Thickness-Dependent Growth Behaviors of Sputtered Amorphous InGaZnO Films Depending on the Substrates and Sputtering Conditions

The deposition behavior and thin-film transistor (TFT) characteristics of amorphous indium–gallium–zinc oxide (a-IGZO) films on two gate-insulating films, thermally oxidized SiO2, and atomic layer deposited SiO2 were investigated. The a-IGZO films generally showed decreasing deposition rate and In content with increasing growth time, which was more severe on the atomic layer deposited SiO2 than on thermally oxidized SiO2. This phenomenon was attributed to the resputtering effect, which is more prominent on the atomic layer deposited SiO2. In atoms with weaker bonding were removed, causing alterations in the composition ratio and increased defects within the thin film, thereby degrading its electrical properties. Furthermore, resputtering persisted throughout the entire film growth, increasing (decreasing) the defect (film) density in the upper layer of the deposited a-IGZO film. Such a variation induced the film-thickness-dependent electrical properties of the fabricated TFTs. An increased carrier concentration and mobility were achieved from the thicker film due to the increased electron density in the lower layer, constituting the carrier accumulation layer. Adding oxygen gas to the sputtering atmosphere mitigated substrate dependency but degraded the TFT performances.