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In order to study the photoemission performance of InGaAs photocathodes, experiments of Cs,O activation, multiple recaesiation, and degradation are performed on a reflection-mode InGaAs photocathode. The photocurrent curves during Cs,O activation, recaesiation, and degradation are measured and analyzed. Based on the quantum efficiency formula of InGaAs photocathodes, the critical performance parameters were obtained by fitting the experimental curves. Results show that Cs-only activation results in a positive electron affinity surface and Cs,O activation leads the surface to a negative electron affinity. Recaesiations can make the degraded InGaAs photocathode recover to a good level. Meanwhile, the spectral response and life time of InGaAs photocathode become smaller and smaller as the recaesiation times increase. [ABSTRACT FROM AUTHOR]

In 0.53 Ga 0.47 As is a new near-infrared material for negative electron affinity photocathodes. Geometry optimization combined generalized gradient approximation based on the density functional theory is adopted to calculate the bulk In 0.53 Ga 0.47 As. Lattice constant of bulk In 0.53 Ga 0.47 As after geometry optimization is compared with the theoretical one based on the Vigard law, which proves the reliability of the parameters of the geometry optimization. The energy bands, electronic structures and optical properties of the bulk ternary In 0.53 Ga 0.47 As for near-infrared photocathode are analyzed. Electron transition way which is closely related with the photoemission is analyzed from the energy bands and the density of electrons. The dielectric function and the optical absorption coefficient are made clear based on the partial density of states. It is found that optical absorption coefficient of bulk In 0.53 Ga 0.47 As is bigger than that of GaAs in the near-infrared band, which is in keeping with the experiments. The analysis provides a theoretical foundation to design a better near-infrared negative electron affinity photocathode from the point of microscopic atoms. [ABSTRACT FROM AUTHOR]

The quantum efficiency of the blue–green transmission-mode GaAlAs photocathode has been studied. Two transmission-mode GaAlAs photocathodes with different structures are prepared, and the quantum efficiency curves are measured. We use the quantum efficiency formula to fit the experimental curves, and obtain the performance parameters of photocathodes such as the electron diffusion length, the back interface recombination velocity, and the surface electron escape probability. The effects of the Al compositions, the thickness of emission layer, and the electron diffusion length on quantum efficiency are investigated. The results show that both of transmission-mode GaAlAs photocathodes are sensitive to the blue–green light. The peak quantum efficiency of one photocathode appears at about 565 nm, while that of another photocathode appears at about 535 nm. The Al composition of emission layer plays a major role on the peak position of quantum efficiency of transmission-mode GaAlAs photocathode. Besides, the thickness of emission layer and the Al composition of window layer also have a large influence on the quantum efficiency. [ABSTRACT FROM AUTHOR]

Abstract: To investigate the quantum efficiency of high performance transmission-mode InGaAs photocathode, the quantum efficiency curve is fitted by using the multilayer optical thin film theory and the first principle calculation. A series of the performance parameters are obtained with the relative error less than 5%. It indicates that the thickness of the emission layer is 0.7–0.9μm, the thickness of the window layer is 0.1–0.3μm. Meanwhile the results from the first principle calculation are proved to be reliable as well. In addition, the long-wave response will increase and the highest integral sensitivity will be obtained when the thickness of the emission layer is the optimum value. For the InGaAs photocathode, the back interface recombination velocity mainly leads to the low integral sensitivity, which is caused by the material lattice mismatch. [Copyright &y& Elsevier]

The 20-inch photomultiplier tube has been extensively applied in neutrino detection, for example, in the Super-K, Daya Bay, and JUNO detectors. In order to achieve a high quantum efficiency, two types of structures, namely variably-doped and uniformly-doped structures, are proposed in this study. It is found that the maximum QE of the variably-doped K-Cs-Sb cathode is about 33%, as compared to the maximum QE of 25% for the uniformly-doped cathode. These high QE bialkali photocathodes of the 20-inch photomultiplier tubes were delivered to JUNO, and the average detection efficiency (410 nm) of 6384 mass-produced PMTs is found to be 30.1%. [ABSTRACT FROM AUTHOR]

To improve the quantum efficiency of AlGaN photocathode, various surfaces cleaning techniques for the removal of alumina and carbon from AlGaN photocathode surface were investigated. The atomic compositions of AlGaN photocathode structure and surface were measured by the X-ray photoelectron spectroscopy and Ar + ion sputtering. It is found that the boiling KOH solution and the mixture of sulfuric acid and hydrogen peroxide, coupled with the thermal cleaning at 850 °C can effectively remove the alumina and carbon from the AlGaN photocathode surface. The quantum efficiency of AlGaN photocathode is improved to 35.1% at 240 nm, an increase of 50% over the AlGaN photocathode chemically cleaned by only the mixed solution of sulfuric acid and hydrogen peroxide and thermally cleaned at 710 °C. [ABSTRACT FROM AUTHOR]

In 0.53 Ga 0.47 As is a perfect III–V compound semiconductor for the photoemissive layer of the infrared-extension negative electron affinity photocathode. It is the key step for the formation of negative electron affinity that the cesium atoms and oxygen atoms activate the photocathode surface alternately. Geometry optimizations based on the first principles have been carried out for the In 0.53 Ga 0.47 As (1 0 0) β 2 (2 × 4) surfaces with a cesium atom adsorbed on 8 different possible sites named as D, D′, T 2 , T 2 ′, T 3 , T 3 ′, T 4 and T 4 ′. The surfaces characteristics have been investigated before and after adsorption from the point of negative electron affinity formation. Meanwhile, the surface atom structure, the adsorption energy, work function, surface energy bands, charge transfer and the dipole generation of the 8 different adsorption surfaces have been compared to each other. The work function and the surface energy bands have been analyzed in detail, which are closely related with the photoelectrons escaping from the surface. The surface work functions are all decreased in varying degrees and energy band bends all appear at the 8 different adsorption sites due to the surface charge transfer and the dipole formation. In conclusion, T 2 and T 3 are the favorable adsorption sites relatively. The surfaces with a cesium atom adsorbed on these two sites are most stable and have much lower work functions, which generates reasonable energy band bend and is benefit for the electrons escape. The theoretical calculations and analysis will guide the design of the negative electron affinity In 0.53 Ga 0.47 As photocathode. [ABSTRACT FROM AUTHOR]

Abstract: The roles of cesium (Cs) and oxides in the processing of gallium aluminum arsenide (GaAlAs) photocathodes have been investigated. The Cs/O activation, Cs/O reactivation and degradation experiments are performed on GaAlAs photocathode. The activated photocurrent, degraded photocurrent and quantum efficiency curves are measured using a self-developed multi-information measurement system. We use the quantum efficiency formula to fit the experimental quantum efficiency curves, and obtain the performance parameters of the photocathode. The results show that the oxide gases play an important role in the degradation of the photocathode, and the Cs atoms could react with the oxides on the degraded surface and the residual oxide gases. Besides, the quantum efficiency of the degraded GaAlAs photocathode could be restored to a good level after Cs/O reactivation. [Copyright &y& Elsevier]

Abstract: The spectral response has been used to evaluate the chemical cleaning for Ga1−x Al x As photocathode by an on-line spectral response measurement system. The spectral response curves of Ga1−x Al x As photocathodes treated by different chemical cleaning methods are measured and analyzed in detail. We use the quantum efficiency formulas to fit the experimental curves transforming from the spectral response curves, and obtain the related performance parameters such as the surface electron escape probability, the back-interface recombination velocity, the electron diffusion length, and the thickness of the etching GaAs layer. The results show that the GaAs photocathode cleaned by the HF solution could obtain a good photoemission effect, while the Ga0.37Al0.63As photocathode could be well cleaned by the solution of sulfuric acid and hydrogen peroxide. [Copyright &y& Elsevier]

Abstract: The conventional quantum efficiency formula for the reflection-mode GaAs photocathode is not applicable to the photocathode with a thin emission layer. The revised quantum efficiency formula has been solved from one-dimension continuity equations, in which the electrons generated in the GaAlAs buffer layer are considered. According to the revised formula, we analyze the impact of some relational performance parameters on the quantum efficiency, such as the thickness of emission layer, the interface recombination velocity etc. Besides, we use the revised and conventional formula to fit the experimental curves of two photocathodes which have the different-thickness emission layers respectively. The results show that compared with the conventional quantum efficiency formula, the revised formula is more suitable to the reflection-mode GaAs photcathode with a thin emission layer, and the GaAlAs buffer layer takes part in the photoemission. [Copyright &y& Elsevier]

We report a systematic theoretical investigation on the activation mechanism of near-infrared In 0.53 Ga 0.47 As photocathodes. Adsorption energies, dipole moments, work functions, density of states, band structures and electron affinity of Cs/O co-adsorption on (001) surface of In 0.53 Ga 0.47 As photocathodes are investigated. First-principles calculation results indicate that the structural stability is greatly enhanced when oxygen atoms adsorb on the Cs-covered surface. The incorporation of oxygen atom helps lower the work function further to achieve the true NEA state. Moreover, two adatom-induced surface dipoles, namely [Csn+-In 0.53 Ga 0.47 Asn− and [Cs+-O2--Cs+, are also adopted to explain the relationship between the adatoms and the substrate. The HOMO and LUMO levels can further move downward and the band bending region is enlarged after Cs/O co-activation. Meanwhile, new energy bands appear in the deep valence band due to the joint effect of Cs 5s, Cs 5p, O 2s and O 2p state electrons. Finally, Cs/O activation experiments are carried out and the photocurrent curves during activation procedure are recorded. The change of photocurrent is closely related to the work function variation, which affects the photoemission of the photocathodes. In order to clearly show the electron affinity, the variation of surface barrier height of InGaAs photocathodes with Cs/O adlayer is also given. • The structural stability of Cs-adsorbed In 0.53 Ga 0.47 As (001) surface is greatly enhanced after oxygen adsorption. • The additional Oxygen atom helps lower the work function further to achieve the true NEA state. • The HOMO and LUMO levels can further move downward and the band bending region is enlarged after Cs/O co-activation. • The change of photocurrent is closely related to the work function variation. [ABSTRACT FROM AUTHOR]