Electret element and vibration power generating device using the same

1. An electret element comprising a substrate, an electrically-conductive electrode, and an electret layer comprising a first dielectric layer and a second dielectric layer, wherein:

the electret layer is capable of holding electrical charge;

the electrically-conductive electrode is on a surface of the substrate;

the first dielectric layer is on a surface of the electrically-conductive electrode and has, in a surface opposite to an electrically-conductive electrode-side surface, at least one concave portion having a bottom face, a side wall and a top face;

the second dielectric layer covers the entirety of the bottom face and at least a part of the side wall; and

a bottom Ec of a conductive band of the second dielectric layer is lower than a bottom Ec of a conductive band of the first dielectric layer, or a top Ev of a valence band of the second dielectric layer is higher than a top Ev of a valence band of the first dielectric layer, wherein

a thickness of the first dielectric layer between the bottom face and the surface of the electrically-conductive electrode is thinner than a thickness of the first dielectric layer between the top face and the surface of the electrically-conductive electrode.

2. The electret element according to claim 1 , wherein the electret layer further comprises at least one dielectric layer and the at least one dielectric layer is between the electrically-conductive electrode and the first dielectric layer.

3. The electret element according to claim 1 , wherein the electrical charge is held in the electret layer and the electrically-conductive electrode is connected to:

a potential which is of a same polarity as that of the electrical charge and has an absolute value being smaller than the absolute value of a surface potential of the electret layer; or

a potential which is of a polarity different from that of the electrical charge.

4. The electret element according to claim 1 , wherein the electrically-conductive electrode is connected to an earth potential.

5. The electret element according to claim 1 , wherein the first dielectric layer is a silicon oxide film and the second dielectric layer is a film selected from the group consisting of a silicon nitride film, an aluminum oxide film, a hafnium oxide film and a titanium oxide film.

6. The electret element according to claim 1 , further comprising a third dielectric layer, wherein the substrate is an electrically-conductive substrate and the third dielectric layer is between the electrically-conductive substrate and the electrically-conductive electrode.

7. The electret element according to claim 6 , wherein the electrically-conductive substrate is a silicon substrate, the third dielectric layer is a silicon oxide film and the electrically-conductive electrode consists of polycrystalline silicon.

8. The electret element according to claim 1 , wherein the substrate is an insulative substrate.

9. The electret element according claim 8 , wherein the insulative substrate is a glass substrate or a sapphire substrate, and the electrically-conductive electrode consists of a material selected from the group consisting of polycrystalline silicon, aluminum and copper.

10. The electret element according to claim 1 wherein an electrically-conductive substrate is substituted for the substrate and the electrically-conductive electrode, and the first dielectric layer is on a surface of the electrically-conductive substrate.

11. The electret element according to claim 10 , wherein the electrical charge is held in the electret layer and the electrically-conductive substrate is connected to:

a potential which is of a same polarity as that of the electrical charge and has an absolute value being smaller than the absolute value of a surface potential of the electret layer; or

a potential which is of a polarity different from that of the electrical charge.

12. The electret element according to claim 10 , wherein the electrically-conductive substrate is connected to an earth potential.

13. The electret element according to claim 1 , wherein a depth from the top face to the bottom face of the at least one concave portion is larger than a thickness of the second dielectric layer.

14. The electret element according to claim 1 , wherein the at least one concave portion includes a plurality of the concave portions, and an interval between two adjacent concave portions of the plurality of concave portions is larger than a thickness of the first dielectric layer.

15. The electret element according to claim 1 , further comprising a moisture-proof layer, wherein the moisture-proof layer covers:

a surface of the second dielectric layer; or

the surface of the second dielectric layer and a region of the surface of the first dielectric layer, which region is not covered with the second dielectric layer.

16. A vibration power generating device which is provided with the electret element according to claim 1 .

17. The electret element according to claim 1 , wherein the side wall is inclined or curved.

18. The electret element according to claim 1 , further comprising an interface between the first dielectric layer and the second dielectric layer, wherein the interface extends in a direction away from the electrically-conductive electrode at the side wall of the at least one concave portion, such that in operation, when the electrical charge tries to move along the interface between the first dielectric layer and the second dielectric layer in the direction away from the electrically-conductive electrode, the electrical charge is drawn to the electrically-conductive electrode by electrostatic force.

19. The electret element according to claim 1 , further comprising an interface between the first dielectric layer and the second dielectric layer, wherein, in operation, electrostatic force attracts the electrical charge to the bottom region of the at least one concave portion.

20. The electret element according to claim 1 , wherein a top region of the at least one concave portion is wider than a bottom region of the at least one concave portion.

21. The electret element according to claim 1 , wherein an interface between the electrically-conductive electrode and the first dielectric layer is flat.