Semiconductor Photodetectors Research Articles

Hot carrier-based metal–semiconductor photodetectors: from quantum efficiency models to structural innovations

Abstract Hot carrier photodetectors have emerged as a promising technology in modern photonics. They show immense potential for affordable, high-performance photodetection in various applications due to their simple fabrication and extended spectral range through sub-bandgap operation. This review provides a comprehensive analysis of hot carrier photodetectors, elucidating their operation principles based on internal photoemission. By examining the evolution of quantum efficiency models for hot carrier generation and transport, we offer a comparative analysis of recent advances in these models. We also outline various design strategies and emphasize their role in improving the overall device efficiency. The paper concludes with a summary of performance milestones achieved by different hot carrier photodetectors and a discussion on future directions.

Reconfigurable Homojunction Phototransistor for Near-Zero Power Consumption Artificial Biomimetic Retina Function.

Semiconductor photodetectors integrating preliminary signal-processing functions play a vital role in artificial biomimetic retina systems. Herein, we propose a tungsten diselenide (WSe2) phototransistor with a dual-layer gate dielectric and an asymmetric graphene insert structure. This phototransistor exhibits a bidirectional self-powered photocurrent by controlling the gate voltage via the formation of reconfigurable p+-p and n-p homojunctions in the channel from the asymmetric graphene insert. At the same time, the nonvolatile electron and hole stored in the dual-layer gate dielectric are generated using a temporary gate voltage, which can replace the gate voltage to regulate the channel charge. Moreover, the photocurrent shows a linear relation with the temporary programming gate voltage. The phototransistor exhibits a rectification ratio of >4 orders of magnitude without the gate voltage, indicating its significant capability to operate in a fully self-powered mode with near-zero power consumption. Based on the device characteristics, we successfully simulate the biological functions of the photoreceptor layer and bipolar cell layer in the retinal receptive field. The identification of the object motion direction in the receptive field can be realized by integrating three programmable devices on the chip. Furthermore, edge enhancement of the image is achieved by independently modulating the light response of each pixel in the sensor by varying the programming gate voltage. This study will promote the developing progress of future artificial biomimetic retina systems.

SOA-based reservoir computing using upsampling.

We introduce a new, to the best of our knowledge, approach to reservoir computing based on upsampling and modulation, utilizing a semiconductor optical amplifier (SOA) and photodetector as nonlinear elements without conventionally used delay loop. We demonstrated the 400-step prediction capability of the proposed scheme for the Mackey-Glass (MG) time series test.

Physical Properties of an Efficient MAPbBr3/GaAs Hybrid Heterostructure for Visible/Near-Infrared Detectors.

Semiconductor photodetectors can work only in specific material-dependent light wavelength ranges, connected with the bandgaps and absorption capabilities of the utilized semiconductors. This limitation has driven the development of hybrid devices that exceed the capabilities of individual materials. In this study, for the first time, a hybrid heterojunction photodetector based on methylammonium lead bromide (MAPbBr3) polycrystalline film deposited on gallium arsenide (GaAs) was presented, along with comprehensive morphological, structural, optical, and photoelectrical investigations. The MAPbBr3/GaAs heterojunction photodetector exhibited wide spectral responsivity, from 540 to 900 nm. The fabrication steps of the prototype device, including a new preparation recipe for the MAPbBr3 solution and spinning, will be disclosed and discussed. It will be shown that extending the soaking time and refining the precursor solution's stoichiometry may enhance surface coverage, adhesion to the GaAs, and film uniformity, as well as provide a new way to integrate MAPbBr3 on GaAs. Compared to the pristine MAPbBr3, the enhanced structural purity of the perovskite on GaAs was confirmed by X-ray Diffraction (XRD) upon optimization compared to the conventional glass substrate. Scanning Electron Microscopy (SEM) revealed the formation of microcube-like structures on the top of an otherwise continuous MAPbBr3 polycrystalline film, with increased grain size and reduced grain boundary effects pointed by Energy-Dispersive Spectroscopy (EDS) and cathodoluminescence (CL). Enhanced absorption was demonstrated in the visible range and broadened photoluminescence (PL) emission at room temperature, with traces of reduction in the orthorhombic tilting revealed by temperature-dependent PL. A reduced average carrier lifetime was reduced to 13.8 ns, revealed by time-resolved PL (TRPL). The dark current was typically around 8.8 × 10-8 A. Broad photoresponsivity between 540 and 875 nm reached a maximum of 3 mA/W and 16 mA/W, corresponding to a detectivity of 6 × 1010 and 1 × 1011 Jones at -1 V and 50 V, respectively. In case of on/off measurements, the rise and fall times were 0.40 s and 0.61 s or 0.62 s and 0.89 s for illumination, with 500 nm or 875 nm photons, respectively. A long-term stability test at room temperature in air confirmed the optical and structural stability of the proposed hybrid structure. This work provides insights into the physical mechanisms of new hybrid junctions for high-performance photodetectors.

Enhanced SWIR Light Detection in Organic Semiconductor Photodetectors through Up-Conversion of Mid-Gap Trap States.

Shortwave-infrared (SWIR) photodetectors are vital for many scientific and industrial applications including surveillance, quality control and inspection. In recent decades, photodetectors based on organic semiconductors have emerged, demonstrating potential to add real value to broadband and narrowband imaging and sensing scenarios, where factors such as thermal budget sensitivity, large area aperture necessity, cost considerations, and lightweight and conformal flexibility demands are prioritized. It is now recognized that the performance of organic photodetectors (OPDs), notably their specific detectivity, is ultimately limited by trap states, universally present in disordered semiconductors. This work adopts an approach of utilizing these mid-gap states to specifically create a SWIR photo-response. To this end, this work introduces a somewhat counter-intuitive approach of "trap-doping" in bulk heterojunction (BHJs) photodiodes, where small quantities of a guest organic molecule are intentionally incorporated into a semiconducting donor:acceptor host system. Following this approach, this work demonstrates a proof-of-concept for a visible-to-SWIR broadband OPD, approaching (and, to some extent, even exceeding) state-of-the-art performance across critical photodetector metrics. The trap-doping approach is, even though only a proof-of-concept currently, broadly applicable to various spectral windows. It represents a new modality for engineering photodetection using the unconventional strategy of turning a limitation into a feature.

Hydrogel Based UV Photodetector Based on Polarization of Free Water Molecules

AbstractConductive hydrogels retain the intrinsic softness and elasticity that are hallmark features of traditional hydrogels, and they also assimilate the electrical properties of the embedded conductive materials. Conventional epitaxial growth techniques for heterojunctions require precise lattice matching between adjacent materials, significantly limiting technological advancements in photodetector innovation. The integration of conductive hydrogels with semiconductors to fabricate photodetectors presents a feasible solution that bypasses the traditional lattice matching constraints inherent in the epitaxial growth of heterojunction semiconductor photodetectors. Herein, device integration is enhanced by developing a transparent conductive hydrogel, positioned between graphene and gallium nitride, which effectively mitigates the fluidity issues of the polar liquid complicating device encapsulation. A comprehensive analysis of the photodetection characteristics of the graphene/conductive hydrogel/gallium nitride heterostructure is conducted, thereby establishing its structural framework. Flexible transparent devices are further prepared based on PEDOT/Alg (Ca2+) and demonstrate their potential application in 360° omnidirectional photodetection. This work introduces a novel approach for the enhanced integration of hydrogel‐based spectrally selective and flexible transparent photodetectors.

High performance infrared photodetectors based on CdS film activated by lanthanide-doped upconverting nanoparticles

Cadmium sulfide (CdS) semiconductors are well known due to their low cost, easy production and excellent photoelectric properties. However, the photodetectors based on CdS respond only to green light at the wavelength of 540 nm and have negligible response to infrared light. Here, we report the high performance infrared photodetectors based on CdS film activated by lanthanide doped upconverting nanoparticles. The composite film integrates a CdS semiconductor substrate with a cutting-edge photon upconversion layer comprising upconversion nanoparticles meticulously arranged within photonic crystal arrays. The CdS semiconductor substrate can respond to the green light of 540 nm, while the photon converters can efficiently convert infrared light at 980 nm to green light at 540 nm. Compared with the single CdS film, the photocurrent amplitude of the composite film is enhanced by 1302 % under the same irradiation of 980 nm infrared light, while the photoresponsivity (R) and the external quantum efficiency (EQE) are enhanced by 14 times and 140 times, respectively. This confirms that the photon up-converters play a key role for enhancing the responsibility of the CdS film to 980 nm infrared light. This finding opens a way to convert conventional semiconductor photodetectors from visible to infrared region.

Ultraviolet-sensitive and power-efficient oxide phototransistor enabled by nanometer-scale thickness engineering of InZnO semiconductor and gate bias modulation

Amorphous oxide semiconductor photodetectors (PDs) are promising ultrasensitive and power-efficient ultraviolet (UV) PDs because they generate low dark current in the dark and exhibit high photoresponse under UV irradiation owing to their superior UV absorption and photocarrier transport characteristics. Herein, we demonstrate UV-sensitive and power-efficient oxide phototransistors through the nanometer-scale engineering of oxide semiconductors and appropriate modulation of gate bias conditions. The dark current and photocurrent of an oxide phototransistor exhibit a trade-off relationship in terms of the thickness of the oxide semiconductor film. Ultrathin InZnO is disadvantageous for fabricating UV-sensitive PDs because of its low photoresponse. In contrast, excessively thick InZnO is disadvantageous for fabricating power-efficient UV PDs owing to its high dark current. However, the InZnO film with an optimal film thickness of 8 nm can simultaneously provide the advantages of both ultrathin and excessively thick cases owing to its low intrinsic carrier concentration and sufficient UV absorption depth. Consequently, an InZnO phototransistor with high UV-sensing performance (Smax = 1.25 × 106), low-power operation capability (Idark = ∼10−13A), and excellent repeatability is realized by using an 8-nm-thick InZnO semiconductor and applying appropriate gate bias modulation (constant gate bias for maximized photosensitivity and temporal positive bias pulse for persistence photocurrent elimination).

Influence of Annealing on Thin Film/Substrate Interface and Vacuum Ultraviolet Photoconductivity of Neodymium Fluoride Thin Films

AbstractHigh photon energy vacuum ultraviolet radiation (VUV, 100−200 nm wavelength) is challenging to detect. It easily degrades conventional silicon and semiconductor photodetectors. Fluoride photodetectors can be the answer, but the correlation between fabrication parameters and photodetector performance is not known. Here, the effect of annealing is investigated on the characteristics of neodymium trifluoride thin film/quartz substrate interface and NdF3 photoconductivity within the VUV. Thin films are deposited on unheated and heated (600 °C) substrates with post‐deposition annealing. Dark current of films on unheated substrates decreases by as much as 1/10 as resistance increases from 1 −12 TΩ after annealing. Dark current of films on heated substrates increases even after annealing, resulting in similar photo and dark currents of ≈303.7 nA and poor detectors. Fluorine diffuses from the film to the substrate during deposition, exacerbated by substrate heating but not by annealing. Fluorine diffusion degrades crystallinity near the interface, increasing the dark current. Fluorine diffusion is absent when MgF2 is used as the heated substrate. Unannealed NdF3/MgF2 detector on 600 °C‐heated substrate and 600 °C‐annealed NdF3/SiO2 detector on unheated substrate exhibit similar resistances of ≈14 TΩ. Considering the film/substrate interface and annealing is crucial when developing VUV photodetectors.

Operation of an Electrical-Only-Contact Photonic Integrated Chip for Quantum Random Number Generation Using Laser Gain-Switching

We present the results of the detected voltage distribution of a quantum random number generator (QRNG) based on a photonic integrated circuit comprising a semiconductor laser, delay interferometer and photodetector. We find that the integrated QRNG system behaves as expected for a QRNG from discrete gain-switched laser sources, especially exhibiting all of the peculiarities of the random voltage distribution and behaving as previously demonstrated for a discrete optical component setup. The biggest advantage of having all of the components integrated into a single chip is that only electrical connections are needed to operate the system, without the need for tricky and expensive optical alignment to external circuitry. We supply results showing that a random bit stream created from the random numbers passes the NIST statistical test suite tests, thus demonstrating the feasibility to generate random numbers via quantum means at gigabit/s rates from a single photonic integrated circuit. All of our results are backed by numerical simulations.

Insight on the structural, electronic and optical properties of Zn, Ga-doped/dual-doped graphitic carbon nitride for visible-light applications

The density functional theory (DFT) was applied for the first time to study the doping and co-doping of Ga and Zn metals on graphitic carbon nitride (g-C3N4). The doping of these metal impurities into g-C3N4 leads to a significant decrease in the bandgap energy. Moreover, the co-doping leads to even lower bandgap energy than either individual Zn or Ga-doped g-C3N4. The theoretical electronic and optical properties including the density of state (DOS), energy levels of the frontier orbital, excited state lifetime, and molecular electrostatic potential of the doped and co-doped g-C3N4 support their application in UV–visible light-based technologies. The quantum mechanical parameters (energy band gap, binding energy, exciton energy, softness, hardness) and dipole moment exhibit higher values (ranging from 1.36 to 4.94 D) compared to the bare g-C3N4 (0.29 D), indicating better solubility in the water solvent. The time-dependent DFT (TD-DFT) calculations showed absorption maxima in between the UV–Vis region (309–878 nm). Additionally, charge transfer characteristics, transition density matrix (TDM), excited state lifetime and light harvesting efficiency (LHE) were investigated. Overall, these theoretical studies suggest that doped and co-doped g-C3N4 are excellent candidates for electronic semiconductor devices, light-emitting diodes (LEDs), solar cells, and photodetectors.

W18O49 Nanowire Arrays for Plasmonic Photodetector with Electromediated Broadband Photoresponse

Plasmonic semiconductors offer significant advantages for the development of photodetectors with a tunable wide spectral response. Herein, we developed a plasmonic semiconductor photodetector based on the W18O49 nanowire array (NWA) to achieve a tunable broadband photodetector by adjusting the plasmonic absorption intensity of the W18O49 NWA. The plasmonic absorption intensity could be mediated through adjusting the free electron density of the W18O49 NWA by applying the external voltage on the W18O49 NWA-loaded electrode in HCl electrolyte. When the applied voltage decreased from +2 to −2 V, the plasmonic absorption intensity of the W18O49 NWA gradually enhanced due to the increased electron density. As a result, the responsivity and detectivity of the W18O49 NWA-based photodetector in the visible–NIR region were dramatically increased. Especially, upon 940 nm illumination, the responsivity and detectivity could reach ∼47.2 mA W–1 and ∼8.22 × 109 Jones, respectively, which is much higher than the corresponding values (∼2.9 mA W–1 and ∼4.53 × 109 Jones) obtained by the W18O49 NWA-based photodetector with a low plasmonic absorption. This work proposes a promising strategy to construct broadband-tunable photodetectors by using low-cost nonmetallic plasmonic nanostructures.

Nanomechanical absorption spectroscopy of 2D materials with femtowatt sensitivity

Nanomechanical spectroscopy (NMS) is a recently developed approach to determine optical absorption spectra of nanoscale materials via mechanical measurements. It is based on measuring changes in the resonance frequency of a membrane resonator vs. the photon energy of incoming light. This method is a direct measurement of absorption, which has practical advantages compared to common optical spectroscopy approaches. In the case of two-dimensional (2D) materials, NMS overcomes limitations inherent to conventional optical methods, such as the complications associated with measurements at high magnetic fields and low temperatures. In this work, we develop a protocol for NMS of 2D materials that yields two orders of magnitude improved sensitivity compared to previous approaches, while being simpler to use. To this end, we use mechanical sample actuation, which simplifies the experiment and provides a reliable calibration for greater accuracy. Additionally, the use of low-stress silicon nitride membranes as our substrate reduces the noise-equivalent power to fW , comparable to commercial semiconductor photodetectors. We use our approach to spectroscopically characterize a 2D transition metal dichalcogenide (WS2), a layered magnetic semiconductor (CrPS4), and a plasmonic super-crystal consisting of gold nanoparticles.

Ultraviolet Photodetectors: From Photocathodes to Low-Dimensional Solids.

The paper presents the long-term evolution and recent development of ultraviolet photodetectors. First, the general theory of ultraviolet (UV) photodetectors is briefly described. Then the different types of detectors are presented, starting with the older photoemission detectors through photomultipliers and image intensifiers. More attention is paid to silicon and different types of wide band gap semiconductor photodetectors such as AlGaN, SiC-based, and diamond detectors. Additionally, Ga2O3 is considered a promising material for solar-blind photodetectors due to its excellent electrical properties and a large bandgap energy. The last part of the paper deals with new UV photodetector concepts inspired by new device architectures based on low-dimensional solid materials. It is shown that the evolution of the architecture has shifted device performance toward higher sensitivity, higher frequency response, lower noise, and higher gain-bandwidth products.

Temperature dependence of electrical conductivity and variable hopping range mechanism on graphene oxide films

The rapid development of optoelectronic applications for optical-to-electrical conversion has increased the interest in graphene oxide material. Here, graphene oxide films (GOF) were used as source material in an infrared photodetector configuration and the temperature dependence of the electrical conductivity was studied. GOF were prepared by the double-thermal decomposition (DTD) method at 973 K, with a fixed carbonization temperature, in a pyrolysis system, under a controlled nitrogen atmosphere, over quartz substrates. Graphene oxide films were mechanically supported in a photodetector configuration on Bakelite substrates and electrically contacted with copper wires and high-purity silver paint. Morphological images from the GOF’s surface were taken employing a scanning electron microscope and observed a homogeneous surface which favored the electrical contacts deposition. Vibrational characteristics were studied employing Raman spectroscopy and determined the typical graphene oxide bands. GOF were used to discuss the effect of temperature on the film’s electrical conductivity. Current–voltage (I–V) curves were taken for several temperatures varying from 20 to 300 K and the electrical resistance values were obtained from 142.86 to 2.14 kΩ. The GOF electrical conductivity and bandgap energy (Eg) were calculated, and it was found that when increasing temperature, the electrical conductivity increased from 30.33 to 2023.97 S/m, similar to a semiconductor material, and Eg shows a nonlinear change from 0.33 to 0.12 eV, with the increasing temperature. Conduction mechanism was described mainly by three-dimensional variable range hopping (3D VRH). Additionally, measurements of voltage and electrical resistance, as a function of wavelength were considered, for a spectral range between 1300 and 3000 nm. It was evidenced that as the wavelength becomes longer, a greater number of free electrons are generated, which contributes to the electrical current. The external quantum efficiency (EQE) was determined for this proposed photodetector prototype, obtaining a value of 40%, similar to those reported for commercial semiconductor photodetectors. This study provides a groundwork for further development of graphene oxide films with high conductivity in large-scale preparation.

Optical Neural Network With Complementary Decomposition to Overcome the Phase Insensitive Constrains

Photonic integrated circuits have potentials to advance artificial neural networks by providing ultra-fast and high power efficient computation resources. In many of optical neural network (ONN) architectures, data are encoded in light amplitudes, using magnitude and phase to denote the absolute value and the sign. However, the phase insensitivity of optical components such as saturable absorbers, semiconductor optical amplifiers and photodetectors can lead to limitations in ONN designs, including nonlinear activation and inference results readout. In this work, we propose a complementary decomposition approach to overcome the phase insensitive problem. Based on that, we further present a optical ReLU function implementation. The proposed designs can scale to multi-layer neural networks by cascading linear and nonlinear units. These optical neural networks do not require repeated bidirectional digital-analog conversions and optical-electronic conversions between each layers as well as reference signals for homodyne detection. These features could provide a potential implementation of compact, scalable, high efficiency ONNs. We evaluate the concept with a machine learning task. The numerical results demonstrate that the proposed design can tackle the aforementioned phase insensitive problem.

Energy Level Prediction of Organic Semiconductors for Photodetectors and Mining of a Photovoltaic Database to Search for New Building Units.

Due to the large versatility in organic semiconductors, selecting a suitable (organic semiconductor) material for photodetectors is a challenging task. Integrating computer science and artificial intelligence with conventional methods in optimization and material synthesis can guide experimental researchers to develop, design, predict and discover high-performance materials for photodetectors. To find high-performance organic semiconductor materials for photodetectors, it is crucial to establish a relationship between photovoltaic properties and chemical structures before performing synthetic procedures in laboratories. Moreover, the fast prediction of energy levels is desirable for designing better organic semiconductor photodetectors. Herein, we first collected large sets of data containing photovoltaic properties of organic semiconductor photodetectors reported in the literature. In addition, molecular descriptors that make it easy and fast to predict the required properties were used to train machine learning models. Power conversion efficiency and energy levels were also predicted. Multiple models were trained using experimental data. The light gradient boosting machine (LGBM) regression model and Hist gradient booting regression model are the best models. The best models were further tuned to achieve better prediction ability. The reliability of our designed approach was further verified by mining the photovoltaic database to search for new building units. The results revealed that good consistency is obtained between experimental outcomes and model predictions, indicating that machine learning is a powerful approach to predict the properties of photodetectors, which can facilitate their rapid development in various fields.

Engineering graphitic carbon nitride for next-generation photodetectors: a mini review.

Semiconductor photodetectors, as photoelectric devices using optical-electrical signal conversion for detection, are widely used in various fields such as optical communication, medical imaging, environmental monitoring, military tracking, remote sensing, etc. Compared to the conventional photodetector materials including silicon, III-V semiconductors and metal sulfides, graphitic carbon nitride (g-C3N4) as a metal-free polymeric semiconductor, has many advantages such as low-price, easy preparation, efficient visible light response, and relatively good thermal stability. In the meantime, the polymer characteristics also endow the g-C3N4 with good mechanical properties. Apart from being used for photo(electro)catalysts during the past decades, the potential use of g-C3N4 in photodetectors has attracted great research interests very recently. In this review, we first briefly introduce the structure and properties of g-C3N4 and the key performance parameters of photodetectors. Then, combining the very recent progress, the review focuses on the active materials, fabrication methods and performance enhancement strategies for g-C3N4 based photodetectors. The existing challenges are discussed and the future development of g-C3N4 based photodetectors is also forecasted.

Broadband ultraviolet plasmonic enhanced AlGaN/GaN heterojunction photodetectors with close-packed Al nanoparticle arrays.

Plasmonic metallic nanostructures could concentrate optical fields into nanoscale volumes and support efficient light scattering and absorption, which therefore stimulates the continuing development of advanced plasmonic-assisted semiconductor photodetectors. In this work, by fabricating Al nanoparticle (NP) arrays in AlGaN surface using the AAO template transferring method, significant broadband ultraviolet (UV) photoresponse enhancement was demonstrated on AlGaN/GaN heterojunction photodetectors. By deliberately designing the close-packed Al NP arrays, the broadband UV plasmonic resonance with large optical field absorption and strong interface field enhancement are enabled, hence, the highest responsivity exceeding 8.1 A W-1 and maximum external quantum efficiency of 3500% was obtained at the resonance wavelength 292 nm, revealing more than 80 times the excellent enhancement in responsivity. Specifically, owing to coupling among NPs at the Al/AlGaN interface, the smaller size Al NP array exhibits an excellent photoresponse enhancement encompassing the entire UV band compared to the relatively larger size Al NP array. In addition, different photoresponse enhancements depending on the applied bias were observed. The Al NPs detector also demonstrates a fast photoresponse with a rise time of around 60 ms and a relatively long fall time of 1.42 s. This work could be of great significance for gaining a low and efficient approach to achieve plasmonic-empowered heterojunction broadband UV detectors.

Focus on the Avalanche Breakdown Characteristic of Si- and InP-Based APDs Irradiated by Fast Neutrons.

The Si- and InP-based APDs as the most important weak light semiconductor photodetectors to have achieved commercial success and are widely used in irradiation environments. Investigating the influencing mechanism of neutron irradiation on the above two types of APDs is of scientific and practical importance. In this paper, the dark current and gain characteristics of Si- and InP-based APDs around breakdown voltage were analyzed in detail before and after irradiation. The increase of dark current and the decrease of gain were observed for both the neutron irradiated Si- and InP-based APDs. Generation centers induced by neutrons are responsible for the increased dark current. The decrease of gain can be attributed to the increase of multiplied dark current and the change of electric field distribution in APD. The Si-based APD exhibits soft breakdown with the breakdown voltage reduced by ~8 V under the neutron fluence of 1.0 × 1012 cm-2, while the soft breakdown occurs along with a small change of breakdown voltage of ~1.5 V under the neutron fluence of 1.0 × 1013 cm-2 for InP-based APD. The difference in the change of breakdown voltage probably occurs because the Si-based APD uses p-doped Si as the multiplication layer, in which the neutron induced carrier removing effect cannot be ignored to keep the electric field distribution away from the optimal state. Therefore, using an intrinsic multiplication layer in APD is helpful to improve the neutron radiation resistance. The findings here are not only useful for the radiation hardened design of APD, but also deepen the understanding of irradiation mechanism.