Controllable epitaxial growth method for crystal phase structure and magnetic properties of iron telluride thin film

Abstract

This invention discloses an epitaxial growth method for iron telluride thin films with controllable crystal phase structure and magnetic properties, belonging to the field of iron chalcogenide thin film preparation technology. This invention achieves the directional selective preparation of tetragonal and hexagonal FeTe thin films by precisely matching the substrate crystal symmetry with the growth temperature window. Compared to the limitations of existing CVD methods, which rely on specific precursors, generate harmful gases, and are difficult to prepare large-area epitaxial films, this invention features a simple process, requires no special precursors, has a highly controllable growth process, good repeatability, and produces thin films with high crystallinity, uniform composition, and controllable thickness. The two crystal phases have significantly different magnetic properties, providing a material selection for antiferromagnetic pinning layers and ferromagnetic storage media in spintronic devices, effectively promoting the research and application in low-dimensional magnetic devices, high-density magnetic storage, and spin valves.

Description

Technical Field

[0001] This invention belongs to the field of iron chalcogenide thin film preparation technology, and particularly relates to an epitaxial growth method for iron telluride thin films with controllable crystal phase structure and magnetic properties. Background Technology

[0002] Iron chalcogenides (FeX, X=S, Se, Te, etc.) are a research hotspot in the interdisciplinary field of materials science and condensed matter physics due to their simple crystal structure, rich electrical and magnetic properties, and some overlap with the category of iron-based superconductors. Among them, iron telluride (FeTe), as a typical iron chalcogenide, exists in two stable structural phases: tetragonal and hexagonal. The tetragonal FeTe (t-FeTe) has a PbO-type layered structure with intrinsic antiferromagnetic order and a Nair temperature of about 70 K; the hexagonal FeTe (h-FeTe) has a non-layered structure with ferromagnetic order and a Curie temperature of about 220 K.

[0003] Since the electrical and magnetic properties of FeTe films vary significantly depending on their structural phase, and the antiferromagnetic and ferromagnetic properties can be directionally switched by controlling the crystal phase, the controllable and directional growth of FeTe films with different crystal phases has become a prerequisite for the study of the material's properties and its practical applications. Currently, the controllable phase growth of FeTe is mainly achieved through chemical vapor deposition (CVD). In 2020, Kang et al. first reported the work on the controllable growth of FeTe nanosheets using CVD: using FeCl2 precursor, selective growth of FeTe nanosheets was achieved on SiO2 / Si substrates by controlling the growth temperature in an Ar / H2 mixed atmosphere. While this technology enables the controllable preparation of FeTe, it has certain limitations: First, it relies on specific metal precursors (such as FeCl2), which are costly to prepare and difficult to store. Second, the growth process generates harmful gases (such as HCl), increasing the need for exhaust gas treatment. Third, this technology can only prepare FeTe materials in nanosheet form and cannot achieve large-area, thick-film epitaxial growth, and the film has weak adhesion to the substrate. Therefore, this method is difficult to adapt to the fabrication requirements of existing industrial spintronic devices, limiting the practical application of FeTe-based functional materials.

[0004] Pulsed laser deposition (PLD) technology has been widely used in the preparation of various thin film materials due to its ease of operation, monitorable deposition process, and ability to produce high-quality epitaxial thin films. However, a mature phase-controlled growth process has not yet been developed in the research on PLD preparation of FeTe thin films. Therefore, developing an easy-to-implement and reproducible method for the phase-controlled preparation of FeTe thin films using PLD is of great practical value for the research and application of FeTe material. Summary of the Invention

[0005] To overcome the shortcomings of the prior art, this invention provides an epitaxial growth method for iron telluride thin films with controllable crystal phase structure and magnetic properties. Based on pulsed laser deposition technology, tetragonal or hexagonal FeTe thin films are prepared by precisely controlling process parameters such as substrate selection and growth temperature. The aim is to obtain FeTe thin films with two crystal phases, one with a single composition, controllable phase structure, and excellent magnetoelectric properties. This achieves precise tuning of magnetic properties according to crystal structure. Furthermore, the thin film preparation method is simple, highly controllable, and has good repeatability.

[0006] The epitaxial growth method for controlling the crystal structure and magnetic properties of iron telluride thin films according to the present invention includes the following steps:

[0007] S1: The pretreated substrate is placed on the heating stage of the pulsed laser deposition apparatus, and the FeTe polycrystalline target is placed on the target stage of the pulsed laser deposition apparatus and rotated at a constant speed by the rotating mechanism.

[0008] S2: Vacuum treatment is performed inside the cavity of the pulsed laser deposition device;

[0009] S3: Heat the substrate to a preset temperature and maintain it at a constant temperature;

[0010] S4: A KrF excimer laser is used to irradiate the target surface to generate a high-temperature plasma plume. The plume migrates towards the substrate along the direction perpendicular to the target surface and deposits to form a FeTe thin film.

[0011] S5: After deposition is completed, maintain the vacuum level of the pulsed laser deposition device cavity, and slowly lower the substrate product with the deposited FeTe thin film to room temperature at a specific rate. The oriented FeTe thin film can then be obtained by taking it out.

[0012] The target and the substrate are arranged face-to-face at a specific distance; the thickness of the FeTe film is proportional to the deposition time in step S4; the substrate is either an MgO substrate or an Al2O3 substrate with a crystal structure that matches the FeTe crystal structure; the MgO substrate has a tetragonal lattice structure with a lattice constant close to that of tetragonal FeTe, and a tetragonal FeTe film is deposited thereon; the Al2O3 substrate has a hexagonal lattice structure with a lattice constant close to that of hexagonal FeTe, and a hexagonal FeTe film is deposited thereon.

[0013] Preferably, in step S2, the chamber is evacuated to 5×10⁻⁶ using a pump unit. -7 mbar ~ 5×10 -6 mbar.

[0014] Preferably, when the substrate is MgO, the substrate is heated to 290-310°C and held at that temperature for 5-10 minutes.

[0015] Preferably, when the substrate is Al2O3, the substrate is heated to 590-610°C and held at that temperature for 5-10 minutes.

[0016] Preferably, the KrF excimer laser has a wavelength of 248 nm, a laser energy of 250–270 mJ, and a pulse frequency of 2 Hz.

[0017] Preferably, the target stage of the pulsed laser deposition apparatus rotates at a constant speed of 5 r / min, and the distance between the target and the substrate is 5 to 7 cm.

[0018] Preferably, in step S1, the substrate pretreatment includes ultrasonically cleaning the substrate sequentially with acetone, anhydrous ethanol, and deionized water for 5-10 minutes each, followed by drying with nitrogen.

[0019] Preferably, the pulsed laser deposition apparatus is provided with a focusing lens, and the KrF excimer laser is focused by the focusing lens and irradiates the target surface at an angle of 40 to 50°.

[0020] Preferably, the substrate formed by depositing the FeTe thin film is slowly cooled to room temperature at a rate of 1°C / min.

[0021] This invention also provides an iron telluride thin film, prepared using the above-described epitaxial growth method. The film is a single-phase FeTe thin film grown by directional epitaxy, wherein: when deposited on an MgO substrate at a deposition temperature of 290–310°C, it is a tetragonal FeTe thin film with antiferromagnetism at a Nell temperature of approximately 70 K; when deposited on an Al2O3 substrate at a deposition temperature of 590–610°C, it is a hexagonal FeTe thin film with out-of-plane easy-axis ferromagnetism at a Curie temperature of approximately 220 K.

[0022] Compared with the prior art, the beneficial effects of the present invention are:

[0023] This invention provides an epitaxial growth method for iron telluride thin films with controllable crystal phase structure and magnetic properties. By precisely matching the substrate crystal symmetry with the growth temperature window, the method achieves the directional selective preparation of tetragonal and hexagonal FeTe thin films. Compared to existing CVD methods, which rely on specific precursors, generate harmful gases, and are difficult to prepare large-area epitaxial films, this invention offers a simple process, requires no special precursors, provides highly controllable growth, and exhibits good repeatability. The resulting films have high crystallinity, uniform composition, and controllable thickness. The two crystal phases have significantly different magnetic properties, providing a material selection for antiferromagnetic pinning layers and ferromagnetic storage media in spintronic devices, effectively promoting the research and application in low-dimensional magnetic devices, high-density magnetic storage, and spin valves. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the pulsed laser deposition instrument used in the method of the present invention.

[0025] Figure 2 This is a schematic diagram illustrating the steps of the epitaxial growth method for iron telluride thin films according to the present invention.

[0026] Figure 3 This is a schematic diagram of the two-phase crystal structure of the iron telluride thin film prepared by the method of the present invention.

[0027] Figure 4 The XRD and Raman characterization results are for the tetragonal FeTe thin film prepared in Example 1 of this invention.

[0028] Figure 5 The results are RT and Hall test results of the tetragonal FeTe thin film prepared in Example 1 of this invention.

[0029] Figure 6 The results show the SQUID magnetic characterization of the tetragonal FeTe thin film prepared in Example 1 of this invention.

[0030] Figure 7 The XRD and Raman characterization results are for the hexagonal FeTe thin film prepared in Example 2 of this invention.

[0031] Figure 8 The results are RT and Hall test results of the hexagonal FeTe thin film prepared in Example 2 of this invention.

[0032] Figure 9 The results show the SQUID magnetic characterization of the hexagonal FeTe thin film prepared in Example 2 of this invention. Detailed Implementation

[0033] To further illustrate the technical solution of the present invention, preferred embodiments are described below in conjunction with examples. However, it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, and not for limiting the scope of the claims. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0034] like Figure 1 As shown, during system operation, the FeTe polycrystalline target is placed on the target stage 1 of the pulsed laser deposition device and rotated at a uniform speed by the rotating mechanism 2 to ensure uniform laser ablation. The substrate is fixed on the heating stage 3 and heated to a predetermined temperature by the heating device 4 and then kept at that temperature. The target and the substrate are arranged face-to-face with a distance of 5-7 cm, with the substrate on top and the target on the bottom. The KrF excimer laser is focused by the focusing lens 5 and irradiates the target surface at an angle of 40-50°, generating a high-temperature plasma plume (orange-red). This plume migrates upwards along the direction perpendicular to the target surface and deposits to form a FeTe thin film. The vacuum chamber evacuation port 6 is connected to a molecular pump and a mechanical pump to control the vacuum level of the vacuum chamber 7. The observation window 8 is used for visual observation of the deposition process.

[0035] The epitaxial growth method for controlling the crystal structure and magnetic properties of the iron telluride thin film includes the following steps:

[0036] S1: The pretreated substrate is placed on the heating stage of the pulsed laser deposition apparatus, and the FeTe polycrystalline target is placed on the target stage of the pulsed laser deposition apparatus and rotated at a constant speed by the rotating mechanism.

[0037] S2: Vacuum treatment is performed inside the cavity of the pulsed laser deposition device;

[0038] S3: Heat the substrate to a preset temperature and maintain it at a constant temperature;

[0039] S4: A KrF excimer laser is used to irradiate the target surface to generate a high-temperature plasma plume. The plume migrates towards the substrate along the direction perpendicular to the target surface and deposits to form a FeTe thin film.

[0040] S5: After deposition is completed, maintain the vacuum level of the pulsed laser deposition device cavity, and slowly lower the substrate product with the deposited FeTe thin film to room temperature at a specific rate. The oriented FeTe thin film can then be obtained by taking it out.

[0041] The target and the substrate are arranged face-to-face at a specific distance; the thickness of the FeTe film is proportional to the deposition time in step S4; the substrate is a MgO substrate or an Al2O3 substrate whose crystal structure matches that of FeTe.

[0042] It should be noted that, in order to achieve the directional and controllable growth of tetragonal and hexagonal FeTe films, the key process parameters have been optimized in this invention, and the mechanism of action is as follows:

[0043] This invention achieves phase selection by matching the substrate crystal structure with the FeTe crystal phase. The substrate can be MgO or Al2O3. The MgO substrate has a tetragonal lattice structure, and its lattice constant is close to that of the tetragonal FeTe phase; the Al2O3 sapphire substrate has a hexagonal lattice structure, and its lattice constant is close to that of the hexagonal FeTe phase. High matching degree helps reduce interfacial stress and suppress lattice mismatch, thereby improving the crystallinity quality of the prepared thin film and ensuring the stability of the thin film during subsequent device fabrication.

[0044] The vacuum degree for thin film preparation is preferably 5 × 10⁻⁶. -7 mbar: On the one hand, the high vacuum environment eliminates gases such as O2 and H2O, avoiding the oxidation of Fe and Te; on the other hand, the very few gas molecules in the cavity reduce the scattering and collision of the plasma plume generated by the laser on the target surface during its migration to the substrate.

[0045] The target rotates at a constant speed of 5 r / min: the uniform rotation of the target ensures that the laser beam acts evenly on the target surface, avoiding local over-bombardment that could form grooves and improving the lifespan of the target; at the same time, since the laser is focused on a fixed point and the continuously rotating target constantly provides new surfaces, the composition of the plasma plume generated at each moment is uniform, thus ensuring that the composition of the prepared epitaxial film remains uniform.

[0046] After deposition, the film is slowly cooled at a rate of 1℃ / min: slow cooling can release the stress inside the thin film crystal, avoid the damage to the microstructure of the thin film caused by rapid cooling, and at the same time, the cooling process provides sufficient relaxation time for the atoms, thereby optimizing the lattice arrangement inside the FeTe thin film.

[0047] Example 1: PLD growth of tetragonal FeTe thin films

[0048] like Figure 2 As shown, the epitaxial growth method for iron telluride thin films of the present invention mainly includes the following steps:

[0049] Step 1, Substrate Preparation: MgO substrate was selected and ultrasonically cleaned sequentially in acetone, anhydrous ethanol, and deionized water for 10 minutes each. After cleaning, it was dried with nitrogen gas. The treated substrate was placed into the pulsed laser deposition cavity, and the cavity pressure was first evacuated to 1×10⁻⁶ using a mechanical pump. -2 mbar, then turn on the molecular pump to further evacuate the vacuum until it reaches 5 × 10 mbar. -7 mbar.

[0050] Step 2, substrate heating: Heat the MgO substrate to 300℃ and hold for 10 min.

[0051] Step 3, Pulsed laser deposition of thin film: Maintain the vacuum and isothermal conditions of Steps 1 and 2, use a KrF excimer laser with a wavelength of 248 nm, focus the laser onto the FeTe polycrystalline target in the cavity through a lens, adjust the angle between the target and the laser beam to 40°, set the laser energy to 260 mJ, and the laser pulse frequency to 2 Hz, and start the deposition of the thin film. The deposition time is 60 min. During the process, control the FeTe target to rotate at a constant speed of 5 r / min, and the target-substrate distance is 5-7 cm.

[0052] Step 4, Obtain the finished product: After deposition, maintain the vacuum level in the vacuum chamber and slowly lower the finished product to room temperature at a rate of 1℃ / min. The directionally grown tetragonal FeTe film can then be obtained.

[0053] The structure of the prepared tetragonal FeTe thin film was characterized by X-ray diffraction (XRD) and Raman spectroscopy, as follows: Figure 4 As shown, the characterization results indicate that the film is a pure tetragonal FeTe film with no impurity phases, and the sharp characteristic peaks indicate good crystallinity.

[0054] Furthermore, electrical transport tests were conducted using a comprehensive low-temperature, high-magnetic-field physical property measurement system. For example... Figure 5 As shown, the resistance-temperature (RT) characteristic curve exhibits a distinct inflection point at 70 K, indicating that this temperature accompanies the intrinsic antiferromagnetic phase transition of tetragonal FeTe, consistent with existing research. The Hall test shows a linear characteristic, and the Hall coefficient changes sign near 70 K, which also stems from the antiferromagnetic phase transition of the thin film.

[0055] Furthermore, the thin film was magnetically characterized using a superconducting quantum interference device (SQUID). For example... Figure 6 As shown, the in-plane magnetization intensity depends on temperature, indicating a clear inflection point at 70 K, which corresponds to the Nell temperature of tetragonal FeTe.

[0056] The above test results demonstrate that the FeTe thin film with a Nair temperature of 70 K and exhibiting antiferromagnetism at low temperatures was successfully prepared using the method provided by this invention. Furthermore, the thin film has high crystallinity and can be used as an antiferromagnetic nailing layer in devices such as spin valves.

[0057] Example 2: PLD growth of hexagonal FeTe thin films

[0058] The only difference from Example 1 is that:

[0059] Step 1, Substrate Preparation: An Al2O3 substrate was selected and sequentially ultrasonically cleaned in acetone, anhydrous ethanol, and deionized water for 10 minutes each. After cleaning, it was dried with nitrogen gas. The treated substrate was then placed into the pulsed laser deposition cavity, and the cavity pressure was first evacuated to 1×10⁻⁶ using a mechanical pump. -2 mbar, then turn on the molecular pump to further evacuate the vacuum until it reaches 5 × 10 mbar. -7 mbar.

[0060] Step 2: Substrate heating: Heat the Al2O3 substrate to 600℃ and hold for 10 min.

[0061] The remaining steps are the same as in Example 1.

[0062] The same testing protocol as in Example 1 was used to characterize the obtained hexagonal FeTe thin film structurally, perform electrical transport tests, and conduct magnetic characterization. Figure 7 As shown, X-ray diffraction and Raman spectroscopy results indicate that the film is a pure hexagonal FeTe film with no impurities and high crystallinity.

[0063] Furthermore, in the electrical transmission test results, such as Figure 8 As shown, the RT curve of the prepared hexagonal FeTe thin film exhibits monotonicity, indicating that the film possesses metallic conductivity, and an inflection point appears near 220 K, corresponding to the Curie temperature of the hexagonal FeTe. The Hall test results show a typical anomalous Hall hysteresis loop shape, indicating that the film has good out-of-plane easy-axis ferromagnetism.

[0064] Furthermore, the dependence of magnetization on an external magnetic field (MH) was tested using a superconducting quantum interference device at 20 K. Figure 9 As shown, the results also exhibit hysteresis loop characteristics, further demonstrating that the thin film prepared at low temperature has good ferromagnetism.

[0065] The above test results demonstrate that the FeTe thin film with a Curie temperature of 220 K and exhibiting out-of-plane easy axis ferromagnetism at low temperatures was successfully prepared using the method provided by this invention. Furthermore, the thin film has high crystallinity and can be used as a magnetic storage medium.

[0066] It should be noted that, Figures 3-9 The structural and physical property characterization shown was achieved using a conventional X-ray diffractometer, Raman spectrometer, low-temperature high magnetic field integrated physical property measurement system, and superconducting quantum interference device. The standard test conditions used in the tests were all conventional settings in this field.

[0067] The above tests and characterizations confirm that the tetragonal and hexagonal FeTe films prepared by the method of this invention both possess the characteristics of single crystalline phase, high crystallinity, and uniform composition. By controlling the substrate type and growth temperature, precise switching of magnetic properties can be achieved, providing an effective material preparation method for the development of spintronic devices and magnetic storage media.

[0068] Meanwhile, all raw materials or reagents mentioned above that are not detailed are commercially available products, and all process steps or methods not mentioned in detail are process steps or methods known to those skilled in the art; the MgO and Al2O3 substrates used are commercial single-crystal substrates.

[0069] The present invention and its embodiments have been described above illustratively. This description is not restrictive, and the figures shown are only one embodiment of the present invention; the actual structure is not limited thereto. Therefore, if those skilled in the art are inspired by this description and design similar structures and embodiments without departing from the spirit of the present invention, such designs should fall within the protection scope of the present invention.

Claims

1. A method for the epitaxial growth of iron telluride thin films with controllable crystal phase structure and magnetic properties, characterized in that, Includes the following steps, S1: The pretreated substrate is placed on the heating stage of the pulsed laser deposition apparatus, and the FeTe polycrystalline target is placed on the target stage of the pulsed laser deposition apparatus and rotated at a constant speed by the rotating mechanism. S2: Vacuum treatment is performed inside the cavity of the pulsed laser deposition device; S3: Heat the substrate to a preset temperature and maintain it at a constant temperature; S4: A KrF excimer laser is used to irradiate the target surface to generate a high-temperature plasma plume. The plume migrates towards the substrate along the direction perpendicular to the target surface and deposits to form a FeTe thin film. S5: After deposition is completed, maintain the vacuum level of the pulsed laser deposition device cavity, and slowly lower the substrate product with the deposited FeTe thin film to room temperature at a specific rate. The oriented FeTe thin film can then be obtained by taking it out. The target and the substrate are arranged face-to-face at a specific distance; the thickness of the FeTe film is proportional to the deposition time in step S4; the substrate is either an MgO substrate or an Al2O3 substrate with a crystal structure that matches the FeTe crystal structure; the MgO substrate has a tetragonal lattice structure with a lattice constant close to that of tetragonal FeTe, and a tetragonal FeTe film is deposited thereon; the Al2O3 substrate has a hexagonal lattice structure with a lattice constant close to that of hexagonal FeTe, and a hexagonal FeTe film is deposited thereon.

2. The epitaxial growth method for controlling the crystal phase structure and magnetic properties of iron telluride thin films according to claim 1, characterized in that, In step S2, the chamber is evacuated to 5×10⁻⁶ using a pump unit. -7 mbar ~ 5×10 -6 mbar.

3. The epitaxial growth method for controlling the crystal phase structure and magnetic properties of iron telluride thin films according to claim 1, characterized in that, When the substrate is MgO, the substrate is heated to 290-310°C and held at that temperature for 5-10 minutes.

4. The epitaxial growth method for controlling the crystal phase structure and magnetic properties of iron telluride thin films according to claim 1, characterized in that, When the substrate is Al2O3, the substrate is heated to 590-610°C and held at that temperature for 5-10 minutes.

5. The epitaxial growth method for controlling the crystal phase structure and magnetic properties of iron telluride thin films according to claim 1, characterized in that, The KrF excimer laser has a wavelength of 248 nm, a laser energy of 250–270 mJ, and a pulse frequency of 2 Hz.

6. The epitaxial growth method for controlling the crystal structure and magnetic properties of iron telluride thin films according to claim 1, characterized in that, The target stage of the pulsed laser deposition apparatus rotates at a constant speed of 5 r / min, and the distance between the target and the substrate is 5-7 cm.

7. The epitaxial growth method for controlling the crystal phase structure and magnetic properties of iron telluride thin films according to claim 1, characterized in that, In step S1, the substrate pretreatment includes ultrasonically cleaning the substrate with acetone, anhydrous ethanol, and deionized water for 5-10 minutes each, and then drying it with nitrogen.

8. The epitaxial growth method for controlling the crystal phase structure and magnetic properties of iron telluride thin films according to claim 1, characterized in that, The pulsed laser deposition apparatus is equipped with a focusing lens, and the KrF excimer laser is focused by the focusing lens and irradiates the target surface at an angle of 40 to 50°.

9. The epitaxial growth method for controlling the crystal structure and magnetic properties of iron telluride thin films according to claim 1, characterized in that, The substrate formed by depositing the FeTe thin film is slowly cooled to room temperature at a rate of 1°C / min.

10. An iron telluride thin film prepared by any one of the methods described in claims 1-9, characterized in that, The thin film is a single-phase FeTe thin film grown by directional epitaxy, wherein: When deposited on an MgO substrate at a deposition temperature of 290–310 °C, it forms a tetragonal FeTe thin film with antiferromagnetism at a Nell temperature of around 70 K. When deposited on an Al2O3 substrate at a deposition temperature of 590–610 °C, it forms a hexagonal FeTe thin film with out-of-plane easy-axis ferromagnetism at a Curie temperature of around 220 K.

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