Early stage of Cs activation mechanism for In0.53Ga0.47As (0 0 1) β2 (2 × 4) surfaces: Insights from first-principles calculations

The negative electron affinity (NEA) InxGa1−xAs photocathodes are of particular importance as a fundamental building block for the near infrared sensitivity image intensifier. However, a comprehensive understanding is still lacking of the activation mechanism in order to form NEA state. Here we perform a detailed theoretical research on the early stage of Cs adsorption on the As-rich β2(2 × 4) reconstruction of In0.53Ga0.47As (0 0 1) surface by using first-principles calculations within the framework of density functional theory. Adsorption energies, work functions, dipole moments, band structures and density of states of In0.53Ga0.47As (0 0 1) β2 (2 × 4) surface corresponding to different Cs coverages are investigated. Calculation results indicate that Cs adsorption on In0.53Ga0.47As (0 0 1) β2(2 × 4) surface becomes more and more difficult as Cs coverage increases. Cs-induced [Csn+-In0.53 Ga0.47Asn-] dipoles are perpendicular to the substrate, which will decrease the work function and make the electrons easier to escape to the vacuum. However, excessive Cs adatoms will lead to the dipoles repelling each other, and some electrons return back to the substrate due to the electrostatic depolarization, resulting in the appearance of “Cs-kill” phenomenon and the work function rises again. Consequently, the optimal Cs coverage corresponding to the peak value of the InGaAs photocurrent should be considered in order to prevent the absence of the “Cs-kill” phenomenon. Meanwhile, there emerge new valence bands induced by Cs adsorption ranging from −25 eV to −22 eV and from −13 eV to 9 eV. Due to the combined effect of Cs 5s and 5p state electrons, the Fermi level moves into the conduction band, leading to the adsorption models showing more metallic properties.