Thin-film transistor having tapered organic etch-stopper layer

1. A thin-film transistor comprising:

a gate electrode above a substrate;

a gate insulating layer above the gate electrode;

a semiconductor layer that is opposed to the gate electrode with the gate insulating layer therebetween;

an etch-stopper layer above the semiconductor layer and comprising an organic material; and

a source electrode and a drain electrode that are opposed to each other, each of the source electrode and the drain electrode having at least a portion located above the etch-stopper layer;

wherein the etch-stopper layer includes an altered layer having at least a portion contacting the semiconductor layer, the altered layer being generated by alteration of a surface layer of the etch-stopper layer in a region exposed from the source electrode and the drain electrode, and

a relational expression of Log 10 Nt≦0.0556θ+16.86 is satisfied where Nt (cm −3 ) represents a defect density of the semiconductor layer and θ (°) represents a taper angle of an edge portion of the region of the etch-stopper layer exposed from the source electrode and the drain electrode.

2. The thin-film transistor according to claim 1 ,

wherein the altered layer is a layer generated by dry etching performed to pattern the semiconductor layer.

3. The thin-film transistor according to claim 1 ,

wherein the altered layer has a thickness of at least 30 nm.

4. The thin-film transistor according to claim 2 ,

wherein the altered layer has a density higher than a density of a non-altered layer which is a portion of the etch-stopper layer that is not altered by the dry etching.

5. The thin-film transistor according to claim 4 ,

wherein the altered layer has a chlorine concentration which is at least ten times higher than a chlorine concentration of the non-altered layer.

6. The thin-film transistor according to claim 4 ,

wherein the altered layer has a carbon concentration which is at most one hundredth of a carbon concentration of the non-altered layer.

7. A method for manufacturing a thin-film transistor, the method comprising:

preparing a substrate;

forming a gate electrode above the substrate;

forming a gate insulating layer above the gate electrode;

forming a semiconductor film above the gate insulating layer;

forming, above the semiconductor film, an etch-stopper layer comprising an organic material;

forming a source electrode and a drain electrode that are opposed to each other, each of the source electrode and the drain electrode having at least a portion located above the etch-stopper layer; and

forming a semiconductor layer patterned, by performing dry etching on the semiconductor film;

wherein in the forming of a semiconductor layer, the dry etching (i) alters, into an altered layer, a surface layer of the etch-stopper layer in a region exposed from the source electrode and the drain electrode and (ii) etches the etch-stopper layer to have a taper angle at an edge portion of the exposed region, and

a relational expression of Log 10 Nt≦0.0556θ+16.86 is satisfied where Nt (cm −3 ) represents a defect density of the semiconductor layer and θ (°) represents the taper angle.

8. The method for manufacturing the thin-film transistor according to claim 7 ,

wherein the altered layer has a thickness of at least 30 nm.

9. The method for manufacturing the thin-film transistor according to claim 7 ,

wherein the altered layer has a density higher than a density of a portion of the etch-stopper layer that is not altered by the dry etching.

10. The method for manufacturing the thin-film transistor according to claim 9 ,

wherein the altered layer has a chlorine concentration which is at least ten times higher than a chlorine concentration of the non-altered layer.

11. The method for manufacturing the thin-film transistor according to claim 9 ,

wherein the altered layer has a carbon concentration which is at most one hundredth of a carbon concentration of the non-altered layer.