The effect of ITO and Mo electrodes on the properties and stability of In-Ga-Zn-O thin film transistors

The effect of ITO and Mo electrodes on the electrical properties and stability of In-Ga-Zn-O (IGZO) thin film transistors (TFTs) are investigated. While the field effect mobility values of the devices employing ITO and Mo electrodes are similar, the former exhibit smaller threshold voltage (V_th) and subthreshold swing (SS). It is suggested that the relatively large workfunction of Mo (4.7 eV) compared to that of ITO (4.4 ~ 4.5 eV) induces a large Schottky barrier at the Mo/IGZO junction, which prohibits the effective injection of electrons from the metal into the IGZO semiconductor. The workfunction of IGZO is usually reported to be approximately 4.5 eV. The device stability of the two types of TFTs under negative bias stress (NBS) and positive bias stress (PBS) is similar, which implies that the degradation of the devices under bias stress is mainly affected by the trapping of carriers at the IGZO/gate insulator interface. In the presence of illumination, the devices using optically transparent ITO electrodes allow the penetration of a more abundant concentration of photons into the IGZO active layer, and thus undergo larger V_th shifts under negative bias illumination stress (NBIS). However, under positive bias illumination stress (PBIS), the TFTs using ITO exhibit smaller positive V_th shifts. The latter phenomenon is suggested to result from the excess photo-induced electrons in the bulk that counter the effect of electron trapping near the IGZO/gate insulator boundary.