Photoconductive Gain Research Articles

Room-temperature infrared photoluminescence and broadband photodetection characteristics of Ge/GeSi islands on silicon-on-insulator

In this study, molecular beam epitaxial growth of strain-driven three-dimensional self-assembled Ge/GeSi islands on silicon-on-insulator (SOI) substrates, along with their optical and photodetection characteristics, have been demonstrated. The as-grown islands exhibit a bimodal size distribution, consisting of both Ge and GeSi alloy islands, and show significant photoluminescence (PL) emission at room temperature, specifically near optical communication wavelengths. Additionally, these samples were used to fabricate a Ge/GeSi islands/Si nanowire based phototransistor using a typical e-beam lithography process. The fabricated device exhibited broadband photoresponse characteristics, spanning a wide wavelength range (300-1600 nm) coupled with superior photodetection characteristics and relatively low dark current (∼ tens of pA). The remarkable photoresponsivity of the fabricated device, with a peak value of ∼11.4 A W-1(λ∼ 900 nm) in the near-infrared region and ∼1.36 A W-1(λ∼ 1500 nm) in the short-wave infrared (SWIR) region, is a direct result of the photoconductive gain exceeding unity. The room-temperature optical emission and outstanding photodetection performance, covering a wide spectral range from the visible to the SWIR region, showcased by the single layer of Ge/GeSi islands on SOI substrate, highlight their potential towards advanced applications in broadband infrared Si-photonics and imaging. These capabilities make them highly promising for cutting-edge applications compatible with complementary metal-oxide-semiconductor technology.

Sensitive direct converting thin film x-ray detector utilizing β-Ga2O3 fabricated via MOCVD

Ga2O3 has been considered as one of the most suitable materials for x-ray detection, but its x-ray detection performance is still at a low level due to the limitation of its quality and absorbance, especially for hard x-ray. In this work, the effects of growth temperature and miscut angle of the sapphire substrate on the crystal quality of Ga2O3 thin films were investigated based on the MOCVD technique. It was found that the crystal growth mode was transformed from island growth to step-flow growth using miscut sapphire substrates and increasing growth temperature, which was accompanied by the improvement of the crystal quality and the reduction of the density of trapped states. Ga2O3 films with optimal crystal quality were finally prepared on a 4° miscut substrate at 900 °C. The x-ray detector based on this film shows good hard x-ray response with a sensitivity of 3.72 × 105μC·Gyair−1·cm−2. Furthermore, the impacts of Ga2O3 film crystal quality and trap density on the x-ray detector were investigated in depth, and the mechanism of the photoconductive gain of the Ga2O3 thin-film x-ray detector was analyzed.

Vertically Stacked Broadband GNIF‐MoS2/p‐Ge Photodetector for Dark Current Suppression, High Photoresponse, and Ultrafast Transient Response

AbstractThe proposed model structure, featuring a gold (Au) nano‐island film (GNIF) integrated with a vertically stacked van der Waals heterojunction and offering an elegant platform for high‐performance, efficient, and sensitive photodetection across a broad spectral range, is designated as GNIF‐MoS₂/p‐Ge(MoS2 = Molybdenum disulfide, p‐Ge = p type germanium). The GNIF is fabricated via ultrathin film deposition, based on the surface dewetting properties of MoS2. The as‐fabricated photodetector (PD), offering ≈20 times reduction in dark current and characterized by wavelength‐dependent high responsivity (R(λ)), photoconductive gain (G(λ)), and detectivity (D(λ)), respond to a broad spectral range from visible light (400 nm) to short wave infrared (SWIR) (1600 nm). The ultrahigh transient response (τr) is found to be ≈2.5 and 16 µs for the 470 (visible light) and 1550 (SWIR) nm wavelengths, respectively, resulting in 3‐dB bandwidths of up to ≈48 kHz, which is considered high for such devices. To understand the inherent mechanisms of broadband detection and the high photoresponse and ultrafast transient response of PDs, a meticulous investigation is conducted on the wavelength‐dependent behaviors, depletion width changes, and material properties. The results provide valuable insights and a basis for the construction of suitable PDs based on nanometer‐thin metal films, 2D semiconductors, and a 3D hybrid structure.

Photovoltage-Driven Photoconductor Based on Horizontal p-n-p Junction.

The photoconductive gain theory demonstrates that the photoconductive gain is related to the ratio of carrier lifetime to carrier transit time. Theoretically, to achieve higher gain, one can either prolong the carrier lifetime or select materials with high mobility to shorten the transit time. However, the former slows the response speed of the device, while the latter increases the dark current and degrades device sensitivity. To address this challenge, a horizontal p-n-p junction-based photoconductor is proposed in this work. This device utilizes the n-region as the charge transport channel, with the charge transport direction perpendicular to the p-n-p junction. This design offers two advantages: (i) the channel is depleted by the space charge layer generated by the p and n regions, enabling the device to maintain a low dark current. (ii) The photovoltage generated in the p-n junction upon light absorption can compress the space charge layer and expand the conductive path in the n-region, enabling the device to achieve high gain and responsivity without relying on long carrier lifetimes. By adopting this device structure design, a balance between responsivity, dark current, and response speed is achieved, offering a new approach to designing high-performance photodetectors based on both traditional materials and emerging nanomaterials.

C60 Functionalized Single‐Walled Carbon Nanotube‐based Phototransistor for Efficient Detection of Visible Light Under Appropriate Gate Electrostatics

AbstractThe current study concerns highly reliable visible light sensing by C60 fullerene functionalized single‐walled carbon nanotube (SWCNT) based phototransistor. The absorbance of visible light (532 nm) increases significantly due to the formation of an interlink between the SWCNT network and C60 clusters. Photogenerated excess electrons in SWCNT are trapped by the C60 clusters and increase the effective hole concentrations in the SWCNT channel, which eventually improves the photoconductive gain. C60‐SWCNT channel is further integrated into the back gated field effect transistor (FET) structure in which 90 nm SiO2 dielectric thickness is used. The responsivity toward visible light is further enlarged with appropriate negative gate electrostatic. In order to ensure the morphological and structural behavior of C60‐SWCNT, various microscopic and spectroscopic characterizations are performed. The C60‐SWCNT phototransistor exhibits responsivity of 1.219 A W−1 at Vgs = – 21 V. The detectivity and rise/fall time of C60‐SWCNT came around to be 2.34 × 1010 Jones, 86.42 ms/3.35 ms at the same Vgs. The maximum external quantum efficiency (EQE) of the C60‐SWCNT phototransistor is very high, ≈274%. In the overall study, a comprehensive discussion is introduced on the effect of variable gate potential on the performance of the C60‐SWCNT phototransistor.

A nanophotonic approach to signal-to-noise ratio of quantum dot infrared photodetectors with surface plasmonic excitation

A nanophotonic approach to the signal-to-noise ratio (SNR) of an InAs-based quantum dot infrared photodetector (QDIP) with surface plasmonic excitation is reported. A 100 nm-thick Au film, perforated with a 3.1 μm-period, 2-dimensional square array of holes, referred to as a metal photonic crystal (MPC), is employed as a plasmonic coupler for it. Under the irradiance on the MPC integrated atop the QDIP, the fundamental surface plasma wave (SPW) is excited along their interface at ∼10.3 μm in wavelength. It carries the near-field that interacts with the quantum dots (QDs) under the interface. Depending on the presence of the coupler, the QDIP generates two different current–voltage (I-V) characteristics; one from the QDs normally working without MPC and the other from those coupled to the SPW near-field with it. The two I-V’s unfold the signal and noise current of each case with photoconductive gain. In their relation, generation-recombination and shot noises which are correlated with the detector signal current are directly affected by the plasmonic coupling but other sources including thermal noise are not relevant to it. Relying on these differences, the I-V analysis allows the SNR of each case and shows ∼20× enhancement by the plasmonic coupling. It reveals that most of the noise current is attributed to thermal fluctuations inherent to the quantum confinement of the QDIP. The SNR lower than the quantum efficiency in plasmonic enhancement is addressed.

A comparative study of broadband PbS quantum dots/graphene photodetectors with monolayer and bilayer graphene

Colloidal quantum dots (QDs)/graphene nanohybrids provide a unique platform to design photodetectors of high performance. These photodetectors are quantum sensors due to the strong quantum confinement in QDs for spectral tunability, and in graphene for high charge mobility. Quantitatively, the high carrier mobility of graphene plays a critical role to enable high photoconductive gain and understanding its impact on the photodetector performance is imperative. Herein, we report a comparative study of PbS QDs/graphene nanohybrids with monolayer and bilayer graphene for broadband photodetection ranging from ultraviolet, visible, near-infrared to short-wave infrared spectra (wavelength: 400 nm–1750 nm) to determine if a specific advantage exists for one over the other. This study has revealed that both the monolayer and bilayer graphene grown in chemical vapor deposition can provide a highly efficient charge transfer channel for photo-generated carriers for high broadband photoresponse.

High-performance uncooled PbSe/CdSe nanostructured mid-infrared photodetector with tunable cutoff wavelength

Room-temperature (RT) high-performance mid-wavelength infrared (MWIR) Lead Selenide (PbSe)/Cadmium Selenide (CdSe) heterostructure nanocrystal photoconductors are designed and fabricated on commercial silicon dioxide on silicon (SiO2/Si) wafer via vapor phase deposition. Tunable absorption edges at 3.75 and 4.0 μm are demonstrated with different sizes of the nanostructure. The devices are annealed in oxygen to make the thin film much more sensitive to MWIR light. The detectors are etched by the reactive ion etching method to define an active area of 17.5 × 20 μm2. All devices exhibit external quantum efficiencies exceeding 100%, a clear indication of photoconductive gain. 1/f noise is the dominating noise source, and it follows Hooge's empirical relation for a homogeneous semiconductor. RT peak specific detectivity (D*) of 2.17 × 1010 and 1.61 × 1010 Jones is achieved for pixels with absorption edge at 3.75 and 4 μm, respectively.

Study on the enhanced performance of visible photodetector based on hybrid structure between reduced graphene oxide and modified ZnO nanorods

Photodetectors based on Zinc oxide (ZnO) have been widely researched due to their significant advantages such as simple fabrication process, high stability, etc. However, the application range of these devices is still limited in the ultraviolet (UV) region, and their reported performance is not as high as desired. Therefore, this study introduces a simple fabrication process of a photodetector based on the hybrid structure of reduced graphene oxides and zinc oxide nanorods (ZnO NRs) decorated silver nanoparticles (Ag NPs). In this RGO/ZnO–Ag structure, the ZnO–Ag serves as a light absorption complex to generate charge carriers through the surface plasmon resonance (SPR) effect and RGO acts as a transport layer driving the photocarriers for the enhancement of photocurrent. The results show that by decorating Ag NPs onto ZnO NRs, the absorption range of ZnO was widened toward longer wavelengths (the optical band gap reduced from 3.2 eV to 3.12 eV). Besides, by incorporating an RGO transport layer, not only the response and recovery time of the ZnO–Ag device were shortened but also its performance parameters were significantly enhanced (the responsivity increased from 5.32 to 6.58 A W−1 and the photoconductive gain rose from 16.92 to 20.70). This reveals the promise of our innovation in hybrid structure of transport layer and modified ZnO for practical photodetector applications.

Performance Characteristics of an Infrared Photodetector Using Intersuband Junctions in Quantum Wells Based on Gallium Nitride

A simulation procedure for analyzing the electrical and optical characteristics of an AlGaN/GaN intersubbandquantum well middle-wavelength infrared photodetector is presented. The photoconductive gain spectrumwas simulated by coupling the drift-diffusion and capture-escape models in the active region of the devicestructure and by ignoring the contribution of radiative emission. It was shown that the photodetector at zero biasis sensitive over a spectral range from 4 to 6 μm, with the peak absorption occurring at 4.64 μm. The dependenceof the available photocurrent on both the wavelength and the angle of incidence of an unpolarized monochromaticbeam of light was also evaluated. An assessment of the dark current characteristics was estimated at varioustemperatures.

Superhigh gain InGaN/GaN visible-light photodetector using polarization heterointerface barrier and single-carrier superlattices

Visible-light detection with high sensitivity and strong wavelength selectivity is highly desired in emerging applications. Here, we demonstrate a high-performance visible-light photodetector with an active region composed of a polarization induced barrier and single-carrier superlattices (SCSLs). The barrier at SCSLs/GaN heterointerface brings both a low dark current and a high gain originating from the photoinduced barrier reduction effect. Meanwhile, the designed InGaN/GaN SCSLs allow the photoelectrons in the quantum wells to escape, but photogenerated holes are weakly localized, thus generating the additional photoconductive gain. The resulting devices exhibited a super-high gain of 7.8 × 104, a large detectivity of 1.2 × 1016 jones, and a relatively fast response speed with rise/falling time of 2.5/89.6 ns. Also, a 400/500-nm rejection ratio greater than 3 × 105 was shown at 1 V, indicating excellent wavelength selectivity.

Spray-lithography of hybrid graphene-perovskite paper-based photodetectors for sustainable electronics

Paper is an ideal substrate for the development of flexible and environmentally sustainable ubiquitous electronic systems. When combined with nanomaterial-based devices, it can be harnessed for various Internet-of-Things applications, ranging from wearable electronics to smart packaging. However, paper remains a challenging substrate for electronics due to its rough and porous nature. In addition, the absence of established fabrication methods is impeding its utilization in wearable applications. Unlike other paper-based electronics with added layers, in this study, we present a scalable spray-lithography on a commercial paper substrate. We present a non-vacuum spray-lithography of chemical vapor deposition (CVD) single-layer graphene (SLG), carbon nanotubes (CNTs) and perovskite quantum dots (QDs) on a paper substrate. This approach combines the advantages of two large-area techniques: CVD and spray-coating. The first technique allows for the growth of SLG, while the second enables the spray coating of a mask to pattern CVD SLG, electrodes (CNTs), and photoactive (QDs) layers. We harness the advantages of perovskite QDs in photodetection, leveraging their strong absorption coefficients. Integrating them with the graphene enhances the photoconductive gain mechanism, leading to high external responsivity. The presented device shows high external responsivity of ∼520 A W−1 at 405 nm at <1 V bias due to the photoconductive gain mechanism. The prepared paper-based photodetectors (PDs) achieve an external responsivity of 520 A W−1 under 405 nm illumination at <1 V operating voltage. To the best of our knowledge, our devices have the highest external responsivity among paper-based PDs. By fabricating arrays of PDs on a paper substrate in the air, this work highlights the potential of this scalable approach for enabling ubiquitous electronics on paper.

Stable Ga2O3 soft x-ray detector with ultrahigh responsivity

Soft x-ray detectors play crucial roles in biology, chemistry, and lithography. Current soft x-ray detectors suffer from insufficient responsivity (R), excessively large cell area, and limited stability. Here, the β-Ga2O3 soft x-ray detector is constructed, and the effects of varying the sensitive layer thickness and voltage on the soft x-ray detection characteristics of the device are explored. Meanwhile, the mechanism of the multiplication ionization process from soft x-ray and the photoconductivity gain on the photoresponse performance of the device are analyzed. The device obtains the R up to 3.05 × 103 A/W under 300 eV soft x-ray irradiation at the synchrotron beamline, which is about 1.19 × 104 times higher than that of the conventional device. The Ga2O3 device also maintains stable operation under long-term irradiation and multicycle switching. These results indicate that Ga2O3 is an ideal candidate material for soft x-ray detection, which has great potential for applications such as imaging of biological cells.

Activation of the Photosensitive Potential of 2D GaSe by Interfacial Engineering.

Two-dimensional (2D) gallium selenide (GaSe) holds great promise for pioneering advancements in photodetection due to its exceptional electronic and optoelectronic properties. However, in conventional photodetectors, 2D GaSe only functions as a photosensitive layer, failing to fully exploit its inherent photosensitive potential. Herein, we propose an ultrasensitive photodetector based on out-of-plane 2D GaSe/MoSe2 heterostructure. Through interfacial engineering, 2D GaSe serves not only as the photosensitive layer but also as the photoconductive gain and passivation layer, introducing a photogating effect and extending the lifetime of photocarriers. Capitalizing on these features, the device exhibits exceptional photodetection performance, including a responsivity of 28 800 A/W, specific detectivity of 7.1 × 1014 Jones, light on/off ratio of 1.2 × 106, and rise/fall time of 112.4/426.8 μs. Moreover, high-resolution imaging under various wavelengths is successfully demonstrated using this device. Additionally, we showcase the generality of this device design by activating the photosensitive potential of 2D GaSe with other transition metal dichalcogenides (TMDCs) such as WSe2, WS2, and MoS2. This work provides inspiration for future development in high-performance photodetectors, shining a spotlight on the potential of 2D GaSe and its heterostructure.

Infrared Imaging Using Thermally Stable HgTe/CdS Nanocrystals.

Transferring nanocrystals (NCs) from the laboratory environment toward practical applications has raised new challenges. HgTe appears as the most spectrally tunable infrared colloidal platform. Its low-temperature synthesis reduces the growth energy cost yet also favors sintering. Once coupled to a read-out circuit, the Joule effect aggregates the particles, leading to a poorly defined optical edge and large dark current. Here, we demonstrate that CdS shells bring the expected thermal stability (no redshift upon annealing, reduced tendency to form amalgams, and preservation of photoconduction after an atomic layer deposition process). The electronic structure of these confined particles is unveiled using k.p self-consistent simulations showing a significant exciton binding energy of ∼200 meV. After shelling, the material displays a p-type behavior that favors the generation of photoconductive gain. The latter is then used to increase the external quantum efficiency of an infrared imager, which now reaches 40% while presenting long-term stability.

Colloidal quantum dots and two‐dimensional material heterostructures for photodetector applications

AbstractPhotodetectors (PDs) are optoelectronic devices that convert optical signals into electrical responses. Recently, there has been a tremendous increase in research interest in PDs based on colloidal quantum dots (QDs) and two‐dimensional (2D) material heterostructures owing to the strong light‐absorption capacity and the well‐adjustable band gap of QDs and the superior charge carriers transfer ability of 2D materials. In particular, the heterojunction formed between QDs and 2D materials can effectively enhance the separation and transport of photogenerated charge carriers, which is expected to establish PDs with ultrahigh photoconductive gain, high responsivity, and detectivity. This review aimed to summarize the state‐of‐the‐art advances in the research of QDs/2D material nanohybrid PDs, including the device parameters, architectures, working mechanisms, and fabrication technologies. The progress of hybrid PDs based on the heterojunction of QDs with different 2D materials, along with their innovative applications, are comprehensively described. In the end, the challenges and feasible strategies in future research and development are briefly proposed.

Sensitive organic/inorganic polarized photodetectors enhanced by charge transfer with image sensing capacity.

Organic photodetectors (OPDs) have attracted increasing attention in the future wearable sensing and real-time health monitoring, due to their intrinsic features including the mechanical flexibility, low-cost processing and cooling-free operations; while their performances are lagging as the results of inferior carrier mobility and small exciton diffusion coefficient of organic molecules. Graphene exhibits the great photoresponse with wide spectral bandwidth and high response speed. However, weak light absorption and the absence of a gain mechanism have limited its photoresponsivity. Here, we report a sensitive organic/inorganic phototransistor with fast response speed by coupling PTCDA organic single crystal with the monolayer graphene. The long range exciton diffusion in highly ordered π-conjugated molecules, efficient exciton dissociation and charge transfer at the PTCDA/graphene heterointerfaces, and the high mobility of graphene enable a high responsivity (8 × 104A/W), short response time (220 µs) and excellent specific detectivity (>1011 Jones), which is higher than the level of commercial on-chip device. This interfacial photogating effect is verified by the high-resolution spatial photocurrent mapping experiment. In addition, the high sensitivity to polarization is clear and the ultrahigh photoconductive gain enables a near-infrared (NIR) response for 980 and 1550 nm. Finally, high-speed visible and NIR imaging applications are successfully demonstrated. This work suggests that high quality organic single crystal/graphene is a promising platform for future high performance optoelectronic systems and imaging applications.

Photo‐Induced Electrical Gating with the Inserting of an Interfacial Carrier Blocking Layer for Organic/Graphene Phototransistors

AbstractAlthough ultrahigh photoconductive gain is achieved in organic semiconductor‐sensitized graphene phototransistors, the response speed is limited by the photo‐induced carrier injection at the organic/graphene interface. In this work, an insulating hexagonal boron nitride layer is inserted between the organic heterostructure and graphene to block the interfacial carrier transport. Above the h‐BN layer, the organic heterostructure is adopted as the light‐absorbing layer, and the photoresponse of graphene is realized through electrical gating of the atop h‐BN dielectric layer, by the accumulated carriers in the organic heterostructure. Under this “photo‐induced electrical gating” mechanism, the recombination of nonequilibrium carriers takes place only within the organic heterostructure. Furthermore, monolayer perylene‐3,4,9,10‐tetracarboxylic dianhydride (PTCDA) is deposited on graphene before transfer of the h‐BN, by which the interfacial encapsulated bubbles are effectively reduced. As a step further, the trap states are pre‐filled with background light illumination, then the carrier recombination follows the band‐to‐band pathway. Finally, the response time of the organic/h‐BN/graphene phototransistor is reduced to 7.15 ms, and a high photoresponsivity is achieved even though the carrier multiplication is sacrificed, which is reconciled by the improvement of the device quantum efficiency.

Pixel Geometry Effect on PbS Quantum Dot/Graphene Nanohybrid Broadband Photodetectors

AbstractPhotodetectors based on colloidal quantum dots (QD)/graphene nanohybrids are quantum sensors due to strong quantum confinement in both QD and graphene. The optoelectronic properties of QD/graphene nanohybrids are affected by the quantum physics that predicts a high photoconductive gain and hence photoresponsivity (R*) depending on the pixel length (L) as R*∝L−2. Experimental confirmation of the effect of the pixel geometric parameters on the optoelectronic properties of the QD/graphene photodetector is therefore important to elucidate the underlying quantum physics. Motivated by this, an array of PbS QDs/graphene nanohybrid photodetectors are designed with variable QD/graphene pixel length L and width (W) in the range of 10–150 µm for a study of R*, noise, and specific detectivity (D*) in a broad spectrum of 400–1500 nm. Intriguingly, R* exhibits a monotonic decreasing trend of 1/L2 while being independent of W, confirming experimentally the theoretical prediction. Interestingly, this geometric effect on the photoresponsivity seems to be partially compensated by that in noise, leading to D* independent of L and W at wavelengths in the ultraviolet‐visible‐near infrared range. This result sheds light on the quantum physics underlying the optoelectronic process in QD/graphene nanohybrids, which is important to the design of high‐quality QD/graphene photodetectors and imaging systems.

Uncooled High Detectivity Mid-Infrared Photoconductor Using HgTe Quantum Dots and Nanoantennas.

Using a metal/insulator/metal (MIM) structure with a gold nanoantenna array made by electron beam lithography, the responsivity of a HgTe colloidal quantum dot film is enhanced in the mid-infrared. Simulations indicate that the spatially averaged peak spectral absorption of an 80 nm film is 60%, enhanced 23-fold compared to that of the same film on a bare sapphire substrate. The field intensity enhancement is focused near the antenna tips, being 20-fold 100 nm away, which represents only 1% of the total area and up to 1000-fold at the tips. The simulated polarized absorption spectra are in good agreement with the experiments, with a strong resonance around 4 μm. A responsivity of 0.6 A/W is obtained at a 1 V bias. Noise measurements separate the 1/f noise from the generation-recombination white noise and give a spatially averaged photoconductive gain of 0.3 at 1 V bias. The spatially averaged peak detectivity is improved 15-fold compared to the same film on a sapphire substrate without an MIM structure. The experimental peak detectivity reaches 9 × 109 Jones at 2650 cm-1 and 80 kHz, decreasing at lower frequencies. The MIM structure also enhances the spatially averaged peak photoluminescence of the CQD film by 16-fold, which is a potential Purcell enhancement. The good agreement between simulations and measurements confirms the viability of lithographically designed nanoantenna structures for vastly improving the performance of mid-IR colloidal quantum dot photoconductors. Further improvements will be possible by matching the optically enhanced and current collection areas.