Effect of H and OH desorption and diffusion on electronic structure in amorphous In-Ga-Zn-O metal-oxide-semiconductor diodes with various gate insulators

Metal-oxide-semiconductor (MOS) diodes with various gate insulators (G/Is) were characterized by capacitance–voltage characteristics and isothermal capacitance transient spectroscopy (ICTS) to evaluate the effect of H and OH desorption and diffusion on the electronic structures in amorphous In–Ga–Zn–O (a-IGZO) thin films. The density and the distribution of the space charge were found to be varied depending on the nature of the G/I. In the case of thermally grown SiO2 (thermal SiO2) G/Is, a high space-charge region was observed near the a-IGZO and G/I interface. After thermal annealing, the space-charge density in the deeper region of the film decreased, whereas remained unchanged near the interface region. The ICTS spectra obtained from the MOS diodes with the thermal SiO2 G/Is consisted of two broad peaks at around 5 × 10−4 and 3 × 10−2 s before annealing, while one broad peak was observed at around 1 × 10−4 s at the interface and at around 1 × 10−3 s in the bulk after annealing. Further, the trap density was considerably high near the interface. In contrast, the space-charge density was high throughout the bulk region of the MOS diode when the G/I was deposited by chemical vapor deposition (CVD). The ICTS spectra from the MOS diodes with the CVD G/Is revealed the existence of continuously distributed trap states, suggesting formations of high-density tail states below the conduction band minimum. According to secondary ion mass spectroscopy analyses, desorption and outdiffusion of H and OH were clearly observed in the CVD G/I sample. These phenomena could introduce structural fluctuations in the a-IGZO films, resulting in the formation of the conduction band tail states. Thin-film transistors (TFTs) with the same gate structure as the MOS diodes were fabricated to correlate the electronic properties with the TFT performance, and it was found that TFTs with the CVD G/I showed a reduced saturation mobility. These results indicate that the electronic structures in the a-IGZO films were strongly influenced by the nature of the G/Is as well as the process conditions. It is concluded that controlling of the natures of the G/I such as film density and hydrogen content in the films is critical to obtain high-performance electronic devices using a-IGZO.