산화아연 기반 박막트랜지스터의 바이어스 스트레스에 대한 안정성 향상을 위한 연구

Abstract

Recently, there has been increasing interest in amorphous oxide semiconductor-based thin film transistors (TFTs) for application in the next generation display industry. In particular, applications in high resolution and large size liquid crystal displays (LCD) have been realized. In order to replace a-Si TFTs using a low cost process, it is favorable to maintain the current process except for the a-Si channel layer deposition. However, the research trends of amorphous oxide TFTs investigation have mostly focused on the device performance with a unit cell. Also, it is certainly necessary to obtain stable device characteristics under various bias, temperature, and light conditions, among others. The guarantee of device stability under external environments is considered to be an important subject for the buoyant development of amorphous oxide TFTs. Zinc oxide-based thin film transistors (TFTs) are of great interest for application in next generation flat panel displays backplane. Most research has been based on amorphous indium-gallium-zinc oxide (IGZO) TFTs, because of their reproducibility, uniformity, low process temperature, high transparency, and surface smoothness. In IGZO system, gallium acts as a suppressor of carrier generation in IZO system due to its high oxygen binding energy and indium acts as an enhancer of mobility due to its large 5s orbital. Similarly, there are many approaches for substitution of Ga and In in ZnO system, using AlZnSnO, ZrInZnO, and GaSnZnO. First, in this study, we report the fabrication of high performance amorphous IGZO-TFTs with a SiNX gate insulator treated using electron cyclotron resonance (ECR) remote O2 plasma. It was found that the O2 plasma treatment of the gate insulator dramatically improved the a-IGZO TFT device performance and bias stability compared to the devices with an untreated-SiNX gate insulator. Second, in order to higher performance and more stable ZnO-based TFTs, brand-new Hafnium-Zinc-Tin-Oxide (HZTO) thin film was employed as an active channel layer. It was expected that HZTO-TFTs would have more stable electrical characteristics under gate bias stress condition because the binding energy of Hf-O is greater than that of Zn-O. Also, HZTO thin films can be deposited with amorphous phase because Sn has a large 5s orbital like In. Also, density of states (DOS) in subgap of channel layer can be controlled from the additional Hf and Sn atoms in ZnO system. Modified DOS in subgap of HZTO, which would enhance the electrical performance and bias stability. The fabrication of the oxide-based TFTs with HZTO active channel layer was reported with excellent stability. Application of HZTO thin films as an active channel layer improved the TFT device performance and bias stability, as compared to intrinsic ZnO-based TFTs. The excellent negative bias temperature stress (NBTS) stability of the device was analyzed using the HZTO and intrinsic ZnO TFTs transfer curves acquired at a high temperature (473 K). Also, in order to evaluate precisely DOS in subgap of channel layer, capacitance-(voltage, frequency) [C-(V,f)] method was introduced. In summary, the large mobility in the amorphous structure comes from the fact that large spherical orbitals such as Sn form electron transport paths, and the strong ionic bonds do not form high-density electron traps in the bandgap. The low-density subgap trap is responsible for the small S value and high stability of ZnO-based TFTs under bias stress conditions. To improve the stability of electrical properties, incorporation of Hf ions, which form stronger chemical bonds with oxygens, is important.