Organic X-Ray Image Sensors Using a Medium Bandgap Polymer Donor with Low Dark Current

The development of portable X-ray detectors is necessary for diagnosing fractures in unconscious patients in emergency situations.However,this is quite challenging because of the heavy weight of the scintillator and silicon photodetectors.The weight and thickness of X-ray detectors can be reduced by replacing the silicon layer with an organic photodetectors.This study presents a novel bithienopyrroledione-based polymer donor that exhibits excellent photodetection properties even in a thick photoactive layer(~700 nm),owing to the symmetric backbone and highly soluble molecular structure of bithienopyrroledione.The ability of bithienopyrroledione-based polymer donor to strongly suppress the dark current density(Jd~10−10 A cm^(−2))at a negative bias(−2.0 V)while maintaining high responsivity(R=0.29 A W−1)even at a thickness of 700 nm results in a maximum shot-noise-limited specific detectivity of D_(sh)^(*)=2.18×10^(13)Jones in the organic photodetectors.Printed organic photodetectors are developed by slot-die coating for use in X-ray detectors,which exhibit D_(sh)^(*)=2.73×10^(12)Jones with clear rising(0.26 s)and falling(0.29 s)response times upon X-ray irradiation.Detection reliability is also proven by linear response of the X-ray detector,and the X-ray detection limit is 3 mA.

Strain-induced the dark current characteristics in InAs/GaSb type-Ⅱ superlattice for mid-wave detector

Type-Ⅱsuperlattice(T2SL)materials are the key element for infrared(IR)detectors.However,it is well known that the characteristics of the detectors with the T2SL layer are greatly affected by the strain developed during the growth process,which determines the performance of IR detectors.Therefore,great efforts have been made to properly control the strain effect and develop relevant analysis methods to evaluate the strain-induced dark current characteristics.In this work,we report the strain-induced dark current characteristics in InAs/GaSb T2SL MWIR photodetector.The overall strain of InAs/GaSb T2SL layer was analyzed by both high-resolution X-ray diffraction(HRXRD)and the dark current measured from the absorber layer at the elevated temperatures(≥110 K),where the major leakage current component is originated from the reduced minority carrier lifetime in the absorber layer.Our findings indicate that minority carrier lifetime increases as the tensile strain on the InAs/GaSb T2SL is more compensated by the compressive strain through‘InSb-like’interface,which reduces the dark current density of the device.Specifically,tensile strain compensated devices exhibited the dark current density of less than 2×10^-5 A/cm^2 at 120 K,which is more than one order of magnitude lower value compared to that of the device without tensile strain relaxation.

Scalability of dark current in silicon PIN photodiode

The mechanism for electrical conduction is investigated by the dark temperature-dependent current–voltage characteristics of Si PIN photodiodes with different photosensitive areas.The characteristic tunneling energy E（00） can be obtained to be 1.40 me V,1.53 me V,1.74 me V,1.87 me V,and 2.01 me V,respectively,for the photodiodes with L = 0.25 mm,0.5 mm,1 mm,1.5 mm,and 2 mm by fitting the ideality factor n versus temperature curves according to the tunneling-enhanced recombination mechanism.The trap-assisted tunneling-enhanced recombination in the i-layer plays an important role in our device,which is consistent with the experimental result that area-dependent leakage current is dominant with the side length larger than 1 mm of the photosensitive area.Our results reveal that the quality of the bulk material plays an important role in the electrical conduction mechanism of the devices with the side length larger than 1 mm of the photosensitive area.

Influence of Different Surface Modifications on the Photovoltaic Performance and Dark Current of Dye-Sensitized Solar Cells

The TiO2 nanoporous film photoelectrode, as a crucial component of dye-sensitized solar cells, has been investigated. The photovoltaic properties and the dark current were studied by two surface modification methods. One was to apply a compact layer between the conductive glass substrate and nanoporous TiO2 film. Another was to produce TiO2 nanoparticles among the microstructure by TICl4 treatment. A suitable concentration and number of times for TICl4 treatment were found in our experiment. The dark current is suppressed by surface modifications, leading to a significant improvement in the solar cells performance. An excessive concentration of TICl4 will produce more surface states and introduce a larger dark current reversely. The dye is also regarded as a source of charge recombination in dark to some extent, due to an amount of surface protonations introduced by the interracial link in the conductive glass substrate/dye interface and dye/TiO2 interface.

NEW DARK CURRENT SUPPRESSION CMOS READOUT CIRCUIT WITH NOVEL CDS STRUCTURE FOR LARGE FORMAT QWIP FPA

A new Dark Current Suppression (DCS) CMOS readout circuits for large format Quantum-Well-Infrared Photo-detector (QWIP) Focal-Plane-Array (FPA) with novel Correlated-Double-Sampling (CDS) structure based on dynamic source-follower are proposed, which can overcome the drawbacks of the present techniques, such as sensitive to the non-uniformity of the QWIP materials, poor readout noise features, low frame frequency, limited injection efficiency and dynamic range, etc. The dummy is adopted to realize dark current suppression, while the cascode current mirror (with current ratio of 1:10) can increase charge sensitivity and reduce integration time. Through the novel CDS structure, the output waveform is boxcar, and the frame frequency is increased. Simulation results demonstrate that, in high background sense, the proposed DCS circuit can suppress the dark current, achieve good readout performance, such as low power consumption, high charge sensitivity, high resolution, large dynamic range, and insensitive to the non-uniformity of the QWIP materials.

New dark current component of InGaAs/InP HPDs confirmed by DLTS

The dark current of In_(0.47) Ga_(0.53) As/InP heterojunction photodiodes (HPDs) was analysed. We found that there exists a new dark current component──deep level-assisted tunnelling current.DLTS was used to measure the In_(0.47)Ga_(0.53)As/InP HPDs. An electronic trap which has a thermal activation energy of O.44 eV, level concentration of 3.10×10￣(13)cm￣(-3) and electronic capture cross section of 1.72×10￣(12)cm￣2 has been found.It's existence results in the new tunnelling current.

Dark Current Compensation and Sensitivity Adjustment on Gallium Arsenide Linear Array Detector for X-Ray Imaging

For the last several years, the linear array x-ray detector for x-ray imaging with gallium arsenide direct conversion sensitive elements has been developed and tested at the In-stitute for High Energy Physics. The array consists of 16 sensitive modules. Each module has 128 gallium arsenide (GaAs) sensitive elements with 200 μm pitch. Current article describes two key program procedures of initial dark current compensation of each sensitive element in the linear array, and sensitivity adjustment for alignment of strip pattern in the raw image data. As a part of evaluation process a modular transfer function (MTF) was measured with the slanted sharp-edge object under RQA5 technique as it described in the International Electrotechnical Commission 62220-1 standard (high voltage 70 kVp, additional aluminium filter 21 mm) for images with compensated dark currents and adjusted sensitivity of detector elements. The 10% level of the calculated MTF function has spatial resolution within 2 - 3 pair of lines per mm for both vertical and horizontal orientation.

CIS生产工艺对暗电流(Dark Current)性能的影响

研究工艺对CIS图像传感器(CMOS image sensor)的影响。通过隔离注入的优化、沉积薄膜膜质的优化、干法刻蚀工艺的优化及热制程的优化可减少硅氧界面载流子与声子群的散射,可大大减少Si-SiO2界面附近陷阱,从而降低CIS传感器的暗电流(Dark Current,DC)。实验数据表明,暗电流可改善30%~82.5%,可适用于不同像素尺寸(0.7~18μm)的CIS产品。

Optimizing theπ-Bridge of Non-fullerene Acceptors to Suppress Dark Current in NIR Organic Photodetectors

Recently,the rapid development of non-fullerene acceptors(NFAs)has laid the foundation for performance improvements in near-infrared(NIR)organic photodetectors(OPDs).However,reducing the bandgap of NFAs to achieve strong absorption in the shorter-wave region usually leads to increased dark current density(J_(d))and decreased responsivity(R),severely limiting the detectivity(D*)of NIR-OPDs.To date,it remains challenging to manipulate the J_(d) of NIR-OPDs through rational structure engineering of NFAs.Herein,three NIR-NFAs,namely bis(2-decyltetradecyl)4,4′-(2′,7′-di-tert-butylspiro[cyclopenta[2,1-b:3,4-b′]dithiophene-4,9′-fluorene]-2,6-diyl)bis(6-(((Z)-1-(dicyanomethylene)-5,6-difluoro-3-oxo-1,3-dihydro-2H-inden-2-ylidene)methyl)thieno[3,4-b]thiophene-2-carboxylate)(TSIC-4F),bis(2-decyltetradecyl)6,6′-(2′,7′-di-tert-butylspiro[cyclopenta[2,1-b:3,4-b′]dithiophene-4,9′-fluorene]-2,6-diyl)bis(4-(((Z)-1-(dicyanomethylene)-5,6-difluoro-3-oxo-1,3-dihydro-2H-inden-2-ylidene)methyl)thieno[3,4-b]thiophene-2-carboxylate)(STIC-4F),and 2,2′-((2Z,2′Z)-(((2′,7′-di-tert-butylspiro[cyclopenta[2,1-b:3,4-b′]dithiophene-4,9′-fluorene]-2,6-diyl)bis(2,3-bis(5-(2-butyloctyl)thiophen-2-yl)thieno[3,4-b]pyrazine-7,5-diyl))bis(metha-neylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile(TPIC-4F),were designed using the thieno[3,4-b]thiophene(TT)and thieno[3,4-b]pyrazine(TPy)derivatives as theπ-bridge.Owing to the intramolecular S-S and S-N interactions,STIC-4F and TPIC-4F exhibited smaller backbone distortions than TSIC-4F.A significantly red-shifted absorption with a peak at 1015 nm was observed in TPIC-4F film,larger than that(ca.960 nm)for TSIC-4F and STIC-4F films.Moreover,OPDs operating in a photovoltaic mode were successfully fabricated,and TPIC-4F-based OPDs achieved the lowest J_(d) of 3.18×10^(-8) A/cm^(2) at-0.1 V.Impressively,although TPIC-4F-based OPDs exhibited the lowest R,higher shot-noise-limited specific detectivity(D_(sh)*)in 1000-1200 nm could be achieved due to its lowest J_(d).This study underscored the effectiveness of optimizing theπ-bridge structure of NFAs to suppress J_(d),ultimately attaining higher D_(sh)*in the NIR region.

Metal–organic framework wafer enabled fast response radiation detection with ultra-low dark current

Semiconductive metal–organic frameworks(MOFs)have attracted great interest for the electronic applications.However,dark currents of present hybrid organic–inorganic materials are 1000–10,000 times higher than those of commercial inorganic detectors,leading to poor charge transportation.Here,we demonstrate a ZIF-8(Zn(mim)_(2),mim=2-methylimidazolate)wafer with ultra-low dark current of 1.27 pA·mm^(-2) under high electric fields of 322 V·mm^(-1).The isostatic pressing preparation process provides ZIF-8 wafers with good transmittance.Besides,the presence of redox-active metals and small spatial separation between components promotes the charge hopping.The ZIF-8-based semiconductor detector shows promising X-ray detection sensitivity of 70.82μC·Gy^(-1)·cm^(-2) with low doses exposures,contributing to superior X-ray imaging capability with a relatively high spatial resolution of 1.2 lp·mm^(-1).Simultaneously,good peak discrimination with the energy resolution of~43.78%is disclosed when the detector is illuminated by uncollimated 241Am@5.48 MeVα-particles.These results provide a broad prospect of MOFs for future radiation detection applications.

Mid-wavelength nBn photodetector with high operating temperature and low dark current based on InAs/InAsSb superlattice absorber

In this paper,we demonstrate nBn InAs/InAsSb type II superlattice(T2SL)photodetectors with AlAsSb as the barrier that targets mid-wavelength infrared(MWIR)detection.To improve operating temperature and suppress dark current,a specific Sb soaking technique was employed to improve the interface abruptness of the superlattice with device passivation using a SiO_(2) layer.These result in ultralow dark current density of 6.28×10^(-6)A/cm^(2)and 0.31 A/cm^(2)under-600 mV at 97 K and297 K,respectively,which is lower than most reported InAs/InAsSb-based MWIR photodetectors.Corresponding resistance area product values of 3.20×10^(4)Ω·cm^(2)and 1.32Ω·cm^(2)were obtained at 97 K and 297 K.A peak responsivity of 0.39 A/W with a cutoff wavelength around 5.5μm and a peak detectivity of 2.1×10^(9)cm·Hz^(1/2)/W were obtained at a high operating temperature up to 237 K.

Influence of hydrogenation on the dark current mechanism of HgCdTe photovoltaic detectors

The influence of hydrogenation on the dark current mechanism ofHgCdTe photovoltaic detectors is studied. The hydrogenation is achieved by exposing samples to a H2/Ar plasma atmosphere that was produced during a reactive ion etching process. A set of variable-area photomask was specially designed to evaluate the hydrogenation effect. It was found that the current-voltage characteristics were gradually improved when detectors were hydrogenated by different areas. The fitting results of experimental results at reverse bias conditions sustained that the improvement of current-voltage curves was due to the suppression of trap assisted tunneling current and the enhancement of minority lifetime in the depletion region. It was also found that the dominative forward current was gradually converted from a generation-recombination current to a diffusion current with the enlargement of the hydrogenation area, which was infered from the ideality factors by abstraction of forward resistance-voltage curves of different detectors.

Dark current modeling of thick perovskite X‑ray detectors

Metal halide perovskites(MHPs)have demonstrated excellent performances in detection of X-rays and gamma-rays.Most studies focus on improving the sensitivity of single-pixel MHP detectors.However,little work pays attention to the dark current,which is crucial for the back-end circuit integration.Herein,the requirement of dark current is quantitatively evaluated as low as 10^(−9)A/cm^(2)for X-ray imagers integrated on pixel circuits.Moreover,through the semiconductor device analysis and simulation,we reveal that the main current compositions of thick perovskite X-ray detectors are the thermionic-emission current(J_(T))and the generation-recombination current(J_(g-r)).The typical observed failures of p-n junctions in thick detectors are caused by the high generation-recombination current due to the band mismatch and interface defects.This work provides a deep insight into the design of high sensitivity and low dark current perovskite X-ray detectors.

Simulation of the effects of defects in low temperature Ge buffer layer on dark current of Si-based Ge photodiodes

The influence of defects in low temperature Ge layer on electrical characteristics of p-Ge/i-Ge/n-Si and n-Ge/i-Ge/p-Ge photodiodes（PDs） was studied.Due to a two-step growth method,there are high defect densities in low-temperature buffer Ge layer.It is shown that the defects in low-temperature Ge layer change the band diagrams and the distribution of electric field,leading to the increase of the total dark current for p-Ge/i-Ge/n-Si PDs,whereas these defects have no influence on the dark current for n-Ge/i-Ge/p-Ge PDs.As a complement,a three-dimensional simulation of the total current under illumination was also performed.

Dark current mechanism of unpassivated mid wavelength type II InAs/GaSb superlattice infrared photodetector

We investigate the dark current mechanism for an unpassivated mid wavelength(MW) type II InAs/GaSb superlattice infrared photodetector by doing the variablearea diode tests. The bulk resistance-area product and the resistivity due to the surface current are determined to be17.72 X cm2 and 704.23 X cm at 77 K, respectively. It is found that for all the mesa sizes used, the dark current is dominated or predominated by the surface component, and with scaling back the mesa size, the surface current increases while the bulk component decreases. The activation energy is determined to be 145 meV for the temperature range around 140–280 K, while it is 6 meV when temperature is below 100 K. It is also found that the dark current is dominated by the generation-recombination current for the MW device when temperature is between140 and 280 K.

碲镉汞PIN结构雪崩器件的I区材料晶体质量研究

本文对中波红外PIN结构的碲镉汞(HgCdTe)雪崩器件关键雪崩区域的材料晶体质量进行研究。通过在实验材料上对PIN结构雪崩器件的全过程工艺模拟,采用微分霍尔、微分少子寿命等测试手段进行材料表征,评估获得了关键雪崩区域的真实材料晶体质量。研究发现,现有优化工艺下雪崩区域的晶体质量良好,拟合材料的SRH寿命最好能达到20.7μs,可达到原生材料SRH寿命的相当水平,满足高质量中波碲镉汞雪崩器件的研制要求。同时,我们以获得的雪崩区域SRH寿命为基础,对HgCdTe APD结构器件进行相应2维数值模拟,获得理论最优的暗电流密度8.7×10^(-10) A/cm^(2)。

nBn结构长波红外碲镉汞器件优化设计

分析了Type-I型能带对nBn结构碲镉汞器件性能的影响。通过理论计算获得了势垒层组分、掺杂浓度与能带带阶的关系,确定了nBn结构长波器件吸收层掺杂浓度与暗电流的关系。优化了nBn结构长波红外碲镉汞器件的掺杂浓度、势垒层与吸收层之间的组分过渡,建立了二维器件仿真模型并对nBn结构长波红外碲镉汞器件的能带结构进行了计算,结果表明器件结构参数的优化可以有效降低器件工作所需的开启电压,同时在吸收层内几乎不会形成耗尽区,从而有效抑制SRH产生-复合电流及隧穿电流。计算了器件结构参数优化后的长波红外碲镉汞nBn器件暗电流的变温特性,器件工作温度达到110 K以上。为高性能势垒结构长波红外碲镉汞器件的研制提供了理论依据。

具有组分梯度的HgCdTe探测器在激光测量中的潜在应用

该文将组分梯度引入HgCdTe探测器结构设计中,提出了一种可以降低PN结附近热激发载流子浓度的方法。建立暗电流机制模型,分析高温下暗电流成份,分析结果表明,降低热激发载流子浓度对结区的影响是提高器件工作温度的关键。利用组分梯度在PN结附近构建不同的电场,不同电场下样品暗电流和噪声电流随温度的变化曲线表明,构建的电场越强,降低结区附近热激发载流子浓度的效果越明显。通过数据分析,提出构建103 V/cm量级的组分梯度内建电场可抑制热激发载流子向结区的扩散运动,有效地降低结区附近热激发载流子浓度。

Investigation on p-type doping of PBn unipolar barrier InAsSb photodetectors

The lattice-matched XBn structures of InAsSb,grown on GaSb substrates,exhibit high crystal quali⁃ty,and can achieve extremely low dark currents at high operating temperatures(HOT).Its superior performance is attributed to the unipolar barrier,which blocks the majority carriers while allowing unhindered hole transport.To further explore the energy band and carrier transport mechanisms of the XBn unipolar barrier structure,this pa⁃per systematically investigates the influence of doping on the dark current,photocurrent,and tunneling character⁃istics of InAsSb photodetectors in the PBn structure.Three high-quality InAsSb samples with unintentionally doped absorption layers(AL)were prepared,with varying p-type doping concentrations in the GaSb contact layer(CL)and the AlAsSb barrier layer(BL).As the p-type doping concentration in the CL increased,the device’s turn-on bias voltage also increased,and p-type doping in the BL led to tunneling occurring at lower bias voltages.For the sample with UID BL,which exhibited an extremely low dark current of 5×10^(-6) A/cm^(2).The photocurrent characteristics were well-fitted using the back-to-back diode model,revealing the presence of two opposing space charge regions on either side of the BL.

Double-ended passivator enables dark-current-suppressed colloidal quantum dot photodiodes for CMOS-integrated infrared imagers

Lead sulfide(PbS)colloidal quantum dot(CQD)photodiodes integrated with silicon-based readout integrated circuits(ROICs)offer a promising solution for the next-generation short-wave infrared(SWIR)imaging technology.Despite their potential,large-size CQD photodiodes pose a challenge due to high dark currents resulting from surface states on nonpassivated(100)facets and trap states generated by CQD fusion.In this work,we present a novel approach to address this issue by introducing double-ended ligands that supplementally passivate(100)facets of halidecapped large-size CQDs,leading to suppressed bandtail states and reduced defect concentration.Our results demonstrate that the dark current density is highly suppressed by about an order of magnitude to 9.6 nA cm^(2) at -10 mV,which is among the lowest reported for PbS CQD photodiodes.Furthermore,the performance of the photodiodes is exemplary,yielding an external quantum efficiency of 50.8%(which corresponds to a responsivity of 0.532 A W^(-1))and a specific detectivity of 2.5×10^(12) Jones at 1300 nm.By integrating CQD photodiodes with CMOS ROICs,the CQD imager provides high-resolution(640×512)SWIR imaging for infrared penetration and material discrimination.