Monochromatic Photo-Field Electron Emission Sources

Laser-pulsed photo-field-emission sources with high coherence and brightness are needed for time-resolved electron microscopy. Our ongoing work explores the possibility of using sharpened semiconductor electron emitters to achieve this goal. Intrinsic GaAs field-emission sources have been prepared from cleaved needles that are clipped into refractory metal holders. These needles are chemically sharpened and surface cleaned by field-desorption and electron-bombardment heating. Field emission I-V curves have been analyzed, with and without laser illumination, which demonstrate a range of metallic and semiconductor characteristics. He-Ne laser illumination has been observed to increase field emission currents by more then an order of magnitude. The band structure of a semiconductor can be used to create a lower bound on the energy of photo-excited field-emitted electrons, producing a beam whose energy width is E=Eg-h$\nu $ (bandgap Eg). Energy analysis is planned using a hemispherical analyzer, aimed at achieving an energy spread less than the 0.26 eV of conventional W tips. Calculations are underway to understand complications arising from surface effects and bulk transport. NNSA award DE-PS52-05NA funds this research.