Recent developments of low-emittance electron guns for accelerator are reviewed. In the accelerator field, DC biased triode thermionic gun (Pierce type gun) has been widely used and is still conventional. On the other hand, because of strong demands on the high brightness electron beam by FEL and other advanced accelerator concepts based on linear accelerator, the low emittance beam generation becomes one of the most important issue in the accelerator science. The R&D effort is “accelerated” by two technological innovations, photo-cathode and RF gun. They made a large improvement on the beam emittance. After the explanations on the technical and physical aspects of the low emittance electron beam generation, advanced electron sources for accelerators are reviewed.

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  A photoinjector which can generate a high brightness, low emittance and short duration electron bunches is required for future light source accelerators and applications such as electron microscopes. A DC high voltage gun which has a semiconductor photocathode in a high DC electric field is one of the technology choices for the generation of high quality electron beam. Many techniques to the development of semiconductor based photoemission gun are introduced in briefly.

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  Two ultrashort electron-bunch photocathode RF guns in picosecond (ps) and femtosecond (fs) time region are reviewed. The ps gun was developed and used successfully to generate an ultralow-emittance and high-charge electron beam with normalized emittance of 1.2 mm-mrad at bunch charge of 1 nC. The low-emittance electron beam has been used successfully to produce a 100-fs high-charge electron single bunch with a booster linear accelerator and a magnetic bunch compressor. An ultrafast pulse radiolysis with time resolution of 240 fs was constructed using the fs electron beam for the study of ultrafast reactions in radiation chemistry and biology. The fs gun, developed under the KEK/Osaka University collaboration with a new structure cavity and many improvements, was succeeded in producing directly a near-relativistic 100-fs electron beam with energy of 1-3 MeV. Femtosecond time-resolved electron diffraction has been constructed using the fs RF gun for the study of photon-induced phase transition in materials. Some experimental results of pulse radiolysis and MeV electron diffraction in fs time region were reported and discussed.

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  A laser-excited photocathode RF gun is one of the most reliable high-brightness electron beam sources for XFELs. Several 3D laser shaping methods have been developed as ideal photocathode illumination sources at SPring-8 since 2001. To suppress the emittance growth caused by nonlinear space-charge forces, the 3D cylindrical UV-pulse was optimized spatially as a flattop and temporally as squarely stacked chirped pulses. This shaping system is a serial combination of a deformable mirror that adaptively shapes the spatial profile with a genetic algorithm and a UV-pulse stacker that consists of four birefringent α-BBO crystal rods for temporal shaping. Using this 3D-shaped pulse, a normalized emittance of 1.4 π mm mrad was obtained in 2006. Utilizing laser's Z-polarization, Schottky-effect-gated photocathode gun was proposed in 2006. The cathode work functions are reduced by a laser-induced Schottky effect. As a result of focusing a radially polarized laser pulse with a hollow lens in vacuum, the Z-field (Z-polarization) is generated at the cathode.

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  We have reviewed new phenomena in physics related with development of single-atom electron sources. A collimated electron beam was emitted from the single-atom situated at the top of the nano-pyramids. The following three topics have been discussed. (1) High brightness of electron beam: High-density electrons come out of the source, and overlapping of wave functions presumably produces new phenomena including anti-bunching of electrons in vacuum. Energy spectra showed characteristic features of single-atom electron sources; additional shoulders appeared in the normal spectra. 3) Stable electron emission originates from the field evaporation.

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  High-brightness electron gun technologies for high resolution electron microscopes are reviewed. High performance electron beam apparatuses today are equipped with either Schottky emission or field emission type cathodes, both of which have sharply etched tips for electric field enhancement that promotes electron emission. One of the key elements in these pointed cathodes is a proper control of the tip geometry. It substantially affects the emitter optics and performance. It is shown that the geometry is dictated by the faceting of the tip, which is in turn determined by the Equilibrium Crystal Shape (ECS). The ECS is the tip geometry that minimizes the surface free energy and dependent on the emitter operation environment. By proper choice of the tip field and temperature, one can control the degree of faceting and achieve optically desirable tip geometries.

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  A low-emittance electron gun has been developed for the x-ray free electron laser facility at SPring-8 that was named as SACLA. The gun uses a single-crystal cerium hexaboride (CeB₆) cathode as a thermionic emitter to produce a low-emittance electron beam. A pulsed high-voltage of 500 kV is applied on the cathode to obtain more than 1 A peak current without emittance degradation. A normalized emittance of the beam was measured to be 0.6 mm mrad. This value is satisfied with a requirement of SACLA. The CeB₆ electron gun is briefly reviewed in this paper.

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  A reaction model is discussed for in-situ boron-doped silicon chemical vapor deposition that uses Si₂H₆ and B₂H₆. First, the elementary reactions were studied by using an ab initio molecular orbital method and the first-principle molecular dynamics method. These showed that the B₂H₆ decomposes into BH₃ easily and that BH₃ has a high probability of sticking to silicon film. Therefore, the boron was doped in the silicon film by BH₃. The desorption energy of H₂ terminated silicon film surface was lower due to the boron doping in the silicon film. Then, an increase in the boron concentration of the silicon film increased the sticking probability of the Si₂H₆. Next, a deposition model was made on the basis of these results, and reaction constants were determined by using experimental results. Finally film thickness distribution was simulated by using a flow-diffusion-reaction analysis and an integrated method on the above deposition model. The result shows that our reaction model can replicate experimental step coverage.

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  An elemental species that bonds oxide and metal at the interface of electronic devices is crucial to the electric properties of the devices. In this study, interface between alumina and metal in general is discussed from a viewpoint of thermodynamics. By introducing a concept of chemical equilibrium and developing a new way of estimating bonding energy at the interface, a novel numerical formula to predict a stable interface bonding species at a metal/alumina interface is proposed. The effectiveness of the formula was examined by experimental results in references. It was revealed that the prediction by the formula agreed quite well with the results reported in the references. The formula uses only basic quantities of pure elements and the formation enthalpy of oxides. Therefore it can be applied for most of metals in the periodic table and is useful for material screening in developing interfaces with particular functions.

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