Method for the controlled-growth synthesis of hexagonal polytype crystalline phases and metastable materials thus obtained

Abstract

The invention relates to a substrate in the form of a stack structure, comprising: - a growth substrate consisting of a solid material having a stabilized hexagonal crystalline structure comprising a surface with prismatic a-plane or m-plane orientation, and - a layer made of a group III-V semiconductor material having a metastable hexagonal crystalline structure, in the form of a thin layer having a thickness of up to 1 μm and being formed by epitaxial growth on the surface of the solid material.

Description

[0001] METHOD FOR SYNTHESIS BY CONTROLLED GROWTH OF HEXAGONAL POLYTYPIC CRYSTALLINE PHASES AND METASTABLE MATERIALS THUS OBTAINED.

[0002] The present invention relates to the fields of metastable materials with hexagonal polytypical crystalline phases, and in particular to the field of semiconductor materials. The invention also relates to the synthesis, by epitaxial growth, of metastable polytypical crystalline phases of semiconductor materials exhibiting, in particular, photonic functionalities.

[0003] Crystalline phase control is an original and efficient way to modify the physical properties of materials, in particular the band structure and phonon dispersion of semiconductors, which makes it possible to bring new functionalities to these materials.

[0004] Polytypism occurs in close-packed structures. Polytypes differ according to the stacking period of the layers along the stacking axis. A polytype is defined by the notation NX, where N is the period (number of layers in the unit cell) and X is the crystal symmetry (C for cubic, H for hexagonal, R for rhombohedral).

[0005] Tetravalent semiconductors can potentially exhibit different polytypes. The 3C diamond cubic and 2H and 4H hexagonal structures differ only in the stacking sequence of the tetrahedra along the axes. <111> Or <0001> structures, with stacking sequences ABC for cubic and AB for hexagonal 2H for example and ABCB for hexagonal 4H.

[0006] The majority of semiconductors, including silicon (Si), germanium (Ge), and gallium arsenide (GaAs), crystallize in the diamond cubic phase, a polytype denoted 3C. These materials do not possess other polytypes in their natural state under standard conditions.

[0007] In contrast, highly ionic materials such as CdS and GaN have a stable hexagonal phase denoted 2H.

[0008] The entire field of micro and nanoelectronics is based on the properties of Si and Ge with 3C cubic symmetry. These group IV semiconductor materials have many advantageous properties, but do not allow light emission due to the band gap, commonly referred to by its English equivalent as "indirect band gap".

[0009] Photonics, on the other hand, relies primarily on light emission in III-V semiconductors such as InP, GaAs, and InAs, which exhibit a direct band gap. The reduction of symmetry from the cubic (3C) polytype to the hexagonal (2H and 4H) polytypes leads to band folding of the electronic structure, which can modify the nature and / or width of the band gap. Thus, for example, Ge in 2H or 4H structures exhibits a direct band gap.

[0010] In the publication, "Direct-bandgap emission from hexagonal Ge and SiGe alloys", E. Fadaly et al. Nature 580 (2020) 205, it is demonstrated that Ge and SiGe alloys with a hexagonal 2H structure exhibit excellent light emission capabilities in the mid-infrared with a tunable wavelength between 1.8 and 4 pm for a Si concentration of up to 40%.

[0011] Based on these highlighted optical properties, it is understood that the hexagonal structure of Si(i-X )Ge x The variable composition of silicon (x) allows for the development of photonic functionalities using group IV semiconductor materials, which are non-toxic. Furthermore, silicon is far more abundant than group III elements such as aluminum, gallium, and indium, and group V elements such as nitrogen, phosphorus, arsenic, and antimony, which are currently used in photonics.

[0012] The hexagonal 2H and 4H phases of Si(i- X )Ge x This could provide a direct gap with mid-infrared (MIR) light emission within a certain Si concentration range. Si-2H, on the other hand, exhibits reduced thermal conductivity.

[0013] It is therefore of interest to be able to propose a synthesis process for polytypes of semiconductor materials and more particularly for Si(i- X )Ge xhexagonal structure with a view to using their photonic properties to create photonic components or devices.

[0014] For binary III-V materials such as GaAs and GaP, vapor-liquid-solid (VLS) catalyzed growth is an established method for altering the stacking sequence along the growth axis of a nanowire and thus controlling the 3C and 2H polytype by adjusting the growth conditions. However, this method remains limited to the synthesis of nanowires. And in the case of nanowires made from group IV materials, this growth process does not result in a controlled modification of the polytype.

[0015] Current methods for synthesizing the hexagonal phase of Si and Ge can be grouped into three categories based on different mechanisms:

[0016] - stress-induced phase transformation (hydrostatic or shear),

[0017] - phase transformation by thermal annealing of metastable phases (BC8, allo-Ge or porous Si24 zeolite), - and, more recently developed, the synthesis of nanostructures by epitaxy on a GaAs or GaP nanowire with a hexagonal structure.

[0018] An inherent drawback of the first two processes is that the material obtained is polycrystalline and often presents a mixture of various phases with a high defect density which prohibits integration into electronic and photonic devices.

[0019] Phase transformation in Si and Ge nanowires enables the fabrication of 3C / 2H heterostructures along the nanowire, as described in the publication "Novel Heterostructured Ge Nanowires Based on Polytype Transformation" by L. Vincent et al., Nano Letters Vol. 14 No. 8 (2014) DOI 10.1021 / nl502049a, and "Shear-driven phase transformation in silicon nanowires" by L. Vincent et al., Nanotechnology Vol. 29 No. 12 (2018) DOI 10.1088 / 1361-6528 / aaa738. However, in the case of this phase transformation process in nanostructures, the resulting volumes are nanometric.

[0020] The controlled growth of GaAs-2H / SiGe-2H core / shell structures was developed by Eindhoven University of Technology (TU / e) and enabled the demonstration of the light emission capacity of Si(i- X )Ge x -2H, as described in particular in document US-A- 2022 / 0135878 which describes the optical properties of Si(i- X )Ge x-2H and the manufacture of optoelectronic devices or components. In US-A-20220135878, a manufacturing process for a Si(i-) light-emitting device is described. X )Ge x -2H from III-V binary compound nanowires with a hexagonal structure fabricated according to the following steps: deposition of metallic catalysts such as gold onto a substrate, growth of III-V nanowires while controlling the 2H hexagonal structure, removal of the metallic catalyst. A GaAs-2H nanowire is thus formed and then used as a support for the radial growth of a Si(i-) shell X )Ge x to form a core (GaAs) / shell (SiGe) structure. The hexagonal crystal structure of GaAs-2H is replicated in the Si(i-) shell X )Ge x .

[0021] It has been demonstrated that the epitaxy, namely the reproduction of the crystal structure layer by layer, of Si(i- X )Ge x-2H can occur on the prismatic m-plane lateral faces of GaAs-2H or GaP-2H nanowires, as can be seen in particular in the publication "Growth-Related Formation Mechanism of I3-Type Basal Stacking Fault in Epitaxially Grown Hexagonal Ge-2H" L. VINCENT et al. Advanced Materials and Interfaces, DOI: 10.1002 / admi.202102340.

[0022] On these faces with a specific orientation of plane m, the AB AB stacking sequence of tetrahedra corresponding to the hexagonal 2H structure of GaAs nanowires can be reproduced in the Si(i- X )Ge x .

[0023] As demonstrated in the article "Hexagonal silicon-germanium nanowire branches with tunable composition" A. Li et al. Nanotechnology 34 (2023), GaAs-2H and GaP-2H nanowires can also be used as a support for the catalyzed growth of Si(i-) nanobranchs. X )Ge x which, following the growth axis <l-100>, exhibit a hexagonal 2H crystal structure perfectly epitaxially on the GaAs-2H support. However, once again the volumes generated are nanometric.

[0024] None of the described processes allows for the implementation of a synthesis process by epitaxy of Si(i- X )Ge x hexagonal on a bulk substrate enabling large-scale synthesis in order to implement the realization of functional devices for photonics, thermoelectrics or quantum technologies.

[0025] It is therefore of interest to be able to propose a method for synthesizing a hexagonal phase crystal structure by epitaxy on a suitable substrate, making it possible to obtain, for example, Si(i- X )Ge x in hexagonal phase in the form of thin films or nanowires.

[0026] For thin-film epitaxy, the first criterion for substrate selection is its hexagonal crystalline structure and orientation. The second criterion is the lattice parameter, which must be close to that of the layer to be epitaxially grown. The third criterion is the surface energy of the substrate relative to that of the layer to be grown, to promote layer-by-layer growth. For the growth of catalyzed nanowires, chemical compatibility with the catalyst must also be considered.

[0027] However, no substrate exists that meets all these criteria. A group III-V semiconductor material such as GaAs, which would be in lattice match with germanium, does not exist in the hexagonal phase as a bulk substrate but can only be synthesized by the vapor-liquid-solid mechanism in the form of nanowires as seen previously.

[0028] The present invention therefore aims to provide a substrate enabling the epitaxial growth of semiconductor materials with a metastable hexagonal crystalline structure, and more particularly of group IV semiconductor materials such as Si(i- X )Ge( X ), as well as a synthesis process by epitaxy (oriented growth) of group IV semiconductor materials such as Si(i- X )Ge( X ) of metastable hexagonal crystalline structure either in thin film or in the form of catalyzed nanowires.

[0029] The invention first of all relates to a substrate characterized in that it comprises: a) a growth support made of a bulk material having a stabilized hexagonal crystalline structure having a prismatic orientation surface of plane a or m, and b) a thin layer of a group III-V semiconductor material having a metastable hexagonal crystalline structure, said thin layer having a thickness of up to 1 pm and being formed by epitaxial growth on said surface of said bulk material.

[0030] According to the invention, the bulk material comprises a prismatic orientation surface of plane a or m and thus serves as a support for the epitaxial growth of the thin film of group III-V semiconductor material having a metastable hexagonal crystalline structure in lattice agreement with the prismatic orientation surface of plane m or plane a of the bulk material serving as growth support.

[0031] Advantageously, said bulk material with stabilized crystalline structure is chosen from materials having a stabilized hexagonal crystalline structure, for example of type 2H (wurtzite) or type 4H.

[0032] Advantageously, when said bulk material has a wurtzite-type hexagonal 2H structure, the III-V thin-film semiconductor material also has a wurtzite-type 2H structure.

[0033] Advantageously, when said material has a hexagonal 4H crystal structure, the III-V semiconductor material formed as a thin film also has a hexagonal 4H crystal structure.

[0034] According to one embodiment, the bulk material with a hexagonal crystalline structure of the growth support is chosen from group II-VI semiconductor materials such as ZnS and Zn(ix)Cd. x S with x ranging from 0 to 1.

[0035] The bulk material forming the growth support is thus advantageously chosen to have a lattice parameter close to that of the III-V group semiconductor material that we wish to grow on it to form said substrate.

[0036] Such a bulk material allows in particular the growth by epitaxy of the group III-V semiconductor material, from a growth surface of plane m or a, of said bulk material respecting the hexagonal stacking sequence.

[0037] Epitaxy of a thin film of a group III-V semiconductor material such as GaAs or GaP is thus possible on a bulk group II-IV semiconductor material, conforming to the hexagonal stacking sequence. The group III-V semiconductor material is preferably GaAs with a hexagonal structure, and in particular GaAs-2H or GaAs-4H. It can also be chosen from the ternary compounds (In,Ga)As or (Al,Ga)As, which possess a non-centrosymmetric hexagonal crystal structure, thus offering new properties to explore and exploit, including nonlinear optics.

[0038] Another example of a thin-film group III-V semiconductor material is hexagonal structure GaP, such as GaP-2H, and GaP-4H.

[0039] According to one embodiment, the substrate according to the invention consists, as a bulk material, of hexagonal ZnS, stabilized having a prismatic orientation surface of plane a or m and of a thin layer of GaAs having a prismatic orientation surface of plane a or plane m, and replicating the hexagonal crystalline structure of ZnS.

[0040] This thin layer of GaAs, also called the "buffer" layer, therefore has a crystalline structure identical to that of the bulk hexagonal phase ZnS material.

[0041] The invention has as its second object a method for synthesizing a substrate as defined according to the first object of the invention, said method being characterized in that it comprises at least one epitaxial growth step of a thin layer of a group III-V semiconductor material, on a prismatically oriented surface of plane m or plane a of a bulk material of stabilized hexagonal crystalline structure.

[0042] According to a first embodiment of said process, the epitaxial growth step is a vapor phase epitaxy step (known by the acronym VPE in English for "Vapor Phase Epitaxy"), using gaseous precursors of said III-V group semiconductor material, to form a thin layer, i.e. a layer with a thickness of about 10 nm to 1 pm, typically 500 nm, for example, on the bulk material of stabilized hexagonal crystalline structure having a prismatic orientation surface of m plane or a plane and serving as a growth support.

[0043] According to this first embodiment and by way of example, on the prismatically oriented surface of plane m or a of a bulk ZnS material, for example, of stabilized hexagonal crystalline structure, an epitaxial growth of a thin layer of GaAs of metastable hexagonal crystalline structure is carried out using appropriate gaseous precursors such as, for example, tertiary butyl arsine (TB As), trimethylgallium (TMGa) or any other precursor.

[0044] On a substrate that would be composed only of the group II-VI semiconductor material, such as ZnS, Zn(i. X )CD x S, vapor-catalyzed growth of nanowires via a Vapor-Liquid-Solid (VLS) mechanism does not ensure epitaxy with this substrate. Si and Ge nanowires always grow in the cubic phase.

[0045] Similarly, with vapor-phase epitaxy (VPE) using suitable gaseous precursors such as GeEL, SiFL, or any other suitable precursor, on the surface of a Group II-VI material, the direct growth of Si and Ge thin films is not possible due to the inherent selectivity of the reaction in this process. Growth by other processes based on the projection of atomic species, such as MBE, allows epitaxy, but island formation is predominant.

[0046] According to a second embodiment, the epitaxial growth step of said synthesis process is a molecular beam epitaxy step (known by the acronym MBE in English for "Molecular Beam Epitaxy") using sources of said group III-V semiconductor material.

[0047] According to a particular embodiment of the invention, the process may further comprise a step of preparing the prismatically oriented surface of the hexagonal crystalline bulk material forming the growth support, in plane a or plane m. This preparation step may, for example, be carried out by chemically mechanical polishing (CMP) of the surface with a very dilute 0.01% bromine:methanol (B-CEEOH) solution. This step removes organic contaminants and surface oxides from the bulk material. This preparation step may also be followed by a complementary vacuum annealing step at a low temperature (for example, 450°C) for 20 minutes, for example, to eliminate volatile bromine residues.

[0048] The bulk material with a stabilized hexagonal crystalline structure, which forms the epitaxial growth support for the thin film, therefore has a surface orientation favorable to the transfer of the crystalline arrangement of said bulk material to the thin film, thus allowing the hexagonal crystalline structure to be replicated in the directions <l-100>(plan m) and<l l-20> (plan a) reproducing the polytype of said solid material used as a growth support.

[0049] Thus, advantageously, by choosing as the epitaxial growth surface the surface of a bulk material with a stabilized hexagonal crystalline structure, oriented in the m-plane or a-plane, the following epitaxial growth <l-100>Or<l l-20> promotes the replication of the stacking sequence of a metastable hexagonal structure of the III-V group semiconductor material constituting the thin film, whereas growth on a basal plane named c could only allow the synthesis of the stable cubic sequence phase of this material.

[0050] The substrate as defined according to the first object of the invention can be used for the synthesis of other materials, in particular group IV semiconductor materials, by epitaxial growth on the surface of said thin film of a group III-V semiconductor material having a metastable hexagonal crystal structure. Indeed, said thin film then presents a surface having an m or a plane orientation favorable to the resumption of epitaxial growth of a group IV semiconductor material, such as, for example, Si(i- X )Ge x , with a metastable hexagonal crystal structure in the directions <l-100>(plane m) and <11-20> (plane a), either in the form of a thin film or in the form of nanowires, said thin film or nanowires reproducing the polytype of the thin film of said substrate, In this case, said thin film of said substrate preferably has a thickness of about 10 to 100 nm.

[0051] The invention therefore has as its third object the use of a substrate as defined according to the first object of the invention, for the synthesis of group IV semiconductor materials, by epitaxial growth on the surface of said thin film of a group III-V semiconductor material having a metastable hexagonal crystalline structure.

[0052] The substrate as defined according to the first object of the invention can also be used for analytical purposes to study the piezo-phototronic or nonlinear optical properties of this material. In this case, said thin film preferably has a thickness of approximately 100 to 1000 nm.

[0053] As previously stated, the substrate as defined according to the first object of the invention, which has a thin layer of group III-V semiconductor material with a hexagonal crystal structure, for example GaAs (also called a "buffer" layer), allows the epitaxial growth of group IV semiconductor materials such as Si(i- X )Ge x in their hexagonal form in thin layers or in the form of nanowires.

[0054] The substrate as defined according to the first object of the invention therefore forms a support of hexagonal structure and specifically oriented plane m or a.

[0055] Advantageously, the epitaxy of single-crystal thin films and catalyzed nanowires of group IV semiconductor materials such as hexagonal phase germanium is obtained on the substrate having the "buffer" layer according to the invention.

[0056] The invention therefore has as its fourth object a method for synthesizing a group IV semiconductor material, characterized in that it comprises, on a substrate as defined according to the first object of the invention, at least one epitaxial growth step of a group IV semiconductor material in the form of a thin film or nanowires having a metastable hexagonal crystalline structure.

[0057] It is therefore possible to propose volumes and morphologies of Si(i-) materials X )Ge x hexagonal phase allowing a complete characterization of the physical properties of these materials in hexagonal form so as to be able to develop their use for optical, electronic and quantum devices based on these materials.

[0058] According to a first embodiment of said process, said substrate has a prismatic orientation surface of plane m and the epitaxial growth step is a vapor-catalyzed epitaxy according to a Vapor Liquid Solid (VLS) mechanism to form nanowires in a group IV semiconductor material with hexagonal crystalline structure.

[0059] According to a second embodiment of said process, said substrate has a prismatic orientation surface of plane m or plane a and the epitaxial growth step comprises vapor phase epitaxy, using gaseous precursors of the group IV semiconductor material, or molecular beam epitaxy using sources of said group IV semiconductor material, to form a thin layer of said group IV semiconductor material.

[0060] The orientation of a bulk material having a surface of plane m or a is determining for the epitaxy of the thin film of a group III-V semiconductor material (the "buffer" layer) on which the stacking sequence of the hexagonal phase of the group II-IV material such as Si(i- X )Ge x , or of the Ge.

[0061] A substrate as defined according to the first object of the invention and having a prismatic orientation surface of plane m is preferably used to carry out nanowire growth.

[0062] A substrate as defined according to the first object of the invention and having a prismatic orientation surface of plane m or plane a can be used to achieve growth in the form of a thin film.

[0063] The invention therefore has as its fifth object a group IV semiconductor material, characterized in that it has a metastable hexagonal crystalline structure, in the form of a thin film or nanowires, said thin film and said nanowires being obtained by the synthesis process as defined according to the fourth object of the invention.

[0064] Said Group IV semiconductor material is preferably Ge-4H, SiGe-4H, Ge-2H or Si(i- X )Ge x -2H (x being between 0 and 1) depending on the structure of the substrate used.

[0065] The materials Si(i- X )Ge x -2H or 4H obtained by the process as defined according to the fifth object of the invention open up possibilities of use in photonic devices such as light-emitting diodes, lasers, for example.

[0066] The emission wavelength of Si(i- X )Ge x hexagonal is located in the mid-infrared (MIR) region. This area is particularly interesting because of the atmospheric transparency windows available for laser applications such as the detection and analysis of gases or chemical pollutants, LIDAR telemetry, and infrared telecommunications.

[0067] Furthermore, in nanowires, the reduction in thermal conductivity of Si(i- X )Ge x -2H is suitable for the use of these nanostructures for applications in thermoelectricity.

[0068] Other features and advantages of the present invention will become clear from the examples in the description that follows and with reference to the accompanying figures, which depict:

[0069] [Fig. 1] a schematic view of a substrate 10 according to the invention comprising a hexagonal solid material 1 (growth support) with orientation <l-100>(i.e. plane m) or of orientation <11-20> (i.e. plane a) and a thin layer 2 of GaAs epitaxially on the surface of said bulk material.

[0070] [Fig. 2a] and [Fig. 2b] a schematic representation of the atomic packing of the hexagonal structure respectively 2H and 4H of type AB AB and ABCB along the direction <0001> i.e. axis c;

[0071] [Fig. 3] a schematic view of catalyzed Si(i- 3 nanowires X )Ge x hexagonal epitaxially grown on a substrate 10 according to the invention, each of the nanowires 3 being topped with a droplet 4 of gold catalyst. In the case of the catalyzed growth of nanowires 3 on the substrate 10, only a bulk material 1 of plane m is relevant, allowing the nanowires 3 to be perpendicular to the substrate 10.

[0072] [Fig. 4] a schematic representation of the principle of catalyzed growth of nanowires 3 epitaxially grown on a substrate 10 according to the invention comprising a thin layer 2 of GaAs-2H epitaxially grown on a bulk material 1 of 2H structure with a (1-100) plane m surface. Each of the nanowires 3 is topped with a droplet 4 of gold catalyst. The diagram shows the alignment of the atomic stacking of the epitaxially grown structures on top of each other.

[0073] [Fig. 5] a schematic representation of the principle of epitaxy of a 5-layer of Si(i- X )Ge x - 2H on a substrate 10 according to the invention comprising a thin layer 2 of GaAs-2H, as a "buffer" layer, epitaxially on a bulk material 1, growth support, of 2H structure of surface (1-100) plane m. The layer 5 has a surface (1-100) ie of plane m;

[0074] [Fig. 6] A HAADF-STEM (High Angle Annular Dark-Field-Scanning Transmission Electron Micrograph) of a substrate 10 according to the invention, consisting of a thin layer 2 of GaAs on a bulk material 1 of ZnS stabilized in a hexagonal 4H structure, and a chemical map of said substrate 10 obtained by energy-dispersive X-ray spectroscopy. The graph represents the linear distribution of chemical elements perpendicular to the GaAs / ZnS interface, as indicated by the arrow in the corresponding image.

[0075] [Fig. 7] a high-resolution HAADF-STEM micrograph of the GaAs / ZnS-4H interface of substrate 10 in Figure 6;

[0076] [Fig. 8] an enlarged micrograph of figure 7 of the GaAs / ZnS interface showing the transfer of the ABCB atomic stacking of the hexagonal 4H structure into the GaAs layer;

[0077] [Fig. 9a] and [Fig. 9b] respectively show a top view and a 45° inclined view of Ge nanowires obtained on a 10 GaAs-4H / ZnS-4H substrate as shown in Figure 6. In the case of an m-plane substrate, the nanowires are vertical on the surface and grow along the orientation <l-100>. Thus, viewed from above, the vertical nanowires in epitaxy with the substrate are visible as white dots.

[0078] [Fig. 10a] a bright-field STEM micrograph showing the crystal structure of a Ge-4H nanowire; [Fig. 10b] an enlarged view of Figure 10a showing the 4H structure in the Ge nanowire;

[0079] [Fig. 11] a HAADF-STEM micrograph of a nanowire comprising an axial Ge-4H / Si(i- X )Ge x -4H of composition approximately x=0.55 with below a curve showing the linear distribution of chemical elements along the nanowire. The base of the nanowire is composed of pure Ge, the Si(i-) part X )Ge x has an atomic concentration x = 0.55;

[0080] [Fig. 12] A HAADF-STEM micrograph with EDX chemical mapping of a Ge layer on a GaAs-4H / ZnS-4H substrate as shown in Figure 6:

[0081] [Fig. 13] a HAADF-STEM micrograph of the Ge-4H layer on a GaAs-4H / ZnS-4H substrate with, superimposed on it, a curve giving the linear distribution of chemical elements along the Ge / GaAs / ZnS stack corresponding to figure 12;

[0082] [Fig. 14] a high-resolution STEM micrograph showing the interface of the Ge layer on the GaAs-4H and allowing visualization of the crystal structure transfer from the GaAs-4H substrate to the Ge-4H;

[0083] [Fig. 15] a Fourier transform of figure 14 of the GaAs-4H zone and the Ge-4H zone showing the alignment of the structures and the hexagonal structure.

[0084] A substrate 10 according to the invention and represented in figure 1 comprises a bulk material 1 of stabilized hexagonal crystalline structure, made of ZnS or Zn(i. X )CD x S is stabilized in the hexagonal phase, 4H or 2H, and oriented along a surface with plane m or a. On this bulk material 1, a thin layer 2, up to 1 pm thick, of a III-V binary semiconductor material exhibiting a metastable hexagonal crystal structure is formed by epitaxy. This thin layer 2 is, for example, GaAs. To achieve lattice matching with the GaAs, the bulk material 1 used as a growth support can be a ternary compound ZnCdS.

[0085] Figure 2a shows the atomic reconstruction of the AB AB stacking sequence of the 2H hexagonal structure. Figure 2b shows the atomic reconstruction of the ABCBA stacking sequence of the 4H hexagonal structure along the direction <0001> Thus, if the bulk material 1 used has a 2H (or 4H) structure, the thin layer 2 deposited on a surface of plane m or plane a, perpendicular to the orientation <0001> will have a hexagonal 2H (or 4H) structure.

[0086] Example of implementation 1

[0087] To enable the formation of a metastable hexagonal crystal structure group III-V semiconductor material as a thin film up to 1 pm thick, the surface of the bulk growth support material, which is a group II-VI semiconductor, must have minimal roughness and be free of defects and contamination. This hexagonal crystal structure bulk material also exhibits a prismatic orientation surface with either an a-plane or an m-plane.

[0088] Due to the thermal desorption of II-VI materials, thermal annealing of the surface to deoxidize it at temperatures above 600°C results in significant roughness, which is unacceptable for the crystalline quality of the epitaxial layer. The process developed for preparing the surface of the bulk II-VI growth substrate material begins with chemical-mechanical polishing (CMP) using a very dilute 0.01% bromine:methanol (B-CFLOH) solution to remove organic contaminants and surface oxide. Any remaining bromine residues are volatile and can be removed by vacuum annealing at a low temperature (450°C) for 20 minutes. Using vapor-phase epitaxy (VPE) or molecular beam epitaxy (MBE), a III-V semiconductor material is grown to form a thin layer of said material, such as GaAs.This epitaxial growth is carried out on a prismatically oriented surface with plane a or m of the bulk material 1, the growth support. A thin film of a group III-V semiconductor material with a metastable hexagonal crystal structure is thus formed, in the form of a thin film up to 1 pm thick. The combination of the bulk material 1 and the thin film 2 constitutes a substrate 10, enabling the synthesis, by epitaxial growth, of a new material with a metastable hexagonal crystal structure (see Figure 1).

[0089] Thus, for example, by UHV-VPE (“Ultra High Vacuum” VPE) with the precursors tertiarybutylarsine (TBAs) and trimethylgallium (TMGa), a GaAs thin film 2 is fabricated, constituting a group III-V semiconductor material with a metastable hexagonal crystal structure, in the form of a thin film up to 1 pm thick according to the invention. If the thin film 2 serves as a “buffer” layer for resuming epitaxy, the deposited thickness will be reduced to approximately 50 nm.

[0090] This vapor-phase epitaxy is implemented in an ultra-high vacuum reactor, and the growth of the GaAs thin film was carried out at 550°C under a pressure of 0.666 x 0' 3 mbar (5xl0' 3 Torr) with respective TBAs and TMGa fluxes of 30 sccm and 3 sccm (standard cubic centimeters per minute) (i.e., a V / III ratio of 10). Growth was carried out for 1 minute. The deposited thickness is 70 nm, corresponding to a deposition rate of 1.1 nm / s under the conditions described. On a plane m surface of a bulk ZnS-4H material 1, the GaAs adopts the hexagonal 4H phase of said bulk material 1 (Figure 8). A substrate 10 is thus formed, consisting of a bulk ZnS-4H material 1 and the GaAs-4H thin film 2. This relaxed GaAs-4H thin film has lattice parameters such as a = 3.989 ± 0.0009 Å and c = 13.004 ± 0.0028 Å. This group III-V semiconductor material with a metastable hexagonal crystal structure, in the form of a thin film 2, constitutes a substrate for the epitaxy of a group IV semiconductor material.

[0091] Figure 6 represents an example of this substrate 10 according to the invention comprising a thin layer 2 of GaAs on a bulk material 1 of ZnS stabilized in a hexagonal 4H structure and a mapping of said substrate 10 by energy-dispersive X-ray spectroscopy allowing visualization of the different elements Ga, As, Zn and S composing the substrate 10. Similarly, this figure illustrates the GaAs-4H semiconductor material.

[0092] Figure 7 allows visualization of the interface between the GaAs-4H layer and the bulk material 1 of ZnS-4H of the substrate of Figure 6, while the enlargement of Figure 8 allows visualization of the transfer of the hexagonal 4H structure from the ZnS into the thin layer 2 of GaAs.

[0093] Example of implementation 2

[0094] From the substrate 10 formed in fabrication example 1, Si(i-) nanowires are produced. X )Ge x The thin GaAs layer formed serves as a substrate for the growth, catalyzed by the VLS (Vapor Liquid Solid) mechanism, of Si(i-) nanowires. X )Ge x which reproduce the same hexagonal structure atomic stacking when the growth axis is along the <1 - 100> direction, for example SiGe-2H (figure 3 and 4).

[0095] To produce these nanowires 3, droplets of gold catalyst 4 are obtained by depositing 3 nm of gold under ultra-high vacuum and thermal dewetting onto the substrate at 400°C for 25 minutes. This process makes it possible to obtain droplets 4 with a diameter between approximately 30 nm and 100 nm.

[0096] Thanks to the m-planar surface GaAs thin layer 2 (buffer layer), the Si(i-) nanowires 3 X )Ge x grow perpendicular to substrate 10 and are of identical hexagonal structure to substrate 10 (figure 3 and 4).

[0097] Figure 3 schematically represents 3 Si(i-) nanowires X )Ge x of hexagonal structure epitaxially grown by catalyzed growth on a substrate 10 according to the invention comprising a thin layer 2 of GaAs obtained by epitaxy on a bulk material 1 of hexagonal structure with a (1-100) plane m surface. The nanowires 3 have a growth axis <l-100>The surface area of ​​the plane m of the substrate 10 allows for the production of vertical nanowires 3.

[0098] Figure 4 schematically represents the principle of epitaxy by catalyzed growth of Si(i-) nanowires. X )Ge x from figure 3.

[0099] According to the invention, Ge nanowires 3 are obtained catalyzed on a substrate 10 such as that shown in figures 7 and 8, the structure of the thin layer 2 of GaAs is of type 4H, itself epitaxially on a bulk material 1 of ZnS of 4H structure of surface (1-100) plane m.

[0100] The Ge nanowires 3 in Figures 9a, 9b, 10a and 10b are obtained by VLS growth with digermane (Ge2H2O) as a precursor under a pressure of 6.66 x 0' 2 mbar (5xlO' 2 Torr) at 350°C. The total length of nanowires 3 is 2.5 pm for a growth of 11 minutes.

[0101] Example of implementation 3

[0102] Figure 11 shows an example of the fabrication of nanowires 3 containing a Ge / Si(i-) heterostructure X )Ge x The base of the Ge nanowire 3 is obtained as in embodiment example 2 with 4 minutes of growth. Then the Si(i- X )Ge x is produced with silane (SiEL) and germane (GeEL) precursors in a ratio of 1 / 1 at 6.66xl0' 2 mbar (5xl0' 2 Torr) and at 450°C for 15 minutes. The total length of the nanofilament thus obtained is 800 nm and the composition of Si(i- X )Ge x is x=0.55.

[0103] Example of implementation 4

[0104] Figure 5 schematically represents the principle of epitaxy of a 5-layer of Si(i- X )Ge x -2H on a substrate 10 according to the invention comprising a thin layer 2 of GaAs- 2H, as a "buffer" layer, epitaxially deposited on a bulk material 1 of 2H structure with a surface (1-100) plane m. The layer has a surface (1-100) plane m.

[0105] According to the stacking principle shown in Figure 5, a thin layer 5 of Ge as shown in Figures 12 to 14 can also be deposited on the GaAs-4H / ZnS-4H substrate 10 according to the invention.

[0106] The Ge 5 thin film is obtained by UHV-VPE with digermane (Ge2H2O). The Ge 5 thin film shown in Figure 12 is obtained at 350 °C and under a pressure of 6.66 x 10⁻¹⁰. 3 mbar (5xl0' 3 Torr) and a flux of 20 sccm. The EDX chemical mapping of Figure 12 reveals a 150 nm Ge thin layer 5 on a GaAs / ZnS substrate 10.

[0107] Figure 13 shows a curve of the linear distribution of chemical elements along the Ge / GaAs / ZnS stack (atomic percentage as a function of the distance from the layer shown in Figure 12).

[0108] Figure 14 shows, on a high-resolution STEM micrograph, the crystal structure transfer from the GaAs-4H buffer layer to the Ge-4H layer on a GaAs-4H / ZnS-4H substrate. A Fourier transform in Figure 15 of the GaAs and Ge regions confirms the hexagonal 4H structure and shows the alignment of the (0004) planes of GaAs-4H and Ge-4H.

[0109] The examples given above demonstrate that epitaxy in the form of a thin film 2 of a group III-V semiconductor material on a bulk material 1 with a hexagonal crystalline structure, such as ZnS-4H or 2H, having a prismatic surface with an m or a plane, promotes control of the hexagonal phase in the epitaxially grown layer. This group III-V semiconductor material, obtained as a thin film 2 according to the invention, forms, with the bulk material 1 on whose surface it was epitaxially grown, a substrate 10 that allows the synthesis of group IV semiconductor materials in the form of a metastable hexagonal structure.

Claims

1. DEMANDS 1. Substrate characterized in that it comprises: a) a growth support made of a bulk material having a stabilized hexagonal crystal structure having a prismatic orientation surface of plane a or m, and b) a thin layer of a group III-V semiconductor material having a metastable hexagonal crystal structure, said thin layer having a thickness of up to 1 pm and being formed by epitaxial growth on said surface of said bulk material.

2. Substrate according to claim 1, characterized in that the bulk material of hexagonal crystalline structure of the growth support is chosen from conductive materials of group II-IV.

3. Substrate according to claim 2, characterized in that said conductive materials of group II-IV are selected from ZnS, and Zn(i. X )CD x S, with x ranging from 0 to 1.

4. Substrate according to any one of the preceding claims, characterized in that the group III-V semiconductor material is selected from hexagonal GaAs, hexagonal ternary (In.Ga)As and (Al.Ga)As compounds, and hexagonal GaP.

5. A method for synthesizing a substrate as defined in any one of claims 1 to 4, said method being characterized in that it comprises at least one epitaxial growth step of a thin layer of said material of a group III-V semiconductor material, on a prismatically oriented surface of plane m or plane a of a bulk material of stabilized hexagonal crystalline structure.

6. A method according to claim 5, characterized in that the epitaxial growth step is a vapor-phase epitaxy step, using gaseous precursors of said group III-V semiconductor material 7. A method according to claim 5, characterized in that the epitaxial growth step is a molecular beam epitaxy step using sources of said group III-V semiconductor material.

8. Use of a substrate as defined in any one of claims 1 to 4, for the synthesis of group IV semiconductor materials, by epitaxial growth on the surface of said thin film of a group III-V semiconductor material having a metastable hexagonal crystal structure.

9. A method for synthesizing a group IV semiconductor material, characterized in that it comprises, on a substrate as defined according to any one of claims 1 to 4, at least one epitaxial growth step of a group IV semiconductor material in the form of a thin film or nanowires having a metastable hexagonal crystalline structure.

10. Synthesis process according to claim 9, characterized in that said substrate has a prismatic orientation surface of plane m and the epitaxial growth step is a vapor-phase catalyzed epitaxy according to a Vapor-Liquid-Solid mechanism to form nanowires in a group IV semiconductor material of hexagonal crystalline structure.

11. Synthesis process according to claim 9, characterized in that said substrate has a prismatic orientation surface of plane m or plane a and the epitaxial growth step comprises vapor phase epitaxy, using gaseous precursors of the group IV semiconductor material, or molecular beam epitaxy using sources of said group IV semiconductor material, to form a thin layer of said group IV semiconductor material.

12. Semiconductor material of group IV, characterized in that it has a metastable hexagonal crystalline structure in the form of nanowires obtained by the synthesis process as defined in claim 9 or 10.

13. Semiconductor material of group IV, characterized in that it has a metastable hexagonal crystalline structure in the form of a thin film obtained by the synthesis process as defined in claim 9 or 11.

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