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J. Phys. Chem. C 2007, 111, 10814-10817
Growth Mechanism of TiSi Nanopins on Ti5Si3 by Atmospheric Pressure Chemical Vapor Deposition Jun Du,*,†,‡ Piyi Du,† Peng Hao,† Yanfei Huang,† Zhaodi Ren,† Gaorong Han,† Wenjian Weng,† and Gaoling Zhao† State Key Laboratory of Silicon Materials, Department of Materials Science and Engineering, Zhejiang UniVersity, Hangzhou 310027, China, and Department of Chemical Engineering, Nanchang UniVersity, Nanchang 330029, China ReceiVed: February 6, 2007; In Final Form: April 13, 2007
High-density single crystalline orthorhombic TiSi nanopins were successfully synthesized on Ti5Si3 by atmospheric pressure chemical vapor deposition, using SiH4 and TiCl4 as the precursors. The growth mechanism was also investigated in detail. The results show that the maximum density of TiSi nanopins is obtained at the deposition temperature of 730 °C. TiSi nanopins grow along with [110] direction. The dimensions of quadrate tip of nanopins increase with deposition time, and the shape of the nanopin changes with the concentration of (SiH4 + TiCl4). A possible growth process is proposed which is well consistent with the experimental results.
1. Introduction
2. Experimental Methods
Due to their ultralow resistivity and high thermal stability, metal silicides have been widely used in the fabrication of gate, contact, and interconnect metallization in ultra-large-scale integration technologies.1 As device dimensions shrink, much attention has been paid on applications of nanostructures in nanoscale electronics and optoelectronics fabrication as ideal building blocks.2,3 They have the potential to reach higher device densities that the conventional semiconductor technology cannot provide.4 It is known that one-dimensional nanomaterials are commonly fabricated by chemical vapor deposition (CVD),5 physical vapor deposition (PVD),6,7 and hydrothermal synthesis,8 etc. Among all the fabrication methods, CVD and PVD seem to be efficient way to grow high-density one-dimensional nanomaterials on large scale substrate. Recently, one-dimensional nanostructures of metal silicides, such as CoSi2,9 ErSi2,10 NiSi,11 Pt6Si5,12 TiSi2,13 and some rare-earth silicides,14 have been successfully prepared by PVD and the solid reaction method. Furthermore, the one-dimensional nanomaterials are being sought for applications. Similar to the titanium silicide nanowires formed on Si by the solid reaction method, it was observed that efficient, homogeneous, and stable field emission was obtained,13 exhibiting a potential future of application in vacuum microelectronic industry, particularly flat panel displays. Since atmospheric pressure CVD (APCVD) is a cost-effective process for continuous large-scale production and is probably used for depositing on required substrate, it is important to find a new way of APCVD to grow titanium silicide nanostructures for the coming use such as for glass based field emission devices (FED). Beyond technology applications, the growth mechanism of titanium silicide nanopins must be studied in detail. In this paper, TiSi nanopins were successfully synthesized on Ti5Si3 base layer by APCVD and the growth mechanism of TiSi nanopins was investigated in detail.
TiSi nanopins and Ti5Si3 thin films were both prepared on borosilicate glass substrates in a hot-wall quartz reaction chamber. SiH4 and TiCl4 were used as the silicon precursor and the titanium precursor, respectively. Nitrogen was used as the carrier gas, which flowed through two bubblers with liquid TiCl4 to transport the Ti precursor to the reaction chamber.15 The deposition temperature ranged from 690 to 750 °C with the deposition time of 1 to 36 min. The molar ratio of SiH4/TiCl4 was between 1 and 1.5. The total gas flux and the flux of (SiH4 + TiCl4) in total gases were 1000 SCCM (SCCM denotes cubic centimeter per minute at STP) and 0-24 SCCM, respectively. Initially, SiH4 and TiCl4 were simultaneously entered to deposit Ti5Si3 thin films on borosilicate glass substrates when the molar ratio of SiH4/TiCl4 equals 1. The thickness of Ti5Si3 thin films was about 600 nm. Then SiH4 and TiCl4 were introduced to grow the TiSi nanopin on Ti5Si3 thin films when the molar ratio of SiH4/TiCl4 was equal to 1.5. The flux of (SiH4 + TiCl4) was kept at 20 SCCM initially for 5-20 min, and then gradually decreased from 20 SCCM to zero in 5-20 min. N2 was introduced into reaction chamber when the temperature was decreased from deposition temperature to room temperature. After that, a light blue layer was formed on the surface of the glass substrates. Field emission scanning electron microscopy (FESEM, SIRON, FEI) was used to identify the morphology of the asdeposited material. X-ray diffractometry (XRD) was performed to characterize the phase structure of the samples. Transmission electron microscope (TEM) and high-resolution TEM (HRTEM) images were obtained on a JEOL-2010 HRTEM using an acceleration voltage of 200 KV. The size of the nanopins was measured by the TEM. To prepare a TEM specimen, sample pieces were scraped from the glass substrate, suspended in ethanol, and dropped on a carbon copper grid.
* Corresponding author. Tel: 86-571-87952324. Fax: 86-571-87952341. E-mail:
[email protected]. † Zhejiang University. ‡ Nanchang University.
3. Results and Discussion Figure 1 shows titanium silicide thin films and nanopins prepared on glass substrate at different temperatures with the
10.1021/jp071019s CCC: $37.00 © 2007 American Chemical Society Published on Web 07/04/2007
Growth Mechanism of TiSi Nanopins
J. Phys. Chem. C, Vol. 111, No. 29, 2007 10815 TABLE 1: The Gibbs Free Energies of the Substances ∆G (kJ/mol) SiH4(g) TiCl4(g) SiCl4(g) HCl(g) H2(g) Ti5Si3(s) TiSi(s) Si(s)
Figure 1. (a) FESEM micrograph of Ti5Si3 thin films prepared on glass at 690 °C. (b) FESEM micrograph of TiSi nanorods formed at 710 °C. FESEM micrographs of TiSi nanopins formed at (c) 730 and (d) 750 °C, respectively. The nanopins were grown for 10 min using 12 SCCM SiH4, 8 SCCM TiCl4, and 980 SCCM N2, and then the flux of (SiH4 + TiCl4) gradually decreased from 20 SCCM to zero in 5 min.
900 K
1000 K
1100 K
-171.679 -1122.279 -998.990 -272.044 -129.070 -855.912 -193.483 -25.793
-199.320 -1169.670 -1043.681 -294.173 -145.536 -902.366 -204.421 -30.390
-227.806 -1218.129 -1089.433 -316.618 -162.305 -951.215 -215.949 -35.252
690 °C, as shown in Figures 1 and 2a. When the deposition temperature increases to 710 °C, the kinetic energy of the atoms increases, and the TiSi crystalline phase begins to form. At this temperature, the TiSi nucleation rate is faster than the growth rate of TiSi; thus, a large amount of short TiSi nanorods rather than nanopins forms. When the deposition temperature increases, the growth rate of the TiSi crystalline phase keeps increasing, but the nucleation rate decreases. Therefore, the TiSi nanopins exhibit a constant increase in the size, but a decrease in distribution density [Figures 1c,d]. Actually, the formation process of the nanopins can be explained as follows: When the molar ratio of SiH4/TiCl4 equals 1 and 1.5, the main reactions, which have the lowest Gibbs free energies, are shown below:
5TiCl4(g) + 5SiH4(g) ) Ti5Si3(s) + 2SiCl4(g) + 12HCl(g) + 4H2(g) (1) Figure 2. Characterization of TiSi nanopins. (a) XRD patterns of the samples prepared by APCVD at different temperatures measured in counts per second (CPS). (b) HRTEM image of a TiSi nanopin. (c) Electron diffraction pattern of the TiSi nanopin.
molar ratio of SiH4/TiCl4 of 1.5. It can be observed that the thin films deposit on the glass substrate and no nanopin forms at 690 °C. However, a large amount of short nanorods appears at 710 °C. When the growth temperature increases to 730 °C, the dense nanopins are formed. With the growth temperature increasing continuously to 750 °C, the density of nanopins decreases even though the nanopins still appear in the SEM micrograph. The nanopins are about 0.7-1 micrometer long in total, with a quadrate tip of about 200 nm long and a 50 × 50 nm2 square. Their XRD patterns are shown in Figure 2a. From the XRD patterns of the sample deposited at 690 °C, the diffraction peaks can be readily indexed to a hexagonal structure with cell constants of a ) 0.743 and c ) 0.517 nm, demonstrating that the thin film is Ti5Si3. The others are the XRD patterns of nanorods or nanopins on Ti5Si3 thin films, Except for the diffraction peaks of Ti5Si3, the other diffraction peaks can be readily indexed to an orthorhombic structure with lattice constants of a ) 0.3610, b ) 0.4965 and c ) 0.6500 nm. It is obvious that the TiSi crystalline phase has been formed. The HRTEM image and the electron diffraction pattern of the nanopin are shown in Figure 2b,c. The lattice spacings of the nanopin, which are 0.32, 0.29, and 0.22 nm, are consistent with the d-spacings of (002), (110), and (112) crystallographic planes of TiSi shown in Figure 2c. The nanopin is confirmed to be a single-crystal phase of TiSi. The growth direction of TiSi nanopin is [110]. It is understood that atoms cannot move together to fulfill the nucleation when the deposition temperature is not high enough, which explains why the TiSi crystalline phase fails to form on Ti5Si3 thin films at a relatively low temperature of
2TiCl4(g) + 3SiH4(g) ) 2TiSi(s) + SiCl4(g) + 4HCl(g) + 4H2(g) (2) The Gibbs free energies16 of the substances are shown in Table 1. The Gibbs free energies of the reactions at the temperature of about 730 °C are ∆rG1 ) -256.998 kJ/mol and ∆rG2 ) -274.060 kJ/mol. Therefore, Ti5Si3 forms on the glass when the molar ratio of SiH4/TiCl4 equals 1. When the molar ratio of SiH4/TiCl4 equals 1.5, TiSi is most likely formed with the lowest ∆rG2. In fact, TiSi can form in one-dimensional nanostructure on the Ti5Si3 base-layer in this case. It is likely a vapor-solid growth process dominates the growth of TiSi nanopins. The schematic diagram of formation of TiSi nanopins is shown in Figure 3a. When the molar ratio of SiH4/TiCl4 increases from 1 to 1.5, the concentration of SiH4 will increase and some SiH4 will be decomposed into Si and H2. So Si incorporates into Ti5Si3, and the highly active Ti-Si nanoislands form. It is understood that the fusing temperature gets lower as the size of the particle gets smaller. Furthermore, considering that the energies of nanoislands are very high, the quasi-liquid Ti-Si alloy can be formed at a lower temperature,13 as shown in Figure 3b. According to the Gibbs free energies16 of the substances, Ti5Si3 and Si can form TiSi, which causes the initial nucleation of TiSi. The reaction is as follows:
Ti5Si3(s) + 2Si(s) ) 5TiSi(s)
(3)
The Gibbs free energy of the reaction 3 at the temperature of 730 °C is ∆rG3 ) -58.96 kJ/mol. During reaction 2 between TiCl4 and SiH4, continuous feeding of Ti and Si atoms into the quasi-liquid Ti-Si alloy leads to 1D growth of the TiSi crystal. TiSi nanoparticles are pushed up, and the nanorods are formed eventually. When the flux of (SiH4 + TiCl4) begins to decrease,
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Figure 5. Length of quadrate tip as a function of growth time at 730 °C. The error bar is (5%.
Figure 3. (a) Schematic diagram of formation of TiSi nanopins. (b) TiSi grown on quasi-liquid alloy.
Figure 6. FESEM micrographs of TiSi nanopins formed at 730 °C with different end procedures. The nanopins were grown using 12 SCCM SiH4, 8 SCCM TiCl4, and 980 SCCM N2 for 15 min; then the flux of the resource gas gradually decreased from 20 SCCM to zero in (a) 5, (b) 10, (c) 15, and (d) 20 min.
Figure 4. FESEM micrographs of TiSi nanopins formed on glass at 730 °C. The nanopins were grown using 12 SCCM SiH4, 8 SCCM TiCl4 and 980 SCCM N2 for (a) 5, (b) 10, (c) 15, and (d) 20 min, and then the flux of (SiH4 + TiCl4) gradually decreased from 20 SCCM to zero in 5 min.
the formation of TiSi decreases. With the flux of (SiH4 + TiCl4) decreasing gradually to zero, the size of the nanorods decreases gradually. Consequently, the nanopins form. The growth process can be defined as self-induced growth. As discussed above, there are two conclusions: one is that TiSi islands are pushed up, and the other is that the size of the nanopin changes with the flux of (SiH4 + TiCl4). To affirm the conclusions, two series of experiments have been done. Figure 4 shows TiSi nanopins formed at 730 °C for different growth times. The quadrate tip dimensions of nanopins increase with growth time. The relationship between quadrate tip length and growth time is shown in Figure 5. As shown in Figure 5, the quadrate tip length of nanopins increases linearly from 50 to 500 nm as the growth time increases from 5 to 20 min, and the tail end length of nanopins exhibits almost no change. It is obvious that the quadrate tip of TiSi nanopins grows with the increase in deposition time. To prove the second conclusion, at the end stage of the reactions, the time in which the reaction gases flux decreased to zero is changed from 5 to 20 min. The results show in Figure 6. As shown in Figure 6a, the difference between the quadratetip and tail end is very distinct as the time is 5 min.
Figure 7. Variation in length of tail end with growth time at 730 °C. The error bar is (5%.
With the time increasing gradually from 10 to 15 min, the change between the quadrate tip and tail end is gradual and inexplicit, as shown in Figure 6b,c. With the time increasing to 20 min, the change is ambiguous, as shown in Figure 6d. Moreover, as shown in Figure 7, the length of the tail end of nanopins increases with time. It further confirms the fact that the growth of tail end of TiSi nanopins is under the control of the concentration of SiH4 and TiCl4. Consequently, the experimental results nicely match the nanopins growth mechanism discussed above. It also further proves the growth mechanism of TiSi nanopins. 4. Conclusions In summary, we first prepared the single crystalline orthorhombic TiSi nanopins on Ti5Si3 layer by APCVD using SiH4 and TiCl4 as the precursors. The maximum density of TiSi
Growth Mechanism of TiSi Nanopins nanopins is obtained at the deposition temperature of 730 °C. TiSi nanopins grow along with [110] direction. The dimensions of quadrate tip of nanopins increases with deposition time, and the shape of the nanopin changes with the concentration of (SiH4 + TiCl4). The quadrate TiSi nanorods form when the flux of (SiH4 + TiCl4) is fixed. With the flux of (SiH4 + TiCl4) decreasing gradually to zero, the size of the nanorods decreases gradually. Consequently, the nanopins form. It is well consistent with the experimental results. The growth mechanism presented in this paper expands our understanding of growing nanostructures, and we believe that the method may be adaptable to the preparation of other nanopins. The application of nanomaterials is now a step closer. Acknowledgment. This work is financially supported by the National Natural Science Foundation of China (Grant No. 50672084). References and Notes (1) Lee, J.; Reif, R. J. Electron. Mater. 1991, 20, 331. (2) Derycke, V.; Martel, R.; Appenzeller, J.; Avouris, Ph. Nano Lett. 2001, 1, 453.
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