Infrared p-on-n photodetector with hgte nanocrystals and an optimized architecture for coupling the read-out circuit

Abstract

Infrared photodetector (10, 20) comprising a bottom end electrode (110, 210) for collecting electrons, a top end electrode (120, 220) for collecting holes, a hole transport layer (105), and an infrared photon absorption layer (104, 204) for generating an electrical signal, the infrared photon absorption layer (104, 204) being positioned between bottom end electrode (110, 210) and the hole transport layer (105), the hole transport layer (105) being positioned between the top end electrode (120, 220) and the infrared photon absorption layer (104, 204), and the top end electrode (120, 220) comprising an outer layer (108, 208) made of transparent oxide conductor and an inner layer (107) made of a thin layer of metal positioned between the outer layer (108, 208) and the hole transport layer (105). The infrared photodetector (10, 20) further comprises a spacer (106) positioned between the inner layer (107) of the top end electrode (120, 220) and the hole transport layer (105), said spacer (106) being optically transparent and having a band gap larger than a band gap of the infrared photon absorption layer (104, 204).

Description

Title of the invention: Infrared p-on-n photodetector with HgTe nanocrystals and an optimized architecture for coupling the read-out circuitTechnical field

[0001] The field of the invention relates to the design of optoelectronic devices, particularly those relying on semiconductor materials. Specifically, it relates to the use of colloidally prepared semiconductor such as colloidal quantum dots (CQDs).

[0002] In particular, the invention relates to a device used for the detection of infrared light, and in which the active layer consists in particular of HgTe nanocrystals. More precisely, the invention concerns the design of a photodiode absorbing up to 5 pm.Background of the invention

[0003] Infrared detectors are based on different component geometries. Photoconductors, phototransistors and photodiodes are the most common. The latter is often the one that leads to the highest signal-to-noise ratio. Typically, photodiodes are based on a vertical stack of layers.

[0004] When a photodiode is made from colloidal nanocrystals, a photodiode 1 is generally based on a stack of six layers (different ordering of the layers can be possible), as shown in Figure 1 :- a mechanical substrate 2 with optical transparency constraints,- a partially transparent and conductive contact 3, often made of transparent oxide such as ITO (for "tin-doped indium oxide"),- an electron transport layer 4 for electron extraction,- a layer 5 which absorbs the incident photons and whose role is to generate an electrical signal,- a hole transport layer 6, which is used to extract the holes,- a top contact 7, possibly made of metal, which is used as a charge collector and generally also as a backside mirror.

[0005] Thanks to major progress relative to their growth, CQDs appear as a viable alternative to epitaxially grown semiconductor for optoelectronics. Their first massmarket application has been focused on their bright photoluminescence properties, and the progress relative to the control of their surface chemistry has enabled using them as electrically active medium as well. Among their potential applications, their use for infrared sensor is currently one of the most active field. Their growth appears more cost-effective than that of conventional narrow band gap semiconductor, while lifting off constrains related to the coupling with the read-out integrated circuit.

[0006] Two material platforms are now mature enough for being integrated at the imager level: PbS and HgTe. The former has benefited from earlier development relative to solar cell and photodiode based on PbS are now well-established thanks to the combination of halide and thiol capped CQDs in order to form the required p-n junction that is the basis of the photodiode. On the other hand, HgTe did not benefit from this transfer from solar cell and its device integration appears not as mature regarding the fabrication of the photodiode. As a trick, imager based on photoconductive operation were the first reported with this material. This requires that the read-out integrated circuit enables the application of an inplane electric field, but such operation mode also simplifies the fabrication (possibly down to a single layer deposition and thus favor high-quality layer formation. It remains that photoconductive operation still requires the application of an electric field and therefore promotes the dark current. As a result, the signal-to-noise ratio so far obtained is typically one decade below the one achieved with diode.

[0007] In parallel to development at the imager level, progress relative to HgTe CQD- based diodes have also been obtained. Ackerman et al., in ACS Nano 2018, 12, 7264 optimized this layer, proposed the first high-efficiency diode based on the combination of a HgTe CQD as absorbing layer coupled with an Hg treated Ag2Te CQD layer acting as a hole transport layer. This diode stack operates optimally in the mid-wave infrared in which the HgTe CQD tends to have a weak n-type behavior. This leads to the formation of a p-n junction once coupled with the Hg treated Ag2Te CQD layer. In the short-wave infrared, the HgTe CQD exhibits a more p-type behavior, and the diode performs better once coupled to an additional electron extraction layer (ETL). Examples of such ETLs include Bi2Se3, CdSe or SnO2. At the single diode pixel, these stacks present a highlyrectifying current-voltage (l-V) characteristic and an external quantum efficiency above 40 %. Nevertheless, their transfer to the read-out integrated circuit (ROIC) is not straightforward.

[0008] The first bottleneck lies in the current top contact, which is made of a metallic electrode while the illumination goes through the substrate. This setup is incompatible with a ROIC, which requires a top illumination and therefore a top transparent electrode. However, attempts to sputter ITO (tin-doped indium oxide) as a top transparent electrode led to a systematic loss of the diode behavior for the HgTe / Ag2Te stack. Furthermore, the challenge is not limited to the fabrication of this top transparent contact, since the bottom electrode appears equally challenging. In a p-on-n configuration, the bottom electrode eases the electron extraction and is ideally made from a low-work function material. A low work function metal electrodes could be used but they suffer from a high tendency for oxidation and can form amalgam with mercury, even in HgTe CQD form. In summary, transitioning a diode stack designed for single-pixel devices to a complex multipixel system, such as a ROIC, requires a comprehensive redesign of the electrode architecture.

[0009] In the article from R. Alchaar et al. Published in Applied Physics Letters 2023, 123, 051108, a vertically staked diode is described with an electron transport layer, a hole transport layer, a top end electrode, and an infrared photon absorption layer for generating an electrical signal, the infrared photon absorption layer being positioned between the electron transport layer and the hole transport layer being positioned between the contact layer and the infrared photon absorption layer, and the top end electrode comprising an outer layer made of transparent oxide conductor and an inner layer made of a thin layer of metal positioned between the outer layer and the hole transport layer.Object and summary of the invention

[0010] The invention aims to design an infrared sensor with an active layer made of CQDs and an optimized device geometry to ensure the design of infrared imager. To reach this goal, the invention proposes an optimized diode stack that ease the coupling to a read-out integrated circuit. In particular, the invention proposes an updated version of the HgTe CQD-based diode, with an electrode configurationthat simultaneously maximizes the optical absorption while optimizing the electrical properties.

[0011] The invention is an infrared photodetector, in particular for wavelengths between 1 to 5 pm, comprising a bottom end electrode for the collecting electrons, a top end electrode for collecting holes, a hole transport layer, and an infrared photon absorption layer for generating an electrical signal, the infrared photon absorption layer being positioned between bottom end electrode and the hole transport layer, the hole transport layer being positioned between the top end electrode and the infrared photon absorption layer, and the top end electrode comprising an outer layer made of transparent oxide conductor and an inner layer made of a thin layer of metal positioned between the outer layer and the hole transport layer.

[0012] In a general feature of the invention, the infrared photodetector further comprises a spacer positioned between the inner layer of the top end electrode and the hole transport layer, said spacer being optically transparent in a spectral range between 1 to 5 pm and having a band gap larger than the band gap of the infrared photon absorption layer.

[0013] This architecture allows having a vertically stacked photodiode with p-on-n stack structure while addressing the challenge of optimizing contact. The spacer is necessary to allow depositing the outer layer made of transparent oxide conductor, such as tin-doped indium (ITO) or alumina-doped Zinc Oxide (AZO) films, as a top electrode on the inner layer made of a thin layer of metal without damaging the active part of the diode, in particular the hole transport layer and the infrared photon transport layer.

[0014] In a first particular embodiment of the infrared photodetector, the photon absorption layer can comprise nanocrystals of a compound selected from HgS, HgSe, HgTe, PbS, PbSe, PbTe, Ag2S, Ag2Se, Ag2Te, InAs, InGaAs and InSb and mixture thereof, and heterostructure thereof and alloy thereof.

[0015] In a particular embodiment of the infrared photodetector, the photon absorption layer can comprise HgTe nanocrystals and doped version of it, the doping being obtained from strategy such as ligand exchange, cation exchange or extrinsic impurities introduction.

[0016] In a preferred embodiment, the hole transport layer may comprise Ag2Te nanocrystals, and in a particular embodiment, it can also comprise Ag2Te nanocrystals, a modified version of it, or p-type HgTe.

[0017] In another particular embodiment of the infrared photodetector, the infrared photodetector may comprise a silicium or silica substrate, said bottom end electrode being positioned between said silicium or silica substrate and said infrared photon absorption layer.

[0018] In a particular embodiment of the infrared photodetector, the bottom end electrode comprises a lower layer made of a thin layer of metal and an upper layer made of transparent oxide conductor made of alumina-doped zinc oxide (AZO) for mid infrared wavelengths or tin-doped indium (ITO) for short infrared wavelengths, said upper layer being positioned between the lower layer and the infrared photon absorption layer. The ITO layer is used to reduce the electrode work function.

[0019] In another particular embodiment of the infrared photodetector, the lower layer of the bottom end electrode may have a thickness between 20 and 200 nm, and preferably between 50 and 100 nm.

[0020] In another particular embodiment of the infrared photodetector, the spacer may have a thickness comprised between 1 nm and 1 pm, and preferably between 10 nm and 100 nm.

[0021] In another particular embodiment of the infrared photodetector, the inner layer of the top end electrode may be made of gold, platinum or palladium. The noble metals are used to ensure a chemical stability with the readout circuit.

[0022] In another particular embodiment of the infrared photodetector, the infrared photon absorption layer may be made of HgTe nanocrystals with an optical band edge between 1 and 5 pm.

[0023] In another particular embodiment of the infrared photodetector, the infrared photon absorption layer may have a thickness 200 and 1000 nm, the hole transport layer may have a thickness between 50 and 100 nm, and the inner layer of the top end electrode may have a thickness between 5 and 10 nm.

[0024] In another particular embodiment of the infrared photodetector, the spacer may have a thickness less than 200 nm and is made of a p-type material.

[0025] Having a spacer which is a p-type layer with a suited band alignment allows avoiding a filtering of the photogenerated hole. Three candidates are well-suited for this role: a thin layer of PbS CQD or WO3or CuSCN.

[0026] In another particular embodiment of the infrared photodetector, the electron transport layer may comprise nanocrystals of a compound selected from SnC>2, ZnO, CdS, CdSe, AZO, CuO, CuO2, CU2O3, ZrO2, and mixture thereof and heterostructure thereof and alloy thereof.

[0027] In another particular embodiment of the infrared photodetector, the outer layer of the contact layer can be made of Al doped ZnO (AZO).

[0028] In another particular embodiment of the infrared photodetector, the photodetector may be a photodiode, or a photoconductor or a phototransistor.

[0029] Another aspect of the invention proposes an imaging device comprising a plurality of infrared photodetectors as defined here above and configured to form an image and coupled to a read-out integrated circuit.

[0030] Another aspect of the invention proposes a camera device comprising a plurality of infrared photodetectors as defined here above and configured to form a camera and coupled to an electronic system, an optical system, and a temperature control system.Brief description of the drawings

[0031] The drawings attached are schematic and are here to illustrate the following description which is given as non-limiting examples. On these drawings, elements or parts which are identical are referenced with the same reference signs.

[0032] [Fig. 1 ], already described, is a perspective view of an infrared photodetector from the prior art,

[0033] [Fig. 2] is a schematic sectional view of an infrared photodetector according to a first embodiment of the invention.

[0034] [Fig. 3] is a schematic sectional view of an infrared photodetector according to a second embodiment of the invention.

[0035] [Fig. 4] represents three different current-voltage characteristics under dark conditions (solid lines) and illumination (dashed lines) with a 1 .55 pm laser diode for three different diode stacks.

[0036] [Fig. 5] is a current-voltage characteristics for the optimized SWIR diode stack.

[0037] [Fig. 6] shows a spectral response of the optimized SWIR diode stack.Detailed description of the embodiments

[0038] As illustrated in Figure 2, a shortwave infrared photodetector 10 according to the invention, such as a photodiode, comprises, stacked vertically and successively from bottom B to top T :- a mechanical substrate 100 with generally optical transparency constraints, in Silicium or in Silica (SiO2),- a bottom-end electrode 1 10 comprising a first partially transparent and conductive layer 101 made of Gold, and, on top of said layer of Gold 101 , a partially transparent and conductive contact later 102 made of ITO or fluorinedoped tin oxide (FTO),- an electron transport layer 103 made of SnO2or CdSe,- an infrared photon absorption layer 104 made of HgTe nanocrystals with a size of 8 nm,- a hole transport layer 105 made of Ag2Te nanocrystals,- a spacer 106 made of WO3 or PbS or CuSCN, and- a top-end electrode 120 comprising a second partially transparent and conductive layer 107 made of Gold topped by a second partially transparent and conductive contact later 108 made of ITO or FTO.

[0039] As illustrated in Figure 3, a mid-infrared photodetector 20 according to the invention, such as a photodiode, comprises, stacked vertically and successively from bottom B to top T :- a mechanical substrate 100 with generally optical transparency constraints, in Silicium or Silica (SiO2),- a bottom-end electrode 210 comprising a first partially transparent andconductive layer 101 made of Gold, and, on top of said layer of Gold 101 , a partially transparent and conductive contact later 202 made of AZO,- an infrared photon absorption layer 204 made of HgTe nanocrystals with a size of 12 nm,- a hole transport layer 105 made of Ag2Te nanocrystals,- a spacer 106 made of WO3or PbS, and- a top-end electrode 220 comprising a second partially transparent and conductive layer 107 made of Gold topped by a second partially transparent and conductive contact later 208 made of AZO.

[0040] Thanks to the architecture of the infrared photodetector, the vertical geometry diode may be illuminated by the substrate side or by the top end electrode side.

[0041] The combination of HgTe and Hg treated Ag2Te nanocrystal layers is used to form a p-n junction, which can be assisted by the presence of an electron transport layer on the HgTe side. The electron transport layer may be chosen to be a semiconductor which conduction band is resonant in energy with the one from HgTe nanocrystals.

[0042] The thickness of this electron transport layer may be between 10 to 300 nm and preferably between 20 to 100 nm range. The electron transport layer can be made of SnO2and doped version of it, CdSe, Bi2S3or Bi2Se3.

[0043] The electrode used on the electron extraction side is chosen to have a work function matching in energy the conduction band of the nanocrystals used to absorb. The work function from the electrode used to extract electron may be below 4.5 eV and preferably below 4.3 eV.

[0044] Infrared photon absorbing layer 104, 204: material, spectral window

[0045] The nanocrystals used for the infrared photon absorbing layer 104, 204, and / or for the hole transport layer 105, are nanocrystal solids made from a disordered array of nanocrystal with a packing density above 50 %.

[0046] The nanocrystals from which the infrared photon absorbing layer 104 or 204 is a semiconductor. It can be made of are for example quantum dots, nanosheets, nanoplatelets, nanoplates, nanocubes, nanodisks, nanoparticles, nanowires,nanorods, nanopowder, nanotubes, nanoribbons, nanotetrapods, nanobelts, nanowires, nanoneedles, nanoballs, or combination thereof.

[0047] The nanocrystal solid used as infrared photon absorbing layer 104, 204 is a semiconductor with a band gap below 1 .5 eV, and / or absorbing in the near infrared or in the short-wave infrared (below 2.5 pm), or in the mid-wave infrared (3-5 pm range), or in the long-wave infrared (8-12 pm range).

[0048] The infrared absorption occurring in the nanocrystals solid may rely on interband absorption or, in another embodiment, on intraband absorption

[0049] In one embodiment, the nanocrystal solid is made of mercury chalcogenides, lead chalcogenide or silver chalcogenides. In another embodiment, the nanocrystal solid can be made of HgTe nanocrystals which size range from 3 to 20 nm and more preferably from 5 to 15 nm. In a particular embodiment, the nanocrystal solid is made of HgTe nanocrystals which size is chosen to generate an optical band edge between 1 and 5 pm.

[0050] In another embodiment, the nanocrystal solid may be made of III -V semiconductor such as InAs or InSb and their alloy.

[0051] The nanocrystal solid may be used as an infrared active layer from a diode to emit light or absorb light.

[0052] In one embodiment, the nanocrystal solid is used as infrared absorbing layer with an absorption coefficient between 100 cm'1and 105cm'1at the band edge, and more preferably between 103cm'1and 104cm'1. It may have a thickness above 100 nm and below 10 pm, and preferably between 200 nm and 1000 nm.

[0053] In one embodiment, the nanocrystal solid used as an infrared absorbing layer has a thickness chosen to generate a light resonance such as a Fabry Perot cavity, and may be capped with their native ligand, or experience a ligand exchange procedure to render the film photoconductive.

[0054] The nanocrystal solid obtained from the ligand exchange procedure may present a reduced absorption relative to the C-H bond compared to the untreated film, and in a particular embodiment, the reduction of the C-H absorption resulting from the ligand exchange may be larger than 50 % of the initial (i.e. untreated film) value, preferentially larger than 80 % and even more preferably above 90 %.

[0055] In one embodiment, the mobility of the carrier in the nanocrystal solid is above 10'4cm'2.V1s'1, preferentially larger 10'2cm'2.V1s'1and even more preferentially larger 10’1cm’2.V’1s’1.

[0056] In one embodiment, the density of nanocrystals in the resulting layer is in the range of 1016-1020cm'3and preferably in the range 1017-1019cm'3.

[0057] Further, the nanocrystals may be electrically undoped and / or have a Fermi level at least 100 meV away from the valence and conduction band.

[0058] The infrared photodetector may be integrated into a photoconductive device or a photovoltaic device.

[0059] In another one embodiment, the electrode used to extract electron (i.e., the bottom end electrode 110, 210), is made of a low work function metal that does not form amalgam with Hg, neither some alloy. It may be made of a transparent conductive oxide such as tin-doped indium oxide or fluorine-doped tin oxide or alumina-doped zinc oxide.

[0060] On the other hand, the electrode used to extract hole, i.e. the top end electrode 120, 220, may be made of a large work function metal which may be above 4.5 eV and more preferably larger 4.7 eV. It may be made of transparent conductive oxide such as tin-doped indium oxide or fluorine-doped tin oxide or alumina-doped zinc oxide, or with a 2D material such as graphene. It may have a thickness below 100 nm and may be made of a thin metal layer with a thickness below the metal skin depth, i.e. a thickness below 100 nm.

[0061] It can also be made of metallic grid, for example which geometry is further chosen to generate a resonance of the light, and in particular at a wavelength below the nanocrystal band edge, and / or chosen to generate multiple resonances at wavelengths below the nanocrystal band edge.

[0062] The spacer 106.

[0063] The spacer 106 is preferentially made of a crystal solid such as transition metal oxides, or lead chalcogenides. In particular, it can be made of PbS nanocrystal, for example with a 1 .2 eV band gap, or of a WO3layer or a CuSCN layer.

[0064] In another embodiment, the spacer 106 may be made of a semiconductor which band gap is larger than the one from the nanocrystals film used to absorb, i.e. from the infrared photon absorption layer 104, 204.

[0065] The spacer layer may be deposited with a technique which does not damage the beneath nanocrystal stack, for example a technique relying on thermal evaporation, or on spin-coating, blade coating, drop casting, dip coating.

[0066] The thickness of the spacer 106 may be smaller than 200 nm, preferentially smaller than 100 nm, and even more preferentially smaller than 80 nm.

[0067] The spacer layer is preferentially made of a p-type material. It allows for a diode behaviour of the photodetector.

[0068] The spacer 106 may induce a rectification behaviour in the current-voltage characteristic of the diode. The rectification factor goes from 1 for the initial stack (i.e., without the spacer layer) to a value preferentially larger than 2.

[0069] It may also induce an open-circuit voltage in the current-voltage characteristic of the diode. The open-circuit voltage may be larger than 1 / 5 of the optical band gap divided by the elementary charge, preferably larger than 1 / 4 of the optical band gap divided by the elementary charge and even more preferably larger than 1 / 3 of the optical band gap divided by the elementary charge.

[0070] It may induce an open-circuit voltage in the current-voltage characteristic of the diode. The open-circuit voltage may go from 0 mV for the initial stack (i.e., without spacer layer) to a value preferentially larger than 50 mV, and even more preferentially larger than 100 mV.

[0071] Bottom end electrode 110, 210: mirror (metal or dielectric) and work function

[0072] The bottom end electrode forms the top layer from a read-out integrated circuit. The lower layer 101 in metal does not form amalgam with mercury. The bottom end electrode may have a work function lower than the infrared photon absorption layer 104, 204, in particular lower than 4.4 eV.

[0073] If multiple layers are used to build the bottom end electrode 110, 210, the layer in contact with upper layer 102, 202 is chosen to match its work function in energy with an offset below 0.5 eV and more preferably below 100 meV

[0074] In one embodiment, the combination of several layers used to build the bottom electrode have thickness for each layer chosen to generate an optical cavity such as a Fabry- Perot resonator.

[0075] Top end electrode 120, 220

[0076] The top end electrode 120, 220 is built on top of the spacer 106 which is on top of the p-n junction formed by the hole transport layer 105 and the infrared photon absorption layer 104, 204.

[0077] The layer in contact with the layer beneath the top end electrode 120, 220 is chosen to match its work function in energy. If multiple layers are used to build the top end electrode 120, 220, the layer in contact with the layer underneath is chosen to match its work function in energy with an offset below 0.5 eV and more preferably below 100 meV.

[0078] The top electrode may be made of a combination of conductive layers than can be a high work function metal (above 4.5 eV) and / or transparent conductive oxide.

[0079] In one embodiment, the combination of several layers used to build the top electrode have thickness for each layer chosen to generate an optical cavity such as a Fabry-Perot resonator.

[0080] Applications: mono pixel, multi pixel, TOF LIDAR

[0081] The infrared photodetector 10, 20 might be built on a mechanical substrate that is partly transparent at the wavelength where nanocrystal absorb light and more specifically at the nanocrystal band edge energy.

[0082] A device comprising one or several infrared photodetector(s) might be built on a read-out integrated circuit, and might include multiples pixel, such as lines of pixel including from 2 to 20000 pixels, or even an array of pixel from 2x2 pixels to 20000x20000 pixels.

[0083] The read-out integrated circuit might have a VGA format, or a megapixel format, or multi megapixel format. It might be used to make imaging, or conduct time of flight (TOF) measurement, or used as part of a light detection and ranging (LIDAR) setup, or operate in photoconductive mode, or in photovoltaic mode

[0084] The read-out integrated circuit on which the device is built might rely on the application of an out plane electric field, or of an in plane electric field.

[0085] The device can be operated at room temperature, or at temperature below room temperature ranging from 4 K to 350 K and preferably from 80 K to 300 K.

[0086] On figure 4 are three different current-voltage characteristics under dark conditions (solid lines) and illumination (dashed lines) with a 1 .55 pm laser diode for a diode stack made of (a) Au / SnO2 / HgTe / Hg:Ag2Te / ITO, (b) Au / SnO2 / HgTe / Hg:Ag2Te / WO3 / ITO, and (c) Au / ITO / SnO2 / HgTe / Hg:Ag2Te / WO3 / Au / ITO. In other words, graphic representation a is for a diode design without any protection layer, graphic representation b is for a diode design with a protection layer, and graphic representation c is for a diode design with a protection layer and hybrid electrodes.

[0087] Figure 5 is a current-voltage characteristics for the optimized SWIR diode stack. The detector is illuminated at different output power with a 1 .55 pm laser diode.

[0088] Figure 6 shows a spectral response of the optimized SWIR diode stack, i.e. a photocurrent spectrum for the optimized SWIR diode.

[0089] PbS nanocrystals used as spacer

[0090] 0.9 g of PbO are introduced in a 100 mL three neck flask with 3 g of oleic acid and 47 g of octadecene. The flask is degassed under vacuum at 120°C for 2 hours. Meanwhile, in an air free glove box, a mixture of 420 pL of bis(trimethylsilyl)sulfide and 10 mL of octadecene is prepared in a 20 mL vial, then introduced into a 20 mL syringe. The atmosphere of the flask is switched to Ar and the temperature is set equal to 90°C. The bis(trimethylsilyl)sulf ide solution is quickly injected and the solution turns dark while the temperature drops to 80°C. After 8 min at 80°C, the reaction is stopped by removing the heating mantle and prompt cooling of the flask by addition of a mixture of heptane and oleic acid. The nanoparticles are then precipitated by addition of ethanol. The formed pellet is redispersed in toluene. A second step of cleaning is repeated. Finally, the pellet is redispersed in toluene with a 50 mg.nT1concentration. The solution iscentrifuged to remove any colloidally unstable material. Finally, the solution is filtered on a 0.22 gm PTFE filter.

[0091] HqTe NCs synthesis with band edge at 6000 cm~1

[0092] In a 50 mL three-neck flask, 540 mg of HgCI2 and 50 mL of oleylamine are degassed under a vacuum at 1 10°C. At this stage, the solution is yellow and clear. Meanwhile, 2 mL of trioctylphosphine telluride (TOP:Te) (1 M) is extracted from the glovebox and is mixed with 8 mL of oleylamine. The atmosphere is switched to N2, and the temperature is set at 57°C. The preheated TOP:Te solution is quickly injected, and the solution turns dark after 1 min. After 3 min, 10 mL of a mixture of 20% dodecanethiol (DDT) in toluene is injected, and a water bath is used to quickly decrease the temperature. The content of the flask is split in 4 tubes, and methanol is added. After centrifugation, the formed pellets are redispersed in one centrifuge tube with 10 mL of toluene. The solution is precipitated a second time with absolute ethanol. Again, the formed pellet is redispersed in 8 mL of toluene. At this step, the nanocrystals are centrifuged in pure toluene to remove the lamellar phase. The solid phase is discarded and the supernatant filtered.

[0093] HqTe NCs synthesis with band edge at 2000 cm1

[0094] In a 100mL three-neck flask, 543 mg of HgCI2 and 50 mL of oleylamine were degassed under vacuum at 1 10°C. Meanwhile, 2 mL of TOP:Te (1 M) was extracted from the glove box and mixed with 8 mL of oleylamine. After the flask atmosphere was switched to N2 and the temperature stabilized at 120°C, the TOP:Te solution was quickly injected. After 3 mins, 10 mL of a mixture of DDT in toluene (10% of DDT) was injected, and a water bath was used to quickly decrease the temperature. The content of the flask was split over 3 centrifuge tubes, and methanol was added. After centrifugation, the formed pellet was redispersed in one centrifuge tube with chloroform. The solution was precipitated a second time using ethanol. The formed pellet was redispersed in chlorobenzene. At this step, the nanocrystals were centrifuged in pure toluene to get rid of the lamellar phase. The solid phase was discarded.

[0095] Gold and ITO deposition

[0096] 1.1 mm thick glass substrates (30x30 mm2) are washed with acetone for 3 minutes in ultrasound bath. They are rinsed with acetone and isopropanol. Then, they are dried under an N2 flow and further physically cleaned using a 02 plasma for 5 minutes. The electrodes are patterned with optical lithography. Ti-prime is spin-coated (4,000 rpm for 30 seconds) on glass substrates and then baked at 1 10°C for 1 minute. The resist AZ5214E is spin-coated (4,000 rpm for 30 seconds) on glass substrates and then baked at 110°C for 1 minute. The substrates are exposed to UV for 1 .5 second with the adapted mask. Then, they are baked for 2 minutes at 1 10°C and then re-exposed to UV for 40 seconds without mask. Samples are developed for 20 s in AZ726MIF and then cleaned in pure water. The substrates are dried with N2and cleaned with O2plasma for 1 minute. 10 nm Cr + 40 nm Au are deposited by thermal evaporation. 40 nm ITO are then deposited by sputtering. The substrates are collected and the lift-off is done by placing them in an acetone bath for 3 to 4 minutes. The samples are washed with acetone and I PA and dried with N2. Finally, the samples are cleaned with O2plasma for 5 minutes.

[0097] SnO2deposition

[0098] 200 pL of SnO2colloidal solution in water (15%) is diluted with 1 mL of 48 mM NH4CI solution in water. This solution is denoted “highly-doped”. 200 pL of SnO2colloidal solution in water (15%) is diluted with 1 mL of 24 mM NH4CI solution in water. This solution is denoted “lightly-doped”. 100 pL of the “lightly-doped” solution is spin-coated at 4,000 rpm for 30 seconds. The same procedure is done with the solution “highly-doped”. After that, the samples are dried on a hot plate at 70°C for 1 hour.

[0099] HqTe 6k ink

[0100] In a test tube, 1 .5 mL of HgTe 6k nanocrystals (presenting a band edge at around 6000 cm'1and an O.D. = 0.9 at 400 nm diluted by 500), 600 pL of exchange solution (30 mg HgCI2+ 2 mL mercapthoethanol + 18 mL N,N- Dimethylformamide DMF and 400 pL of DMF) are added. The tube is stirred with vortex and sonicated for 3 minutes. Then, toluene is added and centrifuged for 3 minutes. The tube is emptied and the pellet is conserved. The pellet is spread on the walls of the tube by tapping it, and dried under vacuum for about 3 minutes.and centrifuged. Finally, short vortex and centrifugations are done to remove any aggregates before the spin-coating.

[0101] Aq2Te NCs synthesis

[0102] In a 25 mL three-neck flask, 34 mg of AgNO3(0.2 mM), 5 mL of oleylamine, and 0.5 mL of oleic acid are degassed at 70°C under a vacuum until the AgNO3 is completely dissolved, and the solution becomes clear. Under nitrogen, 0.5 mL of trioctylphosphine is injected into the solution. Then the temperature is raised to 160°C. At 160°C the solution becomes orange. Then, 0.1 mL of trioctylphosphine telluride (1 M) is injected into the solution, and the reaction is quenched after 10 min with a water bath. The nanocrystals are precipitated with methanol and redispersed in chlorobenzene. At this step, 500 pL of dodecanethiol is added. The washing step is repeated one more time, and finally, the nanocrystals are redispersed in hexane / octane (9:1 ) solution.

[0103] Aq2Te deposition

[0104] The solution is prepared from a Ag2Te batch. 400 pL of Ag2Te is added in a test tube with some drops of methanol. The tube is stirred and centrifuged, the supernatant is removed. 300 pL of chlorobenzene, 100 pL of dodecanethiol and some drops of methanol are added in the test tube. The tube is stirred and centrifuged, the supernatant is removed. 400 pL of chlorobenzene is added in the tube, with some drops of methanol. The tube is stirred and centrifuged, the supernatant is removed. 800 pL of a solution of hexane / octane (9:1 ) is added and the tube is stirred and sonicated for 3 minutes. The solution is filtered with a 0.22 pm PTFE filter and placed in a brown vial. Then, in the glove box, the following recipe is done on the substrates: 50 pL of the Ag2Te solution is spin-coated, a few drops of a HgBr2solution (10 mM in methanol) are spin-coated (wait 15 seconds before starting) and a few drops of IPA are spin-coated twice to wash the substrates. These steps are repeated a second time. Then, the substrates are dipped in a solution of 1 ,2 ethanedithiol in acetonitrile (1%) for 30 seconds and washed in a solution of acetonitrile for 10 seconds. The spin-coatings are done at 1 ,500 rpm for 30 seconds.

[0105] WO3 spacer deposition

[0106] A commercial solution of tungsten oxide nanoparticle ink (WO3-x, solid content 2.4 - 2.6%, Sigma-Aldrich) is filtered through a 0.22 pm PTFE filter. Then, 40 pL of this solution is spin-coated on the samples in a glove box at 2,500 rpm for 30 seconds. The speed can be adjusted to modify the thickness of the layer.

[0107] PbS spacer deposition

[0108] PbS nanocrystals solution in toluene is spin-coated on top of the diode stack at a controlled speed to obtain a 30-50 nm thin film. Then, the latter is dipped in a solution of 1% in mass of ethanedithiol in acetonitrile for 30 s and washed in acetonitrile. Once the film is dried, a second layer of PbS nanocrystals is deposited and the ligand exchange step is repeated.

[0109] ITO deposition

[0110] A thin layer of gold (5 nm) with the shadow mask is deposited by thermal evaporation prior to ITO. Then, 50 nm of ITO is deposited by sputtering at a working pressure of 4x1 O'3mbar, a power of 1 W / cm2, under 40 seem of Argon gas. Those deposition are performed with a shadow mask.

[0111] Optimized SWIR diode fabrication.

[0112] The sample is an array of gold pixels on a Si / SiO2substrate. It is initially cleaned with IPA (isopropanol), dried with a N2gun, and followed by a 5 min exposure to an O2plasma. Then, an adhesion primer (TI-PRIME) is spin-coated onto the substrate and annealed for 1 min at 110°C before AZ5214E photo-resist is spin-coated and baked at 110°C for 1 min. A MJB4 mask-aligner is used to expose the sample to UV light for 1.5 s through a lithography mask. Then, the sample is baked 2 min at 110°C to invert the resist and flood-exposed for 40 s. The sample is dipped 30 s in a solution of AZ726 developer before being rinsed in pure water for 15 s. The patterned substrate is dried and cleaned with 1 min of oxygen plasma to remove resist residues. A 50 nm layer of ITO is deposited by magnetron sputtering. Finally, lift-off is performed in an acetone bath for 10 minutes, then rinsed with IPA, and exposed to a final 5 mins O2plasma. The electron transport layer of SnO2, prepared in both “lightly-doped” and “highly- doped” forms, are spin-coated onto the sample as described previously. The active layer of HgTe nanocrystals (8 nm size), from a HgTe ink, is then depositedto reach a thickness that can be controlled between 100-500 nm and is left to dry under vacuum for 2 hours. After, the hole transport layer of Ag2Te is deposited as previously outlined. A WO3spacer layer is spin-coated onto the sample at a speed of 2500 rpm for 30 seconds. Using a shadow mask to define the pixel array, a 5 nm layer of gold is thermally evaporated, followed by the deposition of a 50 nm layer of ITO via magnetron sputtering to complete the fabrication.

[0113] Optimized MWIR diode fabrication

[0114] The sample is an array of gold pixels on a Si / SiO2substrate. It is initially cleaned with IPA (isopropanol), dried with a N2gun, and followed by a 5 min exposure to an O2plasma. Then, an adhesion primer (TI-PRIME) is spin-coated onto the substrate and annealed for 1 min at 110°C before AZ5214E photo-resist is spin-coated and baked at 110°C for 1 min. A MJB4 mask-aligner is used to expose the sample to UV light for 1 .5 s through a lithography mask. Then the sample is baked 2 min at 110°C to invert the resist and flood-exposed for 40 s. The sample is dipped 30 s in a solution of AZ726 developer before being rinsed in pure water for 15 s. The patterned substrate is dried and cleaned with 1 min of oxygen plasma to remove resist residues. A 50 nm layer of AZO is deposited by magnetron sputtering. Finally, lift-off is performed in an acetone bath for 10 minutes, then rinsed with IPA, and exposed to a final 5 mins O2plasma. The active layer of HgTe nanocrystals (12 nm size), from a HgTe ink, is then deposited to reach a thickness that can be controlled between 100-500 nm and is left to dry under vacuum for 2 hours. After, the hole transport layer of Ag2Te is deposited as previously outlined. A WO3spacer layer is spin-coated onto the sample at a speed of 2500 rpm for 30 seconds. Using a shadow mask to define the pixel array, a 5 nm layer of gold is thermally evaporated, followed by the deposition of a 50 nm layer of AZO via magnetron sputtering to complete the fabrication.

Claims

Claims

1. Infrared photodetector (10, 20) comprising a bottom end electrode (110, 210) for collecting electrons, a top end electrode (120, 220) for collecting holes, a hole transport layer (105), and an infrared photon absorption layer (104, 204) for generating an electrical signal, the infrared photon absorption layer (104, 204) being positioned between bottom end electrode (110, 210) and the hole transport layer (105), the hole transport layer (105) being positioned between the top end electrode (120, 220) and the infrared photon absorption layer (104, 204), and the top end electrode (120, 220) comprising an outer layer (108, 208) made of transparent oxide conductor and an inner layer (107) made of a thin layer of metal positioned between the outer layer (108, 208) and the hole transport layer (105), characterized in that the infrared photodetector (10, 20) further comprises a spacer (106) positioned between the inner layer (107) of the top end electrode (120, 220) and the hole transport layer (105), said spacer (106) being optically transparent in a spectral range between 1 to 5 pm and having a band gap larger than a band gap of the infrared photon absorption layer (104, 204).

2. Infrared photodetector (10, 20) according to claim 1, wherein the hole transport layer (105) and the infrared photon absorption layer (104, 204) comprise nanocrystals of a compound selected from HgS, HgSe, HgTe, PbS, PbSe, PbTe, Ag2S, Ag2Se, Ag2Te, InAs, InGaAs and InSb and mixture thereof, and heterostructure thereof and alloy thereof.

3. Infrared photodetector (10, 20) according to claim 2, wherein the infrared photon absorption layer (104, 204) comprises HgTe nanocrystals.

4. Infrared photodetector (10, 20) according to any of claims 2 or 3, wherein the hole transport layer (105) comprises Ag2Te nanocrystals.

5. Infrared photodetector (1) according to any of claims 1 to 4, further comprising a silicium or silica substrate (100), said bottom end electrode (110, 210) being positioned between said silicium or silica substrate (100) and said infrared photon absorption layer (104, 204).

6. Infrared photodetector (10, 20) according to any of claims 1 to 5, wherein the bottom end electrode (110) comprises a lower layer (101) made of a thin layer of metal and an upper layer (102, 202) made of transparent oxide conductor made of Alumina-doped zinc oxide (AZO) for mid infrared wavelengths or Tin-doped Indium Oxide (ITO) for near infrared wavelengths, said upper layer (102, 202) being positioned between the lower layer (101) and the infrared photon absorption layer (104).

7. Infrared photodetector (10, 20) according to claim 6, wherein the lower layer (101) of the bottom end electrode (110, 210) has a thickness between 50 and 100 nm.

8. Infrared photodetector (10, 20) according to any of claims 1 to7, wherein the spacer (106) has a thickness comprised between 1 nm and 1 pm, and preferably between 10 nm and 100 nm.

9. Infrared photodetector (10, 20) according to any of claims 1 to8, wherein the inner layer (107) of the top end electrode (120, 220) is made of gold, platinum or palladium.

10. Infrared photodetector (10, 20) according to any of claims 1 to9, wherein the infrared photon absorption layer (104, 204) is made of HgTe nanocrystals with an optical band edge between 1 and 5 pm.

11. Infrared photodetector (10, 20) according to any of claims 1 to10, wherein the infrared photon absorption layer (104, 204) has a thickness 200 and 1000 nm, the hole transport layer (105) has a thickness between 50 and 100 nm, and the inner layer (107) of the top end electrode (120, 220) has a thickness between 5 and 10 nm.

12. Infrared photodetector (10, 20) according to any of claims 1 to11, wherein the spacer (106) has a thickness less than 200 nm and is made of a p-type material.

13. Infrared photodetector (10) according to any of claims 1 to 12, further comprising an electron transport layer (103) with nanocrystals of a compound selected from SnCh, ZnO, CdS, CdSe, AZO, CuO, CuO2, CU2O3, ZrO2, and mixture thereof and heterostructure thereof and alloy thereof, the electron transport layer (103) being positioned between the bottom end electrode (110) and the infrared photon absorption layer (104).

14. Infrared photodetector (20) according to any of claims 1 to 12, wherein the outer layer of the contact layer is made of Al doped ZnO (AZO).

15. Infrared photodetector (1) according to any of claims 1 to 14, wherein the photodetector is a photodiode.

16. Imaging device, characterized in that it comprises a plurality of photodetectors (1) according to any one of claims 1 to 15, configured to form an image, and coupled to a readout circuit.

17. Camera device, characterized in that it comprises a plurality of photodetectors (1) according to any one of claims 1 to 15, configured to form a camera, and coupled to an electronic system, an optical system, and a temperature control system.

Images