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Metallic photocathodes have drawn attention due to their outstanding performances of ultrafast photoelectric response and long operational lifetime. However, due to their high work function and the large number of scattering events, metallic photocathodes typically are driven by ultraviolet laser pulses and characterized by low intrinsic quantum efficiency (QE). In this work, a new type of Mie-type silver (Ag) nano-sphere resonant structure fabricated on an Ag/ITO composite substrate is used to enhance the photocathode QE, where Mie scattering resonance is used to enhance the local density of optical state and then to improve the light absorption and electron transporting efficiency in Ag nano-spheres. The cesium (Cs) activation layer is also used to lower the electron work function and then to excite photoemission in the visible waveband for Ag photocathode. The optical characteristics of Ag nano-sphere arrays are analyzed by using finite difference time domain method. For the investigated Ag nano-sphere array, theoretical results show that Mie-type electric dipole resonance modes can be obtained over the 400–600 nm waveband by adjusting the sphere diameter, and the large resonance-enhanced absorption can be achieved in nanospheres at the resonance wavelength. The Ag nano-spheres are fabricated on the Ag/ITO substrate by magnetron sputtering and annealing process, then the Cs activation layer is deposited on surface, and finally QE is measured in an ultra-high vacuum test apparatus. Experimental results show that over 0.35% of QE is obtained for Ag nano-sphere particle (with a diameter of 150 nm) at a wavelength of 425 nm, and the wavelength positions of QE maxima are in agreement with Mie resonance for corresponding geometry predicted from the computational model. Given these unique optoelectronic properties, Ag nanophotonic resonance structured photocathodes represent a very promising alternative to photocathodes with flat surfaces that are widely used in many applications today.

The gradient-doping structure is first applied to prepare the transmission-mode GaAs photocathode and the integral sensitivity of the sealed image tube achieves 1420 μA/lm. This paper studies the inner carrier concentration distribution of the gradient-doping transmission-mode GaAs photocathode after molecular beam epitaxy (MBE) growth using the electrochemical capacitance-voltage profiling. The results show that an ideal gradient-doping structure can be obtained by using MBE growth. The total band-bending energy in the gradient-doping GaAs active-layer with doping concentration ranging from 1 × 1019 cm−3 to 1 × 1018 cm−3 is calculated to be 46.3 meV, which helps to improve the photoexcited electrons movement toward surface for the thin epilayer. In addition, by analysis of the band offsets, it is found that the worse carrier concentration discrepancy between GaAs and GaAlAs causes a lower back interface electron potential barrier which decreases the amount of high-energy photoelectrons and affects the short-wave response.

The photocurrent and spectral response characteristics of gallium arsenide (GaAs) are obtained by a multi-information measurement system, and the evolution of the photocurrent versus the Cs:O flux ratio is investigated. The experimental results show that the photocurrent increases approximately exponentially after the first exposure to Cs until a maximum sensitivity is reached, the detailed evolution process and the ultimate photocurrent are different for different samples. These differences are analysed, and according to the process of coadsorption of Cs and oxygen on GaAs, an equation is presented to explain the increase of photocurrent.

Dislocation densities of two hydride vapor phase epitaxy-grown hexagonal GaN samples, which are Si doped and unintentionally doped respectively, are determined by triple-axis X-ray diffractometry and van der Pauw variable temperature Hall-effect measurement. The dislocation densities of these two samples should be at the same level from the X-ray testing, the -FWHM (full width at half maximum) values of all corresponding reflections for these two samples are almost the same. But from the Hall-effect measurements, the dislocation density values should be different from each other remarkably, because the unintentionally doped sample belongs to Mott transition material, while the Si-doped one does not. This fact indicates that the X-ray testing is perhaps inaccurate under some conditions, although the triple-axis X-ray diffractometry is a highly suitable technique for discriminating different kinds of structural defects such as edge and screw dislocations that lead to characteristic broadening of symmetric and asymmetric Bragg reflection. The experimental result obtained so far (say, for hot-electron bolometer) shows that the dislocation density value from mobility fitting model is in good accordance with that from -FWHM fitting using Srikant method. The anomaly that the dislocation density from -FWHM fitting is much lower than that from mobility fitting for the same sample (sample 59#), indicates that dislocations located in grain boundary may not be tested by triple-axis X-ray diffractometry. According to mosaic model, the layer is assumed to consist of single crystallites, called mosaic blocks, which are assumed to be slightly misoriented with respect to each other. The out-of-plane rotation of the block perpendicular to the surface normal is of the mosaic tilt, and the in-plane rotation around the surface normal is of the mosaic twist. The average absolute values of tilt and twist angles are directly related to the FWHM values of the corresponding distributions of crystallographic orientations. So, the X-ray testing can determine the average orientation of the grains with the same interplanar distance, excluding the information about the grain boundary at which X-ray cannot interfere because of disdortion of lattice. The experimental results and calculation analyses indicate that the dislocation density value from Srikant model is accurate when the ratio of twist angle to tilt angle exceeds 2.0, or the magnitude of the lateral coherence length is larger than 1.5 m.

An analytical model for electron mobility in a class of wurtzite n-GaN, whose carrier concentration is over 1018 cm-3 (Mott's critical limit), is developed. With the dislocation density and two donor levels serving as the important parameters, the proposed model can accurately predict the electron mobility as a function of temperature. The edge and screw dislocation densities in two samples, which are respectively grown on sapphire (001) by metal organic chemical vapor deposition and hydride vapor phase epitaxy, are determined by using this model which is discussed in detail. It is shown that the data-fitting of H-T characteristic curve is a highly suitable technique for accurately determining the edge and screw dislocation densities in n-GaN films. Quantitative analyses of donor concentration and donor activation energy indicate that the impurity band occurs when the carrier concentration is under 1017 cm-3, much lower than the critical carrier concentration of Mott transition (1018 cm-3). Such a behavior can also be confirmed by the results from solving the Boltzmann transport equation by using the Rode iterative method. Another anomaly is that the dislocation density in Mott transition material perhaps is lower than that of material with carrier concentration under 1018 cm-3. This fact indicates that the cause of Mott transition should not be the shallow donor impurities around dislocation lines, but perhaps the deeper donor impurities or other defects. In the theoretical model calculation, two transition characteristics together with the donor distribution and its energy equilibrium are taken into account. Based both on the Mott transition and the H-like electron state model, the relaxation energies for the shallow-donor defects along the screw and edge dislocation lines are calculated by using an electrical ensemble average method. Besides, an assumption that should be made is that there are 6 shallow-donor defect lines around one dislocation line. The research results show that the Hall mobility should be taken as the live degree of the ionizing energy for the shallow-donor defects along the dislocation line. The experimental results indicate that our calculation function can be best fit by the experimental curve, with the values of dislocation density being between our model and others determined by X-ray diffraction or by chemical etching method, which are all in good agreement with each other. The method reported can be applied to the wurtzite n-GaN films grown by various preparation technologies under any condition, with the peak-mobility temperature about or over 300 K, whose Hall mobility near 0 K perhaps is over 10 cm2/(Vs) and even 100 cm2/(Vs).

The modulation transfer function (MTF) of graded band-gap AlGaAs/GaAs transmission-mode photocathodes is numerically solved from the two-dimensional continuity equations. According to the MTF model, we calculate the theoretical MTF of graded band-gap and uniform band-gap transmission-mode photocathodes, and analyze the effects of Al composition, wavelength of incident photon, and thickness values of AlGaAs and GaAs layer on the resolution. The calculated results show that compared with the uniform band-gap photocathode, the graded band-gap structure can increase the resolution of photocathode evidently. If the spatial frequency f ranges from 100 to 500 lp·mm-1, the increase of resolution is more pronounced. Let f=200 lp·mm-1, the resolution of graded band-gap photocathode generally increases 150%-260%. The resolution improvement of graded band-gap photocathode is attributed to the built-in electric field. While too high built-in electric field will influence the spectral response of long-wavelength photons due to higher Al composition in the AlGaAs/GaAs photocathodes.

Aiming to inverstigate the effect of surface potential barrier on electron escape probability of negative electron affinity GaN photocathode, the electron escape probability is calculated by using the Boltzmann distribution and the method of transfer matrix based on Airy function. It is found that barrier I is a key influencing factor of electron escape probability, while barrier II has a limited influence. The activation photocurrent curve of the transmission-mode GaN photocathode is measured by using our built activation and evaluation experimental system of NEA GaN photocathode. The obvious increase of electron escape probability can be achieved mainly by activating Cs only. The increase of electron escape probability is not large in Cs/O activation process with only Cs activated sufficiently. The theoretical calculations are in good agreement with the photocurrent curves from experimental test. The reason is that the contribution of activating only Cs to the reducing of vacuum level for obtaining NEA state is much larger than that of activating Cs/O.

The stability of a reflection-mode GaAs photocathode has been investigated by monitoring the photocurrent and the spectral response at room temperature. We observe the photocurrent of the cathode decaying with time in the vacuum system under the action of Cs current, and find that the Cs atoms residing in the vacuum system are helpful in prolonging the life of the cathode. We examine the evolution and analyse the influence of the barrier on the spectral response of the cathode. Our results support the double dipolar model for the explanation of the negative electron affinity effect.

The degradation of activated GaAs vacuum electron source as a function of time has been investigated by using X-ray photoelectron spectroscopy (XPS).We found that the cathode surface element content changes with time and the significant decrease in sensitivity of electron source is mainly due to the change of dipole direction caused by the adsorption of harmful gases on the cathode surface.Based on the above results,we deduced the degradation model of GaAs electron source through analyzing the adsorption process of harmful gases on the surface in vacuum system.The model reveals the exponential degradation rule of GaAs electron sources and the inverse relationship between lifetime and pressure.The theoretical results are in full agreement with the experimental fact.

The difference between the diffusion drift length of photoelectrons in exponential-doping GaAs photocathode and that in uniform-doping GaAs photocathode is studied. According to quantum equations, the optimized thickness of transmission-mode exponential-doping GaAs photocathode is simulated to be 20 μm. Two transmission-mode exponential-doping GaAs samples with the thickness of 16 and 20 μm are activated by (Cs,O) alternation technique. Integral sensitivities of the two samples are 1228 and 1547 μA/lm, respectively. The ratio of integral sensitivities of the two samples is 0796∶1, which agrees with the simulation result.

Aimming at the decay tendency of reflection-mode negative electron affinity (NEA) GaN photocathode and the different decay speeds of quantum efficiency corresponding to the different wave bands, and referring to the decay tendency of quantum efficiency curve provided by foreign authors for reflection-mode NEA GaN photocathode, the quantum efficiency decay mechanism for reflection-mode NEA GaN photocathode was studied. The surface model ［GaN (Mg) : Cs］: O-Cs for GaN photocathode after being activated with cesium and oxygen was used. And the change of surface barrier in the decay course of quantum efficiency was considered. The reduction of the effective dipole quantity is the basic reason causing quantum efficiency reduction. And it is the change of surface I, II barrier shape that causes the difference of dropping speeds of quantum efficiencies corresponding to different wave bands.

The modulation transfer function （MTF） of transmission-mode exponential-doping photocathodes has been solved from the 2-dimensional continuity equations. According to the MTF equation，we calculated and analyzed the theoretical resolution characteristic of the exponential-doping photocathodes. The calculated results show that，compared with the uniform-doping photocathodes，the exponential-doping structure can increase the resolution of photocathodes evidently. For the spatial frequency franges between 100 and 400 lp/mm，the increase of resolution is more pronounced. Let f=200 lp/mm，the resolution of exponential-doping photocathodes generally increases by 20%—50%. The resolution improvement of exponential-doping photocathodes，as well as the quantum efficiency improvement，is also attributed to the built-in electric field.

The photocurrent curves of reflection-mode GaAs photocathodes as a function of time, when were illuminated by white light with an intensity of 0, 33 and 100lx, respectively, were measured using a multi-information measurement system. The calculated lifetimes of cathodes are 320, 160 and 75min, respectively, showing that the stability of cathodes degraded with the increase of light intensity. The lifetime of cathode, illuminated by white light with an intensity of 100lx, while no photocurrent was being drawn during the illumination, was 100min. Through comparison, we found that the influence of illumination on cathodes stability is greater than that of photocurrent. The quantum-yield curves of cathodes as a functions of time, when illuminated by white light with an intensity of 33lx, were measured also. The measured results show that the shape of the yield curves changes with increasing illumination time due to the faster quantum-yield degradation rate of low energy photons. Based on the revised quantum-efficiency equations for the reflection-mode cathodes, the variation of yield curves are analyzed to be due to the intervalley diffusion of photoelectrons and the evolution of the surface potential barrier profile of the photocathodes during degradation process.

Exponential-doping photocathodes, in which from the GaAs bulk to the surface the doping concentration is distributed exponentially from high to low, can form a stable built-in electric field in the active layer, and the electric field facilitates the excited photoelectron emission. The quantum yield formulas of both reflection-mode and transmission-mode of exponential-doped photocathodes have been solved from the 1-diemnsion continuity equations, in which the build-in electric field is considered. According to these formulas, we calculate the theoretical quantum yield of the exponential-doped photocathodes. The calculated results show that the exponential-doping structure can increase the quantum yield of photocathodes significantly. To compare with the uniformly-doped photocathodes, the integral sensitivity of the reflection-mode exponential-doped photocathodes increases by nearly 20%, and for transmission-mode photocathodes the increase is more than 30%. The performance improvements of exponential-doping photocathodes are mainly attributed to the built-in electric field, the photoelectrons driven by the field move towards the cathode surface by way of diffusion and drift, accordingly, decrease the influence of the back-interface recombination velocity on photoemission and increase the equivalent electron diffusion length of cathodes.