Abstract
In the present study, the electronic structure and film composition at the back channel surface of amorphous In-Ga-Zn-O (a-IGZO) thin films were characterized to discuss the origin of the threshold voltage shift in the a-IGZO TFTs under light negative bias and temperature stress (LNBTS) test. Photoelectron yield spectroscopy revealed that surface gap states were observed near the valence band maximum, but were not responsible for the hump observed in the transfer curve. By using photoelectron spectroscopy for chemical analysis (ESCA), it was found that the film composition was changed as it was approaching to the surface; the content of In increased while that of Ga decreased. A increase in oxygen deficiency was also observed near the back channel surface. In accordance with the change, downward band bending was detected by ultraviolet photoelectron spectroscopy near the surface (up to 5 ∼ 7 nm) region. According to cathodoluminescence (CL) measurements, two broad bands were observed at around 1.8 and 2.4 eV. The gap states causing the luminescence were mainly located near the back channel surface. It is considered that the trap states due to the oxygen deficiency and/or the Zn interstitials could cause the hump effect in the transfer curve of a-IGZO TFTs.
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