The correlation between the crystal structure and valence band structure of a (Ga x In 1−x ) 2 O 3 solid solution system was investigated by using combinatorial synthesis. At a low Ga content of (Ga x In 1−x ) 2 O 3 with a single-phase cubic In 2 O 3 crystal structure, a surface electron accumulation layer (SEAL), which is an important electrical phenomenon in In 2 O 3 , was confirmed. When the Ga content increased to approximately x = 0.4, mixed crystal structures of Ga 2 O 3 and In 2 O 3 were produced. Above x = 0.5, the dominant valence band structure was attributed to Ga 2 O 3 , the SEAL disappeared, and the sheet resistance increased greatly by 5 orders of magnitude or more. The in-gap state and valence band structure of the (Ga x In 1−x ) 2 O 3 solid solution system were strongly affected by Ga 2 O 3 ; however, the valence band maximum position shifted to a higher binding energy.
In this paper, the InGaZnO-based thin-film transistors (TFT) with the sputtered high-k SrTa 2 O 6 (STA) as a gate insulator are fabricated. The IGZO-TFT with STA show low voltage operation below 5 V with good TFT performances, i.e., filed effect mobility of 11.1 cm 2 V À1 s À1 , threshold voltage of 0.6 V, sub-threshold swing of 163 mV/decade, and on/off ratio of 7.8 Â 10 9 . Compared to the thermally grown SiO 2 gate insulator, the sub-threshold swing is improved by use of STA gate insulator. In addition, the high oncurrent and low gate leakage current are obtained. These characteristics are attributable to the sufficient electrical properties of STA, i.e., high dielectric constant of 30 and low leakage current density of 2.6 Â 10 À8 A cm À2 at 300 kV cm À1 .
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