Fabrication and characterization of boron-related nanowires
Microelectronics Journal 34 (2003) 463–470 www.elsevier.com/locate/mejo
Fabrication and characterization of boron-related nanowires J.Z. Wua,*, S.H. Yuna,b, A. Dibosa, Do-Kyung Kimc, M. Tidrowd a Department of Physics and Astronomy, University of Kansas, Lawrence, KS 66045, USA Department of Material Physics, IMIT, Royal Institute of Technology SE-16640 Kista, Sweden c Department of Material Chemistry, Royal Institute of Technology, SE-14400 Stockholm, Sweden d Missile Defense Agency, 7100 Defense Pentagon, Washington, DC 20301-7100, USA b
Abstract A thermal vapor transport process has been employed for fabrication of boron-related (boron, boron – silicon alloys, and MgB2) nanowire films on Au-coated Si and MgO substrates. Tangled polycrystalline as well as aligned single-crystalline boron nanowires (BNWs) have been obtained and their growth mechanism investigated at different growth temperatures, cooling rates, and vapor sources. The growth temperature was found critical to the nucleation of the BNWs and different growth modes were observed in different temperature ranges. The temperature ramping rate in the cooling process after the high-temperature growth of the BNWs was found crucial for the formation of crystalline structures and for controlling the alignment of BNWs. We have observed that slow cooling at 1 – 5 8C/min resulted in non-textured BNWs, while fast cooling at ,90 8C/min induced crystallization of the non-textured BNWs. We have also found that the alignment of the BNWs depended on the cooling rate. At a slow cooling rate the BNWs were heavily tangled while at a higher cooling rate they aligned well with the normal of the substrate. By manipulating the growth parameters, we have obtained two types of nanowire junctions. One was via fusing two nanowires together and the other, via joining them with another material. q 2003 Elsevier Science Ltd. All rights reserved. Keywords: Superconductivity; Thermal vapor transport process; Nanowires
1. Introduction The recent discovery of superconductivity above 40 K in MgB2 [1] has triggered an intensive interest in boron and boron-related compounds. Boron is the third lightest element in the solid form, but has a high melting temperature near 2300 8C. Its hardness is very close to the diamond. These unique features make boron and boronrelated compounds attractive for various applications including high-temperature lightweight coatings and hightemperature semiconductor electronic devices [2,3]. Boron typically forms three-center electron-deficient bonds and B12 icosahedron serves as the building block for many boron-related compounds. In the bulk or thin film forms, Boron has low electrical conductivity in the semiconductor regime [2,3]. High conductivity in the metal regime, however, may be achievable in novel boron structures. Recent theoretical calculations suggested that boron nanostructures of layered, tubular and fullerene-like solids built from elemental subunits may exhibit novel physical * Corresponding author. Tel.: þ 1-858-534-0403; fax: þ1-858-534-2232. E-mail address: [email protected] (J.Z. Wu).
properties [4]. For example, boron nanotubes may have metallic density of states (DOS) [5] that may lead to higher electrical conductivity in boron nanotubes than in carbon nanotubes [6,7]. Such one-dimensional systems as boron nanotubes and nanowires hence are very promising for applications in field emission devices and other hightemperature electronics. In addition, superconducting MgB2 can be obtained by diffusing Mg into pure Boron [8,9]. In such an ex situ process, pure boron samples serve as the ‘precursor’ for the MgB2 to be formed via Mg diffusion. It is hence necessary to grow Boron nanowires (BNWs) to obtain superconducting MgB2 nanowires [10].
2. Experiments Films of BNWs were prepared on single-crystal (100) Si and MgO substrates using the thermal vapor transport process [10]. Before BNWs growth, some substrates were sputter coated with 5– 20 nm thick Au thin films. Boron (purity 99.99%), B2O3 (purity 99.98%), iodine (purity 99.7%), and Si-powder (purity 99.99%), were used as the source materials. Two types of source pellets were utilized.
0026-2692/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0026-2692(03)00074-0
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One was made mixtures of boron (20 mg), iodine (0.2 mg), and Si (0.1 mg) and the other, boron and boron oxide with approximately 40 wt.%. The source pellet was placed inside the quartz tube, together with the substrates with/without Au coating. The source was in the higher temperature zone while the substrate, in a slightly lower temperature one to allow condensation of the decomposed boron vapor on the substrate. The quartz tube was evacuated to about 10 mTorr, torch sealed and then heated to the processing temperature in the range of 600– 1100 8C (for the source) for 1 – 60 min. The temperature difference between the source and the film was in the range of 100 –200 8C during the BNWs growth. After the BNW growth, the samples were cooled down to room temperature using two different cooling rates of 5 8C/ min. and 90 8C/min, respectively. The morphology of the as-synthesized sample was examined using field-emission scanning electron microscopy (SEM). The chemical composition and distribution were examined using energy dispersive X-ray spectroscopy (EDS) attached to the SEM. Some BNWs were removed from the substrates and placed on Cu grids for analysis using transmission electron microscopy and selected area electron diffraction (TEM/SAED). X-ray powder diffraction (XRD) was used to study the structure of the BNWs films.
3. Results and discussions 3.1. Morphology Fig. 1 depicts a typical SEM image of a BNW films prepared, respectively, on a 5 nm thick Au-coated Si (Fig. 1(a)) and MgO (Fig. 1(b)) substrates. Both samples were annealed at 1100 8C for 30 min. and then cooled slowly at
about 5 8C/min to room temperature. Dense and tangled BNWs can be observed on both samples. These BNWs are several tens to hundreds of micrometers in length and 30– 500 nm in diameter. The diameter of the BNWs seems to correlate with the thickness of the Au catalyst layer and thinner Au catalyst layer resulted in smaller diameters of the BNWs. The growth rate of the BNWs depends sensitively on the growth temperature. In the temperature range we have studied in this experiment, we have found BNW growth at as low as 600 8C at very low growth rate and as high as 1200 8C at extremely high growth rate. Although BNWs have been obtained on both MgO and Si substrates, the surface morphology of the substrate looks very different after the BNWs growth. Fig. 2 depicts a close look of substrates used for BNWs shown in Fig. 1. The MgO (Fig. 2(a)) substrate became very rough after the BNW growth, while Si (Fig. 2(b)) substrate remained smooth. A chemical composition study using SEM/EDS revealed incorporation of Mg and Si into the BNWs when high processing temperatures above 1000 8C were employed. Fig. 3 shows the SEM picture (Fig. 3(a)) of and an EDS line map of chemicals (Fig. 3(b)) on a single BNW grown on Si substrates. The atomic ratio between B and Si is nearly 1:1 along the BNW. Similar result was also obtained on BNWs grown on MgO substrates as shown in Fig. 4. We have found that the cooling rate determines the alignment of the BNWs. when the BNW sample was cooled with the slow cooling rate of 5 8C/min., tangled BNWs were formed throughout the substrate surface. When the sample was cooled with larger cooling rate of 90 8C/min, the alignment of the BNWs to the normal of the substrate was significantly improved and BNWs were parallel to the normal of the substrate [11]. This alignment change was accompanied by the structural change in the BNWs, as we will detail in Section 3.2.
Fig. 1. SEM images of boron nanowire films synthesized on MgO (a) and Si (b) substrates, respectively, at 1100 8C for 30 min followed by slow cooling at 5 8C/min. 5 nm thick Au films were coated on the substrates before the nanowire growth. Iodine and pure boron were used as the vapor source.
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Fig. 2. A closer look at the substrate surface after the boron nanowire films were synthesized on MgO (a) and Si (b) substrates, respectively, at 1100 8C for 30 min followed by slow cooling at 5 8C/min. 5 nm thick Au films were coated on the substrates before the nanowire growth. Iodine and pure boron were used as the vapor source.
3.2. Structure XRD analysis of most of the BNWs obtained using slow cooling process shows many low-intensity peaks suggesting a polycrystalline structure for these BNWs [11]. These diffraction peaks can be indexed to an orthorhombic structure. The average lattice parameters of the BNWs calculated using the observed d values of the XRD spectra are ˚ , b ¼ 7.1 A ˚ , and c ¼ 5.4 A ˚ . Although the XRD a ¼ 9.2 A
data suggested that the nanowires formed are mainly BNWs, no preferred orientation can be identified in this sample. When high cooling rate was employed, only few XRD peaks of (211), (030), (420), and (520) were observed with much enhanced intensity. Among these peaks, (211) is dominant. Using the peak intensity for the (211), (030), (420), and (520), the estimated volume portion of the (211)-orientation phase is more than 60%. This suggests that recrystallization occurs during the fast cooling, which in turn may cause strain along
Fig. 3. SEM image (a) and EDS line-scan (b) on an isolated single BNW made on Si substrate with B2O3 in the vapor source at 1100 8C for 30 minutes. The points (x-axis) in (b) are defined along the BNW from root to tip in (a).
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Fig. 4. SEM/EDS line-scan on an isolated single BNW made on MgO substrate with B2O3 in the vapor source at 1100 8C for 30 min.
the BNWs and lead to better alignment of the BNWs. To confirm the crystallinity on a single BNW, some of BNWs were removed from the BNW films and placed on Cu grids for TEM/SAED study. The SAED patterns on a single BNW made under the same conditions as in Fig. 1(a) and (b), respectively, are shown in Fig. 5(a) and (b) for slow cooling and fast cooling. It is clearly seen that the BNW made in slow cooling have an amorphous structure, while that made via fast cooling, single crystalline structure. In general, the XRD and SAED results suggest that fast cooling promoted nucleation and growth of crystalline structures in the BNWs, consistent with the observation that non-textured materials can be single-crystallized by rapid quenching process [12,13] because the abrupt temperature and pressure gradient restrain
the growth of nuclei during solidification. Since the formation of the crystalline structure and the BNW alignment occur simultaneously during the quench, we speculate that the alignment is caused by the crystallization-induced strain along the BNW. In addition, the mechanical property of the BNW may also change during this transition from polycrystalline phase to single crystalline phase. Further investigation of the changes in BNWs physical properties during such a transition is necessary to confirm this. 3.3. Nucleation mechanism The growth mechanism of nanowires has attracted much attention since it is critical to reach a fine control of
Fig. 5. Selective area electron diffraction (SAED) patterns of BNWs synthesized on Si substrates at 1100 8C for 30 min followed, respectively, by slow cooling at 5 8C/min and fast cooling at 90 8C/min
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nanowire growth, such as nucleation sites, diameters, growth orientations and alignments of nanowires. Several mechanisms have been reported for vapor phase growth of nanowires [14,15]. Two most popular ones include the wellaccepted vapor-liquid-solid (VLS) process and the recently reported oxide-assisted growth (OA). The VLS process was proposed in early 1960s for the growth of large singlecrystal whiskers [16], in which an anisotropic crystal growth results from the presence of liquid alloy/solid interfaces. Metal catalysts are found critical in creating such a liquid alloy/solid interface and may be incorporated via many different ways, such as by mixing the metal catalyst into the material source of nanowires to be grown or by coating a metal catalyst buffer on the substrates. In the OA process, on the other hand, the nanowire nucleation was believed to be the precipitation of the nanoparticles, clothed by an oxide shell, under certain temperature gradients [17,18]. The major differences between nanowires fabricated in the metal catalyzed VLS process and in the OA process are the composition and structure of nanowire end tips. In the former case, the tips of nanowire have either spherical or hemispherical shapes that contain the metal catalyst. When a metal catalyst buffer is coated on the surface of the substrate, this catalyst will be carried over from the nucleation site on the substrate to the tip of the nanowire. In the latter case, the nanowire end tips may have different shapes and do not contain any catalytic elements. In growth of BNWs, we have found both VLS and VO growth modes, as well as a hybridized VLS þ VO growth, depending on the vapor source and the growth temperature. When pure boron is used as the vapor source (with small amount of iodine, with or without Si), BNWs nucleate via VLS at temperatures . 1000 8C. No BNWs were obtained at lower temperatures. It was also found that Si is not necessary in nucleation of BNWs. Fig. 6 shows the SEM
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picture of a BNW sample quenched zero minute (Fig. 6(a)) and five minutes (Fig. 6(b)), respectively, after processed at 1100 8C. At the initial stage of the BNW nucleation, dense short stubs were formed as shown in Fig. 6(a). Shortly afterwards, development of BNWs from those stubs can be observed (Fig. 6(b)). The diameter of the wires seems much smaller than that of the stub, while it is not clear whether the former is proportional to the latter. In general, the thicker the Au catalyst layer, the larger the diameter of the BNW in the Au thickness range of 1– 20 nm we have tested in this experiment. This is consistent with what has been reported on other nanowires [14,15]. According to the VLS growth mechanism, catalytic nanoparticles are commonly seen at the free end of nanowire, with a diameter similar to the nanowire diameter. While the nanoparticles often form in hemispheric or spherical shape at the free end of nanowire in the VLS growth mode, the end tips of the BNWs appeared to be sharp and fauceted. The EDX study of the tips revealed that Au and Si were dominant, suggesting that the growth of the BNW in this experiment was indeed via the VLS mode or at least dominated by the VLS growth [10]. In this experiment, we have found that Au played a critical role in nucleation of the BNWs. When the Au layer was removed, no BNWs were formed. The tip shape of BNWs obtained in the experiment, however, does not consist with the popularly reported hemispheric or spherical tip shape in the VLS mode, indicating some subtle differences existed and modified the tip shape. When B2O3 was added to the pure boron source, the nucleation of the BNW was modified dramatically. The most distinct difference is that BNWs can be obtained at much lower temperatures near 600 8C. The nucleation mechanism experiences a transition from VO mode in temperature range of 600 to , 800 8C to a hybridized VO þ VLS mode at higher temperatures [19]. In the low
Fig. 6. SEM picture showing early growth stage of a BNWs film grown on a 20 nm thick Au coated Si substrate. The sample was (a) quenched immediately after it was heated to 1100 8C, or (b) growth for 5 min at 1100 8C. The end tips of the boron wires appear to be sharp.
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Fig. 7. SEM of boron nanowires synthesized at (a) 800 8C and (b) 1100 8C for 30 min, respectively, on Au coated Si substrates. B2O3 was employed in the vapor source at 1100 8C. A distinctive feature of these boron nanowires is their bundle-type of morphology that is not the case without B2O3. The boron nanowires have flat tips when grown at lower temperature (a) with no Au detectable on the tip. Only boron and oxygen were observed on the tip. At higher growth temperature, the boron nanowires have spherical tips containing a large amount of Au (see (a), also Figs. 3 and 4).
temperature range, individual BNWs nucleated from pores developed on the Au layer but no Au can be detected at the BNW’s tips. The BNW’s tips have a flat shape. Similar tip morphology was also reported earlier when B2O3 was
included in a sputtering target for growth of BNWs using magnetron sputtering. It was argued, however, that B2O3 does not play any role in nucleation of BNWs [20]. Oxygen was detected along the BNWs, suggesting VO mode was the
Fig. 8. (a) SEM of inverted Y-junctions made by fusing BNWs together during the nanowire growth. (b) Proposed model for growth procedure of the inverted Y-junctions.
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Fig. 9. (a) SEM of BNW joints made by evaporation of SiOx onto the boron nanowires. (b) Cartoon of a cross joint.
dominant nucleation mechanism. In the high temperature range, bundles of BNWs, instead of individuals, were formed. This bundle feature is different from most individual nanowires formed in VLS mode. There are typically several tens of BNWs in each bundle, each with spherical shaped tips (see Figs. 3 and 4) that contained catalytic Au. The diameter of the tip is much bigger than that of the B-nanowires, in contrast to the same or at least similar tip and nanowire dimensions reported for other nanowires. To clarify the chemical composition distribution of the B-nanowires elemental scans were carried out on many single B-nanowires removed from the bundles using SEM/EDS. Figs. 3 and 4 show the typical elemental scanning profile of a single B-nanowire. Several elements were observed on this B-nanowire including B, Au, O and Si (on Si substrate) or Mg (on MgO substrate). Besides a uniform distribution of oxygen along the wire, a peak distribution of the oxygen was observed near the boundary between the BNW stem and tip. In the proposed OA mechanism, oxide vapor phase acts as a key factor that enhances the nucleation and growth of nanowires. The formation of bundles might be due to creation of additional nucleation sites on each individual Au nanoprecipitate by boron oxide vapor, a hybridization of VLS and VO growth modes. On the other hand, in the sample grown at # 800 8C, the OA mechanism seems to be dominant for the formation of the BNWs (Fig. 7). 3.4. Nanowire junctions Nanowires are very promising for applications in electronic devices, such as nanowire diodes and transistors. The success towards electronic device applications is still limited due to the difficulties in fabrication of these devices. In a preliminary study, two types of nanowire junctions have been obtained recently in our group. One is the inverted Y-
junctions obtained via fusing two nanowires together during their growth (see Fig. 8) and the other, nanowires joints made by evaporating SiOx to the BNWs (see Fig. 9). The mechanism to form the inverted Y-junction is still under investigation. Since all of them have large size tips, we speculate that when the nearby BNWs are close enough so that their tip may touch during the growth, the melt in the tip may join and form a new tip of larger size (see Fig. 8(b)). In fact, the SEM study of these inverted Y-junctions reveal multi-stage fusing of BNWs. In another word, each of these inverted Y-junctions consists of many of smaller dimension inverted Y-junctions. This interesting property may be used to form junction networks for nanoscale devices. In fabrication of the nanowire joints, SiOx was evaporated to the BNWs during the BNW growth. As shown in Fig. 9(a), many beads were formed on the BNWs and SEM/EDS confirmed the composition of the beads of SiOx. Many joints were formed via the SiOx joints. Characterization of the electrical properties of these junctions will be reported elsewhere.
4. Summary In conclusion, the effects of fast cooling on the boron nanowire crystallization and alignment have been investigated. We have found that a rapid cooling process can remarkably enhance both crystallinity and alignment of the BNWs. When the BNW films were cooled slowly (at about 1 –5 8C/min) from the processing temperature of around 1100 8C, the BNWs formed were nontextured and heavily tangled. At a higher cooling rate of 90 8C/min, singlecrystalline BNWs were obtained with the (211) predominant orientation. In addition, the alignment of the BNWs with the normal of the substrate was obtained via fast cooling, which we speculate is caused by the strain induced
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in the BNWs during the transition from non-textured to single-crystalline BNWs.
Acknowledgements This research is supported in part by NSF, and DOE.
References [1] J. Nagamatsu, N. Nakagawa, T. Muranaka, Y. Zcnitani, J. Akimitsu, Nature 410 (2001) 63. [2] A.K. Hagenlocher, Proceedings of the Conference on Boron, Asbury Park, New Jersey, 1959, p. 128. [3] A.A. Berezin, AIP Conference Proceedings on Boron-Rich solids, Albuquerque, New Mexico, 1985, p. 224. [4] I. Boustani, A. Rubio, J.A. Alonso, Chem. Phys. Lett. 311 (1999) 21. [5] I. Boustani, A. Quandt, E. Hernandez, A. Rubio, J. Chem. Phys. 110 (1999) 3176. [6] S. Iijima, Nature 354 (1991) 56.
[7] R. Saito, M. Fujita, G. Dresselhaus, M.S. Dresselhaus, Phys. Rev. B 46 (1992) 1804. [8] S. Jin, H. Mavoori, C. Bower, R.B. van Dover, Nature 411 (2001) 563. [9] P.C. Canfield, D.F. Finnermore, S.L. Bud’ko, J.E. Ostenson, G. Lapertot, C.E. Cunningham, C. Petrovic, Phys. Rev. Lett. 86 (2001) 2423. [10] Y. Wu, B. Messer, P. Yang, Adv. Mater. 13 (2001) 1487. [11] S.H. Yun, A. Dibos, J. Wu, Appl. Phys. Letts. (2003) submitted for publication. [12] S.H. Yun, U.O. Karlsson, B.J. Jo¨nsson, L.D. Madsen, J. Mater. Res. 14 (1999) 3181. [13] D. Li, A. Wang, B. Yao, B. Ding, Z. Hu, J. Mater. Res. 11 (1996) 2685. [14] M.S. Dresselhaus, G. Dresselhaus, Ph. Avouris (Eds.), Carbon Nanotubes, Springer, Berlin, 2001. [15] Y. Wu, P. Yang, J. Am. Chem. Soc. 123 (2001) 3165. [16] A.P. Levitt (Eds.), Whisker Technology, Wiley-Interscience, New York, 1970. [17] S.T. Lee, N. Wang, Y.F. Zhang, Y.H. Tang, MRS Bulletin 24 (1999) 36. [18] W.S. Shi, H.Y. Peng, N. Wang, C.P. Li, L. Xu, C.S. Lee, R. Kalish, S.T. Lee, J. Am. Chem. Soc. 123 (2001) 11095. [19] S.H. Yun, A. Dibos, J. Wu, preprint [20] L. Cao, Z. Zhang, L. Sun, C. Gao, M. He, Y. Wang, Y. Li, X. Zhang, G. Li, J. Zhang, W. Wang, Adv. Mater. 13 (2001) 1701.
Fabrication and characterization of boron-related nanowires J.Z. Wua,*, S.H. Yuna,b, A. Dibosa, Do-Kyung Kimc, M. Tidrowd a Department of Physics and Astronomy, University of Kansas, Lawrence, KS 66045, USA Department of Material Physics, IMIT, Royal Institute of Technology SE-16640 Kista, Sweden c Department of Material Chemistry, Royal Institute of Technology, SE-14400 Stockholm, Sweden d Missile Defense Agency, 7100 Defense Pentagon, Washington, DC 20301-7100, USA b
Abstract A thermal vapor transport process has been employed for fabrication of boron-related (boron, boron – silicon alloys, and MgB2) nanowire films on Au-coated Si and MgO substrates. Tangled polycrystalline as well as aligned single-crystalline boron nanowires (BNWs) have been obtained and their growth mechanism investigated at different growth temperatures, cooling rates, and vapor sources. The growth temperature was found critical to the nucleation of the BNWs and different growth modes were observed in different temperature ranges. The temperature ramping rate in the cooling process after the high-temperature growth of the BNWs was found crucial for the formation of crystalline structures and for controlling the alignment of BNWs. We have observed that slow cooling at 1 – 5 8C/min resulted in non-textured BNWs, while fast cooling at ,90 8C/min induced crystallization of the non-textured BNWs. We have also found that the alignment of the BNWs depended on the cooling rate. At a slow cooling rate the BNWs were heavily tangled while at a higher cooling rate they aligned well with the normal of the substrate. By manipulating the growth parameters, we have obtained two types of nanowire junctions. One was via fusing two nanowires together and the other, via joining them with another material. q 2003 Elsevier Science Ltd. All rights reserved. Keywords: Superconductivity; Thermal vapor transport process; Nanowires
1. Introduction The recent discovery of superconductivity above 40 K in MgB2 [1] has triggered an intensive interest in boron and boron-related compounds. Boron is the third lightest element in the solid form, but has a high melting temperature near 2300 8C. Its hardness is very close to the diamond. These unique features make boron and boronrelated compounds attractive for various applications including high-temperature lightweight coatings and hightemperature semiconductor electronic devices [2,3]. Boron typically forms three-center electron-deficient bonds and B12 icosahedron serves as the building block for many boron-related compounds. In the bulk or thin film forms, Boron has low electrical conductivity in the semiconductor regime [2,3]. High conductivity in the metal regime, however, may be achievable in novel boron structures. Recent theoretical calculations suggested that boron nanostructures of layered, tubular and fullerene-like solids built from elemental subunits may exhibit novel physical * Corresponding author. Tel.: þ 1-858-534-0403; fax: þ1-858-534-2232. E-mail address: [email protected] (J.Z. Wu).
properties [4]. For example, boron nanotubes may have metallic density of states (DOS) [5] that may lead to higher electrical conductivity in boron nanotubes than in carbon nanotubes [6,7]. Such one-dimensional systems as boron nanotubes and nanowires hence are very promising for applications in field emission devices and other hightemperature electronics. In addition, superconducting MgB2 can be obtained by diffusing Mg into pure Boron [8,9]. In such an ex situ process, pure boron samples serve as the ‘precursor’ for the MgB2 to be formed via Mg diffusion. It is hence necessary to grow Boron nanowires (BNWs) to obtain superconducting MgB2 nanowires [10].
2. Experiments Films of BNWs were prepared on single-crystal (100) Si and MgO substrates using the thermal vapor transport process [10]. Before BNWs growth, some substrates were sputter coated with 5– 20 nm thick Au thin films. Boron (purity 99.99%), B2O3 (purity 99.98%), iodine (purity 99.7%), and Si-powder (purity 99.99%), were used as the source materials. Two types of source pellets were utilized.
0026-2692/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0026-2692(03)00074-0
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One was made mixtures of boron (20 mg), iodine (0.2 mg), and Si (0.1 mg) and the other, boron and boron oxide with approximately 40 wt.%. The source pellet was placed inside the quartz tube, together with the substrates with/without Au coating. The source was in the higher temperature zone while the substrate, in a slightly lower temperature one to allow condensation of the decomposed boron vapor on the substrate. The quartz tube was evacuated to about 10 mTorr, torch sealed and then heated to the processing temperature in the range of 600– 1100 8C (for the source) for 1 – 60 min. The temperature difference between the source and the film was in the range of 100 –200 8C during the BNWs growth. After the BNW growth, the samples were cooled down to room temperature using two different cooling rates of 5 8C/ min. and 90 8C/min, respectively. The morphology of the as-synthesized sample was examined using field-emission scanning electron microscopy (SEM). The chemical composition and distribution were examined using energy dispersive X-ray spectroscopy (EDS) attached to the SEM. Some BNWs were removed from the substrates and placed on Cu grids for analysis using transmission electron microscopy and selected area electron diffraction (TEM/SAED). X-ray powder diffraction (XRD) was used to study the structure of the BNWs films.
3. Results and discussions 3.1. Morphology Fig. 1 depicts a typical SEM image of a BNW films prepared, respectively, on a 5 nm thick Au-coated Si (Fig. 1(a)) and MgO (Fig. 1(b)) substrates. Both samples were annealed at 1100 8C for 30 min. and then cooled slowly at
about 5 8C/min to room temperature. Dense and tangled BNWs can be observed on both samples. These BNWs are several tens to hundreds of micrometers in length and 30– 500 nm in diameter. The diameter of the BNWs seems to correlate with the thickness of the Au catalyst layer and thinner Au catalyst layer resulted in smaller diameters of the BNWs. The growth rate of the BNWs depends sensitively on the growth temperature. In the temperature range we have studied in this experiment, we have found BNW growth at as low as 600 8C at very low growth rate and as high as 1200 8C at extremely high growth rate. Although BNWs have been obtained on both MgO and Si substrates, the surface morphology of the substrate looks very different after the BNWs growth. Fig. 2 depicts a close look of substrates used for BNWs shown in Fig. 1. The MgO (Fig. 2(a)) substrate became very rough after the BNW growth, while Si (Fig. 2(b)) substrate remained smooth. A chemical composition study using SEM/EDS revealed incorporation of Mg and Si into the BNWs when high processing temperatures above 1000 8C were employed. Fig. 3 shows the SEM picture (Fig. 3(a)) of and an EDS line map of chemicals (Fig. 3(b)) on a single BNW grown on Si substrates. The atomic ratio between B and Si is nearly 1:1 along the BNW. Similar result was also obtained on BNWs grown on MgO substrates as shown in Fig. 4. We have found that the cooling rate determines the alignment of the BNWs. when the BNW sample was cooled with the slow cooling rate of 5 8C/min., tangled BNWs were formed throughout the substrate surface. When the sample was cooled with larger cooling rate of 90 8C/min, the alignment of the BNWs to the normal of the substrate was significantly improved and BNWs were parallel to the normal of the substrate [11]. This alignment change was accompanied by the structural change in the BNWs, as we will detail in Section 3.2.
Fig. 1. SEM images of boron nanowire films synthesized on MgO (a) and Si (b) substrates, respectively, at 1100 8C for 30 min followed by slow cooling at 5 8C/min. 5 nm thick Au films were coated on the substrates before the nanowire growth. Iodine and pure boron were used as the vapor source.
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Fig. 2. A closer look at the substrate surface after the boron nanowire films were synthesized on MgO (a) and Si (b) substrates, respectively, at 1100 8C for 30 min followed by slow cooling at 5 8C/min. 5 nm thick Au films were coated on the substrates before the nanowire growth. Iodine and pure boron were used as the vapor source.
3.2. Structure XRD analysis of most of the BNWs obtained using slow cooling process shows many low-intensity peaks suggesting a polycrystalline structure for these BNWs [11]. These diffraction peaks can be indexed to an orthorhombic structure. The average lattice parameters of the BNWs calculated using the observed d values of the XRD spectra are ˚ , b ¼ 7.1 A ˚ , and c ¼ 5.4 A ˚ . Although the XRD a ¼ 9.2 A
data suggested that the nanowires formed are mainly BNWs, no preferred orientation can be identified in this sample. When high cooling rate was employed, only few XRD peaks of (211), (030), (420), and (520) were observed with much enhanced intensity. Among these peaks, (211) is dominant. Using the peak intensity for the (211), (030), (420), and (520), the estimated volume portion of the (211)-orientation phase is more than 60%. This suggests that recrystallization occurs during the fast cooling, which in turn may cause strain along
Fig. 3. SEM image (a) and EDS line-scan (b) on an isolated single BNW made on Si substrate with B2O3 in the vapor source at 1100 8C for 30 minutes. The points (x-axis) in (b) are defined along the BNW from root to tip in (a).
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Fig. 4. SEM/EDS line-scan on an isolated single BNW made on MgO substrate with B2O3 in the vapor source at 1100 8C for 30 min.
the BNWs and lead to better alignment of the BNWs. To confirm the crystallinity on a single BNW, some of BNWs were removed from the BNW films and placed on Cu grids for TEM/SAED study. The SAED patterns on a single BNW made under the same conditions as in Fig. 1(a) and (b), respectively, are shown in Fig. 5(a) and (b) for slow cooling and fast cooling. It is clearly seen that the BNW made in slow cooling have an amorphous structure, while that made via fast cooling, single crystalline structure. In general, the XRD and SAED results suggest that fast cooling promoted nucleation and growth of crystalline structures in the BNWs, consistent with the observation that non-textured materials can be single-crystallized by rapid quenching process [12,13] because the abrupt temperature and pressure gradient restrain
the growth of nuclei during solidification. Since the formation of the crystalline structure and the BNW alignment occur simultaneously during the quench, we speculate that the alignment is caused by the crystallization-induced strain along the BNW. In addition, the mechanical property of the BNW may also change during this transition from polycrystalline phase to single crystalline phase. Further investigation of the changes in BNWs physical properties during such a transition is necessary to confirm this. 3.3. Nucleation mechanism The growth mechanism of nanowires has attracted much attention since it is critical to reach a fine control of
Fig. 5. Selective area electron diffraction (SAED) patterns of BNWs synthesized on Si substrates at 1100 8C for 30 min followed, respectively, by slow cooling at 5 8C/min and fast cooling at 90 8C/min
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nanowire growth, such as nucleation sites, diameters, growth orientations and alignments of nanowires. Several mechanisms have been reported for vapor phase growth of nanowires [14,15]. Two most popular ones include the wellaccepted vapor-liquid-solid (VLS) process and the recently reported oxide-assisted growth (OA). The VLS process was proposed in early 1960s for the growth of large singlecrystal whiskers [16], in which an anisotropic crystal growth results from the presence of liquid alloy/solid interfaces. Metal catalysts are found critical in creating such a liquid alloy/solid interface and may be incorporated via many different ways, such as by mixing the metal catalyst into the material source of nanowires to be grown or by coating a metal catalyst buffer on the substrates. In the OA process, on the other hand, the nanowire nucleation was believed to be the precipitation of the nanoparticles, clothed by an oxide shell, under certain temperature gradients [17,18]. The major differences between nanowires fabricated in the metal catalyzed VLS process and in the OA process are the composition and structure of nanowire end tips. In the former case, the tips of nanowire have either spherical or hemispherical shapes that contain the metal catalyst. When a metal catalyst buffer is coated on the surface of the substrate, this catalyst will be carried over from the nucleation site on the substrate to the tip of the nanowire. In the latter case, the nanowire end tips may have different shapes and do not contain any catalytic elements. In growth of BNWs, we have found both VLS and VO growth modes, as well as a hybridized VLS þ VO growth, depending on the vapor source and the growth temperature. When pure boron is used as the vapor source (with small amount of iodine, with or without Si), BNWs nucleate via VLS at temperatures . 1000 8C. No BNWs were obtained at lower temperatures. It was also found that Si is not necessary in nucleation of BNWs. Fig. 6 shows the SEM
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picture of a BNW sample quenched zero minute (Fig. 6(a)) and five minutes (Fig. 6(b)), respectively, after processed at 1100 8C. At the initial stage of the BNW nucleation, dense short stubs were formed as shown in Fig. 6(a). Shortly afterwards, development of BNWs from those stubs can be observed (Fig. 6(b)). The diameter of the wires seems much smaller than that of the stub, while it is not clear whether the former is proportional to the latter. In general, the thicker the Au catalyst layer, the larger the diameter of the BNW in the Au thickness range of 1– 20 nm we have tested in this experiment. This is consistent with what has been reported on other nanowires [14,15]. According to the VLS growth mechanism, catalytic nanoparticles are commonly seen at the free end of nanowire, with a diameter similar to the nanowire diameter. While the nanoparticles often form in hemispheric or spherical shape at the free end of nanowire in the VLS growth mode, the end tips of the BNWs appeared to be sharp and fauceted. The EDX study of the tips revealed that Au and Si were dominant, suggesting that the growth of the BNW in this experiment was indeed via the VLS mode or at least dominated by the VLS growth [10]. In this experiment, we have found that Au played a critical role in nucleation of the BNWs. When the Au layer was removed, no BNWs were formed. The tip shape of BNWs obtained in the experiment, however, does not consist with the popularly reported hemispheric or spherical tip shape in the VLS mode, indicating some subtle differences existed and modified the tip shape. When B2O3 was added to the pure boron source, the nucleation of the BNW was modified dramatically. The most distinct difference is that BNWs can be obtained at much lower temperatures near 600 8C. The nucleation mechanism experiences a transition from VO mode in temperature range of 600 to , 800 8C to a hybridized VO þ VLS mode at higher temperatures [19]. In the low
Fig. 6. SEM picture showing early growth stage of a BNWs film grown on a 20 nm thick Au coated Si substrate. The sample was (a) quenched immediately after it was heated to 1100 8C, or (b) growth for 5 min at 1100 8C. The end tips of the boron wires appear to be sharp.
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Fig. 7. SEM of boron nanowires synthesized at (a) 800 8C and (b) 1100 8C for 30 min, respectively, on Au coated Si substrates. B2O3 was employed in the vapor source at 1100 8C. A distinctive feature of these boron nanowires is their bundle-type of morphology that is not the case without B2O3. The boron nanowires have flat tips when grown at lower temperature (a) with no Au detectable on the tip. Only boron and oxygen were observed on the tip. At higher growth temperature, the boron nanowires have spherical tips containing a large amount of Au (see (a), also Figs. 3 and 4).
temperature range, individual BNWs nucleated from pores developed on the Au layer but no Au can be detected at the BNW’s tips. The BNW’s tips have a flat shape. Similar tip morphology was also reported earlier when B2O3 was
included in a sputtering target for growth of BNWs using magnetron sputtering. It was argued, however, that B2O3 does not play any role in nucleation of BNWs [20]. Oxygen was detected along the BNWs, suggesting VO mode was the
Fig. 8. (a) SEM of inverted Y-junctions made by fusing BNWs together during the nanowire growth. (b) Proposed model for growth procedure of the inverted Y-junctions.
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Fig. 9. (a) SEM of BNW joints made by evaporation of SiOx onto the boron nanowires. (b) Cartoon of a cross joint.
dominant nucleation mechanism. In the high temperature range, bundles of BNWs, instead of individuals, were formed. This bundle feature is different from most individual nanowires formed in VLS mode. There are typically several tens of BNWs in each bundle, each with spherical shaped tips (see Figs. 3 and 4) that contained catalytic Au. The diameter of the tip is much bigger than that of the B-nanowires, in contrast to the same or at least similar tip and nanowire dimensions reported for other nanowires. To clarify the chemical composition distribution of the B-nanowires elemental scans were carried out on many single B-nanowires removed from the bundles using SEM/EDS. Figs. 3 and 4 show the typical elemental scanning profile of a single B-nanowire. Several elements were observed on this B-nanowire including B, Au, O and Si (on Si substrate) or Mg (on MgO substrate). Besides a uniform distribution of oxygen along the wire, a peak distribution of the oxygen was observed near the boundary between the BNW stem and tip. In the proposed OA mechanism, oxide vapor phase acts as a key factor that enhances the nucleation and growth of nanowires. The formation of bundles might be due to creation of additional nucleation sites on each individual Au nanoprecipitate by boron oxide vapor, a hybridization of VLS and VO growth modes. On the other hand, in the sample grown at # 800 8C, the OA mechanism seems to be dominant for the formation of the BNWs (Fig. 7). 3.4. Nanowire junctions Nanowires are very promising for applications in electronic devices, such as nanowire diodes and transistors. The success towards electronic device applications is still limited due to the difficulties in fabrication of these devices. In a preliminary study, two types of nanowire junctions have been obtained recently in our group. One is the inverted Y-
junctions obtained via fusing two nanowires together during their growth (see Fig. 8) and the other, nanowires joints made by evaporating SiOx to the BNWs (see Fig. 9). The mechanism to form the inverted Y-junction is still under investigation. Since all of them have large size tips, we speculate that when the nearby BNWs are close enough so that their tip may touch during the growth, the melt in the tip may join and form a new tip of larger size (see Fig. 8(b)). In fact, the SEM study of these inverted Y-junctions reveal multi-stage fusing of BNWs. In another word, each of these inverted Y-junctions consists of many of smaller dimension inverted Y-junctions. This interesting property may be used to form junction networks for nanoscale devices. In fabrication of the nanowire joints, SiOx was evaporated to the BNWs during the BNW growth. As shown in Fig. 9(a), many beads were formed on the BNWs and SEM/EDS confirmed the composition of the beads of SiOx. Many joints were formed via the SiOx joints. Characterization of the electrical properties of these junctions will be reported elsewhere.
4. Summary In conclusion, the effects of fast cooling on the boron nanowire crystallization and alignment have been investigated. We have found that a rapid cooling process can remarkably enhance both crystallinity and alignment of the BNWs. When the BNW films were cooled slowly (at about 1 –5 8C/min) from the processing temperature of around 1100 8C, the BNWs formed were nontextured and heavily tangled. At a higher cooling rate of 90 8C/min, singlecrystalline BNWs were obtained with the (211) predominant orientation. In addition, the alignment of the BNWs with the normal of the substrate was obtained via fast cooling, which we speculate is caused by the strain induced
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in the BNWs during the transition from non-textured to single-crystalline BNWs.
Acknowledgements This research is supported in part by NSF, and DOE.
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