Photoelectron spectroscopy study of the K-covered ZnO(1 0 1̄ 0) surface; annealing-induced changes in the electronic structure and the chemical composition

The electronic structure and the chemical composition of the K-covered ZnO(1 0 1̄ 0) surface at temperatures between 300 and 1200 K are investigated by X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy. Adsorption of K on ZnO(1 0 1̄ 0) at room temperature results in the formation of a two-dimensional disordered K overlayer and induces 0.2 eV downward bending of the substrate’s bands going from the bulk to the surface. Upon annealing the K-covered surface, initial downward bending turns to upward bending with maximum bending of 0.5 eV at 700-800 K. The thermally induced migration of the bulk O atoms and the resultant increase in the number of the O atoms on the surface is responsible for upward bending on the annealed surface. The accumulated O atoms interact with the predeposited K atoms on the surface to form non-stoichiometric K-O complexes with the O/K atomic ratio being 1.6-1.8 in the temperature range between 600 and 1000 K.