Photoelectron spectroscopic investigations and exoelectron emission of CVD diamond surfaces modified with oxygen and potassium

The surface chemistry of oxygen and alkali (K) treated CVD diamond surfaces was studied with X-ray induced photelectron spectroscopy (XPS) and UV-light induced photoelectron spectroscopy (UPS). The oxygen modification was achieved ex-situ using oxidizing acids (hydrochloric and nitric acid) or in-situ by evaporation of K from a SAES getter source. XPS analysis of the C 1s and O 1s spectra demonstrated that the acid treatment leads to a considerably uptake of oxygen (or OH groups). Owing to the presence of surface oxygen the C 1s peaks splits into two components with binding energies of 284.2 (diamond) and 287.3 eV (carbon bonded to oxygen). The adsorption of K was followed with XPS and UPS (HeI) spectroscopy. With XPS the evaluation of the K 2p states demonstrates that the amount of K which can be accommodated at 300 K depends on the oxygen precoverage; the maximum obtainable K coverage increases with the surface oxygen concentration. In the presence of K the C 1s state from the C-O bonding shifts to lower binding energy, 286.4 eV. With UPS a broad unstructured emission probably from carbon 2p+2s states is detected between 3.5-10 eV below E(F) from the untreated diamond surface. The emission between 0.0-2.5 eV was fairly low (band gap of diamond), it could be considerably reduced by the acid treatment of the sample. This is suggested to be owing to the removal of graphitic or amorphous carbon impurities. Owing to the presence of oxygen the broad emission peak shifted to higher binding energy (from 6.8 to about 8.0 eV). Low doses of K lead to a reduction of the 2p+2s emission from carbon and oxygen, with a center of the reduction at 5.0 eV. For higher K coverages new K-induced states evolves. One state is located in the band gap of diamond near E(F); further peaks were found at 3.5, 7.0 and about 14 eV below E(F). K covered diamond surfaces showed a strong exoelectron emission after oxygen exposure in vacuum.