PHOTODETECTOR WITH COUPLED FABRY-PEROT RESONATORS

Description

technical field

[0001] This description relates to a photodetector and an image sensor which includes such photodetectors. Previous technique

[0002] Infrared light detectors typically rely on semiconductors as light-absorbing materials. These semiconductors are fabricated epitaxially. This growth method results in high manufacturing costs, due to the use of an ultra-high vacuum growth environment and the tight meshing requirements between the substrate and the semiconductor.

[0003] Furthermore, the cost of infrared imagers is also due to the coupling step between the detection circuit, which includes the light-absorbing layer, and the readout circuit, which is generally implemented using CMOS technology. This coupling between the two circuits is achieved using indium beads. Each of these beads must connect the active layer to a pixel of the CMOS readout circuit. The efficiency of this step is limited, which generates an additional cost. Moreover, this procedure becomes increasingly complex as the pixel size decreases. While small pixel sizes are desirable for improving image quality, current dimensions (10-15 µm) are limited by this coupling step between the two circuits.

[0004] It is therefore desirable to use alternative materials to reduce the cost of infrared components. In the spectral range targeted by this invention, i.e., for wavelengths greater than 1 µm, conductive polymers are not a viable alternative due to the strong coupling between the exciton and molecular vibrations. Other possible alternatives include nanocrystals of semiconductors with narrow band gaps, such as lead sulfide (PbS) or mercury telluride (HgTe), or two-dimensional materials like graphene.

[0005] In materials like nanocrystals, a compromise is necessary. Their granular nature means that transport occurs via nearest-neighbor hops between nanoparticles. This transport mechanism is associated with charge carrier mobility values ​​that are lower than in bulk materials. This results in a short carrier scattering length, typically 50 nm to 100 nm. This scattering length is shorter than the absorption length of the electromagnetic field, which is several micrometers. Transport is therefore only efficient at small sizes, but a thick film is required to absorb most of the incident light. One strategy to overcome this limitation is to introduce a light resonator into the light detector. Its role is to concentrate the incident light onto a thin semiconductor layer of optimal thickness for charge collection.

[0006] Several strategies (metal-insulator-metal cavity, Fabry-Pérot resonance, plasmonic resonator, etc.) have been proposed to improve light-matter coupling in nanoparticle films, thereby enhancing the component's absorption. In the article "Near Unity Absorption in Nanocrystal Based Short Wave Infrared Photodetectors Using Guided Mode Resonators" by Audrey Chu et al., ACS Photonics 6, 2553 (2019), the authors propose introducing a mirror into the component to allow a double pass of incident light through the absorbing layer. This increases absorption by a factor of almost two. Furthermore, the authors add a grating that generates an optical mode in which light propagates along the substrate, also resulting in multiple passes of light through the film.

[0007] This type of strategy, based on a periodic grating, suffers from two limitations. The resulting detector exhibits a strong angular dependence, which is unfavorable for integration into an imager. Furthermore, for optimal operation, the grating must be nearly infinite, requiring a large number of grating periods to be included in each pixel. This latter point is incompatible with the previously mentioned objective of pixel size reduction. It is therefore advantageous to develop new light resonator geometries that are compatible with the pixel sizes used in imagers and that also exhibit a reduced angular dependence in their response.

[0008] Furthermore, it is also known to form radiation-absorbing nanostructures, each consisting of a pair of coupled Fabry-Pérot resonators. Each pair of coupled resonators exhibits, in addition to the respective individual resonances of each resonator, a coupling resonance that produces a value close to unity for the absorption coefficient of incident radiation. In such nanostructures with coupled Fabry-Pérot resonators, the two resonators of each pair can each be formed by a trench in the surface of a metallic substrate, as described in the article entitled "Cooperative optical trapping in asymmetric plasmon nanocavity arrays" by Ling Guo et al., Optics Express 31324, Vol. 23, No. 24, November 2015, or in the article entitled "High-quality-factor double Fabry-Perot plasmonic nanoresonator" by B. Fix et al., Optics Letters, Vol. 42, No. 24, December 15, 2017, pp. 5062-5065.In these structures, the standing wave components that form inside the trenches propagate perpendicularly to the substrate surface. For this reason, the corresponding Fabry-Pérot resonators are called vertical-axis resonators. However, it is also known, notably from document WO 2020 / 002330, to form other coupled Fabry-Pérot resonator nanostructures in which the standing wave components inside the resonators propagate parallel to the substrate surface. Such other Fabry-Pérot resonators are therefore called horizontal-axis resonators.

[0009] Another important aspect of the invention is generating a component with a reconfigurable spectral response. Generally, the response of an infrared detector is determined by the nature of its active layer. In the case of nanoparticles, the size of the individual building blocks determines the cutoff wavelength. Changing the cutoff wavelength therefore requires changing the active material. An alternative strategy is to have the spectral response also influenced by the presence of the light resonator. Audrey Chu et al., in ACS Photonics 6, 2553 (2019), demonstrated that the spectral response of the material could be adjusted by the lattice period while retaining the same active material. A further degree of reconfigurability would be the ability to change the spectral response after the component has been manufactured. This type of active component currently relies on phase-change materials or MEMS technology.Recently, Dang et al., in the article published in Nano Letters 21, 6671 (2021) entitled "Bias Tunable Spectral Response of Nanocrystal Array in a Plasmonic Cavity," demonstrated that it was possible to obtain a shift in the spectral response by applying a voltage. In this article, the effect obtained is still small. One of the challenges of this invention is to use this concept to obtain infrared detectors whose spectral response is largely reconfigurable after the component has been fabricated. Technical problem

[0010] From this situation, one aim of the present invention is to propose a new photodetector structure which provides high optical absorptions, and which simplifies the manufacture of each photodetector.

[0011] In particular, an object of the invention may be that the photodetector structure is compatible with the use of a layer of photoconductive nanocrystals deposited from a colloidal solution.

[0012] An ancillary objective of the invention is that each photodetector can have reduced lateral dimensions, typically less than 15 µm and preferably below 5 µm), to enable the realization of high-resolution image sensors.

[0013] Another secondary objective of the invention is that each photodetector remains effective in detecting radiation whose direction of incidence varies within a wide angular sector.

[0014] Finally, yet another aim of the invention is to provide a photodetector whose spectral detection characteristics can be easily modified, and / or that the photodetector is reconfigurable according to its application or between two successive sequences of use of the photodetector. Summary of the invention

[0015] To achieve at least one of these goals, or another, a first aspect of the invention proposes a new photodetector which comprises: a substrate, which is reflective to electromagnetic radiation incident on the photodetector; portions of electrodes, which are supported by the substrate, and which have respective surfaces opposite the substrate, called upper surfaces, and situated at a common level of separation from the substrate; portions of an electrically insulating material, which are situated between the portions of electrodes and the substrate, so as to electrically insulate each portion of electrode from the substrate; and at least one portion of a photoconductive material, which is arranged to be in electrical contact with two of the adjacent portions of electrodes.

[0016] When using this photodetector, at least two of the electrode portions and the substrate are intended to collect a photodetection current.

[0017] In the photodetector of the invention, a first and a second electrode portion, which are adjacent to each other, define a volume parallel to the substrate. During operation of the photodetector, radiation enters this volume to be reflected by the substrate, forming a first Fabry-Pérot resonator between the substrate and the upper surfaces of the electrode portions. Similarly, the second electrode portion and a third electrode portion, located on one side of the second electrode portion opposite the first electrode portion, define another volume parallel to the substrate. During operation of the photodetector, radiation also enters this volume to be reflected by the substrate, forming a second Fabry-Pérot resonator between the substrate and the upper surfaces of the electrode portions.In other words, the first and second Fabry-Pérot resonators are designed to generate standing wave components that propagate perpendicularly to the substrate when the photodetector is in use. They are therefore of the vertical axis type, according to the terminology used by those skilled in the art presented above.

[0018] The photodetector of the invention also has the following characteristics / 1 / to / 3 / : / 1 / a width of the first Fabry-Pérot resonator, measured between the first and second electrode portions parallel to the substrate, is different from a width of the second Fabry-Pérot resonator, measured between the second and third electrode portions also parallel to the substrate, so that the first and second Fabry-Pérot resonators have respective values ​​of individual resonance wavelength, effective for the radiation incident on the photodetector, which are different, with respective values ​​of an individual resonance quality factor of these first and second Fabry-Pérot resonators such that, on an axis of wavelength of the incident radiation, the following intervals of individual resonances: [λ ri ·(1-3 / Qi) ;λri ·(1+3 / Qi)], have an overlap, where i is equal to 1 or 2 to denote the first or second Fabry-Pérot resonator, respectively, and λri and Qi are respectively the wavelength and quality factor values ​​of the individual resonance of Fabry-Pérot resonator i. In other words, the two Fabry-Pérot resonators have individual resonance wavelengths that are different but not too far apart. Furthermore, these two resonators are distinguished by their respective cavity widths, which is particularly easy to achieve, notably by using a masking process; / 2 / a sum of the widths of the first and second Fabry-Pérot resonators with the width of the second portion of electrode, measured parallel to the substrate between the volumes of the first and second Fabry-Pérot resonators, is adapted to produce a coupling between the first and second Fabry-Pérot resonators, by being less than a resonance wavelength value relative to the coupling, called the coupling resonance wavelength, which is effective for the radiation incident on the photodetector, and which results from an interference between at least three waves among which: a first wave which comes from a reflection of the incident radiation on the substrate; a second wave which emerges from the first Fabry-Pérot resonator, and which results from a superposition of several wave components among which at least one of these wave components has made at least one round trip inside the volume of the second Fabry-Pérot resonator;and a third wave that emerges from the second Fabry-Pérot resonator, and which results from another superposition of several other wave components, among which at least one of these other wave components has made at least one round trip within the volume of the first Fabry-Pérot resonator; and / 3 / the photoconducting material is absorbing for the coupling resonance wavelength, and the portion of this photoconducting material is located in or on at least one of the volumes of the first and second Fabry-Pérot resonators.

[0019] Thanks to the coupling resonance exhibited by the photodetector of the invention for the detected radiation, its optical absorption is very high. Indeed, the coupling resonance concentrates the radiation within at least a portion of the photoconductive material, significantly increasing the probability that a photon of the radiation will be absorbed. For this reason, the photoconductive material can be of a type compatible with a deposition process that uses a colloidal solution of nanocrystals of this material, in particular a spin-coating process. Such a process reduces the manufacturing cost of the photodetector, firstly because the photoconductive material can be deposited on the substrate inexpensively, and secondly because the readout circuit can be used as a substrate for the photoconductive material.The step of assembling the detection circuit to the reading circuit can thus be avoided.

[0020] Furthermore, since the structure of the photodetector of the invention can be limited to two Fabry-Pérot resonators with dimensions smaller than the wavelength of the radiation to be detected, the photodetector can have very small lateral dimensions. As a result, an image sensor made from photodetectors according to the invention can provide very high spatial resolution and function as a high-resolution sensor.

[0021] Thanks again to the structure of the photodetector of the invention, its detection efficiency is maintained within a wide angular sector for the direction of incidence of the radiation to be detected.

[0022] The portion of the photoconductive material may be located at least partially within or on the volume of the first and second Fabry-Pérot resonators with the greatest or smallest width, measured parallel to the substrate. It may also be located at least partially within or on both volumes of the first and second Fabry-Pérot resonators.

[0023] In simpler embodiments of the invention, the portions of the electrically insulating material can be parts of a continuous layer of this insulating material extending through the volumes of the first Fabry-Pérot resonator and the second Fabry-Pérot resonator, in addition to extending between the substrate and each electrode portion. It is then unnecessary to etch the layer of insulating material.

[0024] Alternatively, in addition to the first, second, and third electrode portions, the substrate can also be in contact with the portion of photoconductive material, thus forming an additional electrode portion. Specifically, the substrate can be in contact with the portion of photoconductive material because one or more parts of the latter are contained within the volume of at least one of the first and second Fabry-Pérot resonators. In this case, at least two of the first, second, and third electrode portions can be electrically short-circuited to form a first photodetection current collection electrode, and the substrate can be used to form a second photodetection current collection electrode. Alternatively, the photodetection current can be collected between any two subsets of the electrode portions, with these portions being electrically short-circuited within each subset.

[0025] In general, for the invention, the photodetector may further include a polarization circuit adapted to apply, during operation of the photodetector, a variable voltage between two of the electrode portions that collect the photodetection current, and optionally to vary this voltage between two successive uses of the photodetector. Thanks to such a variable polarization voltage, the detection sensitivity of the photodetector, and more generally its sensitivity spectrum, can be modified, and in particular adapted to different applications. Indeed, the variable polarization voltage makes it possible to increase the collection efficiency by the electrode portions of the electrical charges that are created by radiation in the photoconductive material.The bias voltage can vary between 0 V (volts) and 10 V, but values ​​less than or equal to 1 V may advantageously be sufficient. Advantageously, such a photodetector can be adapted so that the absorption value of the radiation at at least one wavelength varies by at least 30%, preferably at least 50%, and even more preferably at least 90%, between a first use of the photodetector with no electrical voltage applied by the bias circuit between the two electrode portions, or during which the electrical voltage applied by the bias circuit is zero, and a second use of the same photodetector during which the electrical voltage applied by the bias circuit is non-zero.

[0026] In one embodiment, the electric fields applied to operate the component are less than 100 kV.cm⁻¹, and preferably below 30 kV.cm⁻¹.

[0027] More generally, for the invention, the photodetector may also include a reconfiguration circuit adapted to electrically select and connect at least two of the electrode and substrate portions of the photodetector in order to collect the photodetection current through those selected electrode and substrate portions. These portions may vary among several photodetection current collection modes, each associated with different photodetector sensitivity spectra relative to the incident radiation. Indeed, each mode may prioritize collecting photodetection current through a different portion of the photoconductive material than another mode, and each portion of the photoconductive material may be the concentration point of the radiation to be detected for a different wavelength.Thus, the photodetector of the invention can be reconfigured simply and instantaneously between two successive uses. For example, one photodetection current collection mode can use the first and second electrode portions to collect the current, and another mode can use the second and third electrode portions. The first collection mode can therefore correspond to the coupling resonance created by the structure of the photodetector of the invention, while the other collection mode can instead correspond to the individual resonance of one of the Fabry-Pérot resonators, this individual resonance and the coupling resonance corresponding to different wavelength values ​​for the radiation to be detected.Possibly, for at least one or all of the collection modes, each electrode portion not used to collect photodetection current can be short-circuited by the reconfiguration circuit with one of the selected electrode portions. Alternatively, an electrode portion not used to collect photodetection current can be at a floating potential.

[0028] Advantageously, the photodetector may possess at least one of the following additional characteristics, separately or in combination with several of them: The substrate may have a flat surface that extends continuously under the portions of the insulating material and under the volumes of the first and second Fabry-Pérot resonators; the substrate may include a photodetector readout circuit; each portion of the photoconductive material may be part of a layer of this photoconductive material that extends continuously over the volumes of the first and second Fabry-Pérot resonators and over the electrode portions; the photodetector may include several pairs of coupled first and second Fabry-Pérot resonators, with first, second and third electrode portions associated with each pair and electrically connected to accumulate photodetection currents that come from each pair when using the photodetector.In the case of such a photodetector with several pairs of coupled Fabry-Pérot resonators, a repetition step of a pattern of the pairs of coupled Fabry-Pérot resonators on the substrate is less than the coupling resonance wavelength; the photodetector may have lateral dimensions which are between 1 µm (micrometer) and 1 cm (centimeter), preferably between 1 µm and 100 µm, in particular less than 15 µm, measured parallel to the substrate; the volumes of the first and second Fabry-Pérot resonators, as well as the width of the second electrode portion, may be dimensioned so that the coupling resonance wavelength is between 1 µm and 12 µm, preferably between 1 µm and 2.5 µm; The photoconductive material can be selected to have a band gap width, called "gap" in English, which is less than 0.8 eV (electrovolt).This limit corresponds to photodetectors that are effective for wavelength values ​​of the radiation to be detected that are greater than approximately 1 µm. In particular, the photoconductive material can be based on lead sulfide (PbS), mercury telluride (HgTe), or graphene; and each portion of the photoconductive material can be made up of agglomerated nanocrystals. Such nanocrystals can be deposited from a colloidal solution.

[0029] A second aspect of the invention proposes an image sensor which includes a matrix arrangement of photodetectors, each photodetector conforming to the first aspect of the invention presented above.

[0030] When each image sensor photodetector comprises multiple pairs of coupled Fabry-Pérot resonators, with electrode portions associated with each pair and electrically connected to accumulate photodetection currents from each pair during photodetector operation, the number of coupled Fabry-Pérot resonator pairs within each photodetector may be five or fewer. Alternatively, or in combination, each photodetector may have an individual photodetector size, measured along a direction of juxtaposition of the first and second coupled Fabry-Pérot resonator pairs, that is less than or equal to ten times a wavelength value of the radiation corresponding to the photodetector's maximum detection sensitivity.

[0031] Finally, a third aspect of the invention proposes a method for manufacturing a photodetector that conforms to the first aspect of the invention, according to which the portions of photoconductive material are obtained from the deposition of a colloidal solution incorporating nanocrystals of the photoconductive material, followed by drying of the deposited colloidal solution. In particular, the portions of photoconductive material can be obtained using a spin-coating deposition process. Brief description of the figures

[0032] The features and advantages of the present invention will become clearer in the following detailed description of non-limiting embodiments, with reference to the accompanying figures, among which: [ Fig. 1a ] is a cross-sectional view of a photodetector according to the invention; [ Fig. 1b ] corresponds to [ Fig. 1a] for a variant embodiment of the photodetector; [ Fig. 1c ] corresponds to [ Fig. 1a ] for another embodiment of the photodetector; [ Fig. 1d ] corresponds to [ Fig. 1a ] for yet another variant of the photodetector implementation; [ Fig. 2 ] is a plan view of a photodetector conforming to any one of [ Fig. 1a ]-[ Fig. 1d ] ; ] Fig. 3a ] is an absorption spectral diagram for a photodetector that conforms to [ Fig. 1b ] ; ] Fig. 3b ] is a detection response spectral diagram for a photodetector that conforms to [ Fig. 1b ] And [ Fig. 2 ] ; ] Fig. 4a ] corresponds to [ Fig. 2 ] for another method of grouping electrode portions; [ Fig. 4b ] corresponds to [ Fig. 3b ] for the photodetector of [ Fig. 4a ] ; ] Fig. 5a ] also corresponds to [ Fig. 2 ] for yet another method of grouping the electrode portions; [ Fig. 5b ] corresponds [ Fig. 3b ] for the photodetector of [ Fig. 5a ] ; ] Fig. 6 ] corresponds to [ Fig. 1a ] for an improvement of the invention; and [ Fig. 7 ] is a perspective view of an image sensor according to the invention. Detailed description of the invention

[0033] For clarity, the dimensions of the elements shown in these figures do not correspond to actual dimensions or ratios of actual dimensions. Furthermore, some of these elements are represented only symbolically, and identical references shown in different figures designate identical elements or elements with identical functions.

[0034] In accordance with the particular embodiment shown in [ Fig. 1a], a substrate 1 of the photodetector 100 has a continuous, flat, and reflective upper surface S for radiation R to be detected which is incident on this surface. For this purpose, the surface S of the substrate 1 can be formed by a continuous metallic layer 11, which is supported by a base portion 10 of the substrate 1. This base portion 10 can be at least partially made of silica, quartz, calcium fluoride (CaF₂), undoped silicon (Si), undoped germanium (Ge), zinc selenide (ZnSe), zinc sulfide (ZnS), potassium bromide (KBr), lithium fluoride (LiF), alumina (Al₂O₃), potassium chloride (KCl), barium fluoride (BaF₂), cadmium telluride (CdTe), sodium chloride (NaCl), cesium bromide (CsBr), gallium arsenide (GaAs), magnesium fluoride (MgF₂), or thallium bromoiodide (Br₂O₃). 3-x I x Tl), in particular.Alternatively, the base portion 10 of the substrate 1 may incorporate a readout circuit for the photodetector 100, in particular such a readout circuit implemented using CMOS technology. The metallic layer 11 may be made of gold (Au), silver (Ag), or aluminum (Al), in particular, or of an alloy, or be a superposition of several elementary metallic layers.

[0035] The substrate 1 is covered by a continuous insulating layer 2, for example a layer of silica (SiO 2 ) or alumina (Al 2 O 3 ), over the metallic layer 11. In particular, the insulating layer 2 may be made of alumina and have a thickness e 2 of about 50 nm (nanometer) measured parallel to the direction D 1 which is perpendicular to the surface S of the substrate 1.

[0036] The thickness of the insulating layer 2 can be between 10 nm and 10 µm, preferably between 30 nm and 5 µm.

[0037] Three electrode portions, designated 3a, 3b, and 3c respectively, are formed on the insulating layer 2. They can be obtained from a continuous metallic layer, for example, a layer of gold, silver, or aluminum, which is then etched to form gaps between adjacent electrode portions. Alternatively, electrode portions 3a, 3b, and 3c can be deposited using a lift-off process, where a resin pattern is first formed on the insulating layer 2, then the electrode material is deposited, and the resin is subsequently dissolved, simultaneously removing the electrode material from the areas of the resin pattern. The common thickness e3 of electrode portions 3a, 3b, and 3c can be approximately 100 nm, along the direction D1. Each portion of electrode 3a, 3b, 3c is thus electrically isolated from the metallic layer 11 by the insulating layer 2.

[0038] Finally, a layer 4 of a photoconductive material is deposited in the gaps between the electrode portions 3a, 3b, and 3c, so as to be in contact with the two electrode portions on each side of each of these gaps. In the embodiment of [ Fig. 1a ], the photoconductive material layer 4 also continuously covers the three electrode portions 3a, 3b and 3c.

[0039] In possible embodiments of the photodetector 100, the photoconductive material of layer 4 can be a two-dimensional material such as graphene or a transition metal chalcogenide such as molybdenum sulfide (MoS₂), molybdenum selenide (MoSe₂), molybdenum telluride (MoTe₂), tungsten sulfide (WS₂), tungsten selenide (WSe₂), tungsten telluride (WTe₂), or their alloys or heterostructures. Agglomerated nanocrystals of these transition metal chalcogenides can be deposited over the electrode portions 3a, 3b, and 3c from colloidal solutions of these nanocrystals, using a centrifugal deposition process, so as to fill the inter-electrode gaps.

[0040] In other possible embodiments of the photodetector 100, the photoconductive material of layer 4 can be a conductive polymer such as a mixture of poly(3,4-ethylenedioxythiophene) and sodium poly(styrene sulfonate), designated PDOT-PSS, or such as poly(3-hexylthiophene-2,5), designated P3HT. These conductive polymers can also be deposited over the electrode portions 3a, 3b, and 3c using a centrifugal deposition process, again in such a way as to fill the inter-electrode gaps.

[0041] In yet other possible embodiments of the photodetector 100, the photoconductive material of layer 4 can consist of nanocrystals of silicon (Si), germanium (Ge), cadmium sulfide (CdS), cadmium selenide (CdSe), cadmium telluride (CdTe), zinc sulfide (ZnS), zinc selenide (ZnSe), zinc telluride (ZnTe), mercury sulfide (HgS), mercury selenide (HgSe), mercury telluride (HgTe), lead sulfide (PbS), lead selenide (PbSe), lead telluride (PbTe), copper indium sulfide (CuInS₂), copper indium selenide (CuInSe₂), and silver indium sulfide (AgInS₂). ), indium silver selenide (AgInSe 2), copper II sulfide (CuS), copper I sulfide (Cu 2 S), silver sulfide (Ag 2 S), silver selenide (Ag 2 Se), silver tellurium (Ag 2 Te), indium nitride (InN), indium phosphide (InP), arsenide indium (InAs), indium antimonide (InSb),indium sulfide (In 2 S 3), cadmium phosphide (Cd 3 P 2), zinc phosphide (Zn 3 P 2), cadmium arsenide (Cd 3 As 2), zinc arsenide (Zn 3 As 2), zinc oxide (ZnO), aluminum nitride (AlN), aluminum phosphide (AlP), aluminum arsenide (AlAs), aluminum antimonide (AlSb), gallium nitride (GaN), gallium phosphide (GaP), gallium arsenide (GaAs), gallium antimonide (GaSb), iron sulfide (FeS 2 ), titanium oxide (TiO 2 ), bismuth sulfide (Bi 2 S 3 ), bismuth selenide (Bi2Se3), bismuth telluride (Bi2Te3), molybdenum sulfide (MoS2), tungsten sulfide (WS2), vanadium oxide (VO2), cesium lead chloride (CsPbCl3), cesium lead bromide (CsPbBr3), cesium lead iodide (CsPbI3), methylammonium lead iodide or MAPI (CH3NH3PbI3), formamidinium lead iodide or FAPI (N2H4PbI3),of their alloys or heterostructures. Such nanocrystals can be spherical or tetrahedral, or shaped like platelets, rods, wires, or tripods, etc. Layer 4 can then be deposited by centrifugation from a solution of the nanocrystals used. In this solution, the nanocrystals can be coated with ligands such as carboxylic acids, amines, thiols, or phosphines. Alternatively, they can be coated with ionic ligands such as S 2- (sulfide), OH- (hydroxide), HS- (hydrosulfide), Se 2- (selenide), NH 2- (amide), Te 2- (tellurium), SCN- (thiocyanate), Cl- (chloride), Br- (bromide), I- (iodide), Cd 2+ (cadmium), NH 4+ (ammonium), Hg 2+ (mercury), Zn 2+ (zinc) and Pb 2+ (lead).

[0042] According to another embodiment, the nanocrystals used to form a photoconductive film are coated with a mixture of organic and inorganic ligands, such as mercaptoethanol and mercurous chloride (HgCl2) solubilized in dimethylformamide.

[0043] Typically, the photoconductive material layer 4 can have a thickness e4 of approximately 80 nm, measured parallel to the direction D1, above the electrode portions 3a, 3b, and 3c, in addition to filling the inter-electrode gaps. Depending on the photoconductive material used, its electrical carriers can have a mobility ranging from approximately 10⁻⁴ cm² .V⁻¹ .s⁻¹ (square centimeters per volt per second) to approximately 50 cm² .V⁻¹ .s⁻¹, preferably between approximately 10⁻³ cm² .V⁻¹ .s⁻¹ and approximately 10 cm² .V⁻¹ .s⁻¹. Its optical refractive index can range from 1 to 4, more particularly from 1.3 to 3.So, layer 4 of this photoconductive material can be absorbing between 200 nm and 15 µm, preferentially between 1 µm and 5 µm, and even more preferentially between 1 µm and 2.5 µm, with an absorption coefficient value per unit thickness of layer 4 which is between 100 cm⁻¹ and 10000 cm⁻¹, and more particularly between 1000 cm⁻¹ and 5000 cm⁻¹.

[0044] In photodetector 100, the separation between electrode sections 3a and 3b, and between electrode sections 3b and 3c, is essential for the photodetector's operation. Each constitutes a vertical Fabry-Pérot resonator, meaning that the propagation directions of the standing wave components are parallel to the direction D1. In the figures, FP1 designates the Fabry-Pérot resonator located between electrode sections 3a and 3b, and FP2 designates the one located between electrode sections 3b and 3c. According to the direction D 1, each of the Fabry-Pérot resonators FP1 and FP2 is limited on one side by the metallic layer 11, and on the other side by a straight extension between the upper surfaces of the electrode portions, opposite the substrate 1, over the inter-electrode separation spaces.Transversely, that is, along the direction D2, each of the Fabry-Pérot resonators FP1 and FP2 is bounded by the edges of the electrode portions. Thus, in the embodiment of [. Fig. 1a The volume of each Fabry-Pérot resonator FP1, FP2 includes a portion of the insulating layer 2 that is located at the right of the corresponding inter-electrode separation space, along the direction D1. It also includes a portion of the inter-electrode space filling that is made up of the photoconductive material used for layer 4. In this case, the phase coincidence relation for each Fabry-Pérot resonator FP1, FP2, established for two propagation directions that are parallel to the direction D1 but in opposite directions, takes into account the superposition of the material of the insulating layer 2 and the photoconductive material.

[0045] Furthermore, the two Fabry-Pérot resonators FP1 and FP2 have different widths, measured parallel to the direction D2. For example, the width of resonator FP1, denoted W1, can be 400 nm, and that of resonator FP2, denoted W2, can be 200 nm. The width of each resonator contributes to the effective value of each refractive index involved in the phase coincidence relation for that resonator FP1, FP2, so that the two resonators FP1 and FP2, taken separately, have different resonance wavelengths, called individual resonance wavelengths. The width of the portion of electrode 3b, between the two resonators FP1 and FP2, is denoted r1. For the embodiment of [ Fig. 1a ], r 1 can be equal to 200 nm.

[0046] In the embodiment variant illustrated by [ Fig. 1bThe photoconductive material layer 4 is discontinuous and has a uniform thickness between the locations within each of the resonators FP1 and FP2 and the locations above each of the electrode portions 3a, 3b, and 3c. The insulating layer 2 remains continuous over the entire surface S in the photodetector 100, with a thickness e2 that can still be approximately 50 nm. The thickness es of each electrode portion 3a, 3b, and 3c can still be approximately 100 nm, and the thickness e4 of the photoconductive material layer 4 can be approximately 80 nm everywhere. The portions of layer 4 that are located in each of the resonators FP1 and FP2 are still in contact with the adjacent electrode portions: the portion of layer 4 that is located in resonator FP1 is in contact with electrode portions 3a and 3b, and that that is located in resonator FP2 is in contact with electrode portions 3b and 3c.The numerical values ​​of the widths W1, W2 and r1 can remain identical to those cited in connection with [. Fig. 1a ].

[0047] The method of implementation of [ Fig. 1c ] corresponds to that of [ Fig. 1b by removing the insulating layer 2 inside each of the resonators FP1 and FP2. Such selective removal of the material from the insulating layer 2 can be carried out in one of the ways well known to those skilled in the art, so it is not necessary to describe it here. The following numerical values ​​can be adopted: e2 = 120 nm, e3 = 70 nm and e4 = 140 nm, the numerical values ​​of W1, W2 and r1 being able to remain identical with respect to the embodiments of [ Fig. 1a ] And [ Fig. 1b ]. For the thickness values ​​just given, the portions of layer 4 located in each of the FP1 and FP2 resonators are still in contact with the adjacent electrode portions. In the embodiment of [ Fig. 1c The metallic layer 11 of substrate 1 is in contact with the portions of photoconductive material 4 located in each of the resonators FP1 and FP2. Therefore, the metallic layer 11 can be used as an additional electrode portion, in addition to the electrode portions 3a, 3b, and 3c. The advantage of adding an additional electrode, particularly in this way, will be explained later in this description.

[0048] Finally, in the implementation of [ Fig. 1d], the inter-electrode spaces, which are located between electrode portions 3a and 3b in resonator FP1, and between electrode portions 3b and 3c in resonator FP2, are filled with planarizing resin up to the level of the upper surface of electrode portions 3a, 3b, and 3c. The photoconductive material layer 4 is then parallel-sided and continuously covers electrode portions 3a, 3b, and 3c as well as the resin portions 5. The insulating layer 2 can again be continuous throughout the entire photodetector 100. The following numerical values ​​can be adopted for the embodiment of [ Fig. 1d ] : e 2 = 50 nm, e 3 = 130 nm, e 4 = 140 nm, W 1 = 400 nm, W 2 = 1050 nm, r 1 = 725 nm, and layer 4 can be made up of a sheet of graphene.

[0049] For the method of implementation of [ Fig. 1band the associated numerical values, and when the photoconductive material of layer 4 is mercury telluride (HgTe), the Fabry-Pérot resonator FP1 has an individual resonance wavelength λ1, effective for incident R radiation in the direction of the surface S, which is approximately 1650 nm (nanometers), with an individual resonance quality factor Q1 of approximately 5, and the Fabry-Pérot resonator FP2 has an individual resonance wavelength λ2 of approximately 1550 nm, with an individual resonance quality factor Q2 of approximately 5. The individual resonance interval [λr1 ·(1-3 / Q1); λr1 ·(1+3 / Q1)] of the FP1 resonator is [660 nm; 2640 nm], and that [λ r2 ·(1-3 / Q 2 ) ; λ r2 ·(1+3 / Q 2 )] of the FP2 resonator is [620 nm ; 2480 nm]. These two intervals therefore overlap between 660 nm and 2480 nm.Similar individual resonances exist for the modes of realization of [. Fig. 1a ], [ Fig. 1c ] And [ Fig. 1d ].

[0050] In general, the portion of electrode 3b, which separates the two Fabry-Pérot resonators FP1 and FP2, has a width, measured along the direction D2 and denoted r1, that is small enough for these two resonators to be coupled. Under the conditions just described in connection with [ Fig. 1b], and again when the photoconductive material of layer 4 is lead sulfide (PbS), the two Fabry-Pérot resonators FP1 and FP2 exhibit a coupling resonance with a resonance wavelength value, called the coupling resonance wavelength, of approximately 1.55 µm, with an associated quality factor, called the coupling resonance quality factor, of approximately 15. This coupling resonance is produced by interference between the following three waves, for each monochromatic component of the radiation R: a part of the radiation R which is reflected on the surface S of the substrate 1, i.e. reflected by the metallic layer 11. This part of the radiation which is reflected only once is designated by the reference OR0 in the figures; a first additional wave, noted OR1, which emerges from the Fabry-Pérot resonator FP1, and which results from a superposition of several wave components of which at least one has made a round trip inside the Fabry-Pérot resonator FP2. In other words, the amplitude of the additional wave OR1 depends on the coupling between the resonator FP1 and the free space from which the radiation R originates. In addition, at least one component of this additional wave OR1 propagated in the resonator FP2, making at least one round trip parallel to the direction D1, then crossed the intermediate space between the two resonators FP1 and FP2, before being retransmitted into free space by the resonator FP1.Additional wave components, which may also contribute to the supplementary wave OR1, may have undergone any combination of successive round trips within the two resonators FP1 and FP2, traversing the intermediate space between resonators FP1 and FP2 at each passage between a round trip in one of the resonators FP1 or FP2 and a round trip in the other resonator, before each being retransmitted into free space by resonator FP1; and a second supplementary wave, denoted OR2, which emerges from the Fabry-Pérot resonator FP2, and which results from a superposition of several other wave components, at least one of which has undergone a round trip within the Fabry-Pérot resonator FP1. In other words, the amplitude of the supplementary wave OR2 depends on the coupling between resonator FP2 and the free space from which the radiation R originates.Furthermore, at least one component of the OR2 supplementary wave propagated through resonator FP1, making at least one round trip parallel to the direction D1, then traversed the intermediate space between the two resonators FP1 and FP2, before being retransmitted into free space by resonator FP2. Similar to the OR1 supplementary wave, other additional wave components, which may also contribute to the OR2 supplementary wave, may have made any combination of round trips through the two resonators FP1 and FP2, traversing the intermediate space between resonators FP1 and FP2 at each passage between a round trip in one of the resonators FP1 or FP2 and a round trip in the other resonator, before each being retransmitted into free space by resonator FP2. The two additional waves OR1 and OR2 are due to the coupling that exists between the two Fabry-Pérot structures FP1 and FP2. Thus, for a particular value of the wavelength of the radiation R, the reflected wave OR0, the first additional wave OR1, and the second additional wave OR2 form a constructive interference that contributes to constituting a total reflected wave OR, which is the object of the coupling resonance. When the wavelength of the radiation R is equal to the wavelength value of the coupling resonance, the absorption coefficient of the photodetector 100 is approximately equal to 1, and it is less than 0.2 outside the coupling resonance interval. A criterion for the sufficiency of the coupling between the resonators FP1 and FP2, for the photodetectors 100 of [ Fig. 1a ]-[ Fig. 1d ], is that the sum of the widths W 1 +r 1 +W 2 is less than the value of the coupling resonance wavelength, denoted λ c .

[0051] Each of the 100 photodetectors of [ Fig. 1a ]-[ Fig. 1d ] can be reproduced several times along the direction D 2 by forming a repeating pattern M with a repetition pitch which is designated by p. A new photodetector 101 is thus obtained, which consists of several elementary photodetectors 100 arranged electrically in parallel. Fig. 2[ ] shows such a photodetector 101 which is made up of five elementary photodetectors 100 joined by their electrode portions 3a / 3c. All the electrode portions 3a / 3c and 3b extend longitudinally along the direction D 3 . The electrode portions 3a / 3c belong to electrode 3 1 of the photodetector 101 thus obtained, and the electrode portions 3b belong to its electrode 3 2 . By way of illustration, the repetition pitch p inside the photodetector 101 can be equal to 1 µm, for example when the width r 2 of each electrode portion 3a / 3c along the direction D 2 is equal to 200 nm, or when W 1 = 400 nm, W 2 = 200 nm and r 1 = 200 nm. In this case, the photodetector 101 can have lateral dimensions L along the directions D 2 and D 3 which are on the order of 6 µm.Such a photodetector 101 produces a photo-detection current which is greater than that of each of the elementary photodetectors 100, substantially in a ratio equal to the number of elementary photodetectors 100 which are grouped in the photodetector 101.

[0052] The diagram of [ Fig. 3a ] shows the spectral absorption of a photodetector 101 which is made up of a very large number of repetitions of the pattern M when this pattern is the elementary photodetector 100 of [ Fig. 1b The following numerical values ​​were again adopted for this example: r₁ = r₂ = 200 nm, W₁ = 400 nm, W₂ = 200 nm, p = 1 µm, e₂ = 50 nm, e₃ = 100 nm and e₄ = 80 nm. The value λc of the coupling resonance wavelength remains approximately equal to 1.5 µm, and is associated with a value Qc of the coupling resonance quality factor that is approximately 15. In the diagram of [ Fig. 3aThe horizontal axis represents the wavelength values ​​for monochromatic radiation R, denoted λ and expressed in micrometers (µm), and the vertical axis represents the spectral absorption values, denoted A and expressed as a percentage (%). Spectral absorption is greater than 80% for the value λc of the coupling resonance wavelength, and less than 20% outside the coupling resonance range [λc · (1-3 / Qc); λc · (1+3 / Qc)]. Furthermore, the value λc of the coupling resonance wavelength varies by less than 0.1 µm in absolute value when the incidence of radiation R in the plane of directions D1 and D2 varies by ±25° (degrees) with respect to direction D1. Simultaneously, the absorption value A(λc) for the coupling resonance wavelength λc varies by less than 10%.Such small variations in the values ​​of λc and A(λc) provide the photodetector 101 with a high angular tolerance in its photodetection efficiency. Finally, the coupling resonance wavelength λc varies little with the number of elementary photodetectors 100 arranged in parallel to constitute the photodetector 101: it varies by approximately 0.025 µm between five and an infinite number of elementary photodetectors 100.

[0053] In general, a photodetector that conforms to the invention may have the following additional characteristics: The photodetector can be effective between 200 nm and 15 µm for the wavelength λ of the R radiation, and more particularly between 1 µm and 5 µm, especially between 1 µm and 2.5 µm, depending on the photoconductive material used; the photo-detection current can be between 1 µA.W -1< (microampere per watt) and 1 kA.W -1< (kiloampere per watt), more particularly between 1 mA.W -1< (milliampere per watt) and 5 AW -1< (microampere per watt), and preferably between 100 mA.W -1< and 2 A.W -1< , expressed per unit power of radiation R; the response time of the photodetector may be less than 40 ms (millisecond), more particularly less than 1 ms, and preferably less than 10 µs (microsecond); the specific detectivity of the photodetector may be greater than 10 8< cm·Hz 1 2< ·W -1< (centimeter times hertz to the power of one and a half per watt: unit also called jones), more particularly greater than 10 9< cm·Hz 1 / 2< ·W -1< , and preferably greater than 10 10< cm·Hz 1 / 2< ·W -1< ; and the operating temperature of the photodetector may be greater than 80 K (kelvin), preferably greater than 150 K, and even more preferably greater than 200 K. .

[0054] The photodetection efficiency of a photodetector according to the invention, when the incident radiation R has a wavelength λ equal to that of the coupling resonance λc, is provided by the two Fabry-Pérot resonators FP1 and FP2, which are coupled to each other. Indeed, for this wavelength, the photodetector concentrates the radiation at the portion of the photoconductive material of the resonator with the larger width W1 or W2. For the photodetector of [ Fig. 1b ], the radiation is more precisely concentrated at the upper part of the portion of the photoconductive material, which is furthest from substrate 1, inside the FP1 resonator. For the photodetector of [ Fig. 1cThe radiation is concentrated both at the top of the portion of photoconductive material in the FP1 resonator and at the lower corners of this portion. Generally, the energy density of the radiation is multiplied by a factor of more than 10, or even more than 15, at these locations in the larger portion of the photoconductive material of the resonator, compared to the energy density of the radiation along its optical path before reaching the photodetector. Thanks to this concentration of radiation, a significantly greater number of electrical charges are generated in the photoconductive material, resulting in increased photodetection efficiency and sensitivity.

[0055] In a photodetector that conforms to [ Fig. 1cThe surface S of substrate 1 is electrically conductive and in contact with the portions of photoconductive material 4 contained within resonators FP1 and FP2, while being electrically isolated from electrodes 31 and 32. It is then possible to collect the photodetection current through any two of the following: electrode 31, electrode 32, and the conductive layer 11, which acts as an additional electrode. The selection of the two electrodes actually used to collect the photodetection current during photodetector operation can be achieved using a reconfiguration circuit. Such a reconfiguration circuit can connect the portion of electrode 31, electrode 32, and conductive layer 11 not used to collect the photodetection current to one of the other two, or leave it at a floating potential.

[0056] The photodetector can be further enhanced by a biasing circuit, which is arranged to apply an adjustable voltage between the two electrodes used to collect the photodetection current. The use of such a biasing circuit is known to those skilled in the art, so it is unnecessary to describe it in more detail here. Generally, for the same pair of electrodes, the efficiency of collecting the electrical charges generated by the radiation R in the photoconductive material layer 4 increases with the absolute value of the biasing voltage. An additional advantage of a photodetector according to the invention is that the biasing voltage values ​​to be applied between the electrodes can be less than 10 V, or even less than 1 V.Such voltage values ​​can therefore be transmitted by an integrated electronic circuit which is made using one of the existing technologies. The electric field which is thus created by the polarizing electrical circuit in the photoconductive material can be between 0 and 100 kV·cm⁻¹ (kilovolts per centimeter), and more particularly less than 20 kV·cm⁻¹.

[0057] The diagram of [ Fig. 3b ] shows the spectral detection response of a photodetector 101 conforming to [ Fig. 1b ] And [ Fig. 2The horizontal axis of this diagram represents wavenumber values, equal to the inverse of the wavelength λ, denoted σ and expressed in cm⁻¹, and its vertical axis represents photodetection current values, denoted Iph and expressed in arbitrary units (au), which are obtained when the radiation R has a constant intensity and a propagation direction parallel to the direction D1. The two electrodes used to collect this photodetection current are electrodes 31 and 32 as shown in [ Fig. 2], and the conductive layer 11 is left at a floating potential. A variable bias voltage is further applied between the two electrodes 31 and 32, the values ​​of which are indicated with reference to each curve in the diagram, from 10 mV (millivolts) to 1000 mV. When this bias voltage is zero or low, the photodetector exhibits a detection efficiency resulting from the coupling resonance as described above, with a detection maximum for a value of approximately 6500 cm⁻¹ of the wavenumber σ. This detection maximum corresponds to the concentration of radiation in those of the largest Fabry-Pérot resonators, i.e., the FP1 resonators of [ Fig. 1b ] And [ Fig. 2]. When the bias voltage is increased, an additional detection contribution appears, the maximum of which is located around 5800 cm⁻¹. This additional contribution, which varies at 5800 cm⁻¹ from 0.12 to 1.0 in the axis system of the diagram of [ Fig. 3b ], between the values ​​10 mV and 1000 mV for the bias voltage, corresponds to a more efficient collection of charges in the portions of photoconductive material 4 which are located in the smallest Fabry-Pérot resonators, i.e. the FP2 resonators of [ Fig. 1b ] And [ Fig. 2 ]. Its spectral position, around 5800 cm -1, corresponds approximately to the individual resonance of the Fabry-Pérot FP2 resonators which produces a concentration of radiation in them.

[0058] In photodetector 101 of [ Fig. 4a], the electrode portions that are intermediate between the Fabry-Pérot resonators FP1 and FP2 are connected to electrodes 31 and 32 such that for each resonator FP2, the two electrode portions 3b and 3c that are contiguous to this resonator are short-circuited to each other and connected either to electrode 31 or to electrode 32, alternately between two successive FP2 resonators. The coupled resonators still have the configuration shown in [ Fig. 1b ], with the width W2 of the FP2 resonators being less than that W1 of the FP1 resonators. The photodetection current is still collected between the two electrodes 31 and 32, and the variable polarization voltage is applied between them. Under these photodetection current collection conditions, its spectral variations become those of the diagram of [ Fig. 4bDue to the electrode configuration, the charges generated by the R radiation in the FP1 resonators (the largest) are efficiently collected, corresponding to the detection peak located around 6500 cm⁻¹. The detection peak located around 6000 cm⁻¹ corresponds to the photoconductive material 4 outside the FP1 resonators. Both peaks are strongly amplified by the bias voltage.

[0059] Conversely, in photodetector 101 of [ Fig. 5a ] which also possesses the constitution of the M motif shown in [ Fig. 1b], it is the portions of electrodes 3a and 3b that are contiguous to each resonator FP1 that are short-circuited to each other and connected to one of the two electrodes 31 and 32 alternately between two successive resonators FP1. As before, the resonators FP1 have a width W1 that is greater than that W2 of the resonators FP2. The photodetection current is still collected between the two electrodes 31 and 32, and the variable bias voltage applied likewise between them. Under these new conditions of photodetection current collection, its spectral variations are those of the diagram of [ Fig. 5b Due to the electrode configuration, only the charges generated by the R radiation in the FP2 resonators (the narrowest ones) are collected. The detection peak is then located mainly around 6000 cm⁻¹ and strongly exacerbated by the bias voltage.

[0060] [ Fig. 6Figure ] is a cross-sectional view of yet another photodetector according to the invention. One motif of this other photodetector consists of more than two, for example three, Fabry-Pérot resonators placed side by side with resonator widths that differ in pairs. These three Fabry-Pérot resonators are designated FP1, FP2, and FP3, with their respective resonator widths W1, W2, and W3. For example, the width W3 is greater than the width W2, which is itself greater than the width W1. The electrode portions are designated 3a, 3b, 3c, and 3d, and the other reference numerals have the same meanings as before.The width of electrode portion 3b is sufficiently small to allow Fabry-Pérot resonators FP1 and FP2 to be coupled according to the invention, and the width of electrode portion 3c is similarly sufficiently small to allow Fabry-Pérot resonators FP2 and FP3 to be coupled. Therefore, a first photodetection current that can be collected between electrode portions 3a and 3b exhibits a sensitivity spectrum, depending on the wavelength of the radiation to be detected, resulting from the coupling between Fabry-Pérot resonators FP1 and FP2. A second photodetection current that can be collected between electrode portions 3c and 3d exhibits a different sensitivity spectrum resulting from the coupling between Fabry-Pérot resonators FP2 and FP3.A third photodetection current, in addition to the two previous ones, can be collected between electrode sections 3b and 3c. Its sensitivity spectrum results from the coupling of the Fabry-Pérot resonator FP2 with each of the other two, namely Fabry-Pérot resonators FP1 and FP3. It can then be advantageous to adjust a bias voltage applied between electrode sections 3b and 3c to fine-tune the sensitivity spectrum of this third photodetection current. The first, second, and third photodetection currents are collected simultaneously, thus providing three distinct pieces of information on the spectral composition of the detected radiation.

[0061] In fact, as can be seen from the description above, the multiplicity of electrical connection methods for the electrode segments, the various possibilities for selecting the electrode pairs used to collect the photodetection current, and the bias voltage all allow the photodetector to be reconfigured to modify its spectral sensitivity characteristic. The photodetector can therefore be adapted to its specific application, or provide measurements of the same radiation using several detection modes.

[0062] [ Fig. 7[The following appears to be a separate, unrelated entry:] ] shows an image sensor that conforms to the invention. This image sensor, which is designated globally by reference numeral 110, comprises an array of photodetectors 100 or 101, all of the same type, for example, one of the types described above. In particular, this array of photodetectors may have dimensions ranging from 4 x 4 to 16,384 x 12,288 photodetectors, more particularly from 320 x 200 to 16,384 x 12,288 photodetectors. The pitch of the photodetectors in this array may be between 1 µm and 1 cm, preferably less than 100 µm. When the photoconductive material is deposited using a spin-deposition process, the photodetectors can be formed directly on a readout circuit of the image sensor 110. For example, this readout circuit can be manufactured using CMOS technology. This reading circuit, which is designated by reference 102 in [ Fig. 7], then constitutes the basic substrate 10 of all the photodetectors 100 / 101. In this case, a set 103 of electrical connection layers can be interposed between the photodetectors 100 / 101 and the readout circuit 102. These electrical connections link the electrode portions of each photodetector to a readout cell that is dedicated to that photodetector and contained within the readout circuit 102. Furthermore, the readout circuit 102 can advantageously incorporate the reconfiguration circuit and the electrical biasing circuit as introduced above. They are designated in [ Fig. 7 ] by reference 104 for the reconfiguration circuit, and by reference 105 for the electrical biasing circuit.

[0063] A method for manufacturing a photodetector according to the invention is now described in detail, by way of example. First, a method for obtaining a precursor of colloidal solution is provided, as well as three examples of colloidal solutions of photoconductive nanocrystals. TOP:Te precursor molar solution (1 M)

[0064] A quantity of 6.35 g (grams) of tellurium (Te) powder was mixed with 50 mL (milliliters) of TOP (tri-octylphosphine) in a three-necked flask. This flask was maintained under vacuum at room temperature for 5 minutes, then its temperature was increased to 100°C. Degassing was carried out for an additional 20 minutes at this temperature. The atmosphere was replaced with nitrogen (N₂) and the temperature was adjusted to 275°C. The solution was stirred until it became clear and orange. The flask was then cooled to room temperature, and the color changed to yellow. Finally, this solution was transferred to a nitrogen-filled glove box for storage. Example 1 : synthesis of HgTe nanocrystals with a band gap of 6000 cm⁻¹

[0065] In a 100 mL three-necked flask, 540 mg (milligrams) of mercuric chloride (HgCl₂) and 50 mL of oleylamine were degassed under vacuum at 110°C. At this stage, the solution was yellow and clear. Meanwhile, 2 mL of the 1 M TOP:Te precursor molar solution were extracted from the glove box and mixed with 8 mL of oleylamine. The atmosphere was replaced with nitrogen, and the temperature was set to 57°C. The TOP:Te solution was rapidly injected into the three-necked flask and turned dark after 1 minute. After 3 minutes, 10 mL of a DDT (dodecanethiol) solution in toluene (10% DDT by volume) was further injected into the three-necked flask, and a cold water bath was used to rapidly lower its temperature. The contents of the second three-necked flask were divided into four tubes and methanol was added to each. After centrifugation, the resulting precipitates were redispersed into a single tube with 10 mL of toluene.The solution was precipitated a second time with ethanol. Again, the precipitate formed was redispersed in 8 mL of toluene. At this stage, the nanocrystals were centrifuged in pure toluene to remove the lamellar phase. The solid phase was removed, and the supernatant was filtered using a 0.2 µm polytetrafluoroethylene (PTFE) filter. Example 2 : synthesis of HgTe nanocrystals with a band gap of 4000 cm⁻¹

[0066] In a 100 mL three-necked flask, 540 mg of mercuric chloride (HgCl₂) and 50 mL of oleylamine were degassed under vacuum at 110°C. At this stage, the solution was yellow and clear. Meanwhile, 2 mL of the 1 M TOP:Te precursor molar solution were extracted from the glove box and mixed with 8 mL of oleylamine. The atmosphere was replaced with nitrogen, and the temperature was set to 86°C. The TOP:Te solution was rapidly injected into the three-necked flask and turned dark after 1 minute. After 3 minutes, 10 mL of a DDT (dodecanethiol) solution in toluene (10% DDT by volume) was further injected into the three-necked flask, and a cold water bath was used to rapidly lower its temperature. The contents of the three-necked flask were divided into four tubes, and methanol was added to each. After centrifugation, the precipitates formed were redispersed in a single tube with 10 mL of toluene. The solution was precipitated a second time with ethanol.The resulting precipitate was again redispersed in 8 mL of toluene. At this stage, the nanocrystals were centrifuged in pure toluene to remove the lamellar phase. The solid phase was removed, and the supernatant was filtered using a 0.2 µm polytetrafluoroethylene (PTFE) filter. Example 3: Synthesis of PbS nanocrystals with a band gap of 6000 cm⁻¹

[0067] In a three-necked flask, 300 mg of lead chloride (PbCl₂) and 7.5 mL of oleylamine are degassed at room temperature and then at 110°C for 30 minutes. During this time, 30 mg of sulfur (S) powder are mixed with 7.5 mL of oleylamine until completely dissolved by stirring under ultrasonic conditions, resulting in a clear, orange solution. Then, under a nitrogen atmosphere at 160°C, this sulfur solution is rapidly added to the three-necked flask. After 15 minutes, the reaction is quickly stopped by adding 1 mL of oleic acid and 9 mL of hexane. The nanocrystals are precipitated by the addition of ethanol, centrifuged, and then redispersed in toluene. This washing step is repeated once more. The nanocrystal solution in toluene is then centrifuged to remove the unstable phase. The supernatant is precipitated with methanol and then redispersed in toluene.Finally, the PbS nanocrystal solution in toluene is filtered using a 0.2 µm polytetrafluoroethylene, or PTFE, filter.

[0068] The fabrication of the photodetector with coupled Fabry-Pérot resonators, according to the invention, is now described and comprises the following steps 1 to 5: Step 1 : formation of a mirror to create the reflective layer

[0069] Silica-coated silicon substrates, measuring 12 mm x 14 mm, are cleaned with acetone and isopropanol. They are placed in an acetone bath and subjected to ultrasonic cleaning for 5 minutes. They are then rinsed with acetone and isopropanol, and dried under a nitrogen stream. These substrates are then cleaned with an oxygen (O2) plasma for 5 minutes. An adhesion promoter, for example TI Prime® from MicroChemicals®, is applied by spin-coating, for example at 4000 rpm for 30 seconds, and baked at 120°C for 2 minutes. Resin, for example reference AZ 5214, is then deposited by spin-coating, for example at 4000 rpm for 30 seconds, and then annealed at 110°C for 90 seconds. The substrates are then exposed to ultraviolet (UV) radiation through a mask for 1.5 seconds, and then annealed at 125°C for 2 minutes.A second irradiation with ultraviolet radiation is then performed, for example for 40 seconds, without a mask. The resin is developed in a developer, for example the AZ 726 MIF model, for 30 seconds and then rinsed with deionized water for 15 seconds. Each substrate is then cleaned with an oxygen plasma for 5 minutes. A first layer of titanium (Ti), 3 nm thick, followed by a second layer of gold (Au), 80 nm thick, are deposited using a thermal evaporator, preferably with sample rotation. Finally, a third layer of aluminum (Al), 5 nm thick, is deposited, again using a thermal evaporator. The resin is then removed by soaking each sample in acetone for 1 hour. The substrates are then rinsed with acetone and isopropanol and dried under a nitrogen stream.The mirror thus obtained on each silicon base substrate is intended to constitute the reflective layer 11 which has been mentioned in connection with [. Fig. 1a ]-[ Fig. 1d ]. Step 2 : application of the insulating layer

[0070] A 50 nm thick layer of alumina (Al₂O₃) is deposited by the ALD process, for "atomic layer deposition", on each substrate. This layer is intended to constitute the insulating layer 2 that was mentioned in connection with [ Fig. 1a ]-[ Fig. 1d ]. Step 3 : formation of macroscopic zones of electrical contact

[0071] The substrates are rinsed with acetone and isopropanol and then dried under a nitrogen stream. An adhesion promoter, for example TI Prime® supplied by MicroChemicals®, is deposited by spin-coating, for example at 4000 rpm for 30 seconds, and then baked at 120°C for 2 minutes. AZ 5214 resin is then deposited by spin-coating, for example at 4000 rpm for 30 seconds, and then annealed at 110°C for 90 seconds. Each substrate is then exposed to ultraviolet radiation through a mask for 1.5 seconds and then annealed at 125°C for 2 minutes. A second exposure to ultraviolet radiation is then performed for 40 seconds, without a mask. The resin is developed in the AZ 726 MIF developer for 30 seconds and then rinsed with deionized water for 15 seconds. Each substrate is then cleaned with an oxygen plasma for 5 minutes.A 3 nm thick layer of titanium, followed by a 150 nm thick layer of gold, is deposited by thermal evaporation, preferably with substrate rotation. The resin is then removed by soaking the sample in acetone for 1 hour. The substrates are then rinsed with acetone and isopropanol, and dried under a nitrogen stream. Step 4 : electron beam lithography

[0072] The substrates are rinsed with isopropanol and then dried under a nitrogen stream. A layer of pure A6-grade polymethyl methacrylate (PMMA) is deposited by spin-coating at 400 rpm for 5 seconds, then at 4000 rpm for 30 seconds, and baked at 180°C for 2 minutes. A 10 nm layer of aluminum is then deposited using an electron beam evaporator. The aluminum deposition rate is set at 0.1 nm·s (nanometers per second), and sample rotation is enabled in the evaporator.

[0073] Each substrate is then transferred to an electron beam lithography (EBLT) device. Electron beam lithography is performed with a current of 12 pA (picoamperes) and a total dose of 200 µC·cm² (microcoulombs per square centimeter). The substrate is then immersed for 15 seconds in a potassium hydroxide (KOH) solution of 40 g in 100 mL of water, rinsed with water, and then dried under a nitrogen stream. This removes the aluminum layer. The PMMA resin is developed using a methyl isobutyl ketone (MIBK:isopropanol, or IPA) solution at a 1:3 by volume ratio for 45 seconds, followed by rinsing in pure isopropanol for 20 seconds. Each substrate is then cleaned with an oxygen plasma for 2 minutes. It is then transferred to the electron beam evaporator. A 3 nm thick layer of titanium, then an 80 nm thick layer of gold, are deposited, with respective deposition rates of 0.1 nm·s-1 and 0.2 nm·s-1.The resin is then removed by immersing each substrate in acetone at 40°C for at least 2 hours. The metallic portions that have been formed on each substrate are the electrodes 31 and 32 mentioned in connection with [. Fig. 2 ], [ Fig. 4a ] And [ Fig. 5a The substrates are then observed by scanning electron microscopy, with parameters of 8 mm and 5 kV, and then the electrodes are electrically controlled. Step 5 : nanocrystal deposition

[0074] One mL of a HgTe nanocrystal solution with a band gap of 6000 cm⁻¹ (720 meV) in toluene and an optical density of 0.9 at 400 nm is mixed with one mL of a ligand exchange solution consisting of 9 mL of dimethylformamide, 1 mL of mercapthoethanol, and 15 mg of HgCl₂. Three successive cleaning steps are performed with hexane. The nanocrystals are then precipitated with toluene. After centrifugation, the supernatant is removed, and the pellet is dried under vacuum for 15 minutes. The pellet is then redispersed in 170 µL of pure dimethylformamide. This ink is then deposited by spin-coating onto each substrate at 2000 rpm (acceleration 200 rpm / s, rotation time 120 s). Previously, the substrate was exposed to an oxygen plasma for 4 minutes.

[0075] It is understood that the invention can be reproduced by modifying minor aspects of the embodiments described in detail above, while retaining at least some of the advantages mentioned. In particular, all numerical values ​​provided are for illustrative purposes only and may be changed depending on the application.

Claims

1. A photodetector (100, 101) comprising: - a substrate (1), which is reflective to electromagnetic radiation incident on the photodetector (100, 101); - electrode portions (3a, 3b, 3c), which are supported by the substrate (1), and which have respective surfaces facing away from the substrate, referred to as upper surfaces and located at a common level of spacing from said substrate; - portions of an electrically insulating material, which are located between the electrode portions (3a, 3b, 3c) and the substrate (1), so as to electrically insulate each electrode portion from the substrate; and - at least one portion of a photoconductive material, which is arranged to be in electrical contact with two of the electrode portions (3a, 3b, 3c) which are adjacent, at least two of the electrode portions (3a, 3b, 3c) and the substrate (1) being intended to collect a photodetection current when the photodetector (100, 101) is in use, characterized in that a first (3a) and a second (3b) of the electrode portions (3a, 3b, 3c) which are adjacent delimit therebetween, parallel to the substrate (1), a volume into which, when the photodetector (100, 101) is in use, the radiation penetrates in order to be reflected by the substrate, forming a first Fabry-Perot resonator (FP1) between said substrate and the level of the upper surfaces of the electrode portions, and in that the second electrode portion (3b) and a third (3c) of the electrode portions (3a, 3b, 3c), which is located on a side of said second electrode portion opposite said first electrode portion, delimit therebetween, parallel to the substrate (1), another volume into which, when the photodetector (100, 101) is in use, the radiation also penetrates to be reflected by the substrate, forming a second Fabry-Perot resonator (FP2) between said substrate and the level of the upper surfaces of the electrode portions, the first and second Fabry-Perot resonators (FP1, FP2) being designed to generate standing-wave components that propagate perpendicular to the substrate (1) when the photodetector (100, 101) is in use, and in that the photodetector (100, 101) has the following features / 1 / to / 3 / : / 1 / a width of the first Fabry-Perot resonator (FP1), measured between the first and second electrode portions parallel to the substrate (1), is different from a width of the second Fabry-Perot resonator (FP2), measured between the second and third electrode portions also parallel to the substrate, so that the first and second Fabry-Perot resonators have respective individual resonance wavelength values, effective for the radiation incident on the photodetector (100, 101), which are different, with respective values of an individual resonance quality factor of the first and second Fabry-Perot resonators such that, on one wavelength axis of the incident radiation, the following individual resonance intervals: [λri·(1-3 / Qi); λri·(1+3 / Qi)], have an overlap, where i is equal to 1 or 2 to designate the first or second Fabry-Perot resonator, respectively, and λri and Qi are respectively the wavelength and quality factor values of the individual resonance of the Fabry-Perot resonator i; / 2 / a sum of the widths of the first and second Fabry-Perot resonators (FP1, FP2) with the width of the second electrode portion (3b), measured parallel to the substrate (1) between the volumes of the first and second Fabry-Perot resonators, is adapted to produce a coupling between said first and second Fabry-Perot resonators, by being less than a resonance wavelength value relative to the coupling, known as the coupling resonance wavelength, which is effective for the radiation incident on the photodetector (100, 101), and which results from interference between at least three waves, including: - a first wave resulting from the reflection of incident radiation on the substrate (1); - a second wave emerging from the first Fabry-Perot resonator (FP1), resulting from a superposition of several wave components, among which at least one of said wave components has made at least one round trip within the volume of the second Fabry-Perot resonator (FP2); and - a third wave emerging from the second Fabry-Perot resonator (FP2), resulting from another superposition of several other wave components, among which at least one of said other wave components has made at least one round trip within the volume of the first Fabry-Perot resonator (FP1); and / 3 / the photoconductive material is absorbent for the coupling resonance wavelength, and the portion of said photoconductive material is located in or on at least one of the volumes of the first and second Fabry-Perot resonators (FP1, FP2).

2. The photodetector (100, 101) according to claim 1, wherein the substrate (1) comprises a photodetector readout circuit (102).

3. The photodetector (100, 101) according to claim 1 or 2, wherein the portions of electrically insulating material are parts of a continuous layer (2) of said insulating material which extends across the volumes of the first and second Fabry-Perot resonators (FP1, FP2), in addition to extending between the substrate (1) and each electrode portion (3a, 3b, 3c).

4. The photodetector (100, 101) according to claim 1 or 2, wherein the substrate (1) is also in contact with the portion of photoconductive material, in addition to the first, second and third electrode portions (3a, 3b, 3c), so as to form an additional electrode portion.

5. The photodetector (100, 101) according to any one of the preceding claims, further comprising a biasing electrical circuit (105) which is adapted to apply, during use of the photodetector, an electrical voltage between two of the electrode portions (3a, 3b, 3c) which collect the photodetection current, said biasing electrical circuit being further adapted to vary said electrical voltage between two successive uses of the photodetector, so as to modify a sensitivity spectrum, in particular a detection sensitivity, of said photodetector.

6. The photodetector (100, 101) according to claim 5, adapted so that a radiation absorption value at least at one wavelength value varies by at least 30%, preferably at least 50%, even more preferably at least 90%, between a first use of the photodetector with no electrical voltage applied by the biasing electrical circuit (105) between the two electrode portions, or during which said applied electrical voltage is zero, and a second use of said photodetector during which said applied electrical voltage is non-zero.

7. The photodetector (100, 101) according to any one of the preceding claims, further comprising a reconfiguration circuit (104) which is adapted to select and electrically connect at least two of the electrode portions (3a, 3b, 3c) and substrate (1) of the photodetector in order to collect photodetection current by those electrode and substrate portions which are selected, those electrode and substrate portions which are selected varying between several modes of photodetection current collection, which are associated with different respective spectra of photodetector sensitivity to incident radiation.

8. The photodetector (100, 101) according to any one of the preceding claims, wherein each portion of the photoconductive material is part of a layer (4) of said photoconductive material which extends continuously over the volumes of the first and second Fabry-Perot resonators (FP1, FP2) and over the electrode portions (3a, 3b, 3c).

9. The photodetector (100, 101) according to any one of the preceding claims, comprising a plurality of pairs of coupled first and second Fabry-Perot resonators (FP1, FP2), with first, second and third electrode portions (3a, 3b, 3c) associated with each pair and electrically connected to accumulate photodetection currents which arise from each pair when the photodetector is in use.

10. The photodetector (100, 101) according to any one of the preceding claims, having lateral dimensions which are between 1 µm and 1 cm, preferably between 1 µm and 100 µm, measured parallel to the substrate (1).

11. The photodetector (100, 101) according to any one of the preceding claims, wherein the volumes of the first and second Fabry-Perot resonators (FP1, FP2), as well as the width of the second electrode portion, are dimensioned so that the coupling resonance wavelength is between 1 µm and 12 µm.

12. The photodetector (100, 101) according to any one of the preceding claims, wherein the photoconductive material is selected to have a bandgap which is less than 0.8 eV.

13. The photodetector (100, 101) according to any one of the preceding claims, wherein each portion of photoconductive material consists of agglomerated nanocrystals.

14. An image sensor (110), comprising a matrix arrangement of photodetectors, each photodetector (100, 101) being in accordance with any one of the preceding claims.

15. The image sensor (110) according to claim 14, wherein each photodetector (100, 101) is individually in accordance with claim 9 and has an individual photodetector size, measured along a direction of juxtaposition of the pairs of coupled first and second Fabry-Perot resonators (FP1, FP2), which is less than or equal to ten times a wavelength value of the radiation corresponding to a maximum detection sensitivity of the photodetector.

16. A method of manufacturing a photodetector (100, 101), said photodetector being in accordance with any one of claims 1 to 13, according to which the portions of photoconductive material are obtained from a deposition of a colloidal solution which incorporates nanocrystals of the photoconductive material, followed by drying of the deposited colloidal solution.