Titanium silicides are suitable for EFE applications, due to their high melting points, low work functions (3.71 - 4.53 eV), and high conductivities.1,28 Thus EFE properties of the samples obtained in this study were investigated. The results are shown in Figure 6. Detailed EFE properties, such as turn-on field Eo, which is defined as the electric field required to generate a current density (J) of 10 Acm-2, and field enhancement factor , which is deduced from the Folwer-Nordheim (F-N) plot (the inset of Figure 6 showing the ln(JE-2)-(E-1) relationship, are summarized in Table 1.36 Sample I demonstrates the best EFE performance among our samples. The Eo of I is low, only 5.25 Vm-1, while the Jmax (at an applied voltage 1100 V) is high, 0.48 mAcm-2. The low Eo value is associated with its high NW aspect ratio. The high Jmax
could be attributed to its high NW density, low TiSi resistivity, and low sheet resistance, which is 8.6 × 10-2 /□. I is 876, calculated from the equation
= 3/2/e, where is the work function of TiSi, 3.99 eV, while e is the effective work-function derived from the slope of the F-N plot of I shown in the inset of Figure 3.17.77 Other samples from this study did not perform equally well because they lack the proper combination of NW density, aspect ratio, and overall NW/thin film resistance. For example, II could not emit electrons even under the highest possible field applied. This is because II was a partially formed TiSi2 film lacking 1-D nanostructures for emission. Sample III shows an Eo 8.5 Vm-1 which is higher than I’s value because III has shorter NWs and consequently, a lower aspect ratio. The same reason is also applicable to explain IV’s poor performance. Sample V has an Eo
5.4 Vm-1, which is slightly higher than I’s result, but a Jmax only a third of the value of I. This is attributed to V’s low NW density, high Ti5Si3 resistivity, and high sheet
resistance, 1.9 × 10-1 /□. In contrast, I is composed of high density, high aspect ratio and vertically aligned crystalline TiSi NWs. These ensure that nearly all NWs in I could emit electrons effectively. The result is that I is one of the best titanium silicide NW emitting materials reported so far.5,6
Figure 3.17 EFE current density as a function of applied electric field of samples I, and III–V. Inset shows their corresponding Fowler-Nordheim plots.
3.4 Conclusions
In conclusion, TiSi NWs were grown vertically on a C54-TiSi2 film employing a unique CVD process. Disproportionation of gaseous TiClx subhalides, formed by reacting TiCl4(g) and Ti(s) at high temperatures, provided Ti atoms while the Si substrate supplied Si atoms. Presence of an amorphous TiSi2-x interlayer was observed between the NWs and the film. This interlayer, probably existed as a quasi-liquid thin film during the growth, appears to be the key factor to assist the development of 1-D nanostructures. Varying the reaction conditions, such as time and temperature, would modulate reactivity and diffusion rate of the Ti and Si containing reactive species and limit the number of nucleation sites. This would influence not only composition and morphology but also physical and chemical properties of the product. Growth direction of the TiSi NWs was determined to be along [010]. The phenomenon could be attributed to the presence of strong Si-Si bonds along TiSi’s b axis. This would lower the energy and provide high stability for the growth of TiSi in [010]. The vertically grown TiSi NWs demonstrates superior EFE properties with a low Eo 5.25 Vm-1 and a high 876. The high performance is attributed to TiSi’s low work function, growth of high density, high aspect ratio, and vertically aligned 1D nanostructures, and the existence of a highly conductive TiSi2 film below. Our study has provided a new route to grow unusual TiSi NWs which cannot be fabricated by other means so far. The synthesis will offer opportunities to study other physical and chemical properties of this unique material in the future.
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Chapter 4
Chemical Vapor Deposition of Ti
xSi
yFilm and Single Crystalline C49 TiSi
2Nanoplates
4.1 Introduction
In addition to one-dimension nanostructures, two-dimensional nanostructures also show unique physical and chemical properties for future applications.1-11 There are some synthetic strategies to obtained two-dimensional nanostructures. For example, Ag nanoplates were synthesized with capping agent to confine the crystal growth into two dimensions.1 Second, self-assembly block copolymer was used as template to synthesize ZnSe nanoplates.2 In addition, under proper reaction condition, nanoplates could be obtained with compounds that have anisotropic structure nature.3,4 In chapters 2 and 3, we have investigated the preparation of titanium silicide samples through a unique CVD process without using the hazardous SiH4 gas and catalyst for one-dimensional nanostructures. TiClx, generated by reaction between TiCl4 vapor and Ti metal, was used as the precursor to grow titanium silicide thin films or NWs on Si substrate.12-14 Also, in this chapter, we will demonstrate the synthesis of C49 TiSi2 nanoplates on Si substrate via our CVD process under low TiCl4 vapor pressure condition. The characterization and the growth mechanism of the two-dimensional nanostructure will be discussed below. Moreover, influence of the reaction temperature and time on the morphologies, crystal structures, and film growth rate of the titanium silicide samples are investigated in detail to better understand the reaction process.
4.2 Experimental Section
4.2.1 Synthesis Procedure
A summary of reaction conditions and experimental observations is listed in Table 4.1. The detail reaction procedure is described below. Ti powders (0.3 g) were placed at the highest temperature zone at 1173 K. Si (100) wafers with size of 0.7 x 1.5 cm2, cleaned by RCA process were placed at a low temperature zone at 723 – 1073 K downstream. After the chamber was evacuated to lower than 2 mtorr, TiCl4 immersed in a 298 K water bath was vaporized into the reaction chamber at a pressure of 20 mtorr, which is manipulated by a needle valve. After 30 min to 6 h, the supply of TiCl4 was stopped and the reaction system was cooled to room temperature.
Samples of gray or black thin films deposited on Si substrates were obtained. By changing the reaction condition, thin film samples with different morphologies and crystal structure could be obtained. On the other hand, when the TiCl4 was changed to a 273 K ice bath and the partial pressure was reduced to 10 mtorr. After 30 min, C49-TiSi2 nanoplates on Si substrate could be obtained at 973 K.
4.2.2 Characterization Instrument
The SEM and EDX data were collected using JEOL JSM-6330F and Hitachi S-4700I at 15 kV. The TEM, SAED, and high-resolution transmission electron microscopic (HRTEM) images and EDX were acquired on a scanning transmission electron microscope (STEM) JEOL JEM-3000F at 300 kV. The X-ray photoelectron spectroscopy (XPS) measurement were carried out using a Perkin-Elmer PHI 1600 spectrometer with Mg K (1253.6 eV) radiation. The sample surface was cleaned by an Ar+ ion sputter gun. The XRD studies were carried out using a MAC MXP-18 and a BRUKER AXS D8 ADVANCE with Cu K1 radiation.
4.3 Results and Discussion
All reactions were performed in a low pressure reactor as shown in Figure 2.1.
TiCl4, the precursor, was vaporized into the reaction chamber at a controlled partial pressure. Close to the precursor inlet, Ti powders were placed at the heating zone at 1173 K. Si (100) wafers were placed at the center heating zone at 723 - 973 K. After 30 min to 6 h, deposition of a gray layer on the substrates was observed. A summary of the representative samples prepared in this study is listed in Table 2.1.
Table 4.1 Summary of Samples
a Thickness / lateral dimension of the nanoplates. b Estimated from the SEM image.
4.3.1 Titanium Silicide Films Grown at Different Temperatures
Figure 4.1 shows SEM images of samples synthesized at different temperatures for 60 min. The growth temperature of sample I, II, III, IV, and V are 723 K, 773K, 873 K, 973 K, and 1073 K, respectively. SEM images of I, Figure 4.1a and b display a thin film with small grains on the top and a thickness of 100 nm. Figure 4.1c and d demonstrate that II has rough film surface and the film thickness is raised to 550 nm.
III, prepared at 873 K, shows a thickness of 1.8 – 2 m and an uneven surface. As shown in Figure 4.1g and h, IV obtained at 973 K for 60 min also displays a 2.6 m- thick film and scarce and short nanowires on it. Figure 4.1i is a low magnification top-view image of V, revealing the presence of high density Two-dimensional nanostructures on the deposited product. Figure 4.1j displays a side-view image of V, indicating that the product deposited on the substrate is composed of a film (thickness 10.5 m) and above it, a layer of NWs pointing upward. The growth rate of the thin film, 1.6 – 170 nm/min, was evaluated from the cross-sectional SEM images. As the data shown in table 4.1, the growth rates of the sample synthesized for 60 min (I-V) increase with the reaction temperature. At 723 K, I, the growth rate was very low (1.6 nm/min). When the reaction temperature was raised to 1073 K, the deposition rate increased notably to a rate of 170 nm/min. As a consequence, the growth rate was increased with the reaction temperature.
XPS studies were performed to verify the chemical state of the Ti and Si in the samples. XPS spectra of sample IV grown at 973 K for 60 min are shown as a typical example, since the spectra of the samples are quite similar. Figure 4.2a shows the high-resolution spectra of Ti. The signals of Ti 2p1/2 and Ti 2p3/2are observed at 460.4 eV and 454.4 eV, respectively. These are close to the values of Ti-Si binding energy.
Figure 4.1 Top-view and side-view SEM images of samples grown at different reaction temperatures for 60 min. (a)-(b) 723 K (I), (c)-(d) 773 K (II), (e)-(f) 873 K (III), (g)-(h) 973 K (IV), and (i)-(j) 1023 K (V).
Figure 4.2 High-resolution XPS spectra of the film grown at 973 K for 60 min. (a) Ti 2p1/2 and Ti 2p3/2 electrons and (b) Si 2P electron.
The binding energy of Si 2p electrons, Figure 4.2b, shows one binding energy of 99.0eV and could be assigned to Si-Ti binding energy. Thus, the chemical states of the Ti and Si in the samples are determined to be Ti-Si bonding.14
The crystal structures of the films were examined by XRD. In Figure 4.3, sample I, synthesized at 723 K for 60 min showing two broad peaks at 2 = 40.8o and 50.9o marked with blue circles were assigned to the {131} and {002} reflections of orthorhombic C49-TiSi2 (JCPDS 10-0225), which suggested that the film was composed mainly of orthorhombic C49-TiSi2 phase. Sample II displays one strong diffraction peak at 2 = 33o, which could be assigned to be the Si {020} reflection, which results from the Si substrate used. Four peaks around 2= 39.1o, 42.3o, 43.5o, and 50o marked with gray circles could be assigned to the {311}, {040}, {022}, and {331} planes of C54-TiSi2 (JCPDS 35-0785), respectively. In addition, two minor reflections at 2= 40.8o and 50.9o marked with blue circles corresponds well with the {131} and {002} planes of C49-TiSi2. Thus, we conclude that II, synthesized at 773 K for 60 min, is composed mainly of C54-TiSi2 and a portion of C49-TiSi2. As the reaction temperature was further raised to 873 K and 973 K, for sample III and IV, respectively, only diffraction patterns of C54-TiSi2 could be observed. Therefore, III
Figure 4.3 XRD patterns of samples prepared at different temperatures for 60 min (I-V).
and IV are determined to be C54-TiSi2 films. As mention in chapter 3, sample fabricated at 1073 K for 60 min, V, contains TiSi NWs with a preferred growth orientation in [020] direction on the top of a C54-TiSi2 film. The presence of pure C49-TiSi2 phase at low reaction temperature of 723 K, mix phases of C54 and C49 TiSi2 at 773 K, and pure C54 TiSi2 at temperatures higher than 873 K imply that the C49-TiSi2 is a low temperature metastable phase before the formation of C54-TiSi2. This result coincides well with the previous research on titanium silicide.15
4.3.2 Titanium Silicide Film Grown at Different Time
Influence of the reaction time on the samples is discussed in this section. Figure 4.4 demonstrates the top-view and cross-sectional SEM images of the samples fabricated at 973 K with different reaction time, which are 30 min, 60 min, 180 min,
and 360 min for sample VI, IV, VII, and VIII, respectively. The SEM images (Figure 4.4a and b) of sample VI, which was grown at 973 K for 30 min, displays a film with an uneven surface and a film thickness of 2.2 m. When the growth time was 60 min, sample IV was obtained. As shown in Figure 4.4c and d, there are few NWs on a layer of thin film with a thickness of 2.6 m. Figure 4.4e and f display the image of sample VII, obtained after the reaction was carried out for 180 min at 973 K. Growth of some thread-like NWs with lengths 0.5 - 3 m on top of a thin film with a thickness of 3.5
m is observed. When the reaction time was increased to 360 min, as shown in Figure 4.4g and h sample VIII, abundant thread-like NWs with lengths 2 – 5 m were grown on a film with thickness of 6 m. Thus, we can conclude that only a thin film was grown initially. When the reaction time was extended to a certain period of time, growth of 1-D nanostructure was started. As the reaction time increased, density and length of the NWs raised accordingly. Besides, the film growth rate was decreased with the lengthening of reaction time, which were 73.3 nm/min and 16.6 nm/min for sample synthesized at 30 min and 360 min, respectively. As discussed in chapter 3, In our CVD system, the TiClx molecular, generated through the reaction between TiCl4(g)
and the Ti powders at 1173 K, could react directly with the Si substrate to form titanium silicides and SiClx(g) byproduct. The SiClx could decompose and serve as another source of Si atom for the silicide formation. As the reaction progressed, the presence of the relatively inert C54-TiSi2 film could impede the reaction between the TiClx and the Si substrate as well as the production of SiClx. Consequently, the deposition rate of the TiSi2 thin film decreased with the lengthening of the reaction time.
Figure 4.4 Top-view and side-view SEM images of samples grown at 973 K for 30 - 360 min. (a)-(b) 30 min (VI), (c)-(d) 60 min (IV), (e)-(f) 180 min (VII), (g)-(h) 360 min (VIII).
Figure 4.5 displays the XRD patterns of the sample synthesized at 973 K for different reaction time. All of them show four diffraction peaks marked by gray circles at 2 = 39.1o, 42.2o, 43.2o, and 49.7o, which indicates that these samples are mainly composed of orthorhombic C54-TiSi2 (JCPDS 35-0785) thin film. For sample
obtained for 30 min, VI, a diffraction peak at 2 = 33o, which is assigned to Si {020}
plane. The Si diffraction was resulted from the Si substrate used. Only diffraction peaks of C54-TiSi2 are observed for sample synthesized at 30 and 60 min. Therefore, IV and VI are determined to be C54-TiSi2 films. As the reaction time increased more than 180, in addition to the C45-TiSi2 diffraction peaks, a weak diffraction peak at 2 = 34.8o is shown. This could be assigned to the reflection by by {002} plane of Ti5Si3 (JCPDS 78-1429), the main component of the NWs in VII and VIII. In addition, the XRD data coincided well with the TEM data of the Ti5Si3 NWs, which reveals that the NWs is grown preferentially along [002] axis of Ti5Si3.
Figure 4.5 XRD patterns of samples synthesized at 973 K for different time.
4.3.3 Synthesis of C49 TiSi
2Nanoplates
4.3.3.1 Characterization of the C49 TiSi
2Nanoplates
Two-dimensional titanium silicide nanostructures spontaneously formed by reacting TiClx(g), producing by reaction between TiCl4(g) and Ti metal at 1173 K, with Si substrate at 973 K for 30 min. Unlike the samples discussed previously, the TiCl4(g)
partial pressure in this study was reduced to 10 mtorr by keeping the TiCl4(l) precursor in an ice bath at 273 K. Figure 4.6 shows the SEM image and XRD of sample IX, grown on Si wafer at 973 K for 30 min as the TiCl4 was immersed in an ice bath.
Top-view SEM image of IX, Figure 4.6a, reveals numerous thin plate structures on the Si substrate. Inset is the EDX spectrum taken at the center part in Figure 1a, which implying that the nanoplates are composed of Ti and Si only. In addition, Ti and Si atom% estimated from the EDX are 34 % and 66 %. Figure 4.6b indicates that the thickness and lateral dimension of the nanoplates are about 30 – 100 nm and 0.5 - 5
m. In addition to the plate-like structures, some net-like structures could be observed.
Enlarged-view of the nanoplate is shown in Figure 4.6c. The nanoplates are composed of arrays of one-dimensional nanostructures in two directions, which are perpendicular to one another, with diameters of 20 – 50nm. Figure 4.6d demonstrates the XRD pattern of IX. Diffraction peaks at 2 = 33o, which is assigned to Si {020}
plane, results from the Si substrate. In addition, there are two sets of diffraction peaks could be observed. Minor diffraction peaks at 2= 39.1o, 43.5o, and 50o marked with
plane, results from the Si substrate. In addition, there are two sets of diffraction peaks could be observed. Minor diffraction peaks at 2= 39.1o, 43.5o, and 50o marked with