31st Annual SSRL Users' Meeting

The activation of InP(100) by Cs and O (Cs/O) co-deposition is critical to the achievement of Negative Electron Affinity (NEA) for applications in infrared detectors and photo-cathodes. The physical and electronic structure of the Cs/O co-adsorbed layer on InP(100) and its role are realizing a NEA state where the vacuum level is brought below the conduction band minimum in the bulk, is not yet fully understood. In order to better understand the surface chemistry, Synchrotron Radiation Photoemission Spectroscopy (SR-PES) measurements have been undertaken to follow the evolution of the valence band and core levels at different photon energies and at different incidence angles. Fig. 1(a) shows the In4d and Cs5p core levels at kinetic energies of 68 and 74 eV respectively for different times after activation. The remaining higher kinetic energy features comprise the oxygen HOMO and are dominated by two principal peaks at 80.5 (B) and 82.5 eV (A) respectively. A good activation, with high quantum efficiency is characterized with a large A to B intensity ratio. With time this ratio drops. Interestingly, the trend of peak A is paralled by the value of quantum efficiency. Fig. 1(b) shows that two principal features are present in the corresponding O1s spectra and that the relative intensity behavior of these peaks follows that of A and B. Fig. 2(a) shows the angular dependence of the interaction of the incident linearly polarized E field with the oxygen HOMO. Peak A is greatly diminished for S polarization and enhanced for P polarization. This polarization dependent behavior along with the extent of the chemically shifted O1s component leads us to believe that after activation at room temperature, the resulting Caesium oxide/InP(100) layer contains molecular oxygen peroxide species (corresponding to peak A) sandwiched between Cs layers and oriented parallel to the surface normal. This polar layer gives rise to the surface dipole which results in the achievement of a NEA surface. We propose that with time the peroxide species evolves to superoxide species and also oxidation of the underlying bulk InP occurs at the same time due to unavoidable adsorption of oxygen from the residual pressure. This assignment is tentative and difficult to evaluate conclusively at room temperature.

Fig.1 (a) Valence band spectra at hv=85eV (b) O1s

Fig.2 (a) Valence band spectra at hv=70eV spectra at hv=610eV at different decaying times at different incident angle (b) beam configurations