High Brightness and High Polarization Electron Source for Spin-LEEM

A point-like emission mechanism is required for a GaAs polarized electron source to produce an electron beam with high brightness. This is realized by changing the direction of injection laser from a front-side to a back-side of the photocathode. Based on this concept, a 20 keV gun (JPES-1) was constructed and a transmission photocathode including an active layer of a GaAs-GaAsP superlattice was installed. It produced a laser spot diameter as small as 1.3 μm for the 760∼810 nm laser wavelength, and the brightness of ∼2x107 A·cm-2·sr-1 corresponding to a reduced brightness of ∼1.0x107 A·cm-2·sr-1·V-1 was obtained for an extracted current of 5.3 μA. This brightness is still smaller than those of W-field-emitters, but one order of magnitude higher than those of LaB6 emitters. The peak polarization of ∼90% was achieved at the same time by the photocathode which has an inserted GaAs thin layer between the GaAsP substrate and the GaAaP buffer layer. It demonstrated that the strain-control of the GaAsP buffer layer is indispensable to achieve the highest polarization. The charge density lifetime of 1.8x108 C/cm2 was observed for an extracted current of ∼3 μA. The second gun system (JPES-2) was already constructed and mounted to a LEEM apparatus operated at OECU.