Negative electron affinity at the Cs/AlN(0001) surface

The effects of cesium (Cs) adsorption on band bending and electron affinity at the AlN(0001)-1×1 surface are investigated via ultraviolet and x-ray photoemission spectroscopy. The movement of the Fermi level indicates an initial interaction between Cs and empty surface states, followed by an increase in band bending presumably linked to metallization. The electron affinity, χ, of the clean AlN surface is positive and equal to 1.9±0.3 eV. The Cs-surface dipole layer decreases χ by 2.6±0.3 eV, leading to evidence of true negative electron affinity at the surface of this important material.