ELECTRICAL STABILITY ENHANCEMENT OF THE AMORPHOUS

Abstract

Improving the reliability of oxide thin film transistor (TFTs) without performance degradation is critically related to the back-channel surface properties. In this sense, a variable application of metal layer on the a-IGZO back-channel can become the significant method leading the improvement of electrical stabilities of oxide TFT. This study shows that such a hypothesis is verified by forming Au nanoparticles or ultrathin Ti on the a-IGZO back-channel. First, a significant improvement in various electrical instabilities of amorphous indium gallium zinc oxide (a-IGZO) thin film transistor (TFT) is demonstrated by implanting Au nanoparticles (NPs) on the a-IGZO back-channel. This TFT showed the enhanced stability of threshold voltage (Vth) under ambient humidity, illumination stress, and a-IGZO thickness variation tests. Application of back-channel Au NPs to a-IGZO TFT is regarded to control the surface potential, to lead reversible carrier trap/injection, and to increase incident UV light absorption by local surface plasmon. Au NPs are formed by e-beam evaporation and therefore, this technique can be applicable to the TFT manufacturing process. Second, electrical stability is enhanced through a Ti oxide (TiOx) layer on the a-InGaZnO (IGZO) back channel; this TiOx layer acts as a surface polarity modifier. Ultrathin Ti metal deposited on the a-IGZO existed as a TiOx thin film, resulting in oxygen cross-binding with the a-IGZO surface. The electrical properties of a-IGZO TFTs with TiOx depend on the surface polarity change and electronic band structure evolution, which varies with TiOx thickness from 0.95 to 3.65 nm. TiOx thinner than 1.87 nm on the a-IGZO back channel led to enhancement in the ambient and illumination stability without a remarkable degradation of electrical properties or an increase in contact resistance between the channel and source/drain electrode. This result indicates that TiOx on the back channel serves as not only a good passivation layer protecting the channel from ambient molecules or process variables but also a control layer of TFT device parameters.