Study on Characteristics with Doping Concentration for n-type In-Ga-Zn-O Thin Film Transistors and p-type Cu-O Thin Film

Abstract

Given the close relation between TFTs and display, it is not strange that the most significant application of oxide TFTs is on display, mostly LCDs and OLEDs. Recently, AOS TFTs have been intensively researched as a promising alternative to conventional a-Si-based TFTs which has low mobility characteristic of less than 1 cm2/(V∙s), and poly-Si-based TFTs, which has high cost. AOS TFTs are very attractive candidates for a-Si:H TFT Oxide TFTs can achieve a high mobility, high aperture ratio, low cost, and low power consumption. However, for use in display device, oxide TFTs need to achieve the critical device performances of high mobility characteristic of more than 8 cm2/(V∙s) and enhanced stability on electric, luminance and thermal humidity. Furthermore, in order to being applicable to the TFT manufacturing process, oxide TFT should be fabricated on 8th generation glass and meet the uniformity specification due to being connected with the yield. This work aims to research and develop the key parameters for realizing mass production of AOS TFTs such as high stability, high drain-source current, and low leakage current. At the former part of this dissertation, we adopted to fabricate n-type IGZO TFTs by using the co-deposition of silicon on an a-IGZO back channel for achieving required high stability under NBIS and thermal humidity, which can avoid the reduction of drain-source current reduction with double active layers. Si plays a powerful role as a suppressor with high M-O bonding strengths and low standard electric potential. Then we achieve the Vth shift 0.95 V under thermal humidity and NBIS without drain current drop using SiOX co-deposition. Additionally, we fabricated copper oxide thin films which are known to show p-type conductivity and are attracting renewed interest as promising semiconductor material for wide range of optoelectronic devices. There are two common forms of copper oxide: Cu2O and CuO. There are fewer p-type semiconducting oxides. Furthermore, it is hard to achieve an TFTs having a p-type semiconductor channel with keeping a large mobility. p-type TFTs have an advantage in driving OLEDs because a TFT must supply hole current to the anode on the bottom side of a OLED. In p-type semiconductors, it does not affect the drain current of a p-channel TFT in the saturation mode, not like n-type TFTs. In this work, we investigated the stability of n-type a-IGZO TFT with Si doped back-channel layer under thermal humidity and NBIS and the band gap of Cu-oxides with annealing temperature and absorption coefficient. The stability of Si doped a-IGZO TFTs was achieved from the role of Si in the back-channel. The Si in the back-channel plays a role as an important suppressor because it has high metal-oxide bonding strength and low standard electric potential. Additionally, SiOX on the back-channel’s surface might reduce the adsorption/desorption interactions because of the reduced number of interaction sites for H2O and O2. Resistance, lattice and grain size was investigated as a function of annealing temperature of copper oxide. The copper oxide films kept to Cu2O structure at annealing temperature up to 200 °C, but following annealing at 300 °C, the formation of Cu2O with cubic structure followed by the formation of CuO with monoclinic structure. Resistance of copper oxide film increased with increasing annealing temperature between 200 and 300 °C, which is transition region from Cu2O to CuO. As a result, we simultaneously obtained a high stable Si doped a-IGZO TFT and the band-gap of Cu2O and CuO. Threshold voltage shift of Si doped a-IGZO TFTs under Thermal humidity and NBIS was 0.06 and 0.95 V, that of a-IGZO TFTs was 2.45 and 1.52 V. In addition, we clearly obtained that the band-gap of Cu2O and CuO was calculated by absorption coefficient, 2.31 and 1.28 eV.