Method for producing field effect transistor, field effect transistor, display device, image sensor, and X-ray sensor

1. A method of fabricating a field effect transistor, the method comprising:

a first step of forming a first oxide semiconductor film on a gate insulation layer disposed on a gate electrode;

after the first step, a second step of forming a second oxide semiconductor film on the first oxide semiconductor film, the second oxide semiconductor film differing in cation composition from the first oxide semiconductor film and being lower in electrical conductivity than the first oxide semiconductor film;

after the second step, a third step of applying a heat treatment at over 300° C. in an oxidizing atmosphere;

after the third step, a fourth step of forming a third oxide semiconductor film on the second oxide semiconductor film, the third oxide semiconductor film differing in cation composition from the first oxide semiconductor film and being lower in electrical conductivity than the first oxide semiconductor film;

after the fourth step, a fifth step of applying a heat treatment in an oxidizing atmosphere; and,

between the fourth step and the fifth step or after the fifth step, an electrode formation step of forming a source electrode and a drain electrode on the third oxide semiconductor film.

2. The method according to claim 1 , wherein, during the second step and the third step, a thickness of the second oxide semiconductor film and the temperature of the heat treatment in the third step are adjusted such that, if the thickness of the second oxide semiconductor film is represented by Z (nm), the heat treatment temperature is represented by T (° C.) and a diffusion distance of oxygen into the second oxide semiconductor film and the first oxide semiconductor film is represented by L (nm), the relational expression 0<Z<L=8×10 −6 ×T 3 −0.0092×T 2 +3.6×T−468±0.1 is satisfied.

3. The method according to claim 2 , wherein, during the second step and the third step, the thickness of the second oxide semiconductor film and the heat treatment temperature are adjusted such that the relational expression Z≦L−3.0 is satisfied.

4. The method according to claim 2 , wherein, during the second step and the third step, the thickness of the second oxide semiconductor film and the heat treatment temperature are adjusted such that the relational expression L−15.0≦Z is satisfied.

5. The method according to claim 2 , wherein, during the second step and the third step, the thickness of the second oxide semiconductor film and the heat treatment temperature are adjusted such that the relational expression L−11.0≦Z≦L−8.0 is satisfied.

6. The method according to claim 1 , wherein, in the fourth step, the third oxide semiconductor film is formed such that a cation composition ratio thereof is the same as a cation composition ratio of the second oxide semiconductor film.

7. The method according to claim 1 , wherein, during the second step and the fourth step, a total thickness of the second oxide semiconductor film and the third oxide semiconductor film is adjusted to be more than 10 nm and less than 70 nm.

8. The method according to claim 1 , wherein the first oxide semiconductor film, the second oxide semiconductor film and the third oxide semiconductor film each includes at least one of In, Ga, and Zn.

9. The method according to claim 8 , wherein the first oxide semiconductor film includes In, and an In composition proportion of the first oxide semiconductor film is higher than an In composition proportion of the second oxide semiconductor film.

10. The method according to claim 8 , wherein the second oxide semiconductor film includes Ga, and a Ga composition proportion of the second oxide semiconductor film is higher than a Ga composition proportion of the first oxide semiconductor film.

11. The method according to claim 8 , wherein the first oxide semiconductor film, the second oxide semiconductor film and the third oxide semiconductor film are each non-crystalline.

12. The method according to claim 8 , wherein the heat treatment temperatures in the third step and the fifth step are adjusted to be less than 600° C.

13. The method according to claim 1 , wherein, in the first step, the first oxide semiconductor film is formed such that a thickness thereof is less than 10 nm.

14. The method according to claim 1 , wherein a composition of the first oxide semiconductor film is represented with In(a) Ga(b) Zn(c) O(d), and (a, b, c, d>0).

15. The method according to claim 14 , wherein the composition of the first oxide semiconductor film is a composition in the range represented by {c≦3/5, b>0, b≧3a/7−3/14, b≧9a/5−53/50, b≦−8a/5+33/25, b≦91a/74−17/40}, in which a+b+c=1.

16. The method according to claim 15 , wherein the composition of the first oxide semiconductor film is a composition in the range represented by {b≦17a/23−28/115, b≧3a/37, b≧9a/5−53/50, b≦1/5}, in which a+b+c=1.

17. The method according to claim 16 , wherein the composition of the first oxide semiconductor film is a composition in the range represented by {b≦7a/13−11/65, b>3a/37, b≦−2a+11/10}, in which a+b+c=1.

18. The method according to claim 1 , wherein a composition of the first oxide semiconductor film is represented with In(x) Zn(1−x) O(y), and (y>0, 0<x<1).

19. The method according to claim 18 , wherein the composition of the first oxide semiconductor film is a composition in the range represented by 0.4≦x≦0.75.

20. The method according to claim 19 , wherein the composition of the first oxide semiconductor film is a composition in the range represented by 0.4≦x≦0.5.

21. The method according to claim 1 , wherein a composition of the second oxide semiconductor film is represented with In(e) Ga(f) Zn(g) O(h), and (e, f, g, h>0).

22. The method according to claim 21 , wherein the composition of the second oxide semiconductor film is a composition in the range represented by 0.250<f/(e+f)≦0.875.

23. The method according to claim 1 , wherein, in each of the first step, the second step and the fourth step, the film is formed using a film formation technique that produces plasma.

24. The method according to claim 1 , wherein the fifth step is carried out after the electrode formation step.

25. A bottom gate, top contact-type field effect transistor comprising:

a gate electrode;

a gate insulation layer over the gate electrode;

a channel layered film in which electrons travel, structured over the gate insulation layer; and

a source electrode and drain electrode formed over the channel layered film, wherein

the channel layered film is structured by first, second and third oxide semiconductor films in this order from the side at which the gate insulation layer is provided, each oxide semiconductor film including In Ga and Zn,

an In content proportion of the first oxide semiconductor film is higher than in the second and third oxide semiconductor films, and

a lattice defect density of the second oxide semiconductor film is smaller than a lattice defect density of the third oxide semiconductor film.

26. A bottom gate, top contact-type field effect transistor comprising:

a gate electrode;

a gate insulation layer over the gate electrode;

a channel layered film in which electrons travel, structured over the gate insulation layer; and

a source electrode and drain electrode formed over the channel layered film, wherein

the channel layered film is structured by first, second and third oxide semiconductor films in this order from the side at which the gate insulation layer is provided, each oxide semiconductor film including In, Ga and Zn,

an In content proportion of the first oxide semiconductor film is higher than in the second and third oxide semiconductor films, and

an oxygen inclusion density in the second oxide semiconductor film is greater than an oxygen inclusion density of the third oxide semiconductor film.

27. A display device comprising field effect transistors fabricated by the method according to claim 1 .

28. A bottom emission-type display device comprising:

a substrate;

field effect transistors fabricated by the method according to claim 1 , which are disposed on the substrate; and

organic electroluminescent elements over the field effect transistors, the organic electroluminescent elements being electrically connected to the field effect transistors,

wherein light emitted from the organic electroluminescent elements is emitted from the side of the display device at which the substrate is disposed.

29. An image sensor comprising field effect transistors fabricated by the method according to claim 1 .

30. An X-ray sensor comprising field effect transistors fabricated by the method according to claim 1 .