Roles of Oxygen Interstitial Defects in Atomic-Layer Deposited InGaZnO Thin Films with Controlling the Cationic Compositions and Gate-Stack Processes for the Devices with Subμm Channel Lengths

Abstract

Roles of oxygen interstitial defects located in the In-Ga-Zn-O (IGZO) thin films prepared by atomic layer deposition were investigated with controlling the cationic compositions and gate-stack process conditions. It was found from the spectroscopic ellipsometry analysis that the excess oxygens increased with increasing the In contents within the IGZO channels. While the device using the IGZO channel with an In/Ga ratio of 0.2 did not show marked differences with the variations in the oxidant types during the gate-stack formation, the device characteristics were severely deteriorated with increasing the In/Ga ratio to 1.4, when the Al₂O₃ gate insulator (GI) was prepared with the H₂O oxidants (H₂O-Al₂O₃) due to a higher amount of excess oxygen in the channel. Additionally, during the deposition process of the Al-doped ZnO (AZO) gate electrode (GE) replacing from the indium-tin oxide (ITO) GE, the thermal annealing effect at 180 °C facilitated the passivation of oxygen vacancy and the strengthening of metal-oxygen bonding, which could stabilize the TFT operations. From these results, the gate-stack structure employing O₃-processed Al₂O₃ GI (O_3-Al₂O₃) and AZO GE (OA) was suggested to be most suitable for the device using IGZO channel with a higher In content. On the other hand, the device employing H₂O-Al₂O₃ GI and AZO GE exhibited larger negative shifts of threshold voltage (<i>V</i>_TH) under positive-bias-temperature stress (PBTS) condition than the device employing O_3- Al₂O₃ GI and ITO GE due to larger hydrogen contents within the gate stacks. Anomalous negative shifts of <i>V</i>_TH were accelerated with increasing the In contents of the IGZO channel. When the channel length of the fabricated device were scaled down to submicrometer regime, the OA gate stacks successfully alleviated the short-channel effects.