LEED-Auger characterization of GaAs during activation to negative electron affinity by the adsorption of Cs and O

A combination of low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) has been used to study the formation of the negative electron affinity (NEA) condition on surfaces of p-type, degenerate, (100) and (111) GaAs. Activation to NEA is achieved by adsorbing Cs and O onto atomically clean GaAs in repetitive cycles of first Cs and then O. Before activation, the clean GaAs surfaces exhibit their characteristic LEED patterns. However, once obtained, there is no significant correlation between the quality of these LEED patterns and the final activation. The adsorption of both Cs and O during activation to NEA is amorphous. Auger measurements have shown that the first photoemission maximum occurs after the adsorption of about a half monolayer of Cs. The initial O adsorption occurs on the GaAs surface between the Cs atoms. The adsorbed O interacts strongly with Cs at any stage during the activation. Peak photosensitivities, after completion of the Cs and O adsorptions, were in the range 400 to 1100 $\text{μA}\text{lumen}$. The final activation does not correlate with the quantity of Cs and O on the surface. The temperature dependence of the photosensitivity of NEA GaAs (100) activated at ?170°C has a broad maximum at about ?50°C and a subsidiary maximum at about 160°C. In addition, the photoemission at ?170°C can be either increased or decreased by having heated the sample up to 200°C, even though no Cs or O desorption has taken place. These results can be traced to changes in work function rather than to changes in bulk properties. While the LEED patterns from clean GaAs show no structural changes with temperature, such changes are observed when Cs is on the surface. It is suggested that changes both in photoemission and in LEED patterns are due to the temperature-induced mobility of Cs on GaAs. An atomic model for the NEA surface is discussed in terms of a layer of Cs and O atoms about 10 Å thick on the GaAs.