Research On Optical And Photoemission Performances Of Transmission-mode GaAs Photocathode

The researches on the optical property and photoemission performance theories of the transmission-mode GaAs photocathodes, structural design and performance test softwares, performance evaluation of the MBE-grown photocathodes, and structural design and experiments of the MOCVD-grown photocathodes with different response properties were carried out in this thesis.Considering the lack of researches on the optical property, the calculation formula of optical property for the transmission-mode GaAs photocathode was derived based on thin film optical theory, where the photocathode module consisted of the glass substrate, the Si3N4antireflection layer, the Ga1-xAlxAs window layer and the GaAs active layer. The influences on the optical property curves corresponding to different thicknesses, refractive indexes and extinction coefficients were analyzed excluding the glass substrate. Besides, the quantum efficiency formula was deduced by taking into account the optical property. Following that, the relation between the absorptivity for optical property and the quantum efficiency for photoemission performance was discussed. Furthermore, the influence of optical property on the photoemission performance was investigated.The software on the structural design of the transmission-mode photocathode was developed on the basis of the theoretical calculation of the optical and photoemission performance. After entering the characteristic parameters of the wideband or narrowband response photocathodes, the structural result and the theoretical curves could be obtained by the software auto-calculation. The software on testing the optical and photoemission performance of the photocathodes was developed on the basis of the relationship between the optical property and photoemission performance. By means of inputting wavelength, structure parameters, experimental curves etc, the relating performance parameters could be computed by the software, which consequently realizes the automatic performance measurement for the transmission-mode photocathodes and affords a non-contact thickness measurement method.With the application of the performance measurement software, the experimental reflectivity and transmittance curves of transmission-mode GaAs photocathodes in varied structures grown by MBE were well fitted, and the thickness of each thin film in the cathode module was obtained, which reveals that a transition layer with the low Al component was presented between the window layer and the active layer for the MBE-grown GaAs photocathode. The experimental quantum efficiency curves were fitted by the formula modified with the optical property, and the photoemission performance of the various photocathodes were evaluated, which illustrates that the structure of the MBE-grown GaAs photocathode should be revised due to the low shortwave response and the poor spectral characteristic parameters. In addition, the experimental quantum efficiency curve of the high performance GaAs photocathode from American ITT Company was effectively fitted in order to gain the optimal structure ranges. The result can be used to guide the material design and cathode preparation of the domestic GaAs photocathodes.In view of the low shortwave response and the poor spectral characteristic of the MBE-grown photocathode, a wideband response standard GaAs, an extended blue GaAs and a narrowband response GaAlAs photocathodes were designed. Three kinds of transmission-mode module structures, Al components, thicknesses and doping distributions were calculated separately according to the required wideband or narrowband spectral response. The three designed photocathodes were grown by MOCVD, and their measured optical properties and spectral response curves were fitted by use of the performance measurement software. It is found that the MOCVD-grown wideband response GaAs photocathodes obtain better performance parameters than the MBE-grown ones. The highest integral sensitivity of the extended blue GaAs photocathode achieves1980μA/lm, meanwhile the narrowband response GaAlAs photocathode realizes the highest spectral response at the expected wavelength of532nm. The highest integral sensitivity of the standard GaAs photocathode achieves2320μA/lm, which is in accord with the released sensitivity level of American ITT Company.