Analysis of IGZO Thin-Film Transistors by XPS and Relation With Electrical Characteristics

The fabrication process and electrical characteristics of bottom-gate Indium Gallium Zinc Oxide (IGZO) thin-film transistors (TFTs) are reported in details. The influence of post-annealing ambient (Oxygen or Nitrogen) is studied. It has been found that characteristics of TFTs strongly depend on annealing conditions. TFTs with Oxygen annealing exhibit standard TFT characteristics. In this case, we have obtained a mobility of 7.2 cm²/V·s, a subthreshold swing of 0.3 V/decade, high I_on/I_off of 10⁷ and low leakage current of the order of 10^-13 at V_gs = -20 V. In the meantime, TFTs without a post-annealing or with Nitrogen annealing exhibited poor characteristics; more particularly the channel could not be depleted in the reverse mode. To understand the origins of this phenomenon, IGZO films from these devices have been analyzed by X-Ray Photoelectron Spectroscopy (XPS). Experimental results show that IGZO layers after annealing in N₂ have higher concentration of oxygen vacancies. This is consistent with our electrical results since it is assumed that conduction in IGZO films is the result of oxygen vacancies.