CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to foreign French patent application No. FR 1911450, filed on Oct. 15, 2019, the disclosure of which is incorporated by reference in its entirety.

FIELD OF THE INVENTION

The field of the invention is that of the hetero-integration of semiconductor materials on silicon substrates, and relates more specifically to the epitaxying of two-dimensional gallium selenide (2D GaSe) with a single orientation on [111]-oriented silicon substrate, which may be denoted by Si(111).

Among the numerous applications of the invention, mention may be made of microelectronics, photonics, sensors, radiofrequency-related technologies, microsystems (MEMS), components for the Internet of Things and more broadly anything to do with “more-than-Moore” applications or what are referred to as “derivative” technologies, that is to say technologies which incorporate several functions on a single silicon chip.

BACKGROUND

Silicon (Si) is the semiconductor at the foundation of micro-nanoelectronics. Its highly mature technology for the manufacture of integrated circuits (IC) has experienced impressive progress in terms of miniaturization and improvement of performance.

However, other semiconductors exhibit complementary (electronic, optical, mechanical, etc.) properties which may be advantageous to implement on silicon-based chips.

In this context, a novel class of materials has recently come into being: “2D materials”. What is meant by “2D material” is a material consisting of a sheet (or of a stack of sheets) a few atoms thick. The term “two-dimensional” material is also used. These materials are electronically saturated, so the stacking of several of these sheets takes place via van der Waals bonding. Among the 2D materials, some are semiconductors. Their very low thickness gives rise to quite unique electronic, optoelectronic and mechanical properties. A material which is not structured as one or more sheets but has ionic or covalent bonds throughout its volume may also be referred to as a “3D material”, for example a traditional semiconductor material.

The monolithic hetero-integration of 2D materials by epitaxying on a silicon substrate is a very promising avenue for adding novel (electronic, optical, photonic, mechanical, biological, etc.) functionalities to CMOS (complementary metal-oxide-semiconductor) integrated circuits of highly mature silicon-based technology. It also makes it possible to overcome the troublesome issue of dislocations in the crystal lattice of more conventional (Ge, GaN, etc.) semiconductors epitaxied on silicon.

Among all 2D materials, gallium selenide, GaSe, is a is a very interesting material for epitaxying on a [111]-oriented silicon substrate in that the dangling bonds on the silicon surface may be passivated electronically by an atomic layer of gallium (Ga) and an atomic layer of selenium (Se) in order to form a stable and electronically passivated Si—Ga—Se interface that allows the subsequent growth of the two-dimensional GaSe material (referred to as “2D GaSe”) by van der Waals epitaxy. The atomic bilayer of gallium and of selenium may be formed by using the technique of molecular beam epitaxy (MBE) which consists in projecting one or more molecular beams towards the substrate.

The expression “electronically passivated”, or “electronic passivation”, means removing dangling bonds (or reactive Si—H bonds in a hydrogen-containing environment). The expression “electronically saturated surface” may also be used.

An atomic layer of gallium thus bonded to an atomic layer of selenium may be referred to as an “atomic bilayer” and may be seen as a half-sheet of the “2D GaSe” material. Specifically, 2D GaSe refers to two-dimensional gallium selenide (as a sheet) made up of four atomic layers in the sequence Se—Ga—Ga—Se.

However, the formation of the Si—Ga—Se interface may result in a surface with two types of domains which are mirror images of one another, resulting in an antiphase defect as illustrated inFIGS. 1A(orientation 1) and1B (orientation 2).

The subsequent epitaxial growth of the 2D GaSe material on this Si—Ga—Se interface, and more specifically on this surface with mirroring domains 5, may also result in the formation of grains of GaSe with two orientations at 180° with respect to one another, as illustrated inFIG. 2. As these grains of GaSe coalesce, crystal defects will form (grain boundaries), and these crystal defects may harm the optical, electronic, thermal and mechanical properties of the 2D GaSe material.

In general, the techniques for growing 2D GaSe used in the prior art are sensitive to the different orientations of the passivated Si(111) surface. The layers of 2D GaSe thus obtained thus always contain two types of domains, mirror images of one another, separated by grain boundaries, resulting in crystal defects.

SUMMARY OF THE INVENTION

The invention aims to provide a process for epitaxying gallium selenide on a [111]-oriented silicon substrate without the aforementioned drawbacks of the prior art.

More particularly, it aims to provide a process for epitaxying 2D gallium selenide on a [111]-oriented silicon substrate which makes it possible to decrease the defects generated at the interface between said silicon substrate and said epitaxied layer of 2D GaSe, in particular to decrease the formation of antiphase boundaries, and which is industrially applicable, that is to say straightforward to implement on a large scale (200 and 300 mm substrate) and with a high manufacturing yield.

A process allowing these drawbacks to be overcome is a process for epitaxying GaSe on a [111]-oriented silicon substrate, characterized in that it comprises:

a step of selecting a [111]-oriented silicon substrate, said substrate resulting from cutting a silicon bar in a miscut direction which is one of the three [11-2] crystallographic directions, the miscut angle being smaller than or equal to 0.1°, the obtained surface of the substrate forming a vicinal surface exhibiting a plurality of terraces and at least one step between two terraces;
a passivation step consisting of depositing an atomic bilayer of gallium and of selenium on the vicinal surface of the silicon substrate so as to form a passivated vicinal surface made of silicon-gallium-selenium, said passivated vicinal surface exhibiting a plurality of passivated terraces and at least one passivated step between two passivated terraces;
a step of forming a layer of two-dimensional GaSe by epitaxy on the passivated vicinal surface, said formation step comprising a step of nucleation from each passivated step and a step of lateral growth on the passivated terraces from the nuclei obtained in the nucleation step.

The expression “miscut angle being smaller than or equal to 0.1°” is understood to mean that the absolute value of the miscut angle is smaller than or equal to 0.1°, in other words the miscut angle is between −0.1° and +0.1°, the values −0.1° and +0.1° included.

The expression “lateral growth” is understood to mean that each nucleus grows from the step in an opposite direction to said step and along the main plane of the terrace.

The process combines precisely selecting an Si(111) substrate which has been cut according to precise specifications, allowing a vicinal surface to be obtained, with conditions for epitaxially growing the GaSe that allow preferential nucleation of the 2D GaSe by alignment from the steps on the vicinal surface.

A vicinal surface is a surface on which the atoms are no longer distributed in a plane but are organized in a set of regularly spaced steps whose height is a multiple of the distance between two successive planes of atoms in the crystal lattice of the material. This surface appears as a succession of terraces and steps between the terraces. The steps are ideally rectilinear and parallel, depending on the conditions under which the material was cut.

The process thus comprises the selection of a [111]-oriented silicon substrate from a cylindrical bar of [111]-oriented silicon. The [111]-oriented face of the silicon is a dense plane. The vicinal surface of the substrate is obtained from said cylindrical bar of [111]-oriented silicon by cutting it along a crystal plane that is slightly misoriented with respect to the dense [111] plane, that is to say whose miscut angle is smaller than or equal to 0.1°.

The selection step may comprise a step of controlling these specifications for cutting the selected substrate. The step of controlling the substrate may comprise a step of measuring the substrate using X-ray diffraction which makes it possible to obtain the misorientation characteristics of the substrate.

The cut must be made so as to orient the direction of alignment of the steps, thus allowing the growth of the 2D GaSe to be determined by these steps in the step of forming the layer of 2D GaSe. This allows preferential nucleation of the 2D GaSe from the steps rather than within the terraces. Specifically, the orientation of the layer of 2D GaSe will be dictated by the direction of alignment of the steps.

In the step of forming the layer of 2D GaSe by epitaxy, for example during the nucleation step or thereafter, it is possible to add a step of controlling the alignment of the GaSe nuclei against the steps. This step of controlling the alignment may comprise a step of measurement using a high-resolution scanning transmission electron microscope (STEM) which makes it possible to check for the presence of covalent bonds of the GaSe atoms at the steps.

Ultimately, the combination of the step of selection and the step of formation of the layer of 2D GaSe makes it possible to obtain a layer of 2D GaSe of single orientation, making it possible to overcome the issue of the grain boundaries. This layer of 2D GaSe, with a very low rate of defects, may subsequently be used as an active layer for devices on silicon with novel (electronic, optoelectronic, mechanical, etc.) functionalities.

According to one embodiment, the process further comprises a step of treating the vicinal surface so as to flatten the terraces of said vicinal surface until a substantially zero roughness is obtained on the terraces, said treatment step being carried out after the substrate selection step and before the passivation step.

The treatment step may consist in heat-treating the silicon substrate at high temperature, preferably higher than 800° C. The pressure during the heat treatment is preferably lower than 600 Torr.

According to one embodiment, the passivation step consists of metal-organic chemical vapour deposition (MOCVD) with metal-organic precursors. In particular, the metal-organic precursors may be trimethylgallium (TMGa) and diisopropyl selenide (DIPSe). The partial pressure of TMGa is preferably between 1 and 200 mTorr. The passivation step is preferably carried out at a temperature of between 400 and 650° C. The passivation step is preferably carried out for a duration of between 2 and 30 seconds.

According to one embodiment, the nucleation step consists of metal-organic chemical vapour deposition with metal-organic precursors. The partial pressure of the gallium precursor is preferably lower than or equal to 50 mTorr. The nucleation step is preferably carried out at a temperature of between 400 and 650° C. The nucleation step is preferably carried out for a duration of between 2 and 30 seconds.

According to one embodiment, the lateral growth step consists of metal-organic chemical vapour deposition with metal-organic precursors. The partial pressure of the gallium precursor is preferably between 0.5 and 5 mTorr. The lateral growth step is preferably carried out at a temperature of between 570 and 650° C.

According to one embodiment, the process further comprises a step of removing the oxide from the vicinal surface, said deoxidation step being carried out after the substrate selection step and before the passivation step.

According to one embodiment, the process further comprises an additional step of epitaxying at least one 2D material other than 2D GaSe or at least one 3D semiconductor material, said additional step of epitaxy being formed on the layer of 2D GaSe.

Thus, the layer of 2D GaSe obtained by means of the process according to the invention may be used as a high-quality buffer layer for the subsequent growth of other materials, with moreover a low rate of defects. The at least one 2D material other than 2D GaSe or the at least one 3D material may be chosen from the III-VI materials such as InSe, GaS, GaTe or a combination of III-VI materials, the III-V materials such as GaN, InGaN, GaAs, GaSb or a combination of III-V materials, the II-VI materials such as CdHgTe or a combination of II-VI materials, or the IV-IV materials such as SiGe or Ge—Ge or a combination of IV-IV materials.

The process makes it possible to carry out epitaxy by advancing from each step of a vicinal surface of silicon Si(111) so as to obtain a layer of 2D GaSe of single orientation without grain boundaries.

The process ultimately makes it possible to achieve a single orientation of the 2D GaSe without antiphase defects, and to do so using an industrially applicable process that can be carried out on a large scale, with a high yield, in particular by means of MOCVD on a substrate compatible with standard micro/nanomanufacturing means of a silicon technological platform.

Another subject of the invention is the structure obtained by means of the epitaxying process, comprising:a [111]-oriented silicon substrate, the surface of which is a passivated vicinal surface exhibiting passivated terraces and at least one passivated step between two passivated terraces;a layer of 2D GaSe formed on the passivated terraces, and exhibiting a single orientation without antiphase boundaries.

The miscut angle of the vicinal surface of the [111]-oriented silicon substrate is smaller than or equal to 0.1°.

Preferably, the [11-2] direction of the silicon surface is aligned with the [11-20] direction of the GaSe.

