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The anti-surfactant effect of silane on the facets-controlled growth of GaN nanorods by MOCVD
Superlattices and Microstructures 96 (2016) 234e240
Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
The anti-surfactant effect of silane on the facets-controlled growth of GaN nanorods by MOCVD J.Z. Li a, Z.Z. Chen a, *, S.F. Li b, Q.Q. Jiao a, Y.L. Feng a, S.X. Jiang a, Y.F. Chen a, T.J. Yu a, B. Shen a, G.Y. Zhang a, b a
State Key Laboratory of Artificial Microstructure and Mesoscopic Physics, School of Physics, Peking University, Beijing 100871, China Dongguan Institute of Optoelectronics, Peking University, Bldg No.1, Technology & Innovation Park Songshanhu, 523808, Dongguan, China
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 April 2016 Received in revised form 20 May 2016 Accepted 23 May 2016 Available online 24 May 2016
N-polar GaN nanorods were selective area grown by continuous mode metalorganic chemical vapor deposition (MOCVD) under a Ga-rich and high silane flow condition. The interruption comparing with continuous supply of silane flow was performed to study the role of silane flux. High resolution scanning electron microscopy (SEM), x-ray diffraction (XRD), cathodoluminescence (CL) and x-ray photoelectron spectroscopy (XPS) measurements were performed. The enhanced vertical growth rate was achieved as 42 mm/h and sharp smooth m-plane, r-plane and c-plane facets were obtained for the nanorods with high silane flux. SieN bonds were clarified to be formed on the surface of the nanorod by XPS spectra. The silane acting as anti-surfactant was suggested to explain the diffusion and incorporation of the species on the facets of GaN nanorods. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Recently, GaN based nanorods have attracted increasing attention due to promising applications in solid-state lighting [1,2], visible light communication [3,4], and water photolysis [5]. Bottom-up and top-down are two main kinds of methods to fabricate the GaN-based nanorods. Threading dislocation free, nonpolar facet active layer, high growth rate and 3D LED structure make bottom up method more attractive to the industry [6e8]. Hersee et al. reported the growth of high-quality GaN nanorods and uniform nanorod arrays using and the selective area growth (SAG) with a pulsed process [9]. The growth rate is comparable to that of the conventional one, and the pulsed flux control may be difficult for many metal organic chemical vapor deposition (MOCVD) system. Bergbauer et al. performed a continuous flux growth of GaN nanorods directly on sapphire using SAG [10]. They achieved a high growth rate as 25 mm/h for N-polar GaN nanorod. In the continuous growth mode a lot of parameters were studied on the morphology and uniformity of nanorods, such as V/III ratio, temperature, polarity, carrier gas and doping, etc [11e14]. Growth kinetics and mass transport mechanisms of GaN nanorods are demonstrated by some reports [1,3,8]. The silane is often used to form a vertical nanorod. However, the role of Si doping on nanorod growth is still under debate [14]. Generally, Si acts as a shallow donor in GaN. Koester et al. found early that a high Si-dopant concentration about 1019 cm3 promoted vertical growth in MOCVD [14]. According to results of density functional theory (DFT), the catalytic role of the Ga
* Corresponding author. E-mail address: [email protected] (Z.Z. Chen). http://dx.doi.org/10.1016/j.spmi.2016.05.034 0749-6036/© 2016 Elsevier Ltd. All rights reserved.
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bilayer was suspected with Si incorporation. Tessarek et al. reported that the formation of a SiNx layer prevents the growth on m-planes, enhances the mobility of atoms, and promotes the vertical growth [13]. However, there are few reports similar to their transmission electron microscopy (TEM) images for SiNx layer [15]. SiNx is formed on the sidewall with necessary condition, which is not likely within the growth window for GaN nanorods. Wang et al. reported that the silane flux does not influence the diffusion length of species on substrate, while has much more effect on that of the sidewall [8]. The vertical growth rate of nanorods increases 3 times when the silane flux increases from 16.5 to 165 nmol/min. In our previous work, GaN growth on nano-scale patterned sapphire substrates (NPSS) was also controlled by silane during the 3D growth of GaN islands. The high silane flux suppressed the lateral growth [16]. It is well known the silane flux is the key parameter for growing high quality GaN nanorods, while it should be further clarified how the silane affects the diffusion and incorporation of species on different planes. In this paper, a continuous-flux selective area growth of GaN nanorods was performed on SiO2 mask/sapphire substrate by MOCVD. To investigate the effect of silane injection on the growth and optical properties, the silane flux was changed in second growth stage after the GaN nanorods had formed. The morphology and crystal quality were characterized by scanning electron microscopy (SEM), x-ray diffraction (XRD). Cathodoluminescence (CL) spectra at room temperature were performed to study the optical property. X-ray photoelectron spectroscopy (XPS) was used to investigate the influence of silane flux on the binding energies of Ga, N, O and Si atoms, which were corresponded to the chemistry of these atoms. Meantime, the effect of silane injection on the optical properties and growth mechanism of the N-polar GaN nanorods was discussed. 2. Experimental Firstly 70 nm-thick SiO2 layer was deposited on sapphire substrate by plasma-enhanced chemical vapor deposition (PECVD). Then nanoimprint lithography was adopted to transfer the nano holes pattern onto the SiO2 layers, followed by inductively coupled plasma (ICP) etching to open the holes in the SiO2 mask layer. The holes distributed hexagonally on the mask. The diameter is 700 nm, and the pitch is 1.5 mm. After using the acetone and ethanol to chemical cleaning, the templates were introduced into a 3 2 inch FT CCS Thomas Swan MOVPE system for GaN nanorods growth. A thermal baking step was employed at 1100 C for 15 min in a H2 carrier environment. Then a nitridation step was performed on the growth templates at temperature of about 1080 C with 4000 sccm NH3 and N2 carrier gas, and the total pressure is set to be 95 torr. Next, the temperature directly ramped up to 1060 C and the trimethylgallium (TMGa) flow was introduced into the reactor for GaN nanorods growth. The V/III ratio was kept as low as about 50. The flow ratio of H2/N2 carrier gas was kept as 2:1. Silane was injected into the reactor with the flux of 40 sccm simultaneously. The Sample A was grown under the above conditions for 500 s. In order to research the effects of silane on GaN nanorods growth further, another 500 s step growth was performed for three samples under the same growth conditions except the silane flux. Sample B, C and D corresponded to the silane flux of 0, 20 and 40 sccm, respectively. The morphology and crystal structure of the GaN nanorods were evaluated by the SEM (Nova Nano SEM 430) and XRD (Bruker D8, Cu Ka1 X-ray source l ¼ 1.5406 Å) measurements. CL measurements were performed by the FEI Quanta 200F equipped on the SEM instrument. XPS (Axis Ultra) was measured to find the binding energy of the main elements and study the chemical reactions on the growth surface. 3. Results and discussion The SEM image for SiO2 mask on sapphire substrate is shown in Fig. 1a. The pitch and size of the holes array are 1500 and 700 nm. Under the above growth conditions, GaN nanorods will be grown selectively in the holes. Fig. 1b and c shows the side- and top-view of SEM images of Sample D. The GaN hexagonal nanorods were formed with vertical sidewall, which can be assigned as {1e100} m-planes according to the reference edge of substrate. The top of the nanorod is composed of smooth c- and r-planes. The average diameter and height of the nanorods are 852 nm and 7.47 mm, respectively. The aspect ratio of the nanorod is about 8.8. Fig. 1d shows 2q-u scan curves of XRD for Sample AeD. Each curve clearly reveals a strong diffraction peak of the GaN (0002), which indicates that wurtzite GaN nanorods were preferentially grown in the [0001] direction [17]. The precise data for Sample AeD are 34.56, 34.54, 34.55 and 34.56 , respectively. The corresponding lattice constant c parameters are 0.5186, 0.5189, 0.5188 and 0.5186 nm, which indicate the little tensile strain in GaN nanorods. The higher silane flux leads to more strain relaxation. Contrary to the two dimensional growth on the heterogeneous substrate, the nanorod growth seems likely to strain relax because there is less lateral confinement. In order to obtain the details of GaN nanorods morphologies, SEM images are obtained with 20 -tilted angle, as shown in Fig. 2. The statistics of geometrical sizes are listed in the Table 1 from plenty of SEM images. In Fig. 2a, it is observed that the nucleation is occurred at top-right corner of the holes consistently, which may be result from the flow direction. The top is flat rather than pyramidal shape, which means the N-polar GaN growth at the first stage [1,11]. For Sample B without silane supply in the second step, the rods are fat and short. Most of the neighbouring nanorods coalesced to the irregular micro-rods. Compared with the geometrical size of Sample A, both of the lateral and vertical growth rates were about 7e8 mm/h, and the aspect ratio was reduced to 1.8. The vertical growth rates in the second stage are 14 and 42 mm/h for Sample C and D, respectively. When the silane is supplied in the second step, the vertical growth rate is rapidly increased while the lateral growth rate is suppressed. Under such high silane flux, the species from the sidewall impingements and substrate diffusion
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Fig. 1. a: SEM image for the SiO2 mask on the sapphire substrate; b: side view and c: top view of SEM images of GaN nanorods, respectively; d: XRD 2q/uscan curves for four GaN nanorod samples.
contributed to the vertical growth rate instead of the lateral growth. The growth rate can increase further when the substrate surface area is not large. However, the crystal quality will be degraded when the silane increases more [8]. The magnified images of the top side of the nanorods for Samples B and D are shown in Fig. 2e and f, respectively. For Sample B, the c-plane facets are rough on the top side, while r-plane facets are smooth. There is less r-plane area than the cplane one on the top of the single rods. For Sample C with the moderate silane flux, a few c-plane facets are not smooth, and they are minority in the whole top facets. The rough c-plane facets appearance can be attributed to less diffusion ability under such low V/III ratio. So the growth rate becomes high and the growth deviates from the mass transport limited process, which leads to rough surface. The diffusion lengths on m- and r-planes are long enough for species to find their equilibrium positions, which leads to the smooth sidewall and r-plane facets. For Sample D, both r-plane and c-plane facets are smooth, and the c-plane facets are minority on the top of the nanorods too. Silane plays a role of the antisurfactant for the species [13,15]. The Ga and N atoms can reach to its equilibrium position, which leads to the smooth facets. For the evolution of nanorod top side with the silane flux, the Wulff growth theory reveals that the planes remain with low growth rate [18]. With high silane flux, species diffusion abilities on all the planes are enhanced simultaneously. The surface mass transportation is strengthened for all of the facets growth. However, the nucleation energy will be different for different planes. Due to the effect of the dangling bonds, the c-plane may be with the smallest nucleation energy and highest growth rate. Thus the c planes tend to disappear with increasing silane flux. Comparing, the growth rate on r-plane seems not lower obviously than that on c-plane for Sample B. So the c-plane facets are still dominant on the top of the un-coalesced nanorods. It indicates that the silane flux suppresses the r- and m-plane facets growth significantly due to the antisurfactant effect. A high silane flux also strongly influences optical properties of GaN nanorods. The optical properties of the GaN nanorods are characterized by room temperature CL, as shown in Fig. 3. The typical panchromatic CL image of one single nanorod from Sample D is demonstrated in the insert. GaN near-band edge emission (NBE) and yellow defect luminescence (YL) are included in the spectra, and located at 363 and 580 nm, respectively. For Sample A and D with 40 sccm silane flux, NBE is
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Fig. 2. (aed) 20 -tilted SEM images of Sample A, B, C and D, respectively; Enlarge view of top part for (e) Sample B and (f) Sample D.
much stronger than YL. With the growth time increase, the NBE becomes stronger and the YL is suppressed more. For Sample B, the YL dominates in PL spectrum. As to Sample C with 20 sccm silane flux, NBE is a bit stronger than YL. As discussed above, the antisurfactant effect of silane leads to high crystal quality since the ratio of NBE to YL increased. The intensity of the YL is associated to lattice defects, especially to Ga vacancy and O impurity complex. Substitution Si impurities are found to be most likely candidates for the donor defects because of the strikingly low formation energies for
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Table 1 The geometrical sizes of the GaN rods arrays by different conditions. Item
Sample A
Sample B
Sample C
Sample D
Stage I (500 s) silane flux Stage II (500 s) silane flux Diameter Height Aspect ratio
40 sccm e 695 nm 1.54 mm 2.2
40 sccm 0 sccm 1392 nm 2.61 mm 1.8
40 sccm 20 sccm 811 nm 3.44 mm 4.2
40 sccm 40 sccm 852 nm 7.47 mm 8.8
Fig. 3. Room temperature CL spectra of single GaN nanorod from Sample A, B, C and D with an acceleration voltage of 15 kV. Inset: the panchromatic CL image of one nanorod from Sample D.
SiGa with high silane flux. As we known, under n-type conditions the Ga vacancy is the dominant native defect; all other native defects are much higher in energy, implying negligible concentrations [19]. Using state-of-the-art first-principles calculations, Neugebauer et al. found that the concentration of the VGa-ON complex was at least an order of magnitude smaller than that of SiGa [20], which means the YL could be suppressed further, as the CL spectra shown. Meanwhile, the line width of NBE becomes wider as a consequence of the lattice disorder induced by the impurities. In this work, Ga vacancies are less likely to form under low V/III ratio conditions. Some Si atoms occupy Ga vacancies, which will be discussed in detail later. Moreover, an additional peak appears around 389 nm. It may correlates with a certain donor-acceptor pair (DAP) like SiGa-VGa, which needs verified further. Fig. 4 shows the high-resolution XPS spectra for GaN nanorods grown with different silane flux. There are two peaks assigned as Si 2p and Ga 3p in Fig. 4a according to the handbook of XPS [21]. With the silane flux increasing, the Si 2p peak is stronger and binding energy is lower than those without silane flux. Si 2p peaks located at 102.1 and 102.2 eV for Sample C and D are related to nitride. Si 2p peak at 102.5 eV for Sample B is partly corresponding to silica, for the O impurity is much higher than other samples. The typical binding energy for SieO is 103.3 eV. Ga and N mainly come from the GaeN bonds in the GaN crystal. As to Ga 3p peaks, there are 0.3 eV shifts for Sample C and D compared to Sample B. For N 1s peaks at
Fig. 4. High resolution XPS spectra of (a) Si 2p and Ga 3p, (b) N 1s, (c) O 1s for GaN nanorods of Sample B, C and D.
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394.5 eV, þ0.3 eV shifts are observed contrarily as shown in Fig. 4. The main reason should also include the contributions of NeSi bonding states arising from SiGa located in Ga atom sites within the crystal, as revealed by the Si 2p spectra. The shoulder peaks at 392 and 389 eV cannot be found in N 1s contents in the handbook [21]. They may belong to certain shell electrons of other elements. The intensity of the O 1s peak for Sample C and D are much weaker than that for Sample B. The concentration of the VGa-ON complex is suppressed which is consisted with the above CL results. The silane flux leads to the chemical environment change in the lattice in two possible ways. One is that Si atoms institutes Ga atoms heavily, which makes the lattice shrink as calculation from the XRD data above. The heavily Si doping causes the wide NBE in CL, which means the nanorod become defective [8]. However, the NBE in our work is strengthened which indicates the crystal quality is improved. Actually, it’s hard to replace the Ga atom within the crystal for Si, even the Si/Ga reach to 0.6 when the silane flux is 40 sccm. Agekyan et al. reported that the typical concentration of Si incorporated into gallium nitride was about 1019 cm3) [22], and the concentrations just increase slightly as the silane flux increase heavily, as shown in Fig. 4a. The moderate doping level was suspected, which should be clarified in the future work. And the other way is that the silane acts as the anti-surfactant during the growth, which has a significant impact on the growth processes of the GaN nanorods. When the silane flux is high, a little part of Si atoms incorporates into the lattice, while most of Si atoms accumulate on the surface. The silane or its decomposition products cover GaN nanorods surface to prevent Ga incorporating easily, which means Ga atom has a larger diffusion length on the nanorods surface. The diffusion length makes Ga and N atoms find their equilibrium positions-exact lattice sites properly. On the contrary, Ga atoms will incorporate into the lattice on the c plane of nanorods top part without silane injection. The presence of high silane flux leads to smooth of the surface for all facets of GaN nanorods owing to the elimination of step bunching for nanorods growth [23]. The excellent mobility of the species on the surfaces contributes to the strain relaxation and high crystal quality as the growth mode described above. The understanding of anti-surfactant effects during the GaN nanorods SAG MOCVD growth is still poor. The influence of high silane flux on the sticking coefficient and the related Schwoebel barrier height as well as the nucleation barrier at the step edge need further study [23]. However, the high silane flux acts as anti-surfactant during MOCVD growth could be an important tool used for the control of nanorods structures and materials properties. 4. Conclusion In summary, the SAG of N-polar GaN nanorods has been realized by a continuous growth mode with high silane flux in MOCVD. The vertical growth rate was enhanced to 42 mm/h under 40 sccm silane flux and the lateral growth was almost suppressed. As to the optical properties, the silane injection enhanced the NBE emission and suppressed the YL luminescence intensity, for the silane could decrease the Ga vacancies and O impurity, which are responsible for the YL in GaN. Strong Si 2p peaks in the XPS spectra were assigned to SieN bonds formed on the surface. SiNx full coverage and Si heavily doping were excluded by SEM and CL results. The antisurfactant role of silane flux is suggested. The diffusion lengths on the m-plane and rplane are strengthened more than c-plane. The Ga and N species would find their exact sites in the lattice on different facets. High vertical growth rate, strain relaxation and few defects were realized with high silane flux. Acknowledgements This work was supported by projects of Natural Science Foundation of China under Nos. 61334009, 60876063, 61076012 and National Key Basic Research Special Foundation of China under Nos. TG2011CB301905, TG2013CB328705. This work was also supported by Guangdong Innovative Research Team Program (No. 2009010044). References [1] S.F. Li, A. Waag, GaN based nanorods for solid state lighting, J. Appl. Phys. 111 (2012) 071101. [2] J.K. Huang, C.Y. Liu, T.P. Chen, et al., Enhanced light extraction efficiency of GaN-based hybrid nanorods light-emitting diodes, IEEE J. Sel. Top. Quantum Electron. 21 (2015) 1. [3] N. Mohsen, D.F. Feezell, Optical properties of plasmonic light-emitting diodes based on flip-chip III-nitride core-shell nanowires, Opt. Express 22 (2014) 29445e29455. [4] S.C. Zhu, Z.G. Yu, L.X. Zhao, et al., Enhancement of the modulation bandwidth for GaN-based light-emitting diode by surface plasmons, Opt. Express 23 (2015) 13752. [5] S.Z. Fan, B. AlOtaibi, Z.T. Mi, et al., High efficiency solar-to-hydrogen conversion on a monolithically integrated InGaN/GaN/Si adaptive tunnel junction photocathode, Nano Lett. 15 (2015) 2721e2726. [6] H.M. Kim, Y.H. Cho, H. Lee, et al., High-brightness light emitting diodes using dislocation-free indium gallium nitride/gallium nitride multiquantumwell nanorod arrays, Nano Lett. 4 (6) (2004) 1059. [7] T. Onuma, H. Amaike, S.F. Chichibu, et al., Quantum-confined Stark effects in the m-plane In0.15Ga0.85N/GaN multiple quantum well blue lightemitting diode fabricated on low defect density freestanding GaN substrate, Appl. Phys. Lett. 91 (2007) 181903. [8] X. Wang, A. Waag, Growth kinetics and mass transport mechanisms of GaN columns by selective area metal organic vapor phase epitaxy, J. Appl. Phys. 115 (2014) 163104. [9] S.D. Hersee, X.Y. Sun, X. Wang, The Controlled Growth of GaN Nanowires, Nano Lett. 6 (2006) 1808. [10] W. Bergbauer, M. Strassburg, A. Waag, et al., Continuous-flux MOVPE growth of position-controlled N-face GaN nanorods and embedded InGaN quantum wells, Nanotechnology 21 (2010) 305201. [11] S.F. Li, S. Fuendling, A. Waag, Polarity and its influence on growth mechanism during MOVPE growth of GaN Sub-micrometer rods, Cryst. Growth Des. 11 (2011) 1573. [12] Y.-T. Lin, T.-W. Yeh Dr., et al., Catalyst-Free GaN nanorods synthesized by selective area growth, Adv. Funct. Mater 24 (2014) 3162e3171. [13] C. Tessarek, M. Heilmann, The role of Si during the growth of GaN Micro- and nanorods, Cryst. Growth Des. 14 (2014) 1486e1492.
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[14] R. Koester, J.S. Hwang, Self-assembled growth of catalyst-free GaN wires by metaleorganic vapour phase epitaxy, Nanotechnology 21 (2010) 015602. [15] K. Pakula, R. Bozek, et al., Reduction of dislocation density in heteroepitaxial GaN: role of SiH4 treatment, J. Cryst. Growth 267 (2004) 1e7. [16] J.Z. Li, Z.Z. Chen, Silane controlled three dimensional GaN growth and recovery stages on a cone-shape nanoscale patterned sapphire substrate by MOCVD, CrystEngComm 17 (2015) 4469e4474. [17] V. Ramesh, A. Kikuchi, Strain relaxation effect by nanotexturing InGaN/GaN multiple quantum well, J. Appl. Phys. 107 (2010) 114303. [18] A. Kelly, G.W. Groves, P. Kidd, in: Crystallography and Crystal Defects, Wiley & Sons Ltd, West Sussex, England, 2000, p. p 391. [19] T. Mattila, R.M. Nieminen, Point-defect complexes and broadband luminescence in GaN and AlN, Phys. Rev. B 55 (1997) 15. € rg, G. Chris, Gallium vacancies and the yellow luminescence in GaN, Appl. Phys. Lett. 69 (1996) 4. [20] N. Jo [21] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Pekin-Elmer Corporation, Minnesota, US, 1992. [22] V.F. Agekyan, E.V. Borisov, Optical and electrial properties of GaN: Si_Based microstructures with a wide range of doping levels, Phys. Solid State 57 (2015) 4. [23] G.B. Stringfellow, Organometallic Vapor-phase Epitaxy: Theory and Practice, USA, 1999, pp. 141e143.
Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
The anti-surfactant effect of silane on the facets-controlled growth of GaN nanorods by MOCVD J.Z. Li a, Z.Z. Chen a, *, S.F. Li b, Q.Q. Jiao a, Y.L. Feng a, S.X. Jiang a, Y.F. Chen a, T.J. Yu a, B. Shen a, G.Y. Zhang a, b a
State Key Laboratory of Artificial Microstructure and Mesoscopic Physics, School of Physics, Peking University, Beijing 100871, China Dongguan Institute of Optoelectronics, Peking University, Bldg No.1, Technology & Innovation Park Songshanhu, 523808, Dongguan, China
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 April 2016 Received in revised form 20 May 2016 Accepted 23 May 2016 Available online 24 May 2016
N-polar GaN nanorods were selective area grown by continuous mode metalorganic chemical vapor deposition (MOCVD) under a Ga-rich and high silane flow condition. The interruption comparing with continuous supply of silane flow was performed to study the role of silane flux. High resolution scanning electron microscopy (SEM), x-ray diffraction (XRD), cathodoluminescence (CL) and x-ray photoelectron spectroscopy (XPS) measurements were performed. The enhanced vertical growth rate was achieved as 42 mm/h and sharp smooth m-plane, r-plane and c-plane facets were obtained for the nanorods with high silane flux. SieN bonds were clarified to be formed on the surface of the nanorod by XPS spectra. The silane acting as anti-surfactant was suggested to explain the diffusion and incorporation of the species on the facets of GaN nanorods. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Recently, GaN based nanorods have attracted increasing attention due to promising applications in solid-state lighting [1,2], visible light communication [3,4], and water photolysis [5]. Bottom-up and top-down are two main kinds of methods to fabricate the GaN-based nanorods. Threading dislocation free, nonpolar facet active layer, high growth rate and 3D LED structure make bottom up method more attractive to the industry [6e8]. Hersee et al. reported the growth of high-quality GaN nanorods and uniform nanorod arrays using and the selective area growth (SAG) with a pulsed process [9]. The growth rate is comparable to that of the conventional one, and the pulsed flux control may be difficult for many metal organic chemical vapor deposition (MOCVD) system. Bergbauer et al. performed a continuous flux growth of GaN nanorods directly on sapphire using SAG [10]. They achieved a high growth rate as 25 mm/h for N-polar GaN nanorod. In the continuous growth mode a lot of parameters were studied on the morphology and uniformity of nanorods, such as V/III ratio, temperature, polarity, carrier gas and doping, etc [11e14]. Growth kinetics and mass transport mechanisms of GaN nanorods are demonstrated by some reports [1,3,8]. The silane is often used to form a vertical nanorod. However, the role of Si doping on nanorod growth is still under debate [14]. Generally, Si acts as a shallow donor in GaN. Koester et al. found early that a high Si-dopant concentration about 1019 cm3 promoted vertical growth in MOCVD [14]. According to results of density functional theory (DFT), the catalytic role of the Ga
* Corresponding author. E-mail address: [email protected] (Z.Z. Chen). http://dx.doi.org/10.1016/j.spmi.2016.05.034 0749-6036/© 2016 Elsevier Ltd. All rights reserved.
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bilayer was suspected with Si incorporation. Tessarek et al. reported that the formation of a SiNx layer prevents the growth on m-planes, enhances the mobility of atoms, and promotes the vertical growth [13]. However, there are few reports similar to their transmission electron microscopy (TEM) images for SiNx layer [15]. SiNx is formed on the sidewall with necessary condition, which is not likely within the growth window for GaN nanorods. Wang et al. reported that the silane flux does not influence the diffusion length of species on substrate, while has much more effect on that of the sidewall [8]. The vertical growth rate of nanorods increases 3 times when the silane flux increases from 16.5 to 165 nmol/min. In our previous work, GaN growth on nano-scale patterned sapphire substrates (NPSS) was also controlled by silane during the 3D growth of GaN islands. The high silane flux suppressed the lateral growth [16]. It is well known the silane flux is the key parameter for growing high quality GaN nanorods, while it should be further clarified how the silane affects the diffusion and incorporation of species on different planes. In this paper, a continuous-flux selective area growth of GaN nanorods was performed on SiO2 mask/sapphire substrate by MOCVD. To investigate the effect of silane injection on the growth and optical properties, the silane flux was changed in second growth stage after the GaN nanorods had formed. The morphology and crystal quality were characterized by scanning electron microscopy (SEM), x-ray diffraction (XRD). Cathodoluminescence (CL) spectra at room temperature were performed to study the optical property. X-ray photoelectron spectroscopy (XPS) was used to investigate the influence of silane flux on the binding energies of Ga, N, O and Si atoms, which were corresponded to the chemistry of these atoms. Meantime, the effect of silane injection on the optical properties and growth mechanism of the N-polar GaN nanorods was discussed. 2. Experimental Firstly 70 nm-thick SiO2 layer was deposited on sapphire substrate by plasma-enhanced chemical vapor deposition (PECVD). Then nanoimprint lithography was adopted to transfer the nano holes pattern onto the SiO2 layers, followed by inductively coupled plasma (ICP) etching to open the holes in the SiO2 mask layer. The holes distributed hexagonally on the mask. The diameter is 700 nm, and the pitch is 1.5 mm. After using the acetone and ethanol to chemical cleaning, the templates were introduced into a 3 2 inch FT CCS Thomas Swan MOVPE system for GaN nanorods growth. A thermal baking step was employed at 1100 C for 15 min in a H2 carrier environment. Then a nitridation step was performed on the growth templates at temperature of about 1080 C with 4000 sccm NH3 and N2 carrier gas, and the total pressure is set to be 95 torr. Next, the temperature directly ramped up to 1060 C and the trimethylgallium (TMGa) flow was introduced into the reactor for GaN nanorods growth. The V/III ratio was kept as low as about 50. The flow ratio of H2/N2 carrier gas was kept as 2:1. Silane was injected into the reactor with the flux of 40 sccm simultaneously. The Sample A was grown under the above conditions for 500 s. In order to research the effects of silane on GaN nanorods growth further, another 500 s step growth was performed for three samples under the same growth conditions except the silane flux. Sample B, C and D corresponded to the silane flux of 0, 20 and 40 sccm, respectively. The morphology and crystal structure of the GaN nanorods were evaluated by the SEM (Nova Nano SEM 430) and XRD (Bruker D8, Cu Ka1 X-ray source l ¼ 1.5406 Å) measurements. CL measurements were performed by the FEI Quanta 200F equipped on the SEM instrument. XPS (Axis Ultra) was measured to find the binding energy of the main elements and study the chemical reactions on the growth surface. 3. Results and discussion The SEM image for SiO2 mask on sapphire substrate is shown in Fig. 1a. The pitch and size of the holes array are 1500 and 700 nm. Under the above growth conditions, GaN nanorods will be grown selectively in the holes. Fig. 1b and c shows the side- and top-view of SEM images of Sample D. The GaN hexagonal nanorods were formed with vertical sidewall, which can be assigned as {1e100} m-planes according to the reference edge of substrate. The top of the nanorod is composed of smooth c- and r-planes. The average diameter and height of the nanorods are 852 nm and 7.47 mm, respectively. The aspect ratio of the nanorod is about 8.8. Fig. 1d shows 2q-u scan curves of XRD for Sample AeD. Each curve clearly reveals a strong diffraction peak of the GaN (0002), which indicates that wurtzite GaN nanorods were preferentially grown in the [0001] direction [17]. The precise data for Sample AeD are 34.56, 34.54, 34.55 and 34.56 , respectively. The corresponding lattice constant c parameters are 0.5186, 0.5189, 0.5188 and 0.5186 nm, which indicate the little tensile strain in GaN nanorods. The higher silane flux leads to more strain relaxation. Contrary to the two dimensional growth on the heterogeneous substrate, the nanorod growth seems likely to strain relax because there is less lateral confinement. In order to obtain the details of GaN nanorods morphologies, SEM images are obtained with 20 -tilted angle, as shown in Fig. 2. The statistics of geometrical sizes are listed in the Table 1 from plenty of SEM images. In Fig. 2a, it is observed that the nucleation is occurred at top-right corner of the holes consistently, which may be result from the flow direction. The top is flat rather than pyramidal shape, which means the N-polar GaN growth at the first stage [1,11]. For Sample B without silane supply in the second step, the rods are fat and short. Most of the neighbouring nanorods coalesced to the irregular micro-rods. Compared with the geometrical size of Sample A, both of the lateral and vertical growth rates were about 7e8 mm/h, and the aspect ratio was reduced to 1.8. The vertical growth rates in the second stage are 14 and 42 mm/h for Sample C and D, respectively. When the silane is supplied in the second step, the vertical growth rate is rapidly increased while the lateral growth rate is suppressed. Under such high silane flux, the species from the sidewall impingements and substrate diffusion
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Fig. 1. a: SEM image for the SiO2 mask on the sapphire substrate; b: side view and c: top view of SEM images of GaN nanorods, respectively; d: XRD 2q/uscan curves for four GaN nanorod samples.
contributed to the vertical growth rate instead of the lateral growth. The growth rate can increase further when the substrate surface area is not large. However, the crystal quality will be degraded when the silane increases more [8]. The magnified images of the top side of the nanorods for Samples B and D are shown in Fig. 2e and f, respectively. For Sample B, the c-plane facets are rough on the top side, while r-plane facets are smooth. There is less r-plane area than the cplane one on the top of the single rods. For Sample C with the moderate silane flux, a few c-plane facets are not smooth, and they are minority in the whole top facets. The rough c-plane facets appearance can be attributed to less diffusion ability under such low V/III ratio. So the growth rate becomes high and the growth deviates from the mass transport limited process, which leads to rough surface. The diffusion lengths on m- and r-planes are long enough for species to find their equilibrium positions, which leads to the smooth sidewall and r-plane facets. For Sample D, both r-plane and c-plane facets are smooth, and the c-plane facets are minority on the top of the nanorods too. Silane plays a role of the antisurfactant for the species [13,15]. The Ga and N atoms can reach to its equilibrium position, which leads to the smooth facets. For the evolution of nanorod top side with the silane flux, the Wulff growth theory reveals that the planes remain with low growth rate [18]. With high silane flux, species diffusion abilities on all the planes are enhanced simultaneously. The surface mass transportation is strengthened for all of the facets growth. However, the nucleation energy will be different for different planes. Due to the effect of the dangling bonds, the c-plane may be with the smallest nucleation energy and highest growth rate. Thus the c planes tend to disappear with increasing silane flux. Comparing, the growth rate on r-plane seems not lower obviously than that on c-plane for Sample B. So the c-plane facets are still dominant on the top of the un-coalesced nanorods. It indicates that the silane flux suppresses the r- and m-plane facets growth significantly due to the antisurfactant effect. A high silane flux also strongly influences optical properties of GaN nanorods. The optical properties of the GaN nanorods are characterized by room temperature CL, as shown in Fig. 3. The typical panchromatic CL image of one single nanorod from Sample D is demonstrated in the insert. GaN near-band edge emission (NBE) and yellow defect luminescence (YL) are included in the spectra, and located at 363 and 580 nm, respectively. For Sample A and D with 40 sccm silane flux, NBE is
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Fig. 2. (aed) 20 -tilted SEM images of Sample A, B, C and D, respectively; Enlarge view of top part for (e) Sample B and (f) Sample D.
much stronger than YL. With the growth time increase, the NBE becomes stronger and the YL is suppressed more. For Sample B, the YL dominates in PL spectrum. As to Sample C with 20 sccm silane flux, NBE is a bit stronger than YL. As discussed above, the antisurfactant effect of silane leads to high crystal quality since the ratio of NBE to YL increased. The intensity of the YL is associated to lattice defects, especially to Ga vacancy and O impurity complex. Substitution Si impurities are found to be most likely candidates for the donor defects because of the strikingly low formation energies for
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Table 1 The geometrical sizes of the GaN rods arrays by different conditions. Item
Sample A
Sample B
Sample C
Sample D
Stage I (500 s) silane flux Stage II (500 s) silane flux Diameter Height Aspect ratio
40 sccm e 695 nm 1.54 mm 2.2
40 sccm 0 sccm 1392 nm 2.61 mm 1.8
40 sccm 20 sccm 811 nm 3.44 mm 4.2
40 sccm 40 sccm 852 nm 7.47 mm 8.8
Fig. 3. Room temperature CL spectra of single GaN nanorod from Sample A, B, C and D with an acceleration voltage of 15 kV. Inset: the panchromatic CL image of one nanorod from Sample D.
SiGa with high silane flux. As we known, under n-type conditions the Ga vacancy is the dominant native defect; all other native defects are much higher in energy, implying negligible concentrations [19]. Using state-of-the-art first-principles calculations, Neugebauer et al. found that the concentration of the VGa-ON complex was at least an order of magnitude smaller than that of SiGa [20], which means the YL could be suppressed further, as the CL spectra shown. Meanwhile, the line width of NBE becomes wider as a consequence of the lattice disorder induced by the impurities. In this work, Ga vacancies are less likely to form under low V/III ratio conditions. Some Si atoms occupy Ga vacancies, which will be discussed in detail later. Moreover, an additional peak appears around 389 nm. It may correlates with a certain donor-acceptor pair (DAP) like SiGa-VGa, which needs verified further. Fig. 4 shows the high-resolution XPS spectra for GaN nanorods grown with different silane flux. There are two peaks assigned as Si 2p and Ga 3p in Fig. 4a according to the handbook of XPS [21]. With the silane flux increasing, the Si 2p peak is stronger and binding energy is lower than those without silane flux. Si 2p peaks located at 102.1 and 102.2 eV for Sample C and D are related to nitride. Si 2p peak at 102.5 eV for Sample B is partly corresponding to silica, for the O impurity is much higher than other samples. The typical binding energy for SieO is 103.3 eV. Ga and N mainly come from the GaeN bonds in the GaN crystal. As to Ga 3p peaks, there are 0.3 eV shifts for Sample C and D compared to Sample B. For N 1s peaks at
Fig. 4. High resolution XPS spectra of (a) Si 2p and Ga 3p, (b) N 1s, (c) O 1s for GaN nanorods of Sample B, C and D.
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394.5 eV, þ0.3 eV shifts are observed contrarily as shown in Fig. 4. The main reason should also include the contributions of NeSi bonding states arising from SiGa located in Ga atom sites within the crystal, as revealed by the Si 2p spectra. The shoulder peaks at 392 and 389 eV cannot be found in N 1s contents in the handbook [21]. They may belong to certain shell electrons of other elements. The intensity of the O 1s peak for Sample C and D are much weaker than that for Sample B. The concentration of the VGa-ON complex is suppressed which is consisted with the above CL results. The silane flux leads to the chemical environment change in the lattice in two possible ways. One is that Si atoms institutes Ga atoms heavily, which makes the lattice shrink as calculation from the XRD data above. The heavily Si doping causes the wide NBE in CL, which means the nanorod become defective [8]. However, the NBE in our work is strengthened which indicates the crystal quality is improved. Actually, it’s hard to replace the Ga atom within the crystal for Si, even the Si/Ga reach to 0.6 when the silane flux is 40 sccm. Agekyan et al. reported that the typical concentration of Si incorporated into gallium nitride was about 1019 cm3) [22], and the concentrations just increase slightly as the silane flux increase heavily, as shown in Fig. 4a. The moderate doping level was suspected, which should be clarified in the future work. And the other way is that the silane acts as the anti-surfactant during the growth, which has a significant impact on the growth processes of the GaN nanorods. When the silane flux is high, a little part of Si atoms incorporates into the lattice, while most of Si atoms accumulate on the surface. The silane or its decomposition products cover GaN nanorods surface to prevent Ga incorporating easily, which means Ga atom has a larger diffusion length on the nanorods surface. The diffusion length makes Ga and N atoms find their equilibrium positions-exact lattice sites properly. On the contrary, Ga atoms will incorporate into the lattice on the c plane of nanorods top part without silane injection. The presence of high silane flux leads to smooth of the surface for all facets of GaN nanorods owing to the elimination of step bunching for nanorods growth [23]. The excellent mobility of the species on the surfaces contributes to the strain relaxation and high crystal quality as the growth mode described above. The understanding of anti-surfactant effects during the GaN nanorods SAG MOCVD growth is still poor. The influence of high silane flux on the sticking coefficient and the related Schwoebel barrier height as well as the nucleation barrier at the step edge need further study [23]. However, the high silane flux acts as anti-surfactant during MOCVD growth could be an important tool used for the control of nanorods structures and materials properties. 4. Conclusion In summary, the SAG of N-polar GaN nanorods has been realized by a continuous growth mode with high silane flux in MOCVD. The vertical growth rate was enhanced to 42 mm/h under 40 sccm silane flux and the lateral growth was almost suppressed. As to the optical properties, the silane injection enhanced the NBE emission and suppressed the YL luminescence intensity, for the silane could decrease the Ga vacancies and O impurity, which are responsible for the YL in GaN. Strong Si 2p peaks in the XPS spectra were assigned to SieN bonds formed on the surface. SiNx full coverage and Si heavily doping were excluded by SEM and CL results. The antisurfactant role of silane flux is suggested. The diffusion lengths on the m-plane and rplane are strengthened more than c-plane. The Ga and N species would find their exact sites in the lattice on different facets. High vertical growth rate, strain relaxation and few defects were realized with high silane flux. Acknowledgements This work was supported by projects of Natural Science Foundation of China under Nos. 61334009, 60876063, 61076012 and National Key Basic Research Special Foundation of China under Nos. TG2011CB301905, TG2013CB328705. This work was also supported by Guangdong Innovative Research Team Program (No. 2009010044). References [1] S.F. Li, A. Waag, GaN based nanorods for solid state lighting, J. Appl. Phys. 111 (2012) 071101. [2] J.K. Huang, C.Y. Liu, T.P. Chen, et al., Enhanced light extraction efficiency of GaN-based hybrid nanorods light-emitting diodes, IEEE J. Sel. Top. Quantum Electron. 21 (2015) 1. [3] N. Mohsen, D.F. Feezell, Optical properties of plasmonic light-emitting diodes based on flip-chip III-nitride core-shell nanowires, Opt. Express 22 (2014) 29445e29455. [4] S.C. Zhu, Z.G. Yu, L.X. Zhao, et al., Enhancement of the modulation bandwidth for GaN-based light-emitting diode by surface plasmons, Opt. Express 23 (2015) 13752. [5] S.Z. Fan, B. AlOtaibi, Z.T. 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