Mean transverse emission energy and surface topography on GaAs (Cs,O) photocathodes

Abstract

A study has been made of the relationship between the facetting and surface structure observed upon (111)B and (100)GaAs photocathode surfaces and the mean transverse emission energy of the photoelectrons emitted from these cathodes. It is concluded that the mean transverse emission is largely independent of the degree of facetting and the presence of surface structure; however, a relationship exists between the temperature at which the cathode surface is cleaned and the mean transverse emission energy.

In situ surface study of the activating layer on GaAs (Cs, O) photocathodes

“…The scattering process will tend to mask variations in ET due to variations in surface topography and this is in agreement with the results of earlier work (Rodway and Bradley 1984). In the Si ( C S , 0) system the layer is highly ordered and 1, would be expected to be long.…”

“…It is possible to make an estimate of the quantities of caesium and oxygen in a singletemperature layer from the time needed for the activation. It has previously been reported (Rodway 1984) that the caesium coverage at the initial photo-emission maximum is 3 of a monolayer. It has also been reported (van Bommel et a1 1978) that the sticking coefficient for caesium up to this first maximum is unity.…”

“…The results reported here were obtained using co-evaporation of caesium and oxygen rather than alternate deposition as our studies have shown no difference in the final activation between the two methods and co-evaporation is better suited to the study of the work function variation with layer thickness. The caesium coverage at the initial photo-emission maximum has previously been established as 3 of a monolayer (Rodway (1984)) (where a monolayer is defined as 7.9 X l O I 4 atoms cm") and this figure has been used.to correct all the results given here to a constant incident caesium flux (in this case 1 monolayer in 12 min) throughout the work. Oxygen admission was typically at a partial pressure of 1-5 X lo-" Torr.…”

“…We have therefore examined some of the cathode changes involved in the decay process. Although long-term life tests and tests at high caesium overpressures cannot easily be carried out in a research chamber it was possible to obtain useful information on the quantities of activant involved in the decay by using the channel calibration previously obtained (Rodway 1984). Figure 12(a) shows typical decays for single-and two-temperature activated cathodes in a chamber that has been passivated by the admission of oxygen at a pressure of 1 X lo-' Torr for 1 h prior to activation to reduce the caesium overpressure.…”

“…Figure 12(b) records the restoration of photosensitivity after decay by exposing the photocathode to CS. The CS flux was known (Rodway 1984) and thus the equivalent CS loss during decay could be derived ( figure 12(c)). This shows that the (CS, 0) layer is very sensitive to deviations from the optimum stoichiometry and that only a small percentage of the layer appears to control the photosensitivity.…”