[en] In order to research the spectral response variation of a negative electron affinity (NEA) photocathode in the preparation process, we have done two experiments on a transmission-type GaAs photocathode.First, an automatic spectral response recording system is described, which is used to take spectral response curves during the activation procedure of the photocathode. By this system, the spectral response curves of a GaAs:Cs-Ophotocathode measured in situ are presented. Then, after the cathode is sealed with a microchannel plate and a fluorescence screen into the image tube, we measure the spectral response of the tube by another measurement instrument. By way of comparing and analyzing these curves, we can find the typical variation in spectral-responses.The reasons for the variation are discussed. Based on these curves, spectral matching factors of a GaAs cathode for green vegetation and rough concrete are calculated. The visual ranges of night-vision goggles under specific circumstances are estimated. The results show that the spectral response of the NEA photocathode degraded in the sealing process, especially at long wavelengths. The variation has also influenced the whole performance of the intensifier tube

[en] Highlights: • In order to preparative the transmission-mode AlGaN photocathodes in the future, the photocathode has a thin emission layer, and the lattice mismatch at the back interface was eased by varying Al composition of AlGaN sub-layer. • The boiling KOH solution and the mixture of sulfuric acid and hydrogen peroxide can effectively remove the alumina and carbon from the AlGaN photocathode surface, respectively. • The quantum efficiency of the activated AlGaN photocathode has up to 35.1% and improved by 50% from that of cleaned by traditional cleaning technology. - Abstract: 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

[en] We designed two transmission-mode GaAs/AlGaAs photocathodes with different Al_xGa_1−xAs layers, one has an Al_xGa_1−xAs layer with the Al component ranging from 0.9 to 0, and the other has a fixed Al component 0.7. Using the first-principle method, we calculated the electronic structure and absorption spectrum of Al_xGa_1−xAs at x = 0, 0.25, 0.5, 0.75 and 1, calculation results suggest that with the increase of the Al component, the band gap of Al_xGa_1−xAs increases. Then we activated the two samples, and obtained the spectral response curves and quantum efficiency curves; it is found that sample 1 has a better shortwave response and higher quantum efficiency at short wavelengths. Combined with the band structure diagram of the transmission-mode GaAs/AlGaAs photocathode and the fitted performance parameters, we analyze the phenomenon. It is found that the transmission-mode GaAs/AlGaAs photocathode with variable Al component and various doping structure can form a two-stage built-in electric field, which improves the probability of shortwave response photoelectrons escaping to the vacuum. In conclusion, such a structure reduces the influence of back-interface recombination, improves the shortwave response of the transmission-mode photocathode. (paper)

[en] The surface chemical compositions, atomic concentration percentage and layer thickness after 'high-temperature' single-step activation and 'high-low temperature' two-step activation were obtained using quantitative analysis of angle-dependent X-ray photoelectron spectroscopy (XPS). It was found that compared to single-step activation, the thickness of GaAs-O interface barrier had a remarkable decrease, the degree of As-O bond became much smaller and the Ga-O bond became dominating, and at the same time the thickness of (Cs, O) layer also had a deduction while the ratio of Cs to O had no change after two-step activation. The measured spectral response curves showed that a increase of 29% of sensitivity had been obtained after two-step activation. To explore the inherent mechanisms of influences of the evolution of GaAs(Cs, O) surface layers on photoemission, surface electric barrier models based on the experimental results were built. By calculation of electron escape probability it was found that the decrease of thickness of GaAs-O interface barrier and (Cs, O) layer is the main reasons, which explained why higher sensitivity is achieved after two-step activation than single-step activation

[en] In this article, a novel x-ray image intensifier is introduced. It is mainly composed of an x-ray intensifying screen and a low-light-level (L³) image intensifier. In order to obtain a bright enough image, the spectral compatibility among three different combinations of the x-ray intensifying screen and the photocathode is analyzed. The comparison indicates that a (Zn,Cd)S:Ag screen is superior to either a CaWO₄ screen or a Gd₂O₂S:Tb screen when combined with the Super S₂₅ photocathode and spectral compatibility analysis is a useful guide when designing an optoelectronic imaging device

[en] The whole process including photoelectron excitation, transportation from bulk to surface and escape to vacuum by traversing surface barrier was analyzed in detail. Photoelectron excitation relates to the band structure of photocathode material and the absorption coefficient of the material. In the bulk of GaN photocathode, the photoelectrons mainly are transited into Γ valley first, when the energy is great enough, the photoelectrons can scatter into higher M-L valley or A valley from Γ valley. The electrons excitated in conduction band will move from bulk to surface by diffusing or drifting. The diffuse length for GaN photocathode is about 3μm by calculating. After the thermal electrons of Γ valley move into surface band bend area, they will drift to the surface because of the electric field of band bend area. The transmission coefficient relates to the incident electron energy, the height and width of the surface potential. The quantum yield formulas of NEA GaN photocathode were gotten by solving the diffuse equation of non-equilibrium carriers. (copyright 2012 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

[en] Gallium arsenide (GaAs) nanowire array (NWA) photocathodes have unique electrical and optical properties. Based on studies about photon absorption, band structure, and electron transport properties of GaAs nanowire, a photoemission model for GaAs NWA photocathodes is established. According to the model, we simulate and analyze the photocurrent, spectral response, and absorption properties of ordered GaAs NWA photocathodes. The results present a very interesting phenomenon; the photocurrent and spectral response peak at incident angles of 20° and 30°, respectively. These special properties of NWA cathodes differentiate them from their thin film counterparts. We also analyze the effects of nanowire length and diameter on the photocurrent of NWA cathodes, and find the optimum height of the nanowires is 10 μm. This study shows that NWAs exhibit higher absorbance and excellent charge transport. Thus, GaAs NWA photocathodes are excellent candidates for electron sources. (paper)

[en] Highlights: • Adsorption energy of Cs adsorption on Al_0.25Ga_0.75N(0 0 0 1) surface increases as the increasing of Cs coverage. • Electrons transfer from Cs adatoms to substrate during Cs adsorption process, meanwhile the transfer efficiency decreases as Cs coverage increases. • The length of Ga-N bond in the first and second bilayers increases after Cs adsorption. • There appear new energy bands at −25 to −23 eV and −14 to −10 eV, which were induced by Cs 5s and Cs 5p state electrons respectively. - Abstract: We investigate cesium adsorption on Al_0.25Ga_0.75N(0 0 0 1) surface at different coverages using first principle method based on density functional theory. Adsorption energies, atomic structure, Mulliken charge distribution, electron transfer, band structures, and density of states of the adsorption systems corresponding to different Cs coverages were obtained. Total-energy calculations show that cesium adsorption on Al_0.25Ga_0.75N(0 0 0 1) surface is more and more difficult as the increase of cesium coverage. A single cesium adatom is preferred to locate at the top of Ga atom (T_Ga). Meanwhile, it is not the most stable configuration when two cesium atoms were located on the top of two Ga neighbors at the same time. This is mainly because the distance of Cs adatoms is so small that repulsive force between adatoms rises. At low coverage, electrons transfer from Cs adatom to Ga atoms on the topmost and second topmost bilayers. Meanwhile, the efficiency of electron transfer decreases as the increasing of Cs coverage. There appear new bands at −25 to −23 eV and −14 to −10 eV, which were caused by Cs 5s and Cs 5p state electrons. Under the joint effect of Cs 5s and 5p state electrons, density of states at Fermi level increases, and the adsorption surfaces show more metal properties. Electrons transferring from Cs adatoms to Al_0.25Ga_0.75N substrate induces dipole moment, which is useful to lower work function. What is more, there exists an optimum of cesium coverage to obtain the lowest work function

[en] In order to study the activation process of Ga_1−xAl_xN photocathodes theoretically, models of Cs adsorption on Ga(Mg)_0.75Al_0.25N (0 0 0 1) surface are built, then the atomic structure, electronic structure, adsorption energy, work function, dipole moment and optical properties of the models are calculated. All calculations are carried out using Cambridge Serial Total Energy Package (CASTEP) based on first principle. Results show that Cs adsorption on Mg doping Ga_1−xAl_xN (0 0 0 1) surface can reduce work function of the surface, and the favorite adsorption site is on the top of p-type impurity. Cs adsorption on p-type Ga_1−xAl_xN (0 0 0 1) surface can produce the structure of p-type bulk with n-type surface, which is helpful to surface band bend downward and decrease electron affinity seriously. The absorption coefficient of Cs adsorption system is less than that of the clean surface. Theoretical study of Cs adsorption on Ga_1−xAl_xN (0 0 0 1) surface can help to improve activation technology of Ga_1−xAl_xN photocathodes.

[en] The adsorption characteristics and change in work function of Cs on a (2 × 2) GaN(0 0 0 1) surface with a coverage from 1/4 to 1 monolayer (ML) have been investigated using density functional theory with a plane-wave ultrasoft pseudopotential method based on first-principles calculations. The results show that the most stable positions of Cs adatoms on GaN(0 0 0 1) surfaces are at N-bridge and H3 sites for 1/4 ML coverage. As the Cs atomic coverage is increased, adsorption energy and stability reduce and achieve saturation when the Cs adatom coverage is 3/4 ML. The transfer of Cs6s electrons to Ga atoms in the outermost layer decreases the work function of the system.