Design and research of new type photoemitters of polarized electrons

Exploitable results

The goal of the present INTAS project was the development of new generation III-V-semiconductor photocathodes that may emit spinpolarised electrons with a degree of polarisation above 50%. Application of elastic strain to the emissive cathode layer or growth of a superlattice III-V structure are the most promising measures. The participating laboratories supplied various new semiconductor heterostructures applying Liquid-Phase-Epitaxy- and MOCVD-techniques respectively. Strained layers of InGaAsP from Novosibirsk possess a very high P2 xY = 0.552 x 0.03 = 9 x 10-3 (P = Polarisation, Y = Quantum Yield) not reached by any other known cathode up to now. A unique technique developed in Novosibirsk allows to fabricate transparent InGaAs-photocathodes that may be of great importance for light detection applications. Strained InGaP-cathodes are successfully working in the NIKHEF-source of polarised electrons in Amsterdam. Strained layers of GaAsP from St. Petersburg emit electrons polarised to 75% if irradiated with circularly polarised light of a wavelength around 835 nm. The quantum efficiency may be as high as 0.4%. Cathodes of this type are successfully run in the source of polarised electrons MAMI in Mainz. A new resonance phenomenon has been observed in the spinpolarisation of photoelectrons emitted from strained semiconductor photocathodes with DBR-backing. Strained GaAlAs-GaInAs and other superlattice crystals may show superior quality factors in the near future. But samples from Ioffe Institute tested are not better than the simple strained layer cathodes yet. Several participating groups will continue joint R&D of superlattice structures beyond the duration of the present project. A diffusion model developed by A. Subashiev and collaborators very effectively describes light absorption and electron emission in semiconductors. A computer code for Monte-Carlo numerical calculations of the electron dynamics in thin layers was developed. Both accelerator groups participating in the project, NIKHEF and MAMI, took great advantage from the co-operation with the Russian participants in Novosibirsk and in St. Petersburg respectively. The source of polarised electrons at NIKHEF was built, attached, and run with the help of Professor Terekhov and his co-workers, the MAMI source uses very successfully cathodes developed in St. Petersburg. In both cases the collaboration will continue beyond the end of the present project. Publication activity was very lively. There are 20 papers in international research journals that are related to the work done in the present project, 33 contributions to international and 4 to national conferences.

Exploitable results

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