Highly Efficient Organic Ultraviolet Photodetectors Based on Re(I) Complexes

High response organic ultraviolet photodetectors（UV-PDs） were demonstrated with 4,4＇,4”- tris[3-methyl-pheny（phenyl）amino]triphenylamine（m-MTDATA） and two Re（Ⅰ） complexes, （bathocuproine）- Re（CO）3CI（Re-BCP） and （bathophenanthroline）Re（CO）3Cl（Re-Bphen） to act as the electron donor and acceptor, re- spectively. UV-PDs have the configuration of indium tin oxide（ITO）/m-MTDATA（25 nm）/m-MTDATA：Re-complex （25-35 nm）/Re-complex（20 nm）/LiF（l nm）/Al（200 nm） with different blend layer thicknesses of 25, 30 and 35 nm. The optimized PD based on Re-Bphen offers a corrected-dark photocurrent up to 760μA/cm^2 at -10 V, corresponding to a response of 310 mA/W which is among the best values reported for organic UV-PDs. Excellent electron transport ability makes for such high photo-to-electron conversion.

Ultraviolet Photodetector based on Sr_(2)Nb_(3)O_(10) Perovskite Nanosheets

Liquid-phase exfoliation was employed to synthesize Sr_(2)Nb_(3)O_(10) perovskite nanosheets with thicknesses down to 1.76 nm.Transmission electron microscopy(TEM),atomic force microscope(AFM),X-ray photoelectron spectrometer(XPS),and other characterization techniques were used to evaluate the atomic structure and chemical composition of the exfoliated nanosheets.A UV photodetector based on individual Sr_(2)Nb_(3)O_(10) nanosheets was prepared to demonstrate the application of an ultraviolet(UV) photodetector.The UV photodetector exhibited outstanding photocurrent and responsivity with a responsivity of 3×10^(5) A·W^(-1) at 5 V bias under 280 nm illumination,a photocurrent of 60 nA,and an on/off ratio of 3×10^(2).

Optical and electrical modulation in ultraviolet photodetectors based on organic one-dimensional photochromic arrays

Organic photochromic materials have drawn considerable attention for their potential applications in large-scale and low-cost optoelectronics owing to unique tunable physicochemical properties.For organic photodetectors,photochromic materials have realized optical and electrical engineering of semiconductor layers,which incorporate not only tunable performance,but also functionalities to optoelectronic devices.However,the essential challenge is to assemble large-area photochromic micro-and nanostructure arrays with controllable geometry and precise alignment,which restricts the integration of multifunctional optoelectronic devices.Herein,we fabricate organic photochromic one-dimensional(1D)arrays via a feasible solution process through the confined crystallization of organic molecules.By modulating and controlling the photoisomerization behaviors,these 1D photochromic arrays possess broad spectral tunability,which ensure tunable photoresponse.Furthermore,we investigate the crystallographic transition and electronic performance variation of these 1D photochromic arrays.By adjusting the dwell time of ultraviolet(UV)irradiation,the UV photochromic photodetectors realize tunable and repeatable responsivity from 85.6 to 0.709 mA/W.Our work provides new possibilities for optical and electrical engineering of photochromic microwires towards the integration of multifunctional optoelectronic devices.

Back-illuminated Al_xGa_(1-x)N-based dual-band solar-blind ultraviolet photodetectors

Back-illuminated AlxGal-xN-based dual-band solar-blind ultraviolet （UV） photodetectors （PDs） are realized by a three-terminal n-i-p-i-n heterojunction structure which is grown on sapphire substrate by metal organic chemical vapor deposition （MOCVD）. The two p-i-n junctions contained in the heterojunction structure can work separately and independently. Working in the photovoltaic mode, the PDs display peak responsivity of ～10.8 mA/W at 242 nm and ～5.0 mA/W at 257 nm, respectively. The two junctions with different size, whose diameters are 500 μm and 800 μm, exhibit almost the same leakage current of ～1.3× 10-9 A at a reverse bias of 10 V. Therefore, dark current densities of the two junctions are close to 6.6 × 10-7 A/cm2 and 2.6 × 10-7 A/cm2 at -10 V respectively.

AlGaN metal-semiconductor-metal ultraviolet photodetectors on sapphire substrate with a low-temperature AlN buffer layer

Unintentionally doped AlGaN thin films are grown on c-plane(0001) sapphire substrate by metal-organic chemical vapor deposition, and low-temperature AlN is deposited onto sapphire substrate used as a bu?er layer. AlGaN metal-semiconductor-metal ultraviolet photodetectors with Ni/Au interdigitated contact electrodes are then fabricated by lift-off technology. The dark current of the AlGaN photodetectors is 5.61×10-9 A at 2-V applied bias and the peak response occurrs at 294 nm.

A Self-Powered Fast-Response Ultraviolet Detector of p–n Homojunction Assembled from Two ZnO-Based Nanowires

Nowadays, fabrication of micro/nano-scale electronic devices with bottom-up approach is paid much research attention. Here, we provide a novel micro/nano-assembling method, which is accurate and efficient, especially suitable for the fabrication of micro/nano-scale electronic devices. Using this method, a self-powered ZnO/Sb-doped ZnO nanowire p–n homojunction ultraviolet detector(UVD) was fabricated, and the detailed photoelectric properties were tested. At a reverse bias of -0.1 V under UV light illumination, the photoresponse sensitivity of the UVD was 26.5 and the rise/decay time of the UVD was as short as 30 ms. The micro/nano-assembling method has wide potential applications in the fabrication of specific micro/nano-scale electronic devices.

Dual-wavelength ultraviolet photodetector based on vertical(Al,Ga)N nanowires and graphene

Due to the wide application of UV-A(320 nm–400 nm)and UV-C(200 nm–280 nm)photodetectors,dual-wavelength(UV-A/UV-C)photodetectors are promising for future markets.A dual-wavelength UV photodetector based on vertical(Al,Ga)N nanowires and graphene has been demonstrated successfully,in which graphene is used as a transparent electrode.Both UV-A and UV-C responses can be clearly detected by the device,and the rejection ratio(R254 nm/R450 nm)exceeds35 times at an applied bias of-2 V.The short response time of the device is less than 20 ms.Furthermore,the underlying mechanism of double ultraviolet responses has also been analyzed systematically.The dual-wavelength detections could mainly result from the appropriate ratio of the thicknesses and the enough energy band difference of(Al,Ga)N and Ga N sections.

A self-powered sensitive ultraviolet photodetector based on epitaxial graphene on silicon carbide

A self-powered graphene-based photodetector with high performance is particularly useful for device miniaturization and to save energy.Here,we report a graphene/silicon carbide（SiC）-based self-powered ultraviolet photodetector that exhibits a current responsivity of 7.4 m A/W with a response frequency of over a megahertz under 325-nm laser irradiation.The built-in photovoltage of the photodetector is about four orders of magnitude higher than previously reported results for similar devices.These favorable properties are ascribed to the ingenious device design using the combined advantages of graphene and SiC,two terminal electrodes,and asymmetric light irradiation on one of the electrodes.Importantly,the photon energy is larger than the band gap of SiC.This self-powered photodetector is compatible with modern semiconductor technology and shows potential for applications in ultraviolet imaging and graphene-based integrated circuits.

Design and Performance of an AlGaN-Based p-i-n Ultraviolet Photodetector

An AIGaN-based back-illuminated ultraviolet p-i-n detector is designed and its performance is analysed both simulately and experimentally. The width of p- and i-regions has been optimized to the best theoretic values. It is indicated that the changing of responsivity with increase of bias can not be attributed to the expansion of depletion layer as it is believed, but to two reasons： 1） the effect of GaN/AlGaN heterojunction barrier to block the electrons decreases with higher bias and 2） the recombination rate of excess carriers decreases due to the building up of an electric field in depletion region. At zero bias, the simulated responsivity reaches its maximum of 0.12 A/W with quantum efficiency of 55.1%. The measured peak responsivity is more than 0.09 A/W with quantum efficiency greater than 41.6%. The experimental data are almost consistent with the results of the simulation.

High response Schottky ultraviolet photodetector formed by PEDOT:PSS transparent electrode contacts to Mg_(0.1)Zn_(0.9)O

In this paper, we report a Schottky ultraviolet photodetector based on poly（3,4-ethylenedioxy-thiophene）poly（styrenesulfonate）（PEDOT：PSS） transparent electrode contacts to Mg0.1Zn0.9O. The I-V characteristic curves of the device are measured in the dark condition and under the illumination of a 340-nm UV light. The device shows a typical rectifying behavior with a current rectification ratio of 103 at ±2 V, which exhibits a good Schottky behavior. The phototo-dark current ratio is high, which is 1×103at-4 V. A peak response of 0.156 A/W at 340 nm is observed. The device also exhibits a wide response from 250 nm to 340 nm, with a response larger than 0.1 A/W. It covers the UV-B region（280 nm-320 nm）, which makes the device very suitable for the detection of UV-B light.

Effect of surface oxygen vacancy defects on the performance of ZnO quantum dots ultraviolet photodetector

The slower response speed is the main problem in the application of ZnO quantum dots(QDs)photodetector,which has been commonly attributed to the presence of excess oxygen vacancy defects and oxygen adsorption/desorption processes.However,the detailed mechanism is still not very clear.Herein,the properties of ZnO QDs and their photodetectors with different amounts of oxygen vacancy(VO)defects controlled by hydrogen peroxide(H_(2)O_(2))solution treatment have been investigated.After H_(2)O_(2) solution treatment,VO concentration of ZnO QDs decreased.The H_(2)O_(2) solution-treated device has a higher photocurrent and a lower dark current.Meanwhile,with the increase in VO concentration of ZnO QDs,the response speed of the device has been improved due to the increase of oxygen adsorption/desorption rate.More interestingly,the response speed of the device became less sensitive to temperature and oxygen concentration with the increase of VO defects.The findings in this work clarify that the surface VO defects of ZnO QDs could enhance the photoresponse speed,which is helpful for sensor designing.

Graphene/SrTiO3 interface-based UV photodetectors with high responsivity

Strontium titanate(SrTiO3),which is a crucial perovskite oxide with a direct energy band gap of 3.2 eV,holds great promise for ultraviolet(UV)photodetection.However,the response performance of the conventional SrTiO3-based photodetectors is limited by the large relative dielectric constant of the material,which reduces the internal electric field for electron-hole pair separation to form a current collected by electrodes.Recently,graphene/semiconductor hybrid photodetectors by van-der-Waals heteroepitaxy method demonstrate ultrahigh sensitivity,which is benefit from the interface junction architecture and then prolonged lifetime of photoexcited carriers.Here,a graphene/SrTiO3 interface-based photodetector is demonstrated with an ultrahigh responsivity of 1.2×106 A/W at the wavelength of 325 nm and∼2.4×104 A/W at 261 nm.The corresponding response time is in the order of∼ms.Compared with graphene/GaN interface junctionbased hybrid photodetectors,∼2 orders of magnitude improvement of the ultrahigh responsivity originates from a gain mechanism which correlates with the large work function difference induced long photo-carrier lifetime as well as the low background carrier density.The performance of high responsivity and fast response speed facilitates SrTiO3 material for further efforts seeking practical applications.

A novel integrated ultraviolet photodetector based on standard CMOS process

A novel integrated ultraviolet（UV） photodetector has been proposed, which realizes a high UV selectivity by combining a conventional UV-selective photodiode with an extra infrared（IR） photodiode. The IR photodiode is designed for compensating the photocurrent response of the UV photodiode in the infrared band and is 15 times smaller than the UV one. The integrated photodetector has been fabricated in a 0.35 μm standard CMOS technology. Some critical performance indices of this new structure photodetector, such as spectral responsivity, breakdown voltage, quenching waveform, and transient response, are measured and analyzed. Test results show that the complementary UV–IR photodetector has a maximum spectral responsivity of 0.27 A·W-1 at the wavelength of 400 nm. The device has a high UV selectivity of 3000,which is much higher than that of the single UV photodiode.

Vertical Schottky ultraviolet photodetector based on graphene and top–down fabricated GaN nanorod arrays

GaN has been widely used in the fabrication of ultraviolet photodetectors because of its outstanding properties.In this paper,we report a graphene–GaN nanorod heterostructure photodetector with fast photoresponse in the UV range.GaN nanorods were fabricated by a combination mode of dry etching and wet etching.Furthermore,a graphene–GaN nanorod heterostructure ultraviolet detector was fabricated and its photoelectric properties were measured.The device exhibits a fast photoresponse in the UV range.The rising time and falling time of the transient response were 13 and 8 ms,respectively.A high photovoltaic responsivity up to 13.9 A/W and external quantum efficiency up to 479%were realized at the UV range.The specific detectivity D*=1.44×10^(10) Jones was obtained at–1 V bias in ambient conditions.The spectral response was measured and the highest response was observed at the 360 nm band.

High Responsivity Organic Ultraviolet Photodetector Based on NPB Donor and C60 Acceptor

We report fabrication and characterization of organic heterojunction UV detectors based on N,N＇-bis（naphthalen- 1-y1）-N,N＇-bis （phenyl） benzidine （NPB） and fullerene C60. The effects of different thicknesses of NPB and C60 layers are studied and compared. Notably, the optimal thicknesses of electron acceptor C60 and electron donor NPB are 40 nm and 80 nm, respectively. The J V characteristic curves of the device demonstrate a three-order- of-magnitude difference when illuminated under a 350nm UV light and in the dark at -0.5 V. The device exhibits high sensitivity in the region of 320-380nm with the peak located around 35Onm. Especially, it shows excellent photo-response characteristic with a responsivity as high as 315 mA/W under the illumination of 192μW.cm 2 350nm UV light at -5 V. These results indicate that the NPB/C60 heterojunction structure device might be used as low-cost low-voltage UV photodetectors.

Ultraviolet irradiation dosimeter based on persistent photoconductivity effect of ZnO

It is essential to determine the accumulative ultraviolet(UV)irradiation over a period of time in some cases,such as monitoring UV irradiation to the skin,solar disinfection of water,photoresist exposure,etc.UV colorimetric dosimeters,which use dyes'color change to monitor the amount of UV exposure,have been widely studied.However,the exposure data of these UV colorimetric dosimeters can hardly be converted to digital signals,limiting their applications.In this paper,a UV dosimeter has been proposed and demonstrated based on the persistent photoconductivity(PPC)in zinc oxide microwires(ZnO MWs).The PPC effect usually results in high photoconductivity gain but low response speed,which has been regarded as a disadvantage for photodetectors.However,in this work,the unique characteristics of the PPC effect have been utilized to monitoring the accumulative exposure.We demonstrate that the photocurrent in the ZnO MWs depends on the accumulative UV exposure due to the PPC effect,thus the photocurrent can be utilized to determine the UV accumulation.The dosimeter is immune to visible light and exhibits a photoconductive gain of 2654,and the relative error of the dosimeter is about 10%.This UV dosimeter with electrical output is reusable and convenient to integrate with other electronic devices and may also open a new application area for the PPC effect.

Large-scale and high-quality III-nitride membranes through microcavity-assisted crack propagation by engineering tensile-stressed Ni layers

Epitaxially grown III-nitride alloys are tightly bonded materials with mixed covalent-ionic bonds.This tight bonding presents tremendous challenges in developing III-nitride membranes,even though semiconductor membranes can provide numerous advantages by removing thick,inflexible,and costly substrates.Herein,cavities with various sizes were introduced by overgrowing target layers,such as undoped GaN and green LEDs,on nanoporous templates prepared by electrochemical etching of n-type GaN.The large primary interfacial toughness was effectively reduced according to the design of the cavity density,and the overgrown target layers were then conveniently exfoliated by engineering tensile-stressed Ni layers.The resulting III-nitride membranes maintained high crystal quality even after exfoliation due to the use of GaN-based nanoporous templates with the same lattice constant.The microcavity-assisted crack propagation process developed for the current III-nitride membranes forms a universal process for developing various kinds of large-scale and high-quality semiconductor membranes.

A self-powered ultraviolet photodetector with van der Waals Schottky junction based on TiO_(2) nanorod arrays/Au-modulated V2CTx MXene

A self-powered ultraviolet photodetector(UV PD)with van der Waals(vdW)Schottky junction based on TiO_(2) nanorod arrays/Au-modulated V2 CTx MXene is reported.The Schottky junction enables the device to operate in self-powered mode.The dangling bond-free surface of V2 CTx MXene reduces the charge recombination at the junction interface.Meanwhile,V2 CTx MXene,with the work function(WF)increasing to 5.35 eV,forms a hole transport layer by contacting with Au electrode,which facilitates the carrier extraction.The electron lifetime in the device has prolonged to 8.95μs.As a result,the responsivity and detectivity of the PD have achieved 28 mA/W and 1.2×10^(11) cm Hz1/2/W(340 nm,65 mW/cm2,0 V),respectively.In addition,the presence of the Au electrode prevents the vanadium from coming into contact with oxygen and oxidizing,preserving the properties of the V2 CTx films.After 180 days of exposure to the atmosphere,the device performance remained at a particularly high level,indicating enhanced durability.This work points out an effective approach to modulate the properties of V2 CTx to obtain the high performance and stability of the UV PD.

Characterization of vertical Au/β-Ga_2O_3 single-crystal Schottky photodiodes with MBE-grown high-resistivity epitaxial layer

High-resistivity β-Ga203 thin films were grown on Si-doped n-type conductive β-Ga203 single crystals by molecular beam epitaxy （MBE）. Vertical-type Schottky diodes were fabricated, and the electrical properties of the Schottky diodes were studied in this letter. The ideality factor and the series resistance of the Schottky diodes were estimated to be about 1.4 and 4.6 x 10^6 %. The ionized donor concentration and the spreading voltage in the Schottky diodes region are about 4 x 10^18 cm-3 and 7.6 V, respectively. The ultra-violet （UV） photo-sensitivity of the Schottky diodes was demonstrated by a low-pressure mercury lamp illumination. A photoresponsivity of 1.8 A/W and an external quantum efficiency of 8.7 x 10%2% were observed at forward bias voltage of 3.8 V, the proper driving voltage of read-out integrated circuit for UV camera. The gain of the Schottky diode was attributed to the existence of a potential barrier in the i-n junction between the MBE-grown highly resistive β-Ga203 thin films and the n-type conductive β-Ga203 single-crystal substrate.

Bolometric effect in a waveguide-integrated graphene photodetector

Graphene is an alternative material for photodetectors owing to its unique properties.These include its uniform absorption of light from ultraviolet to infrared and its ultrahigh mobility for both electrons and holes.Unfortunately,due to the low absorption of light,the photoresponsivity of graphene-based photodetectors is usually low,only a few milliamps per watt.In this letter,we fabricate a waveguide-integrated graphene photodetector.A photoresponsivity exceeding0.11 A·W （-1） is obtained which enables most optoelectronic applications.The dominating mechanism of photoresponse is investigated and is attributed to the photo-induced bolometric effect.Theoretical calculation shows that the bolometric photoresponsivity is 4.6 A·W （-1）.The absorption coefficient of the device is estimated to be 0.27 dB·μm （-1）.