Structural relaxation in amorphous oxide semiconductor, a-In-Ga-Zn-O

Amorphous In-Ga-Zn-O (a-IGZO) is expected as a backplane transistor material to drive next-generation flat-panel and flexible displays. It has been elucidated that thermal annealing even at low temperatures <200 °C reduces deep subgap defects and those at ≥300 °C further improve device characteristics, stability, and uniformity. These temperatures are much lower than the reported crystallization temperature (TX ∼ 600 °C). In this work, we investigate effects of thermal annealing on the structural and optical properties of a-IGZO thin films. We performed classical molecular dynamics simulation (CMD) and optical interference analyses including spectroscopic ellipsometry (SE). CMD reproduced the x-ray diffraction pattern of a-IGZO and exhibited a glass transition. Experimentally, it was found that TX depends largely on deposition methods and conditions, probably due to different chemical compositions. Sputter-deposited a-IGZO films exhibited onset TX ∼ 600 °C and crystalline volume fraction XC increased linearly from 600 °C. 1.2% of film densification occurred even at <TX, and crystallization caused larger densification, which is consistent with the film density measured by x-ray reflectivity spectroscopy. Bandgap increased in two temperature regions; i.e., (i) at <400 °C due to structural relaxation and (ii) at >600 °C due to crystallization. High-temperature in situ SE measurements did not detect a symptom of a glass transition temperature (Tg) presumably because the TX is close to Tg similar to the case of amorphous metals.