The semiconductor field-emission photocathode

The recently developed large-area field-emission photocathode is described. It consists of a finely spaced array of point emitters fabricated by etching of p-type silicon or other semiconductor. Uniform emission over areas of 6-7 cm²have been obtained. For Si, the spectral response extends from 0.4 to 1.1 µm. Quantum yields of 25 percent at 0.86 µm have been measured, which is about five times the value reported for the extended S-20 photocathode and comparable to the best III-V photoemitters. Calculations indicate that quantum yields of up to 40 percent at 0.86 µm and 28 percent at 0.9 µm are attainable with the present photocathode structures. For low dark current densities, photocathode cooling to temperatures approaching 77 K must be employed at present. The dark current is shown to be dominated by surface-generated electrons in the space-chargeregion of the emitters. Effects of phosphorus gettering and annealing treatments on dark current are discussed, and the spatial frequency response of the device is determined. The results of a computer study show that the field intensification factor of p-semiconductor field emitters behaves quite differently from that of metallic emitters.