A near-normal-pressure growth method for TMDC materials and MBE device

Abstract

The present invention discloses a near-normal-pressure growth method for TMDC materials and an MBE device for near-normal-pressure growth of TMDC materials. The near-normal-pressure growth of TMDC materials in the MBE device comprises the following steps: S1: introducing ambient gas to achieve an adjustable gas atmosphere at near-normal pressure within the cavity; S2: using a fiber-coupled laser heater to heat the target substrate to the target material growth temperature; S3: opening the baffle of the dual-temperature-zone thermal evaporation source, then turning on the excimer laser, and using a 248nm KrF excimer pulse laser to bombard a transition metal element target material to achieve near-normal-pressure growth of the TMDC material, and using a two-stage differential reflection high-energy electron diffractometer for real-time monitoring. The present invention provides a new approach to preparing TMDC materials, improves the main structure of the MBE device, and adds two new dimensions of thin film growth control: pressure and atmosphere, thereby achieving near-normal-pressure growth of TMDC materials.

Description

Technical Field

[0001] The present invention relates to the field of TMDC material preparation, and more particularly to a near-normal-pressure growth method for TMDC materials and an MBE device for near-normal-pressure growth of TMDC materials. Background Art

[0002] Since the great success of graphene research, it has stimulated the development of various types of two-dimensional (2D) atomic crystals. One important example is a class of layered transition metal dichalcogenides (TMDCs). TMDCs have a variety of unique electrical, optical, chemical and mechanical properties, making them a popular material for the development of a new generation of microelectronics based on topological effects. TMDCs include a large class of layered materials, whose chemical formula can generally be represented by MX2, where M can be a transition metal of group IV, group V, group VI, etc., and X represents a chalcogen element. Because TMDC materials have rich atomic combinations and rich phases, including 1T, 2H, 3R phases and T d , T' phase, and the material types include semiconductors (such as MoS2, WSe2), metals (such as 1T-MoTe2) and superconductors (such as NbS2, NbSe2), etc., so it has attracted widespread research attention. On the other hand, TMDC crystals have a variety of rich intrinsic defects, such as vacancies, adsorbed atoms, grain boundaries and substitutional impurities. The existence of these defects has a significant impact on the optical and electrical properties of TMDCs, making the development of various defect engineering strategies crucial for realizing materials with new properties and high-performance devices. The rational design of defects can promote carrier transport, and the induction of effective doping can modulate the energy band and phase structure in TMDCs, providing a new platform for the design of new electronic and optoelectronic devices. On this basis, due to the nature of weak van der Waals interlayer interactions, these materials provide unique opportunities for establishing high-quality and atomically sharp heterostructures.

[0003] Developing reliable methods for preparing high-quality van der Waals layered TMDCs is the key to advancing the basic research and application of these materials. Current preparation methods include "top-down" mechanical exfoliation transfer method and thin film transfer method (wet transfer) and "bottom-up" thin film preparation method. The "top-down" mechanical exfoliation transfer method and thin film transfer method (wet transfer) for preparing TMDC materials are relatively simple and are not limited to a specific selected substrate, but have disadvantages such as small transfer sample size, low repeatability, and inability to produce on a large scale. At the same time, problems such as weak interface coupling and interface contamination may affect subsequent related research on TMDC materials. In comparison, the "bottom-up" thin film preparation method (vacuum equipment such as chemical vapor deposition (CVD), pulsed laser deposition (PLD), and molecular beam epitaxy (MBE)) is an alternative method for obtaining large-scale, high-purity van der Waals layered TMDCs. However, according to current reports, the growth pressure is mainly concentrated in atmospheric pressure (100kPa) and ultra-high vacuum (<10 -6 The main problem is that the background vacuum in atmospheric CVD is achieved only by mechanical pumps, or even directly connected to the atmosphere. Therefore, the low environmental cleanliness in CVD leads to inevitable impurities and defects in the prepared thin films, and the resulting product is impure. While ultra-high vacuum (UHV) MBE uses molecular pumps or ion pumps to achieve extremely high vacuum, greatly improving the cleanliness of the growth environment and reducing the introduction of impurities, the source supply method changes from directly placing a large amount of precursor material to evaporate in CVD to slowly providing source material vapor through an evaporation source in MBE. This change limits the increase in substrate temperature. High substrate temperature will lead to severe desorption of the source material, making it difficult to control the nucleation rate and making it difficult to obtain large-area single crystals. Therefore, it is urgent to explore near-ambient pressure growth atmospheres to develop a growth method for TMDC materials that overcomes these drawbacks. Summary of the Invention

[0004] The purpose of the present invention is to provide a near-normal-pressure growth method for TMDC materials and an MBE device for near-normal-pressure growth of TMDC materials, so as to overcome the limitations of the prior art methods for preparing TMDC materials.

[0005] In order to solve the above problems, the present invention adopts the following technical solutions:

[0006] According to a first aspect of the present invention, an MBE apparatus for near-normal-pressure growth of TMDC materials is provided, comprising: an MBE chamber; a five-dimensional sample stage for placing samples; a two-stage vacuum pump system for obtaining an ultrahigh vacuum background pressure through a mechanical pump and a molecular pump; a dual-temperature-zone thermal evaporation source, which employs dual temperature zones at the bottom and mouth of a crucible to increase the effective beam current of the source material; an excimer laser for bombarding a corresponding target material with a 248nm KrF excimer pulse laser; and a gas path connection port, wherein the gas path connection port connects the gas path to the MBE chamber via a leak valve, and argon, hydrogen, or oxygen can be introduced into the MBE chamber to achieve 10 -7 An adjustable inert, reducing, or oxidizing atmosphere at near-ambient pressures of Pa to 10kPa; a fiber-coupled laser heating device for localized heating of the substrate with adjustable heating and cooling rates; and a two-stage differential reflection high-energy electron diffractometer to enable real-time monitoring of the near-ambient pressure growth of TMDC samples.

[0007] Preferably, a one-dimensional linear drive is provided between the dual-temperature zone thermal evaporation source and the MBE chamber to control the distance between the thermal evaporation source port and the substrate, so as to reduce the negative effects of molecular beam free path reduction and scattering under high pressure.

[0008] According to a second aspect of the present invention, a method for near-normal-pressure growth of TMDC materials is provided, wherein near-normal-pressure growth of TMDC materials is performed in the MBE device, comprising the following steps: S1: introducing ambient gas: before introducing the ambient gas, closing the gate valve between the MBE cavity and the molecular pump to make the MBE cavity completely sealed, and the vacuum degree is <10 -9 The space is filled to ensure the cleanliness of the cavity environment, and then pure argon, hydrogen, or oxygen is introduced to achieve 10 -7 Pa ~ 10kPa near atmospheric pressure adjustable inert, reducing, or oxidizing atmosphere, when reaching the target pressure, close the leak valve to stop injection; S2: use a fiber-coupled laser heater to heat the target substrate to the corresponding TMDC material growth target temperature; S3: after the target substrate reaches the target temperature, open the baffle for providing the dual-temperature zone thermal evaporation source of chalcogen elements, then turn on the excimer laser, and use 248nm KrF excimer pulse laser to bombard the transition metal element target, so as to achieve TMDC material in 10 -7 The material is grown at near-normal pressure within the range of Pa to 10 kPa, and a two-stage differential reflection high-energy electron diffractometer is used to monitor the growth of the TMDC material in real time during the growth process.

[0009] Step S1 includes: using a capacitive film pressure gauge connected to the MBE cavity to calibrate the pressure in the MBE cavity, and closing the leakage valve when the target pressure is reached.

[0010] It should be understood that in step S2, the specific growth target temperatures of various TMDC materials vary, and the overall range is between 400 and 700°C.

[0011] Step S3 includes: before opening the baffle of the dual-temperature zone thermal evaporation source, it is necessary to preheat the dual-temperature zone thermal evaporation source to the target temperature. The crucible bottom temperature depends on the evaporation temperature of different source materials and ranges from 300 to 600°C. The crucible mouth temperature is 100°C higher than the crucible bottom temperature.

[0012] Step S3 also includes: adjusting the beam current of the source material by adjusting the pulse frequency, energy and focusing degree of the laser.

[0013] Step S3 also includes: controlling the distance between the dual-temperature zone thermal evaporation source port and the target substrate to reduce the negative effects of molecular beam free path reduction and scattering under high pressure.

[0014] Preferably, the chalcogen element is selected from any one of Se, Te and S.

[0015] Preferably, the transition metal element is selected from any one of transition metals of Group IV, Group V, Group VI, etc.

[0016] It should be understood that the near-normal-pressure MBE device used in the present invention is improved based on the existing ultra-high vacuum MBE device. Since the existing MBE device can only work in an ultra-high vacuum atmosphere, the present invention has made structural improvements to it in order to adapt to the near-normal-pressure working environment.

[0017] At the same time, according to the near-normal-pressure growth method of a TMDC material provided by the present invention, improvements are mainly made in the following four aspects, which are described in detail as follows.

[0018] 1) Control the atmosphere and pressure inside the device. Based on the vacuum acquisition method in the ultra-vacuum MBE device, the MBE cavity is connected to a two-stage vacuum pump system, and the ultra-high vacuum background pressure is obtained by a mechanical pump and a molecular pump. In addition, the gas line is connected to the MBE cavity through a leak valve, and argon (Ar), hydrogen (H2), and oxygen (O2) can be introduced into the MBE cavity to achieve a pressure range of 10 - 7 Pa~10kPa adjustable inert, reducing, and oxidizing atmosphere. It should be understood that hydrogen and oxygen can be introduced as process gases to reduce the reaction barrier of certain materials.

[0019] 2) Changing the source material supply method. As the pressure in the MBE chamber increases, the free path of the molecular beam gradually decreases and scattering gradually increases, resulting in a significant decrease in the beam density reaching the substrate surface. To achieve the required beam density for growth, it is necessary to re-optimize the beam source furnace design, increase the beam source furnace output, or change the source material supply method, as explained below:

[0020] a. For low-melting-point, high-vapor-pressure materials (such as tellurium (Te), selenium (Se), and sulfur (S), the standard single-zone evaporation source is replaced with a dual-zone thermal evaporation source. This secondary heating of the source port improves the effective beam flux of the source material and avoids clogging of the evaporation source caused by material deposition at the evaporation source outlet. Furthermore, a one-dimensional linear drive is added between the thermal evaporation source and the MBE chamber to control the distance between the thermal evaporation source port and the substrate, thereby reducing the negative effects of reduced molecular beam free path and scattering at high pressures.

[0021] b. For high-melting-point, low-vapor-pressure materials (such as tungsten W and tantalum Ta), electron beam evaporation sources are commonly used in UHVMBE. However, these evaporation sources can only operate in an ultra-high vacuum environment. Therefore, the present invention improves upon this method by using pulsed laser deposition (PLD). A 248nm KrF excimer pulsed laser is used to bombard the target material. The beam flux of the source material is adjusted by adjusting the pulse frequency, laser energy, and focus.

[0022] 3) Changed substrate temperature control. Traditional sample stages use heating filaments. Under high pressure, the high thermal conductivity makes it difficult to arbitrarily control the substrate heating and cooling rates, leading to an increase in temperature throughout the cavity. The present invention uses a fiber-coupled laser heating device to locally heat the substrate, with an infrared thermometer providing feedback on the heating temperature. The temperature range can range from room temperature to thousands of degrees Celsius, and the heating and cooling rates are adjustable. Furthermore, laser-focused localized heating can reduce negative effects such as cavity temperature increases caused by heat conduction.

[0023] 4) Use a two-stage differential reflection high-energy electron diffractometer (RHEED) real-time detection system. The reflection high-energy electron diffractometer (RHEED) used for in-situ growth monitoring in the existing UHVMBE device will affect the service life of the RHEED filament in a high-pressure gas atmosphere, and the scattering and refraction of the electron beam by gas molecules will affect the RHEED diffraction pattern. Therefore, the working environment pressure of the RHEED electron gun filament must be better than 10 -6 This device uses a two-stage differential reflection high-energy electron diffractometer, and the working pressure under the two-stage differential can reach 10 Torr, so as to realize the real-time monitoring of the growth of TMDC samples under near-normal pressure.

[0024] In summary, in view of the limitations of the current TMDC material preparation methods, the present invention creatively provides a new idea for TMDC material preparation for the first time. By improving the main structure of the MBE equipment, two new dimensions of film growth control are added: pressure and atmosphere. Chemical processes are introduced into the kinetically controlled physical deposition process to achieve coordinated regulation of the kinetic and thermodynamic processes of film growth, and explore the unknown experimental parameter space between ultra-high vacuum and normal pressure range - near-normal pressure atmosphere. This breaks the limiting factors of traditional technology for preparing TMDC materials and even other materials, and realizes the preparation of a variety of TMDC materials and their heterogeneous structures by near-normal pressure methods, which helps us explore novel quantum phenomena. Therefore, the near-normal pressure growth method of TMDC materials and the near-normal pressure MBE device provided by the present invention have huge application potential in this field. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 A schematic diagram of a near-normal-pressure MBE apparatus according to a preferred embodiment of the present invention;

[0026] Figure 2 For example Figure 1 A perspective view of the near-atmospheric pressure MBE device shown;

[0027] Figure 3 : is the reflection high energy electron diffraction (RHEED) diffraction pattern of the sample in Example 1, wherein a is the double-layer graphene substrate; b, c, and d are the RHEED diffraction patterns of the sample 5 minutes, 10 minutes, and 15 minutes after the start of growth, respectively;

[0028] Figure 4 The morphology of the sample in Example 1 was characterized by scanning tunneling microscopy (STM);

[0029] Figure 5 The RHEED diffraction patterns of the 1T'-WTe2 sample prepared in Example 1 before and after annealing at different pressures are: -8 Before (upper left) and after (lower left) annealing under 50 Pa ultra-high vacuum; before (upper right) and after (lower right) annealing under 50 PaAr atmosphere. DETAILED DESCRIPTION

[0030] The present invention will be further described below with reference to specific examples. It should be understood that the following examples are only used to illustrate the present invention and are not intended to limit the scope of the present invention.

[0031] Combine Figure 1 、 Figure 2As shown, a near-normal-pressure MBE device provided according to a preferred embodiment of the present invention includes: a five-dimensional sample stage 1, a dual-temperature zone thermal evaporation source 2, an excimer laser 3, a gas path connection port 4, a fiber-coupled laser heating device 5, a two-stage differential reflection high-energy electron diffractometer 6, and an MBE cavity 7, and a two-stage vacuum pump system consisting of a mechanical pump 8 and a molecular pump 9.

[0032] It should be understood that the near-normal-pressure MBE apparatus used in the present invention is an improvement on the existing ultra-high vacuum MBE apparatus. Specifically, the five-dimensional sample stage 1 is used to place the sample; the dual-temperature zone thermal evaporation source 2 uses a dual temperature zone at the crucible bottom and crucible mouth to increase the effective beam current of the source material; the excimer laser 3 uses a 248nm KrF excimer pulse laser to bombard the corresponding target material; the gas path connection port 4 connects the gas path to the MBE chamber through a leak valve, allowing argon, hydrogen, or oxygen to be introduced into the MBE chamber to achieve 10 -7 An inert, reducing, or oxidizing atmosphere can be adjusted at near-normal pressure of Pa~10kPa; a fiber-coupled laser heating device 5 performs local heating of the substrate with an adjustable heating and cooling rate; a two-stage differential reflection high-energy electron diffractometer 6 can realize real-time monitoring of the near-normal-pressure growth of TMDC samples; a two-stage vacuum pump system obtains ultra-high vacuum background pressure through a mechanical pump 8 and a molecular pump 9, and measures the vacuum degree in the cavity in real time through a vacuum gauge 10.

[0033] According to this preferred embodiment, a one-dimensional linear drive is further provided between the dual-temperature zone thermal evaporation source 2 and the MBE chamber 7 to control the distance between the thermal evaporation source port and the substrate to reduce the negative effects of the reduction in molecular beam free path and scattering under high pressure.

[0034] Example 1: Near-normal-pressure growth of WTe2 materials

[0035] 1.1 Preparation of double-layer graphene substrate

[0036] The single-side polished 4H-SiC substrate was fixed on a tantalum sample holder with a tantalum wire and placed on the sample stage of a near-normal-pressure molecular beam epitaxy device. -9 mbar) using a fiber-coupled laser heating system to process 4H-SiC.

[0037] The first step is to preheat the substrate surface. Use an infrared thermometer to calibrate the substrate surface temperature and maintain it at 650-700°C for 60 minutes to remove impurities such as water vapor brought from the external atmospheric environment.

[0038] In the second step, the laser heating power is increased to raise the substrate temperature to 1350°C-1400°C, maintain for 60 seconds, and then cool it down to 650°C and maintain for 60 seconds. This is one cycle, and a total of 60 cycles are performed.

[0039] After natural cooling, a double-layer graphene substrate was obtained. The results were characterized by reflection high energy electron diffraction (RHEED). Figure 3 As shown in a in .

[0040] 1.2 Preparation of 1T'-WTe2 single crystal thin film

[0041] The first step is to introduce the atmosphere. Before introducing the atmosphere, close the gate valve between the chamber and the molecular pump to make the chamber completely closed and the vacuum degree <10 -9 The purpose is to completely cut off the connection with the outside atmosphere and ensure the cleanliness of the chamber environment. Then, open the leak valve to introduce pure argon (Ar). Use a capacitive film pressure gauge connected to the chamber to calibrate the chamber pressure. When it reaches 100 Pa, close the leak valve and stop the gas injection.

[0042] In the second step, a fiber-coupled laser heater is used to heat the double-layer graphene substrate to 500-520°C. Experiments have shown that high-quality layered epitaxial films can be obtained within this temperature range. If the temperature is too high, the desorption rate of the Te element is high, making it difficult to form a stoichiometric 1T'-WTe2 film. If the temperature is too low, the W and Te elements that reach the substrate surface are not easy to migrate, resulting in an excessively high nucleation rate and serious grain boundary defects, making it difficult to form a large-area single crystal film.

[0043] In the third step, after the substrate reaches the target temperature, the baffle of the dual-temperature zone evaporation source for providing the Te element is opened (the beam source furnace has been heated to the target temperature in advance, the bottom of the crucible is 400°C, and the crucible mouth is 500°C), and then the excimer laser is turned on, the energy is adjusted to 100mJ, the frequency is 10Hz, and the pulsed laser is used to bombard the W element target to provide the W element required for 1T'-WTe2. During the growth process, a differential reflection high-energy electron diffractometer is used to monitor the sample growth in real time. The RHEED diffraction patterns of the sample 5 minutes, 10 minutes, and 15 minutes after the start of growth are as follows: Figure 3 As shown in b, c, and d.

[0044] 1.3 Morphological characterization of 1T'-WTe2 single crystal thin films

[0045] The sample was transferred to a scanning tunneling microscope (STM) device using a connected vacuum interconnection device (the sample was always in a vacuum environment during the entire process) for surface morphology characterization and analysis. After characterization and analysis, a flat crystal surface was obtained, such as Figure 4 As shown, the maximum diameter of the single crystal is 70nm.

[0046] The RHEED diffraction patterns of 1T'-WTe2 samples before and after annealing at different pressures are shown in Figure 2. Figure 5 As shown, the left picture is 10 -8The right image shows the 1T'-WTe2 before and after annealing at 460°C in a 50Pa ultrahigh vacuum atmosphere. The right image shows the 1T'-WTe2 before and after annealing at 510°C in a 50PaAr atmosphere. Clearly, the stable temperature of 1T'-WTe2 increases at near-ambient pressure. This demonstrates that the growth window (especially the temperature window) of 1T'-WTe2 at near-ambient pressure has been expanded.

[0047] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of the present invention. Various modifications are possible. Any simple, equivalent changes and modifications made in accordance with the claims and description of the present invention are within the scope of protection of the patent claims. Anything not fully described in this invention is conventional technology.

Claims

1. A near-normal pressure growth method for TMDC materials, characterized in that: In an MBE device, TMDC materials are grown at near-normal pressure, the MBE device comprising: an MBE cavity; a five-dimensional sample stage for placing samples; a two-stage vacuum pump system, which obtains an ultra-high vacuum background pressure through a mechanical pump and a molecular pump; a dual-temperature zone thermal evaporation source, which uses a crucible bottom and a crucible mouth dual temperature zone to improve the effective beam current of the source material; an excimer laser, which uses a 248nm KrF excimer pulse laser to bombard the corresponding target material; and a gas path connection port, which connects the gas path to the MBE cavity through a leak valve, and can introduce argon, hydrogen, or oxygen into the MBE cavity to achieve 10 -7 An adjustable inert, reducing, or oxidizing atmosphere at near-ambient pressure (Pa-10kPa); a fiber-coupled laser heating device for localized heating of the substrate with adjustable heating and cooling rates; and a two-stage differential reflection high-energy electron diffractometer for real-time monitoring of near-ambient pressure growth of TMDC materials. The near-ambient pressure growth method comprises the following steps: S1: Introduce atmosphere gas: Before introducing atmosphere gas, close the gate valve between the MBE cavity and the molecular pump to make the MBE cavity completely closed with a vacuum degree of <10 -9 The space is filled to ensure the cleanliness of the cavity environment, and then pure argon, hydrogen, or oxygen is introduced to achieve 10 -7 Adjustable inert, reducing, or oxidizing atmosphere at near-normal pressure of Pa~10kPa. When the target pressure is reached, close the leak valve and stop injection. S2: Use a fiber-coupled laser heater to heat the target substrate to the target temperature corresponding to the TMDC material growth; S3: After the target substrate reaches the target temperature, the baffle of the dual-temperature zone thermal evaporation source for providing chalcogen elements is opened, and then the excimer laser is turned on to bombard the transition metal element target with a 248nm KrF excimer pulse laser, thereby achieving TMDC material in 10 -7 The TMDC material growth is carried out at near-normal pressure in the range of Pa~10kPa. During the growth process, a two-stage differential reflection high-energy electron diffractometer is used to monitor the growth of the TMDC material in real time. The distance between the thermal evaporation source outlet and the substrate is controlled by a one-dimensional linear drive between the dual-temperature zone thermal evaporation source and the MBE cavity to reduce the negative effects of the reduction of the free path of the molecular beam and scattering under high pressure.

2. The near-normal pressure growth method of TMDC material according to claim 1, characterized in that Step S1 includes: using a capacitive film pressure gauge connected to the MBE cavity to calibrate the pressure in the MBE cavity, and closing the leakage valve when the target pressure is reached.

3. The near-normal-pressure growth method of TMDC material according to claim 1, characterized in that: In step S2, the specific growth target temperatures of various TMDC materials vary, ranging from 400 to 700°C.

4. The near-normal pressure growth method of TMDC material according to claim 1, characterized in that Step S3 includes: before opening the baffle of the dual-temperature zone thermal evaporation source, it is necessary to preheat the dual-temperature zone thermal evaporation source to the target temperature. The crucible bottom temperature depends on the evaporation temperature of different source materials and is between 300 and 600°C. The crucible mouth temperature is 100°C higher than the crucible bottom temperature.

5. The near-normal-pressure growth method of TMDC material according to claim 1, characterized in that: Step S3 also includes: adjusting the beam current of the source material by adjusting the pulse frequency, energy and focusing degree of the laser.

6. The near-normal-pressure growth method of TMDC material according to claim 1, characterized in that: The chalcogen element is selected from any one of Se, Te, and S.

7. The near-normal-pressure growth method of TMDC material according to claim 1, characterized in that: The transition metal element is selected from any one of Group IV, Group V, and Group VI transition metals.

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