Abstract
p-GaAs (110) surfaces activated to negative electron affinity (NEA) have been examined with photoelectron spectroscopy. A typical activated GaAs surface is found to consist of both a layer of oxygen bonded to GaAs and a (Cs+,O−2) layer. The GaAs-O layer was not anticipated prior to this work. A GaAs-O-Cs dipole plus the polarization of a Cs+-O−2-Cs+ sandwich layer is proposed to explain the NEA condition based on the structure of activated surfaces found in this work. The identification of the O-GaAs bonding layer explains the better yield achieved by the two-step activation process compared to that achieved by a single-step process. Possible optimization of the activation process by forming the O-GaAs layer and the (Cs+,O−2) layer is also discussed. Adsorption of OH from the residual gas in an ultrahigh vacuum chamber is identified as one degradation mechanism of the GaAs photocathodes.
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