CN108933193B

CN108933193B - Transfer method and application of ferromagnetic semiconductor film

Info

Publication number: CN108933193B
Application number: CN201710385970.1A
Authority: CN
Inventors: 袁恺; 王海龙; 姚笑寒; 赵建华; 戴伦; 叶堉
Original assignee: Peking University
Current assignee: Peking University
Priority date: 2017-05-26
Filing date: 2017-05-26
Publication date: 2020-02-11
Anticipated expiration: 2037-05-26
Also published as: CN108933193A

Abstract

The invention discloses a transfer method and application of a ferromagnetic semiconductor film, which realizes the transfer of a (Ga, Mn) As film on any substrate.The transferred (Ga, Mn) As thin film can be combined with other two-dimensional layered materials (such As semi-metal graphene and semiconductor MoS) with different physical properties ₂NbSe superconductor ₂Etc.) to build magnetic heterostructures by van der waals forces, can be used to fabricate novel spin devices. The invention can improve the compatibility of the (Ga, Mn) As film and the traditional silicon process integration technology, and has important significance for expanding the application of the spintronics.

Description

Transfer method and application of ferromagnetic semiconductor film

Technical Field

The invention relates to the technical field of semiconductor corrosion technology and film transfer and the field of semiconductor spintronics, in particular to a ferromagnetic semiconductor film transfer method and a method for constructing a van der Waals heterojunction by adopting the method.

Background

The silicon-based Complementary Metal Oxide Semiconductor (CMOS) field effect transistor process has been developed to the 14nm technology node, which is expected to reach its limit soon, and new information devices need to be found to continue moore's law. Information storage and processing with higher density, higher speed and lower power consumption can be achieved using spintronics (spintronics) developed to manipulate the degree of freedom of electron spins in semiconductors. Finding a suitable spin injection source is key to the implementation of semiconductor spintronics. The magnetic semiconductor has three advantages as a spin injection source: (1) the problem of mismatching with the conductivity of the semiconductor can be avoided; (2) the method is compatible with the existing semiconductor process; (3) has higher spin injection efficiency. In 1996, after a magnetic semiconductor (Ga, Mn) As was successfully prepared, it has been widely used As a representative magnetic semiconductor in the field of semiconductor spintronics.

The low-temperature molecular beam epitaxy (LT-MBE) technology is mainly adopted for preparing ferromagnetic semiconductor (Ga, Mn) As thin films at present. Molecular beam epitaxy techniques require (Ga, Mn) As thin films grown on lattice-matched substrates, such As GaAs, (In, G)a) As, and the like. Due to the existence of the substrate, the flexible application of the (Ga, Mn) As thin film spin device is greatly limited. If the (Ga, Mn) As thin film can be peeled off from the growth substrate and transferred to any substrate, the method has profound significance for the basic and application research based on the (Ga, Mn) As thin film. The transferred (Ga, Mn) As thin film can be combined with other two-dimensional layered materials (such As semi-metal graphene and semiconductor MoS) with different physical properties ₂NbSe superconductor ₂Etc.) to build magnetic heterostructures by van der waals forces, can be used to fabricate novel spin devices. By the transfer technology, the compatibility of the (Ga, Mn) As film and the traditional silicon process integration technology can be improved, and the method has important significance for expanding the application of the spintronics.

Disclosure of Invention

The invention aims to provide a method for stripping and transferring ferromagnetic semiconductor (Ga, Mn) As grown by Molecular Beam Epitaxy (MBE) from a GaAs substrate, which can be used for constructing Van der Waals heterostructures of other two-dimensional materials and provides convenience for further carrying out the novel physical characteristic research of the heterostructures based on the (Ga, Mn) As.

The technical scheme of the invention is as follows:

a method for transferring a (Ga, Mn) As ferromagnetic semiconductor thin film, comprising the steps of:

1) obtaining a (Ga, Mn) As film with an (Al, Ga) As sacrificial layer on a GaAs substrate by adopting a solid-state low-temperature molecular beam epitaxial growth method (LT-MBE);

2) uniformly spin-coating a layer of trichloroethylene solution containing sealing wax on the surface of the (Ga, Mn) As film, and forming the sealing wax film As a supporting layer after curing and annealing treatment;

3) cleaving the sample into a desired shape and size to expose a cross-section of the (Al, Ga) As sacrificial layer;

4) soaking a sample in a low-concentration hydrofluoric acid solution, and separating the (Ga, Mn) As film from the substrate by utilizing selective corrosion of hydrofluoric acid on the (Al, Ga) As;

5) by SiO ₂Taking out the (Ga, Mn) As film on the Si substrate, airing, and dissolving and removing sealing wax on the surface of the (Ga, Mn) As film by using trichloroethylene; after being cleaned, the mixture is put on SiO ₂Mesh of/SiObtaining a (Ga, Mn) As film sample on the target substrate.

The invention further provides a method for constructing a van der waals heterojunction based on the stripped and transferred (Ga, Mn) As ferromagnetic semiconductor thin film, which comprises the following steps:

2) uniformly spin-coating a layer of trichloroethylene solution containing sealing Wax on the surface of the (Ga, Mn) As film, and forming an Apiezon Wax W film As a supporting layer after curing and annealing treatment;

5) by SiO ₂Taking out the (Ga, Mn) As film on the Si substrate, airing, and dissolving and removing sealing wax on the surface of the (Ga, Mn) As film by using trichloroethylene; after being cleaned, the mixture is put on SiO ₂Obtaining a (Ga, Mn) As film sample on a/Si target substrate;

6) on the micromanipulation stage, SiO is coated with a polypropylene carbonate (PPC) film ₂The (Ga, Mn) As film on the/Si substrate is stuck up and put on other two-dimensional materials, e.g. BN, MoS ₂And, a van der waals heterostructure is constructed.

In the step 1), the Al component in the (Al, Ga) As is 20-80%.

The sealing Wax in the step 2) is black Wax-like substance, namely, an ashezon W type vacuum sealing Wax (Apiezon Wax W), and the sealing Wax is a general chemically inert Wax compound under the brand of English ashezo (ashezon), has corrosion resistance to some common corrosive solutions (such As hydrofluoric acid, nitric acid and acetic acid), can play a supporting role on thinner and brittle materials, is commonly used As a protective coating in the microelectronic industry, and can support a stripped (Ga, Mn) As film in the technology to prevent the stripped (Ga, Mn) As film from being broken in the solution.

In the above step 2), factors affecting the film quality of the exfoliated (Ga, Mn) As are considered: the sealing wax ApiezonWax W needs to be solidified in the air at normal temperature for more than half an hour to be completely solidified on the surface of the film; the annealing is carried out for 0.5-1 hour at about 100 ℃, and the stress of the Apiezon Wax W protective layer is mainly considered to be adjusted by the treatment, so that the supporting effect is more obvious.

In the step 4), selective etching of (Al, Ga) As is mainly considered when selecting the etching solution. And selecting a dilute solution with the percentage concentration of hydrofluoric acid between 6% and 10% as the corrosive liquid. Can ensure the quality of the stripped (Ga, Mn) As film, and H generated by the reaction if the concentration of the hydrofluoric acid solution is too high ₂The bubbles prevent the corrosive liquid from entering the (Al, Ga) As thin film layer, and thus complete corrosion of (Al, Ga) As is difficult to achieve. On the contrary, if the concentration of the hydrofluoric acid solution is too low, the etching time is too long, and the quality of the (Ga, Mn) As is affected by contacting the hydrofluoric acid for a long time due to the thinness of the (Ga, Mn) As. The (Ga, Mn) As film was then rinsed off the substrate with a disposable plastic dropper.

In the step 5), the surface with the Apiezon Wax W protective layer is upward when the film is fished, otherwise, the (Ga, Mn) As film falls off from the target substrate when the Apiezon Wax W protective layer is removed after the film is dried.

Transfer to SiO Using superconducting Quantum interferometer (SQUID) ₂Samples of the/Si substrate were magnetically characterized. The measurement results show that the (Ga, Mn) As thin film after stripping and transferring still maintains ferromagnetism before transferring, and the Curie temperature is about 80K.

In the step 6), the heterojunction construction method is a dry transfer method of a clean interface. The polypropylene carbonate (PPC) is an organic polymer and is characterized in that the solidification and liquefaction process can be controlled by temperature, so that the viscosity of the PPC on a sample can be controlled. The whole transfer process is completed based on a microscope and a micromanipulation platform. SiO transferred with (Ga, Mn) As film ₂the/Si substrate was placed on a small hotplate under the microscope lens. A thin film of polypropylene carbonate (PPC) supported by a small piece of Polydimethylsiloxane (PDMS) was attached to a glass slide and held under a micromanipulation arm. Under a microscope, PPC is slowly attached to SiO ₂(Ga, Mn) As film on/Si substrate while hot plate is heated and then the micromanipulation arm is liftedTaking up the As film; subsequently, the (Ga, Mn) As thin film on the PPC is aligned and brought into contact with the two-dimensional material on the target substrate from which the two-dimensional material is peeled, and the hot plate is heated, at which time the PPC melts and adheres to the target substrate. And then soaking the PPC on the target substrate by using acetone to remove the PPC, so that the Van der Waals heterostructure of the (Ga, Mn) As thin film and the two-dimensional material can be obtained. The two-dimensional materials can be repeatedly picked up by controlling the temperature in the step, so that the stacking of multiple layers of two-dimensional materials is realized, and the stacking is not limited to the stacking of two-dimensional materials.

The invention has the advantages that:

the invention realizes the transfer of (Ga, Mn) As on any substrate. The transferred (Ga, Mn) As thin film can be combined with other two-dimensional layered materials (such As semi-metal graphene and semiconductor MoS) with different physical properties ₂NbSe superconductor ₂Etc.) to build magnetic heterostructures by van der waals forces, can be used to fabricate novel spin devices. By the transfer technology, the compatibility of the (Ga, Mn) As film and the traditional silicon process integration technology can be improved, and the method has important significance for expanding the application of the spintronics.

Drawings

FIG. 1 transfer of (Ga, Mn) As thin films to SiO ₂Photo optics behind/Si substrate, wherein the silver part on the substrate is (Ga, Mn) As thin film;

FIG. 2 transfer to SiO ₂Optical microscope photograph of (Ga, Mn) As thin film after/Si substrate;

FIG. 3 transfer to SiO Using SQUID ₂(a) M-H curve and (b) M-T curve obtained by measuring (Ga, Mn) As film of/Si substrate, the Curie temperature of which is about 80K;

FIG. 4 shows (a) M-H curve and (b) M-T curve obtained by SQUID measurement of (Ga, Mn) As thin film before transfer, the Curie temperature of which is about 90K;

FIG. 5. schematic dry transfer of a (Ga, Mn) As/two-dimensional material heterostructure with (a) schematic of the PPC thin film produced and (b) schematic of the dry transfer apparatus; wherein: 1-a silicon wafer; 2-adhesive tape; 3-microscope sample stage; 4-glass slide; 5-PDMS supporting block; 6-PPC; 7- (Ga, Mn) As thin film samples; 8-SiO ₂a/Si substrate; 9-hot plate; 10-a temperature controller; 11-A micromanipulation platform; 12-MoS ₂A sample; 13-MoS ₂SiO of the sample ₂a/Si substrate;

FIG. 6 (Ga, Mn) As/MoS ₂Optical microscope photograph of heterostructure in which (a) is transferred to SiO ₂A thin film behind the Si substrate; (b) on SiO by mechanical stripping ₂Obtaining few-layer MoS on a/Si substrate ₂(ii) a (c) (Ga, Mn) As on PPC after (Ga, Mn) As is taken up from the original substrate by PPC film; (d) placing (Ga, Mn) As to the target MoS ₂The van der waals heterojunction formed thereon.

Detailed Description

The following is a description of the specific examples (Ga, Mn) As/MoS ₂The method of fabricating the heterostructure and the accompanying drawings are further illustrative of the present invention, but do not limit the scope of the invention in any way.

1. Growing a (Ga, Mn) As film with a (Al, Ga) As sacrificial layer on a GaAs substrate by using an LT-MBE technology. The method comprises the following specific steps: a1000 nm thick sacrificial layer of (Al, Ga) As is grown on a GaAs substrate with an Al content of about 80% by the LT-MBE technique, followed by the subsequent growth of 20nm thick (Ga, Mn) As.

2. And preparing a supporting layer Apiezon Wax W solution. The method comprises the following specific steps: 5g of Apiezon Wax W is dissolved in 20mL of trichloroethylene, stirred, dissolved sufficiently and then placed in a refrigerator for cold storage.

3. Apiezon Wax W was spin coated on the surface of the (Ga, Mn) As sample As a support. The method comprises the following specific steps: and cleaning the epitaxial layer sample according to the sequence of acetone, trichloroethylene, acetone, ethanol and deionized water, and drying. Subsequently, a solution of Apiezon Wax W in trichloroethylene was spin coated uniformly onto the sample epitaxial layer surface at a parameter of 2000 rpm, then cured in air for half an hour and annealed on a hot plate at 100 ℃ for half an hour. After the sample is sufficiently cooled, the sample is cleaved into the desired shape and size, exposing a cross-section of the (Al, Ga) As sacrificial layer for corrosion.

4. Soaking the sample by using low-concentration hydrofluoric acid, and separating the (Ga, Mn) As film from the substrate by utilizing the selective corrosion of the hydrofluoric acid on the (Al, Ga) As. The specific operation steps are as follows: 5mL of 40% HF solution and 20mL of deionized water were mixed to prepare an etching solution with a concentration of 8%. The etching time is 40 h. During the etching reaction, slight rolling and lifting of the epitaxial film were observed. When the reaction lasts for about 24-40h, the epitaxial film is slightly damaged, and a plastic dropper can be used for flushing the epitaxial film away from the surface of the substrate.

5. By SiO ₂Taking out the (Ga, Mn) As film from the/Si substrate, and airing. The surface was dissolved with trichloroethylene to remove Apiezon Wax W. The specific operation steps are as follows: when the epitaxial film is separated from the surface of the substrate, the epitaxial film is fished up by using a silicon wafer or a glass slide and is put into deionized water to be cleaned for 5 minutes, and then SiO is used ₂And fishing out the silicon substrate. Excess water was blotted with filter paper, left to stand for several hours, and then heated with a hot plate at 110 ℃ for 5min to allow the (Ga, Mn) As to be sufficiently adsorbed on the surface of the substrate. Subsequently, the sample is placed into trichloroethylene for soaking for 5min to dissolve Apiezon Wax W on the surface, and then the sample is washed by acetone, ethanol and deionized water to obtain the product transferred to SiO ₂(Ga, Mn) As thin film samples of/Si substrate. The resulting (Ga, Mn) As samples were relatively intact, approximately 3X 3mm in size ². The optical physical pictures and optical micrographs of the resulting (Ga, Mn) As samples correspond to FIGS. 1 and 2, respectively.

6. The M-H and M-T curves of the samples before and after transfer were measured using SQUID, respectively. The measurement results show that the transition to SiO occurs ₂The (Ga, Mn) As sample of the/Si substrate still has ferromagnetism, and the Curie temperature of the (Ga, Mn) As sample is about 80K and is basically consistent with the Curie temperature of the (Ga, Mn) As sample before transfer. It can be seen that the ferromagnetic properties of the sample before transfer are substantially retained by the sample after transfer. The measurements of the SQUID are shown in figures 3 and 4.

7. A film of polypropylene carbonate (PPC) required for dry transfer was made. With reference to fig. 5(a), the specific operation steps are as follows: 6g of polypropylene carbonate (PPC) was dissolved in 40mL of anisole, stirred at 160 ℃ for 4 hours with a magnetic stirrer, and then stored at room temperature in a brown bottle. A clean silicon wafer 1 is cut into a square with the depth of 1cm, an adhesive tape 2 is attached to the surface of the clean silicon wafer, a small hole is cut in the center of the adhesive tape 2, the silicon wafer is exposed, and the diameter of the silicon wafer is about 3 mm. Then, PPC was spin-coated on the surface thereof at a rotation speed of 3000 rpm. Finally drying at 75 ℃ for 5 minutes.

8. A Polydimethylsiloxane (PDMS) support block required for dry transfer was fabricated. The specific operation steps are as follows: 30mL of prepolymer A solution and 1.7mL of curing agent B solution were mixed thoroughly and placed in a 15cm diameter petri dish to form a film (a silicon wafer was placed on the bottom to make the film uniform and flat) with a thickness of about 1 mm. Placing the mixture into a vacuum drying tower for vacuumizing and storing for one night to allow gas to escape. Followed by baking in an oven at 75 ℃ for 3 hours. The prepared PDMS was cut into small pieces (3 mm. times.3 mm) and adhered several times with a transparent adhesive tape to remove surface dirt. Followed by a 3 minute oxygen plasma clean. Each PDMS pellet was then back-mounted on a glass slide and oven baked at 75 ℃ for 1 hour. Finally, the front side of the PDMS was cleaned with tape.

9、(Ga,Mn)As/MoS ₂And (4) preparing the heterostructure. The method is described by combining the attached fig. 5(b) and fig. 6, and the specific operation steps are as follows: in two pieces of SiO ₂(Ga, Mn) As thin film sample 7 after transfer and MoS obtained by mechanical stripping were obtained on a/Si substrate ₂Sample 12 corresponds to (a) and (b) of FIG. 6, respectively. And tearing off the adhesive tape 2 coated with the PPC6 by spin from the silicon wafer 1, adhering the PPC with the middle circular hole part to the PDMS supporting block 5, and using PDMS as the support of the PPC. The slide 4 is fixed on the micromanipulation platform 11. SiO to transfer the (Ga, Mn) As thin film sample 7 ₂the/Si substrate 8 is adhered to a small hot plate 9 and is integrally placed on a microscope sample stage 3. After PPC6 was aligned with the (Ga, Mn) As sample, the platform was lowered to bring the PPC into close proximity with the sample slowly while the sample was heated to 60 ℃ with a hot plate using thermostat 10 for 1 minute, and then cooled to room temperature. The (Ga, Mn) As sample was transferred from the substrate to the PPC by slowly raising the platform, corresponding to FIG. 6 (c). Will subsequently carry MoS ₂SiO of the sample ₂the/Si substrate 13 is attached to a small hot plate and the whole is placed on a microscope sample stage. (Ga, Mn) As samples on PPC were aligned to MoS ₂And then, lowering the operating platform to enable the PPC to be slowly attached to the sample, and simultaneously heating the sample to 90 ℃ by using a temperature controller, wherein the PPC is gradually melted and attached to the substrate. The platform was then raised so that PPC remained with (Ga, Mn) As at the same time in the MoS ₂On the sample. Finally, soaking the substrate in acetone, ethanol and deionized water to remove PPC, and obtaining (Ga, Mn) As/MoS ₂The heterostructure corresponds to (d) of fig. 6.

Although the present invention has been described with reference to the preferred embodiments, it is not intended to be limited thereto. Those skilled in the art can make numerous possible variations and modifications to the present invention, or modify equivalent embodiments, using the methods and techniques disclosed above, without departing from the scope of the present invention. Therefore, any simple modification, equivalent change and modification made to the above embodiments according to the technical essence of the present invention are still within the scope of the protection of the technical solution of the present invention, unless the contents of the technical solution of the present invention are departed.

Claims

1. A method for transferring a ferromagnetic semiconductor thin film comprises the following steps:

1) obtaining a (Ga, Mn) As film with an (Al, Ga) As sacrificial layer on a GaAs substrate by adopting a solid-state low-temperature molecular beam epitaxial growth method;

3) cleaving the (Ga, Mn) As thin film with the supporting layer into a desired shape and size to expose the cross section of the (Al, Ga) As sacrificial layer;

4) soaking the substrate in a hydrofluoric acid solution, and separating the (Ga, Mn) As film with the supporting layer from the substrate by utilizing the selective corrosion of hydrofluoric acid on the (Al, Ga) As;

5) by SiO ₂The (Ga, Mn) As film with the supporting layer is fished out and dried by the Si substrate, and the sealing wax on the surface of the (Ga, Mn) As film is dissolved and removed by trichloroethylene; after being cleaned, the mixture is put on SiO ₂Obtaining the (Ga, Mn) As film on the/Si substrate.

2. The transfer method according to claim 1, wherein in the above step 1), (Al, Ga) As has an Al component of 20% to 80%.

3. The transfer method according to claim 1, wherein in the step 2), the sealing Wax is vacuum sealing Wax Apiezon Wax W, the curing time is 0.5 hours or more, and the annealing is performed at about 100 ℃ for 0.5 to 1 hour.

4. The transfer method according to claim 1, wherein in step 4) the percentage concentration of the hydrofluoric acid solution is between 6% and 10%.

5. The transfer method as claimed in claim 1, wherein in the step 4), SiO is used ₂The sealing wax is arranged on one side of the Si substrate, and the Ga, Mn As film is fished out from the Si substrate.

6. A method for forming a van der Waals heterojunction in a ferromagnetic semiconductor thin film, comprising the step of transferring the ferromagnetic semiconductor thin film according to claim 1 to SiO ₂Obtaining (Ga, Mn) As film on a/Si substrate, and then using a polypropylene carbonate film to remove SiO ₂The (Ga, Mn) As film is stuck on the/Si substrate and put on the two-dimensional material, and the polypropylene carbonate film is removed to obtain the Van der Waals heterostructure of the (Ga, Mn) As film and the two-dimensional material.

7. The method of claim 6, wherein the poly (propylene carbonate) film is supported by a small piece of polydimethylsiloxane, affixed to a slide and held under a micromanipulation arm, and the SiO transferred with the (Ga, Mn) As film ₂the/Si substrate is placed on a hot plate under a microscope lens, and under the microscope, the polypropylene carbonate is slowly attached to the SiO ₂Heating a (Ga, Mn) As film on a/Si substrate by a hot plate, and lifting a micromanipulation arm to pick up the (Ga, Mn) As film; the (Ga, Mn) As film on the polypropylene carbonate is then aligned and brought into contact with the two-dimensional material on the substrate.

8. The method of claim 6, wherein the polypropylene carbonate film is removed with an acetone bubble.

9. The method of construction of claim 6 wherein the two-dimensional materialIs semi-metal graphene, semiconductor MoS ₂Or superconductor NbSe ₂。