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.     

High-photosensitivity polymer thin-film transistors based on poly（3-hexylthiophene）

Polymer thin-film transistors(PTFTs) based on poly(3-hexylthiophene) are fabricated by the spin-coating process,and their photo-sensing characteristics are investigated under steady-state visible-light illumination.The photosensitivity of the device is strongly modulated by gate voltage under various illuminations.When the device is in the subthreshold operating mode,a significant increase in its drain current is observed with a maximum photosensitivity of 1.7×10 3 at an illumination intensity of 1200 lx,and even with a relatively high photosensitivity of 611 at a low illumination intensity of 100 lx.However,when the device is in the on-state operating mode,the photosensitivity is very low:only 1.88 at an illumination intensity of 1200 lx for a gate voltage of-20 V and a drain voltage of -20 V.The results indicate that the devices could be used as photo-detectors or sensors in the range of visible light.The modulation mechanism of the photosensitivity in the PTFT is discussed in detail.     

Slow-rise and fast-drop current feature of ultraviolet response spectra for ZnO-nanowire film modulated by water molecules

This study describes the fabrication of ZnO-nanowire films by electro-chemical anodization of Zn foil. The ZnO films are characterized by field emission scanning electron microscopy, X-ray diffraction patterns, and transmission electron microscopy, respectively. The ultraviolet (UV) photo-response properties of the surface-contacted ZnO film are studied through the current evolution processes under different relative humidities. Unlike the usually observed current spectra of the ZnO films, the drop time is shorter than the rise time. The photo-conductivity gain G and the response time τ are both increased with the increase of the applied bias. The photo-conductivity gain G is lowered with the increase of the environmental humidity, while the response time τ is increased. These results can be explained by considering three different surface processes: 1) the electron-hole (e-p) pair generation by the UV light illumination, 2) the following surface O₂^- species desorption, and 3) the photo-catalytic hydrolysis of water molecules adsorbed on the ZnO surface. The slow-rise and fast-drop current feature is suggested to originate from the sponge-like structure of the ZnO nanowires.     

Investigation of Chemical-Vapour-Deposition Dialnond Alpha-Particle Detectors

Diamond films with [100] texture were prepared by a hot-silament chemical vapour deposition technique to fabricate particale detectors.the response of detectors to 5.5MeV^241 am particles is studied.The photocurrent increases linearly and then levels off with voltage,and 7nA is obtained at bias voltage of 100V.The timedependent photocurrent initially increases rapidly and then tends to reach saturation.Furthermore,a little increase of the dark-current after irradiation can be accounted for by the release of the charges captured by the trapping centres at low energy levels during irradiation.An obvious peak of the pulse height distribution can be observed,associated with the energy of 5.5MeV.     

Photocurrent analysis of organic photovoltaic cells based on CuPc/C60 with Alq3 as a buffer layer

The performance of an organic photovoltaic(OPV) cell based on copper phthalocyanine CuPc/C60 with a tris(8-hydroxyquinolinato) aluminum(Alq3) buffer layer has been investigated.It was found that the power conversion efficiency of the device was 1.51% under illumination with an intensity of 100 mW/cm2,which was limited by a squareroot dependence of the photocurrent on voltage.The photocurrent-optical power density characteristics showed that the OPV cell had a significant space-charge limited photocurrent with a varied saturation voltage and a three quarters power dependence on optical power density.Also,the absorption spectrum was measured by a spectrophotometer,and the results showed that the additional Alq3 layer has a minor effect on photocurrent generation.     

Degradation of current–voltage and low frequency noise characteristics under negative bias illumination stress in InZnO thin film transistors

The instabilities of indium–zinc oxide thin film transistors under bias and/or illumination stress are studied in this paper. Firstly, illumination experiments are performed, which indicates the variations of current–voltage characteristics and electrical parameters(such as threshold voltage and sub-threshold swing) are dominated by the stress-induced ionized oxygen vacancies and acceptor-like states. The dependence of degradation on light wavelength is also investigated. More negative shift of threshold voltage and greater sub-threshold swing are observed with the decrease of light wavelength.Subsequently, a negative bias illumination stress experiment is carried out. The degradation of the device is aggravated due to the decrease of recombination effects between ionized oxygen vacancies and free carriers. Moreover, the contributions of ionized oxygen vacancies and acceptor-like states are separated by using the mid-gap method. In addition, ionized oxygen vacancies are partially recombined at room temperature and fully recombined at high temperature. Finally, low-frequency noise is measured before and after negative bias illumination stress. Experimental results show few variations of the oxide trapped charges are generated during stress, which is consistent with the proposed mechanism.     

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.     

Polymer waveguide thermo-optical switch with loss compensation based on NaYF_4: 18% Yb~(3+), 2% Er~(3+) nanocrystals

A polymer waveguide thermo-optical switch with loss compensation based on NaYF_4: 18% Yb~(3+), 2% Er~(3+)nanocrystals, fabricated by traditional semiconductor processes, has been investigated. NaYF4: 18% Yb~(3+), 2% Er~(3+)nanocrystals were prepared by a pyrolysis method. The morphology and luminescent properties of the nanocrystals were characterized.The nanocrystals were doped into SU-8 as the core material of an optical waveguide amplifier. The size of the device was optimized for its optical and thermal fields as well as its transmission characteristics. The device was fabricated on a silica substrate by spin coating, photolithography, and wet etching. The insertion loss of the switch device is～15 dB. The rise and fall times of the device are 240 μs and 380 μs, respectively, as measured by application of a 304 Hz square wave voltage. The extinction ratio of the device is about 14 dB at an electrode-driving power of 7 mW. When the pump light power is 230 mW and the signal light power is 0.1 mW, the loss compensation of the device is 3.8 dB at a wavelength of1530 nm. Optical devices with loss compensation have important research significance.     

Photoresponse and trap characteristics of transparent AZO-gated AlGaN/GaN HEMT

AZO-gated and Ni/Au-gated AlGaN/GaN HEMTs are fabricated successfully,and an excellent transparency of AZOgated electrode is achieved.After a negative gate bias stress acts on two kinds of the devices,their photoresponse characteristics are investigated by using laser sources with different wavelengths.The effect of photoresponse on AZO-gated electrode device is more obvious than on Ni/Au-gated electrodes device.The electrons are trapped in the AlGaN barrier of AZO-gated HEMT after it has experienced negative gate bias stress,and then the electrons can be excited effectively after it has been illuminated by the light with certain wavelengths.Furthermore,the trap state density D_T and the time constantτ_T of the AZO-gated Schottky contact are extracted by fitting the measured parallel conductance in a frequency range from10 kHz to 10 MHz.The constants of the trap range from about 0.35 μs to 20.35 μs,and the trap state density increased from1.93×l0~(13)eV 1·cm~2 at an energy of 0.33 eV to 3.07×10~(11) eV~1·cm~2 at an energy of 0.40 eV.Moreover,the capacitance and conductance measurements are used to characterize the trapping effects under different illumination conditions in AZO-gated HEMTs.Reduced deep trap states' density is confirmed under the illumination of short wavelength light.     

Electroluminescence of double-doped diamond thin films

A new electroluminescence device is fabricated by microwave plasma chemical vapour deposition system and electron beam vapour deposition system. It is comprised of highly doped silicon/diamond/boron/nitrogen-doped diamond/indium tin oxide thin films. Effects of process parameters on morphologies and structures of the thin films are detected and analysed by scanning electron microscopy, Raman spectrometer and x-ray photoelectron spectrometer. A direct-current (DC) power supply is used to drive the electroluminescence device. The blue light emission with a luminance of 1.2 cd·m 2 is observed from this double-doped diamond thin film electroluminescence device at an applied voltage of 105 V.     

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

High-resistivity β-Ga_2O_3 thin films were grown on Si-doped n-type conductive β-Ga_2O_3 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 about1.4 and 4.6×10~6 Ω.The ionized donor concentration and the spreading voltage in the Schottky diodes region are about4×10~(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 of8.7 ×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-Ga_2O_3 thin films and the n-type conductive β-Ga_2O_3 single-crystal substrate.     

Enhanced photoresponse performance in Ga/Ga_2O_3 nanocomposite solar-blind ultraviolet photodetectors

In the present work, we explore the solar-blind ultraviolet(UV) photodetectors(PDs) with enhanced photoresponse,fabricated on Ga/Ga_2O_3 nanocomposite films. Through pre-burying metal Ga layers and thermally post-annealing the laminated Ga2 O3/Ga/Ga_2O_3 structures, Ga/Ga_2O_3 nanocomposite films incorporated with Ga nanospheres are obtained. For the prototype PD, it is found that the photocurrent and photoresponsivity will first increase and then decrease monotonically with the thickness of the pre-buried Ga layer increasing. Each of all PDs shows a spectrum response peak at 260 nm, demonstrating the ability to detect solar-blind UV light. Adjustable photoresponse enhancement factors are achieved by means of the surface plasmon in the nanocomposite films. The PD with a 20 nm thick Ga interlayer exhibits the best solar-blind UV photoresponse characteristics with an extremely low dark current of 8.52 p A at 10-V bias, a very high light-to-dark ratio of ～ 8 × 10~5, a large photoresponsivity of 2.85 A/W at 15-V bias, and a maximum enhancement factor of ～ 220. Our research provides a simple and practical route to high performance solar-blind UV PDs and potential applications in the field of optoelectronics.     

Nonlinear two-photon absorption properties induced by femtosecond laser with the films of two novel anthracene derivatives

Two novel anthracene derivatives containing 4-vinylpyridine (FPEA) and 2-vinylpyridine (TPEA) poly(methyl methacrylate) films are prepared on quartz glass substrates. Their nonlinear absorption properties are investigated by using a 120-fs, 800-nm Ti:sapphire femtosecond pulsed laser operating at a 1-kHz repetition rate. The unique nonlinear absorption properties of these new compounds are observed by utilizing a Z-scan system. These two-photon absorption (TPA) properties are proven by the two-photon fluorescence excited at 800 nm. The FPEA and TPEA films have nonlinear TPA coefficients of 0.164 and 0.148 cm/GW and the TPA cross sections of 3.345 × 10-48 and 3.081 × 10-48 cm4 ·s/photon, respectively. The influence of the chemical structures on the nonlinear TPA properties of the compounds is also discussed. The highly nonlinear TPA activities of the films implied that the new anthracene derivatives are suitable materials with promising applications in super-high-density three-dimensional data storage and nano- or microstructure fabrication.     

Photodarkening in Amorphous As2S3 Thin Films

The photodarkening effect in amorphous As2S3 films is studied.The optical absorption edge shifts to a lower energy after illumination ar the bandgap light of 514.5nm wavelength by an argon laser.The shift in well annealed films can be recovered by annealing at 180℃ for 1 h but in un-annealed films it is irreversible.In addition,it was found that the magnitude of photodarkening ΔE increased with the increase of illumination light intensity and illumination time.The reversibility of photodarkening in amorphous As2S3 films can be applied in optical memories.     

Slow-rise and fast-drop current feature of ultraviolet response spectra for ZnO-nanowire film modulated by water molecules

This study describes the fabrication of ZnO-nanowire films by electro-chemical anodization of Zn foil.The ZnO films are characterized by field emission scanning electron microscopy,X-ray diffraction patterns,and transmission electron microscopy,respectively.The ultraviolet(UV) photo-response properties of the surface-contacted ZnO film are studied through the current evolution processes under different relative humidities.Unlike the usually observed current spectra of the ZnO films,the drop time is shorter than the rise time.The photo-conductivity gain G and the response time τ are both increased with the increase of the applied bias.The photo-conductivity gain G is lowered with the increase of the environmental humidity,while the response time τ is increased.These results can be explained by considering three different surface processes:1) the electron-hole(e-p) pair generation by the UV light illumination,2) the following surface O2-species desorption,and 3) the photo-catalytic hydrolysis of water molecules adsorbed on the ZnO surface.The slow-rise and fast-drop current feature is suggested to originate from the sponge-like structure of the ZnO nanowires.     

Effects of annealing rate and morphology of sol–gel derived ZnO on the performance of inverted polymer solar cells

The effects of annealing rate and morphology of sol–gel derived zinc oxide(ZnO)thin films on the performance of inverted polymer solar cells(IPSCs)are investigated.ZnO films with different morphologies are prepared at different annealing rates and used as the electron transport layers in IPSCs.The undulating morphologies of ZnO films fabricated at annealing rates of 10 C/min and 3 C/min each possess a rougher surface than that of the ZnO film fabricated at a fast annealing rate of 50 C/min.The ZnO films are characterized by atomic force microscopy(AFM),optical transmittance measurements,and simulation.The results indicate that the ZnO film formed at 3 C/min possesses a good-quality contact area with the active layer.Combined with a moderate light-scattering,the resulting device shows a 16%improvement in power conversion efficiency compared with that of the rapidly annealed ZnO film device.     

The simulation of temperature dependence of responsivity and response time for 6H-SiC UV photodetector

In this paper the temperature dependence of responsivity and response time for 6H-SiC ultraviolet (UV) photodetector is simulated based on numerical model in the range from 300K to 900K. The simulation results show that the responsivity and the response time of device are less sensitive to temperature and this kind of UV photodetector has excellent temperature stability. Also the effects of device structure and bias voltage on the responsivity and the response time are presented. The thicker the drift region is, the higher the responsivity and the longer the response time are. So the thickness of drift region has to be carefully designed to make trade-off between responsivity and response time.     

Dual-channel phase-shifting interferometry for microscopy with second wavelength assistance

Dual-channel phase-shifting interferometry for simultaneous phase microscopy is presented. Red and blue light beams are used for microscope illumination. A 45 tilted beamsplitter replicates the object and reference waves in red light together with the object wave in blue light into two parallel beams. The two resulting quadrature phase-shifting interferograms in red light and the object waves in blue light are generated in the two channels. The two interferograms are recorded simultaneously by a color charge-coupled device (CCD) camera, and can be separated via RGB components of the recorded color patterns without crosstalk. As a result, the phase of tested specimen can be retrieved. The feasibility of the proposed method is demonstrated by test performed on a microscopic specimen.     

P-Type Nitrogen-Doped ZnO Films Prepared by In-Situ Thermal Oxidation of Zn_3N_2 Films

P-type nitrogen-doped ZnO films are prepared successfully by in-situ thermal oxidation of Zn_3N_2 films.The prepared films are characterized by x-ray diffraction,non-Rutherford backscattering(non-RBS) spectroscopy,xray photoelectron spectroscopy,and photoluminescence spectrum.The results show that the Zn_3N_2 films start to transform to ZnO at 400°C and the total nitrogen content decreases with the increasing annealing temperature.The p-type films are achieved at 500℃ with a low resistivity of 6.33Ω·cm and a high hole concentration of+8.82 × 10~(17) cm~(-3),as well as a low level of carbon contamination,indicating that the substitutional nitrogen(N_O) is an effective acceptor in the ZnO:N film.The photoluminescence spectra show clear UV emissions and also indicate the presence of oxygen vacancy(V_O) defects in the ZnO:N films.The p-type doping mechanism is briefly discussed.     

Photophysical characteristics of polyaniline with photochromic azobenzene side groups

Photochromic characteristics and optical molecular reorientation in conducting polymer such as polyaniline derivatives containing photochromic azobenzene moieties (PAPNPAPOA) in side chain are studied. Changes in the UV—vis absorption, refractive index, thickness, contact angle and morphology of these films after irradiation of a linearly polarized light with a wavelength of 365nm are measured. The trans-cis isomerization of PAPNPAPOA is proved irreversible even after withdrawing the UV light for a long time. That the structure of main-chain attends by the trans-cis isomerization of side-chain is confirmed by the absorption and the solution colour during the UV irradiation. These effects are discussed by taking the trans-cis isomerization of azobenzene into consideration.