Method for preparing a heteroepitaxial junction thin film

Abstract

A method for preparing a heteroepitaxial homojunction thin film, comprising: (1) forming a sacrificial layer on an initial substrate to obtain a sacrificial layer / initial substrate structure; (2) forming a self-supporting thin film on the sacrificial layer to obtain a self-supporting thin film / sacrificial layer / initial substrate structure; (3) placing the self-supporting thin film / sacrificial layer / initial substrate structure in a solution capable of dissolving the sacrificial layer until the sacrificial layer is dissolved, so that the self-supporting thin film is separated from the initial substrate to obtain the self-supporting thin film; (4) transferring the self-supporting thin film onto a target substrate in a manner that the self-supporting thin film does not completely cover the target substrate to obtain a self-supporting thin film / target substrate structure in which the self-supporting thin film does not completely cover the target substrate; and (5) forming a target thin film on the self-supporting thin film / target substrate to obtain the heteroepitaxial homojunction thin film; wherein the self-supporting thin film and the target substrate are made of the same material and have different crystal faces. The method is flexible, simple and can be used to prepare the heteroepitaxial homojunction thin film in a universal manner.

Description

Technical Field

[0001] This invention belongs to the field of materials. Specifically, this invention relates to a method for preparing homojunction thin films with heteromorphic planes. Background Technology

[0002] Thin film materials, such as oxide films, possess a wealth of physical properties, including superconductivity, magnetoelectricity, and multiferroicity, which are of great significance in condensed matter physics. Compared to single thin film materials, heterojunctions couple different materials, stress conditions, or crystal planes, inducing even more novel physical properties, which are of great importance in both experimental research and practical applications.

[0003] Currently, the mainstream method for obtaining heterojunctions is through heteroepitaxial growth, which involves epitaxially growing another thin film material on top of an existing material, resulting in high quality. However, this method has many limitations. For example, heteroepitaxial growth requires a very high degree of lattice fit, and many of its physical properties may be suppressed due to the clamping effect of the substrate. Furthermore, materials with significantly different growth conditions are difficult to couple together via heteroepitaxial growth.

[0004] Among the diverse types of heterojunctions, heterojunctions with different crystal planes are an emerging research direction. A heterojunction is a thin film containing more than one crystal plane orientation, which may result in electrical transport and electromagnetic properties different from those of a single-crystal thin film. Epitaxial growth of thin films places high demands on the substrate; previously, thin films could only be grown on a single substrate, limiting the grown film to a single crystal phase. Currently, no existing technology discloses how to prepare heterojunctions with different crystal planes.

[0005] Therefore, there is an urgent need for a method to prepare homojunction thin films with heteromorphic surfaces. Summary of the Invention

[0006] The purpose of this invention is to provide a universal method for preparing heterojunction thin films with heteromorphic surfaces. The heterojunction thin films prepared by the method of this invention can exhibit different electrical transport and electromagnetic properties compared to single-crystal thin films.

[0007] The above-mentioned objective of the present invention is achieved through the following technical solution.

[0008] In the context of this invention, the term "same material" refers to materials having the same crystal structure (i.e., the same atomic stacking) and the same elemental composition.

[0009] In the context of this invention, the term "lattice fit" refers to a lattice mismatch rate between materials that is less than or equal to 5%. For example, lattice fit between material A and material B means that (lattice constant of material A - lattice constant of material B) / lattice constant of material A ≤ 5%, where the lattice constant of material A is greater than that of material B. If the lattice constant of material B is greater than that of material A, then lattice fit between material A and material B means that (lattice constant of material B - lattice constant of material A) / lattice constant of material B ≤ 5%.

[0010] This invention provides a method for preparing homojunction thin films with heteromorphic planes, comprising the following steps:

[0011] (1) A sacrificial layer is formed on the initial substrate to obtain a sacrificial layer / initial substrate structure;

[0012] (2) A self-supporting thin film is formed on the sacrificial layer of the sacrificial layer / initial substrate structure to obtain a self-supporting thin film / sacrificial layer / initial substrate structure;

[0013] (3) Place the self-supporting film / sacrificial layer / initial substrate structure in a solution that can dissolve the sacrificial layer until the sacrificial layer dissolves, so that the self-supporting film is separated from the initial substrate to obtain a self-supporting film.

[0014] (4) The self-supporting film is transferred onto the target substrate in a manner that does not completely cover the target substrate, so as to obtain a self-supporting film / target substrate structure in which the self-supporting film does not completely cover the target substrate;

[0015] (5) A target thin film is formed on the self-supporting thin film / target substrate structure where the self-supporting thin film does not completely cover the target substrate, that is, a heterojunction thin film with different crystal planes is obtained.

[0016] The self-supporting thin film and the target substrate are made of the same material but have different crystal planes.

[0017] The method of this invention can be universally applied to prepare and transfer self-supporting thin films. The self-supporting thin film and the target substrate only need to be made of the same material but with different crystal planes. Various high-quality heterojunctions with different crystal planes can be grown using the method of this invention.

[0018] Preferably, in the method described in this invention, the self-supporting thin film, the sacrificial layer, and the initial substrate are lattice-matched to each other.

[0019] Preferably, in the method described in this invention, the self-supporting thin film is STO, the sacrificial layer is SAO, and the initial substrate is LSAT or STO.

[0020] Preferably, in the method described in this invention, the self-supporting thin film, the sacrificial layer, and the initial substrate have the same crystal planes as each other.

[0021] Preferably, in the method described in this invention, the self-supporting thin film, the target substrate, and the target thin film are lattice-matched to each other.

[0022] Preferably, in the method of the present invention, the method further includes, in step (3), before placing the self-supporting film / sacrificial layer / initial substrate structure in a solution capable of dissolving the sacrificial layer, forming a protective layer on the self-supporting film to obtain the protective layer / self-supporting film / sacrificial layer / initial substrate structure;

[0023] Steps (3) and (4) are respectively performed by methods including the following steps:

[0024] (3) Place the protective layer / self-supporting film / sacrificial layer / initial substrate in a solution that can dissolve the sacrificial layer until the sacrificial layer dissolves, so that the protective layer / self-supporting film is separated from the initial substrate, and a protective layer / self-supporting film structure is obtained.

[0025] (4) The protective layer / self-supporting film structure is transferred to the target substrate in such a way that the self-supporting film does not completely cover the target substrate to obtain a protective layer / self-supporting film / target substrate structure in which the self-supporting film does not completely cover the target substrate; then, the protective layer in the protective layer / self-supporting film / target substrate is removed to obtain a self-supporting film / target substrate structure.

[0026] Preferably, in the method described in this invention, the protective layer is selected from one or more of PDMS, heat-release tape, PMMA, and metal.

[0027] Preferably, in the method described in this invention, the metal is selected from one or more of gold, silver, and platinum.

[0028] Preferably, in the method of the present invention, the formation of a sacrificial layer on the initial substrate in step (1), the formation of a self-supporting thin film on the sacrificial layer in step (2), and the formation of a target thin film on the self-supporting thin film / target substrate structure in step (5), where the self-supporting thin film does not completely cover the target substrate, are each formed independently by a method selected from pulsed laser deposition, molecular beam epitaxy, chemical vapor deposition, or magnetron sputtering.

[0029] In a specific embodiment of the present invention, pulsed laser deposition is used to implement the method of the present invention, which includes the following steps: (1) preparing the growth target and the original substrate: the target of the sacrificial layer and the target of the self-supporting film are loaded into the cavity, and the original substrate is attached to the vacuum chamber; the chamber is pumped to 1×10⁻⁶ using a mechanical pump and a molecular pump.-7 To achieve the vacuum level required for thin film growth, adjust the oxygen pressure required for growth, such as 3 × 10⁻⁶. -2 (1) Heat the target material to the target material and then raise the temperature to the film growth temperature, such as 700℃. (2) Clean the target material: Adjust the laser frequency and energy, such as 3Hz 100mW, and let the laser bombard the target material first. Clean the target material surface for 5 minutes. At this time, close the substrate baffle and do not grow the film. (3) Grow the film: First adjust to the target material of the sacrificial layer, open the substrate baffle, start growing the sacrificial layer, and start timing at the same time. Control the thickness by the growth time. After the sacrificial layer is finished, close the substrate baffle and switch to the target material of the self-supporting film to start growing the self-supporting film. (4) Obtain the self-supporting film: Take out the sample after growing the sacrificial layer and the self-supporting film and place it in a solution that can dissolve the sacrificial layer, such as deionized water. After the sacrificial layer is completely dissolved, the self-supporting film floats on the water surface. Alternatively, cover the surface of the self-supporting film with a protective layer. The protective layer carries the self-supporting film and floats on the water surface. (5) Transfer the self-supporting film to the target substrate: The self-supporting film can be directly retrieved from the target substrate. This process should be handled with extra care to avoid damaging the self-supporting film. After retrieval, carefully blow dry the self-supporting film. Alternatively, the protective layer / self-supporting film is first retrieved and then attached to the target substrate. Finally, the protective layer is removed by heating or dissolving. (6) Growth of the target film, i.e., the heterojunction film: Repeat steps (1) to (3), except that the target material is replaced with the target material of the target film, and the sample attached to the chamber is a combination of self-supporting film / target substrate.

[0030] In specific embodiments of the present invention, the sacrificial layer is typically, but not limited to, soluble in liquids such as water and acids. For example, Sr3Al2O6 (abbreviated as SAO) is soluble in deionized water, and La... 0.67 Sr 0.33 MnO3(LSMO, lattice constant is approximately) It dissolves in dilute hydrochloric acid, etc. The sacrificial layer also requires high-quality epitaxial growth, necessitating the selection of a substrate with a suitable lattice constant. For example, SAO and LSMO can both be grown on (La,Sr)(Al,Ta)O3(LSAT, lattice constant). High-quality epitaxy on the substrate. At the same time, in order to ensure that the sacrificial layer can react efficiently with the solution, the sacrificial layer needs to be of sufficient thickness. For example, SAO grown to 10 nm can dissolve quickly in water.

[0031] In a specific embodiment of the present invention, the self-supporting thin film is epitaxially grown on a sacrificial layer, and the lattice constant of the self-supporting thin film must be compatible with that of the sacrificial layer. The SAO has a relatively large unit cell with a lattice constant of [missing value]. However, it still has a cubic structure, which is consistent with the four-period repeating STO (the lattice constant of a single STO unit cell is ). Four times the STO lattice constant is The mismatch rate between SAO and SAO is only 1.4%, which is still within the lattice fit range, so STO films can be epitaxially grown well on SAO films. Similarly, the mismatch rate between SAO and the four-cycle repeating LSAT substrate is only 2%, so SAO can also be epitaxially grown well on LSAT substrates.

[0032] Meanwhile, since both the sacrificial layer and the self-supporting film are epitaxially grown, their crystal planes are consistent with the substrate. For example, to obtain a self-supporting SrTiO3 (STO, lattice constant) with a (001) crystal plane... For thin films, the substrate should be selected with a similar lattice constant and both being (001) crystal planes, such as LSAT (001) substrates.

[0033] In a specific embodiment of the present invention, the target substrate should be selected as a substrate of the same material as the self-supporting thin film but with a different crystal plane. For example, if the self-supporting thin film is STO with a (001) crystal plane, then the target substrate should be selected as a single crystal substrate with a (110) or (111) crystal plane.

[0034] In a specific embodiment of the present invention, the target thin film still needs to maintain a small lattice mismatch with the materials of the self-supporting thin film and the target substrate. For example, if the self-supporting thin film and the target substrate are STO with (001) and (110) crystal planes respectively, then the target thin film can be selected as SrRuO3 (SRO, lattice constant). ) and BiFeO3 (BFO, lattice constant) )wait.

[0035] In specific embodiments of the present invention, when the self-supporting film is extremely thin, with a thickness as low as a few unit cell layers or a few nanometers, the film is extremely fragile and is likely to be damaged due to uneven stress or fluctuations in the liquid surface during the dissolution of the sacrificial layer. Therefore, an additional protective layer is needed to protect the self-supporting film before dissolving the sacrificial layer. This protective layer includes, but is not limited to, PDMS (polydimethylsiloxane), heat-release tape, PMMA (polymethyl methacrylate), or metals such as gold or silver.

[0036] PDMS is a mild, flexible, and highly adhesive polymer film that can form good contact with self-supporting films. Thermal release tape has strong adhesion at room temperature, but this adhesion rapidly disappears upon heating to a certain temperature, making it suitable for protecting self-supporting films and releasing them onto the target substrate. PMMA is a polymer that can be spin-coated onto the surface of a self-supporting film to form a protective layer. After transfer to the target substrate, the PMMA can be washed away with acetone, leaving only the target substrate. Alternatively, the protective layer can be a metal such as gold or silver, which can be removed later using a metal etchant.

[0037] In a specific embodiment of the present invention, a self-supporting thin film is transferred to a target substrate. The most direct method involves detaching the self-supporting thin film from its original substrate, retrieving it with the target substrate, carefully blowing away surface moisture, and then slowly drying it to thoroughly remove any liquid residue, especially the liquid between the self-supporting thin film and the target substrate. However, direct retrieval has several drawbacks: firstly, the relative position between the self-supporting thin film and the target substrate is difficult to control during retrieval, making precise positioning challenging; secondly, significant liquid level fluctuations during retrieval can damage the self-supporting thin film. A more preferable method is to use a protective layer. First, the protective layer carrying the self-supporting thin film is retrieved from the liquid and dried. Due to the protective layer, the self-supporting thin film remains largely undamaged during this process. Then, the self-supporting thin film is attached to the target substrate, forming a protective layer / self-supporting thin film / target substrate structure. Baking is then performed to enhance the contact between the self-supporting thin film and the target substrate. Finally, the protective layer is removed to complete the transfer of the self-supporting thin film to the target substrate. This transfer process is more non-destructive and can be further refined by additional steps, such as placing the protective layer carrying the self-supporting film on a displacement platform (x, y, z axes and rotation around the z axis) that allows for precise control of the spatial orientation. This allows for more precise control of the relative position between the self-supporting film and the target substrate.

[0038] Pulsed laser deposition (PLD) is particularly adept at growing high-quality single-crystal perovskite oxide films. This is crucial for growing high-quality sacrificial layers with accurate stoichiometry and for epitaxial growth of the same film on different crystal planes. Therefore, the following examples will focus more on PLD-grown film systems.

[0039] This invention involves first growing a sacrificial layer on a substrate and then growing a self-supporting thin film. The sacrificial layer is typically soluble in certain liquids such as water or acids; dissolving the sacrificial layer yields the self-supporting thin film. The self-supporting thin film is not limited by a substrate and can be precisely transferred to other target substrates, providing abundant possibilities for constructing various heterostructures.

[0040] The present invention has the following beneficial effects:

[0041] This invention achieves a self-supporting film / target substrate composite structure by transferring a self-supporting film onto a target substrate with different crystal planes. This structure allows the film grown on the substrate to grow along two different crystal planes, forming a heterojunction film. A distinct grain boundary exists at the interface between the two crystal planes, and the heterojunction film exhibits different electrical transport and electromagnetic properties compared to single-crystal films.

[0042] The preparation method of the present invention is flexible and simple because the self-supporting film and the target substrate can be combined in any way, and the thickness, position and angle of the self-supporting film relative to the target substrate can be adjusted arbitrarily. Attached Figure Description

[0043] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings, wherein:

[0044] Figure 1 This is a schematic diagram illustrating the direct transfer and growth of a heterojunction from a self-supporting thin film according to a specific embodiment of the present invention.

[0045] Figure 2 This is a schematic diagram of a protective layer-assisted self-supporting thin film transfer and growth of a heterojunction according to a specific embodiment of the present invention.

[0046] Figure 3 The XRD patterns are of the heterojunction thin films obtained in Examples 1 and 2 of the present invention.

[0047] Figure 4 This is an RSM diagram of the heterojunction thin film obtained in Example 1 of the present invention. Detailed Implementation

[0048] The present invention will be further described in detail below with reference to specific embodiments. The embodiments given are only for illustrating the present invention and are not intended to limit the scope of the present invention.

[0049] Example 1

[0050] LaSrCoO3 (LSCO) heterostructure homojunctions were prepared by pulsed laser deposition (PLD).

[0051] First, a water-soluble sacrificial layer SAO (Sr3Al2O6) is grown on the original LSAT ((La,Sr)(Al,Ta)O3) substrate. The growth conditions are: background vacuum 5 × 10⁻⁶. -8 Torr, oxygen pressure during growth 5×10 -2The laser was used to grow a single-crystal STO (SrTiO3) (001) film for 10,000 pulses at a frequency of 3 Hz and an energy of 100 mJ. A self-supporting STO film was then grown on the SAO layer under the same conditions as the SAO layer, except for 5,000 pulses. After growth, a heat-release tape (single-sided adhesive, release temperature 120°C) was applied to the outermost STO (001) surface for protection. The heat-release tape / STO (001) / SAO / LSAT was carefully immersed in deionized water for a sufficient time until the SAO layer was completely dissolved. The heat-release tape, carrying the STO layer, was completely separated from the LSAT substrate. The heat-release tape was removed, with the adhesive side carrying the STO layer facing upwards, and the surface moisture was carefully blown dry and air-dried in clean air for 30 minutes. Prepare an STO(110) single-crystal substrate. Slowly and firmly adhere the STO(001) side of the previously thoroughly dried thermal release tape to the STO(110) single-crystal substrate, pressing it firmly. Then transfer the thermal release tape / STO(001) / STO(110) to a heating plate and heat it to the release temperature of the thermal release tape, maintaining pressure throughout the heating process. Once the release temperature is reached, the adhesiveness of the thermal release tape disappears, releasing the carried single-crystal STO(001) film onto the single-crystal STO(110) target substrate. The area of ​​the single-crystal STO(110) target substrate is larger than the area of ​​the single-crystal STO(001) self-supporting film. Next, grow an LSCO film on the STO(001) / STO(110) substrate (the lattice constant of conventional LSCO is...). The growth conditions were: 5×10⁻⁶ vacuum at the bottom. -8 The growth oxygen pressure was 0.1 Torr, the laser frequency / energy was 3 Hz and 100 mJ, and the growth was performed over 5000 pulses. Since part of the LSCO film was epitaxially grown on the STO(001) film, an LSCO(001) film was epitaxially grown, while the other part of the LSCO film was epitaxially grown on the STO(110) substrate, an LSCO(110) film was epitaxially grown. That is, two crystal planes, (001) and (110), appeared in the same LSCO film layer, forming an LSCO heterostructure homojunction.

[0052] Example 2

[0053] The preparation method of this embodiment is the same as that of Example 1, except that the STO (110) target substrate in Example 1 is replaced with a single-crystal STO (111) target substrate. That is, the preparation of the self-supporting thin film STO (001) is the same as in Example 1, except that the same steps are used to transfer the STO (001) thin film to the single-crystal STO (111) target substrate with heat-release tape, and LSCO is epitaxially grown on the obtained STO (001) / STO (111) to obtain an LSCO thin film containing heterocrystalline planes of (001) and (111).

[0054] Example 3

[0055] LaSrCoO3 heterostructure homojunctions were prepared by pulsed laser deposition.

[0056] First, an acid-soluble sacrificial layer La is grown on the original STO(001) substrate. 0.8 Sr 0.2 MnO3(LSMO, lattice constant is approximately) The growth conditions were: 5×10⁻⁶ vacuum at the bottom. -8 The STO (SrTiO3) film was grown under the following conditions: oxygen pressure of 0.1 Torr, laser frequency / energy of 3 Hz and 100 mJ, for 10,000 pulses. A single-crystal STO (SrTiO3) (001) film was then grown on the LSMO sacrificial layer as a self-supporting film, with the same growth conditions as the sacrificial layer, except for 5,000 pulses. After growth, a heat-release tape (single-sided adhesive, release temperature 120°C) was applied to the outermost STO surface for protection. The heat-release tape / STO (001) / LSMO / STO (001) was carefully immersed in dilute hydrochloric acid (0.3 vol%) for a sufficient time until the LSMO layer was completely dissolved. The heat-release tape carrying the STO layer was completely separated from the STO substrate. The heat-release tape was removed, with the adhesive side carrying the STO layer facing upwards, and the surface moisture was carefully blown dry and air-dried in clean air for 30 minutes. Prepare an STO(110) single-crystal substrate. Slowly and firmly adhere the STO(001) side of the previously thoroughly dried thermal release tape to the STO(110) single-crystal substrate, pressing it firmly. Then transfer the thermal release tape / STO(001) / STO(110) to a heating plate and heat it to the release temperature of the thermal release tape, maintaining pressure throughout the heating process. Once the release temperature is reached, the adhesiveness of the thermal release tape disappears, releasing the carried single-crystal STO(001) film onto the single-crystal STO(110) target substrate. The area of ​​the single-crystal STO(110) target substrate is larger than the area of ​​the single-crystal STO(001) self-supporting film. Next, grow an LSCO film on the STO(001) / STO(110) substrate (the lattice constant of conventional LSCO is...). The growth conditions were: 5×10⁻⁶ vacuum at the bottom. -8 The growth oxygen pressure was 0.1 Torr, the laser frequency / energy was 3 Hz and 100 mJ, and the growth was performed over 5000 pulses. Since part of the LSCO film was epitaxially grown on the STO(001) film, an LSCO(001) film was epitaxially grown, while the other part of the LSCO film was epitaxially grown on the STO(110) substrate, an LSCO(110) film was epitaxially grown. That is, two crystal planes, (001) and (110), appeared in the same LSCO film layer, forming an LSCO heterostructure homojunction.

[0057] Example 4

[0058] The preparation method of this embodiment is the same as that of Embodiment 1, except that the protective layer in Embodiment 1 is replaced with PDMS instead of heat-release tape. The specific implementation plan of this embodiment includes the following steps: Prepare a self-supporting thin film STO(001) as in Embodiment 1, but use PDMS as the protective layer. After the PDMS carrying STO(001) is separated from the original substrate and the moisture is removed, use a common displacement mechanism used in microscopes and optical platforms to hold a glass slide or other transparent carrier, attach the side of the PDMS without the self-supporting film to the glass slide, and fix the target substrate STO(110) directly below the glass slide. At this time, the PDMS carrying the STO(001) film is suspended above the target substrate, and the relative position and angle of the STO(001) film relative to the target substrate can be adjusted by adjusting the displacement mechanism. After determining the angle, lower the height of the glass slide until the PDMS contacts the target substrate, forming a structure of glass slide / PDMS / target film / target substrate. Then, in-situ heating is performed. The viscosity of PDMS disappears after the temperature reaches a certain level. After sufficient heating, PDMS separates from the glass slide and the target substrate, leaving a self-supporting thin film / target substrate. Epitaxial films are then grown on the self-supporting thin film / target substrate to obtain a heterojunction with controllable grain boundary positions and angles.

[0059] Example 5

[0060] The preparation method in this embodiment is the same as in Embodiment 1, except that a protective layer is not used. That is, a 40nm target film is grown after the sacrificial layer is grown. Of course, in other embodiments, a target film larger than 40nm can be grown to ensure it is strong enough to withstand the dissolution of the sacrificial layer without breaking. The film is placed in a solution, and after the sacrificial layer dissolves, the self-supporting film is suspended in the solution. At this point, the self-supporting film is directly retrieved using the target substrate, dried, and then the target film is epitaxially grown. Because there is no protective layer containing organic components, the surface of the self-supporting film is relatively clean, but the absence of wrinkles or stress concentration cannot be guaranteed.

[0061] Example 6

[0062] The preparation method of this embodiment is the same as that of Embodiment 1, except that the protective layer in Embodiment 1 is replaced with PMMA instead of heat-release tape. The specific implementation of this embodiment includes the following steps: After the self-supporting film is grown, a layer of PMMA 950A5 film is spin-coated onto its surface at 4000 rpm / min to provide protection. After the sacrificial layer dissolves, the PMMA film carrying the self-supporting film floats in the solution. Due to its strong surface tension, the PMMA film flattens on the liquid surface, and the self-supporting film does not wrinkle or experience excessive stress concentration. At this point, a flat PMMA film / self-supporting film can be retrieved using the target substrate. It is then thoroughly dried and heated for a sufficient time to ensure a tight bond between the self-supporting film and the target substrate. Subsequently, the PMMA / self-supporting film / target substrate is placed in an acetone solution to remove the PMMA film, leaving the self-supporting film / target substrate. A heterojunction can then be epitaxially grown.

[0063] Example 7

[0064] The preparation method of this embodiment is the same as that of Embodiment 1, except that the protective layer in Embodiment 1 is replaced by a layer of silver vapor-deposited instead of a heat-release tape. The thickness of this metal layer cannot be too thin. The thickness of this protective layer is 50 nm. Of course, in other specific embodiments, it can be greater than 50 nm. After dissolving the sacrificial layer, a metal / self-supporting thin film suspended in the liquid is obtained. This film is then retrieved from the target substrate and thoroughly dried. Subsequently, the metal / self-supporting thin film / target substrate is immersed in a metal etchant to remove the metal, thus obtaining the self-supporting thin film / target substrate. Finally, a homojunction with heterogeneous crystal planes is formed in the epitaxial layer.

[0065] Result characterization:

[0066] Figure 3 The images show the XRD patterns of the heterojunction films obtained in Examples 1 and 2. From bottom to top, they are: LSCO grown directly on a single-crystal STO (001) substrate; LSCO grown on an STO (111) substrate with a portion of the self-supporting STO (001) transferred (Example 2); and LSCO grown on an STO (110) substrate with a portion of the self-supporting STO (001) transferred (Example 1).

[0067] Figure 4 The image shows the RSM (Reciprocal space map) of the heterojunction thin film obtained in Example 1. Figure 4 The RSM diagrams of films obtained by growing LSCO on an STO(110) substrate with partially self-supported STO(001) attached (Example 1) and by growing LSCO directly on an STO(001) substrate (LSCO-STO(001)) are shown, where the prefix "FS" indicates self-supporting.

[0068] In the XRD pattern, the marked peaks are substrate peaks (e.g., *, #, ▽), and the dashed lines represent LSCO (001) peaks. LSCO grown on a self-supporting STO (001) thin film / single-crystal STO (111) substrate simultaneously showed peaks for both LSCO (001) and LSCO (111) (Example 2), indicating the successful fabrication of a heterojunction of LSCO. The thin film prepared in Example 1 simultaneously showed peaks for both LSCO (001) and LSCO (110). The RSM pattern of the LSCO thin film grown on a self-supporting STO (001) / single-crystal STO (110) substrate (Example 1) simultaneously showed signals for LSCO and STO on the (110) crystal plane and the (001) and (002) crystal planes, also indicating the presence of (110) and (001) crystal planes in the LSCO thin film.

Claims

1. A method for preparing a homojunction thin film with heteromorphic planes, comprising the following steps: (1) A sacrificial layer is formed on the initial substrate to obtain a sacrificial layer / initial substrate structure; (2) A self-supporting thin film is formed on the sacrificial layer of the sacrificial layer / initial substrate structure to obtain a self-supporting thin film / sacrificial layer / initial substrate structure; (3) Place the self-supporting film / sacrificial layer / initial substrate structure in a solution that can dissolve the sacrificial layer until the sacrificial layer dissolves, so that the self-supporting film is separated from the initial substrate to obtain a self-supporting film; (4) The self-supporting film is transferred onto the target substrate in a manner that does not completely cover the target substrate, so as to obtain a self-supporting film / target substrate structure in which the self-supporting film does not completely cover the target substrate; (5) A target thin film is formed on the self-supporting thin film / target substrate structure where the self-supporting thin film does not completely cover the target substrate, that is, a heterojunction thin film with different crystal planes is obtained. The self-supporting thin film and the target substrate are made of the same material but have different crystal planes; The method further includes, in step (3), forming a protective layer on the self-supporting film before placing the self-supporting film / sacrificial layer / initial substrate structure in a solution capable of dissolving the sacrificial layer to obtain the protective layer / self-supporting film / sacrificial layer / initial substrate structure; Steps (3) and (4) are respectively performed by methods including the following steps: (3) Place the protective layer / self-supporting film / sacrificial layer / initial substrate structure in a solution that can dissolve the sacrificial layer until the sacrificial layer dissolves, so that the protective layer / self-supporting film is separated from the initial substrate, and a protective layer / self-supporting film structure is obtained. (4) The protective layer / self-supporting film structure is transferred to the target substrate in such a way that the self-supporting film does not completely cover the target substrate to obtain a protective layer / self-supporting film / target substrate structure in which the self-supporting film does not completely cover the target substrate; then, the protective layer in the protective layer / self-supporting film / target substrate structure is removed to obtain a self-supporting film / target substrate structure.

2. The method according to claim 1, wherein, The self-supporting thin film, the sacrificial layer, and the initial substrate are lattice-matched to each other.

3. The method according to claim 1, wherein, The self-supporting thin film is STO, the sacrificial layer is SAO, and the initial substrate is LSAT or STO.

4. The method according to claim 1, wherein, The self-supporting thin film, the sacrificial layer, and the initial substrate have the same crystal planes.

5. The method according to claim 1, wherein, The self-supporting thin film, the target substrate, and the target thin film are lattice-matched to each other.

6. The method according to claim 1, wherein, The protective layer is selected from one or more of PDMS, heat-release tape, PMMA, and metal.

7. The method according to claim 6, wherein, The metal is selected from one or more of gold, silver and platinum.

8. The method according to claim 1, wherein, The formation of the sacrificial layer on the initial substrate in step (1), the formation of the self-supporting thin film on the sacrificial layer in step (2), and the formation of the target thin film on the self-supporting thin film / target substrate structure where the self-supporting thin film does not completely cover the target substrate in step (5) are each formed independently by a method selected from pulsed laser deposition, molecular beam epitaxy, chemical vapor deposition, or magnetron sputtering.

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