Amorphous Indium-Gallium-Zinc-Oxide (a-IGZO) has been widely used in the flat-panel display as the backplane [1]. In recent years, IGZO-TFTs have attracted attention for capacitor-free next-generation memory applications owing to their ultra-low off-current. They require ultra-thin (several nanometers), threshold voltage stability, and strict requirements for precise control of carrier concentration; however, the carrier generation mechanism in AOS remains still elusive. Here, we conducted Hall effects on a-IGZO under various conditions as a function of temperature to elucidate the concentration and energy levels of thermally active shallow donors. In general, the activation energy in n-type semiconductors is negatively correlated with the donor concentration as shown in the following equation  E_a(N_D )=E_a(0)-α(N_D)^(1/3), where E_a(0), α and (N_D )^(1/3)  indicate activation energy for band conduction at very low doping level, constant for geometrical factors, average distance between ionized donors. Conventional semiconductors and oxide semiconductors have α>0, which indicate increase of activation energy is due to interaction between ionized donors.  However, if there is positive correlation or both of positive and negative correlations occurs for varying film thickness and post-annealing process, it would lead to have novel picture in AOS for TFTs. In this study, we measured temperature dependent Hall effect in various a-IGZO thin films and Hard X-ray Photoelectron Spectroscopy (HAXPES) to observe the electronic states of a-IGZO. Fig. 1 shows the relationship between N_D and E_a calculated from the temperature dependence of the Hall effect. When the P_O2 during sputtering was reduced to increase the V_O, a decrease in E_a was observed with an increase in N_D, as shown in the blue plot in Fig. 1. This is the behavior typically seen in common n-type semiconductors. The linear relationship between N_D^(1/3)  and E_a, indicating that this relationship is due to the interaction between V_O’s. By increasing V_O by lowering P_O2, as shown in Fig. 1(a and b), average separation between donors is getting closer, causing the top of the occupied orbital become shallower due to orbital interaction. This Ea shift continues until the donor levels become shallow near the CBM, eventually reaches degenerate state. However, interestingly, when ultra-thin a-IGZO films, required for next generation memory applications, were fabricated, the E_a showed a positive correlation with defect concentration, as shown in the red plot in Fig 1. Since the relationship between N_D and E_a is opposite to donor-donor interactions, it is possible that an unoccupied acceptor level located just below the CBM, interact with a donor as shown in Fig. 1(c and d), results in a deepening of the occupied orbitals. This means the increase in E_a with increasing N_D. In the talk, we present these new findings and their responsible model along with influence on TFT performance.

[1]Hosono, H., & Kumomi, H. (Eds.). (2022). Amorphous oxide semiconductors: IGZO and related materials for display and memory. John Wiley & Sons.