Negative Electron Affinity Photocathodes on Non- (001) Surfaces

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Negative electron affinity (NEA) photocathodes have high sensitivity in a wide range of wavelength, and are practically Important. There are strong demands for sensitivity and lifetime improvements. In this research project, new NEA photocathode formed on high index surfaces and structure of the NEA surface are examined. In addition to (001) surface conventionally used for NEA GaAs photocathodes, GaAs photocathodes are fabricated on (111)A and (311)A surfaces. As a result, a critical temperature is found in heat cleaning process, and higher sensitivity is obtained on non-(001) surfaces. In order to investigate the surface structure of NEA substrates, growth by MBE and ex situ, atomic scale imaging by STM are carried out using MBE-STM combined system in Imperial College, University of London. Based on the experience, the characteristic terrace structures on (111)A and (311)A are clearly observed by our own made MBE-STM system. Clarification of the surface structure and NEA mechanism wou … More ld contribute to further improvements of NBA photocathodes.In addition to the collaboration on NBA photocathodes on high index surfaces, we found out common interest on opto-electronic device fabrication, which has been studied independently in both institutes, especially semiconductor laser with quantum structure. As a basic research of opto-electronic device fabrication, we have grown InGaAs/GaAs multi quantum well (MQW) structure using molecular beam epitaxy (MBE). The maximum PL intensity is obtained for the sample grown under V/III = 3.8. Growth and evaluation on our patterned susbstares by STM-MBE combined system In Imperial College indicates potentialities of the atomic scale In-situ observation on prepatterned substrates. We have also shown the formation of pyramid structures on the square mesa by the growth of MBB.The above mentioned results, we believe, are the first step for opto-electronic device application and worth carrying a stage further. Professor Joyce have suggested us to extend our collaborative activities. Less