RU2768948C1 - METHOD FOR FORMING EPITAXIAL HETEROSTRUCTURES EuO/Ge - Google Patents

Abstract

FIELD: manufacturing technology.

SUBSTANCE: invention relates to methods of forming epitaxial EuO/Ge heterostructures, which can be used in spintronic devices. Method of forming epitaxial EuO/Ge heterostructures involves deposition of metal atoms on a germanium substrate in a stream of molecular oxygen by molecular beam epitaxy, wherein surface of Ge(001) substrate is pre-cleaned from natural oxide layer, and surface phases Eu are formed on it, which are sub-monolayer coatings of europium atoms, then at substrate temperature T_S=20÷150 °C, europium is deposited at pressure P_Eu=(0.1÷100)⋅10^-8 Torr of flux of europium atoms (F_Eu) in flow of oxygen F_O2 with relative value 2≤F_Eu/F_O2≤2.2 to formation of EuO film with thickness less than 10 nm.

EFFECT: formation of epitaxial EuO/Ge heterostructures with an atomic-sharp interface without using buffer layers.

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Description

Technical field

The invention relates to methods for forming epitaxial heterostructures, namely EuO/Ge, which can be used in spintronics devices, in particular when creating spin-polarized current injectors or spin filters.

State of the art

The performance of modern electronic devices based on classical semiconductor technological platforms is close to reaching the fundamental limit. This fact stimulates the search and development of new materials and systems capable of providing further progress in the field of microelectronics due to quantitatively superior characteristics, or due to the development of new principles of functioning.

In this context, germanium is attracting more and more attention as a material that, in comparison with traditional silicon, has several times higher charge carrier mobilities. Due to this, on the basis of Ge, it is possible to create transistors with more advanced operating parameters. However, the creation of devices based on it, which have a significant potential for further development, is impossible without the development of new operating concepts.

One of the areas of electronics that can provide a basis for creating such devices is spintronics, which uses the effects associated with the presence of spin in electrons. However, germanium is a non-magnetic material, therefore, the spin polarization of carriers must be created in it from the outside.

Of the many methods available for generating nonequilibrium spin polarization, the most technologically appropriate is the injection of spin-polarized carriers from a magnetic injector contact. However, the use of standard magnetic metal contacts-injectors to Ge turns out to be inefficient: due to the so-called conductance mismatch problem, the degree of spin polarization of carriers in semiconductor Ge turns out to be extremely low.

The introduction of a thin (~2 nm) dielectric layer between a metal spin-injection contact and a semiconductor increases the efficiency of spin injection, but even in this case it does not reach the optimal value for spintronic devices.

At present, the creation of spin-injection contacts from magnetic semiconductor materials is considered to be the most promising. The conductivity of such contacts has a value close to the conductivity of the base material (Ge), moreover, it can be tuned to the conductivity of Ge by doping the contact. One of the most promising materials for semiconductor ferromagnetic injector contacts is europium oxide (EuO), which has almost 100% spin polarization of carriers in the ferromagnetic state.

The necessary conditions for the efficient injection of spin-polarized carriers from EuO-based contacts are the high crystalline quality of the EuO films, which ensures that they retain their transport and magnetic properties, as well as the sharpness (at the monolayer level) of the EuO/Ge interface, which prevents the scattering of charge carriers with a change in spin direction. , which negatively affects the final spin polarization. The creation of such structures is a complex technological problem.

The present invention proposes a method for creating EuOYGe(001) heterostructures with an atomically sharp interface, which makes it possible to grow EuO epitaxial films exhibiting full magnetic properties and exhibiting semiconductor transport behavior.

A known method for producing thin oxide films of SrHfO ₃ on a Ge(0001) substrate by atomic layer deposition (article "Atomic layer deposition of crystalline SrHfO ₃ directly on Ge (001) for use as a high-k dielectric""Atomic layer deposition of crystalline SrHfO ₃ directly on Ge (001) for high-k dielectric applications)) (DOI: 10.1063/1.4906953)). The film is formed on a Ge substrate cleaned of the natural oxide layer and showing a 2×1 surface reconstruction at the substrate temperature T _s = 200°C. At the end of the growth procedure, the film is annealed to a temperature T _s =650÷850°C. The transfer of the above temperature conditions to the case of growth of EuO leads to the formation of a large number of undesirable phases at the interface.

The closest in technical essence to the claimed invention is a method for producing thin BaTiO ₃ oxide films on a Ge(001) substrate by molecular beam epitaxy (article "Atomic and electronic structure of the interface ferroelectric BaTiO ₃ /Ge(001)""Atomic and electronic structure of the ferroelectric BaTiO ₃ /Ge(001) interface)) (DOI: 10.1063/1.4883883). The formation procedure begins with the deposition of half of the monolayer of Sr atoms on the surface of the Ge(001) substrate, from which the natural oxide layer has been removed. Further synthesis is carried out by successive deposition of Ba and Ti atoms in a stream of oxygen atoms at the substrate temperature T _s =650°C. The use of similar conditions during the formation of EuO initiates the uncontrolled formation of side phases at the interface.

The technical problem to be solved by the claimed invention is the formation of EuO/Ge epitaxial heterostructures with an atomically sharp interface that are promising for spintronic devices.

Disclosure of the essence of the invention

The technical result of the claimed invention is the formation of EuO/Ge heterostructures with an atomically sharp interface without the use of buffer layers using molecular beam epitaxy.

To achieve the technical result, a method is proposed for the formation of EuO/Ge epitaxial heterostructures, which includes the deposition of metal atoms on a germanium substrate in a flow of molecular oxygen by molecular beam epitaxy, while the surface of the Ge(001) substrate is pre-cleaned from a layer of natural oxide or is cleaned from a layer of natural oxide. oxide and form on it any of the surface phases of Eu, after which, at a substrate temperature T _S =20÷150°C, europium is deposited at a pressure of a flow of europium atoms (Ф _Eu ) P _Eu =(0.1÷100)⋅10 ^-8 torr in an oxygen flow F _O2 with a relative value of 2≤F _Eu /F _O2 ≤2.2 until the formation of a EuO film with a thickness of less than 10 nm.

Eu surface phases are understood as periodic structures formed by submonolayer coatings of Eu atoms on the surface of a Ge substrate.

In installations of molecular beam epitaxy, there is usually an ambiguous interpretation of the substrate temperatures. In the present invention, the substrate temperature is determined from a thermocouple reading. All cell temperatures are also indicated by thermocouple measurements. The flow pressure is the pressure measured with a Bayard-Alpert ionization gauge in the substrate position.

Brief description of the drawings

On FIG. Figure 1 shows high-energy electron diffraction (HEED) patterns of (a) a cleaned 2×1 reconstructed Ge(001) substrate surface and (b) a 1×3 Eu FS formed on Ge(001).

On FIG. Figure 2 shows characteristic RHEED patterns of EuO films: (a) 3.5 nm thick formed on a cleaned 2×1 reconstructed Ge(001) surface and (b) 7 nm thick formed on 1×3 Eu FS on Ge(001).

On FIG. 3 shows 8-20 x-ray diffraction patterns of SiO _x /EuO/Ge(001) structures with a 3.5 nm thick EuO film formed on a cleaned 2×1 reconstructed Ge(001) surface (gray) and 7 nm formed on PF 1 ×3 Eu on Ge(001) (black).

On FIG. Figure 4 shows dark-field transmission electron microscopy (TEM) images of a cross section of SiO _x /EuO/Ge(001) structures along the [110] zone axis of the Ge(001) substrate and EuO film: (a) a 3.5 nm thick EuO film formed on a purified reconstructed 2×1 Ge(001) surface, (b) a 7 nm thick EuO film formed on the 1×3 Eu FS on Ge(001).

On FIG. Figure 5 shows the magnetic properties of EuO films 3.5 nm thick formed on a cleaned 2×1 reconstructed Ge(001) surface (grey) and 7 nm thick formed on a 1×3 Eu FS on Ge(001) (black), obtained using a SQUID magnetometer: (a) temperature dependences of normalized magnetization in a magnetic field B=10 mT applied parallel to the film surface, (b) corresponding field dependences of magnetizations at temperature T=2 K.

Implementation of the invention

Example 1

The Ge(001) substrate is placed in an ultrahigh vacuum chamber (residual vacuum Р<1⋅10 ^-10 Torr). Then, to remove the natural oxide layer from the surface of the substrate, it is heated to a temperature T _s =650°C. The fact of cleaning the surface from oxide is established in situ using RHEED: a 2×1 surface reconstruction is observed (Fig. 1(a)). After that, the substrate is cooled to a temperature T _s =20÷150°С, and the damper of the Eu cell heated to such a temperature (~400°С) is simultaneously opened to ensure the pressure of the flow of Eu atoms P _Eu =(0.1÷100) ⋅10 ^-8 Torr, and a molecular oxygen supply valve, the pressure of which is set to such a value as to ensure the ratio of real fluxes of Eu atoms (Ф _Eu ) and oxygen molecules (Ф _O2 ) in the range 2≤Ф _Eu /Ф _o0 ≤2.2 (in our case, the ratio Ф _Eu /Ф _O2 = 2 is observed at the ratio of pressures measured using a manometer P _Eu / P _O2 ≈ 10; at the ratio of real flows Ф _Eu / Ф _O2 = 2 and at the pressure of the flow of atoms Eu P _Eu = 1 ⋅10 ^-8 Torr, the film growth rate is ≈ 0.1 nm/min). The process of formation of the EuO film, carried out after the opening of the flows, lasts until the required thickness is less than 10 nm, after which the damper of the Eu cell and the oxygen valve are closed. In this case, a short delay in closing the Eu damper is preferable to bind the residual background oxygen and prevent oxidation of the film surface (the delay time is determined by the time integral of the background oxygen pressure).

To prevent the effect of air on EuO when the sample is taken out of the chamber after growth is complete, the film is covered with a continuous protective layer, for example, silicon oxide SiO _x or Al with a thickness of more than 2 nm.

The crystalline state of the film is controlled in situ using RHEED. The diffraction pattern of the EuO film during growth is shown in FIG. 2(a). They correspond to the epitaxial growth of a smooth film. The lateral lattice parameter of EuО determined from the distance between the reflections has the value а=5.17±0.05 Å, which agrees with the value known for bulk crystals.

X-ray diffraction studies of the fabricated structures (FIG. 3) show that the EuO films are epitaxial and have the same (001) orientation as the germanium substrate. There are no inclusions of overoxidized phases or metallic Eu in the films. The vertical lattice parameter of EuО determined from the position of the peak is a=5.229±0.015 Å. The obtained value is close to the lattice parameter of bulk EuO samples of the cubic system

The results of the study of samples using TEM (Fig. 4(a) and (b)) confirm the formation of epitaxial EuO films, the absence of foreign phases in the bulk of the films, as well as the atomic sharpness of the interfaces.

Magnetic measurements made with a SQUID magnetometer (Fig. 5) show that the magnetic properties of the EuO films are close to those of bulk samples. The determined ferromagnetic transition temperature (Fig. 5(a)) T _c ≈ 68 K coincides with the value for bulk crystals, indicating the absence of defects in the films, in particular oxygen vacancies, leading to its shift. The saturation magnetic moment (Fig. 5(b)), within the experimental error, is M _s =7 μ _B /Eu, indicating the absence of inclusions of the overoxidized phases of Eu ₃ O ₄ and Eu ₂ O ₃ .

Transport measurements performed on SiO _x /EuO/Ge(001) samples show that EuO films are semiconductors, which indicates their stoichiometry.

Example 2

The method is implemented as in Example 1, except that after the stage of cleaning the substrate, an additional procedure for the formation of Eu PF is performed. In this example, the corresponding procedure for 1×3 Eu FS on Ge(001) is given. To form this FS, the substrate temperature is set to T _s = 410°С, and ≈2/3 monolayer of Eu atoms is deposited by opening the damper of the Eu cell heated to such a temperature (~400°С) to provide pressure for the flow of Eu atoms P _Eu =(0.5÷10)⋅10 ^-8 Torr (the specific settling time depends on the set flow; at flow pressure P _Eu =1⋅10 ^-8 Torr, the settling time is ≈ 70 s). The RHEED image observed at the end of the PF formation procedure is shown in Fig. 1(b).

The structural and magnetic properties of the films are shown in Fig. 2(b), 3, 4(b) and FIG. 5, respectively, and are close to the properties of films made according to the procedure of Example 1. The slightly lower value of the vertical lattice parameter of EuO: a=5.168±0.014 Å, which can be calculated from the position of the peak in FIG. 3 is explained by the greater thickness of the film and, as a consequence, the greater relaxation of stresses in it caused by the difference in the lattice parameters of EuO and Ge.

Example 3

The method is implemented as in Examples 1 and 2, except that to clean the germanium substrate from natural oxide, the substrate is etched in an aqueous solution of NH ₄ OH (25%)/H ₂ O (1:4) before loading into a vacuum chamber.

Exceeding the limits of the described growth regimes can lead to the formation of higher oxides Eu ₂ О ₃ or Eu ₃ O ₄ , a polycrystalline EuО film, the accumulation of excess Eu, or a critical deterioration in the structural quality of the grown film.

Thus, the invention allows the synthesis of EuO films on Ge(001) substrates. These films:

- are epitaxial;

- have atomically sharp EuO/Ge and protective layer/EuO interfaces;

- are ferromagnetic;

- are semiconductor.

Such films can be in demand in spintronic devices, in particular, in the creation of spin-polarized current injectors or spin filters.

Claims (1)

A method for the formation of epitaxial EuO/Ge heterostructures, which includes the deposition of metal atoms on a germanium substrate in a flow of molecular oxygen by molecular beam epitaxy, characterized in that the surface of the Ge(001) substrate is pre-cleaned from the natural oxide layer, or is cleaned from the natural oxide layer and formed there are surface Eu phases on it, which are submonolayer coatings of europium atoms, after which, at a substrate temperature T _S =20÷150°C, europium is deposited at a pressure P _Eu =(0.1÷100)⋅10 ^-8 Torr of the flow of europium atoms (Ф _Eu ) in an oxygen flow Ф _O2 with a relative value of 2≤Ф _Eu /Ф _O2 ≤2.2 until the formation of an EuO film with a thickness of less than 10 nm.

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