Method and structure for single crystal acoustic resonator devices using thermal recrystallization

1. A method for fabricating an acoustic material stack, the method comprising:

providing a substrate having a substrate surface region;

forming a nucleation layer overlying the substrate surface region;

forming a strained single crystal piezoelectric layer overlying the nucleation layer and having a strain condition, wherein forming the strained single crystal piezoelectric layer includes an epitaxial growth process on the nucleation layer to modulate the strain condition in the strained single crystal piezoelectric layer to improve one or more piezoelectric properties of the strained single crystal piezoelectric layer;

depositing a polycrystalline piezoelectric layer overlying the strained single crystal piezoelectric layer; and

performing a thermal treatment on the polycrystalline piezoelectric layer overlying the strained single crystal piezoelectric layer; thereby recrystallizing the polycrystalline piezoelectric layer to exhibit characteristics of a single crystal piezoelectric layer in order to form a hybrid thermally recrystallized polycrystalline and single crystalline piezo stack.

2. The method of claim 1 wherein the substrate is selected from one of the following: a silicon substrate, a silicon-on-insulator (SOI) substrate, a sapphire substrate, a silicon carbide substrate, a GaN bulk substrate, a GaN template, an AlN bulk, an AlN template, and an Al x Ga 1-x N template.

3. The method of claim 1 wherein at least one of the nucleation layer, the strained single crystal piezoelectric layer, and the polycrystalline piezoelectric layer includes materials or alloys having at least one of the following: AlN, AlGaN, GaN, InN, InGaN, AlInN, AlInGaN, AlScN, GaScN, BAlN, BN, AlYN, BAlScN, and AlYScN.

4. The method of claim 1 wherein

the epitaxial growth process is selected from one of the following: metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), hydride vapor phase epitaxy (HVPE), and atomic layer deposition (ALD).

5. The method of claim 4 wherein the nucleation layer is characterized by nucleation growth parameters including temperature, pressure, thickness, growth rate, gas phase ratio of reactant species, use of surfactant species, and impurity concentration; and

wherein the epitaxial growth process is configured by the nucleation growth parameters to modulate the strain condition in the strained single crystal piezoelectric layer to improve one or more piezoelectric properties of the strained single crystal piezoelectric layer, the one or more piezoelectric properties including acoustic velocity.

6. The method of claim 4 wherein the strained single crystal piezoelectric layer is characterized by piezoelectric parameters including thickness and temperature; and

wherein the epitaxial growth process is configured by the piezoelectric layer parameters to modulate the strain condition in the strained single crystal piezoelectric layer to improve one or more piezoelectric properties of the strained single crystal piezoelectric layer, the one or more piezoelectric properties including acoustic velocity.

7. The method of claim 1 wherein at least one of the strained single crystal piezoelectric layer and the polycrystalline piezoelectric layer is oriented in the (0001) or (000-1) crystallographic direction.

8. The method of claim 1 further comprising doping at least one of the strained single crystal piezoelectric layer and the polycrystalline piezoelectric layer, wherein the doping includes the introduction of one or more impurity species during growth (in-situ) and includes bulk doping, delta doping, or co-doping processes, wherein the one or more impurity species includes at least one of the following: silicon (Si), magnesium (Mg), carbon (C), oxygen (O), erbium (Er), rubidium (Rb), strontium (Sr), scandium (Sc), beryllium (Be), molybdenum (Mo), zirconium (Zr), Hafnium (Hf), vanadium (Va), and yttrium (Y); wherein the one or more impurity species has impurity concentration ranging from 1E+10 to 1E+21 per cubic centimeter and ranging between 0.1% to 50% atomic composition.

9. The method of claim 1 further comprising doping at least one of the strained single crystal piezoelectric layer or the polycrystalline piezoelectric layer, wherein the doping includes the introduction of one or more impurity species post growth (ex-situ) and includes ion implantation, chemical treatment, surface modification, diffusion, or co-doping processes, wherein the one or more impurity species includes at least one of the following: silicon (Si), magnesium (Mg), carbon (C), oxygen (O), erbium (Er), rubidium (Rb), strontium (Sr), scandium (Sc), beryllium (Be), molybdenum (Mo), zirconium (Zr), Hafnium (Hf), vanadium (Va), and yttrium (Y); wherein the one or more impurity species has impurity concentration ranging from 1E+10 to 1E+21 per cubic centimeter and ranging between 0.1% to 50% atomic concentration.

10. The method of claim 1 wherein the strained single crystal piezoelectric material has a thickness from 10 nm to 10 um, is characterized by a defect density less than 10 12 defects/cm 2 , and has an x-ray rocking curve full width at half maximum (FWHM) less than or equal to 1 degree; and

wherein the polycrystalline piezoelectric layer is characterized by a thickness of 10 nm to 10 um.

11. The method of claim 1 wherein performing the thermal treatment includes heating the polycrystalline piezoelectric layer at a temperature greater than 1000 degrees Celsius.

12. The method of claim 1 wherein performing the thermal treatment includes heating the polycrystalline piezoelectric layer with nitrogen/ammonia overpressure.

13. The method of claim 1 wherein forming the polycrystalline piezoelectric layer includes using the strained single crystal piezoelectric layer as a seed substrate.

14. The method of claim 1 further comprising forming an acoustic resonator device from the hybrid thermally recrystallized polycrystalline and single crystalline piezo stack.

15. The method of claim 14 wherein forming the acoustic resonator device includes forming a bulk acoustic wave resonator device configured within a filter device.

16. A method for fabricating an acoustic material stack, the method comprising:

providing a substrate having a substrate surface region;

forming a nucleation layer overlying the substrate surface region and being characterized by nucleation growth parameters;

forming a strained single crystal piezoelectric layer overlying the nucleation layer and having a strain condition and piezoelectric layer parameters, wherein forming the strained single crystal piezoelectric layer includes an epitaxial growth process configured by nucleation growth parameters and piezoelectric layer parameters to modulate the strain condition in the strained single crystal piezoelectric layer to improve one or more piezoelectric properties of the strained single crystal piezoelectric layer;

depositing a polycrystalline piezoelectric layer overlying the strained single crystal piezoelectric layer; and

performing a thermal treatment on the polycrystalline piezoelectric layer overlying the strained single crystal piezoelectric layer; thereby recrystallizing the polycrystalline piezoelectric layer to exhibit characteristics of a single crystal piezoelectric layer in order to form a hybrid thermally recrystallized polycrystalline and single crystalline piezo stack.

17. The method of claim 16 wherein the substrate is selected from one of the following: a silicon substrate, a silicon-on-insulator (SOI) substrate, a sapphire substrate, a silicon carbide substrate, a GaN bulk substrate, a GaN template, an AlN bulk, an AlN template, and an Al x Ga 1-x N template.

18. The method of claim 16 wherein at least one of the nucleation layer, the strained single crystal piezoelectric layer, and the polycrystalline piezoelectric layer includes materials or alloys having at least one of the following: AlN, AlGaN, GaN, InN, InGaN, AlInN, AlInGaN, AlScN, GaScN, BAlN, BN, AlYN, BAlScN, and AlYScN.

19. The method of claim 16 wherein forming the polycrystalline piezoelectric layer includes using the strained single crystal piezoelectric layer as a seed substrate; and

wherein performing the thermal treatment includes heating the polycrystalline piezoelectric layer at a temperature greater than 1000 degrees Celsius with nitrogen/ammonia overpressure.

20. A method for fabricating an acoustic material stack, the method comprising:

providing a substrate having a substrate surface region;

forming a nucleation layer overlying the substrate surface region and being characterized by nucleation growth parameters;

forming a strained single crystal piezoelectric layer overlying the nucleation layer and having a strain condition and piezoelectric layer parameters, wherein forming the strained single crystal piezoelectric layer includes an epitaxial growth process configured by nucleation growth parameters and piezoelectric layer parameters to modulate the strain condition in the strained single crystal piezoelectric layer to improve one or more piezoelectric properties of the strained single crystal piezoelectric layer; wherein the strained single crystal piezoelectric material has a thickness from 10 nm to 10 um and is characterized by a defect density less than 10 12 defects/cm 2 , and an x-ray rocking curve full width at half maximum (FWHM) less than or equal to 1 degree;

depositing a polycrystalline piezoelectric layer overlying the strained single crystal piezoelectric layer, wherein the polycrystalline piezoelectric layer is characterized by a thickness of 10 nm to 10 um; and

performing a thermal treatment on the polycrystalline piezoelectric layer overlying the strained single crystal piezoelectric layer; thereby recrystallizing the polycrystalline piezoelectric layer to exhibit characteristics of a single crystal piezoelectric layer in order to form a hybrid thermally recrystallized polycrystalline and single crystalline piezo stack.