The Fabrication and Characterization of ZnO Thin-Film Transistors Using Hf-based High-k Gate Dielectrics

• Main Authors: Chin-Chuan Huang, 黃金川
• Other Authors: Shui-Jinn Wang
• Format: Others
• Language: zh-TW
• Published: 2009
• Online Access: http://ndltd.ncl.edu.tw/handle/83895442367891146864

碩士 === 國立成功大學 === 微電子工程研究所碩博士班 === 97 === ZnO-based TFTs have attracted considerable attention due to their potentials toward a new driving and pixel switch component for liquid crystal display and novel applications such transparent and flexible electronics. However, most of ZnO TFTs suffered from high threshold voltage (VT), poor subshreshold swing (SS), and high operation voltage, still setting a limit on their applications. These issues mainly result from the use of low dielectric materials (such as SiO2), which usually leads to poor gate control on the channel current. To overcome the problems originated from using low-k materials, high-k materials need to be adopted as the gate dielectrics for the ZnO TFTs. Among high-k materials, Hafnium oxide (HfO2) was proposed as a good choice for its high dielectric constant and wide bandgap. However, its drawback of high charge trap density as in the CMOS application may retard its further application in the field of TFTs. Therefore, in this thesis, attempts have been made to integrate lanthanum (La) and titanium (Ti) into HfO2 to form HfLaO and HfTiO as the gate dielectrics for the fabrication of ZnO TFTs . In the first part of the thesis, the influence of different ZnO thicknesses on the TFT characteristics was investigated. Tantalum nitride (TaN) and hafnium lanthanum oxide (HfLaO) were employed as the gate electrode and gate insulator, respectively for ZnO thin film transistors. MIM capacitors with TaN/HfLaO/Al structure were also fabricated to evaluate the dielectric constant of high-k�nmaterial and device mobility. The physical properties and compositions of ZnO film were confirmed by SEM, XRD, and XPS analysis. From the highest capacitance of 5 fF/um^2 and the physical thickness of 30 nm, we estimated the dielectric of HfLaO was 25. Ids-Vds and Ids-Vgs characteristies of TFTs showed that ZnO deposited for 1 min might be the best condition for the channel layer. The Ion/Ioff ratio, threshold voltage, subthreshold swing, and mobility obtained from the fabricated ZnO-TFTs with TaN(50 nm)/HfLaO(30 nm)/Al(300 nm) structure were 10^5, 0.28 V, 0.26V/dec, and 3.5 cm^2/V-sec, respectively. In the second part of the thesis, the similar research scheme was applied except the HfLaO was replaced by hafnium titanium oxide (HfTiO) as the gate insulator. Different post deposition annealing (PDA) temperatures for HfTiO gate dielectrics were made to examine the temperature influences on device characteristics. The physical properties and compositions of HfTiO high-k film were also confirmed by XRD and XPS analysis. From the highest capacitance of 7 fF/um^2 and the physical thickness of 50 nm, we estimated this HfTiO dielectric has a even higher k value of 40 than HfLaO which should much benefit the driving current of ZnO TFTs. Experimental results showed the best-behaviored characteristics of ZnO-TFTs were obtained at optimum PDA temperature of 500℃ for HfTiO gate dielectrics. The Ion/Ioff ratio, threshold voltage, subthreshold swing, and mobility obtained from the fabricated ZnO-TFTs with TaN(50 nm)/HfTiO(50 nm)/Al(300 nm) structure were 10^5, 0.34 V, 0.23V/dec, and 2.1 cm^2/V-sec, respectively. In summary, our detailed preliminary experimental results revealed that both HfLaO and HfTiO gate dielectrics work well for future ZnO-TFTs.