Indium Zinc Oxide Thin Film Transistor And A High Work Function Tco Film

Active Matrix Organic Light Emitting Diode (AMOLED) displays have been considered as the next generation displays technologies due to its advantages such as much thinner and lighter weight, quick response, high contrast ratio, wide viewing angle and low power consumption. Currently, amorphous silicon(a-Si) TFT and polycrystalline silicon(p-Si) TFT are two main technologies to determine which pixels get turned on to form an picture. Usually, the field effect mobility for a-Si TFT is less than 1 cm2/V·s and can not be used to the fast, large-area and high definition display. Besides, a-Si thin film is opaque and light-sensitive, which results in the decrease of aperture ratio and requires the black matrix. Therefore, power consumption will be raised for the increase of light intensity so as to obtain enough brightness. Low-temperature polycrystalline silicon (LTPS) TFT technology is also attractive for the high field effect mobility and demonstrates advantages like fast response, high brightness and high definition. Nevertheless, LTPS TFT faces problems, such as ununiformity, complicated process and high cost. Moreover, it is not transparent and the process temperature is still high for organic substrates, thus unsuitable for flexible displays.The main work principle for OLED can be defined as three processes:carrier injection, carrier transport, recombination and radiation degradation. The carrier injection can be explained that the carrier enters the organic layer through the interface between the electrode and the organic layer. The control of this process has direct effects on work voltage, efficiency and lifetime of the device. Thus, it is rather important to choose appropriate electrode material and make certain modification for improving the carrier injection and balancing the process. In fact, it can be referred from the development of OLED that the improvement of OLED efficiency and stability attributes much to the improvement of electrode. The high work function of the anode helps to reduce the potential barrier between the anode and hole transport layer (HTL), with better hole injection and OLED performance.As to above issues, the investigations of indium zinc oxide (IZO) semiconductor thin films and transparent conductive oxide (TCO) thin films with high work function were carried out. Transparent oxide semiconductors (TOS), such as In2O3 and ZnO, have both high carrier mobility and high transparency in the visible region. Low temperature process and uniformity are also their advantages. The substitution of the TFT with TOS for silicon-based TFT in TFT LCD or AMOLED may improve the pixel aperture ratio and brings higher brightness with lower power consumption. Such TFT can be prepared at room temperature, implying the potential application in flexible displays. Compared with the introduction of buffer materials for hole injection, CuPc for an instance, the fabrication of TCO thin films with high work function needs only an additional target and is compatible with the current OLED production, meeting industry concerns. Main research work and achievements are concluded as follows.The transparent amorphous In-Zn-O (a-IZO) semiconductor thin films have been prepared on glass substrates using direct current reactive magnetron sputtering with In/Zn alloy targets. The influence of deposition parameters such as In/Zn ratio and oxygen partial pressure on electrical and optical properties of a-IZO thin films have been investigated in detail. The result shows that by adjusting oxygen partial pressure during the deposition, the resisitivity of a-IZO thin film can be changed from 10-3 to 106 ohm-cm with the transmittance of over 80% in the visible light region. Moreover, the prepared a-IZO thin films have smooth surfaces with the root mean square roughness less than 1 nm.Amorphous IZO-based TFTs with both top-gate structure and bottom-gate structure are prepared. The a-IZO channel layer was deposited by dc reactive magnetron sputtering at room temperature. SiO2 dielectric layer was prepared by pulsed plasma deposition (PPD) method. The top-gate IZO-TFT demonstrates the threshold voltage of -2.4 V and mobility of 0.25 cm2 V-1s-1 and the corresponding values are 0.94 V and 5.2 cm2 V-1s-1 for bottom-gate IZO-TFT with the on-off ratio-104. The top-gate IZO-TFT using TaaO5 dielectric layer prepared by dc reactive magnetron sputtering shows the threshold voltage of 0.22 V and mobility of 1.1 cm2 V-1s-1.The thought of preparing TFT with oxide semiconductor channel layer and organic dielectric layer was proposed. The polyvinyl pyrrolidone dielectric layer was prepared at low temperature with good transparency and insulation by dip coating and spin coating. The hybrid IZO-TFT with inorganic channel layer and organic dielectric layer has been successfully prepared. The mobility, threshold voltage and on-off ratio of the device are 7.8 cm2V-1s-1,-4.8 V and 3.9 X 105, respectively. It is also found that IZO thin film prepared at high oxygen partial pressure shows lower carrier mobility and the TFT device with it as the channel displays lower field effect mobility accordingly.Poly-4-vinylphenol was also used as the dielectric layer and prepared at room temperature by spin coating with the transmittance over 85% in the visible region. The top-gate IZO TFT with poly-4-vinylphenol dielectric layer works in the depletion mode. The threshold voltage, mobility and on-off ratio of the device are-1.5 V,3.3 cm2V-1s-1 and 105, respectively. When the organic layer was properly cured, the mobility of the TFT is remarkably enhanced to 25.4 cm2V-1s-1 with the threshold voltage of 3.8 V. The device performance are improved because the baking helps to make the organic layer more compact, remove the residual component of the solvent, and better the interface between the dielectric layer and the channel layer.Transparent conductive tungsten-doped indium oxide (IWO) thin films with low resistivity and high transparency in the visible region are deposited. The experimental parameters are optimized. For example, the oxygen partial pressure significantly influences performance of IWO films. Further, the effects of post-annealing on the electrical and optical properties of IWO films are studied. It is found that proper post-annealing can reduce thin film resistivity and IWO film exhibits the minimum resistivity of 2.2×10-4Ω·cm. The carrier mobility reaches 63.5 cm2 V-1s-1 and the transmission is 83.2% in visible region.Platinum and tungsten codoped indium oxide (In2O3:Pt,W) thin layers with high work function were then prepared and studied in detail. On IWO layer, In2O3:Pt,W film of a few nanometers was deposited. The work function can be modulated with still high transparency in the visible region by adjusting the platinum content and the thickness of In2O3:Pt,W thin film. The chemical valence of elements and the morphology are characterized by X-ray photoelectron spectroscopy (XPS) and Atomic Force Microscope (AFM), respectively. The root mean square roughness of In2O3:Pt,W thin film is less than 7nm. The work function of thin films are characterized using Ultraviolet Photoemission Spectroscopy (UPS). It was confirmed that the work functions 3φof IWO and IWO/In2O3:Pt,W thin films are 4.7 and 5.5 eV, respectively. The resistivity of IWO/In2O3:Pt,W double layers film reaches 5.7×10-4 ohm-cm and the average visible light transmission of 82.7% was obtained. It is confirmed that the high work function elements is effective for the improvement of TCO thin film work function, this provide valuable ideas for the study of high work function TCO thin films. An OLED device with structure of IWO/In2O3:Pt,W/NPB/Alq3/LiF/Al was fabricated. When the voltage is 14 V, the current density and the brightness is 1600 mA/cm2 and 2.5×104 cd/m2, respectively. While the current density and the brightness of the OLED with ITO anode is 950 mA/cm2 and 7.2×103 cd/m2. This verifies OLED with the high work function anode has low working voltage.