Effects of residual hydrogen in sputtering atmosphere on structures and properties of amorphous ln-Ga-Zn-O thin films

摘要

We investigated the effects of residual hydrogen in sputtering atmosphere on subgap states and carrier transport in amorphous In-Ga-Zn-0 (a-IGZO) using two sputtering systems with different base pressures of ~10~(-4) and 10~(-7)Pa (standard (STD) and ultrahigh vacuum (UHV) sputtering, respectively), which produce a-IGZO films with impurity hydrogen contents at the orders of 10~(20) and 10~(19)cm~(-3), respectively. Several subgap states were observed by hard X-ray photoemission spectroscopy, i.e., peak-shape near-valence band maximum (near-VBM) states, shoulder-shape near-VBM states, peak-shape near-conduction band minimum (near-CBM) states, and step-wise near-CBM states. It was confirmed that the formation of these subgap states were affected strongly by the residual hydrogen (possibly H_2O). The step-wise near-CBM states were observed only in the STD films deposited without O_2 gas flow and attributed to metallic In. Such step-wise near-CBM state was not detected in the other films including the UHV films even deposited without O_2 flow, substantiating that the metallic In is segregated by the strong reduction effect of the hydrogen/H_2O. Similarly, the density of the near-VBM states was very high for the STD films deposited without O_2. These films had low film density and are consistent with a model that voids in the amorphous structure form a part of the near-VBM states. On the other hand, the UHV films had high film densities and much less near-VBM states, keeping the possibility that some of the near-VBM states, in particular, of the peak-shape ones, originate from -OH and weakly bonded oxygen. These results indicate that 2% of excess O_2 flow is required for the STD sputtering to compensate the effects of the residual hydrogen/H_2O. The high-density near-VBM states and the metallic In segregation deteriorated the electron mobility to 0.4 cm~2/(V s).