Enhanced Photoemission Performance of InGaN Inclined Nanowire Array.

We propose to improve the solar energy utilization by using InGaN inclined nanowire array photocathodes. We first study vertical nanowire array. On the basis of vertical nanowire array, we study inclined nanowires by changing the inclination angle of nanowires. The inclined nanowires exhibit higher quantum efficiency at larger period and larger inclination angle. However, the infinite expansion of period will cause its performance to degrade. The quantum efficiency of inclined nanowires with a period of 175 nm and an inclination angle of 5.35° is as high as 80.2% when the incident light angle is irradiated at 5°. In addition, applying an electric field can improve the collection efficiency of inclined nanowires and help them maintain a high collection efficiency over a longer wavelength range. The design principles proposed in this work will provide a theoretical reference for the performance improvement of InGaN photocathodes.

Enhanced blue-green response of nanoarray AlGaAs photocathodes for underwater low-light detection.

Underwater optical communication and low-light detection are usually realized via blue-green laser sources and blue-green light-sensitive detectors. Negative-electron-affinity AlGaAs photocathode is an ideal photosensitive material for ocean exploration due to its adjustable spectrum range, long working lifetime, and easy epitaxy of materials. However, compared with other photocathodes, the main problem of AlGaAs photocathode is its low quantum efficiency. Based on Spicer's three-step photoemission model, nanoarray structures are designed on the surface of AlGaAs photocathode to improve its quantum efficiency from two aspects of optical absorption and photoelectron transport. Through simulation, it is concluded that the cylinder with diameter of 120 nm and height of 600 nm is the best nanoarray structure, and its absorptance is always greater than 90% in the 445∼532 nm range. Moreover, the absorptance and quantum efficiency of the cylinder nanoarray AlGaAs photocathode are less affected by the incident angle. When the angle of incident light reaches 70°, the minimum absorptance and quantum efficiency are still 64.6% and 24.9%. In addition, the square or hexagonal arrangement pattern of the nanoarray has little effect on the absorptance, however, a reduction in the overall emission layer thickness will decrease the absorptance near 532 nm.

Optical absorption enhancement in inhomogeneous InGaN nanowire arrays photocathode.

In the development of surface structures, nanowire arrays (NWAS) have been widely studied because of their trapping effect. In this paper, the finite difference time domain (FDTD) method is used to simulate homogeneous and inhomogeneous NWAS. We studied the influence of the structural parameters of InGaN NWAS and inhomogeneous arrays on optical response properties. The optical response includes light absorptivity and cutoff wavelength sensitivity. The simulation results show that the inhomogeneous NWAS can increase the effective transmission distance of light on the surface, thus greatly improving the optical absorption capacity of InGaN NWAS. We can obtain high sensitivity of cut-off wavelength by adjusting the structural parameters of the side nanowires. We find that by reducing the diameters and heights of the side nanowires, a higher light absorption rate can be obtained, which is a 5% improvement compared to uniform NWAS. Therefore, the research in this paper can provide some theoretical reference for the experiment and preparation of InGaN photocathodes.

Comparison of activation behavior of Cs-O and Cs-NF3-adsorbed GaAs(100)-ß2(2 × 4) surface: From DFT simulation to experiment.

To clarify the performance differences between Cs-O and Cs-NF3-activated GaAs photocathodes, the changes in adsorption characteristics with Cs coverage for the Cs-O and Cs-NF3-adsorbed GaAs(100)-ß2(2 × 4) surfaces were investigated by first-principles calculation based on density function theory. The simulation results show that under the same Cs coverage, the Cs-NF3-adsorbed GaAs surface is more stable than the Cs-only and Cs-O-adsorbed surfaces. In the case of small Cs coverage, the Cs-O-adsorbed GaAs surface exhibits a lower work function than Cs-NF3 does. When the Cs coverage reaches one monolayer, however, the Cs-NF3 adsorption model possesses a stronger dipole moment resulting in an even lower work function. According to theoretical prediction guidance, the Cs-O and Cs-NF3 activation experiments adopting the excessive Cs deposition recipe were performed for preparing GaAs photocathodes. The experimental results show that the Cs-NF3-activated GaAs photocathode has a higher quantum efficiency in the whole waveband and better emission stability under white light illumination than the Cs-O-activated photocathode, which agrees with the differences in work function and adsorption energy between the two adsorption model types. It is concluded that the Cs-NF3 activation recipe using excessive Cs supply is preferred to enhance photoemission performance of GaAs photocathode.

In situ characterization of thermal cleaned surface for preparing superior transmission-mode GaAs photocathodes.

Considering that it is impractical to utilize in situ surface diagnostic means to determine the surface cleanliness of transmission-mode GaAs photocathodes in the vacuum device manufacturing process, the thermal desorption technique with the aid of the quadrupole mass spectrometer during the thermal cleaning process is employed to in situ characterize the thermal cleaned surface. The desorption behaviors for various impurity gases during the thermal cleaning process are analyzed. The experimental results show that the amount of desorbed impurity gases varies due to the different heat treatment temperatures. Through the verification of Cs/O activation and quantum efficiency measurement, it is found that the desorption behaviors of the specific impurity gases including AsH3 and As2 are crucial to surface cleanliness of transmission-mode GaAs photocathodes, which relate to the final photoemission capability. This simple and reliable criterion provides an effective way to guide the thermal cleaning process of transmission-mode GaAs photocathodes, and the desorption behaviors assist in in situ evaluation of surface cleanliness.

Effect of graded bandgap structure on photoelectric performance of transmission-mode AlxGa1-xAs/GaAs photocathode modules.

The graded bandgap AlxGa1-xAs/GaAs photocathode with graded composition and exponential doping structure has shown great potential for improving photoemission capability. In order to better study the performance of transmission-mode AlxGa1-xAs/GaAs photocathode with the complex graded bandgap structure, the experimental optical properties and quantum efficiency are measured by comparison with uniform composition and exponential doping Al0.7Ga0.3As/GaAs photocathode. The theoretical optical properties of the multilayer AlxGa1-xAs/GaAs photocathode modules are calculated by matrix formula on the basis of thin-film optical principles. The effect of cathode thickness and aluminum proportion on optical properties are analyzed by simulation. The results show that these parameters have complicated effects on the optical properties. Different parameters are presented as the changes of peak and valley of the optical property curves. Meanwhile, the emission layer has a significant effect on the absorptivity values of the photocathode modules, which will obviously influence photoemission performance. By using the optical properties via calculation, a better fit of the experimental data with the theoretical model can be achieved, which would make reasonable guidance for further investigation of these complex graded bandgap photoemitters.

Comparison of degradation and recaesiation between GaAs and AlGaAs photocathodes in an unbaked vacuum system.

The lifetime and reliability of a photocathode during operation are always raised problems and the photocathode performance depends on the vacuum condition. With the purpose of investigating the stability and reliability of a GaAs-based photocathode in a harsher vacuum environment, reflection-mode exponential-doped GaAs and AlGaAs photocathodes are metalorganic vapor-phase epitaxial grown and then (Cs, O) activated inside an unbaked vacuum chamber. The degraded photocurrents are compared after activation and recaesiations between GaAs and AlGaAs photocathdoes under illumination with an equal initial photocurrent and an equal optical flux, respectively. It is found that the performance on degradation and recaesiations between GaAs and AlGaAs photocathodes are different. In the unbaked vacuum system, the stability of an AlGaAs photocathode after (Cs, O) activation is always better than that of a GaAs photocathode. After multiple recaesiations, the photocurrent decay curves of the AlGaAs photocathode are nearly coincident, which means a nearly constant operational lifetime. Moreover, operational lifetime of an AlGaAs photocathode is longer than that of a GaAs photocathode, which further illuminates that AlGaAs photocathodes are superior to GaAs photocathodes in stability and repeatability under markedly harsher vacuum conditions.

[Identification of Chemical Constituents of the Leaves from Acanthopanax senticosus by UPLC-Q-TOF-MS/MS].

Objective: To analyze the chemical compositions of the leaves from Acanthopanax senticosus. Methods: Rapid identification of chemical constituents in the leaves of Acanthopanax senticosus by UPLC-Q-TOF-MS / MS. The chemical constituents were identified and speculated by using Peakview data processing software, the retention time, exact relative molecular mass, and cleavage fragments of MS / MS were detected. Chromatography-mass spectrometry conditions were as follows, the analysis was performed on Waters BEH C18column( 100 mm × 2. 1 mm,1. 7 µm) in gradient elution with a mobile phase of 0. 1% formic acid aqueous solution and 0. 1%formic acid acetonitrile, the flow rate was at 0. 3 m L / min, the data was collected by the negative and positive ion mode using ESI ion source. Results: 30 compounds were identified and speculated by the standards and compounds of MS / MS, the references and Chemispider database. Conclusion: This method is fast, sensitive and comprehensive with the rapid identification of chemical constituents in the leaves of Acanthopanax senticosus, which will provide the evidences for perfecting the quality standard, and clarify the efficacy material base of the leaves of Acanthopanax senticosus.

Research on quantum efficiency for reflection-mode InGaAs photocathodes with thin emission layer.

In order to understand the photoemission mechanism of the reflection-mode InGaAs photocathode with a thin emission layer, the formula describing reflection-mode quantum efficiency is revised by solving the one-dimensional continuity equation, in which the electrons generated in the GaAs buffer layer are considered. Compared with the conventional formula, the revised formula is proved to be more suitable for the reflection-mode InGaAs photocathode with a thin emission layer. In experiment, the InGaAs sample goes through two-step surface preparation including a wet chemical cleaning process and a heat treatment process. Then the sample is activated by Cs/O and the experimental quantum efficiency curves are measured simultaneously every other hour. The measured results show that the shapes of the quantum efficiency curves degrade with time because of the contamination of residual gases in the vacuum system. All the quantum efficiency curves are well fitted by the revised formula.

Photoemission performance of thin graded structure AlGaN photocathode.

In order to research a high-efficiency AlGaN photocathode, the AlGaN photocathodes with varied Al composition (0.68 and 0.4) and uniform Al composition (0.24) were grown. The photocathodes were activated by Cs adsorption and received their spectral response via multi-information system. Results show that the absorption rate of the AlGaN photocathode with varied Al composition is half of the AlGaN photocathode with uniform Al composition, but the quantum efficiency of the photocathode with varied Al composition is approximately 29% higher than that of the photocathode with uniform Al composition. The built-in field within the emission layer of the AlGaN photocathode with varied Al composition is much higher than that of the photocathode with uniform Al composition, which is the main factor that promotes the photoelectron movement toward the photocathode surface and improves photoemission performance of the AlGaN photocathode.

Effect of Cs adsorption on the photoemission performance of GaAlAs photocathode.

The effect of Cs adsorption on the photoemission performance of a reflection-mode GaAlAs photocathode in an ultrahigh vacuum chamber has been investigated. The experiments for Cs/O activation, multiple recaesiation, and degradation are performed on a GaAlAs photocathode. Meanwhile, the Cs/O activated and recaesiated photocurrent curves, degraded photocurrent curves, and spectral response curves are measured and analyzed. Besides, the performance parameters of the photocathodes are obtained by using the formula to fit with the experimental quantum efficiency curves. The results show that the Cs atoms not only make the atomically clean surface form the negative electron affinity, but also make the degraded photocathode recover to a good level. The quantum efficiency and the lifetime of GaAlAs photocathode become lower with increasing the recaesiation times.

[Photoemission characteristic of transmission-mode extended blue GaAs photocathodes].

Based on the theoretical models of computing the photocathode optical performance, quantum yield and integral sensitivity, the photoemission characteristics of the domestic and ITT's transmission-mode extended blue GaAs photocathodes, namely the cathode optical properties and performance parameters, were respectively investigated. The compared results show that the integral sensitivity of the domestic transmission-mode extended blue photocathode has achieved 2,100 microA x lm(-1), still falling behind the ITT's integral sensitivity of 2,750 µA x m(-1). The reasons for the difference in quantum yield curves are that, on one hand, the thickness of GaAlAs window-layer and the Al mole fraction play a critical role in the short-wavelength response, especially in the extended blue region. On the other hand, the cathode performance parameters such as electron diffusion length and back interface recombination velocity work on the long-wavelength and short-wavelength response. All these factors are subject to the backwardness of basic industrial manufacturing level.

Comparison of structure and performance between extended blue and standard transmission-mode GaAs photocathode modules.

Extended blue and standard transmission-mode GaAs photocathode modules were prepared, respectively, by metal organic chemical vapor deposition. The experimental reflectivity, transmissivity, and spectral response curves were measured and compared separately. The integral sensitivities are 1980 µA/lm and 2022 µA/lm for both the modules. By use of the revised quantum yield formula, the experimental spectral response curves are fitted to obtain the structure parameters. The fitted results show that the Ga(1-x)Al(x)As window layer with varied aluminum components is beneficial to improve extended blue GaAs photocathode module. In addition, the layer-thickness and aluminum component in the window layer determine the extended blue performance, while the thickness of the GaAs active layer settles the long-waveband performance for the transmission-mode GaAs photocathode module.