A SIMS study on positive and negative ions sputtered from graphite by mass-separated low energy Ne +, N 2+ and N + ions

Abstract

Temperature effect on positive and negative ion yields sputtered from graphite by low energy Ne +, N 2 + and N + ions was studied using a special mass-separated low energy ion beam system and secondary ion mass spectrometry (SIMS) measurements. The origin of dominant positive and negative ions was discussed according to the obtained temperature effect. It was found that C + ion emission was enhanced by nitrogen ion bombardment compared with Ne + bombardment and decreased to the same level as Ne + bombardment at elevated temperature. The enhancement effect was attributed to adsorption or deposition of nitrogen to form weakly bound C δ+–CN δ− species on the surface. No chemical enhancement effect was observed on C 2 − emission during nitrogen ion bombardment. The C 2 − yield increased with temperature during Ne + and nitrogen ion bombardment and was assigned to originate from carbon network of graphite as a consequence of physical sputtering. During nitrogen ion bombardment, CN − ions dominated both the positive and negative ion emission. CN − ion yield during 800 eV N 2 + and N + bombardment decreased at elevated temperature, whereas during 100 eV N 2 + bombardment, it increased with temperature. CN − yield as a function of kinetic energy of primary ions also exhibited difference from that of C + and C 2 −. Three possible channels for CN − emission have been proposed. At higher primary energy (several hundred eV and above), physical sputtering can partly account for CN − emission. At lower primary energy (below 100 eV), it is attributed to chemical etching of surface carbon atoms by energetic nitrogen atoms and ion-induced desorption of CN species.