HgTe Nanocrystals Research Articles

Short-Wave Infrared Detection and Imaging Employing Size-Customized HgTe Nanocrystals.

HgTe nanocrystals (NCs) possess advantages including tunable infrared absorption spectra, solution processability, and low fabrication costs, offering new avenues for the advancement of next-generation infrared detectors. In spite of great synthetic advances, it remains essential to achieve customized synthesis of HgTe NCs in terms of industrial applications. Herein, by taking advantage of a high critical nucleation concentration of HgTe NCs, a continuous-dropwise (CD) synthetic approach that features the addition of the anion precursors in a feasible drop-by-drop fashion is demonstrated. The slow reaction dynamics enable size-customized synthesis of HgTe NCs with sharp band tails and wide absorption range fully covering the short- and mid-infrared regions. More importantly, the intrinsic advantages of CD process ensure high-uniformity and scale-up synthesis from batch to batch without compromising the excitonic features. The resultant HgTe nanocrystal photodetectors show a high room-temperature detectivity of 8.1 × 1011 Jones at 1.7µm cutoff absorption edge. This CD approach verifies a robust method for controlled synthesis of HgTe NCs and might have important implications for scale-up synthesis of other nanocrystal materials.

Focal plane array based on HgTe nanocrystals with photovoltaic operation in the short-wave infrared

Thanks to their remarkable spectral tunability across the entire infrared range, HgTe nanocrystals present a unique platform for designing infrared optoelectronic devices. While in recent years most of the significant advances in this domain have been made on devices at the single-pixel level, there is a growing trend toward exploring the potential of this material for imaging applications. However, until recently, focal plane arrays based on HgTe colloidal nanocrystals have been limited to the photoconductive mode, which is inherently associated with a large dark current. In this work, we demonstrate a diode stack compatible with a readout integrated circuit whose back-end processing has been optimized to ensure compatibility with a complete diode stack deposition. The diode design is also optimized to generate a Fabry–Pérot cavity in which 50% of the light is effectively absorbed at the band edge. Finally, taking benefit from the full video graphics array format, high-resolution images are taken.

Investigations of defect levels in different sized HgTe nanocrystals based photovoltaic devices using thermal admittance spectroscopy

HgTe nanocrystals (NCs) have unique properties that make them suitable for optoelectronic devices in the mid-wave infrared (MWIR) and short-wave infrared regions. However, electrically active defects can trap charge carriers, reducing their mobility and diffusion length, which degrades the NCs’ optical and electrical properties. In this study, we used the thermal admittance spectroscopic (TAS) method to analyze defects in HgTe NC-based photovoltaic devices. The ITO/HgTe/Al device structure was used to study the defect levels in HgTe nanocrystals and the effect of these traps on transport properties. Using low-temperature I–V measurements, we calculated the trap activation energy as 0.14 eV, and the transport was found to occur mainly through these trap states. From the TAS measurements, the trap activation energy obtained was 0.14 eV, and the concentration of trap level was 3.16×1016cm−3eV−1. We have also fabricated ITO/TiO2/HgTe/Au and fluorine doped tin oxide/TiO2/HgTe/MoO3/Au devices and did TAS measurements to understand the effect of adding electron and hole extraction layer on the formation of defect levels. Intriguingly, a distinct reversal in the capacitance–frequency (C–F) behavior is observed at different temperatures, leading to positive slopes in the Arrhenius plot. This peculiar phenomenon is attributed to the size-dependent doping effects within the HgTe nanocrystals. The outcomes of this study shed light on the significance of understanding and quantifying electrically active defects in HgTe nanocrystals for the advancement of NC-based optoelectronic devices.

Coherent THz wave emission from HgTe quantum dots

Mercury telluride (HgTe) nanocrystals (NCs) are very promising for THz technology as they exhibit broad THz absorption resonances and a carrier lifetime of a few picoseconds as well as being easily fabricated using solution synthesis. In this work, we show their light emission properties in the THz spectral range, up until now unexplored, and show how THz pulse generation can be used for microscopic insights into these NCs. In particular, we report on coherent THz emission from large HgTe NCs excited by linearly polarized optical pulses via second-order nonlinear effects. The peak emission frequency is tunable from 0.4 to 0.8 THz by varying incident angles of optical pulses from 0° to 45°. Our results reveal that the THz emission is induced by transient photocurrents arising from both photogalvanic and photon drag effects. By pushing the light emission of colloidal quantum dots down to the THz spectral range, our study expands the application fields of NCs, especially toward the development of easily integrable and tunable THz emitters and quantum THz devices.

Vanishing Confinement Regime in Terahertz HgTe Nanocrystals Studied under Extreme Conditions of Temperature and Pressure.

While HgTe nanocrystals (NCs) in the mid-infrared region have reached a high level of maturity, their far-infrared counterparts remain far less studied, raising the need for an in-depth investigation of the material before efficient device integration can be considered. Here, we explore the effect of temperature and pressure on the structural, spectroscopic, and transport properties of HgTe NCs displaying an intraband absorption at 10 THz. The temperature leads to a very weak modulation of the spectrum as opposed to what was observed for strongly confined HgTe NCs. HgTe NC films present ambipolar conduction with a clear prevalence of electron conduction as confirmed by transistor and thermoelectric measurements. Under the application of pressure, the material undergoes phase transitions from the zinc blende to cinnabar phase and later to the rock salt phase which we reveal using joint X-ray diffraction and infrared spectroscopy measurements. We discuss how the pressure existence domain of each phase is affected by the particle size.

Broadband Enhancement of Mid‐Wave Infrared Absorption in a Multi‐Resonant Nanocrystal‐Based Device (Advanced Optical Materials 9/2022)

The back-cover image depicts an infrared sensor based on HgTe nanocrystals. Incoming infrared light is focused into the nanocrystal film thanks to the presence of the photonic structure, which generates multiple resonances and enhanced sensitivity of the sensor over a broad optical band in the infrared. For further details, see article number 2200297 by Angela Vasanelli, Emmanuel Lhuillier, and co-workers.

Colloidal II–VI—Epitaxial III–V heterostructure: A strategy to expand InGaAs spectral response

For short wave infrared (SWIR) sensing, InGaAs is the leading technology combining high carrier mobility, high homogeneity, and complete control over the n-to-p doping. In the meanwhile, numerous alternative materials have tried to compete with InGaAs. Among them, colloidal nanocrystals with narrow bandgap can address the current issue in designing cost-effective sensors for the SWIR range. Rather than putting these two materials against each other, we design here a synergistic duo in which HgTe nanocrystals are used to broaden the spectral range of InGaAs while lifting the lattice matching constraints. We propose a diode geometry where a p-type HgTe NC array is coupled with n-type InGaAs wires, which are used as high mobility (μ > 1000 cm2 V−1 s−1) minority carrier extractors. This approach also demonstrates that Van der Waals heterostructures are not limited to graphene-like materials, and that bulk-like III–V semiconductors can also be light sensitized by colloidal nanoparticles. This work paves the way toward further synergies between epitaxially grown and colloidally grown semiconductors for infrared detection.

Correlating Structure and Detection Properties in HgTe Nanocrystal Films.

HgTe nanocrystals (NCs) enable broadly tunable infrared absorption, now commonly used to design light sensors. This material tends to grow under multipodic shapes and does not present well-defined size distributions. Such point generates traps and reduces the particle packing, leading to a reduced mobility. It is thus highly desirable to comprehensively explore the effect of the shape on their performance. Here, we show, using a combination of electron tomography and tight binding simulations, that the charge dissociation is strong within HgTe NCs, but poorly shape dependent. Then, we design a dual-gate field-effect-transistor made of tripod HgTe NCs and use it to generate a planar p-n junction, offering more tunability than its vertical geometry counterpart. Interestingly, the performance of the tripods is higher than sphere ones, and this can be correlated with a stronger Te excess in the case of sphere shapes which is responsible for a higher hole trap density.

Synthesis of HgTe colloidal quantum dots for infrared photodetector

Subject to the complexity and high cost of traditional semiconductor materials and manufacturing processes, the features of facile solution process with low cost have made the colloidal quantum dots (CQDs) more competitive, in which HgTe nanocrystals can be tuned as a potential in infrared region and achieve a great photoresponsivity. Herein, we reported the synthesis and characterization of HgTe CQDs materials by a facile solution process, and explored the relationship between the nanocrystal size and the reaction temperature and time, as well as the effect of the ligand exchange on the thickness of the HgTe CQDs film. The as-fabricated photoconductive device showed a high photoresponse even under a weak light, which provides a direction for the next-generation infrared photodetectors.

Gate tunable vertical geometry phototransistor based on infrared HgTe nanocrystals

Infrared nanocrystals are promising building blocks for the design of low-cost infrared sensors. Vertical geometry diode is, among possible geometries, the one that has led to the best performance so far. However, this geometry suffers from a lack of tunability after its fabrication, slowing down possible improvements. Here, we demonstrate gate control on a vertical diode in which the active layer is made of HgTe NCs absorbing in the extended short-wave infrared (2.5 μm). To reach this goal, we take advantage of the electrostatic transparency of graphene, combined with the high capacitance LaF3 ionic glass to design a gate tunable photodiode. The latter behaves as a work function-tunable electrode which lets the gate-induced electric field tune the carrier density within the nanocrystal film. In particular, we show that the gate allows to tune the band profile leading to more efficient charge extraction and thus an enhanced photoresponse (×4 compared to the device with a floating gate). This work also demonstrates that photoelectron extraction can still be improved in the existing diode, by better controlling the doping profile of the stack.

Electroluminescence from HgTe Nanocrystals and Its Use for Active Imaging.

Mercury telluride (HgTe) nanocrystals are among the most versatile infrared (IR) materials with the absorption of lowest energy optical absorption which can be tuned from the visible to the terahertz range. Therefore, they have been extensively considered as near IR emitters and as absorbers for low-cost IR detectors. However, the electroluminescence of HgTe remains poorly investigated despite its ability to go toward longer wavelengths compared to traditional lead sulfide (PbS). Here, we demonstrate a light-emitting diode (LED) based on an indium tin oxide (ITO)/zinc oxide (ZnO)/ZnO-HgTe/PbS/gold-stacked structure, where the emitting layer consists of a ZnO/HgTe bulk heterojunction which drives the charge balance in the system. This LED has low turn-on voltage, long lifetime, and high brightness. Finally, we conduct short wavelength infrared (SWIR) active imaging, where illumination is obtained from a HgTe NC-based LED, and demonstrate moisture detection.

Reconfigurable 2D/0D p-n Graphene/HgTe Nanocrystal Heterostructure for Infrared Detection.

Nanocrystals are promising building blocks for the development of low-cost infrared optoelectronics. Gating a nanocrystal film in a phototransistor geometry is commonly proposed as a strategy to tune the signal-to-noise ratio by carefully controlling the carrier density within the semiconductor. However, the performance improvement has so far been quite marginal. With metallic electrodes, the gate dependence of the photocurrent follows the gate-induced change of the dark current. Graphene presents key advantages: (i) infrared transparency that allows back-side illumination, (ii) vertical electric field transparency, and (iii) carrier selectivity under gate bias. Here, we investigate a configuration of 2D/0D infrared photodetectors taking advantage of a high capacitance ionic glass gate, large-scale graphene electrodes, and a HgTe nanocrystals layer of high carrier mobility. The introduction of graphene electrodes combined with ionic glass enables one to reconfigure selectively the HgTe nanocrystals and the graphene electrodes between electron-doped (n) and hole-doped (p) states. We unveil that this functionality enables the design a 2D/0D p-n junction that expands throughout the device, with a built-in electric field that assists charge dissociation. We demonstrate that, in this specific configuration, the signal-to-noise ratio for infrared photodetection can be enhanced by 2 orders of magnitude, and that photovoltaic operation can be achieved. The detectivity now reaches 109 Jones, whereas the device only absorbs 8% of the incident light. Additionally, the time response of the device is fast (<10 μs), which strongly contrasts with the slow response commonly observed for 2D/0D mixed-dimensional heterostructures, where larger photoconduction gains come at the cost of slower response.

Near- to Long-Wave-Infrared Mercury Chalcogenide Nanocrystals from Liquid Mercury

HgTe nanocrystals are currently the most promising colloidal material for infrared detection, combining broadly tunable infrared absorption and photoconductive properties. Current syntheses lead to...

Near Unity Absorption in Nanocrystal Based Short Wave Infrared Photodetectors Using Guided Mode Resonators

Nanocrystals appear as versatile building blocks for the design of low-cost optoelectronic devices. The design of infrared sensors based on nanocrystals is currently facing a key limitation: the short carrier diffusion length resulting from hopping transport makes that only a limited part of the incident light is absorbed. In order to enhance the device absorption, we use Guided Mode Resonance (GMR). The method appears to be quite versatile and is applied to both PbS and HgTe nanocrystals presenting respectively cut-off wavelengths at 1.7 and 2.6 µm. The designed electrodes present a large enhancement of the material responsivity around a factor of ≈250, reaching external quantum efficiency of 86% for PbS and 340% for HgTe. This increase of the response can be deconvoluted in a factor of 3 for the enhancement of the absorption and a factor of 80 for the photocurrent gain. The method can also be suited to finely tune the cut-off wavelength of the material thanks to geometrical parameters at the device level. The obtained devices are now only limited by the material noise.

Designing Photovoltaic Devices Using HgTe Nanocrystals for Short and Mid‐Wave Infrared Detection

HgTe nanocrystals offer a unique combination of broadly tunable optical absorption in the infrared from 1 to 100 μm and photoconductive properties. Herein, some of the recent developments related to their integration into photodiodes are discussed. Concepts such as the development of colloidal unipolar barrier, development of ink to achieve strongly absorbing and conductive film, and the recent proposition of intraband photodiode are also discussed.

HgTe Nanocrystals for SWIR Detection and Their Integration up to the Focal Plane Array.

Infrared applications remain too often a niche market due to their prohibitive cost. Nanocrystals offer an interesting alternative to reach cost disruption especially in the short-wave infrared (SWIR, λ < 1.7 μm) where material maturity is now high. Two families of materials are candidate for SWIR photoconduction: lead and mercury chalcogenides. Lead sulfide typically benefits from all the development made for a wider band gap such as the one made for solar cells, while HgTe takes advantage of the development relative to mid-wave infrared detectors. Here, we make a fair comparison of the two material detection properties in the SWIR and discuss the material stability. At such wavelengths, studies have been mostly focused on PbS rather than on HgTe, therefore we focus in the last part of the discussion on the effect of surface chemistry on the electronic spectrum of HgTe nanocrystals. We unveil that tuning the capping ligands is a viable strategy to adjust the material from the p-type to ambipolar. Finally, HgTe nanocrystals are integrated into multipixel devices to quantize spatial homogeneity and onto read-out circuits to obtain a fast and sensitive infrared laser beam profile.

HgTe Nanocrystal Inks for Extended Short‐Wave Infrared Detection

AbstractShort‐wave infrared (IR) detection is currently driven by InGaAs technology which has limited the perspective of cost effectiveness and consequently slows the development of IR sensors. Since organic electronics are ineffective in this wavelength range, an alternative to conductive polymers is the use of colloidal quantum dots (CQDs) which exhibit strongly tunable IR absorption. In this paper, the extended short‐wave IR (2.5 µm cut‐off) is focused on to expand the capabilities of InGaAs, while using HgTe nanocrystals as the active material. Previous devices based on this material suffer from a low responsivity due to weak absorption (few percents). The integration of HgTe nanocrystals is presented in ink form to build thick (up to 600 nm), strongly absorbing nanoparticle films. This ink is integrated into a diode, allowing one to boost the responsivity by two orders of magnitude and the detectivity by one order of magnitude compared to previous HgTe devices at the same wavelength. Detectivity reaches 3 × 109 Jones, while the time response is found to be 370 ns for room temperature and 0 V bias operation. Finally, the material is integrated into a focal plane array which is used to determine laser beam profile.

Field-Effect Transistor and Photo-Transistor of Narrow-Band-Gap Nanocrystal Arrays Using Ionic Glasses.

The gating of nanocrystal films is currently driven by two approaches: either the use of a dielectric such as SiO2 or the use of electrolyte. SiO2 allows fast bias sweeping over a broad range of temperatures but requires a large operating bias. Electrolytes, thanks to large capacitances, lead to the significant reduction of operating bias but are limited to slow and quasi-room-temperature operation. None of these operating conditions are optimal for narrow-band-gap nanocrystal-based phototransistors, for which the necessary large-capacitance gate has to be combined with low-temperature operation. Here, we explore the use of a LaF3 ionic glass as a high-capacitance gating alternative. We demonstrate for the first time the use of such ionic glasses to gate thin films made of HgTe and PbS nanocrystals. This gating strategy allows operation in the 180 to 300 K range of temperatures with capacitance as high as 1 μF·cm-2. We unveil the unique property of ionic glass gate to enable the unprecedented tunability of both magnitude and dynamics of the photocurrent thanks to high charge-doping capability within an operating temperature window relevant for infrared photodetection. We demonstrate that by carefully choosing the operating gate bias, the signal-to-noise ratio can be improved by a factor of 100 and the time response accelerated by a factor of 6. Moreover, the good transparency of LaF3 substrate allows back-side illumination in the infrared range, which is highly valuable for the design of phototransistors.

Short Wave Infrared Devices Based on HgTe Nanocrystals with Air Stable Performances

Colloidal quantum dots (CQDs) are candidates of interest for the design of low cost IR detector, especially in the short wave infrared (SWIR; 0.8–3 μm), where the vicinity of the visible range makes the high cost of available technologies even more striking. HgTe nanocrystals are among the most promising candidates to address SWIR since their spectrum can be tuned all over this range while demonstrating photoconductive properties. However, several main issues have been swept under the rug, which prevents further development of active materials and devices. Here we address two central questions, which are (i) the stability of the device under ambient air condition and (ii) the reduction of dark current. Encapsulation of HgTe CQDs is difficult because of their extreme sensitivity to annealing, we nevertheless demonstrate an efficient encapsulation method based on a combination of O2 and H2O repellant layers leading to stability over >100 days. Finally, we demonstrate that the dark current reduction can be o...

HgTe, the Most Tunable Colloidal Material: from the Strong Confinement Regime to THz Material

ABSTRACTHgTe nanocrystals are extremely interesting materials to obtain a highly tunable absorption spectrum in the infrared range. Here, we discuss the two extreme cases of strongly confined and barely confined HgTe nanocrystals. We discuss the synthesis and optoelectronic properties of HgTe 2D nanoplatelets where the confinement energy can be as large as 1.5 eV. This material presents enhanced (mostly narrower) light emitting properties compared to spherical nanocrystals emitting at the same wavelength. Moreover, absorption spectra, majority carriers and time response can be tuned by carefully choosing the surface chemistry and applying a well-chosen gate bias. HgTe can also be used to explore the effect of vanishing confinement and to obtain quasi bulk properties with tunable absorption in the THz, up to 150 µm.