FR3148651A1 - OPTOELECTRONIC DEVICE FOR DEFORMATION ON DEMAND OF A SEMICONDUCTOR CRYSTALLINE PORTION OPTICALLY COUPLED TO A WAVEGUIDE - Google Patents

Abstract

The invention relates to an optoelectronic device (1) for on-demand deformation of a semiconductor crystalline portion (10), comprising: a deformation structure (20), comprising: a structured thin layer (23) having two arms (23.1) suspended above a cavity; the semiconductor crystalline portion (10), resting on the two arms (23.1); a waveguide (2), optically coupled to the semiconductor crystalline portion (10); an electrical deformation device, adapted to deform the semiconductor crystalline portion (10), comprising: an electrical source, adapted to generate an electrical control signal; and an electrical circuit (31, 32, 33, 34; 35, 36), adapted to generate in the arms (23.1), in response to the electrical control signal, an electric field or a temperature field, inducing a deformation of the arms (23.1) and therefore of the semiconductor crystalline portion (10). Figure for the abstract: Fig. 1C

Description

OPTOELECTRONIC DEVICE FOR DEFORMATION ON DEMAND OF A SEMICONDUCTOR CRYSTALLINE PORTION OPTICALLY COUPLED TO A WAVEGUIDE

The field of the invention is that of optoelectronic devices, such as emitting diodes, photodiodes and optical modulators, comprising a semiconductor crystalline portion that can be stressed on demand, in a controlled and reversible manner, by suspended arms. The invention finds an application in integrated photonic circuits, in particular in the context of so-called silicon photonic technology.

STATE OF THE PRIOR ART

In various optoelectronic applications, it may be advantageous to use a semiconductor crystalline portion having a desired mechanical stress, for example in tension. This is the case in particular for certain light sources whose active zone (optical amplification or semiconductor junction) has, outside of mechanical stresses, an indirect energy band structure, this being then made direct by the application of a sufficient mechanical stress in tension. The semiconductor crystalline portion may be a crystalline material based on germanium, such as for example germanium tin Ge _1-x Sn _x .

In this respect, document EP3745473A1 describes an optoelectronic device, for example a laser diode, comprising a semiconductor crystalline portion, optically coupled to a waveguide, and stressed by a deformation structure along a main axis. The deformation structure is formed of a support substrate, a thin interlayer, and a thin semiconductor layer. The thin interlayer is etched locally to form a cavity, and the thin semiconductor layer is structured to form two arms suspended above the cavity and connected to each other by a central part. The latter is an optically active zone, and here forms the gain medium of the laser diode. It is located in an optical cavity delimited by two Bragg mirrors made in the structured thin layer.

In this example, the central part is tensioned by the tension arms along their longitudinal axis. As explained in particular in the article by Süess et al. entitled Analysis of enhanced light emission from highly strained germanium microbridges , Nature Photon. 7 , 466-472 (2013), due to the shape of the arms in the principal plane (parallel to the support substrate), the suspension of the tension arms above the support substrate leads to an increase in the residual tension of the central part of the structured thin layer. Thus, when it is made from germanium, the central part can then have sufficient mechanical tension along the principal axis to make its band structure direct.

Thus, the final value of the mechanical tension undergone by the central part remains constant and cannot be subsequently adjusted. It essentially depends on the dimensions of the tensioning arms and in particular on the aspect ratio ‘length to width’ of the latter. It is therefore not possible to control on demand the value of the mechanical stresses undergone by the central part, which would make it possible to optimize the performance of the optoelectronic device, whether during operation of the device, or to take into account the type of optoelectronic device (emissive diode, photodiode, etc.).

The invention aims to remedy at least in part the drawbacks of the prior art, and more particularly to propose an optoelectronic device suitable for deforming, on demand, in a controlled and reversible manner, a semiconductor crystalline portion optically coupled to a waveguide.

• une structure de déformation d’une portion cristalline semiconductrice, comportant un empilement de : un substrat support ; puis une couche mince intercalaire délimitant une cavité dans un plan parallèle à celui du substrat support ; puis une couche mince structurée, reposant sur la couche mince intercalaire, comportant au moins deux bras suspendus au-dessus de la cavité, s’étendant longitudinalement suivant un même axe principal ;
• la portion cristalline semiconductrice, reposant sur les deux bras, et s’étendant longitudinalement suivant l’axe principal ;
• un guide d’onde, couplé optiquement à la portion cristalline semiconductrice.

For this, the object of the invention is an optoelectronic device:

• a structure for deforming a semiconductor crystalline portion, comprising a stack of: a support substrate; then a thin intercalary layer delimiting a cavity in a plane parallel to that of the support substrate; then a structured thin layer, resting on the thin intercalary layer, comprising at least two arms suspended above the cavity, extending longitudinally along the same main axis;
• the semiconductor crystalline portion, resting on the two arms, and extending longitudinally along the main axis;
• a waveguide, optically coupled to the semiconductor crystalline portion.

According to the invention, the optoelectronic device comprises an electrical deformation device, adapted to deform the semiconductor crystalline portion on demand along the main axis, comprising: an electrical source, adapted to generate an electrical control signal; and an electrical circuit, connected to the electrical source, and adapted to generate in the arms, in response to the electrical control signal, an electrical field or a temperature field, inducing a deformation, along the main axis, of the arms and therefore of the semiconductor crystalline portion.

Some preferred but non-limiting aspects of this optoelectronic device are as follows.

The semiconductor crystalline portion may have a central portion located between two end portions which rest on the arms, the central portion having an average width less than that of the end portions.

The semiconductor crystalline portion may have a central portion located between two end portions which rest on the arms, the central portion having an average width less than that of the arms.

The support substrate can be made from silicon, and the semiconductor crystalline portion can be made from germanium.

The structured thin layer can be made of a piezoelectric material. The electrical circuit can then comprise two lower and upper electrodes, in the form of thin layers, located on either side of the arms along an axis of thickness of the structured thin layer, and connected to the electrical source to generate, in response to the electrical control signal, an electric field in the arms inducing a deformation of the latter by inverse piezoelectric effect.

The optoelectronic device may comprise a thin protective layer made of a material inert to hydrofluoric acid, covering a free surface of the arms.

The optoelectronic device may comprise two thin bonding layers, made from a metallic material, in contact with each other, and located between the semiconductor crystalline portion and the arms.

The electrical circuit may comprise metal tracks, resting on and in thermal contact with the arms, and connected to the electrical source to generate, in response to the electrical control signal, a temperature field in the arms inducing a deformation of the latter by thermal expansion.

The waveguide may rest on the arms, and be made of the same material as that of the semiconductor crystalline portion and be in physical continuity with it. Alternatively, the waveguide may be integrated into the support substrate.

• réalisation d’un premier empilement comportant la couche mince structurée recouverte par une première couche mince de collage ;
• réalisation d’un deuxième empilement comportant une couche mince cristalline semiconductrice destinée à former la portion cristalline semiconductrice et recouverte par une deuxième couche mince de collage ;
• report et collage du deuxième empilement sur le premier empilement, par mise en contact des première et deuxième couches minces de collage.

The invention also relates to a method of manufacturing an optoelectronic device according to any one of the preceding characteristics, comprising the following steps:

• production of a first stack comprising the structured thin layer covered by a first thin bonding layer;
• production of a second stack comprising a thin crystalline semiconductor layer intended to form the crystalline semiconductor portion and covered by a second thin bonding layer;
• transfer and bonding of the second stack onto the first stack, by bringing the first and second thin bonding layers into contact.

• préalablement à l’étape de report, structuration d’une couche mince de déformation du premier empilement pour former la couche mince structurée et les bras ;
• dépôt d’une couche mince de protection, réalisée en un matériau inerte à un agent de gravure utilisé lors d’une étape ultérieure de suspension des bras, de manière à recouvrir une surface libre des bras.

The process may include the following steps:

• prior to the transfer step, structuring a thin deformation layer of the first stack to form the structured thin layer and the arms;
• depositing a thin protective layer, made of a material inert to an etching agent used during a subsequent step of suspending the arms, so as to cover a free surface of the arms.

The method may include the following step: production of a stack formed from the support substrate, then the intercalary thin layer, then a thin deformation layer intended to form the structured thin layer, then a thin semiconductor layer intended to form the semiconductor crystalline portion, the semiconductor thin layer being produced by epitaxy from the thin deformation layer.

• réalisation d’un empilement formé du substrat support, puis de la couche mince intercalaire, puis de la couche mince structurée, puis de la portion cristalline semiconductrice ;
• gravure chimique d’une partie de la couche mince intercalaire située sous les bras, de manière à réaliser une cavité au-dessus de laquelle les bras sont suspendus.

The process may include the following steps:

• production of a stack formed from the support substrate, then the thin intercalary layer, then the structured thin layer, then the semiconductor crystalline portion;
• chemical etching of a part of the thin interlayer located under the arms, so as to create a cavity above which the arms are suspended.

Other aspects, aims, advantages and characteristics of the invention will appear better on reading the following detailed description of preferred embodiments thereof, given by way of non-limiting example, and made with reference to the appended drawings in which:

Figures 1A, 1B and 1C are schematic and partial views, respectively in longitudinal section, in top view and in perspective, of an optoelectronic device according to a first embodiment where the electromechanical actuator is of the piezoelectric type;

Figures 2A, 2B and 2C are schematic and partial views, respectively in longitudinal section, in top view and in perspective, of an optoelectronic device according to a second embodiment where the electromechanical actuator is of the thermal type;

Figures 3A to 3G illustrate different steps of a method of manufacturing an optoelectronic device similar to that illustrated in at 1C;

Figures 4A to 4F illustrate different steps of a method of manufacturing an optoelectronic device similar to that illustrated in at 2C;

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

In the figures and in the remainder of the description, the same references represent identical or similar elements. Furthermore, the different elements are not shown to scale so as to enhance the clarity of the figures. Furthermore, the different embodiments and variants are not mutually exclusive and may be combined with each other. Unless otherwise indicated, the terms “substantially”, “approximately”, “of the order of” mean to within 10%, and preferably to within 5%. Furthermore, the terms “between … and …” and equivalents mean that the limits are included, unless otherwise indicated.

The invention relates to an optoelectronic device adapted to deform on demand, in a controlled and reversible manner, a semiconductor crystalline portion, the latter being optically coupled to a waveguide to receive or transmit an optical mode thereto. Such an optoelectronic device can be a light source (laser diode or light-emitting diode), a photodetector, or even an optical modulator. Depending on the applications, the waveguide can belong to or be coupled to an integrated photonic circuit, or can be coupled to an external optical fiber.

In general, the optoelectronic device comprises an electrical deformation device adapted to deform, in a controlled and reversible manner, suspended arms of a deformation structure on which the semiconductor crystalline portion rests. The electrical deformation device and the deformation structure together form an electromechanical actuator.

The deformation structure comprises a structured thin layer having two arms suspended above a support substrate, which extend longitudinally along a principal axis. The semiconductor crystalline portion rests on the two arms such that deformation of the arms along the principal axis causes deformation of the semiconductor crystalline portion along the same axis.

The electrical deformation device comprises: an electrical source adapted to generate an electrical control signal; and an electrical circuit, connected to the electrical source, and adapted to generate, in response to the electrical control signal, an electric field or a temperature field in the arms, which causes the deformation of the arms along the main axis, and therefore of the semiconductor crystalline portion along the same axis.

The semiconductor crystalline portion then passes, in a controlled and reversible manner, from a first state at rest of mechanical constraints, in the absence of the electrical control signal, where it can be relaxed or can present residual constraints, to a second state of mechanical constraints different from the first state.

By constrained portion, it is meant that the semiconductor crystalline portion undergoes mechanical stresses in tension or compression, leading to a deformation of the meshes of its crystalline network. Thus, the portion is constrained in tension when it undergoes a mechanical stress which tends to stretch the meshes of the network in a given direction. In the context of the invention, the semiconductor crystalline portion is intended to be more or less constrained in tension or compression, and preferably in tension, along the main axis which corresponds to the longitudinal axis of the arms.

The material of the semiconductor crystalline portion, when subjected to mechanical stresses generated by the electromechanical actuator, therefore exhibits modified optical and/or electrical properties, for example in terms of refractive index at a given wavelength and band gap energy. In particular, in the case of a semiconductor crystalline portion made from germanium undergoing mechanical tension, the material may exhibit a reduced band gap energy, in particular that associated with the Γ valley (or direct valley). The band gap energy can be estimated as a function of the tensile strain, as described in the case of a germanium layer in the publication by Guilloy et al. entitled Germanium under high tensile stress: Nonlinear dependence of direct band gap vs strain , ACS Photonics 2016, 3, 1907-1911. The mechanical tensile stress undergone by the semiconductor crystalline portion may be sufficient for the energy band structure to become direct.

By direct or substantially direct band structure, we mean that the energy minimum E _BC,L of the conduction band of the valley L (or indirect valley) is greater than or substantially equal to the energy minimum E _BC, Γ of the conduction band of the valley Γ (or direct valley), in other words: ΔE = E _BC,L - E _BC, Γ 0. The semiconductor crystalline portion can be made from germanium, the energy band structure of which is indirect in the relaxed state, in other words ΔE < 0, and becomes direct when it undergoes sufficient tensile deformation.

Note, however, that the semiconductor crystalline portion can be made of a material whose band structure is not necessarily made direct by the deformation generated by the electromechanical actuator. Thus, in the case of a photodetector, the semiconductor crystalline portion can be made of germanium whose value of the forbidden energy band varies according to the deformation generated (regardless of whether its band structure is direct or indirect). Furthermore, in the case of a light source, the semiconductor crystalline portion can be made of Ge _1-x Sn _x with x at least equal to 6% so that its band structure is naturally direct.

As detailed below, in a first embodiment illustrated in the at 1C, the electromechanical actuator is of the piezoelectric type, that is to say that it ensures a controlled deformation of the arms by an inverse piezoelectric effect. Furthermore, in a second embodiment illustrated in the at 2C, the electromechanical actuator is of the thermal type, that is to say that the controlled deformation of the arms corresponds to a thermal expansion of the latter.

Figures 1A and 1B are schematic and partial views, in cross-section and in top view ( ), of an optoelectronic device 1 according to the first embodiment where the deformation of the arms 23.1 is obtained by an inverse piezoelectric effect. The is a perspective view of the optoelectronic device 1 where certain elements are omitted for the sake of clarity.

In this example, the optoelectronic device 1 is adapted to emit coherent monochromatic light radiation (laser diode). It comprises an optical cavity oriented along the main axis and delimited by two optical reflectors 3, in which is located the gain medium formed by a central part 11 of the semiconductor crystalline portion 10. However, the optoelectronic device 1 can be a light-emitting diode, a photodiode, or even an optical modulator.

Here and for the remainder of the description, a three-dimensional direct reference XYZ is defined, where the XY plane (main plane) is parallel to the plane of the support substrate 21, where the X axis is oriented along the longitudinal axis of the arms 23.1 and of the semiconductor crystalline portion 10 (main axis), and where the +Z direction is oriented from the support substrate 21 towards the semiconductor crystalline portion 10. Furthermore, the terms “lower” and “upper” are understood as relating to an increasing positioning as one moves away from the support substrate 21 along the +Z direction.

In general, the optoelectronic device 1 comprises: the semiconductor crystalline portion 10; a waveguide 2 optically coupled thereto; and an electromechanical actuator adapted to generate a deformation along the main axis, at the request (in a controlled and reversible manner) of the arms 23.1, and therefore of the semiconductor crystalline portion 10, here by inverse piezoelectric effect.

The electromechanical actuator is formed: of a deformation structure 20 which comprises deformation arms 23.1 on which the semiconductor crystalline portion 10 rests; and of an electrical deformation device adapted to generate in the arms 23.1, in response to an electrical control signal, an electric field inducing by reverse piezoelectric effect a mechanical deformation of the latter, and therefore a deformation of the semiconductor crystalline portion 10 along the main axis.

The deformation structure 20 is formed of a stack of the support substrate 21, a thin interlayer 22, and a structured thin layer 23 made of a piezoelectric material, which comprises two arms 23.1 which extend longitudinally along the same main axis. The semiconductor crystalline portion 10 rests on these two arms 23.1 so that the deformation of the arms 23.1 (here by inverse piezoelectric effect) induces the deformation of the semiconductor crystalline portion 10. This inverse piezoelectric effect is the physical phenomenon of deformation of the crystalline structure of the piezoelectric material, in expansion or compression, in response to the application of an electric field passing through it. In a known manner, the stress field T in the piezoelectric material depends on the electric field E and the piezoelectric coefficient e. The stress tensor [T] can thus be written, in the absence of an applied external force: [T] = –[e][E].

The support substrate 21 provides support for the intercalary thin layer 22, the structured thin layer 23 and the semiconductor crystalline portion 10. It can be made from silicon, for example silicon and/or a silicon oxide or nitride. However, it can be made from a material chosen from sapphire, borosilicate, glass, quartz or any other suitable material. It can have a thickness of the order of a few microns to a few hundred microns. Furthermore, the support substrate 21 can include active or passive optical elements (modulators, multiplexers, coupling networks, etc.), in particular in the context of so-called silicon photonic technology.

The intermediate thin layer 22 ensures the spacing of the arms 23.1 of the structured thin layer 23 with respect to the support substrate 21, and delimits the cavity in the XY plane. It is preferably made from silicon, for example from a silicon oxide or nitride, or even from alumina Al ₂ O ₃ , among others. It rests on the support substrate 21, and can have a thickness of the order of a few tens of nanometers to a few microns. It defines laterally, in the XY plane, a cavity above which the arms 23.1 of the structured thin layer 23 are suspended.

The structured thin layer 23 rests on the intercalary thin layer 22. It is formed of a holding portion 23.2, which rests on the intercalary thin layer 22, and arms 23.1 suspended above the cavity. The arms 23.1 are elongated along the main axis X, that is to say that they have an average width along the Y axis less than the length along the Z axis. They extend longitudinally, coaxially, along the main axis X, so that this axis forms the main deformation axis of the semiconductor crystalline portion 10.

The arms 23.1 have a length Lb and an average width <lb>, which is less than the width of the cavity. The width is here the dimension along the transverse axis Y. The arms 23.1 may have a shape in the XY plane that is substantially rectangular, with possibly a progressive or abrupt decrease (as illustrated in the and 1C) of the local width towards the semiconductor crystalline portion 10. In this example, the arms 23.1 have a local width lb which passes, from the holding part 23.2, a first substantially constant value lb1, then decreases abruptly to a second substantially constant value lb2. Preferably, to effectively induce a deformation of the semiconductor crystalline portion 10, the average value <lb> of the width of the arms 23.1 is greater than the average value of the semiconductor crystalline portion 10, and preferably greater than the average value <lpc> of the width of the central part 11 of the semiconductor crystalline portion 10 (where the gain medium is located).

Note that the arms 23.1 may be physically distinct from each other along the main axis X, as illustrated in the (no continuity between the arms 23.1 by the material(s) from which they are formed). Alternatively (not shown), they may be connected to each other by a central portion of the structured thin layer 23, this central portion extending continuously from one arm to the other under the semiconductor crystalline portion 10 and preferably having a local width at most equal to that of the semiconductor crystalline portion 10.

In this embodiment where the on-demand deformation of the arms 23.1 is obtained by an inverse piezoelectric effect, the structured thin layer 23 is made from a piezoelectric material, preferably lead zirconate titanate PbZrTiO ₃ (PZT), but other materials may be used, such as BaTiO ₃ , AlN, ZnO, LiNbO ₃ , Pb(NbO ₃ ) ₂ , PbTiO ₃ , Pb(Mg _0.33 Nb _0.66 )O ₃ , Pb(Sc _0.5 Ta _0.5 )O ₃ or any other suitable piezoelectric material. It has a thickness of the order of a few microns, for example between 0.5 and 2 µm. This is a thin layer deposited on the intercalary thin layer 22 by microelectronic deposition techniques, for example by a sol-gel method or by spraying. It is therefore distinguished from piezoelectric substrates whose thickness is several hundred microns or even several millimeters.

The semiconductor crystalline portion 10 is made of at least one crystalline, and preferably monocrystalline, semiconductor material. This material may be chosen in particular from the elements of column IV of the periodic table, such as germanium Ge, silicon Si, and from the compounds formed from these elements, for example GeSn, SiGe and SiGeSn. It may also be chosen from the III-V compounds comprising elements from columns III and V of the periodic table, such as for example InP and GaInAs, or even from the II-VI compounds, such as for example CdHgTe.

Preferably, the semiconductor crystalline portion 10 is made from a semiconductor material whose band structure, in the absence of sufficient strain deformation, is indirect. The strain deformation of the semiconductor crystalline portion 10, induced by the electromechanical actuator, may then be sufficient to make its band structure direct. The semiconductor crystalline portion 10 is preferably made from germanium. By “made from germanium”, it is meant that the semiconductor crystalline portion 10 is formed mainly from germanium or its compounds. In this example, the semiconductor crystalline portion 10 is made from GeSn.

The semiconductor crystalline portion 10 preferably has an elongated shape along the main axis X. It rests on the arms 23.1, such that the deformation of the arms 23.1 along the main axis X causes that of the semiconductor crystalline portion 10 along the same axis. It comprises a central part 11, which here forms the gain medium of the laser diode, and two end parts 12, which rest on the arms 23.1. In this example where the arms 23.1 are distinct from each other (no continuity of material), the central part 11 is directly suspended above the support substrate 21 without resting on the arms 23.1, while the end parts 12 rest on the arms 23.1. To improve the mechanical strength of the semiconductor crystalline portion 10 on the arms 23.1 on the one hand, and to optimize the deformation thereof, the central portion 11 preferably has an average width less than that of the end portions 12. Furthermore, the central portion 11 here has a length Lpc and a width lpc here substantially constant. Preferably, as indicated above, so as to optimize the deformation induced by the arms 23.1 in the semiconductor crystalline portion 10, its average width <lpc> is less than the average width <lb> of the arms 23.1. The semiconductor crystalline portion 10 has a thickness along the Z axis which can be between a few hundred nanometers and a few microns, for example between 100 nm and 2 µm approximately.

The optoelectronic device 1 comprises an electrical deformation device, adapted to generate an electric field in the arms 23.1 of the structured thin layer 23, inducing a deformation of the arms 23.1 (by inverse piezoelectric effect), and therefore of the semiconductor crystalline portion 10, along the main axis X. This electrical deformation device comprises an electrical source (not shown) and an electrical circuit adapted to generate the electric field.

The electrical source is adapted to generate an electrical control signal, for example an electrical potential difference between two electrodes 31, 32, so as to cause a deformation in compression or tension, by inverse piezoelectric effect, of the structured thin layer 23 along the main axis X.

The electrical circuit is connected to the electrical source. It comprises two lower electrodes 31 and upper electrodes 32, which are in the form of thin conductive layers located on either side of the structured thin layer 23 along the vertical axis Z. The electrodes 31, 32 are made of an electrically conductive material, for example CrAu, Pt/TiO2, among others, and have a thickness for example of the order of 100 nm. The electrical circuit may comprise contact pads 33, 34, which ensure the connection of the lower electrodes 31 and upper electrodes 32 to the electrical source. The contact pads 34 here pass through the structured thin layer 23 to come into contact with the lower electrodes 31 (without contacting the upper electrode 32).

Finally, the optoelectronic device 1 comprises a waveguide 2 optically coupled to the semiconductor crystalline portion 10. In this example where the optoelectronic device 1 is a laser diode, the waveguide 2 is adapted to receive light radiation from the semiconductor crystalline portion 10 (see dotted arrow on the figure). ). The waveguide 2 is here a linear portion made of the same material as that of the semiconductor crystalline portion 10, and ensuring physical continuity with the latter. It may have a width different from that of the semiconductor crystalline portion 10. Note that an optical insulation dielectric layer (not shown) may be located between the waveguide 2 and the upper electrode 32 to limit optical losses and thus improve the performance of the waveguide 2.

Alternatively, as illustrated by the , the waveguide 2 may be an integrated waveguide located in the support substrate 21. The optical coupling is then an evanescent coupling, where the optical mode from the semiconductor crystalline portion 10 is coupled first to the underlying structured thin layer 23 then to the integrated waveguide 2. This type of coupling is similar to that described in the document EP3462555A1.

The waveguide 2 may belong to or be optically coupled to a photonic circuit of the optoelectronic device 1, located for example at least partly in the support substrate 21. The photonic circuit may comprise active and/or passive optical elements. Alternatively or in addition, the waveguide 2 may be optically coupled to an external optical fiber by means of a diffraction grating.

In this example where the optoelectronic device 1 is a laser diode, two optical reflectors 3 are produced here in the semiconductor crystalline portion 10, in end parts 12 located on either side of its central part 11 where the gain medium is located. The optical reflectors 3 thus delimit the optical cavity along the main axis X. Also, one of the optical reflectors 3 has a reflectivity substantially equal to 100%, while the other optical reflector 3 (here located between the central part 11 and the waveguide 2) has a reflectivity less than 100%, so as to allow the directional emission of an optical mode at the wavelength of the optical cavity (Fabry-Pérot). In this example, the optical reflectors 3 are cube corner structures, as described in the article by Zabel et al. entitled Top-down method to introduce ultra-high elastic strain , J. Mater. Res., 2017, 32 (4), 726-736. Alternatively, they may be Bragg mirrors made by partial or total localized etching, depending on the thickness, of the semiconductor crystalline portion 10, as described for example in the document EP3745473A1 cited above. It should also be noted that it is advantageous for the optical reflectors 3 to be located in a suspended part of the semiconductor crystalline portion 10 (i.e. which are not perpendicular to the arms 23.1), so as to improve the confinement of the mode in the optical cavity.

Note that the deformation structure 20 may comprise additional thin layers. Thus, a thin protective layer 41 extends so as to cover a free surface of the arms 23.1, and extends here on the upper face and sides of the arms 23.1. It extends here on the upper electrode 32 (but it can be located between the upper face of each arm 23.1 and the upper electrode 32). It is made of a material inert here to a chemical agent used during a chemical attachment making it possible to produce the cavity and the suspension of the arms 23.2. It makes it possible to protect the arms 23.1 during this chemical attack, here with hydrofluoric acid (HF) in the vapor phase. It can be made of amorphous silicon with a thickness of approximately 60 nm, and can be deposited in a conformal manner by plasma-enhanced chemical vapor deposition (PECVD).

Furthermore, thin layers of bonding 42 (42.1 and 42.2, cf. ) may be located between and in contact with the semiconductor crystalline portion 10 and the arms 23.1. They may be made of a metallic material, for example aluminum, gold Au or its compounds, among others. As described below, these thin bonding layers 42 ensure the bonding (here by thermocompression) of the semiconductor crystalline portion 10 on the arms 23.1 of the structured thin layer 23.

Finally, as illustrated by the , a thin encapsulation layer 47 can be deposited so as to cover the semiconductor crystalline portion 10 as well as the deformation structure 20. It can be made of amorphous silicon with a thickness of a few tens of nanometers deposited by PECVD.

In operation, the electrical source generates an electrical control signal, here a bias voltage between the two lower 31 and upper 32 electrodes. An electric field is then generated within the arms 23.1 of the structured thin layer 23, the field lines of which extend substantially parallel to the vertical axis Z. The electric field induces a deformation of the arms 23.1 along the vertical axis Z, and also, by Poisson effect, a deformation in the XY plane and therefore along the main axis X due to the elongated shape of the arms 23.1 along this axis. The deformation of the arms 23.1 along the main axis X therefore causes that of the semiconductor crystalline portion 10 along the same axis. Note that the intensity of the deformation is proportional to that of the electric field and therefore to that of the electrical control signal.

Also, the electromechanical actuator is able to deform the semiconductor crystalline portion 10, on demand (in a controlled and reversible manner). The deformation range of the arms 23.1 and therefore of the semiconductor crystalline portion 10 can be significant, for example of the order of several percent, with a control voltage of the order of ten to a few tens of volts, for example of the order of 10 to 20V. The optoelectronic device 1 has an architecture allowing high integration of the different elements on a reduced surface of the support substrate 21. It also allows effective optical coupling to be achieved between the semiconductor crystalline portion 10 and the waveguide 2. All this is obtained in particular by the fact that the deformation structure 20 comprises thin layers produced by conventional microelectronics techniques (deposition, lithography, etching, etc.). Furthermore, as described below, it can be made from silicon on insulator (SOI) or germanium on insulator (GeOI) substrates, with possibly a transfer and bonding step (here by thermocompression).

For example, the arms 23.1 may have a constant width lb2 of 50 µm over a length of approximately 300 µm. The semiconductor crystalline portion 10 may have a central portion 11 having a constant width lpc of 1.5 µm over a length Lpc of approximately 8 µm. The end portions 12 which rest on the arms 23.1 may have a length of approximately 20 µm and a constant width of approximately 40 µm. The central portion 11 of the semiconductor crystalline portion 10 may have non-zero tensile stresses in the absence of a control voltage applied by the electromechanical actuator (U=0V), for example of the order of -1.5% for the uniaxial deformation ε _xx due to the residual stresses present in the materials of the stacks and which are released during under-etching. Numerical simulation studies, of the finite element method type, carried out using the COMSOL Multiphysics tool show that the central part 11 of the semiconductor crystalline portion 10 can reach tensile deformations of approximately 3% with very high deformation rates, for example here of the order of 0.2% per volt applied.

In the case here where the optoelectronic device 1 is a laser diode, the modification on demand of the state of mechanical constraints of the semiconductor crystalline portion 10, and in particular that of the central part 11, makes it possible to tune in a controlled manner the gap of the material and therefore the wavelength of the emission band. It is then possible to shift the optical gain zone over a large wavelength range and therefore to tune or detune it with the spectral distribution of the permitted modes of the cavity. Furthermore, the contribution of free carriers in the gain medium can be carried out by optical pumping or by electrical pumping. In the latter case, the central part 11 comprises a semiconductor junction, for example of the pin type, and electrodes are in electrical contact with it.

In the case where the optoelectronic device 1 is a photodetector, the central part 11 of the semiconductor crystalline portion 10 comprises a semiconductor junction, for example of the pin type. Its deformation by the electromechanical actuator makes it possible to modulate the cut-off wavelength in a controlled manner. Due to its architecture, the optoelectronic device 1 can occupy a very small surface area, for example on a silicon substrate, much smaller than that of conventional macroscopic spectrometers. It is then possible to find the shape of the spectrum of the incident radiation on the basis of the measurement of the photoconductivity of the illuminated semiconductor crystalline portion 10 and under increasing mechanical tension. The optoelectronic device 1 then forms a spectrometer of a technology different from those of the FTIR type (analysis in the reciprocal space of the spectrum) or of the grating type (diffraction in the real space of the components of the light according to their frequency).

Finally, note that the optoelectronic device 1 can be an optical modulator. The material of the semiconductor crystalline portion 10 can be made opaque to the wavelength of the guided mode, due to a reduction in its band gap energy due to sufficient deformation by means of the electromechanical actuator. The optical modulator can behave, for example, as an optical switch, controllable by actuation of the deformation of the arms 23.1 and therefore of the semiconductor crystalline portion 10.

Figures 2A and 2B are schematic and partial cross-sectional views ( ) and top view ( ), of an optoelectronic device 1 according to the second embodiment, where the deformation of the semiconductor crystalline portion 10 is induced by a thermal expansion of the arms 23.1 controlled by the electromechanical actuator. is a perspective view of the optoelectronic device 1 where certain elements are omitted for the sake of clarity.

In this example, the optoelectronic device 1 is also a laser diode, but the waveguide 2 is an integrated guide located in the support substrate 21. It belongs to an integrated photonic circuit comprising waveguides and possibly active and/or passive optical elements.

Here, the deformation structure 20 is similar to that of the first embodiment in that it also comprises a support substrate 21, a thin intercalary layer 22 delimiting a cavity in the XY plane, and a structured thin layer 23 comprising the arms 23.1 suspended above the cavity.

The support substrate 21 is similar to that of the and is distinguished therefrom in that it comprises the integrated waveguide 2. The latter can be produced from a thin layer of silicon, preferably monocrystalline and having a residual voltage. It is surrounded by a sheath made of a silicon oxide. The support substrate 21 can thus be formed from a strained SOI substrate. Furthermore, the thin intercalary layer 22 is here identical to that of the Stacking 21, 22 and 23 can also be made from a GeOI substrate.

The structured thin layer 23 is here made of a material whose thermal expansion coefficient is sufficient to impose a desired deformation of the semiconductor crystalline portion 10. In addition, it has optical properties, for example in terms of refractive index at the wavelength of the optical mode, allowing optical coupling between the semiconductor crystalline portion 10 and the integrated waveguide 2. It can also be adapted to produce the semiconductor crystalline portion 10 by epitaxy. Thus, in this example, the semiconductor crystalline portion 10 is produced by epitaxy from the material of the structured thin layer 23. The latter can thus be made of germanium Ge, and the semiconductor crystalline portion 10 can be made of germanium tin GeSn.

The electromechanical actuator is here adapted to generate a thermal expansion of the arms 23.1, and therefore a deformation of the latter along the main axis X, which consequently induces a deformation of the semiconductor crystalline portion 10 along the same main axis X.

For this, the electrical source is here adapted to generate a control electric current, and the electrical circuit here comprises metal tracks 35 extending over and in thermal contact with the arms 23.1. These metal tracks 35 are made of a metallic material, for example Pt or TiN with a thickness of 200 nm, and are connected to the electrical source by contact pads 36. A thin layer 24 is located between the structured thin layer 23 on the one hand, and the metal tracks 35 and the electrodes 36 on the other hand. It is made of an electrically insulating and thermally conductive material, for example AlN. The semiconductor crystalline portion 10 can be covered by it. The flow of the control electric current in the metal tracks 35 results in heating of the latter by Joule effect, and therefore of the arms 23.1, which causes their thermal expansion along, in particular, the main axis X. In other words, the flow of the control electric current generates a temperature field in the arms 23.1 which induces a thermal expansion of the latter. The thermal expansion of the arms 23.1 along the main axis X therefore causes the deformation of the semiconductor crystalline portion 10 along the same axis.

Also, in operation, the electromechanical actuator makes it possible to impose a deformation on demand, in a controlled and reversible manner, on the semiconductor crystalline portion 10 along the main axis X. Stopping the flow of the control electric current leads to a return to the initial temperature of the arms 23.1, and therefore to their initial mechanical stress, thus canceling the deformation of the semiconductor crystalline portion 10 imposed by the electromechanical actuator.

In this example, the optoelectronic device 1 being a laser diode, the central part 11 (gain medium) of the semiconductor crystalline portion 10 is located in an optical cavity delimited by two optical reflectors 3. The latter can be Bragg mirrors (not shown) formed in the integrated waveguide 2. The optical mode (see dotted arrow on the ) is formed in the gain medium and oscillates in the optical cavity, i.e. in the semiconductor crystalline portion 10, then is emitted out of the optical cavity to circulate in the integrated waveguide 2.

Here, the germanium-based material of the structured thin layer 23 may not be sensitive to the chemical etching agent used to partially etch the intercalary thin layer 22 and form the cavity. Also, a protective thin layer 41 covering the free surface of the structured thin layer 23 (of the arms 23.1 in particular) is not necessary.

Figures 3A to 3G illustrate steps of an example of a method for manufacturing an optoelectronic device 1 according to the first embodiment (deformation by inverse piezoelectric effect) similar to that of at 1C. This process is given here as an example and several modifications can be made.

In reference to the , a first stack of continuous thin layers is produced from the support substrate 21. Here, we start with an SOI substrate, formed of a silicon substrate (support substrate 21) several hundred microns thick, a buried oxide layer (intercalary thin layer 22), and a thin silicon layer 43 (optional). A conductive thin layer 31c intended to form the lower electrode 31, for example made of CrAu, Pt, or Pt/TiO2, with a thickness of approximately 100 nm, is then deposited, then a piezoelectric thin layer 23c intended to form the structured thin layer 23, for example made of PZT with a thickness of a few microns, and finally a conductive thin layer 32c intended to form the upper electrode 32.

The thin silicon layer 43, although optional, is advantageous insofar as it makes it possible to improve the symmetry of the thin layers on either side of the piezoelectric thin layer 23, and in particular of the arms 23.1 when they are produced and suspended. Indeed, they will comprise the thin silicon layer 43 at their lower face, and a thin protective layer 41, here made of amorphous silicon, at their upper face (cf. ). Preferably, these two thin silicon layers 41, 43 have a substantially identical thickness, for example of the order of a few tens of nanometers, for example 60 nm.

In reference to the , a through opening is made opening onto the intermediate thin layer 22, by localized etching of the upper conductive thin layer 32c (thus forming the upper electrode 32), of the piezoelectric thin layer 23c (thus forming the structured thin layer 23 with the arms 23.1), of the lower conductive thin layer 31c (thus forming the lower electrode 31), and here of the silicon thin layer 43.

Here, the thin protective layer 41 is deposited in a conformal manner, by PECVD, in amorphous silicon 60 nm thick, so as to cover the arms 23.1 and in particular the sides of the latter. This thin protective layer 41 will ensure protection of the piezoelectric material during the chemical etching of a portion of the intermediate thin layer 22 during the production of the cavity and the suspension of the arms 23.1. The portion of the thin protective layer 41 located in contact with the intermediate thin layer 22 is removed. Then, a full-plate conformal deposition of a thin bonding layer 42.1 is carried out, here a metallic material such as aluminum.

In reference to the , a second stack is produced comprising a buffer substrate 44, a thin seed layer 45, for example here in germanium with a thickness of a few microns, here approximately 2.5 µm, epitaxially grown from the buffer substrate 44, then a thin layer 46 intended to form the semiconductor crystalline portion 10, here in germanium tin with a thickness of between a few tens of microns to a few microns, for example here approximately 500 nm, epitaxially grown from the thin seed layer 45. A thin bonding layer 42.2 is deposited, here a metallic material such as aluminum.

In reference to the , the second stack is postponed to bring the two thin bonding layers 42.1, 42.2 into contact with each other. The assembly of the two stacks is here carried out by thermocompression, for example at a temperature of approximately 300°C and a pressure of 5MPa for 30 min. Another type of bonding remains possible, for example a molecular bonding of the oxide/oxide type.

In reference to the , the buffer substrate 44 is removed, for example by grinding , so as to free the thin seed layer 45. The grinding comprises a mechanical polishing step followed by selective wet or dry etching. The thin seed layer 45 is then removed by selective etching with etching stop on the thin crystalline semiconductor layer 46. The latter is then structured to form the crystalline semiconductor portion 10 (cf. ), with the central part 11 and the end parts 12, as well as, here, the waveguide 2. A lateral part of the thin bonding layers, denoted here 42, located around the semiconductor crystalline portion 10 and the waveguide 2, is then made free. This part is then removed, which makes free the lateral part of the thin protective layer 41 not covered by the semiconductor crystalline portion 10, as well as the part of the thin intercalary layer 22 located in the cavity.

In reference to the , a portion of the thin intercalary layer 22 is etched, here by chemical etching with HF in the vapor phase so as to form the cavity. This partial etching causes the suspension of the arms 23.1. The cavity is delimited in the -Z direction by the support substrate 21 and in the XY plane by the non-etched thin intercalary layer 22. The semiconductor crystalline portion 10, and in particular its central portion 11, has a first state of mechanical stresses. It can be relaxed, or, as here, have a slight residual tension.

In reference to the , the optical reflectors 3 are produced (which may have been produced earlier), here cube corners, then a thin encapsulation layer 47 (optional) is deposited in a conformal manner, for example an oxide, covering the semiconductor crystalline portion 10. Finally, the contact pads are produced for each of the arms 23.1, with a first contact pad 33 which comes into contact with the upper electrode 32, and a second contact pad 34 which comes into contact with the lower electrode 31. The contact pads 33, 34 are connected to the electrical source. An optoelectronic device 1 is thus obtained having an integrated architecture, here in silicon technology, comprising an electromechanical actuator (here of the piezoelectric type) making it possible to deform the semiconductor crystalline portion 10 on demand, in a controlled and reversible manner, over a large deformation range.

Figures 4A to 4F illustrate steps of a method of manufacturing an optoelectronic device 1 according to the second embodiment (deformation by thermal effect) identical or similar to that of at 2C. This process is given here as an example and several modifications can be made.

In reference to the , a stack is produced formed of a support substrate 21, a thin intercalary layer 22, a thin seed layer 23c intended to form the structured thin layer 23, and a thin layer 46 intended to form the semiconductor crystalline portion 10. The support substrate 21 here comprises an integrated waveguide 2, which belongs to an integrated photonic circuit. This support substrate 21 may have been produced from an SOI substrate, where the waveguide 2 was formed from the thin silicon layer of the SOI. The thin intercalary layer 22 is here a layer of silicon oxide, the thin seed layer 23c is here made of germanium, and the semiconductor crystalline layer 46 is made of tin germanium.

In reference to the , the thin layer 46 is first structured so as to form a mask intended for the production of the arms 23.1 of the structured thin layer 23. To do this, the thin layer 46 is locally etched, anisotropically, with a selective etching stop on the Ge layer 23c (or a time etching). The optical reflectors (not shown here) can be produced during this step. Note that the right part of the is a top view of the stack, and the left part is a cross-sectional view along line AA.

In reference to the , the 23c layer is locally etched over its entire thickness, selectively with GeSn, by isotropic dry etching. The Ge under-etching is illustrated on the right part of the by dotted lines. This gives the structured thin layer 23 which has arms 23.1.

In reference to the , the semiconductor crystalline portion 10 is then produced by anisotropic localized etching of the layer 46 over its entire thickness selectively with Ge. This gives the semiconductor crystalline portion 10 in GeSn which rests on the ends of the arms 23.1 in Ge. The optical reflectors (if any) can be located in a part of the semiconductor crystalline portion 10 which does not rest directly on the arms 23.1, thus improving the confinement of the optical mode in the optical cavity.

In reference to the , the contact pads 36 and the metal tracks 35 are produced on the thin layer 24 (here in AlN). The contact pads 36 are connected to the electrical source and the metal tracks are in thermal contact with the arms 23.1.

In reference to the , a portion of the thin intercalary layer 22 is etched, here by chemical etching with HF in the vapor phase. This partial etching results in the suspension of the arms 23.1 above a cavity delimited in the -Z direction by the support substrate 21 and in the XY plane by the non-etched thin intercalary layer 22. The semiconductor crystalline portion 10, and in particular its central portion 11, has a first state of mechanical stresses. It can be relaxed, or, as here, have a slight residual tension.

Thus, an optoelectronic device 1 is obtained having an integrated architecture, here in silicon technology, comprising an electromechanical actuator (here of the thermal type) making it possible to deform the semiconductor crystalline portion 10 on demand, in a controlled and reversible manner, over a large deformation range.

Particular embodiments have just been described. Different variants and modifications will appear to those skilled in the art.

Claims (13)

Optoelectronic device (1), comprising:

• a deformation structure (20) of a semiconductor crystalline portion (10), comprising a stack of:
  • a support substrate (21); then
  • a thin intercalary layer (22) delimiting a cavity in a plane parallel to that of the support substrate (21); then
  • a structured thin layer (23), resting on the intermediate thin layer (22), comprising at least two arms (23.1) suspended above the cavity, extending longitudinally along the same main axis;
• the semiconductor crystalline portion (10), resting on the two arms (23.1), and extending longitudinally along the main axis;
• a waveguide (2), optically coupled to the semiconductor crystalline portion (10);
• characterized in that it comprises an electrical deformation device, adapted to deform on demand the semiconductor crystalline portion (10) along the main axis, comprising:
  • an electrical source, suitable for generating an electrical control signal;
  • an electrical circuit (31, 32, 33, 34; 35, 36), connected to the electrical source, and adapted to generate in the arms (23.1), in response to the electrical control signal, an electric field or a temperature field, inducing a deformation, along the main axis, of the arms (23.1) and therefore of the semiconductor crystalline portion (10).

Optoelectronic device (1) according to claim 1, wherein the semiconductor crystalline portion (10) has a central portion (11) located between two end portions (12) which rest on the arms (23.1), the central portion (11) having an average width less than that of the end portions (12). Optoelectronic device (1) according to claim 1 or 2, wherein the semiconductor crystalline portion (10) has a central portion (11) located between two end portions (12) which rest on the arms (23.1), the central portion (11) having an average width less than that of the arms (23.1). Optoelectronic device (1) according to any one of claims 1 to 3, in which the support substrate (21) is made from silicon, and the semiconductor crystalline portion (10) is made from germanium. Optoelectronic device (1) according to any one of claims 1 to 4, in which the structured thin layer (23) is made of a piezoelectric material, and in which the electrical circuit comprises two lower (31) and upper (32) electrodes, in the form of thin layers, located on either side of the arms (23.1) along an axis of thickness of the structured thin layer (23), and connected to the electrical source to generate, in response to the electrical control signal, an electric field in the arms (23.1) inducing a deformation of the latter by inverse piezoelectric effect. Optoelectronic device (1) according to claim 5, comprising a thin protective layer (41), made of a material inert to hydrofluoric acid, covering a free surface of the arms (23.1). Optoelectronic device (1) according to claim 5 or 6, comprising two thin bonding layers (42.1, 42.2), made from a metallic material, in contact with each other, and located between the semiconductor crystalline portion (10) and the arms (23.1). Optoelectronic device (1) according to any one of claims 1 to 4, in which the electrical circuit comprises metal tracks (35), resting on and in thermal contact with the arms (23.1), and connected to the electrical source to generate, in response to the electrical control signal, a temperature field in the arms (23.1) inducing a deformation of the latter by thermal expansion. Optoelectronic device (1) according to any one of claims 1 to 8, in which the waveguide (2) rests on the arms (23.1), is made of the same material as that of the semiconductor crystalline portion (10) and is in physical continuity with the latter; or in which the waveguide (2) is integrated into the support substrate (21). Method of manufacturing an optoelectronic device (1) according to any one of claims 5 to 7, comprising the following steps:

• production of a first stack comprising the structured thin layer (23) covered by a first thin bonding layer (42.1);
• production of a second stack comprising a thin crystalline semiconductor layer (46) intended to form the crystalline semiconductor portion (10) and covered by a second thin bonding layer (42.2);
• transfer and bonding of the second stack onto the first stack, by bringing the first and second thin bonding layers (42.1, 42.2) into contact.

Manufacturing method according to claim 10, comprising the following steps:

• prior to the transfer step, structuring a thin deformation layer (23c) of the first stack to form the structured thin layer (23) and the arms (23.1);
• depositing a thin protective layer (41), made of a material inert to an etching agent used during a subsequent step of suspending the arms (23.1), so as to cover a free surface of the arms (23.1).

Method of manufacturing an optoelectronic device (1) according to any one of claims 8, comprising the following step:

• production of a stack formed from the support substrate (21), then from the intermediate thin layer (22), then from a thin deformation layer (23c) intended to form the structured thin layer (23), then from a thin semiconductor layer (46) intended to form the semiconductor crystalline portion (10), the thin semiconductor layer (46) being produced by epitaxy from the thin deformation layer (23c).

Method of manufacturing an optoelectronic device (1) according to any one of claims 1 to 9, comprising the following steps:

• production of a stack formed from the support substrate (21), then from the thin intercalary layer (22), then from the structured thin layer (23), then from the semiconductor crystalline portion (10);
• chemical etching of a portion of the thin intercalary layer (22) located under the arms (23.1), so as to produce a cavity above which the arms are suspended.