Properties And Preparation Of Titanium Silicide Thin Films And Nanowires By APCVD

Due to their low resistivity and high thermal stability, titanium silicides have been widely used in ultra large scale integration (ULSI) technologies. As device dimensions shrink, using titanium silicide nanostructures in nanoscale electronics and optoelectronics as ideal building blocks has been drawing more attention. Compared to conventional semiconductor, nanostructures have the potential to reach higher device densities. One-dimensional nanomaterials can be fabricated by several methods. Among all the fabrication methods, Chemical vapor deposition (CVD) is considered to be efficient for growing high-density one-dimensional nanomaterials on large scale substrate. Especially, atmospheric pressure CVD (APCVD) is economical for continuous large scale production and is practical for depositing on required substrate. To find a new APCVD way to grow titanium silicide nanowires under the self-induced effect of as-deposted homogeneous silicide thin films is very important for promoting applications of the nanowires to the microelectronic devices. This new method can grow the nanowires on the devices in situ. That is to say, the functional blocks of nanowires can be directly integrated into the microdevices and microcircuit. It will blaze a brand-new development of the applicatins of nanowires to the microelectronic device. Furthermore, the self-induced method can grow the nanowires on the different kinds of substrates, which represents a rapidly expanding area of the application of silicide nanowires. On the other hand, titanium silicide thin films have high reflectivity on low-frequency electromagnetic wave because of its low resistivity, and it has good solar shielding window properties. Therefore, a new type of coating glass which combined the functions of solar control and low-E will be exploited if titanium silicide thin film is successfully prepared as a glass coating. The new type of coating glass will attract considerable attention due to its high performances and low cost.In this thesis, the thin films and nanowires of titanium silicides were prepared on the glass substrate by APCVD, using SiH₄ and TiCl₄ as precursors. The phase structure and compositions were identified by XRD and EDX. The surface morphology and thickness were observed by FESEM and TEM. The sheet resistance and optical behaviors of the thin films were measured using the four point probe method and UV-VIS spectrometer, respectively. The silicide phase formations were studied. The formation and growth of TiSi nanowires were also clarified. The results show that the titanium silicide thin films have been successfully prepared on the glass by APCVD using SiH₄ and TiCl₄ as the precursors. TiSi₂ thin films are formed with the face-centered orthorhombic structure. The maximal content of TiSi₂ is obtained when the molar ratio of SiH₄/TiCl₄ is 3 and the deposition temperature is about 700℃.TiCl₄(g)+3SiH₄(g)=TiSi₂(s)+6H₂(g)+SiCl₄(g) is the main reaction. Due to the effect of the amorphous substrate, the amorphous layer is formed initially on glass substrate and TiSi₂ crystalline phase grows finally on top of it. The thickness and content of TiSi₂ crystalline phase increases with the increase of deposition time. The growth rate and thus the size of TiSi₂ crystalline phase increase with increasing deposition temperature. The stack density decreases with increasing deposition temperature because the rapid growth at higher temperature causes the more irregular shape of crystalline particles. The maximal stack density of the crystalline phase is obtained at the deposition temperature of 700℃and the molar ratio of SiH₄/TiCl₄ of 3. The expressions of the stack density of crystalline phase as functions of the deposition conditions are theoretically deduced. The equation can be written aswhen K₁=0.85 and K₂ = 4.0×10^-4, it is consistent well with the experimental results.Ti₅Si₃ thin films were also successfully deposited on glass by APCVD. Ti₅Si₃ is obtained when the ratio of SiH₄ /TiCl₄ is 1. The main reaction is 5TiCl₄(g)+5SiH₄(g)=Ti₅Si₃(s)+2SiCl₄(g)+12HCl(g)+4H₂(g). Ti₅Si₃ thin films are formed with the hexagonal structure. The growth rate and thus the size of Ti₅Si₃ crystalline phase increase with increasing deposition temperature. The stack density decreases with increasing deposition temperature. The thickness and content of Ti₅Si₃ crystalline phase increases with the increase in deposition temperature.The resistivity of thin films is dependent on the crystalline formation. It is controlled with the kinds of crystalline phase, the crystalline phase size and the stack density, respectively. The resistivity of Ti₅Si₃ thin films is higher than that of TiSi₂ thin films. The content of crystalline phase increases and thus the resistivity decreases with the increase of stack density of TiSi₂ crystalline phase. The theoretical expression of the resisitivity as the functions of the deposition conditions can be written as follows, when K₁=0.85, K₂ = 4.0×10^-4 and K₄=0.05, it is consistent well with the experimentalresults. The stack density increases and thus the resistivity of TiSi₂ thin films decreases with the increase of time. The growth rate and the content of crystalline phase increase with temperature, thus the resistivity decreases. The minimal resistivity is obtained at 700℃. The stack density decreases and thus the resistivity increases with increasing deposition temperature because the rapid growth at higher temperature causes the more irregular shape of crystalline particles. The resistivity of Ti₅Si₃ thin films is also dependent on the formation and stack density of Ti₅Si₃ crystalline phase. The resistivity of Ti₅Si₃ thin films decreases with increasing of stack density of Ti₅Si₃ crystalline phase, the minimum resistivity of the Ti₅Si₃ thin films is 4.5×10^-4Ω·cm.The TiSi₂ thin films have the same transmittance between 400nm and 750nm. The resistivity decrease and thus the absorption and dispersion increase with the increase of stack density of TiSi₂ crystalline phase. That is to say, the transmittance of TiSi₂crystalline films is smaller than that of amorphous films. The Ti₅Si₃ thin films have the same and maximum transmittance between 600nm and 700nm. Because the resistivity of Ti₅Si₃ thin films is higher than that of TiSi₂ thin films, the absorption of Ti₅Si₃ thin films is smaller, the transmittance of Ti₅Si₃ thin films is higher than that of TiSi₂ thin films. The infrared reflection relys much on the phase formation in the films.The IR-reflectance of the thin films increases with the decrease in the resistivity of the thin films. The reflectance of the TiSi₂ thin films increases when the wavelength increases to 5000nm, and then the reflectance is almost costant with increasing of wavelength. The IR-reflectance of the TiSi₂ thin films is about 0.95 with increasing light wavelength to 25000nm. The IR-reflectance of the Ti₅Si₃ thin films increased with the wavelength. The reflectance increases little when the wavelength is larger than 7000nm, The IR-reflectance of the Ti₅Si₃ thin films increased to about 0.98 with increasing light wavelength to 25000nm.The high-density single crystalline orthorhombic TiSi nanowires were first prepared on Ti₅Si₃ layer at 700℃by APCVD, using SiH₄, and TiCl₄, as the precursors. The procedure includes two steps. First, Ti₅Si₃ thin films are deposited with the molar ratio of SiH₄/TiCl₄ of 1. Then, the molar ratio of SiH₄/TiCl₄ increases from 1 to 1.5, TiSi nanowires forms on the Ti₅Si₃ layer. The nanowires with diameters of 15-40nm and lengths of about 5 micrometers were obtained when the growth time is 15 min. The TiSi nanowires grow along [110] direction of the orthorhombic structure. The formation process of TiSi nanowires includes: Initially, Ti₅Si₃ thin films form when the ratio of SiH₄/TiCl₄ is 1, then Si forms on Ti₅Si₃ thin films when the ratio of SiH₄/TiCl₄ increases to 1.5. Si incorporates into Ti₅Si₃ to form the quasi-liquid Ti-Si alloy. After the quasi-liquid alloy becomes supersaturated with TiSi, TiSi will precipitate on the surface of quasi-liquid alloy to form TiSi nanoparticles, and then TiSi will precipitate on the interface between quasi-liquid alloy and TiSi nanoparticles. Continuous feeding of Ti and Si atoms into the quasi-liquid alloy leads to the formation of nanowires. The surface free energy per unit area of TiSi crystal decreases spontaneously with the increase in length along [110] direction of the crystal. Finally, TiSi nanowires form. The length of TiSi nanowires increases with the time. The growth rate and the length increase with the increase of temperature from 670 to 700℃. With the temperature increasing continuously, the radial growth rate increases gradually. The quadrate nanorods form at 730℃. With the temperature increasing continuously to 750℃, the growth rate is higher in the every direction, there are no nanowires, and the particles form.The single crystalline orthorhombic TiSi nanowire bundles were also successfully prepared with the different concentrations of (SiH₄+TiCl₄) of the second process. TiSi nanowires bundles, which reveal that the several parallel nanowire are tightly bound together, grow along [110] direction of the orthorhombic structure. The quantities and the dimension of the nanowires bundles increase with the concentration of (SiH₄+TiCl₄). The nanowire bundles with the lengths of about 2 micrometers and the diameters about 40-80nm were obtained when the concentration of (SiH₄+TiCl₄) is 3.33% and the molar ratio of SiH₄/TiCl₄ is 1.5. In addition, high-density single crystalline orthorhombic TiSi nanorods were also successfully prepared on Ti₅Si₃ layer at 730℃via the two-step method. The quadrate nanorods are approximately 0.5μm long and 40×40 nm² in area. The TiSi nanorods grow along the [110] direction of the orthorhombic TiSi crystal. The quantities and dimensions of TiSi nanopins increase with the concentration of (SiH₄+TiCl₄). The density of the nucleus increases, so the neighboring nanorods are parallel when the concentration of (SiH₄+TiCl₄) is higher than 2.0%In addition, high-density single crystalline orthorhombic TiSi nanopins were also first prepared on Ti₅Si₃ layer via the new three-process method. Based on the first two steps, the nanorods form. In the third step, the concentration of (SiH₄+TiCl₄) decreases gradually to zero, TiSi nanopins form. TiSi nanopins also grow along [110] direction of the orthorhombic structure. The temperature has an important influence on the growth of TiSi naopins. The atoms can not gather together to fulfill the nucleation when the temperature is not high enough, which explains why the nanopins fail to form until the temperature is above 710℃. A large amount of nanopins form when the temperature increases. With the temperature increasing continuously, the growth rate increases, but the nucleation rate switchs to decrease. Therefore, the nanopins exhibit a constant increase in the dimensions, but a decrease in distribution density. The maximal density of TiSi nanopins is obtained at the deposition temperature of 730℃. The nanopins are 0.7-1μm long in total, with the quadrate tip of about 200 nm long and 40×40 nm² in area. The growth process involves that Ti-Si quasi-liquid alloy formed on Ti₅Si₃ layer, the continuous feeding of Ti and Si form TiSi nanorods on Ti₅Si₃ layer. When the flux of (SiH₄+TiCl₄) begins to decrease, the formation of TiSi decreases, the size of the nanorods decreases gradually. Consequently, the nanopins form. The dimension of nanopins is controlled by the change of the concentration of (SiH₄+TiCl₄) in the third step. Because the growth of nanopins tip is controlled by the second step and the growth of tail end is dependent on the third step. The theoretical expression of the growth rate of the third step as the functions of the depositiontime and the concentration of (SiH₄+TiCl₄) was established as v =k₃,C₀ⁿ,(1-t/t_g)ⁿ,when k₃= 1.05×10¹⁵ and n=3, it was well consistent with the experimental results.TiSi rocket-shaped nanowires were also successfully prepared at 700℃via the three step method. Based on the first two steps, TiSi nanowires form. In the third step, the growth time is 5min, the molar ratio of SiH₄/TiCl₄ is 1.5 and the concentration of (SiH₄+TiCl₄) fixes to a higher value. TiSi rocket-shaped nanowires, which reveal that the several same nanowire sections are tightly bound parallel round the tip of the central nanowire, were obtained. TiSi central nanowires are more than 3 micrometers long with diameters within between 15nm and 30nm. The parallel nanowire sections are about 150nm long with the diameter about 10nm. The TiSi rocket-shaped nanowires grow along the [110] direction of the orthorhombic TiSi crystal. To form TiSi rocket-shaped nanowires, the optimal concentration of (SiH₄+TiCl₄) in the third step is about 3.33%. Their formations are based on both the excess of TiSi nanoparticles precipitated on the tip of nanowires and the inducing effect of TiSi nanowires. In the third step, the quantities of Ti and Si increase with the concentration of (SiH₄+TiCl₄). Because the nanowires have formed in the second step, the excess of Ti and Si atoms will precipitate on the tip of nanowires with the increase of (SiH₄+TiCl₄). Some Ti and Si atoms will form new TiSi nanoparticles. These new TiSi nanoparticles will form the new nanowires under the inducing effect of TiSi central nanowire. Finally, the rocket-shaped TiSi nanowires form. The growth process can be defined as self-assembled growth.The growth mechanism that we report here expands our understanding of growing nanostructures, and the method may be adaptable to the preparation of other nanostructure. It is believed that a way to grow titanium silicide nanowires on large scale glass substrate without metal catalysts by APCVD for promoting application of large scale flat panel display will be taken into account, and the application for the thin film capacitor with super-high charge storage will be induced with a special thin film configuration of the conductive nanowires planted into the dielectric matrix.