Aluminum nitride film, piezoelectric device, resonator, filter, and multiplexer

1. An aluminum nitride film, wherein

aluminum nitride crystal grains that contain a metal element, which is different from aluminum and substitutes for aluminum, are main crystal grains of a polycrystalline film formed of a plurality of crystal grains, and

a concentration of the metal element in a grain boundary between the aluminum nitride crystal grains in at least one region of a first region and a second region, which correspond to both end portions of the polycrystalline film in a film thickness direction of the polycrystalline film, is higher than a concentration of the metal element in a center region of the aluminum nitride crystal grain in the at least one region, and is higher than a concentration of the metal element in a grain boundary between the aluminum nitride crystal grains in a third region located between the first region and the second region in the film thickness direction of the polycrystalline film.

2. The aluminum nitride film according to claim 1 , wherein

the metal element is scandium.

3. The aluminum nitride film according to claim 1 , wherein

the aluminum nitride crystal grains contain at least one of titanium, zirconium, and hafnium that substitute for aluminum, and at least one of magnesium, calcium, strontium, and zinc that substitute for aluminum, and

the metal element is at least one of the at least one of titanium, zirconium, and hafnium, and the at least one of magnesium, calcium, strontium, and zinc.

4. The aluminum nitride film according to claim 1 , wherein

the concentration of the metal element in the grain boundary between the aluminum nitride crystal grains in the third region is equal to or less than 10% of the concentration of the metal element in the grain boundary between the aluminum nitride crystal grains in the at least one region.

5. The aluminum nitride film according to claim 1 , wherein

the concentration of the metal element in the grain boundary between the aluminum nitride crystal grains in the at least one region is equal to or greater than 1.1 times the concentration of the metal element in the center region of the aluminum nitride crystal grain in the at least one region.

6. The aluminum nitride film according to claim 1 , wherein

a concentration of the metal element in a grain boundary between the aluminum nitride crystal grains in the first region and a concentration of the metal element in a grain boundary between the aluminum nitride crystal grains in the second region are respectively higher than a concentration of the metal element in a center region of the aluminum nitride crystal grain in the first region and a concentration of the metal element in a center region of the aluminum nitride crystal grain in the second region, and are higher than the concentration of the metal element in the grain boundary between the aluminum nitride crystal grains in the third region.

7. The aluminum nitride film according to claim 1 , wherein

the aluminum nitride crystal grains have a wurtzite-type crystal structure in which the plurality of crystal grains are oriented in a c-axis.

8. A piezoelectric device comprising:

a substrate;

an aluminum nitride film in which aluminum nitride crystal grains that contain a metal element, which is different from aluminum and substitutes for aluminum, are main crystal grains of a polycrystalline film formed of a plurality of crystal grains, and a concentration of the metal element in a grain boundary between the aluminum nitride crystal grains in at least one region of a first region and a second region, which correspond to both end portions of the polycrystalline film in a film thickness direction of the polycrystalline film, is higher than a concentration of the metal element in a center region of the aluminum nitride crystal grain in the at least one region, and is higher than a concentration of the metal element in a grain boundary between the aluminum nitride crystal grains in a third region located between the first region and the second region in the film thickness direction of the polycrystalline film, the aluminum nitride film being located on the substrate; and

a first electrode and a second electrode facing each other across at least a part of the aluminum nitride film in the film thickness direction, the first electrode being in contact with the first region, the second electrode being in contact with the second region.

9. A resonator comprising:

a substrate;

an aluminum nitride film in which aluminum nitride crystal grains that contain a metal element, which is different from aluminum and substitutes for aluminum, are main crystal grains of a polycrystalline film formed of a plurality of crystal grains, and a concentration of the metal element in a grain boundary between the aluminum nitride crystal grains in at least one region of a first region and a second region, which correspond to both end portions of the polycrystalline film in a film thickness direction of the polycrystalline film, is higher than a concentration of the metal element in a center region of the aluminum nitride crystal grain in the at least one region, and is higher than a concentration of the metal element in a grain boundary between the aluminum nitride crystal grains in a third region located between the first region and the second region in the film thickness direction of the polycrystalline film, the aluminum nitride film being located on the substrate; and

a first electrode and a second electrode facing each other across at least a part of the aluminum nitride film in the film thickness direction, the first electrode being in contact with the first region, the second electrode being in contact with the second region.

10. A filter comprising:

the resonator according to claim 9 .

11. A multiplexer comprising:

the filter according to claim 10 .