High Brightness and High Polarization Electron Source for Spin-LEEM

Nakanishi, T.; Yamamoto, N.; Mano, A.; Nakagawa, Y.; Konomi, T.; Yamamoto, M.; Okumi, S.; Jin, X.; Ujihara, T.; Takeda, Y.; Ohshima, T.; Kohashi, T.; Yasue, T.; Koshikawa, T.; Saka, T.; Kato, T.
August 2009
AIP Conference Proceedings;8/4/2009, Vol. 1149 Issue 1, p1148
Academic Journal
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 ∼2×107 A·cm-2·sr-1 corresponding to a reduced brightness of ∼1.0×107 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 1.8×108 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.


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