Graphene-based rectifying antenna

Abstract

The invention relates to a rectifier antenna comprising: an antenna (10); and a Metal-Insulator-Graphene (MIG) diode (20), electrically coupled to the antenna (10), comprising a metallic portion (21) and a graphene conductive portion (24), between which is located at least one insulating portion (22; 23). The metallic portion (21), the insulating portion(s) (22; 23), and the graphene conductive portion (24) form a vertical stack of thin film portions. Furthermore, the antenna (10) is formed of a continuous graphene thin film, a portion of which forms the graphene conductive portion (24) of the diode (20).

Description

TECHNICAL FIELD

[0001] The field of the invention is that of graphene-based diode-based rectenna electromagnetic radiation detectors, in other words, rectifier antennas, also called "rectennas," comprising an electromagnetic radiation-absorbing antenna coupled to a MIG (metal-insulator-graphene) diode. The invention finds application particularly in the fields of infrared or terahertz imaging, thermography, gas detection, and energy conversion. PREVIOUS STATE OF THE ART

[0002] Infrared and terahertz detectors that operate at room temperature can be, for example, bolometer-type thermal detectors. These may consist of an absorbing membrane suspended above a substrate containing a readout circuit, which itself contains a thermistor material whose electrical resistance varies with temperature. However, the performance of these thermal detectors is generally limited by their thermal time constant, which can be on the order of tens of milliseconds.

[0003] Infrared and terahertz detectors can also be rectifier devices comprising a detection antenna coupled to a diode, also called "rectifying antennas" or "rectennas," where the diode can be of the MIM (metal-insulator-metal) or MIIM (metal-insulator-insulator-metal) type. These rectennas can exhibit a much faster response time than bolometer-type thermal detectors, since the transit time of electrons by tunneling through the insulating thin layer of the diode is on the order of femtoseconds to nanoseconds.

[0004] There Figure 1AFigure A illustrates the schematic diagram of a rectenna A1, here in the case of an energy conversion application. It consists of an antenna A10 adapted to absorb the incident electromagnetic radiation, and a rectifier element A20 such as a diode, here of the MIM type, electrically coupled to the antenna A10. A DC filter A2 is generally connected in parallel with the diode A20 to retain only the DC component of the rectified AC signal. The operating principle of such a rectenna A1 is as follows: the antenna A10 absorbs the incident electromagnetic radiation and converts it into a high-frequency electrical signal, which is transferred to the input of the diode A20. The diode A20 rectifies the AC electrical signal, and then the DC filter A2 retains only the DC component of the rectified electrical signal, to supply it here to an electrical load A3.

[0005] There figure 1BThis illustrates an example of an energy band diagram for a MIM diode rectenna, here in the case of an optical sensing application. Such a rectenna is described in particular in the article by Grover & Moddel entitled "Applicability of Metal / Insulator / Metal (MIM) Diodes to Solar Rectennas," IEEE Journal of Photovoltaics, Vol. 1, no. 1, pp. 78-83, 2011. The MIM diode consists of two metallic layers (metals M1 and M2) with a single insulating layer (electrical insulator I) between them. The diagram depends in particular on the work functions ΨM1 and ΨM2 of metals M1 and M2, the electron affinity φl of the insulator I, and the bias voltage VD applied to the MIM diode. Electrons can cross the energy barrier via different conduction mechanisms, for example by Fowler-Nordheim tunneling or by direct tunneling, depending in particular on the heights φL and φR of the energy barriers.These different types of conduction mechanisms are described in particular in Chiu's article entitled A Review on Conduction Mechanisms in Dielectric Films, Advances in Materials Science and Engineering, vol. 2014, Article ID 578168, 18 pages, 2014. In this example, tunneling conduction is of the Fowler-Nordheim type.

[0006] As indicated in the Grover & Moddel 2011 article, in terms of performance, the MIM diode is sought to exhibit a high responsiveness β, which corresponds to a measure of the rectified DC signal as a function of the incident power. It can be determined from the diode's I(V) characteristic using the relation: β = I" / (2I'), where I' and I" are the first and second derivatives of the electric current as a function of the voltage I(V), at the bias voltage VD. Furthermore, the diode is also sought to have a low dynamic resistance to achieve good impedance matching with the antenna.

[0007] However, it appears that for a MIM diode—that is, a diode with a single insulating layer between two metallic layers—optimizing responsiveness leads to a degradation of the dynamic resistance value, and vice versa. Therefore, it does not seem possible to optimize both the responsiveness and the dynamic resistance of an MIM diode simultaneously. Yet, it appears that a MIIM diode—that is, a diode with two insulating layers having different electronic affinities—overcomes this constraint, making it possible to configure the MIIM diode to exhibit both high responsiveness and low dynamic resistance. This is particularly true when the MIIM diode allows charge carrier conduction via resonant tunneling.

[0008] In this respect, the figure 1CThis illustrates an example of an energy band diagram of an infrared rectenna with an MIIM diode configured to allow conduction of charge carriers, here electrons, by resonant tunneling. This type of energy band diagram is described in particular in the article by Grover & Moddel entitled "Engineering the current-voltage characteristics of metal-insulator-metal diodes using double-insulator tunnel barriers", Solid-State Electron. 67, 94-99 (2012), as well as in that of Belkadi et al. entitled "Demonstration of resonant tunneling effects in metal-double-insulator-metal (MI2M) diodes", Nat Commun 12, 2925 (2021).

[0009] Resonant tunneling occurs when electrons pass through insulating layers via a right-angled quantum well located between the two insulating layers. Electrons with energies corresponding to the energy levels of the quasi-bound states in the quantum well can traverse the insulating layers and reach the M2 metallic layer while minimizing reflection, thus producing a higher electric current than in the case of non-resonant tunneling. Note that, in this example, the electrons traverse the first insulating layer via Fowler-Nordheim tunneling and the second insulating layer via direct tunneling. The authors have shown that by using insulators with different electron affinities, it is possible to configure a resonant tunneling MIIM diode to achieve both high responsiveness and low dynamic resistance, which is not possible with a standard MIM diode.

[0010] Furthermore, the document by Hemmetter et al. entitled Terahertz Rectennas on Flexible Substrates Based on One-Dimensional Metal-Insulator-Graphene Diodes, ACS Appl. Electron. Mater. 2021, 3, 3747-3753, describes a THz rectenna with a butterfly antenna ( bow-tie (in English) in aluminium electrically coupled to a MIG type diode with a so-called "1D" junction.

[0011] In this rectenna, the MIG-type diode is not formed from a vertical stack of thin films, but is structured in the plane of the substrate, where two metallic portions (one of Ti and the other of Ni) are located on either side of a mesa in which a thin graphene layer is embedded. An insulating thin layer of TiO₂ extends along one side of the mesa and is in contact with one end of the graphene layer and the titanium metallic portion. Thus, the nickel and graphene form the cathode of the diode, and the titanium forms the anode. The two parts of the butterfly antenna extend above the MIG diode and come into contact with the two metallic portions.

[0012] However, there is a need to improve certain aspects of a MIG-type diode rectenna, whether for infrared or terahertz sensing applications, or for energy conversion applications. DESCRIPTION OF THE INVENTION

[0013] The invention aims to remedy at least in part the drawbacks of the prior art, and more particularly to offer a diode-based MIG-type rectenna with improved performance.

[0014] For this purpose, the object of the invention is a rectifier antenna for detecting electromagnetic radiation, comprising: an antenna adapted to absorb electromagnetic radiation; and a Metal-Insulator-Graphene (MIG) diode, electrically coupled to the antenna, comprising a metallic portion and a graphene conductive portion, between which is located at least one insulating portion.

[0015] According to the invention, the metallic portion, the insulating portion(s), and the graphene conductive portion form a vertical stack of thin film portions. Furthermore, the antenna is formed from a continuous thin graphene layer, a portion of which forms the graphene conductive portion of the diode.

[0016] Some preferred but not exhaustive aspects of this rectifying antenna are as follows.

[0017] The thickness of the antenna, and therefore of the conductive portion made of graphene, can be between 1 and 10 monolayers.

[0018] The antenna can be square or rectangular in shape.

[0019] The conductive portion made of graphene can be located above the insulating portion(s) and the metallic portion.

[0020] The rectifying antenna may include a reflective layer, covered by an intermediate insulating layer on which the antenna rests, forming a quarter-wave cavity with respect to the electromagnetic radiation to be absorbed.

[0021] The rectifier antenna may include connection pads intended to bias the diode, a first connection pad being located under and in contact with the metallic portion, and a second connection pad being located under and in contact with the antenna.

[0022] The insulating portion(s) can be made of Al 2 O 3 , ZrO 2 , HfO 2 , ZnO , SiO 2 , or HfAlO .

[0023] The insulating portion(s) may have a thickness between 0.5 and 4nm.

[0024] The diode can have dimensions ranging from 20×20nm to 100×100nm.

[0025] The rectifier antenna may include an electrical source adapted to apply a non-zero bias voltage VD to the diode.

[0026] The antenna can be adapted to absorb electromagnetic radiation in the infrared or terahertz range.

[0027] The invention also relates to an optoelectronic device, comprising a rectifier antenna array according to any one of the preceding characteristics, identical to each other.

[0028] The invention also relates to a method for manufacturing a rectifier antenna according to any one of the preceding characteristics, comprising the following steps: fabrication of the metallic portion of the diode; fabrication of at least one insulating portion, on and in contact with the metallic portion; fabrication of a graphene layer extending on and in contact with the insulating portion; localized etching of the graphene layer, thus forming the antenna, the part of which in contact with the insulating portion forms the graphene conductive portion.

[0029] The manufacturing process may include the following steps: fabrication of a first and a second connection pad in an intermediate insulating layer; the metallic portion rests on and in contact with the first connection pad, and the second connection pad is flush with the intermediate insulating layer; the graphene layer is made so as to be on and in contact with the insulating portion, and to extend over the intermediate insulating layer to come into contact with the second connection pad. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Other aspects, objectives, advantages, and features of the invention will become clearer upon reading the following detailed description of preferred embodiments thereof, given by way of non-limiting example, and made with reference to the accompanying drawings in which: there Figure 1A The figure already described is a schematic view of a diode rectifier antenna coupled to an antenna, according to a prior art example; figure 1BThe figure already described illustrates an example of an energy band diagram of a diode MIM rectifier antenna, based on a prior art example; figure 1C The figure already described illustrates an example of an energy band diagram of a MIIM diode rectifier antenna, based on a prior art example; figure 2A and the figure 2B are schematic and partial views, in perspective ( fig.2A ) and in cross-section ( fig.2B ), of a graphene-based rectifying antenna according to one embodiment; the figure 3 illustrates an evolution of the absorption rate of the rectenna antenna according to one embodiment; the figures 4A to 4J illustrate different stages of a manufacturing process for a graphene-based rectifier antenna according to one embodiment. DETAILED DESCRIPTION OF SPECIFIC METHODS OF IMPLEMENTATION

[0031] In the figures and throughout the description, the same reference numerals represent identical or similar elements. Furthermore, the various elements are not drawn to scale to ensure clarity. Moreover, the different embodiments and variants are not mutually exclusive and may be combined. Unless otherwise stated, the terms "approximately," "around," and "in the order of" mean within 10%, and preferably within 5%. Furthermore, the terms "between ... and ..." and equivalents mean inclusive of the bounds, unless otherwise specified.

[0032] The invention relates to a graphene-based rectenna-type electromagnetic radiation detector, in other words, a rectifier antenna comprising a graphene antenna adapted to absorb electromagnetic radiation coupled to a MIG-type diode. In the following description, such a rectifier antenna is referred to as a "rectenna." It can function as an infrared or terahertz detector, or even as a power converter, depending in particular on whether the diode is biased, or not, to a non-zero voltage VD.

[0033] In general, the rectenna antenna can be adapted to absorb electromagnetic radiation of interest across a spectral range, notably from infrared to terahertz. Thus, the antenna can be configured to absorb in the near-infrared (SWIR) range. Short Wavelength IR (in English) corresponding to a spectral range from approximately 0.8 to 2.7 µm; in the mid-infrared (MWIR, for Middle Wavelength IR(in English) corresponding to a spectral range from approximately 3 to 5 µm; in the far-infrared (LWIR, for Long Wavelength IR (in English) corresponding to a spectral range from approximately 7 to 14µm; or even in the terahertz range, which has a spectral range from approximately 0.1 to 1mm (approximately 0.3 to 3THz).

[0034] According to the invention, the rectenna comprises a thin conductive layer of graphene that extends continuously and forms both the antenna and the graphene conductive portion of the diode. The diode is a vertical MIG-type diode insofar as it is formed from a vertical stacking of thin film portions, namely a metallic portion, at least one insulating portion, and the graphene conductive portion.

[0035] There figure 2A and the figure 2B are schematic and partial views, in perspective ( fig.2A ) and in cross-section ( fig.2B ), of a rectenna 1 according to an embodiment.

[0036] We define here and for the rest of the description a three-dimensional direct frame XYZ, where the XY plane is parallel to the principal plane of the antenna 10 of the rectenna 1, and where the Z axis is oriented along the thickness of the MIG type diode 20 of the rectenna 1. Moreover, the terms "lower" and "upper" are understood as being relative to an increasing positioning along the +Z direction.

[0037] In this example, the Rectenna 1 is an infrared detector whose antenna is adapted to absorb in the MWIR and / or LWIR infrared. Of course, the Rectenna 1 can be configured to absorb in other infrared spectral ranges, even in the terahertz range.

[0038] Preferably, the described rectenna 1 belongs to a matrix of unit rectennas, identical to each other, connected to an electrical control and reading circuit (ROIC) responsible for biasing the diodes 20 and reading the generated electrical signals.

[0039] In general, the rectenna 1 comprises an antenna 10, a MIG-type diode 20, and a DC filter (not shown). Here, it includes a power source (not shown) to bias the MIG-type diode to a non-zero voltage VD (optical detection application). However, in the case of an energy conversion application (e.g., solar cell), the MIG-type diode can be unbiased or biased to zero voltage and can be connected to an electrical load.

[0040] The antenna 10 is adapted to absorb the incident electromagnetic radiation, here an LWIR infrared radiation, and is electrically connected to the MIG type diode 20 to transmit to it the electrical signal generated in response to the absorption of the infrared radiation of interest.

[0041] Antenna 10 is formed of a thin conductive layer made of graphene, with a thickness, for example, between 1 and 10 monolayers, preferably between 1 and 5 monolayers, and preferably between 1 and 3 monolayers. A graphene monolayer has a thickness of approximately 0.34 nm. The graphene is n-type doped such that antenna 10 has a resistance per square approximately equal to the impedance of free space, within 10% or even 5%, thus optimizing the absorption of the radiation to be detected. Since the impedance of free space is approximately 377 Ω, antenna 10, thus formed of a doped graphene monolayer, can have a resistance per square of approximately 360 Ω / sq. n-type doped graphene is obtained through doping with N2 or NH3, and can have a doping level between 1 at.% and 10 at.% with NH3 as an example.

[0042] The antenna 10 extends continuously over a thick insulating layer 37, and in a substantially planar manner (allowing, possibly, for the thickness of the insulating layer(s) 22, 23 of the MIG-type diode 20). It is therefore located entirely on one side of the MIG-type diode 20, here above it.

[0043] Antenna 10 is a patch antenna, as it consists of a conductive layer (here, graphene) resting on a thick insulating layer. In this example, a reflective layer 35 is located beneath antenna 10 and forms a resonant optical cavity with it. Antenna 10 can have all sorts of shapes, for example, rectangular or square, circular, triangular, elliptical, among others. It can also be spiral, serpentine, or even butterfly-shaped. In this example, antenna 10 is advantageously rectangular or square, so as to be virtually insensitive to the polarization of the incident light.

[0044] Antenna 10 can have a width W such that W = λ 0 2 2 ε r + 1 where λ₀ is the resonance wavelength associated with the optical cavity, and where ε₀r is the dielectric constant of the insulating layer of thickness 37 that fills the optical cavity. We can define an effective dielectric constant ε₀r,eff such that ε r , eff = ε r + 1 2 + ε r − 1 2 × 1 + 12 h W − 0 , 5 , where h is the height of the optical cavity (vertical distance between the antenna and the reflecting layer). Finally, the antenna 10 can have a length L defined from the following relation: L = λ 0 ε r , eff 2 − h × 0.824 ε r , eff + 0.3 W h + 0.264 ε r , eff − 0.258 W h + 0.8 .

[0045] The MIG-type diode 20 is adapted to receive the high-frequency AC electrical signal generated by the antenna 10 in response to the absorption of the electromagnetic radiation of interest, here in the MWIR and / or LWIR, and to rectify it to provide a rectified AC electrical signal.

[0046] The diode 20 is formed from a vertical stack of thin film portions, namely a metallic portion 21, at least one insulating portion 22, 23, and the graphene conductive portion 24, which is part of the continuous graphene thin film that forms the antenna 10. In this example, the diode 20 has two distinct insulating portions 22, 23.

[0047] Furthermore, the graphene conductive portion 24 (and therefore the antenna) is located above the insulating portions 22, 23 and the metallic portion 21. Also, in this example, the diode 20 comprises, arranged along the +Z direction: the lower metallic portion 21, a first insulating portion 22 which extends over and in contact with the lower metallic portion 21, a second insulating portion 23 which extends over and in contact with the first insulating portion 22, and finally the upper graphene conductive portion 24.

[0048] Note that, in diode 20, each of the portions 21, 22, 23, 24 extends parallel to the XY plane continuously and is continuously in contact with the neighboring portion(s) over its entire surface. This distinguishes it from the case where a lower portion would be formed by distinct lines or grooves, such that the adjacent upper portion would not be in contact, over its entire surface, with the lower grooved portion.

[0049] The upper conductive portion 24 is therefore made of graphene and is part of the continuous thin layer that forms the antenna 10. There is thus no discontinuity or separation between the antenna 10 and the conductive portion 24. The conductive portion 24 therefore has the same thickness as the antenna 10. In this example, the thickness is equal to a monolayer, or approximately 0.34 nm. The work function ΨG of the graphene is approximately 4.5 eV.

[0050] The lower metallic portion 21 is made of at least one metal M whose work function is denoted ΨM. This metal can be chosen from among Ti, TiN, W, Al, Ni, Au, Pt, Cr, AlCu, among others. It can also be graphene (which, for simplicity, is considered here to be equivalent to metal M).

[0051] In the case of an optical detection application, the metal M can be chosen to be the same as or different from graphene in terms of work function. However, in the case of an energy conversion application, the work functions ΨG and ΨM are different. Preferably, the work function ΨM is greater than ΨG, for example by at least 5% or even at least 10%. Thus, the upper conductive portion 24 can be made of graphene (ΨG = 4.5 eV) and the lower metallic portion 21 of Ni (ΨM = 5.15 eV), Au (ΨM = 5.3 eV), or Pt (ΨM = 5.4 eV), approximately.

[0052] At least one insulating portion 22, 23 is located between the lower metallic portion 21 and the upper conductive graphene portion 24. Here, it extends over the thick insulating layer 37, over an area that may be limited to the dimensions of the diode 20, or, as in this case, over a larger area.

[0053] In the case where only one insulating portion is present, it is then made of the same electrically insulating material I with electron affinity φ and dielectric constant εr, and is in contact with the lower metallic portion 21 and the upper conductive graphene portion 24. The dielectric constant corresponds to the relative permittivity.

[0054] In the case where two insulating portions are present, an insulating portion 23 is located in contact with the upper conductive portion 24 made of graphene, and is made of an electrically insulating material I1 with electron affinity φ1 and dielectric constant εr1. The insulating portion 22 is located between the insulating portion 23 and the lower metallic portion 21, and is made of an electrically insulating material I2 with electron affinity φ2 and dielectric constant εr2. Preferably, the insulators I1 and I2 have a relative difference in electron affinity Δφ / φ2 = |φ1 -φ2| / φ2 of at least 10%, or even 50%, 100%, or more.

[0055] The insulating material(s) can be chosen from aluminum oxide, hafnium, zirconium, silicon, zinc, copper, nickel, among others, such as Al₂O₃, HfO₂, HfAlO, ZrO₂, ZnO, CuO, NiO, SiO₂. For example, diode 20 can be formed by stacking the following materials: graphene / ZnO / HfO₂ / metal. Other insulating pairs can be used, such as Al₂O₃ / HfO₂, ZnO / HfO₂, ZnO / Al₂O₃, ZnO / HfAlO, among others. Preferably, the insulating materials are chosen so that they exhibit a high dielectric constant (material high-k ), such as Al 2 O 3 , HfO 2 , ZrO 2 , ZnO, in order to increase the asymmetry of the diode and reduce its dynamic resistance.

[0056] It should also be noted that the insulating portion(s) 22, 23 preferably have a thickness between approximately 0.2 and 4 nm, and preferably between 0.5 and 2 nm. The insulating portions 22, 23 may have the same thickness or different thicknesses.

[0057] The insulating portion(s) 22, 23 are thin film portions, meaning they were fabricated using conventional microelectronic techniques, including chemical deposition (CVD, ALD, etc.) and physical deposition (PVD, etc.), among others. In the case of a silicon oxide insulating portion with a thickness preferably between 0.5 and 2 nm, it is preferably fabricated by ion beam deposition (IBD). Ion Beam Deposition (in English) at room temperature, so that its interface has few or no defects such as precursors, thus preserving its electronic qualities.

[0058] As previously mentioned, the antenna 10 rests on a thick insulating layer 37, for example made of silicon oxide, which vertically separates it from an underlying reflective layer 35, made for example of a metal. This vertical separation is on the order of λc / 4n, where n is the refractive index of the medium separating the antenna 10 from the reflective layer 35. This forms a quarter-wave cavity which improves the absorption rate of the antenna 10.

[0059] Diode 20 is biased by means of two connection pads 39.1 and 39.2 located below diode 20 and antenna 10, extending vertically into the thick insulating layer 37. One connection pad 39.1 is located below the MIG-type diode 20 and makes electrical contact with the lower metallic portion 21, and another connection pad 39.2 is located below antenna 10 (and offset in the XY plane relative to diode 20) and makes electrical contact with it. Connection pad 39.2 can be located near an edge of antenna 10. In this example, diode 20 is located at the center of antenna 10, but alternatively, it could be offset.

[0060] Preferably, the stack of diode 20 has dimensions in the XY plane between 20 nm × 20 nm and 100 nm × 100 nm, for example, 50 nm × 50 nm. The antenna 10 can be square with sides of 4.5 µm, and the rectenna antennas can be arranged periodically at 5 µm intervals. The antenna 10 and the upper conductive portion 24 made of graphene can be a single layer thick (0.34 nm), the adjacent insulating portion 23 can be made of HfO₂ with a thickness of 0.5 nm, the next insulating portion 22 can be made of Al₂O₃ with a thickness of 0.5 nm, and the lower metallic portion 21 can be made of nickel. The connection pads 39 can be made of metals such as copper, tungsten, among others, and the thick insulating layer 37 can have a thickness of approximately 1 to 2µm, depending on whether absorption in the MWIR and / or the LWIR is preferred.

[0061] Rectenna 1 also includes a DC filter (not shown), electrically connected to diode 20, to filter the rectified AC electrical signal and retain only the DC component. The filter is then electrically connected to a circuit for reading the electrical signal (optical detection application) or to an electrical load (energy conversion application).

[0062] An electrical source can be used to electrically bias the diode 20 to a bias voltage VD. For example, the lower metallic portion 21 is grounded, while the upper conductive portion 24, made of graphene, is held at an electrical potential UD, which can be zero or non-zero depending on the intended application. The electrical voltage VD can be between approximately 0 and 0.3 V. In the case of a MIIM diode (with two different insulators), the bias is positive between 0 and +0.3 V, and preferably around a bias of +0.1 V to achieve maximum responsiveness.

[0063] Recall here that the performance of the Rectenna 1 depends in particular on the dynamic resistance RD of diode 20, the asymmetry As D of the I(V) characteristic, and the responsiveness β. The dynamic resistance RD can be determined from the relation RD = 1 / I', where I' is the first derivative of the electric current I as a function of the electric voltage V, at the bias voltage VD. The asymmetry As D can be determined from the relation As D = |I f / I r |, at the bias voltage VD, where I f is the value of the electric current in forward and I r is that of the electric current in reverse. Finally, the responsiveness β has been defined previously and can be determined by the relation: β = I" / (2I').

[0064] Thus, the rectenna 1 comprises an antenna 10 electrically coupled to a MIG-type diode 20, where the same continuous thin graphene layer forms both the antenna 10 and the conductive portion 24 of the diode 20. The fact that the antenna 10 is formed from a thin graphene layer allows for the efficient absorption of the electromagnetic radiation of interest, particularly in the MWIR and LWIR ranges. Furthermore, the fact that the same continuous thin graphene layer forms both the antenna 10 and one of the conductive portions of the diode 20 allows for the efficient transfer of electrons excited by the light absorbed by the antenna 10 to the diode 20.

[0065] Furthermore, in the case where antenna 10 is a rectangular patch antenna and preferably square, it is almost insensitive to the polarization of the electromagnetic radiation of interest, which improves broadband absorption.

[0066] It should also be noted that the work function of graphene can be modified, and more specifically reduced, by applying a suitable treatment. This can involve more precisely treating the free surface of the upper conductive portion 24 of graphene, so as to locally decrease the work function of the graphene and thus lower the potential barrier between the upper conductive portion 24 of graphene and the adjacent insulating portion 23. This then leads to increased responsiveness and a reduction in the dynamic resistance of the diode 20. Such a treatment can be a surface treatment leading to the adsorption of molecules such as NO₂, N₂H₂, NH₃, among others. It can also be an ultraviolet treatment.

[0067] There figure 3illustrates an example of the evolution of the absorption rate A of the graphene antenna 10 as a function of the wavelength of the incident electromagnetic radiation, here in the infrared range including MWIR (3-5µm) and LWIR (8-14µm).

[0068] In this example, we consider a detection device comprising a matrix of rectennas 1 similar to that of the fig.2B where the graphene antennas 10 are arranged periodically at 6µm intervals. Each antenna 10 is formed of a 0.34nm thick graphene monolayer with a side dimension of 5×5µm, the graphene being n-type doped so that the antenna 10 has a resistance per square of approximately 360Ω / sq. The antenna 10 rests on a 1µm thick SiO2 layer 37, which forms an optical cavity with an underlying reflective layer 35.

[0069] The graphene antenna 10 exhibits very good absorption in the MWIR and LWIR bands (particularly in the MWIR band due to the size of the optical cavity). Furthermore, it is possible to adjust the size of the optical cavity to optimize absorption in the LWIR band, while maintaining good absorption in the MWIR band, and minimizing the (already low) parasitic absorption by the SiO2 of the optical cavity.

[0070] THE figures 4A to 4J illustrate different stages of a manufacturing process for a rectenna 1 according to one embodiment. The rectenna 1 is adapted here to absorb electromagnetic radiation of interest in the MWIR and LWIR. Only a single rectenna is shown here, but preferably, the process involves the fabrication of an array of identical rectennas on the same reading substrate 31.

[0071] With reference to the fig.4A, a reading substrate 31 is provided, containing a reading and control circuit (ROIC), for example of CMOS type, adapted here to electrically bias the diode and to read the rectified electrical signal from the rectenna in response to the absorption of light radiation.

[0072] The reading substrate 31 is covered with a lower insulating layer 32, made of an insulating material such as silicon oxide. First conductive vias 33 are then made through the lower insulating layer 32 and connected to connection pads (not shown) of the reading substrate 31, followed by first electrical connection pads 34 in contact with the conductive vias 33.

[0073] With reference to the fig.4BNext, the reflector 35 and intermediate connection pads 36 are fabricated. This is achieved by depositing a layer of at least one metallic material, which is then structured by lithography and localized etching to form the reflector 35 and the connection pads 36. The metallic layer can be a multilayer, for example, of the Ti / TiN / AlCu or Ti / TiN / AlSi type. The AlCu or AlSi layer can be at least 80 nm thick, for example, 300 nm. The connection pads 36 are in electrical contact with the connection pads 34.

[0074] With reference to the fig.4CAn insulating intermediate layer 37, made of an insulating material such as silicon oxide, is deposited to define the quarter-wave cavity with the antenna. SiO₂ can be deposited by plasma-enhanced physical vapor deposition (PECVD), with a thickness, for example, between 0.3 and 2 µm, or for example, 1.2 µm, at a deposition temperature of 400°C or less to comply with the thermal budget of the CMOS readout circuit. A chemical-mechanical polishing step can then be performed to remove excess material and flatten the insulating intermediate layer 37 to a thickness of approximately 1 µm (the optical cavity has a thickness of λ / 4n, where n is the refractive index of the medium forming the optical cavity). Secondary conductive vias 38 are then made through the insulating intermediate layer 37, making contact with the connection pads 36.

[0075] With reference to the fig.4DNext, the upper connection pads 39 (39.1, 39.2) and the lower metallic portion 21 of the diode are fabricated. This is achieved by depositing a layer of at least one metallic material, which is then structured by lithography and etching. The layer is preferably formed from a metallic multilayer, for example, Ti / AlCu / TiN. This multilayer forms both the connection pads 39.1, 39.2 and the lower metallic portion 21 (here, TiN) of the diode. More precisely, pad 39.1 is covered by the metallic portion 21, while pad 39.2 has a metallic portion (here, TiN). The intermediate AlCu layer is preferably thicker than 80 nm to ensure the quality and continuity of the layer. The connection pad 39.1 is used to bias the diode, and the connection pad 39.2 is used to bias the antenna 10.A new insulating layer is then deposited, for example, a TEOS-type silicon oxide, which is planarized to expose the upper surface of the metal portion 21 of the diode as well as that of the connection pad 39.2. In the figure, the insulating layer surrounding the vias 38 and the connection pads 39 (and therefore the metal portion 21) in the XY plane is always labeled "37". Its thickness helps define the size of the optical cavity.

[0076] With reference to the fig.4ENext, a first insulating thin layer 40 is deposited on the free face of the stack, extending over and in contact with the metallic portion 21. This insulating thin layer 40 is locally etched to leave the free face of the connecting pad 39.2 free. The portion of the insulating thin layer 40 that is on and in contact with the metallic portion 21 forms the insulating portion 22. The remainder of the insulating thin layer 40 can extend over the thick insulating layer 37, opposite the reflector 35. In this example, the insulating thin layer 40, and therefore the insulating portion 22, is made of Al₂O₃ with a thickness between 0.5 and 2 nm, and here 0.5 nm. It can be deposited by atomic layer deposition (ALD) at approximately 200°C to 400°C.

[0077] With reference to the fig.4F, a layer 44 of graphene, here a monatomic layer, is produced on an insulating layer 43, here in a silicon oxide, resting on a support substrate 42. The graphene can be initially deposited on a copper substrate (not shown) by a high-temperature chemical vapor deposition technique, for example at about 1000°C, and then transferred onto the oxide layer 43.

[0078] With reference to the fig.4G A layer 45 of a polymer such as polymethyl methacrylate (PMMA), or a layer of adhesive, is deposited onto the graphene layer 44. This layer 45 will form a flexible handle for manipulating the graphene layer 44.

[0079] With reference to the fig.4HThe substrate 42 and the oxide layer 43 are removed, for example by wet etching of the oxide layer 43 with hydrofluoric acid (HF) in the vapor phase. This yields a PMMA layer 45 on which the graphene layer 44 rests. Note that this technique for fabricating and transferring a graphene layer is described in particular in the article by Lee et al. entitled "Multilayered Graphene Electrode using One-Step Dry Transfer for Optoelectronics," Current Optics and Photonics, Vol. 1, Issue 1, pp. 7-11 (2017).

[0080] With reference to the fig.4I , we transfer the graphene layer 44 onto the stack, so that it comes into contact with the insulating portion 22 (and more broadly with the insulating layer 40), the thick insulating layer 37, and the connecting pad 39.2.

[0081] With reference to the fig.4JThe PMMA layer 45 is then removed, for example with a solvent such as acetone, to expose the upper surface of the graphene layer 44. The graphene layer is then etched locally to form the antenna 10. The portion of the antenna 10 in contact with the insulating portion 22 forms the upper conductive portion 24 made of graphene.

[0082] Thus, a rectenna 1 is obtained, consisting of an antenna 10 and a MIG-type diode 20, where a single continuous thin layer forms both the antenna 10 and the graphene conductive portion 24 of the diode 20. The diode 20 is biased by means of connecting pads 39.1 and 39.2, one located below the diode 20 and the other below the antenna 10, which are connected to the underlying readout circuit by conductive vias. Furthermore, the antenna 10 is vertically spaced from a reflector 35, thus forming a quarter-wave cavity that optimizes the absorption of the light radiation of interest (here, MWIR and / or LWIR). Also, not only does the graphene antenna 10 efficiently absorb the electromagnetic radiation of interest (in the case where it is a patch antenna not sensitive to the polarization of light), but electrons are optimally transferred into the diode 20, insofar as the same continuous thin layer forms the antenna 10 and the conductive portion 24 of the diode 20.The Rectenna 1 therefore exhibits high performance, in terms of antenna absorption, but also in terms of diode responsiveness and dynamic resistance.

[0083] Specific embodiments have just been described. Different variations and modifications will be apparent to those skilled in the art.

Claims

1. Rectifying antenna (1) for detecting electromagnetic radiation, comprising: ∘ an antenna (10), adapted to absorb electromagnetic radiation; ∘ a Metal-Insulator-Graphene (MIG) diode (20), electrically coupled to the antenna (10), comprising a metallic portion (21) and a conductive portion (24) made of graphene, between which is located at least one insulating portion (22; 23); ∘ characterized in that : • the metallic portion (21), the insulating portion(s) (22; 23) and the conductive portion (24) in graphene form a vertical stack of thin film portions; • the antenna (10) is formed of a continuous thin film in graphene, part of which forms the conductive portion (24) in graphene of the diode (20).

2. Rectifying antenna (1) according to claim 1, wherein the thickness of the antenna (10) and therefore of the conductive portion (24) in graphene is between 1 and 10 monolayers.

3. Rectifying antenna (1) according to claim 1 or 2, wherein the antenna (10) has a square or rectangular shape.

4. Rectifying antenna (1) according to any one of claims 1 to 3, wherein the graphene conductive portion (24) is located above the insulating portion(s) (22, 23) and the metallic portion (21).

5. Rectifying antenna (1) according to any one of claims 1 to 4, comprising a reflective layer (35), covered with an intermediate insulating layer (37) on which rests the antenna (10), forming a quarter-wave cavity with respect to the electromagnetic radiation to be absorbed.

6. Rectifying antenna (1) according to any one of claims 1 to 5, comprising connection pads (39.1, 39.2) for biasing the diode (20), a first connection pad (39.1) being located under and in contact with the metallic portion (21), and a second connection pad (39.2) being located under and in contact with the antenna (10).

7. Rectifying antenna (1) according to any one of claims 1 to 6, wherein the insulating portion(s) (22, 23) are made of Al2O3, ZrO2, HfO2, ZnO, SiO2, or HfAlO.

8. Rectifying antenna (1) according to any one of claims 1 to 7, wherein the insulating portion(s) (22, 23) have a thickness between 0.5 and 4nm.

9. Rectifying antenna (1) according to any one of claims 1 to 8, wherein the diode (20) has dimensions between 20×20nm and 100×100nm.

10. Rectifying antenna (1) according to any one of claims 1 to 9, comprising an electrical source adapted to apply a bias voltage V D non-zero at diode (20).

11. Rectifying antenna (1) according to any one of claims 1 to 10, wherein the antenna (10) is adapted to absorb electromagnetic radiation in the infrared or terahertz range.

12. Optoelectronic device, comprising a rectifier antenna array (1) according to any one of the preceding claims, identical to each other.

13. Method for manufacturing a rectifier antenna (1) according to any one of claims 1 to 11, comprising the following steps: ∘ making the metallic portion (21) of the diode (20); ∘ making at least one insulating portion (22; 23), on and in contact with the metallic portion (21); ∘ making a graphene layer (44) extending on and in contact with the insulating portion (22; 23); ∘ localized etching of the graphene layer (44), thus forming the antenna (10), the part of which in contact with the insulating portion (22; 23) forms the conductive portion (24) in graphene.

14. Manufacturing method according to claim 13, comprising the following steps: ∘ making a first and a second connection pad (39.1; 39.2) in an intermediate insulating layer (37); ∘ the metallic portion (21) rests on and in contact with the first connection pad (39.1), and the second connection pad is flush with the intermediate insulating layer (37); o the graphene layer (44) is made so as to be on and in contact with the insulating portion (22; 23), and to extend over the intermediate insulating layer (37) to come into contact with the second connection pad (39.2).

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