Crystallization behavior of amorphous indium-gallium-zinc-oxide films and its effects on thin-film transistor performance

Amorphous indium-gallium-zinc oxide (a-IGZO) films were deposited by DC magnetron sputtering and post-annealed in air at 300-1000 C for 1 h to investigate the crystallization behavior in detail. X-ray diffraction, electron beam diffraction, and high-resolution electron microscopy revealed that the IGZO films showed an amorphous structure after post-annealing at 300 C. At 600 C, the films started to crystallize from the surface with c-axis preferred orientation. At 700-1000 C, the films totally crystallized into polycrystalline structures, wherein the grains showed c-axis preferred orientation close to the surface and random orientation inside the films. The current-gate voltage (Id-Vg) characteristics of the IGZO thin-film transistor (TFT) showed that the threshold voltage (Vth) and subthreshold swing decreased markedly after the post-annealing at 300 C. The TFT using the totally crystallized films also showed the decrease in Vth, whereas the field-effect mobility decreased considerably.