Charlie Gréboval

Eva Izquierdo

Claire Abadie

Adrien Khalili

Mariarosa Cavallo

Audrey Chu

Tung Huu Dang

Huichen Zhang

Xavier Lafosse

Michael Rosticher

Xiang Zhen Xu

Armel Decamps-Mandine

Abdelkarim Ouerghi

Mathieu G Silly

Sandrine Ithurria

Emmanuel Lhuillier

HgTe Nanocrystal-Based Photodiode for Extended Short Wave Infrared Sensing with Optimized Electron Extraction and Injection

Keywords: short wave infrared, detection, HgTe, nanocrystals, photodiode

Thanks to their narrow band gap nature and fairly high carrier mobility, HgTe nanocrystals are of utmost interest for optoelectronics beyond the telecom window (λ>1.55 µm). In particular, they offer an interesting cost-effective alternative to the well-developed InGaAs technology. However, in contrast to PbS, far less work has been dedicated to the integrating this material in photodiodes. In the short-wave infrared, HgTe NCs have a more p-type character than in the mid wave infrared, thus promoting the development of new electron transport layers with an optimized band alignment. As for perovskites, HgTe NCs present a fairly deep band gap with respect to vacuum. Thus, we were motivated by the strategy developed for perovskite solar cells, for which SnO2 has led to the best performing devices. Here, we explore the following stack made of SnO2/HgTe/Ag2Te in which the SnO2 and Ag2Te layers behave as electron and hole extractors respectively. Using X-ray photoemission, we show that SnO2 presents a nearly optimal band alignment with HgTe to efficiently filter the hole dark current while letting the photoelectrons flow. The obtained I-V curve exhibits an increased rectifying behavior, and the diode stack presents a high internal efficiency for the diode (above 60%) and an external quantum efficiency that is mostly limited by the absorption magnitude. Furthermore, we tackle a crucial challenge for the transfer of such diode onto readout circuits which prevents back-side illumination. We also demonstrate that the diode stack is reversible with a partly transparent conducting electrode on top while preserving the device's responsivity. Finally, we show that such SnO2 layer is also beneficial for electron injection and leads to enhanced electroluminescent signal as the diode is operated under forward bias. This work is an essential step toward the design of a focal plane array with an HgTe NCs-based photodiode.

INTRODUCTION

Thanks to their cost-effective fabrication, nanocrystals (NCs) [START_REF] Rogalski | Progress in Quantum Dot Infrared Photodetectors[END_REF] raise interest in the design of optoelectronic devices. This is especially true in the spectral range where silicon cannot be used and for which alternative materials, generally epitaxially grown semiconductors, have to be integrated. This is typically the case of infrared, a spectral range where the device cost is prohibitive for many applications [START_REF] Hafiz | Colloidal Quantum Dots for Thermal Infrared Sensing and Imaging[END_REF] . Over the past decade, significant progresses have been made on integrating NCs at the focal plane array (FPA) level. [START_REF] Pejović | Infrared Colloidal Quantum Dot Image Sensors[END_REF][START_REF] Chu | HgTe Nanocrystals for SWIR Detection and Their Integration up to the Focal Plane Array[END_REF][START_REF] Buurma | MWIR Imaging with Low Cost Colloidal Quantum Dot Films[END_REF][START_REF] Bossavit | Optimized Infrared LED and Its Use in an All-HgTe Nanocrystal-Based Active Imaging Setup[END_REF][START_REF] Chatterjee | Uncooled Mid-Wave Infrared Focal Plane Array Using Band Gap Engineered Mercury Cadmium Telluride Quantum Dot Coated Silicon ROIC[END_REF] Lead sulfide NCs have reached the global market, but the spectral range of this material mostly overlaps that of existing InGaAs technology. HgTe nanocrystals [START_REF] Gréboval | Mercury Chalcogenide Quantum Dots: Material Perspective for Device Integration[END_REF][START_REF] Zhang | Advances of Sensitive Infrared Detectors with HgTe Colloidal Quantum Dots[END_REF][START_REF] Liu | Synthesis of HgTe Colloidal Quantum Dots for Infrared Photodetector[END_REF][START_REF] Grotevent | Colloidal HgTe Quantum Dot/Graphene Phototransistor with a Spectral Sensitivity Beyond 3 µm[END_REF] , with their even narrower band gap, open up a new degree of freedom to explore longer wavelengths [START_REF] Zhang | Spray-Stencil Lithography Enabled Large-Scale Fabrication of Multispectral Colloidal Quantum-Dot Infrared Detectors[END_REF] , at least up to mid-wave infrared and possibly even further in the THz range. 13- 15 However, HgTe-based devices are still in their early stage of development. In particular, there are only a few studies dedicated to photodiode structures that have been reported. [START_REF] Nam | Near-Infrared-Sensitive Bulk Heterojunction Solar Cells Using Nanostructured Hybrid Composites of HgTe Quantum Dots and a Low-Bandgap Polymer[END_REF][START_REF] Günes | Hybrid Solar Cells Using HgTe Nanocrystals and Nanoporous TiO2 Electrodes[END_REF][START_REF] Ackerman | Fast and Sensitive Colloidal Quantum Dot Mid-Wave Infrared Photodetectors[END_REF] The Heiss' group proposed a diode stack inspired by solar cells where the absorbing HgTe layer is coupled to a TiO2 electron transport layer (ETL). [START_REF] Günes | Hybrid Solar Cells Using HgTe Nanocrystals and Nanoporous TiO2 Electrodes[END_REF] The resulting device presents a rectifying I-V curve, but the photocurrent was modest because of the poor band alignment between the two materials, so that TiO2 also filters the photoelectrons, as later shown by Jagtap et al. [START_REF] Jagtap | Short Wave Infrared Devices Based on HgTe Nanocrystals with Air Stable Performances[END_REF][START_REF] Jagtap | Design of Unipolar Barrier for Nanocrystal Based Short Wave Infrared Photodiode[END_REF] In the mid-wave infrared (MWIR), Ackerman et al. [START_REF] Ackerman | Fast and Sensitive Colloidal Quantum Dot Mid-Wave Infrared Photodetectors[END_REF] introduced a hole transport layer based on a Hg cation exchanged layer of Ag2Te NC. The efficiency of this layer in the MWIR is related to the n-type nature of HgTe NCs in this spectral range. As a result, an efficient pn junction is formed at the HgTe/Ag2Te interface. When shorter wavelengths are targeted, HgTe acquires a more p-type [START_REF] Jagtap | Design of Unipolar Barrier for Nanocrystal Based Short Wave Infrared Photodiode[END_REF] character, and thus the HgTe-Ag2Te junction is mostly a p-p' junction. This leads to a drop in performance for wavelengths below 2.5 µm [START_REF] Ackerman | Colloidal Quantum Dot Photodiodes for Extended Short-Wave Infrared Detection[END_REF] (short wave infrared or SWIR range). A possible strategy is to introduce a gate at the level of the electron extracting electrode [START_REF] Gréboval | Gate Tunable Vertical Geometry Phototransistor Based on Infrared HgTe Nanocrystals[END_REF] , but such an approach is technologically demanding and difficult to implement at the FPA level. Tang et al. proposed to use Bi2Se3 flakes as an electron transport layer [START_REF] Tang | Towards Infrared Electronic Eyes: Flexible Colloidal Quantum Dot Photovoltaic Detectors Enhanced by Resonant Cavity[END_REF] ; however, the large lateral extension of the flakes makes it difficult to build a pinhole-free film from this material. Recently Khalili et al have proposed an original diode design based on HgTe NCs coupled to epitaxially grown InGaAs wires used as electron extractor, however this approach loses the benefit of the solution processability. [START_REF] Khalili | Colloidal II-VI-Epitaxial III-V Heterostructure: A Strategy to Expand InGaAs Spectral Response[END_REF] It is important to stress that all reported diode stacks terminate with a metallic mirror. While such strategy is compatible with back-side illumination, it is incompatible with the presence of a readout integrated circuit (ROIC). Thus, it is of utmost interest to develop a new carrier extraction layer that matches the constraints raised by narrow band gap materials and enables a reversed fabrication. To design an efficient electron transport layer (ETL) that aims to be coupled with a SWIR absorbing HgTe NCs, the following constraints have to be fulfilled: (i) the conduction band has to be resonant with the one of the absorbing layer, (ii) the valence band has to be strongly offset to prevent hole dark current injection. Both constraints imply that the band gap of this layer has to be significantly larger than the one of the absorbing layer, (iii) the material has to be n-type to prevent further hole injection, (iv) the material should be processable at a fairly low temperature to avoid any sintering of the HgTe NCs. The latter process will shift the cut-off wavelength while increasing the dark current. Lastly, (v) the top contact has to be (partly) transparent to be ROIC compatible.

Infrared absorbing mercury chalcogenides NCs 8 combine a narrow band gap nature with a fairly large work function of the bulk material. [START_REF] Nethercot | Prediction of Fermi Energies and Photoelectric Thresholds Based on Electronegativity Concepts[END_REF] This makes the conduction band fairly deep with respect to the vacuum level. This observation explains why TiO2 is a sub-optimal ETL for HgTe NC-based diodes. Interestingly, the same issue is faced by perovskite materials used in solar cells. As a result, a large interest has been focused on tin oxide. [START_REF] Ye | Perovskite Solar Cells: Doped Bilayer Tin(IV) Oxide Electron Transport Layer for High Open-Circuit Voltage Planar Perovskite Solar Cells with Reduced Hysteresis[END_REF][START_REF] Yoo | Efficient Perovskite Solar Cells via Improved Carrier Management[END_REF][START_REF] Yi | Bilayer SnO2 as Electron Transport Layer for Highly Efficient Perovskite Solar Cells[END_REF][START_REF] Zhu | Simultaneous Contact and Grain-Boundary Passivation in Planar Perovskite Solar Cells Using SnO2-KCl Composite Electron Transport Layer[END_REF][START_REF] Chen | SnO2-Based Electron Transporting Layer Materials for Perovskite Solar Cells: A Review of Recent Progress[END_REF][START_REF] Jiang | 2: A Wonderful Electron Transport Layer for Perovskite Solar Cells[END_REF] SnO2 presents a deeper band gap with respect to the vacuum level compared to other oxide materials such as ZnO or TiO2. In this paper, we study the potential of SnO2 as ETL coupled to HgTe NCs for SWIR sensing. We demonstrate, using photoemission, that SnO2 presents a nearly optimal band alignment with HgTe with a resonant conduction band. We then conduct a deep structural investigation of the diode stack and identify the optimal thickness to maximize the absorption. An interesting feature of this diode is the lack of 1/f noise [START_REF] Liu | 1/f Noise in Semiconductor and Metal Nanocrystal Solids[END_REF][START_REF] Lai | Low-Frequency (1/f) Noise in Nanocrystal Field-Effect Transistors[END_REF] , which is generally prevailing in most devices based on NCs. In the final part of the paper, we demonstrate that the diode stack is compatible with a reversed contact while maintaining most of the performance. This result is an important step toward the transfer of the diode onto focal plane arrays. Finally, we also text the benefit of this ETL for charge injection as the diode is used as a light emitting diode and show an increase of the relative EQE of almost a factor 10.

DISCUSSION AND RESULTS

To study the potential of SnO2 as an electron transport layer coupled to HgTe NCs absorbing in the short-wave infrared, we start by fabricating a diode based on the following stack glass/FTO/doped SnO2/HgTe/Ag2Te/gold, where FTO stands for fluorine-doped tin oxide. This layer is a conventional transparent conductive oxide prepared by chemical vapor deposition. A moderate thickness of 80 nm is chosen to avoid excessive absorption in the short-wave infrared range.

The absorbing layer is made of HgTe NCs grown using the Keuleyan's procedure. [START_REF] Keuleyan | Synthesis of Colloidal HgTe Quantum Dots for Narrow Mid-IR Emission and Detection[END_REF] Transmission electron microscopy (TEM) can be seen in Figure 1h, and it shows a branched aspect for the particles. The HgTe NCs present a band edge at 1.7 µm (0.72 eV) according to absorption and photoluminescence spectra (Figure 1e). Following ligand exchange and thin-film processing, the band edge will shift to bring the cut-off wavelength around 2 µm, as later discussed. The top part of the diode is made of Hg cation exchanged Ag2Te NCs behaving as a hole extracting layer as proposed by Ackerman et al. [START_REF] Ackerman | Fast and Sensitive Colloidal Quantum Dot Mid-Wave Infrared Photodetectors[END_REF] The 8-10 nm Ag2Te NCs (see TEM in Figure 1g) also present a narrow band gap nature and their infrared absorption spectrum displays a cut-off around 3 µm, see Figure 1d. For the ETL layer, we use commercially available SnO2 particles (see Figure 1i for TEM) that are later doped on the surface using NH4Cl. The formation of this layer follows the one developed for perovskites solar cells by the Reiss' group. [START_REF] Ye | Perovskite Solar Cells: Doped Bilayer Tin(IV) Oxide Electron Transport Layer for High Open-Circuit Voltage Planar Perovskite Solar Cells with Reduced Hysteresis[END_REF] Their absorption spectrum shows a cutoff wavelength at around 300 nm corresponding to a band gap of 4.07 eV, see Figure 1f and S6. This layer is only annealed at 70 °C to dry the water used as a solvent. TEM and energy dispersive X-ray spectroscopy on the side of the diode enable a clear identification of the different layers, see Figure 1a-c. While Sn, Hg, and Au are forming well-identified layers, with no noticeable interdiffusion, the Cl (Figure 1c) is located all over the structure, consistent with its use as a capping ligand for both SnO2 and HgTe layers. More surprisingly, and in spite of the thin character of the Ag2Te layer (20-30 nm), the EDX profile suggests that Ag atoms diffuse all over the film thickness, as suggested by Ackerman et al. The benefit for this specific diode stack, whose schematic is shown in Figure 2b and S5, is better revealed by determining its band alignment throughout X-ray photoemission, see Figure S2-3 for photoemission measurements and Figure 2a for the diode band structure. Interestingly, the band alignment is close to the targeted one with a quasi-resonant conduction band between doped SnO2 and HgTe NCs, while the wide band gap of SnO2 ensures a large offset of the valence band and thus an efficient dark hole current filtering. We also checked using field-effect-transistor measurements (Figure S4) that the SnO2 layer only presents electron conduction.

To further understand the value of the ETL layer, we fabricated two diodes, one with and one without. Their I-V characteristics are given in Figure 2d andc, respectively. In the absence of the ETL layer, the dark I-V curve is barely rectifying at room temperature, in particular in the case of thick HgTe layer. However, under illumination, a clear open-circuit voltage can be observed, see Figure 2c. Note, that the curve presents some hysteresis that is likely due to the presence of free ions coming from the ligand exchange procedure. In the presence of the SnO2 layer (Figure 2d), the I-V curve displays a genuine rectifying behavior, with two orders of magnitude for the current asymmetry between forward and reverse operation. We observe As a result, the photoinduced current modulation is drastically increased by two orders of magnitude for the zero-bias operation of the diode.

Figure 3 Absorption within the FTO/doped:SnO2/HgTe/Ag2Te/Au stack. a. Simulated absorption map in NCs only (i.e., excluding losses in contacts) as a function of the wavelength and thickness of the HgTe layer. b. Dispersion map (absorption as a function of in plane wavevector and total wavevector) for the diode stack for a 550 nm thickness of the HgTe layer. c. Absorption spectra of the diode for various thicknesses of the HgTe layer. d. Spatial map of the absorption of the diode stack for a 550 nm thickness of the HgTe layer

Beyond the band alignment, the device absorption is another parameter driving the diode stack performance through the control of the layer thickness. Using rigorous coupled-wave analysis (RCWA), we simulate the device absorption properties. The inputs of these simulations are the complex optical index of each layer measured by ellipsometry, see Figure S9. Figure 3a shows the absorption spectra for various thicknesses of the HgTe layer. Note that in this graph, we only simulate the absorption occurring in the NC (i.e., excluding the losses in metals) since only this contribution leads to the generation of photocurrent. We distinguish at λ=1.7 µm a stronger absorbing feature that can be associated with the band edge of the HgTe NCs. We also observe a series of tilted lines corresponding to various resonances of the absorption. The nature of these resonances is better revealed by the dispersion map, see Figure 3b. All features appear dispersionless which is consistent with the formation of a Fabry-Pérot cavity. This mode leads to the formation of standing waves in the NC film, as shown on the absorption map, see Figure 3d. However, obtaining a significant absorption (above 50% at the band edge) requires the formation of fairly thick films above 500 nm for the HgTe layer, see Figure 3a andc. From a previous study, it has been demonstrated that such thick films are difficult to fabricate and may not lead to the best performance (in particular, high VOC) due to the presence of pinholes. For a more typical thickness of 250 nm (see Figure 1a), the absorption is more in the 30-35% range, which is still higher than the value obtained for a free-standing layer of HgTe NC with the same thickness. This depicts the cavity effect resulting from the diode stack. Figure 4 shows the performance of this diode. In Figure 4a, the spectral response has been measured and compared with the one of epitaxially matched InGaAs grown on InP. The diode presents a 2 µm cut-off wavelength (1.75 µm peak response), which genuinely expands the response toward longer wavelengths with respect to InGaAs. Under blackbody illumination (i.e., broadband illumination), the measured response is in the range from 0.2 to 0.5 A.W -1 , with the best performing device exhibiting a response of 0.8 A.W -1 , see the inset of Figure 4b and S7. The typical external quantum efficiency is in the 20-30% range. Since the absorption is ≈1/3, this diode stack presents a high internal efficiency of 60-90 %, consistent with the determined diode band alignment. The response time of the diode (from exponential fit of the decay part of the curve) has been measured to be around 300 ns, see Figure 4c. The latter can be reduced to 200 ns under the application of a moderate reverse bias, see the inset of Figure 4c andS8.

A striking feature of this diode is the lack of 1/f noise even at moderate signal frequency, see Figure 4b. The detectivity at room temperature and under 0 V operation falls in the 3-4 x10 10 Jones (inset of Figure 4b, which is fairly competitive without being the best for HgTe-based diodes operating at this wavelength. Likely the device absorption is becoming a clear limitation to the diode performance, and introducing a light management strategy [START_REF] Zhu | Integrated Plasmonic Infrared Photodetector Based on Colloidal HgTe Quantum Dots[END_REF][START_REF] Chen | Photocurrent Enhancement of HgTe Quantum Dot Photodiodes by Plasmonic Gold Nanorod Structures[END_REF][START_REF] Chen | Integration of Colloidal Quantum Dots with Photonic Structures for Optoelectronic and Optical Devices[END_REF][START_REF] Tang | Thermal Imaging with Plasmon Resonance Enhanced HgTe Colloidal Quantum Dot Photovoltaic Devices[END_REF][START_REF] Dang | Bias Tunable Spectral Response of Nanocrystal Array in a Plasmonic Cavity[END_REF][START_REF] Gréboval | Ferroelectric Gating of Narrow Band-Gap Nanocrystal Arrays with Enhanced Light-Matter Coupling[END_REF] would be a realistic next step to this work. A complete comparison of the diode performance is given in Table 1. At this stage, the use of a top gold electrode still prevents the transfer of such a diode to a focal plane array. This is why we test the robustness of this diode stack toward a reversed fabrication where the metallic mirror is on the substrate while the top contact is semi-transparent. We fabricated the following diode stack glass/gold/SnO2/HgTe/Ag2Te/gold/ITO, see Figure 5a for a schematic of the diode. Though not so usual in silicon foundries, gold can easily be deposited as a top layer onto a ROIC and presents the advantage of a high thermal and chemical stability. The attempt to replace this layer with a lower work function such as silver leads to a layer without a shiny reflective aspect due to transformation of Ag to silver chalcogenides in the presence of HgTe NCs. The active part of the diode remains the same as the one depicted in the first part of this paper. The top contact is made of a very thin (few nm) layer of gold used for its appropriate work function. This layer is then further coated by ITO (50-60 nm) deposited by sputtering. The layer thickness is chosen so that the sheet resistance of the contact remains low while ensuring a good IR transparency. In this configuration, the I-V curve remains rectifying, and the open-circuit voltage presents the same magnitude as the non-reversed diode, see Figure 5b andc. The responsivity of the diode in the reversed configuration is slightly reduced to the 0.1-0.2 A.W -1 range. To finish we have also tested the benefit of introducing this SnO2 layer as electron injector while the diode is operated under forward bias to generate an electroluminescent signal. To confirm the presence of an electroluminescent signal we have image the LED under operation through a high pass filter that transmits photons above 1400 nm using an InGaAs camera (cut off wavelength 1.65 µm). As the diode is turned-on, we see a bright signal coming from the pixel, see Figure 6c andd.

In this regime of operation, the I-V curve of the LED with and without the SnO2 layer almost superimposed, see Figure 6a. However, the LED with the SnO2 layer is typically 10 times brighter under low current, see Figure 6b.

CONCLUSION

We demonstrate that SnO2 is a well-suited electron extraction layer for HgTe NCs absorbing in the short-wave infrared. SnO2 and HgTe present a nearly optimal band alignment that ensures an efficient filtering of the hole dark current while letting the photoelectrons flow. The resulting I-V curve presents an enhanced rectifying behavior including at room temperature. This band alignment enables a high internal efficiency in the 60 to 90% range. The diode performance is now limited by the diode absorption that will likely require the introduction of a light management strategy, although the device absorption is already increased compared to the free-standing layer. Another striking benefit of this diode stack is the absence of 1/f noise which will limit the drift of the performance over time. We also have demonstrated that the diode is sufficiently robust to be reversed and that the performance is maintained while the diode stack is coupled with a partly transparent electrode. The next challenge is to transfer this structure onto a focal plane array to design a low-noise infrared imaging camera. Last, we have also shown that the introduction of the SnO2 layer is benefitial to increase the electroluminescence signal.

Methods

Chemicals: Mercury chloride (HgCl2, Sigma-Aldrich, 99%), Mercury compounds are highly toxic. Handle them with special care. Tellurium powder (Te, Sigma-Aldrich, 99.99%), silver nitrate (Alfa Aesar, 99.9%), SnO2 nanoparticles (15% dispersion in water, Alfa Aesar), trioctylphosphine (TOP, Alfa Aesar, 90%), oleylamine (OLA, Acros, 80-90%), dodecanethiol (DDT, Sigma-Aldrich, 98%), ammonium chloride (Alfa Aesar, 98 %) ethanol absolute (VWR), methanol (VWR, >98%), isopropanol (IPA, VWR), hexane (VWR, 99%), octane (Carlo erba, 99%), 2-mercaptoethanol (MPOH, Merck, >99%), N,N dimethylformamide (DMF, VWR), N-methylformamide (NMF, Alfa Aesar, 99%), toluene (VWR, 99.8%). All chemicals are used without further purification except oleylamine that is centrifuged before use.

M TOP:Te precursor:

In a three neck flask, we have mixed Tellurium powder (2.54 g) in 20 mL of trioctylphosphine. The flask is degassed under sub mbar pressure at room temperature for 5 min and then the temperature is increased to 100 °C. The degassing of the flask is conducted for 20 additional minutes. The atmosphere is switched to nitrogen and the temperature is raised to 275 °C.

The solution turns to a clear orange coloration. The heating mantle is removed and the temperature of the flask cools down. Its color changes to yellow. Finally, this solution TOPTe is used as stock solution and stored into a glove box for storage.

HgTe NCs synthesis with band-edge at 6000 cm -1 : In a 50 mL three-neck flask, we have mixed 540 mg of HgCl2 salt together with 50 mL of oleylamine. The flask is then degassed under reduced pressure at 110 °C. The solution is yellow and clear. Then, 2 mL of TOP:Te (1 M) are extracted from the glove box and are mixed with 8 mL of oleylamine. The atmosphere is replaced by nitrogen and the temperature tuned to 57 °C. The pre-heated TOP:Te solution is injected in the three neck flask and the solution turns dark after 1 min. After 3 min, we inject 10 mL of a mixture of 10% DDT in toluene and a water bath is used to prompt the drop of temperature. The content of the flask is split in 4 tubes and MeOH is added. After centrifugation, the pellets are redispersed using 10 mL of toluene in a single centrifuge tube. The solution is precipitated again using absolute EtOH. The formed pellet is redispersed in 8 mL of toluene. At this step, the NCs are centrifuged in fresh toluene to remove the lamellar phase. The solid phase is discarded by centrifugation and the supernatant filtrated.

Ag2Te CQD synthesis: In a 50 mL three neck flask, 170 mg of AgNO3 are mixed together with 25 mL of oleylamine and 2.5 mL of oleic acid. The flask is then degassed under vacuum at room temperature for 15 min and then at 70 °C for 10min. The atmosphere is replaced to nitrogen and 2.5 mL of TOP are added into the solution. Then, the temperature of the flask is raised to 160 °C. After 30 min, the solution becomes orange. At this step, 0.5 mL of TOP:Te (1M) is injected and the reaction is quenched after 10 min with a water bath. The crude solution was stored in a freezer at -20 °C. When needed, 800 µL of the unfrozen crude solution are precipitated with MeOH. After centrifugation the formed pellet is redispersed in a mixture containing 200 µL DDT and 600 µL of chlorobenzene. Then, the NCs are precipitated by addition of methanol. After centrifugation the pellet is dispersed using 800 µL of chlorobenzene. Again, the NCs are precipitated by addition of MeOH and centrifugation. The pellet is this time dispersed in 1.6 mL of a (9:1 volume ratio) mixture of hexane:octane.

SnO2:

The pristine solution (SnO2-P) is obtained by diluting 400 µL of commercial solution with 0.8 mL of distilled water. For lightly doped SnO2 (SnO2-L), we dilute 200 µL of commercial solution with 1 mL of 24 mM NH4Cl solution in water. For heavily doped SnO2 (SnO2-H), we dilute 200 µL of commercial solution with 1 mL of 48 mM NH4Cl solution in water.

Infrared spectroscopy: we use a Thermo-Fischer iS50 in attenuated total reflection (ATR) mode. The spectra are averaged over 32 times and acquired with a 4 cm -1 resolution. To measure photocurrent spectrum, a Femto DLPCA 200 current amplifier, is used to bias and magnify the output current from the sample. The output signal is then fed into the FTIR electronics.

Transmission electron microscopy on NCs:

A dilute NC solution is drop-casted onto a copper grid covered with an amorphous carbon film. This grid is degassed under secondary vacuum to reduce future contamination. A JEOL 2010F microscope, operated at 200 kV, is used for acquisition of pictures.

Electronic microscopy image on device:

The TEM slice is prepared using a FEI Helios NanoLab600i dual beam SEM FIB fitted with an EasyLift micromanipulator for in situ lift-out. The ion column is operated at 30 kV for all the steps, except for final cleaning of the sample, for which tensions of 5 kV and 2 kV are used. The beam current varies in the 15 pA to 47 nA range. The electron beam is used to deposit a 20 nm thick carbon layer over an area of 20 μm x 2 μm. The ion beam is then used to deposit a 3 μm thick layer Pt over the same area. The slice is partially cut out and a Pt needle is fixed. The sample is lifted out in situ and then attached to a copper support grid. Thinning of the sample can then be processed using a gradual cross-section cleaning milling pattern, with a final milling beam current of 80 pA. Finally, the 5 kV and 2 kV cleaning steps are carried out on each side. The TEM images are performed with a JEOL probe corrected ARM200F TEM. The gun is a cold FEG used with a high voltage of 200kV. The slice is observed with a Gatan ultrascan 2kx2k Camera in TEM mode. The EDS spectrum and mapping used a CENTURIO-X SDD from JEOL. All the STEM images are done using BF and HAADF STEM detectors.

HgTe ink:

We dissolve 30 mg of HgCl2 salt in a solution of 2 mL of mercaptoethanol and 18 mL of DMF. This sokution is called exchange solution in the following of this paragraph. The solution of HgTe NCs is processed to obtain ligand exchange. At each step, the solution is stirred with a vortex and sonicated. 900 µL of HgTe NCs solution are mixed with 1 mL of the exchange solution. 5mL of hexane and 2 mL of DMF are added to the solution. The top phase is discarded, and the bottom solution is washed twice more adding 5 mL of hexane each time and removing the top phase every time. Then, toluene is added up to induce precipitation and the solution is centrifugated for 2 min at 6000 rpm. The supernatant is discarded and the formed pellet is dried under nitrogen flow. The dry pellet is then redispersed in 175 µL of DMF to reach a thickness of about 100 nm. This solution is called ink. Before ink deposition, the substrate is placed in oxygen plasma cleaner for 2 min to promote adhesion. The ink is spin-coated on the substrate at 4000 rpm (at 2000 rpm.s -1 acceleration) during 5 minutes. This gives a homogeneous deposit between 100 nm and 200 nm.

FTO etching:

The surface of FTO coated (80 nm) glass (1.1 mm thick) substrate, purchased from Solems, is cleaned by sonication within an acetone bath. The substrate is rinsed with acetone, then isopropanol and dried using N2 flow. A final cleaning is made using an O2 plasma for 5 minutes. An adhesion primoter (Ti PRIME) is spin-coated on the cleaned substrate and baked for 2 min at 120 °C. AZ5214E resist is spin-coated and baked at 110 °C for 90 s. A standard photolithography process is performed using mask aligner for exposing the substrates to UV light for 20 s through a lithography mask. The resist is developed using AZ 726 developer for 30 s. reaction is stopped by dipping the sample into a deionized water bath. The sample is exposed one more time to an O2 plasma to remove residual resists at the interface with the substrate. The FTO is then etched for 7 min using RIE, the etching gas is SF6 (pressure: 7 mTorr, 25 sccm SF6, Power = 70 W). After etching the resist is lift-off by dipping the film into acetone for a several hours. The film is finally rinsed with a flow of acetone and then isopropanol, before being dried under nitrogen flow.

Diode with FTO/doped:SnO2/HgTe/Ag2Te/Au stack: The FTO-coated glass substrates (see above procedure used to define patterns) are sequentially cleaned in acetone and isopropanol. The substrates are exposed to ozone plasma for 10 min. The SnO2-L solution is deposited onto the patterned FTO substrate via spin-coating. The SnO2-H solution is deposited onto the SnO2-L layer. The film is annealed on a hot plate at 70 °C for 1h. The HgTe nanocrystals ink is deposited onto the SnO2 film via spin coating. The thickness of the film is tuned with spin coating speed and ink concentration in DMF solvent. On top of HgTe ink film, Ag2Te nanocrystals layer is spin-coated at 2000 rpm followed by HgCl2 treatment. This layer is then cation exchanged, to do so we perform a a HgCl2 treatment. 50 µL of HgCl2 diluted in methanol (10 mM) is dropped onto HgTe film and spindried after 15 s. Then the film is rinsed using isopropanol. This procedure is repeated one more time.

Finally, an ethanedithiol ligand-exchange is performed by dipping the film in a solution made of 1% ethanedithiol in acetonitrile for 30 s. The film is then rinsed with fresh acetonitrile. An top gold (80 nm) electrode is deposited with thermal evaporation under a vacuum of ≈5x10 -6 mbar with a rate of 3 Å/s. To ensure a high degree of homogeneity, the substrate holder is spinned during the deposition.

Reversed photodiode based on Au/doped:SnO2/HgTe/Ag2Te/Au/ITO stack. We start with a glass substrate. The surface of the glass substrate (1.1 mm thickness) is cleaned by sonication in acetone. The substrate is rinsed with acetone, then isopropanol and dried with a N2 gun. A final cleaning is made using an O2 plasma. An adhesion primer (Ti PRIME) is spin-coated onto the substrate and baked at 120 °C for 2 min. AZ5214E is spin-coated and baked at 110 °C for 90 s. The substrate is exposed under UV through a pattern mask (the same as for FTO patterning) for 1.5 s. The film is then baked at 127 °C for 2 min in order to invert the resist. Then a 40 s flood exposure is performed. The resist is developed using a bath of AZ726 for 32 s and the sample is rinsed in pure water and dried with N2. We then deposit 5 nm of chromium layer and 80 nm of gold layer using thermal evaporation. The lift-off is performed by dipping the film in acetone for 1 hour. The film is finally rinsed with a flow of acetone and then isopropanol, before being dried under nitrogen flow. The substrate is exposed to ozone plasma for 10 min. The SnO2-L solution is deposited onto the patterned gold substrate via spin-coating. The SnO2-H solution is deposited onto the SnO2-L layer. The film is annealed on a hot plate at 70 °C for 1h. The HgTe nanocrystals ink is deposited onto the SnO2 film via spin coating. The thickness of the film is tuned with spin coating speed and ink concentration in DMF solvent. On top of HgTe ink film, a Ag2Te nanocrystals layer is spin-coated at 2000 rpm followed by HgCl2 treatment. For HgCl2 treatment, 50 µL of HgCl2 methanol (10 mM) solution is dropped onto HgTe film and spin-dried after 15 s. Then the film is rinsed with isopropanol. This procedure is repeated one more time. Finally, an ethanedithiol ligand-exchange is performed by dipping the film in 1% ethanedithiol in acetonitrile solution for 30 s and rinsed with pure acetonitrile. A 3 nm Au layer is deposited with thermal evaporation under a vacuum of ≈5x10 -6 mbar at the rate of 3 Å/s. Finally, 50-60 nm of ITO is deposited by sputtering from a PLASSYS (MP800S), the deposition speed is set at 0.36 nm.s -1 , a 0.4 Pa partial pressure of Ar (Ar flux=40 sccm) is used to generate the plasma and the radio frequency power is set to 1.1 W.cm -2 .

Photodiode characterization:

The devices are operated in room conditions of pressure and temperature, unless specified. The sample is plugged in a sample holder that enable connexion of all pixel without further touching the diode. The sample holder is connected to a Keithley 2634 used as a source-meter that applied bias and measure current. For measurements under illumination, we use two types of sources: a blackbody (omega BB4B) typically operated at 600 °C used as a broad band source and a 1.55 µm laser diode. For the time-resolved measurements, we use a pulsed laser (CNI laser) at 1573 nm. The pulses are 1.2 ns long and repeated every 10 ms (100 Hz). A fast (<50 ps rise time) InGaAs photodiode is used to trigger the signal. An oscilloscope (Rohde & Schwarz, RTE 1102) acquires the outcoming signal through a 50 Ω resistor, which is proportional to IDS.

Electroluminescence characterization: for EL the same diode stack as for the detector is used. The diode is biased using a Keitley 2400. The light signal is collected by an InSb detector cooled at nitrogen temperature. the relative EQE is the ratio of the photocurrent measurement on the InSb sensor divided by the current flowing within the diode. To image the diode, we used a VGA format InGaAs sensor (model 1601) from New Imaging technologies.

ASSOCIATED CONTENT

Supporting Information include (i) procedure and results for X-ray photoemission measurements, (ii) Field effect transistor fabrication and characterization, (iii) ellipsometry measurements and their use in RCWA simulation.

Figure 1

 1 Figure 1 Diode structural properties. a. Transmission electron microscopy image of the FTO/ doped:SnO2/HgTe/Ag2Te/Au stack. False colors have been superimposed. b. (resp c.) Signal from EDX normalized by the value at their maximum for Au, Hg, Sn and Si (resp. Ag and Cl). d. (resp g.) Absorption spectrum (resp. TEM image) of Ag2Te NCs. e. (resp h.) Absorption and photoluminescence (resp TEM image) of HgTe NCs. f. (resp i.) Absorption spectrum (resp TEM image) of SnO2 NCs.

18

 18

Figure 2

 2 Figure 2 Diode electronic structure. a. Band diagram of the FTO/doped:SnO2/HgTe/Ag2Te/Au stack as revealed from photoemission. b Schematic of the diode stack: FTO/doped:SnO2/HgTe/Ag2Te/Au. c. I-V curves in the dark and under illumination (λ=1.55 µm -0.6 W.cm -2 ) for an FTO/HgTe/Ag2Te/Au stack. d. I-V curves in the dark and under illumination (λ=1.55 µm -0.6 W.cm -2 ) for an FTO/ doped:SnO2/HgTe/Ag2Te/Au stack.

Figure 4

 4 Figure 4 Performance of the FTO/doped:SnO2/HgTe/Ag2Te/Au stack. a. Photoresponse spectrum of the diode compared with InGaAs alloy. b. Noise current spectral density as a function of the signal frequency for a diode with various sizes. The inset provides the responsivity (to a blackbody at 600 °C) and specific detectivity of the diode. The error bar corresponds to the standard deviation. d. Photocurrent at 0 V as a function of time while a 1 ns pulse of light at 1573 nm illuminates arriving at t=0 on the device. The pixel size is 50 µm by 1 mm. Inset. Decay time of the temporal response as a function of the applied reverse bias.

Figure 5

 5 Figure 5 Reversed diode made of Au/doped:SnO2/HgTe/Ag2Te/Au/ITO stack a. Schematic of the reversed diode based on the Au/doped:SnO2/HgTe/Ag2Te/Au/ITO stack. b. I-V curve in the dark and under illumination for the inverted diode at room temperature. c. Open-circuit voltage as a function of irradiance for illumination by a laser diode at 1.55 µm, for the reversed diode.

Figure 6

 6 Figure 6 Benefit of SnO2 layer for electroluminescence. a. IV curves of the LED stack with and without SnO2 layer under forward bias. b. Relative EQE of the LED stack with and without SnO2 layer under large forward bias. Image of the diode collected through a filter at 1.4 µm on a InGaAs camera while the LED is off (c.) and on (d.). The diode is operated at room temperature under 5 mA in this case.

Table 1

 1 Figures of merit relative to light sensors operating in the SWIR and based on HgTe NCs. (PC: photoconductive, PT: phototransistor, PD: photodiode)

	Cut-off λ (µm)	Operating mode	Responsivity (A.W -1 )	Response time	Detectivity (jones)	Operating (K) temperature	Specific feature	Ref.
	2.5	PC	0.1	10 µs	3.5 x 10 10	230	As2S3 surface chemistry
	2.5	PC	1000	20 µs	2 x 10 12	200	Nanotrench
	2.5	PC	150	1.5 ms	6 x 10 8	80	HgTe decorated graphene channel
	2.4	PC	0.9	264 µs decay time	8 x 10 9	300	Spray coating with patterning
	2.4	PC	0.22	2.2 ms	3.5 x 10 8	300	Multicolor pixel
	2.5	PT	6.5 x10 -3	10 µs	10 9	220	Graphene electrode
	2.5	PT	2.0 x10 -3	14 µs	10 12	30	STO gate+resonator
	2	PT	<0.5	≈10 µs	3 x 10 10	300	SiO2 back gate
	2.4	PT	1	1.5 µs	10 10	300	Hybrid Polymer: HgTe
	2.5	PT	-	15 ms	-	300	Doped-graphene/ HgTe
	2.5	PT	0.08	10 µs	>10 8 <10 10	250	Planar pn junction based on dual gate
	2.5	PD	2.5 x10 -3	370 ns	3 x 10 9	300	HgTe ink
					3 x 10 10
				260 ns	(without cavity)
	2.5	PD	0.25	decay		300	Flexible substrate
				time	7.5 x 10 10
					(with cavity)
				1.4 µs
	2.2	PD	1	decay	6 x 10 10	300	HgCl2 treatment
				time
	1.8	PD	0.13	110 ns	2 x 10 10	300	With resonator
	2.5	PD	0.6	-	4 x 10 11	85	Resonator grating + Fabry Pérot
	2.4	PD	0.45	13 ns	10 10	300	Si/graphene/HgTe
	2.5	PD	0.28	2.5 µs	6 x 10 10	300	Bi2Se3/HgTe/Ag2Te

ACKNOWLEDEGMENTS

We thank New Imaging technologies for providing the InGaAs camera. The project is supported by ERC starting grant blackQD (grant n° 756225). We acknowledge the use of clean-room facilities from the "Centrale de Proximité Paris-Centre" and the french RENATECH network. This work has been supported by the Region Ile-de-France in the framework of DIM Nano-K (grant dopQD). This work was supported by French state funds managed by the ANR within the Investissements d'Avenir programme under reference ANR-11-IDEX-0004-02, and more specifically within the framework of the Cluster of Excellence MATISSE and also by the grant IPER-Nano2 (ANR-18CE30-0023-01), Copin (ANR-19-CE24-0022), Frontal (ANR-19-CE09-0017), Graskop (ANR-19-CE09-0026) and NITQuantum (ANR-20-ASTR-0008-01), Bright (ANR-21-CE24-0012-02) and MixDferro (ANR-21-CE09-0029).

COMPETING INTEREST

The authors declare no competing financial interest.


Charlie Gréboval

David Darson

Victor Parahyba

Rodolphe Alchaar

Claire Abadie

Vincent Noguier

Simon Ferré

Eva Izquierdo

Adrien Khalili

Yoann Prado

Pierre Potet

Emmanuel Lhuillier

Photoconductive focal plane array based on HgTe quantum dots for fast and costeffective short-wave infrared imaging

Keywords: short wave infrared, detection, HgTe, quantum dots, imaging

INTRODUCTION

Infrared (IR) sensing is driven by epitaxially grown semiconductors such as InGaAs for the shortwave infrared (SWIR), InSb and HgCdTe for the mid-wave infrared (MWIR) and mostly HgCdTe in the long-wave infrared (LWIR). Heterostructures of III-V semiconductors, the so-called quantum well infrared photodetectors (QWIP) and type-II superlattices complete the family of quantum detectors in this spectral range. In all cases, the material growth accounts for a significant part of the focal plane array (FPA) final cost. In addition, the coupling of this active layer to the readout integrated circuit (ROIC) is complex. In general, indium bumps ensure the electrical coupling between the absorbing semiconductor and the ROIC. This step adds cost, reduces the production yield, and complicates the pixel size reduction, although pixel pitches close to the diffraction limit provide higher image quality. [START_REF] Lee | IEEE International Electron Devices Meeting (IEDM)[END_REF] Because these technologies are already established, their roadmaps do not offer a clear route to substantial cost reduction. Therefore, new materials have to be considered. In the short and mid wave infrared, the strong vibrations occurring in conductive polymers prevent their use as light sensors. Colloidal quantum dots (QDs) now offer an interesting alternative. [START_REF] Nakotte | [END_REF] Over the past two decades, a lots of effort have been dedicated, first to push their absorption toward longer wavelengths [3][4][5] and later to integrate them into functional devices such as LEDs [6][7][8][9] , solar cells 10 and IR sensors. [START_REF] Nakotte | [END_REF][11][12][13][14] For the latter application, most of the efforts have been focused on the design of single pixel devices, but there are few reports relative to image sensors. 12,[START_REF] Ciani | Infrared Technology and Applications XLII[END_REF][START_REF] Buurma | Optical Sensing, Imaging, and Photon Counting: Nanostructured Devices and Applications[END_REF][START_REF] Chu | [END_REF][18][19] Several companies (such as SWIR Vision [START_REF]SWIR Vision[END_REF][START_REF] Gregory | [END_REF][START_REF] Hinds | Infrared Technology and Applications XLVI[END_REF] , ST Microelectronics [START_REF] Steckel | [END_REF] , IMEC [START_REF] Lee | IEEE International Electron Devices Meeting (IEDM)[END_REF][24][25][26] , Emberion [START_REF] Emberion | [END_REF] , QDIR, Osram) are now developing or even commercializing nanocrystal-based cameras.

The majority of the attention has been drawn to two materials: PbS and HgTe. Lead sulfide has initially been developed as a material for solar cells. [START_REF] Nozik | [END_REF] Its band gap can easily be tuned to match the optimal value for single junction solar cells. In addition, the observation of multi exciton generation 29 at moderate pump energies has been seen as a path to overcome the Shockley-Queisser limitation. This widespread application has resulted in a large amount of effort spent on this material, both on the material side to improve stability, doping control and carrier mobilities 30 and on the device side to design optimized photodiodes. 31,32 Alternatively, HgTe is a gap-less semiconductor in bulk form, allowing to explore absorption at wavelengths beyond the PbS band gap, especially in the MWIR. Significant progresses have also been made for this material integration, including demonstration of multicolor sensors 33,34 , coupling to light resonators [35][36][37][38][39] , or high carrier mobility films. 40 When it comes to focal plane arrays (FPAs), most efforts have so far been focused on vertical geometry, which requires a multilayer deposition. Moreover, photodiode stacks have received far less attention in the case of HgTe, and all reported diodes currently have a top metallic contact (ie non transparent) that hinders the coupling to a ROIC. Here, we explore a simpler planar geometry, thanks to a specifically designed readout circuit enabling in plane bias application. Furthermore, the device can be obtained in a single step of fabrication which opens the door to very low-cost infrared imaging setups.

DISCUSSION AND RESULTS

The material of interest is made of HgTe QDs, the latter are grown using a scaled-up version (x2.5 in quantity at constant concentration) of the Keuleyan's procedure. 41 The QDs are then processed under an ink form to cap them with short ligands 42 (here a combination of short thiols and HgCl2 salt) making them charge conduction compatible. This process tends to redshift the absorption spectrum, see Figure S1. The particle size is chosen so that after ligand exchange the band-edge matches the one of InGaAs, as shown in Figure 1a. According to transmission electron microscopy, the particles have a branched aspect with a characteristic size in the 6-8 nm range, see Figure 1b.

For the readout integrated circuit (ROIC), we use a modified version of the New Imaging Technologies (NIT) ROIC (model 1601 Quantum) allowing for bias application within the plane and presenting a flatter top surface compared to its regular version used to hybridize InGaAs photodiode arrays. The ROIC has a VGA format (640x512 pixels, see Figure 1d) with a 15 µm pixel pitch. The ROIC is then fully packaged onto a ceramic and electronically connected to it through wire bonding, see Figure 1c andd. To form an image at room temperature, this sensor is then introduced into a compact camera with a SWIR objective which couples the FPA to an electronic controller, see Figure 1e. Our strategy enables the design of a camera from a single fabrication step. An image obtained from this camera operated at room temperature without a thermal management strategy is shown in Figure 1f. The associated video is provided as video S1 and the lack of ghost effect should be noted. To better reveal the operation of our FPA, we have conducted electromagnetic simulations to quantify the device absorption and electrostatic simulations to unveil the field and current mapping. We use an atomic force microscopy (AFM) image for the ROIC topology (Figure 2a) as input to propose a model of the ROIC. The electrodes are made of a final top gold layer, while the surface is based on Si3N4 as the dielectric. The HgTe QD film thickness is in the 200-400 nm range depending on the ink concentration and the spin coating speed. The formed film is conformal to the substrate, see Figure S2.

As stated, the ROIC configuration enables to bias differently two consecutive pixels forming a checkerboard pattern, see the potential map in Figure 2b as well as the static electric field and current density on Figure S4. The typical electric field is around ≈1 kV.cm -1 with a limited inhomogeneity resulting from the tip effect in the vicinity of the electrodes. We have simulated the absorption spectrum of the FPA (see figure S1 for complex optical index used as input) and found that absorption at the band edge reaches 30%, see Figure 2d. Though the exact absorption localization, especially considering tip effects, can strongly depend on the exact geometry chosen to model the pixel, it appears that the absorption is mostly located on the top of the gold electrode, see Figure 2c. The latter acts as a backside mirror, allowing for a second pass of the light. This condition is ideal for charge collection in a QD film where the conduction is driven by hopping. Indeed, the short diffusion length resulting from the limited carrier mobility makes charge collection more efficient in the vicinity of the electrode 42 . In the next stage, we quantified the FPA's dark current as a function of temperature and image integration time. For this purpose, a second camera environment has been built where FPA cooling is now possible thanks to a multi-stage Peltier, see Figure S5. The signal output of the camera is digitized over 16 bits. Figure 3a shows the average charge count as a function of integration time for various temperatures in dark condition. At -4°C, we notice a clear saturation of the dark counts that corresponds to the full filling of the pixel's capacitance. This limits the integration time below 10 ms. As the temperature is further reduced, the saturation regime shifts toward higher integration times (>200 ms for operation at -32 °C). Fitting the thermal dependence of the dark current (slopes of Figure 3a) with an Arrhenius law which assumes that the current magnitude is given by I(T) =I0.exp(-Ea/kbT), with Ea an activation energy, kb the Boltzmann constant and T the temperature. We can extract an activation energy of 520 meV, see Figure 3b. In other words, the dark current drops by one order of magnitude every 30 K, which is consistent with the observed shift of the time integration threshold to reach the ROIC capacitance full filling. This reduction of the average dark current is also well highlighted by the histogram of the charge counts for all the pixels of the FPA and an exposure time of 10 ms, see Figure 3c. We can notice that not only lower temperatures reduce the average dark current but also the linewidth of the distribution. We investigate the photocurrent signal coming from the FPA to quantify its external quantum efficiency (EQE). Using the following procedure, we measure for a given wavelength the number of charges within the ROIC capacitance as a function of the incident power, see Figure 4a andS6-7. We perform the same measurement using the same readout circuit coupled to an InGaAs photodiode array with a known absolute EQE (90% in our case), see Figure S6. We then compare the relative slope of the QD FPA and the InGaAs one to determine the absolute EQE of the former. We also have checked that the signal is linear as a function of the integration time up to 100 ms (operating temperature is -32 °C in this case, see Figure 4c), that is the duration over which the capacitance is mostly fulfilled by the dark current as shown in Figure 3. Obtained EQE values are reported in Figure 4b and shows typical EQE in the 4-5% range. Since the absorption of the band edge has been determined around 30%, the internal quantum efficiency is around 15 %. This value is certainly modest compared to the best performing single pixel devices (with EQE reaching 80% for diodes 32 and even higher for devices with gain 43 ). On the other hand, there are very few reported EQE values at the FPA level 18 , especially based on HgTe QDs. Buurma et al [START_REF] Buurma | Optical Sensing, Imaging, and Photon Counting: Nanostructured Devices and Applications[END_REF] reported a 0.64% EQE, for a 320x256 pixel array with a 30 µm pixel pitch, but the cut-off wavelength was also much longer (5 µm) and operation was conducted at cryogenic temperature (100 K). The main reason for the modest EQE is the relatively large electrode spacing, which is highly detrimental in the case of hopping conduction. For PbS QD based FPAs, EQE have been reported up to 60% but at a shorter wavelength (1.4 µm cut-off wavelength 19,[START_REF] Steckel | [END_REF]44,[START_REF]Acuros® CQD®1920/1920L GigE SWIR Camera[END_REF] ) and drop around 15% for a 2 µm cut-off [START_REF] Allen | [END_REF] much closer to our device. It is important to stress that all these values based on PbS NCs relie on device geometries that are more complicated to fabricate than our single step FPA. The modest EQE compared to InGaAs certainly prevents the use of such sensor for applications with limited incident photon flux, however as our image prove it, it is already compatible with active imaging and industrial vision (trash sorting, detection of bruise of food…) applications.

The focal plane array can be operated full frame up to 200 frames per second (fps, see in Figure 4d) and even faster operation can be achieved only selecting a part of the image. A video at 340 fps is proposed as figure S8 and video S4 on a 640 x 200 pixels region of interest. In spite of its basic fabrication the sensor can image all over the NIR and SWIR range. Raw images are processed according to the procedure described in Figure S9. Basically, two reference images are collected respectively in the dark and under homogeneous illumination. The dark image will be used as offset, while the picture under illumination is used to generate a gain map. All images are then treated using this procedure. To finish, we have tried to quantify the cost of such sensors. For chemicals, we have used the cost charged at the laboratory level without considering any possible cost disruption relative to bulk order. Additional costs include amortization of the equipment, safety equipment, workforce. The cost of a nanocrystal growth has been estimated around 160 € per batch to obtain 16 mL of concentrated

nm f.

solution ([HgTe QD] ≈80 mg.mL -1 ). 77% of the growth cost actually relates to the workforce as it has been already pointed out in the case of QDs dedicated to solar cells. 47 This result further emphasizes the need for scaled up syntheses, since the time per synthesis is driving the final cost. As the QD solution is transformed into an ink, its value per liter increases by a factor ≈5 which mostly reflects the increase of concentration (x 3.4) of the ink with respect to the pristine solution, now reaching 55 k€ per liter. To functionalize one ROIC, we use 30 µL of ink, meaning that 130 devices can be prepared for a batch of QD (4 mL of ink). The cost of the QD film on the ROIC has been estimated around 16 € per FPA, while the sensor cost is around 70 €. The former value has to be compared with the cost relative to InGaAs sensor estimated around 600 €. This highlights the potential of NC for cost reduction, particularly if scale effects (larger NC batches and wafer scale fabrication) start to be considered. For sure, this cost excludes amortization of research behind this development. It remains that the QD cost becomes a negligible part of the final camera, since for sake of comparison, the cost of a SWIR objective is around 500 €. Certainly, the fact that the technology relies of Hg based material may also appears as an obvious limitation. But it should be pointed that the amount of HgTe per sensor is below 8 mg, while including the waste resulting from spin coating deposition. Further optimization of the deposition process, likely using inkjet 14 or spray 48 deposition, can further reduce this amount around 1 mg. It is thus important to notice that actual amount of toxic compound remains very low in such sensor.

CONCLUSION

We propose a strategy to build a cost-effective focal plane array based on QDs using a single step fabrication. The device relies on a ROIC that enables in plane electric field application and an absorbing layer made of HgTe QDs presenting a band gap similar to the one of InGaAs (λcut-off =1.7 µm). Homogeneous images over the VGA format array can be obtained and EQE reaches the 4-5 % range for a device only cooled by a Peltier stage. In this condition, the FPA is operated in its linear regime and dark current is not prevailing as long as the integration time remains below 100 ms. Finally, we estimate of the cost of such sensor and demonstrate that the overall cost is driven by labor costs, stressing the need for scaled-up syntheses that minimize manpower cost per mass of produced QDs. The cost of a film of QDs onto a ROIC has been determined in the 10-20 € range, for a ≈70 € cost per sensor, meaning that the active layer cost becomes almost negligible on the overall camera cost. This result raises new perspectives to broadly spread infrared sensor beyond the current niche market

METHODS

Chemicals: Mercury chloride (HgCl2, Sigma-Aldrich, 99%), Mercury compounds are highly toxic. Handle them with special care. tellurium powder (Te, Sigma-Aldrich, 99.99%), trioctylphosphine (TOP, Alfa Aesar, 90%), oleylamine (OLA, Acros, 80-90%), dodecanethiol (DDT, Sigma-Aldrich, 98%), ethanol absolute anhydrous (VWR), methanol (VWR, >98%), isopropanol (IPA, VWR), hexane (VWR, 99%), octane (Carlo erba, 99%), 2-mercaptoethanol (MPOH, Merck, >99%), N,N dimethylformamide (DMF, VWR), N-methylformamide (NMF, Alfa Aesar, 99%), toluene (VWR, 99.8 %). All chemicals are used without further purification except oleylamine that is centrifuged before use.

1 M TOP:Te precursor: 6.38 g of Te powder is mixed in 50 mL of TOP in a three neck flask. The flask is kept under vacuum at room temperature for 5 min and then the temperature is raised to 100 °C. Furthermore, degassing of flask is conducted for the next 20 min. The atmosphere is switched to nitrogen and the temperature is raised to 275 °C. The solution is stirred until a clear orange coloration is obtained. The flask is cooled down to room temperature and the color change to yellow. Finally, this solution is transferred to a nitrogen filled glove box for storage.

HgTe NCs synthesis with band-edge at 6000 cm -1 : In a 250 mL three neck flask, 1360 mg of HgCl2 (5 mmol) and 125 mL of oleylamine are degassed under vacuum at 110 °C. Meanwhile 5 mL of TOP:Te (1 M) are extracted from glove box and mixed with 5 mL of oleylamine. After the atmosphere is switched to N2 and the temperature stabilized at 56 °C, the TOP:Te solution is quickly injected. After 3 min, 10 mL of a mixture of DDT in toluene (25% of DDT) are injected and a water bath is used to quickly decrease the temperature. The content of the flask is split over 7 centrifugation tubes (50 mL) and MeOH is added (up to 40 mL). After centrifugation the formed pellet is redispersed in two centrifugation tubes with toluene. The solution is precipitated a second time with absolute EtOH (up to 40 mL). Again, the formed pellet is redispersed in toluene. At this step the nanocrystals are centrifuged in pure toluene to get rid of the lamellar phase. The solid phase is discarded. The stable phase is transferred in a vial after filtration (0.22 µm) with a volume of toluene V = 16 mL. The optical density of the NC solution is measured to be 0.32 at 400 nm after a 5000x dilution.

HgTe ink: 16 mL of HgTe CQD solution in toluene (2.23 OD at 400 nm after a 500x dilution) is mixed with 26 mL of exchange solution containing 2.6 mL of DDT, 39 mg of HgCl2 and 23.4 mL of DMF.

Ligand exchange is performed during 3 min in a sonication bath. Then, the solution is precipitated by adding 40 mL of toluene and centrifuged at 6000 rpm for 3 min. Supernatant is discarded and the QDs are dried 10 min under vacuum before being redispersed in 4 mL of DMF. Finally, the ink is centrifuged 3 min at 6000 rpm and filtered through a 0.22 µm PTFE filter. The optical density of the ink is measured to be 0.92 at 400 nm after a 5000x dilution, which is roughly 3 times more concentrated than the initial solution.

For infrared spectroscopy, we use a Fischer Nicolet iS50 in attenuated total reflection (ATR) mode.

The spectra are averaged over 32 acquisitions and have a 4 cm -1 resolution.

Transmission electron microscopy (TEM): For TEM pictures, a drop of QD solution is dropcasted on a copper grid covered with an amorphous carbon film. The grid is degassed overnight under secondary vacuum. A JEOL 2010F is used at 200 kV for acquisition of pictures.

Spectroscopic ellipsometry:

The spectroscopic ellipsometry measures the changes in the polarization state between the incident and the reflected light and is characterized by the angles ψ and Δ.

𝜌 = 𝑟

𝑝 𝑟 𝑠 = | 𝑟 𝑝 𝑟 𝑠 | 𝑒 𝑖(𝛿 𝑝 -𝛿 𝑠 ) = tan ψ 𝑒 𝑖∆
where rp and rs are the reflection coefficients of p and s polarized light respectively and where 𝛿 𝑝 .and 𝛿 𝑝 are the phase shifts in reflection in p and s polarizations, respectively. The measurements are performed on a V-VASE ellipsometer (J.A. Woollam) in the 350-2500 nm range with steps of 10 nm and with angles of incidence of 55° and 65°.

Focal plane array fabrication:

A ROIC packaged in a CLCC is cleaned in a O2 plasma cleaner for 10 min. The ROIC is then placed on a spincoater and 30 µL of HgTe ink is spincoated at 1000 rpm and an acceleration of 500 rpm.s -1 for 60 s and a drying step at 2000 rpm (1000 rpm.s -1 ) for 120 s. The film is further dried for 2 hours under primary vacuum.

Electromagnetic simulations are conducted using COMSOL, a software using Finite Element Method to solve Maxwell's equations. The array of resonators is modelled using RF module in 2D geometry. It is one unit cell and Floquet periodic boundary conditions are used to describe the periodicity. On both sides (top and bottom), we define perfectly matched layers (PML) to absorb all outgoing waves and prevent nonphysical reflections. The absorption calculated comes from the function "emw.Qe" which is the power density dissipated in W.m -3 . On top of the resonator inside air, a port condition is used to define the incident wave, either in TE or TM polarization. This port is a periodic port where the orders of diffraction are automatically calculated by the software. A specific mesh is used for QD elements where the maximum element size is 20 nm. For all the rest, a predefined "Extremely fine" mesh is used, which means that the maximum element size is 210 nm and the minimum is 6 nm, except for the PML where a mapped mesh is used with a distribution of 12 elements. The inputs of the simulations are the refraction index of the materials which come from ellipsometry measurements for HgTe NCs (see Figure S1), from ref 49 for gold and we use n=2 and k=0 for Si3N4 50 , where n is the refraction index and k the extinction coefficient.

Electrostatic simulations are also conducted using COMSOL. The array of resonators is modeled using AC/DC module in 3D geometry. Four electrodes are inside a unit cell and the periodic boundary conditions of the type "Continuity" are used. The main inputs of this simulation are the electrical conductivities, calculated experimentally for HgTe NCs (1.54x10 -4 S/m), taken from COMSOL database for gold (44x10 6 S/m) and from ref 51 for silicon nitride (1x10 -13 S/m). Two diagonal gold blocks are set to 0 V and the two other diagonals gold blocks are set to 1 V. The 4 borders are meshed identically side by side (with a copy face) to prevent any non-physical asymmetry. For all the rest, a predefined "Extra fine" mesh is used, which means that the maximum element size is 1 um and the minimum is 45 nm.

ASSOCIATED CONTENT

Supporting Information include ellipsometric measurements, simulation of the electric field and current density distribution, additional data relative to EQE determination, imaging at high frame rate, procedure for image correction, additional infrared imaging using band pass filters.

We also provide four videos. Video S1 depicts a living scene that it is taken while the sun is the only light source. Video S2 depicts the same series of vials containing solvents as shown in Figure 5a, while the Si wafer/ITO substrate are moved in front of the scene. Video S3 depicts the same series of vials containing solvents as shown in Figure 5a, while a soldering iron (at 400 °C) is moved in front to show passive imaging of warm objects. Video S4 shows the high frame rate imaging of a chopper wheel while the chopper is in phase or out of phase with respect to the acquisition of the FPA.

Figure 1

 1 Figure 1 Nanocrystal-based photoconductive ROIC. a. Absorption spectra of HgTe QDs used in the quantum dot-based SWIR camera and a typical InGaAs photodetector. b. TEM picture of HgTe QDs with a band edge at 6000 cm -1 . The particles have a tripodic shape and a size of 8 nm. The inset is a high-resolution image of the same particles. c. Scheme of the device that is plugged in the camera based on a film of HgTe QDs. d. Picture of the ROIC in the CLCC (ceramic lead less chip carrier) after deposition of the QD film. The 640 x 512 pixels matrix is represented with the white arrows. e. Picture of the SWIR camera with a Computar M1614-SW objective. f. Picture of a building (Arabic fine art museum of Paris) taken with the QD-based SWIR camera shown in part e.

Figure 2

 2 Figure 2 Modeled optical and electrical properties of the ROIC. a. Atomic force microscopy picture of one pixel from the ROIC before QD deposition. b. Electric potential simulation of the ROIC whose pixels are biased to 0 and 1 V alternatively. c. Electromagnetic absorption map at 1.7 µm of a pixel coated with a 400 nm HgTe QD film. d. Simulated absorption spectrum integrated from the absorption map in b. In red the absorption in the QD film and in black the losses in the gold electrode.

Figure 3

 3 Figure 3 Dark current conduction of the ROIC. a. Average value of dark images as a function of exposure time for various temperatures. b. Dark current decrease as a function of the cooling of the detector. The black line represents an Arrhenius exponential decay. Uncorrected histograms of dark images taken at -32 °C (c.), -20 °C (d.) and -4 °C (e.) for a 10 ms exposure time.

Figure 4

 4 Figure 4 Determination of the quantum efficiency and fast operation of the ROIC. a. Average value (measured in analog digital units = magnitude of the digitalized number of electrons collected over 16 bits) of homogeneously illuminated pictures, taken at different relative powers in a 50 nm spectral range centred on 1570 nm for various exposure times and measured at -32 °C. b. External quantum efficiencies of the HgTe QD camera as a function of the wavelength and the exposure time for two different devices. The external quantum efficiency is relative to the one of a 1601 InGaAs FPA from New Imaging Technologies. c. Evolution of the detector response as a function of the exposure time at 940 nm and 1570 nm. The response is extracted from a linear fit of the average

Figure 5

 5 shows the imaging of vials containing various solvents transparent in the visible. To better highlight the change of contrast between visible and SWIR range, a visible transparent glass substrate coated with ITO (tin-doped indium oxide) and a reflective undoped Si wafer have been added to the front line, see Figure 5a. As we switched to SWIR imaging using the QD based FPA, we can now see in Figure 5b, S11 and video S2 through the Si wafer, while the water vial and the ITO/glass substrate now turn opaque. A more systematic study of the spectral dependence of this scene is shown in Figure 5d-f and S10. Blackbody radiation can also be detected using the HgTe QD-based sensor as revealed by the large contrast obtained by passively imaging the soldering iron, see Figure 5c

Figure 5 SWIR

 5 Figure 5 SWIR imaging using HgTe QD-based FPA. a. Visible picture (smartphone camera) of a scene with four vials containing from left to right: tetrachloroethylene (TCE), toluene, acetone and water (H2O). In front of the vials, an ITO covered glass slide and a two inches silicon wafer are placed. b. Same scene as in a. taken with the HgTe QD based FPA. c. Image of the tip of a soldering iron heated at 400 °C. d-f. Scene composed of vials containing from left to right: TCE, toluene, acetone, isopropyl alcohol (IPA) and water imaged through a 1170 nm band pass filter (d.), a 1270 nm bandpass filter (e.) and a 1570 nm bandpass filter (f.), All the SWIR pictures are taken through 50 nm spectral width bandpass filters and with an exposure time of 50 ms at -32 ° C.

table 1

 1 Costs associated with the different steps of fabrication of a QD-based sensor.

	Step	Cost	Amount	Cost for this	Comment justification
				step in €
		OLA	0,13 L	10,4	80 €.L -1
		TOP	5 mL	5	1000 €.L -1
		DDT	10 mL	0,2	20 €.L -1
		HgCl2	1,36 g	0,9928	0.73 €.g -1
	growth	Te alcohol solvent Non-polar	0,63 g 1 L 0.1 L	3,2949 2 2.5	5.23 €.g -1 2 €.L -1 25 €.L -1
	colloidal QD	Filter Glassware Equipment	1 1 1	1 1.5 5	1€/filter Schlenk line+50 € for 3 neck flask that can be used 100 times: 1.5 €/use Balance, centrifuge, hood -10k€ amortization over 5 years, with 400 use per year: 5€/use
					Assuming 70 k€ annual cost working 1500 hours
		Workforce	2.5 hours	125	a year: 50€.h -1
		Safety	6 gloves	1.8	0.3€/glove
		Pipette			0.5€ each
		cone	6	3

Cost per synthesis in € 161.7 Cost per L of QD solution in € 10105.5 Ink preparation

			ROIC	1	6.62	1000 € for a 8 inches wafer sliced in 151 dies
			Packaging	1	50	Bonding, ceramic package…
				QD cost per sensor in €	≈16
				Cost per sensor in €	≈73
			HgCl2	0,039 g	0,02847	0.73 €.g -1
			DMF	23.4 mL	0,234	10 €.L -1
			MpOH	2.6 mL	0,3276	126 €.L -1
			QD	16 mL	161.7
			Safety	4	1.2	0.3€/glove
			Pipette,			0.5€ each
			cone	6	3
						Assuming 70 k€ annual cost working 1500 hours
			Workforce	1	50	a year : 50€.h -1
				Cost per ink preparation in €	216.5
				Cost per ink liter in €	54120
			QD film	30 µL	1.62	54199.4 €.L -1
	Film	deposition	Deposition Pipette cones	1 6	3 3	spin coating + hood -6000€ over 5 year; used 400 times a year: 3€ per use 0.5€ each Assuming 70 k€ annual cost working 1500 hours a
			Workforce	0.16 h	8.33	year: 50€.h -1

ACKNOWLEDEGMENTS

We thank Bruno Gallas for the use of ellipsometer and Herve Cruguel for conducting AFM measurements. The project is supported by ERC starting grant blackQD (grant n° 756225). We acknowledge the use of clean-room facilities from the "Centrale de Proximité Paris-Centre". This work has been supported by the Region Ile-de-France in the framework of DIM Nano-K (grant dopQD). This work was supported by French state funds managed by the ANR within the Investissements d'Avenir programme under reference ANR-11-IDEX-0004-02, and more specifically within the framework of the Cluster of Excellence MATISSE and also by the grant IPER-Nano2 (ANR-18CE30-0023-01), Copin (ANR-19-CE24-0022), Frontal (ANR-19-CE09-0017), Graskop (ANR-19-CE09-0026), NITQuantum (ANR-20-ASTR-0008-01), Bright (ANR-21-CE24-0012-02) and MixDferro (ANR-21-CE09-0029).

AUTHOR CONTRIBUTION

EL, VP, PT design the project. VP, VN and SF design the ROIC. CG, DD, VP, VN and SF conducted measurements. YP, CG and EI synthetize the nanocrystals. CP, EI and RA functionalize the ROIC. CA and AK conducted electromagnetic simulation. EL and CG write the manuscript with inputs and feedback from all authors

COMPETING INTEREST

The authors declare no competing financial interest.