Showing papers on "Photoconductivity published in 2020"
TL;DR: This work proposes and realizes high-performance waveguide photodetectors based on bolometric/photoconductive effects by introducing an ultrathin wide silicon−graphene hybrid plasmonic waveguide, which enables efficient light absorption in graphene at 1.55 μm and beyond.
Abstract: Graphene has attracted much attention for the realization of high-speed photodetection for silicon photonics over a wide wavelength range. However, the reported fast graphene photodetectors mainly operate in the 1.55 μm wavelength band. In this work, we propose and realize high-performance waveguide photodetectors based on bolometric/photoconductive effects by introducing an ultrathin wide silicon−graphene hybrid plasmonic waveguide, which enables efficient light absorption in graphene at 1.55 μm and beyond. When operating at 2 μm, the present photodetector has a responsivity of ~70 mA/W and a setup-limited 3 dB bandwidth of >20 GHz. When operating at 1.55 μm, the present photodetector also works very well with a broad 3 dB bandwidth of >40 GHz (setup-limited) and a high responsivity of ~0.4 A/W even with a low bias voltage of −0.3 V. This work paves the way for achieving high-responsivity and high-speed silicon–graphene waveguide photodetection in the near/mid-infrared ranges, which has applications in optical communications, nonlinear photonics, and on-chip sensing. The use of a silicon−graphene plasmonic waveguide has enabled the realization of fast and sensitive photodetectors that operate at the wavelength of 2 µm. In order to satisfy the demands for the applications in optical communication and optical sensing, there is the need to extend silicon photonics to wavelengths beyond 1.55 µm. However, it is a challenge to create high-performance photodetectors at these wavelengths. Now, Daoxin Dai and coworkers from Zhejiang University and Southeast University in China have proposed and realized a silicon−graphene hybrid plasmonic waveguide photodetector that operates at 2 µm with a responsivity of ~70 mA/W and a 3-dB bandwidth over 20 GHz. In this design, efficient light absorption in graphene is enabled by using a hybrid plasmonic waveguide with a wide thin silicon ridge core and a metal cap that serves as a signal electrode.
125 citations
TL;DR: A nonlinear photoconductive sampling method to measure electric field wave-forms in the infrared, visible and ultraviolet spectral ranges is demonstrated, providing sub-fs temporal precision in reconstructing the sub-cycle electronic response of a solid state structure.
Abstract: The sub-cycle interaction of light and matter is one of the key frontiers of inquiry made accessible by attosecond science. Here, we show that when light excites a pair of charge carriers inside of a solid, the transition probability is strongly localized to instants slightly after the extrema of the electric field. The extreme temporal localization is utilized in a simple electronic circuit to record the waveforms of infrared to ultraviolet light fields. This form of petahertz-bandwidth field metrology gives access to both the modulated transition probability and its temporal offset from the laser field, providing sub-fs temporal precision in reconstructing the sub-cycle electronic response of a solid state structure. Characterization of light pulses is important in order to understand their interaction with matter. Here the authors demonstrate a nonlinear photoconductive sampling method to measure electric field wave-forms in the infrared, visible and ultraviolet spectral ranges.
75 citations
TL;DR: In this paper, the effects of hydrogen and hydroxyl passivation on the structure, electrical and optical properties of SiCNWs were investigated by analyzing the distribution of electronic states and the polarity of chemical bonds.
73 citations
TL;DR: In this paper, the effects of the reaction temperature, reaction atmosphere, and oxygen vacancy (Ov) on the photoconductivity of hydrogenated ST-01 TiO2 were studied and it was shown that photothermal coupling assisted the detrapping of electrons at the Ov surface and promoted electron transfer to CO2.
57 citations
TL;DR: It is demonstrated that it is indeed possible to enhance both the conductivity and photoconductivity of a p-type semiconductor rr-P3HT that is ultrastrongly coupled to plasmonic modes and shows a modified spectral response due to the formation of the hybrid polaritonic states.
Abstract: During the past decade, it has been shown that light-matter strong coupling of materials can lead to modified and often improved properties which has stimulated considerable interest. While charge transport can be enhanced in n-type organic semiconductors by coupling the electronic transition and thereby splitting the conduction band into polaritonic states, it is not clear whether the same process can also influence carrier transport in the valence band of p-type semiconductors. Here we demonstrate that it is indeed possible to enhance both the conductivity and photoconductivity of a p-type semiconductor rr-P3HT that is ultrastrongly coupled to plasmonic modes. It is due to the hybrid light-matter character of the virtual polaritonic excitations affecting the linear response of the material. Furthermore, in addition to being enhanced, the photoconductivity of rr-P3HT shows a modified spectral response due to the formation of the hybrid polaritonic states. This illustrates the potential of engineering the vacuum electromagnetic environment to improve the optoelectronic properties of organic materials.
56 citations
TL;DR: A high-performance ultrabroadband photodetector is demonstrated with detecting range from UV to LWIR based on air-stable nonlayered ultrathin Fe3 O4 nanosheets synthesized via a space-confined chemical vapor deposition (CVD) method and demonstrates the high sensitivity for light with any light intensities.
Abstract: The ultrabroadband spectrum detection from ultraviolet (UV) to long-wavelength infrared (LWIR) is promising for diversified optoelectronic applications of imaging, sensing, and communication. However, the current LWIR-detecting devices suffer from low photoresponsivity, high cost, and cryogenic environment. Herein, a high-performance ultrabroadband photodetector is demonstrated with detecting range from UV to LWIR based on air-stable nonlayered ultrathin Fe3 O4 nanosheets synthesized via a space-confined chemical vapor deposition (CVD) method. Ultrahigh photoresponsivity (R) of 561.2 A W-1 , external quantum efficiency (EQE) of 6.6 × 103 %, and detectivity (D*) of 7.42 × 108 Jones are achieved at the wavelength of 10.6 µm. The multimechanism synergistic effect of photoconductive effect and bolometric effect demonstrates the high sensitivity for light with any light intensities. The outstanding device performance and complementary mixing photoresponse mechanisms open up new potential applications of nonlayered 2D materials for future infrared optoelectronic devices.
55 citations
TL;DR: The direct evidence of CT mechanism in 2D Cu-THQ MOFs and the correlation of temporal and spatial behaviors of charge carriers with their photoconductivity is reported by combining three advanced spectroscopic methods, including time resolved optical and X-ray absorption spectroscopy and terahertzSpectroscopy.
Abstract: Conductive metal organic frameworks (MOFs) represent a promising class of porous crystalline materials that have demonstrated potential in photo-electronics and photocatalytic applications. However, the lack of fundamental understanding on charge transport (CT) mechanism as well as the correlation of CT mechanism with their structure hampered their further development. Herein, we report the direct evidence of CT mechanism in 2D Cu-THQ MOFs and the correlation of temporal and spatial behaviors of charge carriers with their photoconductivity by combining three advanced spectroscopic methods, including time resolved optical and X-ray absorption spectroscopy and terahertz spectroscopy. In addition to Cu-THQ, the CT in Cu/Zn-THQ after incorporating Zn2+ guest metal was also examined to uncover the contribution of through space pathway, as the presence of the redox inactive 3d10 Zn2+ is expected to perturb the long range in-plane CT. We show that the hot carriers in Cu-THQ generated after photoexcitation are highly mobile and undergo fast localization to a lower energy state (cool carriers) with electrons occupying Cu center and holes in ligands. The cool carriers, which have super long lifetime (>17 ns), are responsible for the long-term photoconductivity in Cu-THQ and transport through the O-Cu-O motif with negligible contribution from interlayer ligand π-π stacking, as incorporation of Zn2+ in Cu-THQ significantly reduced photoconductivity. These unprecedented results not only demonstrate the capability to experimentally probe CT mechanism but also provide important insight in the rational design of 2D MOFs for photoelectronic and photocatalytic applications.
50 citations
TL;DR: The vertical architecture combining perovskite crystal with highly conductive graphene offers opportunities to fulfill the synergistic effect of perovkite and 2D materials, is thus promising for developing high-performance electronic and optoelectronic devices.
Abstract: Organolead trihalide perovskites have attracted significant attention for optoelectronic applications due to their excellent physical properties in the past decade. Generally, both grain boundaries in perovskite films and the device structure play key roles in determining the device performance, especially for horizontal-structured device. Here, the first optimized vertical-structured photodetector with the perovskite single crystal MAPbBr3 as the light absorber and graphene as the transport layer is shown. The hybrid device combines strong photoabsorption characteristics of perovskite and high carrier mobility of flexible graphene, exhibits excellent photoresponse performance with high photoresponsivity (≈1017.1 A W-1 ) and high photodetectivity (≈2.02 × 1013 Jones) in a low light intensity (0.66 mW cm-2 ) under the actuations of 3 V bias and laser irradiation at 532 nm. In particular, an ultrahigh photoconductive gain of ≈2.37 × 103 is attained because of fast charge transfer in the graphene and large recombination lifetime in the perovskite single crystal. The vertical architecture combining perovskite crystal with highly conductive graphene offers opportunities to fulfill the synergistic effect of perovskite and 2D materials, is thus promising for developing high-performance electronic and optoelectronic devices.
43 citations
TL;DR: These findings provide a route to fabricate high performance and wide detection range p-type metal oxide photodetectors and show the optimal response in the near-infrared region, owing to the high absorption coefficient.
Abstract: Photodetectors based on p-type metal oxides are still a challenge for optoelectronic device applications. Many effects have been paid to improve their performance and expand their detection range. Here, high-quality Cu1-xNixO (x = 0, 0.2, and 0.4) film photodetectors were prepared by a solution process. The crystal quality, morphology, and grain size of Cu1-xNixO films can be modulated by Ni doping. Among the photodetectors, the Cu0.8Ni0.2O photodetector shows the maximum photocurrent value (6 × 10-7 A) under a 635 nm laser illumination. High responsivity (26.46 A/W) and external quantum efficiency (5176%) are also achieved for the Cu0.8Ni0.2O photodetector. This is because the Cu0.8Ni0.2O photosensitive layer exhibits high photoconductivity, low surface states, and high crystallization after 20% Ni doping. Compared to the other photodetectors, the Cu0.8Ni0.2O photodetector exhibits the optimal response in the near-infrared region, owing to the high absorption coefficient. These findings provide a route to fabricate high-performance and wide-detection range p-type metal oxide photodetectors.
41 citations
TL;DR: In this article, an ultraviolet (UV) photovoltaic photodetector based on graphene/ZnO heterostructure was fabricated and investigated, and a simple surface treatment was conducted by soaking the as-grown ZnO film in H2O2 solution at room temperature.
39 citations
TL;DR: Efficient photoconductivity multiplication in few-layer 2H-MoTe2 is reported as a direct consequence of an efficient steplike carrier multiplication with near unity quantum yield and high carrier mobility in MoTe2.
Abstract: We report efficient photoconductivity multiplication in few-layer 2H-MoTe2 as a direct consequence of an efficient steplike carrier multiplication with near unity quantum yield and high carrier mobility (∼45 cm2 V-1 s-1) in MoTe2. This photoconductivity multiplication is quantified using ultrafast, excitation-wavelength-dependent photoconductivity measurements employing contact-free terahertz spectroscopy. We discuss the possible origins of efficient carrier multiplication in MoTe2 to guide future theoretical investigations. The combination of photoconductivity multiplication and the advantageous bandgap renders MoTe2 as a promising candidate for efficient optoelectronic devices.
TL;DR: In this article, a self-powered inorganic perovskite photodetector with a simplified configuration of ITO/CsPbBr3/Ag was presented.
Abstract: Perovskite photodetectors, with the merits of excellent light absorption, high carrier mobility and fast light response, are attracting tremendous interest for their promising application in optical communication, sensing and imaging. However, the requirement of a power supply for photoconductive detectors or the usage of expensive charge transport layers for photovoltaic detectors will pronouncedly increase the whole costs and process complexity. Herein, we demonstrate a self-powered inorganic perovskite photodetector with a novel and simplified configuration of ITO/CsPbBr3/Ag. Since no electron and hole transport layers are employed, it is highly necessary for the CsPbBr3 layer to prevent direct contact between the anode and cathode. Hence, we develop a sequential evaporation technique to fabricate a high-quality CsPbBr3 photosensitive film, which is more controllable than the traditional solution-processing route. The evaporated CsPbBr3 film obtains a much higher crystallinity, phase-purity, coverage and light harvesting capability than the conventionally solution-processed counterparts. The evaporated CsPbBr3 also exhibits larger grain sizes with fewer grain boundaries, enabling a faster charge transport and shorter photoresponse time of 54 ms for the photodetectors. After modification by a thin polymethyl methacrylate (PMMA) layer, the trap states and leakage current pathways in the CsPbBr3 film are substantially decreased, yielding an improved carrier lifetime and suppressed recombination loss. The PMMA-modified photodetectors achieve an ultrafast response time of 3.8 μs and an ultrahigh on/off ratio of 3.5 × 104, which are the best performance among all the reported charge-transport-layer-free perovskite photodetectors and surpass most of the perovskite photodetectors even with charge transport layers. The as-fabricated CsPbBr3 detectors also present superior stability after being stored in ambient air for over 1000 h. Moreover, our devices are successfully applied in a home-made visible light communication system as light receivers to transmit audio signals. Our work solves the contradiction between high detection performance and structure simplification, promoting practical applications of cost-effective, ultrafast and stable inorganic perovskite photodetectors in the optoelectronic field.
TL;DR: Observations are understood on the basis of a phenomenological model in which strong coupling alters the Onsager dissociation of polarons from their dopant counterions by effectively increasing the thermalization length of the photoexcited charge carrier.
Abstract: We investigate the potential for cavity-modified electron transfer in a doped organic semiconductor through the photocurrent that arises from exciting charged molecules (polarons). When the polaron optical transition is strongly coupled to a Fabry-Perot microcavity mode, we observe polaron polaritons in the photoconductivity action spectrum and find that their magnitude depends differently on applied electric field than photocurrent originating from the excitation of uncoupled polarons in the same cavity. Crucially, moving from positive to negative detuning causes the upper and lower polariton photocurrents to swap their field dependence, with the more polaronlike branch resembling that of an uncoupled excitation. These observations are understood on the basis of a phenomenological model in which strong coupling alters the Onsager dissociation of polarons from their dopant counterions by effectively increasing the thermalization length of the photoexcited charge carrier.
TL;DR: It is shown that electrons generated in the SCL flow through the IGZO layer, which enables the phototransistor to detect visible light and exhibited excellent durability and reversibility.
Abstract: Visible light can be detected using an indium–gallium–zinc oxide (IGZO)-based phototransistor, with a selenium capping layer (SCL) that functions as a visible light absorption layer. Selenium (Se) exhibits photoconductive properties as its conductivity increases with illumination. We report an IGZO phototransistor with an SCL (SCL/IGZO phototransistor) that demonstrated optimal photoresponse characteristics when the SCL was 150 nm thick. The SCL/IGZO phototransistor exhibited a photoresponsivity of 1.39 × 103 A/W, photosensitivity of 4.39 × 109, detectivity of 3.44 × 1013 Jones, and external quantum efficiency of 3.52 × 103% when illuminated by green light (532 nm). Ultraviolet–visible spectroscopy and ultraviolet photoelectron spectroscopy analysis showed that Se has a narrow energy band gap, in which visible light is absorbed and forms a p–n junction with IGZO so that photogenerated electron–hole pairs are easily separated, which makes recombination more challenging. We show that electrons generated in ...
TL;DR: This work proposes a different approach for highly responsive graphene-based photodetectors with orders of magnitude lower dark current levels, which makes the device appealing for applications where high responsivity and low power consumption are required.
Abstract: Graphene-based photodetectors have shown responsivities up to 108 A/W and photoconductive gains up to 108 electrons per photon. These photodetectors rely on a highly absorbing layer in close proximity to graphene, which induces a shift of the graphene chemical potential upon absorption, hence modifying its channel resistance. However, due to the semimetallic nature of graphene, the readout requires dark currents of hundreds of microamperes up to milliamperes, leading to high power consumption needed for the device operation. Here, we propose a different approach for highly responsive graphene-based photodetectors with orders of magnitude lower dark-current levels. A shift of the graphene chemical potential caused by light absorption in a layer of colloidal quantum dots induces a variation of the current flowing across a metal-insulator-graphene diode structure. Owing to the low density of states of graphene near the neutrality point, the light-induced shift in chemical potential can be relatively large, dramatically changing the amount of current flowing across the insulating barrier and giving rise to an alternative gain mechanism. This readout requires dark currents of hundreds of nanoamperes up to a few microamperes, orders of magnitude lower than that of other graphene-based photodetectors, while keeping responsivities of ∼70 A/W in the infrared, almost 2 orders of magnitude higher than that of established germanium on silicon and indium gallium arsenide infrared photodetectors. This makes the device appealing for applications where high responsivity and low power consumption are required.
TL;DR: In this article, the carrier lifetime of Zener-Klein transistors was shown to be 30πps for photoexcited carriers at low density and energy, using mid-infrared photoconductivity measurements.
Abstract: Graphene/hBN heterostructures are promising active materials for devices in the THz domain, such as emitters and photodetectors based on interband transitions. Their performance requires long carrier lifetimes. However, carrier recombination processes in graphene possess sub-picosecond characteristic times for large non-equilibrium carrier densities at high energy. An additional channel has been recently demonstrated in graphene/hBN heterostructures by emission of hBN hyperbolic phonon polaritons (HPhP) with picosecond decay time. Here, we report on carrier lifetimes in graphene/hBN Zener-Klein transistors of ~30 ps for photoexcited carriers at low density and energy, using mid-infrared photoconductivity measurements. We further demonstrate the switching of carrier lifetime from ~30 ps (attributed to interband Auger) down to a few picoseconds upon ignition of HPhP relaxation at finite bias and/or with infrared excitation power. Our study opens interesting perspectives to exploit graphene/hBN heterostructures for THz lasing and highly sensitive THz photodetection as well as for phonon polariton optics. Long carrier lifetimes are beneficial for graphene-based optoelectronics, but carrier recombination processes in graphene possess sub-picosecond characteristic times. Here, the authors report carrier lifetimes ~30 ps at low energy in graphene/hBN Zener-Klein transistors, attributed to interband Auger processes.
03 Aug 2020
TL;DR: In this article, the authors developed ZnO/p-Si photodetectors by atomic layer deposition (ALD) on laser-microstructured silicon, and investigated their electrical and optical behavior, demonstrating high performance.
Abstract: We develop ZnO/p-Si photodetectors by atomic layer deposition (ALD) of ZnO thin films on laser-microstructured silicon, and we investigate their electrical and optical behavior, demonstrating high ...
TL;DR: This study shows a potential application of both bulk crystal and individual flakes of CrPS4 as active components in light detection, when introduced as ink-printable moieties with a large benefit for manufacturing.
Abstract: Layered semiconductors have attracted significant attention due to their diverse physical properties controlled by composition and the number of stacked layers. Herein, large crystals of the ternary layered semiconductor chromium thiophosphate (CrPS4 ) are prepared by a vapor transport synthesis. Optical properties are determined using photoconduction, absorption, photoreflectance, and photoacoustic spectroscopy exposing the semiconducting properties of the material. A simple, one-step protocol for mechanical exfoliation onto a transmission electron microscope grid is developed, and multiple layers are characterized by advanced electron microscopy methods, including atomic resolution elemental mapping confirming the structure by directly showing the positions of the columns of different elements' atoms. CrPS4 is also liquid exfoliated, and in combination with colloidal graphene, an ink-jet-printed photodetector is created. This all-printed graphene/CrPS4 /graphene heterostructure detector demonstrates a specific detectivity of 8.3 × 108 (D*). This study shows a potential application of both bulk crystal and individual flakes of CrPS4 as active components in light detection, when introduced as ink-printable moieties with a large benefit for manufacturing.
TL;DR: In this paper, the authors used near-field scanning microwave microscopy (SMM) to image spatial variations in photoconductivity in MoS2-WS2 lateral multijunction heterostructures using photon energy-resolved narrowband illumination.
Abstract: The optical and electronic properties of 2D semiconductors are intrinsically linked via the strong interactions between optically excited bound species and free carriers. Here we use near-field scanning microwave microscopy (SMM) to image spatial variations in photoconductivity in MoS2-WS2 lateral multijunction heterostructures using photon energy-resolved narrowband illumination. We find that the onset of photoconductivity in individual domains corresponds to the optical absorption onset, confirming that the tightly bound excitons in transition metal dichalcogenides can nonetheless dissociate into free carriers. These photogenerated carriers are most likely n-type and are seen to persist for up to days. Informed by finite element modeling we reveal that they can increase the carrier density by up to 200 times. This persistent photoconductivity appears to be dominated by contributions from the multilayer MoS2 domains, and we attribute the flake-wide response in part to charge transfer across the heterointerface. Spatial correlation of our SMM imaging with photoluminescence (PL) mapping confirms the strong link between PL peak emission photon energy, PL intensity, and the local accumulated charge. This work reveals the spatially and temporally complex optoelectronic response of these systems and cautions that properties measured during or after illumination may not reflect the true dark state of these materials but rather a metastable charged state.
TL;DR: The GBIPs with the b-P and b-As BLs can operate at longer radiation wavelengths than the infrared GL-based IPs comprising the BLs made of other van der Waals materials and can also compete with all kinds of the far-infrared photodetectors.
Abstract: We develop the device models for the far-infrared interband photodetectors (IPs) with the graphene-layer (GL) sensitive elements and the black Phosphorus (b-P) or black-Arsenic (b-As) barrier layers (BLs). These far-infrared GL/BL-based IPs (GBIPs) can operate at the photon energies ℏ
Ω smaller than the energy gap, ΔG, of the b-P or b-As or their compounds, namely, at ℏ
Ω≲2Δ
G/3 corresponding to the wavelength range λ≳(6−12) μm. The GBIP operation spectrum can be shifted to the terahertz range by increasing the bias voltage. The BLs made of the compounds b-AsxB1−x with different x, enable the GBIPs with desirable spectral characteristics. The GL doping level substantially affects the GBIP characteristics and is important for their optimization. A remarkable feature of the GBIPs under consideration is a substantial (over an order of magnitude) lowering of the dark current due to a partial suppression of the dark-current gain accompanied by a fairly high photoconductive gain. Due to a large absorption coefficient and photoconductive gain, the GBIPs can exhibit large values of the internal responsivity and dark-current-limited detectivity exceeding those of the quantum-well and quantum-dot IPs using the intersubband transitions. The GBIPs with the b-P and b-As BLs can operate at longer radiation wavelengths than the infrared GL-based IPs comprising the BLs made of other van der Waals materials and can also compete with all kinds of the far-infrared photodetectors.
TL;DR: In this article, a reduction of PtS e 2 channel conductance is observed under exposure to light, which is explained by a photogating effect caused by photo-charge accumulation in SiO2 and at the Si/SiO 2 interface.
Abstract: PtS e 2 ultrathin films are used as the channel of back-gated field-effect transistors that are investigated at different temperatures and under super-continuous white laser irradiation. The temperature-dependent behavior confirms the semiconducting nature of multilayer PtS e 2, with p-type conduction, a hole field-effect mobility up to 40 c m 2 V − 1 s − 1, and significant gate modulation. Electrical conduction measured along different directions shows isotropic transport. A reduction of PtS e 2 channel conductance is observed under exposure to light. Such a negative photoconductivity is explained by a photogating effect caused by photo-charge accumulation in SiO2 and at the Si / SiO 2 interface.
TL;DR: The results reveal the intrinsic time and length scales of photocarriers in van der Waals materials, providing the guidance for implementing nanooptoelectronic devices based on 2D semiconductors.
Abstract: The optoelectronic properties of atomically thin transition-metal dichalcogenides are strongly correlated with the presence of defects in the materials, which are not necessarily detrimental for certain applications. For instance, defects can lead to an enhanced photoconduction, a complicated process involving charge generation and recombination in the time domain and carrier transport in the spatial domain. Here, we report the simultaneous spatial and temporal photoconductivity imaging in two types of WS2 monolayers by laser-illuminated microwave impedance microscopy. The diffusion length and carrier lifetime were directly extracted from the spatial profile and temporal relaxation of microwave signals, respectively. Time-resolved experiments indicate that the critical process for photoexcited carriers is the escape of holes from trap states, which prolongs the apparent lifetime of mobile electrons in the conduction band. As a result, counterintuitively, the long-lived photoconductivity signal is higher in chemical-vapor deposited (CVD) samples than exfoliated monolayers due to the presence of traps that inhibits recombination. Our work reveals the intrinsic time and length scales of electrical response to photoexcitation in van der Waals materials, which is essential for their applications in optoelectronic devices.
TL;DR: In this paper, an alternative analytical expression was proposed to improve the accuracy of the estimated value of the real photoconductivity, while remaining simple to implement experimentally, in the regime of optically thin samples.
Abstract: The analysis of terahertz transmission through semiconducting thin films has proven to be an excellent tool for investigating optoelectronic properties of novel materials. Terahertz time-domain spectroscopy (THz-TDS) can provide information about phonon modes of the crystal, as well as the electrical conductivity of the sample. When paired with photoexcitation, optical-pump-THz-probe (OPTP) technique can be used to gain an insight into the transient photoconductivity of the semiconductor, revealing the dynamics and the mobility of photoexcited charge carriers. As the relation between the conductivity of the material and the THz transmission function is generally complicated, simple analytical expressions have been developed to enable straightforward calculations of frequency-dependent conductivity from THz-TDS data in the regime of optically thin samples. Here, we assess the accuracy of these approximated analytical formulas in thin films of highly doped semiconductors, finding significant deviations of the calculated photoconductivity from its actual value in materials with background conductivity comparable to 102Ω− 1cm− 1. We propose an alternative analytical expression, which greatly improves the accuracy of the estimated value of the real photoconductivity, while remaining simple to implement experimentally. Our approximation remains valid in thin films with high dark conductivity of up to 104Ω− 1cm− 1 and provides a very high precision for calculating photoconductivity up to 104Ω− 1cm− 1, and therefore is highly relevant for studies of photoexcited charge-carrier dynamics in electrically doped semiconductors. Using the example of heavily doped thin films of tin-iodide perovskites, we show a simple experimental method of implementing our correction and find that the commonly used expression for photoconductivity could result in an underestimate of charge-carrier mobility by over 50%.
TL;DR: A dual-functional ultraviolet (UV) photodetector with a large UV-to-visible rejection ratio is presented, in which interdigitated finger-type two-dimensional graphene electrodes are introduced to an AlGaN/GaN heterostructure to facilitate the development of a single device that can achieve multiple purposes ofPhotodetection.
Abstract: A dual-functional ultraviolet (UV) photodetector with a large UV-to-visible rejection ratio is presented, in which interdigitated finger-type two-dimensional graphene electrodes are introduced to an AlGaN/GaN heterostructure. Two photocurrent generation mechanisms of photovoltaic and photoconductive dominances coexist in the device. The dominance of the mechanisms changes with the induced bias voltage. Below a threshold voltage, the device showed fairly low responsivities but fast response times, as well as a constant photocurrent against the induced bias. However, the opposite characteristics appeared with high bias voltage. Specifically, above the threshold voltage, the device showed high responsivities with additional gain, but slow rise and recovery times. For instance, the responsivity of 10.9 A/W was observed with the gain of 760 at the induced bias voltage of 5 V. This unique multifunctionality enabled by the combination of an AlGaN/GaN heterostructure with graphene electrodes facilitates the development of a single device that can achieve multiple purposes of photodetection.
TL;DR: In this paper, the effect of surface ligands, morphology, and boundaries on charge-carrier transport in thin films fabricated with high-quality colloidal synthesis of nanocrystals is disentangled.
Abstract: Metal‐halide perovskites (MHP) are highly promising semiconductors for light‐emitting and photovoltaic applications. The colloidal synthesis of nanocrystals (NCs) is an effective approach for obtaining nearly defect‐free MHP that can be processed into inks for low‐cost, high‐performance device fabrication. However, disentangling the effects of surface ligands, morphology, and boundaries on charge‐carrier transport in thin films fabricated with these high‐quality NCs is inherently difficult. To overcome this fundamental challenge, terahertz (THz) spectroscopy is employed to optically probe the photoconductivity of CsPbBr3 NC films. The vibrational and optoelectronic properties of the NCs are compared with those of the corresponding bulk polycrystalline perovskite and significant deviations are found. Charge‐carrier mobilities and recombination rates are demonstrated to vary significantly with the NC size. Such dependences derive from the localized nature of charge carriers within NCs, with local mobilities dominating over interparticle transport. It is further shown that the colloidally synthesized NCs have distinct vibrational properties with respect to the bulk perovskite, exhibiting blue‐shifted optical phonon modes with enhanced THz absorption strength that also manifest as strong modulations in the THz photoconductivity spectra. Such fundamental insights into NC versus bulk properties will guide the optimization of nanocrystalline perovskite thin films for optoelectronic applications.
TL;DR: Here, Cd3As2 nanoplate/multilayer MoS2 heterojunction photodetector was fabricated and achieved a quite high responsivity of 2.7×103 A/W at room temperature and exhibits a short response time of in broad spectra region from ultraviolet to short-wavelength-infrared.
Abstract: High responsivity, fast response time, ultra-wide detection spectrum are pursuing goals for state-of-art photodetectors. Cd3As2, as a three-dimensional (3D) Dirac semimetal, has a zero bandgap, high light absorption rate in broad spectral region, and higher mobility than graphene at room temperature. However, photoconductive detectors based Cd3As2 suffer low quantum efficiency due to the absence of high built-in field. Here, a Cd3As2 nanoplate/multilayer MoS2 heterojunction photodetector was fabricated which achieved a quite high responsivity of 2.7 × 103 A W-1 at room temperature. The photodetector exhibits a short response time of in broad spectra region from ultraviolet (365 nm) to short-wavelength-infrared (1550 nm) and reached 65 μs at 650 nm. This work provides a great potential solution for high-performance photodetector and broadband imaging by combining 3D Dirac semi-metal materials with semiconductor materials.
TL;DR: B-site doping of organic-inorganic perovskites is an effective way to improve its optoelectronic properties as discussed by the authors, but it is rarely reported for single crystals.
Abstract: B-site doping of organic–inorganic perovskites is an effective way to improve its optoelectronic properties; however, it is rarely reported for perovskite single crystals. Here, we report the B-sit...
TL;DR: The large value of the high frequency dielectric constants-together with the appreciable magnitude of absorption coefficients and refractive indices, small values of effective masses of conducting electrons and holes, and the indirect nature of the bandgap transitions, suggest that cubic A2AgCrBr6 and Cs2Ag CrCl6 may likely be a set of optoelectronic materials for subsequent experimental characterizations.
Abstract: With an interest to quest for transition metal-based halogenated double perovskites AB′B″X6 as high performance semiconducting materials for optoelectronics, this study theoretically examined the electronic structures, stability, electronic (density of states and band structures), transport (effective masses of charge carriers), and optical properties (dielectric function and absorption coefficients, etc.) of the series A2AgCrBr6 (A = K, Rb, Cs) using SCAN + rVV10. Our results showed that A2AgCrBr6 (A = Rb, Cs), but not K2AgCrBr6, has a stable perovskite structure, which was revealed using various traditionally recommended geometry-based indices. Despite this reservation, all the three systems were shown to have similar band structures, density of states, and carrier effective masses of conducting holes and electrons, as well as the nature of the real and imaginary parts of their dielectric function, absorption coefficient, refractive index, and photoconductivity spectra. The small changes observed in any specific property of the series A2AgCrBr6 were due to the changes in the lattice properties driven by alkali substitution at the A site. A comparison with the corresponding properties of Cs2AgCrX6 (X = Cl, I) suggested that halogen substitution at the X-site can not only significantly shift the position of the onset of optical absorption found of the dielectric function, absorption coefficient and refractive spectra of Cs2AgCrCl6 and Cs2AgCrI6 toward the high- and low-energy infrared regions, respectively; but that it is also responsible in modifying their stability, electronic, transport, and optical absorption preferences. The large value of the high frequency dielectric constants—together with the appreciable magnitude of absorption coefficients and refractive indices, small values of effective masses of conducting electrons and holes, and the indirect nature of the bandgap transitions, among others—suggested that cubic A2AgCrBr6 (A = Rb, Cs) and Cs2AgCrCl6 may likely be a set of optoelectronic materials for subsequent experimental characterizations.