride nanowires by introducing hydrogen plasma Tung-Hsien Wu a, Franklin Chau-Nan Hong a, b, c, d,⁎ a
Department of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 70101, Taiwan Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan 70101, Taiwan d NCKU Research Center for Energy Technology and Strategy, National Cheng Kung University, Tainan 70101, Taiwan b c
a r t i c l e
i n f o
Available online 26 March 2012 Keywords: Plasma enhanced chemical vapor deposition Gallium nitride Nanowires Hydrogen plasma Polarity
a b s t r a c t In this study, we have found that the lateral homoepitaxial growth on GaN nanowires is suppressed by introducing hydrogen gas into the plasma-enhanced chemical vapor deposition (PECVD) apparatus for the growth of GaN nanowires. The formation of GaHx (x = 2, 3) species due to the reaction between gallium atoms and hydrogen plasma is shown to decrease the amount of excess gallium atoms adsorbed on GaN nanowire surfaces, which results in the elimination of nucleation on the nanowire surface and thus improves the surface smoothness of the nanowire. The stacked-cone nanostructures appear under low hydrogen or hydrogen-less conditions, but completely disappear under high hydrogen conditions in the PECVD system. The mechanism of the elimination of lateral growth on the nanowire surface is further proposed. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Gallium nitride is one of the most widely used materials for electronics and optoelectronics applications due to its direct energy bandgap, high electron mobility, and tunable bandgap. Gallium nitride nanowires are excellent building blocks for the applications in nanophotonics [1,2]. Since the synthesis of GaN nanowires using the reaction confined by carbon nanotubes at 1997 [3], various kinds of GaN nanostructures have been reported, such as quantum dots [4,5], nanobelts [6,7], stacked-cones [7–9], zigzag [10], and hexagonal prism-like islands [6], etc. The formation of such nanostructures is understood in terms of the formation energies of different facets and the diffusivity of adatoms on different crystal planes [6,7]. The chemical reaction for the growth of GaN nanowire is shown below: Ga þ N→GaN One would expect that the reaction should be influenced by availability of nitrogen and Ga species on the growth surface. The presence of excess Ga in the gas phase results in the homoepitaxial growth on GaN nanowires [7]. Additionally, Ga-rich conditions result in the improvement of lateral growth rate, while N-rich conditions suppress the lateral growth. In other words, the presence of excess Ga adatoms on nanowire surfaces is one of the necessary conditions to develop
the laterally grown nanostructures. Consequently, we can control the lateral homoepitaxial growth rate by tuning the concentration of Ga adatoms on the nanowire surface. The presence of hydrogen in the gas phase during the growth reduces the wettability of Ga with GaN surface, and thus reduces the Ga diffusion on the GaN nanowire surface and restricts the formation of GaN nuclei, which might lead to 1-dimensinoal growth [6]. The reaction between active hydrogen atoms and Ga atoms tends to form gallium hydrides, and the free energy of gallium hydride is lower than that of elemental gallium in a temperature range between 830 and 1030 °C (the well-known temperature range adopted for the growth of GaN). In other words, the gallium species prefer to form gallium hydride (GaHx, x = 2, 3) rather than in the form of elemental gallium within this temperature range [11]. Hou and Hong also reported that the presence of small amount of hydrogen results in the formation of gallium hydride species and suppresses the surface diffusion of Ga adatoms on substrate surfaces [12]. In this study, the hydrogen plasma is introduced into the growth system to react with excess gallium atoms adsorbed on GaN nanowires, thus forming gallium hydride in the gas phase to suppress the lateral homoepitaxial growth. This reports the controlling of the amount and Ga adatoms on GaN nanowires during the growth period by introducing hydrogen plasma into the plasma-enhanced chemical vapor deposition (PECVD) apparatus. 2. Experimental details
⁎ Corresponding author at: Department of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan. Tel./fax: + 886 6 2757575 62681 218. E-mail address: [email protected] (F.C.-N. Hong). 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2012.03.052
GaN nanowires were grown in a 38 mm inner-diameter quartztube inserted in a three-temperature-zone furnace. The nitrogen
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and hydrogen plasma were generated by means of dielectric-barrier discharge using a 25 kHz high-frequency power supply (Creating Nanotechnology Inc.) [13]. The metallic Ga (99.999%, 0.148 g) placed in an alumina crucible was positioned in the central zone of the quartz tube as the Ga source, and the Si(100) substrate was placed in the downstream of the tube. Prior to the growth of GaN nanowires, a droplet of 0.01 M ethanol solution of nickel nitrate hexahydrate (Ni(NO3)2·6H2O) was dropped onto the substrate to be dried out as the catalyst for the vapor–solid–solid (VSS) growth. In order to ensure the high vaporization rate of gallium droplets, gallium was always sealed in flowing argon inside a 15 mm diameter quartz tube to prevent the nitridation of Ga droplets by active nitrogen species in N2 plasma. Gallium vapor carried by an argon flow of 250 sccm (standard cubic centimeters per minute) was introduced into the tube reactor to mix with N2 (99.9999%). The base pressure and the growth pressure were 0.13 Pa and 266.64 Pa, respectively. To investigate the effect of hydrogen on the growth of GaN nanowires, 5%, 10%, 15%, and 20% of H2 in N2 was used with a total flow rate maintained at 200 sccm. The substrate temperature was increased at a rate of 30 °C min − 1 from room temperature to the reaction temperature, 900 °C. To investigate the effect of Ga vapor pressure on the growth of GaN nanowires, the Ga temperature was increased at a rate of 30 °C min − 1 from room temperature to 850 and 900 °C. Initiation of the GaN nanowire growth is always introduced by turning the N2 Plasma on after Ga source reaching the preset temperature with a growth period fixed at 60 min. The average growth rate of the nanowires, not affected by varying the hydrogen content in the gas during growth, was about 8 μm/h. After the completion of growth, the furnace was cooled down to room temperature by ambient air at an average ramping rate of 7.5 °C/min. The samples were characterized with a variety of techniques, including scanning electron microscopy (SEM: JEOL JSM-6700 F) and transmission electronic microscopy (TEM: FEI E.O Tecnai F20 G2) with the selected area electron diffraction (SAED). SEM and TEM were operated at 10 kV and 200 kV respectively. For TEM analysis, the specimen was prepared by dispersing the GaN nanowires in
ethyl ethanol by ultrasonic agitation, and then dropping them onto the carbon film on a copper grid. Photoluminescence spectra of GaN nanowires were measured at room temperature by a spectrometer (HORIBA Jobin Yvon Triax 550) equipped with confocal microscope using a 325 nm line of a He–Cd laser as the excitation source. 3. Results Fig. 1 shows the 45°-tilting SEM images of the GaN micro-/nanostructures grown using various contents of hydrogen in the gas phase with the Ga source temperature fixed at 900 °C. Many lateral growth GaN structures were observed on the substrate, including stacked-cones, zigzag, sub-micrometer (sub-μm) rods, and nanowires. The rod-like clusters on the substrate were resulted from the aggregation of nickel nitrate during the temperature-ramping process. It is obvious that the amount of stacked-cones, zigzag, and sub-μm rods decreases as the hydrogen content in the gas phase increases. Since the density of lateral-grown GaN nanowires decreased as the active hydrogen content increases, the introduction of hydrogen plasma resulted in decrease of lateral growth on the GaN nanowires surfaces. The mechanism for the elimination of lateral growth on nanowire surface will be discussed in the following section. The 45°-tilting SEM images in Fig. 2 show the GaN nanowires grown under different contents of hydrogen in the gas phase with the Ga source temperature kept at 850 °C. When the hydrogen content was below 10%, as shown in Fig. 2-(a), a few stacked-cone nanostructures appeared on the substrate, while smooth and straight GaN nanowires were obtained as the hydrogen content was higher than 10% (Fig. 2-(b) and (d)). In addition, the low Ga vapor pressure in the gas phase, controlled by lowering the Ga source temperature, would result in straight GaN nanowires without lateral growths. The average growth rate of nanowires, not varied with the hydrogen content in the gas phase, was around 8 μm/h. Fig. 3 shows the crosssectional SEM image of the interface between Si(100) substrate and a GaN nanowires grown under 5% H2. The well bonded interface between substrate and nanowires is utterly important for device
Fig. 1. SEM images of the GaN nanowires grown under various contents of hydrogen in the gas phase with the Ga source temperature fixed at 900 °C. The hydrogen contents in the gas phase were (a) 5%, (b) 10%, (c) 15% and (d) 20%, respectively.
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Fig. 2. The 45°-tilting SEM images of GaN nanowires grown under various contents of hydrogen in the gas phase with the Ga source temperature fixed at 850 °C. The hydrogen contents in the gas phase were (a) 5%, (b) 10%, (c) 15%, and (d) 20%, respectively. Stacked–cone nanostructures appeared in (a) and (b), and smooth and straight GaN nanowires appeared in (c) and 2(d).
fabrication. Fig. 4 shows the room-temperature continuous-wave photoluminescence spectrum of GaN nanowires grown under 20% H2. The photoluminescence spectra of GaN nanowires grown under other hydrogen contents also showed similar result. The main peak at 360 nm was attributed to the near-band-edge emission [21]. The peak at 650 nm and around 720 nm indicated the second order emission of He–Cd CW laser and GaN, respectively. Besides, the broad yellow luminescence around 550 nm was not obvious at all in our PL spectra, indicating the high crystal quality of GaN nanowires. Fig. 5 presents the TEM images and the corresponding SAED patterns of two GaN nanostructures. The insets at the upper and the lower right corners in Fig. 5 show the low-magnification TEM images and the corresponding SAED patterns, respectively. High-resolution TEM (HRTEM) image and the corresponding selected-area electron diffraction pattern shown in Fig. 5(a) confirmed that the straight nanowires
observed in the HRTEM images, indicating the high crystal quality of nanowires. The adjacent lattice distance was ~0.276 nm along the growth axis, which corresponded to the d-spacing between the neighboring 1010 planes of wurtzite GaN, and further confirmed the b1010 > crystallographic growth direction of the nanowires. HRTEM image and the corresponding SAED patterns shown in Fig. 5 (b) confirmed that the nanowires with stacked–cone structure were single-crystalline wurtzite structure grown along the b0001> orientation. The adjacent lattice distance was ~ 0.517 nm along the growth axis, which corresponded to the d-spacing between neighboring (0001) planes of wurtzite GaN, and further confirmed the b0001> crystallographic growth direction of the nanowires. Besides, the pyran o midal structure was enclosed by {0001} basal planes and six 1122
were single-crystalline wurtzite structure grown along b1010 > orientation. No defects, stacking faults and amorphous shells were
Fig. 3. The cross-sectional SEM image of the interface between Si(100) substrate and a GaN nanowires grown under 5% H2.
Fig. 4. Room-temperature continuous-wave photoluminescence spectrum of GaN nanowire grown under 20% H2.
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Fig. 5. The bright-field HRTEM images and the corresponding SAED patterns of two GaN nanostructures. (a) The inset at the upper right corner shows the low-magnification TEM image of a straight GaN nanowire. The inset at the lower right corner is the corresponding SAED pattern of the nanowire recorded along [0001]zone axis. (b) The inset at theh upperi right corner shows the low-magnification TEM image of a GaN nanowire with stacked–cone structure close to the tip of the nanowire. The SAED pattern is recorded, along 0110 zone axis, at the red circle marked in the low-magnification image.
sidewalls. Low-magnification TEM images show that each GaN nanostructure was terminated with a faceted nickel nanoparticle at the tip, indicating the growth of GaN nanowires via catalyst-induced VSS mechanism [14,15]. The results that stacked–cone nanowires terminated with a faceted nickel nanoparticle at the tip are different from those reported by Cai et al. [7], as the authors pointed out that the stacked–cone nanowires grew out from the catalytic droplet on the substrate. The GaN nanowires grown along b1010 > and b0001> directions n o tend to have {0001} and 1120 planes as sidefaces, respectively. The mobility of Ga adatoms on different crystallographic planes is affected by the difference in binding strength [16]. According to the density of n o dangling bonds of {0001} and 1120 planes, 11.4 and 14.0 nm − 2, respectively [17], the mobility of Ga adatoms on {0001} planes is n o higher than that on 1120 planes. This phenomenon makes the nun o cleation on 1120 facets easier than that on {0001} facets, and leads n o to the lateral growth structure and the smooth surface on the 1120 and the {0001} facets, respectively. Hou and Hong reported that the introduction of a small amount of hydrogen into the growth system resulted in the formation of gallium
hydride species (GaHx , x = 2, 3) which did not prefer to adsorb on the substrate surface and tended to desorb from the surface [12]. Additionally, the free energy of gallium hydride is lower than that of elemental gallium at 850 and 900 °C in our PECVD system [11]. Therefore, the gallium species prefer to form gallium hydride rather than elemental gallium in this temperature range. Fig. 6 shows the mechanism we proposed for explaining the elimination of lateral growth of GaN nanowires by introducing hydrogen into the PECVD apparatus. In summary, we suggest that the active hydrogen species may react with the Ga atoms in the gas phase and the Ga atoms adsorbed on the GaN nanowire surface forming gallium hydride species. Such reactions lead to the decrease of the concentration of Ga adatoms on the nanowire surface, thus eliminating the lateral growth. With the hydrogen content below 10%, the lateral growth starts to occur due to the excess Ga atoms on the nanowire surface, more likely for a high Ga vapor pressure. With the hydrogen content above 10%, the lateral growth of nanowires will be eliminated particularly at a low Ga vapor pressure. Several groups have reported that hydrogen can etch GaN via Ga– H and N–H formation at elevated temperature [18,19]. Compared with GaN low index crystallographic planes with a low density of dangling bonds, the high index crystallographic planes with a high density of dangling bonds are easier to react with hydrogen [20]. n o Since the density of dangling bonds on 1122 planes (17.8 nm − 2) n o is higher than that on 1120 planes (14 nm − 2), hydrogen atoms n o react with the surface Ga atoms more easily on the 1122 planes at a high temperature, thus converting them into the thermodynamn o ically more stable 1120 planes, which are the surfaces of smooth GaN nanowires. The lateral growth nanostructures on the nanowire can thus be etched away by hydrogen revealing the smooth nanowire n o surfaces, 1120 .
Fig. 6. The growth model of GaN nanowire under hydrogen plasma: Hydrogen atoms react with the adsorbed Ga atoms on the GaN nanowire surface to form GaHx (x = 2, 3) species (purple arrow), which tend to desorb from the surface (red arrow).
To investigate the effect of hydrogen addition in the plasma on the lateral growth during the growth of GaN nanowires, we grew GaN nanowires under 20% hydrogen content in the first 30 min and under 0% hydrogen for the next 30 min by maintaining the Ga source temperature at 850 °C. During growth period, the total flow rate was fixed at 200 sccm. The 45°-tilting SEM images in Fig. 7 show that the nanostructures due to lateral growth appeared only appeared close to the tips of GaN nanowires.
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Fig. 7. FE-SEM images obtained by tilting the samples 45° with respect to the normal of sample showing the morphologies of the GaN nanowires on Si(100) substrates grown using 20% H2 in the first 30 min and 0% H2 in the next 30 min.
4. Conclusions In summary, we controlled the behavior of the lateral growth on the GaN nanowire surface by introducing hydrogen into the PECVD apparatus. Due to different properties of various crystallographic planes, GaN nanowires grown along b0001 > and b1010 > directions developed the stacked–cone and straight nanowires structures, respectively. Moreover, the stacked–cone structures appeared under low hydrogen or hydrogen-less conditions but completely disappeared under high hydrogen concentrations in the PECVD system. The mechanism for the elimination of lateral growth by hydrogen addition is due to the formation of gallium hydrides (GaHx, x = 2, 3) for decreasing the excess Ga adatoms on the nanowire surface as well as the etching of high index planes into the thermodynamically stable low index planes, which are the surface of smooth nanowires. Acknowledgments The authors gratefully acknowledge the financial support from the National Science Council of Taiwan under grant NSC-100-2221-E006-147 and NSC-99-2221-E-006-197-MY3, the Aim for the Top University Project from NCKU and the Central Taiwan Science Park under grant number: 302202501. References [1] W. Guo, M. Zhang, A. Banerjee, P. Bhattacharya, Nano Lett. 10 (2010) 3355. [2] R. Armitage, K. Tsubaki, Nanotechnology 21 (2010) 7. [3] W.Q. Han, S.S. Fan, Q.Q. Li, Y.D. Hu, Science 277 (1997) 1287.
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