Thin film transistor, method of producing the same, electrooptic apparatus, and sensor
A thin film transistor includes: a substrate; and, on the substrate, an oxide semiconductor film which serves as an active layer and contains In, Ga, and Zn, a gate electrode, a gate insulating film, a source electrode, and a drain electrode, wherein, when a molar ratio of In, Ga, and Zn in the oxide semiconductor film is expressed as In:Ga:Zn=(2.0−x):x:y, wherein 0.0<x<2.0 and 0.0<y, the distribution of y in the thickness direction of the oxide semiconductor film is such that the oxide semiconductor film has a region at which a value of y is larger than that at a surface of the oxide semiconductor film at a side closer to the substrate and that at a surface of the oxide semiconductor film at a side farther from the substrate.
1. A thin film transistor comprising:
a substrate; and, on the substrate,
an oxide semiconductor film which serves as an active layer and comprises In, Ga, and Zn, a gate electrode, a gate insulating film, a source electrode, and a drain electrode,
wherein, when a molar ratio of In, Ga, and Zn in the oxide semiconductor film is expressed as In:Ga:Zn=(2.0−x):x:y, wherein 0.0<x<2.0 and 0.0<y, the distribution of y in the thickness direction of the oxide semiconductor film is such that the oxide semiconductor film has a region in which a value of y is larger than that at a surface of the oxide semiconductor film at a side closer to the substrate and larger than that at a surface of the oxide semiconductor film at a side farther from the substrate.
2. The thin film transistor according to claim 1 , wherein an absolute maximum value of y over an entire thickness of the oxide semiconductor film is 1.8 or more.
3. The thin film transistor according to claim 1 , wherein an absolute maximum value of y over an entire thickness of the oxide semiconductor film is 5.0 or less.
4. The thin film transistor according to claim 1 , wherein a thickness of a region in which the value of y is at least 90% of an absolute maximum value thereof over an entire thickness of the oxide semiconductor film is less than one-third of the entire thickness of the oxide semiconductor film.
5. The thin film transistor according to claim 1 , wherein a center of a region in which the value of y is at least 90% of an absolute maximum value thereof over an entire thickness of the oxide semiconductor film is positioned at a side closer to the gate insulating film than the center of the oxide semiconductor layer in a thickness direction thereof.
6. The thin film transistor according to claim 1 , wherein a region in which the value of y is at least 90% of an absolute maximum value thereof over an entire thickness of the oxide semiconductor film is included in a region ranging from 1 nm to 30 nm from a surface of the oxide semiconductor film at a side closer to the gate insulating film in a thickness direction of the oxide semiconductor film.
7. The thin film transistor according to claim 1 , wherein at least a part of the oxide semiconductor film which includes a region in which the value of y is at least 90% of an absolute maximum value thereof over an entire thickness of the oxide semiconductor film is formed by a series of consecutive film-forming processes during which the semiconductor film is not exposed to the atmosphere.
8. The thin film transistor according to claim 1 , wherein at least a part of the oxide semiconductor film which includes a region in which the value of y is at least 90% of an absolute maximum value thereof over an entire thickness of the oxide semiconductor film is amorphous.
9. The thin film transistor according to claim 1 , wherein the oxide semiconductor film comprises:
a first region which includes the surface of the oxide semiconductor film at a side closer to the substrate and in which the value of y is from 0.8 to 1.2;
a second region which includes the surface of the oxide semiconductor film at a side farther from the substrate and in which the value of y is from 0.8 to 1.2; and
a third region which includes a region in which y takes an absolute maximum value over an entire thickness of the oxide semiconductor film and in which the value of y is 1.8 or more.
10. The thin film transistor according to claim 9 , wherein at least one of a change in the value of y from the first region to the third region and a change in the value of y from the third region to the second region is continuous.
11. The thin film transistor according to claim 1 , wherein the thin film transistor is a bottom-gate thin film transistor.
12. A method of producing the thin film transistor of claim 1 , the method comprising:
forming at least a part of the oxide semiconductor film which includes a region in which the value of y is at least 90% of an absolute maximum value thereof over an entire thickness of the oxide semiconductor film by a series of consecutive film-forming processes during which the oxide semiconductor film is not exposed to the atmosphere.
13. An electrooptic apparatus comprising the thin film transistor of claim 1 .
14. A sensor comprising the thin film transistor of claim 1 .