CN114457415A

CN114457415A - PLEES preparation system of laser pulse enhanced molecular beam epitaxy system

Info

Publication number: CN114457415A
Application number: CN202210090243.3A
Authority: CN
Inventors: 王暄; 郑玥乾
Original assignee: Harbin University of Science and Technology
Current assignee: Harbin University of Science and Technology
Priority date: 2022-01-25
Filing date: 2022-01-25
Publication date: 2022-05-10

Abstract

The invention discloses a PLEES preparation system of a laser pulse enhanced molecular beam epitaxy system, relating to the technical field of nano-scale films; the preparation method comprises the following steps: bi with a purity of 99.95%₂Te₃Splicing the target material and a Te target material with the purity of 99.99% according to the ratio of 1:1 to form a mixed target material, and placing the mixed target material in an epitaxial chamber; scattering the mixed target material into a plasma beam by using laser beams generated by an excimer laser, and depositing the scattered Bi atoms and Te atoms on the substrate on the other side to gradually grow a nano-scale film; the mixed target material can excellently solve the problem of atom vacancy caused by insufficient supplement of Te atoms in the growth process of the film; ensuring the correct element proportion of each epitaxial film and Bi₂Te₃Stable growth of crystals; the operation process is simplified, the production cost is reduced, the growth speed of the film is improved, and the growth quality of the film can be ensured.

Description

PLEES preparation system of laser pulse enhanced molecular beam epitaxy system

Technical Field

The invention belongs to the technical field of nano-scale films, and particularly relates to a PLEES preparation system of a laser pulse enhanced molecular beam epitaxy system.

Background

The study of topological insulators is not open to the development of thin film technology, since, to date, all confirmation of the properties of topological insulators, including the various hall effects, is observed on thin film sample materials. Meanwhile, as the research on the topological insulator materials in the laboratory is gradually advanced, the higher requirements on how to prepare the high-quality topological insulator thin film are put forward. The current common methods for preparing topological insulators, especially second generation topological insulator materials, are as follows: magnetron sputtering, Chemical Vapor Deposition (CVD), Metal Organic Chemical Vapor Deposition (MOCVD), Molecular Beam Epitaxy (MBE), and the like. The method is characterized in that a proper experimental material is found, corresponding to the most proper synthesis mode, the optimal experimental conditions are used, so that the high-quality topological insulator is obtained, and the prepared high-quality sample material is significant in exploring the singular performance of the high-quality sample material.

A common method for preparing a topological insulator in a laboratory is MBE (molecular beam epitaxy), and the system has the principle that different targets are heated by a beam source furnace to generate beam sources, and the beam sources are not used and are finally deposited on a substrate by mixing according to corresponding proportions. The quality of the film prepared by the method is high, but the complicated system structure and operation mode, high manufacturing cost and slow growth speed always limit the development of the preparation. Other devices such as chemical vapor deposition, magnetron sputtering and the like have improved efficiency, but are easy to cause element loss in the preparation process or the quality after film formation cannot reach the optimal state due to the device structure and principle problems.

Thin film materials refer to thin metal or organic layers with thicknesses ranging from a single atom to several millimeters. Electronic semiconductor functional devices and optical coatings are the main applications of thin film technology. The growth of thin films is an important process in semiconductor manufacturing. Thin film growth techniques can be broadly classified into physical methods and chemical methods. Common thin film growth techniques include: thermal oxidation, physical vapor deposition, and chemical vapor deposition.

Currently, MBE is used for the preparation of high quality nanoscale topological insulator films. The MBE generates a beam source (beam source) by heating a pure target in a beam source furnace (beam source furnace), and sprays different target beam sources into a cavity (cavity) for epitaxial growth, thereby preparing the topological insulator film. In order to produce high quality nanoscale topological insulator films, it is necessary to fill in excess missing elements during film growth. For example, preparation of Bi from MBE₂Te₃And when the nano topological insulator thin film is used, the Te element needs to be supplemented. The flow velocity of the beam source of various elements needs to be accurately controlled and monitored in real time, so that the cost of system equipment and the preparation difficulty are greatly increased, and the preparation production efficiency is reduced. However, other Bi₂Te₃The preparation method of the nanometer topological insulator thin film, such as CVD and magnetron sputtering, can only ensure the supplement of Te element of the epitaxial film at the outermost layer even if annealing is carried out in the atmosphere of Te element because the element proportion and the growth condition can not be accurately controlled in the preparation process, and a plurality of holes still exist in the epitaxial film, thereby causing the uneven quality of the thin film.

Disclosure of Invention

To solve the problems in the background art; the invention aims to provide a PLEES preparation system of a laser pulse enhanced molecular beam epitaxy system.

The PLEES preparation system of the laser pulse enhanced molecular beam epitaxy system comprises the following preparation methods: bi of 99.95% purity₂Te₃Splicing the target material and the Te target material with the purity of 99.99% according to the ratio of 1:1 to form a mixed target material, and placing the mixed target material in an epitaxial chamber; the laser beam generated by the excimer laser is utilized to scatter the mixed target material to form a plasma beam, and the dispersed Bi atoms and Te atoms are deposited on the substrate at the other side to gradually grow a nano-scale film.

Preferably, the substrate is selected to be single-side polished Al₂O₃Materials, since a clean substrate surface is conducive to thin film growth, the substrate is subjected to standard cleaning procedures prior to use.

Preferably, the substrate cleaning process comprises: ultrasonic cleaning is respectively carried out in alcohol and acetone solution, each time, the alcohol is ultrasonically cleaned for 10 minutes, and then deionized water washing is carried out; then, the substrate is dried by nitrogen, and is placed in a substrate holder to be preheated to a certain temperature.

Preferably, the vacuum degree of the epitaxial chamber is controlled to be 10^-7Pa to avoid influence of other air ions; irradiating the target material rack rotating at a constant speed by laser to enable the target material rack to uniformly hit the mixed spliced target material, and recording the operation time of the laser; after the preparation is finished, the substrate is cooled to a certain temperature for in-situ annealing.

Compared with the prior art, the invention has the beneficial effects that:

firstly, the mixed target material can perfectly solve the problem of atom vacancy caused by insufficient supplement of Te atoms in the growth process of the film.

Secondly, putting pure Te and Bi on the same target material rack₂Te₃Mixing and splicing the target materials according to the proportion of 1: 1; meanwhile, in the synthesis process, laser is alternatively hit on the two targets at a constant speed; therefore, in the whole preparation process, excessive Te element can be accompanied with the growth of the film, and Te atom holes are continuously supplemented, so that the correct element proportion of each epitaxial film and Bi are ensured₂Te₃Stable growth of the crystal; the method simplifies the operation process, reduces the production cost, greatly improves the growth speed of the film by matching with the working principle of the laser molecular beam epitaxy equipment, and simultaneously can ensure the growth quality of the film.

Drawings

For ease of illustration, the invention is described in detail by the following detailed description and the accompanying drawings.

FIG. 1 is a schematic structural view of the present invention;

FIG. 2 is a schematic view of the target installation of the present invention;

FIG. 3 is a diffraction pattern diagram of a film grown by the PLEES system of the present invention under RHEED;

FIG. 4 is a schematic diagram of the Figure S1.RHEED system of the present invention monitoring the growth of the thin film in real time;

FIGS. 5 and 6 are surface topography and cross-sectional topography diagrams of sample films under AFM in accordance with the present invention;

FIG. 7 is a graph of the average height difference at the cross-section, dz being 69.82nm, i.e., the thickness of the film sample, in accordance with the present invention;

FIG. 8 is a schematic view of SEM images of the present invention, wherein a-d are surface topography images of the film prepared by mixing the target materials under magnification of 5000, 10000, 20000, 50000 times, respectively;

FIG. 9 is a statistical view of the elemental atoms of a film according to the present invention;

FIG. 10 is a graph of GIXRD measurements in accordance with the present invention;

FIG. 11 is a diagram of a cell structure according to the present invention;

FIG. 12 is a diagram showing the comprehensive physical properties of a sample film subjected to PPMS in the present invention.

Detailed Description

In order that the objects, aspects and advantages of the invention will become more apparent, the invention will be described by way of example only, and in connection with the accompanying drawings. It is to be understood that such description is merely illustrative and not intended to limit the scope of the present invention. The structure, proportion, size and the like shown in the drawings are only used for matching with the content disclosed in the specification, so that the person skilled in the art can understand and read the description, and the description is not used for limiting the limit condition of the implementation of the invention, so the method has no technical essence, and any structural modification, proportion relation change or size adjustment still falls within the range covered by the technical content disclosed by the invention without affecting the effect and the achievable purpose of the invention. Moreover, in the following description, descriptions of well-known structures and techniques are omitted so as to not unnecessarily obscure the concepts of the present invention.

It should be noted that, in order to avoid obscuring the present invention with unnecessary details, only the structures and/or processing steps closely related to the scheme according to the present invention are shown in the drawings, and other details not so relevant to the present invention are omitted.

As shown in fig. 1, the following technical solutions are adopted in the present embodiment: the preparation method comprises the following steps: bi of 99.95% purity₂Te₃Splicing the target material and the Te target material with the purity of 99.99% according to the ratio of 1:1 to form a mixed target material, and placing the mixed target material in an epitaxial chamber; the mixed target material is scattered into plasma beams by using laser beams generated by an excimer laser (COMPEXPro 201 KrF excimer laser produced by Coherent), and the dispersed Bi atoms and Te atoms are deposited on the other side substrate to gradually grow a nano-scale film.

FIG. 1 is a PLEES system configuration diagram: the laser provides energy, the epitaxial chamber creates a 10-7Pa grade high vacuum environment, the camera records and photographs diffraction patterns formed by the RHEED (reflective high-energy electron diffractometer) in real time, and the RHEED system is connected with a computer to record the diffraction intensity of X rays.

FIG. 2 is a schematic view Bi of target mounting₂Te₃The target material and the pure Te target material are mixed and installed according to the proportion of 1: 1.

SubstrateAl selected as single-side polished₂O₃(001) Materials, since a clean substrate surface is conducive to thin film growth, the substrate is subjected to standard cleaning procedures prior to use. Ultrasonic cleaning the substrate in alcohol and acetone solution respectively for 10 minutes each time, and then washing with deionized water; then, the substrate is dried by nitrogen, and is placed in a substrate holder to be preheated to a certain temperature. The vacuum degree of the epitaxial chamber is controlled to be 10^-7Pa to avoid other air ion effects. The laser irradiates the target material frame rotating at a constant speed to enable the target material frame to be uniformly hit on the mixed spliced target material, and the operation time of the laser is recorded. After the preparation is finished, the substrate is cooled to a certain temperature for in-situ annealing. The laser energy frequency will affect the atomic density and kinetic energy of the plasma beam, the laser operation time will affect the film thickness, and the annealing temperature and time will affect the film crystallinity and surface morphology. In order to find the best experimental conditions, a plurality of different sets of experimental conditions are tested to find the best experimental conditions. In the preparation process, a reflection type high-energy electron diffractometer (RHEED) is used for monitoring the growth condition of the film; a distinct diffraction pattern was found to appear on the Reflective High Energy Electron Diffractometer (RHEED) as shown in figure 3. In the process of growing the film, four points are randomly selected and diffraction patterns of the four points are collected. Although the oscillation image is influenced under the influence of laser, the oscillation curve shows obvious periodic change, which proves that the film grows stably.

As shown in FIG. 4, A is the diffraction intensity monitored at four randomly selected positions in the diffraction region, and B is the variation curve of the X-ray diffraction intensity of four monitoring points with time, which is periodically changed, and the results show that Bi₂Te₃The film grows in atomic layer sequence at uniform rate.

When obvious diffraction patterns and obvious periodic oscillation curves are found in the synthesis process, the prepared film is characterized. AFM (olympus AC240) was used to characterize the film surface morphology and thickness. The surface morphology and the element composition of the film are measured by adopting a JEOL JSM-7800F Primes super-resolution field emission scanning electron microscope/energy spectrometer (SEM/EDS), and the energy spectrum acceleration voltage is 15 kV. The film unit cell structure and crystallinity were characterized using a multifunctional X-ray diffractometer model D8 DaVinci (D8 Bruker Advance Da Vinci). Grazing incidence X-ray diffraction (GIXRD) was carried out at a voltage of 40kV, a current of 40mA and an X-ray incidence angle of 3 degrees. Lastly, a comprehensive physical property testing system (PPMS) is adopted to characterize the electrical properties of the synthesized film. The surface resistivity under zero field is plotted against the temperature in the temperature range of 3-400K. Meanwhile, in the magnetic field range of-6T to 6T, a Hall resistance curve changing with the magnetic field and a resistance change rate curve changing with the magnetic field at room temperature are drawn.

PLEES puts strict requirements on the preparation process and various experimental conditions. In the preparation process, the high-quality nano topological insulator film can be prepared only by accurately controlling various parameters including laser energy, laser frequency, laser duration, substrate temperature, annealing temperature and annealing time. In order to optimize preparation parameters, the following 16-group comparison experiment is carried out according to the 6 experimental conditions needing to be regulated, the laser frequency is set to be 2Hz, and due to the excessively low laser frequency, Te atoms are not sufficiently supplemented when the target is swept by laser, so that atomic holes are generated in the thin film due to the loss of the Te atoms in the growth process. Similarly, the laser energy should be set to 150mJ, too low laser energy cannot break the molecules into atomic states, and too high energy makes the kinetic energy of the formed ion beam too high to recombine Bi on the substrate₂Te₃A molecule. The laser duration also has a significant effect on the film thickness. When the laser duration is below 20min, the film is too thin, and RHEED diffraction fringes and GIXRD diffraction peaks are not obvious; when the laser duration is 30min or more, the film stably grows, and a significant diffraction peak can be observed. Simultaneously selecting a first group of data to prepare the nano-scale Bi₂Te₃A topological insulator film.

Wherein the optimal substrate temperature is 400 ℃ and the optimal annealing temperature is 280 ℃. The film growth is not favored when the temperature is too high or too low, and the fundamental reason is that 1) Bi atoms and Te atoms are recombined into Bi by virtue of covalent bonds₂Te₃Molecules, which rely on van der waals forces to form a unit cell structure; 2) with temperature directly influencing atoms and moleculesEnergy, thereby affecting cell formation. Therefore, the substrate temperature and the annealing temperature need to be controlled within a precise temperature range. The optimal annealing time is determined to be 3h, the annealing is not complete, so that a good cell structure cannot be formed, and the excessive annealing causes Te atoms to be deleted to form cavities. Finally, the preparation of Bi by PLEES is determined₂Te₃Parameters of the nanoscale topological insulator thin film: the laser energy is 150mJ, the laser frequency is 2Hz, the laser time is more than 30min, the substrate temperature is 400 ℃, the annealing temperature is 280 ℃, and the annealing time is 3 h.

The optimized preparation parameters are utilized to successfully prepare the nano-scale Bi₂Te₃Topological insulator film, and characterizing the surface morphology. As in AFM images, the thin film edge cross section showed a layered structure, demonstrating that Bi₂Te₃Crystallizing in a multilayer structure. These layered structures result from the van der Waals bonding properties of the material, which results also from the Bi content during annealing₂Te₃The grains are steadily grown. The cross-sectional film thickness (dz) is 69.82 nm; since the preparation time of the thin film was 30min, the average growth rate of the thin film was about 140 nm/h. Compared with the growth rate of 2.2nm/h on MBE equipment, the preparation scheme greatly improves the film preparation efficiency. AFM images are shown in fig. 5, 6, and 7:

the surface characteristics of the prepared film were further characterized using SEM. As shown in fig. 8, by SEM images of different magnifications, it was observed that the surface of the prepared film was dense and smooth, and had no cracks or holes. The grain edge on the film is triangular or truncated hexagonal, and the grain size is about 300 nm. The triangular character of the grains indicates that the product has triple symmetry.

Three are cut out on the surface of the prepared film as shown in FIG. 9Different regions (pt1, pt2 and pt3), as shown in the following figures, and elemental analysis was performed using EDS; the elemental atom statistics of the film in the three regions are shown as elemental atom ratios in fig. 9B-D. Sampling point element ratio Bi: te is 2: 3. Thus, the target material structure is improved by the PLEES method, and Bi reaching the expected element proportion is prepared₂Te₃A film.

	Te-L	Bi-M
			Base(1)_pt1	57.47	42.53
Base(1)_pt2	58.77	41.23
			Base(1)_pt3	58.01	41.99

The result of GIXRD measurement is compared with the standard PDF card, the characterization result is shown in fig. 10, the matching degree of diffraction peaks is intact, and the lattice constants calculated are that a is 0.260nm, b is 0.438nm, and c is 3.05 nm; when the preferred orientation in the (001) direction is observed from the main peak and the 2 θ angle is only in the range of 10 to 80 ° (00L, L is 3n, and n is an integer), Bi is proved₂Te₃Rhombohedral structure of the unit cell, corresponding to Bi₂Te₃Hexahedral lamellar structureConsistently, it belongs to R-3m space group, orthorhombic system. The structure of the unit cell is shown in FIG. 11, Bi₂Te₃The bonding is carried out in a sequence of Te-Bi-Te-Bi-Te, atoms are bonded by covalent bonds, and molecules are bonded by Van der Waals force.

As shown in fig. 12, the sample film was subjected to PPMS comprehensive physical property test, and showed peculiar topological properties; where D shows a significant weak anti-localization effect, typical of topological insulator features.

It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrative embodiments, and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Furthermore, it should be understood that although the present description refers to embodiments, not every embodiment may contain only a single embodiment, and such description is for clarity only, and those skilled in the art should integrate the description, and the embodiments may be combined as appropriate to form other embodiments understood by those skilled in the art.

Claims

1. The PLEES preparation system of the laser pulse enhanced molecular beam epitaxy system is characterized in that: the preparation method comprises the following steps: bi of 99.95% purity₂Te₃Splicing the target material and the Te target material with the purity of 99.99% according to the ratio of 1:1 to form a mixed target material, and placing the mixed target material in an epitaxial chamber; the laser beam generated by the excimer laser is utilized to scatter the mixed target material to form a plasma beam, and the dispersed Bi atoms and Te atoms are deposited on the substrate at the other side to gradually grow a nano-scale film.

2. PL of a laser pulse enhanced molecular beam epitaxy system according to claim 1EES preparation system, its characterized in that: the substrate is selected to be single-side polished Al₂O₃Materials, since a clean substrate surface is conducive to thin film growth, the substrate is subjected to standard cleaning procedures prior to use.

3. The PLEES preparation system of laser pulse enhanced molecular beam epitaxy system according to claim 2, wherein: the substrate cleaning process comprises the following steps: ultrasonic cleaning is respectively carried out in alcohol and acetone solution, each time, the alcohol is ultrasonically cleaned for 10 minutes, and then deionized water washing is carried out; then, the substrate is dried by nitrogen, and is placed in a substrate holder to be preheated to a certain temperature.

4. The PLEES preparation system of laser pulse enhanced molecular beam epitaxy system according to claim 1, wherein: the vacuum degree of the epitaxial chamber is controlled to be 10^-7Pa to avoid influence of other air ions; irradiating the target material rack rotating at a constant speed by laser to enable the target material rack to uniformly hit the mixed spliced target material, and recording the operation time of the laser; after the preparation is finished, the substrate is cooled to a certain temperature for in-situ annealing.