| Summary: | 碩士 === 國立成功大學 === 材料科學及工程學系碩博士班 === 97 === As the design of the electrodes on Thin Film Transistor (TFT) plays a curial role in the determination of TFT electrical properties, we have investigated the differences in electrical properties of TFTs fabricated with different types of metals, such as Al, Ti, Ta, and Ni, used as source and drain electrodes along with various designs of channel width to length ratios (W/L). The structure of the TFTs was designed as bottom-type, in which the gate was made with p-type Si (100). In addition, 500 nm thick SiO2 serving as the insulator was thermally grown on to the gate. 50 nm thick active layer material, Zinc Oxide (ZnO) was prepared by magnetron sputtering prior to the growth of the metals on the ZnO surface by E-beam Evaporation.
The crystal structure of fabricated ZnO thin film was identified by grazing incident angle x-ray diffractometer (GIAXRD). X-ray photoelectron spectroscopy (XPS) was applied for the composition and chemical bonding analysis of the interface between metals and ZnO layer. Rutherford backscattering spectroscopy (RBS) was utilized to examine the composition of ZnO thin film. The cross-sectional images and microstructure of the ZnO thin film were observed through high-resolution transmission electron microscopy (HRTEM). The optical transmittance of the ZnO thin films with various thicknesses were measured by UV-vis. spectrophotometer and their energy band gaps were calculated.
The experimental results of GIXRD and HRTEM reveal that the ZnO thin film is polycrystalline structure. With RBS, we confirmed that the O/Zn ratio for 50 nm ZnO is around 0.95, so the conducting mechanism through oxygen vacancies of the ZnO thin film was implicated. XPS analysis showed the oxidation behaviors at the interface of different metals and ZnO layer with the support of HRTEM cross-sectional images. From the results of UV-vis., the transmittance of the ZnO thin film was measured around 85%~90% and energy band gap was calculated.
The electrical properties of the TFT, such as Saturation Mobility( ), Saturation Current(ISat), Threshold Voltage(VTH), and On current and Off current ratio (On/Off ratio) were measured by Agilent 4156C; The contact resistance on samples of ”metals(Al, Ti,Ta,Ni)/ZnO(50nm)/SiO2(500nm)/Si” were measured and calculated through Transmission Line Model (TLM).
The results of the contact resistance measurement and calculation of the Al/ZnO、Ti/ZnO、Ta/ZnO are 0.0029( )、1.252( )、0.315( ) respectively , the contact resistance of Ni/ZnO was not able to be measured due to its metal work function(5.15eV) is 0.65eV higher than the fabricated ZnO(4.5eV), making the contact a non-ohmic contact(shocktty contact).
With various designs of channel width and length ratios (W/L), a series of electrical properties were summarized. If the metal material is the same, the , ISat , On/Off ratio all increased with only the VTH went down when the distance of the electrodes was shortened. If the electrode width and conducting channel length ratio (W/L) is fixed, the ISat and On/Off ratio of Al electrodes TFTs are higher than that of Ti electrodes TFTs’, while the and VTH of Al electrodes TFTs are lower than that of Ti electrodes TFTs’. The failure of the Ta electrodes is possibly due to the degradation of the properties of metal electrodes during its fabrication process, and the non-ohmic Ni/ZnO also leads to the failure of its TFT electrical properties.
|
|---|
