The variety of semiconductor materials has been extended in various directions, for example, to very wide bandgap materials such as oxide semiconductors as well as to amorphous semiconductors. Crystalline β-Ga 2 O 3 is known as a transparent conducting oxide with an ultra-wide bandgap of~4.9 eV, but amorphous (a-) Ga 2 O x is just an electrical insulator because the combination of an ultra-wide bandgap and an amorphous structure has serious difficulties in attaining electronic conduction. This paper reports semiconducting a-Ga 2 O x thin films deposited on glass at room temperature and their applications to thin-film transistors and Schottky diodes, accomplished by suppressing the formation of charge compensation defects. The film density is the most important parameter, and the film density is increased by enhancing the film growth rate by an order of magnitude. Additionally, as opposed to the cases of conventional oxide semiconductors, an appropriately high oxygen partial pressure must be chosen for a-Ga 2 O x to reduce electron traps. These considerations produce semiconducting a-Ga 2 O x thin films with an electron Hall mobility of~8 cm 2 V − 1 s − 1 , a carrier density N e of~2 × 10 14 cm − 3 and an ultra-wide bandgap of~4.12 eV. An a-Ga 2 O x thin-film transistor exhibited reasonable performance such as a saturation mobility of~1.5 cm 2 V − 1 s − 1 and an on/off ratio 410 7 .

We fabricated amorphous oxide semiconductor films, a-(Ga 1-x Zn x )O y , at room temperature on glass, which have widely tunable band gaps (E g ) ranging from 3.47-4.12 eV. The highest electron Hall mobility~7 cm 2 V −1 s −1 was obtained for E g =~3.8 eV. Ultraviolet photoemission spectroscopy revealed that the increase in E g with increasing the Ga content comes mostly from the deepening of the valence band maximum level while the conduction band minimum level remains almost unchanged. These characteristics are explained by their electronic structures. As these films can be fabricated at room temperature on plastic, this achievement extends the applications of flexible electronics to opto-electronic integrated circuits associated with deep ultraviolet region.

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