Audio resonator

Abstract

This acoustic resonator comprises a first electrode (101), a piezoelectric element (102) and a second electrode (103). The first electrode (101) is constituted by a semiconductor doped with impurities. The piezoelectric element (102) is constituted by a piezoelectric semiconductor and formed on the first electrode (101). The piezoelectric semiconductor is, for example, a nitride semiconductor. The second electrode (103) is formed on the piezoelectric element (102). The second electrode (103) can be constituted by a metal or a semiconductor doped with impurities.

Description

[Technical Field]

[0001] The present invention is based on the bulk acoustic resonance In a container Regarding. [Background technology]

[0002] Conventional bulk acoustic resonators have a structure in which a piezoelectric material is sandwiched between two metal electrodes. In this case, the piezoelectric material is formed on one of the metal electrodes, and the other metal electrode is formed on the formed piezoelectric material. However, epitaxial growth, which allows for high-quality film formation, has been difficult when forming a piezoelectric material film on one of the metal electrodes. For this reason, the piezoelectric material is usually formed on one of the metal electrodes by sputtering. [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] J. Wu et al., "A New Class of High-Overtone Bulk Acoustic Resonators Using Lithium Niobate on Conductive Silicon Carbide", IEEE Electron Device Letters, vol. 42, no. 7, pp. 1061-1064, 2021. Summary of the Invention [Problem to be solved by the invention]

[0004] However, when a piezoelectric film is formed on a metal electrode using the sputtering method, epitaxial growth of a single crystal is difficult, and the piezoelectric film becomes a polycrystalline thin film, resulting in a problem of reduced film quality. Furthermore, epitaxial growth of a piezoelectric film on a metal electrode using the sputtering method requires the growth of a seed layer on the metal electrode, which complicates the manufacturing process.

[0005] In recent years, bulk acoustic resonators have been fabricated by bonding a lithium niobate single crystal onto an impurity-doped semiconductor substrate instead of using a metal lower electrode (Non-Patent Document 1). This technique has the problem that foreign matter or cavities may be present at the interface between the lithium niobate single crystal and the semiconductor layer, potentially reducing the piezoelectric efficiency and Q value.

[0006] The present invention has been made to solve the above problems, and has as its object to make it possible to form an acoustic resonator using a high-quality piezoelectric material. [Means for solving the problem]

[0007] The acoustic resonator according to the present invention comprises a first electrode made of a semiconductor doped with impurities, a piezoelectric body made of a piezoelectric semiconductor formed on the first electrode, and a second electrode formed on the piezoelectric body.

[0008] The modulator of the present invention comprises a first electrode made of a semiconductor doped with impurities, a piezoelectric body made of a piezoelectric semiconductor formed on the first electrode, a second electrode formed on the piezoelectric body, and a semiconductor optical element stacked on the second electrode.

[0009] The acousto-optic modulator of the present invention comprises a first electrode made of a semiconductor doped with impurities, a piezoelectric body made of a piezoelectric semiconductor formed on the first electrode, a second electrode formed on the piezoelectric body, and an acousto-optic medium laminated on the first electrode or the second electrode.

[0010] The acousto-optic modulator according to the present invention comprises a first electrode made of a semiconductor doped with impurities, a piezoelectric body made of an acousto-optic medium formed on the first electrode, and a second electrode formed on the piezoelectric body. [Effects of the Invention]

[0011] As described above, according to the present invention, a piezoelectric body made of a piezoelectric semiconductor is formed on a first electrode made of a semiconductor doped with impurities, so that an acoustic resonator made of a high-quality piezoelectric body can be formed. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a cross-sectional view showing the configuration of an acoustic resonator according to a first embodiment of the present invention. [Figure 2] FIG. 2 is a cross-sectional view showing the configuration of another acoustic resonator according to the first embodiment of the present invention. [Figure 3A] FIG. 3A is a characteristic diagram showing a bulk acoustic resonance mode generated by applying a high frequency wave to the acoustic resonator according to the first embodiment of the present invention. [Figure 3B] FIG. 3B is a characteristic diagram showing a bulk acoustic resonance mode that occurs when a high frequency wave is applied to the acoustic resonator according to the first embodiment of the present invention. [Figure 3C] FIG. 3C is a characteristic diagram showing the relationship between the thickness of the piezoelectric body 102 and the center frequency at which it can be excited. [Figure 4] FIG. 4 is a cross-sectional view showing the configuration of a modulator according to a second embodiment of the present invention. [Figure 5] FIG. 5 is a cross-sectional view showing the configuration of an acousto-optic modulator according to a third embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0013] An acoustic resonator according to an embodiment of the present invention will now be described.

[0014] [Embodiment 1] First, an acoustic resonator according to a first embodiment of the present invention will be described with reference to FIG. 1. This acoustic resonator includes a first electrode 101, a piezoelectric body 102, and a second electrode 103. The first electrode 101 is made of a semiconductor doped with impurities. The piezoelectric body 102 is made of a piezoelectric semiconductor and is formed on the first electrode 101. The piezoelectric semiconductor is, for example, a nitride semiconductor. The second electrode 103 is formed on the piezoelectric body 102. The second electrode 103 can be made of a metal or a semiconductor doped with impurities.

[0015] A piezoelectric body 102 is sandwiched between a first electrode 101 and a second electrode 103, and for example, the piezoelectric body 102 is formed on and in contact with the first electrode 101, and the second electrode 103 is formed on and in contact with the piezoelectric body 102. By applying a high-frequency voltage between the first electrode 101 and the second electrode 103, acoustic vibrations corresponding to the frequency of the applied high-frequency voltage can be generated.

[0016] For example, the piezoelectric body 102 can be formed by epitaxially growing a piezoelectric semiconductor on the first electrode 101. When the second electrode 103 is a metal, the second electrode 103 can be formed by depositing a metal material on the piezoelectric body 102 by a sputtering method or the like. When the second electrode 103 is a semiconductor, the second electrode 103 can be formed by growing the semiconductor on the piezoelectric body 102 by a well-known CVD method or the like.

[0017] For example, the first electrode 101 can be made of n-type silicon carbide doped with N as an impurity. The piezoelectric body 102 can be made of aluminum nitride (AlN). The second electrode 103 can be made of Ti and Al. An acoustic resonator can be obtained by epitaxially growing c-axis oriented aluminum nitride on the first electrode 101 made of n-type silicon carbide using a known metal organic chemical vapor deposition (MOCVD) method to form a 700 nm thick piezoelectric body 102, and then depositing Ti and Al on the piezoelectric body 102 to form the second electrode 103.

[0018] The first electrode 101 is either floating ground or grounded. When grounded, an ohmic electrode can be fabricated by forming a film of a metal such as Ni or Ti, which forms an alloy with silicon carbide, on the ground surface of the first electrode 101 and then heat-treating it at a temperature of 800 to 1200°C. The thickness of the n-type silicon carbide substrate used as the first electrode 101 is 244 μm, but this can be made thinner by polishing or etching. It is also possible to make it thicker by using substrates of different thicknesses.

[0019] When the first electrode 101 is floating-grounded or grounded as described above, three second electrodes 103a, 103b, and 103c are formed on the piezoelectric body 102, with the second electrodes 103a and 103c serving as ground terminals and the second electrode 103b serving as a signal terminal, to which a high-frequency GSG probe is electrically connected. When a high frequency is applied in this state, a bulk acoustic resonance mode centered around 4 GHz is obtained, as shown in (a) of FIG. 3A.

[0020] Furthermore, as shown in Figure 3B, which is an enlarged view of a portion of Figure 3A, a cavity mode with peaks every 26.8 MHz was observed. This frequency interval corresponds to the acoustic mode propagating back and forth through the bulk acoustic resonator. This experimental result shows that applying a high-frequency voltage between second electrode 103a, second electrode 103b, and second electrode 103c and first electrode 101 made of n-type silicon carbide causes piezoelectric body 102 to vibrate through the inverse piezoelectric effect of AlN piezoelectric body 102, and this vibration propagates through first electrode 101. This result demonstrates that a bulk acoustic resonator can be realized by constructing first electrode 101 from n-type silicon carbide and piezoelectric body 102 from AlN.

[0021] On the other hand, when a non-doped silicon carbide substrate is used, AlN is epitaxially grown on this substrate to form a piezoelectric body, and a metal electrode is then formed on top of this, it was confirmed that bulk acoustic resonance mode does not occur, as shown in Figure 3A (b). This result shows that it is essential that the first electrode be made from a doped semiconductor layer.

[0022] Note that the piezoelectric body 102 can be composed of a piezoelectric semiconductor such as AlN, Al x Ga 1-x N (0 < x < 1), GaN, Sc x Al 1-x N (0 < x < 1), Sc x Ga 1-x N (0 < x < 1), etc. When Sc is included, the crystal structure changes from hexagonal to cubic as the composition x increases. In the case of the cubic crystal structure, the inverse piezoelectric effect can be maximized by forming a (111)-oriented piezoelectric semiconductor.

[0023] Here, the relationship between the excitable center frequency f c and the thickness t of the piezoelectric semiconductor and the longitudinal wave sound velocity vL is expressed by the formula "f c = vL / (2t) · n (n is an odd number)". For example, when the piezoelectric body 102 is composed of AlN, the relationship between the thickness of the piezoelectric body 102 and the excitable center frequency is as shown in FIG. 3C. The excitable center frequency can also be adjusted by the second electrode 103. Note that FIG. 3A is the result of an acoustic resonator in which the second electrode 103 made of metal is formed on the piezoelectric body 102, so it is different from the result of FIG. 3C in the single piezoelectric semiconductor.

[0024] It has also been clarified by experiments that the excitable center frequency can be changed by changing the effective area of the acoustic resonator.

[0025] According to Embodiment 1, since the piezoelectric semiconductor is epitaxially grown on the first electrode 101 composed of an impurity-doped semiconductor, it is possible to form the piezoelectric body 102 from a high-quality piezoelectric semiconductor. As a result, it is possible to improve the Q value, piezoelectric coefficient, and electromechanical coupling constant. In addition, since this type of epitaxial growth is carried out in a reduced-pressure film formation chamber, it is also possible to prevent the occurrence of foreign matter mixing and voids at the interface between the first electrode 101 and the piezoelectric body 102.

[0026] Further, according to Embodiment 1, since the substrate for growing the piezoelectric semiconductor that is the piezoelectric body 102 is the first electrode 101, there is no need to separately form an electrode made of metal, and the manufacturing process of the acoustic resonator can be simplified.

[0027] By the way, when the second electrode 103 is composed of an impurity-doped semiconductor and the piezoelectric body 102 is composed of a nitride semiconductor, a transparent acoustic resonator can be obtained. Examples of the piezoelectric semiconductor include AlN, Al x Ga 1-x N(0 < x < 1), GaN, Sc x Al 1-x N(0 < x < 1), Sc x Ga 1-x N(0 < x < 1), etc. Also, the first electrode 101 and the second electrode 103 can be composed of doped - AlN, doped - AlGaN, doped - GaN, etc. These materials can be combined in various ways. [[ID=…]] [[ID=…]]

[0028] [[ID=…]] By making it a transparent acoustic resonator, the interaction between the acoustic mode and, for example, charges, spins, elastic waves, excitons, color centers in the crystal can be optically and detailedly observed. [[ID=…]] Further, by forming all of the first electrode 101, the piezoelectric body 102, and the second electrode 103 from the same kind of nitride semiconductor, an acoustic resonator with completely acoustic impedance matching can be obtained, and high - efficiency propagation of the acoustic mode can be achieved. For example, the first electrode 101 / piezoelectric body 102 / second electrode 103 can be doped - GaN / GaN / doped - GaN, doped - AlN / AlN / doped - AlN, doped - AlGaN / AlGaN / doped - AlGaN, etc. [[ID=…]] [[ID=…]]

[0029] [[ID=…]] [Embodiment 2] Note: There are some consecutive tags like - and - in the original text which seem to be placeholders without specific content for translation. I've left them as they are in the translation. If there's more context or specific instructions regarding these tags, it would be helpful for a more accurate translation.Next, a modulator according to a second embodiment of the present invention will be described with reference to Fig. 4. This modulator includes a first electrode 101 made of a semiconductor doped with impurities, a piezoelectric body 102 made of a piezoelectric semiconductor formed on the first electrode 101, a second electrode 103 formed on the piezoelectric body 102, and a semiconductor optical element 120 stacked on the second electrode 103.

[0030] For example, a semiconductor optical element 120, which is a semiconductor laser, can be formed by epitaxially growing a first semiconductor layer 121 made of an n-type semiconductor, an active layer 122 made of a semiconductor, and a second semiconductor layer 123 made of a p-type semiconductor in sequence on a second electrode 103 made of a semiconductor doped with impurities.

[0031] The acoustic resonator formed by the first electrode 101, the piezoelectric body 102, and the second electrode 103 modulates the energy band in the active layer 122 of the semiconductor optical element 120, thereby enabling modulation of the laser output. The acoustic resonator and the semiconductor optical element 120 can be integrated into a modulator, which allows for miniaturization and high efficiency of the modulator. Furthermore, by thinning the piezoelectric body 102, it is possible to increase the modulation frequency. Furthermore, by using a nitride semiconductor, the high speed of sound allows for higher acoustic mode frequencies, and the wide band gap allows for shorter wavelength laser light, all at the same time.

[0032] [Embodiment 3] Next, an acousto-optic modulator according to a third embodiment of the present invention will be described with reference to Fig. 5. This acousto-optic modulator includes a first electrode 101, a piezoelectric body 102 formed on the first electrode 101, a second electrode 103 formed on the piezoelectric body 102, and an acousto-optic medium 131 laminated on the first electrode 101 or the second electrode 103. The acousto-optic medium 131 can be made of a crystal such as gallium phosphide, tellurium dioxide, indium phosphide, quartz, or germanium.

[0033] In this example, the first electrode 101 is laminated on the acousto-optic medium 131 in contact therewith. For example, an acousto-optic modulator can be formed by bonding the first electrode 101 of an acoustic resonator to the acousto-optic medium 131. The acoustic resonator serves as a high-frequency transducer.

[0034] High-frequency bulk acoustic waves generated from the acoustic resonator propagate through the acoustooptic medium 131. The propagating bulk acoustic waves periodically modulate the refractive index of the acoustooptic medium 131. When laser light is incident on the acoustooptic medium 131 whose refractive index is periodically modulated in this manner, the laser light is diffracted by the periodic change in refractive index, and laser light whose frequency is shifted by the frequency of the bulk acoustic wave is output.

[0035] It should be noted that by configuring the piezoelectric body 102 from an acousto-optic medium, the acoustic resonator alone can achieve the same effect as described above.

[0036] As described above, according to the present invention, a piezoelectric body made of a piezoelectric semiconductor is formed on a first electrode made of a semiconductor doped with impurities, making it possible to form an acoustic resonator made of a high-quality piezoelectric body.

[0037] It should be noted that the present invention is not limited to the embodiments described above, and it is clear that many modifications and combinations can be made by a person having ordinary knowledge in the art within the technical concept of the present invention. [Explanation of symbols]

[0038] 101...first electrode, 102...piezoelectric body, 103...second electrode.

Claims

[Claim 1] a first electrode made of a semiconductor doped with impurities; a piezoelectric body made of a piezoelectric semiconductor formed on the first electrode; a second electrode formed on the piezoelectric body; a semiconductor optical element stacked on the second electrode; Equipped with the second electrode is made of a semiconductor doped with impurities; The piezoelectric semiconductor is a nitride semiconductor.

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