Photoemission properties and hydrogen surface coverage of CVD diamond films

摘要

UV assisted photoelectron emission measurements are carried out using a xenon lamp as the excitation source.Free-standing diamond surfaces treated at diverse experimental conditions give rise to varying photocurrent intensities.In order to gain a better understanding of these phenomena,different surfaces are prepared:as-grown,hydrogenated,oxidized and annealed.The hydrogenated film is obtained from optimised treatment in hydrogen plasma.The highest emission photocurrent is obtained with the hydrogenated diamond film in agreement with in situ surface conductivity values.Moreover,in situ photoelectric measurements performed in a vacuum chamber demonstrate that the emission photocurrent increases when the sample is exposed to air.Indeed,it is now agreed that the formation of an adsorbate layer on hydrogenated surfaces leads to a decrease of the work function.Two electrons emission thresholds are observed:the highest energy emission is close to the bandgap value (5.5 eV)and the lowest is located near 4.5 eV corresponding to defect levels in the bandgap.Both emissions are dependent on the surface coverage but the main striking result is the increase of the intensity of the 4.5 eV emission for hydrogenated diamonds in contact with air.Optical absorption experiments are compared with the photoconductivity measurements.Laser excited photoluminescence at 514 nm revealed defects corresponding to a 690 nm broad band emission sensitive to the surface coverage.