Dark Current Generation Research Articles

Characterization and Calibration of a Photodiode Based Pyranometer for Solar Energy Applications

<p>In the domain of solar science, a pyranometer assumes a pivotal position, functioning as an indispensable tool for appraising solar irradiance across the abbreviated wavelengths of the solar spectrum, spanning from 300 to 3000 nanometers. Conventional pyranometers typically employ intricate and labour intensive thermopiles. These devices rely on arrays of thermocouples interconnected either serially or in parallel and necessitate rare earth minerals for efficient operation. In this investigation, we advocate for an innovative methodology employing photodetectors, specifically photodiodes and phototransistors, to surmount the limitations linked with traditional pyranometers. Photodiodes, commonly utilized for this purpose, proffer numerous benefits, including accelerated response times, customizable spectral range selection, and cost-effectiveness. Nonetheless, they cannot entirely supplant thermopiles owing to impediments such as the generation of dark current in the absence of illumination, temperature-dependent functionalities, and saturation at elevated irradiance levels. To counteract these constraints, we have devised a pioneering design of a photodiode-based pyranometer that incorporates a data acquisition system and correction mechanism. This innovative approach furnishes a feasible resolution for autonomous solar radiation measurement, furnishing enhanced precision and dependability.</p>

Black Arsenic Phosphorus Mid-Wave Infrared Barrier Detector with High Detectivity at Room Temperature.

The barrier structure is designed to enhance the operating temperature of the infrared detector, thereby improving the efficiency of collecting photogenerated carriers and reducing dark current generation, without suppressing the photocurrent. However, the development of barrier detectors using conventional materials is limited due to the strict requirements for lattice and band matching. In this study, a high-performance unipolar barrier detector is designed utilizing a black arsenic phosphorus/molybdenum disulfide/black phosphorus van der Waals heterojunction. The device exhibits a broad response bandwidth ranging from visible light to mid-wave infrared (520nm to 4.6 µm), with a blackbody detectivity of 2.7× 1010 cmHz-1/2 W-1 in the mid-wave infrared range at room temperature. Moreover, the optical absorption anisotropy of black arsenic phosphorus enables polarization resolution detection, achieving a polarization extinction ratio of 35.5 at 4.6 µm. Mid-wave infrared imaging of the device is successfully demonstrated at room temperature, highlighting the significant potential of barrier devices based on van der Waals heterojunctions in mid-wave infrared detection.

Design study and spectroscopic performance of SOI pixel detector with a pinned depleted diode structure for X-ray astronomy

We have been developing silicon-on-insulator (SOI) pixel detectors with a pinned depleted diode (PDD) structure, named “XRPIX”, for X-ray astronomy. The PDD structure is formed in a thick p-type substrate, to which high negative voltage is applied to make it fully depleted. A pinned p-well is introduced at the backside of the insulator layer to reduce a dark current generation at the Si-SiO2 interface and to fix the back-gate voltage of the SOI transistors. An n-well is further introduced between the p-well and the substrate to make a potential barrier between them and suppress a leakage current. An optimization study on the n-well dopant concentration is necessary because a higher dopant concentration could result in a higher potential barrier but also in a larger sense-node capacitance leading to a lower spectroscopic performance, and vice versa. Based on a device simulation, we fabricated five candidate chips having different n-well dopant concentrations. We successfully found out the best n-well design, which suppressed a large leakage current and showed satisfactory X-ray spectroscopic performance. Too low and too high n-well dopant concentration chips showed a large leakage current and degraded X-ray spectroscopic performance, respectively. We also found that the dependency of X-ray spectroscopic performance on the n-well dopant concentration can be largely explained by the difference in sense-node capacitance.

Monolithically Integrable SQW High Speed Waveguide Photodiode With InGaAs/InP Graded Bandgap QW and SL Cladding

A monolithically integrable p-i-n waveguide photodiode (WGPD) has been theoretically analyzed and experimentally demonstrated. The p-i-n WGPD comprises of a single quantum-well absorbing layer, graded band gap quantum well architecture along with superlattice cladding designed through electrical, optical, and RF optimizations. Better tolerance and responsivity are achieved by integrating fiber-matched waveguide to the WGPD using a laterally tapered waveguide interconnect. A 30 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ \mu$</tex-math></inline-formula> m long WGPD exhibits a low dark current density of 5.31 mA/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$cm^{2}$</tex-math></inline-formula> at −3 V due to the existence of near mid-gap traps. The electrons and holes tunneling at higher electric fields further enhance the dark current generation. The WGPDs with 3 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> m mesa width and lengths of 10, 20, 30, 40, and 50 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> m are fabricated, studied electrically and optically, for a maximum responsivity of 0.56 A/W. Instrument limited 20 GHz 3-dB bandwidths at 1550 nm is demonstrated, which are better than any PDs with i-QW architecture. 3-dB bandwidth of over 100 Gbps is estimated from the measured scattering parameter and simulation. This device is suitable for low dark current, high responsivity, and high bandwidth performance, allowing integration of other optical components with a high-speed photodiode, without any regrowth requirements.

Applicability of Large-Area Single-Photon Counting Detectors Timepix for High-Resolution and High-Contrast X-Ray Imaging of Biological Samples

High-resolution X-ray imaging techniques, usually known as microradiography and micro-computed tomography (CT), have become highly required and frequently used tools for biology, biomedical and preclinical research. State-of-the-art micro-CT scanners are capable of achieving a spatial resolution of few micrometers or even less, thanks to the constant development of compact microfocus X-ray sources together with simultaneous progress in detector technologies. The current standard in X-ray detection is a digital imaging device containing read-out circuitry coupled with a scintillation sensor. Such detectors are available in a variety of different sizes, and are easy to use and relatively affordable. Nevertheless, the mentioned technology suffers from inherent limitations like, for example, undesirable generation of dark current, that compromise the quality of the provided data. This work demonstrates the applicability of Timepix large-area hybrid-pixel photon-counting detectors (PCDs) for high-resolution X-ray imaging in biology research. Photon-counting detection technology provides dark-current-free quantum-counting operation. Therefore, an enhanced contrast-to-noise ratio of the acquired data is achieved. Furthermore, the biased semiconductor sensor achieves almost ideal point-spread function (PSF) resulting in images with high spatial-resolution. Both above-mentioned features make PCDs to be excellent tools for high-resolution X-ray imaging especially for samples with low intrinsic absorption contrast. We evaluated the imaging performance of large-area Timepix detectors compared to a state-of-the-art flat-panel detector dedicated for high-resolution X-ray imaging. The presented data demonstrate the versatility of the used detectors as it covers a wide range of samples from laboratory animals to single-cell marine organisms.

Tunable Quantum Tunneling through a Graphene/Bi2Se3 Heterointerface for the Hybrid Photodetection Mechanism.

Graphene-based van der Waals heterostructures are promising building blocks for broadband photodetection because of the gapless nature of graphene. However, their performance is mostly limited by the inevitable trade-off between low dark current and photocurrent generation. Here, we demonstrate a hybrid photodetection mode based on the photogating effect coupled with the photovoltaic effect via tunable quantum tunneling through the unique graphene/Bi2Se3 heterointerface. The tunneling junction formed between the semimetallic graphene and the topologically insulating Bi2Se3 exhibits asymmetric rectifying and hysteretic current–voltage characteristics, which significantly suppresses the dark current and enhances the photocurrent. The photocurrent-to-dark current ratio increases by about a factor of 10 with the electrical tuning of tunneling resistance for efficient light detection covering the major photonic spectral band from the visible to the mid-infrared ranges. Our findings provide a novel concept of using tunable quantum tunneling for highly sensitive broadband photodetection in mixed-dimensional van der Waals heterostructures.

Minimization of bandstructure dependent dark current in InAs/GaAs quantum dot photodetectors

The paper presents a theoretical model which illustrates the electron capture and emission dynamics in InAs/GaAs quantum dot (QD) photodetector and describes the bandstructure dependent dark current generation. Result of the study predicts the possibility of excessive dark current that depends on the different rates of thermal and phonon assisted tunneling emissions from the quantized energy states. Variation of QD size can alter the quantized states and control the emission rates. Our findings shows that high dark current generated in a QD at a particular temperature and bias voltage can be reduced significantly by changing the size. Hence, optimum QD size is proposed that generates minimum the dark current at different temperatures of operation and applied bias. • Proposes a theoretical model on dark current generation in InAs/GaAs quantum dot photodetectors. • Illustrates modification of quantum dot size for altering the bandstructure. • Illustrates bandstructure dependent dark current generation mechanism at different voltages and operating temperatures. • Proposes optimizing the quantum dot size for minimizing dark current at different voltages and operating temperatures. • This work is a guideline for minimizing dark current and improve the performance of quantum dot photodetectors.

CMOS Image Sensors and Plasma Processes: How PMD Nitride Charging Acts on the Dark Current.

Plasma processes are known to be prone to inducing damage by charging effects. For CMOS image sensors, this can lead to dark current degradation both in value and uniformity. An in-depth analysis, motivated by the different degrading behavior of two different plasma processes, has been performed in order to determine the degradation mechanisms associated with one plasma process. It is based on in situ plasma-induced charge characterization techniques for various dielectric stack structures (dielectric nature and stack configuration). A degradation mechanism is proposed, highlighting the role of ultraviolet (UV) light from the plasma in creating an electron hole which induces positive charges in the nitride layer at the wafer center, and negative ones at the edge. The trapped charges de-passivate the SiO2/Si interface by inducing a depleted interface above the photodiode, thus emphasizing the generation of dark current. A good correlation between the spatial distribution of the total charges and the value of dark current has been observed.

Integration of an InSb photodetector on Si via heteroepitaxy for the mid-infrared wavelength region

In this study, InSb p-i-n photodetectors with In0.82Al0.18Sb barrier layers were grown on a (100) 6° offcut Si substrate by heteroepitaxy via an AlSb/GaSb buffer. Based on an interfacial misfit array growth mode, the dislocations at the GaSb/Si and InSb/AlSb interfaces accommodated the lattice mismatch. The In0.82Al0.18Sb barrier layer increased the 77 K R0A of the detector. From 180 K to 300 K, the generation-recombination mechanism dominated the dark current generation in the detector and surface leakage became dominant below 120 K. The detector exhibited a 77 K responsivity of 0.475 A/W and a Johnson-noise-limited detectivity of 3.08 × 109 cmHz1/2W-1 at 5.3 µm.

2-4 a-Si : H p-i-n(SI sub.)アバランシェフォトダイオードの光電流増倍および暗電流発生機構の検討

2-4 a-Si : H p-i-n(SI sub.)アバランシェフォトダイオードの光電流増倍および暗電流発生機構の検討

Modelling of high-temperature dark current in multi-quantum well structures from MWIR to VLWIR

In this paper, a model to calculate the dark current of quantum well infrared photodetectors at high-temperature regime is presented. The model is derived from a positive-definite quantum probability-flux and considers thermionic emission and thermally-assisted tunnelling as mechanisms of dark current generation. Its main input data are the wave functions obtained by time-independent Schrodinger equation and it does not require empirical parameters related to the transport of carriers. By means of this model, the dark current of quantum well infrared photodetectors at high-temperature regime is investigated with respect to the temperature, the barrier width, the applied electric field and the position of the first excited state. The theoretical results are compared with experimental data obtained from lattice-matched InAlAs/InGaAs, InGaAsP/InP on InP substrate and AlGaAs/GaAs structures with rectangular wells and symmetric barriers, whose absorption peak wavelengths range from MWIR to VLWIR. The corresponding results are in a good agreement with experimental data at different temperatures and at a wide range of applied electric field.

Enhancement in multicolor photoresponse for quaternary capped In0.5Ga0.5As/GaAs quantum dot infrared photodetectors implanted with hydrogen ions

In(Ga)As/GaAs-based quantum dot infrared photodetectors (QDIPs) are among the most efficient devices in the mid-wavelength infrared and long-wavelength infrared regions for various defense and space application purposes. Considering the importance of the results reported so far on In(Ga)As/GaAs QDIPs, here we had tried to develop a post-growth method for enhancing QDIP characteristics using both low energy and high energy light ion (hydrogen) implantations. The field-assisted tunneling process of dark current generation was suppressed due to the hydrogen ion implantation, even at a very high operational bias. A stronger multicolor photo-response was obtained for devices implanted with low energy hydrogen ions. From experimental results, we proposed a device model which explains the improved QDIPs performance caused by hydrogen ion implantation.

Increasing peak detectivity (D*) of In 0.5 Ga 0.5 As/GaAs quantum dot infrared photodetectors by up to two orders with high‐energy proton implantation

In(Ga)As/GaAs-based quantum dot infrared photodetectors (QDIPs) are among the most efficient devices for use in the mid- and long-wavelength infrared regions, making them suitable for various defence and space applications. Considering previously reported results on In(Ga)As/GaAs QDIPs, a post-growth method is investigated to improve QDIP characteristics using high-energy proton implantation. It was found that implantation suppressed the field-assisted tunnelling of dark current generation, which decreased the dark current density by three orders, whereas the peak detectivity (D*) in the implanted devices increased by up to two orders of magnitude, from 6.1 × 108 to 1.0 × 1010 cm-Hz1/2/W at 77 K.

Simulation of dark current and dark current-induced background photons in the Thomson scattering X-ray source

A model of dark current generation in the photocathode radio-frequency (RF) gun is established in the Thomson scattering X-ray source, and dark current transport and losses along the beamline are simulated. A velocity bunching cavity is added between the RF gun and the first linac to achieve the longitudinal compression of the photoelectron bunches. Given the longitudinal acceleration and the transverse focusing of the bunching cavity, the dark current electrons with bunching are approximately three times more than those without bunching, and this condition aggravates the harm to the operation of the photoinjector. Numerous dark current electrons around the electron–laser interaction section hit against the pipe inner wall and two laser focusing mirrors, producing a large number of background photons. A simulation of the bremsstrahlung process using an MCNP code is presented, showing that the background photon yield is less than 2.1% of the scattering photon yield, which is acceptable for our application.

Quantum-dot-based single-photon avalanche detector for mid-infrared applications

A single-photon detector is presented with quantum dot (QD) layers in its absorption region. The proposed detector is a three-terminal device in which a QD-based absorption region is integrated with an avalanche multiplication region through a tunneling barrier and the applied bias of each region can be controlled separately. The mid-infrared single photons (&#x3BB;=3&#x223C;5&#x2009;&#x2009;&#x3BC;m) can be absorbed in QDs and the photogenerated electrons are drifted to the avalanche region to trigger an avalanche and generate an output pulse. Since the absorption region consists of doped QD layers, it is expected to have higher orders of dark count. However, by separately controlling the bias and keeping the electric field of absorption region low, the dark current of this region can be reduced. Our simulations predict a single-photon detection efficiency (SPQE) of about 0.7 at T=50&#x2009;&#x2009;K for the proposed detector. At higher temperatures, the dark count rate increases and results in reduced SPQE. To increase the operation temperature, resonant tunneling barriers (RTB) are included in the absorption region to inhibit the thermally excited electrons from contributing to the dark current generation. Our results show that the SPQE for the RTB-based device is about 0.65 at T=77&#x2009;&#x2009;K, which is approximately equivalent to the SPQE of the device without RTB at T=50&#x2009;&#x2009;K.

Analyzing the Radiation Degradation of 4-Transistor Deep Submicron Technology CMOS Image Sensors

This paper presents a radiation degradation study on 4-Transistor (4T) complementary metal-oxide-semiconductor (CMOS) image sensors designed in standard 0.18-μm technology. The significant contribution of this paper is a systematic evaluation of the X-ray radiation effects on image sensors from the individual device level, to the pixel level and to the level of the entire sensor. The major degradation parameters of the sensor have been analyzed. This paper also includes test structures of varying geometries of in-pixel MOSFETs, pinned photodiodes (PPD), and transfer gates (TG). Characterization was performed during different X-ray doses up to 109 krad. The major degradation-an increase in the dark signal-is analyzed by modifying the TG charge transfer time and integration time. The PPD and the TG are the elements most sensitive to the dark signal of the sensor. The radiation-related dimensional effects on the sensors are also evaluated, which show different results compared to 3T pixels. The transfer-gate length influences the dark signal due to not only the electric field variation in the TG channel but also the local defect generations. In-pixel MOSFETs are used to identify the origin of increases in radiation-induced dark signal. Shallow trench isolation (STI) oxides are responsible for the radiation degradation of the sensor. A slight degradation of the quantum efficiency was observed after radiation in the short-wavelength region. Basic hardening-by-design techniques are also presented. The discussion results of the radiation-related dimensional effects on the sensors together with the STI effect can be used as a guideline for future layout designs of radiation-tolerant sensors. Identifying the pixel dark current origin can help to determine where and how to suppress the pixel dark current generation more effectively.

RecoveringSwift-XRT energy resolution through CCD charge trap mapping

The X-ray telescope on board the Swift satellite for gamma-ray burst astronomy has been exposed to the radiation of the space environment since launch in November 2004. Radiation causes damage to the detector, with the generation of dark current and charge trapping sites that result in the degradation of the spectral resolution and an increase of the instrumental background. The Swift team has a dedicated calibration program with the goal of recovering a significant proportion of the lost spectroscopic performance. Calibration observations of supernova remnants with strong emission lines are analysed to map the detector charge traps and to derive position-dependent corrections to the measured photon energies. We have achieved a substantial recovery in the XRT resolution by implementing these corrections in an updated version of the Swift XRT gain file and in corresponding improvements to the Swift XRT HEAsoft software. We provide illustrations of the impact of the enhanced energy resolution, and show that we have recovered most of the spectral resolution lost since launch.

Numerical analysis of long wavelength infrared HgCdTe photodiodes

We present a detailed investigation of the performance limiting factors of long and very long wavelength infrared (LWIR and VLWIR) p on n Hg 1− x Cd x Te detectors through numerical simulations at 77 K incorporating all considerable generation–recombination (G–R) mechanisms including trap assisted tunneling (TAT), Shockley–Read–Hall (SRH), Auger and radiative processes. The results identify the relative strengths of the dark current generation mechanisms by numerically extracting the contribution of each G–R mechanism to the detector characteristics with various cut-off wavelengths ( λ c ) and practically achievable material parameters. The results show that the dominant sensitivity degrading trap level depends on the detector cut-off wavelength being ∼0.7 E g for LWIR HgCdTe sensors ( λ c = ∼10 μm) instead of 0.5 E g which is generally believed to be the most efficient R–G level. TAT related 1/ f noise dominates the sensor noise even under small reverse bias voltages at a trap density as low as 1 × 10 14 cm −3 for sensors with λ c > 11 μm. Considering the fact that trap densities below this level are rarely reported for HgCdTe material, exceptionally trap-free material is required to achieve desirable imaging performance with these sensors. Simulation results show that Auger mechanism has twofold effect on the sensitivity of the sensor by increasing the dark current and decreasing the photo current of the detector.

Antimonide superlattice complementary barrier infrared detector (CBIRD)

The nearly lattice-matched InAs/GaSb/AlSb (antimonide) material system offers tremendous flexibility in realizing high-performance infrared detectors. Antimonide-based superlattice infrared absorbers can be customized to have cutoff wavelengths ranging from the short-wave infrared (SWIR) to the very long-wave infrared (VLWIR). They can be used in constructing sophisticated heterostructures to enable advanced infrared photodetector designs. In particular, they facilitate the construction of unipolar barriers, which can block one carrier type but allow the un-impeded flow of the other. Unipolar barriers are used to implement the barrier infrared detector (BIRD) design for increasing the collection efficiency of photo-generated carriers, and reducing dark current generation without impeding photocurrent flow. We report our recent efforts in achieving state-of-the-art performance in antimonide superlattice based long-wavelength infrared photodetectors using a complementary barrier infrared detector (CBIRD) design.

Exposure Time Dependence of Dark Current in CCD Imagers

In this paper, we present a systematic study on the rate of dark current generation of two scientific charge-coupled device imagers. The dark current in both imagers was measured for exposure times from 5 to 7200 s at a constant temperature. As one would expect, the majority of pixels show a linear increase in dark count with exposure time. However, we found distinct groups of pixels that show a nonlinear dark current dependence versus exposure time well below saturation. Since the dark count is often assumed to scale linearly with exposure time, these pixels can pose a problem during dark current correction. We also discuss what could cause some pixels to produce a dark count that is linear versus exposure time whereas others do not.