Gradient-doped ferroelectric thin film device and preparation method thereof

Abstract

This application provides a gradient-doped ferroelectric thin film device and its fabrication method, relating to the field of optoelectronic device technology. The gradient-doped ferroelectric thin film device comprises, from bottom to top, a substrate, a buried oxide layer, a buffer layer, and a ferroelectric thin film material layer; the ferroelectric thin film material layer is doped with impurity ions, with the doping concentration of the impurity ions increasing sequentially from bottom to top. This application, by employing a bottom-up incremental gradient doping design, optimizes the interface defects and internal stress of the ferroelectric thin film, overcomes the problem of lattice mismatch between ordinary ferroelectric thin films and the substrate layer, achieves improved crystal quality and enhanced electro-optic coefficient, and effectively improves the electro-optic effect.

Description

Technical Field

[0001] This application relates to the field of optoelectronic device technology, and in particular to a gradient-doped ferroelectric thin film device and its fabrication method. Background Technology

[0002] Currently, the mainstream fabrication methods for ferroelectric thin film devices mainly include ion knife single-crystal cutting and exfoliation, radio frequency magnetron sputtering, pulsed laser deposition, chemical solution deposition, and molecular beam epitaxy. Two core technical bottlenecks remain in the fabrication of high-performance ferroelectric thin film devices: first, lattice mismatch and thermal expansion coefficient mismatch, which easily lead to defects such as oxygen vacancy enrichment, and stress accumulation can easily cause thin film cracks; second, the problem of improving the intrinsic electro-optic coefficient of the material.

[0003] The asymmetry of crystal structure is an important source of electro-optic response, and improving spontaneous lattice polarization is a key approach to optimizing the electro-optic coefficient of materials. Therefore, there is an urgent need to propose targeted optimized fabrication methods for ferroelectric thin film devices. Summary of the Invention

[0004] In view of this, this application proposes a gradient-doped ferroelectric thin film device and its fabrication method. By inducing lattice distortion in the ferroelectric thin film device through layered gradient doping, the intrinsic spontaneous polarization and domain wall activity of the crystal are enhanced, the interface defects and internal stress relaxation of the ferroelectric thin film are optimized, the crystal quality and electro-optic coefficient are enhanced, and the macroscopic electro-optic response is improved.

[0005] The first aspect of this application provides a gradient-doped ferroelectric thin film device, comprising: a substrate 1, a buried oxide layer 2, a buffer layer 3, and a ferroelectric thin film material layer 4 stacked sequentially from bottom to top; the ferroelectric thin film material layer 4 is doped with impurity ions, and the doping concentration of the impurity ions increases sequentially from bottom to top.

[0006] Furthermore, an embedding layer is provided above the ferroelectric thin film material layer 4.

[0007] Furthermore, the materials of the ferroelectric thin film material layer 4 include lead zirconate titanate and barium titanate.

[0008] Furthermore, the materials of buffer layer 3 include lanthanum oxycarbonate and strontium titanate; the thickness of buffer layer 3 is 5nm~15nm.

[0009] Furthermore, the thickness of the ferroelectric thin film material layer 4 is 100 nm to 1 μm.

[0010] Furthermore, the impurity ions are lanthanide ions, including La 3+ 、Nd 3+ Pr 3+ 、Sm 3+ The doping concentration of impurity ions ranges from 1% to 10%.

[0011] The second aspect of this application provides a method for fabricating a gradient-doped ferroelectric thin film device, comprising: step S1, selecting metal ion sources corresponding to the ferroelectric thin film material layer and the buffer layer, and dissolving them respectively in a precursor solvent to obtain a precursor solution for the ferroelectric thin film material layer and a precursor solution for the buffer layer; step S2, pretreating the substrate 1 and growing a buried oxide layer 2 on the substrate 1 using an oxygen implantation isolation process; step S3, dropping the precursor solution of the buffer layer onto the surface of the buried oxide layer 2, and then heating the surface of the buried oxide layer 2 with air to form a buffer layer 3; step S4, using the precursor solution of the ferroelectric thin film material layer to perform multiple growths on the buffer layer 3, performing a pre-annealing treatment after each growth until the growth reaches a preset target thickness, thereby obtaining a gradient-doped ferroelectric thin film device.

[0012] Furthermore, the precursor solvents include 2-methoxyethanol and ethylene glycol.

[0013] Furthermore, in step S4, the thickness of each growth in the multiple growth is 50nm~100nm; the multiple growth process includes chemical solution deposition, radio frequency magnetron sputtering and pulsed laser deposition.

[0014] Furthermore, a pre-annealing treatment is performed after each growth, including: using a layered annealing method, high-temperature annealing is performed after the first growth, with a temperature range of 550℃~650℃; except for the first growth, low-temperature annealing is performed after each subsequent growth, with a temperature range of 250℃~350℃.

[0015] By adopting the above technical solution, this application proposes a gradient-doped ferroelectric thin film device and its fabrication method, which solves the problems of existing fabrication methods being unable to simultaneously meet the requirements of low cost, high compositional accuracy, and lattice mismatch. It can be used to fabricate ferroelectric thin film devices with uniform density, no cracks, high transparency, and high electro-optic effect. Simultaneously, it overcomes the lattice mismatch problem between ordinary ferroelectric thin films and the substrate layer, effectively improving the electro-optic effect, and can be used to fabricate electro-optic modulators with high electro-optic coefficients. Attached Figure Description

[0016] Figure 1 A schematic diagram illustrating the structure of a gradient-doped ferroelectric thin-film device according to an embodiment of this application is shown.

[0017] Figure 2A A flowchart illustrating a method for fabricating a gradient-doped ferroelectric thin-film device according to an embodiment of this application is shown schematically.

[0018] Figure 2B The diagram schematically illustrates the structural diagrams of each stage of the fabrication method of a gradient-doped ferroelectric thin film device according to an embodiment of this application;

[0019] Figure 3The doping gradient diagram of the ferroelectric thin film material layer according to an embodiment of this application is schematically shown;

[0020] Figure 4 A schematic diagram of a waveguide cross-section fabricated from a gradient-doped ferroelectric thin-film device according to an embodiment of this application is shown.

[0021] Explanation of reference numerals in the attached figures:

[0022] 1-Substrate; 2-Buried oxide layer; 3-Buffer layer; 4-Ferroelectric thin film material layer. Detailed Implementation

[0023] The embodiments of this application will now be described with reference to the accompanying drawings. However, it should be understood that these descriptions are exemplary only and are not intended to limit the scope of this application. In the following detailed description, numerous specific details are set forth to provide a thorough understanding of the embodiments of this application for ease of explanation. However, it will be apparent that one or more embodiments may be implemented without these specific details. Furthermore, descriptions of well-known structures and technologies are omitted in the following description to avoid unnecessarily obscuring the concepts of this application.

[0024] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of this application. The terms “comprising,” “including,” etc., as used herein indicate the presence of features, steps, operations, and / or components, but do not exclude the presence or addition of one or more other features, steps, operations, or components.

[0025] All terms used herein (including technical and scientific terms) have the meanings commonly understood by those skilled in the art, unless otherwise defined. It should be noted that the terms used herein are to be interpreted in a manner consistent with the context of this specification, and not in an idealized or overly rigid way.

[0026] Secondly, this application provides a detailed description in conjunction with schematic diagrams. When detailing the embodiments of this application, for ease of explanation, the cross-sectional views illustrating the device structure may be partially enlarged, not adhering to the usual scale. Furthermore, the schematic diagrams are merely examples and should not limit the scope of protection of this application. In addition, actual fabrication should include three-dimensional spatial dimensions of length, width, and depth.

[0027] Figure 1 A schematic diagram of a gradient-doped ferroelectric thin-film device according to an embodiment of this application is shown.

[0028] like Figure 1 As shown, this application provides a gradient-doped ferroelectric thin film device, comprising: a substrate 1, a buried oxide layer 2, a buffer layer 3, and a ferroelectric thin film material layer 4 stacked sequentially from bottom to top; the ferroelectric thin film material layer 4 is doped with impurity ions, and the doping concentration of the impurity ions increases sequentially from bottom to top.

[0029] In this embodiment, substrate 1 serves as the supporting substrate for the gradient-doped ferroelectric thin film device, providing a stable platform for the subsequent growth of each functional layer, and possesses good mechanical strength, thermal stability, and crystal compatibility.

[0030] For example, the materials of substrate 1 include, but are not limited to, silicon (Si), sapphire (Al2O3), magnesium oxide (MgO), and aluminum nitride (AlN).

[0031] The buried oxide layer 2 achieves electrical insulation isolation between the substrate and the upper structure, reduces the impact of substrate impurity diffusion on ferroelectric properties, and improves the interface flatness of the device.

[0032] For example, the materials of the buried oxide layer 2 include, but are not limited to, silicon dioxide (SiO2), silicon nitride (Si3N4), and silicon oxynitride (SiON).

[0033] In this embodiment, the material of the buffer layer 3 includes lanthanum oxycarbonate (La2O2CO3) and strontium titanate (SrTiO3); the thickness of the buffer layer 3 is 5nm~15nm.

[0034] The buffer layer 3 serves to alleviate the lattice mismatch and thermal expansion coefficient difference between the substrate / buried oxide layer and the ferroelectric thin film layer, reduce the stress during the thin film growth process, and improve the crystallinity and orientation consistency of the ferroelectric thin film.

[0035] In this embodiment, the ferroelectric thin film material layer 4 comprises barium titanate (BaTiO3) and lead zirconate titanate (Pb(Zr)). x Ti 1-x )O3). Among them, for lead zirconate titanate (Pb(Zr) x Ti 1-x The polarization intensity of the ferroelectric thin film material layer 4 can be controlled by adjusting the Zr / Ti ratio.

[0036] In this embodiment, the thickness of the ferroelectric thin film material layer 4 is 100nm~1μm.

[0037] Preferably, the thickness of the ferroelectric thin film material layer 4 is 200nm~300nm.

[0038] In this embodiment, the impurity ions are lanthanide ions, including La 3+ 、Nd 3+ Pr 3+ 、Sm 3+ The doping concentration of impurity ions ranges from 1% to 10%.

[0039] For example, in the ferroelectric thin film material layer 4, the doping concentration of impurity ions near the buffer layer 3 is low, which can avoid lattice stress mismatch and suppress hole defect states; while the top layer is highly doped to enhance lattice structure distortion and improve the electro-optic coefficient.

[0040] In this embodiment, an embedding layer is also provided above the ferroelectric thin film material layer 4.

[0041] The embedded layer mainly serves as the upper protective and isolation structure of the ferroelectric thin film material layer 4, and can achieve the following functions: isolate water vapor, oxygen and impurity ions in the external environment to avoid performance degradation of the ferroelectric thin film material layer 4; reduce physical / chemical damage to the ferroelectric thin film material layer 4 by subsequent device fabrication processes (such as photolithography and etching); provide a planarized surface interface to lay the foundation for the top electrode or multilayer integrated structure of the device, and at the same time, it can help regulate the stress state of the ferroelectric thin film material layer 4 and maintain its polarization stability.

[0042] The materials used in the embedding layer include, but are not limited to, silicon dioxide (SiO2), silicon nitride (Si3N4), aluminum oxide (Al2O3), and silicon oxynitride (SiON).

[0043] Figure 2A A flowchart illustrating a method for fabricating a gradient-doped ferroelectric thin-film device according to an embodiment of this application is shown schematically. Figure 2B The diagram schematically illustrates the structural diagrams of each stage of the fabrication method of a gradient-doped ferroelectric thin film device according to an embodiment of this application.

[0044] like Figure 2A and Figure 2B As shown, the structural diagrams of each stage of the fabrication method of the gradient-doped ferroelectric thin film device provided in this application embodiment include... Figure 2B (a) to (c) in the text specifically include Figure 2A Steps S1 to S4 in the process.

[0045] In step S1, the metal ion sources corresponding to the ferroelectric thin film material layer and the buffer layer are selected and dissolved in the precursor solvent respectively to obtain the precursor solution of the ferroelectric thin film material layer and the precursor solution of the buffer layer.

[0046] In this embodiment, the precursor solvent includes, but is not limited to, 2-methoxyethanol and ethylene glycol, and should be selected based on the solubility of the metal ion source.

[0047] For example, ferroelectric thin film material layers contain lanthanum ions (La). 3+ ) as the metal ion source, ethylene glycol as the precursor solvent, to prepare La 3+ A PLZT (lanthanum-doped lead zirconate titanate) precursor solution with a content of 1% to 10%.

[0048] The buffer layer also uses La3+ Using ethylene glycol as a precursor solvent and lanthanum nitrate (La(NO3)3) buffer layer precursor solution as a metal ion source, a precursor solution was obtained.

[0049] In step S2, the substrate 1 is pretreated and an oxygen-injection isolation process is used to grow a buried oxide layer 2 on the substrate 1.

[0050] For example, the substrate 1 is cleaned to remove impurities and moisture, and then dried.

[0051] Furthermore, an oxygen implantation isolation process is employed, in which an oxide layer (such as a SiO2 buried oxide layer on substrate 1) is formed inside or on the surface of the substrate using oxygen implantation technology, thereby obtaining buried oxide layer 2, see [link to documentation]. Figure 2B The substrate 1 and buried oxide layer 2 are shown in (a).

[0052] The core function of the buried oxide layer 2 is to isolate the substrate 1 from the subsequently grown buffer layer 3, preventing elements in the substrate (such as Si) from diffusing into the buffer layer 3 and functional layers such as the ferroelectric thin film material layer 4. At the same time, it improves the insulation and flatness of the substrate surface, providing a high-quality substrate interface for subsequent thin film growth.

[0053] In step S3, the precursor solution of the buffer layer is dropped onto the surface of the buried oxygen layer 2, and then the surface of the buried oxygen layer 2 is heated with air to form the buffer layer 3.

[0054] For example, when a precursor solution containing lanthanum nitrate (La(NO3)3) is heated in air, the oxygen and carbon dioxide in the air react chemically with the heated lanthanum nitrate (La(NO3)3) to form a lanthanum oxycarbonate (La2O2CO3) solid film as buffer layer 3.

[0055] Specifically, a precursor solution containing lanthanum nitrate (La(NO3)3) for the buffer layer is spin-coated onto the surface of the buried oxide layer 2. A layered spin-coating method is used. After spin-coating, the precursor solution containing lanthanum nitrate (La(NO3)3) for the buffer layer on the surface of the buried oxide layer 2 is subjected to air heating treatment. After natural cooling, a solid thin film containing lanthanum oxycarbonate (La2O2CO3) with a thickness of 5 nm to 15 nm is formed as the buffer layer 3. See this application for details. Figure 2B (b) in the middle.

[0056] In step S4, the precursor solution of the ferroelectric thin film material layer is used to grow multiple times on the buffer layer 3. After each growth, a pre-annealing treatment is performed until the growth reaches the preset target thickness, thereby obtaining a gradient-doped ferroelectric thin film device.

[0057] For example, using a gradient-doped PLZT (lanthanum-doped lead zirconate titanate) precursor solution as the precursor solution for the ferroelectric thin film material layer, growth is carried out with a low-doped bottom layer and a high-doped top layer. 3+ The doping of La through a charge compensation mechanism enables... 3+ It can replace Pb 2+ At this point, the excess positive charge can offset the negative charge deficiency of oxygen vacancies, reducing the number of oxygen vacancies at the interface and decreasing the obstacle of defects to the subsequent ordered growth of polarized domains. See this application for details. Figure 2B As shown in (c) in the figure.

[0058] Specifically, the reason for using a low-doped bottom layer is: La 3+ There is a difference in ionic radius between it and lead ions (La) 3+ (With a smaller radius), doping induces distortion of the oxygen octahedrons in the lead zirconate titanate lattice, causing adaptive stretching of the a-axis in the lattice parameters of the tetragonal lead zirconate titanate, making it closer to the lattice parameters of the lanthanum oxycarbonate (La2O2CO3) buffer layer, reducing the interlayer lattice mismatch, and preventing the interfacial stress caused by the mismatch from being transmitted to the upper film.

[0059] The reason for high doping in the top layer is that it can be achieved through high La content. 3+ Doping enhances the electro-optic coefficient of materials, thereby improving the macroscopic electro-optic response.

[0060] In this embodiment, after multiple growths, the preset target thickness of the ferroelectric thin film material layer 4 is 200nm~300nm.

[0061] In step S4 of this embodiment, the thickness of each growth in multiple growth processes is 50nm~100nm; the growth process includes chemical solution deposition, radio frequency magnetron sputtering and pulsed laser deposition.

[0062] Specifically, a single thickness of 50nm to 100nm can disperse the internal stress during the growth process, and the subsequent annealing process can further release the stress, ensuring the density and integrity of the film and solving the problem of frequent cracking in the fabrication of ordinary ferroelectric thin film devices.

[0063] In this embodiment, a pre-annealing process is performed after each growth, including: using a layered annealing process, high-temperature annealing is performed after the first growth, with a temperature range of 550℃ to 650℃; except for the first growth, low-temperature annealing is performed after each subsequent growth, with a temperature range of 250℃ to 350℃.

[0064] Specifically, high-temperature annealing at 550℃~650℃ after the first growth can promote the full growth of grains in the bottom ferroelectric thin film material layer 4 (near the buffer layer 3), improve crystallinity and orientation consistency, and reduce grain boundary defects during the growth of the upper layer. This provides structural support for the spontaneous polarization enhancement of gradient doped ferroelectric thin film devices and indirectly improves the electro-optic coefficient of the devices.

[0065] The subsequent low-temperature annealing at 250℃~350℃ ensures the stability of the gradient doping and avoids the diffusion of interlayer ions. At the same time, low-temperature annealing can accurately preserve the gradient characteristics of increasing concentration from bottom to top while maintaining the existing crystal quality, reducing the damage of high temperature to the microstructure of the ferroelectric thin film material layer 4 (such as the decrease in uniformity caused by excessive grain growth).

[0066] In steps S1 to S4 above, by precisely controlling the doping concentration of the precursor solution of the ferroelectric thin film material layer 4, and combining multiple spin coating and annealing processes, a gradient distribution of doping concentration is achieved, which effectively improves the dielectric and ferroelectric properties of the ferroelectric thin film material layer 4 and reduces the leakage current density.

[0067] Figure 3 The diagram schematically illustrates the doping gradient of a ferroelectric thin film material layer according to an embodiment of this application.

[0068] like Figure 3 As shown, when the precursor solution of the prepared ferroelectric thin film material layer is deposited on the buffer layer 3, a layered gradient doping method is used.

[0069] For example, the doping concentration of the bottom layer can be set to 2%, and then increased by 2% for each layer upwards, with the doping concentration of the top layer set to 10%.

[0070] The main purpose of using layered gradient doping is to avoid the accumulation of lattice stress in the thin film as the thickness increases, which could trigger a lattice phase transition during subsequent annealing or cooling, disrupt the ordered orientation of polarization domains, weaken the electro-optic response, and cause microcracks to form on or inside the thin film when the stress accumulates to a critical value.

[0071] This application achieves this by adjusting the doping concentration of doped ions in the ferroelectric thin film material layer layer by layer, such as La. 3+ The doping concentration is increased by growing the film in a gradient manner to reduce the degree of lattice distortion and stress concentration effect. At the same time, the lower temperature annealing process of the top layer further alleviates the structural damage during stress release and ensures the compactness and integrity of the film.

[0072] Figure 4 A schematic diagram of a waveguide cross-section fabricated from a gradient-doped ferroelectric thin-film device according to an embodiment of this application is shown.

[0073] In this embodiment, Figure 4 Cross-sectional view of a waveguide fabricated from PLZT (lanthanum-doped lead zirconate titanate).

[0074] Specifically, the waveguide has an overall thickness of 300 nm and an etching depth of 150 nm. The surface exhibits excellent smoothness with no obvious particle accumulation or cracks, indicating that atomic-level uniform spreading was achieved during the growth process.

[0075] The excellent performance of the aforementioned waveguide structure lays a solid foundation for the application of gradient-doped ferroelectric thin film devices in fields such as electro-optic modulation and optical signal transmission. It not only ensures the optical field confinement capability, but also reduces optical loss due to the excellent surface and interface quality.

[0076] Those skilled in the art will understand that the features described in the various embodiments of this application can be combined and / or combined in various ways, even if such combinations or combinations are not explicitly described in this application. In particular, the features described in the various embodiments of this application can be combined and / or combined in various ways without departing from the spirit and teachings of this application. All such combinations and / or combinations fall within the scope of this application.

[0077] The embodiments of this application have been described above. However, these embodiments are merely illustrative and not intended to limit the scope of this application. Although various embodiments have been described above, this does not mean that the measures in the various embodiments cannot be used advantageously in combination. Without departing from the scope of this application, those skilled in the art can make various substitutions and modifications, all of which should fall within the scope of this application.

Claims

1. A gradient-doped ferroelectric thin-film device, characterized in that, include: The substrate (1), buried oxide layer (2), buffer layer (3), and ferroelectric thin film material layer (4) are stacked sequentially from bottom to top. The ferroelectric thin film material layer (4) is doped with impurity ions, and the doping concentration of the impurity ions increases sequentially from bottom to top; the impurity ions are lanthanide ions, including La 3+ 、Nd 3+ Pr 3+ 、Sm 3+ The doping concentration of the impurity ions ranges from 1% to 10%; wherein the doping concentration of the bottom layer is set to 2%, increasing by 2% for each layer upwards, with the doping concentration of the top layer set to 10%. The ferroelectric thin film material layer (4) is made of lead zirconate titanate and barium titanate; The material of the buffer layer (3) includes lanthanum oxycarbonate and strontium titanate; the thickness of the buffer layer (3) is 5 nm to 15 nm.

2. The gradient-doped ferroelectric thin film device according to claim 1, characterized in that, An embedding layer is also provided above the ferroelectric thin film material layer (4).

3. The gradient-doped ferroelectric thin film device according to claim 1, characterized in that, The thickness of the ferroelectric thin film material layer (4) is 100nm~1μm.

4. A method for fabricating a gradient-doped ferroelectric thin-film device, characterized in that, include: Step S1: Select the metal ion source corresponding to the ferroelectric thin film material layer and the buffer layer and dissolve them in the precursor solvent respectively to obtain the precursor solution of the ferroelectric thin film material layer and the precursor solution of the buffer layer. The metal ion source corresponding to the buffer layer is lanthanum nitrate, and the precursor solution of the buffer layer is a lanthanum nitrate buffer layer precursor solution. Step S2: The substrate (1) is pretreated and an oxygen injection isolation process is used to grow a buried oxide layer (2) on the substrate (1). Step S3: The precursor solution of the buffer layer is dropped onto the surface of the buried oxygen layer (2), and then the surface of the buried oxygen layer (2) is heated with air to form a solid film containing lanthanum carbonate as a buffer layer (3). Step S4: The precursor solution of the ferroelectric thin film material layer is used to grow the buffer layer (3) multiple times. A layered gradient doping method is adopted to form a ferroelectric thin film material layer (4) with the impurity ion doping concentration increasing from bottom to top. The impurity ions are lanthanide ions, including La 3+ 、Nd 3+ Pr 3+ 、Sm 3+ The doping concentration of the impurity ions ranges from 1% to 10%; wherein the doping concentration of the bottom layer is set to 2%, increasing sequentially upwards by 2% for each layer, with the top layer having a doping concentration of 10%; a pre-annealing process is performed after each growth until the preset target thickness is reached to obtain the gradient-doped ferroelectric thin film device; the pre-annealing process after each growth includes: using a layered annealing method, employing high-temperature annealing after the first growth, with the high-temperature annealing temperature range being 550℃ to 650℃; except for the first growth, low-temperature annealing is performed after each subsequent growth, with the low-temperature annealing temperature range being 250℃ to 350℃.

5. The method for fabricating a gradient-doped ferroelectric thin film device according to claim 4, characterized in that, The precursor solvent includes 2-methoxyethanol and ethylene glycol.

6. The method for fabricating a gradient-doped ferroelectric thin-film device according to claim 4, characterized in that, In step S4: The thickness of each growth in the multiple growth processes is 50nm~100nm; The multiple growth process includes chemical solution deposition, radio frequency magnetron sputtering, and pulsed laser deposition.

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