A Study on High-Performance Oxide Transistor via Crystallization and Instability Mechanism Identification for Display Application

Abstract

The display industry has rapidly evolved from bulky cathode-ray tube to flat panel display such as liquid crystal and organic light-emitting displays. Now, new form factor such as curved panel, foldable, bendable, flexible begin to play a crucial role to create a new product in emerging display-related market. Simultaneously, the ever-increasing demand on the vivid, high quality image quality necessitates the high-pixel density, fast driving, and large-area capability in terms of backplane electronics. Since the discovery of amorphous indium gallium zinc oxide (a-IGZO) by Prof. Hosono in 2004, metal oxide thin-film transistors (TFTs) have attracted a huge attention due to their high mobility, low off-state current, good uniformity, and reasonable stability as a next-generation backplane electronic. World-wide intensive research and development led the commercialization of IGZO TFTs into high-end LCD and OLED displays, replacing the conventional amorphous Si TFTs. Field-effect mobility of a-IGZO TFTs, which is used as a pixel transistor in current OLED TV, is known to be ~10 cm2/Vs. However, the novel oxide TFTs with high mobility (>30 cm2/Vs) and good stability need to be developed to drive the advanced OLED display with an ultra-high-pixel density and high frame rate. For this purpose, various channel material systems such as indium zinc oxide, indium zinc tin oxide and indium gallium zinc tin oxide have been investigated. General trend is that the high indium fraction in the channel layer allows the high carrier mobility in the resulting transistors. However, the oxygen vacancy (VO) defects are easily created for such an In-rich metal oxide channel due to relatively weak bond of In-O, leading to the increasing off-state current and notorious NBIS instability. In this thesis, the high-temperature crystallization behavior of indium zinc tin oxide (IZTO) was examined to investigate the feasibility as an alternative oxide channel layer in TFTs. Major finding was that the different thickness resulted in the versatile phases including amorphous, low, and high-crystalline structure after annealing at 700 ℃. In particular, the carefully controlled crystallization of IZTO allowed the resulting TFTs to have the high mobility of 39.2 cm2/Vs, excellent ION/OFF ratio of 9.7 x 108 and better stability against the external gate bias thermal stress duration compared to the counter part of amorphous IZTO film. This promising performance is explained by the crystallization-induced lattice ordering. In second part, the degradation mechanism for self-aligned a-IGZO TFTs was examined under gate bias thermal stress duration. Depending on the oxygen partial pressure during IGZO sputtering, the different physical models against the external bias-thermal-stress was suggested. Beside the conventional oxygen vacancy model, the new degradation model involving the cation vacancy and hydrogen impurity was identified in the TFTs with the excessive oxygen concentration in the IGZO channel layer on basis of the experimental and theoretical calculation results. In the last part, the short channel effect for the self-aligned oxide TFTs with the different oxygen concentration in IGZO channel layer was studied. The native threshold voltage (VTH) shift for IGZO TFTs with increasing drain voltage (VDS) becomes severe with decreasing the channel length (L) from 10 to 2 μm. It can be suppressed by increasing oxygen concentration in IGZO layer, which was attributed to the reduced concentration of VO defect near the n+ drain of the a-IGZO region. In contrast, the hot carrier effect (HCS)-induced degradation in terms of VTH was accelerated for devices with increasing oxygen concentration in IGZO, presumably due to the creation of excessive oxygen-originated defects. The rationale for these observations is discussed with regard to the increasing local electric field near the drain junction, which was calculated by technology computer-aided design (TCAD) simulation. It can be concluded that an acceptable compromise between short channel effect and HCS-induced degradations can be achieved by choosing an intermediate oxygen concentration in IGZO film.