Preparation method of tungsten ditelluride film based on temporary Van der Waals buffer layer

Abstract

The invention discloses a preparation method of a tungsten ditelluride film based on a temporary Van der Waals buffer layer. The method comprises the following steps: growing a WTe2 thin film by adopting a low-temperature co-deposition method, annealing to form a Bi2Te3 / WTe2 vertical heterojunction, continuously raising the temperature, annealing to gradually decompose a Bi2Te3 buffer layer until the Bi2Te3 buffer layer disappears, and finally forming the high-quality WTe2 thin film on a substrate. The Bi2Te3 film serves as a temporary buffer layer to provide an atomic-scale flat surface for WTe2 growth, and compared with a WTe2 film directly grown on a substrate, the method has the advantages that the quality is excellent, the coverage degree is high and continuous, the thickness is uniform, the orientation is ordered, and no stress exists, so that the method has obvious advantages when the Bi2Te3 is used as the buffer layer to grow the WTe2 film. The method can be popularized to growth of a thin film material which is poor in quality and grows on a buffer layer directly, and favorable conditions are created for high-quality preparation and large-scale application of the thin film.

Description

Technical Field

[0001] The present invention belongs to the field of condensed matter physics, and specifically relates to a method for epitaxially growing a high-quality, large-area, uniformly thick 1T'-WTe2 film (WTe2 film for short) by constructing a temporary van der Waals buffer layer. Background Art

[0003] With the advancement of integrated circuit technology, the size of electronic devices has approached the physical limit. The electrons on both sides of the insulating layer at the extreme thickness will experience quantum tunneling, which will affect the transmission of information. There are edge states protected by time reversal symmetry at the edge of the thin film of the two-dimensional topological insulator, and the transmission of electrons at its edge is not easily disturbed. The key to the practical application of two-dimensional topological insulators is that the body band gap is greater than the thermal excitation energy corresponding to room temperature (26meV). At present, experiments have confirmed that the body band gap of WTe2 thin film can reach 70meV. Therefore, optimizing film growth, achieving high-quality, large-area, uniformly covered single-layer films, and increasing the operating temperature of the quantum spin Hall effect are important links in applying topological devices to real life.

[0004] WTe2 is a Weyl semimetal belonging to transition metal chalcogenides (TMDs). It has huge unsaturated magnetoresistance, superconductivity, ferroelectric reversal, nonlinear Hall effect and other phenomena that have attracted widespread attention from scientists. The films grown by molecular beam epitaxy are of high purity and the thickness and composition of the films can be precisely controlled. However, the high melting point of transition metals makes their fluidity when grown on substrates low. At the optimal growth temperature (above 250°C), the adhesion of the film to the substrate is not high. The low mobility of W and the low reaction rate of W and Te elements can easily lead to the formation of W clusters, which in turn affects the continuity of the film. Growing a single-layer WTe2 film by molecular beam epitaxy is a huge challenge, so a new method that can achieve large-area uniform thickness films is urgently needed. Summary of the invention

[0006] The purpose of the present invention is to address the shortcomings of the prior art and to solve the quality problem of WTe2 thin films, and to provide a new method for preparing large-area uniform thickness thin films based on a temporary van der Waals buffer layer. The purpose of introducing Bi2Te3 is to provide a buffer layer without dangling bonds to increase the reaction rate of W and Te and reduce the influence of substrate lattice mismatch, and a low-temperature deposition method is used to increase the coverage, and a high-quality single-layer WTe2 thin film with almost full coverage and uniform thickness is successfully prepared.

[0007] The technical solution adopted by the present invention to solve the above-mentioned problems is: A method for preparing a tungsten ditelluride film based on a temporary van der Waals buffer layer, characterized in that a Bi2Te3 film is epitaxially grown on a substrate surface, and a 1T'-WTe2 film is epitaxially grown on the film surface, and the specific steps are as follows: Step 1: Substrate treatment: Place the substrate in acetone and deionized water for ultrasonic cleaning for 5-10 minutes, blow dry with nitrogen, place it in the rapid injection chamber of the molecular beam epitaxy equipment, and then transfer it into the preparation chamber, use DC heating to degas to 200-600°C for 2-12 hours; then anneal to 400-900°C for 1-2 hours to obtain a substrate with reconstruction and uniform step distribution; Step 2: Growth of Bi2Te3 buffer layer: Bi and Te are used as evaporation sources to grow Bi2Te3 buffer layer on the treated substrate at room temperature, and the substrate temperature is controlled to 240~250℃ for annealing for 5~10min; Step 3: Growth of 1T'-WTe2 thin film: Using W and Te as evaporation sources, control the substrate temperature to -196~-186℃ on the Bi2Te3 buffer layer to grow 1T'-WTe2 thin film, then raise the temperature to 250~375℃ and anneal for 1~2h to obtain a 1T'-WTe2 thin film with a uniform thickness of 4mm2~12mm2, referred to as WTe2 thin film.

[0008] Furthermore, the vacuum degree of the injection chamber is 1×10-6 ~ 1×10-8 Torr, and when the vacuum degree is better than 1×10-7 Torr, the sample is introduced into the preparation chamber, and the vacuum degree of the preparation chamber is 1×10-9 ~ 1×10-11 Torr.

[0009] Furthermore, the substrate described in step 1 is STO (111), Graphene, Mica or Al2O3.

[0010] Furthermore, in step 2, Bi and Te are used as evaporation sources, and both Bi and Te are K-cell evaporation sources; the Bi beam current is 0.70 ~ 0.90 Å / min, and the Te beam current is 1.10 ~ 1.30 Å / min; the growth time of the Bi2Te3 buffer layer is 10 ~ 15 min, the growth vacuum is 5×10-9 ~ 1×10-11 Torr, and the annealing is carried out under a Te beam current of >9Å / min.

[0011] Furthermore, the Bi2Te3 buffer layer in step 2 has a thickness of 2 to 4 nm.

[0012] Furthermore, in step 3, W and Te are used as evaporation sources, wherein the W evaporation source is an electron beam evaporation source, and the Te evaporation source is a K-cell evaporation source; the W beam current is 0.10 ~ 0.20 Å / min, and the Te beam current is 1.50 ~ 1.90 Å / min; the growth time of the 1T'-WTe2 film is 7 ~ 20 min, the growth vacuum is 5×10-9 ~ 1×10-11 Torr, and the annealing is carried out under a Te beam current of >12Å / min.

[0013] Furthermore, the 1T'-WTe2 film described in step 3 has a thickness of 0.7 ~1.4 nm.

[0014] Compared with the prior art, the present invention has the following advantages: First, molecular beam epitaxial growth technology is used, and van der Waals epitaxy is adopted, which has low requirements for lattice matching. The probability of stress or dislocation at the interface of highly lattice-mismatched heterostructures is greatly reduced. The film growth can be precisely controlled to prepare high-quality 1T'-WTe2 films with precise chemical composition.

[0015] Second, the low-temperature co-deposition method is used to grow 1T'-WTe2 thin films to achieve large-area uniform thickness film growth, greatly improving the coverage of the film.

[0016] Third, the Bi2Te3 buffer layer provides a very flat surface, which enables the WTe2 film to evenly cover its surface before annealing, improving the reaction rate between W and Te and reducing the generation of W clusters.

[0017] Fourth, a Bi2Te3 / WTe2 vertical heterojunction is constructed, in which Bi2Te3 acts as a temporary van der Waals buffer layer, which then decomposes and disappears during the annealing process of WTe2. This method can be applied to the growth of thin film materials with poor growth quality on a certain substrate but better growth quality on the buffer layer, laying a good foundation for the high-quality preparation and large-scale application of thin films. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 A schematic diagram of the structure of the 1T'-WTe2 epitaxial film provided in Example 1, Example 2, and Example 3 of the present invention; Figure 2 Reflection high-energy electron diffraction images of the STO (111) substrate provided in Example 1 of the present invention and step (1) of the comparative example before degassing and after annealing; Figure 3 Large-scale scanning tunneling microscope surface image and height distribution of the STO (111) substrate provided in Example 1 of the present invention and step (1) of the comparative example; Figure 4Reflection high-energy electron diffraction images of the Bi2Te3 buffer layer before and after annealing provided in step (2) of Example 1 of the present invention; Figure 5 A large-scale scanning tunneling microscope surface image of the Bi2Te3 buffer layer provided in step (2) of Example 1 of the present invention; Figure 6 Reflection high-energy electron diffraction images of the 1T'-WTe2 thin film grown at low temperature with a temporary buffer layer provided in step (3) of Example 1 of the present invention before and after annealing; Figure 7 Scanning tunneling microscope surface image and height distribution of the 1T'-WTe2 thin film grown at low temperature with a temporary buffer layer provided in step (3) of Example 1 of the present invention; Figure 8 This is the scanning tunneling spectrum of the 1T'-WTe2 thin film grown at low temperature with a temporary buffer layer provided in step (3) of Example 1 of the present invention.

[0019] Fig. 9 Reflection high-energy electron diffraction images of the 1T'-WTe2 thin film grown without a temporary buffer layer provided in step (2) of the comparative example of the present invention before and after annealing; Fig.10 Scanning tunneling microscope surface image of the 1T'-WTe2 thin film grown without a temporary buffer layer provided in step (2) of the comparative example of the present invention; Fig.11 This is the scanning tunneling spectrum of the 1T'-WTe2 thin film grown without a temporary buffer layer provided in step (2) of the comparative example of the present invention. Specific implementation methods

[0020] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments to facilitate a better understanding of the present invention, but the present invention is not limited to the following embodiments. Example

[0021] A method for preparing a tungsten ditelluride film based on a temporary van der Waals buffer layer, the specific steps are as follows: (1) Select the STO (111) substrate provided by Hefei Kejing Company, ultrasonicate the substrate in acetone for 5 minutes and in deionized water for 10 minutes; blow dry with dry nitrogen and then transfer it into the fast injection chamber. When the vacuum degree of the injection chamber is better than 1×10-7 Torr, transfer it into the preparation chamber for degassing and annealing. Control the substrate to degas at 550℃ for 12 hours, wait for the environment to restore the back vacuum, heat it to 820℃ and keep it for 60 minutes to obtain (√13x√13) surface reconstruction. The step height of the STO (111) substrate with reconstruction and uniform step distribution is 0.225 nm, and the surface roughness is about 100 pm.

[0022] See also Figure 2 Reflection high-energy electron diffraction images of the STO (111) substrate provided for this step before degassing (a) and after annealing (b). The diffraction fringes show that (√13x√13) reconstruction occurs on the substrate surface. Compared with before degassing, the diffraction fringes are brighter and sharper.

[0023] See also Figure 3 The large-scale scanning tunneling microscope surface image (a) and height distribution (b) of the STO (111) substrate provided for this step show that the substrate has an atomically flat surface and uniformly distributed steps. The area of ​​the illustrated area is 600nm×600nm, the step height is 0.225~0.9nm, and the roughness is about 100pm.

[0024] (2) Growth of Bi2Te3 buffer layer, Bi beam current is 0.84Å / min, Te beam current is 1.225Å / min, The growth vacuum was 2.8×10-10 Torr, the growth time was 11 min, and then the substrate temperature was controlled to 240°C. Annealing was performed at a high Te beam current (>9Å / min) for 5 min to obtain a Bi2Te3 film with a thickness of about 2 nm.

[0025] See also Figure 4 Reflection high-energy electron diffraction images of the Bi2Te3 buffer layer before (a) and after (b) annealing provided for this step. The sharp stripes in the images indicate that the grown Bi2Te3 buffer layer film has good quality and high crystallinity.

[0026] See also Figure 5 The large-scale scanning tunneling microscope surface image of the Bi2Te3 buffer layer provided for this step shows an area of ​​800nm×800nm. The Bi2Te3 film is continuous and defect-free. The film surface is very flat, with a surface undulation of less than 80pm.

[0027] (3) Growth of 1T'-WTe2 thin film, W beam current is 0.11Å / min, Te beam current is 1.65Å / min, The growth vacuum was 4.1×10-10 Torr, the growth time was 12 min, and then the substrate temperature was controlled to 250°C for annealing for 20 min under high Te beam current (>12Å / min), and then the temperature was raised to 300°C for annealing for 30 min, and finally maintained at 375°C for 60 min. After completing the entire annealing process, a 1T'-WTe2 film with a thickness of about 0.7 nm was obtained.

[0028] See also Figure 1 This is a schematic diagram of the structure of the 1T'-WTe2 epitaxial film provided in this embodiment. The Bi2Te3 / WTe2 film grown at low temperature is annealed. After annealing, the Bi2Te3 film gradually decomposes and disappears.

[0029] See also Figure 6 The reflection high-energy electron diffraction images of the 1T'-WTe2 film grown at low temperature with a temporary buffer layer provided in this step before (a) and after (b) annealing. There is no obvious diffraction pattern before annealing, indicating that WTe2 has not crystallized to form a film; after annealing, the diffraction fringes change significantly, indicating that it has formed a high-quality and high-crystallinity film.

[0030] See also Figure 7 Scanning tunneling microscope surface image and height distribution of 1T'-WTe2 thin film grown at low temperature with a temporary buffer layer provided for this step. Figure 7 (a) is a large-scale image, and the illustrated area is 300nm×300nm. The thin film grown by this method has high coverage and is continuous, with a smooth surface and no defects. Figure 7 (b) is the first-order differential diagram of the film. The area of ​​the diagram is 50nm×50nm. It can be clearly observed that the film is oriented in an orderly manner, consisting of three domains oriented at 120° and without stress. Figure 7 (c) shows the height distribution of the film. The film height is about 7.0Å and the thickness is uniform, with almost no W clusters or other crystalline phases generated.

[0031] See also Figure 8 The scanning tunneling spectrum of the 1T'-WTe2 film grown at low temperature with a temporary buffer layer provided for this step. The differential conductivity shows the distribution of the localized electronic state density of the film. When the Bi2Te3 buffer layer exists, WTe2 has a large band gap. After the Bi2Te3 buffer layer disappears after annealing, the WTe2 film has no band gap, indicating that the Bi2Te3 buffer layer will cause the band gap size of the single-layer WTe2 to change. Example

[0032] A method for preparing a tungsten ditelluride film based on a temporary van der Waals buffer layer, the specific steps are as follows: (1) Select the Al2O3 substrate provided by Hefei Kejing Company, ultrasonicate the substrate in acetone for 5 minutes and in deionized water for 5 minutes; blow dry with dry nitrogen and then transfer it into the fast injection chamber. When the vacuum degree of the injection chamber is better than 1×10-7 Torr, transfer it into the preparation chamber for degassing and annealing. Control the substrate to degas at 220℃ for 12 hours, wait for the environment to restore the back vacuum, and then heat it to 560℃ for 75 minutes to obtain a reconstructed Al2O3 substrate with uniform step distribution, and its step height is 0.22 nm.

[0033] (2) Growth of Bi2Te3 buffer layer, Bi beam current is 0.828Å / min, Te beam current is 1.23Å / min, The growth vacuum was 2.9×10-10 Torr, the growth time was 11 min, and then the substrate temperature was controlled to 240°C, and annealing was performed at a high Te beam current (>9Å / min) for 5 min to obtain a Bi2Te3 film with a thickness of about 2 nm.

[0034] (3) Growth of 1T'-WTe2 thin film, W beam current is 0.16Å / min, Te beam current is 1.66Å / min, The growth vacuum was 3.1×10-10 Torr, the growth time was 12 min, and then the substrate temperature was controlled to 250°C for annealing for 20 min under high Te beam current (>12Å / min), and then the temperature was raised to 300°C for annealing for 30 min, and finally maintained at 375°C for 60 min. After completing the entire annealing process, a 1T'-WTe2 film with a thickness of about 0.7 nm was obtained. Example

[0035] A method for preparing a tungsten ditelluride film based on a temporary van der Waals buffer layer, the specific steps are as follows: (1) Select the Mica substrate provided by Hefei Kejing Company, transfer it into the fast sample chamber after cleavage, and wait for the vacuum degree of the sample chamber to be better than 1×10-7 Torr before transferring it into the preparation chamber for degassing and annealing. Control the substrate to degas at 200℃ for 12h, wait for the environment to restore the back vacuum, and then heat it to 400℃ and keep it for 60min to obtain a reconstructed Mica substrate.

[0036] (2) Growth of Bi2Te3 buffer layer, Bi beam current is 0.83Å / min, Te beam current is 1.215Å / min, The growth vacuum was 2.0×10-10 Torr, the growth time was 11 min, and then the substrate temperature was controlled to 240°C, and annealing was performed at a high Te beam current (>9Å / min) for 5 min to obtain a Bi2Te3 film with a thickness of about 2 nm.

[0037] (3) Growth of 1T'-WTe2 thin film, W beam current is 0.142Å / min, Te beam current is 1.58Å / min, The growth vacuum was 2.5×10-10 Torr, the growth time was 19 min, and then the substrate temperature was controlled to 250°C for annealing for 20 min under high Te beam current (>12Å / min), and then the temperature was raised to 300°C for annealing for 30 min, and finally maintained at 375°C for 60 min. After completing the entire annealing process, a 1T'-WTe2 film with a thickness of about 1.4 nm was obtained.

[0038] Comparative Example The preparation method of a tungsten ditelluride film grown under low Te beam conditions without a temporary van der Waals buffer layer comprises the following specific steps: (1) Select the STO (111) substrate provided by Hefei Kejing Company, ultrasonicate the substrate in acetone for 5 minutes and in deionized water for 10 minutes; blow dry with dry nitrogen and then transfer it into the fast injection chamber. When the vacuum degree of the injection chamber is better than 1×10-7 Torr, transfer it into the preparation chamber for degassing and annealing. Control the substrate to degas at 550℃ for 12 hours, wait for the environment to restore the back vacuum, heat it to 820℃ and keep it for 60 minutes to obtain (√13x√13) surface reconstruction. The step height of the STO (111) substrate with reconstruction and uniform step distribution is 0.225 nm, and the surface roughness is about 100 pm.

[0039] (2) Directly grow 1T'-WTe2 thin film with W beam current of 0.172Å / min, Te beam current of 1.08Å / min, growth vacuum of 8.0×10-10 Torr, growth time of 9min, then control the substrate temperature to 250℃ for annealing for 20min at high Te beam current (>12Å / min), then raise the temperature to 300℃ for annealing for 30min, and after completing the entire annealing process, obtain 1T'-WTe2 thin film with a thickness of about 0.7nm.

[0040] See also Fig. 9 Reflection high-energy electron diffraction images of the 1T'-WTe2 film grown without a temporary buffer layer provided in this step before (a) and after (b) annealing. The absence of diffraction stripes before annealing indicates that WTe2 has not yet crystallized into a film, and the bright stripes after annealing indicate that it has formed a highly crystalline film.

[0041] See also Fig.10 Scanning tunnelling microscopy surface image of 1T'-WTe2 thin film grown without temporary buffer layer provided for this step. Fig.10 (a) is a large-scale image, the area of ​​the illustrated area is 200nm×200nm, and the film quality is poor. Fig.10 (b) is the first-order differential diagram of the film. The area of ​​the illustrated area is 50nm×50nm, and the domain orientation of the WTe2 film is random.

[0042] See also Fig.11 The scanning tunneling spectrum of the 1T'-WTe2 film grown without a temporary buffer layer provided in this step shows that the WTe2 film directly grown on the STO substrate has a band gap and has different band gap values ​​in different regions, which means that the directly grown film has stress and the stress is uneven in different regions.

[0043] The above is only a preferred embodiment of the present invention, which mainly describes the technical concept and features of the present invention. Its purpose is to allow people familiar with the art to understand the content of the present invention and implement it. Without departing from the creative concept of the present invention, equivalent improvements and changes to the present invention should all fall within the scope of protection of the present invention.

Claims

1. A method for preparing a tungsten ditelluride thin film based on a temporary van der Waals buffer layer, characterized in that: The specific steps are as follows: Step 1: Substrate treatment: Place the substrate in acetone and deionized water for ultrasonic cleaning for 5-10 minutes, blow dry with nitrogen, place it in the rapid injection chamber of the molecular beam epitaxy equipment, and then transfer it into the preparation chamber. Use DC heating to degas to 200-600°C for 2-12 hours; then anneal to 400-900°C for 1-2 hours to obtain a substrate with reconstruction and uniform step distribution; Step 2: Growth of Bi2Te3 buffer layer: Bi and Te are used as evaporation sources to grow Bi2Te3 buffer layer on the treated substrate at room temperature, and the substrate temperature is controlled to 240~250℃ for annealing for 5~10min; Step 3: Growth of 1T'-WTe2 thin film: Using W and Te as evaporation sources, control the substrate temperature to -196~-186℃ to grow 1T'-WTe2 thin film on the Bi2Te3 buffer layer, then raise the temperature to 250~375℃ and anneal for 1~2h to obtain a 1T'-WTe2 thin film with a uniform thickness of 4mm2~12mm2, referred to as WTe2 thin film.

2. The method for preparing a tungsten ditelluride thin film according to claim 1, characterized in that: In step 1, the vacuum degree of the injection chamber is 1×10-6 ~ 1×10-8 Torr, and when the vacuum degree is better than 1×10-7 Torr, the sample is introduced into the preparation chamber, and the vacuum degree of the preparation chamber is 1×10-9 ~ 1×10-11 Torr.

3. The method for preparing a tungsten ditelluride thin film according to claim 1, characterized in that: The substrate described in step 1 is STO (111), Graphene, Mica or Al2O3.

4. The method for preparing a tungsten ditelluride thin film according to claim 1, characterized in that: In step 2, Bi and Te are used as evaporation sources, and both Bi and Te are K-cell evaporation sources; the Bi beam current is 0.70 ~ 0.90 Å / min, and the Te beam current is 1.10 ~1.30 Å / min; the growth time of the Bi2Te3 buffer layer is 10 ~ 15 min, the growth vacuum is 5×10-9 ~1×10-11 Torr, and the annealing is carried out under a Te beam current of >9Å / min.

5. The method for preparing a tungsten ditelluride thin film according to claim 1, characterized in that: The Bi2Te3 buffer layer described in step 2 has a thickness of 2~4nm.

6. The method for preparing a tungsten ditelluride thin film according to claim 1, characterized in that: In step 3, W and Te are used as evaporation sources, wherein the W evaporation source is an electron beam evaporation source, and the Te evaporation source is a K-cell evaporation source; the W beam current is 0.10 ~ 0.20 Å / min, and the Te beam current is 1.50 ~ 1.90 Å / min; the growth time of the 1T'-WTe2 film is 7 ~ 20 min, the growth vacuum is 5×10-9 ~ 1×10-11 Torr, and the annealing is carried out under a Te beam current of >12Å / min.

7. The method for preparing a tungsten ditelluride thin film according to claim 1, characterized in that: The 1T'-WTe2 film described in step 3 has a thickness of 0.7 ~1.4 nm.

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