Study on the Zn-Sn-O field effect transistors for the application to transparent and flexible display devices

Abstract

In recent years, the display devices focused on user experience such as curved, flexible and transparent features. Since conventional silicon-based thin film transistors (TFTs) have a limit in flexibility and transparency, it is necessary to develop new materials with these properties. Therefore, amorphous oxide semiconductors (AOSs) such as InGaZnOx (IGZO) and ZnSnOx (ZTO) have attracted attention as a new active-layer materials of the TFTs in display devices due to the need for channel materials that have flexibility and productivity. ZTO is especially taking the spotlight as it has none of expensive rare-earth elements as well as high mobility and superior productivity. ZTO has superior electrical properties, but there are still problems to be solved to apply them to the flexible devices. To fabricate a flexible device, it is necessary to introduce polymer substrates with thermal resistance, such as polyethylene naphthalate (PEN) or poly arylate (PAR), which resist heats up to 200 °C. Whereas conventional process for fabrication of ZTO TFTs need a thermal treatment over 350 °C, which is excessively high than the heat resistance temperature of the polymer substrate. Therefore, it is essential to develop a process which can achieve the transistor properties of oxide semiconductor even at low temperature under the limit of polymer substrate. Also, there is a lack of understanding of the phenomenon that occurs when a flexible display device using the ZTO exposed to external stress. Generally, the external stresses applied to the flexible display devices are the illumination from the back light unit and the mechanical stress. In this dissertation, the studies on the ZTO TFTs for the application to transparent and flexible devices were conducted. First, the experiments carried out to lower the maximum process temperature in order to make the ZTO devices applicable to the polymer substrates which have process temperature limit of 200 °C or less. Conventional ZTO should be annealed at 350 °C or higher after the deposition using an RF sputtering system. This annealing process makes atomic bonds tight and enhances to react the oxygen vacancies with oxygen. Instead of post annealing process, the inter-bonding among atoms and relief of oxygen vacancy occurred through heating the substrate and adding oxygen in the process chamber. As a result, it was possible to fabricate devices that show excellent electrical characteristics with field effect electron mobilities of 5.8 – 27.1 cm2 V–1 s–1, Ion/Ioff of 108, and subthreshold swing of 0.15~1.7 V dec-1, which are similar to those of the devices fabricated by the conventional process, even though lowering the process temperature by more than 150 °C. As a result of measuring the thin film density by X-ray reflectometry, the films deposited at room temperature had the density of 5.5 g cm-3, 5.7 g cm-3 and 5.9 g cm-3 after deposition, post-heat treatment at 200 °C and 350 °C, respectively. The more dense films were formed as the annealing temperature increased. In the case of the in-situ annealing deposition, the density of 5.9 and 6.0 g cm-3 was observed at the 150 ° and 200 ° processes, respectively. It was confirmed that a dense thin film can be formed even at a low temperature through the in-situ annealing deposition. In addition, the surface roughness using an atomic force microscope was also improved with in-situ annealing. The ZTO thin film deposited at room-temperature after post-annealing at 150 °C had a root mean square roughness of 1.81nm, whereas the thin films after in-situ annealing had the roughness of 0.33 ~ 0.43nm, which are improved more than four times better than that of the conventional post annealing method. Transmission electron microscopy analysis showed the effects of in-situ annealing process indisputably. In the samples deposited at room temperature, a complete amorphous phase was observed even with the post deposition annealing at 350 °C. These results inform that the ZTO clusters deposited on the heated substrate have a sufficient kinetic energy due to high temperature and be movable filling the physical vacancies and forming the crystalline phase which stabilize the electrical properties of ZTO TFTs . Therefore, it has been confirmed that the density increase and the improvement of the roughness occurs together. These results show that the ZTO clusters deposited on the heated substrate have sufficient kinetic energy due to thermal energy and move to fill the physical pores, and thus the density increases and the roughness decreases. The ultimate goal of lowering the process temperature was to fabricate the ZTO TFT devices on polymer substrate which have transparency and flexibility. ZTO-TFTs possessing transparency and flexibility were successfully fabricated on the 125 μm thick PEN substrates. They showed electrical properties with a mobility of 9.7 cm2 V–1 s–1, Ion/Ioff of 108, and subthreshold swing of 0.28 V dec-1. The devices on the polymer substrates performed good enough to be applied to commercial electronic devices. Secondly, the effects of external stress on the properties of flexible ZTO TFTs were studied. As mentioned above, ZTO TFTs are widely used in display devices based on the advantages such as transparency and flexibility. When applied to a flexible display device, the ZTO TFTs are exposed to the illumination stress from the back light unit and the mechanical stress caused by the bending. Therefore, it is very important to analyze the reliability of the devices under such stress conditions. The device was fixed in convex and concave jigs with a radius of curvature of 20 mm, and performed reliability analysis by irradiating the light of 450 and 500 nm wavelength under the condition of mechanical stress. As a result, when light was illuminated on the convex jig, the threshold voltage of the TFT moved more toward lower voltage with the tensile stress. Tensile stress tended to deteriorate the reliability of the ZTO TFT devices. On the contrary, when the compressive stress was applied to the devices by fixing on the concave jig, the threshold voltage shift was lowered in the stress state. To analyze this phenomenon, the way for applying different stress states to the equally deposited ZTO thin films on a transparent PEN substrate was studied, and a method of independently differentiating stress without changing the chemical composition was suggested. The optical band gap measurement using UV-vis spectrometer and elemental analysis using ultra-violet photoelectron spectrometer were performed to analyze the valence band maximum (VBM) and work function. As a result, when the compressive stress was applied up to 0.6 %, the bandgap and VBM increased. It is found that the energy differences between conduction band minimum and fermi level (EF) decrease with increasing compression. And the reliability of the devices have correlation with the applied mechanical stress which means that it can be possible to enhance the photo-bias stability by adopting appropriate mechanical stress.