Dark Current Density Research Articles

Breaking the Size Limitation of Directly-Synthesized PbS Quantum Dot Inks Toward Efficient Short-wavelength Infrared Optoelectronic Applications.

PbS quantum dots (QDs) are promising building blocks for solution-processed short-wavelength infrared (SWIR) devices. The recently developed direct synthesis of semi-conductive PbS QD inks has substantially simplified the preparation processing and reduced the material cost, while facing the challenge to synthesize large-size QDs with absorption covering the SWIR region. Herein, we for the first time realize a low-cost, scalable synthesis of SWIR PbS QD inks after an extensive investigation of the reaction kinetics. Finally, based on these PbS SWIR QD inks, the solar cell demonstrates a record-high power conversion efficiency (PCE) of 1.44 % through an 1100 nm cutoff silicon filter and the photodetector device shows a low dark current density of 2×10-6 A cm-2 at -0.8 V reverse bias with a high external quantum efficiency (EQE) of 70 % at ≈1300 nm. Our results realize the direct synthesis of low-cost and scalable SWIR QD inks and may accelerate the industrialization of consumer SWIR technologies.

Breaking the Size Limitation of Directly‐Synthesized PbS Quantum Dot Inks Toward Efficient Short‐wavelength Infrared Optoelectronic Applications

AbstractPbS quantum dots (QDs) are promising building blocks for solution‐processed short‐wavelength infrared (SWIR) devices. The recently developed direct synthesis of semi‐conductive PbS QD inks has substantially simplified the preparation processing and reduced the material cost, while facing the challenge to synthesize large‐size QDs with absorption covering the SWIR region. Herein, we for the first time realize a low‐cost, scalable synthesis of SWIR PbS QD inks after an extensive investigation of the reaction kinetics. Finally, based on these PbS SWIR QD inks, the solar cell demonstrates a record‐high power conversion efficiency (PCE) of 1.44 % through an 1100 nm cutoff silicon filter and the photodetector device shows a low dark current density of 2×10−6 A cm−2 at −0.8 V reverse bias with a high external quantum efficiency (EQE) of 70 % at ≈1300 nm. Our results realize the direct synthesis of low‐cost and scalable SWIR QD inks and may accelerate the industrialization of consumer SWIR technologies.

Large-area flexible colloidal-quantum-dot infrared photodiodes for photoplethysmogram signal measurements

Epitaxially grown photodiodes are the foundation of infrared photodetection technology; however, their rigid structure and limited area scaling limit their use in advanced applications. Colloidal-quantum-dot (CQD) infrared photodiodes have increased active areas through solution processing, and are thus potential candidates for large-area flexible photodetection, but these large-area photodiodes have disadvantages such as large dark current density, poor homogeneity, and poor stability. Therefore, this study established a fabrication strategy for large-area flexible CQD photodiodes that involves introducing polyimide to CQD ink to improve CQD passivation, monodisperse ink persistence, and film morphology. The resulting CQD photodiodes exhibited reduced dark current density and improved homogeneity and work stability. Furthermore, the as-prepared photodiodes exhibited a detectivity (D*) of greater than 1013 Jones, which was higher than other reported CQD photodetectors. The CQD photodiodes developed in this study can be used for wearable photoplethysmogram (PPG) signal measurement under ambient light at reduced cost and power consumption.

Low-temperature spatial atomic layer deposited Ga2O3 films for high-performance flexible deep ultraviolet photodetector

This research aims to prepare amorphous gallium oxide (α-Ga2O3) films for use in flexible deep ultraviolet photodetectors. The films are deposited using high-speed spatial atomic layer deposition (SALD) at different substrate temperatures. The experimental results show that the saturating deposition rate at 145–155 °C is four to sixteen times higher than that of temporal ALD. Moreover, under the saturated SALD conditions the deposited film has less carbon impurities, oxygen-related defects, and greater compactness than under non-saturated conditions. Finally, a high-performance flexible deep ultraviolet α-Ga2O3 photodetector with fast decay time of 161 µs and low dark current density of 6.8 × 10-10 A/cm2 is achieved.

Silicon four element p-i-n photodiode with improved characteristics

This article presents the results of the development of silicon coordinate p-i-n photodiodes (PD) with improved parameters. The technological possibilities of reducing the gaps between the responsive areas of multi-element PDs were studied. PDs with reduced gaps were modeled, samples were made according to the models, the parameters of the obtained PDs and the influence of various technological factors on their values were investigated. During research, it was established that the factor that limits the possibility of reducing the gaps is the insulation resistance between the responsive elements. The decrease in the insulation resistance between the PD elements was a consequence of the formation of inversion leakage channels at the Si-SiO2 interface, which is characteristic of high-resistance p-type silicon, the conductivity of which increased with a decrease in the width of the gaps. To increase the resistance of the gaps between the platforms and reduce the influence of inversion layers, it was decided to form regions of the restriction of the leakage channels between the REs – highly doped regions isotypic with the substrate material (p+-type). They will prevent the movement of current carriers that are generated in the inversion layers on the surface of the substrate. Four-element p-i-n photodiodes with 31 μm gaps between the sites were made. The proposed PD is not significantly inferior in parameters to analogues, but it is the sample with the smallest gaps between RE among photodiodes of the same type and size in the world market. The improvement of the insulation resistance between the REs made it possible to reduce the level of dark current of the PD by twice compared to serial products, with a slight decrease in responsivity due to the introduction of additional thermal operations into the technological route. In particular, the dark current density of the RE of experimental PDs was 40-80 nA/cm2, compared to 80-250 nA/cm2 for commercial samples. Impulse current monochromatic sensitivity at a wavelength of 1064 nm for experimental samples was 0.41-0.44 A/W, and for commercial ones - 0.45-0.48 A/W.

MAPb(Br1–xClx)3 Hybrid Perovskite Materials for Direct X-ray Detection

The study and development in recent years of hybrid (organic–inorganic) halide perovskite materials have given them an unprecedented opportunity for direct ionizing radiation detection, given their large attenuation coefficient and sufficient charge carrier mobility lifetime product. The use of single crystals, considered as model materials, allows us to investigate their intrinsic properties. Characterizations under X-ray illumination of detector devices based on methylammonium lead tribromide (MAPbBr3) single crystals, obtained by optimized growths, show good sensitivity but high dark current density. To improve this critical parameter, while using MAPbBr3 as the base material, we employ anion engineering within the halide elements. We present here mixed halide perovskite crystals, with bromide partially replaced with chloride, obtained through optimized growths using modified inverse temperature crystallization in dimethylformamide, leading to high-quality single crystals of the general formula MAPb(Br1–xClx)3. Six chlorine contents are targeted and carefully determined experimentally via energy-dispersive X-ray analysis and X-ray powder diffraction. For each composition, several crystals are synthesized and used to prepare X-ray detection devices. Their optoelectronic properties are determined under standard X-ray medical conditions and hint at the existence of an optimal composition. MAPb(Br0.85Cl0.15)3 exhibits the best sensitivity with a value of S ≈ 3 μC mGyair–1 cm–2 for RQA5 spectral quality and the lowest dark current density with a value of Jdark ≈ 22 nA mm–2, both recorded at a 50 V mm–1 electric field. This sensitivity value doubles our own MAPbBr3 single crystal device and is higher than that of CsI(Tl)- or a-Se-based flat panels. The present work broadens the benefits and drawbacks of employing halide engineering in perovskite materials to improve the optoelectronic performance under high-energy radiation.

High-gain low-excess-noise MWIR detection with a 3.5-µm cutoff AlInAsSb-based separate absorption, charge, and multiplication avalanche photodiode

Mid-wavelength infrared (MWIR) detection is useful in a variety of scientific and military applications. Avalanche photodiodes can provide an advantage for detection as their internal gain mechanism can increase the system signal-to-noise ratio of a receiver. We demonstrate a separate absorption, charge, and multiplication avalanche photodiode using a digitally grown narrow-bandgap Al0.05InAsSb absorber for MWIR detection and a wide bandgap Al0.7InAsSb multiplier for low-excess-noise amplification. Under 2-µm illumination at 100 K, the device can reach gains over 850. The excess noise factor of the device scales with a low k-factor of ∼0.04. The unity-gain external quantum efficiency of the device attains a peak of 54% (1.02 A/W) at 2.35 µm and maintains an efficiency of 24% (0.58 A/W) at 3 µm before cutting off at ∼3.5 µm. At a gain of 850, the device has a gain-normalized dark current density of 0.05 mA/cm2. This device achieves gains more than double that of the state-of-the-art InAs detectors and achieves gain-normalized dark current densities over two orders of magnitude lower than that of a previously reported MWIR Al0.15InAsSb-based detector.

Suppressed Trap Density Leads to Versatile p‐i‐n Heterojunction Photodiode with Enhanced Photovoltaic/Photodetection Dual‐Function

AbstractMultifunctional integration of optoelectronic devices within a single photodiode is in high demand for next‐generation on‐chip multifunctional integration and Internet of Things applications. Owing to the rapidity of nucleation‐crystallization that is associated with the solution method, large trap state densities (Nt), and consequently, severe dark current densities, as well as recombination losses are generally induced, which are detrimental to the detectivity (D*) of organic photodetectors (OPDs) and to the open‐circuit voltage (VOC) of organic photovoltaics (OPVs). Herein, a versatile p‐i‐n heterojunction organic photodiode with optimized vertical phase distribution and rational solid‐state packing is fabricated via a layer‐by‐layer (LBL) deposition procedure entailing the introduction of a rylene‐fullerene hybrid as a morphological modulator. This precisely controlled photodiode displayed suppressed Nt (2.5 × 1016 cm−3), low‐lying Urbach energy Eu (23.2 meV), as well as synergistically reduced series resistance, dark current, and sub‐bandgap radiative/non‐radiative recombination losses. Consequently, this ternary‐pseudo‐bilayer‐type photodiode exhibits excellent dual‐function performance with an outstanding D* of 9.48 × 1011 Jones in self‐powered OPD mode and a surprisingly suppressed energy loss of 0.538 eV in OPV mode. This study provides important insights into the mechanism and effects of trap density and energy disorder suppression in solution‐processed multifunctional integrated photoelectric conversion diodes.

High-Performance Colloidal Quantum Dot Photodiodes via Suppressing Interface Defects.

PbS colloidal quantum dot (CQD) infrared photodiodes have attracted wide attention due to the prospect of developing cost-effective infrared imaging technology. Presently, ZnO films are widely used as the electron transport layer (ETL) of PbS CQDs infrared photodiodes. However, ZnO-based devices still suffer from the problems of large dark current and low repeatability, which are caused by the low crystallinity and sensitive surface of ZnO films. Here, we effectively optimized the device performance of PbS CQDs infrared photodiode via diminishing the influence of adsorbed H2O at the ZnO/PbS CQDs interface. The polar (002) ZnO crystal plane showed much higher adsorption energy of H2O molecules compared with other nonpolar planes, which could reduce the interface defects induced by detrimentally adsorbed H2O. Based on the sputtering method, we obtained the [002]-oriented and high-crystallinity ZnO ETL and effectively suppressed the adsorption of detrimental H2O molecules. The prepared PbS CQDs infrared photodiode with the sputtered ZnO ETL demonstrated lower dark current density, higher external quantum efficiency, and faster photoresponse compared with the sol-gel ZnO device. Simulation results further unveiled the relationship between interface defects and device dark current. Finally, a high-performance sputtered ZnO/PbS CQDs device was obtained with a specific detectivity of 2.15 × 1012 Jones at -3 dB bandwidth (94.6 kHz).

Vertical‐type 3D/Quasi‐2D n‐p Heterojunction Perovskite Photodetector

AbstractThis research demonstrates a state‐of‐the‐art vertical‐transport photodetector with an n‐type 3D MAPbI3/p‐type quasi‐2D (Q‐2D) BA2MA2Pb3I10 perovskite heterojunction. This structure introduces a ≈0.6 V built‐in electric field at the n‐p junction that greatly improves the characteristics of the perovskite photodetector, and the presence of Q‐2D perovskite on the surface improves the life. The electrical polarities of the 3D and the Q‐2D perovskite layers are simply controlled by self‐constituent doping, making clearly defined n‐p characteristics. Doctor‐blade coating is used to fabricate the photodetector with a large area. The Q‐2D materials with highly oriented (040) Q‐2D (n = 2,3) planes are near the surface, and the (111) preferred planes mixed with high index Q‐2D materials (n = 4,5) are found near the 3D/Q‐2D interface. The stacking and interface are beneficial for carrier extraction and transport, yielding an external quantum efficiency of 77.9%, a carrier lifetime long as 295.7 ns, and a responsibility of 0.41 A W−1. A low dark current density of 6.2 × 10−7 mA cm−2 and a high detectivity of 2.82 × 1013 Jones are obtained. Rise time and fall time are fast as 1.33 and 10.1 µs, respectively. The results show the application potential of 3D/Q‐2D n‐p junction perovskite photodetectors.

2 MeV proton irradiation effect on the performance of InAs/GaSb type-II superlattice long-wave infrared detectors.

In this work, we investigated the effect of 2 MeV proton irradiation on the performance of InAs/GaSb type-II superlattice long-wave infrared detectors by combining ground-based irradiation experiments with Stopping and Range of Ions in Matter (SRIM) computer simulations. We found that irradiation with 2 MeV protons significantly increases the dark current density of the detector by more than two orders of magnitude. At the same time, the quantum efficiency decreases by an order of magnitude. Moreover, proton irradiation degrades the device's performance by raising bulk and surface leakage currents. As the mesa size of the device increases, the proportion of surface leakage current in the total dark current drops. When the mesa size is smaller, the detector irradiation damage is greater. In addition, simulations indicate that the amount of damage produced by 2 MeV proton irradiation rises as irradiation fluences increase. The trends of the irradiation damage produced by the SRIM simulation results and the photocurrent measurements after proton irradiation are consistent, whereas the dark current measurements are identical only at small fluences.

Connecting electrical current conduction and Urbach energy in doped BiFeO3 thin-films

The present study explores the influence of Co2+ and Mn4+ ions on the optical and electrical properties of bismuth ferrite thin films prepared on indium tin oxide-coated glass substrates. In particular, a correlation between the Urbach energy tails and the current conduction is established. Doping-dependent variations in Urbach energy tail widths, optical bandgap energies and J-V curves of the fabricated films are investigated. The Urbach energy tail width of Co-doped BiFeO3 is found to increase by 36%, while that of Mn-doped one is found to reduce by 15%. The optical bandgap of Co-doped and Mn-doped BiFeO3 was found to reduce by 44% and 40% respectively compared to undoped BiFeO3. A correlation between the induced defect states and the optical parameters has been observed. Analysis of J - V characteristics reveals the dependence of the current conduction upon the valence state of the dopant. In particular, the dark current density decreased by almost one order of magnitude with Mn-doping and increased by more than one order with Co-doping. A defect-dependent space-charge-limited conduction (SCLC) mechanism is proposed to understand the J - V curve in the Co-doped and Mn-doped BFO samples. A direct variation of the leakage current density with the Urbach energy suggests that Urbach energy studies could be considered as an alternative indicator of the leakage current density in thin-film BiFeO3-based devices.

High performance InAs0.91Sb0.09 MWIR detectors with an AlAs1-ySby graded barrier

InAs0.91Sb0.09 barrier detectors are an important candidate in realizing high operating temperature middle wavelength infrared (MWIR) photodetectors. Here, an AlAs1-ySby graded barrier layer with linear Sb composition varying from y = 1 to 0.92 is utilized to improve the extraction of photo-generated minority carriers from an InAs0.91Sb0.09 absorber layer. With the modification of valance band misalignment between the absorber and barrier layers, although a hole barrier between AlAs0.08Sb0.92 and InAs0.91Sb0.09 layers exists, the photo-generated holes can be effectively extracted. At 150 K and under zero bias voltage, the maximum quantum efficiency of the devices with a graded barrier reaches ∼56 %, which is much higher than that of the device with a uniform AlAs0.08Sb0.92 barrier. Benefiting from the low dark current density at small reverse bias voltages, a maximum specific detectivity (D*) ∼6.19 × 1011 cm⋅Hz1/2/W is realized at zero bias voltage, which is 50 % higher than the maximum D* ∼4.26 × 1011 cm⋅Hz1/2/W of the uniform barrier device operated at −150 mV bias voltage.

Solution‐Processed AgSbS2 Thin Films for Photodetection

AbstractTernary chalcogenide, silver antimony sulfide (AgSbS2), has emerged with great potential for optoelectronic applications, thanks to its excellent optical properties, facile processability and superior stability. However, high‐performance AgSbS2‐based photodiodes have not been realized mainly due to the large dark current caused by the poor morphology and device leakage. Here, compact AgSbS2 and porous AgSbS2 thin films are fabricated mainly via modulating the sol–gel processes of the precursors. After optimizing the Ag content and the thermal annealing temperature of the AgSbS2 films, AgSbS2 photodiodes with excellent performance metrics, including low dark current density, decent specific detectivity, and superior stability, are achieved. Furthermore, the optimized AgSbS2‐based photodetectors are also introduced for heart rate detection, which exhibit excellent sensitivity and great potential for real applications.

Highly Efficient Ternary Near-Infrared Organic Photodetectors for Biometric Monitoring.

Near-infrared (NIR) small-molecule acceptors that absorb at wavelengths of up to 1000 nm are attractive for applications in organic photodetectors (OPDs) and biometrics. In this study, we incorporated IEICO-4F as the third component for PffBT4T-2OD:PC71BM-based OPDs to provide an efficient NIR response while greatly suppressing the leakage current at reverse bias. By varying the blend ratio and thickness (250-600 nm), we obtained an NIR OPD displaying an ultralow dark-current density (JD = 2.62 nA cm-2), ultrahigh detectivity [D* = 7.2 × 1012 Jones (850 nm)], high sensitivity, and photoresponsivity covering the region from the ultraviolet to the NIR. We used tapping-mode atomic force microscopy, optical microscopy, grazing-incidence wide-angle X-ray scattering, and contact angle measurements to investigate the effect of IEICO-4F on the performance of the ternary OPDs. The low compatibility of PffBT4T-2OD and IEICO-4F, originating from weak intermolecular interactions, allowed us to manipulate the degree of phase separation between the donor and acceptor in the ternary blends, leading to an optimized blend morphology featuring efficient charge separation, transport, and collection. To demonstrate its applicability, we integrated our OPD with two light-emitting diodes and used the system for precisely calculated transmissive pulse oximetry.

Strain‐Tolerant, High‐Detectivity, and Intrinsically Stretchable All‐Polymer Photodiodes

AbstractA skin‐like photodiode (PD) that is stretchable and skin‐conformable is crucial to opening the next‐generation wearable electronics for optical biometric monitoring, biomedical imaging, and others. To achieve reliable PD characteristics under large deformation, stretchable PDs with high detectivity and high mechanical stretchability must be developed. Herein, intrinsically stretchable polymer‐based PDs (is‐PPDs) comprising all‐polymeric constituent layers are demonstrated. In particular, elastomeric photoactive layers consisting of an elastomer with p‐/n‐type semiconducting polymers and conducting polymer‐based stretchable transparent electrodes with modulated work functions improve both the mechanical stability and the detectivity (D*) of is‐PPDs. Accordingly, is‐PPDs show excellent D* over 1013 Jones with a suppressed dark current density of 0.1 nA cm−2 before and after 100% stretching. The proposed is‐PPDs record high‐quality and stable photoplethysmography signals at the wrist with outward extension.

Research on Electro-Optical Characteristics of Infrared Detectors with HgCdTe Operating at Room Temperature.

This paper presents a thorough analysis of the current-voltage characteristics of uncooled HgCdTe detectors optimized for different spectral ranges. HgCdTe heterostructures were grown by means of metal-organic chemical vapor deposition (MOCVD) on GaAs substrates. The obtained detector structures were measured using a Keysight B1500A semiconductor device analyser controlled via LabVIEW for automation. The experimental characteristics were compared with numerical calculations performed using the commercial platform SimuAPSYS (Crosslight). SimuAPSYS supports detector design and allows one to understand different mechanisms occurring in the analysed structures. The dark current density experimental data were compared with theoretical results at a temperature of 300 K for short, medium, and long wavelength infrared ranges. The dark current density of detectors optimized for different wavelengths was determined using various generation-recombination mechanisms. Proper matching between experimental and theoretical data was obtained by shifting the Shockley-Read-Hall carrier lifetime and the Auger-1 and Auger-7 recombination rates. Exemplary spectral responses were also discussed, giving a better insight into detector performance. The matching level was proven with a theoretical evaluation of the zero-bias dynamic resistance-area product (R0A) and the current responsivity of the designed detectors.

Dark current in monolithic extended-SWIR GeSn PIN photodetectors

Monolithic integration of extended short-wave infrared photodetectors (PDs) on silicon is highly sought-after to implement manufacturable, cost-effective sensing and imaging technologies. With this perspective, GeSn PIN PDs have been the subject of extensive investigations because of their bandgap tunability and silicon compatibility. However, due to growth defects, these PDs suffer a relatively high dark current density as compared to commercial III–V PDs. Herein, we elucidate the mechanisms governing the dark current in 2.6 μm GeSn PDs at a Sn content of 10 at. %. It was found that in the temperature range of 293–363 K and at low bias, the diffusion and Shockley–Read–Hall (SRH) leakage mechanisms dominate the dark current in small diameter (20 μm) devices, while combined SRH and trap assisted tunneling (TAT) leakage mechanisms are prominent in larger diameter (160 μm) devices. However, at high reverse bias, the TAT leakage mechanism becomes dominant regardless of the operating temperature and device size. The effective non-radiative carrier lifetime in these devices was found to reach ∼100–150 ps at low bias. Owing to TAT leakage current, however, this lifetime reduces progressively as the bias increases.

Superior Performances of Self-Driven Near-Infrared Photodetectors Based on the SnTe:Si/Si Heterostructure Boosted by Bulk Photovoltaic Effect.

The upsurge of new materials that can be used for near-infrared (NIR) photodetectors operated without cooling is crucial. As a novel material with a small bandgap of ≈0.28eV, the topological crystalline insulator SnTe has attracted considerable attention. Herein, this work demonstrates self-driven NIR photodetectors based on SnTe/Si and SnTe:Si/Si heterostructures. The SnTe/Si heterostructure has a high detectivity D* of 3.3 × 1012 Jones. By Si doping, the SnTe:Si/Si heterostructure reduces the dark current density and increases the photocurrent by ≈1 order of magnitude simultaneously, which improves the detectivity D* by ≈2 orders of magnitude up to 1.59×1014 Jones. Further theoretical analysis indicates that the improved device performance may be ascribed to the bulk photovoltaic effect (BPVE), in which doped Si atoms break the inversion symmetry and thus enable the generation of additional photocurrents beyond the heterostructure. In addition, the external quantum efficiency (EQE) measured at room temperature at 850nm increases by a factor of 7.5 times, from 38.5% to 289%. A high responsivity of 1979mA W-1 without bias and fast rising time of 8 µs are also observed. The significantly improved photodetection achieved by the Si doping is of great interest and may provide a novel strategy for superior photodetectors.

Scalable fabrication of self-assembled GeSn vertical nanowires for nanophotonic applications.

In this work, scalable fabrication of self-assembled GeSn vertical nanowires (NWs) based on rapid thermal annealing (RTA) and inductively coupled-plasma (ICP) dry etching was proposed. After thermal treatment of molecular-beam-epitaxy-grown GeSn, self-assembled Sn nanodots (NDs) were formed on surface and the spontaneous emission from GeSn direct band was enhanced by ∼5-fold. Employing the self-assembled Sn NDs as template, vertical GeSn NWs with a diameter of 25 ± 6nm and a density of 2.8 × 109cm-2 were obtained by Cl-based ICP dry etching technique. A prototype GeSn NW photodetector (PD) with rapid switching ability was demonstrated and the optoelectronic performance of Ge NW PD was systematically studied. The GeSn NW PD exhibited an ultralow dark current density of ∼33 nA/cm2 with a responsivity of 0.245 A/W and a high specific detectivity of 2.40 × 1012cmHz1/2W-1 at 1550nm under -1V at 77K. The results prove that this method is prospective for low-cost and scalable fabrication of GeSn NWs, which are promising for near infrared or short wavelength infrared nanophotonic devices.