New method for oriented growth of a ferroelectric thin film of a rhombohedral ferroelectric alloy

Abstract

The invention relates to a method for growing a ferroelectric film (11) of a rhombohedral ferroelectric alloy on a substrate (10), in which a growth sub-layer (21) formed by a stack of crystalline sheets bonded together by van der Waals bonds is disposed on the substrate, and then the rhombohedral crystalline ferroelectric alloy is deposited, in the form of crystalline grains (12), on the growth sub-layer, at a suitable temperature with a component that provides segregation between the crystalline grains.

Description

DESCRIPTION New method for oriented growth of a ferroelectric thin film of a rhombohedral ferroelectric alloy technical field

[0001] The present invention relates to the technical field of thin film deposition processes. In particular, the invention relates to a growth method applicable to any type of substrate, which makes it possible to obtain the growth of a crystalline ferroelectric thin film of a rhombohedral ferroelectric alloy having ferroelectric domains organized along the direction normal to the surface of the substrate on which the growth is carried out.

[0002] Technological background of the invention and technical problem

[0003] The rhombohedral a-GeTe crystalline phase (space group R3m) is a ferroelectric semiconductor with high spin-charge conversion capacity, i.e., it is a semiconductor with strong Rashba coupling. The ferroelectric GeTe alloy is also denoted a-GeTe. The rhombohedral a-GeTe crystalline phase is particularly interesting for all applications exploiting the ferroelectricity of a thin-film material. The ability to control the orientation of the ferroelectric domains is very advantageous for applications requiring the reversal of the orientation of the ferroelectric domains, for example, by applying an electric field when the GeTe layer is placed between two electrodes, as notably in FESO (FerroElectric SpinOrbit devices) devices [1].

[0004] Spin-charge interconversion induced by spin-orbit (SO) phenomena has opened new avenues for the design of low-power spin-logic architectures. Recently, Intel proposed a device called MESO [2] (Magneto-Electric Spin-Orbit). The write process requires magnetization reversal, while the read component relies on the spin-charge interconversion signal resulting from SO phenomena.

[0005] Recent studies in spintronics have led to the discovery of spin-charge inter-conversion phenomena in ferroelectric materials [1, 3], as well as the possibility of controlling the room-temperature spin-charge interconversion by switching the ferroelectric state in Rashba ferroelectric semiconductors (FERSCs) such as rhombohedral GeTe

[0001] . These materials exhibit a sign change in spin-to-charge conversion by reversing the ferroelectric polarization of the material, which led to the proposal of the FerroElectric Spin-Orbit (FESO) device. This device exhibits the same important characteristics as the MESO device, such as non-volatile information control for in-memory computing. The output signal is electrically controlled, but there is no magnetization switching, thus eliminating the need for an efficient magnetoelectric material as in the MESO write block. Indeed, the output signal is controlled directly by the ferroelectric state and does not require reversing the magnetic state of a spin-injection layer.Its overall simplicity makes FESO a valid alternative to MESO as an information-based device and "Beyond-CMOS" spin logic.

[0006] To date, room-temperature proofs of concept for the use of this type of film in FESO-type devices have been carried out using ferroelectric GeTe films, i.e., rhombohedral α-GeTe films deposited on small samples by molecular beam epitaxy (or MBE for "Molecular Beam Epitaxy") [4]. Thus, in this context, the large-scale fabrication of such a device remains a challenge, particularly for the industrial production of the active ferroelectric GeTe layer with organized ferroelectric domains to allow their reversal by the application of an electric field.

[0007] Furthermore, thin films of crystalline rhombohedral α-GeTe, in which the c-axis in hexagonal indexing, coinciding with the longest diagonal of the rhombohedron and corresponding to the pseudo-cubic direction

[0111] , has an out-of-plane orientation, are very promising for spintronic and ferroelectric devices. Spontaneous polarization of α-GeTe occurs along the pseudo-cubic axis, leading to the formation of four ferroelastic variants and three possible polarization switching between domains at 71°, 109°, or 180° [4(b)]. Thus, in crystalline GeTe grains, the alignment of all the long diagonals (pseudo-cubic indexing

[0111] or hexagonal indexing

[0001] of the rhombohedral structure) perpendicular to the substrate surface maximizes the polarization potential of the rhombohedral a-GeTe thin film, since the applied electric field is always perpendicular to the surface of the substrate on which the thin film is deposited, in the applications envisaged.

[0008] Non-volatile nanometric ferroelectric domains were observed in oriented rhombohedral α-GeTe films 23 nm thick deposited by molecular beam epitaxy (MBE) using the conductive tip of an atomic force microscope in PFM (piezo-response force microscopy) mode [5]. Reversible writing of (11 1 ) and (-1 -1 -1 ) ferroelectric domains, illustrated by ferroelectric hysteresis in such cases, was successfully achieved, revealing the potential of rhombohedral α-GeTe for ferroelectric memories.Although in films deposited by MBE there are only 5 possible orientations in the plane of the a and b axes of GeTe, it would be possible to obtain a similar ferroelectric potential with only rhombohedral a-GeTe crystalline grains having their c axis oriented out of the plane, and more precisely perpendicular to the plane of the substrate on which these crystalline grains are deposited, and exhibiting a random texture in the ab plane, since the ferroelectric potential of rhombohedral a-GeTe thin films corresponds only to the physical displacement of Ge atoms along the c axis.

[0009] Therefore, a new process compatible with industrial applications allowing the manufacture of thin films, with control of the growth of crystalline grains of a rhombohedral ferroelectric alloy oriented with their c-axis, perpendicular to the surface of the substrate on which the growth is carried out, would constitute a major technological advance for spintronic and ferroelectric applications. Summary of the invention

[0010] In this context, the present invention aims to provide a new method for achieving controlled growth of crystalline grains of a rhombohedral ferroelectric alloy along the desired orientation, that is, with their c-axis, and therefore their pseudo-cubic axis <111>, oriented perpendicularly or substantially perpendicularly to the surface of the substrate on which the growth is carried out. The invention also proposes such a method that is suitable for industrial implementation and for the deposition of thin films. large surface area. The process according to the invention is also perfectly suited to the controlled growth, along this orientation, of crystalline grains of a ferroelectric alloy, for example, rhombohedral α-GeTe, Gei- x Sn x Te rhombohedral, or GeSei- x You x rhombohedral.

[0011] The invention relates to a method for growing a ferroelectric layer formed of crystalline grains of a rhombohedral ferroelectric alloy, optionally doped, on a substrate. The method comprises the following steps: a) providing a substrate; b) providing on the substrate a growth sublayer formed of a stack of crystalline sheets linked together by van der Waals bonds; c) depositing the rhombohedral crystalline ferroelectric alloy in the form of crystalline grains onto the growth sublayer present in step b), the ferroelectric alloy optionally being doped, and the deposition being carried out in the presence of a component that does not insert itself into the crystalline grains of the ferroelectric alloy. The deposition in step c) is carried out at a temperature suitable for obtaining the deposition and crystallization of the crystalline grains of the rhombohedral ferroelectric alloy, optionally doped.with the component that segregates between the crystalline grains.

[0012] Within the scope of the invention, a method is proposed for producing a ferroelectric layer formed of crystalline grains of a rhombohedral ferroelectric alloy, optionally doped, with growth along the desired orientation on any type of substrate. Advantageously, in the method according to the invention, during step c), the growth of each crystalline grain occurs along a preferred orientation of the crystallographic axis c, perpendicular or substantially perpendicular to the surface of the substrate on which the growth sublayer is deposited, and more particularly perpendicular or substantially perpendicular to the surface of the growth sublayer. "Segregated" means that the component that does not fit within the crystalline grains of the rhombohedral ferroelectric alloy is inserted between the crystalline grains of the optionally doped rhombohedral ferroelectric alloy.In other words, this component accumulates at the crystalline grain boundaries of the possibly doped rhombohedral ferroelectric alloy, depending on the composition of the resulting ferroelectric layer.

[0013] Compared to rhombohedral a-GeTe, a Gei- alloy x Sn x Rhombohedral Gei- is potentially as interesting as, or even more interesting than, a-GeTe because this alloy is less conductive and has an even higher Rashba power due to the presence of Sn. Compared to rhombohedral a-GeTe, in a Gei- alloy x Sn xIn the rhombohedral lattice, Sn replaces some of the Ge in the rhombohedral unit cell. Indeed, Ge and Sn can substitute for each other in the rhombohedral phase. The advantage in this case is that Sn is heavier than Ge, which increases the spin-charge interconversion power of the material and therefore the Rashba effect. In the process of the invention, when the growth involves a ferroelectric layer formed of crystalline grains of a Gei- alloy x Sn x For a rhombohedral structure, possibly doped, the value of x is greater than 0 and, preferably, less than or equal to 0.7. This preferred value of x applies to the entire description that follows concerning the growth of a ferroelectric layer formed from crystalline grains of a Gei- alloy. x Sn x The rhombohedral tetrahedral, possibly doped. It can be added that the GeSei- alloy x You x rhombohedral is even more electrically insulating than Gei- x Sn xRhombohedral te

[0014] In the process according to the invention, growth is carried out on a growth sublayer formed from a stack of crystalline sheets bonded together by van der Waals forces. This growth sublayer does not disrupt the growth of the crystalline grains of the rhombohedral ferroelectric alloy, depending on the thin film to be deposited, and acts as a growth orientation sublayer. This sublayer is compatible with the ferroelectric alloy, particularly in terms of lattice parameters and chemical interaction. It can, in particular, form a van der Waals gap or pseudo-van der Waals gap with the deposited thin film of the ferroelectric alloy, or even form an alloy with it. In particular, the growth sublayer comprises Te or Se and is typically a growth sublayer of As2Te3, Sb2Te3, E Tes, As2Se3, Sb2Se3 or E Ses, and typically a growth sublayer of Sb2Te3.

[0015] In particular, step b) is carried out by depositing the growth sublayer, which consists of a stack of crystalline sheets bonded together by van der Waals forces. It is also possible to deposit an amorphous sublayer, which will crystallize during the temperature increase to the temperature required for step c), this crystallization leading to the formation of the sublayer of growth formed by the stacking of crystalline sheets linked together by van der Waals bonds.

[0016] Advantageously, prior to step b), the substrate surface is passivated with Te or Se, in particular using a Te or Se flux, and this is followed by the deposition of a growth sublayer of As₂Tea, Sb₂Tea, E₂Tes, As₂Se₃, Sb₂Se₃, or E₂Ses onto the passivated substrate surface. The As₂Te₃, Sb₂Te₃, E₂Tes, As₂Se₃, Sb₂Se₃, or E₂Ses growth sublayer deposited on the passivated substrate surface is either in an amorphous form that is then crystallized to form a stack of crystalline sheets linked by van der Waals bonds, or deposited directly as a stack of crystalline sheets linked by van der Waals bonds.

[0017] According to some embodiments, the ferroelectric alloy comprises Sn.

[0018] According to certain embodiments which can be combined with the previous ones, the ferroelectric alloy comprises Te.

[0019] Advantageously, the ferroelectric alloy also includes Ge.

[0020] According to a first particular embodiment, the ferroelectric alloy corresponds to a rhombohedral GeSe1-xTex alloy possibly doped.

[0021] In this case, step c) can, in particular, be carried out from one or more targets leading in the ferroelectric layer to an atomic composition of Ge, Se and Te in which Se+Te represents 50 to 75 atomic %, preferably 50 to 60 atomic %, and in particular 50 to 55 atomic %.

[0022] In particular, according to a first variant of the implementation of this first embodiment, the component that does not fit into the GeSe1-xTex crystalline grains is Ge or Se and step c) leads to the formation of crystalline grains and rhombohedral GeSe1-xTex with amorphous Ge or Se (or both) which segregate(s) between said crystalline grains.

[0023] In particular, the growth of a ferroelectric layer of a rhombohedral GeSel-xTex alloy, possibly doped, can be obtained with Ge or Se segregated between said crystalline grains, and step c) is carried out from: - a GeSetTeu target with t+u such that the atomic % Se+Te varies from 50 to 75%, preferably from 50 to 60% atomic, and in particular from 50 to 55% atomic, and being typically 50%; - of a Ge target, a Se target and a Te target, a GeSe1-xTex target and a Se target, or - a stoichiometric GeSe target, a stoichiometric GeTe target and a Se and / or Te target.

[0024] According to a second embodiment of this first method, step c) is carried out in the presence of a target of a component other than Ge, Se, or Te that segregates between said crystalline grains. In particular, the component that segregates between the crystalline grains can be chosen from C, SiNx, SiOx, GeN, Al2O3, SiC, Si, and any other component that does not incorporate into a rhombohedral crystalline phase of GeSe1-xTex.

[0025] In the first embodiment, the deposited rhombohedral GeSe1-xTex alloy can be doped with a dopant, in particular As, Bi or Sb, and step c) can be carried out using a dopant target or a Ge, Se and / or Te target incorporating the dopant.

[0026] According to a second particular embodiment, the ferroelectric alloy corresponds to the rhombohedral a-GeTe possibly doped, or to a rhombohedral Ge1 - xSnxTe alloy possibly doped.

[0027] In this case, step c) can be carried out from one or more targets leading in the ferroelectric layer to an atomic composition of Ge and Te or Ge, Sn and Te in which Ge or Ge+Sn, respectively, represents 50 to 75% atomic, preferably 50 to 60% atomic, and in particular 50 to 55% atomic.

[0028] Thus, in the process according to the invention, in the case of the growth of a ferroelectric layer formed of rhombohedral a-GeTe crystalline grains optionally doped, step c) can be carried out from one or more targets leading in the ferroelectric layer to an atomic composition in Ge and Te in which Ge represents from 50 to 75 atomic%, preferably from 50 to 60 atomic%, and in particular from 50 to 55 atomic%.

[0029] In the process according to the invention, in the case of the growth of a ferroelectric layer formed of crystalline grains of a Gei-xSn alloy xThe rhombohedral Te possibly doped, step c) can be carried out from one or more targets leading in the ferroelectric layer to an atomic composition of Ge, Sn and Te in which Ge+Sn represents 50 to 75% atomic, preferably 50 to 60% atomic, and in particular 50 to 55% atomic.

[0030] According to a first embodiment of the second method of the invention, the component that does not insert itself into the crystalline grains of a-GeTe or Gei- x Sn x Te is Ge or Sn and step c) leads to the formation of crystalline grains of a-GeTe or Gei- x Sn x Rhombohedral te, with amorphous Ge or Sn which segregates between said crystalline grains.

[0031] In this first variant, in the case of the growth of a ferroelectric layer formed of rhombohedral α-GeTe crystalline grains, possibly doped with Ge that segregates between said crystalline grains, the component that does not insert itself into the α-GeTe crystalline grains is Ge, and step c) leads to the formation of rhombohedral α-GeTe crystalline grains, with amorphous Ge at the grain boundaries. Specifically, step c) can be carried out starting from: - of a Ge target y Te with y such that the atomic % of Ge varies from 50 to 75%, preferably from 50 to 60% atomic, and in particular from 50 to 55% atomic, and typically having a GeTe composition (so-called stoichiometric composition or with the atomic % of Ge equal to 50%), of a Ge target and a Te target, or - of a stoichiometric GeTe target and a Ge target.

[0032] In this first variant, in the case of the growth of a ferroelectric layer formed from crystalline grains of a Gei- alloy x Sn x The rhombohedral component, possibly doped, which does not fit into the crystalline grains of Gei- x Sn x Te is Ge or Sn and step c) leads to the formation of Gei- crystalline grains x Sn x Rhombohedral tetrahedral, with amorphous Ge or Sn segregating between the crystalline grains. In particular, step c) can be carried out starting from: - of a GetSn target u Te with t+u such that the atomic % Ge+Sn varies from 50 to 75%, preferably from 50 to 60% atomic, and in particular from 50 to 55% atomic and being typically 50%, of a Ge target, an Sn target and a Te target, a Gei- target x Sn x Te and a target Ge, or - a stoichiometric GeTe target, a stoichiometric SnTe target and a Ge and / or Sn target.

[0033] According to a second embodiment of the second method of the invention, step c) is carried out in the presence of a target of a component other than Ge, Sn, or Te, which segregates between said crystalline grains. In particular, the component that segregates between said grains is chosen from C, SiNx, SiOx, GeNx, GeOx, AlOx, MgO, Pt, SiC, Si, and any other component that does not incorporate into a rhombohedral crystalline phase of α-GeTe or Gei α-Sn α-Te.

[0034] Regardless of the embodiment or implementation variant, in the process according to the invention, the deposition in step c) can be carried out using any suitable deposition technique. In particular, step c) is performed by a vaporization technique, specifically sputtering. In particular, during this vaporization step, the desired ferroelectric layer is deposited onto the growth sublayer by vaporizing one or more targets adapted to the composition of the ferroelectric layer to be deposited, which correspond in particular to the targets described above.

[0035] Advantageously, in step c), the substrate bearing the growth sublayer is heated to a temperature sufficient to obtain the formation of crystalline grains of the rhombohedral ferroelectric alloy (for example, rhombohedral α-GeTe alloy or rhombohedral Gei xSn xTe alloy, or GeSei- xTe alloyx possibly doped), but which remains below 400°C, in particular the substrate carrying the growth underlayer is heated to a temperature in the range of 180 to 300 °C.

[0036] In the process according to the invention, when the deposited rhombohedral ferroelectric alloy is doped with a dopant, in particular As, Bi, or Sb, then step c) is carried out using a dopant target or a Ge, Sn, Se, and / or Te target incorporating the dopant. Thus, the targets described above are modified or supplemented by those skilled in the art to obtain the desired doping. By Ge, Sn, Se, and / or Te target, we mean a target that contains one or more of these elements, or even all three. Of course, the incorporation of the dopant into a target made of or containing Sn or Se relates to the case of growing a ferroelectric layer formed of rhombohedral Gei x Sn x Te crystalline grains or rhombohedral GeSei x Te.

[0037] The process according to the invention may include one or more steps subsequent to step c). In particular, the process according to the invention includes a step d), subsequent to step c), of depositing on the ferroelectric layer an oxidation protection layer, said protection layer preferably being a dielectric layer.

[0038] It is also possible that the process according to the invention includes a step d), subsequent to step c), of depositing a dielectric protection layer on the ferroelectric layer, followed by a step e) of depositing an electrically conductive layer.

[0039] The process according to the invention can be implemented on any type of substrate. For example, the substrate may be based on or composed of crystalline silicon, amorphous silicon, silicon dioxide, mica, CaF2, SiNx, SiOC, SiCN, MgO, aluminum oxide, magnesium oxide, TiNx, WSi, W, TiSiN, TaN, CoFe(Pt), CoFe, Au, Al, a ferromagnetic alloy, carbon, or a polymer resistant to the heating temperature applied during step c). In the case where the substrate is a conductive layer, for example TiN, WSi, W, TiSiN, TaN, etc., the substrate can act as an electrode in the intended applications. Detailed description of the invention

[0040] Other features, details and advantages of the invention will become apparent from the description provided with reference to the accompanying figures given for illustrative purposes, which represent, respectively:

[0041] Figure 1 is a schematic illustration, without regard to the respective dimensions of the different layers for clarity, showing the orientation of the c-axis, corresponding to the pseudo-cubic axis <111>, of the rhombohedral a-GeTe grains 12 in a thin film 11 produced on a growth sublayer 21, itself deposited on the surface of a substrate 10, according to the process of the invention. The x-axis, perpendicular to the plane of the substrate 10, is parallel to the c-axis, corresponding to the pseudo-cubic axis <111>, of the rhombohedral a-GeTe grains 12 of the thin film 11. The y- and z-axes are parallel or in the plane of the substrate 10 and the growth sublayer 21. For clarity, grain boundaries are not shown. An identical figure illustrates the growth of a Gei- thin film 11. x Sn x Te formed of 12 grains of Gei- x Sn x Rhombohedral te

[0042] Fig. 2 is a schematic illustration, without regard to the respective dimensions of the different layers for greater visibility, showing different stages of a process according to the invention.

[0043] Figure 3 shows the XRD diagram of the rhombohedral a-GeTe layer obtained on the single-crystal Si (100) substrate known as "Monitor" covered with an Sb2Te3 growth layer, as in Example 1.

[0044] Fig. 4a and Fig. 4b present the rocking curve acquired on one of the Bragg 001 diffraction peaks of the rhombohedral a-GeTe layer obtained in Example 1, respectively for the "Monitor" Si substrate and the Si substrate covered with a SiO2 layer (500 nm), substrates themselves covered with an Sb2Te3 growth layer.

[0045] Figure 4c shows the Bragg 006 peak diffraction curves of the rhombohedral a-GeTe layer obtained in Example 1, for a SiO2 substrate (100 nm) covered with an Sb2Te3 growth sublayer (solid line curve) or without an Sb2Te3 growth sublayer (dashed line curve).

[0046] Figure 5 presents the STEM-HAADF analysis of the rhombohedral a-GeTe layer obtained in Example 1, in the case of the Si substrate “Monitor” with an Sb2Te3 sublayer.

[0047] The [Fig.6] is an image obtained by piezo-response force microscopy (PFM, over a range of X shown in abscissa of 1.50 pm) in the case of an amorphous silicon substrate, covered with an Sb2Te3 growth layer, on which the growth of a rhombohedral a-GeTe thin layer was carried out, and then covered with a protective SiN layer.

[0048] Figures 7a and 7b present the XRD diagram of the rhombohedral a-GeTe layer obtained in Example 2, respectively for the Si Monitor substrate and the Si substrate coated with a SiO2 layer (100 nm), using a complementary Te target.

[0049] [Fig.8] presents the XRD diagram of the rhombohedral ferroelectric GeTe and GeSe1-xTex ferroelectric layers obtained on the single-crystal Si(100) substrate known as "Monitor" covered with an Sb2Te3 growth layer, in example 1.

[0050] Figures [Fig.9a], [Fig.9b], [Fig.9c] and [Fig.9d] show the rocking curves acquired on one of the Bragg 006 diffraction peaks of the rhombohedral GeTe layer and the rhombohedral GeSe1-xTex layers obtained in Example 1, for the Si(100) substrate deoxidized by spraying and coated with an Sb2Te3 growth layer.

[0051] In the context of this invention, a grain is conventionally defined as a crystalline region having a periodic and uniform atomic organization. As shown in Figure 1, the process according to the invention leads, in particular, to the formation of a ferroelectric thin film 11 composed of crystalline grains 12 of a rhombohedral ferroelectric alloy (for example, rhombohedral a-GeTe), schematically represented by blocks in this figure, in which the crystallographic orientation corresponds to the c-axis perpendicular to the plane of the substrate 10 (called the growth surface), itself parallel to the surface of the growth sublayer 21. It should be noted that although a slight misorientation is possible, the surface of the growth sublayer 21, on which the ferroelectric thin film 11 is deposited, is generally parallel to the plane of the substrate 10 (called the growth surface) on which it is itself deposited.The c-axis of the crystalline grains 12 is therefore parallel to the x-axis shown. Within the scope of the invention, 11 refers to the preferred orientation for the growth of crystalline grains 12, corresponding to the orientation of the crystallographic axis c perpendicular to the growth surface or substantially perpendicular to the growth surface. This notion of preferred orientation means that there is a certain tolerance regarding the orientation of the crystallographic axis c of a crystalline grain 12 of a rhombohedral ferroelectric alloy (for example, rhombohedral α-GeTe or a Gei- alloy). x Sn x The rhombohedral, or an alloy of GeSei- x You x , ...) in the thin layer 1 1 relative to the surface of the substrate 10: for each crystalline grain 12 of the rhombohedral ferroelectric alloy, the crystallographic axis c of the unit cell of the crystalline grain 12 forms an angle with the surface of the substrate 10, on which the thin layer of the rhombohedral ferroelectric alloy is deposited, which is constant and approximately equal to 90°, that is to say, it is equal to 90° ± 10° at most, or even 90° ± 5° at most, or even 90° ± 3° at most, or even equal to 90°. In other words, the crystallographic axis c of each crystalline grain 12 is parallel to the x-axis shown in Figure 1 or forms with it an angle of ± 10° at most, or even ± 5° at most more, or even + / - 3° at most. However, there may be variability between the angle formed by the crystallographic axis c and the surface of the substrate 10, from one crystalline grain 12 to another. This variability will generally be + / - 10° at most, or even + / - 5° at most, or even + / - 3° at most. The orientation and possible distribution can be obtained by X-ray diffraction (XRD). The c-axis of the crystalline grains 12 of the rhombohedral ferroelectric alloy thin film 11 therefore has a preferred orientation. On the other hand, the crystallographic orientation can change from one crystalline grain 12 to another; that is to say, the a and b axes can have different orientations, as is the case in Figure 1, with the c-axis maintaining this same orientation parallel to or close to the x-axis.In the thin layer 11, the grain boundaries, not shown in Figure 1, have a different composition from the rhombohedral a-GeTe crystalline grains 12 (in the case of a ferroelectric layer formed from rhombohedral a-GeTe crystalline grains 12) or Gei-. x Sn x Te (in the case of a ferroelectric layer formed of crystalline grains 12 of Gei- x Sn x Rhombohedral ferroelectric alloy). However, the thin film 11 has a composition that is predominantly rhombohedral ferroelectric alloy or doped rhombohedral ferroelectric alloy, the grain boundaries representing less than 25 atomic percent and generally 1 to 20 atomic percent, typically 1 to 5 atomic percent, of the resulting thin film 11, which can therefore be properly described as ferroelectric. For short, despite the presence of a component other than the rhombohedral ferroelectric alloy (e.g., GeTe or Gei- x Sn x Te or GeSei- x Youx depending on the case) at the grain boundaries, in the following description, the thin layer 1 1 may simply be called the thin layer of the rhombohedral ferroelectric alloy (GeTe (or Gei- x Sn x (Te respectively)) or ferroelectric layer of (GeTe or Gei- x Sn x Te respectively). For example, the crystalline grains 12 of rhombohedral a-GeTe can be composed of GeTe or doped GeTe. The crystalline grains 12 of Gei- x Sn x Rhombohedral tetrahedrals can be composed of a Gei- alloy x Sn x Te or an alloy of Gei- x Sn xDoped Gei- x Sn x Te rhombohedral, this will cover a Ge-i- alloy rhombohedral xSrixTe, but also a Gei-xSn alloy x The rhombohedral doped, unless it is explicitly stated that this cannot be the case.

[0052] For example, a rhombohedral ferroelectric alloy may contain Te. For example, a rhombohedral ferroelectric alloy may contain SiTe, PbTe, or MnTe. A ferroelectric alloy may contain Sn. For example, a rhombohedral ferroelectric alloy may contain SnSe, SnS, or SnSe-i-xTex. For example, a rhombohedral ferroelectric alloy may contain GeSe-i-xTex, Ge-i-xSixTe, Ge-i-xPbxTe, Ge-i-xMnxTe, or Gei- y Sn y Sei-xTe x The previously mentioned rhombohedral ferroelectric alloys can be doped with As, Bi or Sb.

[0053] The detailed description below is given for the growth of a rhombohedral α-GeTe ferroelectric layer. However, it is equally applicable to the growth of a Gei-xSn ferroelectric layer. xrhombohedral Te, or of a ferroelectric alloy comprising Sn, or of a ferroelectric alloy comprising Te, or of a ferroelectric alloy comprising GeSe-i-xTex.

[0054] In this case, it is not rhombohedral a-GeTe crystalline grains that are deposited, but grains of a Gei-xSn alloy xRhombohedral Te, or a ferroelectric alloy comprising Sn, or a ferroelectric alloy comprising Te, or a ferroelectric alloy comprising GeSe-i-xTex. Of course, the targets and deposition conditions detailed later in the description will be adapted by those skilled in the art to obtain the desired composition. Within the framework of the process of the invention, the growth of the rhombohedral α-GeTe crystalline grains 12 of the thin film 11 in the desired orientation can be achieved on any type of substrate 10 with the use of a growth sublayer 21 located between the substrate 10 and the thin film 11.It is on this growth sublayer 21, composed of crystalline sheets 22 linked together by van der Waals bonds, that the growth of rhombohedral α-GeTe is carried out, and it is this growth sublayer 21 that will contribute to giving the desired orientation to the crystalline grains 12 of rhombohedral α-GeTe in the thin layer 11. In the context of the invention, "crystalline sheet" means a crystalline layer whose thickness corresponds to a single plane of atoms, parallel to the surface of the substrate on which the sublayer is deposited, or to several parallel planes of atoms forming a crystalline block, which are also parallel to the surface of the substrate on which the growth sublayer is deposited. In the case of crystalline sheets corresponding to a single plane of atoms, these are called two-dimensional sheets. Such two-dimensional sheets are present, notably, when the subshell is formed of M0S2, for example. In the case of crystalline sheets formed of several atomic planes, called crystal blocks, each atomic plane contains atoms of the same element, as is the case, for example, with Sb2E3 where each block is composed of 5 atomic planes. Within the growth subshell 21, the crystalline sheets 22, whether in the form of crystal blocks or not, are linked together by van der Waals bonds between two planes of atoms belonging to each of the adjacent crystalline sheets. These crystalline sheets can be in the form of two-dimensional crystalline sheets or crystal blocks. Van der Waals bonds can be, for example, Te-Te, Se-Se, or SS bonds in chalcogenides.The large spacing between the crystalline sheets linked by these van der Waals bonds is generally attributed to the existence of a van der Waals vacancy plane, which is called a van der Waals gap or pseudo-van der Waals gap (vdW). For the purposes of this description, the vdW gap and pseudo-vdW gap are considered equivalent and separate two crystalline sheets linked by vdW bonds. This particular structural form is observed notably in phase-change materials (PCMs), chalcogenides, and more generally in materials exhibiting a so-called "metavalent" chemical bonding mechanism. These materials are frequently described as 2D materials linked by van der Waals bonds. This description can nevertheless be broadened because the distance between planes across the vdW gap is generally shorter than twice the van der Waals radius of the atoms constituting these planes.For example, in the case of tellurium, the Te-Te distance across the van der Waals gap is about 10% shorter than twice the van der Waals radius of Te. Thus, in the stacking that constitutes subshell 21, the crystalline sheets can be separated by van der Waals gaps or pseudo-van der Waals gaps, which occur, in particular, as a spacing between two pure planes of the same type of atom. The bonds between atoms across the pseudo-van der Waals gap are typically shorter and stronger than ordinary van der Waals bonds. At the interface with the rhombohedral α-GeTe layer, the growth subshell 21 can form an alloy or be separated from it by a van der Waals gap or pseudo-gap. The materials in the following non-exhaustive list typically adopt a crystalline sheet structure with gaps or pseudo-gaps of vdW described above: tellurium, antimony, the. binary alloys Ge-Sb, Ga-Sb, Ge-Te, Ge-Se, GeSe2, Sb-Te, in particular Sb2Se3, Sb-Se, in particular Sb2Se3, As-Se, in particular As2Se3, As-Te, in particular As2Te3, Sn-Se, Sn-Te, Bi-Se, in particular Bi2Se3, Bi-Te, in particular Bi2Te3, Ga-Te, Ga-Se, Pb-Te, In-Se, in particular In2Se3, Pb-Se, Zr-Te, Zr-Se, Ti-Te, in particular TiTe2, Ti-Se, in particular TiSe2, Si-Te, Al-Te, Mo-Te, in particular MoTe2, Mo-Se, in particular MoSe2, Mo-S, in particular MoS2, W-Te, in particular WTe2, WS, in particular WS2, W-Se, in particular WSe2, ternary alloys Ge-Sb-Te, Ge-Se-Te, Ag-Sb-Te, Ge-Bi-Te, Ge-Sb-Se, quaternary Ge-Sb-Se-Te or even Ge-Bi-Sb-Te-Se. A person skilled in the art would choose such a growth sublayer formed of crystalline sheets linked together by van der Waals forces (vdW gap or pseudo-gap layer structure) which does not disrupt the growth of the crystalline grains of a-GeTe or the Gei-xSn alloy xTe and acts as a growth orientation sublayer. This growth sublayer 21 is compatible with α-GeTe or the rhombohedral Gei-xSnxTe alloy, depending on the desired ferroelectric layer, particularly in terms of lattice parameters and chemical interaction. Preferably, the growth sublayer 21 is a crystalline sheet layer corresponding to a stacking of chalcogenide crystalline blocks, specifically As2Te3, Sb2ε3, or Bi2Te3.

[0055] As illustrated in Figure 2, the process according to the invention comprises a step a) of providing a substrate 10, followed by a step b) in which the growth sublayer 21 is deposited onto the substrate. The growth sublayer 21 can be obtained in one or more steps. It is possible to form an amorphous deposit on the upper surface of the substrate 10, which will then crystallize upon subsequent heating to form the stack of crystalline sheets linked together by van der Waals forces. It is also possible, and simpler, to directly deposit the growth sublayer 21 in the form of the stack of crystalline sheets linked together by van der Waals forces.According to a preferred embodiment, the process of the invention further includes, before step b), i.e. before the formation of the growth sublayer 21, a step of passivating the substrate 10 with tellurium or antimony of the surface on which the growth sublayer 21 is formed. This makes it easier to form the growth sublayer 21 by so-called vdW epitaxy, where growth takes place in the form of crystalline sheets. 22 oriented parallel to the surface of the substrate and separated by gaps or pseudo-gaps of vdW, to form a stack.

[0056] As for the substrate 10, it can be made of any type of material. It can be monolayer or multilayer. It can be a substrate 10 based on or made of crystalline silicon, amorphous silicon, silicon oxide, glass, mica, CaF2 or any standard deposition substrate... The substrate can also be made of or coated with another type of material, including TiN, SiN, WSi, W, TiSiN, TaN, SiOC, SiCN, Pt, CoFe(Pt), Au, Cu, Al, other metallic or magnetic deposits, carbon, or a polymer, as well as any material resistant to the heating temperature applied to the substrate 10 during the deposition step c) of the growth process according to the invention, including vaporization techniques such as physical vapor deposition.The substrate can also be a stack of several layers of these types of material, notably a layer of amorphous silicon covered by a layer of amorphous silicon oxide, which will then correspond to the surface on which the growth sublayer 21, and then the ferroelectric thin layer 11, will be deposited. The substrate 10 typically has a thickness of 100 to 1000 pm, and for example 100, 150, 200, or 300 pm. The substrate 10 is generally in the form of a disk with a diameter commonly used in microelectronics.

[0057] The substrate may have a flat surface or a step-like or pattern-like surface, which will ultimately result in a planar polarized stack rather than a vertically polarized one. The growth sublayer 21 is deposited on the so-called upper surface of the substrate 10, as illustrated in Figures 1 and 2. When the substrate 10 includes steps or patterns, the orientation of the crystalline grains of the ferroelectric layer 11 is determined locally with respect to the surface of the substrate on which it is deposited, which itself carries the intermediate growth sublayer 11.

[0058] For the implementation of step c) of deposition, particularly by vaporization, the substrate 10 carrying the growth sublayer 21 and the target(s) necessary to obtain the desired deposit are conventionally positioned in a deposition chamber, in particular a vaporization chamber. Within the scope of the invention, the vaporization carried out in step c) of the target(s) used can be performed using any suitable technique, particularly for industrial-scale implementation. This may involve of a chemical vapor deposition (CVD) or physical vapor deposition (PVD) technique. Sputtering and pulsed laser deposition (PLD) techniques are particularly suitable. Within the scope of this invention, the use of magnetron sputtering is preferred, and in particular, direct current (DC) or pulsed DC sputtering, and radio-frequency magnetron sputtering. In this type of technique, a cold plasma is created in a vacuum chamber that constitutes the vaporization chamber and comprises the substrate 10 carrying the growth sublayer 21 and the target(s), which is / are the source of GeTe, possibly doped, to be deposited and the source of the grain boundary component that will accumulate between the GeTe grains.Ions, usually argon, are bombarded onto the target(s) acting as the cathode, vaporizing atoms from the target which are then deposited onto the substrate to form a thin layer. In magnetron sputtering, a magnetic field is created near the target(s) to increase the plasma density and thus the sputtering efficiency. In the radio frequency version, the continuous power supply to the cathode is replaced by a high-frequency supply, typically at 13.56 MHz, and the anode-cathode polarization is reversed at high frequency.

[0059] Within the framework of the invention, it has been shown that three parameters are important for achieving controlled growth in the desired orientation: the composition of the deposited film 11 and therefore of the vaporized target(s), the use of a growth sublayer 21 on the surface of the substrate 10 on which growth is carried out, and the temperature at which the substrate 10 bearing the growth sublayer 21 is heated. Indeed, it has been observed that if, during step c), the heating is not carried out at a sufficient temperature, it is not possible to obtain a rhombohedral crystalline α-GeTe phase, but rather an amorphous or disordered crystalline phase of GeTe is obtained. As is known, this temperature must not, however, be too high to achieve deposition.However, simply choosing the heating temperature of substrate 10 is not sufficient to obtain the desired crystallographic orientation, as will be shown in the examples that follow. The origin of the preferential grain orientation. Rhombohedral α-GeTe crystals with their c-axis perpendicular or nearly perpendicular to the deposition surface, which in the example illustrated in Figure 1 is also perpendicular or nearly perpendicular to the substrate surface, is also related to the composition of the films actually deposited and therefore to that of the starting target(s). It was observed that when a target with a stoichiometric GeTe composition was used, the overall composition of the ferroelectric thin film obtained in step c), particularly by sputtering, had a composition with a slight Te deficit. Without being linked by any specific mechanism of action, it is assumed that this deficit results from the greater volatility of Te during vaporization or from its desorption from the substrate surface when the latter is heated for the growth of the oriented rhombohedral α-GeTe thin film.This slight excess of Ge causes, during growth, the segregation of an excess phase of amorphous Ge at the joints of the rhombohedral a-GeTe grains which then induces by effect of stress the orientation of the ferroelectric domains of the crystalline grains 12 of rhombohedral a-GeTe with their axis c perpendicular or substantially perpendicular to the surface of the substrate 10 and of the growth sublayer 21.The inventors also demonstrated that using an additional pure Te target to compensate for Te loss, in addition to the stoichiometric GeTe target—a conventional practice for maintaining stoichiometry—when both targets were vaporized simultaneously, caused the excess Ge phase to disappear. This resulted in a loss of the correct orientation of the ferroelectric domains of the rhombohedral α-GeTe grains along the c-axis, which is perpendicular or substantially perpendicular to the surface of the growth sublayer 21 and the substrate 10, and a rotation of the ferroelectric domains. The effect of stress and the orientation of the ferroelectric domains of the rhombohedral α-GeTe crystalline grains 12, with their c-axis perpendicular or substantially perpendicular to the surface of the growth sublayer 21, can be achieved with a component other than Ge that does not insert itself into the crystalline phase of rhombohedral α-GeTe.

[0060] In this context, several variations in the implementation of the process according to the invention can be envisaged. In a first variation, amorphous Ge segregates at the boundaries between the oriented rhombohedral a-GeTe grains deposited on the growth sublayer 21. In this case, the deposit on the growth sublayer 21 in step c) can be carried out as follows: - perform the deposition during step c), in particular by vaporization, from a single target of stoichiometric GeTe composition, or more generally from a target composed of Ge and Te, having a Ge composition vTe with v such that Ge represents at least 50 atomic percent, in particular more than 50 atomic percent and at most 75 atomic percent, preferably more than 50 atomic percent and at most 60 atomic percent, and in particular more than 50 atomic percent and at most 55 atomic percent, or carry out the deposition in step c), in particular by vaporization, from a target of stoichiometric GeTe composition, and a Ge target, or - carry out the deposition during step c), in particular by vaporization, from a Ge target and a Te target.

[0061] In these different cases, the deposition conditions, and in particular the vaporization of the targets when there are several, are adjusted by a person skilled in the art to obtain a ferroelectric layer 1 1 of atomic composition in Ge and Te in which Ge represents at least 50% atomic, in particular more than 50% atomic and at most 75% atomic, preferably more than 50% atomic and at most 60% atomic, and in particular more than 50% atomic and at most 55% atomic.

[0062] In a second variant, a component (other than Ge) that does not integrate into the GeTe crystalline grains segregates at the boundaries between the oriented rhombohedral α-GeTe grains. In this case, the deposition on the growth sublayer 21 in step c) can be carried out as follows: - carry out the deposition during step c), in particular by vaporization, from a target of composition GewTe with w such that Ge represents less than 50 atomic % and from a target of a component that does not fit into the crystalline grains of a-GeTe, or - carry out the deposition during step c), in particular by vaporization, from a Ge target, of a Te target and of a target of a component which does not fit into the crystalline grains of a-GeTe.

[0063] In these cases, the deposition conditions, and in particular the vaporization of the targets, are adjusted by a person skilled in the art to obtain a ferroelectric layer 11 with an atomic composition of Ge and Te in which Ge represents 50% atomic, i.e., a stoichiometric GeTe composition. A component that does not fit into the crystalline grains of α-GeTe can, in particular, be C, SiNx (notably SiaIX), SiOx (especially S1O2), GeN, Al2O3, ...or any other material that does not incorporate into the ferroelectric rhombohedral α-GeTe crystalline phase (especially SiC, Si, ...). It forms an amorphous or crystalline deposit at the boundaries between the oriented rhombohedral α-GeTe grains.

[0064] In all cases, when doped α-GeTe is desired, the target(s) used in step c) above may be supplemented by a target containing the desired dopant element, such as As, Bi, or Sb, or alternatively, a target with the composition Ge(v or w)Te. The Te or Ge target may be doped with the desired amount of dopant (generally representing less than 10% atomic percentage of the α-GeTe). A person skilled in the art will adjust the deposition parameters, particularly the vaporization parameters, to achieve the desired doping level, including the power applied to each target.

[0065] In the case where the process according to the invention is applied to the growth of rhombohedral crystalline Gei-xSnxTe, all the above applies. In this case, the deposition will be carried out with a target containing Sn in the proportions or under the conditions allowing the growth of the desired layer. The component that segregates between the crystalline grains and forms the grain boundaries may be amorphous Ge or Sn, or a component that does not insert itself into the crystalline grains of rhombohedral Gei-xSnxTe. Again, this component may, in particular, be C, SiNx (especially SialSk), SiOx (especially SiO2), GeN, Al2O3, SiC, Si, etc. It forms an amorphous or crystalline insertion at the boundaries between the deposited oriented rhombohedral crystalline Gei-xSnxTe grains.

[0066] In the case where a doped material is sought, dopants such as As, Bi or Sb in ferroelectric GeTe or rhombohedral Gei xSnxTe alloys located on the GeTe-SnTe pseudo-binary line of the Ge-Sn-Te ternary phase diagram are of interest because they will also allow the same ferroelectric properties to be obtained, but with lower electronic conductivity in the Ge-Sn-Te rhombohedral alloys or by charge compensation via n-induced doping by As, Bi and / or Sb and compensating for the intrinsic p-doping of GeTe due to Ge vacancies in the crystal lattice of the a-GeTe phase.

[0067] Typically, a person skilled in the art will adjust the parameters in step c) to obtain the desired composition. In particular, depending on the device of Depending on the vaporization method used, and in particular the sputtering device, a person skilled in the art will adjust the pressure in the vaporization chamber, the gas flow used to perform the vaporization, the target-substrate distance, and, in the case of magnetron sputtering, the strength of the magnetic field and, in the case of radio-frequency magnetron sputtering, the radio frequency power as well. All these parameters depend on the equipment used. The heating temperature of the substrate bearing the growth sublayer during vaporization is the most suitable for obtaining the growth of rhombohedral a-GeTe or Gei- alloy crystalline grains. x Sn xThe potential will depend on the vaporization technique used, as well as the vaporization device used within the same technique, and the composition of the targets and therefore the grain boundaries that will be inserted between the crystalline grains of a-GeTe or the Gei- alloy. x Sn x Rhombohedral Te and constraining them favoring the orientation of ferroelectric domains. In particular, the substrate heating temperature used during vaporization is the most suitable for obtaining the growth of crystalline grains of a-GeTe or the Gei- alloy. x Sn xThe rhombohedral α-GeTe will be adjusted by a person skilled in the art, depending on the deposition setup, and in particular the sputtering method used. Most often, a heating temperature above 180°C and below 330°C will be suitable, specifically a temperature in the range of 180 to 300°C, advantageously a temperature in the range of 200 to 250°C. Such a preferred temperature range is particularly suitable, especially when the grain boundaries are formed of Ge (i.e., especially when the rhombohedral α-GeTe source target is of the Ge type). v(as previously defined) and that the vaporization technique is a sputtering technique. In any case, with knowledge of the invention and therefore of the importance of the substrate heating temperature and depending on the composition of the selected target(s), a person skilled in the art can conduct some evaluation tests to adjust the substrate heating temperature, depending on the vaporization device used, to obtain the desired crystallographic orientation and ferroelectric polarization.

[0068] Figure 2 schematically illustrates different stages of a growth process according to the invention. The substrate 10 available in stage a) can correspond to any substrate as previously described. The substrate 10 may be subjected to a preliminary cleaning and / or deoxidation step (in particular by treatment with an alkaline solution, typically potassium hydroxide or sodium hydroxide) or acid (typically hydrofluoric acid or nitric or sulfuric acid) or solvent (alcohol, acetone, ...) or spraying of its surface by bombardment with Ar+ ions depending on the nature of the substrate, rinsing then drying, classically implemented prior to the deposition of any thin films.

[0069] In step b), the sublayer 21, formed by a stack of crystalline sheets 22 bonded together by van der Waals forces, is deposited onto the upper surface of the substrate 10, most often after prior preparation, notably by passivation with tellurium or antimony, particularly when the sublayer 21 is As₂Te₃, Sb₂Te₃, or E₂Tes. This can be achieved by flash vaporization of Te to saturate the surface of the substrate 10, resulting in an atomic plane of Te on the surface of the substrate 10 before depositing a growth sublayer 21 of As₂Te₃, Sb₂Te₃, or E₂Tes, among others. The sublayer 21 is generally a few nanometers thick (typically 1 to 10 nm for Sb₂Tes). This sub-layer 21 can be formed in step b) by a prior vaporization step from a suitable target in the same device as that used in step c) or in a different device.A co-spraying method using Sb and Te or Sb2Te3 and Te targets, by adjusting the power applied to each target, can be employed. The use of a Te-enriched Sb2Te3 target is also possible. This allows for compensation of a potential tellurium deficiency during the formation of sublayer 21.

[0070] When the sublayer 21 is a sublayer of As2Te3, Sb2Te3 or Bi2Te3 is present under the thin layer 1 1, the latter can also be used to obtain an As, Sb or Bi doping, respectively, of the thin layer 1 1, by carrying out an adapted diffusion annealing after deposition of the thin layer 1 1, in particular at an annealing temperature greater than or equal to 300°C.

[0071] Finally, after step c), a step d) can be included to protect the thin film 1 formed. In particular, a protective layer 30 against oxidation can be deposited during step d), as illustrated in Figure 2. Examples of such protective layers 30 include a conductive layer, such as a layer of TiN, WSi, W, TiSiN, TaN, or PtCoFe (with or without Pt), Au, Al, any ferromagnetic alloy, or a dielectric or insulating layer or tunnel barrier for injecting a spin-polarized current, such as a layer of SiN, SiOx, SiOC, SiCN, MgO, TiOx, a polymer layer, or any other protective layer commonly used in microelectronics. Again, this protective layer 30 can be formed by a vaporization step from a suitable target in the same device as that used in step c) or in a different device. A dielectric protective layer 30 is advantageous because it allows the thin film 11 to be polarized by the application of an electric field, thus preventing leakage currents through the heavily doped, pseudo-metallic GeTe thin film 11.

[0072] As illustrated in Figure 2, the process according to the invention may include a step (d), subsequent to step (c), of depositing a dielectric protective layer 30 (in particular as previously described) onto the ferroelectric layer 1, followed by a step (e) of depositing an electrically conductive layer 40 (in particular as previously described). The deposition of these protective layers 30, or even of an electrically conductive layer 40, will depend on the intended application.

[0073] The growth process according to the invention makes it possible to obtain thin films 1 1 of ferroelectric rhombohedral a-GeTe or thin films 1 1 of a Gei- alloy x Sn xThe ferroelectric rhombohedral layer with crystalline grains 12 corresponding to ferroelectric domains aligned along the pseudo-cubic axis <11 1> and perpendicular or substantially perpendicular to the surface of the substrate 10, regardless of the chemical nature of the surface of the growth substrate used. Vaporization in step c) leads, on the growth sublayer 21 deposited on the substrate 10, to the segregation of an excess phase of Ge (or Ge or Sn in the case of Gei- x Sn x Te) or a phase of another component that does not fit into the crystalline phase of a-GeTe (or into the crystalline phase of the Gei- alloy x Sn x (Te depending on the case), at the grain boundaries of rhombohedral a-GeTe (or of the Gei- alloy x Sn x (Te depending on the case) which forms during their growth. This segregation at grain boundaries constrains the crystalline grains of a-GeTe (or of the Gei- alloyx Sn x (Te depending on the case) and thus induces their orientation and the formation of ferroelectric domains stabilized with the c-axis (corresponding to the pseudocubic axis <1 1 1 >) of the crystals present in the a-GeTe grains (or of the Ge-i- alloy (xSrixTe depending on the case) rhombohedral perpendicular or substantially perpendicular to the surface of the substrate 10 and to the growth sublayer 21 on which it is deposited. By applying an electric field perpendicular to the surface of the substrate, it is possible to reversibly orient the ferroelectric domains along the

[0111] or [-1 -1 -1] direction.

[0074] Finally, at the end of step c), a ferroelectric thin film 1 1 corresponding to a thin film, specifically with a thickness of 5 to 500 nm (typically 20 nm), is obtained. The coercive field required to reverse the orientation of the ferroelectric domains in the case of a ferroelectric layer of a Gei-xSn alloy x Te will be lower than for a ferroelectric layer of a-GeTe. Thus, a person skilled in the art can adapt the thickness of the ferroelectric layer formed, depending on the material a-GeTe or Gei-xSn x To possibly adapt the necessary coercive scope.

[0075] The ferroelectric domains that correspond to the crystalline grains 12 of rhombohedral a-GeTe or of the Gei-xSn alloy xThe rhombohedral strands, depending on the case, generally have an average size in the plane parallel to the substrate surface of approximately 20 to 100 nm. With the process according to the invention, which can be implemented on an industrial scale, it is possible to produce large-area thin films, particularly those with an area greater than 7500 mm². 2 , and typically from 100,000 to 300,000 mm 2 , or even 636,173 mm 2 .

[0076] As will be shown in the examples that follow, the quality of the thin films obtained with a process according to the invention was checked by X-ray diffraction (XRD). In particular, it was observed that the growth obtained with the process according to the invention led to thin films in which the rhombohedral a-GeTe grains were perfectly oriented: only the 001 diffraction peaks were observed (hexagonal indexing of the rhombohedral structure of a-GeTe), which indicates oriented growth of the a-GeTe crystals with the c-axis perpendicular to the plane of the thin film formed and therefore to the surface of the substrate.

[0077] The manufacture, according to the processes of the invention, of substrates 10 bearing a ferroelectric layer 11 formed of rhombohedral α-GeTe crystalline grains 12 or of a Gei-xSn alloy x The rhombohedral shape, depending on the case, and deposited on a The intermediate growth sublayer 21 present on the surface of the substrate is of interest in a large number of applications: in CMOS microelectronic technologies, the fabrication of FESO devices, spintronic, ferroelectric, ferromagnetic, multiferroic devices for applications in logic, computing, neuromorphic computing, memory computing, resistive memory...

[0078] Implementation examples

[0079] Example 1

[0080] The deposition of 100 nm a-GeTe thin films composed of crystalline grains with their pseudo-cubic axis <1 1 1> perpendicular to the substrate surface (called a-GeTe(1 1 1) film) was carried out either on a 200 mm diameter silicon substrate previously deoxidized by sputtering with Ar+ ions (called Si Monitor), or on a 200 mm diameter silicon substrate coated with 500 nm of SiO2 obtained by thermal oxidation of Si. In this example, vaporization was performed using a substrate heating temperature of 250°C, followed by the deposition of a 5 nm growth sublayer of Sb2Ie3 by co-sputtering a stoichiometric Sb2Ie3 target and a pure Te target, and then the deposition of the a-GeTe(1 1 1) film by sputtering a target of GeTe stoichiometry.Figure 3, which shows the XRD pattern obtained with the Si Monitor substrate, highlights that in the resulting thin film, the rhombohedral a-GeTe grains were perfectly oriented: only the 001 diffraction peaks were observed (hexagonal indexing of the rhombohedral structure of a-GeTe), indicating oriented growth of the a-GeTe crystals with the c-axis perpendicular to the plane of the thin film formed and therefore to the substrate surface. The full width at half maximum (FWHM) of the rocking curve acquired on the 001 Bragg diffraction peaks shown in Figures 4a and 4b, for both substrates, confirms the excellent out-of-plane orientation of the crystallites in the film with low mosaicity around 3°.The STEM-HAADF analyses shown in Figure 5, in the case of the Si Monitor substrate with Sb2Te3 sublayer, also confirm the high structural quality of the films, showing ferroelectric domains inside the a-GeTe crystalline grains and aligned along the pseudo-cubic <11 1 > axis perpendicular to the substrate.

[0081] Finally, a new test was performed under similar conditions, but to produce a 40 nm thin film with the same target, using a 200 mm silicon (100) substrate deoxidized on its surface by Ar+ sputtering. The resulting a-GeTe thin film was then coated in situ by sputtering a 10 nm thick SiN protective layer. Piezo-response force microscopy (PFM) demonstrated the possibility of writing (Figure 6) and highlighting the polarization state in the ferroelectric domains of the material in an upward or downward orientation depending on the direction of the applied electric field, with a 180° phase shift measured between the two programmed areas (analogously with Figure 5). It was thus demonstrated that the obtained a-GeTe film is indeed ferroelectric and that the domains are reversible using an electric field.

[0082] In the absence of an Sb2Te3 layer, a deposition carried out under the same conditions and from the same GeTe stoichiometric target did not allow the growth of a-GeTe with all the crystalline grains in the desired orientation, as shown in Figure 4c. In Figure 4c, the intensity of the Bragg diffraction peak on one of the 006 lines of the rhombohedral a-GeTe deposited on SiO2 without a sublayer is very low compared to that obtained for a rhombohedral a-GeTe film deposited under the same conditions, but on a 5 nm Sb2Te3 growth sublayer, indicating a very high proportion of a-GeTe grains not oriented along the c-axis perpendicular to the substrate in the absence of a growth sublayer.

[0083] For example, the deposition of 100 nm thin films of GeSe-i-xTex alloys with different proportions of Se and composed of rhombohedral crystalline grains with their pseudocubic axes <1 1 1 > perpendicular to the surface of the substrate was carried out on a silicon substrate of 200 mm in diameter previously deoxidized by spraying with Ar+ ions. Vaporization was carried out using a substrate heating temperature of 200°C, deposition of a 5 nm Sb2Te3 growth sublayer by co-spraying a stoichiometric Sb2Te3 target and a pure Te target, followed by deposition of the Ge-Se-Te based alloy film by co-spraying two GeTe and GeSe targets respectively with relative deposition rates adjusted to vary the composition of the resulting alloy (with 0%, 5%, 15% and 25% Se respectively as an atomic % in the alloy).

[0084] The XRD diagram, illustrated in Figure 8, shows that in the thin films obtained, the rhombohedral grains are perfectly oriented: only the 001 diffraction peaks are observed (hexagonal indexing of the rhombohedral structure), indicating oriented crystal growth with the c-axis perpendicular to the plane of the thin film formed and therefore to the substrate surface. The full width at half maximum (FWHM) of the rocking curve acquired on the 001 Bragg diffraction peaks confirms the excellent out-of-plane orientation of the crystallites in the film with low mosaicity around 3°.

[0085] Example 2

[0086] The use of a 5 nm Sb2Ïe3 sublayer deposited on an amorphous silicon substrate (according to the conditions described in [6a]) also enabled the obtaining of a rhombohedral a-GeTe thin film with grains oriented along the pseudo-cubic axis <1 11 > perpendicular to the substrate.

[0087] GeTe thin layers of 50 nm thickness were able to be deposited at different substrate heating temperatures between 180°C and 300°C on a 5 nm thick Sb2Ïe3 sublayer as described in the literature [6b] to promote orientation.

[0088] By vaporizing a stoichiometric GeTe target without adding Te, no misoriented crystalline grains were detected by XRD for any GeTe thin films deposited on a 5 nm Sb2Te3 growth sublayer: again, well-ordered α-GeTe ferroelectric domains were obtained within the crystalline grains, aligned along the pseudo-cubic <1 11> axis perpendicular to the substrate. However, when vaporization was performed with the addition of a Te target, the excess Ge that accumulated at the grain boundaries was lost, as was the preferred orientation of the rhombohedral α-GeTe, as shown in Figures 7a and 7b. References:

[0089]

[0001] Room-temperature ferroelectric switching of spin-to-charge conversion in germanium telluride. Nature electronics 4 (2021) 740-747.

[0090] [2] Scalable energy-efficient magnetoelectric spin-orbit logic, Nature 565 (2018) 35-42.

[0091] [3] Non-volatile electric control of spin-charge conversion in a SrTiO3 Rashba system, Nature 580 (2020) 483-486.

[0092] [4] (a) A. Giussani, K. Perumal, M. Hanke, P. Rodenbach, H. Riechert, R. Calarco, physica status solidi (b) 2012, 249, 1939; (b) B. Croes, F. Cheynis, P. Müller, S. Curiotto, F. Leroy, Polar surface of ferroelectric nanodomains in GeTe thin films, Phys. Rev. Materials 2022, 6, 064407.

[0093] [5] C. Rinaldi, S. Varotto, M. Asa, J. Slawihska, J. Fujii, G. Vinai, S. Cecchi, D. Di Santé, R. Calarco, I. Vobornik, G. Panaccione, S. Picozzi, R. Bertacco, Nano Lett. 2018, 18, 2751.

[0094] [6] (a) F. Hippert, P. Kowalczyk, N. Bernier, C. Sabbione, X. Zucchi, D. Térébénec, C. Mocuta and P. Noé, J. Phys. D: Appl. Phys., 2020, 53, 154003. (b) V. Sever, N. Bernier, D. Térébénec, C. Sabbione, J. Paterson, F. Castioni, P. Quéméré, A. Jannaud, J.-L. Rouvière, H. Roussel, J.-Y. Raty, F. Hippert and P. Noé, physica status solidi (RRL) - Rapid Research Letters, 2024, 2300402.

Claims

DEMANDS 1. A method for growing a ferroelectric layer (11) formed of crystalline grains (12) of a rhombohedral ferroelectric alloy, optionally doped, on a substrate (10), said method comprising the following steps: a) providing a substrate (10), b) providing on the substrate a growth sublayer (21) formed of a stack of crystalline sheets linked together by van der Waals bonds, c) depositing rhombohedral crystalline ferroelectric alloy in the form of crystalline grains (12) onto the growth sublayer (21) present in step b), the ferroelectric alloy optionally being doped, and the deposition being carried out in the presence of a component that does not insert itself into the crystalline grains of the ferroelectric alloy, the deposition in step c) being carried out at a temperature suitable for obtaining the deposition and crystallization of the crystalline grains (12) of the alloy rhombohedral ferroelectric, possibly dopedwith the component that segregates between the crystalline grains (12).

2. Method according to claim 1, characterized in that the growth sublayer (21 ) of step b) is a sublayer (21 ) of As2Te3, Sb2Te3, Bi2Te3, As2Se3, Sb2Se3 or Bi2Se3.

3. Method according to claim 1 or 2, characterized in that it comprises a step of passivating the substrate (10) before step b), followed by the deposition of a sub-layer (21) of As2Te3, Sb2Te3, Bi2Te3, As2Se3, Sb2Se3 or Bi2Se3 on the passivated surface of the substrate (10).

4. A method according to any one of claims 1 to 3, characterized in that the ferroelectric alloy corresponds to a GeSei- alloy x You x rhombohedral, possibly doped.

5. Method according to claim 4, characterized in that step c) is carried out from one or more targets leading in the ferroelectric layer (1 1 ) to an atomic composition of Ge, Se and Te in which Se+Te represents from 50 to 75 atomic %, preferably from 50 to 60 atomic %, and in particular from 50 to 55 atomic %.

6. A process according to any one of claims 1 to 3, characterized in that the ferroelectric alloy corresponds to rhombohedral α-GeTe, optionally doped, or to a Gei- alloy x Sn x The rhombohedral tetralogy, possibly doped.

7. Method according to claim 6, characterized in that step c) is carried out from one or more targets leading in the ferroelectric layer (1 1 ) to an atomic composition in Ge and Te or in Ge, Sn and Te in which Ge or Ge+Sn, respectively, represents from 50 to 75 atomic%, preferably from 50 to 60 atomic%, and in particular from 50 to 55 atomic%.

8. A method according to claim 6 or 7, characterized in that the component that does not insert itself into the crystalline grains of a-GeTe or Gei-xSn x Te is Ge or Sn and step c) leads to the formation of rhombohedral a-GeTe or Ge-i-xSnxTe crystalline grains (12), with amorphous Ge or Sn (or both) segregating between said crystalline grains (12).

9. A method according to any one of claims 6 to 8, characterized in that when the rhombohedral a-GeTe or the Gei-xSn alloy x The deposited rhombohedral Te is doped with a dopant, notably As, Bi or Sb, step c) is carried out using a target of the dopant or a target of Ge, Sn and / or Te incorporating the dopant.

10. A method according to any one of claims 1 to 9, characterized in that during step c), the growth of each crystalline grain (12) takes place along a preferred orientation of the crystallographic axis c perpendicular to the surface of the substrate (10).

11. A method according to any one of claims 1 to 10, characterized in that step c) is carried out by a vaporization technique, in particular by cathodic sputtering.

12. A method according to any one of claims 1 to 11, characterized in that in step c), the substrate (10) bearing the growth sublayer (21) is heated to a temperature sufficient to obtain the formation of crystalline grains (12), with this temperature remaining below 400°C, in particular the substrate (10) bearing the growth sublayer (21) is heated to a temperature in the range of 180 to 300°C.

13. A method according to any one of claims 1 to 12, characterized in that it comprises a step d), subsequent to step c), of depositing on the ferroelectric layer (11) a protective layer (30) against oxidation, said protective layer (30) preferably being a dielectric layer.

14. A method according to any one of claims 1 to 13, characterized in that it comprises a step d), subsequent to step c), of depositing on the ferroelectric layer (11) a dielectric protective layer (30), followed by a step e) of depositing an electrically conductive layer (40).

15. A method according to any one of claims 1 to 14, characterized in that the substrate (10) is based on or made up of crystalline silicon, amorphous silicon, silicon oxide, mica, CaF2, SiN, SiOC, SiCN, MgO, aluminium oxide, magnesium oxide, TiN, WSi, W, TiSiN, TaN, PtCoFe, CoFe, Au, Al, a ferromagnetic alloy, carbon, or a polymer resistant to the heating temperature applied during step c).

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