Electronic properties of cesium-covered GaN(0001) surfaces

Cesium was deposited on clean n-GaN(0001)-1 × 1 surfaces at 150 K using Cs dispensers. X-ray photoemission spectroscopy showed that the Cs-films grow layer by layer. Cesium-induced variations of electronic surface properties were observed using photoemission spectroscopy with monochromatized He I radiation (UPS) and a Kelvin probe (CPD). The UPS data recorded with clean GaN(0001) surfaces give their ionization energy as 6.8 ± 0.1 eV, so that their electron affinity measures 3.35 ± 0.1 eV. The work function of clean GaN(0001) surfaces was determined as 4 ± 0.2 eV. Cesium deposition first reduces the work function by up to 2.7 eV and then increases it until eventually a work function of 2 eV is reached. A simple point-charge model easily explains this decrease of the work function by the formation of surface dipoles. The latter value is characteristic of bulk cesium.