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(Color online) As shown in the inset, a size metal grid is lithographically patterned at the center of the GaN substrate to increase the electrical conductivity of the photocathode. After the sample cleaning, we deposit a CsBr film on top of the GaN film. We show a schematic drawing of the cross section view of the photocathode structure (not in scale). The cathode is mounted on a molybdenum sample holder that is kept at room temperature.
(Color online) We show a photocurrent curve of a photocathode in transmission mode under , laser illumination. The laser spot size is less than . The photoyield increases from zero to larger than (0.4%) in less than . This photoyield is about four times of the maximum photoyield of a cathode. The photoyield decreases with time, it drops below after . Under such a condition, we can operate the CsBr photocathode with more than for at least .
We show the energy diagram of the heterostructure to explain photoemission photocathode. The electron affinity of the CsBr is reduced after the surface becomes Cs rich under illumination. A possible negative electron affinity can be achieved (Ref. 7). Many photoelectrons generated inside the GaN film are injected into the CsBr film. After activation, the energy barrier at the surface is reduced. More electrons can escape from the surface.
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