Negative electron affinity of cesiated p-GaN(0001) surfaces

The adsorption of cesium on clean n- and p-GaN(0001)-1×1 surfaces at 150 K was investigated using x-ray photoemission spectroscopy, photoemission spectroscopy with monochromatized He I radiation ultraviolet photoelectron spectroscopy (UPS) and a Kelvin probe (contact potential difference, CPD). The CPD measurements gave work functions of 3.88±0.15 and 3.6±0.15 eV for clean n- and p-GaN(0001) surfaces, respectively. The widths of UPS energy distribution curves yield an ionization energy of 6.8±0.15 eV. Thus, depletion and inversion layers exist at clean surfaces of n- and p-GaN(0001) surfaces, respectively. As a function of Cs coverage, the work function displays the well-known behavior in that it first decreases, passes through a minimum, and eventually reaches a value of 2.1 eV, the work function of metallic cesium. In the submonolayer coverage regime, the ionization energy decreases by 2.3±0.15 eV. At clean p-GaN(0001) surfaces the vacuum level lies by only 0.3 eV above the conduction-band minimum in the bulk. Already minute amounts of Cs suffice to produce negative electron affinity. The Schottky barrier height of metallic Cs films on n-GaN(0001) is determined as 0.2±0.15 eV. This value is in good agreement with what is predicted by the MIGS and electronegativity model.