Electronic states and effective negative electron affinity at cesiated p-GaN surfaces

The electronic structure of, and the effects of cesium (Cs) and oxygen (O) adsorption on, the p-GaN(001) surface are investigated via photoemission sp ectroscopy. Bulk- and surface-sensitive photoemission measurements, and oxy gen exposure of clean surfaces, demonstrate the existence of filled surface states which extend similar to 0.6 eV above the valence band maximum. The valence band maximum measured after the removal of the surface states gives a downward band bending and electron affinity equal to 1.2 +/- 0.2 and 3.3 +/- 0.2 eV, respectively. The surface dipole layer induced by exposure to oxygen followed by Cs deposition lowers the vacuum level by 2.8 +/- 0.3 eV with respect to the valence and conduction band edges. Under these conditio ns, the vacuum level is approximately 0.7 eV below the conduction band mini mum of the bulk, corresponding to the effective negative electron affinity at this surface. (C) 1999 American Institute of Physics. [S0021-8979(99)044 18-7].