Photoconductive Photodetectors Research Articles

Investigation of the preparation and performance of Ag-modified ZnO nanowall ultraviolet detector

In this paper, silver (Ag) nanoparticle-modified zinc oxide (ZnO) nanowall photoconductive ultraviolet photodetector was prepared by using magnetron sputtering technology, electron beam evaporation method and hydrothermal method. The results showed that ZnO nanowalls exhibited a hexagonal wurtzite structure. ZnO nanowalls grew uniformly perpendicular to the substrate, and metallic Ag nanoparticles were uniformly attached to the surface of ZnO nanowalls, effectively improving the ZnO nanowall absorption capability in the ultraviolet light region. The dark current of the Ag-modified ZnO nanowall UV detector deposited by sputtering for 40[Formula: see text]s was 0.042[Formula: see text]mA, revealing a decrease of 44%. Under the irradiation of ultraviolet light with a wavelength of 325[Formula: see text]nm, the photocurrent reached 3.088[Formula: see text]mA, which increased by four times. The responsivity increased from 27.29 to 101.07[Formula: see text]mA/W, and the sensitivity increased from 8.07 to 73.52. In addition to the good repeatability and stability, the response time was reduced by 64%, and the recovery time was decreased by 50%.

Two-dimensional molybdenum ditelluride waveguide-integrated near-infrared photodetector

Low-cost, small-sized, and easy integrated high-performance photodetectors for photonics are still the bottleneck of photonic integrated circuits applications and have attracted increasing attention. The tunable narrow bandgap of two-dimensional (2D) layered molybdenum ditelluride (MoTe2) from ∼0.83 to ∼1.1 eV makes it one of the ideal candidates for near-infrared (NIR) photodetectors. Herein, we demonstrate an excellent waveguide-integrated NIR photodetector by transferring mechanically exfoliated 2D MoTe2 onto a silicon nitride (Si3N4) waveguide. The photoconductive photodetector exhibits excellent responsivity (R), detectivity (D*), and external quantum efficiency at 1550 nm and 50 mV, which are 41.9 A W−1, 16.2 × 1010 Jones, and 3360%, respectively. These optoelectronic performances are 10.2 times higher than those of the free-space device, revealing that the photoresponse of photodetectors can be enhanced due to the presence of waveguide. Moreover, the photodetector also exhibits competitive performances over a broad wavelength range from 800 to 1000 nm with a high R of 15.4 A W−1 and a large D* of 59.6 × 109 Jones. Overall, these results provide an alternative and prospective strategy for high-performance on-chip broadband NIR photodetectors.

A high responsivity UV - visible dual band photodetector based on SnO2 microwires with RhB surface sensitization

A SnO2 microwire (MW) based dual band photoconductive photodetector without persistent photoconductivity has been achieved. For expansion of the photo response to visible light, a dual band photodetector with UV and visible light response was fabricated via introducing RhB dye molecules. Due to the surface sensitization, photo responsivity of 43.1 A W−1 was obtained under 520 nm light illumination and 10 V bias for the detector made from a dye modified SnO2 MW, which is a 7-fold increase when compared to a photodetector fabricated from sole SnO2 MW (about 5.3 A W−1). The dual band photodetector also showed about double responsivity at 260 nm (about 351.9 A W−1) when compared to the detector made from sole SnO2 MW (about 183.9 A W−1), indicating improved UV light detection. Furthermore, under light excitations at 260 nm and 550 nm, the detectivity of about 2.38 × 1011 Jones and 3.55 × 1010 Jones can be obtained respectively for RhB molecule surface sensitized SnO2 MW at 10 V bias voltage. Through comparison of the electronic structures, the improved device performance in UV and visible light are attributed to the electron transfer mechanism from RhB to SnO2 surface, which can be verified from the measured I - t curves.

Fine control of Ce doped CH3NH3PbBr3 to modulate photoluminescence and carrier characteristics for application in photoconductive photodetectors

Finely tunable amounts of Ce3+ ions into MAPbBr3 can improve the opto-electronic properties of photodetector.

Experimental comparison between Nb2O5- and TiO2-based photoconductive and photogating GFET UV detector

In the present study, by adding graphene to a photoconductive photodetector with a niobium pentoxide (Nb2O5) absorber layer and exploiting the photogating effect, the responsivity of the photodetector is significantly improved. In this photodetector, the Nb2O5 layer detects the light, and the graphene improves the responsivity based on the photogating effect. The photocurrent and the percentage ratio of the photocurrent to dark current of the Nb2O5 photogating photodetector are compared with those of the corresponding photoconductive photodetector. Also, the Nb2O5 photoconductive and photogating photodetectors are compared with titanium dioxide (TiO2) photoconductive and photogating photodetectors in terms of responsivity at different applied (drain-source) voltages and gate voltages. The results show that the Nb2O5 photodetectors have better figures of merit (FOMs) in comparison with the TiO2 ones.

Performance of a tunable photoconductive graphene plasmonic photodetector

Abstract In this paper, the performance of a graphene photodetector is investigated theoretically in the infrared spectral region (8–12 µm). To increase the absorption of infrared radiation in the graphene layer, plasmon–polaritons are excited in the graphene layer by using dielectric grating. Due to the large propagation constants of plasmon–polaritons compared to the propagation constants of the electromagnetic waves in free space, the dielectric grating is required to provide the phase matching condition of plasmon–polaritons excitation. The results show that due to the excitation of plasmon–polaritons in the graphene layer, the infrared wave has been confined to a small reign around the graphene layer with a full width at half maximum (FWHM) of about 8 nm. Increasing in Fermi energy level leads to a shift in the wavelength of the infrared radiation required to excite plasmon–polaritons in the graphene layer towards shorter wavelengths, so that for the Fermi energy levels of 10, 30, 45, and 60 meV the required wavelengths for plasmon–polaritons excitation are 11.6, 10.6, 9.4, and 8.2 µm, respectively. Under the incidence of the infrared radiation with these wavelengths, and at the corresponding Fermi energy levels, the responsivities of the photodetector at peak points are 2.74, 2.39, 2.19, and 2.04 mA/W, respectively. Therefore, this photodetector is tunable where the detection wavelength is changed by tuning the Fermi energy level of the photodetector. In addition, the results indicate that excitation of plasmon–polaritons approximately increases the responsivity by two times compared to the case without the plasmon–polaritons excitation.

A study for the influences of temperatures on ZnGa2O4 films and solar-blind sensing performances

In this work, ZnGa2O4 metal–semiconductor–metal solar-blind ultraviolet (UV) photodetectors were fabricated and discussed by using the radio-frequency (r.f.) magnetron sputtering method with considerations of the influences of growing and measuring temperatures on photoelectric performances in detail. With the increase of growing temperature, the grain size and optical bandgap of the films increased. Ohmic contacts were formed between the films and Ti/Au electrodes for all the samples, and the photoconductive photodetectors shown decent solar-blind photoelectric performances. At 10 V biasing voltage, the devices had fast response speed and large photo-to-dark current ratio of ∼107 at 650 °C. Moreover, we measured the dark current (I dark) and photocurrent (I photo) of these devices in the temperature range from 298 K to 598 K. With the increase of temperature, the I dark increased monotonously, while the I photo first decreased and then increased. Furthermore, this study about the effects of temperatures on the ZnGa2O4 films and photodetectors may well provide a meaningful guide for its relevant developments in the future.

High responsivity graphene-InGaAs near-infrared photodetector realized by hole trapping and its response saturation mechanism.

Graphene is an ideal material for wide spectrum detector owing to its special band structure, but its low light absorption and fast composite of photogenerated carriers lead to a weak response performance. In this paper, we designed a unique photoconductive graphene-InGaAs photodetector. The built-in electric field was formed between graphene and InGaAs, which can prolong the lifetime of photogenerated carriers and improve the response of devices by confining the holes. Compared with graphene-Si structure, a higher built-in electric field and reach to 0.54 eV is formed. It enables the device to achieve a responsivity of 60 AW-1 and a photoconductive gain of 79.4 at 792 nm. In the 1550 nm communication band, the responsivity of the device is also greater than 10 AW-1 and response speed is less than 2 ms. Meanwhile, the saturation phenomenon of light response was also found in this photoconductive graphene heterojunction detector during the experiment, we have explained the phenomenon by the capacitance theory of the built-in electric field, and the maximum optical responsivity of the detector is calculated theoretically, which is in good agreement with the measurement result.

Photodiode working in zero-mode: detecting light power change with DC rejection and AC amplification

We propose a new mode of operation when using a photodiode to extract a variable optical signal from a constant (ambient) background. The basic idea of this 'zero-mode' of operation is to force the photodiode to operate at either zero current or zero voltage. We present possible implementations of this novel approach and provide the corresponding equivalent circuits while also demonstrating experimentally its performance. The gain and bandwidth of the zero-mode photodetector are measured and simulated, and they show highly agreement. The gain compression effect because of the nonlinearity of the forward bias region is also explored. Comparing to the conventional photoconductive photodetector, the zero-mode photodetector is able to obtain higher AC gain and lower noise. With the same component used in the circuit, the measured input referred root mean square noise of zero-mode photodetector is 4.4mV whereas that of the photoconductive mode photodetector is 96.9mV respectively, showing the feasibility of the zero-mode of operation for measuring the small variable light signal under a high power constant background light.

Alloying induced disorder and localized excitonic states in ternary BexZn1−xO thin films

II-VI ternary alloy is a promising material for advanced optoelectronic devices like ultraviolet (UV) photodetectors, light emitting diodes and laser diodes with high efficiency and low fabrication cost. The precise control over the doping element, concentration and crystal quality allows careful tuning of their band structures for novel optoelectronic properties. In this work, we report a prominent two-mode behavior in ternary BexZn1−xO alloy using resonant Raman spectroscopy as a probe. By combining with the excitonic localization and extrinsic Fröhlich interaction, we present a theoretical analysis on the resonant Raman scattering with different Be concentrations and tunable measured temperatures. Frequency blue shifts caused by compressed stress and two-mode phonons (ZnO-like and BeO-like A1(LO) phonons) scattering up to third orders revealed in this novel ternary alloy. In addition, alloying induced disorder and electronic states in the forbidden band result in the broadening of line width and the enhancement of LO-phonon oscillator strength. As a result, a high performance BeZnO based photoconductive type UV photodetector was realized, indicating the potential of BeZnO as a promising material for optoelectronic devices.

High-responsivity molybdenum diselenide photodetector with dirac electrodes

Two-dimensional molybdenum diselenide (MoSe2) has attracted immense attention in the field of next-generation optoelectronic devices for its unique structure and excellent photosensitivity. However, the Schottky barrier and surface defects at the interface between the electrode and MoSe2 channel limit the performance of MoSe2 photodetectors. Here, using Dirac graphene electrodes, a high performance MoSe2 photoconductive photodetector was fabricated via mechanical exfoliation. In contrast with photodetectors using Mo/Au electrodes with a high Schottky barrier of 196.7 meV, the devices using graphene electrodes maintain linear ohmic contacts owing to the alignment of the work function and Fermi-level depinning, thereby resulting in fast transient response time on the order of 30 ms and high responsivity exceeding 103 A/W. Moreover, we simultaneously demonstrate the effects of contact resistance on responsivity, response time, and light–dark current ratio of MoSe2 photodetectors. Further, the hysteresis accompanying the adsorption and desorption of water and oxygen molecules in ambient air was eliminated owing to a high photocurrent level between source and drain. This research unveils the vast potential of proper electrode contact selection to further improve the performance of photodetectors.

Strong photovoltaic effect in high-density InAlAs and InAs/InAlAs quantum-dot infrared photodetectors

We report on a high responsivity at zero bias in infrared photodetectors based on InAlAs quantum dots due to the formation of an Al-rich wetting layer which acts as an electron barrier. This layer has a larger bandgap and results from the low segregation and low surface diffusion of the Al atoms that mostly remain inside the wetting layer. These photovoltaic infrared photodetectors could operate up to 80 K and presented a detectivity similar to the one of a conventional photoconductive photodetector based on InAs quantum dots, despite the lack of optimization for photovoltaic operation. Such InAlAs devices might be good candidates for focal plane arrays that require low dark current, low noise, and low current consumption.

Multilayer InSe-Te van der Waals Heterostructures with an Ultrahigh Rectification Ratio and Ultrasensitive Photoresponse.

Multilayer van der Waals (vdWs) semiconductors have promising applications in high-performance optoelectronic devices. However, photoconductive photodetectors based on layered semiconductors often suffer from sizeable dark currents and high external driving bias voltages. Here, we report vertical van der Waals heterostructures (vdWHs) consisting of multilayer indium selenide (InSe) and tellurium (Te). The multilayer InSe-Te vdWH device shows a record high forward rectification ratio greater than 107 at room temperature. The vdWH device achieves an ultrasensitive and broadband photoresponse photodetector with an ultrahigh photo/dark current ratio over 104 and a high detectivity of 1013 Jones under visible light illumination with weak incident power. Moreover, the vdWH device has a photovoltaic effect and can function as a self-powered photodetector (SPPD). The SPPD is also ultrasensitive to a broadband spectrum ranging from 300 to 1000 nm and is capable of detecting weak light signals. This work offers an opportunity to develop next-generation electronic and optoelectronic devices based on multilayer vdWs materials.

Self-powered ultraviolet photovoltaic photodetector based on graphene/ZnO heterostructure

Compared to ZnO-based photoconductive photodetectors (PDs), photovoltaic PDs possess the advantages of easy fabrication, low dark current, high response speed and possible self-driven ability. In this paper, an ultraviolet (UV) photovoltaic photodetector based on graphene/ZnO heterostructure was fabricated and investigated. A simple surface treatment was conducted by soaking the as-grown ZnO film in H2O2 solution at room temperature. After this processing, a transition from ohmic contact to Schottky contact was observed in graphene/ZnO interface, accompanied with a high-performance photovoltaic behavior for the graphene/H2O2-treated ZnO (denoted as G/H-ZnO) heterostructure. The self-powered Schottky photodetector of G/H-ZnO exhibits a responsivity of 50 μA/W and a short rise/decay time of 32 ms at zero bias. The fast response performance benefits from the rapid separation of photogenerated carriers, which can be attributed to the synergistic effect of pyroelectric potential and built-in electric field at graphene/ZnO interface. This study provides a facile approach for the development of ZnO-based self-powered photovoltaic devices.

Ligand engineering of mid-infrared Ag2Se colloidal quantum dots

Abstract Silver selenide colloidal quantum dots exhibit distinct optical absorption in the mid-wavelength infrared spectral region, which arises from the intraband transition between the first and the second quantum-confined energy levels. The optical absorption coefficient, carrier mobility, and carrier lifetime, which are the three primary parameters that determine the ultimate performance of a photodetector, are expected to be heavily dependent on the surface capping ligands. Herein, we characterize these parameters on silver selenide colloidal quantum dots films chemically treated with ligands selected from literatures that report high-performance quantum dot-based solar cells, photodetectors, transistors, and thermoelectrics. We correlate these results with the mid-infrared responsivities measured from photoconductive photodetectors fabricated from respective ligand-exchanged films. The insights gained from this study may serve as a foundation for enabling future intraband CQD-based MWIR sensing and imaging technologies.

High-Performance Devices Based on InSe-In1-xGaxSe Van der Waals Heterojunctions.

Multilayer InSe is a promising material for high-performance optoelectronic applications because of its small direct band gap and good light absorption. However, as a photoconductive photodetector, multilayer InSe photodetectors endure large dark current and high driving power. In this work, we study the electrical properties of InGaSe alloys and demonstrate the high-performance devices based on multilayer InSe-In0.24Ga0.76Se van der Waals heterojunctions (vdWHs). The electrical properties of InGaSe alloy samples strongly depend on the ratio of In to Ga, and the In0.24Ga0.76Se alloy shows a p-type transport behavior. More importantly, a multilayer InSe-In0.24Ga0.76Se vdWH device is demonstrated as a high-performance forward diode, photodiode, and self-powered photodetector (SPPD). The multilayer InSe-In0.24Ga0.76Se diode shows a high forward rectification ratio of over 103 without gate modulation at room temperature, which is superior to most of the multilayer vdWH devices. Moreover, the vdWH photodiode has a broadband photoresponse spectrum (400-1000 nm) and a high-performance photoresponse. The light switching ratio, detectivity (D*), and responsivity (R) are 103, 1012 Jones, and 49 A W-1 for 400 nm illumination, respectively. Furthermore, the vdWH SPPD also shows a sensitive photoresponse to a broadband spectrum of 400-1000 nm. Our work offers an opportunity for multilayer vdWH device applications in high-performance electronic and optoelectronic devices.

The photovoltaic and photoconductive photodetector based on GeSe/2D semiconductor van der Waals heterostructure

Although plenty of two-dimensional (2D) semiconductor heterostructure photodetectors have been studied, there is still a lack of systematic comparison and analysis about photovoltaic and photoconductive 2D semiconductor photodetectors. Taking advantage of the 2D semiconductor van der Waals heterostructure, this work constructs a photovoltaic (PV) GeSe/MoS2 and a photoconductive (PC) GeSe/graphene photodetector, respectively. The PC GeSe/graphene photodetector achieves relatively higher photoresponsivity (R), where R can reach up to 104 AW−1. The PV GeSe/MoS2 photodetector, by contrast, obtains a faster photoresponse speed. More importantly, the photoresponse properties of the PV GeSe/MoS2 photodetector can remain constant under the reverse bias, due to the minority carrier conduction in its depletion region at this time. The different characteristics of the two type 2D photodetectors are explored in detail, which can play a guiding role in the construction of high-performance photodetectors.

A facile method to synthesize two-dimensional CsPb2Br5 nano-/micro-sheets for high-performance solution-processed photodetectors

In this paper, a facile method to synthesize two-dimensional (2D) all-inorganic CsPb2Br5 nano-/micro-sheets at room temperature and their application as the active layer in solution-processed high-performance photodetectors are presented. The obtained CsPb2Br5 nano-/micro-sheets showed a photoluminescence emission peak at 522 nm with a narrow full-width-at-half-maximum of ∼17 nm, showing its high color purity, and an absorption peak at 307 nm with a shoulder peak at 339 nm. Then, the obtained CsPb2Br5 nano-/micro-sheets were used as the active layer in photoconductive photodetector and a high specific detectivity of 1.0 × 1012 Jones with a photoresponsivity of 0.02 A/W and an ultra-low dark current of ∼10−12 An under 16 μW/cm2 405 nm illumination were obtained from photoconductive photodetector Au/CsPb2Br5(85 μm)/Au. Further, the physical mechanism for the enhanced performance is discussed. Therefore, it provides a new method to suppress dark current for high-performance CsPb2Br5 photodetectors.

High‐Performance Polymer Photodetector Using the Non‐Thermal‐and‐Non‐Ultraviolet–Ozone‐Treated SnO2 Interfacial Layer

In organic or perovskite photovoltaic‐type devices, to improve device performance, the thermal and ultraviolet–ozone (UVO) treatments are usually critical for the metal oxide film fabrication by removing the interfacial layer induced defects. However, the defects might play the opposite role in photoconductive photodetectors, where the defect‐induced photogenerated charge accumulation can lead to a tunneling current. Herein, a high‐performance solution‐processed broadband organic photodetector (OPD) in the visible wavelength is realized and characterized by introducing the non‐thermal‐and‐non‐UVO‐treated SnO2 nanoparticle film between the indium tin oxide (ITO) electrode and the active layer of the poly(3‐hexylthiophene) (P3HT):phenyl‐C61‐butyric‐acid‐methyl‐ester (PC61BM) blend. The untreated SnO2 nanoparticle layer can efficiently block the external charge injection, which considerably reduces the dark current density. Under illumination, the photogenerated charges are accumulated at the interface between the SnO2 layer and the active layer, and thus contribute a tunneling injection, resulting in a significantly high photocurrent. Therefore, the OPD shows a high performance in every figure of merit: external quantum efficiency (EQE) of 1430%, responsivity of 6.97 A W−1, detectivity of 2.29 × 1013 Jones, and −3 dB bandwidth of 2.8 MHz. The solution‐processed high‐performance photodetectors without thermal and UVO treatments are highly compatible with low cost, flexible, and large‐area electronics.

Mechanisms behind slow photoresponse character of Pulsed Electron Deposited ZnO thin films

Semiconducting Zinc Oxide (ZnO) is ideal candidate for ultraviolet (UV) photodetector due to its promising optoelectronic properties. Photoconductive type ZnO photodetectors, which is fabricated in metal-semiconductor-metal configuration, show mostly very high photoconductivity under UV light, but they are plagued by slow photoresponse time as slow as several tens of hours, even more. Most of the studies claimed that atmospheric adsorbates such as water and oxygen create charge traps states on the surface and remarkably increase both the photoconductivity and response time. There are also limited studies, which claim that the defect states acting as hole trap centers prolong response time significantly. However, the underlying physical mechanism is still unclear. Here we study the effects of both adsorbates and defect-related states on the photoresponse character of Pulsed Electron Deposited ZnO thin films. In order to distinguish between these two mechanisms, we have compared the time-dependent photoresponse measurements of bare-ZnO and SiO2 encapsulated-ZnO thin film samples taken under UV light and high vacuum. We show that the dominant mechanism of photoresponse in ZnO is the adsorption/desorption of oxygen and water molecules even when the measurement is performed in high vacuum. After the encapsulation of sample surface by a thin SiO2 layer, the adsorption/desorption rates can significantly improve, and the effects of these molecules partially removed.