RU2680544C1 - Method of cultivation of epitaxial films of europium monoxide on graphene (options) - Google Patents

Abstract

FIELD: manufacturing technology.SUBSTANCE: invention relates to the methods for producing epitaxial thin-film materials, namely films of europium monoxide on graphene, and it can be used to create such spintronics devices as a spin transistor and an injector of spin-polarized carriers. Method of growing europium monoxide EuO epitaxial films on graphene involves the formation of a europium submonolayer with a surface phase (√3 × √3) by molecular beam epitaxy on the surface of a preformed graphene monolayer structure/substrate at the substrate temperature T=20–100 °C and pressure of the flow of atoms of europium P=(1⋅10–1⋅10) Torr, deposition of a layer of europium monoxide EuO at the temperature of the substrate T=20–100 °C, pressure of oxygen flow P=(1⋅10–1⋅10) Torr and pressure flux of europium atoms P=(1⋅10–1⋅10) Torr, satisfying condition 10⋅P≤P≤11⋅P, to achieve the required layer thickness of europium monoxide EuO. In one of the embodiments of the invention after the above operations, a layer of europium monoxide EuO is deposited at the temperature of the substrate T=340–420 °C, oxygen flow pressure P=(1⋅10–1⋅10A) Torr and pressure flux of europium atoms P=(1⋅10–1.5⋅10A) Torr satisfying condition 10⋅P≤P≤15⋅P, to achieve the required total thickness of the layer of europium monoxide EuO. In particular cases of carrying out the invention after deposition of a film of europium monoxide, it is annealed in vacuum within the temperature range of T=490–520 °C.EFFECT: formation of epitaxial stoichiometric films of europium monoxide with the thickness of more than 5 nm with high crystalline perfection without inclusions of phases of higher oxides on graphene is obtained, which allows to obtain magnetic states in graphene to create such technical devices as a single-electron transistor and a spin filter.4 cl, 4 dwg, 4 ex

Description

The invention relates to methods for producing epitaxial thin-film materials, namely, europium monoxide films on graphene, and can be used to create spintronics devices, for example, spin-polarized current injectors.

The EuO / Graphene structure is a promising basis for creating spintronic devices due to the unique properties of materials: EuO is a ferromagnetic insulator with a large absolute value of the magnetic moment per atom, which allows it to be used as an injector of spin-polarized electrons in a conductor. At the same time, graphene is known for its high conductivity and long spin diffusion, which makes it a good conductor of spin current. In addition, the possibility of separation of charge carriers along the spin in graphene due to the proximity effect with ferromagnetic oxide is shown.

The invention is known "Method and equipment for growing single-crystalline oxides, nitrides and phosphides" (patent No. US 7135699 B1), in which a layered structure containing rare earth oxide is formed on various substrates, including silicon, and forms a superlattice. Within the framework of the method, epitaxial layers of europium monoxide can be grown on silicon substrates during the deposition of metal in an oxygen stream. The disadvantage of the invention is the fact that the invention is focused on dielectrics with high dielectric constant, and therefore does not take into account the peculiarities of the growth of semiconductor EuO layers, which require the preservation of the valency of Eu ^{2+ ions} . Meanwhile, growing EuO requires a special approach to prevent the transition of the europium ion to the trivalent state and, at the same time, to maintain epitaxial growth.

A well-known article is “Atomic Layer Deposition of Metal Oxides on Pristine and Functionalized Graphene” (Article DOI: 10.1021 / ja8023059), in which thin films of metal oxides are grown on graphene by atomic layer deposition. The disadvantage of this technique is the inability to obtain epitaxial films with a good crystalline structure.

A well-known article is “Growth of Epitaxial Thin Films of Oxides on Graphene” “Growth of Epitaxial Oxide Thin Films on Graphene” (Article DOI: 10.1038 / srep31511), in which SrTiO ₃ epitaxial thin films are grown by laser ablation on graphene. The disadvantage of the invention is the inability to grow stoichiometric EuO while maintaining the valency of Eu ^{2+ ions} .

The well-known article is “Deposition of a EuO ferromagnetic insulator on graphene” “Integration of the Ferromagnetic Insulator EuO onto Graphene” (Article DOI: 10.1021 / nn303771f), in which EuO epitaxial thin films are grown by molecular beam epitaxy on graphene. The disadvantage of the invention is the presence of a significant amount of europium oxide Eu ₃ O ₄ in the film.

The well-known article “Structure and magnetic properties of ultrathin EuO films on graphene” “Structure and magnetic properties of ultra thin textured EuO films on graphene” (Article DOI: 10.1063 / 1.4821953), in which stoichiometric ultrathin EuO films are produced by molecular beam epitaxy on substrates Graphene / Ir (111). The disadvantage of this invention is the fact that the invention is focused on growing films with a thickness of less than 10 monolayers (~ 2.6 nm), and therefore does not take into account the growth characteristics of films of greater thickness while maintaining stoichiometry.

The invention is known "A method of growing epitaxial films of europium monoxide on silicon" (Patent No. RU 2557394), in which the europium silicide submonolayer is grown by molecular beam epitaxy at a substrate temperature T _s = 640 ÷ 680 ° C and a pressure of the stream of europium atoms P _Eu = ( 1 ÷ 7) ⋅10 ^-8 Torr, after which the deposition is carried out at the substrate temperature T _s = 340 ÷ 380 ° С, oxygen flow pressure P _O2 = (0.2 ÷ 3) ⋅10 ^-8 Torr and europium atom flux pressure P _Eu = (1 ÷ 4) ⋅10 ^-8 Torr, then the deposition is carried out at the substrate temperature T _s = 430 ÷ 490 ° С, oxygen flow with pressure P _O2 = (0.2 ÷ 3) ⋅10 ^-8 Torr, and the flux of europium atoms with a pressure P _Eu = (1 ÷ 7) 10 ^-8 Torr. The procedure also provides for a number of vacuum anneals:

- intermediate annealing after the first stage (T _s = 340 ÷ 380 ° C) of growth, carried out at a temperature T _s = 490 ÷ 520 ° C;

- final annealing in the temperature range T _s = 500 ÷ 560 ° С

The disadvantage of the invention is the impossibility of applying the method for the growth of EuO films on graphene.

Disclosure of invention

The technical result of the present invention is the formation of epitaxial stoichiometric EuO films with a thickness of more than 5 nm with high crystalline perfection without inclusion of higher oxide phases on graphene. The result obtained made it possible to obtain magnetic states in graphene, which was not achieved in previous works and can be used to create such technical devices as a single-electron transistor and a spin filter.

при температуре подложки T_s=20÷100°С и давлении потока атомов европия P_Eu=(1⋅10^-8÷1⋅10^-7) Торр, затем осаждают слой монооксида европия EuO при температуре подложки T_s=20÷100°С, давлении потока кислорода P_O2=(1⋅10^-9÷1⋅10^-8) Торр и давлении потока атомов европия P_Eu=(1⋅10^-8÷1,1⋅10^-7) Торр, удовлетворяющих условию 10⋅P_O2≤P_Eu≤11⋅P_O2 до достижения необходимой толщины слоя монооксида европия EuO.To achieve a technical result, a method is proposed for growing epitaxial films of europium monoxide EuO on graphene, which consists in the fact that the method of molecular beam epitaxy on the surface of a pre-formed structure of a graphene monolayer / substrate forms a submonolayer of europium with a surface phase

at a substrate temperature T _s = 20 ÷ 100 ° С and a pressure of the flux of europium atoms P _Eu = (1⋅10 ^-8 ÷ 1⋅10 ^-7 ) Torr, then a layer of europium monoxide EuO is deposited at a substrate temperature T _s = 20 ÷ 100 ° C, oxygen flux pressure P _O2 = (1⋅10 ^-9 ÷ 1⋅10 ^-8 ) Torr and europium atomic flux pressure P _Eu = (1⋅10 ^-8 ÷ 1.1⋅10 ^-7 ) Torr satisfying condition 10 OP _O2 ≤P _Eu ≤11⋅P _O2 until the required thickness of the layer of europium monoxide EuO is reached.

In addition, after deposition of the film of europium monoxide EuO, it is annealed in vacuum in the temperature range Ts = 490–520 ° С.

при температуре подложки T_s=20÷100°C и давлении потока атомов европия P_Eu=(1⋅10^-8÷1,1⋅10^-7) Торр, затем осаждают слой монооксида европия EuO при температуре подложки T_s=20÷100°С, давлении потока кислорода P_O2=(1⋅10^-9÷1⋅10^-8) Торр и давлении потока атомов европия P_Eu=(1⋅10^-8÷1,1⋅10^-7) Торр, удовлетворяющих условию 10⋅P_O2≤P_Eu≤11⋅P_O2, а затем осаждают слой монооксида европия EuO при температуре подложки T_s=340÷420°С, давлении потока кислорода P_O2=(1⋅10^-9÷1⋅10^-8) Торр и давлении потока атомов европия P_Eu=(1⋅10^-8÷1,1⋅10^-7) Торр, удовлетворяющих условию 10⋅P_O2≤P_Eu≤15⋅P_O2 до достижения необходимой общей толщины слоя монооксида европия EuO.Also, to achieve the same technical result, a method is proposed for growing epitaxial films of europium monoxide EuO on graphene, which consists in the fact that the method of molecular beam epitaxy on the surface of a pre-formed structure of a graphene monolayer / substrate forms a europium submonolayer with a surface phase

at a substrate temperature T _s = 20 ÷ 100 ° C and a pressure of the stream of europium atoms P _Eu = (1⋅10 ^-8 ÷ 1.1⋅10 ^-7 ) Torr, then a layer of europium monoxide EuO is deposited at a substrate temperature T _s = 20 ÷ 100 ° С, oxygen flow pressure P _O2 = (1⋅10 ^-9 ÷ 1⋅10 ^-8 ) Torr and europium atomic flow pressure P _Eu = (1⋅10 ^-8 ÷ 1,1⋅10 ^-7 ) Torr, satisfying condition 10⋅P _O2 ≤P _Eu ≤11⋅P _O2 , and then a layer of europium monoxide EuO is deposited at the substrate temperature T _s = 340 ÷ 420 ° С, oxygen flow pressure P _O2 = (1⋅10 ^-9 ÷ 1⋅10 ^{- 8)} Torr pressure and flow atoms europium ^{_{P Eu = (1⋅10 -8 ÷ 1,1⋅10}} -7) torr satisfying 10⋅P _O2 ≤P _Eu ≤15⋅P _O2 d achieve the necessary total thickness of the layer of europium monoxide EuO.

In addition, after deposition of the film of europium monoxide EuO, it is annealed in vacuum in the temperature range T _s = 490 ÷ 520 ° C.

The described method allows one to grow single-phase epitaxial EuO films on the surface of a graphene monolayer, which cannot be achieved by the methods indicated in the analogues. It should be noted that in the technological process any substrate with a graphene monolayer deposited on it can be used, except for the silicon substrate shown in the examples, which does not degrade under growth conditions. A graphene monolayer can be preliminarily deposited on the surface of various substrates using both growth technologies and film transfer technologies (Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems (DOI: 10.1039 / c4nr01600a)).

In molecular beam epitaxy units, an ambiguous interpretation of substrate temperatures usually takes place. In the present invention, the substrate temperature above 270 ° C is determined by the readings of an infrared pyrometer, below - by the readings of a thermocouple.

The flow pressure is considered to be the pressure measured by the Bayard-Alpert ionization manometer in the position of the substrate. Moreover, the difference in ionization energies for certain materials (2.54 and 12.2 eV for Eu and O _2, respectively) leads to ambiguity in the estimation of real molecular beam flux densities (in units of Atom / m ² ⋅ s) from the readings of such manometers. In our case, it was experimentally established that the real atomic fluxes of Eu and oxygen coincide with the pressure gauge P _Eu = 10⋅P _O2 .

Brief Description of the Drawings

The invention is illustrated by drawings:

после завершения процедуры ее формирования (б).In FIG. Figure 1 shows the images of diffraction of fast electrons on the initial graphene / SiO ₂ / Si surface before annealing (a) and the surface phase in the process of reconstruction of the graphene surface -

after completion of the procedure for its formation (b).

In FIG. Figure 2 gives a characteristic picture of the diffraction of fast electrons on EuO films.

In FIG. Figure 3 presents an θ-2θ X-ray diffraction pattern obtained on a SiO _x / EuO (80 nm) / Graphene / SiO ₂ / Si (100) sample.

In FIG. Figure 4 shows the dependence of the magnetization of the SiO _x / EuO (67 nm) / Graphene / SiO ₂ / Si (100) sample on the temperature, according to which the Curie temperature for EuO in the film is 68.3 K, which corresponds to the data on bulk single crystals and indicates the absence of impurities and oxygen vacancies that increase the temperature of the ferromagnetic transition.

Example 1. The implementation of the method of the invention.

The graphene / SiO ₂ / Si structure is placed in an ultrahigh-vacuum chamber (residual vacuum P ~ 1⋅10 ^-10 Torr). Then, to clean the surface of graphene, the structure is heated to a temperature T _s = 600 ° С. The fact that graphene is purified is established by fast electron diffraction (Fig. 1a). After that, the structure is cooled to T _s = 20 ÷ 100 ° С, then the shutter of the Eu cell is preheated for 20 sec to a temperature providing the flow pressure P _Eu = (1⋅10 ^-8 ÷ 1.1⋅10 ^-7 ) Torr, which corresponds to the coating of a submonolayer with a thickness of 1/6 of the monolayer.

позволяющей провести выращивание стехиометрического EuO.During the deposition of pure Her, intermediate reflections appear in the patterns of fast electron diffraction (Fig. 16). These changes indicate the formation of a periodic submonolayer coating of metallic europium with a surface structure of the type

allowing to grow stoichiometric EuO.

After the surface reconstruction is formed, the flapper of the Eu cell is heated simultaneously to such a temperature as to ensure the pressure of the atom flux Eu P _Eu = 6.2 610 ^-8 Torr, and oxygen, the molecular beam pressure of which is P _O2 = 6⋅10 ^-9 Torr, while ensuring compliance with the condition 10⋅P _O2 ≤P _Eu ≤11⋅P _O2 . The substrate temperature is maintained at the level of T _s = 20 ÷ 100 ° C. The opening of the Eu and oxygen cell is performed for 30 minutes, which corresponds to a EuO film thickness of 20 nm at a film growth rate of 0.66 nm / min at given flows of substances. Control over the state of the film is carried out in situ by diffraction of fast electrons. The diffraction pattern from the EuO film during growth is shown in FIG. 2. Going beyond the limits of the described regime leads to the formation of amorphous or crystalline higher oxides Eu ₂ O ₃ and Eu ₃ O ₄ , or a mixture thereof with EuO.

Since the film is extremely sensitive to oxidation, at the end of growth, the film is closed with a continuous protective layer, for example, Al or SiO _x with a thickness of at least 2 nm.

Studies of the prepared samples using x-ray diffractometry (Fig. 3) showed that EuO films are textured: due to symmetry considerations (integration of the fcc lattice with the hexagonal), the crystallites are laterally rotated relative to each other at an angle of 30 °, the vast majority of them have a normal orientation to the surface, (001). Inclusions with the (111) orientation are also present, however, an estimate from the peak intensities shows that their amount is no more than 10% of the film volume. The positions of the EuO reflections indicate that the cubic syngonium Fm3m corresponds to the crystal lattice of the EuO film.

Example 2

At the end of the deposition of a 20 nm EuO film, the sample is subjected to vacuum annealing at a temperature T _s = 490–520 ° С. The rest of the method is implemented as in Example 1.

Example 3

Upon completion of the deposition of a 20 nm EuO film at a temperature of T _s = 20 ÷ 100 ° C, 30 nm of a EuO film is deposited at a substrate temperature of T _s = 340 ÷ 420 ° C. The rest of the method is implemented as in Example 1.

Example 4

After the deposition of the EuO film is completed, the sample is subjected to vacuum annealing at a temperature T _s = 490–520 ° С. The rest of the method is implemented as in Example 3.

Thus, the possibility of producing a thick EuO epitaxial film without the inclusion of higher oxide phases on the graphene monolayer, which leads to the appearance of magnetic states in it, has been shown, which will make it possible to create spintronics devices, for example, spin filters and spin-polarized current injectors.

Claims (4)

1. A method of growing epitaxial films of europium monoxide EuO on graphene, including the formation of a submonolayer of europium with a surface phase

by molecular beam epitaxy on the surface of a pre-formed graphene monolayer / substrate structure at a substrate temperature T _s = 20-100 ° С and a pressure of the flux of europium atoms P _Eu = (1⋅10 ^-8 -1⋅10 ^-7 ) Torr, layer deposition europium monoxide EuO at a substrate temperature T _s = 20-100 ° C, oxygen flux pressure P _O2 = (1⋅10 ^-9 -1⋅10 ^-8 ) Torr and europium atom flux pressure P _Eu = (1⋅10 ^-8 - 1⋅10 ^-7 ) Torr, satisfying the condition 10⋅P _О2 ≤P _Eu ≤11⋅P _O2 , until the required thickness of the layer of europium monoxide EuO is reached. 2. The method according to p. 1, characterized in that after the deposition of the film of europium monoxide EuO carry out its annealing in vacuum in the temperature range T _s = 490-520 ° C. 3. A method of growing epitaxial films of europium monoxide EuO on graphene, including the formation of a submonolayer of europium with a surface phase

by molecular beam epitaxy on the surface of a pre-formed graphene monolayer / substrate structure at a substrate temperature T _s = 20-100 ° С and a pressure of the flux of europium atoms P _Eu = (1⋅10 ^-8 -1⋅10 ^-7 ) Torr, layer deposition europium monoxide EuO at a substrate temperature T _s = 20-100 ° С, oxygen flow pressure Р _О2 = (1⋅10 ^-9 -1⋅10 ^-8 ) Torr and europium atom flow pressure P _Eu = (1⋅10 ^-8 - 1.1⋅10 ^-7 ) Torr, satisfying the condition 10⋅P _O2 ≤P _Eu ≤11⋅P _O2 , after which a layer of europium monoxide EuO is deposited at a substrate temperature T _s = 340-420 ° С, oxygen flow pressure P _O2 = (1⋅10 ^-9 -1⋅10 ^-8 ) Torr and the pressure of the flux of europium atoms P _Eu = (1⋅10 ^-8 -1.5⋅10 ^-7 ) Torr, satisfying the condition 10⋅P _O2 ≤P _Eu ≤15 ⋅P _O2 , until the required total thickness of the layer of europium monoxide EuO is reached. 4. The method according to p. 3, characterized in that after the deposition of the film of europium monoxide EuO carry out its annealing in vacuum in the temperature range T _s = 490-520 ° C.

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