Unraveling the role of post-annealing in IGZO transistor for memory applications

Abstract

We demonstrate that post-annealing techniques are important for achieving the transfer characteristics of indium‑gallium‑zinc oxide (IGZO) transistors and identify that their role depends on the sputter-deposited IGZO film conditions. The as-fabricated transistor with a thin IGZO channel, HfO₂ gate dielectric, and Mo gate electrode exhibits a constant drain current (I_DS) over gate voltage (V_GS). Although the oxygen (O₂) plasma gas rate is adjusted from 0.2 to 1 sccm with an argon gas rate of 30 sccm during IGZO deposition, the I_DS level was reduced by a factor of 10⁴. Notably, V_GS-controlled transfer behavior of the transistors only starts after post-annealing is performed at temperatures above 300 °C, regardless of which IGZO channel properties are used. More specifically, since oxygen vacancies (V_Os) serve as carriers in the IGZO, annealing in different O₂ gas or air environments to generate or reduce the number of V_Os is found to be optimal for the V_O-rich or V_O-poor channels, respectively. In this study, we reveal that oxidation annealing appears to be a more effective way for achieving improved gate controllability (e.g., subthreshold swing). Accordingly, we further analyze how the V_Os in the IGZO are involved in switching by examining the effect of annealing temperature and gate dielectric materials on the transfer curve. These results indicate that V_Os in the bulk need to be annihilated to lower the off-state I_DS, while a sufficient number of V_Os near the channel and gate dielectric interface should be ensured to responded by V_GS for rapid switching.