| 摘要 |
A phototube (10) comprises a photocathode (30) having photoelectric surface. In a sealed enclosure (20) whose inside is vacuum, the photoelectric cathode (30) and an anode (40) are opposed to each other. Voltages are applied to them through lead pins (51 and 52). The photocathode (30) includes a metallic support plate (31) to which is secured a sapphire plate (32) on which are formed an a-AlN matching layer (33), a p-type GaN active layer (34), and a CsO surface layer (35). The active layer (34) has a dopant concentration that increases from 1 x 10<16> cm<-3> in the surface up to 5 x 10<17> cm<-3> at a depth of 100 nm. The dopant concentration only at the deepest region over a thickness of several nanometers is 1 x 10<18> cm<-3>. The crystallinity of the active layer (34) is improved, and the diffusion length is increased, improving the quantum efficiency and sharp-cut property.
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