Scanning electron microscopic (SEM) images and energy dispersive X-ray spectra (EDX) of the samples were taken with a Hitachi S-4700I and a JEOL JSM-7401F operated at 15 keV. High-resolution transmission electron microscopic (HRTEM) images and EDX were acquired on a scanning transmission electron
microscope (STEM) JEOL JEM-3000F at 300 kV. A cross-sectional TEM sample was prepared using a dimpling and ion milling technique.61 X-ray diffraction (XRD) patterns of the samples were obtained using a Bruker AXS D8 ADVANCE with Cu K1 radiation.
EFE measurements were carried out using a home-built apparatus, composed of a vacuum chamber and a spherical-shaped tungsten probe (diameter 1 mm) as the anode. The sample-to-anode distances were adjustable by a manipulator equipped with a micrometer. All measurements were taken below 1.05 × 10-3 Pa at room temperature. Current-to-voltage (I － V) characteristics were measured using a Keithley 237 or 2410. The maximum applied voltage was 1100 V while the current was restricted to 10 mA. Sheet resistances of the samples were measured by a four-probe method using a Mitsubishi MCP-T600 resistivity meter.
3.3 Results and Discussion
In a typical reaction, TiCl4 was vaporized into a hot-wall LPCVD reactor loaded with Ti powders at the heating zone at 1173 K, which was upstream from Si (100) substrates placed at the center heating zone at 973 – 1073 K. The reaction between TiCl4 and Ti generated gaseous titanium subchlorides TiClx, which would undergo disproportionation to regenerate TiCl4 and to deposit Ti.62 Depending on the reaction time employed, from 10 min to 6 h, growth of products composed of gray to black thin films on the substrates was observed. In Table 3.1, effects of varying the reaction parameters on morphology and phase of the products are summarized. In this report, we will discuss more on characterizations of the samples deposited at 1073 K, including I and III, the vertically grown long and short single crystalline TiSi NWs, respectively. We will not focus on samples grown at 973 K, C54-TiSi2 thin film (IV) and Ti5Si3 NWs (V) because we have reported their fabrications in chapter 4.
3.3.1 SEM and EDX Characterizations
SEM images of I are shown in Figure 3.1. Figure 3.1a is a low magnification top-view image of I, revealing the presence of high density 1-D nanostructures on the deposited product. The EDX spectrum shown in the inset of Figure 3.1a suggests that I is composed of Ti and Si only. A high magnification image in Figure 3.1b shows that the 1-D nanostructures are NWs with diameters of 30－80 nm. We do not observe metal particles on the tips of the NWs. This suggests that the NWs were not grown via a vapor － liquid － solid (VLS) mechanism.63 Figure 3.1c displays a side-view image of I, 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.
An enlarged side-view image of the NWs in Figure 3.1d shows that the NW lengths are 2-5 m.
Figure 3.1 SEM images of I grown on Si. (a) Top-view and EDX (inset), (b) high magnification image, (c) low magnification side-view, and (d) high magnification side-view image.
Figure 3.2 displays the images of II – V, the products prepared at different conditions. When the growth time was 10 min, sample II was obtained. As shown in Figure 3.2a, the Si substrate is partially covered (ca. 20%) by irregularly shaped islands with areas of tens to hundreds of m2. Unlike sample I, we could not find the presence of any 1-D nanostructure on the substrate of II. The rugged surface islands, shown in the side-view SEM image in the inset of Figure 3.2a, imply that they might be formed by etching and deposition reactions involving the Si surface and the gaseous TiClx molecules.64,65 Figure 3.2b shows the image of sample III, obtained after the reaction was carried out for 30 min at 1073 K. Growth of numerous NWs with lengths 0.5－3 m on top of a thin film with a thickness of 8 m is observed.
Comparing samples I - III, we conclude that only a thin film was grown at the early stage of the reaction. After a certain period of time, NWs started to grow on the film.
As the reaction time was lengthened, density and length of the NWs increased accordingly.
The SEM image (Figure 3.2c) of sample IV, which was grown at 973 K for 1 h, shows the presence of few and scattered NWs on a layer of thin film.36 When the growth time was extended, the NWs elongated as well. For example, Figure 3.2d demonstrates the image of sample V, which was grown at 973 K for 6 h. It displays the growth of abundant thread-like NWs, diameter ca. 20 – 50 nm and length up to several micrometers, on top of a thin film. The above observations suggest that, before the growth of 1-D titanium silicide NWs (characterization of the products by XRD and TEM will be discussed below), an adequate reaction time was required to allow the initial deposition of a layer of titanium silicide thin film on the Si substrate.
Figure 3.2 SEM top-view and side-view (inset) of (a) II, (b) III, (c) IV, and (d) V.
3.3.2 XRD Characterizations
XRD patterns of the samples I – V are shown in Figure 3.3. All of them display four diffraction peaks marked by gray circles at 2 = 39.1o, 42.2o, 43.2o, and 49.7o. This set of peaks indicates the presence of orthorhombic C54-TiSi2 (JCPDS 35-0785) as the major product. Since all of the samples have a layer of thin film several micrometers thick, we conclude that all of the thin films are composed of C54-TiSi2. In addition to the C54-TiSi2 pattern, sample I shows a strong peak at 2= 50.3o, which is marked by a blue square in Figure 3.3. This signal is also observed for III, the sample with short NWs, but not for II, which does not show any 1D nanostructure in the SEM image. The peak is assigned to the reflection from {020} plane of orthorhombic TiSi (JCPDS 17-0424). In Figure 3.4a, a detailed XRD scan of I taken at 2= 25 - 38o was demonstrated. The diffraction peaks observed at 2= 32.83o, 35.3o, and 36.93o were indexed to be the reflections from TiSi {201}, {002}, and {210}
planes, respectively. Thus, we conclude that in I, the NWs on top of the thin film, as
Figure 3.3 XRD patterns of I - V. Assignment of peaks: spheres, C54-TiSi2; square:
TiSi (020); diamond: Ti5Si3 (002); triangle: Si (020), this forbidden signal only appears in several samples grown on a specific batch of Si substrates. For clarity, Si (400) at 69.2o is not shown.
shown in Figure 3.1, are composed of TiSi. These data differ slightly from the standard XRD pattern of TiSi. We suggest that unlike randomly oriented TiSi crystals in powders, I contains TiSi NWs with a preferred growth orientation in [020]
direction. This is further confirmed by TEM studies and will be discussed below. All samples also showed a very strong signal at 69.2o. This was assigned to Si {400}
planes from the substrates. For clarity, it is not displayed in Figure 3 (see Figure 3.4b for a pattern of I with the substrate signal). For sample IV, only diffraction peaks of C54-TiSi2 are observed. Accordingly, IV is determined to be a film composed of C54-TiSi2. For sample V, in addition to the pattern of C54-TiSi2, a weak diffraction peak at 2= 34.8o is shown. This is assigned to the reflection by {002} plane of Ti5Si3
(JCPDS 78-1429), the main component of the NWs in V. The XRD observation is in good agreement with the TEM result, which indicates that the preferred growth direction of the NWs is along [002] axis of Ti5Si3.36 We speculate whether isolated Ti
and Si crystals could be grown during the reactions because TiClx and SiClx byproducts were known to deposit thin films in their elemental forms.62,66 However, the XRD results do not show any peak which can be assigned to Ti and Si crystals.
Figure 3.4 (a) Detailed XRD of I from 25o to 38o, (b) XRD of I from 20o to 80o (spheres, C54-TiSi2; square: TiSi; triangles: Si), and (c) XRD of the Si wafer used to grow silicides in samples I – III. 33.2o: Si (200), 56.5o: Si (311), 69.5o: Si (400), 61.7o and 65.9o: unable to assign.
Table 3.2 Assignments of the XRD Peaks in Figure 3.4
a Calculated from ICSD C54-TiSi2 : 96029, TiSi : 43494.
b JCPDS C54-TiSi2 : 35-0785, TiSi : 17-0424. c Not reported.
XRD 2
Crystal Phase (hkl) Calculated Value a JCPDS b
2 Intensity ratio 2 Intensity ratio
30.02 C54-TiSi2 (220) 30.05 9 30.04 10
32.83 TiSi (201) 32.68 32 32.83 20
35.3 C54-TiSi2 (212) 35.5 0 - c -
TiSi (002) 35.8 0 - -
36.93 TiSi (210) 36.92 99 36.91 100
37.38 C54-TiSi2 (020) 37.44 0 - -