Light Illumination Stability Of Amorphous Ingazno Thin Flim Transistors

Amorphous In GaZnO(a-IGZO) thin film transistors(TFTs) have considerable potential for their applications in active matrix liquid crystal display(AMLCD) and active matrix organic light emitting diode(AMOLED) due to their high mobility, good uniformity, transparency, etc. Although a-IGZO TFTs have shown such good performance, their actual applications are still limited by some stability issues such as bias stress, environmental effect, high temperature and light illumination, which may lead to a certain extent of threshold voltage(Vth) shift. In this work, we focus our attention on light illumination stability of a-IGZO TFTs and try to understand the internal physical mechanism, so that we can improve their stability and provide certain reference for the actual mass production.Firstly, an effective fabrication flow of a-IGZO TFTs for this study was built, with which bias stress effect of a-IGZO TFTs was simply investigated. The results showed that passivation-layer thickness evidently influenced bias stress instability of a-IGZO TFTs.Secondly, we deeply investigated light illumination stability of a-IGZO TFTs with various passivation layer thicknesses. In the experiment, light wavelength and passivation-layer thickness were intentionally controlled to ascertain the related two physical mechanisms, i.e., electron-hole pair(EHP) generation and oxygen vacancy(VO) formation. A qualitative model was proposed to effectively compare and distinguish the above two mechanisms in thermal stability of a-IGZO TFTs with passivation layers. With light wavelength decreasing EHP generation became evident where the “threshold wavelength” was between 420 and 400 nm for the a-IGZO TFTs used in this study. Meanwhile, passivation-layer could significantly improve the stability of a-IGZO TFTs under long-wavelength light illumination by suppressing the escape of oxygen atoms to form VO in a-IGZO films. The “threshold thickness” of Al2O3 passivation-layer in our devices was estimated to be about 90 nm. After that, the negative bias illumination stress(NBIS) of a-IGZO TFTs was studied, and the results indicated that the negative bias stress would aggravate the devices’ stability. We attributed the phenomenon to the trap of holes by the trap states at the interface between active layer and gate insulator and/or in active layer.Finally, in the view of the close relationship between light illumination stability of a-IGZO TFTs and the VO in a-IGZO films, it’s quite meaningful to study the light illumination stability of a-IGZO TFTs with various oxygen flow rate(OFR) in active layer. Before preparing devices, we conducted AFM and XPS experiments of IGZO films with different OFR. The results showed that the increasing of OFR would degrade the surface roughness and reduce oxygen vacancies in the films. Then, four a-IGZO TFTs with 0, 1, 2 and 3 sccm OFR in active layer were made to compare their electrical properties and light illumination stability. We found that high OFR would not only worse the TFTs’ electrical performance, but also aggravate their stability under light. Two reasons might account for this result. One was the rougher surface of a-IGZO films caused by higher OFR and the other might be the increase in oxygen content generate more oxygen vacancies. Therefore, it should be better to lower oxygen flow rate during the preparation of IGZO film in the actual production.