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Growth of AlN nanostructures by a rapid thermal process
Solid State Communications 139 (2006) 522–526 www.elsevier.com/locate/ssc
Growth of AlN nanostructures by a rapid thermal process Philippe F. Smet ∗ , Jo E. Van Haecke, Dirk Poelman Department of Solid State Sciences, Ghent University, Krijgslaan 281-S1, 9000 Gent, Belgium Received 28 February 2006; received in revised form 15 June 2006; accepted 14 July 2006 by G. Abstreiter Available online 1 August 2006
Abstract Aluminum nitride nanorods were grown during rapid thermal annealing of multi-layered Al2 S3 /BaS thin films. Depending on the thickness ratio between the BaS and Al2 S3 layers, nanowires or straight nanorods were obtained. Typical dimensions for the nanorods were a diameter in the range of 50–100 nm and a length of 2–5 µm. The nanostructures are formed upon annealing at a relatively low temperature of 900 ◦ C when aluminum evaporates from the thin film, but remains trapped between the thin film surface and the Si wafer, which is used as a support during the annealing. The nitrogen is provided by N2 gas flushed through the annealing chamber. High-resolution transmission electron microscopy showed crystalline, wurtzite-structured AlN nanorods. The growth mechanism in terms of thin film composition, annealing parameters and the role of catalysts is discussed. c 2006 Elsevier Ltd. All rights reserved.
PACS: 81.05.Ys Keywords: A. Nanostructures; A. AlN; A. BaAl2 S4 ; B. Thermal annealing
1. Introduction One-dimensional nanostructures such as nanotubes and nanowires are currently attracting a lot of attention, from both scientific and technological points of view. More specifically, aluminum nitride offers promising properties for future applications, such as field emission displays [1]. AlN nanostructures, both in hexagonal and cubic phases, have already been prepared with several techniques [2], including nitrogen arc discharges [3], gas-source molecular beam epitaxy [4] and thermal chemical vapour deposition [5]. A wide variety of nanostructures have been obtained, whether or not arranged in arrays: nanorods, nanowires, nanorings, nanocones and even Eiffel-tower-shaped nanotips [6]. Cubic AlN nanowires, with diameters of 30–100 nm and lengths up to 700 nm could be prepared with a highly non-equilibrium arcplasma method, at temperatures of about 6000 ◦ C [7]. 15 nm wide hexagonal nanowires resulted from a reaction at 1100 ◦ C of Al powder in a NH3 –N2 atmosphere [8]. The use of Ni as a catalyst yielded narrow and uniform nanowires. Aligned, but polycrystalline nanowires could be grown with the same ∗ Corresponding author. Tel.: +32 9 264 43 53; fax: +32 9 264 49 96.
E-mail address: [email protected] (P.F. Smet). c 2006 Elsevier Ltd. All rights reserved. 0038-1098/$ - see front matter doi:10.1016/j.ssc.2006.07.017
method using a confinement of anodic porous alumina [9]. Recently, AlN nanorings were prepared by evaporating an Al–Mn alloy in a N2 -NH3 atmosphere at 1100 ◦ C [10]. In this paper we report the formation of AlN nanorods and nanowires by a specific process of thermal annealing of a multilayered Al2 S3 /BaS thin film. Europium-doped BaAl2 S4 thin films are currently studied as blue-emitting phosphor layers in thin film inorganic electroluminescent displays [11]. In order to obtain such films, we implemented a multi-layer technique in which a stack of alternating BaS and Al2 S3 thin films is deposited with electron beam evaporation [12]. Up to a substrate temperature of 500 ◦ C, the reaction between the constituent layers is limited, and a post-deposition thermal annealing at 800–900 ◦ C is necessary to obtain crystalline BaAl2 S4 thin films. During electron microscopy investigation, AlN nanostructures could be observed on the thin film surface, depending on the annealing conditions. Several deposition and annealing parameters were varied to optimize the formation of nanostructures and, more specifically, to elucidate the growth mechanism of these structures. 2. Experimental procedure Typical multi-layered Al2 S3 /BaS thin films consist of 23 alternating layers of BaS and Al2 S3 , with a total thickness
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of about 1 µm. The overall stoichiometry can be tuned by changing the thickness ratio of both types of layers. These multi-layered thin films were deposited on a silicon substrate, at a substrate temperature of 400 ◦ C. Top and bottom layers were BaS, because of its lower sensitivity to humidity than Al2 S3 , as exposure of bare Al2 S3 thin films to ambient air immediately leads to degradation. More details on source powder treatment and deposition conditions were reported earlier [12]. If required, metallic Al layers were deposited by electron beam evaporation, while Au layers were deposited by DC sputtering. During the rapid thermal annealing (RTA) process, a sample (typical 2 × 2 cm2 ) is placed onto a Si wafer (acting as a substrate holder), which is rapidly heated (up to 60 ◦ C/s) by an array of tungsten halogen lamps, placed below and above the Si wafer. Pure nitrogen or argon (>99.9999%) is flushed through the chamber, at a pressure of about 1.5 × 105 Pa with a flow rate of 2 slm. An increased flow of nitrogen (15 slm) is used to cool the sample (from 900 to 300 ◦ C in less than a minute). The annealing processes were performed using an AST superheat system 1000, allowing accurate temperature profiling. SEM images were obtained using an FEG-SEM (FEI Quanta 200 F) equipped with EDX (EDAX Genesis 4000) for elemental analysis. HRTEM measurements were performed on an FEI Tecnai G2 TEM including an imaging Cs corrector and on a Jeol JEM-2200FS TEM system equipped with EDX, which allowed elemental analysis during HRTEM work.
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Fig. 1. SEM cross-section of a multi-layered Al2 S3 /BaS thin film (molar ratio 1.2/1) on a Si substrate annealed at 900 ◦ C for 1 min, as seen from an angle grazing at the surface.
3. Experimental results Fig. 1 shows the cross-section of a multi-layered Al2 S3 /BaS thin film (molar ratio 1.2/1) on a Si substrate annealed at 900 ◦ C for 1 min, as seen from an angle grazing at the surface. EDX (energy dispersive x-ray analysis) and HRTEM (high-resolution transmission electron microscopy) analysis revealed that the nanorods were composed of crystalline AlN, with a wurtzite structure. The AlN nanorods are distributed homogeneously over the entire thin film surface and show high uniformity in length and diameter. Most of the nanorods are orientated more or less perpendicular to the thin film surface (Fig. 1), and hence they could be called ‘quasi-aligned’ as the orientation is similar to that of the AlN nanotips reported by Shi et al. [5]. Occasionally, merged nanorods composed of two or three single-crystalline components are formed. Fig. 2 shows HRTEM images of a single-crystalline nanorod with a polycrystalline droplet-like tip. The inset in Fig. 2(b) shows the magnification at the edge of the nanorod. The spacing of 0.27 nm between two adjacent lattice fringes is consistent with that of the (100) planes of hexagonal AlN. Furthermore, no amorphous phase was observed at the outer regions of the nanorods. EDX measurements (not shown), both in SEM and TEM, confirmed the composition to be AlN. The following deposition and annealing parameters were varied to obtain more information on the growth mechanism: the Al2 S3 /BaS molar ratio in the multi-layered thin films, the annealing conditions (sample orientation, temperature and duration) and the influence of catalysts on the AlN formation.
Fig. 2. (a) TEM image of an AlN nanorod. The arrow points to where the image in (b) was taken. The inset in (b) shows an enlarged HRTEM image at the edge of a nanorod.
AlN nanorods can be obtained on annealed multi-layered thin films with an Al2 S3 /BaS molar ratio of 1/1 or higher. For a ratio of 1/1, the nanorods have a typical length of 1 µm and a diameter of 100–150 nm. The diameter is remarkably uniform over the length of the nanorods. Increasing the molar ratio (while keeping the total thin film thickness constant) leads both to higher needle density and to increased length and aspect ratio. For a ratio of 1.5/1, the typical nanorod length is 2–3 µm. A further increase of the Al2 S3 /BaS ratio to 2/1 leads to even more, but less uniform nanostructures. Besides nanorods (with a much higher aspect ratio), nanostructures are formed with a length up to 10 µm and diameter of only 20 nm. As they are curved and do not grow in a single direction, they can be called nanowires (Fig. 3). An annealing temperature of 900 ◦ C is required for the formation of nanostructures. Upon a reduction of the annealing temperature to 875 ◦ C, no nanostructure growth is observed, although Al-rich ‘droplets’ are observed randomly distributed over the thin film surface. Hence, the outdiffusion of Al alone is not a sufficient condition for the nanostructure growth. Annealing temperatures higher than 900 ◦ C (studied
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Fig. 3. SEM cross-section of a multi-layered Al2 S3 /BaS thin film (molar ratio 2/1) on a Si substrate annealed at 900 ◦ C.
up to 1000 ◦ C) do not change the morphology of the formed nanostructures drastically. Increasing the duration of the annealing beyond 1 min only leads to a slight increase in the nanorod’s length. Longer nanostructures can also be obtained by increasing the total multi-layer thickness. The tips of the nanowires are more or less round-shaped and slightly wider than the diameter of the nanorods. They generally consist of multiple crystalline and amorphous regions. TEM–EDX measurements revealed clear compositional differences between the nanorods (i.e. AlN) and the tips. Only a weak nitrogen signal was detected in the tips compared to the nanorods. The main element in the tips is Al, with traces of Ba and occasionally a considerable contribution of Si. It was shown that the presence of Si originates from the wafer which is used as the substrate holder, as annealing on Al2 O3 resulted in the absence of Si in the tips while the nanostructure formation was similar. 4. Discussion The following growth mechanism can be proposed. In a previous study, we observed that the Al2 S3 layers in the multilayered structure were considerably sulfur deficient [13] and that part of the Al was in an unbound (metallic) state. During the annealing process at 900 ◦ C, Al partially diffuses out of the multi-layered structure. Similar effects were observed in sputtered SrGa2 S4 thin films by Nakajima et al., where annealing at 850 ◦ C in Ar resulted in a partial re-evaporation of Ga [14]. In our multi-layered devices, the presence of a large fraction of metallic Al will promote this loss from the thin film. During annealing studies of BaAl2 S4 :Eu thin films deposited with dual source e-beam evaporation, it was noticed by Inoue et al. that an Al2 O3 layer had formed between the BaAl2 S4 :Eu layer and a top ZnS buffer layer, again showing the mobility of Al [15]. The liberated aluminum is then trapped between the thin film surface and the Si wafer (step (a) in Fig. 4). Turning the sample upside down resulted in the absence of nanowires. As no Alrich droplets were observed with SEM on the surface of these
Fig. 4. Schematic representation of the AlN growth mechanism (not to scale). The substrate with the Al2 S3 /BaS thin film lies on the supporting Si wafer. Both are heated by two arrays of tungstens lamps. Nitrogen is flushed through the sample chamber. Three different steps in the growth mechanism are indicated, (a), (b), (c), and referred to in the text.
thin films, the nitrogen flux apparently sweeps the liberated Al away from the thin film surface before any condensation can occur. The presence of the Al-rich tips at the end of the nanostructures supports the vapour–liquid–solid (VLS) growth mechanism, already observed in AlN [9] and InN [16]. A molten droplet of Al is formed on the thin film surface and nitrogen from the process atmosphere is dissolved in the droplet (step (a) in Fig. 4). Once supersaturation is reached, AlN precipitates out of the droplet. The AlN nanowire starts to grow and in this case the nanostructure/droplet interface is pushed away from the thin film surface (step (b) in Fig. 4). The driving force behind the unilateral nanostructure formation is the symmetry breaking liquid–solid interface [17]. Generally speaking, the diameter of the nanostructure in catalytic VLS growth is determined by the size of the catalyst particle [17]. It was indeed observed that a good relationship existed between the diameter of the AlN nanostructures and the diameter of the tip. Furthermore, the diameter of the nanostructures and the tips decreases as the Al2 S3 /BaS ratio increases. Apparently the associated increased outdiffusion of Al leads to the formation of more but also smaller Al droplets on the surface, enhancing the growth of nanostructures with a smaller diameter, finally leading to nanowire growth instead of nanorods. In principle, the nanostructure growth continues as long as there is a supply of aluminium and nitrogen (step (c) in Fig. 4). The outdiffusion of Al and the AlN growth occur very fast, as annealing for 15 s at 900 ◦ C is sufficient for the formation of nanostructures. Increasing the annealing time beyond one minute still leads to a slight increase in the length of the nanowires. This shows that there is a continuous outdiffusion of Al from the multi-layered thin film, although the largest effect takes place at the beginning of the annealing.
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Fig. 5. SEM image of the surface of an Al thin film annealed at 900 ◦ C in nitrogen.
The relatively low formation temperature could point to the presence of a catalyst. Probably Ba acts as a catalyst, as annealed Al2 S3 /CaS multi-layered thin films showed only reduced nanostructure growth. Although it has been reported that Ba can play a catalytic role in the dissociation of nitrogen [18], more detailed research is necessary to clarify whether the nanostructure growth is a purely self-catalytic process or assisted by the presence of (trace amounts of) barium. The occasional presence of Si in the nanostructure tips results from the contact between the Al-rich tips and the supporting Si wafer on which the annealing is performed. Given the process temperature of 900 ◦ C, which is well above the eutectic temperature of Si–Al, Si diffuses from the Si wafer into the Al-rich tips upon contact. It was ruled out that the Si plays a catalytic role, as similar nanostructures could be obtained under Si-free circumstances. As Au is a common catalyst for the synthesis of III–V nanostructures, we investigated its role by depositing a 10 nm thick Au layer on top of the multilayered Al2 S3 /BaS prior to annealing. Although this resulted in a slightly higher nanostructure density, no considerable differences in terms of nanostructure length, diameter or uniformity were observed. One could argue on whether the multi-layer design could not radically be simplified, and still yield nanostructure growth. Experiments with pure Al thin films (in combination with BaS or Au top layers) did not result in the formation of nanostructures. Instead, a thin and granulated AlN top layer can be formed (Fig. 5). Although outdiffusion of Al is the driving force behind the nanostructure formation in the multi-layered Al2 S3 /BaS thin films, this apparently happens in a controlled way in the multi-layered design. This not only limits the number of growth sites, but also provides a controlled, continuous outdiffusion of Al after the nanostructure growth is initiated. Hence the multi-layered design is required for the formation of nanorods and nanowires. In this communication we have shown an alternative method for obtaining crystalline AlN nanostructures and explained
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the underlying growth mechanism. Despite the fact that the nanostructure yield is lower than those from several other methods (see the introduction), this method certainly has some advantages. The processing temperature is relatively low (900 ◦ C) compared to those for several other techniques. Furthermore, the length and aspect ratio of the nanostructures (nanowires or nanorods) can be controlled by variation of the Al2 S3 /BaS ratio. For the nanorods, most of them are aligned (i.e. more or less perpendicular to the sample surface) without the use of templates. We observed a nanorod density in the range of 107 –108 /cm2 , which is very similar to the density obtained by Tang et al. using a mobile nitrogen arc discharge [3]. Also the nanorods’ morphology (length, diameter and ball-like tip) is comparable to that of the work by Tang et al., where a better alignment perpendicular to the surface is obtained. For these nanorods, the field emission properties were studied and it appeared to be a promising field emission material. 5. Conclusions In this paper we presented a reproducible method for obtaining wurtzite AlN nanowires and nanorods at a relatively low temperature of 900 ◦ C, most of the latter being perpendicular to the sample surface. The growth is based on the outdiffusion and trapping of Al upon annealing of a multi-layered Al2 S3 /BaS thin film in a nitrogen atmosphere. The morphology and the growth mechanism of the AlN nanostructures were investigated by varying the deposition and annealing conditions. It appears that AlN nanostructures are formed following a self-catalytic VLS growth mechanism as Al-rich tips were observed at the ends of the nanostructures. Acknowledgments The authors are grateful to B. Freitag and C. Hetherington for the HRTEM images. PFS and JVH both acknowledge the BOF-UGent. This research is partially sponsored by the FWOVlaanderen (Fund for Scientific Research, Flanders, Belgium). References [1] V.N. Tondare, C. Balasubramanian, S.V. Shende, D.S. Joag, V.P. Godbole, S.V. Bhoraskar, Appl. Phys. Lett. 80 (2002) 4813. [2] V.N. Tondare, Nanotechnology 15 (2004) 1388. [3] Y.B. Tang, H.T. Cong, Z.G. Zhao, H.M. Cheng, Appl. Phys. Lett. 86 (2005) 153104. [4] K.A. Bertness, A. Roshko, N.A. Sanford, J.M. Barker, A. Davydov, J. Cryst. Growth 287 (2006) 522. [5] S.C. Shi, C.F. Chen, S. Chattopadhyay, K.H. Chen, L.C. Chen, Appl. Phys. Lett. 87 (2005) 073109. [6] Y.B. Tang, H.T. Cong, Z.G. Chen, H.M. Cheng, Appl. Phys. Lett. 86 (2005) 233104. [7] C. Balasubramanian, V.P. Godbole, V.K. Rohatgi, A.K. Das, S.V. Bhoraskar, Nanotechnology 15 (2004) 370. [8] Q. Wu, Z. Hu, X. Wang, Y. Lu, K. Huo, S. Deng, N. Xu, B. Shen, R. Zhang, Y. Chen, J. Mater. Chem. 13 (2003) 2024. [9] Q. Wu, Z. Hu, X. Wang, Y. Hu, Y. Tian, Y. Chen, Diamond Relat. Mater. 13 (2004) 38.
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[10] J.H. Duan, S.G. Yang, H.W. Liu, J.F. Gong, B. Huang, X.N. Zhao, J.L. Tang, R. Zhang, Y.W. Du, J. Cryst. Growth 238 (2005) 291. [11] X. Wu, D. Carkner, H. Hamada, I. Yoshida, M. Kutsukake, K. Dantini, Digest of the SID International Symposium 2004, Seattle, USA, 2004, p. 1146. [12] P.F. Smet, D. Poelman, R.L. Van Meirhaeghe, J. Appl. Phys. 95 (2004) 184. [13] P.F. Smet, J.E. Van Haecke, R.L. Van Meirhaeghe, D. Poelman, J. Electron Spectrosc. Relat. Phenom. 148 (2005) 91.
[14] H. Nakajima, H. Kominami, T. Aoki, Y. Nakanishi, Y. Hatanaka, J. Lumin. 87–89 (2000) 1146. [15] Y. Inoue, I. Tanaka, Y. Izumi, S. Okamoto, M. Kawanishi, D. Barada, N. Miura, H. Matsumoto, R. Nakano, Jpn. J. Appl. Phys. 40 (2001) 2451. [16] C.N.R. Rao, F.L. Deepak, G. Gundiah, A. Govindaraj, Prog. Solid State Chem. 31 (2003) 5. [17] M. Law, J. Goldberger, P.D. Yang, Annu. Rev. Mater. Res. 34 (2004) 83. [18] S. Hagen, R. Barfod, R. Fehrmann, C.J.H. Jacobsen, H.T. Teunissen, K. St˚ahl, I. Chorkendorff, Chem. Commun. 11 (2002) 1206.
Growth of AlN nanostructures by a rapid thermal process Philippe F. Smet ∗ , Jo E. Van Haecke, Dirk Poelman Department of Solid State Sciences, Ghent University, Krijgslaan 281-S1, 9000 Gent, Belgium Received 28 February 2006; received in revised form 15 June 2006; accepted 14 July 2006 by G. Abstreiter Available online 1 August 2006
Abstract Aluminum nitride nanorods were grown during rapid thermal annealing of multi-layered Al2 S3 /BaS thin films. Depending on the thickness ratio between the BaS and Al2 S3 layers, nanowires or straight nanorods were obtained. Typical dimensions for the nanorods were a diameter in the range of 50–100 nm and a length of 2–5 µm. The nanostructures are formed upon annealing at a relatively low temperature of 900 ◦ C when aluminum evaporates from the thin film, but remains trapped between the thin film surface and the Si wafer, which is used as a support during the annealing. The nitrogen is provided by N2 gas flushed through the annealing chamber. High-resolution transmission electron microscopy showed crystalline, wurtzite-structured AlN nanorods. The growth mechanism in terms of thin film composition, annealing parameters and the role of catalysts is discussed. c 2006 Elsevier Ltd. All rights reserved.
PACS: 81.05.Ys Keywords: A. Nanostructures; A. AlN; A. BaAl2 S4 ; B. Thermal annealing
1. Introduction One-dimensional nanostructures such as nanotubes and nanowires are currently attracting a lot of attention, from both scientific and technological points of view. More specifically, aluminum nitride offers promising properties for future applications, such as field emission displays [1]. AlN nanostructures, both in hexagonal and cubic phases, have already been prepared with several techniques [2], including nitrogen arc discharges [3], gas-source molecular beam epitaxy [4] and thermal chemical vapour deposition [5]. A wide variety of nanostructures have been obtained, whether or not arranged in arrays: nanorods, nanowires, nanorings, nanocones and even Eiffel-tower-shaped nanotips [6]. Cubic AlN nanowires, with diameters of 30–100 nm and lengths up to 700 nm could be prepared with a highly non-equilibrium arcplasma method, at temperatures of about 6000 ◦ C [7]. 15 nm wide hexagonal nanowires resulted from a reaction at 1100 ◦ C of Al powder in a NH3 –N2 atmosphere [8]. The use of Ni as a catalyst yielded narrow and uniform nanowires. Aligned, but polycrystalline nanowires could be grown with the same ∗ Corresponding author. Tel.: +32 9 264 43 53; fax: +32 9 264 49 96.
E-mail address: [email protected] (P.F. Smet). c 2006 Elsevier Ltd. All rights reserved. 0038-1098/$ - see front matter doi:10.1016/j.ssc.2006.07.017
method using a confinement of anodic porous alumina [9]. Recently, AlN nanorings were prepared by evaporating an Al–Mn alloy in a N2 -NH3 atmosphere at 1100 ◦ C [10]. In this paper we report the formation of AlN nanorods and nanowires by a specific process of thermal annealing of a multilayered Al2 S3 /BaS thin film. Europium-doped BaAl2 S4 thin films are currently studied as blue-emitting phosphor layers in thin film inorganic electroluminescent displays [11]. In order to obtain such films, we implemented a multi-layer technique in which a stack of alternating BaS and Al2 S3 thin films is deposited with electron beam evaporation [12]. Up to a substrate temperature of 500 ◦ C, the reaction between the constituent layers is limited, and a post-deposition thermal annealing at 800–900 ◦ C is necessary to obtain crystalline BaAl2 S4 thin films. During electron microscopy investigation, AlN nanostructures could be observed on the thin film surface, depending on the annealing conditions. Several deposition and annealing parameters were varied to optimize the formation of nanostructures and, more specifically, to elucidate the growth mechanism of these structures. 2. Experimental procedure Typical multi-layered Al2 S3 /BaS thin films consist of 23 alternating layers of BaS and Al2 S3 , with a total thickness
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of about 1 µm. The overall stoichiometry can be tuned by changing the thickness ratio of both types of layers. These multi-layered thin films were deposited on a silicon substrate, at a substrate temperature of 400 ◦ C. Top and bottom layers were BaS, because of its lower sensitivity to humidity than Al2 S3 , as exposure of bare Al2 S3 thin films to ambient air immediately leads to degradation. More details on source powder treatment and deposition conditions were reported earlier [12]. If required, metallic Al layers were deposited by electron beam evaporation, while Au layers were deposited by DC sputtering. During the rapid thermal annealing (RTA) process, a sample (typical 2 × 2 cm2 ) is placed onto a Si wafer (acting as a substrate holder), which is rapidly heated (up to 60 ◦ C/s) by an array of tungsten halogen lamps, placed below and above the Si wafer. Pure nitrogen or argon (>99.9999%) is flushed through the chamber, at a pressure of about 1.5 × 105 Pa with a flow rate of 2 slm. An increased flow of nitrogen (15 slm) is used to cool the sample (from 900 to 300 ◦ C in less than a minute). The annealing processes were performed using an AST superheat system 1000, allowing accurate temperature profiling. SEM images were obtained using an FEG-SEM (FEI Quanta 200 F) equipped with EDX (EDAX Genesis 4000) for elemental analysis. HRTEM measurements were performed on an FEI Tecnai G2 TEM including an imaging Cs corrector and on a Jeol JEM-2200FS TEM system equipped with EDX, which allowed elemental analysis during HRTEM work.
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Fig. 1. SEM cross-section of a multi-layered Al2 S3 /BaS thin film (molar ratio 1.2/1) on a Si substrate annealed at 900 ◦ C for 1 min, as seen from an angle grazing at the surface.
3. Experimental results Fig. 1 shows the cross-section of a multi-layered Al2 S3 /BaS thin film (molar ratio 1.2/1) on a Si substrate annealed at 900 ◦ C for 1 min, as seen from an angle grazing at the surface. EDX (energy dispersive x-ray analysis) and HRTEM (high-resolution transmission electron microscopy) analysis revealed that the nanorods were composed of crystalline AlN, with a wurtzite structure. The AlN nanorods are distributed homogeneously over the entire thin film surface and show high uniformity in length and diameter. Most of the nanorods are orientated more or less perpendicular to the thin film surface (Fig. 1), and hence they could be called ‘quasi-aligned’ as the orientation is similar to that of the AlN nanotips reported by Shi et al. [5]. Occasionally, merged nanorods composed of two or three single-crystalline components are formed. Fig. 2 shows HRTEM images of a single-crystalline nanorod with a polycrystalline droplet-like tip. The inset in Fig. 2(b) shows the magnification at the edge of the nanorod. The spacing of 0.27 nm between two adjacent lattice fringes is consistent with that of the (100) planes of hexagonal AlN. Furthermore, no amorphous phase was observed at the outer regions of the nanorods. EDX measurements (not shown), both in SEM and TEM, confirmed the composition to be AlN. The following deposition and annealing parameters were varied to obtain more information on the growth mechanism: the Al2 S3 /BaS molar ratio in the multi-layered thin films, the annealing conditions (sample orientation, temperature and duration) and the influence of catalysts on the AlN formation.
Fig. 2. (a) TEM image of an AlN nanorod. The arrow points to where the image in (b) was taken. The inset in (b) shows an enlarged HRTEM image at the edge of a nanorod.
AlN nanorods can be obtained on annealed multi-layered thin films with an Al2 S3 /BaS molar ratio of 1/1 or higher. For a ratio of 1/1, the nanorods have a typical length of 1 µm and a diameter of 100–150 nm. The diameter is remarkably uniform over the length of the nanorods. Increasing the molar ratio (while keeping the total thin film thickness constant) leads both to higher needle density and to increased length and aspect ratio. For a ratio of 1.5/1, the typical nanorod length is 2–3 µm. A further increase of the Al2 S3 /BaS ratio to 2/1 leads to even more, but less uniform nanostructures. Besides nanorods (with a much higher aspect ratio), nanostructures are formed with a length up to 10 µm and diameter of only 20 nm. As they are curved and do not grow in a single direction, they can be called nanowires (Fig. 3). An annealing temperature of 900 ◦ C is required for the formation of nanostructures. Upon a reduction of the annealing temperature to 875 ◦ C, no nanostructure growth is observed, although Al-rich ‘droplets’ are observed randomly distributed over the thin film surface. Hence, the outdiffusion of Al alone is not a sufficient condition for the nanostructure growth. Annealing temperatures higher than 900 ◦ C (studied
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Fig. 3. SEM cross-section of a multi-layered Al2 S3 /BaS thin film (molar ratio 2/1) on a Si substrate annealed at 900 ◦ C.
up to 1000 ◦ C) do not change the morphology of the formed nanostructures drastically. Increasing the duration of the annealing beyond 1 min only leads to a slight increase in the nanorod’s length. Longer nanostructures can also be obtained by increasing the total multi-layer thickness. The tips of the nanowires are more or less round-shaped and slightly wider than the diameter of the nanorods. They generally consist of multiple crystalline and amorphous regions. TEM–EDX measurements revealed clear compositional differences between the nanorods (i.e. AlN) and the tips. Only a weak nitrogen signal was detected in the tips compared to the nanorods. The main element in the tips is Al, with traces of Ba and occasionally a considerable contribution of Si. It was shown that the presence of Si originates from the wafer which is used as the substrate holder, as annealing on Al2 O3 resulted in the absence of Si in the tips while the nanostructure formation was similar. 4. Discussion The following growth mechanism can be proposed. In a previous study, we observed that the Al2 S3 layers in the multilayered structure were considerably sulfur deficient [13] and that part of the Al was in an unbound (metallic) state. During the annealing process at 900 ◦ C, Al partially diffuses out of the multi-layered structure. Similar effects were observed in sputtered SrGa2 S4 thin films by Nakajima et al., where annealing at 850 ◦ C in Ar resulted in a partial re-evaporation of Ga [14]. In our multi-layered devices, the presence of a large fraction of metallic Al will promote this loss from the thin film. During annealing studies of BaAl2 S4 :Eu thin films deposited with dual source e-beam evaporation, it was noticed by Inoue et al. that an Al2 O3 layer had formed between the BaAl2 S4 :Eu layer and a top ZnS buffer layer, again showing the mobility of Al [15]. The liberated aluminum is then trapped between the thin film surface and the Si wafer (step (a) in Fig. 4). Turning the sample upside down resulted in the absence of nanowires. As no Alrich droplets were observed with SEM on the surface of these
Fig. 4. Schematic representation of the AlN growth mechanism (not to scale). The substrate with the Al2 S3 /BaS thin film lies on the supporting Si wafer. Both are heated by two arrays of tungstens lamps. Nitrogen is flushed through the sample chamber. Three different steps in the growth mechanism are indicated, (a), (b), (c), and referred to in the text.
thin films, the nitrogen flux apparently sweeps the liberated Al away from the thin film surface before any condensation can occur. The presence of the Al-rich tips at the end of the nanostructures supports the vapour–liquid–solid (VLS) growth mechanism, already observed in AlN [9] and InN [16]. A molten droplet of Al is formed on the thin film surface and nitrogen from the process atmosphere is dissolved in the droplet (step (a) in Fig. 4). Once supersaturation is reached, AlN precipitates out of the droplet. The AlN nanowire starts to grow and in this case the nanostructure/droplet interface is pushed away from the thin film surface (step (b) in Fig. 4). The driving force behind the unilateral nanostructure formation is the symmetry breaking liquid–solid interface [17]. Generally speaking, the diameter of the nanostructure in catalytic VLS growth is determined by the size of the catalyst particle [17]. It was indeed observed that a good relationship existed between the diameter of the AlN nanostructures and the diameter of the tip. Furthermore, the diameter of the nanostructures and the tips decreases as the Al2 S3 /BaS ratio increases. Apparently the associated increased outdiffusion of Al leads to the formation of more but also smaller Al droplets on the surface, enhancing the growth of nanostructures with a smaller diameter, finally leading to nanowire growth instead of nanorods. In principle, the nanostructure growth continues as long as there is a supply of aluminium and nitrogen (step (c) in Fig. 4). The outdiffusion of Al and the AlN growth occur very fast, as annealing for 15 s at 900 ◦ C is sufficient for the formation of nanostructures. Increasing the annealing time beyond one minute still leads to a slight increase in the length of the nanowires. This shows that there is a continuous outdiffusion of Al from the multi-layered thin film, although the largest effect takes place at the beginning of the annealing.
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Fig. 5. SEM image of the surface of an Al thin film annealed at 900 ◦ C in nitrogen.
The relatively low formation temperature could point to the presence of a catalyst. Probably Ba acts as a catalyst, as annealed Al2 S3 /CaS multi-layered thin films showed only reduced nanostructure growth. Although it has been reported that Ba can play a catalytic role in the dissociation of nitrogen [18], more detailed research is necessary to clarify whether the nanostructure growth is a purely self-catalytic process or assisted by the presence of (trace amounts of) barium. The occasional presence of Si in the nanostructure tips results from the contact between the Al-rich tips and the supporting Si wafer on which the annealing is performed. Given the process temperature of 900 ◦ C, which is well above the eutectic temperature of Si–Al, Si diffuses from the Si wafer into the Al-rich tips upon contact. It was ruled out that the Si plays a catalytic role, as similar nanostructures could be obtained under Si-free circumstances. As Au is a common catalyst for the synthesis of III–V nanostructures, we investigated its role by depositing a 10 nm thick Au layer on top of the multilayered Al2 S3 /BaS prior to annealing. Although this resulted in a slightly higher nanostructure density, no considerable differences in terms of nanostructure length, diameter or uniformity were observed. One could argue on whether the multi-layer design could not radically be simplified, and still yield nanostructure growth. Experiments with pure Al thin films (in combination with BaS or Au top layers) did not result in the formation of nanostructures. Instead, a thin and granulated AlN top layer can be formed (Fig. 5). Although outdiffusion of Al is the driving force behind the nanostructure formation in the multi-layered Al2 S3 /BaS thin films, this apparently happens in a controlled way in the multi-layered design. This not only limits the number of growth sites, but also provides a controlled, continuous outdiffusion of Al after the nanostructure growth is initiated. Hence the multi-layered design is required for the formation of nanorods and nanowires. In this communication we have shown an alternative method for obtaining crystalline AlN nanostructures and explained
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the underlying growth mechanism. Despite the fact that the nanostructure yield is lower than those from several other methods (see the introduction), this method certainly has some advantages. The processing temperature is relatively low (900 ◦ C) compared to those for several other techniques. Furthermore, the length and aspect ratio of the nanostructures (nanowires or nanorods) can be controlled by variation of the Al2 S3 /BaS ratio. For the nanorods, most of them are aligned (i.e. more or less perpendicular to the sample surface) without the use of templates. We observed a nanorod density in the range of 107 –108 /cm2 , which is very similar to the density obtained by Tang et al. using a mobile nitrogen arc discharge [3]. Also the nanorods’ morphology (length, diameter and ball-like tip) is comparable to that of the work by Tang et al., where a better alignment perpendicular to the surface is obtained. For these nanorods, the field emission properties were studied and it appeared to be a promising field emission material. 5. Conclusions In this paper we presented a reproducible method for obtaining wurtzite AlN nanowires and nanorods at a relatively low temperature of 900 ◦ C, most of the latter being perpendicular to the sample surface. The growth is based on the outdiffusion and trapping of Al upon annealing of a multi-layered Al2 S3 /BaS thin film in a nitrogen atmosphere. The morphology and the growth mechanism of the AlN nanostructures were investigated by varying the deposition and annealing conditions. It appears that AlN nanostructures are formed following a self-catalytic VLS growth mechanism as Al-rich tips were observed at the ends of the nanostructures. Acknowledgments The authors are grateful to B. Freitag and C. Hetherington for the HRTEM images. PFS and JVH both acknowledge the BOF-UGent. This research is partially sponsored by the FWOVlaanderen (Fund for Scientific Research, Flanders, Belgium). References [1] V.N. Tondare, C. Balasubramanian, S.V. Shende, D.S. Joag, V.P. Godbole, S.V. Bhoraskar, Appl. Phys. Lett. 80 (2002) 4813. [2] V.N. Tondare, Nanotechnology 15 (2004) 1388. [3] Y.B. Tang, H.T. Cong, Z.G. Zhao, H.M. Cheng, Appl. Phys. Lett. 86 (2005) 153104. [4] K.A. Bertness, A. Roshko, N.A. Sanford, J.M. Barker, A. Davydov, J. Cryst. Growth 287 (2006) 522. [5] S.C. Shi, C.F. Chen, S. Chattopadhyay, K.H. Chen, L.C. Chen, Appl. Phys. Lett. 87 (2005) 073109. [6] Y.B. Tang, H.T. Cong, Z.G. Chen, H.M. Cheng, Appl. Phys. Lett. 86 (2005) 233104. [7] C. Balasubramanian, V.P. Godbole, V.K. Rohatgi, A.K. Das, S.V. Bhoraskar, Nanotechnology 15 (2004) 370. [8] Q. Wu, Z. Hu, X. Wang, Y. Lu, K. Huo, S. Deng, N. Xu, B. Shen, R. Zhang, Y. Chen, J. Mater. Chem. 13 (2003) 2024. [9] Q. Wu, Z. Hu, X. Wang, Y. Hu, Y. Tian, Y. Chen, Diamond Relat. Mater. 13 (2004) 38.
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