Semiconductor photocathode
In the case of a thick light-absorbing layer 2 , a phenomenon of a decrease in the time resolution occurs. However, when the thickness of the light-absorbing layer 2 is limited, a portion of low electron concentration in one electron group is cut out, and hence overlap regions of adjacent electron concentration distributions decrease. Therefore, by shortening the transit time necessary for the passage of electrons, regions of overlapping electron distributions due to diffusion can also be suppressed. Furthermore, the strength of an electric field within a light-absorbing layer can be increased by thinning the light-absorbing layer. Therefore, the time resolution of infrared rays can be remarkably improved by a synergistic action of these effects. If it is assumed that the time resolution is 40 ps (picoseconds), for example, when the thickness of a light-absorbing layer is 1.3 μm which is nearly equal to the wavelength of infrared, then a possible time resolution is 7.5 ps when this thickness is 0.19 μm.
1. A semiconductor photocathode which includes a light-absorbing layer made of a compound semiconductor absorbing infrared rays and emits electrons in response to the incidence of infrared rays, wherein the light-absorbing layer is formed between an electron transfer layer, which has an energy band gap wider than an energy band gap of this light-absorbing layer, and a semiconductor substrate and the thickness of the light-absorbing layer ranges from 0.02 μm to 0.19 μm inclusive.
2. The semiconductor photocathode according to claim 1 , wherein the light-absorbing layer is thinner than the electron transfer layer.
3. The semiconductor photocathode according to claim 1 , wherein the semiconductor substrate is made of InP, the light-absorbing layer is made of InGaAsP, and the electron transfer layer is made of InP.