Ultraviolet photoemission study of cesium oxide films on GaAs

Ultraviolet photoemission energy distribution curves (EDC’s) have been measured from a GaAs (110) surface covered with Cs oxide layers of varying thickness. There is no evidence of emission from Cs oxide in the EDC’s from GaAs with a surface treatment that produces optimum yield, but structure characteristic of the GaAs is present in the EDC’s. However, EDC’s characteristic of bulk Cs oxide were measured from GaAs with a thick (at least several molecular layers) Cs oxide surface layer, but no structure characteristic of the GaAs was present in the EDC’s. Compared to the GaAs with the optimum surface treatment, this thick Cs oxide film produced yield spectra having a higher photoemission threshold and a lower yield throughout the photon energy range studied (1.4⩽ hν⩽11.6 eV). These measurements indicate that the Cs oxide layer required for activation of GaAs to negative electron affinity is so thin that the Cs oxide layer does not have bulk properties which can be detected by photoemission. Measurements similar to those reported here on narrower-band-gap III-V alloys should determine if the thickness of the Cs oxide layer required for activation increases with decreasing band gap or is independent of band gap.