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Exceptionally long GaN sub-micrometer rods grown by HVPE on a MOCVD-GaN rod template
Journal of Alloys and Compounds 688 (2016) 967e971
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Exceptionally long GaN sub-micrometer rods grown by HVPE on a MOCVD-GaN rod template Yutian Cheng a, Hua Zong a, Jiejun Wu a, *, Peng Liu b, Tong Han a, Tongjun Yu a, **, Xiaodong Hu a, Guoyi Zhang a, b a
Research Center for Wide-Gap Semiconductors, State Key Laboratory for Artificial Microstructures and Mesoscopic Physics, School of Physics, Peking University, Beijing 100871, PR China Sino Nitride Semiconductor CO., LTD, Dongguan 523500, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 April 2016 Received in revised form 8 July 2016 Accepted 12 July 2016 Available online 15 July 2016
The growth mechanism of exceptionally long N-polar GaN sub-micrometer (sub-mm) rods has not been revealed to date. In this work, we investigated the effects of the V/III molar ratio and HCl (diluted in carrier gas) on the lengthening of a MOCVD-GaN rod template by HVPE. It is found that a low V/III molar ratio and HCl (diluted in carrier gas) help form vertical sidewalls and suppress the lateral growth on the top part of exceptionally long GaN sub-mm rods. Simulation results revealed that the low V/III molar ratio leads to reactive species distributing almost exclusively on the top part of GaN rods, which can effectively prevent crystal growth on the bottom of GaN rods. Chlorine ions support the growth by etching the sidewalls. After growth, the diameter and length of each GaN rod are more than 1.5 mm and 70e80 mm, respectively. Finally, an empirical growth model was developed to account for the exceptionally long GaN sub-mm rods under HVPE growth. © 2016 Elsevier B.V. All rights reserved.
Keywords: Hydride vapor phase epitaxy GaN rods V/III molar ratio Chlorine ions
1. Introduction III-nitride materials have been highly important for their application in short wavelength laser diodes, high power electronics and microwave devices since the last three decades [1,2]. Due to the high aspect ratio and large surface-to-volume ratio, IIInitride nano-/sub-micrometer rods have gained greater interest in recent years with their advantages of low threading dislocation [3], relieving lattice and thermal expansion mismatch [4]. These structures have great promise for various applications, for example, lateral strain relaxation can lead to a higher level of indium incorporation during InGaN/GaN multi-quantum-well (MQW) growth [5], the exposed nonpolar planes of GaN rods eliminate the piezoelectric polarization and reduce the quantum confined Stark effect (QCSE) in the quantum-well [6], and the high surface-to-volume ratio can increase the surface charge sensitivity in bio or chemical sensors [7,8]. Moreover, it was shown that the sensitivity of the sensors increases dramatically with the length of the GaN rods [9].
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Wu), [email protected] (T. Yu). http://dx.doi.org/10.1016/j.jallcom.2016.07.115 0925-8388/© 2016 Elsevier B.V. All rights reserved.
Thus, the synthesis of exceptionally long GaN rods has important implications for sensor devices. GaN nano-/sub-micrometer rods are typically grown by metalorganic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE) and hydride vapor phase epitaxy (HVPE) [10e12]. Many methods and growth models such as catalyst-assisted, silane doping and the NH3-pulsed growth mode have been proposed to promote the vertical growth of GaN rods [13e18]. However, these methods also increase the cost and complexity of the experiment. Despite this, there are no reports of growing GaN rods by combining MOCVD and HVPE for the advantages of producing GaN rod arrays with better uniformity and high growth rate, respectively. In this Letter, we reported the growth of GaN rods with record lengths (up to 70e80 mm) by HVPE on a MOCVD-GaN rod template. The effect of V/III molar ratio on the exceptionally long GaN sub-micrometer rods were investigated, and Chlorine ions were introduced in the growth process for the first time. Based on the experiments, a growth model was developed to account for the exceptionally long GaN sub-mm rods under HVPE growth. 2. Experimental Prior to HVPE growth, N-polar GaN rods were synthesized by
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MOCVD self-assembled growth on a sapphire substrate [19]. The diameter and length of each GaN rod are in the ranges of more than 1.5 and 20e35 mm, respectively (Fig. 1). Lengthening of the GaN rods was performed in a home-made HVPE system with a vertical reactor by the reaction of GaCl with NH3. The GaCl was obtained in a Ga source by reacting HCl with Ga at 800e900 C. The NH3 flow rate was held in the range of 0.5e2 slm, while the HCl flow rate was typically 20e100 sccm. The growth rate of GaN rods was varied between 10 and 100 mm/h, with N2 and H2 as carrier gases. The growth was carried out at a low pressure of 330 Torr at 1070 C for all the samples investigated in this work [20,21]. To avoid decomposition of the MOCVD-GaN rod templates, NH3 was kept in the atmosphere during heating to the growth temperature. Specifically, another HCl gas (diluted in carrier gas) was introduced. Our approach to clarifying the growth mechanism of exceptionally long GaN sub-mm rods was shown in detail in Table 1. The morphology of the GaN sub-mm rods was characterized by differential interference contrast microscopy (DICM) and field emission scanning electron microscopy (SEM) operating at 10 kV. Numerical simulation for source gas concentration among GaN rods in the growth zone was conducted using the commercial FLUENT software package. To verify the flow field among the exceptionally long GaN rods and to determine the optimum operating parameters, a threedimensional numerical simulation for the reactant concentration fields was conducted with a commercial CFD code, FLUENT. The simulation consists of solving conservation equations of mass, momentum, energy and species concentrations at a steady state with boundary conditions corresponding to the designed reactor and GaN HVPE growth. The major assumptions for the simulation included a constant susceptor temperature, convectiondetermined wall temperature, neglecting chemical reactions and thermal radiation, ideal gas approximation and the steady state of the system [22]. The boundary conditions are specified as follows. For the four concentric inlets, velocities and species mass fraction are specified for each inlet, which are converted from the given volume flow rate of each inlet. For the outlet, the ‘‘pressure outlet’’ boundary condition is selected, which presumes a fully developed flow at the outlet [23]. For the susceptor, ceiling and side walls, zero velocity, constant temperature, and zero species gradient are assumed. The momentum, heat, and mass transfers of gas mixture in the reactor are governed by four coupled conservation equations: continuity equation (Eqn. (1)) and the energy, momentum, and species conservation equations, which can be generalized into one generic equation (Eqn. (2)):
vr þ V$ðrVÞ ¼ 0 vt
(1)
vðrФÞ þ V$ðrVФÞ ¼ VðGФ VФÞ þ SФ vt
(2)
where GФ ¼ m; k=cp or rDij, corresponding to momentum, energy or species conservation equations, respectively. SФ , called the source term, represents all other terms in each conservation equation and is constant during each iteration step [23]. The above mentioned equations have been installed in the FLUENT code. The corresponding thermophysical parameters, including thermal conductivity k, viscosity u, specific heat cp , binary diffusion coefficient Dij , and thermal diffusion factor ai are selected either by kinetic relations or by mass-weighted gas mixture relations provided by FLUENT [24]. In the modelling study, the following parameters were selected as the base conditions: the reactor diameter D is 160 mm, and the diameters of four separate concentric inlets are 18, 50, 84 and
Fig. 1. Bird's-eye view (30 ) SEM image of GaN rod template by MOCVD selective area growth.
100 mm, respectively. The distance between the nozzle and the susceptor is 105 mm. In the simulation, the growth parameters were based on the experiments, the HCl gas is assumed to be 100% transformed to GaCl gas and the reactions between GaCl and NH3 are not considered. 3. Results and discussions As presented in experimental sets A and B, two phenomena were observed in the GaN rod evolution with respect to the V/III molar ratio. As shown in Fig. 2, the GaN rods transformed into the bulk material when V/III molar ratio was equal to 36. When the HCl flow was maintained at 50 sccm and NH3 was adjusted from 1800 sccm to 600 sccm during the growth, as presented in experiment set B, a disk-like cap formed on each rod (Fig. 3). This indicates that a lower V/III molar ratio promotes the vertical growth of GaN rods and is consistent with the phenomena observed in previous studies [13,25,26]. K. Lekhal et al. showed that local supersaturation was not sufficiently high to initiate nucleation of planar growth on sapphire substrate at a low V/III ratio [13]. In this condition, GaN growth was preferentially promoted and deposited at the GaN rod sites. When the V/III ratio increased, GaN growth was gradually more favoured onto the sapphire surface. It has also been noted in several previous works that hydrogen can passivate the N-terminated surface. A low V/III ratio was suggested to suppress the hydrogen passivation and support higher growth rates on the c-planes of N-polar GaN [26]. However, one of the largest differences between our work and previous studies is the height of the samples. A significant percentage of the reported GaN rods were shorter than 5 mm, whereas the nominal heights of our MOCVDGaN rod template could reach 30 mm and could be lengthened to 70e80 mm by HVPE. It is possible that the different mass transport mechanism is a more important factor for vertical growth of exceptionally long GaN rods, and disk-like caps formed on each rod. Therefore, a three-dimensional numerical simulation based on MOCVD-GaN rod templates was conducted with a commercial Computational Fluid Dynamics (CFD) code, FLUENT. The distribution of the GaCl reactive species molar fraction among the rods was simulated and is shown in Fig. 4. Simulation results indicated that GaCl scattered mainly on the top part of GaN rods but rare on the
Y. Cheng et al. / Journal of Alloys and Compounds 688 (2016) 967e971
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Table 1 Experimental details of experimental sets A, B, C, and D. Experimental set
V/III molar ratio
HCL flow rate [sccm] (diluted in carrier gas)
H2/N2 ratio
A B C D
36 12 12 12
0 0 0 20
1:7 1:7 1:2 1:7
sidewall and substrate. According to the growth model developed by Xue Wang [27], the vertical growth rate is mainly attributed to the top surface impingement and the lateral growth rate is mainly caused by the sidewall impingement through the gas phase diffusion. Because the exceptionally long GaN sub-micrometer rods inhibited the downward diffusion of reactive species, the reactive species could not reach the sapphire substrate and restrained the growth at the bottom of the GaN rods. Thus, the higher species density on the top part led to the higher growth rate and resulted in a disk-like cap on each rod. To suppress the radial growth on the top of the rods, a higher concentration of H2 carrier gas was used for its etching effect of GaN via Ga-H and N-H formation at elevated temperatures (experimental set C) [28,29]. However, the conditions were not improved, because the etching rate was too small compared to that of the HVPE GaN growth. In experimental set D, another 20 sccm HCl (diluted in carrier gas) was introduced. A columnar structure with significant vertical sidewalls and a diameter of approximately 2e3 mm can be observed in Fig. 5. Remarkably, the length of the GaN rods reached approximately 80 mm, and the vertical-to-lateral aspect ratio of sub-micrometer rods ranges from 25 to 36. This phenomenon reflects the fact that HCl has a stronger effect than H2 on etching the disk-like cap formed on each rod. The reaction between chlorine and GaN is generally considered to be described by GaN þ Cl / Ga, GaClx, Gaþ, GaClx, N2 (x ¼ 1, 2, 3), with the etching rate increasing with substrate temperature [30,31]. The thermal etching of chlorine ion appeared to occur just on the top of GaN rods for the same density distribution of HCl and reactive species. When the etching rate of chlorine ion was equal to the lateral growth rate, the lateral growth was suppressed but the net
Fig. 2. Cross-section fluorescence microscopy image of GaN rod template after HVPE growth at V/III molar ratio equal to 36.
Fig. 3. Bird's-eye view (30 ) SEM image of the GaN rod template after HVPE growth at V/III molar ratio equal to 12. GaN rods with disk-like caps can be seen in the inset figure.
vertical growth rate was preserved due to its higher growth rate compared to lateral growth [32,33]. Thus, exceptionally long GaN sub-micrometer rods with significant vertical sidewalls formed. If we prolonged the growth time in the same atmosphere when the GaN rods reached their critical length, the sub-micrometer rods did not continue to grow higher and the top of the GaN rods became thinner due to the higher amount of chlorine ions over the top of the exceptionally long GaN rods. To further elucidate the influence of the V/III ratio and chlorine ions on the growth of the exceptionally long GaN rods, a growth model was developed based on the experiments. All reactive species are transported by gas phase diffusion and impinge on the substrate, on the top surfaces of GaN rods, or on their sidewall. As shown in Fig. 6a, a high V/III ratio leads to reactive species being distributed among the entire GaN rod and thus leads to growth both in the lateral and vertical directions as well as giving rise to the nucleation on the substrate. Consequently, the GaN rods transform to bulk GaN at a high V/III ratio. When a low V/III ratio is used, reactive species are distributed just among the top part of the exceptionally long GaN rods. This leads to the prevention of the growth in the lateral direction on the lower part and maintains the vertical and lateral growth rate on the top part, causing the formation of a disk-like cap on each rod (Fig. 6b). By introducing more
Fig. 4. Numerical simulation of GaCl concentration distribution among GaN rods under a V/III molar ratio of 12.
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suppress the lateral growth in the same place at low V/III molar ratios. Combining these two observations, a growth model based on experiments was proposed for the exceptionally long GaN sub-mm rods under HVPE growth. Acknowledgments This work was supported by the Project of National High Technology of China (No. 2014AA032605), National Key Basic R&D Project of China (No. 2012CB619304, 2011CB301904), Guangdong Innovative Research Team Program(No.2009010044) and National Natural Science Foundation of China (61474003, 61327801, 61376012, 51272008). References
Fig. 5. Bird's-eye view (30 ) SEM image of GaN rod template after HVPE growth at V/III molar ratio of 12 with another 20 sccm HCl (diluted in carrier gas). Inset shows a single GaN rod lying on the substrate.
HCl (diluted in carrier gas) at a low V/III ratio as shown in Fig. 6c, the lengthened GaN rods on the top part have the same diameter as the MOCVD-GaN rods when the etching rate of chlorine ion is equal to the lateral growth rate. Therefore, the GaN rods could be lengthened while maintaining the same diameter as MOCVD-GaN rods. Furthermore, the optical properties of GaN rods were investigated, and the mechanical properties of the exceptionally long GaN rods are under further investigation [19]. 4. Conclusion In summary, the growth of 70e80 mm long GaN rods by HVPE has been demonstrated on an N-polar MOCVD-GaN rod template. The effects of the V/III ratio and chlorine ions on the growth of the exceptionally long GaN rods were investigated. The simulation results indicate that reactive species are distributed almost exclusively on the top part of GaN rods while chlorine ions could
Fig. 6. Schematic growth model of lengthening GaN rods under different HVPE parameters. (a) High V/III ratio. (b) Low V/III ratio. (c) Low V/III ratio and HCl (diluted in carrier gas).
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Exceptionally long GaN sub-micrometer rods grown by HVPE on a MOCVD-GaN rod template Yutian Cheng a, Hua Zong a, Jiejun Wu a, *, Peng Liu b, Tong Han a, Tongjun Yu a, **, Xiaodong Hu a, Guoyi Zhang a, b a
Research Center for Wide-Gap Semiconductors, State Key Laboratory for Artificial Microstructures and Mesoscopic Physics, School of Physics, Peking University, Beijing 100871, PR China Sino Nitride Semiconductor CO., LTD, Dongguan 523500, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 April 2016 Received in revised form 8 July 2016 Accepted 12 July 2016 Available online 15 July 2016
The growth mechanism of exceptionally long N-polar GaN sub-micrometer (sub-mm) rods has not been revealed to date. In this work, we investigated the effects of the V/III molar ratio and HCl (diluted in carrier gas) on the lengthening of a MOCVD-GaN rod template by HVPE. It is found that a low V/III molar ratio and HCl (diluted in carrier gas) help form vertical sidewalls and suppress the lateral growth on the top part of exceptionally long GaN sub-mm rods. Simulation results revealed that the low V/III molar ratio leads to reactive species distributing almost exclusively on the top part of GaN rods, which can effectively prevent crystal growth on the bottom of GaN rods. Chlorine ions support the growth by etching the sidewalls. After growth, the diameter and length of each GaN rod are more than 1.5 mm and 70e80 mm, respectively. Finally, an empirical growth model was developed to account for the exceptionally long GaN sub-mm rods under HVPE growth. © 2016 Elsevier B.V. All rights reserved.
Keywords: Hydride vapor phase epitaxy GaN rods V/III molar ratio Chlorine ions
1. Introduction III-nitride materials have been highly important for their application in short wavelength laser diodes, high power electronics and microwave devices since the last three decades [1,2]. Due to the high aspect ratio and large surface-to-volume ratio, IIInitride nano-/sub-micrometer rods have gained greater interest in recent years with their advantages of low threading dislocation [3], relieving lattice and thermal expansion mismatch [4]. These structures have great promise for various applications, for example, lateral strain relaxation can lead to a higher level of indium incorporation during InGaN/GaN multi-quantum-well (MQW) growth [5], the exposed nonpolar planes of GaN rods eliminate the piezoelectric polarization and reduce the quantum confined Stark effect (QCSE) in the quantum-well [6], and the high surface-to-volume ratio can increase the surface charge sensitivity in bio or chemical sensors [7,8]. Moreover, it was shown that the sensitivity of the sensors increases dramatically with the length of the GaN rods [9].
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J. Wu), [email protected] (T. Yu). http://dx.doi.org/10.1016/j.jallcom.2016.07.115 0925-8388/© 2016 Elsevier B.V. All rights reserved.
Thus, the synthesis of exceptionally long GaN rods has important implications for sensor devices. GaN nano-/sub-micrometer rods are typically grown by metalorganic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE) and hydride vapor phase epitaxy (HVPE) [10e12]. Many methods and growth models such as catalyst-assisted, silane doping and the NH3-pulsed growth mode have been proposed to promote the vertical growth of GaN rods [13e18]. However, these methods also increase the cost and complexity of the experiment. Despite this, there are no reports of growing GaN rods by combining MOCVD and HVPE for the advantages of producing GaN rod arrays with better uniformity and high growth rate, respectively. In this Letter, we reported the growth of GaN rods with record lengths (up to 70e80 mm) by HVPE on a MOCVD-GaN rod template. The effect of V/III molar ratio on the exceptionally long GaN sub-micrometer rods were investigated, and Chlorine ions were introduced in the growth process for the first time. Based on the experiments, a growth model was developed to account for the exceptionally long GaN sub-mm rods under HVPE growth. 2. Experimental Prior to HVPE growth, N-polar GaN rods were synthesized by
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Y. Cheng et al. / Journal of Alloys and Compounds 688 (2016) 967e971
MOCVD self-assembled growth on a sapphire substrate [19]. The diameter and length of each GaN rod are in the ranges of more than 1.5 and 20e35 mm, respectively (Fig. 1). Lengthening of the GaN rods was performed in a home-made HVPE system with a vertical reactor by the reaction of GaCl with NH3. The GaCl was obtained in a Ga source by reacting HCl with Ga at 800e900 C. The NH3 flow rate was held in the range of 0.5e2 slm, while the HCl flow rate was typically 20e100 sccm. The growth rate of GaN rods was varied between 10 and 100 mm/h, with N2 and H2 as carrier gases. The growth was carried out at a low pressure of 330 Torr at 1070 C for all the samples investigated in this work [20,21]. To avoid decomposition of the MOCVD-GaN rod templates, NH3 was kept in the atmosphere during heating to the growth temperature. Specifically, another HCl gas (diluted in carrier gas) was introduced. Our approach to clarifying the growth mechanism of exceptionally long GaN sub-mm rods was shown in detail in Table 1. The morphology of the GaN sub-mm rods was characterized by differential interference contrast microscopy (DICM) and field emission scanning electron microscopy (SEM) operating at 10 kV. Numerical simulation for source gas concentration among GaN rods in the growth zone was conducted using the commercial FLUENT software package. To verify the flow field among the exceptionally long GaN rods and to determine the optimum operating parameters, a threedimensional numerical simulation for the reactant concentration fields was conducted with a commercial CFD code, FLUENT. The simulation consists of solving conservation equations of mass, momentum, energy and species concentrations at a steady state with boundary conditions corresponding to the designed reactor and GaN HVPE growth. The major assumptions for the simulation included a constant susceptor temperature, convectiondetermined wall temperature, neglecting chemical reactions and thermal radiation, ideal gas approximation and the steady state of the system [22]. The boundary conditions are specified as follows. For the four concentric inlets, velocities and species mass fraction are specified for each inlet, which are converted from the given volume flow rate of each inlet. For the outlet, the ‘‘pressure outlet’’ boundary condition is selected, which presumes a fully developed flow at the outlet [23]. For the susceptor, ceiling and side walls, zero velocity, constant temperature, and zero species gradient are assumed. The momentum, heat, and mass transfers of gas mixture in the reactor are governed by four coupled conservation equations: continuity equation (Eqn. (1)) and the energy, momentum, and species conservation equations, which can be generalized into one generic equation (Eqn. (2)):
vr þ V$ðrVÞ ¼ 0 vt
(1)
vðrФÞ þ V$ðrVФÞ ¼ VðGФ VФÞ þ SФ vt
(2)
where GФ ¼ m; k=cp or rDij, corresponding to momentum, energy or species conservation equations, respectively. SФ , called the source term, represents all other terms in each conservation equation and is constant during each iteration step [23]. The above mentioned equations have been installed in the FLUENT code. The corresponding thermophysical parameters, including thermal conductivity k, viscosity u, specific heat cp , binary diffusion coefficient Dij , and thermal diffusion factor ai are selected either by kinetic relations or by mass-weighted gas mixture relations provided by FLUENT [24]. In the modelling study, the following parameters were selected as the base conditions: the reactor diameter D is 160 mm, and the diameters of four separate concentric inlets are 18, 50, 84 and
Fig. 1. Bird's-eye view (30 ) SEM image of GaN rod template by MOCVD selective area growth.
100 mm, respectively. The distance between the nozzle and the susceptor is 105 mm. In the simulation, the growth parameters were based on the experiments, the HCl gas is assumed to be 100% transformed to GaCl gas and the reactions between GaCl and NH3 are not considered. 3. Results and discussions As presented in experimental sets A and B, two phenomena were observed in the GaN rod evolution with respect to the V/III molar ratio. As shown in Fig. 2, the GaN rods transformed into the bulk material when V/III molar ratio was equal to 36. When the HCl flow was maintained at 50 sccm and NH3 was adjusted from 1800 sccm to 600 sccm during the growth, as presented in experiment set B, a disk-like cap formed on each rod (Fig. 3). This indicates that a lower V/III molar ratio promotes the vertical growth of GaN rods and is consistent with the phenomena observed in previous studies [13,25,26]. K. Lekhal et al. showed that local supersaturation was not sufficiently high to initiate nucleation of planar growth on sapphire substrate at a low V/III ratio [13]. In this condition, GaN growth was preferentially promoted and deposited at the GaN rod sites. When the V/III ratio increased, GaN growth was gradually more favoured onto the sapphire surface. It has also been noted in several previous works that hydrogen can passivate the N-terminated surface. A low V/III ratio was suggested to suppress the hydrogen passivation and support higher growth rates on the c-planes of N-polar GaN [26]. However, one of the largest differences between our work and previous studies is the height of the samples. A significant percentage of the reported GaN rods were shorter than 5 mm, whereas the nominal heights of our MOCVDGaN rod template could reach 30 mm and could be lengthened to 70e80 mm by HVPE. It is possible that the different mass transport mechanism is a more important factor for vertical growth of exceptionally long GaN rods, and disk-like caps formed on each rod. Therefore, a three-dimensional numerical simulation based on MOCVD-GaN rod templates was conducted with a commercial Computational Fluid Dynamics (CFD) code, FLUENT. The distribution of the GaCl reactive species molar fraction among the rods was simulated and is shown in Fig. 4. Simulation results indicated that GaCl scattered mainly on the top part of GaN rods but rare on the
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Table 1 Experimental details of experimental sets A, B, C, and D. Experimental set
V/III molar ratio
HCL flow rate [sccm] (diluted in carrier gas)
H2/N2 ratio
A B C D
36 12 12 12
0 0 0 20
1:7 1:7 1:2 1:7
sidewall and substrate. According to the growth model developed by Xue Wang [27], the vertical growth rate is mainly attributed to the top surface impingement and the lateral growth rate is mainly caused by the sidewall impingement through the gas phase diffusion. Because the exceptionally long GaN sub-micrometer rods inhibited the downward diffusion of reactive species, the reactive species could not reach the sapphire substrate and restrained the growth at the bottom of the GaN rods. Thus, the higher species density on the top part led to the higher growth rate and resulted in a disk-like cap on each rod. To suppress the radial growth on the top of the rods, a higher concentration of H2 carrier gas was used for its etching effect of GaN via Ga-H and N-H formation at elevated temperatures (experimental set C) [28,29]. However, the conditions were not improved, because the etching rate was too small compared to that of the HVPE GaN growth. In experimental set D, another 20 sccm HCl (diluted in carrier gas) was introduced. A columnar structure with significant vertical sidewalls and a diameter of approximately 2e3 mm can be observed in Fig. 5. Remarkably, the length of the GaN rods reached approximately 80 mm, and the vertical-to-lateral aspect ratio of sub-micrometer rods ranges from 25 to 36. This phenomenon reflects the fact that HCl has a stronger effect than H2 on etching the disk-like cap formed on each rod. The reaction between chlorine and GaN is generally considered to be described by GaN þ Cl / Ga, GaClx, Gaþ, GaClx, N2 (x ¼ 1, 2, 3), with the etching rate increasing with substrate temperature [30,31]. The thermal etching of chlorine ion appeared to occur just on the top of GaN rods for the same density distribution of HCl and reactive species. When the etching rate of chlorine ion was equal to the lateral growth rate, the lateral growth was suppressed but the net
Fig. 2. Cross-section fluorescence microscopy image of GaN rod template after HVPE growth at V/III molar ratio equal to 36.
Fig. 3. Bird's-eye view (30 ) SEM image of the GaN rod template after HVPE growth at V/III molar ratio equal to 12. GaN rods with disk-like caps can be seen in the inset figure.
vertical growth rate was preserved due to its higher growth rate compared to lateral growth [32,33]. Thus, exceptionally long GaN sub-micrometer rods with significant vertical sidewalls formed. If we prolonged the growth time in the same atmosphere when the GaN rods reached their critical length, the sub-micrometer rods did not continue to grow higher and the top of the GaN rods became thinner due to the higher amount of chlorine ions over the top of the exceptionally long GaN rods. To further elucidate the influence of the V/III ratio and chlorine ions on the growth of the exceptionally long GaN rods, a growth model was developed based on the experiments. All reactive species are transported by gas phase diffusion and impinge on the substrate, on the top surfaces of GaN rods, or on their sidewall. As shown in Fig. 6a, a high V/III ratio leads to reactive species being distributed among the entire GaN rod and thus leads to growth both in the lateral and vertical directions as well as giving rise to the nucleation on the substrate. Consequently, the GaN rods transform to bulk GaN at a high V/III ratio. When a low V/III ratio is used, reactive species are distributed just among the top part of the exceptionally long GaN rods. This leads to the prevention of the growth in the lateral direction on the lower part and maintains the vertical and lateral growth rate on the top part, causing the formation of a disk-like cap on each rod (Fig. 6b). By introducing more
Fig. 4. Numerical simulation of GaCl concentration distribution among GaN rods under a V/III molar ratio of 12.
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suppress the lateral growth in the same place at low V/III molar ratios. Combining these two observations, a growth model based on experiments was proposed for the exceptionally long GaN sub-mm rods under HVPE growth. Acknowledgments This work was supported by the Project of National High Technology of China (No. 2014AA032605), National Key Basic R&D Project of China (No. 2012CB619304, 2011CB301904), Guangdong Innovative Research Team Program(No.2009010044) and National Natural Science Foundation of China (61474003, 61327801, 61376012, 51272008). References
Fig. 5. Bird's-eye view (30 ) SEM image of GaN rod template after HVPE growth at V/III molar ratio of 12 with another 20 sccm HCl (diluted in carrier gas). Inset shows a single GaN rod lying on the substrate.
HCl (diluted in carrier gas) at a low V/III ratio as shown in Fig. 6c, the lengthened GaN rods on the top part have the same diameter as the MOCVD-GaN rods when the etching rate of chlorine ion is equal to the lateral growth rate. Therefore, the GaN rods could be lengthened while maintaining the same diameter as MOCVD-GaN rods. Furthermore, the optical properties of GaN rods were investigated, and the mechanical properties of the exceptionally long GaN rods are under further investigation [19]. 4. Conclusion In summary, the growth of 70e80 mm long GaN rods by HVPE has been demonstrated on an N-polar MOCVD-GaN rod template. The effects of the V/III ratio and chlorine ions on the growth of the exceptionally long GaN rods were investigated. The simulation results indicate that reactive species are distributed almost exclusively on the top part of GaN rods while chlorine ions could
Fig. 6. Schematic growth model of lengthening GaN rods under different HVPE parameters. (a) High V/III ratio. (b) Low V/III ratio. (c) Low V/III ratio and HCl (diluted in carrier gas).
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