Method for preparing palladium ditelluride single crystal film by molecular beam epitaxy method

Abstract

The invention discloses a method for preparing a palladium ditelluride single crystal film by using a molecular beam epitaxy method, and belongs to the technical field of nano material preparation. Comprising the following steps: ultrasonically cleaning a silicon carbide substrate, putting the silicon carbide substrate into a growth cavity, and performing annealing treatment to remove impurities adsorbed on the surface of the substrate; performing flash processing on the annealed silicon carbide substrate for multiple times in a mode of increasing and reducing current to obtain a double-layer graphene surface, and then reducing the temperature of the substrate to a growth temperature; pd and Te are placed in different evaporation sources to be heated, the atomic beam flow rate ratio of Pd to Te is adjusted by controlling the evaporation temperature during growth, Pd and Te grow on a substrate in a co-deposition mode, and baffles of the Pd source and the Te source are opened at the same time; and after the growth is completed, closing the Pd source baffle plate and the Te source baffle plate, and annealing the substrate. According to the method, the preparation conditions of the thin film can be accurately regulated and controlled, so that the controllable growth of the thin film material is achieved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of nanomaterial preparation, and particularly relates to a method for preparing a ditellurium palladium monocrystal thin film by using a molecular beam epitaxy method. BACKGROUND

[0002] In the field of condensed matter physics, topological superconductivity (TSC) has attracted much attention due to its potential to host Majorana fermions. These massless quasi-particles, resulting from the combination of p-wave pairing mechanism of spin-triplet states and topological non-trivial surface states, are topologically protected and have shown great application prospects in fault-tolerant quantum computing, as Majorana bound states are extremely robust against environmental decoherence.

[0003] To realize topological superconductivity, researchers have explored various strategies: for example, using the proximity effect between s-wave superconductor and topological insulator to induce superconductivity; adjusting the electronic structure of topological materials by external doping; applying extreme pressure to modulate the band topology; constructing a topological superconducting platform in a two-dimensional crystal with antisymmetric spin-orbit coupling; and indirectly realizing topological superconducting pairing by means of the superconducting proximity effect of Rashba spin-splitting state under a Zeeman field. However, these methods still face many technical challenges in experimental implementation: the proximity effect is highly dependent on the quality of the interface, while chemical doping easily introduces uncontrollable surface defects and destroys the uniformity of the material, limiting the stable construction of topological superconducting state.

[0004] Therefore, finding a single material that has both intrinsic superconductivity and topological non-trivial surface states (TSS) has become one of the ideal paths to realize topological superconductivity. In recent years, transition metal dichalcogenides (TMDs) have become a research hotspot in this field due to their simultaneous possession of superconductivity and non-trivial band topology. It is particularly worth noting that when bulk transition metal dichalcogenide materials are thinned to the two-dimensional limit, not only can their high-quality crystallinity be preserved, but also a new way is opened up for exploring novel quantum states of matter in low-dimensional systems and their device applications.

[0005] Among them, the topological semimetal type transition metal chalcogenide material palladium ditelluride (PdTe2) is particularly attractive. Its bulk superconductivity has been discovered more than half a century ago, and the superconducting transition temperature is about 1.7 K. In recent years, with the in-depth study of its electronic structure, PdTe2 is confirmed to have intrinsic type-I superconductivity, type-II Dirac fermion and topological nontrivial surface state. More notably, from single-layer to double-layer structure evolution, PdTe2 also exhibits semiconductor-metal phase transition behavior. These characteristics together show that PdTe2 is a potential candidate material for realizing intrinsic topological superconducting state.

[0006] In the past few decades of research on PdTe2 superconductivity and band topological properties, basically all focus on bulk materials, and until recent years there have been related research reports on thin film materials, but the material preparation technology is still not perfect; therefore, it is particularly crucial to experimentally realize a mature method for preparing high-quality PdTe2 single crystal thin film. SUMMARY

[0007] In view of the shortcomings of the prior art, the purpose of the present application is to provide a method for preparing a palladium ditelluride single crystal thin film by molecular beam epitaxy, which solves the problems in the prior art.

[0008] The purpose of the present application can be achieved by the following technical solutions: A method for preparing a palladium ditelluride single crystal thin film by molecular beam epitaxy, comprising the following steps: S1, after ultrasonic cleaning of the silicon carbide substrate, the substrate is placed in a growth chamber and annealed to remove impurities adsorbed on the surface of the substrate; S2, the annealed silicon carbide substrate is subjected to multiple flash treatments by increasing and decreasing the current to obtain a double-layer graphene surface, and then the substrate temperature is reduced to the growth temperature; S3, Pd and Te are heated in different evaporation sources, the atomic beam flow rate ratio of Pd and Te is adjusted by controlling the evaporation temperature during growth, and Pd and Te are grown on the substrate by co-deposition, and the shutters of the Pd source and the Te source are opened at the same time; S4, after growth, the shutters of the Pd source and the Te source are closed, and the substrate is annealed.

[0009] Further, the vacuum degree of the growth chamber is 5x10 -10 mbar.

[0010] Further, in S1, the annealing temperature is 650 DEG C, and the annealing time is 3h.

[0011] Furthermore, the flash processing involves heating the annealed silicon carbide substrate to 1200°C at a heating rate of 60°C / s and maintaining it for 30s, then reducing the substrate temperature to 400°C within 30s.

[0012] Furthermore, the flash processing is performed 20 times.

[0013] Furthermore, during growth, the lower temperature zone of the dual-temperature evaporation source used for Te was 330℃, and the upper temperature zone was 360℃; the middle temperature zone of the evaporation source used for Pd was 1120℃.

[0014] Furthermore, the substrate growth temperature is 225°C.

[0015] Furthermore, the atomic beam current ratio of Pd and Te is 1:30.

[0016] Furthermore, in S4, the substrate annealing temperature is 270°C.

[0017] A palladium ditelluride single-crystal thin film is prepared by the method described above.

[0018] The beneficial effects of this invention are: 1. This invention provides a method for preparing palladium ditelluride single-crystal thin films using molecular beam epitaxy (MBE). This method allows for highly precise control of the film preparation conditions, resulting in controllable growth of the thin film material. Simultaneously, in-situ monitoring using a reflective high-energy electron diffractometer (RHEED) during the film preparation process enables large-scale, atomically flat epitaxial growth with precisely controllable thickness. This method can be further extended to other types of thin film preparation, revealing novel physical properties of the thin film material and providing an important opportunity for in-depth research into the interaction between intrinsic superconductivity and topologically nontrivial electronic states in PdTe2 quantum thin films.

[0019] 2. This invention provides a method for preparing palladium ditelluride single-crystal thin films using molecular beam epitaxy (MBE). Its advantage lies in creating and using an extremely clean growth environment, fundamentally suppressing and reducing impurity contamination during the growth process, and obtaining ultra-high purity thin film samples. In the ultra-high vacuum growth environment, the content of impurity gas molecules (such as oxygen and water vapor) is extremely low, thereby preventing the introduction of impurity atoms during the growth of the palladium ditelluride thin film, resulting in thin film samples with high purity and good crystallinity. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 This is a flowchart of the thin film sample preparation process; Figure 2 A schematic diagram of the growth process for preparing thin film samples; Figure 3 This is a schematic diagram of the structure of the PdTe2 single crystal thin film prepared in this invention; Figure 4 This is a reflection high-energy electron diffraction pattern of the silicon carbide substrate (treated) and the PdTe2 single crystal thin film in this invention; Figure 5 The results are obtained by angle-resolved photoelectron spectroscopy (ARPES) of the PdTe2 single crystal thin film prepared in this invention. Detailed Implementation

[0022] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] Example 1 like Figure 1 As shown, a method for preparing palladium ditelluride single-crystal thin films using molecular beam epitaxy includes the following steps: (1) Before placing the silicon carbide substrate into the MBE cavity, the substrate is ultrasonically cleaned using an alcohol-acetone-alcohol cleaning sequence. (2) Transfer the silicon carbide single crystal substrate to the sample holder in the growth chamber. The base vacuum level of the growth chamber is 1×10⁻⁶. - 10 mbar, the substrate is annealed by DC heating to remove impurities adsorbed on the substrate surface. The annealing temperature is 650℃ and the annealing time is three hours. (3) By rapidly increasing or decreasing the current, the annealed silicon carbide substrate was rapidly heated to 1200℃ at a heating rate of 60℃ / s and maintained for 30 seconds. Then, the substrate temperature was reduced to 400℃ within 30 seconds. This process was repeated 20 times to obtain a large-area bilayer graphene surface. Subsequently, the temperature was reduced to a growth temperature of 225℃. When silicon carbide is heated at high temperature in a vacuum environment, a large number of Si atoms on the surface will desorb, leaving C atoms to rearrange on the surface to form graphene. (4) Growth of PdTe2 single crystal thin film: High-purity Pd and Te are heated in different evaporation sources. By controlling the evaporation temperature during growth, the atomic beam flow rate ratio of Pd and Te is adjusted. During growth, the lower temperature zone of the dual-temperature evaporation source used for Te is 330℃ and the upper temperature zone is 360℃; the temperature of the middle temperature zone evaporation source used for Pd is 1165℃. According to the calibration results of the in-situ quartz crystal oscillator, the atomic beam flow rate ratio of Cr and Te is 1:30. The substrate growth temperature is stabilized at 225℃. The growth method is Pd and Te co-deposition. At the same time, the baffles of Pd source and Te source are opened.

[0024] (5) The average growth rate of the thin film was controlled at about 0.35 ML / min. After epitaxial growth for 30 minutes, a PdTe2 thin film with a thickness of 10 ML was obtained. After the growth was completed, the Pd source and Te source baffles and the sample holder baffle were turned off, and the substrate was heated to 270℃ for annealing to improve the quality of the thin film.

[0025] Figure 2 The diagram below illustrates the molecular beam epitaxy (MBE) growth method used in Example 1. It shows that the entire epitaxial growth process is carried out in an ultra-high vacuum environment. Pd and Te targets are heated by a thermal resistance wire, causing them to evaporate at high temperatures and emit atomic beams in specific directions. In the ultra-high vacuum environment, the two atomic beams converge on the substrate surface without scattering. Simultaneously, at the optimal substrate temperature, Pd and Te atoms adsorb, migrate, and crystallize on the silicon carbide substrate, ultimately achieving the epitaxial growth of the thin film. By controlling the beam size of the evaporated target, the substrate growth temperature, and the epitaxial growth time, atomically precise thin film growth can be achieved. In this embodiment, the growth process for preparing palladium ditelluride single crystal thin films using molecular beam epitaxy is as follows: Figure 1 As shown, the cleaned silicon carbide substrate is first annealed to remove surface impurities. Then, a "flash" treatment is performed on the substrate surface (the annealed silicon carbide substrate is rapidly heated to 1200°C at a heating rate of 60°C / s and held for 30 seconds, then the substrate temperature is reduced to 400°C within 30 seconds) to obtain large-area bilayer graphene. Further growth on the graphene surface can yield atomically flat single-crystal thin films.

[0026] The structure of a single-crystal thin film is as follows Figure 3 As shown, from bottom to top, the layers are: a silicon carbide substrate, a bilayer graphene layer, and a palladium distellide single-crystal thin film.

[0027] The PdTe2 single crystal thin film has a layered crystal structure of CdI2. Each unit cell consists of three layers of atoms: Te-Pd-Te. Pd atoms and Te atoms are bonded within the layers and arranged in a hexagonal structure. Adjacent layers are connected by van der Waals forces. When epitaxially grown on the surface of bilayer graphene, Pd atoms and Te atoms are stacked along the c-axis due to the modulation of the substrate lattice. The crystal orientation of the epitaxial surface is (0001) plane.

[0028] The palladium ditelluride single-crystal thin film has an in-plane hexagonal lattice arrangement and a lattice constant of 4.08 Å. The periodic structure described therein can be characterized by reflection high-energy electron diffraction. Figure 4 This is a reflection high-energy electron diffraction pattern of the silicon carbide substrate (treated) and the PdTe2 single crystal thin film in Example 1. The clear and sharp RHEED fringes indicate that the thin film exhibits an atomically flat layered growth mode.

[0029] Example 2 In this embodiment, the 10ML PdTe2 single crystal thin film prepared in Example 1 was tested; The 10 mL PdTe2 single crystal thin film prepared in Example 1 was tested using angle-resolved photoelectron spectroscopy (ARPES) to detect its specific electronic band structure; the testing process included: (1) The prepared 10ML PdTe2 single crystal thin film was taken out from the sample holder of MBE and then transferred to the ARPES test chamber which is connected to the vacuum of the MBE chamber through the magnetic coupling sample transfer rod. (2) The thin film sample taken from the transfer rod is placed on the ARPES sample holder. The high-precision six-axis ARPES sample holder is moved to the preset test position by motor control to perform band test. (3) In the ARPES band structure test, a laboratory-grade helium lamp was used as the light source, with a photon energy of 21.2 eV, a spot size of 0.7 mm × 0.7 mm, and an energy resolution of 20 meV. The electron energy analyzer used was a DFS30 Analyzer from ScientaOmicron, Sweden. The test temperature was 10 K, and the vacuum degree was 2 × 10⁻⁶. -5 mbar.

[0030] Test results are as follows Figure 5 As shown, where, Figure 5 In the image, (a) and (b) are respectively the ARPES mapping diagrams at the Fermi surface of the PdTe2 single-crystal thin film and along the GM ( k x ), GK ( k yThe energy-momentum dispersion relations along two high-symmetry directions reveal that the entire Fermi surface exhibits metallic characteristics and a very clear six-fold symmetry. Two electron pockets traverse the Fermi level exist around the Γ point, and six sets of clearly dispersed electronic bands also exist at the Brillouin zone boundary near the M point, away from the Γ point. Furthermore, in the ARPES-cut spectra along the two high-symmetry directions M−Γ−M and K−Γ−K, the theoretically calculated topological surface Dirac cone located 1.74 eV below the Fermi level, and the significant electron pockets at the Fermi surface near the Γ point, are clearly observed. Simultaneously, at the Γ point, 0.7 eV below the Fermi level, a bulk Dirac point of type II can be found, where a set of electronic quantum well bands (QWSs) intersects with a set of hole quantum well bands. These band characteristics well reflect the topological properties of the palladium ditelluride single-crystal thin film, demonstrating its great application potential.

[0031] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0032] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention.

Claims

1. A method for preparing palladium ditelluride single-crystal thin films using molecular beam epitaxy, characterized in that, Includes the following steps: S1. After ultrasonic cleaning, the silicon carbide substrate is placed in the growth chamber and annealed to remove impurities adsorbed on the substrate surface. S2, by adding or subtracting current, the annealed silicon carbide substrate is subjected to multiple flash processes to obtain a bilayer graphene surface, and then the substrate temperature is lowered to the growth temperature. S3, Pd and Te are placed in different evaporation sources and heated. By controlling the evaporation temperature during growth, the atomic beam flow rate ratio of Pd and Te is adjusted. Pd and Te are grown on the substrate by co-deposition. At the same time, the baffles of Pd source and Te source are opened. S4. After growth is complete, turn off the Pd source and Te source baffles and anneal the substrate.

2. The method for preparing palladium ditelluride single-crystal thin films using molecular beam epitaxy according to claim 1, characterized in that, The vacuum level of the growth chamber is 5 × 10⁻⁶. -10 mbar.

3. The method for preparing palladium ditelluride single-crystal thin films using molecular beam epitaxy according to claim 1, characterized in that, In S1, the annealing temperature is 650°C and the annealing time is 3 hours.

4. The method for preparing palladium ditelluride single-crystal thin films using molecular beam epitaxy according to claim 1, characterized in that, The flash processing involves heating the annealed silicon carbide substrate to 1200°C at a heating rate of 60°C / s and maintaining it for 30s, then reducing the substrate temperature to 400°C within 30s.

5. The method for preparing palladium ditelluride single-crystal thin films using molecular beam epitaxy according to claim 4, characterized in that, The flash processing is performed 20 times.

6. The method for preparing palladium ditelluride single-crystal thin films using molecular beam epitaxy according to claim 1, characterized in that, During growth, the lower temperature zone of the dual-temperature evaporation source used for Te was 330℃, and the upper temperature zone was 360℃; the middle temperature zone of the evaporation source used for Pd was 1120℃.

7. The method for preparing palladium ditelluride single-crystal thin films using molecular beam epitaxy according to claim 1, characterized in that, The substrate growth temperature is 225℃.

8. The method for preparing palladium ditelluride single-crystal thin films using molecular beam epitaxy according to claim 1, characterized in that, The atomic beam flux ratio of Pd and Te is 1:

30.

9. A method for preparing palladium ditelluride single-crystal thin films using molecular beam epitaxy according to claim 1, characterized in that, In S4, the substrate annealing temperature is 270°C.

10. A palladium distellide single-crystal thin film, characterized in that, It is prepared by the method described in any one of claims 1-9.

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