Synthesis process by controlled growth of hexagonal polytypical crystalline phases and metastable materials thus obtained.

Abstract

The invention relates to a group III-V semiconductor material, which has a metastable hexagonal crystal structure, in the form of a thin film up to 1µm thick. Figure 1.

Description

Title of the invention: Method for the synthesis by controlled growth of hexagonal polytypical crystalline phases and metastable materials thus obtained.

[0001] The present invention relates to the fields of metastable materials of 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, particularly those exhibiting photonic functionalities.

[0002] Control of the crystalline phase is an original and efficient means of modifying 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.

[0003] 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 symmetry of the crystal (C for cubic, H for hexagonal, R for rhombohedral).

[0004] 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 AB for hexagonal 2H for example and AB CB for hexagonal 4H.

[0005] The majority of semiconductors, including silicon Si, germanium Ge and gallium arsenide GaAs, crystallize in the diamond cubic phase, polytype denoted 3C. These materials do not possess other polytypes in the natural state under standard conditions.

[0006] On the other hand, highly ionic materials such as CdS and GaN have a stable hexagonal phase denoted 2H.

[0007] The whole 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 "indirect band gap".

[0008] Photonics on the other hand relies essentially on the emission of light in IILV semiconductors such as InP, GaAs, InAs which have a direct band gap.

[0009] Reducing the symmetry of the cubic polytype (3C) to the hexagonal polytypes (2H and 4H) results in band folding of the electronic structure, which can modify the nature and / or width of the band gap. Thus, for example, Ge with 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 shown 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 demonstrated optical properties, it is understood that the hexagonal structure of Si(lx)Gex with variable composition x makes it possible to envision photonic functionalities with group IV semiconductor materials, which are non-toxic. Furthermore, Si is a material far more abundant than group III elements such as aluminum, gallium, and indium, and those of group V such as nitrogen, phosphorus, arsenic, and antimony, currently used for photonics.

[0012] The hexagonal 2H and 4H phases of Si(i_x)Gex could thus offer a direct band 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 interesting to be able to propose a process for the synthesis of polytypes of semiconductor materials and more particularly of Si(i. X)Gex with hexagonal structure in view of using their photonic properties to make photonic components or devices.

[0014] For binary IILV 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 of group IV materials, this growth process does not result in a controlled modification of the polytype.

[0015] Current processes for synthesizing the hexagonal phase of Si and Ge can be grouped into three groups based on different mechanisms: - stress-induced phase transformation (hydrostatic or shear), - 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.

[0016] An inherent disadvantage 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.

[0017] Phase transformation in Si and Ge nanowires enables the creation 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.

[0018] The controlled growth of GaAs-2H / SiGe-2H core / shell structures was developed by Eindhoven University of Technology (TU / e) and demonstrated the light-emitting capability of Si(i-x)Gex-2H, as described in US-A-2022 / 0135878, which describes the optical properties of Si(i-x)Gex-2H and the fabrication of optoelectronic devices or components. US-A-2022 / 0135878 describes a process for fabricating a Si(i-x)Gex-2H light-emitting device from III-V binary compound nanowires with a hexagonal structure, produced according to the following steps: deposition of metallic catalysts such as gold onto a substrate, growth of IILV nanowires while controlling the hexagonal 2H structure, and 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-x)Gex shell to form a core (GaAs) / shell (SiGe) structure.The hexagonal crystal structure of GaAs-2H is replicated in the Si(i. x)Gex shell.

[0019] It has been demonstrated that epitaxy, namely the reproduction of the crystal structure layer by layer, of Si(i_x)Gex-2H can occur on the prismatic lateral faces of m-plane 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.

[0020] On these faces of specific orientation of plane m, the AB AB stacking sequence of tetrahedra corresponding to the hexagonal structure of GaAs nanowires can be reproduced in the Si(i. x)Gex shell.

[0021] 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_x)Gex nanobranchs which, along 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.

[0022] None of the described processes allows for the implementation of a synthesis process by epitaxy of Si(i. x)Gex,hexagonal on a bulk substrate allowing for large-scale synthesis in order to implement the realization of functional devices for photonics, thermoelectricity or quantum technologies.

[0023] It is therefore interesting 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)Gex in hexagonal phase in the form of thin films or nanowires.

[0024] 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, in order to promote layer-by-layer growth. For the growth of catalyzed nanowires, chemical compatibility with the catalyst must also be considered.

[0025] However, no substrate exists that meets all these criteria. A semiconductor material from group IILV such as GaAs, which would be in lattice agreement 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.

[0026] The present invention therefore aims to provide a substrate enabling the epitaxial growth of semiconductor materials with metastable hexagonal crystalline structure, and more particularly of group IV semiconductor materials such as Si(i_x)Ge(x), as well as the process of synthesis by epitaxy (oriented growth) of group IV semiconductor materials such as Si(ix)Ge(x) with metastable hexagonal crystalline structure either in thin film or in the form of catalyzed nanowires.

[0027] The invention relates first of all to a semiconductor material of group IILV, characterized in that it has a metastable hexagonal crystalline structure, in the form of a thin film having a thickness of up to 1 pm.

[0028] Such a group III-V semiconductor material with a metastable hexagonal structure is in the form of a thin film with a thickness of up to 1 pm obtained by epitaxial growth on a prismatically oriented surface of plane m or plane a of a substrate in a material with a hexagonal crystalline structure, growth support.

[0029] The substrate in hexagonal crystalline structure material, growth support is chosen from group II-VI semiconductor materials such as ZnS, Zn(i. x) CdxS.

[0030] An example of such a group III-V thin-film semiconductor material is hexagonal GaAs, such as GaAs-2H, GaAs-4H, as well as the related ternary compounds (In,Ga)As or (Al,Ga)As, which are also new materials offering new properties to explore and exploit, including nonlinear optics thanks to the non-centrosymmetric hexagonal crystal structure.

[0031] Another example of such a thin-film group III-V semiconductor material is hexagonal structure GaP, such as GaP-2H, GaP-4H.

[0032] The invention also relates to a method for synthesizing a group III-V semiconductor material, characterized in that it comprises at least one epitaxial growth step of a layer of said group III-V semiconductor material, on a prismatically oriented surface of plane m or plane a of a stabilized hexagonal crystalline structure material.

[0033] Advantageously, the epitaxy step of said synthesis process is a vapor phase epitaxy step (known by the acronym VPE in English Vapor Phase Epitaxy), using gaseous precursors of said III-V group semiconductor material, to form a thin layer, i.e. having a thickness of 500 nm, for example, on the hexagonal crystalline structure material "growth support" having a prismatic orientation surface of plane m or plane a.

[0034] The epitaxy 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.

[0035] The stabilized hexagonal crystalline structure material forming the substrate therefore has an orientation of its epitaxial growth surface favorable to the transfer of the crystalline arrangement of the substrate to the thin film and allowing the hexagonal crystalline structure to be replicated in the directions <l-100>(plane m) and <11-20> (plane a) reproducing the polytype of said growth support material.

[0036] Thus, advantageously, by choosing as the surface for epitaxial growth a surface of a material with a stabilized hexagonal crystalline structure, oriented in plane m or plane a, the following epitaxial growth <l-100>or < 11-20> promotes the replication of the stacking sequence of a metastable hexagonal structure of the III-V group semiconductor material, whereas growth on a basal plane named c could only allow the synthesis of the stable cubic sequence phase of this material.

[0037] Such a material according to the invention preferably has a layer thickness of 10 to 100 nm and can be used to serve as a pseudo-substrate for implementing an epitaxy of other materials.

[0038] The material in a thicker layer of 100 to 1000 nm can be used for analytical purposes to study the piezo-phototronic or nonlinear optical properties of this material.

[0039] The invention also relates to a pseudo-substrate for the synthesis of group IV semiconductor materials, characterized in that it comprises at least one group IILV semiconductor material having a metastable hexagonal crystal structure, in the form of a thin film having a thickness of 10 nm to 100 nm.

[0040] Thus, the pseudo-substrate has a bilayer structure, comprising a so-called bulk substrate in a material with a stabilized hexagonal crystalline structure (first layer) and a thin layer of nanometric thickness between 10 nm-100 nm of a semiconductor material from group IILV, said thin layer being formed by epitaxial growth on a prismatically oriented surface of plane a or m of said bulk substrate in a material with a stabilized hexagonal crystalline structure.

[0041] The pseudo-substrate thus formed by the thin layer of the metastable semiconductor material of group IILV on the substrate with a stabilized hexagonal structure then exhibits an orientation of its epitaxial growth surface favorable to the resumption of epitaxy of a Si, |XJGcs material with a metastable hexagonal crystalline structure in the directions <l100>(plane m) and <1L2O> (plane a), either in the form of a thin film or in the form of nanowires, said second thin film or nanowires reproducing the polytype of the substrate,

[0042] Advantageously, the stabilized crystalline structure material forming the substrate is chosen from materials having a stabilized hexagonal crystalline structure, for example of type 2H (wurtzite) or type 4H.

[0043] Advantageously, when said material has a wurtzite-type hexagonal 2H structure, the IILV semiconductor material formed as a thin film also has a wurtzite-type 2H structure.

[0044] Advantageously, when said material has a hexagonal 4H crystal structure, the second IILV semiconductor material formed then has a hexagonal 4H crystal structure.

[0045] Preferably, this stabilized hexagonal crystal structure material is chosen from group II-VI semiconductor compounds, such as ZnS, Zn(i. x)CdxS (x from 0 to 1).

[0046] The material forming the substrate is thus advantageously chosen to have a lattice parameter close to that of the semiconductor material which we wish to grow on it to form the pseudo-substrate.

[0047] Such a material allows in particular the growth by epitaxy of the III-V semiconductor material, from a growth surface of plane m or a, of said first material respecting the hexagonal stacking sequence.

[0048] The epitaxy of a thin layer of III-V semiconductor material such as GaAs, GaP is thus proven possible on the group II-IV semiconductor material and in a compliant manner by respecting the hexagonal stacking sequence.

[0049] The pseudo-substrate according to the invention therefore has a 'bilayer' structure, comprising a substrate of a semiconductor material from group ILVI, forming the first layer, while the second layer consists of the thin film formed on the substrate, said thin film being advantageously a semiconductor material from group III-V such as gallium arsenide (GaAs), gallium phosphide (GaP) of hexagonal crystal structure in the form of a thin film of thickness between 10 nm and 100 nm, with prismatic orientation of plane m or a, which has been grown by epitaxy on the first layer of substrate material.

[0050] According to one embodiment, the pseudo-substrate according to the invention consists, as a first layer, of stabilized hexagonal ZnS having a prismatic orientation surface of plane a or m and of a layer of GaAs having a prismatic orientation surface of plane a or m, formed by epitaxial growth and replicating the hexagonal crystalline structure of ZnS.

[0051] This GaAs layer, called the "buffer" layer, therefore has a crystalline structure identical to the hexagonal phase ZnS substrate and forms with the ZnS substrate the pseudo-substrate for the growth of a material with a hexagonal crystalline structure such as Ge, Si(i_x)Gex, for example.

[0052] The invention therefore also relates to a method for synthesizing a pseudosubstrate according to the invention comprising at least one epitaxial growth step of a layer of the group III-V semiconductor material, on a prismatically oriented surface of plane m or plane a of a stabilized hexagonal crystalline structure material.

[0053] The epitaxial step is a vapor-phase epitaxial step, using suitable gaseous precursors of said IILV group semiconductor material. Thus, epitaxial growth, for example in the vapor phase, is carried out on the prismatically oriented surface of plane m or a of a ZnS substrate, for example, with a hexagonal structure. of a thin layer of GaAs with metastable hexagonal crystal structure using suitable gaseous precursors such as, for example, Tertiarybutylarsine (TBAs) and trimethylgallium (TMGa) or any other precursor.

[0054] The epitaxy step can also be carried out by molecular beam using sources of said group IV semiconductor material.

[0055] The process also includes a step of preparing the prismatically oriented surface of the hexagonal crystalline substrate material, such as an ILVI group semiconductor material, in plane a or plane m. This step comprises chemically mechano-polishing (CMP) with a very dilute 0.01% bromine:methanol (Br2-CH3OH) solution to remove organic contaminants and surface oxides from the material. A further step of annealing under vacuum at low temperature (e.g., 450°C) for 20 minutes, for example, removes volatile bromine residues.

[0056] On a substrate composed solely of group II-VI semiconductor material, such as ZnS or Zn(bx)CdxS, VLS (Vapor-Liquid-Solid) catalyzed growth of nanowires does not ensure epitaxy with this substrate. Si and Ge nanowires always grow in the cubic phase.

[0057] Similarly, by vapor-phase epitaxy (VPE) using suitable gaseous precursors such as GeH4, SiH4, or any other suitable precursor, on an ILVI group material substrate, the direct growth of Si and Ge thin films is not possible due to the reaction selectivity inherent in this process. Growth by other processes based on the projection of atomic species, such as MBE, allows epitaxy, but island formation is predominant.

[0058] The pseudo-substrate according to the invention, which has a layer of group IILV semiconductor material with a hexagonal crystalline structure, for example GaAs, called a buffer layer, allows the growth by epitaxy of group IV semiconductor materials such as Si(i.x)Gex in their hexagonal form in thin films or in the form of nanowires.

[0059] This “bilayer” pseudo-substrate according to the invention therefore forms a support with a hexagonal structure and specifically oriented in the m or a plane.

[0060] 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 pseudo-substrate having the buffer layer according to the invention.

[0061] The invention therefore also relates to a method for synthesizing a group IV semiconductor material, characterized in that it comprises, on a substrate according to the invention, at least one epitaxial growth step of a semi-conductive material Group IV conductor in the form of a thin film or nanowires exhibiting a metastable hexagonal crystalline structure.

[0062] It is thus possible to propose volumes and morphologies of hexagonal phase Si(i_ x)Gex materials 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.

[0063] It is thus proposed an epitaxial growth step comprising epitaxy catalyzed according to the VLS process to form nanowires of group IV semiconductor materials of hexagonal crystalline structure on a pseudo-substrate of prismatic orientation of plane m.

[0064] It is also proposed that the epitaxial growth step includes 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 layer of said group IV semiconductor material on a prismatic surface pseudo-substrate of plane m or plane a.

[0065] The orientation of the first surface substrate material of plane m or a is determining for the epitaxy of the buffer layer of the pseudo-substrate, on which the stacking sequence of the hexagonal phase of Si(i. X)Gex, or of Ge, is then replicated.

[0066] A pseudo-substrate according to the invention having a prismatic orientation surface of plane m is preferably used for nanowire growth. A pseudo-substrate according to the invention having a prismatic orientation surface of plane m or plane a is used for thin-film growth.

[0067] The invention therefore also relates to a group IV semiconductor material, characterized in that it has a metastable hexagonal crystalline structure, in the form of a thin film or nanowires, obtained by the process of synthesizing a group IV semiconductor material according to the invention.

[0068] Said Group IV semiconductor material is Ge-4H and SiGe-4H or Ge-2H and Si(i. x)Gex-2H (x being between 0 and 1) depending on the structure of the substrate used.

[0069] The Si(i_ x)Gex -2H or 4H materials obtained by the process of the invention open up possibilities for use in photonic devices such as light-emitting diodes, lasers, for example.

[0070] The emission wavelength of hexagonal Si(i. x)Gex lies in the mid-infrared (MIR) region. This region is particularly interesting because of the Atmospheric transparency windows are available for laser applications such as gas or chemical pollutant detection and analysis, LIDAR telemetry, and infrared telecommunications.

[0071] Moreover, in nanowires; the reduction in thermal conductivity of Si(i. x) Gex -2H is conducive to the use of these nanostructures for applications in thermoelectricity.

[0072] Other features and advantages of the present invention will become clear from the examples in the following description and with reference to the accompanying figures, which represent:

[0073] [Fig. 1] a schematic view of a substrate according to the invention comprising a solid hexagonal support material 1 of orientation <l-100>(i.e., plane m) or with an orientation <11-20> (i.e., plane a) and a thin layer 2 of GaAs epitaxially placed on top; the whole can constitute a pseudo-substrate 10

[0074] [Fig.2a] and [Fig.2b] a schematic representation of the atomic stacking of the hexagonal structure respectively 2H and 4H of type AB AB and ABCB along the direction <0001> i.e. axis c;

[0075] [Fig. 3] A schematic view of catalyzed hexagonal Si(i_x)Gex nanowires epitaxially grown on a pseudo-substrate 10 according to the invention. In the case of catalyzed growth of nanowires on the pseudo-substrate 10, only the substrate in plane m is relevant, allowing the nanowires to be perpendicular to the substrate.

[0076] [Fig.4] a schematic representation of the principle of catalyzed growth of epitaxially grown nanowires on a pseudo-substrate according to the invention, having a GaAs-2H pseudo-substrate epitaxially grown on a primary substrate of 2H structure with a (1-100) plane m surface. The diagram shows the alignment of the atomic stacking of the epitaxially grown structures on each other;

[0077] [Fig.5] a schematic representation of the principle of the epitaxy of a layer 5 of Si, ix)Gcx-2H on a pseudo-substrate 10 according to the invention comprising a thin layer 2 of GaAs-2H, as a buffer layer, epitaxially grown on a growth support material 1 of structure 2H of surface (1-100) plane m. The layer has a surface (1-100) ie of plane m;

[0078] [[Fig.6] a HAADF-STEM (High angle annular dark-field-) micrograph Scanning Transmission Electron Microscopy (STE) of a pseudo-substrate according to the invention, consisting of a GaAs-on-ZnS layer stabilized in a 4H hexagonal structure, and a chemical map of said substrate 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.

[0079] [Fig.7] a high-resolution HAADF-STEM micrograph of the GaAs / interface ZnS-4H from the substrate in [Fig.6];

[0080] [Fig.8] an enlarged micrograph of [Fig.7] of the GaAs / ZnS interface showing the transfer of the ABCB atomic stacking of the hexagonal 4H structure into the GaAs layer;

[0081] [Fig.9a] and [Fig.9b] respectively a top view of Ge nanowires and a view inclined at 45° of Ge nanowires obtained on a GaAs-4H / ZnS-4H substrate as shown in [Fig. 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.

[0082] [Fig. 10a] a bright-field STEM micrograph showing the crystal structure of a Ge-4H nanowire; [Fig. 10b] an enlarged view of [Fig. 10a] showing the 4H structure in the Ge nanowire;

[0083] [Fig. 11] a HAADF-STEM micrograph of a nanowire comprising an axial Ge-4H / Si(i_x)Gex-4H heterostructure of composition approximately x=0.55 with below a curve giving the linear distribution of chemical elements along the nanowire. The base of the nanowire is composed of pure Ge, the Si(i. X)Gex part has an atomic concentration x = 0.55;

[0084] [Fig. 12] a HAADF-STEM micrograph with EDX chemical mapping of a Ge layer on a GaAs-4H / ZnS-4H pseudo-substrate as shown in [Fig. 6]:

[0085] [Fig. 13] a HAADF-STEM micrograph of the Ge-4H layer on a GaAs-4H / ZnS-4H pseudo-substrate with superimposed on it a curve giving the linear distribution of chemical elements along the Ge / GaAs / ZnS stack corresponding to [Fig. 12];

[0086] [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 pseudo-substrate to the Ge-4H;

[0087] [Fig. 15] a Fourier transform of the [Fig. 14] of the GaAs-4H zone and the Ge-4H zone showing the alignment of the structures and the hexagonal structure.

[0088] A pseudo-substrate 10 according to the invention and shown in [Fig. 1] comprises a stabilized hexagonal crystalline material 1, consisting of ZnS or Zn(bx)CdxS stabilized in the hexagonal, 4H or 2H phase and oriented along a surface of plane m or a. On this substrate material 1, a thin layer 2, up to 1 pm thick, of a binary IILV semiconductor material having a metastable hexagonal crystalline structure is formed by epitaxy. This layer is, for example, GaAs. To achieve mesh agreement with GaAs, the growth support substrate material can be a ternary ZnCdS compound.

[0089] Figure 2a represents the atomic reconstruction of the AB AB stacking sequence of the 2H hexagonal structure. Figure 2b represents the atomic reconstruction of the ABCBA stacking sequence of the 4H hexagonal structure along the direction <0001> Thus, if the substrate 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.

[0090] Example of implementation 1

[0091] To enable the formation of a metastable hexagonal crystal structure group III-V semiconductor material in the form of a thin film up to 1 pm thick, the surface of the growth support material, which is an ILVI group semiconductor, must have minimal roughness and be free from defects and contamination. This hexagonal crystal structure material further exhibits a prismatic orientation surface in the a-plane or m-plane.

[0092] Due to the thermal desorption of ILVI materials, thermal annealing of the surface to deoxidize it at temperatures above 600°C results in very high roughness, which is unacceptable for the crystalline quality of the epitaxial layer. The process developed for preparing the surface of the ILVI growth support material first involves chemical polishing (CMP) with a very dilute 0.01% bromine:methanol (Br2-CH3OH) 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.

[0093] By vapor-phase epitaxy (VPE) or molecular beam epitaxy (MBE), the growth of an IILV semiconductor material is implemented to form a thin film of said material 2 such as GaAs. This epitaxial growth is carried out on a prismatically oriented surface of plane a or m of said growth support material 1. A semiconductor material of the IILV group is thus formed, which has a metastable hexagonal crystal structure, in the form of a thin film up to 1 pm thick which constitutes a substrate 10 allowing the synthesis by epitaxial growth of a new material, having a metastable hexagonal crystal structure, see [Fig. 1].

[0094] Thus, for example, by UHV-VPE (ultra high vacuum VPE) with tertiarybutylarsine (TBAs) and trimethylgallium (TMGa) precursors, a thin layer of GaAs is produced constituting a metastable hexagonal crystal structure IILV group semiconductor material, in the form of a thin layer of thickness up to 1 pm according to the invention. If layer 2 serves as a buffer layer for epitaxial resumption, the deposited thickness will be reduced to approximately 50 nm.

[0095] This vapor-phase epitaxy is carried out in an ultra-high vacuum reactor, and the GaAs layer was grown at 550°C under a pressure of 0.666 x 10³ mbar (5 x 10³ 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 aforementioned conditions. On a surface of plane m of a ZnS-4H material 1 forming the substrate, the GaAs adopts the hexagonal 4H phase of said material 1 ([Fig. 8]). A pseudo-substrate 10 is thus formed, consisting of a ZnS-4H substrate 1 and the GaAs-4H thin layer 2. This relaxed GaAs-4H layer has lattice parameters such as a= 3.989+ / -0.0009Â and c=13.004+ / -0.0028Â.This group IILV semiconductor material, with a metastable hexagonal crystal structure, in the form of a thin film, constitutes the pseudo-substrate for the epitaxy of a group IV semiconductor material. Preferably, the pseudo-substrate 10 is in the form of this layer epitaxially grown on a substrate-forming growth support material.

[0096] Figure 6 represents an example of this material / substrate according to the invention comprising a layer of GaAs on ZnS stabilized in a hexagonal 4H structure and a mapping of said substrate 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.

[0097] Fig. 7 shows the interface between the GaAs-4H layer and the first ZnS-4H material of the substrate in Fig. 6, while the enlargement of Fig. 8 shows the transfer of the hexagonal 4H structure from the ZnS into the GaAs layer.

[0098] Example of implementation 2

[0099] Starting from the pseudo-substrate formed in embodiment 1, Si(i.x)Gex nanowires are produced. The thin layer formed of GaAs 2 serves as a substrate for the growth, catalyzed by the VLS (Vapor Liquid Solid) mechanism, of Si(i.x)Gex nanowires 3, which reproduce the same hexagonal atomic stacking structure when the growth axis is along the direction <l-100>, for example SiGe-2H ([Fig.3] and 4).

[0100] To produce these nanowires 3, droplets of gold catalysts 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 with a diameter between approximately 30 nm and 100 nm.

[0101] Thanks to the m-plane surface GaAs buffer layer, the Si(i. x)Gex nanowires 3 grow perpendicular to the substrate 10 and have a hexagonal structure identical to the substrate 10 ([Fig.3] and 4).

[0102] Figure 3 schematically represents Si(1.X)Gex nanowires 3 with a hexagonal structure epitaxially grown on a substrate 10 according to the invention, the structure of the thin-film GaAs pseudo-substrate 2 of which was obtained by epitaxy on a first material 1 with a hexagonal surface structure (1-100) plane m. 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.

[0103] Figure 4 schematically represents the principle of epitaxy by catalyzed growth of the Si(i.x)Gex nanowires of Figure 3.

[0104] According to the invention, Ge nanowires are obtained catalyzed on a pseudo-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 substrate of a first material 1 of ZnS of 4H structure of surface (1-100) plane m.

[0105] The Ge nanowires in Figures 9a, 9b, 10a and 10b are obtained by VLS growth with a digermane precursor under a pressure of 6.66x102 mbar (5x102 Torr) at 350°C. The total length of the nanowires is 2.5 pm for a growth of 11 minutes.

[0106] Example of implementation 3

[0107] Figure 11 shows an example of the fabrication of a nanowire containing a Ge / Si(i_x)Gex heterostructure. The Ge base of the wire is obtained as in embodiment example 2 with a 4-minute growth period. Then the Si(i_x)Gex portion is fabricated with silane (SiH4) and germane (GeH4) precursors in a 1:1 ratio at 6.66 x 10² mbar (5 x 10² Torr) and at 450°C for 15 minutes. The total length of the wire thus obtained is 800 nm and the composition of the Si(i_x)Gex is x = 0.55.

[0108] Example of implementation 4

[0109] Figure 5 schematically represents the principle of the epitaxy of a Si(i_x)Gex-2H layer 5 on a substrate 10 according to the invention comprising a thin GaAs-2H layer 2, as a buffer layer, epitaxially grown on a growth support substrate material 1 of 2H structure with a surface area of ​​(1-100) plane m. The layer has a surface area of ​​(1-100) plane m.

[0110] According to the stacking principle shown in [Fig.5], a layer of Ge as shown in figures 12 to 14 can also be deposited on the GaAs-4H / ZnS-4H pseudo-substrate according to the invention.

[0111] The Ge layer is obtained by UHV-VPE with digermane (Ge2H6). The Ge layer shown in [Fig. 12] is obtained at 350 °C and under a pressure of 6.66 x 10 3 mbar (5x103 Torr) and a flux of 20 sccm. Chemical mapping by EDX of [Fig. 12] reveals a 150 nm Ge layer on a GaAs / ZnS substrate.

[0112] Fig. 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 Fig. 12).

[0113] 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 pseudo-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.

[0114] The examples given above demonstrate that the epitaxy of a Group IILV semiconductor material on a substrate with a hexagonal crystalline structure, such as ZnS-4H or 2H, having a prismatic surface of pan m or a, promotes control of the hexagonal phase in the epitaxially grown layer. This Group IILV semiconductor material obtained according to the invention forms a pseudo-substrate that allows the synthesis of Group IV semiconductor materials in the form of a metastable hexagonal structure.

Claims

Demands

1. A group III-V semiconductor material characterized in that it has a metastable hexagonal crystal structure, in the form of a thin film of thickness up to Ipm.

2. Material according to claim 1, characterized in that said material is hexagonal structure GaAs such as GaAs-4H, GaAs-2H.

3. Material according to any one of claims 1 or 2, characterized in that the thin film of thickness up to Ipm is obtained by epitaxial growth on a prismatically oriented surface of plane m or plane a of a material of hexagonal crystalline structure, growth support.

4. Material according to claim 3, characterized in that the hexagonal crystalline structure growth support material is selected from group II-VI semiconductor materials such as ZnS, Zn(i_x)CdxS.

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

6. A method according to claim 5, characterized in that it comprises a vapor phase epitaxy step, using gaseous precursors of said group III-V semiconductor material.

7. A method according to claim 6, characterized in that it comprises a molecular beam epitaxy step using sources of said group III-V semiconductor material.

8. Pseudo substrate for the synthesis of group IV semiconductor materials, characterized in that it comprises at least one group III-V semiconductor material according to any one of claims 1 to 4, in the form of a thin film having a thickness of 10 nm to 100 nm.

9. Pseudo-substrate according to claim 8, characterized in that it has a bilayer structure, comprising a so-called bulk substrate made of a material with a stabilized hexagonal crystalline structure (first layer) and a thin layer of nanometric thickness between 10 nm-100 nm of a group III-V semiconductor material, said thin film being formed by epitaxial growth on a prismatically oriented surface of plane a or m of said bulk substrate of stabilized hexagonal crystalline structure material

10. Pseudo-substrate according to claim 9, characterized in that it consists of hexagonal ZnS having a prismatic orientation surface of plane a or m and a layer of GaAs having a prismatic orientation surface of plane a or plane m.

11. A method for synthesizing a pseudo-substrate according to any one of claims 8 to 10, characterized in that it comprises at least one epitaxial growth step of a layer of the group III-V semiconductor material, on a prismatically oriented surface of plane m or plane a of a stabilized hexagonal crystalline structure material.

12. A method according to claim 11, characterized in that the epitaxial growth step is a vapor phase epitaxial step, using gaseous precursors of said group III-V semiconductor material or a molecular beam epitaxial step using sources of said group III-V semiconductor material.

13. A method for synthesizing a group IV semiconductor material, characterized in that it comprises, on a pseudo-substrate according to any one of claims 9 to 11, 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.

14. A method according to claim 13, characterized in that the epitaxial growth step comprises VLS-catalyzed epitaxy, for forming nanowires of group IV semiconductor material with hexagonal crystal structure on a prismatic-oriented surface pseudo-substrate of plane m.

15. A method according to claim 13, characterized in that the epitaxial growth step comprises vapor-phase epitaxy, using gaseous precursors of the group IV semiconductor material, or a molecular beam epitaxy step using sources of said group IV semiconductor material, to form a layer of said group IV semiconductor material on a prismatic surface pseudo-substrate of plane m or plane a.

16. Group IV semiconductor material, characterized in that it has a metastable hexagonal crystalline structure, in the form of nanowires obtained by the process according to claim 13 or 14.

17. A group IV semiconductor material, characterized in that it has a metastable hexagonal crystalline structure, in the form of a thin film obtained by the process according to claim 13 or 1

18. 1 J. Material according to claim 16 or 17, characterized in that it is Si(ix)Gex-4H.

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