Investigation of UV photosensor properties of Al-doped SnO_(2) thin films deposited by sol-gel dip-coating method

Transparent conducting aluminum doped tin oxide thin films were prepared by sol-gel dip coating method with differ-ent Al concentrations and characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), UV-Vis spectrophotometry and photoconductivity study. The variation observed in the properties of the measured films agrees with a difference in the film's thickness, which decreases when Al concentration augments. X-ray diffraction analysis reveals that all films are polycrystal-line with tetragonal structure, (110) plane being the strongest diffraction peak. The crystallite size calculated by the Debye Scher-rer’s formula decreases from 11.92 to 8.54 nm when Al concentration increases from 0 to 5 wt.%. AFM images showed grains uni-formly distributed in the deposited films. An average transmittance greater than 80% was measured for the films and an en-ergy gap value of about 3.9 eV was deduced from the optical analysis. Finally, the photosensitivity properties like current-voltage characteristics, ION/IOFF ratio, growth and decay time are studied and reported. Also, we have calculated the trap depth energy using the decay portion of the rise and decay curve photocurrent.

Interfacial photoconductivity effect of type-Ⅰ and type-Ⅱ Sb2Se3/Si heterojunctions for THz wave modulation

An in-depth understanding of the photoconductivity and photocarrier density at the interface is of great significance for improving the performance of optoelectronic devices. However, extraction of the photoconductivity and photocarrier density at the heterojunction interface remains elusive. Herein, we have obtained the photoconductivity and photocarrier density of 173 nm Sb2Se3/Si(type-Ⅰ heterojunction) and 90 nm Sb2Se3/Si(type-Ⅱ heterojunction) utilizing terahertz(THz) time-domain spectroscopy(THz-TDS) and a theoretical Drude model. Since type-Ⅰ heterojunctions accelerate carrier recombination and type-Ⅱ heterojunctions accelerate carrier separation, the photoconductivity and photocarrier density of the type-Ⅱ heterojunction(21.8×10^(4)S·m^(-1),1.5 × 10^(15)cm^(-3)) are higher than those of the type-Ⅰ heterojunction(11.8×10^(4)S·m^(-1),0.8×10^(15)cm^(-3)). These results demonstrate that a type-Ⅱ heterojunction is superior to a type-Ⅰ heterojunction for THz wave modulation. This work highlights THz-TDS as an effective tool for studying photoconductivity and photocarrier density at the heterojunction interface. In turn, the intriguing interfacial photoconductivity effect provides a way to improve the THz wave modulation performance.

Helicity-dependent photoconductance of the edge states in the topological insulator Bi_(2)Te_(3)

The helicity-dependent photoconductance of the edge states in three-dimensional topological insulator Bi_(2)Te_(3)films is investigated.It is revealed that the helicity-dependent photoconductivity current on the left edge of the Bi_(2)Te_(3)film shows an opposite sign with that on the right edge.In addition,the helicity-dependent photoconductivity current increases linearly with the applied longitudinal electric field,and it reverses the sign with the reversal of the electric field.As the thickness of the Bi_(2)Te_(3)film increases,the helicity-dependent photoconductivity current also increases.Theoretical analysis suggests that the helicity-dependent photo-conductivity current may come from the intrinsic spin orbit coupling(SOC)or the SOC introduced by the chiral impurities or defects.

Transition of photoconductive and photovoltaic operation modes in amorphous Ga_2O_3-based solar-blind detectors tuned by oxygen vacancies

We report on the transition of photovoltaic and photoconductive operation modes of the amorphous Ga_2O_3-based solar-blind photodetectors in metal–semiconductor–metal(MSM) configurations. The conversion from Ohmic to Schottky contacts at Ti/Ga_2O_3 interface is realized by tuning the conductivity of amorphous Ga_2O_3 films with delicate control of oxygen flux in the sputtering process. The abundant donor-like oxygen vacancies distributed near the Ti/Ga_2O_3 interface fascinate the tunneling process across the barrier and result in the formation of Ohmic contacts. As a consequence, the serious sub-gap absorption and persistent photoconductivity(PPC) effect degrades the performance of the photoconductive detectors. In contrast, the photovoltaic device with a Schottky contact exhibits an ultra-low dark current less than 1 pA,a high detectivity of 9.82×10^(12) cm·Hz^(1/2)·W^(-1), a fast response time of 243.9 μs, and a high ultraviolet C(UVC)-toultraviolet A(UVA) rejection ratio of 103. The promoting performance is attributed primarily to the reduction of the subgap states and the resultant suppression of PPC effect. With simple architecture, low fabrication cost, and easy fusion with modern high-speed integrated circuitry, these results provide a cost-effective way to realize high performance solar-blind photodetectors towards versatile practical applications.

Photoconductive semiconductor switch-based triggering with 1 ns jitter for trigatron

Synchronization for multiple-pulse at nanosecond range shows a great value on the power multiplication and synchronous electric fieldsapplications. Nanosecond or sub-ns jitter synchronization is essential for the improved working efficiency of the large amounts of pulse modulesand accurate requirements for the power coherent combining applications. This paper presents a trigger generator based on a laser diodetriggered GaAs photoconductive semiconductor switch (PCSS) with low jitter and compact size characteristics. It avoids the high currentsthat are harmful to high-gain mode PCSSs. In the trigger circuit, a 200 pF capacitor is charged by a microsecond-scale 18 kV pulse and thendischarged via the high-gain mode GaAs PCSS to trigger the high-power trigatron switch. When triggered by the ~10 ns pulse generated by thePCSS, the DC-charged trigatron can operate in the 20e35 kV range with 10 ns rise time and 1 ns delay-time jitter.

Bifacial Silicon Solar Cell Steady Photoconductivity under Constant Magnetic Field and Junction Recombination Velocity Effects

The paper reported a theoretical study on the photoconductivity of a bifacial silicon solar cell under polychromatic illumination and a constant magnetic field effect. By use of the continuity equation in the base of the solar cell maintained in a constant temperature at 300 K, an expression of the excess minority carriers’ density was determined according to the applied magnetic field, the base depth and the junction recombination velocity. From the expression of the minority carriers’ density, the photoconductivity of the solar cell was deduced and which allowed us to predict some recombination phenomena, the use of such solar cell in optoelectronics. The profile of the photoconductivity also permitted us to utilize a linear model in order to determine an electrical capacitance that varied with magnetic field.

Ultrafast Carrier Dynamics and Terahertz Photoconductivity of Mixed-Cation and Lead Mixed-Halide Hybrid Perovskites

Using time-dependent terahertz spectroscopy, we investigate the role of mixed-cation and mixed-halide on the ultrafast photoconductivity dynamics of two different methylammonium(MA) lead-iodide perovskite thin films. It is found that the dynamics of conductivity after photoexcitation reveals significant correlation on the microscopy crystalline features of the samples. Our results show that mixed-cation and lead mixed-halide affect the charge carrier dynamics of the lead-iodide perovskites. In the(5-AVA)_(0.05)(MA)_(0.95) PbI_(2.95) Cl_(0.05)/spiro thin film, we observe a much weaker saturation trend of the initial photoconductivity with high excitation fluence, which is attributed to the combined effect of sequential charge carrier generation, transfer, cooling and polaron formation.

Terahertz radiation of a butterfly-shaped photoconductive antenna(invited)

The terahertz(THz) far-field radiation properties of a butterfly-shaped photoconductive antenna(PCA) were experimentally studied using a home-built THz time-domain spectroscopy(THz-TDS) setup.To distinguish the contribution of in-gap photocurrent and antenna structure to far-field radiation,polarization-dependent THz field was measured and quantified as the illuminating laser beam moved along the bias field within the gap region of electrodes. The result suggests that, although the far-field THz radiation originates from the in-gap photocurrent, the antenna structure of butterfly-shaped PCA dominates the overall THz radiation. In addition, to explore the impact of photoconductive material,radiation properties of butterfly-shaped PCAs fabricated on both low-temperature-grown GaAs(LT-GaAs) and semi-insulating GaAs(Si-GaAs) were characterized and compared. Consistent with previous experiments, it is observed that while Si-GaAs-based PCA can emit higher THz field than LT-GaAs-based PCA at low laser power, it would saturate more severely as laser power increased and eventually be surpassed by LT-GaAs-based PCA. Beyond that, it is found the severe saturation effect of Si-GaAs was due to the longer carrier lifetime and higher carrier mobility, which was confirmed by the numerical simulation.

Synthesis and Photoconductivity of Nanosized Phthalocyanine

Functional phthalocyanine （Pc） compounds of H2Pc, TiOPc, FePc and CIAIPc were synthesized with a yield of 46.7%, 91.2%, 37.4% and 34.0%, respectively. Nanosized TiOPc was synthesized via a one-step sol-gel method and effects of surfactant doses, nucleation temperature on TiOPc particle size and photoconductivity were investigated. When re（PEG）： m（TiOPc） was 0.1 and nucleation temperature was 0℃, the as-obtained TiOPc had the smallest particle size and largest specific surface area, which were 60 nm and 83m^2/g, respectively. TiOPc synthesized under these conditions also exhibits excellent photoconductivity with charging potential V0, dark decay speed Rd and energy for half-discharging of potential E1/2 being 1160 V, 30 V/s and 0.6 1x.s, respectively.

Ligand Exchange of Colloidal ZnO Nanocrystals from the High Temperature and Nonaqueous Approach

Colloidal zinc oxide(ZnO) nanocrystals generated from the high temperature and nonaqueous approache are attractive for use in solution-processed electrical and optoelectronic devices. However, the asprepared colloidal ZnO nanocrystals by this approach are generally capped by ligands with long alkyl-chains,which is disadvantage for solution-processed devices due to hindering charge transport. Here we demonstrate an effective ligand exchange process for the colloidal ZnO nanocrystals from the high temperature and nonaqueous approach by using n-butylamine. The ligand exchange process was carefully characterized. The thin films based on colloidal ZnO nanocrystals after ligand exchange exhibited dramatically enhanced UV photoconductivity.

CARRIER PHOTOGENERATION IN POLY(N-VINYL-CARBAZOLE)-BASED PHOTOCONDUCTIVE THIN-FILM DEVICES

In this paper, photoexcitation processes in the bilayer devices based on inorganic materials and poly（N-vinylcarbazole） （PVK） were investigated. In order to clarify the roles of inorganic materials in photoconductive properties of bilayer devices, TiO2 and ZnS were chosen to combine with PVK. A model for generation of photocurrent （Iph） in single layer device of PVK was obtained. It is deduced that the recombination rate constant （Pcomb） and the ionization rate constant （y） ofexcitons should be considered as the most important factors for Iph. For inorganic materials （TiO2 or ZnS）/PVK bilayer devices, in reverse bias of-4 V, the photocurrent of 115 mA/cm^2 in the TiO2/PVK device was observed, but the photocurrent in the ZnS/PVK device was only 10 mA/cma under the illumination light of 340 nm and the light intensity of 14.2 mW/cm^2. The weaker photocurrent is attributed to the absorption of ZnS within UV region and the energy offset at the interface between PVK and ZnS, which impedes the transport of charge carriers.

Conductive MOFs with Photophysical Properties:Applications and Thin-Film Fabrication

Metal-organic frameworks(MOFs)are a class of hybrid materials with many promising applications.In recent years,lots of investigations have been oriented toward applications of MOFs in electronic and photoelectronic devices.While many high-quality reviews have focused on synthesis and mechanisms of electrically conductive MOFs,few of them focus on their photophysical properties.Herein,we provide an in-depth review on photoconductive and photoluminescent properties of conductive MOFs together with their corresponding applications in solar cells,luminescent sensing,light emitting,and so forth.For integration of MOFs with practical devices,recent advances in fabrication of photoactive MOF thin films are also summarized.

Roles of voltage in semi-insulating GaAs photoconductive semiconductor switch

An experimental study of leakage current is presented in a semi-insulating（SI） Ga As photoconductive semiconductor switch（PCSS） with voltages up to 5.8 kV（average field is 19.3 kV/cm）. The leakage current increases nonlinearly with the bias voltage increasing from 1.2×10^-9 A to 3.6×10^-5A. Furthermore, the dark resistance, which is characterized as a function of electric field, does not monotonically decrease with the field but displays several distinct regimes. By eliminating the field-dependent drift velocity, the free-electron density n is extracted from the current, and then the critical field for each region of n（E） characteristic of PCSS is obtained. It must be the electric field that provides the free electron with sufficient energy to activate the carrier in the trapped state via multiple physical mechanisms, such as impurity ionization, fielddependent EL2 capture, and impact ionization of donor centers EL10 and EL2. The critical fields calculated from the activation energy of these physical processes accord well with the experimental results. Moreover, agreement between the fitting curve and experimental data of J（E）, further confirms that the dark-state characteristics are related to these field-dependent processes. The effects of voltage on SI-Ga As PCSS may give us an insight into its physical mechanism.

Covalent Functionalization of Carbon Nanotube by Tetrasubtituted Amino Manganese Phthalocyanine

The multiwall carbon nanotube (MWCNT) bonded to 2, 9, 16, 23-tetraamino manganese phthalocyanine (TAMnPc) was obtained by covalent functionalization, and its chemical structure was characterized by TEM. The photoconductivity of single-layered photoreceptors, where MWCNT bonded by TAMnPc (MWCNT-b-TAMnPc) served as the charge generation material (CGM), was also studied.

Optimization of wide band mesa-type enhanced terahertz photoconductive antenna at 1550 nm

A mesa-type enhanced InGaAs/InAIAs multilayer heterostructure （MLHS） terahertz photoconductive antenna （PCA） at 1550 nm is demonstrated on an InP substrate. The InGaAs/InAIAs superlattice multilayer heterostructures are grown and studied with different temperatures and thickness ratios of InGaAs/InAIAs. The PCAs with different gap sizes and pad sizes are fabricated and characterized. The PCAs are evaluated as THz emitters in a THz time domain spectrometer and we measure the optimized THz bandwidth in excess of 2 THz.

Negative photoconductivity in low-dimensional materials

In recent years,low-dimensional materials have received extensive attention in the field of electronics and optoelectronics.Among them,photoelectric devices based on photoconductive effect in low-dimensional materials have a broad development space.In contrast to positive photoconductivity,negative photoconductivity(NPC)refers to a phenomenon that the conductivity decreases under illumination.It has novel application prospects in the field of optoelectronics,memory,and gas detection,etc.In this paper,we review reports about the NPC effect in low-dimensional materials and systematically summarize the mechanisms to form the NPC effect in existing low-dimensional materials.

Fabrication and Characteristics of Fast Photo Response ZnO/Porous Silicon UV Photoconductive Detector

Fast response time UV photoconductive detector was fabricated based on ZnO film prepared by thermal chemical spray pyrolysis technique. The ZnO nanofilms are grown on the porous silicon (PS) nanosurface which has drastically reduced the response time of the ZnO UV detector from few seconds to few hundreds of microseconds. The surface functionalization of the ZnO film deposited on porous silicon (PS) layer by polyamide nylon has highly improved the photoresponsivity of the detector to 0.8 A/W. The normalized de-tectivity (D*) of the fabricated ZnO UV detector at wavelength of 385 nm is found to be about 2.12 × 1011 cm Hz1/2 W–1. The ZnO film grown on the porous silicon layer was oriented in the c-axis and it is found to be a p-type semiconductor, which is referred to the compensation of the excess charge carriers in the ZnO film by the nanospikes silicon layer.

Growth, Optical, Mechanical and Dielectric Properties of Glycine Zinc Chloride NLO Single Crystals

Single crystals of Glycine Zinc Chloride (GZC) were grown from aqueous solution by slow evaporation technique. Single crystal X-ray diffraction analysis reveals that the crystal belongs to orthorhombic system with the space group Pna21.The optical transmission study reveals the transparency of the crystal in the entire visible region and the cut off wavelength has been found to be 230 nm. The optical band gap is found to be 3.40eV. The transmittance of GZC crystal has been used to calculate the refractive index (n), the extinction coefficient (K) and the real er and imaginary ei components of the dielectric constant. Mechanical studies were carried out on the as grown crystal. Dielectric constant and dielectric loss measurements were carried out at different temperatures and frequencies. Photoconductivity measurements carried out on the grown crystal reveal the negative photoconducting nature.

Crystal Growth, Structural and Optical Studies of CuGa3Se5 Bulk Compounds

Bulk materials were synthesized by the Bridgman technique using the elements Cu, Ga, Se. These samples were characterized by Energy Dispersive Spectrometry (EDS) to determine the elemental composition, as well as by X-ray diffraction for structure, hot point probe method for type of conductivity. Optical response (Photoconductivity) and Photoluminescence (PL) and PL-excitation (PLE) at temperatures from 4.2 to 77 K were also used to estimate the band-gap energy of Cu-Ga3Se5. They show a nearly perfect stoechiometry and present p-type conductivity. CuGa3Se5 either have an Ordered Defect Chalcopyrite structure (ODC), or an Ordered Vacancy Chalcopyrite structure (OVC). The gap energy obtained by Photoconductivity and Photoluminescence (PL) for the different samples is 1.85 eV. Studying the variation of the gap as a function of the temperature shows that the transition is a D-A type. The defects that appear are probably GaCu.

Optical and Dielectric Studies on L-Valinium Picrate Single Crystal

Single crystals of L-Valinium picrate were grown from aqueous solution by slow evaporation technique. Single crystal X-ray diffraction analysis reveals that the crystal belongs to monoclinic system. The optical transmission study reveals the transparency of the crystal in the entire visible region and the cut off wave length has been found to be 470 nm. The optical band gap is found to be 2.55 eV. The transmittance of L-Valinium picrate crystal has been used to calculate the refractive index (n), the extinction coefficient (K) and both the real (εr) and imaginary (εi) components of the dielectric constant as functions of wavelength. Low dielectric loss at high frequency region is indicative of enhanced optical quality with lesser defects. Photoconductivity measurements carried out on the grown crystal reveal the negative photoconducting nature.